The evolution of spinel-based conductive phase in thick-film NTC thermistors

The aim of the work was to gain some insight into the development of the thick-film NTC thermistor´s electrical characteristics, i.e,. sheet resistivity, beta factor and noise, and microstructural characteristics, during the firing process. 1 kohm/sq. thick-film NTC thermistors were fired at different temperatures. Thermistors were fired at temperatures from 500degC to 950degC, and also for a relatively long time (3 hours) at 950degC to allow reactions to reach or at least to come close to equilibrium. The dried thermistors and the thermistors fired at different temperatures were examined by XRD, SEM, and EDS microanalysis. The semiconductive spinel phase is not present in the dried thermistors but is formed during firing. Higher firing temperatures increased beta factors and significantly increased sheet resistivities. However, the noise indices were relatively low, regardless of the firing conditions. The high sheet resisitivities of samples fired for 3 hours at 950degC are presumably due to partial substitution of the B site ions with Al³⁺ ions in the spinel structures.