Ca3Al2O6: novel low-permittivity microwave dielectric ceramics with abnormally large negative τf

Abstract

Ca3Al2O6 ceramics are synthesized using a standard solid-state reaction method and their microwave dielectric properties are systematically investigated. Pure phases with space group of Pa_3 are confirmed from X-ray diffraction patterns (XRD) and Rietveld refinements. er and Qf values of the present ceramics are mainly determined by the porosity, and the optimal values (er = 14.5, Qf = 24,500 GHz) are achieved when sintered at 1425 °C. Abnormally large negative τf values in the range of − 322 to − 354 ppm/°C are obtained, which are rare and extremely important in adjusting the positive τf values of other material systems. In addition, a large temperature sensitivity of − 4.19 MHz/°C is also confirmed in the present ceramics, which indicates great potential for temperature sensor applications. Moreover, the dielectric spectra measured under various radio frequencies reveal similar positive τe values of 715.5 ppm/°C, which could account for the large negative τf values in the operating temperatures.