Types of Glass-to-Metal Seals

There are many types of glass-to-metal seals. These seals can be categorized by their structures.

Internal seals are those in which the metal, in the form of wire, rod, ribbon, or tube, is surrounded by glass.

External seals are those in which a band, tube, or cup of metal adheres to the edge of a glass insert, which may be a round button or disc or it may be square or any other shape, in which ease the stresses are always greater and more complex than in a simple round button or disc.  Since glass is much stronger in compression than in tension, the differences in coefficients of expansion of the glass and metal components may be somewhat greater in external than in internal seals. Where expansion mismatch occurs, the ratio between the outside diameter of the tube in an external seal and its wall thickness should not be less than 15:1, with a maximum wall thickness of 0.01 inch. Where expansion coefficient differentials are small, as in Kovar-to-metal seals, these restrictions do not apply.

Tubular seal is “inside tubular” when the metal is surrounded by glass and “outside tubular” when the glass is surrounded by metal.  The tubular seals are so-called “Housekeeper” seals. The metal, such as copper or stainless steel, having a widely different expansion coefficient, is formed to a thin edge so that it is able to yield as it expands with heat relative to the glass, and the stresses in the glass do not exceed its tensile strength.

Edge seals are those in which the edge of the metal is embedded in glass. These resemble tubular seals in that they may be either normal edge or “Housekeeper” type, but the geometry and applications differ.

Butt seals are made by sealing a metal disc or ring to the end of a piece of glass tubing.

Window seals may be of any shape, although the round window is subject to less severe and complex stresses.

Combinations of different types of the seals is also possible.

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