Conductive Polymer Composites with Large Positive Temperature Coefficients

Abstract

AbstractConductive polymer composites that contain conductive fillers such as metal powder, carbon black, and other highly conductive particles in a nonconductive polymer matrix have been widely used in electrostatic dissipation (ESD) and electromagnetic interference shielding (EMIS). A special group among electrically conductive polymer composites are conductive polymer composites that have large positive temperature coefficients (PTC), which in some cases are called positive temperature coefficient resistance (PTCR). The resistivity of this kind of composite increases several orders of magnitude in a narrow temperature range. This kind of smart material can change from a conductive material to an insulating material or vice versa upon heating or cooling, respectively. The smartness of this kind material lies in this large PTC amplitude (defined as the ratio of maximum resistivity at the peak or the resistivity right after the sharp increase to the resistivity at 25°C), and also in its reversibility, its ability to adjustment the transition temperature, its low‐temperature resistivity, and high‐temperature resistivity. PTC behavior in a polymer composite was first discovered by Frydman in 1945, but not much attention was paid to it originally. Because Kohler obtained a much higher PTC amplitude from high density polyethylene loaded with carbon black in 1961, this kind of temperature‐sensitive materials has aroused wide research interest and also led to many very useful applications. In this article, the general theories of PTC conductive polymer are introduced. Carbon‐black‐filled conductive polymer composites and their PTC behavior are discussed in more detail, in regard to the effects of fillers, the polymer matrix, and processing conditions, and additives. Applications of this kind of smart temperature‐sensitive material are presented.