Glass-to-metal seal

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).

[citation needed] After complete dissolution of the surface oxides into the glass, further progress of interaction depends on the oxygen activity at the interface.

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.

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.

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.

Copper(I) oxide, however, is wetted by molten glass and partially dissolves in it, forming a strong bond.

To improve the bonding to glass, the oxide layer should be borated; this is achieved by e.g. dipping the hot part into a concentrated solution of borax and then heating it again for certain time.

[2] The copper-to-glass seal should look brilliant red, almost scarlet; pink, sherry and honey colors are also acceptable.

Special compositions of soda-lime glasses that match the thermal expansion of gold, containing tungsten trioxide and oxides of lanthanum, aluminum and zirconium, exist.

[3] Iron is only rarely used for feedthroughs, but frequently gets coated with vitreous enamel, where the interface is also a glass-metal bond.

Iron is prone to creating gas bubbles in glass due to the residual carbon impurities; these can be removed by heating in wet hydrogen.

Due to its low thermal expansion coefficient, matched to glass, tungsten is frequently used for glass-metal bonds.

The thermal expansion coefficient of steel is however fairly different from the glass; like with copper, this can be alleviated by using knife-edge (Houskeeper) seals.

[4] Zirconium wire can be sealed to glass with just little treatment – rubbing with abrasive paper and short heating in flame.

Due to its very low vapor pressure, indium finds use in glass-metal seals used in vacuum technology[8] and cryogenic applications.

This type of seal was used in scientific equipment throughout the 19th century and also in the early incandescent lamps and radio tubes.

If copper is properly oxidised before it is wetted by molten glass a vacuum tight seal of good mechanical strength can be obtained.

Scott a copper plated tungsten wire is immersed for about 30 s in chromic acid and then washed thoroughly in running tap water.

If copper is to be heated in hydrogen-containing atmosphere e.g. a gas flame it needs to be oxygen-free to prevent hydrogen embrittlement.

The keys to success are proper borating, heating of the joint to a temperature as close to the melting point of the copper as possible and to slow down the cooling, at least by packing the assembly into glass wool while it is still red hot.

It is also possible to use silver plating, but then an additional gold layer is necessary as an oxygen diffusion barrier to prevent the formation of iron oxide.

Also, technical iron contains some carbon which forms bubbles of CO when it is sealed to glass under oxidizing conditions.

Both are a major source of problems for the technical enamel coating of steel and make direct seals between iron and glass unsuitable for high vacuum applications.

The oxide layer formed on chromium-containing steel can seal vacuum tight to glass and the chromium strongly reacts with carbon.

Another widely used method to seal through glass with low coefficient of thermal expansion is the use of strips of thin molybdenum foil.

The disadvantage here is that the tip of the edge which is a local point of high tensile stress reaches through the wall of the glass container.

Here the wire is usually matched to the glass which is inside of the bore of a strong metal part with higher coefficient of thermal expansion.

Compression seals can withstand extremely high pressures[a] and physical stress such as mechanical and thermal shock.

If the contact angle is low (good wetting) the surface of the glass is everywhere under compression stress like an enamel coating.