Internal displacement reactions in multicomponent oxides: Part II. Oxide solid solutions of wide composition range

Abstract

As models of internal displacement reactions in oxide solid solutions, the following reactions were studied at 1273 K as a function of time: Fe + Ni_xMg_1-x)O = Ni + (Fe_xMg_1-x)O Fe + (Co_0.5Mg_0.5)O = Co + (Fe_0.5Mg_0.5)O In both reactions, Ni or Co in the starting oxide is displaced by Fe and the γ-(Ni-Fe) or (Co-Fe) alloy is precipitated. In the reaction zone, composition gradients develop in both product phases, viz., the oxide and the alloy precipitate. The Ni (or Co) concentration of the alloy precipitate increases towards the reaction front. In the product oxide, the "inert" Mg diffuses toward the reaction front along with the Fe, while the Ni (or Co) diffusion is in the opposite direction, towards the Fe/boundary. The shape of the composition profiles for Mg and Fe in the product oxide suggests that cross-coefficient terms in the generalized flux equations contribute significantly to the cation flux. The parabolic rate constants of reactions involving Fe/(Ni_xMg_1-x)O decrease by nearly four orders of magnitude when x decreases from 1 to 0.1.