Tacho
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Powder resulting from electroerosion.
I tried a new idea to make aluminum powder. I made strong electric sparks (repeatedly short-circuiting a 100mF capacitor at 170V) under mineral oil,
using aluminum electrodes. A black cloud quickly developed, but I don’t think that’s aluminum. The powder precipitates after an hour or two.
Any idea about what is this black powder?
I tried both silicone oil and mineral oil with the same results.
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Oxydro
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I don't really know, but is it possible it could be a coating of oil on very fine aluminum powder that's making it appear black? If so,
maybe washing with alcohol would help get rid of it.
That's an interesting technique, would be useful if it turned out. Keep trying different liquids, would be my advice. Maybe an inert gas? Under
vacuum? Just tossing out (probably stupid) ideas.
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Mr. Wizard
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This sounds like EDM (electro-discharge machining?). The black could be aluminum dust with surface texture so rough it looks black, or it could be
carbon from the vaporized mineral oil. It could also be a carbide of aluminum. Try letting some precipitate to the bottom,... tic tock, tic tock....
remove it with an eyedropper or syringe, and wash it with acetone or some other polar solvent to remove the oil. The precipitate could then be
tested. Acetylene gas with water might indicate a carbide, hydrogen gas with dilute sodium hydroxide would indicate aluminum, an remaining black
residue might indicate carbon. If you were doing this with steel or iron you could use a magnet. Sounds interesting.
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BromicAcid
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Aluminum carbide actually reacts with water to form methane not acetylene.
I know some of the liquid may have vaporized and formed bubbles but was there any apparent otherwise gas evolution that could lead one to believe that
the aluminum could have actually chemically reacted with something?
Determining if your powder is aluminum should be easy, e.g. look at the post above mine, if it is aluminum there should be a vigorus reaction with
hydroxdie with just a touch of water added.
[Edited on 6/26/2004 by BromicAcid]
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t_Pyro
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I'd tried a similar method before too. The major problem is that the Al powder formed was never too pure- it almost always contained oxides and
other compounds of Al as impurities... The same method works wonders with copper, though! The impurities could depend on what the medium is. Would you
know exactly what the oil was, that you used?
On the other hand, a capacitor bank could be used to create a larger spark, that could be used to vaporise the Al in an inert atmosphere.... The
vaopur, if condensed really fast, would give really fine powder.
[Edited on 27-6-2004 by t_Pyro]
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unionised
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Why don't you think the black powder is aluminium?
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Tacho
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Thank you Mr. Wizard, EDM machining, thats the english name for it!
t_pyro, I don't know its composition, but I think its a short chain paraffin. It's liquid vaseline, I think its the same thing as nujol oil,
pharmacy kind. I would like to hear more about your experiments.
Its not aluminum in mineral oil. It does't react with NaOH after washed with acetone. Just a bit of fizzing, maybe some al, but not much.
The electrodes are eroded by the spark, but it seems to create lots of carbon (that's what I think it is) from the mineral oil. I wonder what are
the residues in the silicone oil, my oil is thick as honey and it takes forever to precipitate, if it will precipitate at all.
Nothing but bubbles seem to happen with distilled water. DCM is too messy, the spark spills it all over.
Would CO2 be inert enough to try a dry version of this idea?
BTW, a spark that does no more than a loud pop in air, does a big bang under the liquid. I changed the flask from glass to plastic because I was
afraid it would break.
[Edited on 28-6-2004 by Tacho]
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Mr. Wizard
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If the precipitate reacts with water (bubbles) it may be aluminum carbide making methane (not acetylene as I speculated). The fizzing with sodium
hydroxide indicates aluminum. Did you try the water first, and then test the residue with sodium hydroxide, or did you try two separate tests?
I don't think CO2 will be any better than the mineral oil you are using, and will oxidize the aluminum more. Kerosene has been used, but it is
combustible. It is much thinner than mineral oil. Try limiting the current in your apparatus; this will cut down the noise and maybe the splashing of
the liquid. Do this by introducing more resistance into the circuit. You should limit the current to 1-2 amps. Try putting a 100 watt light bulb in
series with the gap. There are links for DIY, search for "mini-EDM" systems, and "Cheap and dirty EDM".
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t_Pyro
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| Quote: | Originally posted by Tacho
t_pyro, I don't know its composition, but I think its a short chain paraffin. It's liquid vaseline, I think its the same thing as nujol oil,
pharmacy kind. I would like to hear more about your experiments.
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Actually, my experiments were more related to electrolysis, rather than "sparking".
I accidentally discovered that performing electrolysis of a dil. nitric acid solution with a graphite anode and copper cathode with a very high
current density ( about 8A, with a copper transformer wire as the cathode) eroded the cathode so fast, that it started giving out "clouds"
of copper dust in the electrolyte. After filtering, and dissolving it in conc. sulfuric acid, it gave all the tests for copper, so I think it was
reasonably pure.
Copper can withstand this treatment because it's even below H on the electropositivity series... Al, on the other hand, is just too high up,
resulting in powdered aluminium compunds.
Unfortunately, I have no use for copper powder, so I didn't bother following up the experiment...
[Edited on 29-6-2004 by t_Pyro]
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