Please let us know your experience with glass to metal seal

[Bob Reite]

Most hard vacuum "thermionic valves" aka "vacuum tubes". have a "getter" that continues to get rid of excess gasses after the tube has been sealed off. You may want to consider that for your project.

[steve_rb]

Yes I have noticed this. It looks getter material should be heated after tube sealed? Any experience with getter materials?

[John Futter]

You can get samples from Seas getters in Italy.
They are usually heated by inductive heating of the getter part that is on its own somewhere within the vacuum
Look at any Vacuum tube (ValvE) you will see a circular thing usually near the top of the valve and the mirror metalisation that came from the getter ring after it was heated to cherry red hot.
They are made from Zirconium and various reactive rare earths that are selected to getter various gases ie nitrogen hydrogen oxygen

[Jerry Biehler]

I need the second sort. The one that holds the vacuum for years.

Then all bets are off. You will need a commercial metal-glass transition and probably a getter like other have mentioned, There are getters that are fired electrically, the materials are deposited on a filament and you apply power to evaporate and activate the strip. Of course they need hermetic electrical feedthroughs.

[Richard Hull]

I agree with Jerry. Go commercial. They have years in the biz and have made reputations on good, long lasting seals. Gettering is really important if you hope to hold better than a 10e-5 torr seal forever. Common old receiving tubes are a prime example of how it is done right!

Richard Hull

[steve_rb]

I am OK with getters but no knowledge regarding this "metal-glass transition" thing. If I have got it right you mean all seals above not suitable for sealed tubes and I only have to use metal-glass transition (the one was at the above houskeeper video) right?
In that video copper gets hot and oxidized. Then glass melts and sticks to the copper oxide. Do you mean this sort of seal with "metal-glass transition"?

[Chris Bradley]

Might be easier to describe what your objective is, than the means you think you need to use to achieve it.

[steve_rb]

I am trying to build a custom made vacuum tubes like X-ray tube for a home made simple X-ray diffractometer. Also try to repair a few broken X-ray tube we have (1 target melted, 2 filament burned). Be windows are all OK. I might also give a scintillation detectors a try too.

[Jerry Biehler]

Good quality metal to glass transitions use something like kovar which has a similar CoE to glass and will bond and not shatter when it cools down. http://www.lesker.com/newweb/flanges/fl ... cfm?pgid=0

You can get x-ray tubes pretty cheap on ebay. And be darned careful with that beryllium, that stuff is pretty toxic.

[steve_rb]

Tubes in eBay are not suitable for X-ray diffractometers.
Thank you for your notice. Yes I am aware of Be toxicity.
Anyone has practical experience with Kovar seals?

[Bob Reite]

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?

[steve_rb]

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

[Roberto Ferrari]

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]

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