Microstructure evolution during air bonding of Al₂O₃ to Al₂O₃ joints using bismuth–borate–zinc glass

Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi₂O₃–40B₂O₃–20ZnO (mol.%). The wettability of the glass on the Al₂O₃ substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi₂₄B₂O₃₉ was detected to be the product in the joints sealed at 530–580 °C, while ZnAl₂O₄ was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al₂O₃ substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl₂O₄ increased with increasing holding time.