Microwave dielectric properties of ZrO2 and SnO2 doped Ca5Nb4TiO17 ceramics

In this study, the effects of B-site dopants, such as Sn4+ and Zr4+ ions, on the microwave dielectric properties of the Ca5Nb4TiO17 ceramic were investigated, in contrast to the replacement of A-site ions shown in literature. The sintering temperature of the Ca5Nb4Ti1-xSnxO17 ceramics depended on the Sn4+ content, while the Zr4+ dopant had no impact on the densification of the Ca5Nb4Ti1-xZrxO17 ceramics. Both the Ca5Nb4Ti1-xSnxO17 and Ca5Nb4Ti1-xZrxO17 samples have a dense microstructure with closely packed plate-like grains and little porosity. The relative permittivity εr of the Ca5Nb4Ti1-xSnxO17 ceramics decreased from 45.0 for x = 0 to 30.3 for x = 1.0, because the more highly polarizable TiO2 (α = 15.6 Å3) was replaced by the less polarizable SnO2 (α = 6.70 Å3). The quality factor (Q × f) value increased significantly to 33,542 GHz for x = 0.3, which was primarily caused by the increase in the average grain size. The temperature coefficient of resonance frequency (τf) of the Ca5Nb4Ti1-xSnxO17 ceramics moved toward the positive direction with the Sn4+ substitution. The εr value of the Ca5Nb4Ti1-xZrxO17 ceramics decreased almost linearly from 45.2 for x = 0 to 40.6 for x = 0.5 with an increase in Zr4+ content, due to the lower molecular polarizability of ZrO2 (α = 7.33 Å3). The Q × f and τf values were observed to decrease with an increasing x value, which can be explained by the decreasing degree of ordering.