Microstructural and Microwave Dielectric Properties of Bi 12 GeO 20 and Bi 2 O 3 ‐Deficient Bi 12 GeO 20 Ceramics

Bi 12 GeO 20 ceramics sintered at 800°C had dense microstructures, with an average grain size of 1.5 μm, a relative permittivity (ε r ) of 36.97, temperature coefficient of resonance frequency (τ f ) of −32.803 ppm/°C, and quality factor ( Q × f ) of 3137 GHz. The Bi 12‐ x GeO 20‐1.5 x ceramics were well sintered at both 800°C and 825°C, with average grain sizes exceeding 100 μm for x ≤ 1.0. However, the grain size decreased for x > 1.0 because of the Bi 4 Ge 3 O 12 secondary phase that formed at the grain boundaries. Bi 12‐ x GeO 20‐1.5 x ( x ≤ 1.0) ceramics showed increased Q × f values of >10 000 GHz, although the ε r and τ f values were similar to those of Bi 12 GeO 20 ceramics. The increased Q × f value resulted from the increased grain size. In particular, the Bi 11.6 GeO 19.4 ceramic sintered at 825°C for 3 h showed good microwave dielectric properties of ε r = 37.81, τ f = −33.839 ppm/°C, and Q × f = 14 455 GHz.