Role of Annealing for Improving Hydrogen Storage Properties of Ti-Cr-V Alloy
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0971-Z07-21
Role of Annealing for Improving Hydrogen Storage Properties of Ti-Cr-V Alloy Yasuhiro Munekata1, Kota Washio1, Takanori Suda1, Naoyuki Hashimoto1, Somei Ohnuki1, Hironobu Arashima2, and Hideaki Ito2 1 Graduate School of Engineering, Hokkaido University, Sapporo, Japan 2 Muroran Res. Inst., Japan Steel Works, Muroran, Japan ABSTRACT The Cyclic degradation of Ti-Cr-V, a solid solution type hydrogen storage alloy with Body-Centered Cubic, has been investigated by SEM and TEM. The cyclic test resulted in fine powders with cracking, and then the size was resulted over 100 cycles. Microstructural observation revealed the dislocation density increased more than one order of magnitude during this test, meaning that the decrease in effective storage capacity could depend on the stored lattice defects and strain. To prevent the cyclic degradation, the sub-micron sized particles were annealed for reducing dislocation density. This treatment enables to expand the plateau in hydrogenation process, but it has no effect at de-hydrogenation, that is the plateau returned to the un-improved level. This means that the stability of beta-hydride is strongly depended on the lattice defect and strain level created by the hydrogenation. If the particle size is reduced to less than several hundred nm, the degradation could be improved. INTRODUCTION For practical use of hydrogen storage alloys, higher hydrogen storage capacity and less cyclic degradation are important issues [1]. Especially, the repetition of hydrogenation-dehydrogenation, which greatly affects on hysteresis [2], resulted in decline of the actual storage capacity due to lattice strain in the crystalline. Furthermore, surface contamination by impurity gas leads to decrease in storage rate [3]. These performances can be categorized as extrinsic and intrinsic degradation. The extrinsic degradation induced by surface contamination prevents the dissociation of hydrogen molecule, however, it can be improved by the coating. On the other hand, the intrinsic degradation can be occurred due to repeating of volumetric expansion and shrinkage due to the formation and dissociation of hydrides. The repetition sometime leads to phase segregation by disproportionation reaction, where the amorphization and hardening occur due to high dislocation density with increasing of this reaction[4]. Ti-Cr-V hydrogen storage alloy is a solid solution type [5], and has not stable metallic compound. Therefore, intrinsic degradation and cyclic degradation could be developed easily. However, only limited studies on the degradation have been reported from perspective of microstructure. The purpose of this paper is to investigate the effect of microstructures on the cyclic degradation properties of Ti-Cr-V alloy with large hydrogen storage capacity [5-8]. EXPERIMENTAL Ti24Cr36V40 alloy [7, 8, 10] was made by an arc-melting under Ar atmosphere. The alloy was homogenized at 1673K for 3 hr under Ar atmosphere by floating zone melting (the standard sample was named as FZ). The alloy was crushed by t
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