Sub-nano and Nano-structures of Hydrides of LaNi 5 and its related Intermetallics
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Sub-nano and Nano-structures of Hydrides of LaNi5 and its related Intermetallics Etsuo AKIBA, Kouji SAKAKI, Yumiko NAKANURA Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 Japan ABSTRACT Sub-nano and nano-structures of intermetallics such as LaNi5 and its related alloys were studied by in-situ X-ray and neutron diffraction methods. From the profile shape analysis, changes in lattice strain and crystalline size during hydrogenation/dehydrogenation were estimated, while the crystal (sub-nano) structures were refined by the Rietveld method using diffraction patterns. The crystallite size of the alloys studied did not change during hydrogenation and dehydrogenation. It was found formation of remarkably dense dislocations and vacancies in the lattice of hydrides of LaNi5 and related intermetallics. INTRODUCTION LaNi5 is a representative intermetallic compound which has a reversible hydrogen absorbing property under ambient conditions. The alloy phase transforms into hydride phase through two-phase coexistence region. The lattice parameters and the cell volume increase by 7 % and 24 %, respectively, in the transformation from the alloy phase to the hydride phase, respectively. The hydriding reaction with such large lattice expansion in absorption and contraction in desorption could induce various kinds of lattice defects and strain. We have found that highly dense dislocations were introduced to the lattice of LaNi5 intermetallic hydride at the first hydrogenation and it did not disappear in the following desorption reaction using in-situ X-ray diffraction (XRD) technique [1]. Introduction of dislocation of 1012cm-2 was confirmed by Yamamoto et al. using transmission electron microscopy (TEM) [2]. One of the authors found that both dislocations and vacancies were introduced into the lattice of intermetallics at the first hydrogenation [3]. This paper will present a brief review of these experiments on LaNi5 and its related intermetallic compounds such as Al and Sn substituted LaNi5. In addition, the mechanism of defects formation will be discussed. EXPERIMENT Preparation of alloys An alloy ingot of LaNi5 was prepared by high-frequency induction melting and alloy ingots of LaNi4.75Al0.25 were prepared using the arc melting technique. They were purchased from
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Santoku Metal Industry Co., Ltd. The ingot of LaNi4.75Al0.25 was annealed at 1373 K for 24 h in an Ar atmosphere. Alloy ingots of LaNi4.75Sn0.25 and LaNi4.78Sn0.22 were prepared from high purity metals using the arc melting technique by Robert C. Bowman, Jr. of Jet Propulsion Laboratory, USA [4]. They were annealed at 1223K for ~100h in a purified Ar atmosphere. The ingots were crushed into the particle size under 30 µm for X-ray powder diffraction measurements.
XRD measurements and peak profile analysis In situ X-ray powder diffraction were measured using a horizontal sample stage type diffractometer (Rigaku, RINT-TTR) with a r
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