Glass-to-metal Seal - Copper Tube Seal

Copper Tube Seal

Another possibility to avoid a strong tensile stress when sealing copper through glass is the use of a thin walled copper tube instead of a solid wire. Here a shear stress builds up in the glass-to-metal interface which is limited by the low tensile strength of the copper combined with a low tensile stress. The copper tube is insensitive to high electrical current compared to a Dumet-seal because on heating the tensile stress converts into a compression stress which is again limited by the tensile strength of the copper. Also, it is possible to lead an additional solid copper wire through the copper tube. In a later variant, only a short section of the copper tube has a thin wall and the copper tube is hindered to shrink at cooling by a ceramic tube inside the copper tube.

If large parts of copper are to be fitted to glass like the water cooled copper anode of a high power radio transmitter tube or an x-ray tube historically the Houskeeper knife edge seal is used. Here the end of a copper tube is machined to a sharp knife edge, invented by O. Kruh in 1917. In the method described by W.G. Houskeeper the outside or the inside of the copper tube right to the knife edge is wetted with glass and connected to the glass tube. In later descriptions the knife edge is just wetted several millimeters deep with glass, usually deeper on the inside, and then connected to the glass tube.

If copper is sealed to glass, it is an advantage to get a thin bright red Cu₂O containing layer between copper and glass. This is done by borating. After W.J. Scott a copper plated tungsten wire is immersed for about 30 s in chromic acid and then washed thoroughly in running tap water. Then it is dipped into a saturated solution of borax and heated to bright red heat in the oxidizing part of a gas flame. Possibly followed by quenching in water and drying. Another method is to oxidize the copper slightly in a gas flame and then to dip it into borax solution and let it dry. The surface of the borated copper is black when hot and turns to dark wine red on cooling.

It is also possible to make a bright seal between copper and glass where it is possible to see the blank copper surface through the glass, but this gives less adherence than the seal with the red Cu₂O containing layer. If glass is melted on copper in a reducing hydrogen atmosphere the seal is extremely weak. If copper is to be heated in hydrogen-containing atmosphere e.g. a gas flame it needs to be oxygen-free to prevent hydrogen embrittlement. Copper which is ment to be used as an electrical conductor is not necessarily oxygen-free and contains particles of Cu₂O which react with hydrogen that diffuses into the copper to H₂O which cannot diffuse out-off the copper and thus causes embrittlement. The copper usually used in vacuum applications is of the very pure OFHC (oxygen-free-high-conductivity) quality which is both free of Cu₂O and deoxidising additives which might evaporate at high temperature in vacuum.