Abstract
Some small fraction of large grains was observed in the microstructure of sintered ZnFe2O4, MnZnFe2O4, Fe3O4 (in N2) and MnFe2O4 (in air). This structure is sometimes called "duplex structure". As the sintering proceeded, the number of large grains increased to form the continuous structure of large grains, while the size of large grains did not increase remarkably. On the other hand, the duplex structure was not observed in the ferrites with the composition of NiFe2O4 and CoFe2O4, independent of prefiring and sintering conditions. However, when amounts of Fe2O3 were excess in the composition of the ferrites, duplex-like structures were observed. This structure was also formed in the CoFe2O4 containing platinum powder (Pt black) known as a redox catalyst. From these results, the duplex structures seem to be formed in such ferrites that are easy to vary their compositions or valencies at high temperatures. In addition, the growth of small grains was very slow during the formation of duplex structure. It was considered that the exaggerate grain growth was generated by the local activation of the stabilized grain boundaries, which was caused by the variation of compositions or valencies during the sintering.
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