Ralf Struempler

Nonlinear dielectric composites

Polymer composites with high dielectric constant are widely used as shielding and field grading materials. An improvement of the refractive grading can be expected for a dielectric constant being a nonlinear function of the electric field or the temperature, as it is known for nonlinear resistive field grading. An increase of the dielectric constant in those areas, where the highest electrical fields occur, will result in a homogenization of the field distribution. Such composite materials can be considered as having the smart functions of sensing and actuating. The influence of nonlinear properties of filler material on the resulting dielectric properties have been studied both theoretically and experimentally. Calculations using effective medium theory show how much of the non-linearity of the filler is transferred to the composite. They are compared with experiments on composites containing ferroelectric, semiconducting and varistor-type filler material in a thermoset or thermoplastic matrix. Depending on the filler type, the dielectric constant increases by a factor of up to three, for example, by raising the temperature from 30 degree(s)C to 150 degree(s)C. Such an enhancement can be sufficient to rearrange the field distribution in stressed insulating parts.