Researchers Database

OHFUJI, Hiroaki

    Geodynamics Research Center Professor
Last Updated :2021/01/05

Researcher Information


  • Ph.D.(2005/07 Cardiff University (UK))

J-Global ID

Research Interests

  • 電子顕微鏡   ダイヤモンドアンビルセル   高圧実験   Electron Microscopy   Diamond Anvil Cell   High-Pressure Experiment   

Research Areas

  • Natural sciences / Solid earth science / High Pressure Earth Sciences
  • Natural sciences / Solid earth science / Mineralogy

Academic & Professional Experience

  • 2020/10 - Today  Tohoku UniversityGraduate School of ScienceProfessor
  • 2016/04 - Today  Ehime UniversityGeodynamics Research CenterProfessor
  • 2012/04 - 2016/03  Ehime UniversityGeodynamics Reserch CenterAssociate Professor
  • 2007/04 - 2012/03  Ehime UniversityGeodynamics Research CenterAssistant Professor
  • 2005/10 - 2007/03  Ehime UniversityGeodynamics Research CenterAssistant Professor
  • 2004/11 - 2005/09  Ehime UniversityGeodynamics Research Center研究機関研究員
  • 2004/11 - 2005/09  Ehime UniversityGeodynamics Research CenterPostdoctoral Fellow


  • 2001/09 - 2004/10  Cardiff University  Department of Earth, Ocean and Planetary Sciences
  • 1999/04 - 2001/03  Niigata University  Graduate School of Science and Technology  Earth and Environmental Science
  • 1995/04 - 1999/03  Niigata University  Faculty of Science  Department of Geology

Association Memberships

  • Mineralogical Society of America   American Geophysical Union   Japan Association of Mineralogical Sciences   Japan Geoscience Union   The Japanese Association for Crystal Growth   The Clay Science Society of Japan   

Published Papers

  • Teruki Tsuchiya, Masahiro Fukuda, Hiroaki Ohfuji, Michiaki Yamasaki, Yoshihito Kawamura, Masafumi Matsushita
    Journal of Alloys and Compounds 844 0925-8388 2020/12 [Peer-reviewed]
     Scientific journal 
    © 2020 Elsevier B.V. A Mg80Yb12Cu8 (CY) phase was found in a Mg97Yb2Cu1 alloy quenched after being annealed at 873 K for 10 min; the unit cell dimensions of the CY phase are described as 43×43×2 with respect to those of the original hcp-Mg lattice. The a-axis lattice constant of the CY phase well matches 43 times the a-axis length of the α-Mg phase. In the present work, the c-axis of the CY phase and the dendrite-like α-Mg phase in the precipitate zone are coaxial, i.e., the basal planes are coherent. This result is attributed to the good match between the lattice constants of the fundamental a-axis of the CY phase and the α-Mg phase. According to compression tests, the CY phase is responsible for the increase in strength observed in both the elastic and work-hardening regions in the Mg97Yb2Cu1 alloy. As far as our investigations of Mg97Yb2M1 (M = Al, Ni, Co) alloys, the phase similar to the CY phase have not found. Our discovery of this phase might provide a new path to the discovery of long-period superlattice in various Mg alloys.
  • Tadao Nishiyama, Hiroaki Ohfuji, Kousuke Fukuba, Masami Terauchi, Ukyo Nishi, Kazuki Harada, Kouhei Unoki, Yousuke Moribe, Akira Yoshiasa, Satoko Ishimaru, Yasushi Mori, Miki Shigeno, Shoji Arai
    Scientific Reports 10 (1) 2020/12 [Peer-reviewed]
     Scientific journal 
    © 2020, The Author(s). Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote–blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C–O–H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.
  • T. B. Bekker, K. D. Litasov, A. F. Shatskiy, N. E. Sagatov, P. G. Krinitsin, S. P. Krasheninnikov, I. V. Podborodnikov, S. V. Rashchenko, A. V. Davydov, H. Ohfuji
    Journal of the European Ceramic Society 40 (10) 3663 - 3672 0955-2219 2020/08 [Peer-reviewed]
     Scientific journal 
    © 2020 Elsevier Ltd The Ti-Al-Zr-O system has received significant attention as it hosts a variety of phases of industrial interest. In this paper, new experiments on the Ti-Al-Zr-O system aimed at clarifying phase formation depending on the redox conditions with special attention to the formation of ternary compounds and their results have been reported. For the first time a monophase AlTi2O5 sample with a ratio of titanium oxides TiO2:Ti2O3 = 2:1 was obtained and characterized. Single-crystal X-ray diffraction analysis of β-(ZrxTi1-x)2O4 crystals was carried out. The synthesis of the ternary Ti-Al-Zr compound was performed in an evacuated quartz ampoule and accompanied by the extraction of silicon from the ampoule. The compound has pyrochlore-type cubic structure A24+B23+O7 (Fd-3m) and the composition close to (Ti4+1,47(2)Zr0.41(2)Si0.12(1))2(Al1.15(4)Ti3+0.85(4))2O7. We believe that our study is a starting point for future research as it provides first experimental data on the ternary compound in the Ti-Al-Zr-O system.
  • Dyuti Prakash Sarkar, Ando Jun-Ichi, Kaushik Das, Anupam Chattopadhyay, Gautam Ghosh, Kenji Shimizu, Hiroaki Ohfuji
    Journal of Mineralogical and Petrological Sciences 115 (2) 216 - 226 1345-6296 2020/04 [Peer-reviewed]
     Scientific journal 
    © 2020 Tohoku University. Serpentine mineralogy controls fault rheology in the ocean and continental rift settings to subduction settings and hence can be used to discern the paleo deformational conditions. The Rakhabdev lineament from Rajasthan, India, provides a unique opportunity to understand its tectonic evolution inferred from the deformation microstructures. However, the complexity of surrounding calc-silicate rocks had resulted in a long-driven debate on the origins of these serpentinite rocks. The source rocks of the serpentinites also cannot be determined previously due to complete serpentinization and metasomatism rendering complete alteration of the source rocks. In this study, the serpentinite mineral was analyzed using Raman spectroscopy to accurately characterize its molecular structure. The presence of the antigorite-variety of serpentine mineral indicate towards the origin of Rakhabdev serpentinites in the upper mantle condition. The antigorite serpentinite of Rakhabdev is a hydration product of mantle materials showing high Mg# values obtained from EPMA data. The microstructural and EBSD analysis also indicates two stages of deformation, with deformation of antigorite at upper mantle conditions, followed by their shallow crustal carbonate metasomatism and subsequent deformation of the carbonates, with later stage calcite vein intrusion. This resulted in the appearance of antigorite in contact with calcite, dolomite, talc, tremolite, and chlorite. The exhumation of mantle wedge antigorite serpentinite is, therefore, indicating a paleo-subduction zone culminating in a crustal-scale collision boundary expressed as arcuate discontinuous bodies forming the Rakhabdev lineament.
  • Kanta Fukimoto, Masaaki Miyahara, Takeshi Sakai, Hiroaki Ohfuji, Naotaka Tomioka, Yu Kodama, Eiji Ohtani, Akira Yamaguchi
    Meteoritics and Planetary Science 1086-9379 2020 [Peer-reviewed]
     Scientific journal 
    © The Meteoritical Society, 2020. We investigated the back-transformation mechanisms of ringwoodite and majorite occurring in a shock-melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back-transformed into olivine and enstatite, respectively. Ringwoodite (Fa~18) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, fine-grained polycrystalline olivine becomes dominant instead of ringwoodite. The back-transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface-controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs16–17En82–83Wo1) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back-transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.
  • Hideaki Kawamura, Hiroaki Ohfuji
    High Pressure Research 0895-7959 2020 [Peer-reviewed]
     Scientific journal 
    © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group. Here we report a novel route for synthesizing nano-polycrystalline diamond (NPD) using stearic acid (C18H36O2) as a starting material under high pressure and high temperature. The obtained NPD shows a transparent dark-yellowish color similar to the standard NPD synthesized from graphite and consists of extremely fine diamond grains (∼10 nm). The temperature required for the present synthesis of pure transparent NPD is as low as 1000°C at 13 and 17 GPa, which is surprisingly lower than that for conventional NPD synthesis (1800–2000°C). The amorphous-like, extremely poorly crystalline graphite produced by the thermal decomposition of stearic acid likely provides preferential nucleation sites for diamond and significantly lower the activation energy. The removal of volatile components such as H2O generated through the decomposition from the system is a key to obtain pore-free transparent NPD. Magnesite, MgCO3 and periclase, MgO can be used as an efficient H2O remover through the hydration reaction.
  • Takeshi Sakai, Takehiko Yagi, Ryosuke Takeda, Toshiki Hamatani, Yuki Nakamoto, Hirokazu Kadobayashi, Hideto Mimori, Saori Kawaguchi, Naohisa Hirao, Keitaro Kuramochi, Naoki Ishimatsu, Takehiro Kunimoto, Hiroaki Ohfuji, Yasuo Ohishi, Tetsuo Irifune, Katsuya Shimizu
    HIGH PRESSURE RESEARCH 40 (1) 12 - 21 0895-7959 2020/01 Scientific journal 
    Both micro-paired and conical support type double-stage diamond anvil cells (ds-DAC) were tested using a newly synthesized ultra-fine nano-polycrystalline diamond (NPD). Well-focused X-ray sub-micron beam and the conically supported 2nd stage anvils (micro-anvils) with 10 mu m culet enable us to obtain good quality X-ray diffraction peaks from the sample at around 400 GPa. The relationship between confining pressure and sample pressure depends heavily on the initial height (thickness) of micro-anvils, the difference of a few micrometers leads to a quite different compression path. The conical support type is a solution to retain both enough thickness and strength of micro-anvils at higher confining pressure conditions. All conical support ds-DAC experiments terminated by the failure of the 1st stage anvil instead of 2nd one. The combination of ultra-fine NPD 2nd stage anvil and NPD 1st stage anvil opens a new frontier for measurement of the X-ray absorption spectrum above 300 GPa.
  • K. D. Litasov, H. Kagi, S. A. Voropaev, T. Hirata, H. Ohfuji, H. Ishibashi, Y. Makino, T. B. Bekker, V. S. Sevastyanov, V. P. Afanasiev, N. P. Pokhilenko
    Gondwana Research 75 16 - 27 1342-937X 2019/11 
    © 2019 International Association for Gondwana Research The enigmatic appearance of cuboctahedral diamonds in ophiolitic and arc volcanic rocks with morphology and infrared characteristics similar to synthetic diamonds that were grown from metal solvent requires a critical reappraisal. We have studied 15 diamond crystals and fragments from Tolbachik volcano lava flows, using Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM), synchrotron X-ray fluorescence (SRXRF)and laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS). FTIR spectra of Tolbachik diamonds correspond to typical type Ib patterns of synthetic diamonds. In TEM films prepared using focused ion beam technique, we find Mn-Ni and Mn-Si inclusions in Tolbachik diamonds. SRXRF spectra indicate the presence of Fe-Ni and Fe-Ni-Mn inclusions with Cr, Ti, Cu, and Zn impurities. LA-ICP-MS data show variable but significantly elevated concentrations of Mn, Fe, Ni, and Cu reaching up to 70 ppm. These transition metal concentration levels are comparable with those determined by LA-ICP-MS for similar diamonds from Tibetan ophiolites. Mn-Ni (+Fe)solvent was widely used to produce industrial synthetic diamonds in the former USSR and Russia with very similar proportions of these metals. Hence, it appears highly probable that the cuboctahedral diamonds recovered from Kamchatka arc volcanic rocks represent contamination and are likely derived from drilling tools or other hard instruments. Kinetic data on diamond dissolution in basaltic magma or in fluid phase demonstrate that diamond does not form under the pressures and temperature conditions prevalent within the magmatic system beneath the modern-day Klyuchevskoy group of arc volcanoes. We also considered reference data for inclusions in ophiolitic diamonds and compared them with the composition of solvent used in industrial diamond synthesis in China. The similar inclusion chemistry close to Ni70Mn25Co5 for ophiolitic and synthetic Chinese diamonds scrutinized here suggests that most diamonds recovered from Tibetan and other ophiolites are not natural but instead have a synthetic origin. In order to mitigate further dubious reports of diamonds from unconventional tectonic settings and source rocks, we propose a set of discrimination criteria to better distinguish natural cuboctahedral diamonds from those produced synthetically in industrial environments and found as contaminants in mantle- and crust-derived rocks.
  • Konstantin D. Litasov, Anton F. Shatskiy, Daniil A. Minin, Konstantin E. Kuper, Hiroaki Ohfuji
    High Pressure Research 39 561 - 578 0895-7959 2019/10 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. We present experimental results on melting and subsolidus phase relations in the Ni–Ni2P system at 6 GPa and 900–1600°C. The system has four stable phosphides: Ni3P nickelphosphide, Ni8P3/Ni5P2, Ni12P5 and Ni2P transjordanite. The Ni12P5 phase becomes unstable at 900°C and decomposes into Ni5P2 and Ni2P. The eutectic Ni–Ni3P locates at 950°C and 20 mol% P. Ni3P melts incongruently at 1075°C to Ni5P2 and liquid containing 23 mol% P. Ni8P3 and Ni5P2 form a homogeneity range with crystal structure close to Ni5P2. The Ni5P2–Ni2P eutectic locates at 1175°C and 29 mol%. Ni2P melts congruently at 1250°C. The quenched products of nickelphosphide liquid correspond to the eutectics composition and can serve as indicators for reconstructing the P–T–X conditions recorded in meteoritic and terrestrial Fe–Ni–P assemblages. The Ni–P stoichiometries, which were not observed in the equilibrium phase diagrams, including Ni4P and Ni5P, remain enigmatic.
  • Hiroyuki Kagi, Toshinori Kubo, Ayako Shinozaki, Taku Okada, Hiroaki Ohfuji, Aiko Nakao
    Geochemistry International 57 956 - 963 0016-7029 2019/09 
    © 2019, Pleiades Publishing, Ltd. Abstract: Behavior of nitrogen in the deep Earth still remains unrevealed. Geochemical data suggest that substantial amount of nitrogen could be stored in the deep Earth. In this study, reactions between forsterite (Mg2SiO4) and molecular nitrogen (N2) were investigated at high pressure and high temperature using laser-heating diamond anvil cells (DACs). Pressure in the DAC was estimated from Raman spectra of nitrogen before heating and the initial pressure was set at 5 GPa. Pelleted sample of powder forsterite or a single crystal of forsterite was loaded in the DAC with N2 fluid. A carbon dioxide laser (λ = 10.64 μm, <100 W) and a fiber laser (λ = 1.019 μm, <100 W) were used to heat forsterite in the temperature range from 1300 to 3500 K. An SEM image on the surface of the recovered forsterite crystal after the laser heating showed a stepwise texture which strongly suggests the dissolution of forsterite into the N2 fluid. The EDS chemical mapping showed that Mg-rich area and Si-poor area overlapping each other, which suggests the preferential dissolution of MgO component and its precipitation from the N2 fluid. X-ray diffraction patterns of the powder and single crystal forsterite samples after the reaction showed reflections assignable to orthopyroxene (MgSiO3) and periclase (MgO). The present experimental results indicate that Mg2SiO4 incongruently melts into MgSiO3 and MgO in N2 fluid. Moreover, N1s XPS spectra collected from a single crystal of forsterite after the reaction with N2 fluid revealed three components assignable to NH4+N2, and N3–. The present study provides a new clue to the reaction between forsterite and molecular nitrogen under the upper mantle condition.
  • Naira S. Martirosyan, Konstantin D. Litasov, Sergey S. Lobanov, Alexander F. Goncharov, Anton Shatskiy, Hiroaki Ohfuji, Vitali Prakapenka
    Geoscience Frontiers 10 (4) 1449 - 1458 1674-9871 2019/07 [Peer-reviewed]
     Scientific journal 
    © 2019 China University of Geosciences (Beijing) and Peking University The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70–150 GPa and 800–2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO3 + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D’’ region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
  • A "coarse-grained" metabasite body newly found in the Sambagawa metamorphic rocks in Kumakogen Town, Ehime Prefecture, SW Japan
    Nakata, K, Kusuhashi, N, Saito, S, Ohfuji, H, Nara, M
    The Journal of the Geological Society of Japan 125 (6) 447 - 452 2019/06 [Peer-reviewed]
     Scientific journal
  • Masafumi Matsushita, Kohtaro Masuda, Ryohei Waki, Hiroaki Ohfuji, Michiaki Yamasaki, Yoshihito Kawamura, Yuji Higo
    Journal of Alloys and Compounds 784 1284 - 1289 0925-8388 2019/05 
    © 2019 Elsevier B.V. Under high-temperature and high-pressure conditions, the long-period stacking-ordered structure in Mg85Y9Zn6 transforms to a dual-phase structure consisting of a hexagonal close-packed lattice and a face-centered cubic lattice. After further solidification at high-pressure, unique ultrafine microstructures are formed. The alloy recovered after subjected at 10 GPa and 973 K exhibited a fine lamella structure, and a 0.2% proof stress (σ0.2) of 480 MPa obtained by the compression test. Further the alloy recovered after subjected at 10 GPa and 1273 K, a unique spherulite structure were formed, and its σ0.2 improved to 780 MPa.
  • Tingting Gu, Hiroaki Ohfuji, Wuyi Wang
    American Mineralogist 104 652 - 658 0003-004X 2019/05 
    © 2019 Walter de Gruyter GmbH, Berlin/Boston. The milky appearance shown by certain type IaB diamonds has been subjected to several recent studies, but the origin of this feature is not fully understood. Here several type IaB diamonds with a milky appearance have been studied by cathodoluminescence (CL), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). CL of several hazy type IaB diamonds shows scattered or orientated micro-sized spots or short linear luminescence features. TEM observation revealed that those spots and linear features are caused by dislocation loops that are likely responsible for the hazy appearance of the host diamonds. It is also shown that type IaB diamonds with a cloudy appearance contain nano-sized inclusions with negative crystals of octahedral shape. Some of these negative crystals contain a precipitate that can be explained by a compressed disordered cubic δ-N2 phase observed by high-resolution TEM. In one of the milky IaB diamonds with platelet defects, polycrystalline areas composed of columnar diamond crystals elongated radially in [110], similar to ballas diamond, were revealed by EBSD. Taking into account these observations, it is suggested that the dislocation loops, nano-sized inclusions (negative crystals) and/or characteristic grain boundaries of the radiating fibrous crystals would be the origins for the milky appearance of the type IaB diamonds studied here. Those results add a complementary explanation that accounts for the milkiness of type IaB diamonds studied before.
  • Maki Hamada, Masahide Akasaka, Hiroaki Ohfuji
    Mineralogical Magazine 83 239 - 247 0026-461X 2019/04 
    © 2018 Mineralogical Society of Great Britain and Ireland. K-rich nepheline with a structural formula of A2B6T14T24T34T44O32 (Z = 1) within melilite-olivine nephelinite from Hamada, Shimane Prefecture, Japan, was investigated to clarify its crystal structure and to determine cation distributions in the A and B structural positions of structural channels and tetrahedral T1-T4 sites. The chemical formula of a single-crystal sample was (Na5.437K2.248Mg0.034Ca0.031)Σ7.750(Si8.332Al7.445Fe3+0.158Ti0.009Cr0.005)Σ15.949O32, which results in 65.2, 27.8, 2.1, 3.2 and 1.6 mol.% NaAlSiO4, KAlSiO4, NaFe3+SiO4, □Si2O4 and □0.5(Ca,Mg)0.5AlSiO4 end-member components, respectively, where □ is a vacancy. X-ray diffraction data of a single crystal with dimensions of 0.28 mm × 0.15 mm × 0.05 mm measured at 296 K indicate the space group P63. In the structural refinement, the R1 factor was reduced to 3.69% by taking twinning by merohedry into the refinement. The refinement accounted for 77.7% of the absolute structure and 22.3% of the a and b axes reversed absolute structure. The atomic populations determined in the A and B positions were 1.834 K + 0.166 □ and 5.705 Na + 0.198 K + 0.031 Ca + 0.034 Mg, respectively, implying the substitution of K for Na in the B position. The a and c dimensions are a = 10.0270(3) and c = 8.4027(3) Å. The average <A-O> and <B-O> distances are 3.009 and 2.65 Å, respectively. The substitution of K for Na in the B channel results in increased volume and bond-length distortion of the BO8 polyhedra, which then reduces distortion of the AO9 polyhedra. The average T-O distances indicate that the T1 and T4 sites are essentially filled with Al, whereas the T2 and T3 are filled with Si. Despite the deviation of the O1 oxygen from the triad axis and the combination of K+ ions and vacancies in the hexagonal channels, an incommensurate structure was not observed in the X-ray diffraction data or using the electron diffraction technique.
  • Tatsuaki Sakamoto, Shota Kukeya, Hiroaki Ohfuji
    Materials Science and Engineering A 748 428 - 433 0921-5093 2019/03 Scientific journal 
    © 2019 Elsevier B.V. Microstructure and yield strength at room temperature and 300 °C in Al-5 wt%Y 2 O 3 and Al-5 wt%La 2 O 3 fabricated by mechanical alloying (MA) have been investigated by transmission electron microscopy and compression test. Al-5 wt%Y 2 O 3 and Al-5 wt%La 2 O 3 were fabricated by MA with powders Al, Y 2 O 3 and La 2 O 3 . The alloy powders were consolidated by hot pressing at 550 °C under 400 MPa. Yield strengths in the consolidated Al-5 wt%La 2 O 3 at room temperature and 300 °C were higher than those in the consolidated Al-5 wt%Y 2 O 3 . Vickers microhardness at room temperature in Al-5 wt%La 2 O 3 after the consolidation increased as compared with that before the consolidation. In contrast, Vickers microhardness at room temperature in Al-5 wt%Y 2 O 3 after the consolidation was almost the same as that before the consolidation. X-ray diffraction analyses revealed that in Al-5 wt%La 2 O 3 , an intermetallic compound Al 11 La 3 was formed during the consolidation. Al 11 La 3 was finely dispersed in the matrix after the consolidation. In Al-5 wt%Y 2 O 3 , however, no intermetallic compound was formed after the consolidation. It is noted that higher yield strength in Al-5 wt%La 2 O 3 than Al-5 wt%Y 2 O 3 at room temperature and 300 °C is derived from grain boundary strengthening and dispersion strengthening due to pinning of Al grain growth and retarding of dislocation motion by fine numerous dispersoids. The key factor is the large number density of dispersoids in Al-5 wt%La 2 O 3 , which is due to the in-situ formation of intermetallic precipitates Al 11 La 3 during the consolidation.
  • Daniil A. Minin, Anton F. Shatskiy, Konstantin D. Litasov, Hiroaki Ohfuji
    High Pressure Research 39 50 - 68 0895-7959 2019/01 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Here, we report experimental results on melting and subsolidus phase relations in the Fe–Fe 2 P system at 6 GPa and 900–1600°C. The system has two P-bearing compounds: Fe 3 P and Fe 2 P. X-ray diffraction patterns of these compounds correspond to schreibersite and barringerite, respectively. The Fe–Fe 3 P eutectic appears at 1075°C and 16 mol% P. Schreibersite (Fe 3 P) melts incongruently at 1250°C to produce barringerite (Fe 2 P) and liquid containing 23 mol% P. Barringerite (Fe 2 P) melts congruently at 1575°C. Maximum solid solution of P in metallic iron at 6 GPa is 5 mol%. As temperature increases to 1600°C, the P solubility in the metallic iron decreases to 0.5 mol%, whereas the P content in coexisting liquid decreases to 3 mol%. The composition of quenched phases from Fe–P melt coincides with the compositions of equilibrium phases at corresponding temperature. Consequently, the composition of quenched products of Fe-P melts in meteorites can be used for reconstruction of P–T conditions of their crystallization under ambient or low pressures or during shock melting by impact collisions.
  • Ichida Yoshio, Ohfuji Hiroaki, Irifune Tetsuo, Kunimoto Takehiro
    Proceedings of JSPE Semestrial Meeting 公益社団法人 精密工学会 2019 (0) 648 - 649 2019 [Peer-reviewed]


  • Ichida Yoshio, Ohfuji Hiroaki, Irifune Tetsuo, Kunimoto Takehiro
    Proceedings of JSPE Semestrial Meeting 公益社団法人 精密工学会 2019 (0) 43 - 44 2019 [Peer-reviewed]


