Accelerated burning with reactive metal wires

[tti82phys]

Everyone here knows that fuel-oxidizer mixtures containing finely powdered metals (Al, Mg, Zr, etc.) have a much higher combustion rate than those containing only carbon and sulfur as fuels (black powder, or alternatively a chlorate/perchlorate mix with charcoal). The problem with the flash compositions containing reactive metals is that they are more prone to ignite from friction than the BP-type mixes. So I got an idea to accelerate the burning of a slower pyrotechnical mixture by adding several low-diameter (0.05 mm or less) magnesium or zirconium wires in black powder. Then I suppose the combustion would progress much more rapidly along the surface of the metal wires, igniting the mixture at several places at the same time and producing a quicker "flash" even if only a very small mass-% of the explosive charge is metallic fuel.

 

This kind of accelerated black powder would probably not be as shock sensitive as a composition where all of the fuel is finely powdered metal. There would be a much smaller reactive metal surface area where accidental ignition is most likely to take place.

 

Even simple heat conduction through metal could possibly act to make the combustion front move faster through the fuel oxidizer mixture. Suppose you fill a cardboard tube with BP and ignite it from the top, similar to a "roman candle" firework. Then do the same experiment with a copper rod sticked through the BP charge and compare the time needed for all of it to deflagrate.

 

I haven't made much practical pyrotechnical mixtures after my teenage years, but I was working for some time as a scientist in theoretical combustion physics/engineering research. So that's why this question is of this type instead of being about producing practical fireworks myself. I remember seeing how magnesium filings added to a slower pyro mix made it burn much more rapidly even when it wasn't finely powdered Mg, so my first guess is that Mg wires would cause the same. Has anyone here seen any experiments like this somewhere? One way to ignite a pile of fuel-oxidizer mixture at several points at the same time would be to add small metal filings here and there in the material and put it in a sudden quickly varying magnetic field that causes ignition by inductive heating. I believe that could even make a normally slow burning mix detonate without confinement, but that's a bit difficult experiment to perform by other than professional scientists.

Edited December 14, 2022 by tti82phys

[Crazy Swede]

The way of increasing burn rate through conduction of heat is well known and experiments with very thin metal wires as well as aluminised glass fibres has been performed with success.

 

There is/was fuses on the market that had one central metal wire as heat conductor for more stable and quicker combustion.

 

Using wires and fibres in powder mixtures has a lot of drawbacks though since they make mixing, dosing and pressing very difficult.

[tti82phys]

Thanks for the reply. There definitely is a problem with mixing when adding metal wires in a powder, and the burn rate could be somewhat unpredictable depending on how the wires are positioned inside the mixture. Unless they are fixed in place to opposite edges of a container.

 

If it's possible to convert a charcoal-based pyrotech mixture to a faster burning one with added magnesium wires, you would get around the problem of needing respiratory protection when handling atomized aluminum powder or other metal dusts.

 

Edit: here there's an article (someone's master thesis) that I hadn't noticed before, about increasing the burn rate of solid rocket propellant with the addition of heat conducting wires: https://apps.dtic.mil/sti/pdfs/AD1076429.pdf

Edited December 14, 2022 by tti82phys

[cmjlab]

Just to hazard a guess here - but based on the other items in pyrotechnic comps, that require some form of respirator or protection, that may be why it hasn't been further studied in this field. It's still interesting, and could likely be put to use in some fashion to improve different pyrotechnic devices. I'm curious to hear more.

 

Charles

[tti82phys]

If you can cast a solid pyrotechnic mixture in the shape of rectangular blocks, it's also possible to pile up a few layers of those with 0.005 mm thickness magnesium foil added in between them. That way you would circumvent the problem of imperfect mixing.

[Crazy Swede]

Remember that thin wires or foils of magnesium would be very sensitive to corrosion. Unless treated with dichromate, that is banned today, they would not survive very long and would also be a risk of spontaneous ignition! Edited December 15, 2022 by Crazy Swede

[Arthur]

I have once seen a fuse that had two continuous thin wires along its length. The thermal conductivity of the wires was enough to assist the rate of burn

[tti82phys]

I would like to see an experiment where BP or a KClO3-icing sugar mixture is packed in this type of a steel wire mesh

 

 

and ignited to find out the new burn rate in that situation. The effect of faster heat conduction would be likely to be quite large if there are enough metal wires through the material. Just watch out how large gram amounts you ignite in these experiments - I believe it's really possible that even the normally slow burning mixtures can detonate without confinement if you scale up this experiment too much.

[Crazy Swede]

Heat conduction can accelerate the burn rate to a certain degree but then you reach steady state and nothing more happens. This is connected to the fact that it takes both some heat and time to ignite unreacted pyrotechnic composition and the only thing heat conduction does is the pre-heat the layer below the combustion zone. Also, if too much metal is introduced you both dilute the composition and make it fuel rich.

 

Adding metal to a composition can make it more difficult to ignite, since heat is transferred away from the ignition surface, but it often burns faster once ignited from heat conduction but more important from the higher combustion temperature.

 

On the other hand, surprising effects have been seen with nanomaterials so who knows how deep the rabbit hole goes?

 

The whole discussion has very little practical application though since pyrotechnics needs to be simple to produce and control.

[tti82phys]

The whole discussion has very little practical application though since pyrotechnics needs to be simple to produce and control.

