Saturday, October 24, 2009

Thermite ignition tests

[Disclaimer: I assume no responsibility for use or misuse of this information. It's available elsewhere on the Internet and I'm simply sharing my engineering interpretation of that information. Use these materials with great caution. Do not mix reactants until just before use. Do not mix or ignite them anywhere near flammable materials. Do not look directly at the reaction as harmful UV radiation is emitted. Be careful!]
If you watch Mythbusters then you'll likely have seen their experiments involving thermite. According to Wikipedia Hans Goldschmidt discovered and patented this thermite reaction in 1893 and used it to both cut and weld metal. These powdered mixtures require very high ignition temperatures (2000˚F) but then burn at 4,500˚F swapping oxygen, producing molten metal, and releasing a bunch 'o' heat and UV radiation. Common reactants include elemental aluminum or magnesium with metal oxides like copper (II) oxide (cupric oxide) or iron (III) oxide (ferric oxide).
Many folks in high-power rocketry are successfully using thermite as a source of instant ignition of even the largest rocket motors using a guideline of roughly 1 g thermite per 1000 N•s of total impulse. I'm intrigued both by the reaction itself and its utility in this hobby so I decided to give it a try. I ordered 1 pound each of aluminum powder, cupric oxide, and ferric oxide online. I'm quite certain I overpaid as well so you should definitely compare prices on the Internet(s). :) I was finally able to use my chemical engineering degree and calculated the appropriate mass ratio of the reactants using stoichiometry and atomic and molecular weights. For the aluminum + ferric oxide reaction it's:

Fe2O3 + 2Al → 2Fe + Al2O3 + heat + UV

2 moles of Al = 26.98 g/mol * 2 moles = 53.96 g
1 mole of Fe2O3 = 159.69 g/mol * 1 mole = 159.69 g

So the mass/weight ratio is almost exactly 1:3 Al:Fe2O3

For the aluminum + cupric oxide reaction it's:

3CuO + 2Al → 3Cu + Al2O3 + Heat + UV

CuO: 79.545 g/mol * 3 mol = 238.635 g
Al: 26.981g/mol * 2 mol = 53.962

Mass/weight ratio: 4.4:1 CuO:Al

I set up my video camera intending to share the clips here but the ignitions were so fast that they only show up on about 4 frames at 30 frames per second. As such I've simply grabbed the key frames and assembled sequences for both tests. This camera method also eliminated the need to directly observe the reaction although the emitted UV might have improved my tan a bit. Here a single electric match ignites 0.5 grams of a molar ratio of ferric oxide + aluminum powder thermite:
Ignition was fairly rapid but continued to sputter for about 30 seconds afterward. In this second experiment I again used a single electric match to ignite 0.5 grams of a molar ratio of cupric oxide + aluminum powder thermite:
This second combination appeared to be far more energetic and burned to completion without sputtering. Most folks have been using this cupric oxide+Al combination but I wanted to try the classic ferric oxide+Al combination as well. It's clear that cupric oxide works way better so I now have my empirical answer. Again... exercise caution!!

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