Transport in Chemical Potential Gradients of Multicomponent Oxides

Abstract

The ultimate goal of the investigation of solid-state reactions in materials science is the understanding of the atomistic reaction mechanisms. There­ fore, all the complicated conditions at the phase boundaries and, conse­ quently, the complex phenomenological formalisms in any non-steady state only obscure the atomistic processes. Recent investigations therefore studied transport in multicomponent nonmetallic solid phases, prefer­ entially in multicomponent oxides, under the action of only the metal­ loid (oxygen) potential gradient in true steady state in order to clarify the atomistic interpretation. Model investigations of this kind are also of considerable practical importance in view of the fact that nonmetallic, high-temperature ceramics are often multicomponent systems that normally operate under the action of chemical potential gradients. Oxygen potential gradients are of par­ ticular interest. Solid-state reactions (in inhomogeneous and heterogeneous systems) occur under the action of thermodynamic potential gradients (I). Except in the case of formation of ternary (stoichiometric) oxide compounds from binary reactants at a given oxygen potential (e.g. AO+B203 = AB204), the boundary conditions at the interfaces (phase boundaries) during reac­ tion are not constant. For example, the oxidation of an alloy (A,B) to (A,B)On in oxygen can only be treated formally under quite restrictive simplifications (2) because transport of A and B occurs simultaneously in the lattice of the receding metal and in the growing oxide layer. The respective fluxes are coupled at the common interface. This renders the