So I just tried it, and I think Mr. Wizard just had access to more potent stuff than you can get at the drug store. I added salt and drugstore hydrogen peroxide (3%) to a steel wire cleaning pad in a beaker with a lab thermometer and it gave a modest increase in temperature, maybe 10-15 degrees Celsius in about as many minutes. I think the reaction needs a higher concentration of peroxide to really move. I’ll try it again later when I have some time to make more concentrated H2O2 and report back.
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It was my impression that @shaddack meant Salt water and a power source, though I don’t see quite how that works.
Clarification?
Nope. The chlorides and water accelerate rusting. The rest is manipulating the situation to achieve a thermal runaway.
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When I said a critical mass, it was more like a barrel full of steel wool. It was rather a lot!
I encourage you to get some steel wool and touch it to the contacts on a fresh 9V battery. No other ingredients are needed.
Please do this in a safe space. You’ll quickly see what I mean.
ETA - If you’ve ever had a wire vaporize in your hands, this is closely related and somewhat less disturbing.
Oh, I’ve tried it, I’m just intrigued by what @popobawa4u was proposing, to use a runaway thermal reaction with a few drops of a reagent.
Water has a high specific heat capacity, and if you tried to ignite the wool with a battery in salt water, it would quickly just swallow up the heat created by the battery burning up the thin wire strands of wool. As @shaddack is saying, you want the ions and water to be an efficient transfer medium for electrons. This is a redox reaction, and the oxygen is is being reduced (gaining electrons) while the iron is being oxidized (losing electrons). In the air, this happens slowly, because the transfer of electrons isn’t efficient in a gas medium, in the water, it’s a little quicker, but it’s still fairly slow because of various equilibrium considerations. We use salt ions to facilitate the movement of electrons in solution.
So the solutions lie in pushing that equilibrium in certain directions. If we use a stronger oxidizer than the oxygen in the air, i.e. less stable, the rate of the reaction is faster which allows the energy to be released in a shorter period of time. I also added some small quantity of acid to help things along catalytically. I think I’m on the right track, but again, the water has a high specific heat capacity and there are moles and moles of it sucking up the heat generated by my reaction. So I want less water and more oxidant. If the reaction temperature rises high enough, it causes the rest of the wool to simply catch fire in the air.
This is another way to push the thing into thermal runaway, but I’d want to run some numbers on that, I think you’d need more than a few drops of weak reagent. If it’s strong enough, then the mass wasn’t a factor.
This is a fun way to get a match going when you want distance between yourself and whatever you’re igniting.
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The mass is always a factor.
A reaction that doesn’t noticeably heat a flask in the lab can runaway in a large reactor. (Happened. Lab measurements did not suggest need for beefed up cooling. They built the plant and the reactor walked away in quite a hurry.)
The rate of heat production must be faster than the rate of heat removal. The reactions get faster with temperature, therefore the hotter it is, the more heat per time unit is produced. (Subject to throttling by rate of ingress of the chemicals needed, in this case of gaseous oxygen. Sometimes the limiting factor is the reaction speed, sometimes it is the chemical(s) availability. You can also run out of the chemicals, in this case steel wool, before the reaction has a chance to pick up speed.) The heat removal rate increases with the temperature gradient. If you maneuver the variables - have enough of the materials in a configuration where all the chemicals are in the right ratios and there’s enough of them and the cooling is slow enough which you can help with e.g. ceramic wool - you can have a nice runaway. Fwooosh. Or, if confined, fwoo-kaBOOM!!!
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That show was great…
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A lot of the episodes are on the Youtube channel. Great blast from the past.
Only one I found with steel wool burning didn’t use any liquids. He did throw in some powdered oxidizer to get some kick into it, though…
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Poor choice of words, I meant that in the configuration described the mass of the steel wool wouldn’t be critical. The beaker I used last night had gotten perceptibly warm to the touch after about thirty minutes and I could imagine it getting out of control in larger quantities, but a barrel of dry steel wool is just going to cause me to run out of a few drops of the particular weak reactant mixture I was using last night, well before the reaction got warm enough.
