Oxygen self-diffusion in single and polycrystalline magnesio-ferrites

Abstract

Self-diffusion coefficients of oxygen ion in polycrystalline and single-crystal magnesioferrites have been measured by a gas-solid isotopic exchange technique using18O as a tracer at temperature in the range 975 to 1465° C. A new method was considered for the oxygen volume diffusion in polycrystalline magnesio-ferrite, and its reliability is discussed. The volume diffusion coefficients of polycrystalline magnesio-ferrite (MgO/Fe2O3=0.95 in mole ratio) can be expressed asD=1.51×10−1exp (−78 600/RT) cm2 sec−1 (1135 to 1465° C) andD=1.2×10−7exp (−38 000/RT) cm2sec−1 (975 to 1135° C). The volume diffusion coefficients of the polycrystal in the high temperature range were very close to those of single crystal ferrite of the same composition as the polycrystal. The activation energy of grain-boundary diffusion in this polycrystal was expected to be greater than that of the volume diffusion. A possible interpretation of this unusual behaviour is given in terms of an increased enrichment of Fe2+ ion along the grain boundary with temperature elevation, by which oxygen vacancies increase.