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Glass-to-metal seals are a type of mechanical seal which joins glass and metal surfaces. They are very important elements in the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass-encapsulated semiconductor diodes, reed switches, glass windows in metal cases, and metal or ceramic packages of electronic components.
Properly done, such a seal is hermetic (capable of supporting a vacuum, good electrical insulation, special optical properties e.g. UV lamps). To achieve such a seal, two properties must hold:
- The molten glass must be capable of wetting the metal, in order to form a tight bond, and
- The thermal expansion of the glass and metal must be closely matched so that the seal remains solid as the assembly cools.
Thinking for example about a metal wire in a glass bulb sealing, the metal glass contact can break if the coefficients of thermal expansion (CTE) are not well aligned. For the case that the CTE of the metal is larger than the CTE of the glass, the sealing shows a high probability to break upon cooling. By lowering the temperature, the metal wire shrinks more than the glass does, leading to a strong tensile force on the glass, which finally leads to breakage. On the other hand, if the CTE of the glass is larger than the CTE of the metal wire, the seal will tighten upon cooling since compression force is applied on the glass.
According to all requirements that need to be fulfilled and the strong necessity to align the CTE of both materials, there are only a few companies offering specialty glass for glass-metal sealing, such as SCHOTT AG and Morgan Advanced Materials.