On the temperature coefficient of resonant frequency in microwave dielectrics

Abstract

Developing a material with a temperature coefficient of the resonant frequency (TCF) of zero is probably the most difficult aspect of research into microwave dielectric ceramics. There are a host of high-quality-factor (Q) high-relative-permittivity (ε_r) materials whose TCF is far too high to be usable, notably TiO₂ (ε_r = 104; Q = 14000 at 3GHz: TCF = +427 ppm K⁻¹). The challenge is to find ways of tuning TCF while maintaining values of ε_r and Q suitable for use in microwave components. This paper explains, in terms of octahedral tilting and ε_r dilution, the tunability of TCF or, more precisely, the temperature coefficient of ε_r in Ca and Sr titanate-based ceramics. Examples of the structural changes that occur during tuning are also shown.