TITLE: Applications for Macor in Metal to Ceramic High-Vacuum Seals
One of the most specialised technologies used to manufacture components for the high- and ultra-high-vacuum industry is the production of seals between metals and ceramics.
The most common seal of this type is that found on the base of an average lamp bulb. The properties of glass ceramics such as Macor can be controlled during its production process so that its thermal expansion is tailored to fit that of the metals – tungsten, molybdenum, nickel alloys and stainless steel – adjacent to it.
A larger-scale application of Macor in high-vacuum seals is in power feed-throughs. These are systems that transmit high-voltage or high currents or a combination of the two. If the ceramic surface of the seal faces the open air, it is usually glazed with a surface of high-temperature glass. The purpose of such a coating is to minimise the ceramic’s surface roughness as well as any potential for surface contamination. The glaze is an economic alternative to expensive surface finishing of the ceramic.
Such products can be used in vacuum furnaces, electron beam evaporation and direct-current plasma spluttering. The aim is to design the feed-through in a way that optimises its electrical performance.
Ceramics such as Macor can be made to fit into small areas where space is at a premium or over large spaces. They are especially valuable in those environments where ingress of moisture or other contaminants may become a problem, as this will impact on the overall electrical performance
The seals are created first by heating the two materials in an inert atmosphere. The ceramic melts and wets the metal by flowing over it. The temperature is then reduced to allow the ceramic to nucleate and raised again to control its crystallisation. In this way, the thermal properties of the crystallised glass ceramic match those of the metal underlying it.
However, it is important to understand that the seal remains a compromise, as the metal and ceramic alone have different thermal expansion coefficients. Even the smallest difference between the two thermal expansion coefficients could cause a catastrophic failure of the seal if is subjected to a very steep thermal gradient. Engineers recommend that ceramic to metal seals should not be subjected to a higher thermal gradient than 25 degrees C per minute.
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