Abstract
The effect of precursor salt on the sinterability and properties of Mn-Co-Ni oxide thermistors has been studied using nitrates and sulfates. The starting samples were obtained by firing the nitrates or sulfates of Mn, Co and Ni with molar ratio of 3:2:1, at 1000°. In order to pursue the differences in the two samples fired between 1000°C and 1400°C, their sinterability, phase changes during firing and complex impedance properties were investigated. When sintered at 1100°C in air, the shrinkage rate was greater with sulfate than nitrate. X-ray diffraction analysis revealed the structure change in both samples fired above 1200°C. When fired above 1200°C, phase separation into Ni-rich and Mn-rich phases was observed by EPMA in both samples, and the amount of Ni-rich phase increased with increasing temperature. When fired at 1400°C, a remarkable microstructural difference was observed between nitrates and sulfates sample. In the case of the sample prepared from nitrates, closely linked Mn-rich phase existed in Ni-rich phase matrix. On the other hand, in the case of the sample prepared from sulfates, Mn-rich phase existed isolatedly in Ni-rich phase matrix. In the Cole-Cole plots, different shape of circular paths were observed in the two samples, and the resistivity of the sample from sulfates was greater than that from nitrates. These results correspond to the results of EPMA.