  • Tetsuo Irifune, Chiaki Ueda, Shohei Ohshita, Hiroaki Ohfuji, Takehiro Kunimoto, Toru Shinmei
    High Pressure Research 0895-7959 2019 Scientific journal 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25 GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ∼1700°C at 15 GPa to ∼1900°C at 25 GPa. The NPD having grain sizes less than ∼50 nm was synthesized at temperatures below ∼2000°C at 15 GPa and ∼2300°C at 25 GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10 nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.
  • Ryota Ishii, Rei Fukuta, Fumitaro Ishikawa, Masafumi Matsushita, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune, Mitsuru Funato, Yoichi Kawakami
    High Pressure Research 0895-7959 2019/01 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Deep-ultraviolet continuous-wave photoluminescence spectroscopy is performed for nano-polycrystalline diamond (NPD) synthesized by a high pressure high-temperature technique. NPD exhibits clear deep-ultraviolet emissions, which originate from intrinsic excitonic transitions assisted by a momentum-conserving phonon with a photon. Surprisingly, the peak emission energy is about 30 meV higher than that of the single-crystalline diamond. Raman scattering spectroscopy indicates that the energy difference should originate from the excitonic properties of the NPD and not the phonon. Hence, NPD has a large bandgap compared to single-crystalline diamond.
  • Yanbin Wang, Feng Shi, Hiroaki Ohfuji, Julien Gasc, Norimasa Nishiyama, Tony Yu, Timothy Officer, Toru Shinmei, Tetsuo Irifune
    High Pressure Research 0895-7959 2019/01 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. The use of nanopolycrystalline diamond has allowed a systematic study on deformation of polycrystalline diamond composites (PCDCs). Bulk PCDCs samples containing either Co or SiC as a binding agent were deformed under high pressure and temperature to strains up to 18% at strain rates ∼10−5 s−1. All samples exhibit strong work hardening. The strength of PCDCs depends on the amount and type of binding agents and is consistently weaker than that of diamond single crystals. The weakening may be due to the binder materials, which play an important role in affecting grain boundary structures. In SiC-based PCDC, significant grain fragmentation occurs. Nearly all grain boundaries are wetted by SiC after large deformation, resulting in lower strength. In Co-based PCDC, the microstructure is dominated by dislocations, deformation twins, and separated grain boundaries. The density of deformation twins increases significantly with strain, with the twin domain width reaching as low as 10–20 nm at 14% strain.
  • Rei Fukuta, Naoya Yamamoto, Fumitaro Ishikawa, Fumitaro Ishikawa, Masafumi Matsushita, Masafumi Matsushita, Tsuyoshi Yoshitake, Hiroshi Ikenoue, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune
    Japanese Journal of Applied Physics 57 0021-4922 2018/11 
    © 2018 The Japan Society of Applied Physics. We propose a simple and easy electrode formation technique on nanopolycrystalline diamond (NPD) synthesized by a high-temperature and high-pressure technique. The stronger light absorption nature of NPD than of standard transparent diamond enables the laser irradiation process on NPD. Pulsed laser irradiation using an ArF excimer laser under atmospheric condition drastically reduced the resistance of the NPD surface by forming graphite. Repetition of the laser irradiation reduced the resistance, showing a resistance of 300 Ω for the laser-irradiated area of 100 × 150 μm2. These results indicate the applicability of pulsed laser irradiation to an NPD apparatus having a conductive electrode, as well as electronic device processes.
  • Yasuhide Nakamura, Izumi Iwata, Izumi Iwata, Rie S. Hori, Naomi Uchiyama, Akihiro Tuji, Masaki J. Fujita, Daiske Honda, Daiske Honda, Hiroaki Ohfuji
    Journal of Structural Biology 204 45 - 51 1047-8477 2018/10 
    © 2018 Elsevier Inc. Cross-sections were prepared by ultramicrotome (UM) and focused ion beam (FIB) system in order to examine the skeletal structure of ecologically and geologically important shell-bearing protists: phaeodarians and radiolarians. The elemental composition of the skeleton was clarified by the energy dispersive X-ray spectroscopy, suggesting that the skeletons of both groups are mainly made of amorphous silica (SiO2·nH2O) with other minor elements (Na, Mg, Al, Cl, K, Ca and Fe) and that these two groups have similar elemental composition, compared with other siliceous organisms (diatoms and sponges). However, the structural difference among the two groups was confirmed: phaeodarian skeletons are porous, unlike radiolarians with solid skeletons. It was also revealed that the phaeodarian skeleton contains concentric layered structure with spaces, presumably related to the ontogenetic skeleton formation. The distinction in the skeletal ultrafine structure (porous/solid and non-dense/dense) would reflect the ecological difference among the two groups and could be an effective criterion to determine whether microfossils belong to Radiolaria or Phaeodaria. The UM and FIB combined method presented in this study could be a useful approach to examine the chemical and structural characteristics of unculturable and/or rare microorganisms.
  • Y. Zhou, T. Irifune, T. Irifune, H. Ohfuji, T. Kuribayashi
    Geophysical Research Letters 45 8167 - 8172 0094-8276 2018/08 
    ©2018. American Geophysical Union. All Rights Reserved. Phase relations of Al2SiO5 have been studied by multianvil experiments at pressures of 13–23 GPa and temperatures of 2000–2900 K. Al2SiO5 kyanite was found to transform into two new high-pressure forms of Al2SiO5 (kyanite II and III) at temperatures exceeding 2300–2500 K and pressures of 14–23 GPa: The first phase transition occurs near 14 GPa, and the second occurs near 17 GPa. The new Al2SiO5 phases have triclinic and monoclinic crystal symmetries with zero-pressure densities of 3.876(2) and 3.982(1) g/cm3, respectively, which are significantly denser than kyanite (ρ0 = 3.666 g/cm3) but less dense than the isochemical mixture of Al2O3 corundum and SiO2 stishovite (ρ0 = 4.036 g/cm3). The exceptionally high stability temperatures of the new Al2SiO5 phases suggest that they are unlikely to form in the present mantle but may be found in some impact craters and shocked meteorites and act as important indicators of pressure and temperature for the shock events.
  • Li Lei, Leilei Zhang, Shangpan Gao, Qiwei Hu, Leiming Fang, Xiping Chen, Yuanhua Xia, Xianlong Wang, Hiroaki Ohfuji, Yohei Kojima, Simon A.T. Redfern, Zhi Zeng, Bo Chen, Duanwei He, Tetsuo Irifune
    Journal of Alloys and Compounds 752 99 - 105 0925-8388 2018/07 Scientific journal 
    Binary ε-Fe3N1.098 and ternary ε-Fe2.322Co0.678N0.888 were synthesized as spherical bulk materials using novel high-pressure solid-state metathesis reactions (HPSSM). The structural and magnetic properties of the two nitrides were investigated using neutron powder diffraction (NPD), a vibrating sample magnetometer (VSM), and first-principle calculations. We found that at high pressure and high temperature (HPHT), nitrogen atoms enter the interstitial 2d sites of the nitrogen-rich ε-Fe3N1.098 structure, space group P6322, which were previously reported to be vacant. The stoichiometry of the nitride (N/Me ratio) and the level of disorder have a significant influence on the site occupancies and local magnetic moments in these iron-based nitrides. The substitution of Fe by Co in ε-Fe2.322Co0.678N0.888 did not reduce the mean magnetic moment (μm) per metal atom at the Wyckoff 6 g site. NPD refinements showed that the magnetic moment per Fe atom in ε-Fe3N1.098 (at 1.8 (4) μB) was slightly higher than that of ε-Fe2.322Co0.678N0.888, where the moment per Fe/Co atom is 1.7(2) μB.
  • N. Chertkova, H. Ohfuji, R. Nomura, H. Kadobayashi, T. Irifune
    High Pressure Research 38 337 - 347 0895-7959 2018/07 
    © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group. Using an external heating system, specifically designed for the lever-type diamond anvil cell, we investigated for the first time the phase relationships in the C2H2O4 system at temperatures exceeding 850°C and pressures up to 6.5 GPa. In situ Raman spectroscopy was applied for the characterization of structural features of observed high-temperature phases, which transformed to the black carbon-rich material upon quenching and decompression. Results of this work give insights on the pressure-induced polymerization process of organic compounds at high temperatures.
  • Yoshio Ichida, Hiroaki Ohfuji, Tetsuo Irifune, Takehiro kunimoto, Yohei Kojima, Toru Shinmei
    Journal of the European Ceramic Society 38 (7) 2815 - 2822 0955-2219 2018/07 Scientific journal 
    © 2018 Elsevier Ltd Using a turbostratic pyrolytic boron nitride as a starting material, we synthesized a variety of ultrahard polycrystalline cubic boron nitride (PcBN) as a function of the heating duration changing from 1 to 60 min under a constant temperature and pressure conditions (1950 °C and 25 GPa) using a multi-anvil apparatus. When the heating duration was less than 13 min, ultrafine nano-polycrystalline cBN (U-NPcBN) with the mean grain size of <50 nm was produced. Among these U-NPcBNs those synthesized with 11–13 min were found to have a uniform texture composed purely of cBN (i.e. with no wurzite BN residue) and a Knoop hardness of >53 GPa, which is 20% higher than that of the hardest conventional binderless PcBN in practical use. Furthermore, the PcBNs synthesized with 18–20 min showed a unique nanocrystalline texture composed of relatively coarse grains dispersed in a fine grained matrix and even higher Knoop hardness (54.5–55.2 GPa).
  • D. S. Mikhailenko, O. V. Shchepetova, K. A. Musiyachenko, A. V. Korsakov, Hiroaki Ohfuji, I. V. Pekov
    Doklady Earth Sciences 480 814 - 817 1028-334X 2018/06 
    © 2018, Pleiades Publishing, Ltd. The results of studying an aggregate of graphite-and-diamond crystal in tourmaline 5 μm of the Kokchetav massif by the method of transmission electron microscopy are presented. The detail studies of the interface between the crystals of graphite and diamond have revealed the absence of disordered graphite that is detail partially graphitized diamond. Intense deformation changes in the graphite crystal occurred after it was captured by tourmaline at the regression stage, which led to considerable kinking of the graphite crystal along the a-axis. Thus, the coexistence of graphite and diamond crystals cannot be unambiguously interpreted as a product of partial diamond graphitization. Graphite could have crystallized syngenetic with a diamond crystal or at the retrograde stage in the graphite stability field.
  • Takashi Mizuguchi, Tsutomu Ito, Kota Kimura, Tokiko Kurisaka, Hiroaki Ohfuji
    Zairyo/Journal of the Society of Materials Science, Japan 67 (5) 568 - 573 1880-7488 2018/05 Scientific journal 
    The stress–strain relationship and microstructural evolution of a fine-grained 5083 aluminum alloy produced via friction-stir processing (FSP) during high-temperature tensile deformation were investigated. The FSP of the 5083 aluminum alloy resulted in the formation of a homogeneous fine-grained microstructure. Based on the stress–strain relationship, it was found that the 5083 aluminum alloy exhibited a large elongation, especially at a temperature above 693 K. The stress exponent and the activation energy for deformation, which were determined by the flow stress at a nominal strain of 0.03, were approximately 2.5 and 123 kJ/mol, respectively. These results suggest that grain boundary sliding accommodated by the solute drag motion of dislocations was the rate-controlling process in the early stages of deformation. The largest elongation of 350% occurred at 743 K and an initial strain rate of 1.0 10−3 s−1. In this case, the grain aspect ratio increased with increasing nominal strain, which indicated that equiaxed grains continuously elongated along the tensile axis during high-temperature deformation because of dislocation creep. The value of the stress exponent increased with increasing strain. From our experimental results, the dominant deformation mechanism was determined to change during the tensile test, and the contribution of dislocation creep to the high-temperature deformation increased as the deformation proceeded.
  • Terumi Ejima, Yasuhito Osanai, Masahide Akasaka, Tatsuro Adachi, Nobuhiko Nakano, Yoshiaki Kon, Hiroaki Ohfuji, Jargalan Sereenen
    Minerals 8 (5) 2075-163X 2018/05 Scientific journal 
    The oxidation states of Fe within olivine, orthopyroxene, clinopyroxene, and spinel in a spinel lherzolite xenolith from the Tariat Depression, Mongolia were investigated using57Fe Mössbauer spectroscopy to evaluate the redox condition of the upper mantle from which the Tariat spinel lherzolite xenolith was derived. The purity of separated minerals for the Mössbauer spectroscopic analysis was examined using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. Average Fo and Fe contents of olivine at the core part of the xenolith are 89.9(4) mol % and 0.195(3) atoms per formula unit, respectively. The Fe3+/ΣFe values of the olivine, orthopyroxene, clinopyroxene, and spinel, determined by Mössbauer spectroscopic analysis, are 0.027(2), 0.15(1), 0.26(3), and 0.34(5), respectively. TheMössbauer spectrum of olivine consists of two doublets assigned to Fe2+ at the octahedral sites and one doublet, with I.S. of 0.40(2) mm/s and Q.S. of 0.69(3) mm/s assigned to Fe3+ at the octahedral site. Since the Tariat spinel lherzolite xenolith in this study shows no evidence of metasomatism or thermal alteration, the existence of a small amount of Fe3+ in olivine and the fairly high Fe3+ contents of clinopyroxene, orthopyroxene, and spinel imply that the upper mantle under the Tariat area was in a rather oxidized condition.
  • Hirokazu Kadobayashi, Hisako Hirai, Hiroaki Ohfuji, Michika Ohtake, Yoshitaka Yamamoto
    Journal of Chemical Physics 148 (16) 0021-9606 2018/04 Scientific journal 
    High-temperature and high-pressure experiments were performed under 2-55 GPa and 298-653 K using in situ Raman spectroscopy and X-ray diffraction combined with externally heated diamond anvil cells to investigate the stability of methane hydrate. Prior to in situ experiments, the typical C-H vibration modes of methane hydrate and their pressure dependence were measured at room temperature using Raman spectroscopy to make a clear discrimination between methane hydrate and solid methane which forms through the decomposition of methane hydrate at high temperature. The sequential in situ Raman spectroscopy and X-ray diffraction revealed that methane hydrate survives up to 633 K and 40.3 GPa and then decomposes into solid methane and ice VII above the conditions. The decomposition curve of methane hydrate estimated by the present experiments is > 200 K lower than the melting curves of solid methane and ice VII, and moderately increases with increasing pressure. Our result suggests that although methane hydrate may be an important candidate for major constituents of cool exoplanets and other icy bodies, it is unlikely to be present in the ice mantle of Neptune and Uranus, where the temperature is expected to be far beyond the decomposition temperatures.
  • Keisuke Niwase, Mititaka Terasawa, Shin-Ichi Honda, Masahito Niibe, Tomohiko Hisakuni, Tadao Iwata, Yuji Higo, Takeshi Hirai, Toru Shinmei, Hiroaki Ohfuji, Tetsuo Irifune
    Journal of Applied Physics 123 (16) 1089-7550 2018/04 Scientific journal 
    The super hard material of "compressed graphite" (CG) has been reported to be formed under compression of graphite at room temperature. However, it returns to graphite under decompression. Neutron-irradiated graphite, on the other hand, is a unique material for the synthesis of a new carbon phase, as reported by the formation of an amorphous diamond by shock compression. Here, we investigate the change of structure of highly oriented pyrolytic graphite (HOPG) irradiated with neutrons to a fluence of 1.4 × 1024 n/m2 under static pressure. The neutron-irradiated HOPG sample was compressed to 15 GPa at room temperature and then the temperature was increased up to 1500 °C. X-ray diffraction, high-resolution transmission electron microscopy on the recovered sample clearly showed the formation of a significant amount of quenchable-CG with ordinary graphite. Formation of hexagonal and cubic diamonds was also confirmed. The effect of irradiation-induced defects on the synthesis of quenchable-CG under high pressure and high temperature treatment was discussed.
  • Rei Fukuta, Fumitaro Ishikawa, Akihiro Ishikawa, Kohsuke Hamada, Masafumi Matsushita, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune
    Diamond and Related Materials 84 66 - 70 0925-9635 2018/04 Scientific journal 
    We investigated inherent electronic properties of ultra-hard nano-polycrystalline diamond (NPD) synthesised at 15 GPa and 2300 °C. NPD exhibited a p-type semiconducting property with a resistivity of 1 × 106 and 4 × 102 Ω cm at 400 °C and 800 °C, respectively, with activation energies as high as 1.2 eV. The mobility was 2 cm2/Vs at 800 °C. The observed transport was considered to be attributed to grain boundary. Cathodoluminescence spectra show defect-related peaks with dominant peaks at 2.8 eV close to band-A and peaks below 2 eV, which is possibly related to crystal defects. In contrast, photoluminescence excited with a wavelength of 405 nm resonantly excited peaks having energies lower than 2.1 eV. The large density of crystalline defects such as grain boundaries and impurities in the NPD would result in the characteristic electronic and optical properties.
  • Takeshi Sakai, Takehiko Yagi, Tetsuo Irifune, Hirokazu Kadobayashi, Naohisa Hirao, Takehiro Kunimoto, Hiroaki Ohfuji, Saori Kawaguchi-Imada, Yasuo Ohishi, Shigehiko Tateno, Kei Hirose
    High Pressure Research 38 (2) 107 - 119 0895-7959 2018/04 Scientific journal 
    © 2018 Informa UK Limited, trading as Taylor & Francis Group. We have developed a double stage diamond anvil cell (ds-DAC) technique for reproducible pressure by precisely fabricating 2nd stage anvils using a focused ion beam system. We used 2nd stage micro-anvils made of ultra-fine (< 10 nm) nano-polycrystalline diamond with various shapes and dimensions synthesized from glassy carbon at high pressure and temperature. The X-ray diffraction patterns from the rhenium sample always showed very broad peaks due to large pressure gradients in the culet of the micro-anvils. Deconvolution of the broad 101 diffraction peak results in compression of rhenium to V/V0= 0.633 for the smallest d-spacing. The calculated pressure for this minimum volume varies from 430 to 630 GPa, depending on the choice of the equation of state of rhenium. We conclude that the most likely pressure achieved for the minimum volume of rhenium is in a range of 430–460 GPa based on a calibration using the platinum pressure scale to 280 GPa and the latter value of 630 GPa is unreasonably high, suggesting that the pressures in an earlier study for the equation of state of rhenium would have been significantly overestimated.
  • Leilei Zhang, Ya Cheng, Li Lei, Xianlong Wang, Qiwei Hu, Qiming Wang, Hiroaki Ohfuji, Yohei Kojima, Qiang Zhang, Zhi Zeng, Fang Peng, Zili Kou, Duanwei He, Tetsuo Irifune
    Crystal Growth and Design 18 (3) 1843 - 1847 1528-7505 2018/03 Scientific journal 
    The well-crystallized mixed anions compound cerium oxychloride (CeOCl) was successfully synthesized by a high-pressure solid-state metathesis reaction. The photoluminescence experiment shows that the CeOCl, with a band gap of ∼3.05 eV, has good violet-blue emission properties. And first-principles calculations of the band structures show that CeOCl is an indirect (direct) band gap semiconductor for the spin-up (spin-down) branch. This suggests that the CeOCl can be expected to be a semiconductor material. In addition, in situ high-pressure angle-dispersive X-ray diffraction experiment reveals that the bulk modulus of CeOCl is 52.8(8) GPa, which is close to our first-principles calculations, giving that B0 = 47.6(5) GPa.
  • Nico A. Gaida, Norimasa Nishiyama, Atsunobu Masuno, Ulrich Schürmann, Christopher Giehl, Oliver Beermann, Hiroaki Ohfuji, Jozef Bednarcik, Eleonora Kulik, Astrid Holzheid, Tetsuo Irifune, Lorenz Kienle
    Journal of the American Ceramic Society 101 (3) 998 - 1003 1551-2916 2018/03 Scientific journal 
    Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite (91.4 vol%) and Al2O3 corundum (8.6 vol%) were fabricated at 10 GPa and 1200-1400°C. These materials were obtained by direct conversion from Al2O3-SiO2 glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in-line transmission value of 78% at a wavelength of 645 nm and a sample-thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in-line transmission and grain size is well explained by a grain-boundary scattering model based on a classical theory.
  • Lei Qi, Li Lei, Qiwei Hu, Leilei Zhang, Leihao Feng, Meifang Pu, Hiroaki Ohfuji, Tetsuo Irifune
    Journal of Applied Physics 123 (5) 1089-7550 2018/02 Scientific journal 
    Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.
  • Mititaka Terasawa, Shin-ichi Honda, Keisuke Niwase, Masahito Niibe, Tomohiko Hisakuni, Tadao Iwata, Yuji Higo, Toru Shinmei, Hiroaki Ohfuji, Tetsuo Irifune
    Diamond and Related Materials 82 132 - 136 0925-9635 2018/02 Scientific journal 
    Highly oriented pyrolytic graphite (HOPG) is a unique source material for the synthesis of new types of diamond. It can transform to layered nano-polycrystalline diamond (NPD) under static high pressure and high temperature (HPHT) and to “amorphous diamond” by introducing structural defects by neutron irradiation followed by shock compression. Here, we investigated the structural change of the neutron-irradiated HOPG through a HPHT treatment at 2300 °C and 15 GPa by Raman, X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES) analyses. The recovered sample consists of randomly oriented nanodiamonds (50–100 nm), showing clearly a different microtexture from those of the layered NPD and “amorphous diamond” reported by the previous studies. This is likely as a result of competing effects of the introduction of irradiation-induced defects, which provided the preferential nucleation sites for cubic diamond, and their partial recovery upon annealing during the HPHT treatment. The present result suggests that NPDs with various crystalline structures can potentially be synthesized from neutron-irradiated HOPG by controlling the density and distribution of the defects introduced.
  • Lei-Hao Feng, Qi-Wei Hu, Li Lei, Lei-Ming Fang, Lei Qi, Lei-Lei Zhang, Mei-Fang Pu, Zi-Li Kou, Fang Peng, Xi-Ping Chen, Yuan-Hua Xia, Yohei Kojima, Hiroaki Ohfuji, Duan-Wei He, Bo Chen, Tetsuo Irifune
    Chinese Physics B 27 (2) 2058-3834 2018/02 Scientific journal 
    Tantalum nitride (TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature (HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order BirchMurnaghan equation of state fitted to the x-ray diffraction pressure- volume (PV) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B0 = 369(2) GPa with pressure derivatives of B0 0 = 4 for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result (B0 = 351 GPa), which is close to the recent theoretical calculation result (B0 = 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent TaN bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.
  • Qiwei Hu, Xiaozhi Yan, Li Lei, Qiming Wang, Leihao Feng, Lei Qi, Leilei Zhang, Fang Peng, Hiroaki Ohfuji, Duanwei He
    Physical Review B 97 (1) 2469-9969 2018/01 Scientific journal 
    Pressure induced solid-solid reconstructive phase transitions for graphite-diamond, and wurtzite-rocksalt in GaN and AlN occur at significantly higher pressure than expected from equilibrium coexistence and their transition paths are always inconsistent with each other. These indicate that the underlying nucleation and growth mechanism in the solid-solid reconstructive phase transitions are poorly understood. Here, we propose an elastic-strain dominated mechanism in a reconstructive phase transition, β-LiGaO2 to γ-LiGaO2, based on in situ high-pressure angle dispersive x-ray diffraction and single-crystal Raman scattering. This mechanism suggests that the pressure induced solid-solid reconstructive phase transition is neither purely diffusionless nor purely diffusive, as conventionally assumed, but a combination. The large elastic strains are accumulated, with the coherent nucleation, in the early stage of the transition. The elastic strains along the (100) and (001) directions are too large to be relaxed by the shear stress, so an intermediate structure emerges reducing the elastic strains and making the transition energetically favorable. At higher pressures, when the elastic strains become small enough to be relaxed, the phase transition to γ-LiGaO2 begins and the coherent nucleation is substituted with a semicoherent one with Li and Ga atoms disordered.
  • Hiroaki Ohfuji
    Japanese Magazine of Mineralogical and Petrological Sciences 47 (1) 1 - 12 1349-7979 2018 Scientific journal 
    Here I introduce the crystallization and texturing mechanism of nano-polycrystalline diamond (NPD) in the graphite-diamond direct transformation in laboratory and in nature. Two types of transformation processes, diŠusion-controlled (nucleation and growth) process and diŠusion-less martensitic process are involved in the diamond formation depending on the crystallinity of the initial starting material used. The former process is dominated when using low crystalline graphite/carbon materials and produces a granular texture, while the latter process is favorable when using high crystalline graphite and produces lamellar (layered) texture. Understanding the transformation and texturing mechanism has enabled the texture control of NPD and even opened a window to the production of new varieties of NPDs with novel microtextures. It is also helpful to understand the formation process and origin of the unique microtexture of natural NPD (impact diamond), which was recently identified from the large impact crater in Siberia, Russia. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale are the key factors for accelerating the multiple diamond nucleation and suppressing the overall grain growth to produce the unique nanocrystalline texture of natural NPD.
  • Olga V. Shchepetova, Andrey Korsakov, Denis Mikhailenko, Pavel Zelenovskiy, Vladimir Shur, Hiroaki Ohfuji
    JOURNAL OF RAMAN SPECTROSCOPY 48 (11) 1606 - 1612 0377-0486 2017/11 Scientific journal 
    Confocal 2D and 3D Raman imaging was applied to SiO2 inclusions in kyanite porphyroblasts from diamond-bearing kyanite gneisses of the Kokchetav Massif (Northern Kazakhstan). The study reveals a mineral assemblage of coesite-disordered graphite which is forbidden in ultrahigh pressure-high temperature (UHP-HT) metamorphic complexes. Disordered graphite in coesite inclusions is optically undetectable and has been identified exclusively by Raman spectroscopy. Raman imaging of the forbidden coesite-disordered graphite assemblage fails to detect bands assigned to fluid components (e.g. CO2, N-2, CH4, H(2)Oliq), but the Raman spectra of fluid inclusions in kyanite, garnet and quartz demonstrate the presence of CO2, N-2, CH4, H(2)Oliq and disordered graphite. The disordered graphite most likely formed by precipitation from a C?O?H fluid during the retrograde stage. The fluid was entrapped by kyanite with coesite inclusions near the peak metamorphic conditions. The lack of disordered graphite around diamond crystals in our samples does not support the generally accepted origin of disordered graphite in ultra high pressure metamorphic rocks by partial diamond graphitization during exhumation. Copyright (c) 2017 John Wiley & Sons, Ltd.
  • Zhaodong Liu, Masayuki Nishi, Takayuki Ishii, Hongzhan Fei, Nobuyoshi Miyajima, Tiziana Boffa Ballaran, Hiroaki Ohfuji, Takeshi Sakai, Lin Wang, Svyatoslav Shcheka, Takeshi Arimoto, Yoshinori Tange, Yuji Higo, Tetsuo Irifune, Tomoo Katsura
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 122 (10) 7775 - 7788 2169-9313 2017/10 Scientific journal 
    Phase relations in the system MgSiO3-Al2O3 were investigated at pressures of 27-45GPa and temperatures of 1700, 2000, and 2300K using sintered diamond and tungsten carbide anvils in a multianvil apparatus. The bulk compositions in the MgSiO3-Al2O3 binary system crystallize a phase assemblage of pyrope and corundum at pressures below 27GPa and an assemblage of bridgmanite and corundum at pressures above 27GPa regardless of temperatures. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increases significantly with increasing temperature. The solubility of Al2O3 in bridgmanite increases from 6.7mol% at 1700K to 21.8mol% at 2500K under a constant pressure of 27GPa. Bridgmanite becomes more aluminous with increasing pressure from 27 to 45GPa at a given temperature. The MgSiO3 content in corundum increases with increasing pressure at pressure lower than 27GPa, while it decreases at pressure higher than 27GPa. Our results suggest that bridgmanite can incorporate a considerably higher Al2O3 content than that of the pyrope composition (25mol% Al2O3). The present study further suggests that the entire Al2O3 component is accommodated into bridgmanite in the pyrolite lower mantle. However, Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs in the shallow part of the lower mantle, and therefore, additional phases such as MgAl2O4 with calcium ferrite-type structure are necessary to host the excess Al2O3. Plain Language Summary Here we determined the phase relations in the system MgSiO3-Al2O3 up to 2300K under lower mantle pressures and found that the solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increase with increasing temperature. All Al2O3 can be completely accommodated into bridgmanite in the pyrolite composition, while the Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs, and therefore, additional phases are required to host the excess Al2O3.
  • Tomohiro Ohuchi, Xinglin Lei, Hiroaki Ohfuji, Yuji Higo, Yoshinori Tange, Takeshi Sakai, Kiyoshi Fujino, Tetsuo Irifune
    NATURE GEOSCIENCE 10 (10) 771 - + 1752-0894 2017/10 Scientific journal 
    The occurrence of intermediate-depth and deep earthquakes at depths greater than 60 km in subducting slabs has long puzzled geoscientists. These earthquakes require some mechanism to accelerate the fault movement at high pressures above 1.8 GPa. Localized heating would contribute to faulting, but experimental evidence for this mechanism has been limited to pressures of up to 0.5 GPa. Here we conduct deformation experiments on dry dunite samples at pressures of 1.0 to 2.6 GPa and temperatures of 860 to 1,350 K-conditions close to those for relatively shallow intermediate-depth earthquakes. We observe plastic deformation of the dunite, followed by faulting and acoustic emissions at an accelerated strain rate of about 5 x 10(-5) s(-1) or higher. We find that ultrafine-grained gouge layers containing iron-rich melt films, which is indicative of a very high peak temperature of about 2,110 K along the fault planes. We also observe faulting in wet harzburgite-a dehydration product of antigorite-at natural stress levels of 0.3 to 0.4 gigapascals. We therefore suggest that intermediate-depth earthquakes can be induced by localized heating both in dry and wet subducting slabs, if the background strain rate exceeds a threshold value in the range from 10(-16) to 10(-13) s(-1).
  • Yoshio Ichida, Hiroaki Ohfuji, Tetsuo Irifune, Takehiro Kunimoto, Yohei Kojima, Toru Shinmei
    DIAMOND AND RELATED MATERIALS 77 25 - 34 0925-9635 2017/08 Scientific journal 
    Using a turbostratic pyrolytic boron nitride produced by a chemical vapor deposition process as a starting material, we conducted experiments for synthesizing polycrystalline cubic boron nitride (PcBN) at a temperature of 1600-2600 degrees C and pressure of 25 GPa with a multi-anvil high-pressure apparatus. Moreover, we evaluated the microstructure and hardness characteristics of the synthesized PcBN. The results showed that a complete single-phase PcBN without residual wurtzite BN was produced at 1900 degrees C or higher and a nano-polycrystalline cBN (NPcBN) having a mean grain size smaller than 100 nm could be synthesized at a temperature of 1950 degrees C or lower under a pressure of 25 GPa. Furthermore, we found that a single-phase NPcBN (CGD-NPcBN) having a characteristic microstructure of dispersed relatively coarse grains in a fine polycrystalline structure, which was harder than 53.5 GPa in Knoop hardness, was produced at a temperature range of 1900-1950 degrees C. In particular, at 1950 degrees C, CGD-NPcBN having a Knoop hardness of 54.7 GPa, which is 25% higher than that of the hardest binderless PcBN tool currently in practical use, can be synthesized. In addition, we verified that the Knoop hardness of the single-phase PcBN with a uniform microstructure increased with decreasing mean grain size, which complied with the Hall fetch relationship.
  • Peter Nemeth, Kurt Leinenweber, Hiroaki Ohfuji, Thomas Groy, Kenneth J. Domanik, Istvan J. Kovacs, Judit S. Kovacs, Peter R. Buseck
    EARTH AND PLANETARY SCIENCE LETTERS 469 148 - 155 0012-821X 2017/07 Scientific journal 
    Water-bearing minerals provide fundamental knowledge regarding the water budget of the mantle and are geophysically significant through their influence on the rheological and seismic properties of Earth's interior. Here we investigate the CaO-SiO2-H2O system at 17 GPa and 1773 K, corresponding to mantle transition-zone condition, report new high-pressure (HP) water-bearing Ca-silicates and reveal the structural complexity of these phases. We document the HP polymorph of hartrurite (Ca3SiO5), post-hartrurite, which is tetragonal with space group P4/ncc, a = 6.820(5), c = 10.243(8) angstrom, V = 476.4(8) angstrom(3), and Z = 4, and is isostructural with Sr3SiO5. Post-hartrurite occurs in hydrous and anhydrous forms and coexists with larnite (Ca2SiO4), which we find also has a hydrous counterpart. Si is 4-coordinated in both post-hartrurite and larnite. In their hydrous forms, H substitutes for Si (4H for each Si; hydrogrossular substitution). Fourier transform infrared (FTIR) spectroscopy shows broad hydroxyl absorption bands at 3550 cm(-1) and at similar to 3500-3550 cm(-1) for hydrous post-hartrurite and hydrous larnite, respectively. Hydrous post-hartrurite has a defect composition of Ca2.663Si0.826O5H1.370 (5.84 weight % H2O) according to electron-probe microanalysis (EPMA), and the Si deficiency relative to Ca is also observed in the single-crystal data. Hydrous larnite has average composition of Ca1.924Si0.851O4H0.748 (4.06 weight % H2O) according to EPMA, and it is in agreement with the Si occupancy obtained using X-ray data collected on a single crystal. Superlattice reflections occur in electron-diffraction patterns of the hydrous larnite and could indicate crystallographic ordering of the hydroxyl groups and their associated cation defects. Although textural and EPMA-based compositional evidence suggests that hydrous perovskite may occur in high-Ca-containing (or low silica-activity) systems, the FTIR measurement does not show a well-defined hydroxyl absorption band for this phase, implying the water content, at least in the quenched glass, is below the limit of detection (100-1000 ppm). We conclude that at high pressure, as at ambient pressure, some calcium silicates have a high affinity for H2O and high dehydration temperatures. The thermal stability of these hydrous phases suggests that they could exist along a typical mantle geotherm and thus they might be relevant for understanding the mineralogy and water content of Earth's mantle. (C) 2017 Elsevier B.V. All rights reserved.
  • Hongzhan Fei, Daisuke Yamazaki, Moe Sakurai, Nobuyoshi Miyajima, Hiroaki Ohfuji, Tomoo Katsura, Takafumi Yamamoto
    SCIENCE ADVANCES 3 (6) 355 - 358 2375-2548 2017/06 [Peer-reviewed]
     Scientific journal 
    An opesn question for solid-earth scientists is the amount of water in Earth's interior. The uppermost mantle and lower mantle contain little water because their dominant minerals, olivine and bridgmanite, have limited water storage capacity. In contrast, the mantle transition zone (MTZ) at a depth of 410 to 660 km is considered to be a potential water reservoir because its dominant minerals, wadsleyite and ringwoodite, can contain large amounts of water [up to 3 weight % (wt %)]. However, the actual amount of water in the MTZ is unknown. Given that water incorporated into mantle minerals can lower their viscosity, we evaluate the water content of the MTZ bymeasuring dislocationmobility, a property that is inversely proportional to viscosity, as a function of temperature and water content in ringwoodite and bridgmanite. We find that dislocation mobility in bridgmanite is faster by two orders of magnitude than in anhydrous ringwoodite but 1.5 orders ofmagnitude slower than inwater-saturated ringwoodite. To fit the observed mantle viscosity profiles, ringwoodite in the MTZ should contain 1 to 2 wt % water. The MTZ should thus be nearly water-saturated globally.
  • Terumi Ejima, Mari Yoneda, Masahide Akasaka, Hiroaki Ohfuji, Yoshiaki Kon, Mariko Nagashima, Yoshihiro Nakamuta
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 112 (3) 116 - 126 1345-6296 2017/06 Scientific journal 
    Mineral precipitates within olivine grains are a sensitive recorder of the oxidation conditions of scoria. The crystallization process of precipitates within olivine phenocrysts in andesitic scoria from Kasayama volcano, Hagi, Yamaguchi Prefecture, Japan was investigated. Electron microprobe analysis and Raman spectroscopy were used for mineral identification and electron back-scattered diffraction to determine the crystallographic orientation of the precipitate minerals and host olivine phenocrysts. The scoria in the interior of the Kasayama scoria cone is red-brown, and the outer surface of the cone is black or black with red-brown tint. The olivine phenocrysts (Fo(79-81)) within the black scoria lack precipitate minerals, but those in the black scoria with red-brown tint (Fo(82-85)) contain small amounts of precipitates at their rims, and those in the red-brown scoria (Fo(99)) contain abundant cryptocrystalline precipitates, including hematite and enstatite. Vermicular rods of hematite and enstatite form symplectite zones on the rims, and symplectite domains in the cores of the phenocrysts. The host olivine and the hematite precipitates have a crystallographic relationship of [100](O1)//[000 (1) over bar](Hem), [010](O1)// [10 (1) over bar0](Hem), and [001](O1)//[21 (3) over bar0](Hem), which is characteristic of olivine and precipitate minerals generated by high-temperature oxidation. The maximum oxidation temperature is estimated to be > 800 degrees C. High temperature oxidizing conditions may have been maintained in the inner wall of the scoria cone because the scoria that erupted early in the sequence was deposited in the presence of air and was subsequently covered by black scoria making up the outer wall of the cone.
  • T. Kimura, H. Ohfuji, M. Nishi, T. Irifune
    NATURE COMMUNICATIONS 8 2041-1723 2017/06 Scientific journal 
    Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth's lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials. Significant discrepancies exist between the melting temperatures of MgO determined by laser-heated diamond anvil cell (LHDAC) and those based on dynamic compressions and theoretical predictions. Here we show the melting temperatures in earlier LHDAC experiments are underestimated due to misjudgment of melting, based on micro-texture observations of the quenched samples. The high melting temperatures of MgO suggest that the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are peridotitic, which are stabilized near the core-mantle boundary.
  • Norimasa Nishiyama, Ryo Ishikawa, Hiroaki Ohfuji, Hauke Marquardt, Alexander Kurnosov, Takashi Taniguchi, Byung-Nam Kim, Hidehiro Yoshida, Atsunobu Masuno, Jozef Bednarcik, Eleonora Kulik, Yuichi Ikuhara, Fumihiro Wakai, Tetsuo Irifune
    SCIENTIFIC REPORTS 7 2045-2322 2017/03 Scientific journal 
    Glasses and single crystals have traditionally been used as optical windows. Recently, there has been a high demand for harder and tougher optical windows that are able to endure severe conditions. Transparent polycrystalline ceramics can fulfill this demand because of their superior mechanical properties. It is known that polycrystalline ceramics with a spinel structure in compositions of MgAl2O4 and aluminum oxynitride (gamma AlON) show high optical transparency. Here we report the synthesis of the hardest transparent spinel ceramic, i.e. polycrystalline cubic silicon nitride (c-Si3N4). This material shows an intrinsic optical transparency over a wide range of wavelengths below its band-gap energy (258 nm) and is categorized as one of the third hardest materials next to diamond and cubic boron nitride (cBN). Since the high temperature metastability of c-Si3N4 in air is superior to those of diamond and cBN, the transparent c-Si3N4 ceramic can potentially be used as a window under extremely severe conditions.
  • Hiroaki Ohfuji, Motosuke Nakaya, Alexander P. Yelisseyev, Valentin P. Afanasiev, Konstantin D. Litasov
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 112 (1) 46 - 51 1345-6296 2017/02 Scientific journal 
    This study revealed for the first time the microtexture and crystallographic features of natural polycrystalline diamond, yakutite found in placer deposits in the Siberian Platform, Russia. Yakutite consists of well-sintered nanocrystalline (5-50 nm) diamond and small amount of lonsdaleite showing distinct preferred orientations. Micro-focus X-ray and electron diffractions showed a coaxial relationship between lonsdaleite 100 and diamond 111, suggesting the martensitic formation of yakutite from crystalline graphite. These textural and crystallographic features are well comparable to those of the impact diamonds from the Popigai crater located in the central Siberia and strongly support the idea that yakutite is a product of long-distance outburst from the Popigai crater, which has been inferred merely from the geochemical signatures.
  • Arimoto Takeshi, Irifune Tetsuo, Nishi Masayuki, Ohfuji Hiroaki, Kunimoto Takehiro, Tange Yoshinori
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences 一般社団法人日本鉱物科学会 2017 (0) 112 - 112 2017 [Peer-reviewed]
    Phase relations in the system MgSiO3-FeSiO3 were investigated using Kawai-type multianvil apparatus with sintered diamond anvils up to 64 GPa and 2300 K.Bridgmanite coexists with stishovite and magnesiowüstite for the (Mg0.4Fe0.6)SiO3 starting material in the pressure regions applied in this study.We found that the solubility of FeSiO3 in bridgmanite increases almost linearly with increasing pressure from 20 mol% for 27 GPa to 38 mol% for 60 GPa at 2000 K.
  • Mizuguchi Takashi, Ito Tsutomu, Kimura Kota, Kurisaka Tokiko, Ohfuji Hiroaki, Kimura Masaki
    Journal of Japan Institute of Light Metals 一般社団法人 軽金属学会 67 (8) 361 - 366 0451-5994 2017 

