Some studies on composite dielectric materials

Abstract

Two phase composite dielectrics made of conducting and insulating phases provide the flexibility to alter the dielectric and related properties of the insulating material. By proper choice of the insulating and conducting components, it is possible to realize new materials for electronic applications. With this in mind, the present work examines composites from the viewpoint of their use as variable capacitors, whose capacitance varies with temperature, and the flexibility these materials offer in meeting desired specifications. This thesis demonstrates experimentally that thermal capacitors-whose capacitance decreases with temperature-can be realized using polymers with positive thermalexpansion coefficients. The graphite composition was varied between 5-30% by weight. The particle size of graphite used was 45µm. Graphite was dispersed in molten insulator and solidified, and the samples were cured in a temperaturecontrolled oven. Five samples were prepared for each composition. The following measurements were carried out:

Capacitance at different compositions, temperatures, and frequencies Loss tangent at different temperatures and frequencies for various compositions Breakdown strength as a function of temperature for different compositions

Key findings include:

Capacitance increases with graphite composition in both polystyrene-graphite and wax-graphite composites. Capacitance decreases with temperature; the decrease is larger in wax-graphite composites. Capacitance is nearly constant from 100Hz to 1kHz and decreases gradually from 10kHz to 10MHz. Loss tangent increases with composition and increases with frequency. Loss tangent decreases with temperature in both composites; the decrease is smaller in polystyrene-graphite than in wax-graphite composites. Breakdown strength decreases with increasing composition in both composites, but increases with temperature.

These observations are explained by assuming a composite microstructure consisting of cubic insulating microcells surrounded by uniformly distributed graphite particles. The effective dielectric constant was derived as a function of composition and particle size. The decrease in dielectric constant with temperature is attributed to thermal expansion of the insulating phase, and approximate calculations support this explanation. Variation of loss tangent with composition and temperature was analysed using Debye relaxation models. It was found that the relaxation time depends only on the two constituent phases and is almost independent of composition. Relaxation times decrease with temperature, leading to reduced loss tangent. The thesis covers preparation of composite materials, the measurement methods, and explanations for variations in dielectric constant, loss tangent (tan ), and breakdown voltage with respect to composition, temperature, and frequency. The material is presented in eight chapters:

Chapter 1: Introduction and objectives of the study Chapter 2: Basic definitions of dielectric constant, dielectric loss, complex permittivity, etc. Chapter 3: Literature survey on dielectric constant, loss tangent, and breakdown behaviour of twophase composite materials Chapter 4: Experimental procedures: sample preparation, measurements of capacitance, loss tangent, and breakdown voltage with variation in temperature and frequency Chapters 5-7: Detailed analysis of dielectric constant, loss tangent, and breakdown strength using an appropriate compositestructure model Chapter 8: Conclusions

Part of this work has been published in Journal of Materials Science: Materials in Electronics, Vol. 7, No. 6, 1996, pp. 403-407.