Does the Valence State of an Ion Affect its Diffusivity? - Part I: Oxygen Activity Dependence of the Diffusion of Iron in Alumina-Doped MgO

E. Chen, T.-L. Tsai and R. Dieckmann

Department of Materials Science and Engineering, Cornell University, Bard Hall, Ithaca, NY 14853-1501, U.S.A.

Abstract

To investigate whether a change in the valence state of tracer ions affects their diffusivity or not, the iron tracer diffusion in Al₂O₃-doped MgO, in which 0.5 % of the cations were Al³⁺ ions, has been studied experimentally. Samples were prepared from high purity aluminum and magnesium nitrates using a chemical solution method and from powders of high purity Al₂O₃ and MgO. Because the concentration of the Al³⁺ dopant ions present in the samples was much larger than that of all other impurities, the concentration of the majority point defects, cation vacancies, was determined by the Al³⁺ concentration. Therefore, when changing the oxygen activity, the diffusivity of iron tracer ions can only be altered by changes in their valence state. Measurements of iron tracer diffusion coefficients were performed as a function of the oxygen activity at 1100 and 1200 °C. The experimental results indicate that the mean diffusivity of iron ions in Al₂O₃-doped MgO increases with increasing oxygen activity at both temperatures, suggesting that Fe³⁺ ions diffuse in Al₂O₃-doped MgO faster than Fe²⁺ ions.

Oxygen activity dependence of the iron tracer diffusion coefficient, D*_Fe, at 1100 and 1200 °C.

Solid State Sciences, 10 (6) [2008] 735-745.

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