Please let us know your experience with glass to metal seal

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Bob Reite
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Re: Please let us know your experience with glass to metal s

Post by Bob Reite » Wed Nov 12, 2014 1:43 am

My experience with Kovar seals is in the day of glass transmitting tubes. You don't want them to get too hot or they fail. The glass transmitting tubes seemed to go "gassy" if sitting on the self for too long because the seal was never perfect. That's why in the old days we would alternate the working tube(s) with the spare(s) every six months or so.

For some reason, the metal ceramic seals used in modern transmitting tubes don't have this problem, so I run those till the rig won't make licensed power anymore, install the spare and send the weak tube off to the rebuilder.

Would a ceramic metal seal be suitable for your project?
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

steve_rb
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Re: Please let us know your experience with glass to metal s

Post by steve_rb » Wed Nov 12, 2014 10:28 am

Yes. As a matter of fact ceramic to metal might even be better. Have you got experience with ceramic to metal?

Roberto Ferrari
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Re: Please let us know your experience with glass to metal s

Post by Roberto Ferrari » Tue Nov 18, 2014 10:36 pm

Steve and rest of posters,

I used to repair X-ray tubes and also diffraction X-ray tubes. Now do similar thing with special lamps.
Working with a glassblower, we messed in kovar to glass seals. It is a hard learning.
You will need to follow Dennis Brown' comments about CTE of glasses. Usually you have to do transitions, that means joining a couple of intermediate glasses to arrive to borosilicate.
As mentioned by Richard, recovering parts of radio tubes is wise and exciting.
When talking of wires, be careful with tungsten and molybdenum wires, they are made by pulverimetallurgy and many types doesn't stand high vacuum, due to micro channels between particles. You need a vacuum tube grade.

One point related to rebuilding tubes with beryllium windows. They are brazed to the anode and many times the brazing material gets corrosion...and you get leaks, very hard to correct. My suggestion in that case is to replace the whole window. I never tried alternative procedures -highly experimentals- like electroplating the punctured area or covering it with vaporized metal under vacuum chamber.

Getters come in two main families: evaporable and non-evaporable. Evaporable-ones are activated by RF or electric heating; non-evaporable can be activated in some cases (tube type and design) bombarding them with low pressure noble gas under approx. 300 V dc. That slowly vaporizes the active metal which deposits in the cold areas a film, very reactive to residual gasses. Others only are mounted close to hot spots.
Diffraction tubes in the past used electrically-activaded getters because the position inside metallic anode avoid the RF option. Some X- rays tubes use zirconium or tantalum disks, close to the heated anode, so when hot they start to suck gasses.

Please feel free to ask anything about these subjects.
Regards
Roberto

Roberto Ferrari
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Re: Please let us know your experience with glass to metal s

Post by Roberto Ferrari » Tue Nov 18, 2014 11:19 pm

Steve,

I forgot to comment the hard time that is degassing the X-ray tubes.
You need to do it over the maximum power of the tube, generally taking the anode to dull red. And you do that simultaneously while pumping hard and applying filament and high voltage to the tube... Yes, you got it... generating X-rays.
We used to have an elevated furnace, cubic, approx. 1m by side, shielded with 1/4 " lead and 1/8" SS, with propane heaters and high voltage cables through it.
Everybody in the lab used dosimetric badgets.
Was pretty Frankenstein's lab! High voltage sparks were not uncommon.
I used a modified evacuation system inspired in the 40's, when pumping down transmission tubes. Go to the Electronics magazine from the WW II times, looking at the tube ads. They talk of a "charcoal trap", between the diffusion pump and the tube to be evacuated. Was a cage with charcoal and an electric heater. When starting the system you heat a lot the charcoal and pumps only with the mechanical pump; very slowly the charcoal releases mainly water and gasses retained. Then after the pressure improves, you start the diffusion pump, achieving 10-6 torr, degassing the evacuated tube thoroughly. Then you turn off the charcoal heater and the vacuum improves... two orders of magnitude! I did it replacing the dirty charcoal with molecular sieve pellets. When breaking the vacuum, I used dry nitrogen and after several cycles, the system arrived to 10-9 torr.
Regards
Roberto

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