Dielectric Spectroscopy of Insulator/Conductor Composites
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Robert Ruh Universal Technology Corporation, Beavercreek, Ohio 45431-1600 David S. McLachlan Physics Department, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa
ABSTRACT The dielectric properties of composites are affected in different ways by a number of parameters. These include the electrical properties of both the filler and matrix, the wetting properties of the matrix on the filler, the size and shape of the filler, and the amount of the filler. Most composite systems have been studied via dc resistivity measurements which clearly show the effect of the addition of a second phase to an insulating matrix. However, frequency dependent measurements can provide additional
insight into the mechanisms controlling the electrical response. The frequency dependence of the dielectric properties of composites will be shown. The permittivity and admittance plots of BN/B 4C composites will be given and the relevance of the
trends seen in them will be discussed.
INTRODUCTION Modeling of the electrical properties of composites is desirable, not only in being able to predict the electrical properties, but may also be used to predict the mechanical properties. By using the electrical properties to determine the microstructure, the mechanical properties can therefore be predicted through the microstructural models. Most modeling of electrical properties of composites has been done on the dc conductivity/resistivity. Many mixing models exist which allow for property prediction. Most of these models work better for dilute composites. These models include the following Parallel Model: a.
Series Model:
= •'VU +•cOU'
I__= v,+ v_ am
aj
ac
and Lichteneckers Rule: log a, = vi log a, + vClog a', where a, and a, are the conductivities of the conducting and insulating phase respectively, vc and v, are the volume fractions of the conducting and insulating phases, respectively, and a., is the conductivity of the composite [I]. Another popular method of predicting the properties of composites is percolation theory. Percolation theory is based on the idea that a large change of properties will occur when the second phase is totally connected from one side of the composite to the other. The volume fraction at which this occurs is called the percolation threshold. It depends on many factors including the connectivity of the phases, the size of each phase, the shape of each phase, and the wetting behavior of the phases. Percolation models allow for a large (orders of magnitude) change of properties over a very small concentration range [2]. Recently[3,4], one of the authors has proposed an equation which incorporates the mixing rules[l], most aspects of the Bruggeman Effective Media theory[l], and percolation theory[3,4]. Formerly known as the General Effective Media equation (GEM), the McLachlan equation, which models the complex conductivity (dielectric constant) over the entire volume fraction and a limited frequency range, is: 341 Mat. Res. Soc. Symp. Proc. Vol. 500 © 1998 Materials Research Society
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