CRYSTAL STRUCTURE CHANGE OF THERMISTOR OF Mn-Co-Ni OXIDES IN FIRING PROCESS.

This investigation was carried out to study the change in X-ray diffraction patterns observed in Mn-Co-Ni oxides heat-treated at high temperatures. The samples were prepared by firing the oxides of Mn, Co and Ni with molar ratio of 3. 0 : 1. 9 : 1. 1 between 1000 degree and 1400 degree C. In the X-ray diffraction study, the change of diffraction patterns, which seemed to be due to same crystal structure change, was observed in the samples fired above 1140 degree -1160 degree C. To examine these crystal structure changes, EPMA study was made on the sample heat-treated at 1200 degree C. The distributions of Mn, Co and Ni were heterogeneous and the two phases, Mn-rich and Ni-rich, were observed. The quantitative analyses of Mn, Co and Ni contents were made by EPMA on these two phases. The samples with the same compositions as those of the analytical values were newly prepared by firing at 1200 degree C. X-ray diffraction study on these newly prepared samples proved that the Ni-rich phase had a rock salt structure and that the Mn-rich phase was estimated to have a spinel structure of composition (Co, Mn) left bracket Mn, Co right bracket //2O//4 in which some cations were substituted by Ni. These results proposed that the change in X-ray diffraction patterns was attributed to the phase separation of cubic spinel into tetragonal spinel (Mn-rich) and rock salt (Ni-rich) phases.