Influence of Firing Atmosphere on Crystal Structure of Mn-Co-Ni Oxide for Thermistor Material

Abstract

Phase changes in Mn-Co-Ni oxide in firing process for making thermistor materials were investigated in different atmospheres in the temperature range from 1000° to 1400°C. The starting oxide was prepared from a mixture of Mn, Co ana Ni nitrates with molar ratio of 3.0:1.9:1.1 by calcining at 1000°C for three hours in air. The fired samples employed in this study were obtained by quenching them in water after firing the starting oxide at a desired temperature between 1000° and 1400°C for one hour in different atmospheres of oxygen, air and nitrogen. In the X-ray diffraction analysis, changes in diffraction pattern due to the phase separation and phase transition were observed in each firing atmosphere. In air, the cubic spinel at 1000°C changed to a mixed crystal phase composed of tetragonal spinel and rock salt at 1160°C. Similar crystal phase changes were observed in both oxygen and nitrogen atmospheres. However, the temperatures of crystal phase change were dependent on the concentration of oxygen in the firing atmosphere. In oxygen, the temperature of crystal phase change increased to 1240°C. In nitrogen, on the other hand, the crystal phase change occurred at lower temperature than in air. On the basis of these results, a phase diagram of Mn-Co-Ni oxide with molar ratio of 3.0:1.9:1.1 was proposed, in which the temperature of crystal phase change is correlated with the concentration of oxygen.