Abstract:
Cation tracer diffusion coefficients, DMe*, for Me=Fe, Mn, Co and Ti, were measured using radioactive isotopes in the spinel solid solution (TixFe1-x)3-δO4 as a function of the oxygen activity. Experiments were performed at different cationic compositions (x=0, 0.1, 0.2 and 0.3) at 1100, 1200, 1300 and 1400 °C. The oxygen activity dependence of all data for DMe* at constant temperature and cationic composition can be described by equations of the type DMe*=D°Me[V]. CV·aO22/3+DMe[I]°·aO2-2/3·DMe[V]° and DMe[I]° are constants and CV is a factor of the order of unity which decreases with increasing δ. All log DMe* vs. logaO2 curves obtained for different values of x and for different temperatures go through a minimum due to a change in the type of point defects dominating the cation diffusion with oxygen activity. Cation vacancies prevail for the cation diffusion at high oxygen activities while cation interstitials become dominant at low oxygen activities. At constant values of x, DMe[V]° decreases with increasing temperature while DMe[I]° increases.