Boron Nitride

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Boron Nitride

Postby Joe Jarski » Sun Jan 09, 2011 2:20 am

Doug, since you've tried a lot of materials inside your fusor, I was wondering if you've tried boron nitride and how it survives under hot D+ bombardment. It looks like it saw at least some amount of use by the old school fusor operators and is a little easier to work with than alumina.
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Re: Boron Nitride

Postby Doug Coulter » Sun Jan 09, 2011 11:59 am

Haven't found any to try, and I've heard of it too. So, not a clue whether it works for this. Anyone have a small sample they want tried?
Ah, I see they have it at McMaster, a mere $368 for a 1" by 1' rod. Ow! They don't sell tubing, just various solid pieces. You'd have to bore it out.
You might be able to get away with just a little bit, though, just enough to protect the ends of things, which is where the more conventional materials fail.

I'd think it was pretty robust stuff, but being slippery might mean it's hard to vacuum seal across, dunno.

What seems to happen to every other insulator (other than pure alumina) is the hydrogen, which acts like it's insanely hot (I never liked that particle energy to temperature conversion baloney though) chemically reduces the stuff back to metal(s), replacing the oxygen most insulators have -- just like an ore smelter, but H works better than coke. Nitrogen does tend to bond to things better, but in my experience, chemically it likes itself better than most anything else, really would rather be N2 than in some other compound. Having to displace a couple at a time could make that harder to do, I suppose.

What happens with alumina is some of it gets changed to aluminum hydroxide, which is still an insulator, and subsequently evaporates, so alumina isn't perfect either, just much better than any glass or quartz and most other ceramics I've tried.

FWIW, the reason I don't like the glib math to translate eV to a temperature is that we're talking apples and oranges. Thermal motion is random, all degrees of freedom (including spin for non symmetric molecules). eV implies that all the energy is directed along a single direction vector, not random. So the assumptions that creep in make the conversion wrong in some cases.

As far as nitrogen goes, if you run the math on most high explosives (not gunpowder) where one of the products is N2, you find that more energy percentage than most think is simply due to the N's getting back together, and not that much from burning a fuel in an oxidizer (which also happens more slowly). The really quick HE's, like PETN tend to have shapes that allow the N's to hit easily "for some reason", which in turn releases the other stuff to do its thing.

I guess we'd just have to try some, deliberately put a piece "in harm's way" and see what happens to it. In my case, that could be a pretty easy test to do if I had some. McMaster says you can machine it with carbide tooling, but I'd guess you would have issues with it staying in the chuck if it's slippery, as they say. I have to be pretty careful with graphite, but since it machines so easily, the forces applied are low too, so it's at least possible.

Heck, hit boron hard enough with protons and maybe you get a little free fusion from that!
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: Boron Nitride

Postby Joe Jarski » Sun Jan 09, 2011 1:48 pm

It sounds like it might be worth testing. If it holds up anywhere near as good as alumina then it would be a good alternative for more complex shapes. The expense would be offset by not needing the more specialized tooling to work with alumina. Vacuum sealing is another issue, but not really a concern for what I'm looking at right now.
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Re: Boron Nitride

Postby Doug Coulter » Sun Jan 09, 2011 2:17 pm

Yes, there is no need to copy the mechanical design of existing feedthroughs at all, and in some ways, what I'm doing now is superior mechanically and thermally anyway. Just have the parts that take the punishment be made of the right stuff, which needn't be all that much of it. My little trick with glass and O ring seems to seal fine and has other advantages anyway (could even skip that if I had the correct adhesive, which I've not discovered yet). I'll have to get my hands on some BN and see how it does.

I haven't had much trouble cutting alumina tubing, it's slow, but it gets done. But cutting that sheet like they use for PCB substrate in some things, now that's a bear indeed. I've only drilled one hole in that stuff with success, darnit. Just wipes the diamonds off the bits. It's also hard to slit, even with a special jig in the mill and a ton of light passes.
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Re: Boron Nitride

Postby Bill Fain » Sun Jan 09, 2011 2:23 pm

Hi, We tried to get some Boron Carbide neutron detectors from the University of Nebraska, but they haven't really ramped up into production of them yet. -bill
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Re: Boron Nitride

Postby Doug Coulter » Sun Jan 09, 2011 3:09 pm

You know, for the carbide, it might be as simple as trying with some large grit abrasive that is fairly available (not real cheap, though). I don't think we have any boron in our little kit of cool things at any rate. I saw a link someplace on how to make the nitride in a vac system, but that was for "threads" of it, not the compact material. If we could find a tiny bit of the stuff, exposing it to the nasty environment would be an easy test to do here, then we'd know if the big chunks were worth buying. The alumina from Accuratus isn't so cheap either -- Large minimum prices on each part type, separately. They cut me a deal to sell me 3 pieces of tubing and 3 1/16 rods for over $300 as is. And I still don't have the 1/32 rods, I guess I should call them back and find out why.

I think for pure neutron detection, activation is the way to go -- can't fool it. For fusion detection (assuming DD fuel) then a high threshold situation with a scintillator might be fine for realtime feedback for tuning -- to only catch the fairly rare but super hot gammas from the reaction path that doesn't make neutrons, just gammas. The needed assumption is that nothing we do affects which of the 3 reactions is more likely. I don't know that there is any available information on that, it would actually be kind of hard to find out -- but we might find a way to, since we're fairly well equipped on that.
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Re: Boron Nitride

Postby chrismb » Sun Jan 09, 2011 6:31 pm

On the subject of boron compounds, here's a heads-up on a boron alloy that I pondered re: in place of a boron target. (It seems to me to be sufficiently close to 'pure boron' that the advantages of the material properties might outweigh the disadvantages of the small 'non-boron' percentage!)

http://en.wikipedia.org/wiki/BAM_(material)

Looks a bit specialised at the moment [read; very expensive!] but who knows how common it might become?
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Re: Boron Nitride

Postby Doug Coulter » Sun Jan 09, 2011 8:45 pm

Interesting. Wonder if you can get it at all? We'll have to chase that one down. You wouldn't need too terribly much for deposition.
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Re: Boron Nitride

Postby Joe Jarski » Sun Jan 09, 2011 9:19 pm

I'll have to look into working with alumina some more too. I have most of the grinding equipment and should be able to do quite a bit, but dropping $150-$200 each for a few different styles of diamond wheels is tough. I guess it wouldn't be a single purpose thing though - diamond wheels are great grinding stuff like tungsten too.
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Re: Boron Nitride

Postby Doug Coulter » Sun Jan 09, 2011 9:35 pm

I just used up 3 of the cheap Harbor Freight wheels making two cuts in thick pure alumina for this.
Good thing they are cheap. I have no way of knowing if the commercial/real ones are all that much better....these work fine, and live forever doing glass and quartz after all.
They are still useful after wearing the diamonds off the rim, as you can still use the faces for things. Less than 2 bucks a pop, not so bad.
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