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I’m one of the inventors of this technology - feel free to ask me anything about it. A number of terrific points have been made in the comments already (all basically correct). A few notes:
- We first published the ultralight lattice work in 2011. This is a Boeing-focused piece on innovations that might arise. As you might expect, an engineering-focused company would present a highly edited and conservative projection.
- @japhroaig - I think I remember the fellow. JP? Plus, because of the scalable fabrication, cost-wise this approach is pretty favorable. If cost is the primary driver, this won’t make it. Cost + performance, completely different story. Plus you are totally right - Trusses are age-old - we’re just making them small, scalable and out of nifty materials.
- @frauenfelder Feel free to visit us in Malibu. Hit me up on G+ or here!
- stephen_schenck - totally agree. PR simplifies things. These are the lightest metallic structures although it is difficult to differentiate between materials and structures when the length scale becomes small (say <1mm)
- enkidoodler - not aerogel, but yes it scales. Fabrication time is ~1 min, independent of size. Sort of. The template is ~1 min to form, the metal film is ~5 min, and etching the template (to leave the metal) is ~30-60 min. independent of size.
- tyroney yea. Sophia is way more relaxed and knowledgable than that video would let on.
- omems shaddack Ni forms a protective oxide scale so it does not tend to combust as… spectacularly as Mg. But I’m sure we could provoke something interesting out of the material.
- CLamb shaddack yep - 3D printing is also an option for the template.
Sorry to everyone - I could not ping you due to being a new user.
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Welcome, @wbc. We like smart people here. 
Are there any applications this stuff is currently in use or testing for? (That you can talk about?)
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Thanks for the welcome @slybevel! We are looking at both automotive and aerospace applications - lightweight parts for the most part. We’ve been working also on energy absorption (e.g. in a crash) - Engineering for that property results in better than state-of-the-art absorption for helmet and blast applications. We’ve looked at bioscaffolds and cushioning applications as well.
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What (roughly) are its properties with repeated strain? How often can it ‘bounce back’ to its original shape, with what magnitude compressive strain.
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Heh, yup! It is awesome stuff, and I mean absolutely no disrespect in any way–quite the opposite.
Do you happen to know any of the guys at one drop design?
Eta
Jeez it is such a small world.
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You could try thin alternating layers of nickel and aluminium. These, when pushed a bit, will start reacting and forming nickel aluminide and a considerable amount of heat. Aluminium-palladium system would work even with just two thick layers (Pyrofuze).
(Would this be fast enough to be initiated with a projectile impact, and, possibly with some additional coating, form enough shock wave to push against the projectile, or in case of shaped charges deflect/fragment the jet? With care the alloy/coating system could be engineered so the reaction is not fully self-propagating, so only the hit area would ignite.)
(Another thought: if the intermetallic system propagates the reaction fast enough, it could be used for ignition of an energetic material throughout its volume in a controllable way. This could lead to some interesting applications in e.g. gas generators or pyrosyntheses of bulk nanomaterials. Just a rough idea here…)
Resorbable tissue scaffolds (from biocompatible resorbable alloys, e.g. the magnesium-calcium system) could be also pretty interesting.
And I saw somewhere a microscale 3d-printed guide for growing-together nerve fibers. The microlattice production method could be perhaps useful here too?
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@MarkDow Good question - the answer depends on density (or more precisely on relative density (fraction of metal vs fraction air). For densities >50mg/cc, the material squashes and stays squashed. Irreversible deformation. At densities <10 mg/cc (about 10X the density of air) the material springs back almost 100% but loses about 50% of its strength after the first cycle. Upon repeated cycling (say >100 cycles) the material response stabilizes at about 30% of the original strength. The magnitude of compressive strain is ~50%. As you vary that compressive strain, that springback density changes (smaller strains can be recovered in higher density samples). It’s a neat effect.
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I’m not familiar with one drop design. Sorry! And thanks for the kind words - they were absolutely taken in that spirit.
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@shaddack Cool idea! Our material concept is basically a way of taking any of those thin film technologies and making a 3D structure out of them. Tissue scaffolds are absolutely a cool application that someday should get some attention. We’re not working on that now though.
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