Point defects and grain boundary diffusion in NiO and Fe3O4

The grain boundary diffusion coefficients of Co-57 tracer and nonradioactive Co-59 in polycrystalline NiO films, NiO bicrystal and polycrystalline Fe3O4 specimens were determined in various oxygen pressures at 750 degrees C. The oxygen pressure dependence of the grain boundary diffusion coefficients was found to be similar to the oxygen pressure dependence of the volume diffusion coefficients. This behavior indicates that the same mechanisms of diffusion operate in both cases. In NiO the vacancy mechanism predominates. The ratio of D-gb/D-v for the polycrystalline NiO thick films was found to be approximate to 10(3) at 750 degrees C. For the NiO bicrystals the ratio D-gb/D-v was approximate to 10(5). This difference is believed to stem from the purity and nature of grain boundaries. In Fe3O4 polycrystalline specimens the grain boundary diffusion coefficients of Co-ions increase at low and decrease at high oxygen pressures. This behavior is similar to that of the behavior of the volume diffusion of Co-ions in Fe3O4 as determined previously by Dieckmann and Schmalzried, which indicates that the mechanism of the grain boundary diffusion at low oxygen pressure is the interstitial (or interstitialcy) mechanism and at high oxygen pressure the vacancy mechanism is predominant. The ratio of D-gb/D-v for diffusion of Co-ions in Fe3O4 is approximate to 10(3) at 750 degrees C.