    This work investigated the microstructural evolution occurring during high-temperature tensile tests (743 K, strain rate 1×10−3 s−1) of a friction stir processed polycrystalline solid solution 5083 aluminum alloy, along with the effect of the microstructure change on the room-temperature mechanical properties. Grain refinement was effectively achieved in the stir zone, with the mean observed grain size less than 10 µm. The nominal stress–nominal strain curve indicated that the flow stress drastically increased until a maximum stress value at which point fracture occurred. The value of elongation to failure exceeded 300%; this high ductility can be referred to as superplastic-like elongation. The area fraction of cavity increased with increasing strain. Room-temperature tensile tests at a strain rate of 1×10−3 s−1 revealed that the strength and ductility decreased with an increasing area fraction of cavity, and that the extent to which the strength and ductility are affected depends on the cavity.

  • Youmo Zhou, Tetsuo Irifune, Hiroaki Ohfuji, Toru Shinmei, Wei Du
    PHYSICS AND CHEMISTRY OF MINERALS 44 (1) 33 - 42 0342-1791 2017/01 Scientific journal 
    Lingunite (hollandite with Na contents of 80-85 mol%) discovered in the shock veins of strongly shocked meteorites is an important signature of shock metamorphism. To seek the stability region of lingunite, phase relations in the system KAlSi3O8-NaAlSi3O8 have been investigated by multi-anvil experiments at pressures of 20-23 GPa and temperatures of 1873 and 2273 K. Phase assemblages of hollandite + jadeite + stishovite, hollandite + calcium ferrite-type NaAlSiO4 + stishovite and hollandite single phase have been recovered, depending on the pressure-temperature conditions and the compositions of starting materials. Both pressure and temperature have large effects on the solubility of Na in hollandite, and hollandite with 79 mol% Na, similar to the natural lingunite in terms of Na content, has been firstly synthesized at 22 GPa and 2273 K. The stability region of K0.2Na0.8AlSi3O8 hollandite is comparable to the typical pressure-temperature conditions of the shock veins of strongly shocked meteorites (20-25 GPa and 2273-2500 K).
  • Nico A. Gaida, Norimasa Nishiyama, Atsunobu Masuno, Astrid Holzheid, Hiroaki Ohfuji, Ulrich Schuermann, Christin Szillus, Eleonora Kulik, Jozef Bednarcik, Oliver Beermann, Christopher Giehl, Lorenz Kienle
    JOURNAL OF THE AMERICAN CERAMIC SOCIETY 100 (1) 323 - 332 0002-7820 2017/01 Scientific journal 
    We report the synthesis of alumina/stishovite nano-nano composite ceramics through a pressure-induced dissociation in Al2SiO5 at a pressure of 15.6GPa and temperatures of 1300 degrees C-1900 degrees C. Stishovite is a high-pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from similar to 15 to similar to 750nm. The composite is harder than alumina and the hardness increases with reducing grain size down to similar to 80nm following a Hall-Petch relation. The maximum hardness with grain size of 81nm is 23 +/- 1GPa. A softening with reducing grain size was observed below this grain size down to similar to 15nm, which is known as inverse Hall-Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain-boundary phase.
  • H. Kadobayashi, H. Hirai, H. Ohfuji, Y. Kojima, Y. Ohishi, N. Hirao, M. Ohtake, Y. Yamamoto
    JOINT AIRAPT-25TH & EHPRG-53RD INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, 2015 950 1742-6588 2017 International conference proceedings 
    The phase transition mechanism of methane hydrate from sH to filled-ice Ih structure was examined using a combination of time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device (CCD) camera observation under fixed pressure conditions. Prior to time-resolved Raman experiments, the typical C-H vibration modes and their pressure dependence of three methane hydrate structures, fluid methane and solid methane were measured using Raman spectroscopy to distinguish the phase transitions of methane hydrates from decomposition to solid methane and ice VI or VII. Experimental results by XRD, Raman spectroscopy and CCD camera observation revealed that the structural transition of sH to filled-ice Ih occurs through a collapse of the sH framework followed by the release of fluid methane that is then gradually incorporated into the filled-ice Ih to reconstruct its structure. These observations suggest that the phase transition of sH to filled-ice Ih takes place by a typical reconstructive mechanism.
  • T. Irifune, K. Kawakami, T. Arimoto, H. Ohfuji, T. Kunimoto, T. Shinmei
    Nature Communications 7 2041-1723 2016/12 Scientific journal 
    Transparent ceramics are important for scientific and industrial applications because of the superior optical and mechanical properties. It has been suggested that optical transparency and mechanical strength are substantially enhanced if transparent ceramics with nanocrystals are available. However, synthesis of the highly transparent nano-crystalline ceramics has been difficult using conventional sintering techniques at relatively low pressures. Here we show direct conversion from bulk glass starting material in mutianvil high-pressure apparatus leads to pore-free nano-polycrystalline silicate garnet at pressures above similar to 10 GPa in a limited temperature range around 1,400 degrees C. The synthesized nano-polycrystalline garnet is optically as transparent as the single crystal for almost the entire visible light range and harder than the single crystal by similar to 30%. The ultrahigh-pressure conversion technique should provide novel functional ceramics having various crystal structures, including those of high-pressure phases, as well as ideal specimens for some mineral physics applications.
  • Hiroyuki Kagi, Dmitry A. Zedgenizov, Hiroaki Ohfuji, Hidemi Ishibashi
    GEOCHEMISTRY INTERNATIONAL 54 (10) 834 - 838 0016-7029 2016/10 Scientific journal 
    We report cloudy micro- and nano-inclusions in a superdeep diamond from So-Luiz, Brazil which contains inclusions of ferropericlase (Mg, Fe)O and former bridgmanite (Mg, Fe)SiO3 and ringwoodite (Mg, Fe)(2)SiO4. Field emission-SEM and TEM observations showed that the cloudy inclusions were composed of euhedral micro-inclusions with grain sizes ranging from tens nanometers to submicrometers. Infrared absorption spectra of the cloudy inclusions showed that water, carbonate, and silicates were not major components of these micro- and nano-inclusions and suggested that the main constituent of the inclusions was infrared-inactive. Some inclusions were suggested to contain material with lower atomic numbers than that of carbon. Mineral phase of nano- and micro-inclusions is unclear at present. Microbeam X-ray fluorescence analysis clarified that the micro-inclusions contained transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) possibly as metallic or sulfide phases. The cloudy inclusions provide an important information on the growth environment of superdeep diamonds in the transition zone or the lower mantle.
  • Hidemi Ishibashi, Hiroyuki Kagi, Shoko Odake, Hiroyuki Ohfuji, Hiroshi Kitawaki
    GEOCHEMISTRY INTERNATIONAL 54 (10) 882 - 889 0016-7029 2016/10 Scientific journal 
    Field Emission SEM (FESEM) textural observations, crystal size distribution (CSD) analyses, UV-excited luminescence imaging, and photoluminescence (PL) microspectroscopy excited by 488 nm laser were conducted on two texturally contrasting samples of carbonado, a kind of natural polycrystalline diamond from the Central African Republic (CAR). The investigated carbonado samples A and B show extremely different textures: sample A is made up of faceted crystals accompanied by abundant, small rectangular pores, whereas sample B has a granular texture with coarser crystals and scarce, large pores. Diamond crystals smaller than 2-3 A mu m are enriched in sample A but depleted in sample B. These textural features indicate that sample B diamonds were annealed under thermodynamically stable P-T conditions. The pore characteristics indicate that fluid permeability was higher for sample A than sample B. Photoluminescence (PL) spectra indicate that samples A and B correspond to Group A and B carbonados in the classification of Kagi et al. (1994), respectively, so that sample A reveals emissions from the H3 center without any N-V-0 derived emission at 575 nm, whereas sample B shows emissions from the 3H center and the N-V-0 defect. In addition, UV-excited luminescence images and photoluminescence spectra for sample B indicate that the rims of diamond crystals within several microns of a pore show luminescence features similar to those of Group AB carbonados (Kagi et al., 1994), indicating that this Group AB material was formed from Group B by irradiation from pore-filling, radioactive-element-bearing materials at a low temperature. The extent of the low-temperature irradiation is considered to depend on fluid permeability, and the Group A material was strongly irradiated due to its permeable texture whereas the Group B material was not significantly irradiated due to its less permeable granular texture. These results indicate that Group B carbonados have retained their original PL spectral features produced under high pressures and temperatures at mantle depths.
  • Shang-Pan Gao, Li Lei, Qi-Wei Hu, Lei-Ming Fang, Xian-Long Wang, Hiroaki Ohfuji, Yohei Kojima, Lei-Lei Zhang, Li-Jie Tan, Zhi Zeng, Fang Peng, Duan-Wei He, Tetsuo Irifune
    Gaoya Wuli Xuebao/Chinese Journal of High Pressure Physics 30 (4) 265 - 270 1000-5773 2016/08 Scientific journal 
    Ternary ε-Fe3-xMxN1+δ (M=Co,Ni) were synthesized as spherical bulk materials with variable components through composite high-pressure solid-state metathesis (HPSSM) reactions under 5 GPa and at 1673 K, employing diversity binary metal oxides (Fe2O3, CoO, NiO) and hBN as the reaction precursors. The structural characterizations of the as-prepared iron-based metal nitrides were determined by X-ray powder diffraction (XPD), field-emission scanning electronic microscopy (FE-SEM) and high-resolution transmission electron microscope (HRTEM), etc. First-principles calculations were used to explore the effect of pressure on the reaction enthalpy ΔH in HPSSM. Our results show that the high-pressure confinement environment is favorable for the synthesis of high-quality metal nitrides, and the HPSSM reaction is an effective synthetic route to the bulk iron-based metal nitrides.
  • Taku Fujii, Hiroaki Ohfuji, Toru Inoue
    PHYSICS AND CHEMISTRY OF MINERALS 43 (5) 353 - 361 0342-1791 2016/05 Scientific journal 
    Calcium sulfate (CaSO4), one of the major sulfate minerals in the Earth's crust, is expected to play a major role in sulfur recycling into the deep mantle. Here, we investigated the crystal structure and phase relation of CaSO4 up to similar to 90 GPa and 2300 K through a series of high-pressure experiments combined with in situ X-ray diffraction. CaSO4 forms three thermodynamically stable polymorphs: anhydrite (stable below 3 GPa), monazitetype phase (stable between 3 and similar to 13 GPa) and barite-type phase (stable up to at least 93 GPa). Anhydrite to monazite-type phase transition is induced by pressure even at room temperature, while monazite-to barite-type transition requires heating at least to 1500 K at similar to 20 GPa. The barite-type phase cannot always be quenched from high temperature and is distorted to metastable AgMnO4-type structure or another modified barite structure depending on pressure. We obtained the pressure-volume data and density of anhydrite, monazite-and barite-type phases and found that their densities are lower than those calculated from the PREM model in the studied P-T conditions. This suggests that CaSO4 is gravitationally unstable in the mantle and fluid/melt phase into which sulfur dissolves and/or sulfate-sulfide speciation may play a major role in the sulfur recycling into the deep Earth.
  • Ayako Shinozaki, Hiroyuki Kagi, Hisako Hirai, Hiroaki Ohfuji, Taku Okada, Satoshi Nakano, Takehiko Yagi
    PHYSICS AND CHEMISTRY OF MINERALS 43 (4) 277 - 285 0342-1791 2016/04 Scientific journal 
    Stability and phase relations of coexisting enstatite and H-2 fluid were investigated in the pressure and temperature regions of 3.1-13.9 GPa and 1500-2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO2 grains were found at the margin of the heating hot spot, suggesting that the SiO2 component dissolved in the H-2 fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH4 and H2O molecules formed through the reaction between dissolved SiO2 and H-2. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO2 components of enstatite in H-2 fluid, as well as that observed in the forsterite H-2 system and the quartz H-2 system, implies that H-2-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.
  • T. Ejima, M. Akasaka, T. Nagao, H. Ohfuji
    MINERALOGICAL MAGAZINE 79 (7) 1833 - 1848 0026-461X 2015/12 Scientific journal 
    The oxidation state of Fe and precipitates within olivine phenocrysts from an olivine-basalt from Kuroshima volcano, Goto Islands, Nagasaki Prefecture, Japan, were determined using electron microprobe analysis, Fe-57 Mossbauer spectroscopy, Raman spectroscopy and transmission electron microscopy, to examine the formation process of the Fe-bearing precipitates. The average Fo content of the olivine phenocrysts is 76.2 mol.%. The olivine phenocrysts occasionally have precipitate minerals at their rims, especially on rims near vesicles. The 57Fe Mossbauer spectrum of olivine separates consists of two doublets assigned to Fe2+ at the octahedral M1 and M2 sites, and a Fe3+ doublet at the M1 and M2 sites. The Fe2+: Fe3+ ratio is 90(5): 10(1). The precipitates at the rims of the olivine phenocrysts consist of magnetite and enstatite showing coaxial relations with host olivine, and grow parallel to the olivine c axis. Moreover, clusters consisting of nanoscale domains of a few tens of nm in size occur in the host olivine. Their rounded form and appearance in transmission electron microscope images are similar to those of the magnetite precipitates, but they have an olivine structure and can be regarded as embryos of magnetite within the olivine. The oxidation process of olivine phenocrysts under cooling conditions is: (1) formation of magnetite embryos on the rims of olivine phenocrysts; (2) formation of enstatite-like pyroxene domains by depletion of Fe in olivine due to the generation of magnetite embryos; (3) crystallization of magnetite and enstatite- like pyroxene precipitates.
  • Eleonora Kulik, Norimasa Nishiyama, Atsunobu Masuno, Yan Zubavichus, Vadim Murzin, Evgeny Khramov, Akihiro Yamada, Hiroaki Ohfuji, Hans-Christian Wille, Tetsuo Irifune, Tomoo Katsura
    JOURNAL OF THE AMERICAN CERAMIC SOCIETY 98 (12) 4111 - 4116 0002-7820 2015/12 Scientific journal 
    High pressure and temperature synthesis of compositions made of (Si1-x,Ge-x)O-2 where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7-12 GPa and 1200-1600 degrees C using a Kawai-type high-pressure apparatus. At 12 GPa and 1600 degrees C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile-type phases in the SiO2-GeO2 system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600 degrees C, but not at 1200 degrees C. At lower pressures (7 and 9 GPa) and 1600 degrees C, we observed the decomposition of (Si-0.5,Ge-0.5)O-2 into SiO2-rich coesite and GeO2-rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite-stishovite phase transition pressure in SiO2.
  • Nao Cai, Toru Inoue, Kiyoshi Fujino, Hiroaki Ohfuji, Hisayoshi Yurimoto
    AMERICAN MINERALOGIST 100 (10) 2330 - 2335 0003-004X 2015/10 Scientific journal 
    A new Al-bearing hydrous Mg-silicate that we named as 23 angstrom phase was synthesized at 10 GPa and 1000 degrees C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg7Si2O8(OH)(6)] + Al2O3 + H2O. The chemical composition of this new 23 angstrom phase is Mg11Al2Si4O16(OH)(12), and it contains about 12.1 wt% water. Powder X-ray diffraction and electron diffraction patterns show that this new 23 angstrom phase has a hexagonal structure, with a = 5.1972(2), c = 22.991(4) angstrom, and V = 537.8(2) angstrom(3), and the possible space group is P (6) over bar c2, P6(3)cm, or P6(3)/mcm. The calculated density is 2.761 g/cm(3) accordingly, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. Several experiments revealed that its stability region is very similar to that of phase A. We further confirmed that this new 23 angstrom phase was stable in the chlorite composition at 10 GPa and 1000 degrees C. The present results indicate that this new 23 angstrom hydrous phase will form in an Al-bearing subducting slab, and transport water together with Al into the deep upper mantle or even into the upper part of the transition zone.
  • Hiroaki Ohfuji, Tetsuo Irifune, Konstantin D. Litasov, Tomoharu Yamashita, Futoshi Isobe, Valentin P. Afanasiev, Nikolai P. Pokhilenko
    SCIENTIFIC REPORTS 5 2045-2322 2015/10 Scientific journal 
    Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds. Here we identified natural pure NPD, which was produced by a large meteoritic impact about 35 Ma ago in Russia. The impact diamonds consist of well-sintered equigranular nanocrystals (5-50 nm), similar to synthetic NPD, but with distinct [111] preferred orientation. They formed through the martensitic transformation from single-crystal graphite. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD. A huge amount of natural NPD is expected to be present in the Popigai crater, which is potentially important for applications as novel ultra-hard material.
  • Hiroaki Ohfuji, Masashi Yamamoto
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 110 (4) 189 - 195 1345-6296 2015/08 Scientific journal 
    This study demonstrates the validity of a thin osmium coating for quantitative energy-dispersive spectroscopic (EDS) analysis, particularly for light elements such as O (and potentially C and N) in natural/synthetic minerals. An osmium coating prepared by chemical vapor deposition provides an extremely thin and uniform layer whose thickness can be controlled simply by coating time. Because of the high reproducibility and reliability of the osmium coating process, users have no difficulty in evaluating the actual coating thickness, which enables strict and precise absorption corrections (for the coating layer), even for low-energy characteristic X-rays, which are susceptible to attenuation by the coating layer itself. Our results show that oxygen concentrations in silicate and oxide minerals can be quantified correctly when using the osmium coating, whereas quantification using a carbon coating afforded values that were a few wt% lower than stoichiometry, probably due to the uncertainty of the actual coating thickness (i.e., the absorption correction was incorrect). The ability to accurately quantify oxygen may stimulate new analytical applications, such as the estimation of Fe2+/Fe3+ concentrations and water content in minerals. Furthermore, the Os-coated samples prepared for EDS analysis are also suitable for electron back-scattered diffraction (EBSD) analysis without re-polishing and re-coating, which are usually routine but time-consuming tasks in the case of carbon-coated samples.
  • Hideaki Yasuhara, Naoki Kinoshita, Hiroaki Ohfuji, Manabu Takahashi, Kazumasa Ito, Kiyoshi Kishida
    WATER RESOURCES RESEARCH 51 (7) 5425 - 5449 0043-1397 2015/07 Scientific journal 
    In this study, a series of long-term, intermittent permeability experiments utilizing Berea sandstone and Horonobe mudstone samples, with and without a single artificial fracture, is conducted for more than 1000 days to examine the evolution of rock permeability under relatively high-temperature and confining pressure conditions. Effluent element concentrations are also measured throughout the experiments. Before and after flow-through experiments, rock samples are prepared for X-ray diffraction, X-ray fluorescence, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to examine the mineralogical changes between pre and postexperimental samples, and also for microfocus X-ray CT to evaluate the alteration of the microstructure. Although there are exceptions, the observed, qualitative evolution of permeability is found to be generally consistent in both the intact and the fractured rock samplesthe permeability in the intact rock samples increases with time after experiencing no significant changes in permeability for the first several hundred days, while that in the fractured rock samples decreases with time. An evaluation of the Damkohler number and of the net dissolution, using the measured element concentrations, reveals that the increase in permeability can most likely be attributed to the relative dominance of the mineral dissolution in the pore spaces, while the decrease can most likely be attributed to the mineral dissolution/crushing at the propping asperities within the fracture. Taking supplemental observations by microfocus X-ray CT and using the intact sandstone samples, a slight increase in relatively large pore spaces is seen. This supports the increase in permeability observed in the flow-through experiments.
  • Mitsuhiro Okayasu, Shuhei Takeuchi, Masaki Yamamoto, Hiroaki Ohfuji, Toshihiro Ochi
    The effects of microstructural characteristics (secondary dendrite arm spacing, SDAS) and Si- and Fe-based eutectic structures on the mechanical properties and failure behavior of an Al-Si-Cu alloy are investigated. Cast Al alloy samples are produced using a special continuous-casting technique with which it is easy to control both the sizes of microstructures and the direction of crystal orientation. Dendrite cells appear to grow in the casting direction. There are linear correlations between SDAS and tensile properties (ultimate tensile strength sigma (UTS), 0.2 pct proof strength sigma (0.2), and fracture strain epsilon (f)). These linear correlations, however, break down, especially for sigma (UTS) vs SDAS and epsilon (f) vs SDAS, as the eutectic structures become more than 3 mu m in diameter, when the strength and ductility (sigma (UTS) and epsilon (f)) decrease significantly. For eutectic structures larger than 3 mu m, failure is dominated by the brittle eutectic phases, for which SDAS is no longer strongly correlated with sigma (UTS) and epsilon (f). In contrast, a linear correlation is obtained between sigma (0.2) and SDAS, even for eutectic structures larger than 3 mu m, and the eutectic structure does not have a strong effect on yield behavior. This is because failure in the eutectic phases occurs just before final fracture. In situ failure observation during tensile testing is performed using microstructural and lattice characteristics. From the experimental results obtained, models of failure during tensile loading are proposed. (C) The Minerals, Metals & Materials Society and ASM International 2015
  • Koji Maruyama, Hiroyuki Kagi, Toru Inoue, Hiroaki Ohfuji, Toru Yoshino
    CHEMISTRY LETTERS 44 (4) 434 - 436 0366-7022 2015/04 Scientific journal 
    Pressure-induced crystallization from amorphous calcium carbonate (ACC) was observed with X-ray diffraction at room temperature. Time-resolved diffraction patterns revealed that calcite and vaterite simultaneously crystallized from ACC, without transformation from vaterite to calcite against Ostwald's step rule. The n values of calcite in Avrami modeling were slightly larger than that of vaterite, which was consistent with the slightly larger grain size of calcite compared with vaterite. Microtexture of the recovered samples suggested that vaterite and calcite crystallized by noticeably different mechanisms.
  • Takeshi Sakai, Takehiko Yagi, Hiroaki Ohfuji, Tetsuo Irifune, Yasuo Ohishi, Naohisa Hirao, Yuya Suzuki, Yasushi Kuroda, Takayuki Asakawa, Takashi Kanemura
    REVIEW OF SCIENTIFIC INSTRUMENTS 86 (3) 0034-6748 2015/03 Scientific journal 
    Micron-sized diamond anvils with a 3 mu m culet were successfully processed using a focused ion beam (FIB) system and the generation of high pressures was confirmed using the double stage diamond anvil cell technique. The difficulty of aligning two second-stage micro-anvils was solved via the paired micro-anvil method. Micro-manufacturing using a FIB system enables us to control anvil shape, process any materials, including nano-polycrystalline diamond and single crystal diamond, and assemble the sample exactly in a very small space between the second-stage anvils. This method is highly reproducible. High pressures over 300 GPa were achieved, and the pressure distribution around the micro-anvil culet was evaluated by using a well-focused synchrotron micro-X-ray beam. (C) 2015 AIP Publishing LLC.
  • Taku Fujii, Hiroaki Ohfuji
    MEASUREMENT SCIENCE AND TECHNOLOGY 26 (2) 0957-0233 2015/02 Scientific journal 
    Pressure estimation using the frequency shift of the diamond Raman peak from the anvil culet is readily and widely used in diamond anvil cell experiments along with the conventional ruby fluorescence method. Here, we propose a modified diamond Raman scale particularly designed for pressure measurement below similar to 10 GPa. A series of experiments were conducted using a highly confocal Raman system and H2O, ethanol/methanol mixture and NaCl samples loaded in a rhenium gasket which was pre-indented to 40-60 or 100-110 mu m thick. The result showed that the frequency of the diamond Raman peak from the anvil culet increases linearly with pressure between 1 and 13 GPa, when using a sufficiently pre-indented (40-60 mu m thick) gasket. The frequency shifts are calibrated against the pressure determined by the ruby fluorescence method, which is an alternative pressure scale. In addition, a preliminary measurement at high temperature up to 575 K suggests the potential application of this method for high temperature experiments.
