The microstructure and conductive mechanism of high density polyethylene/carbon black (HDPE/CB) compositewere investigated by positron annihilation lifetime spectroscopy (PALS). The PALS were measured in two series of samples

one with various CB contents in the composites and the other with various γ-irradiation doses in HDPE/CB compositecontaining 20 wt% CB. It was found that CB particles distribute in the amorphous regions

the CB critical content value inHDPE/CB composite is about 16.7 wt% and the suitable γ-irradiation dose for improving the conductive behavior of HDPE/CB composite is about 20 Mrad.The result observed for the second set of samples suggests that γ-irradiation causes not only cross-linking in amorphous regions but also destruction of the partial crystalline structure.Therefore

a suitable irradiation dose

about 20 Mrad

can induce sufficient cross-linking in the amorphous regions without enhancing the decomposition of crystalline structure

so that the positive temperature coefficient(PTC) effect remains while the negative temperature coefficient(NTC) effect is suppressed.A new interpretation of the conductive mechanism

which might provide a more detailed explanation of the PTC effect and the NTC effect has been proposed.