Vacancy diffusion in magnetite-hercynite solid solution

Abstract

To extend the chemical relaxation technique to ternary oxide systems, theoretical analysis was made to obtain an expression for the chemical diffusion coefficient in terms of the vacancy diffusion coefficient. An equation, D̃ = [ C O (C 1 + C 2) ] D v ( 1 2 )( ∂ ln P( O 2) ∂ ln C v ) , was derived. This is similar to the one for the binary oxide system. Chemical relaxation experiments were made on the magnetite-hercynite solid solutions, (Fe 1− y Al y ) 3−δO 4 with y = 0.0, 0.067, 0.133, and 0.20, at temperatures between 1300 and 1400°C. The vacancy diffusion coefficient decreased remarkably with an increase in y. The activation energy was found to be 20.8 ± 3.7, 33.5 ± 4.2, 50.4 ± 3.6, and 66.3 ± 4.5 kcal/mole for y = 0.0, 0.067, 0.133, and 0.20, respectively. A strong dependence on y was also found. The dependence of the vacancy diffusion coefficient on y was interpreted to indicate that the jump frequency of cation vacancies is decreased by the introduction of aluminium ion.