Effect of processing on the dielectric properties of ceramic dielectric resonator materials

The world market for ceramic dielectric resonators is around 0.8BN Ecu. A major use is in cellular communications where there is explosive growth (38million subscribers in 1997 in the US alone). The massive increase in demand has caused severe congestion in the bands allocated to mobile communications (0.9 and 1.8GHz). Far better pre-selection filters are required to handle the increasing volume and one low cost and realistic option is to improve the properties of dielectric resonators by attention to processing. Certain dielectric oxide single crystals display very low loss at microwave frequencies. On cooling the loss is generally observed to drop and Q's of sapphire at 10GHz exceed 10(6) at low temperatures of around 10K. However, single crystals are expensive and the purpose of this research is to explore inexpensive, sintered polycrystalline alternatives. By very careful attention to purity, processing and microstructure Q values approaching those of single crystals have been achieved and in alumina and titania we have achieved the highest Q values ever reported in these materials at microwave frequencies. In this paper Al2O3 and TiO2 have been studied as a model materials for dielectric loss and have given important insights into the role of processing as a means of optimising dielectric properties.