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,Zn2-3x)(ZnxNb2-x)O-7 (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 alpha-beta pyrochlore phase transformation and pure beta pyrochlore structure were obtained. The dielectric constants (epsilon(r)) varied slightly with x increasing. The quality factor Q was significantly improved by Ca substitution. The temperature coefficient of resonant frequency (tau(f)) was negative and increased with x increasing. (Bi3xCa2-3x) (ZnxNb2-x)O-7(x=0.58) ceramic, sintered at 960 degrees C, exhibits the optimal microwave dielectric properties of epsilon(r)similar to 74, Qf value similar to 6,457 (at 4.8 GHz), and tau(f)similar to-143 ppm/degrees C.