Please let us know your experience with glass to metal seal

[Richard Hull]

One of our local HEAS guys, Tim Raney, has worked extensively with both house keeper seals, graded seals, (U glass), and nitrited tunsten rod.. the bulk of them leaked until he started using delecate 20 mil tungsten rod and backing all seals with TorrSeal. Making a good glass to metal seal that is forever proof against leaks is an art for the amateur and a science for the manufacturer.

I have found that using old vacuum tube contacts by first breaking of the tip to kill the vacuum and then diamond sawing the lower half of the tube with contacts away. This will yield a nice feed through once the tube element are cliped away. Melting and joining a good finished, manufactured product to a home built project of glass always works well. This is not a solution for all projects, but will get many jobs done fast and forever.

HV feed throughs are best done with a top part of the classic and commonly found 1B3GT or any anode capped HV rectifier or dead transmitting tube. Why fight city hall when 50 cents will get you a factory done glass to metal sealed tube at a hamfest. Don't over look ceramic tube options.

Richard Hull

[Jerry Biehler]

There is also the method of using indium metal to solder glass to metal. Indium will wet just about anything.

But going from 70mm glass to metal? Yikes. You could do a transition from kovar to glass and then weld the kovar to your tubing like on an ion gauge.

[steve_rb]

Yes I noticed but Hysol 1C looks is good up to 10^-6 but Torr seal is good up to 10^-9. This was the reason I focused on Torr seal.

[Richard Hull]

I have made many glass to metal and ceramic to metal seals with indium and indium-tin alloys. Beware! Low melting point critical. If your project gets hot as in a target electrode, anode cap, or even the envelope, the seal can fail.
Not good. If it stays cool, it is hard to beat an indium to glass seal.

Richard Hull

[steve_rb]

Hmm. Great. I really liked this indium and indium-tin thing. Richard could you please give some more info how I can do this? What critical points should be considered to have a good seal? Also I would like to know which type of metal can be sealed to which type of glass and ceramic?

Also how this seal acts at high voltagees (150 kv) and high temperatures (at least 200 deg C)?

[Carl Willis]

Indium melts at 157 degrees C.

[Richard Hull]

Pure indium has a much higher melting point than tin-indium. As Carl notes, even pure indium melts at a relatively low temp. Seals are made by cleaning the materials to be joined by soaking in very dilute HCl and then rinsing in de-ionized water. Molten indium is painted on the surfaces. Painted with a glass fiber brush. This is, effectively, "tinning" them with indium at this point, molten indium can be used to solder the seal closed. Again, if your item gets hotter than boiling water, forget indium seals.

Interestingly, indium remains molten and has no real boiling vapor pressure until 2000 deg C! A stick of indium can be used to write a conductive line on paper or wood as it is one of the softest metals known. It makes lead look like case hardened steel. There are no reports of indium poisoning or hazardous industrial results. If you are "that type person" some contact dermititis has been reported, as with all metals. A most intriguing metal.

http://www.indium.com/technical-documen ... d-its-uses

Richard Hull

[Jerry Biehler]

Indium is also used as a heat sink junction material especially in places like laser diode module mounts and getting good thermal conductivity on laser crystals or other optics and their mounts. It is also used by places like intel to interface heatsinks to cpus for testing, this way they dont need to mess with pastes.

The alloying of indium solder effects the melting point so you can use two different solders to solder things like laser diode dies to their mounts much easier than using one single melting point solder.

If you need to go to that kind of temp you will probably have to look at an o-ring gland seal.

[Rich Feldman]

Isn't there a fundamental dichotomy among the glass-to-metal seal applications being discussed?

On the one hand, we have apparatus that operate while connected to vacuum pumps.
On the other are sealed devices that are processed once, then expected to hold their vacuum forever.
Both had practical solutions 100 years ago.
Some materials and methods suit one, some the other, some both.

Which sort do you need, steve_rb?

[steve_rb]

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