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Submitted on 1 Jan 1987
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WHY DOES AN EXPLOSIVE EXPLODE ?
S. Odiot, M. Peyrard
To cite this version:
S. Odiot, M. Peyrard. WHY DOES AN EXPLOSIVE EXPLODE ?. Journal de Physique Colloques,
1987, 48 (C4), pp.C4-393-C4-395. �10.1051/jphyscol:1987429�. �jpa-00226668�
JOURNAL DE PHYSIQUE
C o l l o q u e C4, suppl6ment a u n'g, Tome 48, septembre 1987
WHY
DOES AN EXPLOSIVE EXPLODE ?S. ODIOT and M. PEYRARD'
D.R.P., UntversitQ Pierre et Marie Curie, Tour 22, 4 place Jussieu, 75252 Paris Ceder 05, France
'~aboratotre O.R.C., Faculte des Sciences, 6 boulevard Gabriel, 22100 Dijon, France
A t a macroscopic l e v e l , t h e d e t o n a t i o n phenomenon i s w e l l known. I n t h i s aspect, how an e n e r g e t i c m o l e c u l a r c r y s t a l detonates has been d e s c r i b e d p e r f e c t l y . B u t t h e q u e s t i o n as t o why one m o l e c u l a r compound d e t o n a t e s and why a n o t h e r one does n o t i s s t i l l a r e a l fundamental problem i n s p i t e o f t h e t e c h n i c a l p r o g r e s s b o t h i n u l t r a f a s t spectroscopy and i n super computer s i m u l a t i o n s o f m o l e c u l a r e l e c t r o n i c s t a t e s and m o l e c u l a r dynamics.
However, we s h o u l d l i k e t o emphasize t h e f o l l o w i n g f a c t p r e v i o u s l y d i s c u s s e d (PEYRARD
...
paper) i n t h e two dimensional m o l e c u l a r dynamics model : as much as i t i s w e l l e s t a b l i s h e d t h a t t h e e l e c t r o n i c m o l e c u l a r bond s t r u c t u r e i s i n v o l v e d i n t h e d e t o n a t i o n , t h e c r y s t a l s t r u c t u r e seems a l s o v e r y s t r o n g l y i m p l i c a t e d i n t h e phe- nomenon because i t c o n t r o l s t h e energy d i s s i p a t i o n f a c t o r which i s m a i n l y d e t e r m i n - ed by t h e balance between t h e energy r e l e a s e when molecules d i s s o c i a t e and t h e energy t r a n s f e r r e d i n t o t r a n s v e r s e o s c i l l a t i o n s ( 1 ) .As shown i n F i g . 2 and 3 ( c r y s t a l s t r u c t u r e e f f e c t ) b o t h t h e dimensions a and b o f t h e u n i t c e l l a l o n g t h e X and Y axes ( F i g . 1 ) and t h e o o s i t i o n o f t h e i n t r a m o - l e c u l a r bond NC a x i s w i t h t h e shock wave d i r e c t i o n : a a n g l e i n F i g . 1, p l a y a r o l e i n d e t e r m i n i n g i f a 20 l a t t i c e can s u s t a i n a d e t o n a t i o n .
Figure 1
( 1 ) I n t h e case o f nitromethane, i t has been shown by a quantum t h e o r e t i c a l a ~ n r o a c h t h a t no energy i s d i s s i p a t e d i n t o v i b r a t i o n a l modes o f t h e n i t r o m e t h a n e f r a g - ments d u r i n g t h e " a c t i v e " e x c i t e d e l e c t r o n i c s t a t e 1~~ d i s s o c i a t i o n .
Ph. MILLIE e t S. ODIOT, 1987, J. Phys. Chem. 91, 1397.
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987429
JOURNAL D E PHYSIQUE
The l i m i t d e t o n a t i o n v e l o c i t y v depends on t h e r a t i o a/b which a f f e c t s t h e way t h e energy i s shared between l o n g i t u d i n a l and t r a n s v e r s e m o t i o n s . ( F i g . 2)
Figure 2
The o r i e n t a t i o n a o f t h e CN i n t r a m o l e c u l a r bond a g a i n s t t h e shock wave p r o p a g a t i o n a x i s appears as a d e t e r m i n i n g f a c t o r , c o r r e l a t e d t o t h e s e n s i t i v i t y t o shocks a , t o i n d u c e a d e t o n a t i o n wave. ( F i g . 3 ) . Here, f o r nitromethane, a a n g l e must be 45'
- 6 0
a1.1
Figure 3
T h i s l a s t r e s u l t agrees w i t h t h e e x p e r i m e n t a l r e s u l t s showing a n i s o t r o p i c b e h a v i o r s i n PETN and RDX monomolecular c r y s t a l s (see SANIRANT, DUF09T1s oaper and J. DICK, J. Appl. Phys. L e t t . 44, 859 (1984)).To o u r knowledge, n i t r o m e t h a n e i n s o l i d ~ h a s e does n o t e x p l o d e ( S N P U whereas i t does i n l i q u i d phase. I~le th i n k t h a t perhaps i n t h e c r y s t a l t h e C-N f i x e d bond c h a i n s p r e v e n t t h e d e t o n a t i o n and t h a t i n t h e l i q u i d , i f such supposedly f r o z e n c h a i n s would e x i s t , t h e s e a r e i n p e r p e t u a l r e o r i e n t a t i o n and t h u s , t h e a v a l u e s which s u s t a i n t h e d e t o n a t i o n do appear f r e q u e n t l y .
We suggest t h a t i t would be i n t e r e s t i n g :
1
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t o observe by t h e m o l e c u l a r dynamics 2D s i m u l a t i o n s , how t h e d e t o n a t i o n wavep r o p a g a t i o n can be m o d i f i e d b y i n t r o d u c i n g i n t h e c h a i n some v a r i a t i o n i n a,
2
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t o pursue s y s t e m a t i c e x p e r i m e n t a l works i n o r d e r t o p r o v e i f i t does e x i s t a values w h i c h s u s t a i n and a v a l u e s w h i c h do n o t s u s t a i n a d e t o n a t i o n i n a s i n g l e m o l e c u l a r c r y s t a l . ( i f p o s s i b l e ! n i t r o m e t h a n e )A1 1 these m i c r o s c o o i c r e s u l t s would qua1 i t a t i v e l y account f o r t h e c o n n e c t i o n b e t - ween a n i s o t r o o i c d e t o n a t i o n p r o p e r t i e s o f s o l i d s , d i f f e r e n c e s between s o l i d a n d l i q u i d phases p r o p e r t i e s , and e l e c t r o n i c m o l e c u l a r o r c r y s t a l s t r u c t u r e which cannot be e x p l a i n e d by any macroscopic model which i g n o r e s t h e m o l e c u l a r and c r y s t a l s t r u c t u r e s .
These i n v e s t i g a t i o n s c o u l d h e l p t o b e g i n t o answer t h e q u e s t i o n : why does an e x p l o s i v e e x p l o d e ? which, i n c o n c l u s i o n , t o day, has no answer.