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 permanent fuses that do not have to be replaced after an overload or short-circuit operation. An interesting class of materials for this purpose are particulate filled polymer composites with a strong positive temperature coefficient (PTC) of resistivity. If an applied current becomes too high, the PTC element is heated to its critical temperature and trips from the conducting into the insulating state. The dynamic heating of the composite upon current flow is described by a one-dimensional model. It is predicted that the heating of a composite depends on the size of the filler particles. Smaller filler particles should allow a faster heating and, hence, a better limitation of the current. Experimental verification is performed using composite of TiB2 particles in a polyethylene matrix. Commercial TiB2 powders with different particle-size distributions between 1 and 200 mu m were used. The specific resistivity of the composites is small, in the range of 0.01-0.02 Omega cm. Around the melting temperature of the polymer, the resistivity increases 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 characteristics. Samples were prepared having, to a certain degree, the same specific resistivity, cross section, and total resistance. Free parameters were the length, and for some samples, the filler content. Short-circuit experiments show that for decreasing particle size the time until the material trips into the high-resistive state becomes shorter. The best current limitation occurs for composites containing particles in 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 within a further 200 mu s. The experiments are in excellent agreement with the predictions from theory. Due to the low resistance in the cold state and the very fast limitation of electrical currents, PTC elements based on conducting polymers can be highly attractive for power applications. (C) 1997 American Institute of Physics.