Measuring microwave dielectric properties of materials: Theory and applications

The knowledge of the microwave dielectric properties of materials has great significance for scientific and industrial applications. The measurement of the complex permittivity, ε*=ε′−iε″, in this frequency range, can be made using the small perturbation theory. In this method, the resonance frequency and the quality factor of a cavity, with and without a sample, can be used to calculate the complex dielectric permittivity of the material. The design of a cavity resonator implies solving the Maxwell equations inside that cavity, respecting the boundary conditions. This method was applied to characterize the dielectric properties of yttrium ferrites, that were prepared by solid state reaction, and exposed to different heat treatments. Structural and morphological characterizations were also performed. A correlation between these properties is presented. The best results, in terms of energy storage at microwave frequencies, are obtained for the sample heat treated at 1400 °C, with ε´=6.4, at 2.7 GHz, and with losses tangent lower than 10–4, at 300 K.