This is true, it's not likely that most of this has much use for practical fireworks manufacturing, at least immediately. As a scientist I just find this more interesting than the visually spectacular sparkle effects that pyrotechnics are for. The magnesium and zirconium metals participate in the combustion reaction if you add wires of those in pyro compositions (there's a YouTube video, findable by the search engine, where someone shows how zirconium can be ignited with a gas burner similarly to a magnesium ribbon), and that will also speed up the burning of the mixture along with enhanced heat conduction. I'm not sure how large part of flash powder's exceptionally fast burn rate is because of it containing a metallic heat conductor as fuel, and how much because the high reaction temperature of Mg or Al ignites other metal particles from a longer distance. The modelling of heat conduction is also important when considering combustion reactors for energy production. BTW, a framework similar to the image in my previous post could also be made of some exceptionally fast-burning fuse to cause ignition all over the material. Or put a high enough electrical voltage across it to heat it to incandescence at all locations at the same time.

 

It will take months until I will have a possibility to play with this kind of metal wire enhanced mixtures myself, in the case that I care enough to perform that kind of experiments myself. I will tell then what happened if there's something worth of mention.

Edited December 15, 2022 by tti82phys

[cmjlab]

If you are going to go down this road, you should look into the reactive metal wires (where two metals that separately not very spectacular, but when heated to melting temperature and "alloyed" together - have a vigorous exothermic reaction).

 

If studying the effect of different metals (in various forms) on the rate of deflagration of a pyrotechnic composition - then this ought to lead to some interesting data.

 

Edit: Palladium Clad Aluminum Wire was one of the wires I was thinking of (not a bridge wire like is used in detonators).

 

Charles

Edited December 18, 2022 by cmjlab

[Crazy Swede]

Yes, the intermetallic reactions should be of interest for tyi82phys!

 

Pyrofuze (Pa/Al) is the most well known and is extremely energetic. Al/Ni works too but is less spectacular.

[tti82phys]

As a physicist, I'm most interested on questions like how to optimize the geometry of metal wires going through the mass of fuel-oxidizer mixture to produce as high combustion rate as possible. Presumably this would mean minimizing the average distance to closest wire, but I would rather calculate that distance for several alternative geometries and compare the results than solve this as a variational calculus problem. The wire thickness is another important variable and it's not easy to guess the optimal one by any easy considerations. In the situation of a metal rod through a cylindrical roman candle, I would expect that it's best that the rod is less thick at the beginning than at the end to not lose too much of the initial released combustion energy to heating the metal. If some researcher were to attempt to investigate this thorughly, it would take a lot of experiments and burning rate measurements. The point of ignition also has effect on the rate, and igniting the mixture from the middle would clearly be optimal for this purpose.

 

Aluminum wire can't be ignited with a gas burner flame like you can ignite magnesium, as far as I know, and the reason seems to be that it's too good a heat conductor for enough localized heating to start the reaction. So I wouldn't attempt this combustion acceleration with aluminum wires.

Edited December 18, 2022 by tti82phys

[cmjlab]

You are correct, a torch would likely melt an Aluminum wire long before catching it on fire. However, the theory behind the Palladium Clad Aluminum wire is that a strong current is passed through a thin enough Aluminum wire, and will instantly melt the wire (similar to the bridge wires used in detonators - not the bridge wire in an ematch).

 

If that Aluminum wire is clad in a Palladium skin (and there are other combinations as well like the Al/Ni mentioned above), also in a thin enough layer to melt from the heat generated by the current and liquid Aluminum, then the two metals will turn to liquid and react in a exothermic reaction / deflagration. As Swede mentioned above, the better known brand is Pyrofuze.

 

That is the extent of my understanding of theory behind the "reactive" fuse wires, but they would eliminate several of your concerns you listed above (i.e. ignition point, heat sink / heat loss, etc.). I would also imagine that based on the way it functions, the sympathetic heat / deflagration would help to speed the ignition as it continues to react.

 

I'm also not a physicist..... Just a weekend pyro hobbyist with a phone! You (and Swede for sure based on the previous posts of his I've read) have much more knowledge on these things I'm sure! :-)

Edited December 19, 2022 by cmjlab

[Crazy Swede]

...Aluminum wire can't be ignited with a gas burner flame like you can ignite magnesium, as far as I know, and the reason seems to be that it's too good a heat conductor for enough localized heating to start the reaction...

That is only one part of the explanation. The thick, non-porous and tough coating of Al₂O₃ on the surface of aluminium is what keeps it from lighting and propagating as easy compared to magnesium. When heated, the superficial alumina layer quickly gets thicker and can act like a flexible bag, holding liquid aluminium inside that is protected from further oxidation.

 

To get around this problem, a fluorine containing flux additive (CaF₂ for example) can be used to strip the aluminium from its protective layer and lower the activation energy for ignition. (This is a good trick to allow coarse aluminium shavings to ignite from compositions that normally burns at too low temperature for their ignition!)

[tti82phys]

To get around this problem, a fluorine containing flux additive (CaF₂ for example) can be used to strip the aluminium from its protective layer and lower the activation energy for ignition. (This is a good trick to allow coarse aluminium shavings to ignite from compositions that normally burns at too low temperature for their ignition!)

Thanks for telling me this. I will try that some day next year if I can find calcium fluoride easily enough.