Internal displacement reactions in multicomponent oxides: Part III. Solid solutions of ternary oxide compounds
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-STATE displacement reactions inside an oxide matrix are useful in the design and synthesis of metal-ceramic composites with useful physical and mechanical properties. As part of a study of internal displacement reactions between a metal and a multicomponent oxide, previous articles[1,2] (Parts I and II) considered the reactions involving two types of ternary oxides: (1) line compound of narrow homogeneity range and (2) solid solution of wide composition range. The cation concentration profiles in the reaction zone are qualitatively different for the two types of oxides. (1) The displacement reaction inside a line compound is represented as A (metal) BCOmn (line compound) “B” (metal) “ACOmn” (line compound) [1] S.N.S. REDDY, Senior Engineer, is with the Systems and Technology Group, IBM Corporation, Hopewell Junction, NY 12533. Contact e-mail: snreddy@us.ibm.com Manuscript submitted July 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
In Reaction [1], the ternary oxides BCOmn and ACOmn are formed from the binary oxides AOm, BOm, and COn. The meaning of line compound is given by a constant A:C or B:C ratio (1:1 in this case) in the ternary oxides. Also, in the ternary oxides, A (or B) occupies a cation sublattice, which is different from the sublattice for C. The product oxide in Reaction [1], “ACOmn,” is the solid solution, (A,B)COmn, in which (A B):C 1:1. During the reaction, the cation fluxes at a given location in the product oxide are as follows: JA JB and JC 0. As such, in (A,B) COmn, concentration gradients exist only for A and B and not for C. As a model of Reaction [1], the results for the reaction couple Fe/NiTiO3 were reported.[1] (2) The displacement reaction inside an oxide of wide composition range is represented as xA (metal) (BxC1-x)O (solid solution) x“B” (metal) “(AxC1-x)O” (solid solution) [2] For the sake of simplicity, only the monoxides are considered in Reaction [2]. The oxides AO, BO, and CO have the same crystal structure and form complete solid solutions. The product oxide, “(AxC1-x)O,” is (A,B,C)O solid solution VOLUME 36A, NOVEMBER 2005—2993
in which all the cations occupy a single sublattice and their diffusion is strongly correlated. During Reaction [2], at a given location in product oxide, JA JB and JC 0. The direction of JC is determined by the thermodynamics of (A,B,C)O solid solution; for most reactions, JC is in the same direction as JA. Hence, concentration gradients develop for all the cations (A, B, and C) in the product oxide. As models of Reaction [2], the results for the reaction couples Fe/(NixMg1-x)O and Fe/(Co0.5Mg0.5)O were reported.[2] This article considers an internal displacement reaction between a metal and a quaternary solid solution of ternary line compounds, which can be generally represented as x A (metal) (BxC1-x)DOmn (oxide) x“B” (metal) “(AxC1-x)DOmn” (oxide); G3 0 [3] where G3 is the Gibbs energy change for Reaction [3]. The starting oxide, (BxC1-x)DOmn, is a solid solution of the ternary compounds BDOmn a
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