Preparation and microwave dielectric properties of BPO4 ceramics with ultra-low dielectric constant

BPO4 ceramics with a cristobalite-like structure were prepared via solid-state sintering (SSS) and spark plasma sintering (SPS). Significant shrinkage and grain growth were observed with increasing the sintering temperature up to 1200 °C for SSS samples, while the highest relative density is only 74.6% because of the serious sublimation of BPO4, and the microwave dielectric properties of εr = 3.38, Qf = 18,200 GHz and τf = −42.8 ppm/°C were achieved at 14.6 GHz for SSS samples sintered at 1000 °C for 3 h. A much higher relative density of 92.7% was obtained in SPS sample (1000 °C/10 min) with εr = 4.20, Qf = 4,000 GHz, and τf = −20.7 ppm/°C. The Qf value increases by a factor of 6.9 and 5.9 with increasing frequency from 4.6 GHz to 14.6 GHz for SSS and SPS samples, respectively, which implies that the Qf value is dominated by extrinsic factors and has a huge improvement space. The corrected dielectric constant by porosity is only 4.65, indicating that BPO4 ceramic is a promising candidate as the microwave substrate material if the relative density and microstructure can be further improved.