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Nonlinear dielectric properties of particulate barium titanate–polymer composites

Ferroelectric barium titanate particles were incorporated within an acrylic polymer matrix to produce a composite material that exhibits nonlinear dielectric properties at high electric field strengths. Such materials can be employed as a means of electrical stress relief in applications involving divergent electric fields. A method to calculate the field-dependent complex dielectric permittivity from the polarization–electric field (P–E) relationship is presented. The dielectric characteristics of composites prepared using fine and coarse (agglomerated) grades of barium titanate powders are compared. The fine-grained composites exhibited nonlinear dielectric behaviour that conformed to the empirical Rayleigh law; a typical Rayleigh coefficient of 3 × 10−6 m V−1 was obtained for a composite containing 35 vol% of the ferroelectric phase. The coarse-grained composites displayed significantly higher Rayleigh coefficients, up to 7 × 10−6 m V−1, and a tendency for saturation in the P–E loops. The saturation in the dielectric behaviour of the coarse-grained composites is discussed in terms of particle size effects and the development of an improved flux path through the ferroelectric particles by particle–particle contacts.