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.