Lightweight Ceramic Materials as High-Frequency Dielectrics

To eliminate some of the weight problems usually associated with ceramic dielectrics, lightweight (porous) ceramic materials have been developed which show excellent electrical properties at temperatures up to 800°C and a high strength-to-weight ratio. These materials generally are characterized by low-dielectric constant values, low loss, and a fiat temperature vs dielectric constant relation. Some suggested uses for lightweight ceramic materials are fillers for microwave devices, microwave lenses for high temperature use, and low-loss high-temperature insulators. Two types of dielectric materials are discussed: 1) a Wollastonite foam, and 2) a 90 per cent aluminum oxide foam. The transverse strength of these foam materials was found to be approximately 400-450 psi, and this strength could be more than doubled in thin sections, by the application of dense ceramic coatings to the foam material. The thermal expansion of these lightweight materials was 5-6 microinches per inch per °C, and the thermal conductivity was 5 to 7 x l0^-4cal/cm sec °C. The dielectric constant of these foam materials was found to follow closely the Gladstone Dale relation.