Joining simple ceramic components to make more complex shapes is an important aspect of many industrial applications such as Solid Oxide Fuel Cells, electronic packages, sensors, batteries, aerospace components, etc. Joining can be achieved by simple mechanical attachments, adhesives, glass seals, brazing, diffusion bonding, fusion welding, etc. Glass seals are a simpler and cheaper option to achieve strong and vacuum-tight joints. It is also possible to tailor the glass-seal properties such as Coefficient of Thermal Expansion, chemical durability, thermal durability, etc. Therefore, it is one of the more popular choices for joining ceramics. It can also be used for stable low-temperature joining (joining below 700°C while maintaining integrity for temperatures up to 350-400°C) to protect heat-sensitive parts during multi-material assembly.
In the past, most low-temperature joining glasses have been PbO-based glasses. However, due to the toxicity of PbO, alternatives are being researched and some commercial PbO-free low-temperature joining glasses are now available. However, many of these glasses contain Bi₂O₃ and B₂O₃ in significant amounts. However, these glasses have very poor chemical durability in water and chlorinated environment. B₂O₃ is one of the most soluble components of glass and it is used as a tracer for glass leaching in water. Therefore, there is a necessity to research more chemically durable low-temperature sealing glasses. Glasses can be made more chemically durable by including oxides of elements such as Al, Zr, Nb etc. But not all glass compositions are suitable for joining ceramics. The suitable glass composition for joining also depends on the type of ceramic to be joined. Therefore, to effectively tailor glass compositions suitable for joining specific materials, it is important to understand the joining mechanism. The general idea is that many ceramics may have amorphous phases remaining at the grain-boundaries after the sintering process, which may be “wet” by glassy materials introduced at the joint. Joining may also be facilitated by network modifying cations (alkali/alkaline earth) that may diffuse into the ceramic through point defects while heating. To study these two joining mechanisms, 6 different sodium-borosilicate glasses were synthesized, with additives such as Al₂O₃, ZrO₂, Nb₂O₃, CaO and BaO. They were used to join polished and unpolished high-purity alumina ceramics at 700°C. The difference in joining between polished ceramics and unpolished ceramics revealed the significance of the grain boundaries of the ceramics to the joining mechanism. A comparison of the six different glass compositions also revealed the usefulness of certain additives to improve the joining and revealed that the diffusion of alkali and alkaline-earth ceramics is also an important part of the joining mechanism.