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woelen
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CrO3 is very hygroscopic and is a pain to work with. If you take dry CrO3 and want to mix that with Al, then soon, it will become a nasty, very
corrosive, sticky paste. Not something you would want to work with.
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not_important
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Woelen covered most of the problems. The others are CrO3 is more expensive that dichromates, harder to find, and is too reactive for all but rather
small runs.
In several old inorganic prep books they ran either a very small amount of the mixture, or used Cr2O3 and CrO3 together. In either case it was "mix
and very quickly use".
A possible advantage of using K/Na Cr2O7 is that the alkali oxide will act as a flux to some extent.
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chemoleo
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Aside from this... CrO3 is still in +VI oxidation state, just like the chromate. You'll only get a marginal increase in energy per weight, because of
course the Na grabs some of the oxygen.
Other than that, I agree, that the size of the batch needs to be greater. I remember playing with small portions of Cr thermite, and I got a single
mass that did not produce any visible Cr. Plus, I recommend pressing the thermite. Same goes for SiO2 thermites - the bigger the better (Makes
beautiful fireworks too!). Sulphur is needed once again. There's a thread on this where I posted details & pictures.
[Edited on 27-9-2006 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
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not_important
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Well, dichromates are like K2Cr2O7, or in old style K2O . 2 CrO3, so there's just a dilution effect.
Schlessinger's 1962 book "Inorganic Laboratory Preparations", which I believe S.C. Wack has provided to the Library area, has this very preparation in
it. It uses well dried Cr2O3 and K2Cr2O7 with aluminium, and a core of that mixed with BaO2 down the center of the shot to boost the original
ignition.
When I ran this, I didn't do the booster core, but I did mill the Cr2O3 and dichromate well together before mixing in the aluminium.
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Greenhorn82
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I'm starting to get chromium collecting from thermite down to a science.
I take very high mesh (over 1000) Al powder, mix it with Cr2O3 in just a little under a 1:2 ratio (something like *edited for math correction*
32-68%), make a paper tube about 3/4 " in diameter and fill it with the thermite, and then I pack it down very tightly. Then I remove the paper, take
the cylinder of packed thermite, place it on a brick, heat it evenly with two propane torches until the entire thing is glowing (at this point it
could ignite at any moment - it's actually very easy to ignite with a torch, especially with the 1000 mesh Al powder), and then when I'm ready I move
the torch to the top edge of the cylinder which immediately ignites it. The cylinder completely burns up in less than a second, and a few large
globules form at the bottom. The biggest one I've made weighs about 7 grams.
Unfortunately the metal bubbles before it solidifies, so it's not solid and it's easy to break. But I think it's bubbling because of trace amounts of
volatile materials and/or water, so I ordered a graphite mold to burn some thermite in from eBay, and hopefully I'll be able to make a solid piece.
After experimenting, I've learned that chromium thermite mixed with "correct" weights, based on the Cr2O3+2Al --> 2Cr + Al2O3 (about 26% Al and 74%
Cr) doesn't work well. I'm not sure why though.
[Edited on 15-12-2006 by Greenhorn82]
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12AX7
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| Quote: | Originally posted by Greenhorn82
in just a little under a 1:2 ratio (something like 32-78%),
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-68%? 
Maybe reduced CrO acts as a flux...*shrug*
Might be worth trying it with a dash of sodium carbonate and/or silica and various other things to encourage slag.
Tim
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YT2095
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thinking of slag/flux production, how about the coating on welding rods, break that up and pulverise it then put it in the thermit mix.
might be worth a shot 
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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12AX7
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Sure. Mind that it's usually bonded with cellulose, burning it first might be prudent.
Tim
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Waffles
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| Quote: | Originally posted by YT2095
thinking of slag/flux production, how about the coating on welding rods, break that up and pulverise it then put it in the thermit mix.
might be worth a shot |
Generally, the fluxes on welding rods are fluxes intended for use with that particular metal being welded. I'm not saying it WOULDN'T work, but I
think using the recommended fluxes is a better idea.
\"…\'tis man\'s perdition to be safe, when for the truth he ought to die.\"
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