  • Yasui Nozomu, Sougawa Masaya, Hirai Masaaki, Yamamoto Kaoru, Okada Taku, Yamazaki Daisuke, Kojima Yohei, Ohfuji Hiroaki, Kunitsugu Shinsuke, Takarabe Kenichi
    COGENT PHYSICS 2 (1) 2331-1940 2015 [Peer-reviewed]
  • Mitsuhiro Okayasu, Shuhei Takeuchi, Hiroaki Ohfuji
    The mechanical properties and failure characteristics of a cast Mg alloy (AZ91: Mg-Al-8.9-Zn-0.6-Mn-0.2) produced by a heated-mold continuous casting process (HMC) are investigated. In a modification of the original HMC process, the cooling of the liquid alloy by direct water spray is carried out in an atmosphere of high-purity argon gas. The HMC-AZ91 alloy exhibits excellent mechanical properties (high strength and high ductility) that are about twice as high as those for the same alloy produced by conventional gravity casting. The increased material strength and ductility of the HMC sample are attributed to nanoscale and microscale microstructural characteristics. The fine grains and tiny spherical eutectic structures (e.g., Mg17Al12 and Al6Mn) distributed randomly in the matrix of the HMC alloy result in resistance to dislocation movement, leading to high tensile strength. Basal slip on (0001) planes in the relatively organized crystal orientation of the HMC alloy, as well as grain boundary sliding through tiny spherical eutectic structures, results in high ductility. Details of the failure mechanism under static loading in the HMC alloy are also discussed using failure models.
  • Norimasa Nishiyama, Fumihiro Wakai, Hiroaki Ohfuji, Yusuke Tamenori, Hidenobu Murata, Takashi Taniguchi, Masafumi Matsushita, Manabu Takahashi, Eleonora Kulik, Kimiko Yoshida, Kouhei Wada, Jozef Bednarcik, Tetsuo Irifune
    SCIENTIFIC REPORTS 4 2045-2322 2014/10 Scientific journal 
    Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of similar to 100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.
  • Yoshio Kono, Curtis Kenney-Benson, Daniel Hummer, Hiroaki Ohfuji, Changyong Park, Guoyin Shen, Yanbin Wang, Abby Kavner, Craig E. Manning
    NATURE COMMUNICATIONS 5 2041-1723 2014/10 Scientific journal 
    Knowledge of the occurrence and mobility of carbonate-rich melts in the Earth's mantle is important for understanding the deep carbon cycle and related geochemical and geophysical processes. However, our understanding of the mobility of carbonate-rich melts remains poor. Here we report viscosities of carbonate melts up to 6.2 GPa using a newly developed technique of ultrafast synchrotron X-ray imaging. These carbonate melts display ultralow viscosities, much lower than previously thought, in the range of 0.006-0.010 Pa s, which are similar to 2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. As a result, the mobility of carbonate melts (defined as the ratio of melt-solid density contrast to melt viscosity) is similar to 2 to 3 orders of magnitude higher than that of basaltic melts. Such high mobility has significant influence on several magmatic processes, such as fast melt migration and effective melt extraction beneath mid-ocean ridges.
  • Ayako Shinozaki, Hiroyuki Kagi, Naoki Noguchi, Hisako Hirai, Hiroaki Ohfuji, Taku Okada, Satoshi Nakano, Takehiko Yagi
    AMERICAN MINERALOGIST 99 (7) 1265 - 1269 0003-004X 2014/07 Scientific journal 
    Species dissolved in H-2 fluid were investigated in a SiO2-H-2 system. Raman and infrared (IR) spectra were measured at high pressure and room temperature after heating experiments were conducted at two pressure and temperature conditions: 2.0 GPa, 1700 K and 3.0 GPa, 1500K. With the dissolution of quartz, a SiH vibration mode assignable to SiH4 was detected from Raman spectra of the fluid phase. Furthermore, an OH vibration mode was observed at 3260 cm(-1) from the IR spectra at 3.0 GPa. With decreasing pressure, the OH vibration frequencies observed between 3.0 and 2.1 GPa correspond to that of ice VII, and those observed at 1.4 and 1.1 GPa correspond to that of ice VI. These results indicate that the chemical reaction between dissolved SiO2 components and H-2 fluid caused the formation of H2O and SiH4, which was contrastive to that observed in SiO2-H2O fluid. Results imply that a part of H-2 is oxidized to form H2O when SiO2 components of mantle minerals dissolve in H-2 fluid, even in an iron-free system.
  • Kiyoshi Fujino, Daisuke Nishio-Hamane, Takaya Nagai, Yusuke Seto, Yasuhiro Kuwayama, Matthew Whitaker, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 228 186 - 191 0031-9201 2014/03 Scientific journal 
    Spin transition and substitution of Fe3+ in Fe3+AlO3-bearing MgSiO3 perovskite (Pv) and post-perovskite (PPv) were examined up to 200 and 165 GPa, respectively, at room temperature by X-ray emission spectroscopy (XES) and XRD. The results of XES and XRD indicate that in Pv high spin (HS) Fe3+ at the dodecahedral (A) site replaces Al at the octahedral (B) site and becomes low spin (LS) between 50 and 70 GPa with pressure, while in PPv LS Fe3+ occupies the B-site and Al occupies the A-site above 80-100 GPa. The Fe3+ Al coupled substitution seems to be at work in both Pv and PPv. Combining these results on Fe3+ with the recent first-principles calculations on Fe2+ in Pv and PPv, the spin transition and substitution of iron in pyrolitic lower mantle minerals are proposed. Further, their effects on iron-partitioning among the lower mantle minerals are discussed. (C) 2013 Elsevier B.V. All rights reserved.
  • M. Okayasu, H. Fukui, H. Ohfuji, T. Shiraishi
    MATERIALS SCIENCE AND TECHNOLOGY 30 (3) 301 - 308 0267-0836 2014/03 Scientific journal 
    The strain induced martensitic transformation (SMT) of the austenite stainless steel (SUS 304) under cyclic loading and static loading was investigated directly using electron backscattered diffraction. Two different SMT characteristics are observed, which are attributed to the differences of plastic and twinning deformation. The maximum cyclic stress has a strong influence on the SMT. The total area fraction of the Fe-alpha' phase increases significantly when the maximum cyclic load is.80% sigma(UTS). In other words, the SMT is apparently absent when the samples are loaded with less than sigma(max)=70% sigma(UTS), although such samples are fractured completely. Moreover, there is a clear R ratio effect on the SMT. For example, the loading condition R=-1 gives rise to a strong SMT compared to R=0.1 due to the more severe strain caused by the compressive stress. In contrast, no clear frequency effect (1 versus 30 Hz) on the SMT is detected, which may be attributed to the same maximum cyclic stress. Like the SMT characteristics under cyclic loading, the proportion of Fe-alpha' phase shows no clear increase until the sample is loaded statically to a tensile stress,70% sigma(UTS).
  • Shinozaki Ayako, Kagi Hiroyuki, Hirai Hisako, Ohfuji Hiroaki, Okada Taku, Nakano Satoshi, Yagi Takehiko
    Abstracts of Annual Meeting of the Geochemical Society of Japan 一般社団法人日本地球化学会 61 (0) 241 - 241 2014 [Peer-reviewed]
    マントルに相当する高温高圧下において水素流体が共存するケイ酸塩鉱物の相関係、融点に与える影響を明らかにするため、レーザー加熱ダイヤモンドアンビルセルを用いた高温高圧実験を行った。約3GPaから15 GPa, 1400 KでMg2SiO4-H2系ではforsteriteのSiO2成分が水素に溶け込んでpericlase(MgO)が生成した。MgSiO3-H2系でも同様にorthoenstatite中のSiO2成分が溶けてforsteriteとpericlaseが生成した。水素流体へとSiO2成分が溶け、共存するケイ酸塩鉱物の組成が大きく変化することが示された。
  • Nishiyama Norimasa, Fumihiro Wakai, Hiroaki Ohfuji, Yusuke Tamenori, Hidenobu Murata, Takashi Taniguchi, Masafumi Matsushita, Manabu Takahashi, Eleonora Kulik, Kimiko Yoshida, Kouhei Wada, Jozef Bednarcik, Tetsuo Irifune
    Scientific Reports 4 6558  2014 [Peer-reviewed]
     Scientific journal
  • M. Okayasu, K. Ota, S. Takeuchi, H. Ohfuji, T. Shiraishi
    The effects of microstructural characteristics on the mechanical properties of an aluminum alloy (ADC12: Al-Si-11.3-Cu-1.9-Zn-0.8-Fe-0.8-Mg-0.2-Mn-0.2) produced by various casting technologies are studied experimentally and numerically. Six different casting processes are employed: gravity casting, cold-chamber die-casting, hot-chamber die-casting, squeeze casting, twin-rolled continuous casting and heated-mold continuous casting. Microstructural characteristics, dislocation density and defect density vary depending on the casting method, owing to differences in solidification rate, casting pressure and injection speed. The material characteristics of the samples affect their mechanical properties. Multiple regression analysis is carried out to find equations to predict tensile strength using five independent factors: secondary dendrite arm spacing, microporosity rate, diameter of eutectic structures, aspect ratio of eutectic structures and dislocation density. All these factors influence the tensile properties, although to different degrees. The estimated values of tensile strength are in good agreement with experimental results. (C) 2013 Elsevier B.V. All rights reserved.
  • Norimasa Nishiyama, Takashi Taniguchi, Hiroaki Ohfuji, Kimiko Yoshida, Fumihiro Wakai, Byung-Nam Kim, Hidehiro Yoshida, Yuji Higo, Astrid Holzheid, Oliver Beermann, Tetsuo Irifune, Yoshio Sakka, Ken-ichi Funakoshi
    SCRIPTA MATERIALIA 69 (5) 362 - 365 1359-6462 2013/09 Scientific journal 
    Transparent nanocrystalline bulk alumina was obtained at 7.7 GPa and 800 degrees C. The alumina has an average grain size of similar to 150 nm and an in-line transmission of 71% for a wavelength of 640 nm and a thickness of 0.8 mm. The values of microhardness and fracture toughness were found to be 25.5 +/- 0.3 GPa and 2.9 +/- 0.3 MPa m(1/2), respectively. The hardness value is consistent with the previously determined Hall-Petch relation and fracture toughness is independent of grain size. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Mitsuhiro Okayasu, Hironobu Fukui, Hiroaki Ohfuji, Tetsuro Shiraishi
    JOURNAL OF MATERIALS SCIENCE 48 (18) 6157 - 6166 0022-2461 2013/09 Scientific journal 
    In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room temperature were performed around the notch of a dumbbell-shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (sigma (0.2)), some SMTs occurred. However, the area fraction of the Fe-alpha'-martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (sigma (UTS)). After the sigma (UTS) point, the total fraction of the Fe-alpha' phase increased dramatically to the fracture point (sigma (f)). The phase textures of Fe-alpha' and Fe-gamma were almost equal at (sigma (UTS) - sigma (f))/2, and the Fe-alpha' phase was observed over almost the entire measurement area around the notch at the sigma (f) point. However, the area fraction of the Fe-alpha' phase at the sigma (f) point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-alpha' phase was almost the same as that of the Fe-gamma phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around sigma (UTS), and severe plastic deformation before fracture make a strong Fe-alpha' phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter diffraction analysis and finite element analysis.
  • Ayako Shinozaki, Hisako Hirai, Hiroaki Ohfuji, Taku Okada, Shin-ichi Machida, Takehiko Yagi
    AMERICAN MINERALOGIST 98 (8-9) 1604 - 1609 0003-004X 2013/08 Scientific journal 
    High-pressure and high-temperature experiments were carried out in a Mg2SiO4-H-2 system using laser-heated diamond-anvil cells to understand the influence of H-2 fluid on the stability of forsterite. In situ X-ray diffraction experiments and Raman spectroscopic measurements showed the decomposition of forsterite, and formation of periclase (MgO) and stishovite/quartz (SiO2) in the presence of H-2 after being heated in the range between 2.5 GPa, 1400 K and 15.0 GPa, 1500K. Transmission electron microscopic observation of the samples recovered from 15.0 GPa and 1500 K showed that the granular to columnar periclase grains maintained the original grain shape of forsterite, indicating that the periclase crystals crystallized under high temperature. On the other hand, euhedral columnar stishovite crystals were found at the boundaries between residual forsterite grains and reacted periclase. This implies that the SiO2 component was dissolved in H-2 fluid, and that stishovite was considered to have crystallized when the solubility of the SiO2 component became reduced with decreasing temperature. Additional experiment on a SiO2-H-2 system clearly showed the dissolution of quartz in H-2 fluid, while those on a MgO-H-2 system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H-2 fluid to form periclase crystals in the Mg2SiO4-H-2 system, which is different from what was observed in the Mg2SiO4-H2O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.
  • Lei-Ming Fang, Xi-Ping Chen, H. Ohfuji, T. Irifune, Guang-Ai Sun, Bo Chen, Shu-Ming Peng
    Chinese Physics C 37 (8) 1674-1137 2013/08 Scientific journal 
    High pressure pyrolysis of melamine has been attracting great interest recently, due to it being considered as a suitable precursor to realize the g-C3N4 and even superhard C3N4. In this work, we studied the detailed pyrolysis behavior of melamine at 22 GPa. Melamine was stable at 800 °C, and decomposed to diamond in the form of powder at 1500-2000 °C under this pressure condition. At 2000 °C, the pure cubic diamond powders with 0.1-0.5 μm grain size were obtained. The diamond particles exhibited euhedral forms and dispersed to each other, we proposed that these novel features were caused by the presence of liquid N2 and NH3 during diamond formation. The high pressure pyrolysis of melamine may provide a new means of producing micrometer-sized diamond powders. © 2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.
  • Li Lei, Hiroaki Ohfuji, Jiaqian Qin, Xinyu Zhang, Fulong Wang, Tetsuo Irifune
    SOLID STATE COMMUNICATIONS 164 6 - 10 0038-1098 2013/06 Scientific journal 
    The high-pressure phase transitions and Raman phonon behaviors of LiGaO2 were investigated by high-pressure Raman spectroscopy and first-principle calculations. The beta-LiGaO2 was evidenced to undergo irreversible phase transitions to Raman silent gamma-LiGaO2 at similar to 14 GPa, and the high-pressure phase transition from alpha- to beta-phase was found to be kinetically sluggish at room temperature. In addition, mode-Gruneisen parameters, bulk modulus, and pressure dependence of Raman phonon frequencies LiGaO2 were also determined. (C) 2013 Elsevier Ltd. All rights reserved.
  • Kiyoshi Fujino, Daisuke Nishio-Hamane, Yasuhiro Kuwayama, Nagayoshi Sata, Sayaka Murakami, Matthew Whitaker, Ayako Shinozaki, Hiroaki Ohfuji, Yohei Kojima, Tetsuo Irifune, Nozomu Hiraoka, Hirofumi Ishii, Ku-Ding Tsuei
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 217 31 - 35 0031-9201 2013/04 Scientific journal 
    The spin transition and substitution of Fe3+ in Mg0.85Fe0.153+Al0.15Si0.85O3 post-Mg-perovskite (PPv) synthesized at 165-170 GPa, 2100 K were examined by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) during decompression to 37 GPa without annealing. XRD showed that samples were single phase PPv down to 49 GPa, although the samples may have partly become amorphous. The result of XES measurement indicates that Fe3+ is fully low spin (LS) for 165-100 GPa and Fe3+ largely changes from LS to high spin (HS) between 80 and similar to 40 GPa. These results combined with previous reports indicate that Al and LS Fe3+ may occupy the A-(dodecahedral) and B-(octahedral) sites, respectively, as the favorable cation sites for 165-100 GPa in Al-bearing PPv. Based on the present results on Fe3+ and recent theoretical reports on Fe2+, the spin states of iron in lower mantle PPv are discussed. (C) 2013 Elsevier B.V. All rights reserved.
  • 攝津暢浩, 松下正史, 高橋学, 水井洸佑, 岡部永年, 大藤弘明
    材料システム 31 43 - 50 0286-6013 2013/03
  • Terumi Ejima, Masahide Akasaka, Takashi Nagao, Hiroaki Ohfuji
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 108 (1) 25 - 36 1345-6296 2013/02 Scientific journal 
    Oxidation states of Fe and precipitates within olivine in orthopyroxene-olivine-clinopyroxene andesite (Opx-Ol-Cpx andesite) lava from Kasayama volcano, Hagi, Yamaguchi Prefecture, were investigated to reveal the oxidation process of the lava at high temperatures, using electron microprobe analysis, Raman spectroscopy and transmission electron microscopy. Although the Opx-Ol-Cpx andesite lava is generally black in color, in places it has a red-brown surface and reddish-black subsurface. Olivines from the black lava have normal zoning with Fo(68.5-74 9) cores and Fo(64 9-72.9) rims. Olivine in the black lavas with red-brown tint and red-brown lava contains precipitates of Ti-rich hematite, hematite, magnesioferrite and enstatite, and tends to be Mg-rich (cores: Fo(74.1-78.6); rims: Fo(76.4-83.8)) in comparison with black lava. Stronger red coloration of the lavas is related to greater volume of cryptocrystalline precipitates within olivine. This results in increased Mg contents in Olivine. Olivines in red-brown lava are extremely Mg-rich (Fo(91.0-95.4)). By applying the correlation between FeL beta- and FeL alpha-intensity ratio and Fe2+/Fe3+-ratio, small amounts of Fe3+ (0.05 atoms per formula unit at maximum) were invariably detected in olivine from the black lava with red-brown tint. Even in olivine in the black lava, Fe3+ was detected in the rims, although Fe is ferrous in the cores. These facts on the chemical compositions and oxidation state of Fe within olivine phenocrysts and the occurrence of vermicular rod-form titanohematite and magnesioferrite precipitates in olivine provide the evidence for high temperature oxidation, at temperatures above 800 degrees C.
  • Masayuki Nishi, Tomoaki Kubo, Hiroaki Ohfuji, Takumi Kato, Yu Nishihara, Tetsuo Irifune
    EARTH AND PLANETARY SCIENCE LETTERS 361 44 - 49 0012-821X 2013/01 Scientific journal 
    Si4+ + M2+ <-> 2Al(3+) (M=Mg+Fe+Ca) interdiffusion rates in majoritic garnet have been determined at pressure of 17 GPa and temperatures of 1550-1700 degrees C using a Kawai-type high-pressure apparatus. The diffusion profiles were measured across the interface between pyropic garnet and majoritic garnet samples by an analytical transmission electron microscope, and the rate of the Si4+ + M2+ <-> 2Al(3+) interdiffusion was determined to be a Dsi-Al (m(2) s(-1))=2 x 10(-1) exp [-364 (kJ mol(-1))/RT]. We found that the diffusion rate is low compared to the Si diffusion rate in other silicate minerals relevant to the deep mantle. Slow kinetics of the pyroxene-garnet transformation due to the low diffusion rate indicates that large low-density metastable regions may exist in the slab, greatly contributing to the slab stagnation around the transition region. Stagnant slabs possibly stay in this region over 10(8) yr because of the sluggish pyroxene-garnet transformation, but may eventually fall into the lower mantle after the completion of the transformation. (C) 2012 Elsevier B.V. All rights reserved.
  • Mitsuhiro Okayasu, Yuki Ohkura, Tatsuaki Sakamoto, Shuhei Takeuchi, Hiroaki Ohfuji, Tetsuro Shiraishi
    To better understand the fatigue and tensile properties of SPCC steels welded by metal inert gas welding, the mechanical properties of the welded component in several localized regions, e.g., weld metal, heat affected zone (HAZ) and base metal, were investigated. The tensile and fatigue properties of the weld metals were high compared to the other areas (base metal and HAZ) due to the precipitated Ti containing oxide inclusions in acicular ferrite (bainite). Two typical microstructures were mainly observed in the heat affected zones (HAZ): (i) bainite in a ferrite matrix (HAZ-B) and (ii) a ferrite phase with low internal stress (HAZ-A). The hardness of HAZ-B was higher than HAZ-A because of the partially formed bainite structure and precipitated Ti containing oxide inclusions. The mechanical properties of the weld sample were further investigated using test specimens that included all regions, i.e., weld metal, HAZ and base metal (BHW). The tensile and fatigue properties of the BHW sample were found to be lower than those in all other regions, which was influenced by the high internal stress. The mechanical properties were analyzed using microstructural and crystal characteristics, as examined by TEM and EBSD analysis. In addition, the fatigue strength can be precisely evaluated using a modified Goodman diagram. (C) 2012 Elsevier B.V. All rights reserved.
  • Futoshi Isobe, Hiroaki Ohfuji, Hitoshi Sumiya, Tetsuo Irifune
    JOURNAL OF NANOMATERIALS 2013 1687-4110 2013 Scientific journal 
    A new type of polycrystalline sintered diamond has been successfully synthesized by direct conversion from highly oriented pyrolytic graphite at 15GPa and 2300 degrees C. It is optically transparent and consists entirely of layered nanocrystals (50-100nm thick) of cubic diamond, which are tightly bound to each other and have strong [111] preferred orientation along the stacking direction. This nanolayered diamond has excellent indentation hardness (similar to 114GPa in Knoop scale) comparable to the highest values obtained from single crystalline diamond. Furthermore, it is expected to have significantly high wear resistance on both ends of cylindrical sintered compact, since the surfaces are terminated exclusively by the hardest {111} planes of the layered diamond nanocrystals.
  • Leiming Fang, Hiroaki Ohfuji, Tetsuo Irifune
    JOURNAL OF NANOMATERIALS 2013 1687-4110 2013 Scientific journal 
    We developed a novel technique to synthesize nanodiamond powder through the decomposition of graphitic C3N4 under high pressure and high temperature. The nanodiamond obtained by the present method is in an extremely pure form with no sp(2) carbon contaminations. Individual nanodiamond grains are very uniform in size and virtually monodispersed single crystals. The grain size can be controlled from less than 1 nm to several hundred nanometers by adjusting the heating temperature (and also potentially by controlling pressure) used for the synthesis. The present product requires neither post-surface treatment to remove outer shell made of sp(2) carbons nor deglomeration and size classification unlike the case for nanodiamond obtained by the conventional TNT detonation method.
  • Yohei Kojima, Hiroaki Ohfuji
    Diamond and Related Materials 39 1 - 7 0925-9635 2013 Scientific journal 
    The crystal structure of carbon nitride under high pressure and temperature was investigated up to megabar pressures using graphitic C3N 4 as a starting material. It transformed to an orthorhombic phase above 30 GPa and 1600 K, which has a similar unit cell parameters (a = 7.6251(19), b = 4.4904(8), and c = 4.0424(8) Å at 1 atm) to those of reported hydrogen-bearing carbon nitride phases such as C2N 2(NH) and C2N2(CH2). Although the C:N ratio of this orthorhombic phase was carefully determined to be 3:4, FT-IR analysis showed a strong possibility of hydrogen contamination both in the starting and recovered samples. These results suggest that in the studied wide pressure and temperature range, hydrogen-bearing carbon nitride favors the orthorhombic structure with a fundamental composition of C2N 2X where NH, CH2, and even potentially vacancies can be flexibly accommodated in the X site. © 2013 Published by Elsevier B.V.
  • Norimasa Nishiyama, Satoshi Seike, Teruo Hamaguchi, Tetsuo Irifune, Masafumi Matsushita, Manabu Takahashi, Hiroaki Ohfuji, Yoshio Kono
    SCRIPTA MATERIALIA 67 (12) 955 - 958 1359-6462 2012/12 Scientific journal 
    Nanocrystalline bulk stishovite, with pure SiO2 composition, was synthesized under high pressure and temperature. We found that this material has an extremely high fracture toughness (similar to 13 MPa m(1/2)), nearly eight times higher than that of stishovite single crystal. The measured Vickers hardness of this material was similar to 29 GPa and stishovite is known to be one of the hardest oxides. This new material has potential to be used for industrial purposes because of its combination of very high hardness and toughness. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Terumi Ejima, Masahide Akasaka, Takashi Nagao, Hiroaki Ohfuji
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 107 (6) 215 - 225 1345-6296 2012/12 Scientific journal 
    The oxidation state of an olivine phenocryst separate from clinopyroxene-olivine andesitic black scoria with red-brown tint from Kasayama volcano, Hagi, Yamaguchi Prefecture, was determined using Fe-57 Mossbauer spectroscopy, to examine the effect of high temperature oxidation on the oxidation state of Fe. The olivine phenocrysts examined were euhedral to subhedral in form, typically fractured, and about 0.2-1 mm in maximum dimension. Small amounts of Fe-bearing precipitate minerals often occur on the rims of olivine phenocrysts in the black scoria with red-brown tint, and near subhedral Cr-rich magnetite with about 5 mu m in diameter. Olivine phenocrysts lacking such precipitates and inclusions were separated from a sample using an isodynamic separator and handpicking under a binocular microscope. Examination by optical microscopy, electron microprobe analysis, Raman spectroscopy, and high-resolution transmission microscopy proved no precipitates in the separated olivine phenocrysts. Average Fo value and Fe content of the olivines were 81 mol% and 0.36 atoms per formula unit (apfu), respectively. The Fe-57 Mossbauer spectrum of the olivine separate consisted of three doublets assigned to Fe2+ at the M1, Fe2+ at the M2 and Fe3+ at the octahedral sites. The Fe2+:Fe3+-ratio is 95(3):5(1). By applying this value to the average Fe content, Fe2+ and Fe3+ are calculated to be 0.34(1) and 0.018(4) apfu, respectively. Since the Fe3+ within olivine detected in this study is not due to any Fe3+-bearing impurity, Fe3+ could be located at the octahedral site within the olivine structure. The quadrupole splitting value of the Fe3+-Mossbauer doublet [0.53(5) mm/s] in Kasayama olivine is significantly lower than published data for Fe3+ at the M2 site in olivine, and in laihunite. This suggests possible distribution of Fe3+ at the M1 site. Fe3+ within olivine in the black scoria with red-brown tint is considered to have been generated at high temperatures (perhaps above 800 degrees C).
