Hydrogen absorption and desorption by Mg_67-xCa _xNi₃₃ powders prepared by mechanical alloying

Mg_67-xCa_xNi₃₃ elemental powder (0≤x≤20) mixtures were mechanically alloyed by a planetary ball mill, and phase variations after mechanical alloying (MA) and subsequent hydrogenation by gas phase loading at temperatures of 373 and 503 K, and their hydrogen desorption were investigated. The MA leaded to the formation of the Mg ₂Ni-type phase for all powders. All powders containing Ca were induced to form an amorphous-like phase after hydrogenation at 373 K for 168 h, although the binary Mg₆₇Ni₃₃ powder was the α-phase (Mg₂NiH_0.3 hydride). After hydrogenation at 503 K, on the other hand, the low temperature (LT)-phase (Mg₂NiH₄ hydride) formed for all powders. The ternary powders also formed the CaH ₂ hydrides. The temperature for the maximum hydrogen desorption rate increased with increasing Ca concentration in the powders after hydrogenation at both temperatures. The activation energies for hydrogen desorption for the binary powder after hydrogenation at 373 and 503 K were the same, about 92 J mol^-1, and those for Mg₄₇Ni₃₃Ca₂₀ powder at 373 and 503 K were about 168 and 244 J mol^-1, respectively, indicating that substitution of Ca for Mg stabilized the hydrides.