Offering tunable thermal expansion properties, stability under thermal cycling, and resistance to chemical attack; sealing glasses are used to produce reliable hermetic seals between glass, metal, and ceramic components.
What makes sealing glasses so useful?
One of the primary benefits of glass as a sealing material is that its coefficient of thermal expansion (CTE) can be tuned during production. Most materials expand when heated: This means that if two sealed components expand at different rates, the seal between them can be compromised. Adjusting the composition and manufacturing parameters of sealing glasses means their thermal expansion coefficient can be made to match that of a range of different engineering materials. The result is that glass seals remain sealed when components undergo thermal expansion.
This property lends sealing glasses to widespread use, especially in electronic components (such as vacuum tubes, electric discharge tubes, and reed switches), and other glass-to-metal applications.
Specialized sealing glasses can also exhibit a variety of other desirable properties – these include resistance to chemical attack, stability under thermal cycling, and the ability to be laser-bonded. These characteristics render sealing glasses suitable for a range of high-spec applications, which we explore below.
Perovskites (a class of minerals with an ABX3 structure) have garnered a huge amount of attention in recent years as an alternative to silicon in photovoltaic cells. Worldwide efforts are underway to commercialize perovskite solar cells, which promise both low manufacturing costs and high photovoltaic efficiency.1
One of the challenges facing perovskite solar cells is that exposure of perovskites to even small amounts of moisture can cause complete loss of functionality. Hermetic encapsulation of perovskites is therefore essential – both to prevent moisture from getting into the cell, and to prevent lead-containing chemicals from leaking out.
Glass encapsulation of perovskite photovoltaic elements using sealing glasses can ensure that they remain completely sealed from the external environment. Laser-assisted bonding of glass frit ensures long-term hermetic encapsulation, which can protect perovskite crystals from moisture incursion and drastically increase their lifetime.2 Matching the thermal expansion coefficient of the glass to that of the surrounding materials helps to ensure long-term stability of the entire cell.
Metal Ion and Thermal Batteries
As a transition towards renewable energy sources and electric cars increases demand for more efficient and compact energy storage solutions, the battery industry is undergoing radical changes.
Sealing glass plays a vital role in battery technologies, both experimental and commercial. Lithium-ion (Li-ion) batteries, currently the go-to for electric vehicle and renewable energy storage applications, require hermetic seals that exhibit long-term stability at high temperatures and resistance to chemical attack. The same is true of sodium-ion (Na-ion) batteries, which are currently under investigation as a low-cost alternative to lithium-ion technology.
Sealing glasses are the ideal solution for metal ion batteries. As well as offering a wide range of operating temperatures and customizable thermal expansion coefficients, specialized sealing glasses offer resistance to alkali ion corrosion and reliable performance under thermal cycling.
Sealing glass also offers a solution for alternative battery technologies such as molten salt batteries. Molten salt batteries typically rely on sodium salts (such as sodium-sulfur and sodium-nickel chloride) to achieve high energy and power densities. Such batteries are an attractive candidate for large-scale energy storage and industrial applications.
Sealing glasses are widely used in molten salt battery applications, where they provide a high-performance alternative to polymeric or metal seals. Glass seals are capable not only of withstanding harsh chemical conditions, but can easily withstand the required operating temperatures of molten salt batteries, typically in the region of 300-350 C.3,4
High Temperature Sensors
Glass seals are also commonly used as feedthroughs for sensors that are deployed in high-temperature conditions. By providing predictable thermal expansion, glass seals enable sensor feedthroughs to remain tightly sealed under extreme thermal stresses, such as in chemical processing plants and automotive applications. Resistance to corrosion and thermal cycling ensures sealing glass feedthroughs provide reliable hermetic sealing over a long lifetime.
Solid Oxide Fuel Cells (SOFCs)
Solid oxide fuel cells – characterized by the use of a solid electrolyte and high operating temperatures – are attracting attention as a highly efficient and versatile means of producing electrical power from fuel. While SOFCs are hoped to offer low-cost and long-life power generation, their high operating temperatures (typically 500 C to 1000 C) pose significant engineering challenges.
Inside a SOFC, the anode, cathode, and electrolyte must remain hermetically separated. Due to creep, oxidation, and high-temperature corrosion, conventional polymeric gasket seals are unsuitable for use in SOFCs.
In response to demand, Mo-Sci has developed a range of viscous compliant glass seals for SOFC applications. Unlike conventional glass-ceramic seals, which are partially crystallized, these glasses are designed to remain completely vitreous throughout their application. Remaining in an amorphous, glassy state throughout use, these viscous glass seals can heal themselves if any cracking occurs due to thermal cycling. This provides increased resistance to the extreme thermal stresses within SOFCs while still ensuring excellent bonding with metals and ceramics.5 It’s hoped that this revolutionary glass sealing technology will aid the commercialization of SOFCs and enable their widespread use.
High-Performance Sealing Glass Solutions from Mo-Sci
Mo-Sci is a world leader in glass technology. We produce a comprehensive range of high-performance sealing glasses to suit any application, including viscous compliant glasses for high-temperature applications such as SOFCs. To find out more about our glass sealing solutions, or to enquire about our custom development services, get in touch with a member of the Mo-Sci team today.
References and Further Reading
- Extance, A. The reality behind solar power’s next star material. Nature 570, 429–432 (2019).
- Emami, S., Martins, J., Ivanou, D. & Mendes, A. Advanced hermetic encapsulation of perovskite solar cells: the route to commercialization. J. Mater. Chem. A 8, 2654–2662 (2020).
- Lu, X., Xia, G., Lemmon, J. P. & Yang, Z. Advanced materials for sodium-beta alumina batteries: Status, challenges and perspectives. Journal of Power Sources 195, 2431–2442 (2010).
- Smeacetto, F. et al. Glass-ceramic joining material for sodium-based battery. Ceramics International 43, (2017).
- Hsu, J.-H. et al. An alkali-free barium borosilicate viscous sealing glass for solid oxide fuel cells. Journal of Power Sources 270, 14–20 (2014).