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.
steve_rb
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Please let us know your experience with glass to metal seal

Post by steve_rb »

All those have some experience with making glass to metal seals please post your experiences here.
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Dennis P Brown
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Re: Please let us know your experience with glass to metal s

Post by Dennis P Brown »

I have created glass to metal seals that can withstand cooling rates of over 1000C/sec and they have create strong bonds that are vacuum tight with copper wire even as the temperature ranges over a few hundred C. The single trick is to buy the special glasses that match the CTE of the metal. Sounds simple (and is) but these special glasses are generally costly (and some are custom orders only.) There are glasses for iron/steel, copper and even aluminum. Just google for the companies but Schott glass is your best bet. I should add - this should be considered only if you can melt the glass yourself (some are very low temperature melting - under 300 C.) They sell blocks of the glass so you need to make it into the size/shape you require - these are not tubes made of glass.
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Carl Willis
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Re: Please let us know your experience with glass to metal s

Post by Carl Willis »

As a very amateur glassblower with a self-sustaining hobby making decorative plasma tubes, I have made tungsten-borosilicate seals via uranium glass and Houskeeper ribbon seals from copper straight to borosilicate. Both types are challenging to perform reliably in my experience. The Dumet or platinum seal to soft soda-lime glass is easy by comparison and makes a very reliable seal, but you probably don't want to make apparatus out of soda-lime glass.

Given the experience, I strongly recommend hunting down some pre-made lead-ins rather than making them from scratch using materials of unreliable characteristics like tungsten wire. For wire seals, you are looking for a product known commercially as a "lead-in wire", preferably pre-beaded so you don't have to bother with the delicate challenge of prepping the wire and fusing a glass sleeve to it. The tricks when buying pre-beaded lead-ins are finding someone who will sell a small quantity (something less than thousands) and making sure you ask for one specifically for sealing COE33 borosilicate glass. They are all sourced in China or India. I haven't succeeded in finding a stable small-order supplier, but I have had a couple companies send me free "samples" of 50 or 100 leads. I figure these will last me essentially forever, although the sizes aren't appropriate for all jobs. You want to prepare an appropriately-thin-walled flange in your glass to accept the lead-in so that you don't risk overheating it (which will cause the joint to form bubbles and possibly leak). For sealing into larger, heavier walls, you make a maria in a piece of tubing that will slip over the lead-in bead and can be fused to it, and then you prepare a flange on the piece of glassware that will seal to the maria. Seals must be annealed in a kiln.

Borosilicate neon electrodes are another possibility. They are expensive, $5-10 apiece. (Contrast with Dumet-sealed lead glass neon electrodes, which are about $1 each.) They contain reactive gettering materials and are not suitable for some applications.

If you want a metal-tube to glass seal, you should look through the offerings from Larson and MDC / Insulator Seal. These seals are variants of the Houskeeper seal, but are strain-relieved through graded glass transitions. The cheapest and most common such seal is made from borosilicate glass to Kovar. The Kovar can in turn be soldered, brazed, or welded to other metals. It's worth noting that Kovar is embrittled by silver in brazing alloys.

Good luck!
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Re: Please let us know your experience with glass to metal s

Post by prestonbarrows »

Unless you have access to scientific glassblowing equipment and fair skills (I'm guessing this is not the case or you wouldn't be asking), compression fittings are the simplest way to go for smaller diameters.

Example:
http://www.lesker.com/newweb/flanges/ad ... cfm?pgid=0

Just weld/braze onto a flange and you are done.

If you need diameters over an inch or two, you will likely want something pre-made. As mentioned earlier, this usually involves Kovar metal parts since the coeficient of thermal expansion is the same as many glasses/ceramics. Must it be glass or just an insulator? I would suggest going with brazed ceramics if you need large diameters. The ends of the ceramic are plated with metal than brazed directly to Kovar end caps. For large diameters, ceramic is much stronger and cheaper in general than a glass to metal seal.