  • Masayuki Nishi, Tetsuo Irifune, Hiroaki Ohfuji, Yoshinori Tange
    GEOPHYSICAL RESEARCH LETTERS 39 0094-8276 2012/12 Scientific journal 
    The mechanism of the post-garnet transformation in natural single-crystalline pyropic garnet has been examined under large overpressure conditions of similar to 31-43 GPa using the multianvil apparatus with sintered diamond anvils. Intracrystalline nucleation was found to be dominant above 3538 GPa, while only grain-boundary nucleation is responsible at lower pressures. In addition, natural pyropic garnet transformed to a single phase of perovskite without decomposing to multiple phases above 38 GPa. Both intracrystalline nucleation and polymorphic growth of the post-garnet transformation under large overpressure conditions would strongly enhance the transformation kinetics. Consequently, the post-garnet transformation may pwroceed rapidly in the subducting slab at depths near similar to 950 km, which would cause substantial rheological weakening of the subducting slab. The metastable post-garnet transformation may also cause the seismic discontinuities at a depth of 900-1080 km depth, which is indeed observed beneath some subduction zones. Citation: Nishi, M., T. Irifune, H. Ohfuji, and Y. Tange (2012), Intracrystalline nucleation during the post-garnet transformation under large overpressure conditions in deep subducting slabs, Geophys. Res. Lett., 39, L23302, doi:10.1029/2012GL053915.
  • Yoshio Kono, Tetsuo Irifune, Hiroaki Ohfuji, Yuji Higo, Ken-ichi Funakoshi
    GEOPHYSICAL RESEARCH LETTERS 39 0094-8276 2012/12 Scientific journal 
    Compressional (Vp) and shear (Vs) wave velocities of mid-ocean-ridge basalt (MORB) was investigated at in situ high pressure and high temperature conditions of the mantle transition region by using a combination of ultrasonic and in situ x-ray measurements. Both Vp and Vs of MORB are lower than the previously predicted velocities of the major mantle mineral phases. We found that the Vp and Vs of MORB along a typical geotherm are lower by about 2 and 5%, respectively than those of seismological models, and do not match any global and regional seismological models in the deeper parts of the mantle transition region. Thus, the existence of a basaltic layer in this region is unlikely, suggesting that the oceanic crust materials are transported into the lower mantle. Citation: Kono, Y., T. Irifune, H. Ohfuji, Y. Higo, and K. Funakoshi (2012), Sound velocities of MORB and absence of a basaltic layer in the mantle transition region, Geophys. Res. Lett., 39, L24306, doi:10.1029/2012GL054009.
  • A. Kurio, Y. Tanaka, H. Sumiya, T. Irifune, T. Shinmei, H. Ohfuji, H. Kagi
    JOURNAL OF SUPERHARD MATERIALS 34 (6) 343 - 349 1063-4576 2012/11 Scientific journal 
    Wear resistance of nano-polycrystalline diamond (NPD) rods containing various amounts of hexagonal diamond has been tested with a new method for practical evaluation of the wear-resistance rate of superhard ceramics, in addition to the measurements of their Knoop hardness. The wear resistance of NPD has been found to increase with increasing synthesis temperature and accordingly decreasing proportion of hexagonal diamond. A slight increase in Knoop hardness with the synthesis temperature also has been observed for these samples, consistent with the results of the wear-resistance measurements. These results suggest that the presence of hexagonal diamond would not yield any observable increase in both hardness and wear resistance of NPD, contradictory to a recent prediction suggesting that hexagonal diamond is harder than cubic diamond. It is also demonstrated that NPD is superior to single crystal diamond in terms of relatively homogeneous wearing without any significant chipping/cracking.
  • Xi Liu, Hiroaki Ohfuji, Norimasa Nishiyama, Qiang He, Takeshi Sanehira, Tetsuo Irifune
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 117 2169-9313 2012/09 Scientific journal 
    Multianvil experiments with long experimental durations have been made with the anorthite composition CaAl2Si2O8 at pressure-temperature (P-T) conditions of 14-25 GPa and 1400-2400 degrees C. At subsolidus conditions, these experiments demonstrated three phase assemblages, grossular (Gr) + kyanite (Ky) + stishovite (St) at similar to 14 GPa, Gr + calcium-alumino-silicate phase (CAS) + St at similar to 18 GPa, and CAS + CaSiO3-perovskite (CaPv) + St at above similar to 20 GPa, which are related by the reactions Gr + Ky = CAS + St and Gr + St = CAS + CaPv. Following the method of Schreinemakers, we combined our data with the literature data to deduce a P-T phase diagram for a portion of the CaO-Al2O3-SiO2 system at subsolidus conditions, which subsequently helped to solve some long-lasting discrepancies in the high-P behavior of the compositions of anorthite and grossular. The crystal chemistry of the CAS and CaPv solid solutions was examined, and new substitution mechanisms were firmly established. Along the solidus, the melting reaction at similar to 14 GPa is peritectic while that at similar to 22 GPa is eutectic. For both pressures, St is the first phase to melt out and the melt is generally andesitic. For the An composition, its density starts to be significantly higher than the density of pyrolite at similar to 2.5 GPa, a much lower pressure than that for the Or, Ab or Qtz composition (similar to 7.5-10 GPa), so that the An-enriched continental crust material should readily plunge into the upper mantle.
  • Jiaqian Qin, Norimasa Nishiyama, Hiroaki Ohfuji, Toru Shinmei, Li Lei, Duanwei He, Tetsuo Irifune
    SCRIPTA MATERIALIA 67 (3) 257 - 260 1359-6462 2012/08 Scientific journal 
    The Vickers hardness of polycrystalline gamma-boron (gamma-B) was measured using a diamond indentation method. The elastic properties of polycrystalline gamma-B were determined using an ultrasonic measurement method. Under a loading force of up to 20 N our test gave an average Vickers hardness in the asymptotic hardness region around 30.3 GPa. We also measured the hardness and elastic properties of polycrystalline beta-B and cubic boron nitride Ti3SiC2 (PcBN) for comparison. The hardness and elastic properties of polycrystalline gamma-B were found to be very close to those of PcBN. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Hidemi Ishibashi, Hiroyuki Kagi, Haruko Sakuai, Hiroaki Ohfuji, Hirochika Sumino
    AMERICAN MINERALOGIST 97 (8-9) 1366 - 1372 0003-004X 2012/08 Scientific journal 
    Carbonado, a variety of natural polycrystalline diamond whose origin remains unknown, differs notably in the properties from common diamonds of mantle origin. In this study, infrared spectroscopic and microscopic analyses were conducted on carbonado from the Central African Republic. Stepwise heating followed by infrared spectroscopic measurements indicated that liquid H2O is enclosed within diamond single crystals in carbonado. Transmission electron microscope observation revealed a negative crystal that is interpreted as a primary fluid inclusion in a diamond single crystal. Observations by field-emission scanning electron microscope and electron backscatter diffraction analysis show an absence of lattice preferred orientation of diamond crystals, {111} growth steps along grain boundaries, and the crystal-size distribution of diamond similar to those of crystals formed in liquid media. In addition, the redox conditions of carbonado formation is inferred to be similar to 3 log units below the quartz-magnetite-fayalite buffer, which is the prevailing condition in cratonic upper mantle. These lines of evidence suggest that the carbonado crystallized in C-O-H fluid, supporting the hypothesis of a mantle-depth origin of carbonado.
  • Li Lei, Hiroaki Ohfuji, Tetsuo Irifune, Jiaqian Qin, Xinyu Zhang, Toru Shinmei
    JOURNAL OF APPLIED PHYSICS 112 (4) 0021-8979 2012/08 Scientific journal 
    Disorder-activated phonon behaviors of the cubic rocksalt-type semiconductor alloy Li(1-x)/2Ga(1-x)/2MxO (M=Mg, Zn) prepared by high-pressure and high-temperature method has been studied by Raman scattering analysis. The LO Raman phonon in Li(1-x)/2Ga(1-x)/2MxO was found to exhibit a distinct two-mode behavior. The compositional dependence of Raman frequency, peak-width, and intensity has been discussed. A model associated with a composite mode of the Brillouin zone center and edge phonons combined with phonon dispersion curves determined by first-principle calculations were employed to explain the asymmetric broadening of the LO phonon mode. The broadening and asymmetric Raman line-shape in Li(1-x)/2Ga(1-x)/2MxO can be interpreted as a composite mode of the softening Brillouin zone center mode and the Brillouin zone edge mode. (C) 2012 American Institute of Physics. []
  • Hiroaki Ohfuji, Shinsuke Okimoto, Takehiro Kunimoto, Futoshi Isobe, Hitoshi Sumiya, Kazuo Komatsu, Tetsuo Irifune
    PHYSICS AND CHEMISTRY OF MINERALS 39 (7) 543 - 552 0342-1791 2012/07 Scientific journal 
    Nano-polycrystalline diamond (NPD) is a super-hard pure polycrystalline aggregate of nano-diamonds and has a characteristic microtexture composed of a mixture of granular and lamellar crystals. We investigated the origin of the unique microtexture and the influence of the crystallinity of initial graphite sources on the resulting microtexture of NPDs. Polycrystalline graphite rods used for NPD synthesis were found to consist of coke-derived relatively large crystals and pitch-derived nanocrystalline particles. Upon conversion to NPD, the former are converted to cubic and hexagonal diamond mixtures by the martensitic transformation and left a lamellar texture behind, while the latter transform to granular nano-diamonds by diffusion-controlled nucleation and subsequent crystal growth, which initiate preferentially at lattice defects and crystal surfaces. A clear correlation between the crystallite size of the initial graphite and the grain size of the granular nano-diamonds in the NPDs was also found. Our results suggest that the average grain size and the relative abundance of lamellar domains in NPD can potentially be controlled by carefully choosing initial graphite sources based on their crystallinity.
  • D. Spengler, M. Obata, T. Hirajima, L. Ottolini, H. Ohfuji, A. Tamura, S. Arai
    JOURNAL OF PETROLOGY 53 (7) 1477 - 1504 0022-3530 2012/07 Scientific journal 
    Serpentinized massif peridotite in the Xugou, Su-Lu ultrahigh-pressure (UHP) metamorphic belt, eastern China, preserves texturally old (porphyroclastic) ortho- and clinopyroxene with up to two generations of lamellae of garnet, clinopyroxene and Mg-chromite. Their crystallographic orientation with respect to the host pyroxene is consistent with an origin by solid-state exsolution. Comparison of integrated mineral chemistry with simplified and natural chemical datasets suggests that both aluminous precursor pyroxenes were in equilibrium at a minimum pressure of similar to 4 GPa and within a temperature range of about 1300-1500 degrees C. Steep isopleths of Ca in orthopyroxene imply that exsolution occurred during cooling. Al diffusion modelling suggests growth of widely spaced lamellae in orthopyroxene down to about 900 degrees C. Integrated Al contents between wide lamellae record a minimum of 4 GPa pressure during cooling. Compositionally uniform exsolved minerals were formed at 4 center dot 3 +/- 0 center dot 3 GPa and 730 +/- 30 degrees C and reflect a cratonic geotherm with about 33 mW m(-2) surface heat flow. The peridotite matrix mineral assemblage of olivine + orthopyroxene +/- garnet +/- Mg-chromite +/- clinopyroxene +/- phlogopite records strain-induced recrystallization that partially to completely replaced precursor porphyroclasts. The recrystallized minerals lack lamellar exsolution. Recrystallized orthopyroxene, with Al2O3 at 0 center dot 13 wt %, indicates conditions of 5 center dot 5 +/- 0 center dot 3 GPa and 760 +/- 30 degrees C, which are higher-grade metamorphic conditions than those preserved in the chemically equilibrated exsolution microstructures. Both estimates overlap with the range reported for the Early Mesozoic UHP metamorphism in the region (4 center dot 0-6 center dot 7 GPa and 760-970 degrees C). Major element melt models applied to previously published Xugou peridotite data suggest high degrees of melt extraction (30-35 %) in the garnet peridotite stability field (3-4 center dot 5 GPa) until garnet and clinopyroxene exhaustion. Coincidence in pressure and in the order of temperature of equilibration of precursor pyroxenes and peridotite melting implies that peridotite formation occurred at similar to 135 km depth in the subcontinental lithospheric mantle (SCLM) beneath the Archaean North China Craton. Subsequent refertilization, mineral exsolution and chemical re-equilibration during long-term cooling in the SCLM occurred prior to deformation and incorporation of the mantle fragment into the continental crust during UHP metamorphism at a minimum depth of 170 km. Because the Xugou precursor pyroxenes and peridotite formed at depths greater than the regional SCLM (c. 90 km), we infer that the orogenic peridotite massif formed part of the former hanging wall of the Archaean SCLM, which delaminated after the Late Mesozoic.
  • Yongtao Zou, Tetsuo Irifune, Steeve Greaux, Matthew L. Whitaker, Toru Shinmei, Hiroaki Ohfuji, Ryo Negishi, Yuji Higo
    JOURNAL OF APPLIED PHYSICS 112 (1) 0021-8979 2012/07 Scientific journal 
    Elastic wave velocities of synthetic polycrystalline Mg3Al2(SiO4)(3) garnet have been successfully measured to 20 GPa and temperatures up to 1700 K by ultrasonic interferometry combined with energy-dispersive synchrotron x-ray diffraction in a Kawai-type multi-anvil apparatus. Compressional (Vp) and shear (Vs) wave velocities as well as the adiabatic bulk (Ks) and shear (G) moduli exhibit monotonic increase with increasing pressure and decrease with increasing temperature, respectively. Two-dimensional (P-T) linear fittings of the present data yield the following parameters: K-S0 = 170.0(2) GPa, partial derivative Ks/partial derivative P = 4.51(2), partial derivative Ks/partial derivative T = -0.0170(1) GPa/K, G(0) = 93.2(1) GPa, partial derivative G/partial derivative P = 1.51(2), and partial derivative G/partial derivative T = -0.0107(1) GPa/K, which is in good agreement with the earlier results by Brillouin scattering and/or ultrasonic measurements at relatively low P-T conditions. The observed linear pressure and temperature dependence in both Vp and Vs is in contrast to the non-linear behavior of Vp and Vs for majorite garnet with the pyrolite composition, in particular for Vs. (C) 2012 American Institute of Physics. []
  • M. Okayasu, Y. Ohkura, S. Takeuchi, S. Takasu, H. Ohfuji, T. Shiraishi
    The mechanical properties of an Al-Si-Cu alloy (ADC12), produced using various casting technologies, have been examined experimentally. Four different casting processes were employed, including gravity casting (GC), cold-chamber die-casting (CD), twin rolled continuous casting (TRC) and the Ohno continuous casting process (OCC). Although these produced the same Al-Si-Cu aluminum alloy, different mechanical properties were obtained, in particular microstructural characteristics and dislocation density. The microstructure of GC and CD samples was formed mainly with coarse alpha-Al phase and needle-shaped Si and Fe based eutectic structures. In contrast, a fine round alpha-Al phase and tiny eutectic structures were observed for the TRC and OCC samples. Such a change of microstructure was caused by the different casting process parameters, namely injection speed, casting pressure and cooling rate. High internal stress as well as high dislocation density was detected for GC and TRC, caused by the high shrinkage force and high applied rolling force, respectively. Because of the different material properties, the tensile and fatigue strength were altered. A clear Hall-Petch relation with sigma(0.2) = k(y)d(-0.5) + B was obtained, and the fatigue properties were evaluated with the power law dependence sigma(a) = sigma N-f(f)-b. The mechanical properties obtained were also analyzed in relation to the crystal orientation and lattice mis-orientation angle. (C) 2012 Elsevier B.V. All rights reserved.
  • M. Matsushita, S. Nakano, H. Ohfuji, I. Yamada, T. Kikegawa
    Journal of Magnetism and Magnetic Materials 324 (4) 655  0304-8853 2012/02 [Peer-reviewed]
     Scientific journal
  • Nobuhiro Settsu, Masafumi Matsushita, Manabu Takahashi, Masaki Tanimoto, Hiroaki Ohfuji
    Zairyo/Journal of the Society of Materials Science, Japan 61 (2) 197 - 202 0514-5163 2012/02 Scientific journal 
    The effects of brazing filler metal diffusion on the mechanical strength of a ceramic/metal composite are described. First, a specimen of a SUS304/Cu/Si 3N 4 system, including brazing filler metals of AgCu and/or AgCuTi, was prepared by brazing in a vacuum furnace. Next, the vicinity of the joint parts was observed using field emission scanning electron microscopy. The brazing filler metals were found to diffuse into the Cu interlayer and ramify. In addition, a crack due to thermal stress resulting from a mismatch in thermal contraction between the materials present was observed around the AgCuTi/Si 3N 4 boundary. Energy-dispersive X-ray spectroscopic analysis revealed that Ag atoms diffused into the Cu interlayer to form the solid solutions of Ag and Cu. Micro-Vickers hardness tests were conducted on the surface of the Cu interlayer in order to examine the effects of brazing alloy diffusion on the hardening of the Cu interlayer. It was found that the diffusion of the brazing filler metals enhanced the hardness of the Cu interlayer. Finally, numerical simulations were performed to verify the increase in thermal stress due to hardening of Cu, and this behavior was discussed using simple equations. The results of the experiments and numerical simulations showed that the mechanical strength of the SUS304/Cu/Si 3N 4 composite is subjected to the diffusion of brazing filler metals. © 2012 The Society of Materials Science Japan.
  • M. Matsushita, S. Nakano, H. Ohfuji, I. Yamada, T. Kikegawa
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 324 (4) 655 - 655 0304-8853 2012/02
  • Shinozaki Ayako, Hirai Hisako, Ohfuji Hiroaki, Oohashi Kazuki, Okada Taku, Machida Shin-ich, Kagi Hiroyuki, Yagi Takehiko
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences 一般社団法人日本鉱物科学会 2012 (0) 125 - 125 2012 [Peer-reviewed]
    High pressure and temperature experiments of Mg2SiO4-H2 and MgSiO3-H2 samples have been carried out to clarify effect of H2 fluid on phase relation in MgO-SiO2-H2 samples and to characterize silicate composition dissolved in H2 fluids from 2.5 GPa, 1400 K to 15.0 GPa, 1600K using laser heated diamond anvil cells. The phase relations was clarified by X-ray diffraction measurements, Raman spectroscopy, electron microscopic and transmission electron microscopic observation indicated that forsterite and enstatite was decomposed in Mg2SiO4-H2 and MgSiO3-H2 samples, respectively. These phase were dissolute into H2 fluid incongruently.
  • Hiroaki Ohfuji
    Japanese Magazine of Mineralogical and Petrological Sciences 41 (1) 12 - 18 1349-7979 2012 Scientific journal 
    The 3D packing structures of microcrystals in framboidal pyrite have been investigated by morphological observations and crystallographic orientation analyses using the EBSD technique to understand its self-organization process. The packing structures are basically classified into three types: (i) face-centered cubic (fcc), (ii) icosahedral and (iii) random packings. The orientation analyses on many ordered (fcc) framboids revealed that about a half of the microcrystals in a framboid involve high-angle (~ 90°) misorientation. This means that even in morphologically highly ordered framboids, the crystallographic orientations of microcrystals are not uniform, suggesting that the self-organization process of framboidal microcrystals is not crystallographically controlled. We propose a three-step model for the self-organization: (1) simultaneous nucleation of monodispersive pyrite microcrystals in a fixed volume (from precursor FeS), (2) aggregation of the randomly oriented microcrystals driven by surface forces and the reduction in total surface energy associated with both the individual microcrystals and the whole aggregate, resulting in the formation of the framboidal texture, and 3) reorientation of the microcrystals driven by further reduction in surface energy associated with the grain boundaries. © 2012, Japan Association of Mineralogical Sciences. All rights reserved.
  • Jiaqian Qin, Tetsuo Irifune, Haruhiko Dekura, Hiroaki Ohfuji, Norimasa Nishiyama, Li Lei, Toru Shinmei
    PHYSICAL REVIEW B 85 (1) 1098-0121 2012/01 Scientific journal 
    The phase relations in boron have been investigated at high pressure and high temperature using a multianvil apparatus, and the quenched sample has been analyzed by x-ray diffraction, Raman spectra, and transmission electron microscopy. We demonstrate that gamma-B-28 can be synthesized over a wide pressure and temperature range, and alpha-tetragonal B-50 (T-B-50) is obtained at higher temperatures and similar pressures. The phase boundary of the beta-B-106, gamma-B-28, and T-B-50 is determined at pressures between 7.0 and 18GPa and the temperatures of 500-2200 degrees C. The results suggest that T-B-50 might be an intermediate phase formed for kinetic reasons (Ostwald rule) on the way from beta-B-106 to beta-tetragonal B-192 (T-192) and gamma-B-28 to T-192.
  • Hide Yasuhara, Naoki Kinoshita, Hiroaki Ohfuji, Dae Sung Lee, Shinichiro Nakashima, Kiyoshi Kishida
    APPLIED GEOCHEMISTRY 26 (12) 2074 - 2088 0883-2927 2011/12 Scientific journal 
    Examining the evolution of fracture permeability under stressed and temperature-elevated conditions, a series of flow-through experiments on a single rock fracture in granite has been conducted under confining pressures of 5 and 10 MPa, under differential water pressures ranging from 0.04 to 0.5 MPa, and at temperatures of 20-90 degrees C, for several hundred hours in each experiment. Measurements of fluid and dissolved mass fluxes, and post-experimental microscopy, were conducted to constrain the progress of mineral dissolution and/or precipitation and to examine its effect on transport properties. Generally, the fracture aperture monotonically decreased with time at room temperature, and reached a steady state in relatively short periods (i.e., <400 h). However, once the temperature was elevated to 90 degrees C, the aperture resumed decreasing and kept decreasing throughout the rest of the experimental periods. This reduction may result from the removal of the mineral mass from the bridging asperities within the fracture. Post-experimental observations by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM-EDX), revealed the formation of several kinds of secondary minerals such as silica and calcite. However, the precipitated minerals seemed to have had little influence on the flow characteristics within the fracture, because the precipitation was limited to quite local and small areas. The evolving rates and ultimate magnitudes of the fracture aperture are likely to be controlled by the stress exerted over the contacting asperities and temperatures, and by the prescribed flow conditions. Thus, this complex behavior should be attributed to the coupled chemically-and mechanically-induced effect. A coupled chemo-mechano conceptual model, accounting for pressure and free-face dissolutions, is presented in this paper to follow the evolution of the fracture permeability observed in the flow-through experiments. This model addresses the two dissolution processes at the contacting asperities and the free walls within the fractures, and is also capable of describing multi-mineral dissolution behavior. The model shows that the evolution of a fracture aperture (or related permeability) and of element concentrations may be followed with time under arbitrary temperature and pressure conditions. The model predictions for the evolving fracture aperture and elements concentrations show a relatively good agreement with the experimental measurements, although it is not possible to replicate the abrupt reduction observed in the early periods of the experiments, which is likely to be due to an unaccounted mechanism of more stress-mediated fracture compaction driven by the fracturing of the propping asperities. (C) 2011 Elsevier Ltd. All rights reserved.
  • Terumi Ejima, Masahide Akasaka, Hiroaki Ohfuji
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 106 (5) 246 - 254 1345-6296 2011/10 Scientific journal 
