Crystallization and wetting behavior of bismuth–borate–zinc glass and its application in low temperature joining alumina ceramics

Bismuth glass, 50Bi ₂ O ₃ –40B ₂ O ₃ –10ZnO, was designed, prepared and employed to join polycrystalline alumina with 95 wt.% purity (95Al ₂ O ₃ ) in air. Crystallization behavior of the glass was studied. The results showed that Bi ₄ B ₂ O ₉ and Bi ₂₄ B ₂ O ₃₉ were formed in the glass in the heating process. The glass transition temperature (T _g ) and the softening temperature (T _f ) were 385 and 419 °C, respectively. Prior to the joining process, the wettability of the glass on the ceramic was studied. The wetting angle of the glass/95Al ₂ O ₃ system at 700 and 750 °C was 26.6° and 12.6° respectively, exhibiting excellent wetting performance. Joining parameters influencing microstructure and mechanical property of the joint was researched, showing the in-situ formation of Bi ₄ B ₂ O ₉ , Bi ₂₄ B ₂ O ₃₉ and ZnAl ₂ O ₄ crystals in the joints when brazed at 600, 625 and ≥635 °C, respectively. The joint microstructure evolved considerably with temperature increasing (T ≥ 635 °C): (i) ZnAl ₂ O ₄ particles grew steadily (ii) the aggregation of ZnAl ₂ O ₄ particles was more and more serious, as well as the infiltration of the glass melt into the 95Al ₂ O ₃ . The joint shear strength was proved to be a function of temperature, optimized average value of 58 MPa was acquired when the joints were joined at 625 °C.