Mechanically alloyed Mg–Ni–Ti and Mg–Fe–Ti powders as hydrogen storage materials

Abstract

Recent research on coevaporated thin gradient films Mg1−x−zNixTiz have detected, using optical hydrogenography, a compositional region centered around Mg69Ni26Ti5, with low absolute values of the hydride formation enthalpy. These destabilized hydrides would present then sorption kinetics more favorable for their use as hydrogen reservoirs. In order to check if these relevant properties extend to massive samples we attempted an investigation on mechanically alloyed powder mixtures Mg1−x−zNixTiz of similar composition and also on mixtures Mg1−x−zFexTiz, with Fe instead of Ni. Thus, powder mixtures of nominal composition: Mg69M26Ti5, Mg64M26Ti10 and Mg64M31Ti5 (M = Ni, Fe) were milled in a H2 ambient till a steady state, with no further H2 absorption, was attained and the hydriding kinetics was determined from the H2 intake. Each hydrided sample was discharged by increasing the temperature at a given starting H2 pressure and recharged by decreasing the temperature under the same conditions. The resulting P vs T curves were analyzed to extract approximate values of the equilibrium (T,P) data for H2 desorption and absorption. Before and after their cycling in H2 the samples were characterized by X ray diffraction and those containing Fe, by Mössbauer spectroscopy. The results are analyzed in connection with possible mechanisms for the transition metals catalytic action and the influence of the sample microstructure.