B2O3-modified fused silica microwave dielectric materials with ultra-low dielectric constant

B2O3-modified fused silica bulks with the formula of xB2O3-(1 − x)SiO2 (x = 0.05-0.35) were prepared by solid state sintering, and the microwave dielectric properties were investigated. The content of low-cristobalite decreases with increasing x for the samples sintered at 1000 °C, and the high content of low-cristobalite causes cracking for x = 0.05-0.15. The densification temperature decreases as well as the highest relative density with increasing x from 0.2 to 0.35, and interesting microstructural evolution was observed. The highest Qf value was obtained at the sintering temperature significantly higher than that for the highest density. The introduction of B2O3 improves the microwave dielectric properties significantly, and the optimal properties with ultra-low dielectric constant of 3.56, high Qf value of 70,600 GHz and low temperature coefficient of resonant frequency of −11.4 ppm/°C were achieved for x = 0.2 sintered at 1100 °C with the high relative density of 95.3%.