Difference between revisions of "Flash powder"

Revision as of 10:39, 9 July 2016

Flash powder is an energetic composition consisting of a metal fuel and a solid oxidizer. It is commonly used in pyrotechnics as bursting charge for firework shells and report composition. The most widely used mixture consists of 70% potassium perchlorate and fine aluminium powder.

Preparation

Flash powder is prepared simply by mixing the two or three components. Care has to be taken to avoid excessive friction and not risk creating any electrostatic discharges. This means a flash powder cannot be ground after mixing. Components must be ground separately. An effective way to mix flash powders is by gently rolling the finely divided components in a small container for a long period of time, similar to how to a rock tumbler works, but without grinding media.

Particle size

As in all pyrotechnic mixtures, the particle size of the components is of greatest importance. A mixture that hardly burns when the components have a certain particle size may explode violently with the particle size reduced to a tenth. When aluminium is used as fuel, a powder finer than 5 µm should be used for best effect. "German Dark" aluminium (3 µm flake shaped) is generally considered to be the best option. Magnesium and magnalium don't require as fine powders; 20-40 µm is usually appropriate for most applications.

Common flash powder compositions

Potassium perchlorate and aluminium

This is the standard flash powder of the pyrotechnic industry. It combines insensitivity and good stability with great power. Common ratios are 7:3 and 2:1 by mass. Sulfur can be added for increased potential as report composition.

Potassium chlorate and aluminium

The Potassium chlorate flash powder is very similar to the one above, but with considerably higher sensitivity. This flash powder is more brisant than its perchlorate counterpart, so it gives louder reports. A ratio of 70% KClO₃ with 30% Al is close to the stoichiometric. Sulfur and acids must be avoided!

Nitrate and magnesium

Potassium nitrate or sodium nitrate can be used with magnesium powder as a moderately powerful flash powder. The bang from these compositions is not as sharp as those from chlorate and perchlorate based mixtures. Neither are they particularly sensitive, but the sodium nitrate flash powder suffers from the hygroscopic nature of NaNO₃. Ratios can be varied from 6:4 (KNO₃:Mg) to 1:1. The best ratio for NaNO₃/Mg is 45% sodium nitrate and 55% magnesium. Sulfur can be added to these compositions.

Potassium nitrate, aluminium and sulfur

This is one of the slower flash powders in use. To have any success, the aluminium must be very fine and the other ingredients well ground. It is also very insensitive, but 1-2% boric acid must be added to increase its stability, which is otherwise bad in humid air. Commonly used ratios are 5:3:2 (KNO₃:Al:S), 6:3:2 and 5:2:3.

Exotic flash powder compositions

The following mixtures do not find any use in fireworks; they are generally very sensitive and/or unstable, as well as much more powerful than the regular ones listed above. Some may have other interesting properties. Amounts larger than 1 gram should never be prepared!

Ferrocerium and oxidizer

Ferrocerium is an alloy of rare earth metals (mainly cerium and lanthanum), iron and small amounts of magnesium. Finely powdered ferrocerium is a much more reactive fuel than magnesium and aluminium, and therefore mixtures with oxidizers tend to burn much faster than other flash powders. Minimal amounts will self-confine, i.e. explode with a loud bang in open air. This superior reactivity is not without downsides: all mixtures containing ferrocerium are very sensitive to impact and friction, and it will quickly degrade even in dry air.

Potassium permanganate and metal fuel

Potassium permanganate in combination with magnesium or aluminium are exceptionally fast-burning flash powders. They are very unstable however, and are not recommended.

PTFE and magnesium

This flash powder finds its use in IR-flares. It burns quickly and has extraordinary energy content. Its unique property is that it burns faster with increasing magnesium content, up to a certain limit of course. The stoichiometric ratio is 67% PTFE to 33% Mg, but a 1:1 ratio is superior. This mixture can be tricky to prepare and is sensitive to static electricity. Using ytterbium metal instead of magnesium metal will make an emerald-green flame and dramatically increase the infrared output (which is of interest to military forces around the world).

Persulfate and magnesium

Potassium persulfate with magnesium is an extremely powerful flash powder, even more so than permanganate mixtures. Amounts below one gram will explode violently unconfined as long as the magnesium can be called "powder". With very fine magnesium, as little as 5 mg is enough. Despite this, K₂S₂O₈/Mg is not particularly brisant and does not give too sharp reports. It is also surprisingly insensitive. It's stability is unknown, so mixtures should not be stored.

Safety

Flash powders are the most dangerous pyrotechnic compositions due to their relatively high sensitivity and great burn rates. Amounts, handling and storage must be minimized. Care must also be taken to avoid situations that can generate static electricity, something many flash powders are exceedingly sensitive towards.

References

• Dornier335A's personal experience.
• Flash powder, Wikipedia.
• Ferrocerium, Wikipedia.

Relevant Sciencemadness threads

• Flash Powder
• Flash powder detonation
• Rare earth flash powders!
• HE + Flash Powder