    The oxidation state and distribution of Fe in olivine in a lherzolite xenolith from the Oku district, Oki-Dogo Islands, Japan, were investigated. A Iherzolite xenolith collected from a volcanic neck of alkali olivine basalt contains olivine, orthopyroxene, clinopyroxene, and a small amount of spinel. Anhedral anorthite grains less than 0.01 mm across occur as inclusions in clinopyroxene. The olivine grains are nearly homogeneous in composition, with Fo contents of 80.7-81.9 mol%. Pure olivine grains for X-ray diffraction and Fe-57 Mossbauer analyses were separated from the fresh core of the lherzolite xenolith under a binocular microscope. The Mossbauer spectrum of the separated olivines consists of two doublets assigned to Fe2+ at the M1 and M2 sites and a doublet attributed to Fe3+ at the M2 site. The populations of Fe3+ in the olivine derived from the Mossbauer method and FeL beta/FeL alpha-intensity ratios are 0.02 and 0.03 atoms per Formula unite (apfu), respectively. The site occupancies of Fe in the M1 and M2 sites refined using X-ray Rietveld method are 0.151 and 0.206 apfu, respectively. The chemical formula of the olivine based on the refined Fe populations at the octahedral M1 and M2 sites and oxidation state of Fe determined using Mossbauer method is [((M2)(Mg0.792+Fe0.192+Fe0.023+)(M1)(square 0.01Mg0.85Fe0.14+2)](Sigma 2.00)SiO4.
  • Junji Yamamoto, Kazuhiko Otsuka, Hiroaki Ohfuji, Hidemi Ishibashi, Naoto Hirano, Hiroyuki Kagi
    EUROPEAN JOURNAL OF MINERALOGY 23 (5) 805 - 815 0935-1221 2011/10 Scientific journal 
    To assess the capacity of fluid inclusions in mantle minerals for CO2 retention, annealing experiments were conducted for two mantle xenoliths with CO2 inclusions for 8 days at 1000 degrees C under atmospheric pressure and fO(2) of 10(-11) MPa. The results show no marked decrease in the CO2 density of the CO2 inclusions for any examined minerals - olivine, orthopyroxene, clinopyroxene, or spinel. The CO2 density of CO2 inclusions in olivine in the present mantle xenoliths is lower than that in pyroxenes or spinel. Results of previous studies indicate that the low CO2 density in olivine is attributable to plastic deformation of olivine around CO2 inclusions during annealing in ascending magma. Results of this study present fundamental implications for deformation mechanisms that arise from internal pressure of fluid inclusions in silicate minerals. We calculated the stress field in minerals having a CO2 inclusion. Results show a steep stress gradient in the host around the inclusion. Such local stress in the mineral induces a local rise in the density of dislocations around the CO2 inclusions. The orthopyroxene used for this study showed a sparse distribution of dislocations around a CO2 inclusion, whereas olivine showed dense dislocations around CO2 inclusions, implying that the low CO2 density of the CO2 inclusions in olivine results from volume expansion of the CO2 inclusions through plastic deformation of the host mineral during annealing of the xenoliths in ascending magma. In this respect, constancy of CO2 density during the annealing experiments for all minerals is an interesting finding. Regarding olivines, the reduction of internal pressure of the CO2 inclusions or interaction of the dense dislocations possibly inhibits decrepitation or further volume expansion of the CO2 inclusions during annealing experiments. However, pyroxenes and spinel show higher and similar CO2 density, which reflects the resistance to plastic deformation and which indicates the effectiveness of CO2 inclusions in these minerals as a depth probe for mantle xenoliths.
  • Shigeto Hirai, Yohei Kojima, Hiroaki Ohfuji, Norimasa Nishiyama, Tetsuo Irifune, Stephan Klemme, Geoffrey Bromiley, J. Paul Attfield
    PHYSICS AND CHEMISTRY OF MINERALS 38 (8) 631 - 637 0342-1791 2011/09 Scientific journal 
    Raman spectroscopy and heat capacity measurements have been used to study the post-perovskite phase of CaIr0.5Pt0.5O3, recovered from synthesis at a pressure of 15 GPa. Laser heating CaIr0.5Pt0.5O3 to 1,900 K at 60 GPa produces a new perovskite phase which is not recoverable and reverts to the post-perovskite polymorph between 20 and 9 GPa on decompression. This implies that Pt-rich CaIr1-xPtxO3 perovskites including the end member CaPtO3 cannot easily be recovered to ambient pressure from high P-T synthesis. We estimate an increase in the thermodynamic Gruneisen parameter across the post-perovskite to perovskite transition of 34%, of similar magnitude to those for (Mg,Fe)SiO3 and MgGeO3, suggesting that CaIr0.5Pt0.5O3 is a promising analogue for experimental studies of the competition in energetics between perovskite and post-perovskite phases of magnesium silicates in Earth's lowermost mantle. Low-temperature heat capacity measurements show that CaIrO3 has a significant Sommerfeld coefficient of 11.7 mJ/mol K-2 and an entropy change of only 1.1% of Rln2 at the 108 K Curie transition, consistent with the near-itinerant electron magnetism. Heat capacity results for post-perovskite CaIr0.5Rh0.5O3 are also reported.
  • Leiming Fang, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune
    DIAMOND AND RELATED MATERIALS 20 (5-6) 819 - 825 0925-9635 2011/05 Scientific journal 
    The stability and decomposition of graphitic C3N4 (g-C3N4) were studied in the pressure and temperature range of 10-25 GPa and up to 2000 degrees C by multi-anvil experiments and phase characterization of the quenched products. g-C3N4 was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600-700 degrees C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800-900 degrees C and between 22 and 25 GPa. The estimated decomposition curve for g-C3N4 has a positive slope (similar to 0.05 GPa/K) up to similar to 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic C - N framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C3N4 was found in the studied P-T range. (C) 2011 Elsevier B.V. All rights reserved.
  • Steeve Greaux, Norimasa Nishiyama, Yoshio Kono, Laurent Gautron, Hiroaki Ohfuji, Takehiro Kunimoto, Nicolas Menguy, Tetsuo Irifune
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 185 (3-4) 89 - 99 0031-9201 2011/04 Scientific journal 
    High-pressure and high-temperature phase transformations of Ca3Al2Si3O12 grossular garnet were examined at 19-26 GPa and 700-2000 K using Kawai-type multi-anvil apparatus coupled with in situ X-ray diffraction (XRD). Recovered samples were analyzed by a combination of micro-focused X-ray diffraction (mu-XRD) and transmission electron microscopy (TEM). The results show that grossular garnet gradually transforms to an Al-rich CaSiO3 perovskite at 22-26 GPa and 1000-1400K. The transition boundary can be expressed as P (GPa) = -0.0082 X T (K) + 33.05. When the garnet completely disappears, we observed orthorhombic CaSiO3 perovskite with a grossular composition. At 20-24 GPa and temperatures above 1500 K the CAS phase with the composition CaAl4Si2O11 appears to accommodate excess Al from the perovskite along with two distinct populations of Al-bearing CaSiO3 perovskites, with Al content of 3.7 and 10.0 wt% Al2O3, respectively. The pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that Ca-rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660 km depth discontinuity. (C) 2011 Elsevier B.V. All rights reserved.
  • M. Matsushita, S. Nakano, H. Ohfuji, I. Yamada, T. Kikegawa
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 323 (6) 838 - 841 0304-8853 2011/03 Scientific journal 
    We have investigated the pressure variation of the volume and structure of an FCC Fe64Mn36 anti-ferromagnetic Invar alloy. The inclination of the pressure-volume (P-V) curve of the FCC structure becomes discontinuous at a pressure of 4 GPa. According to the bulk modulus at zero pressure estimated by the Birch-Murnaghan equation of state, the pressure between 4 and 10 GPa is 33 GPa larger than that at a pressure below 4 GPa. Considering previous experiments on magnetism at high pressure the Neel temperature at 4 GPa almost decreases to room temperature. These results suggest that the increase in the bulk modulus by 33 GPa can be attributed to the pressure-induced magnetic phase transition from anti-ferromagnetism to paramagnetism. Volume at zero pressure was estimated using the Birch-Murnaghan equation of state. The volume of FCC structure in the anti-ferromagnetic state was 1.17% larger than the volume in the paramagnetic state, namely, the spontaneous magnetostriction was 1.17%. Pressure-induced structural transition from FCC to HCP occurs with an increase in the pressure, especially at up to 5 GPa. The value of c/a is 1.62; this value almost corresponds to that of an ideal HCP structure. The bulk modulus of the HCP structure estimated by the Birch-Murnaghan equation of state is larger than that of the FCC structure, and the volume/atom ratio is smaller than that of the FCC structure. (C) 2010 Elsevier B.V. All rights reserved.
  • EBSD analysis of the Submicron Width Fibber Shaped Grain Copper Fabricated by Drawing
    Masafumi Matsushita, Tomoya Kuji, Hiromitsu Kuroda, Seigi Aoyama, Hiroaki Ohfuji
    Materials Sciences and Application 2 911 - 916 2011 [Peer-reviewed]
     Scientific journal
  • Yoshio Kono, Steeve Greaux, Yuji Higo, Hiroaki Ohfuji, Tetsuo Irifune
    JOURNAL OF EARTH SCIENCE 21 (5) 782 - 791 1674-487X 2010/10 Scientific journal 
    Simultaneous ultrasonic elastic wave velocity and in situ synchrotron X-ray measurements on grossular garnet were carried out up to 17 GPa and 1 650 K. P- and S-wave velocities and bulk and shear modulus showed linear pressure and temperature dependence. These data yielded a pressure derivative of the bulk modulus of 4.42(7) and a shear modulus of 1.27(3), which are in good agreement with those of garnets with variable chemical compositions. Temperature dependence of the bulk modulus of grossular (-1.36x10(-2)GPa/K) is also similar to that of other garnets, while the temperature dependence of the shear modulus of grossular (-1.11x10(-2)GPa/K) is higher than those of magnesium end-member garnets and pyrolitic garnet.
  • Li Lei, Tetsuo Irifune, Toru Shinmei, Hiroaki Ohfuji, Leiming Fang
    JOURNAL OF APPLIED PHYSICS 108 (8) 0021-8979 2010/10 Scientific journal 
    Two new high-pressure phases of LiGaO2 have been discovered by direct compression of orthorhombic wurtzite-type LiGaO2 at pressures between 7 and 22 GPa and the temperatures of 25-2000 degrees C by means of the multianvil apparatus. The new intermediate phase recovered above 13 GPa at room temperature has been determined as a cation-disordered tetragonal structure with lattice parameters a=0.287 63(36) nm and c=0.419 29(117) nm, space group I4/m. The new high-pressure phase recovered above 16 GPa and 1400 degrees C has been refined as a cation-disordered cubic structure with lattice parameter of a=0.411 34(16) nm, space group Fm-3m. Powder x-ray diffraction and Raman scattering spectra observations reveal that besides the previously described polymorphic hexagonal phase, the tetragonal and cubic new phases of LiGaO2 are found to have rocksalt-related structures. Their structures can be labeled as layered-rocksalt- type, deformed-rocksalt- type, and disordered-rocksalt-type, respectively. The relationships between polymorphs of LiGaO2 provide some new insights into the pathways of ternary wurtzite-type semiconductors under high pressure. (C) 2010 American Institute of Physics. [doi:10.1063/1.3487976]
    銅と銅合金 49 56 - 60 1347-7234 2010/08
  • Ikuya Yamada, Yuka Takahashi, Kenya Ohgushi, Norimasa Nishiyama, Ryoji Takahashi, Kohei Wada, Takehiro Kunimoto, Hiroaki Ohfuji, Yohei Kojima, Toru Inoue, Tetsuo Irifune
    INORGANIC CHEMISTRY 49 (15) 6778 - 6780 0020-1669 2010/08 Scientific journal 
    A novel A-site ordered perovskite CaCu3Pt4O12 was synthesized under high pressure and high temperature of 12 GPa and 1250 degrees C. CaCu3Pt4O12 is the first perovskite in which the B site is fully occupied by Pt4+. The crystal structure refinement based on the synchrotron powder X-ray diffraction data shows that CaCu3Pt4O12 crystallizes in the space group Im (3) over bar (cubic) with a lattice constant of a = 7.48946(10) angstrom. The magnetic susceptibility data show the antiferromagnetic transition at T-N = 40 K, which is attributed to the magnetic ordering of Cu2+ spins with S = 1/2.
  • Hiroaki Ohfuji, Takuo Okuchi, Shoko Odake, Hiroyuki Kagi, Hitoshi Sumiya, Tetsuo Irifune
    DIAMOND AND RELATED MATERIALS 19 (7-9) 1040 - 1051 0925-9635 2010/07 Scientific journal 
    Micrometer- to nanometer-scale structures of the cut surfaces of single- and polycrystalline diamonds by a pulsed ultraviolet laser have been thoroughly investigated by scanning and transmission electron microscopy. Within the laser-cut grooves, the processed diamond surfaces are extensively covered with laser-modified debris which consists of complex layered units of graphite with various crystallinities. The units consist of 1) highly oriented graphite, 2) corrugated graphite, and 3) nanocrystalline graphite, which are sequentially located from the surface of the underlying diamond substrate to the center of the grooves. Detailed textural examinations revealed that the highly oriented graphite unit is a product of the initial graphitization of diamond by a solid-state diffusion process, whereas the latter two units are deposition products from the liquid and/or vapor phases of carbon in the later stage. The present study demonstrates that the laser-cutting of diamonds proceeds in a two-step process: 1) extensive graphitization of laser-scanning path and 2) subsequent sublimation of the pre-formed graphite. These processes are basically identical among the three different types of diamonds (single crystal type Ib, single crystal type IIa and nano-polycrystalline aggregate) tested in this study. (C) 2010 Elsevier B.V. All rights reserved.
  • Fujino Kiyoshi, Irifune Tetsuo, Ishii Hirofumi, Hiraoka Nozomu, Tsuei Ku-Ding, Hamane Daisuke, Kuwayama Yasuhiro, Sata Nagayoshi, Murakami Sayaka, Whitaker Matthew, Shinozaki Ayako, Ohfuji Hiroaki, Kojima Yohei
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences 一般社団法人日本鉱物科学会 2010 (0) 108 - 108 2010 [Peer-reviewed]
    Spin state of Fe3+ in Al-bearing post-Mg-perovskite was measured by X-ray emission spectroscopy by reducing pressure from 165 to 35 GPa at room temperature. The result indicates that Fe3+ is low spin from 165 to around 95 GPa and it becomes almost high spin from around 95 to 35 GPa. This indicates that most Fe3+ in Al-bearing post-Mg-perovskite in the lower mantle would be in the low spin state.
  • M. Matsushita, H. Ohfuji
    DIFFUSION IN SOLIDS AND LIQUIDS V, PTS 1 AND 2 297-301 359 - + 1012-0386 2010 International conference proceedings 
    Recrystallization processing of cold-rolled copper after isothermal annealing was investigated using high-resolution electron backscattered diffraction pattern analysis. The fiber texture is obtained by cold rolling with the rolled direction oriented along {111}, and the transverse and nominal directions have a random orientation. An isothermal recrystallization process at 150 degrees C was investigated. Initially, rotations of the orientations occur from {111} to {100} and then small misfit angle boundaries decreased. Accompanying this change, the fiber-shaped grains change to a round shape grain and their sizes decrease. Considering these tendencies, we determined that rotation at subgrain boundaries is activated by isothermal annealing and subboundaries grow the boundary misfit angle >15 degrees. With further annealing, those grains surrounded by grain boundaries greater than 15 degrees expanded. The rolling, transverse, and nominal orientations rotate {100}. Subsequently, a twin boundary appeared, and the fraction of twin boundaries increased.
  • H. Ohfuji, T. Okada, T. Yagi, H. Sumiya, T. Irifune
    INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, JOINT AIRAPT-22 AND HPCJ-50 215 1742-6588 2010 International conference proceedings 
    Laser heating in a diamond anvil cell equipped with nano-polycrystalline diamond anvils (NPDAC) was tested for the first time. NPDAC was found to provide better heating efficiency than the standard DAC with single crystal anvils. This is probably attributed to an order of magnitude lower thermal conductivity of the NPD. High-temperature generation exceeding 4500 K was observed during laser heating of hcp-iron sample at pressure of similar to 100 GPa. The present study demonstrates a promising potential of laser-heated NPDAC for generation of ultra-high temperature under multi-megabar pressure conditions equivalent to those of the Earth's core.
  • H. Ohfuji, T. Okada, T. Yagi, H. Sumiya, T. Irifune
    HIGH PRESSURE RESEARCH 30 (1) 142 - 150 0895-7959 2010 Scientific journal 
    Laser heating in diamond anvil cell (DAC) equipped with nano-polycrystalline diamond (NPD) anvils was tested for the first time using various types of lasers. An NPD-based DAC was found to provide better heating efficiency than a standard DAC with single-crystal anvils, probably due to the order of magnitude lower thermal conductivity of the NPD. We observed high-temperature generation exceeding 5000 and 3500K during laser heating of hcp-Fe sample at pressures of 100 and 170GPa, respectively. The results of the present study demonstrate promising potential of laser-heated nano-polycrystalline DAC for the generation of very high temperature under multi-megabar pressure conditions equivalent to those of the Earth's core.
  • Hiroaki Ohfuji, Kiyoshi Kuroki
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 104 (5) 307 - 312 1345-6296 2009/10 Scientific journal 
    The influence of the local stress state and crystallinity of graphite on the graphite-diamond trans formation process was investigated using a laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics. The samples recovered from the experiments using highly oriented graphite (HOG) have layered structures composed of lonsdaleite and diamond arranged in a coaxial relation, indicative of their martensitic formation from graphite. The compression of HOG perpendicular to the graphite c axis results in a partial fragmentation of the original layered structure, but no clear evidence of diamond formation by the diffusion process is found even in the fragmented regions. On the other hand, the sample recovered from the experiment using mechanically milled graphite powder consists of euhedral diamond nanocrystals (size: 20-50 nm), indicating that the diamond is formed by the diffusion process. The results of the present study suggest that the graphite-diamond transformation process and the resulting microstructures are controlled mainly by the initial structure characteristics (such as crystallinity and crystallite size) of the graphite, while the influence of the local stress state in the graphite under compression is likely less significant for the formation process and the microtexture.
  • Takuo Okuchi, Hiroaki Ohfuji, Shoko Odake, Hiroyuki Kagi, Syohei Nagatomo, Mitsuru Sugata, Hitoshi Sumiya
    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 96 (4) 833 - 842 0947-8396 2009/09 Scientific journal 
    Laser beam micromachining was applied to super-hard nano-polycrystalline diamond (NPD) synthesized by the direct conversion of graphite at high pressure and high temperature. Three types of pulsed lasers were tested: nanosecond near-infrared, nanosecond near-ultraviolet, and femtosecond near-infrared lasers. The latter two were also applied for synthetic single crystal of diamond to compare the results with those of the NPD. It was demonstrated that the nanosecond near-infrared laser was the most efficient device for rough shaping of the NPD, while the ultraviolet and femtosecond lasers give satisfactory results for precise surface finishing of it. The properties of the laser-processed surfaces were analyzed by scanning and transmission electron microscopy, laser scanning microscopy, and micro Raman spectroscopy. These analyses demonstrated that the three types of lasers play different and complementary roles, and that their combination is the best suitable solution for micromachining of the hardest diamond into any desired shapes.
  • K. Watanabe, H. Ohfuji, R. Kitagawa, Y. Matsui
    CLAY MINERALS 44 (2) 177 - 180 0009-8558 2009/06 Scientific journal 
    The mineralogy of the glazed surfaces of Japanese sekishu roof tiles covered by a crustose lichen - Lecidella asema (Nyl.) Knoph & Hertel - was investigated using transmission electron microscopy (TEM). The study sought to identify the Ti-Fe mineral observed as a low concentration of Ti and Fe in a previous study of the glazed surfaces of the same roof tile. The TEM analysis revealed that: (1) a thin layer of the Ti-Fe mineral pseudobrookite exists on the glaze surface; (2) the pseudobrookite consists of well-ordered single crystals, continuously and widely distributed on the glaze surface.
  • Shoko Odake, Hiroaki Ohfuji, Takuo Okuchi, Hiroyuki Kagi, Hitoshi Sumiya, Tetsuo Irifune
    DIAMOND AND RELATED MATERIALS 18 (5-8) 877 - 880 0925-9635 2009/05 Scientific journal 
    We have conducted laser processing of ultrahard nano-polycrystalline and single crystalline diamonds (NPD, SCD, respectively) using nano-pulsed near-ultraviolet laser, and the machining properties were compared through microstructural examinations by SEM, TEM and Raman spectroscopy. The cut depth of the laser-cut grooves was observed to be deeper for the NPD than for the SCD. This is probably due to the lower thermal conductivity feature of NPD, which provides higher absorption efficiency of the laser energy and decreases the laser ablation threshold. TEM cross-section observation showed that the processed grooves in the both types of diamonds are covered with identical laser-modified layers (similar to 1 mu m thick) composed of roughly oriented nanocrystalline graphite. A marked difference was observed between the laser-processed surfaces of NPD and SCD: in the former the diamond-graphite interface is almost linear and undamaged, whereas in the latter the boundary is slightly folded and significantly distorted. These textural features suggest that different laser-machining processes are involved between NPD and SCD in the microscopic scale. Our results demonstrate that pulsed laser can be used even more effectively for the fabrication of nano-polycrystalline diamond than the case for single crystal diamond. (C) 2008 Elsevier B.V. All rights reserved.
  • M. Matsushita, T. Suko, S. Matsuda, H. Ohfuji, H. Ogiyama
    MATERIALS CHEMISTRY AND PHYSICS 114 (2-3) 522 - 524 0254-0584 2009/04 Scientific journal 
    We have investigated the effect of superplastic carburizing on the texture of duplex stainless (DS) alloy. The diffusion velocity of carbon in the superplastic DS alloy is much higher than that in non-superplastic DS alloy that has the same chemical composition. In carburized layer, the segregation of Cr can be observed and Cr-carbides precipitate at the grain boundary, in particularly, the triple-point intersection of grain boundaries. The precipitation of Cr-carbides in stainless alloys is disadvantageous because it causes a decrease in its corrosion resistance. (C) 2008 Elsevier B.V. All rights reserved.
  • Yoshio Kono, Hiroaki Ohfuji, Yuji Higo, Akihiro Yamada, Toru Inoue, Tetsuo Irifune, Ken-ichi Funakoshi
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 103 (6) 429 - 431 1345-6296 2008/12
  • Shinya Matsuda, Manabu Takahashi, Masafumi Matsushita, Nagatoshi Okabe, Hiroaki Ohfuji
    Zairyo/Journal of the Society of Materials Science, Japan 57 (11) 1138 - 1145 0514-5163 2008/11 Scientific journal 
    Sphere indentation tests were performed to clarify damage properties on surface of Advanced Pore-Free SiC (APF-SiC) with excellent damage tolerance. The ring crack initiation strength properties were discussed from fracture mechanics viewpoint based on FEM analysis and detailed observations of growth behaviors of the ring crack and the cone crack. As compared to conventional SiC, the initiation strength was nearly equal to regardless of sphere sizes. However, its scatter was increased with increasing sphere sizes. Clusters are formed by uniting some in multi-micro cracks which occur near surface with increasing the contact load, one of that from which the ring crack occurs. Therefore, the strength and the scatter are controlled by the cluster size and its distribution, and the size depends on the generating crack density. It can be explained by the mechanics model which is expressed by the relationship between strength ratio and crack density based on the energy equilibrium theory. © 2008 The Society of Materials Science.
  • Yoshio Kono, Hiroaki Ohfuji, Yuji Higo, Akihiro Yamada, Toru Inoue, Tetsuo Irifune, Ken-ichi Funakoshi
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 103 (4) 1345-6296 2008/08 [Peer-reviewed]
  • Shinya Matsuda, Manabu Takahashi, Takushi Onoue, Keiji Ogi, Masafumi Matsushita, Nagatoshi Okabe, Hiroaki Ohfuji
    Nihon Kikai Gakkai Ronbunshu, A Hen/Transactions of the Japan Society of Mechanical Engineers, Part A 74 (8) 1156 - 1163 0387-5008 2008/08 Scientific journal 
    In the present study, quench tests were carried out to characterize the fracture behavior of advanced pore-free SiC with damage tolerance during thermal shock. The residual strength of specimens after the quench tests was also measured. It is found that the critical temperature difference and the decrease in residual strength of this material are larger than those of conventional SiC. This is because the fracture is dominated only by flaw size since the wake zone is not generated during thermal shock with high strain rate. The critical temperature difference as well as the crack propagation is calculated with the aid of the unified theory of thermal shock.
