Investigations on low-melting glasses in ZnO–CuO–Bi2O3–B2O3 system for sealing vacuum glasses

The dependence of glass forming ability, structure and properties on the composition of lead-free low-melting glassy solders in the ZnO–CuO–Bi₂O₃–B₂O₃ system was studied in the present paper. X-ray diffractometer (XRD), dilatometer, differential thermal analyzer (DTA) and infrared spectrometer (IR) were used to study the glass-forming ability, thermal properties and structure. The leaching rate method was used to evaluate chemical durability. An experiment utilizing pellet heating on plate glass substrates was conducted to assess the wettability and sealing temperatures(T_s) of solders. The sealing strength of glassy solder was investigated by using the crossover method. Glassy solders with the content of Bi₂O₃ in the range of 60.8–80.8 wt% were prepared by the conventional melting and air quenching technique. They maintain a vitreous state even when heated at temperatures around T_s for as long as 0.5 h except for solders with more than 20 mol% CuO. It is favourable to prepare glassy solders that can be used as edge sealing materials for tempered vacuum glass by using compositions with the contents of B₂O₃≤25 mol%, CuO≤ 20 mol% and Bi₂O₃ in the range of 35∼40 mol% because their comprehensive properties are excellent and meets the requirements for vacuum glass sealing. The water resistance and thermal expansion coefficients of these solders closely match that of plate glass. Their sealing temperatures are below 400 °C and they can adhere very well onto plate glass after sealing.