Sciencemadness Discussion Board

Explosives Initiation Doubt

IanCaio - 19-3-2013 at 07:46

Hey guys,

I was having thermodynamics class in the university this morning, and a doubt came to me about explosives initiation.
I'm making a few assumptions when this question crossed my mind, so feel free to correct me if any of them is wrong:

1 - There are explosives that are sensitive to shock and sensitive to temperature; The ones that are sensitive to temperature but not to shock; and the ones that are sensitive to shock but not to temperature

I'm a little apprehending about the last part (in red)

2 - The shock initiated explosives obtain their energy from pressure that converts itself to "intern" energy

3 - I'm assuming also that what actually initiates the explosion is the total energy transferred to the energetic matter, which should be equal to the explosion reaction activation energy

This is the assumption that I'm most certain of. It's basically part of the foundation of Chemistry kinects.

So to the question:
How does the way that energy comes makes a difference in the initiation if what really matters is the ammount of energy the energetic material gets?

My actuall theory is:
Maybe the pressure transfers energy as friction between the material itself, and maybe the structure (crystals having more friction between them for example) would determine why they are more sensitive to the friction, but after all the total energy obtained to the initiation would be the same as if it was obtained by thermic energy.

So the most sensitive primary explosives would be a combination of low activation energy and high intern friction.


But this would lead me to another question: How can nitroglycerin be so sensitive if its a liquid, and in theory its internal friction should be lowered?

Sorry if I made the post confusing, I'm kind of still assimilating the information. Could anyone more experienced give me a light?

Thanks

watson.fawkes - 19-3-2013 at 07:56

Quote: Originally posted by IanCaio  
How does the way that energy comes makes a difference in the initiation if what really matters is the ammount of energy the energetic material gets?
From the examples cited, you should conclude that it is not the energy input as such that matters, but instead that the means by which it arrives does make a difference. From this you should also conclude that there is not a single activation energy, but rather multiple ones that depend on energy mode.

Ral123 - 19-3-2013 at 11:54

and the ones that are sensitive to shock but not to temperature: lead azide

You can deliver "100x the activation energy" in the form of a firecracker. Usually that wont achieve efficient detonation.
You can deliver "1/100 the activation energy" in the form of micro laser that ensures everything in a microscopic point is heated to 5000C and atomized. That also wont work/it equivalent of high quality No8 cap in anfo.
You can palace normal No8 cap in pressed picric acid. Fast and efficient energy transfer. That ensures the cap and some of it's surroundings will give off their energy(fast). If that "system" has higher diameter then the critical diameter of the explosive, the detonation wave will continue in all the volume of the charge.

IanCaio - 20-3-2013 at 20:46

Thanks for the answers both of you.

So the way this energy gets into the system does make a difference? This is a particular caracteristic of explosives or most reactions follow this "rule"?

Can I conclude that the reaction only needs an activation energy to begin, but some materials convert the thermic energy, or the "pressure energy" more easily into "intern" energy (which would be the one that counts)?

Ral123 - 20-3-2013 at 22:37

You can input energy to initiate a ton of picric acid or much more energy to melt that ton :D
The energy of the material converts to heat and kinetic energy.

watson.fawkes - 21-3-2013 at 05:39

Quote: Originally posted by IanCaio  
So the way this energy gets into the system does make a difference? This is a particular caracteristic of explosives or most reactions follow this "rule"?
This is far more characteristic of explosives than other areas, but it's not unknown elsewhere. For example, radical initiation by UV light relies on photon energy, not heat. In electrochemistry, there are reaction initiated by a potential difference, not heat. In "most" chemistry, though, it's heat that matter, and that's the ordinary subject of thermodynamics.
Quote: Originally posted by IanCaio  
Can I conclude that the reaction only needs an activation energy to begin, but some materials convert the thermic energy, or the "pressure energy" more easily into "intern" energy (which would be the one that counts)?
What evidence to have for concluding that? You're assuming that there's only a single kind of internal energy. Bond vibration, lattice excitation, excited electron states, and others are all forms of internal energy. In thermal equilibrium, there's a stable partition of internal energy between all the different forms. Equilibrium states, however, have little to do with reaction dynamics, which is all about transient states.

There are some very interesting physical chemistry issues here, or maybe they're chemical physics issues. Regardless, there's lots of physics content you'll need to get answers to what you're seeking, and many of the questions may still be open for research. The entry point into the field is called statistical mechanics, which is typical first taught as an upper division undergraduate course.

IanCaio - 21-3-2013 at 07:22

Thanks for the answers again :)
Quote:

Ral123:
You can input energy to initiate a ton of picric acid or much more energy to melt that ton :D
The energy of the material converts to heat and kinetic energy.


Indeed, another example of a primary more easily initiated by shock, but hardly initiated with heat.
Quote: Originally posted by watson.fawkes  
This is far more characteristic of explosives than other areas, but it's not unknown elsewhere. For example, radical initiation by UV light relies on photon energy, not heat. In electrochemistry, there are reaction initiated by a potential difference, not heat. In "most" chemistry, though, it's heat that matter, and that's the ordinary subject of thermodynamics.


Sure! There are reactions activated with UV lights, or even electric potential (i.e.: water electrolisis) that wouldn't be activated with the same energy applyied in heat. Thanks for pointing that out, didn't think of it.
Quote: Originally posted by watson.fawkes  

What evidence to have for concluding that? You're assuming that there's only a single kind of internal energy. Bond vibration, lattice excitation, excited electron states, and others are all forms of internal energy. In thermal equilibrium, there's a stable partition of internal energy between all the different forms. Equilibrium states, however, have little to do with reaction dynamics, which is all about transient states.

There are some very interesting physical chemistry issues here, or maybe they're chemical physics issues. Regardless, there's lots of physics content you'll need to get answers to what you're seeking, and many of the questions may still be open for research. The entry point into the field is called statistical mechanics, which is typical first taught as an upper division undergraduate course.


The problem seens to have a bigger dimension than I imagined at first :P