Effect of magnesium content and quenching rate on the phase structure and composition of rapidly solidified La2MgNi9 metal hydride battery electrode alloy

The phase structure and composition of La2MgNi9 metal hydride battery electrode alloy after rapid solidification at wheel surface speeds of 10.5 m s (1) and 4.2 m s (1), have been investigated by electron microscopy (SEM, TEM) and X-ray diffraction techniques. The alloy is multi-phase structured, containing rhombohedral La2MgNi9 as the main phase with small amounts of LaNi5, La3MgNi14 and LaMgNi4 as secondary compounds. The high vapour pressure and low melting point of magnesium compared with other constituent elements and its eventual loss from the melt prior to rapid solidification, as well as the peritectic mechanism of the formation of La2MgNi9, accounts for the formation of these secondary compounds during cooling of the melt. However, optimisation of the amount of Mg in the melt and the application of low cooling rates during solidification considerably increases the yield of the La2MgNi9 phase, which is required to achieve enhanced electrochemical properties of the metal hydride battery electrode. The lower alloy solidification front velocity and temperature gradient at the quenching wheel speed of 4.2 m s (1), allow sufficient time for atomic diffusion and interaction of phases during the peritectic reaction and promote the formation of a nearly-homogeneous, single phase La2MgNi9 alloy. (C) 2012 Published by Elsevier B.V.