The influence of filler properties on the strong PTC effect of resistivity in polymer based conducting composites
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The influence of filler properties on the strong PTC effect of resistivity in polymer based conducting composites Joachim Glatz-Reichenbach and Ralf Strümpler ABB Corporate Research Ltd., CH-5405 Baden-Dättwil, Switzerland ABSTRACT Polymer based composites are very attractive materials for a plurality of technical applications. For electrical purposes the resistivity of such materials can be tuned over many orders of magnitude from highly insulating (1014 Ωcm) to well conducting (10-3 Ωcm) states. One particular class of polymer based composites even show a strong non-linear reversible change in resistivity with temperature between conduction and insulation. Such a pronounced effect of positive temperature coefficient of resistivity (PTCR) can be technically used, for example, in self regulating heating devices, for temperature sensing or, as a very challenging and important task, for current (i.e. over- as well as short-circuit currents) limiting and interrupting devices. The PTCR-devices act similar like fuses but repetitively, which offers technical and economical benefits. In the present work we investigate how the physical properties of the conducting filler influence the switching characteristics of the PTC material. Experimental results on resistivity and its change under active heating by Joule’s losses during current flow are presented and discussed for different composites, compounded with fillers like carbon black, Ni/Ag, TiB2 or WC/Co. The strong resistance change caused by break-off and separation of particle-particle micro-contacts is driven by the very different thermal expansion coefficients of filler and matrix. It is in particular demonstrated how the heat capacity of filler-particles influences the dynamics of the micro-contact separation.
INTRODUCTION Polymer composites with a strong positive temperature coefficient of resistance (PTCR) [1] are examples for materials, which adapt their structural composition in response to the operating environment or service conditions. They are used, e.g., for self-limited heating systems, which consist commonly of carbon black in a thermoplastic matrix. At low temperature the resistance of the PTCR heater is low allowing a relative large current to flow. When the composite is heated to a critical temperature, the volume expansion near the melting point of the semicrystalline polymer disrupts the conductive filler network, causing the resistance of the composite to increase tremendously. Consequently, the power dissipation is limited and overheating is avoided. Other examples are polymer composites as fault current limiters in circuit protection [2, 3]. At low current, the composite conducts the electricity efficiently, but it becomes current limiting when a sufficiently high current heats it. Commercial devices are available for electronic circuit protection [4], for example protecting against faults by external devices in computers [5] or against short circuit discharge of batteries [6, 7].
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