Preparation and Characterization of Co1.7Ni1.3-xAlxO4 (0.1 ≤ x ≤ 0.5) NTC Ceramics with Low B-Value and High Resistivity

In this paper, a negative temperature coefficient (NTC) thermistor material Co1.7Ni1.3-xAlxO4 (0 <= x <= 0.5) with high resistivity and low B-value is investigated for low temperature applications. The Co1.7Ni1.3-xAlxO4 ceramics consist of a Ni-rich rock salt phase and a tetragonal spinel phase formed by the decomposition of the cubic spinel phase. With increasing Al content, NiO is reabsorbed into the spinel lattice. The added Al does not affect the crystal structure and forms a solid solution with the decomposed phase. Because of the introduction of Al, the number of carriers involved in the conductivity decreases and the hopping distance and hopping frequency both decrease, thus increasing the resistivity. In the studied range, the B-25/50 value increases slightly with increasing Al content from 1711 K to 2496 K, while the resistivity increases from 4219 omega cm (x = 0) to a maximum value of 43484 omega cm (x = 0.3), then decreases to 21233 omega cm (x = 0.4) and finally increases to 25213 omega cm (x = 0.5).