Phase evolution of (Ni, Co, Mn)O₄ during heat treatment with high temperature in situ X-ray diffraction

Phase evolution of (Ni, Co, Mn)O₄ (NMC) for negative thermal coefficient (NTC) thermistor is investigated using high temperature X-ray diffraction. Diffraction patterns were collected during heating and cooling, allowing the observation of chemical reactions in the range of temperature between 25 and 1400 °C. NiO, Mn₃O₄ and CO₃O₄ as the reactant materials were prepared using spray drying. As increasing temperature, phase transformation of Mn₃O₄ to Mn₂O₃ was first observed prior to the formation of NMC spinel structure at 700 °C. Between 1000 and 1200 °C, only spinel NMC was observed without any other phases. Also, grain growth of NMC was observed. Phase transition from cubic to tetragonal was observed during cooling. With further increasing temperature up to 1400 °C, thermal stability of NMC decrease, which was confirmed by NiO separation due to the thermal decomposition. Thermogravimetric and differential thermal analysis are performed on NMC compound and the results support the mechanism of phase formation presented by high temperature XRD data. Also, electrical resistance of NMC is measured as a function of temperature (-40-80 °C) and the factor of thermal sensitivity (B value) is calculated.