Microstructural and Microwave Dielectric Properties of Bi12GeO20 and Bi2O3-Deficient Bi12GeO20 Ceramics

Abstract

Bi12GeO20 ceramics sintered at 800 degrees C had dense microstructures, with an average grain size of 1.5 mu m, a relative permittivity (epsilon(r)) of 36.97, temperature coefficient of resonance frequency (tau(f)) of -32.803 ppm/degrees C, and quality factor (Q x f) of 3137 GHz. The Bi12-xGeO20-1.5x ceramics were well sintered at both 800 degrees C and 825 degrees C, with average grain sizes exceeding 100 mu m for x <= 1.0. However, the grain size decreased for x > 1.0 because of the Bi4Ge3O12 secondary phase that formed at the grain boundaries. Bi12-xGeO20-1.5x (x <= 1.0) ceramics showed increased Q x f values of > 10 000 GHz, although the epsilon(r) and tau(f) values were similar to those of Bi12GeO20 ceramics. The increased Q x f value resulted from the increased grain size. In particular, the Bi11.6GeO19.4 ceramic sintered at 825 degrees C for 3 h showed good microwave dielectric properties of epsilon(r) = 37.81, tau(f) = -33.839 ppm/degrees C, and Q x f = 14 455 GHz.