by Doug Coulter » Fri Oct 24, 2014 3:23 pm
Dave, out of curiosity, were you working with relatively pure Be, or a dispersion strengthened alloy, whatever? I know it's quite dangerous to say, use a diamond saw to slice 1/4" rods of beryllia (not to mention very difficult, tried it, eats diamond saws and water lube). It basically wasn't worth the effort, discounting any risk.
I use Cu/Be a lot here for springs - some of them hold the inner window in my tank, those, I just soldered after a spot weld wasn't quite enough. Any flavor of copper is kinda hard to spot weld...I wasn't crazy enough to go for the TIG.
The rare metals handbook points out that it's one of the few that have actually killed quite a few people. Cd for example, though more poisonous, tends to make you so sick so fast...hardly anyone has ever been killed by it. If you ingest food that had gone through Cd plated hardware and had low Ph, you just throw up before you can absorb much of it. The only Cd death I'm aware of involved welding fumes - no way to throw up your lungs. We have some Be and I've handled it a little, even cutting sheets with scissors to make neutron generators with a polonium alpha source but it was quite dodgy work - the stuff is super brittle. It has a high "neato" factor in nuclear stuff, since a really thin piece is strong enough to hold off a vacuum, yet let even low energy charged particles or X rays go right through...
In yet another case of "I'm lucky to be alive" I had about half a pound of Cd catch on fire in a melt pot I was going to make an alloy in. Good thing I was upwind and outdoors. The resulting ash killed a poison ivy plant with a stem the size of my upper thigh and the tree it was growing on, so not a total waste - it was a good place to dump the stuff. The resulting alloy was a good thermal neutron stopper, but the capture gammas that resulted made me just put it on the shelf instead.
Be interested me for a couple reasons. Being so low in Z, most things go right through - they use it as windows on super-sensitive particle counters, or did, despite it's nearly impossible to braze, weld, glue...funny tempco, nothing wets it, and so on.
But it also takes part in some neutron-boosting reactions, the n,2n variety, as well as reacts with all the other species in a fusor - T, He3, D (all flavors and we have them all), H, you name it. Since it's low Z (really, low A) it's not as hard to overcome the Coloumb barrier as it is with other things. Further, when hit by electrons etc, it doesn't make X rays well at all, the K lines are so low-energy due to the low charge and the loose electrons. Like I said, possibly a lot of magic in a fusor - it would cut X ray production, increase neutrons, act as part of the fuel and so on. I found out the hard way why you don't put high Z coatings on the fusor wall - the X ray production goes WAY up. I had to tear it down and remove the Pd I tried for that reason. Ti might still be a viable wall coating with refinement. I think my last try put it on too thick, and we loaded it up too much. It changed form (TiHx) and wanted to flake off. As well as releasing all the D when it got hot, and we weren't and right now aren't that good at keeping the walls cold. Hard to do, stainless steel is practically speaking, a thermal insulator and we only get to fool with the outside. The possibility of storing "too much" D for decent gas pressure control didn't occur to me till it happened. Ti really soaks it up.
I didn't know anyone machined the stuff at all - my old books talk about arc-melting it (in argon) in cooled copper molds, the outer part of the Be never gets hot, so you get rough ingots, nothing can really hold molten Be like you can with say, steel.
In other words, most of the same fab problems you run into with tungsten - and for the same reasons. Obviously, someone made the 4 mil thick sheet we have a few sq inches of some way. It impressed me as having large brittle crystals, not well held together and was hard to cut without shattering. It looked ground rather than machined.
I'm still liking carbon (graphite in our case so far) for grid ends. It has so many good features. It's both thermally and electrically conductive, doesn't melt (sublimes somewhere above the MP of tungsten - we never go past red/orange heat, not a problem), radiates heat well - almost the only way to get rid of heat in our conditions, and is just soft enough so that for example, with my 20 mil holes and slightly elliptical 20 mil tungsten rods, you have to (but also, can) just force them in and they are then held tightly. Keeping a grid together is no mean feat FWIW - there are gram to ounce levels of repulsion at normal conditions, and under arcing, it's far worse. Many is the time I've had to take the system down and recover loose grid parts from all over the inside of the tank, after giving them quite the torture test before putting them in there. I suppose tempcos matter too. Carbon has one of the lower secondary electron emission numbers of any conductor as well - at least measured the way they did back in the electron tube days with medium voltages and electrons as the projectiles. I can't seem to find any study of the elements on secondary emission from being hit with fast D (or whatever) at all, and it would be very useful to know how things act at our conditions. We need some, but not very many, electrons. I'm pretty sure we don't need many more than we have "free" from ionizing the D in the first place.
I'm even wondering about tungsten carbide we can now get that's supposed to be super round. Will that trick still work? It's going to be a lot of tooling up to use it, expensive. I might have to try and grind threads on one end or something.
It appears the .009" drills don't come in the usual solid carbide with 1/8" shanks, so I have to make a 1mm collet for my drill rig, for example, no small task that needs its own special tools (I made the 1/8" one I have as well, I managed but it was a lotta work). The collets etc I see in places like McMaster may as well not apply - too big, too complex, too expensive, too much inertial mass for these tiny drills...And I kind of doubt I can just drill a 1mm hole in the end of a rod and use setscrews to get the required accuracy...I'll have to actually make a collet (the tiny drills are 1mm shank). We're interested in the WC because if I read the old books right - it has lower electron emission, and this stuff is both tinier than anything else we can find, and very round and stiff - just what we want, I believe.
The one bad thing about carbon is that you can wind up with hydrocarbons if you smash hydrogen into it. We saw some simple ones when our mass spec was in working order, not many but some. It also sputters a little bit. But with our new BN high voltage feedthrough, cleanup of sputtered anything is now a walk in the park - one wipe with 320 grid sandpaper and all is new again. At any rate, what usually happens to any simple hydrocarbons in the tank is that they find their way to something incandescently hot and simply decompose again there, releasing the D and depositing the carbon. That's not all bad, and is probably responsible for the improvement with running of some of our grids - they get a little coating of C and don't emit electrons as well as a result, and do radiate heat better too.
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.