Effect of Al2O3 addition on the microstructure and electrical properties of (Mn0.37Ni0.3Co0.33−xAlx)O4 (0 ≤ x ≤0.03) NTC thermistors

The effect of Al2O3 addition on the microstructure and electrical properties of (Mn0.37Ni0.3Co0.33−xAlx)O4 (0 ≤ x ≤ 0.03) negative temperature coefficient (NTC) thermistors was studied. The (Mn0.37Ni0.3Co0.33−xAlx)O4 ceramics sintered at 1230 or 1260 °C were composed of Ni-rich rock-salt structured phase and Mn-rich tetragonal spinel phase. The added Al2O3 did not affect the crystalline structure and formed a solid solution with the decomposed phases. The relationship between log resistivity (log ρ) and the reciprocal of the absolute temperature (1/T) of the prepared NTC thermistors was nearly linear, indicative of NTC characteristics. It was found that the electrical resistivity, B25/85 constant, and activation energy of the thermistors increased with increasing Al2O3 content.