Dielectric properties of microwave ceramics investigated by infrared and submillimetre spectroscopy

Results of our 10 years study of more than 80 microwave ceramics varying in composition and processing with the aid of infrared reflectivity and submillimetre transmission spectroscopy are reviewed. These techniques are unavoidable for understanding the origin of microwave permittivity and losses. Extrapolation from submillimetre to microwave range is discussed as well as the observed steep overall increase in losses on increasing permittivity. The most important structural parameter for dielectric properties is the packing degree. Ideally packed structures show the lowest permittivity and losses. On lowering the packing degree, permittivity and losses increase and the lattice becomes more anharmonic. Extrinsic part of losses due to defects is discussed on the basis of their temperature and frequency dependences, but no clear quantitative conclusions can be drawn in most cases.