Copper-to-glass seals: Although the CTE difference between glass and copper is large, a satisfactory vacuum seal can be made which utilizes the property of glass to “wet” copper oxide, some of which diffuses into the glass and forms a transition phase. For a satisfactory seal, the oxide layer should be no more than a few tenths of a micron thick and may consist of various oxides.
1. The copper piece is tapered down to a very thin feather edge in order to follow the expansion and contraction of the glass. The feather edge must be smooth and polished.
2. Degreasing and hydrogen-firing at 800°C.
3. The copper piece is usually borated by heating to redness in air and quenching in a concentrated solution of sodium borate. The color of the copper at the seal site should have a uniform deep red to purple sheen.
4. The copper and glass are heated to 1000°C in air and brought together. This is a process called beading
5. Copper-glass seals must be carefully annealed in an oven immediately after glassing
6. The assembly is chemically cleaned to remove the excess copper oxides.
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