  • Yoshio Kono, Hiroaki Ohfuji, Yuji Higo, Akihiro Yamada, Toru Inoue, Tetsuo Irifune, Ken-ichi Funakoshi
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 103 (2) 126 - 130 1345-6296 2008/04 Scientific journal 
    The elastic wave velocities of MORB glass were measured up to 18.7 GPa at 27 degrees C using an ultrasonic technique combined with in situ high-pressure X-ray diffraction and imaging. High-pressure Raman spectroscopy using a diamond anvil cell was also employed to study the local structure of the glass sample up to 20 GPa. Both the elastic wave velocities and Raman frequency at similar to 1600 cm(-1) show significant changes in their pressure dependences at similar to 10 GPa. The pressure derivatives of the P-wave and bulk sound velocities gradually increase with pressure above similar to 10 GPa, whereas the S-wave velocity is almost constant below 10 GPa, above which it increases significantly. The plot of apparent change in the Raman frequency shift versus pressure suggests that these notable changes in elastic properties may be attributed to the local structural changes in the MORB glass. Our observations demonstrate that both the density and adiabatic bulk modulus of the MORB glass increase continuously at pressures above similar to 10 GPa. These apparently conflicting tendencies can be reasonably explained if the zero-pressure density is increased continuously above similar to 10 GPa accompanied by structural modification. Since the fundamental structure of silicate glass is analogous to that of melt, the results are important and aid in understanding the elastic properties of silicate melt at high pressures.
  • T. Irifune, Y. Higo, T. Inoue, Y. Kono, H. Ohfuji, K. Funakoshi
    NATURE 451 (7180) 814 - 817 0028-0836 2008/02 Scientific journal 
    The composition of the mantle transition region, characterized by anomalous seismic- wave velocity and density changes at depths of similar to 400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine- rich 'pyrolite' composition(1,2), whereas others have inferred that it is characterized by pyroxene- and garnet- rich compositions ('piclogite'), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed(3-5). Although the velocities of the olivine polymorphs at these pressures ( wadsleyite and ringwoodite) have been well documented, those of majorite ( another significant high- pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X- ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates, owing to strong nonlinear decreases at high temperature, particularly for shear- wave velocity. We found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region, except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km. In contrast, both of these compositions lead to significantly low shear- wave velocities in the lower part of the region, suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite- rich material supplied by the subducted slabs stagnant at these depths.
  • Nagayoshi Sata, Hiroaki Ohfuji, Kei Hirose, Hisao Kobayashi, Yasuo Ohishi, Naohisa Hirao
    AMERICAN MINERALOGIST 93 (2-3) 492 - 494 0003-004X 2008/02 Scientific journal 
    FeS exhibits extensive polymorphism at high pressure and temperature. All with NiAs-type (B8) or closely related structures. Here we report a new phase transition from FeS VI to CsCl-type (B2) phase (FeS VII) above 180 GPa based on the synchrotron X-ray diffraction (XRD) measurements. A significant volume reduction by 3.0% was observed at the phase transition, due to an increase in the coordination number from six to eight. Present results suggest that a substantial amount of sulfur may be incorporated into an Fe-Ni alloy with bcc structure in the Earth's inner core.
  • Shinya Matsuda, Masafumi Matsushita, Manabu Takahashi, Hiroaki Ohfuji, Nagatoshi Okabe
    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 116 (1349) 126 - 129 1882-0743 2008/01 Scientific journal 
    It had been well known that the fracture of SiC manufactured by conventional reaction sintering method was caused by one of the pores in that. However, according to the recent studies, pore free SiC. so called APF-SiC was developed by improvement of reaction sintering method. As the result, the strength and the damage tolerance ability rise and then the cause of fracture of APF-SiC changed from pores to something else. Therefore we have investigated fracture surface of APF-SiC by using FE-SEM and EDS in order to demonstrate the cause of fracture. As the result, we found the inclusion that consisted of allotropes of Carbon covered with very thin SiO2 and SiN layer. And the size of the inclusion is ten times larger than that of SiC grain. Considering from present results, the inclusion is the one of the cause of fracture of APF-SiC.
  • Shinya Matsuda, Masafumi Matsushita, Manabu Takahashi, Hiroaki Ohfuji, Nagatoshi Okabe
    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 116 (1349) 126 - 129 1882-0743 2008/01 Scientific journal 
    It had been well known that the fracture of SiC manufactured by conventional reaction sintering method was caused by one of the pores in that. However, according to the recent studies, pore free SiC. so called APF-SiC was developed by improvement of reaction sintering method. As the result, the strength and the damage tolerance ability rise and then the cause of fracture of APF-SiC changed from pores to something else. Therefore we have investigated fracture surface of APF-SiC by using FE-SEM and EDS in order to demonstrate the cause of fracture. As the result, we found the inclusion that consisted of allotropes of Carbon covered with very thin SiO2 and SiN layer. And the size of the inclusion is ten times larger than that of SiC grain. Considering from present results, the inclusion is the one of the cause of fracture of APF-SiC.
  • Yoshio Kono, Hiroaki Ohfuji, Yuji Higo, Akihiro Yamada, Toru Inoue, Tetsuo Irifune, Ken Ichi Funakoshi
    Journal of Mineralogical and Petrological Sciences 103 (4) 303  1345-6296 2008 Scientific journal
  • Takeshi Sanehira, Tetsuo Irifune, Toru Shinmei, Hiroaki Ohfuji, Fabrice Brunet, Ken-ichi Funakoshi
    HIGH PRESSURE RESEARCH 28 (3) 335 - 349 0895-7959 2008 Scientific journal 
    Precise density changes across 660 km seismic discontinuity have been determined for pyrolite and mid-oceanic ridge basalt (MORB) compositions at 1873 K at high pressures using a combination of in situ X-ray diffraction on unit cell measurements and chemical composition analyses on the recovered products, using a multiple sample chamber cell assembly. Density jump for the post-spinel transformation in pyrolite was determined to be similar to 9%, which is comparable to that of PREM. The density of MORB is found to be lower than that of pyrolite at the depths of similar to 590-680 km (using Anderson's equation of state for Au), however, it becomes denser than the pyrolite below similar to 680 km. The density cross-over region at similar to 590-680 km may contribute to the stagnation of subducted slabs near the similar to 660 km discontinuity, as suggested by seismological observations as well as earlier experimental studies based on quench experiments and extrapolated in situ X-ray measurements.
  • Yoshio Kono, Yuji Higo, Hiroaki Ohfuji, Toru Inoue, Tetsuo Irifune
    GEOPHYSICAL RESEARCH LETTERS 34 (14) L14308  0094-8276 2007/07 Scientific journal 
    [1] Ultrasonic P- and S-wave velocities of a garnetite sample synthesized from MORB glass at around 17 GPa and 1200 degrees C were measured up to 14.3 GPa at room temperature. The garnetite consists of well-sintered microcrystalline majorite garnet and a minor amount of stishovite. P- and S-wave velocities of the garnetite at room temperature are significantly lower than those of (Mg-0.9, Fe-0.1)(2)SiO4 wadsleyite and ringwoodite, which are the major high-pressure phases in the mantle transition region. Bulk and shear moduli of the garnetite are determined from the P- and S-wave velocities, and those of majorite garnet in the MORB composition (MORB majorite) are calculated using the modal abundance data and elastic properties of stishovite. The MORB majorite is estimated to have quite different bulk and shear moduli (up to 19% and 9%, respectively) from those of Mg end-member garnets with simpler chemical compositions.
  • Hiroaki Ohfuji, Nagayoshi Sata, Hisao Kobayashi, Yasuo Ohishi, Kei Hirose, Tetsuo Irifune
    PHYSICS AND CHEMISTRY OF MINERALS 34 (5) 335 - 343 0342-1791 2007/07 Scientific journal 
    A new polymorph of FeS has been observed at pressures above 30 GPa at 1,300 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. It is stable up to, at least, 170 GPa at 1,300 K. The new phase (here called FeS VI) has an orthorhombic unit cell with lattice parameters a = 4.8322 (17) angstrom, b = 3.0321 (6) angstrom, and c = 5.0209 (8) angstrom at 85 GPa and 300 K. Its topological framework is based on the NiAs-type structure as is the case for the other reported polymorphs (FeS I-V). The unit cell of FeS VI is, however, more distorted (contracted) along the [010] direction of the original NiAs-type cell. For example, the c/b axial ratio is similar to 1.66 at 85 GPa and 300 K, which is considerably smaller than that of orthorhombic FeS II (similar to 1.72) and NiAs-type hexagonal FeS V (=root 3 approximate to 1.73). The phase boundary between FeS IV and VI is expected to be located around 30 GPa at 1,300 K. The phase transition is accompanied by gradual and continuous changes in volume and axial ratios and may be second order. At room temperature, FeS VI becomes stable over FeS III at pressures above 36 GPa. It is, therefore, suggested that the phase boundary of FeS III-VI and/or FeS IV-VI has negative pressure dependence.
  • Akihiro Yamada, Toru Inoue, Satoru Urakawa, Ken-ichi Funakoshi, Nobumasa Funamori, Takumi Kikegawa, Hiroaki Ohfuji, Tetsuo Irifune
    GEOPHYSICAL RESEARCH LETTERS 34 (10) L10303  0094-8276 2007/05 Scientific journal 
    Structure of a hydrous silicate melt with a forsterite-enstatite-H2O composition was investigated by in-situ X-ray diffraction with synchrotron radiation. The high temperature hydrous melt was successfully enclosed in a single-crystal diamond capsule under high pressure, up to 6.5 GPa. Results showed that the first sharp diffraction peak (FSDP), which reflects the intermediate-range structure of the complex melt, shifted drastically toward higher Q ( scattering vector [angstrom(-1)]) at pressure below 2.2 GPa. This observation indicates that the silicate network in the melt was depolymerized by the influence of water molecules and partly replaced by Si-OH units under those high-pressure conditions. In contrast, between 3 - 5 GPa, such a peak shift was not clearly detected, suggesting that the lengthening of the intermediate-range ordering of ...-Si-O-Si-... linkage (i.e., polymerization of silicate network) occurred.
  • Corentin Le Guillou, Fabrice Brunet, Tetsuo Irifune, Hiroaki Ohfuji, Jean-Noel Rouzaud
    CARBON 45 (3) 636 - 648 0008-6223 2007/03 Scientific journal 
    Five precursors covering the whole range of carbon structural organization, i.e. a quasi-amorphous soot (QAS), a raw carbon black (CB), a carbon black heat-treated at 2600 degrees C (HTCB), a polycrystalline graphite (PCG) and a highly oriented pyrolytic-graphite (HOPG) were run at 15 GPa in the 1500-1900 degrees C range between 15 and 60 min. Full transformation into nano-diamonds was not always achieved and the corresponding run products preserved the tracks of the transformation mechanisms which led to diamond formation. These mechanisms and their kinetics were characterized combining X-ray powder diffraction, Raman micro-spectroscopy and high-resolution TEM. Globally, the disordered precursors react faster than the crystalline ones: they achieve higher transformation rates and become transparent more easily. For the spherical CB particles, nano-diamond preferentially nucleates in their centre. The graphitic layers in the QAS directly transform into diamond without any prior graphitization. The crystalline organization is even found to decrease for the graphitized precursors (HTCB and PCG) as evidenced by HRTEM images showing graphite delarnination. These precursors mostly evolve according to a diffusion-limited reconstructive mechanism which initiates at the precursor structural defects. HOPG behaves differently since it mostly transforms into lonsdaleite rather than cubic diamond. (c) 2006 Elsevier Ltd. All rights reserved.
  • Daisuke Yamazaki, Takashi Yoshino, Hiroaki Ohfuji, Jun-ichi Ando, Akira Yoneda
    EARTH AND PLANETARY SCIENCE LETTERS 252 (3-4) 372 - 378 0012-821X 2006/12 Scientific journal 
    Seismic anisotropy is one of the significant features in the D" layer of the Earth and is thought to be derived from the lattice preferred orientation (LPO) of constituent materials or shape preferred orientation (SPO) of heterogeneous materials such as melt and inclusions. Recent experimental and theoretical studies strongly suggest that the D" layer consists mainly of a MgSiO3 Postperovskite phase together with ferro-periclase. To understand the anisotropy in the D" layer, we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline CaIrO3 as an analogue of MgSiO3 and measured the LPO of the post-perovskite phase. Crystallographic orientation analysis of the deformed post-perovskite phase showed strong LPOs with the dominant slip system being [100](010). Calculation of the elastic wave velocities considering the effect of LPOs of post-perovskite and ferro-periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized S-wave is considerably faster than that of vertically polarized S-wave. Thus, the seismic anisotropy observed in the D" layer can be reasonably explained by the LPO of the mixture of post-perovskite and ferro-periclase, where the LPO may result from the horizontal shear flow generated by the mantle convection. (c) 2006 Elsevier B.V. All rights reserved.
  • K. Watanabe, H. Ohfuji, J. Ando, R. Kitagawa
    CLAY MINERALS 41 (4) 819 - 826 0009-8558 2006/12 Scientific journal 
    Elemental behaviour, during the process of weathering of glazed sekishu roof-tiles affected by Lecidea sp. (a crustose lichen), was investigated using optical and fluorescence microscopy, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy. Sekishu roof tiles have an opaque reddish brown glaze on their surfaces which consist of an alkali feldspar-type X-ray amorphous glass recrystallized at 1200 degrees C. Optical and fluorescence microscopy revealed the presence of corrosion pits (at a depth of similar to 50 mu m) at the lichen-glaze interface. Elemental mapping by FE-SEM identified the concentrations of Ti and Fe in the section of the glazed tile analysed. The behaviour of C was correlated with those elements, suggesting the possibility of biomineralization.
  • H. Sumiya, H. Yusa, T. Inoue, H. Ofuji, T. Irifune
    HIGH PRESSURE RESEARCH 26 (2) 63 - 69 0895-7959 2006/06 Scientific journal 
    The conditions and mechanism of formation of nano-polycrystalline diamonds directly from graphite and non-graphitic carbon (carbon black, glassy carbon, C-60 and carbon nanotubes) at high pressure and high temperature have been investigated. The onset temperature for diamond formation at P >= 15 GPa is 1500 - 1600 degrees C for all carbon materials, although the required temperature conditions for pure polycrystalline diamond are T >= 2200 degrees C for graphite and T >= 1600 degrees C for non-graphitic carbon. Polycrystalline diamond forms as a result of simultaneous diffusion and two-step martensitic processes from graphite, whereas it forms only due to diffusion without graphitization or formation of intermediate phases from non-graphitic carbon. Nano-polycrystalline diamonds consisting only of very fine particles (< 10 nm in size) can be obtained from non- graphitic carbon at T similar to 1600 - 2000 degrees C under pressures >= 15 GPa.
  • H Ohfujii, D Rickard, ME Light, MB Hursthouse
    EUROPEAN JOURNAL OF MINERALOGY 18 (1) 93 - 98 0935-1221 2006/01 Scientific journal 
    The crystallographic nature of pyrite framboids, self-assembled multicrystalline pyrite, was investigated by single crystal X-ray diffraction. The results show that even morphologically well-organised framboids are not extreme examples of single crystals but are microcrystalline aggregates. Two dimensional diffraction patterns obtained from well-organised framboids composed of relatively few (similar to 1000) microcrystals, are single-crystal like but detailed analysis shows that they are actually produced by the superimposition of XRD reflections from similarly, but not identically, oriented microcrystals. The number of coherent crystallographic orientations of pyrite crystals estimated from a complete Ewald sphere data set obtained from a larger single framboid, were found to be approximately four to five times fewer than the number of the microcrystals that are actually contained in the framboid. The result shows the presence of multiple single crystal-like domains that are arranged in similar crystallographic orientations. The results show that framboids are not formed by a process in which crystallographic control is a primary factor. Framboid self-assembly is probably caused by the aggregation of a large number of similarly sized and shaped pyrite microcrystals that are nucleated in a limited volume. Self-organisation occurs as these microcrystals adopt a minimum energy configuration.
  • H Ohfuji, D Rickard
    EARTH AND PLANETARY SCIENCE LETTERS 241 (1-2) 227 - 233 0012-821X 2006/01 Scientific journal 
    Direct observation using high resolution transmission electron microscopy reveals that precipitated iron(II) monosulfide, FeS, consists of nanocrystalline mackinawite particles. The individual nanocrystals are laminar rectilinear prisms displaying a continuum of particle sizes from 2 to 5.7 nm in thickness (the direction parallel to the c axis) and from 3 to 10.8 nm in length. The corresponding mean specific surface area is estimated to be to 380 +/- 10 m(2)/g. The d(001) of mackinawite nanocrystals obtained from precipitated FeS and freeze-dried FeS by electron diffraction are 5.19 and 5.08 angstrom, respectively. The effect of water on the nanoparticle structures is indicated by the formation of curved structures and infrequent dislocations in an anhydrous environment. The apparent disorder suggested by absent or weaker lattice spacings is symptomatic of the breakdown of Braggian systematics at these particle sizes. The results confirm that nanoparticulate materials do not behave simply as small fragments of their bulk crystalline counterparts. The results contribute to understanding the behavior of nanoparticulate materials on planetary surfaces and in the biosphere in general and the nature and properties of FeS in anoxic aqueous environments in particular. (c) 2005 Elsevier B.V. All rights reserved.
  • H Ohfuji, AP Boyle, DJ Prior, D Rickard
    AMERICAN MINERALOGIST 90 (11-12) 1693 - 1704 0003-004X 2005/11 Scientific journal 
    The detailed crystal lography of natural pyrite framboids has been determined for the first time using electron backscatter diffraction techniques. The crystallographic ordering of microcrystals correlates positively with morphological ordering; the crystallographic orientations are random in morphologically disordered framboids and are almost ordered in morphologically ordered framboids. Morphologically ordered framboids involve two types of systematic misorientations across the microcrystal boundaries: low-an-le (ca. < 20 degrees) and high-angle (ca. 70-90 degrees) misorientations. The low-angle misorientation probably reflects slight physical misalignment of microcrystals in the packing structure, whereas the hi h-an le misorientation is considered to result from the dichotomy of the pyrite microcrystals having fourfold morphological symmetry but only twofold crystallographic symmetry about < 100 >. Thus, the crystallographic orientation of microcrystals is not uniform, even in highly ordered framboids. This suggests that the self-organization of microcrystals in pyrite framboids is not crystallographically controlled, for example by sequential replication of existing microcrystals, since this would not result in high lattice misorientation angles between adjacent microcrystals. Presumably, the self-organization process is a consequence of the aggregation of multiple equidimensional and equimorphic microcrystals that have nucleated in a fixed volume. We suggest that the regular arrangement of microcrystals occurs by the physical rotation (reorientation) of individual microcrystals, driven by the reduction in surface free energy between neighbors.
  • H Ohfuji, D Rickard
    EARTH-SCIENCE REVIEWS 71 (3-4) 147 - 170 0012-8252 2005/08 Scientific journal 
    \Eleven reported laboratory syntheses of pyrite framboids are critically reviewed. Pyrite framboids are defined as microscopic spheroidal to sub-spheroidal clusters of equidimensional and equimorphic pyrite microcrystals. The synthetic products from the reported studies are critically reviewed based on this definition. Three of the reported studies have produced well-developed, large pyrite framboids clearly comparable to the natural varieties. Although many other studies claimed successful syntheses of pyrite firamboids, they did not report the textural and structural details of the products, which are necessary for the identification. Some of the products are described as pseudo-framboids which consist of a massive internal part with a microcrystalline surface layer, indicating a different formation process from that of framboids. Thus, careful structural examinations are required for reliable identification of synthetic pyrite spherulites. The common factor in the successful syntheses of pyrite framboids is extremely high supersaturation so that the pyrite nucleation rate is significantly greater than the crystal growth rate. Such conditions can be achieved by the addition of S(0), O-2 or by increasing Eh of the systems and by increasing temperature. (c) 2005 Elsevier B.V All rights reserved.
  • J Akai, K Izumi, H Fukuhara, H Masuda, S Nakano, T Yoshimura, H Ohfuji, HM Anawar, K Akai
    APPLIED GEOCHEMISTRY 19 (2) 215 - 230 0883-2927 2004/02 Scientific journal 
    Sources of As in the Ganges sediments and microbial mechanisms of its release in groundwater were examined in the present study, where the authors have systematically examined the pertinent mineral species present in the sediments using XRD, TEM-EDS and EPMA techniques. The results show several As-bearing minerals in the Ganges sediments, in western Bangladesh. Iron-sulfide minerals consist of near-amorphous and/or crystalline precursors of framboidal pyrite and pyrite both of which contain As. Several types of Fe oxyhydroxides (oxides), which contain variable amounts of As were also found in muddy sediments. The content of As increases from Fe-oxides to the precursors of framboidal pyrite and pyrite. Four different chemical forms of As from the core sediments were determined. The sequentially extracted chemical forms are as follows: (1) acid soluble form (As mainly fixed in carbonates), (2) reducible form (As fixed in Fe- and/or Mn-oxides), (3) organic form, (4) insoluble form (As fixed mainly in sulfide and rarely in silicate minerals). Arsenic is dominantly sorbed on to Fe- and/or Mn-oxides, organic forms and sulfide minerals in most samples, although their relative abundances differ in different samples. Geomicrobial culture experiments were carried out to test the hypothesis that microbial processes play a key role in the release of As in groundwater. Batch culture and circulating water system experiments were designed using the sediments from Bangladesh. In the batch experiments, As was released at low Eh values a few days after adding nutrients containing glucose, polypepton and yeast extract, urea and fertilizer under a dominantly N-2 atmosphere. This contrasts with the control experiments without nutrients. Circulating water experiments with sand layer in a N-2 atmosphere showed similar results. These results support the hypothesis that microbial processes mediate the release of As into groundwater under reducing conditions. Glucose and polypepton used in the experiments may correspond to C and N sources, respectively. Younger sediments contain abundant organic matter, which is easily used by bacteria. So, the enhanced bacterial activity may correspond to simulation of accelerated natural diagenetic process using organic matter, or some fertilizer/wastewater effects. (C) 2003 Elsevier Ltd. All rights reserved.
  • H Ohfuji, J Akai
    AMERICAN MINERALOGIST 87 (1) 176 - 180 0003-004X 2002/01 Scientific journal 
    A new type of framboidal pyrite, with icosahedral domains, is described in this study. Examining the microcrystals on sections of framboids from various localities using a scanning electron microscope, we found pentagonal and trigonal patterns, These are made up of rectangular and fan-shaped domains, and octahedral microcrystals are regularly linked by sharing of edges in each domain. These symmetrical arrangements are interpreted to be different sections of icosahedrally arranged framboids which are composed of twenty tetrahedral domains. Thus some pyrite framboids are not spherical, but are fundamentally icosahedral both in appearance and internal structure. The formation of the icosahedral framboids might be related to the initial nucleation rate and the number of microcrystals within each framboid.


