The PTC And NTC Effects Of Conductive Polymer Composites

Polymers incorporating carbon black (CB), graphite or metal particles can be prepared for electrically conductive polymer composites. A more important feature of these kinds of polymer composites is the positive temperature coefficient (PTC) effect of resistance. The effect of thermal volume expansion on PTC effect is studied.By fitting the experimental data into the General Effective Media (GEM) equation, a theoretical percolation curve was obtained. Therefore, a mathematical model was established which is based on the GEM equation and the dilution effect of filler volume fraction due to thermal volume expansion. With this mathematical model, we predicted the PTC effect of two kinds of composites successfully. Our mathematical model and experiment have proved that the abrupt resistivity increase at PTC transition range and the abrupt resistivity increase at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment.The study of resistivity relaxation and agglomeration of CB particles is reported, and it proves that the movement of CB particles is short range and thermal activation. The results reveal that the resistivity is decreased with time under constant temperature, exhibiting a typical relaxation behavior. We find that adding Tris(hydroxymethyl)aminomethane could increase the resistivity relaxation time and prevent the agglomeration of CB particles. As a result, the negative temperature coefficient (NTC) effect of composites is decreased and the electrical stability of the composites is improved.