Effects of WO3 and Ta2O5 Dopants on the Structure, Microstructure, and Microwave Dielectric Properties of Ca5Nb4TiO17 Ceramics

Abstract

Ca5Nb4TiO17 ceramics were doped with WO3 and Ta2O5 to improve their microwave dielectric properties. The substitution of W6+ into Nb5+/Ti4+ sites resulted in the reduction of the sintering temperatures of the Ca5Nb4−1.2xWxTiO17 ceramics to 1450°C for x > 0.3 due to the formation of a second phase, CaWO4. In addition, the densification temperatures of the Ca5Nb4−xTaxTiO17 ceramics increased with Ta5+ content. Some irregular grains of CaWO4 were observed in the microstructures with plate-like grains, which increased with increasing W6+ content in the Ca5Nb4−1.2xWxTiO17 ceramics. All the Ca5Nb4−xTaxTiO17 samples exhibited dense microstructures with closely packed plate-like grains and a few pores. The dielectric constant (er) of the Ca5Nb4−1.2xWxTiO17 ceramics decreased with increasing W6+ content from 45.0 for x = 0 to 36.4 for x = 0.9. This decrease occurred because the more highly polarizable Nb5+ ions were replaced by less polarizable W6+ ions at B-sites, and the formation of the CaWO4 second phase diluted er. The quality factor (Q × f) reached a maximum of 26,478 GHz for x = 0.3 because of the cation distribution and decrease in the volume of cation sites as well as the increase in the average grain size. The CaWO4 second phase caused the temperature coefficient of the resonant frequency (τf) of the Ca5Nb4−1.2xWx TiO17 ceramics to move in the positive direction. For the Ca5Nb4−xTaxTiO17 ceramics, er decreased almost linearly with increasing Ta5+ content from 45.2 for x = 0 to 36.2 for x = 2.5 because of the dampening of the ionic mobility and decrease in the molecular polarizability. The Q × f and τf values decreased with increasing x value.