There are two types of glass seals available at Specialty Seal Group: Compression Seals and Matched Seals. In the design of Compression Seals the expansion coefficients of the steel, glass and ID members are placed in decreasing expansion fashion. This process will maintain the glass under compressive stresses over a wide temperature range. Glass, like ceramic, has little strength when subjected to tension. However, under compression, glass is strong.
If a higher cycle range is needed, you should consider Kovar* Matched Seals. Specialty Seal Group uses a fundamental glass sealing process to manufacture glass seals in which the Kovar metal thermal expansion coefficient matches the glass thermal expansion coefficient. The outcome is a highly effective hermetic seal.
In both Compression and Kovar Seals, the glass is selected to minimize stresses that cause cracks. However, Kovar Seals typically require Corning Glass 7052 or its equivalent, with milky white, gray, and black being the popular color options.
The most common glass for Compression Seals is Corning Glass 9013 or its equivalent.
Nickel-iron 52% is standard conductor material is because of its low thermal expansion in Compression Seals. The specific resistivity of the material is 260OHM-Cir-MIL per foot. Copper cored alloys are selected when higher current capacities are needed. The copper core is ½ of the outside diameter and its resistivity is 71 OHM-CIR-MIL per foot.
Kovar Seals typically use Kovar for the conductor.
Compression Seals' body components can be machined formed stamped, or coined. The materials normally low-carbon steel, leaded steel, stainless steel, or Monel. The external body of a Compression Seal must be thick enough in cross section to maintain compression around the glass and sustain the tensile stresses without yielding. Also, because of high forces around the glass, a tangential stress is set up in this ring. So the ring must be thick enough to sustain tensile stresses. If it is not sufficiently heavy, the glass will crack. General guidelines for ring thickness are:
Kovar Matched Seals do not require heavy external body cross-section thickness. Therefore, Kovar Seals should always be designed with lighter stamped outer bodies.