Relation between microwave and submillimetre losses in selected ceramic materials

We have studied the relation between microwave and submillimetre dielectric losses in almost 100 ceramic materials using submillimetre transmission spectroscopy (150 - 1500 GHz) and in several cases also various microwave techniques. Submillimetre and infrared properties are unavoidable for understanding the origin of microwave permittivity and losses. Extrapolation from submillimetre down to microwave range is discussed and it is demonstrated that for well processed ceramics with not too high permittivity (¿¿ < 100) the extrapolation to microwave range is straightforward: the permittivity is given by the polar phonon contributions and is practically dispersionless, and losses are proportional to frequency below ~ 1012 Hz In high permittivity materials (¿¿ > 100) additional relaxational dispersion is found below the phonon frequency range which is a manifestation of high lattice anharmonicity in these materials. Comparing various materials, the losses at constant frequency increase steeply with permittivity. The most important structural parameter determining the intrinsic dielectric properties (at least in complex perovskites) is the packing degree. Also the extrinsic part of losses due to defects is discussed on the basis of its temperature and frequency dependences. In all investigated cases the extrinsic losses play dominant role at low temperatures and are not negligible at room temperature.