Electrical properties of polyethylene highly filled with carbon
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Electrical properties of polyethylene highly filled with carbon F. A. Modine Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6030
A. R. Duggal General Electric Company, Corporate Research and Development, Schenectady, New York 12301
D. N. Robinson and E. L. Churnetski Development Division, Oak Ridge Y-12 Plant, Oak Ridge, Tennessee 37831-8095
M. Bartkowiak Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6030
G. D. Mahan Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6032 and Department of Physics, University of Tennessee, Knoxville, Tennessee 37996
L. M. Levinson General Electric Company, Corporate Research and Development, Schenectady, New York 12301 (Received 17 November 1995; accepted 25 July 1996)
Carbon-filled polyethylene composites were fabricated and tested to establish the practical lower limit of their electrical resistivity at room temperature and to investigate the trade-offs between low resistivity and the magnitude of the resistance anomaly (i.e., a large positive temperature coefficient of resistivity) that appears when such composites are heated through the polyethylene crystalline melting transition. Carbon blacks with large particle size and low surface area provided low-resistivity composites having large resistance anomalies. The largest resistance anomalies were found in composites that were well mixed, but the room-temperature resistivity also increased in composites that were cycled repetitively through the crystalline-melting transition. A mixture of carbon blacks of two different sizes provided a lower resistance than was found in a material with the same fill of only the coarser black. By controlling the composition and the processing, composites were made with room-temperature resistivities lower than 0.2 ohm cm and resistance changes of at least 2 orders of magnitude. A resistance change of as much as 5 orders of magnitude was obtained for composites with room-temperature resistivities of only 1 ohm cm.
I. INTRODUCTION
Many polymer-based composites can be fabricated to be electrically conductive and to exhibit what is commonly referred to as the positive temperature coefficient of resistance (PTCR) effect.1 In such composites, the electrical resistivity changes by several orders of magnitude for a temperature change of only a few degrees.2 Examples include plastics, epoxies, and rubbers filled with one or more of several metals and/or semiconductors. Because of the large resistance anomaly, these materials are useful in devices that limit electric fault currents. An electric current sufficient to cause significant Joule heating also increases the electrical resistance which in turn limits the current. Because there is a positive feedback mechanism involved in the increase of temperature and resistance, the composites can switch quickly from a low- to a high-resistance state. Polyethylene filled with carbon black is prototypical of composite
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