PHASE-CHANGE MATERIALS FOR THERMAL STABILIZATION OF COMPOSITE THERMISTORS

The objective of this investigation was to develop a triphasic PTC thermistor composite which incorporated a phase capable of absorbing heat at a critical temperature, and thus limiting deleterious effects associated with thermal runaway. The system chosen for study was pentaerythritol incorporated into a carbon black-polyethylene thermistor system. Pentaerythritol exhibits a first order tetragonal to cubic phase transition at 185-degrees-C, with a 1.87 to 3.18 J/degrees-C.g change in specific heat and a 425 J/cm3 heat of transition. Composites with room temperature resistivities as low as 0.1-OMEGA.m, a PTCR effect of up to six orders of magnitude, and reproducible temperature-cycling behavior were developed. The pentaerythritol introduced thermal delays up to 7 min at 185-degrees-C and substantially increased the electrical and mechanical stability of the composites with temperature and voltage cycling. High fields imparted irreversible effects in these composites as reflected by an increase in the room temperature and high temperature resistivity.