Properties of Grain Boundaries in Spinel Ferrites
It is shown that all ferrites with the spinel structure should have approximately the same value of the activation energy for grain boundary migration, irrespective of their composition, provided migration of grain boundaries is not hindered by voids or inclusions. Furthermore, the mean free energy λ of the boundaries (as calculated from the kinetics of crystal growth) doubles when a reducing atmosphere is replaced by an oxidizing one. The transition temperature between the two values of λ varies with oxygen partial pressure. Special attention is paid to the equilibrium segregation of some ions at the grain boundaries of spinel ferrite during cooling. This leads to a displacement of the reduction-oxidation equilibrium at the grain boundary and changes the resistivity and eddy-current losses.
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