Hydrogenation Properties of CaMg₂ Based Alloys

Abstract

Binary C14 Laves phase alloy CaMg₂ is expected to absorb hydrogen more than 6 mass%, when the H/M ratio reaches 2. We have found that the binary CaMg₂ did not absorb hydrogen at ambient temperature and hydrogen pressure but Ni added CaMg_1.8Ni_0.2 which is also C14 Laves phase absorbed 6.0 mass% of hydrogen at room temperature. CaMg_1.8Ni_0.2 formed hydrides of Mg and Ca, and the C14 Laves phase disappeared. Hydrogen desorption was not observed from hydrogenated CaMg_1.8Ni_0.2. (Ca_0.8La_0.2)Mg_2.2Ni_0.1 absorbed 5.1 mass% hydrogen and H/M ratio reached 1.8 at room temperature. After hydrogenation, the lattice of the Laves phase expands 4% in a-axis and 3% in c-axis. The volume expansion is 13%. The Laves phase structure is kept and hydrogen was absorbed in the lattice. The hydride of (Ca_0.8La_0.2)Mg_2.2Ni_0.1 decomposed from 470 K with increase at temperature and hydrogen was released from the products of decomposition from 610 K.