Carbon Black-Polyethylene Composites for PTC (Positive Temperature Coefficient) Thermistor Applications.

摘要

A new class of composite Positive Temperature Coefficient (PT C) thermistors are commercially available. These composite thermistors exhibit a PTC measuring 6-8 orders of magnitude and have several advantages over conventional Barium Titanate 3 thermistors, including lower room temperature resistivity, better thermal shock resistance, and high temperature stability. In this study, composite PTC thermistors are produced from a carbon black/polyethylene composition that exhibited a PTC of resistance measuring 6-8 orders of magnitude at 125 C. The composites are irradiated (gamma source) to induce polymer crosslinking. Crosslinking imparts mechanical integrity and prevents the onset of an NTC effect above the transition temperature. Crosslinking stabilizes the high temperature resistivity to 200 C. The electrical properties of these crosslinked composites are examined as functions of temperature, frequency, and thermal cycling. To eliminate the crosslinking procedure, an insulating mullite phase was incorporated into the diphasic mixture to form a triphasic composite. Addition of the third filler yielded two effects. First, the filler provided sufficient mechanical stabilization of the composite structure to maintain high post-PTC resistance without an NTC effect. Second, the addition of an insulating filler resulted in a shift of the room temperature resistivity caused by a shift in the percolation curve. (KT)