High Breakdown Strength, High Dielectric Constant Composites for High Frequency Applications

Dielectric materials used for high power applications currently tend to be based upon either bulk ceramics or liquids such as ethylene glycol-water mixtures. Both of these approaches suffer problems in use; ceramics have limited strength and are brittle, whilst liquid based materials have chemical compatibility and environmental issues and cannot be used in applications that require a solid-state solution. This paper outlines the design and fabrication of material systems that allow the achievement of high breakdown strength, high dielectric constant, low dielectric losses and good mechanical strength and toughness. The approach that has been taken to achieve this involves the use of novel proprietary methods to fabricate composite structures of sizes ranging from microns to several hundred millimetres. Control of the electrical properties is complex as it is necessary to tailor the dielectric constant, the breakdown strength and the loss component, which inherently have conflicting requirements, and the situation is exacerbated in the high frequency region of interest for our application in the MHz-GHz range. The properties obtained with these materials, on a large scale, have resulted in the achievement of significant increases in the available energy density. Breakdown strengths that were previously only seen in thin-film structures are now possible in large scale bulk structures. This paper gives an overview of the development and recent advances that have been made through the use of different ceramic/matrix combinations and the effect of microstructure and morphology on the electrical properties. Furthermore, the interplay between electrical and mechanical properties and the implications of sample fabrication will be discussed.