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Old 21st March 2011, 11:25 PM  
R.Mackey
Philosopher
 
Join Date: Apr 2006
Posts: 7,854
An explosive is a substance that, when initiated, undergoes an exothermic reaction creating a pressure wave. The reaction is also understood to be self-sustaining, where the pressure of the explosive sets off the rest of the explosive.

There are two types of explosions: Detonation, where the pressure wave created is in fact a shock wave, viz. a sharp discontinuity in pressure that travels greater than the speed of sound (in the explosive, not in the open air); or a deflagration, where the pressure rise has a finite "thickness," travelling approximately at the speed of sound. Whether your explosive detonates or deflagrates depends on whether there is a unique solution of the Rankine-Hugoniot curve (which relates conditions ahead of and behind the shock as it passes through the explosive) and the Rayleigh Line, which describes a state condition where momentum is conserved (required for the reaction to be self-sustaining or better). This solution, the so-called Chapman-Jouget Point, defines the speed of the explosion as it develops in the explosive, and determines whether it is supersonic or merely sonic.

Like explosions, explosives are generally categorized into two types. The first are the so-called "low" explosives, such as black powder, which deflagrate rather than detonate. The second, the "high" explosives, actually detonate. This difference has a profound effect upon the target.

Note that subtle changes in composition or geometry can change how an explosive behaves. Perhaps the best known example is a Fuel-Air Explosive (FAE). If you get the air mixture just right, you can have a true detonation, causing extensive destruction. In general, however, you only get a deflagration, as being slightly fuel-rich slows down the reaction and "smears" the pressure pulse.

If the mixture is way off, it won't explode at all, but merely burn in a big smoky fireball. This burning can still seem like an explosive, but since the reaction is now being driven by thermal effects rather than compression, it is really not an explosive any longer. This also happens in firearms and in your car -- in both of these applications, we want the chemicals to burn, but if conditions are wrong they can detonate (that's what knocking is in your car) and damage the device.

There are two ways to measure the "power" of an explosive. One is by energy content, i.e. the total amount of energy released by the chemical reaction, typically expressed in terms of energy per unit mass -- Joules per kilogram. The other is in terms of the speed of reaction, a quantity sometimes referred to as "brisance." Both of these quantities are determined through experiment. There is no strict correlation between them -- Ammonium nitrate-fuel oil (ANFO) has a higher energy content than TNT, for instance (about 6.3 kJ/g versus 4.6 kJ/g), but TNT has higher brisance with a propagation speed of 6900 m/s vs. 5200 m/s for ANFO. Both are classified as high explosives, but they perform differently. TNT would be more useful for shattering reinforced structures or solid rock, while ANFO is more appropriate for a "cratering charge," or moving large volumes of material.

With that out of the way, let us finally turn to "nanothermite." There is such a thing, as it has been described in about two dozen scientific papers, but it has only been produced in minute quantites. Strictly speaking, nanothermite is not an explosive at all. The reactants undergo a redox reaction but only generate heat, and their reactants are molten metal oxides rather than gases. They generate no increase in pressure whatsoever. Thus, there is no Rankine-Hugoniot curve in the first place.

It has been suggested that nanothermite may be manufactured with a buffer material, and this material -- while not taking any part in the chemical reaction -- might absorb some of the heat of reaction and flash to a gas phase, kind of like a steam explosion. Indeed, laboratory nanothermite does contain trace amounts of nonparticipatory organics in order to form a "xerogel," i.e. a highly porous plastic that contains the reactants in numerous microscopic cavities, however the mass of this substrate is negligible and additional inert matter would be needed for any significant pressure effect.

IF this were done, the resulting hybrid material would still not be classified as an explosive, since the reaction would be self-sustaining only through thermal effects. Thus it would only burn. But it would be unique in that the burning speed could be supersonic.

If we ignore this technicality and measure it like an explosive, we find that nanothermite would be classified as a low explosive. This is because boiling the inert material will be a slow reaction and cannot happen fast enough to support a shock wave. In fact, the presence of the shock wave will itself try to compress the inert material back into a solid... Thus, we must see a gradual pressure rise, just like a low explosive. Paradoxically, the most effective nanothermite "explosive" would be where the reaction speed is tuned, by varying particle size and surface condition, to slow down the reaction front to exactly sonic speed, otherwise the reaction front will actually outrun the "steam" expansion and spread out the pressure front. Very weird stuff.

In terms of energy content, nanothermite has been measured to have only about 30% as much energy as normal thermite. This is because the tiny particles of aluminum invariably acquire an inert oxide coating, and the nanoparticles are so small that the coating is, in fact, a large fraction of their total mass. This leaves nanothermite with an experimentally measured energy content of about 1.5 kJ/g, or much lower than any known true explosive. But that's just for the thermite reactants. We would have to subtract from this number to account for our "steam," which will absorb energy equal to its heat of fusion vaporization. We are not likely to have much left over.

So, in conclusion:
  • Nanothermite is not an explosive at all.
  • Any attempt to make it behave like an explosive will not be powerful by any definition.
  • At best, a nanothermite derivative would be a "low" explosive.
  • The energy content of this derivative is lower than any known explosive.
  • The brisance of this derivative would be < 1 km/s, among the lowest of any known explosive.
  • Figures in the Truth Movement, attempting to claim otherwise, are liars.

Last edited by R.Mackey; 21st March 2011 at 11:55 PM. Reason: Typo, as marked
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