General > General Technical Chat
Glass seals - kovar to borosilica
Drirr:
Hi,
I am trying to make borosilica glass to kovar seals.
Has anyone tried this? Any literature advice?
I was able to make several argon tubes using oxy propane torch, seals seems to be "OK" but bubbles around kovar wire makes me sad. :'(
Kovar wire was preoxydized to grey colour before sealing.
Thanks
LaserSteve:
Bubbles mean gss is entrapped in the Kovar.
Normally the Kovar gets prefired in s Hydrogen or Argon -4% Hydrogen oven to remove the heavy oxides. Then it is lightly oxidized.. This process removes most of the occluded gas.
Nickle plating or a quick flash of copper on the outside of the pin can help.
In my case switching to 7052 or 7056 helped. Then go 7056-to 3320 to 7740. The alkali glass was less susceptible to bubbling.
Strve
james_s:
Could you use off the shelf neon sign electrodes? Most use lead glass but borosilicate electrodes do exist.
LaserSteve:
Bubbling also shows up if you have the wrong oxidation state prior to firing. A good Kovar seal should be "mouse grey" after the glass has cooled. The text is Rosebury. Review of Scientific Instruments has some good one to two page reviews on techniques from thev60s and 70s.
Steve
LaserSteve:
Heat the Kovar in H2 at 800'C for 1 hour, cool in H2.
Alternative, use a mixture H2, 3%, in Argon (known as forming gas) again for an hour at 800'C.
The above strips the oxides with little or no loss in diameter.
Usually we tie a weight to one end of the Kovar and pass enough current through it to take it to an orange red, for straightening.
That straightening process mitigates leaks caused by micro-bends.
Mr. Rosebury was the chief technician at MITs Vacuum Tube Lab. Starting with the RADLAB program of all things. The first 100 pages or so are how to achieve the materials conditions, purity, and cleanliness to make your tube work. It is a handbook of what to do and how to do it, not a boring textbook.
Book:
Handbook of Electron Tube and Vacuum Techniques (AVS Classics in Vacuum Science and Technology) 1993rd Edition
by Fred Rosebury (Author)
Steve
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