Yet again:
kylebisme said:
What has more energy by weight is only part of the equation. As for the rest, I'll give you a clue; consider why the military brings thermite grenades into battle rather than wood grenades.
Yes, why indeed? A good question, and if you really knew the answers, you would be half a step closer to seeing several of the things that are wrong with the Bentham paper.
Because
a) Thermite burns without ambient oxygene, and can't be quelled by water, sand that are effective against wood fires. It is for this reason that we keep saying that
Farrer/Harrit/Jones should have done their DSC test under inert atmosphere
b) the thermite reaction reaches a very high
temperature (and can melt through concrete and steel), despite it releasing less
heat than wood. Do you know the difference between the two? I'll tell you:
Temperature is a statistical thing, something about
local distribution of the kinetic energy of molecules. Loosely speaking, something has a high temperature, if the molecules move very fast on average.
Heat can be described as a form of
energy and is
limited by the universal Law that energy is conserved. Loosely speaking, the heat of an exothermic reaction can be seen as the product of the temperature of the reaction products times their volume (you also have to factor in the conversion of physical states). Now, thermite reaction reaches a lot higher temperature than organic combustible fires because its reaction products are liquid, whereas organics largely end as gasses (CO2 and H2O being the most common). Gasses have a much larger volume than liquids, and this way, the heat of wood fires gets dissipated immediately, resulting in lower
local temperatures of the reaction products.
However, the total
energy release (heat) of the reaction is limited strictly by the enthalpy of the reaction - in other words, the energy stored in chemical bonds. The enthalpy is given in J/g (or more commonly kJ/g, or the equivalent MJ/kg). A reaction with an enthalpy of 3.9 kJ/g cannot possibly release more heat energy than 3.9 kJ/g. If you measure more than that, you have absolute, unambiguous, uncontrovertible proof that your reactants did NOT have an enthalpy of 3.9kJ/g. This is totally, absolutely indepent of the speed of the reaction. Whether it took your sample 1ms to burn or a day - when you burn 1g, you get at most 3.9kJ of energy.
Now, 3.9kJ/g is the enthalpy of the thermite reaction (Fe2O3 + 2Al -> Al2O3 + 2Fe). If you have 1g of perfectly mixed pure thermite, and burn it, the absolut maximum heat you get out of that is 3.9kJ. If you get 6 or 7.5kJ, you immediately know that
your sample wasn't pure thermite. The
main reactants were NOT thermite. You have thus proven that soemthing else, that is not thermite, has burned in your sample. It could be wood (15kJ/g), it could be paper (17kJ/g), it could be plastics (24-38kJ/g), it could even be human tissue (10kJ/g). it coul be epoxy resin, jet fuel, nylon, paraffin, any number of things. But
it can't possibly be thermite. Impossible. The laws of this universe would be broken if those 6 or 7.5kJ/g came from a thermitic reaction.
I hope you get it now.
The speed of the reaction (energy release rate = power) may be an interesting property, and may be used to rule out some materials. For example, I'd guess that human tissue could be ruled out by a very fast reaction because it would first have to cook off the water, and that takes time. The same is probably true for wood. Less so for paper. So I think we can agree very quickly that Harrit/Farrer/Jones did not burn paper or wood in those red-grey chips.
But it may well have been epoxy resin, or some such stuff.
Unfortunately, they didn't even start to analyse the organic matrix. Mind you, they did find organic material in their chips, they mention it in the paper, but pass that info by until the recommendations for future research.
Since they have these dust samples, I wonder why they have not done that research.
