Polymer based composites are very attractive materials for a plurality of technical applications. For electrical purposes the resistivity of such materials can be tuned over many orders of magnitude from highly insulating (1014&cm) to well conducting (10-3&cm) states. One particular class of polymer based composites even show a strong non-linear reversible change in resistivity with temperature between conduction and insulation. Such a pronounced effect of positive temperature coefficient of resistivity (PTCR) can be technically used, for example, in self regulating heating devices, for temperature sensing or, as a very challenging and important task, for current (i.e. over- as well as short-circuit currents) limiting and interrupting devices. The PTCR-devices act similar like fuses but repetitively, which offers technical and economical benefits.

In the present work we investigate how the physical properties of the conducting filler influence the switching characteristics of the PTC material. Experimental results on resistivity and its change under active heating by Joule's losses during current flow are presented and discussed for different composites, compounded with fillers like carbon black, Ni/Ag, TiB2 or WC/Co. The strong resistance change caused by break-off and separation of particle-particle micro-contacts is driven by the very different thermal expansion coefficients of filler and matrix. It is in particular demonstrated how the heat capacity of filler-particles influences the dynamics of the micro-contact separation.