Static and dynamic non-linear behaviors of carbon particle filled polymerswith positive thermal coefficient

Polymers filled with conductive particles like carbon black, short carbon f ibers or microbeads, may exhibit a very large positive thermal coefficient of the electrical resistance when they are heated above room temperature. S uch a behavior is particularly useful in industrial applications for produc ing electrical safety devices. We present a comprehensive study of the elec trical properties of epoxies filled with carbon microbeads when heating is achieved by the Joule effect. In that case, the electrical responses up to a large enough electrical excitation must be non-linear as a result of prog ressive heating of the material. As expected, we have found that it depends on both the 'intrinsic' properties of the sample (essentially a function o f the composition) and the extent of thermal coupling between the investiga ted sample and the external thermal bath (variable heat leak). We have part icularly studied the stationary states under constant applied voltage and t he transients following a step like time dependent voltage. Most of our fin dings agree with the general results of percolation theory combined with th eoretical models of rupture on a random fuse network and with a simple mode l of heat flow through the sample and from the sample to the thermal bath.