Dielectric properties of epoxy-barium titanate-carbon black composites

Frequency-domain dielectric measurements on epoxy-barium titanate-carbon black composites from 100 Hz to 1 MHz and from room temperature to + 200°C are reported. The results are presented mainly in the form of plots of the real and imaginary parts of the relative permittivity versus temperature for different frequencies and of the resistivity versus frequency for different temperatures. The basic composite consisted of 56 vol% barium titanate and 44 vol% epoxy. To this basic composite small amounts of carbon black were added to increase the electrical conductivity of the epoxy matrix, thus reducing the relaxation time and increasing the high frequency permittivity. When the carbon black concentration was near the percolation threshold, high permittivities were obtained even at high frequencies since the relaxation time was reduced to less than a microsecond. A further increase in the carbon black concentration lead to short-circuiting the charge build-up on the barium titanate/epoxy interfaces and loss of the high permittivity. The experimental results are interpreted in terms of the theory of dielectric loss in heterogeneous materials.