ULTRA HIGH VACUUM (U.H.V.) FLANGES
To ensure total reliability under all conditions it is essential that the material employed, the flange design and manufacture are of the highest quality. For the demanding requirements of U.H.V. operation we normally use austenitic stainless steel specification EN1.4307 (304S11). This low carbon material is very suitable for welding with minimum risk of intergranular corrosion. It will withstand repeated high temperature bakeouts with very low outgassing levels and is easily cleaned to the high level of chemical cleanliness required for U.H.V. applications. In addition to good corrosion and oxidation resistance EN1.4307 has a low magnetic permeability (less than 1.1).

For applications where it is necessary to have the lowest level of magnetic permeability and where greater corrosion resistance is required stainless steel specification EN1.4406 (316LN) can be provided. This material will usually have to be produced specially and may incur a significant extra cost.

METALLIZATION
Development of the metallization process dates back to approximately 1935 in Germany where firms such as AEG and Telefunken started to independently investigate this technique for valve construction to replace glass to metal seals. The process was employed extensively in Germany during the early 1940's and then 'exported' to the USA sponsored by their government.

Some 10 years later Ferranti International plc developed their own process, initially using Steatites, to replace the glass to metal seals in microwave valves. After replacing Steatite with stronger high alumina ceramics, the technique was employed in many other applications outside the Company requirements. In 1990 the ceramic-to-metal seal division of Ferranti became an independent Company, Ceramic Seals Limited, thus retaining the knowledge and expertise built up over 30 years primarily in the Defence industry. Metallization is continued on the next page.