IV CHARACTERISTICS OF CARBON BLACK-LOADED CRYSTALLINE POLYETHYLENE

The I-V characteristics of carbon black-loaded crystalline polyethylene were investigated at room temperature and at few degrees above the melting temperature, T(m). The material was chemically cross-linked using silane. It had a large carbon black content of 28 parts per hundred by weight and displayed a strong positive temperature coefficient of resistance (PTCR) effect with a resistivity jump of four orders of magnitude. Logarithmic current-voltage plots were found to be linear with unity slopes at temperatures above and below T(m), indicating good ohmic behaviour. This is in contrast with previous theories which explain the PTCR effect on the basis of electron tunnelling as the current conduction mechanism. A new model capable of explaining both the PTCR effect and the steep reduction in resistance above T(m) is presented, according to which the PTCR effect is the result of the co-operative effect of changes in crystallinity and volume expansion.