Overview
When designing a glass-to-metal seal the selection of the glass and its associated metal is the most important criterion. Some design rules lead to different material pairings depending on:
- the customer’s product design,
- the operating environment,
- hermeticity and insulation requirements.
CTVM will always guide you through the design of your hermetic assembly.
And if you already have a design, we will review it with you to ensure it is compatible with glass-to-metal sealing technologies.
From a quality standpoint, CTVM guarantees full traceability of all sourced and used materials and works exclusively with recognized, high-quality suppliers.
Glass
The glass grades we use vary in composition and form:
- Electronic-grade glass Schott 8250, which we can machine to produce our own preforms when dealing with more “massive” parts, windows, or small production quantities.
- Special sealing glasses, supplied as pressed or sintered preforms, available as small tubes or multi-hole discs. These are divided into two main families:
- Borosilicate based-glasses, with low thermal expansion coefficients
- Soda-lime glasses with high thermal expansion coefficients
Schott 8250 and borosilicate-based glasses are used for matched or mixed seals with Kovar or molybdenum, while soda-lime glasses are used for compression seals with stainless steel or Inconel.
The typical maximum operating temperature range is around 600–700 °C.
Metal
- Hermetic Glass-to-Metal or Ceramic-to-Metal Assemblies
CTVM uses a variety of metallic alloys and metals to produce hermetic glass-to-metal and ceramic-to-metal feedthroughs and assemblies.
The metal directly bonded to the glass must have a thermal expansion coefficient closely matched to that of the glass to reduce the risk of residual stresses within the seal — stresses that could otherwise lead to cracking or failure.
However, in some cases, other design constraints dictate the choice of materials, requiring the seal to be engineered with suitable geometries and glass grades.
When using a ceramic body with metallic pins, only the glass-to-metal interface is critical, since the ceramic itself exhibits very low expansion at sealing temperatures.
The metals most commonly used by CTVM include:
- Iron–nickel-cobalt alloys (such as Kovar, FN48, FN52), which are special steels with thermal expansion coefficients very close to sealing glasses such as Schott 8250 or borosilicate grades.
- Stainless steels 316L and 304L, offering superior corrosion resistance or non-magnetic properties when required.
- Inconel, chosen for higher corrosion resistance and a lower thermal expansion coefficient than stainless steel.
- Tungsten, used for very high temperature applications.
- Filaments and Grids
For these products, we work from drawn and generally annealed wires.
Electron-emission filaments are made from tungsten or iridium.
For sublimation applications, filaments are manufactured from tungsten, molybdenum, or tantalum.
Bayard-Alpert gauge grids can be produced from tungsten or platinum–iridium alloys.