Densification, microstructure evolution, and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg2+ ions replaced by Ca2+ ions on the microwave dielectric properties of newly developed MgZrTa2O8 were investigated. Mg1-xCaxZrTa2O8 (x = 0-1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca2+ significantly inhibited the densification of Mg1-xCaxZrTa2O8, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa2O6. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg2+ ions by the more polarizable Ca2+ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τf values was in accordance with that of relative permittivity. The best composition appeared to be Mg0.9Ca0.1ZrTa2O8, which showed excellent microwave dielectric properties of εr = 22.5, Q × f = 231,951 GHz, and τf = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.