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

[BrokenYugo]

I have zero experience with glasswork past making stirring rods in high school chemistry class, but Youtuber glasslinger (who makes vacuum tubes, where you need a perfect gas tight seal) prefers to use a tungsten feedthrough for borosilicate. This is detailed on their website here: http://tubecrafter.com/tubecrafter_020.htm .

[LaserSteve]

I've had some success with really thin Kovar that has been heat treated and cleaned, then sealed in Pyrex.  I've had good luck with Molybdenum in 7740 Pyrex but it tends to suffer from bubbles.  Thin 306 alloy Stainless Steel  will seal to Pyrex 7740  if properly cleaned, stretched, strain relived and pre-oxidized.   I know your supposed to have the metal match the glass within 7 COE, but sometimes the finer wire will do a good job under compression. 

Attached  is a neat chart:

Chart from Direct Glass‐Metal Seals by F. E. Gifford and Arthur Dolenga
Review of Scientific Instruments 35, 591 (1964)

In the chart, thin tungsten is doing something it should not, sealing over a 30 COE range.  Tungsten is pressed under high temperature and pressure, basically sintered in manufacture.  If not cleaned while hot,  with a nitrite salt, followed by sanding at 300 or 600 grit,  it tends to suffer from leak caused by microcracks in the tungsten.

3320 is  "Canary", aka Uranium Doped Borosilicate. A careful Ebay search will usually find a piece of tubing or rod  suitable for beading your seals and then sealing to 7740.   8250 or 7052 sometimes 7056 shows up as capillary tubes for pulling micropipets for neurology.
It is a much better insulator, being an Aluminosilicate  compared to Pyrex, so serious biologists have created a demand for it.

Steve