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Awards & Honors

  • 2017/09 Japan Association of Mineralogical Sciences Award of Japan Association of Mineralogical Sciences
    受賞者: OHFUJI Hiroaki
  • 2016/06 岩の力学連合会 平成27年岩の力学連合会賞(論文賞)
    受賞者: 大藤 弘明
  • 2012/09 日本鉱物科学会 第12号論文賞
    受賞者: 大藤 弘明
  • 2011/09 日本鉱物科学会 第7回研究奨励賞
    受賞者: 大藤 弘明
  • 2002/09 Universities UK Overseas Research Student Award
    受賞者: OHFUJI Hiroaki

Research Grants & Projects

  • 文部科学省:科学研究費補助金(基盤研究(S))
    Date (from‐to) : 2013 -2017 
    Author : 入舩 徹男, 大藤 弘明, 丹下 慶範, 境 毅, 肥後 祐司
    ・高温高圧下での多結晶体合成手法を確立し、様々な化学組成を持つガーネット、イルメナイト、MgSiO3およびCaSiO3ペロブスカイトの高品質多結晶体試料の合成とキャラクタリゼーションを行った。このうちイルメナイト、アルマンダイン及びメージャライトガーネット、CaSiO3ペロブスカイトに対して、マントル遷移層条件下で放射光X線その場観察及び超音波法を用いた弾性波速度精密測定を行った。これらの結果に基づき、マントル遷移層最下部の化学組成に関する制約を行った。・下部マントル上部に対応する27GPa, 2000K程度までの条件下での超音波測定技術の開発に基づき、上記で合成したMgSiO3ペロブスカイトに対する弾性波速度測定を行った。更に圧力領域を拡張するため、新しく開発されたタングステンカーバイドや、焼結ダイヤモンドアンビルを用いた超音波測定技術の開発を行った。・天然のコンドライト隕石の下部マントル条件下での溶融実験を開始した。50GPa領域までの融解関係および元素分配に関する新しい結果が得られつつあり、今後更に様々な圧力温度領域での実験を行う予定である。・焼結ダイヤモンドアンビルを用いたX線その場観察実験及び急冷回収実験の併用により、下部マントル条件下で新たな高圧型含水相(Phase H)を発見した。この結果に基づき、マントル中の水の大循環、及び最下部マントルの地震学的不均質構造に対する影響を評価した。・ヒメダイヤを用いた様々な研究を開始した。特に放射光を利用したX線吸収実験において興味ある結果が得られている。
  • 文部科学省:科学研究費補助金(基盤研究(B))
    Date (from‐to) : 2014 -2016 
    Author : 大藤 弘明
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(B))
    Date (from‐to) : 2011 -2013 
    Author : Jun-ichi ANDO, Takehiko HIRAGA, Tomoaki MORISHITA, Hiroaki OHFUJI
    In this research, we observed the microstructures of both alpine-type peridotites and peridotite xenoliths in basalt, and analyzed the microchemistry around the dislocations developed in the olivine grains of the studied samples, using optical microscopy, EPMA, EBSD, TEM and ATEM techniques to clarify if the ultra-low strain rate has any effect on the olivine plasticity. We detected Fe-concentration along dislocation line in all observed samples, which was probably caused by Cottrell atmosphere. This result strongly suggests that the effect of ultra-low strain rate on olivine plasticity is very important to understand the dynamics of the upper mantle, which has not been studied so far. In addition, we tried to do the deformation experiments of olivine to support the above conclusion, which has yet to produce any confirming results.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(若手研究(B))
    Date (from‐to) : 2011 -2013 
    Author : Hiroaki OHFUJI
    We performed a series of high pressure and high temperature experiments using a large-volume press to synthesize a pure, single-phase hexagonal diamond sintered body. Our study demonstrated that single-phase hexagonal diamond, which includes neither traces of unreacted graphite starting material nor cubic diamond, can be obtained at 25 GPa and 1100-1500C through direct conversion of highly-oriented, highly-crystalline graphite. The sample consists of extremely thin layered units (a few to several tens of nm) and is as dense as cubic diamond. Since our preliminary hardness measurements provided very high indentation hardness values comparable to or exceeding that of cubic diamond, this nano-sintered body of pure hexagonal diamond would have potential application as a novel ultra-hard material.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(挑戦的萌芽研究)
    Date (from‐to) : 2011 -2013 
    Author : Junji YAMAMOTO, Hiroyuki KAGI, Hiroaki OHFUJI, Hidemi ISHIBASHI
    Dislocations provide a direct proof of plastic deformation of minerals. We observed dislocation structure around fluid inclusions in olivine and orthopyroxene. For olivine, development of edge dislocations with loop shape whose size is analogous to that of an adjacent fluid inclusion indicate that the dislocations result from excess pressure of the fluid inclusion. The generation of loop dislocations would be controlled by yield strength of slip systems of olivine. For orthopyroxene, a few screw dislocations are identified in c-axis. The difference in dislocation density between both minerals suggests the higher yield strength of orthopyroxene. This is verified by the fact that expansion of a fluid inclusion in olivine estimated by dislocation structure can well explain difference in CO2 fluid density between the minerals. The observation of dislocation structure around fluid inclusions in xenoliths is useful to assess relative strength of slip systems among minerals.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(A))
    Date (from‐to) : 2008 -2012 
    Author : Toru INOUE, Norimasa NISHIYAMA, Hiroaki OHFUJI, 河野 義生, Kyoko MATSUKAGE, Akihiro YAMADA, 安東 淳一, Yoshio KONO, Yuji HIGO
    Distribution of water in the Earth’s interiors and the effect of water for phase transition boundaries were clarified, and the water content in the Earth’s interior s was estimated by combining with the seismic data. Moreover, the studies for the dehydration reaction and the equation of state of the slab constituent minerals, and also the study for the effect of water to the mantle mineralogy were carried out. In addition, the study of the compositional dependence for the water content in mantle minerals was carried out. We also tried to determine the water content of magma as a function of pressure and temperature.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(特別推進研究)
    Date (from‐to) : 2008 -2012 
    Author : Tetsuo IRIFUNE, 肥後 祐司, 臼井 祐介, Taku TSUCHIYA, Hiroaki OHFUJI, Norimasa NISHIYAMA, Yoshinori TANGE, Yasuhiro KUWAYAMA, Yuji HIGO, Yusuke USUI, Takahiro ISHIKAWA, Yusuke USUI, L Whitaker MATTHEW, Yu NISHIHARA, Takehiro KUNIMOTO
    Behaviors of iron and iron-bearing minerals have been studiedby both multianvil and diamond anvil cell experiments, in conjunction with ab initio calculations, under the pressure and temperature conditions from the deep lower mantle to the inner core of the Earth. A number of important results such as relevant to the partitioning of iron in the lower mantle and the phase relations of iron in the inner core, as well as technological developments in applying nano-polycrystalline diamond (NPD or HIME-diamond) to various high-pressure apparatus, have been obtained in this study.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(学術創成研究費)
    Date (from‐to) : 2007 -2012 
    Author : Hiroyuki KAGI, 山室 修, 上床 美也, 奥地 拓生, 近藤 忠, 佐々木 重雄, 平井 寿子, 長壁 豊隆, Takuo OKUCHI, Hisako HIRAI, Tadashi KONDO, Toyotaka OSAKABE, Shigeo SASAKI, Osamu YAMAMURO, Yoshiya UWATOKO, Hiroshi ARIMA, Kazuki KOMATSU, Hiroaki OHFUJI
    This research project aimed at developing neutron diffraction measurements at high pressure using a intense spallation neutron source at J-PARC and clarifying crystal structures of hydrogen-bearing materials in the deep earth, interior of planets, and so on. We have developed novel neutron-focusing device for small samples under high pressure, new high-pressure instruments optimized for the intense neutron source. We have opened a new horizon for sciences on earth and planetary materials at high pressure.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(若手研究(B))
    Date (from‐to) : 2008 -2010 
    Author : Hiroaki OHFUJI
    Laser heating in diamond anvil cell (DAC) equipped with nano-polycrystalline diamond (NPD) anvils was tested for the first time using various types of lasers. NPD-based DAC was found to provide better heating efficiency than a standard DAC with single-crystal anvils. We observed high-temperature generation exceeding 5000 K and 3500 K during laser heating of hcp-Fe sample at pressures of ~100 and 170 GPa, respectively.
  • 文部科学省:科学研究費補助金(萌芽研究, 挑戦的萌芽研究)
    Date (from‐to) : 2007 -2009 
    Author : 赤井 純治, 秋山 茂樹, 松岡 篤, 土山 明, 大藤 弘明
    本研究課題は、それぞれの鉱物、構成単位、の形態・構造・組織をミクロに観察し、データを得、記載することにあった。本年度は最終年度として、いくつか、まとめの段階をめざした。○とくにマンガンノジュールの薄い縞状組織の解明については、明確にストロマトライト構造といえるものを、見いだし、また、フラクタル的特徴もX線-CTによる解析、立体形態と構造を記載して。そして、バイオ起源ということを、論文として、準備した。このなかには、生成モデルも含む。一度論文投稿を行ったが、現在まだ受理されていない。これの掲載までめざす。また、これについて、さらに進んだ点を今年8月のIMA(Budapest)にて発表の予定である。○フランボイダル磁鉄鉱、黄鉄鉱についてSEMによる3次元的な形態、構造を徹底的に調べる課題;バクテリアによる生成過程を実験的に追跡し、その形態解析、集合組織を諸手段で解析し、かなりの成功をおさめた。あと一歩の詰めという段階である。あと一歩である。○ストロマトライト構造の相似律;これまで各種ストロマトライトを鉱物学の立場、バイオミネラリゼーョンの立場からみてきた。ストロマトライトのサイズ的次元の異なったものがあるということ、しかもそれが非常に強い相似形態をなしていることであり、この形態的特長をとらえることをめざしたが、なお、最まで詰めることができず、中途段階にある(新潟大 松岡篤)。○隕石中のフランボイダル磁鉄鉱を上記パイライトと同様の視点、また結晶成長の面から解析することも課題としてあげたが、今年度他の課題におわれ、十分進展はしなかった。しかし、前年までの成果がねむっているところなので、機会をえて、大展開する予定である。○隕石中の炭素鉱物の連結構造の解析は、上記、隕石中磁鉄鉱の課題とあわせて展開する。総じて、最初の課題で、発表へむけて具体的に進展しつつあるところである。
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(B))
    Date (from‐to) : 2006 -2008 
    Author : Daisuke YAMAZAKI, Eiji ITO, Tomoo KATURA, Hiroaki OFUJI, Takashi YOSHINO
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(特定領域研究)
    Date (from‐to) : 2004 -2008 
    Author : 入舩 徹男, Norimasa NISHIYAMA, Toru INOUE, Daisuke YAMAZAKI, Junichi ANDO, Tomoo KATSURA, Kenichi FUNAKOSHI, Hiroaki OHFUJI
  • 文部科学省:科学研究費補助金(若手研究(B))
    Date (from‐to) : 2006 -2007 
    Author : 大藤 弘明
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(学術創成研究費)
    Date (from‐to) : 2003 -2007 
    Author : Tetsuo IRIFUNE, 西山 宣正, 阪本 志津枝, 山崎 大輔, 趙 大鵬, Toru INOUE, Taku TSUCHIYA, Hiroaki OHFUJI, Yuji HIGO, Akira YAMADA
    Technical developments have been made in producing higher pressures in multianvil apparatus using sintered diamond (SD) anvils and also for precise measurements of sound velocities of high-pressure phases under the P/T conditions of the mantle transition region. We succeeded to produce pressures as high as〜80 GPa at room temperature using SD anvils, while pressures have been limited to〜60 GPa at the high temperatures relevant to the Earth's mantle. Some new phase transitions have been found in mantle minerals under the conditions of the lower mantle, and precise determination of the P-V-T relations of these and other high-pressure phases have been conducted under the corresponding P, T conditions. On the other hand, precise measurements of sound velocities have been made at pressures to〜20GPa and temperatures to〜1700K, equivalent to those of the depths of the middle part of the mantle transition region. Using this technique; measurements on ringwoodite and majorite in a pyrolite composition, polycrystalline MgO, and the garnetite (majorite + stishovite) in a basaltic composition have been made, which provided important data sets to constrain the mineralogy of the mantle transition region and to provide a new self-consistent pressure scale. In addition to these studies, we have tested the performance of the newly produced nano-polycrystalline diamond (HIME-DIA) for high-pressure devices, and found the great potential of this novel material for anvils for both multianvil apparatus and diamond anvil cell, which yielded the highest pressures of〜110 Gpa and〜20 Gpa, respectively.
  • Ministry of Education, Culture, Sports, Science and Technology:Grants-in-Aid for Scientific Research(基盤研究(B))
    Date (from‐to) : 2003 -2006 
    Author : Toru INOUE, 肥後 祐司, Daisuke YAMAZAKI, Tetsuo IRIFUNE, Jun-ichi ANDO, Yuji HIGO, Hiroaki OHFUJI
    We have clarified the physical properties of deep Earth mantle minerals and the effect of water. Experiments were conducted by in situ X-ray measurements in addition to quench experiments.From our previous studies, it has been clarified that high pressure polymorphs of olivine, wadsleyite and ringwoodite, can contain 2-3 wt% of H_2O in their crystal structures, but the partitioning of H_2O among these minerals has not been clarified yet. In the present study, we have clarified the partitioning of H_2O to synthesize these coexisting phases, and found that mantle transition zone should be a strong water reservoir in the Earth's interior.In addition, the measurements of thermal expansion and elastic properties of hydrous wadsleyite and hydrous ringwoodite were conducted to clarify the H_2O content in the mantle. Combined these results with seismic observation, we estimated the H_2O content in the mantle. Further extensive studies have been conducted with seismologist.We also tried to observe dehydration and physical properties of high pressure hydrous phase with capsulation using relatively light metal capsule by in situ X-ray observation. We succeeded to get X-ray spectra of superhydrous phase B using AgPd capsule, and further improved capsule, which were made by a single crystal diamond sleeve and metal caps, were designed for in situ X-ray experiment. The capsule is quite successful, and the effective acquisition of X-ray spectra under high pressure has been achieved by in situ X-ray experiment. This technique has been adopted to determine hydrous melt structure and time-resolved experiment of dehydration.




  • 2019, the first semester, under graduate, 地学英語
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  • 2019, the first semester, under graduate, 地球科学実験I

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