Example:
http://mpfpi.com/stock-products/breaks/ ... reaks.aspx
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Re: Please let us know your experience with glass to metal s

Post by John Futter »

Steve
You still have not replied to the intended use but I use Hysol 1C or torr seal works a treat to low ten to the minus 8 millibar and will handle 150 degrees C just
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Re: Please let us know your experience with glass to metal s

Post by steve_rb »

I would like to join an about 70mm cylindrical glass to about the same diameter cylindrical metal to make a 60 KV insulation. Room temperature will be OK. I have heard usually some sort of wetting agent should be applied between the glass and metal interface for better sealing/temperature coefficient matching but couldn't find any info about glass to metal seal wetants.
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Re: Please let us know your experience with glass to metal s

Post by Carl Willis »

Hello Steve,

You have described a very challenging seal for a hobbyist to make by himself, and a costly one to obtain from a professional. I think there are preferred alternatives to the glass-metal seal in this situation as you have described it.

A master scientific glassblower would make this seal on a glass lathe by the Houskeeper method, and then anneal the assembly in a kiln. The diameter of 70mm is much too large to attempt without a glass lathe. You are correct that the Houskeeper seal requires a flux, but this is formed in situ in a thin layer by controlled oxidation of the metal (e.g. with boric acid in the case of copper). If your university in Tehran has a first-rate scientific glassblowing shop, you can see if they will do it. However, even a good glass shop would probably try to buy a suitable seal from a specialist manufacturer like those mentioned earlier in the thread. Either way, this will be a very expensive seal.

Ceramic vacuum breaks are available (as Preston mentioned) that are more durable and less expensive than a glass-metal seal in this size and that don't involve the sharp edge used in a Houskeeper seal, which can be a failure point if exposed to high electric fields.

I can't help but wonder if your real needs would be served by one of the standard feedthroughs used on hobby fusors all the time. One reliable approach to pass the higher voltages into a fusor is to use an inexpensive ($200 USD) 20-30 kV feedthrough insulated on the air side with oil.
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Re: Please let us know your experience with glass to metal s

Post by Andrew Robinson »

I know absolutely nothing on this subject, but you guys peaked my curiosity. Discovered this video as a result. Pretty cool example of apparently what Carl was referring to: https://www.youtube.com/watch?v=OWw32BLodjY
I can wire anything directly into anything! I'm the professor!
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Re: Please let us know your experience with glass to metal s

Post by steve_rb »

Interesting Video.
Also I found Varian TorrSeal resin epoxy bonder can stand up to 10^-9 torr. Anyone has used this bonder? How it behaves at 50 KV?
http://www.pchemlabs.com/product.asp?pid=2144
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Re: Please let us know your experience with glass to metal s

Post by John Futter »

steve
i'm not surprised you found torrseal as I had mentioned it to you further up the thread-- note Hysol 1C is exactly the same at a fraction of the price.
I use both up to 150kV with no apparent problems
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Richard Hull
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Re: Please let us know your experience with glass to metal s

Post by 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.

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

Post by 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.
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Re: Please let us know your experience with glass to metal s

Post by 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.
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Richard Hull
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Re: Please let us know your experience with glass to metal s

Post by 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.

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Fusion is the energy of the future....and it always will be
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Re: Please let us know your experience with glass to metal s

Post by 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)?
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Re: Please let us know your experience with glass to metal s

Post by Carl Willis »

Indium melts at 157 degrees C.
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Re: Please let us know your experience with glass to metal s

Post by 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

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Fusion is the energy of the future....and it always will be
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Re: Please let us know your experience with glass to metal s

Post by 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.
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Re: Please let us know your experience with glass to metal s

Post by 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?
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Re: Please let us know your experience with glass to metal s

Post by steve_rb »

I need the second sort. The one that holds the vacuum for years.
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Re: Please let us know your experience with glass to metal s

Post by 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.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Please let us know your experience with glass to metal s

Post by steve_rb »

Yes I have noticed this. It looks getter material should be heated after tube sealed? Any experience with getter materials?
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Re: Please let us know your experience with glass to metal s

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

Post by Jerry Biehler »

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

Post by 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
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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