Reactive diffusion in Ni - RE (RE = Ce, Pr, Nd) binary alloys

An experimental study was carried out on the reactive diffusion in the Ni-RE (RE = Ce, Pr, Nd) binary alloy systems to gain a fundamental knowledge of the reduction-diffusion (RD) method to directly produce powders of nickel-rare earth compounds, RENi₅, for use of hydrogen storage materials. Firstly, phase relations were investigated at temperatures above 1100K in a concentration range of nickel above 55 mol%, using reactive diffusion and metallographic methods. The following compounds were found in the process of reactive diffusion: (a) the Ni-Ce system - CeNi₂, CeNi₃, Ce₂Ni₇ and CeNi₅, (b) the Ni-Pr system - PrNi₂, PrNi₃, Pr₂Ni₇ and PrNi₅, and (c) the Ni-Nd system - NdNi₂, NdNi₃, Nd₂Ni₇, NdNi₄, NdNi₅ and Nd₂Ni₁₅. The compounds of CeNi₅, PrNi₅ and NdNi₅ presented solid solutions and their ranges of solubility were determined by an EPMA analysis. The widest ranges of solubility were 82.0 - 86.4 mol% Ni at 1488K, 81.7 - 85.4 mol% Ni at 1558K and 82.4 - 86.5 mol% Ni at 1573K in the Ni-Ce, Ni-Pr and Ni-Nd systems, respectively. Secondly, interdiffusion coefficients, of the PrNi₅ and NdNi₅ compounds were determined, using a diffusion couple of the Ni/Ni-RE melt, given as follows. PrNi₅: D/m²·s^-1 = 1.61×10^-9 exp (-61.3 kJ·mol^-1/RT) (T = 1423-1513K) NdNi₅: D/m²·s^-1 = 1.85×10^-4 exp (-205 kJ·mol^-1/RT) (T = 1443-1503K).