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 x 10(-6)mV(-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 x 10(-6) mV(-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.