May 19, 2005

Green Explosives 

Yes, this is another long-awaited instalment in my explosive science series.

The idea of non-polluting explosives may seem a nonsense on first sight, but it's not so far fetched. When traditional explosives detonate, they also produce a certain amount of rather nasty chemicals - the black smoke of explosions (at least certain explosions) indicates the presence of particulate, which isn't the healtiest stuff; and there are other substances. The people most exposed to this form of pollution are the soldiers who use ornance for both training and combat, and the civilians in the operations area. Reducing the pollution from military operations is beneficial for both the friendly soldiers and the innocent* civilians (although, like all good intentions, sometimes it can backfire).

Basically (and a bit imprecisely, but bear with me) the energetic part of an explosive molecule are the nitrogen-oxigen and nitrogen-nitrogen bonds, while the carbon atoms work more like "fuel" and produce those toxic chemicals. So the explosives' scientists came up with the idea of producing explosives containing more nitrogen and less carbon - and nitrogen-nitrogen bonds. The ultimate molecule I've seen is constituted by two six-membered rings of nitrogen atoms joined by one vertex: it's almost totally nitrogen in solid form, with only four petty hydrogen atoms at the extremities (if I remember correctly, because I've seen that structure only quite a while ago): upon detonation, it will produce only nitrogen gas (and a little water vapour), that is innocuous. Also compunds containing carbon chains or rings in tensioned configurations contain a lot of energy per unit of mass, because energy is required to constrain the atoms in these unnatural configuarations, and it will released upon detonation. A typical tensioned hydrocarbon is cubane (I think it was experimented as a F1 car fuel) , and indeed octanitrocubane is a powerful explosive.

As a bonus, some of these newer green explosives are also more powerful that the traditional RDX-based ones. The drawbacks (there always are drawbacks) are that these exotic compounds are still difficult to synthetize and at the moment are much more expensive than traditional ones.

Not to mention current primary explosives (those used in blasting caps and initiators) often contain lead, and upon detonation this lead is released as fine particles of metal or its salts - it appears that much of the lead contamination at firing ranges is due to the lead azide of the caps, not to the bullet fragmentation.

Other concerns regard the fact that TNT, HMX etc are toxic by themselves, and their production leaves a great amount of toxic waste and spent but contaminated acidic solutions - although even green explosives may not be the best in this regard. Moreover, destruction of expired ordnance (which is usually done by open-air burning) poses environmental problems.

Other researchers, instead, are considering the use of metal (aluminium) nanoparticles as explosives and propellants, because the size of these particles can be adjusted in order to produce the desired combustion (or detonation) rate. Maybe these formulations are not exactly green in the widest sense, but they may allow to do away with lead and other heavy metals.

Ordinary rocket propellants often contain aluminium powder and ammonium perchlorate, which make a good propellant but are also polluting: better solid propellants would be a bonus. Actually, liquid oxigen/liquid hydrogen is a clean propellant, but those are probably the most dangerous cryogenic liquids to transport, store and use - maybe only liquid fluorine is worse.

*At least, we normally suppose civilians are innocent...


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