Effects of CBS coating on dielectric properties of porous Si₃N₄ ceramic at high temperature and high frequency

Compact moisture proof coating was fabricated with CaO-B₂O₃-SiO₂(CBS) glass ceramic on the porous silicon nitride substrate by sol-gel method. The dielectric constant and dielectric loss from 7.3 GHz to 18.4 GHz were tested from room temperature to 1200°C by applying factor cylindrical cavity working in multimode before and after sealing. The result shows that, after coating, the dielectric constant is slightly higher and the fluctuation of dielectric loss decreases with frequency. At frequency of 9.2 GHz the temperature coefficient of dielectric constant increases after coating. The dielectric loss is decreased by 60% or more at room temperature after coating; the temperature stability of dielectric loss is improved by coating, and the demands of dielectric properties of radome are well satisfied.