On the Current—Voltage Characteristics of Polyethylene—Carbon Black Conductive Composites

In order to obtain the proper electric properties of PTC material, testing the threshold current at constant voltage is essential if the voltage used is not low enough to avoid the influence of Joule heat. It was found that the I-U characteristics of PE—CB composites which contain CB 10.6—12.2 Vol% can be divided into two regions, ohmic and nonohmic, according to the test voltage. The I-U and R-U behavior of the composites can be expressed by equations I=AU^B and R=A^-1U^I-B in which constants A and B reflect the electric conductivity and the nature of electron tranport respectivitely. Ih the nonohmic area, B>1 and the resistance decreases as voltage increases. Because of the great difference between the thermoexpansion of PE and CB, increasing temperature enhances the values of A^-1 and B, the same effect as reducing CB content. As a result of heat treatment, values of A^-1 and B decrease. Probably, heat treatment, by stimulating the irreversible movement of carbon particles, completes and stabilizes the conductive network in the composites, so that the nonohmic component of conductances was decreased whild the ohmic component was inereased.