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.
Glass-to-metal bonds
Glass and metal can bond together by purely mechanical means, which usually gives weaker joints, or by chemical interaction, where the oxide layer on the metal surface forms a strong bond with the glass (the glass itself is about 73% composed of a silicon dioxide (SiO2)) . The acid-base reactions are main causes of interaction between glass-metal in the presence of metal oxides on the surface of metal. After complete dissolution of the surface oxides into the glass, further progress of interaction depends on the oxygen activity at the interface. The oxygen activity can be increased by diffusion of molecular oxygen through some defects like cracks.
Also, reduction of the thermodynamically less stable components in the glass (and releasing the oxygen ions) can increase the oxygen activity at the interface. In other words, the redox reactions are main causes of interaction between glass-metal in the absence of metal oxides on the surface of metal.
For achieving a vacuum-tight seal, the seal must not contain bubbles. The bubbles are most commonly created by gases escaping the metal at high temperature; degassing the metal before its sealing is therefore important, especially for nickel and iron and their alloys. This is achieved by heating the metal in vacuum or sometimes in hydrogen atmosphere or in some cases even in air at temperatures above those used during the sealing process. Oxidizing of the metal surface also reduces gas evolution. Most of the evolved gas is produced due to the presence of carbon impurities in the metals; these can be removed by heating in hydrogen.
The glass-oxide bond is stronger than glass-metal. The oxide forms a layer on the metal surface, with the proportion of oxygen changing from zero in the metal to the stoichiometry of the oxide and the glass itself. A too-thick oxide layer tends to be porous on the surface and mechanically weak, flaking, compromising the bond strength and creating possible leakage paths along the metal-oxide interface. Proper thickness of the oxide layer is therefore critical.
Copper
Metallic copper does not bond well to glass. Copper(I) oxide, however, is wetted by molten glass and partially dissolves in it, forming a strong bond. The oxide also bonds well to the underlying metal. But copper(II) oxide causes weak joints that may leak and its formation must be prevented.