Sciencemadness Discussion Board

Initiation of an energetic material without air pockets

dangerous amateur - 5-6-2015 at 08:46

Imagine you have a casted HE or something that consists of one single or a few large crytals without air pockets.

Since there are no voids to get compressed and superheated, what mechanism makes it possible to detonate such material?

How is the energy delivered to decompose the material?

PHILOU Zrealone - 6-6-2015 at 07:56

Very easy! That's a bigginer question for the short question thread!

--> Heat, friction, shock and/or radiation.

It is all a matter of activation energy and of enthalpy/energy of explosion with relation to enthalpy/energy of formation of the réactants (the explosive or explosive mix).

[Edited on 6-6-2015 by PHILOU Zrealone]

Gargamel - 6-6-2015 at 08:42

I think heat and friction are responsible.

Shock? Doesn't shock rely on air bubbles? As long as shock is only transmitted trough a material it does not do work...?

Dany - 6-6-2015 at 09:55

Quote: Originally posted by dangerous amateur  
Imagine you have a casted HE or something that consists of one single or a few large crytals without air pockets.

Since there are no voids to get compressed and superheated, what mechanism makes it possible to detonate such material?

How is the energy delivered to decompose the material?


The are two mode of initiation and propagation of detonation which operate in an explosive: the Homogenous and the Heterogenous mode.

By an heterogenous explosive we mean an explosive that may contain interstice, voids, air pockets, binders (such as PBX's)...On the other hand a homogenous explosive is one which lacks the above carachteristics. Pure liquid explosives on one hand and single crystal explosive fall into this category.

The initiation and propagation of a detonation in a heterogenous explosive is dominated by the Hot spot mechanism. The hot spot generated at the voids, air pockets location provides high temperature which assure the initiation and subsequent formation of a detonation wave.

In the homogenous mechanism the things are much different than the one seen for the heterogeneous one. It has been seen that for a shocked homogenous explosive little or no reaction occurred. After the shock wave has travel a certain distance in the explosive (i.e. the distance is a function of the peak pressure of the shock wave, the higher the pressure the smaller the distance is) a superdetonation is initiated at the explosive boundary which propagate in the already shocked explosive. After few microseconds, the initial shock wave and the superdetonation initiated at the explosive rear boundary coalesce and form a single detonation wave which travel first at higher speed than that predicted by the Chapman-Jouguet theory. We call the detonation an Overdriven detonation. However, the overdriven detonation will decay back the the normal speed which should operate in a given explosive. The shock wave in a homogenous explosive must be very strong in order to break bonds in explosive molecules. It is widely accepted that the first bond broken by the passage of the detonation wave is the molecule weakest bond. We call this bond the Trigger linkage. The propagation of detonation via transport mechanism such as convection, conduction and radiation is negligible. Transport mechanism dominate the propagation of a deflagration wave and not a detonation wave.

More useful informations can be found here:

http://www.sciencemadness.org/talk/viewthread.php?tid=529&am...

Dany.

dangerous amateur - 24-6-2015 at 02:44

Thank you Dany.
I'm reading...

roXefeller - 1-7-2015 at 18:10

+1 for a good question and another +1 for the responses.

PHILOU Zrealone - 2-7-2015 at 03:35

Other points that may explain initiation without air pockets are:
-crystal latice breaking ; with initiators large crystals often are more sensitive than tiny ones because the chances of a inner stress (error in the latice) increases with the size.
-piezo-electricity --> with pressure high voltage potential develops on sides of the crystals
-tribo-electricity and luminescence --> with crystal smaching/breaking sparks, light and electric voltage develops