Glass-ceramic-to-metal seals

Glass-to-metal seals have been around for many years, with one of the most common uses being lamp bulb seals. A more recent invention is glass-ceramic-to-metal seals.

Properties

Glass-ceramics are polycrystalline ceramic materials prepared by the controlled crystallization of suitable glasses, normally silicates. Depending on the starting glass composition and the heat-treatment schedule adopted, glass-ceramics can be prepared with tailored thermal expansion characteristics. This makes them ideal for sealing to a variety of different metals, ranging from low expansion Tungsten (W) or Molybdenum (Mo) to high expansion stainless steels and nickel-based superalloys.

Glass-ceramic-to-metal seals offer superior properties over their glass equivalents including more refractory behaviour, in addition to their ability to seal successfully to many different metals and alloys. They have been used in electrical feed-through seals for such applications as vacuum interrupter envelopes and pyrotechnic actuators, in addition to many applications where a higher temperature capability than is possible with glass-to-metal seals is required, including solid oxide fuel cells.

Process

In the formation of a glass-ceramic-to-metal seal, the parts to be joined are first heated, normally under inert atmosphere, in order to melt the glass and allow it to wet and flow into the metal parts, in much the same way as when preparing a more conventional glass-to-metal seal. The temperature is then normally reduced into a temperature regime where many microscopic nuclei are formed in the glass. The temperature is then raised again into a regime where the major crystalline phases can form and grow to create the polycrystalline ceramic material with thermal expansion characteristics matched to that of the particular metal parts.

Examples

The white opaque "glue" between the panel and the funnel of a colour TV cathode ray tube is a devitrified solder glass based on the system PbO-ZnO-B
2O
3. While this is a glass-ceramic-to-glass seal, the basic patent of S.A. Claypoole considers glass-ceramic-to-metal seals as well.

References

US 2889952, Claypoole, Stewart A., "Composite article and method", published 1956, issued 1959 (devitrified solder glass)

Glass science topics

Basics	Glass Glass transition Supercooling

Formulation	AgInSbTe Bioglass Borophosphosilicate glass Borosilicate glass Ceramic glaze Chalcogenide glass Cobalt glass Cranberry glass Crown glass Flint glass Fluorosilicate glass Fused quartz GeSbTe Gold ruby glass Lead glass Milk glass Phosphosilicate glass Photochromic lens glass Silicate glass Soda-lime glass Sodium hexametaphosphate Soluble glass Tellurite glass Ultra low expansion glass Uranium glass Vitreous enamel Wood's glass ZBLAN

Glass-ceramics	Bioactive glass CorningWare Glass-ceramic-to-metal seals Macor Zerodur

Preparation	Annealing Chemical vapor deposition Glass batch calculation Glass forming Glass melting Glass modeling Ion implantation Liquidus temperature Sol-gel technique Viscosity Vitrification

Optics	Achromat Dispersion Gradient-index optics Hydrogen darkening Optical amplifier Optical fiber Optical lens design Photochromic lens Photosensitive glass Refraction Transparent materials

Surface modification	Anti-reflective coating Chemically strengthened glass Corrosion Dealkalization DNA microarray Hydrogen darkening Insulated glazing Porous glass Self-cleaning glass Sol-gel technique Toughened glass

Diverse topics	Glass-coated wire Safety glass Glass databases Glass electrode Glass fiber reinforced concrete Glass ionomer cement Glass microspheres Glass-reinforced plastic Glass-to-metal seal Porous glass Prince Rupert's Drops Radioactive waste vitrification Windshield

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