The Effect Of Thermal Volume Expansion On The Electrical Behavior Of Polymeric PTC Composite

The equipment for simultaneously measuring resistance, volume expansion and temperature was constructed. Two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS) were selected, which were loaded with carbon black (CB). Comparisons of the resistivity and thermal volume expansion of the PTC composites revealed that the thermal volume expansion is one of the leading factors for the polymeric PTC transition. The more the thermal volume expansion is, the stronger the PTC effect shows. For the crystalline polymer matrix, the PTC transition temperature is close to the melting point, while for the amorphous polymer matrix, the PTC transition temperature is close to the glass transition temperature.Fitting the experimental data into the general effective media (GEM) equation, a theoretical percolation curve was gained. A mathematical model was proposed which is based on the GEM equation and the dilution effect of filler volume fraction due to thermal volume expansion. By using this model, the contribution of the thermal volume expansion of the matrix to the PTC transition of the composite is quantitatively estimated. The theory and experiment revealed that the conductive mechanism of abrupt resistivity increase at PTC transition region was equivalent as abrupt resistivity increase at the percolation curve close the critical volume fraction. A dilution of the CB volume fraction due to the thermal volumen expansion of the composites is estimated. If the CB volume fraction decrease to the percolation threshold, PTC effect of the composites occur. The resistivity-temperature curve can be predicted by using this mathematical model, which was in good agreement with the experimental data when the CB volume fraction is close to the critical volume fraction.