Volume expansion dependence of ptc effect of carbon black loaded polymer composite

摘要

Carbon black loaded polyethylene has been found to exhibit the effect of positive temperature coefficient (PTC) resistance. It could be widely used in making self-regulating heater and circuit protection devices. However, the mechanism behind the PTC abnormality has not been well established. It is generally believed that the PTC effect is caused by a break-up of the conductive network during sample heating. The network break-up has been modelled by many workers. It was suggested by Kohler [1] that a sudden expansion, which takes place at the crystalline melting point, results in the break-up of the conducting chains with a consequent PTC effect. Kohler's model paid little attention to effects on the micro-scale within the materials and treated the break-up as a uniform separation of the conductive chain units. It works poorly in dealing with the negative temperature coefficient (NTC) resistance behaviour following the PTC transition above the melting point when the volume continues to increase. Ohe and Nitio [2] stressed in their model that the PTC effect is mainly caused by an increasing fluctuation of the gaps between conducting chain units as the melting point is approached. The PTC effect is due to the increasing number of gaps too large to allow electron tunnelling. On the basis of an experimental observation that the carbon black particles are mostly located in the amorphous region rather than crystalline areas, Meyer [3], Voet [4] and Allak et ai. [5] attributed the PTC effect to a cooperation of the crystallinity decrease with the volume expansion. They gave models in more detail to describe the conductive chain change accompanying the crystallinity decrease during the PTC transition and also gave an explanation for the NTC after PTC by assuming a reformation of the conducting network above the melting point. In any of these models, the volume expansion is by any means the key factor causing the PTC effect. However, no experiment has been designed, so far, to directly determine the volume expansion accompanying the PTC transition and to investigate to what extent the resistivity change depends upon the volume expansion.