Glass is a good electrical insulator and it has low thermal conductivity, along with the capacity to withstand high operational pressure and temperature. However, the limitations of glass- to-metal seals become apparent when put to the test under extreme operating parameters. These limitations are twofold: • Material properties of glass — Glass is rigid and brittle, making it prone to cracking under mechanical stresses of high pressure, shock, and vibration, especially in combination with extreme thermal cycling. • Constraints of the manufacturing process — The difficulty of melting glass and its viscosity when molten impose constraints on the types of metals that can be used.
MATCHING THE GLASS WITH THE METAL Glass-to-metal seals are formed through a melting process at high temperatures, often exceeding 900°C, followed by rapid cooling. The creation of a strong, resilient seal requires the matching of the coefficient of thermal expansion (CTE) between the glass and the metal materials. CTE is the extent to which a material expands and contracts when subjected to temperature change. During the heating and cooling process, the rate at which the volume changes of the two materials must remain in sync, otherwise thermally produced stresses are induced, as one material expands at a faster rate than the other(s).
Due to the inherent rigidity and brittleness of glass,
81
Powered by FlippingBook