Effects of Bi3+ substitution on microwave dielectric properties of (Ce1−xBix)0.2Sr0.7TiO3 ceramics

Microwave dielectric ceramics (Ce1−xBix)0.2Sr0.7TiO3 (x = 0–0.08) were prepared by conventional solid-state route. The specimens have been characterized using scanning electron microscopy, X-ray diffraction, Raman and impedance spectroscopy techniques. A single perovskite solid solution with cubic structure can be formed over the entire range of x value. Raman analysis discovers that first-order modes are activated by polar defects. With increasing substitution of Bi3+ for Ce3+ ions from x = 0–0.08, the dielectric constant can be improved from 158 to 200 and the product (Q × f) of quality factor (Q) and response frequency (f) decreases from 9531 to 1751 GHz, while the temperature coefficient of resonant frequency increases slightly from 500 to 620 ppm/°C. The impedance analysis revealed that the (Ce1−xBix)0.2Sr0.7TiO3 ceramics illustrate electrical heterostructure, which includes grains, grain-layers and grain-boundaries. The mechanism of such electrical heterostructure associated with charge compensation is induced by Bi volatilization at elevated temperature. The presence of the heterogeneous microstructure can deteriorate seriously the Q × f value duo to Bi3+ volatilization at higher sintering temperature. An excellent microwave dielectric properties with dielectric constant ~200, modest Q × f value ~1751 GHz and relatively low temperature coefficient of resonant frequency ~621 ppm/°C can be obtained for the (Ce1−xBix)0.2Sr0.7TiO3 with Bi content of x = 0.08, compared with that of (Sr, Ca) TiO3 high-dielectric microwave ceramics. This system will provide a new insight for the fabrication of high-dielectric microwave devices.