Tungsten

Tips and descriptions of materials you use

Tungsten

Postby Starfire » Mon Aug 23, 2010 4:44 pm

Tungsten is heavy for its size and surprising when you lift it cause you expect it to be lighter. :?
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Re: Tungsten

Postby Doug Coulter » Wed Aug 25, 2010 10:16 am

Yes, I guess we need to get the ball rolling on the properties of this useful material. Tungsten has many uses for its density alone (bullets with W cores are used by the military for better ballistics and penetration of armor, for example). We use it in fusors as it makes a nice grid material that will take heat well, in a water free vacuum. It also is pretty easy to seal into pyrex as a glass to metal seal for conductors brought into vacuum systems. I use thin MIG rods (.040") for that, as they are cheap compared to buying tungsten wire from most sources.

Tungsten gets into a very destructive cycle with water when hot even in what would be called a good vacuum otherwise. Among other reasons, this is why Edison didn't start right there with it as light bulb filaments. Tungsten splits water molecules, becomes a volatile oxide that flies off the filament, then recombines with hydrogen on the bulb walls, recreating water to repeat the destructive cycle until your filament burns out. As this works better the higher the temperature, the already thinnest part of the filament is the part that continues to erode the fastest...

This is why quartz halogen bulbs do better. At high temperatures, any tungsten halogen compounds decompose on the hottest part of the filament, replacing the tungsten there...They are always quartz as for those W-H compounds to not simply condense on the bulb, you need a hot bulb. This leads to a non-monotonic life vs temperature curve, unlike plain W in vacuum or inert gas -- they actually have to be hot to live long!

(I'm going to edit this and add scans from Kohl's book and the rare metals handbook that give properties better in the usual tables, and some "how to fabricate with this stuff" info).
Here's a beginning on that edit, a link to an older version of Kohl -- the newer one is better, go buy it, you won't regret that one.


Tungsten has some weird properties vs temperature. Conditions that would anneal most metals make it more brittle than before -- it "work softens" and things like that. The prior history of any you get to do things with is really important to how you're going to work with it without breaking it.

Tungsten-Rhenium alloys are used in type C thermocouples and filaments that need to resist the water cycle. Hideously expensive from Nanmac (note to self, add nanmac to the vendors elsewhere they are good guys, just that this stuff is basically made of gold), but I bought some and am glad I did -- very much more useful here than the pure element for a lot of things. Not only does the Rhenium make it more resistant to troubles, it makes it more ductile and higher in electrical resistance so you can use fatter stuff, or need less current to light off the same size stuff.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Tungsten

Postby Starfire » Thu Aug 26, 2010 7:42 am

Doug - is the Tungsten directly to the glass to seal?
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Re: Tungsten

Postby Doug Coulter » Thu Aug 26, 2010 9:18 am

Yes, to Kimble-33 or Pyrex 7740, there's a bit of a trick, easy to master, on that, I will start a thread under "glass" I suppose and show it off. I do it all the time here, it's easy once you know how, and they are indeed leak tight at ultra high vacuum, and live through thermal cycling fine. There's a slight tempco mismatch that means it works better with 1/16" tungsten or smaller, but up to that size, no problems here with it at all. I have also successfully done Ti to lead glass (the kind used for neon tubes). I am working on quartz-metal seals, but it still looks like the best way there is to steal the seal from a quartz-halogen bulb and weld that into your quartz rig -- not all that expensive. I'll try to get that up here maybe next week, this week is bespoke already.

One thing about tungsten rod and wire -- it tends to have leak channels along its length due to the way it's made. So you either fuse both ends, or braze nickel over them. If you have the welder that uses those rods anyway, the former is trivially easy to do, so that's what I do here. I can then weld or braze on a copper connection to the world as required, or just use a clip lead.

It's a nice trick to make a discharge tube for your vacuum foreline as I suggest elsewhere so you can see what type of gas you're pumping optically -- poor man's mass spectrometer.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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