Effect of Ca substitution on structure and dielectric properties of bismuth-based microwave ceramics

The effect of the chemical substitution of Ca cation on the microstructure and microwave dielectric properties of bismuth-based pyrochlores has been investigated. Broad ranges of solid solutions based on (Bi_3x Zn_2?3x )(Zn _x Nb_2?x )O₇ (x?=?0.56–0.64) were obtained with Ca cation substitution of Zn at A site. The XRD patterns revealed that the substitution of Ca for Zn led to the α–β pyrochlore phase transformation and pure β pyrochlore structure were obtained. The dielectric constants (? _r ) varied slightly with x increasing. The quality factor Q was significantly improved by Ca substitution. The temperature coefficient of resonant frequency (τ _f ) was negative and increased with x increasing. (Bi_3x Ca_2?3x )(Zn _x Nb_2?x )O₇(x?=?0.58) ceramic, sintered at 960 °C, exhibits the optimal microwave dielectric properties of ? _r ?～?74, Qf value ～6,457 (at 4.8 GHz), and τ _f ?～??143 ppm/°C.