On tunneling effects in metal-deposited polyethylene-carbon black and polycarbonate-carbon black systems
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J. Plans Departamento de Fisica Fundamental, Universidad Autonoma de Madrid, Cantoblanco, Madrid, Spain (Received 18 December 1985; accepted 18 April 1986) Electrical conductivity for polyethylene-carbon black, polycarbonate-carbon black composites was measured for conducting compositions well above the percolation limit. Three conductivity methods were employed in our studies: sputtering coated metal electrodes, painted metal electrodes, and the four-point method. Results suggest that while the two latter methods do not modify substantially the material surface, the former method introduces a low conductivity region at the surface. The effect of contact resistance developed during metal coating is discussed in the light of both fluctuation-induced tunneling predictions and "hopping" transport mechanisms. Experimental results favor the tunneling alternative across the evaporated metalcomposite interphase.
I. INTRODUCTION Polymers are commonly dielectric materials. The insulating properties of these materials are well known and make them suitable for many applications.' The addition of conducting solid microadditives (carbon black, carbon fibers, and metallic powders) to insulating polymers raises the conductivity of the composite material to the conducting level of the microadditive.2 The investigation of these materials can shed light on the basic problems of charge transport in composite systems leading to the development of novel light processable materials, amenable to conduct electricity and having potential switching thermal and electric properties.3'4 It is now known that such solid mixtures of electrically conductive additive dispersed in a nonconducting polymeric matrix undergo a discontinuous transition from nonconductor to conductor at a given concentration of the additive.2 The transport mechanism around the critical concentration has been shown to be governed by tunneling of electrons between particles.5 Because of the small size of the potential barriers between aggregates, the thermally activated voltage fluctuations are high enough to modulate the tunneling probability introducing a characteristic temperature variation to the temperature-independent traditional tunneling conductivity.6 Assuming a parabolic potential barrier, it can be proved that the tunneling conductivity for low fields is o),
(1)
where To can be regarded as the limiting temperature for which the fluctuation effects below To become negligible. In previous studies we reported the behavior of conductivity as a function of temperature for polycarbonate 510
J. Mater. Res. 1 (3), May/Jun 1986
http://journals.cambridge.org
(PC)-carbon black and low-density polyethylene (LDPE)-carbon black composites.7'8 These investigations indicated that the value of a depends on whether conductivity was measured by means of metal coating on the surface or through the four-point method. In the former method the electrical current across the sample is measured when an electrical field between electrodes is applied. In the latter, four-point electro
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