Crystal Phase and Thermistor Constant of Mn-Co-Ni Oxide in Cooling Process after Firing

Changes in crystal phase and thermistor constant of Mn-Co-Ni oxide, a thermistor material, in cooling process below 1000°C after firing at 1400°C were studied. The starting oxide was made of nitrates of Mn, Co and Ni with a molar ratio of 3.0:1.9:1.1 by firing at 1400°C. Samples were prepared by annealing the above oxide at various temperatures between 250° and 1000°C for 24 hours followed by quenching into water. Precise X-ray diffraction analysis of quenched samples revealed the good correlation between the thermistor constant B and the fractional ratio of crystal phases, such as cubic spinel, tetragonal spinel of low-temperature form and tetragonal spinel of high-temperature form, in the sample. It seemed that, in the temperature range between 560° and 650°C, there existed very complicated crystal structures in which the fractional ratio of cubic spinel and tetragonal spinel of high-temperature form varied intricately. Therefore, it is assumed that the change in thermistor constant observed at around 550° to 650°C in the previous paper can be attributed to the very complicated changes of fractional ratio of cubic spinel and tetragonal spinel of high-temperature form in annealed samples.