Microstructure and microwave dielectric properties of Ba5Nb4O15–BaWO4 composite ceramics

Microwave dielectric composite ceramics with compositions of (1 − x)Ba5Nb4O15–xBaWO4 (x = 0–1) were prepared by firing the mixture of Ba5Nb4O15 and BaWO4. The sintering temperature of the composite ceramics was reduced from 1400 to 1100 °C with increasing BaWO4 content. XRD patterns indicate that Ba5Nb4O15 and BaWO4 coexisted and no secondary phase was detected in sintered bodies. The composite ceramics have the finer grained microstructure compared with the pure Ba5Nb4O15 ceramics. TEM observation and EDS analysis reveal that a very thin interfacial area (∼60 nm in width) was formed due to the limited solid solution between Ba5Nb4O15 and BaWO4 grains. With increasing BaWO4 (from x = 0 to x = 0.65), the dielectric constants (ɛr) and the temperature coefficient of resonant frequency (τf) of the ceramics decrease from 39.2 to 16.9 and from +78 to −4.3 ppm/°C, respectively, while the quality factors (Q × f value) increase from 26,000 to 56,700 GHz. The samples with x = 0.54–0.65 sintered at 1100 °C exhibit good microwave dielectric properties: ɛr = 21.0–16.9, Q × f = 49,500–56,700 GHz, and τf = 8.9–−4.3 ppm/°C.