Processing Effects on the Performance of Ptc Composite Thermistors

By combining phases, the desirable properties of individual materials may be optimized in a composite. One such group of functional composites are conductive filler -amorphous polymer positive temperature coefficient of resistance (PTC) thermistors. These thermistors have a large increase in electrical resistance near the glass transition temperature of the polymer. Below T_{rm g} the composite resistivity is similar to that of the conductive filler while above T_ {rm g} the resistivity is similar to that of the insulating polymer matrix. The effect of polymer crosslink density, filler particle size, and filler hardness on the thermistor characteristics were studied. For the V_2O _3-polystyrene system a strong correlation between crosslink density and the PTC magnitude was observed. As the crosslink density increased the composite thermal expansion above T_{rm g} decreased. Both particle size and sample thickness were also found to influence the thermistor electrical properties. These two variables were combined in the particle size to sample thickness (d/t) ratio. As the d/t ratio increased the critical volume fraction (V_{rm c}) at which the system percolates decreased. An increase in the PTC transition steepness and magnitude was seen with increasing d/t ratio. A study of the filler hardness showed that materials such as Al_2O_3 and Ni had substantial PTC transitions while softer materials, graphite and Al, showed only minimal PTC transitions. Deformation from processing may prevent the conducting paths from being disrupted and consequently, a PTC transition from occurring. Modelling estimated that the number of filler particle chains which contribute to the thermistor conduction for a 30 vol% 6 μm V_2 O_3-polystyrene composite was 2.4 times 10^5 /cm^2. The number of conducting chains decreased as the filler particle size increased. Thermal expansion of the composite results in the particles of the chain being separated. Decreasing thermal expansion resulted in a PTC transition which was more gradual. Smaller particles increased the number of particles per chain which decreased the size of the gap at a given temperature. These trends, predicted by a model, were observed in the V_2O _3-polystyrene composite system.