Study on brazing sealing and wave transmittance of single crystal Al₂O₃ microwave window

Al₂O₃ is considered a promising material for high-power microwave windows due to its low dielectric loss, excellent mechanical properties, and outstanding corrosion resistance. However, the inherent brittleness and low thermal conductivity pose significant challenges in achieving a dependable metal seal. In this study, vacuum brazing technology was employed to achieve brazing sealing between copper and single crystal Al₂O₃. The interface structure, mechanical properties, and sealing properties of the brazing joint were focused on. The brazed joints exhibited outstanding mechanical properties with an average shear strength of 207 MPa. The sealing performance of the Al₂O₃ window was conclusively determined to be excellent, as evidenced by the helium leakage rate and X-ray testing results. The dielectric properties and standing wave coefficient of Al₂O₃ window were analyzed using a vector network analyzer in combination with a quasi-optical resonator and free space test system. The results indicate that the Al₂O₃ window exhibits an extremely low dielectric loss of 10^-5 magnitude at 95-98 GHz, accompanied by a standing wave coefficient below 2, which satisfies the requirements of high-power microwave windows.