Phase change materials for thermal stabilization of composite thermistors
- PDF / 632,874 Bytes
- 8 Pages / 576 x 792 pts Page_size
- 89 Downloads / 226 Views
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 °C, with a 1.87 to 3.18 J/°C • g change in specific heat and a 425 J/cm3 heat of transition. Composites with room temperature resistivities as low as 0.1 il • 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 °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.
I. INTRODUCTION
The discovery of the positive temperature coefficient of resistance (PTCR) effect in rare earth doped BaTiOs1 directly resulted in the development of a new class of materials capable of a wide variety of functions including circuit protection, self-regulated heating, and monitoring temperature, air flow, and liquid levels. The fundamental property of doped BaTiO3 common to these devices is the approximately six orders of magnitude increase in resistivity which occurs over a small temperature interval near the ferroelectric to paraelectric transition. Although this transition temperature can readily be controlled by chemical composition,2 problems with high room temperature resistivities, (pRT ~ 100 ft-cm), and expensive processing have restricted the use of PTC BaTiO3 in both high current and low cost devices. In recent years these limitations have been overcome through the use of composite materials.3"33 By dispersing a conductive filler in an insulating polymer matrix at a volume fraction in excess of the percolation threshold, a diphasic material with a low room temperature resistivity can be formed. In these systems, the PTC effect is generally believed to be a consequence of the change in volume of the polymer and the associated disruption of the conductive pathways. This is a good example of a device based on a coupled phase transformation; i.e., the phase transition of the polymer results in the metal-to-insulator transition of the composite thermistor. A wide variety of filler systems have been studied, with emphasis placed on carbon black3"24 and transition metal oxide.25"33 The most common system J. Mater. Res., Vol. 6, No. 1, Jan 1991 http://journals.cambridge.org
Downloaded: 13 Mar 2015
studied is carbon black-polyethylene; in this case the PTCR effect is usually attributed to the change in specific volume of the polyethylene at its melting temperature, Tm. The primary drawback of these composites is related to the structural rearrangement (slumping) which
Data Loading...
