FAST CURRENT LIMITATION BY CONDUCTING POLYMER COMPOSITES

The transition of materials from low resistivity to comparatively high resistivity can be utilized for current limitation, enabling permanen t fuses that do not have to be replaced after an overload or short-cir cuit operation. An interesting class of materials for this purpose are particulate filled polymer composites with a strong positive temperat ure coefficient (PTC) of resistivity. If an applied current becomes to o high, the PTC element is heated to its critical temperature and trip s from the conducting into the insulating state. The dynamic heating o f the composite upon current flow is described by a one-dimensional mo del. It is predicted that the heating of a composite depends on the si ze of the filler particles. Smaller filler particles should allow a fa ster heating and, hence, a better limitation of the current. Experimen tal verification is performed using composite of TiB2 particles in a p olyethylene matrix. Commercial TiB2 powders with different particle-si ze distributions between 1 and 200 mu m were used. The specific resist ivity of the composites is small, in the range of 0.01-0.02 Omega cm. Around the melting temperature of the polymer, the resistivity increas es within only 20 degrees C by seven orders of magnitude. In order to verify the expected dependence of the switching dynamic on the filler particle size, the tested elements had comparable electrical character istics. Samples were prepared having, to a certain degree, the same sp ecific resistivity, cross section, and total resistance. Free paramete rs were the length, and for some samples, the filler content. Short-ci rcuit experiments show that for decreasing particle size the time unti l the material trips into the high-resistive state becomes shorter. Th e best current limitation occurs for composites containing particles i n the range of 1-45 mu m. Current limitation starts already after 150 mu s, and a current density of up to 10 kA/cm(2) can be switched off w ithin a further 200 mu s. The experiments are in excellent agreement w ith the predictions from theory. Due to the low resistance in the cold state and the very fast limitation of electrical currents, PTC elemen ts based on conducting polymers can be highly attractive for power app lications. (C) 1997 American Institute of Physics.