Vacuum solder compatibility.

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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Peter Schmelcher
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Vacuum solder compatibility.

Post by Peter Schmelcher »

Some time ago I wondered if lead free solder was vacuum compatible. The answer is yes; at least to 1E-5 mBar at room temperature. This is the ultimate pressure of my setup. The diaphragm roughing pump reaches 0.7mBar and the turbo has a hydrogen compression ratio of 10,000, so this pressure seems reasonable.

I have been refurbishing the setup above. Repairing and replacing missing and damaged parts on a 2000 era bell jar vacuum system bought surplus from a university. It suffered from the inexperienced hands that touched it over the years. I cleaned out glass shards that fortunately never reached the turbo pump. The metering valves were cranked shut so hard that I repeatedly looked for shaft locks. It took both hands to free them, surprisingly however they are fine. I calibrated the new flow behavior using a ground glass syringe and a stop watch, putting the revised flow calibration marks on the handle. FYI when it takes two hands to free them they flow at about 0.25cc/minute. Snug that is soft finger pad tissue with no bone clamping force involved, the flow is about 1.5cc/minute. I would consider this snugness not likely to damage the valve seat, but I don’t actually know. I was not doing anything new to the valves so I had nothing to lose. I still need to remove several radial scratches in the stainless bell jar table.

One night after unbolting a CF flange looking for leaks (finally turned out to be an out gassing L-gasket), I could not re-seal it reusing the same copper gasket. The new gaskets I ordered had not arrived so I took the plunge and filled the knife edge cuts with solder. The solder I used was Multicore Hydro-X 96.5% tin 3% silver and 0.5% copper. The flux core is water soluble, hence the name, so clean up is easy, but the wetting action is so-so, YMMV. On the flip side I used a serious electronic liquid rosin flux that requires smelly Ethel Acetate for clean up. The wetting action with rosin flux is excellent. Lead free electronic solders have higher melting temperatures than older lead solders but not high enough for a proper vacuum bakeout, something that few are likely to do, 100C is fine. The next time I would flux the knife edge cut groove and lay a ring of solder wire in the groove on both sides of the gasket. Then position the assembly on glass and heat the copper and indirectly melt the solder. I haven’t unbolted the gasket yet but I expect all that will be required is flux both sides, reheat, clean, and reuse.

I just started testing a double o-ring bell jar seal. The outer o-ring is 0.103” cross section viton and the inner is 0.064” copper but is made from standard electrical wire. The wire is softened by torch heating and the ends are later soldered together using a scarf joint. The initial jar seal is made with the viton o-ring. Continued pumping squeezes the viton until the pressure differential lowers the bell jar edge onto the copper o-ring, compressing and deforming it. At least in theory, I haven’t visually inspected the copper o-ring sealing surfaces yet. So far the seal is much improved and pumps down fast. During the seal conversion I noticed and wiped down the jar inside with a paper towel to remove the oil residue liberated from the L-gasket. I guess if I had thought about the state of the metering valves I would have expected past solvent cleaning.

-Peter
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Richard Hull
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Re: Vacuum solder compatibility.

Post by Richard Hull »

Thanks for the exposition on clever sealing with solder. I have used pure indium and tin-indium alloys in the past. They work great, as well and will even wet and allow soldering to glass and ceramics..

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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Carl Willis
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Re: Vacuum solder compatibility.

Post by Carl Willis »

Good report on making a "wire seal" with CF flanges.

Occasionally one finds solder being used as you have described on commercial equipment. The Alcatel CF2P Penning ion gauge uses what I suspect is ordinary lead-tin solder wire (no flux) in two face seals, and in repairing the gauge, using a loop of 60-40 unfluxed solder wire produces a reliable seal.

Some older germanium detectors (and possibly a few in current production) used fairly heavy aluminum wire in a special face seal on the cryostat.

-Carl
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David Housley
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Re: Vacuum solder compatibility.

Post by David Housley »

This is interesting, Richard, about soldering glas and ceramic to metal with good wetting...Do you have any more information, or possible links to any more information?
Thanks!
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Richard Hull
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Re: Vacuum solder compatibility.

Post by Richard Hull »

http://www.indium.com/technical-documen ... ion-notes/

I have put this link up before. The Indium Corp of America has these App notes on the use of indium and its many alloys.

I make up my own indium alloys as needed for various tasks. In general 50:50 tin indium is chosen for its increased strength for seals metal to glass or ceramic to metal. Pure indium is often used to wet or "tin" the two Joints and the 50:50 material to actually make the joint, though the 50:50 material will wet OK by itself.

Not good for really high temperature work, of course.

As you will read in the above Apps, surface preparation, (not difficult but absolutely demanded), is where the real work comes in. The indium and alloys do all the rest.

Indium has an almost non-existent vapor pressure at its melting point and you can snort over the molten mix with impunity. No known case of indium process poisoning has been reported! A friendly heavy metal, much like bismuth.

Lead is not even that bad as long as you don't lean over the melt for long periods.

Cadmium in any proportion in a "melt" with lead or any other metal is a real killer though. Its vapor pressure is through the roof as its boiling point is just a few hundred degrees F over its melting point! While I have over 25lbs of cadmium on hand for my alloy work, I know how and when to use it to advantage. To all. Never, ever melt cadmium by itself.

If Cd it is a constituent of a low melting point alloy like Woods metal or Lipowitz metal, (melting point~160 deg F), melt the bismuth first, (zero danger), then put in the tin to alloy with the bismuth. You now have a low melting alloy, and can reduce the melting pot heat. Next put in any lead needed and this will dissolve in the cooler, molten mix. Finally, add the small amount of cadmium required which is usually a 1/2" diameter Cd rod cut to length needed to add the percentage Cd needed. Cadmium floats in this melt! You have just put the cadmium in a melt well below its melting temperature and, thus, its vapor pressure is not all that bad. The little cadmium "log" gets smaller and smaller as you "poke" it around on the melt surface with a carbon rod. The cadmium will not melt, per se. Instead, it "dissolves" or goes into solution much like an aspirin tablet. This greatly reduces its dangerous vapor pressure issue. When the Cd is in solution, the heat on the melt can be completely removed. Now, the low melting alloy can be cast or poured into its mold.

All such alloying efforts demands a respirator with a certified metal fume filter cartridge.

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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