Prof. Katsutoshi Aoki [Visiting Researcher, University of Tokyo, Japan]

Title: Neutron diffraction study of the Fe-H system at high temperatures and high pressures

Time: 10:00 - 11:00 AM, Friday, May 19, 2017

Place: Auditorium Room 410, HPSTAR (Shanghai)

Host: Dr. Duckyoung Kim

Abstract

At high hydrogen pressures (several GPa), metals react with hydrogen (H) to form hydrides, with the hydrogen atoms occupying the interstitial sites of metal lattices. Iron hydrides have been intensively studied for many decades and the diagram of the Fe–H system is well established at temperatures up to 2000 K and pressures up to 20 GPa [1]. Three hydride forms exist: body-centered cubic (bcc)-FeHx, double hexagonal close-packed (dhcp)-FeHx, and face-centered cubic (fcc)-FeHx.

The solubility of hydrogen in bcc Fe is very low (i.e., <10–6 H/Fe) at ambient temperature in hydrogen pressure of 0.1 GPa. The high-temperature phase of fcc-FeHx is a solid solution with x varying with temperature and pressure. The dhcp phase has a stoichiometric composition x = 1 over the whole pressure range measured at room temperature and can be described as a monohydride FeH.

Knowledge of hydrogen composition and site occupancy of H atom is essential for interpreting the physical and chemical properties of hydrides. Neutron diffraction measurement is the most powerful tool to investigate the crystal structures of the Fe-H system and, however, has been reported only for the recovered dhcp FeDx: no in situ measurement has been carried out at high temperature and high pressure.    

The high-pressure neutron diffractometer, PLANET, constructed at the Materials and Life Science Experimental Facility in J-PARC provides a diagnostic method for the hydrogenation of metals occurring at high temperature and high pressure [2]. Since the nucleus of D atom has a scattering length comparable to that of Fe nucleus against neutron beams, neutron diffraction measurement enables us to precisely investigate the crystal structures of iron deuterides, including the location and occupancy of D atoms at the interstitial sites of the Fe metal lattice.

In the HPSTAR seminar, the neutron diffraction results obtained first for fcc-FeDx at 988 K and 6.3 GPa are presented [3] and the latest results obtained by cooling of fcc-FeD under equilibrium and seal-off conditions of hydrogen dissolution. For the equilibrium condition, fcc-dhcp structural transition occurred across the phase boundary in agreement with the proposed phase diagram [1], while for the equilibrium condition, fcc-FeDx decomposed into bcc- and dhcp-FeDx via an intermediate coexisting state of fcc- and hcp-FeDx. Hydrogenation cell developed for in situ neutron diffraction measurement in combination with a 6-axis cubic anvil press is explained. Finally, the future prospectives of the neutron facility, MLF, and the metal-hydride study will be presented.

1. Y. Fukai, et al. J. Alloys. Compd. 348, 105–109 (2003).

2. T. Hattori, et al. MLF Annual Report 2012, 96–97 (2013).

3. A. Machida, et al . Nature Communications 5 (2014) 5063-(1)–(6).

Biography of the Speaker:

Experience and Education

2015- Visiting Researcher,

Graduate School of Science, The University of Tokyo

2012-2015 Senior Scientist,

Institute for Materials Research, Tohoku University

2005-2012 Senior Scientist (2010-2012),

Director of Synchrotron Radiation Research Center, Senior Principal Scientist (2008-2010),

Deputy Director of Synchrotron Radiation Research Center, Principal Scientist (2005-2008),

Japan Atomic Energy Agency (JAEA)

2003 -2005 Deputy Director of Synchrotron Radiation Research Center,, Principal Scientist,

Japan Atomic Energy Research Institute (JAERI)

2001 - 2003 Deputy Director of Materials and Chemical Process Division, Principal Scientist,

National Institute of Advanced Industrial Science and Technology (AIST)

1999 - 2001 Director of Basic Research Division, Principal Scientist, (1999-2001),

Group Leader of High Pressure Chemistry Group, Principal Scientist, (1991-1999)

National Institute of Material and Chemical Research (NIMC)

1978 - 1991 Senior Researcher (1983-1991),

Researcher (1978-1991),

National Chemical Laboratory for Industry (NCLI)

March 1978 Graduated from The University of Tokyo (Institute for Solid State Physics), Doctor of Science

Other Carrier

1995 - 2000 Professor, Cooperative Graduate School, Tsukuba University

1991 - 1991 Visiting Associate Professor, Institute for Solid State Physics, The University of Tokyo

1989 - 1989 Visiting Research Chemist, Department of Chemistry and Biochemistry,

University of California Los Angels, USA

1982 -1983 Visiting Researcher, Max Planck Institute for Solid State Physics, Germany

Awards

Science and Technology Director Award in 1995

Tsukuba Prize in 1996