Substitution effects on the shape memory and hydrogenation properties of TiNi

This study aims to improve shape memory and hydrogen storage properties of TiNi by chemical substitutions in the Ni sub-lattice. The effect of Pd, Cu and Co substitutions on crystal structure, martensitic transformation andhydrogenation properties of TiNi has been studied by structural (X-ray and neutron powder diffraction),calorimetric, solid-gas and electrochemical means. Ab initio DFT calculations were done to highlight electroniceffects on hydrogenation properties. The three substitutions, TiNi1-zMz (M = Pd, Cu and Co; z ≤ 0.5), lead to the formation of pseudobinarycompounds. Substitutions by Pd and Cu increase the unit-cell volume of TiNi, whereas the reverse effect occurs for Co. Martensitic transformation temperatures correlate with volume variations. They increase strongly for M= Pd and slightly for M = Cu, whereas M = Co decreases it. Hydrogenation properties are very sensitive tochemical elements substitution. The capacity decreases strongly for M = Pd, moderately for M = Cu and remainsstable for M = Co. Contrary to expected effect by geometric model, both Pd and Cu substitutions decrease the stability of hydrides. DFT calculations show that electronic rather than geometric effects govern hydride stability for M = Pd. Co substitution induces step-wise formation of hydrides with a multi-plateau behaviour inpressure-composition isotherms. As concerns applications, low amount of Cu substitution (z = 0.2) increases the electrochemical dischargecapacity of TiNi from 150 to 300 mAh/g due to hydride destabilization. This opens new perspectives for using TiNi-based alloys in Ni-MH batteries. In contrast, Pd and high amount of Cu substitution (z = 0.5) are effective to decrease TiNi reactivity towards hydrogen, and therefore attractive for shape memory applications under reductive environment.

کلیدواژه ها:

TiNi ، pseudo-binary compounds ، martensitic transformation ، hydrogenation properties ، Ni-MH batteries