Internal displacement reactions in multicomponent oxides: Part II. Oxide solid solutions of wide composition range
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INTRODUCTION
PREVIOUS studies of solid-state displacement reactions have identified different reaction morphologies and their relationship to the thermodynamic and diffusional transport properties of product phases.[1,2] In the case of simple displacement reactions between a metal and a binary oxide, Rapp et al.[1] have predicted the morphologies and reaction rates from a knowledge of the thermodynamic and diffusion data of product phases; the predictions were verified for the following reaction couples: Cu2O/Ni, Cu2O/Co, Cu2O/Fe, and NiO/Fe. Depending on the diffusional rate control in the product phases, the reactive interface is either stable (layered morphology) or unstable (aggregate morphology). The reactive interface is morphologically stable when the rate-controlling diffusion flux is in the same direction as the interface motion (e.g., Cu2O/Ni and Cu2O/Co); the interface is unstable when it moves against the rate-determining flux (e.g., Cu2O/Fe and NiO/Fe). Tangchitvittaya et al.[2] have studied the reaction mechanism and the development of an interwoven product morphology for the reaction between Fe and NiO (single-crystal oxide). Internal displacement reactions occur inside a metal or ceramic matrix. The only published internal displacement reaction is between MoO2 and Cr in a Ni-base alloy matrix by Shook et al.[3] As part of a study of internal displacement reactions in multicomponent oxides, a previous study (Part I) considered the reaction between a metal and a ternary oxide compound with a narrow homogeneity range.[4] A study of the internal displacement reaction between a metal and an initially S.N.S. REDDY, Senior Engineer and L.B. WIGGINS, Senior Technical Staff Member, are with the Systems and Technology Group, IBM Corporation, Hopewell Junction, NY 12533. Contact e-mail: [email protected] D.N. LEONARD, Graduate Student, is with the Department of Materials Science, North Carolina State University, Raleigh, NC 27695. K.T. JACOB, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India. Manuscript submitted July 13, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
homogeneous single-phase ternary oxide solid solution with a wide composition range, at a constant temperature and pressure, is the subject of this article. In a following article (Part III), oxide solid solutions involving ternary line compounds will be considered.[5] II. AN INTERNAL DISPLACEMENT REACTION IN AN OXIDE SOLID SOLUTION Consider the oxides AO, BO, and CO, which have the same crystal structure and which form solid solutions in the entire composition range. The thermodynamic stability of the oxides is such that, GoCO GoAO GoBO. The oxide (BxC1x)O is not stable when in contact with metal A at elevated temperatures, and the following internal displacement reaction occurs: x A (Bx C1x)O x “B” “(Ax C1x)O”; G1 0 [1] The G1 is the Gibbs energy change for Reaction [1]. During the reaction, cation B is displaced by A in the oxide without changing the crystal structure
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