Percolation properties of metal-filled polymer films, structure and mechanisms of conductivity

Conducting polymer composites are, as a rule, strongly disordered systems. The concepts of the percolation theory and of the fractal geometry are wide ly used to describe the properties of such systems and to establish the qua ntitative relationship between the composite structure and its conductivity . Thorough study of the electrical properties of composites based on these theoretical notions allows to draw some specific conclusions about the stru ctural features of a system. The basic notions of the percolation theory an d of fractal geometry and the application of these theories to the descript ion of electrophysical properties of metal-filled polymer film composite ar e reviewed. The aggregate formation in curing polymer films is studied. The main featur e of the systems studied is the fact that, after the transition of the poly mer matrix to the glassy state, the system becomes "frozen". This makes pos sible the study of aggregates structure at a specified state of film format ion and to determine the fractal parameters of the aggregates. The relation ship between the fractal characteristics of aggregates in dilute dispersion and percolation cluster is demonstrated. The electrophysical properties of metal-filled films are shown to exhibit a number of anomalies near the per colation threshold. The frequency dependence of the conductivity of metal-f illed polymer films is considered. The determination of the mechanism of co nductivity is based on these data. The geometric concepts of the structure of the conducting skeleton is based on the voltage-current sample character istics. It is shown that the frequency dependence of the conductivity is de termined by the interlayers between particles, where the transfer of charge carriers occurs by combining quantum-mechanical tunneling transition and t he mechanism of jumping over localized energy states. The physical nature o f the multiple percolation thresholds in metal-filled polymer films is disc ussed. The influence of external effects on the position of the percolation threshold and on the change of the conductivity mechanism in the composite is also considered. (C) 2000 Elsevier Science S.A. All rights reserved.