Microstructural engineering of microwave dielectric ceramics

Temperature stable, low loss dielectric ceramics find application as resonators in communication systems operating at microwave frequencies. Candidate materials need to exhibit high relative permittivity, high dielectric Q value and near zero temperature coefficient of resonant frequency. Current materials include a range of complex perovskites, predominantly titanate-based, with Q × fo (product of Q value and resonant frequency) values of 250,000 GHz or more. An overview is given in the ways in which the microstructures of microwave dielectric ceramics have been ‘engineered’ at the levels of the grain, grain boundary, sub-grain and the lattice to optimise the critical dielectric parameters. This is primarily accomplished by either (a) modifying the processing conditions, or (b) modifying the powder formulation (from dopants to major compositional changes). The outcomes provide a series of ground rules to maximise the performance of new materials.