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SPECTROSCOPIC STUDIES OF INITIATION AND DETONATION CHEMISTRY
A. Renlund, W. Trott
To cite this version:
A. Renlund, W. Trott. SPECTROSCOPIC STUDIES OF INITIATION AND DETO- NATION CHEMISTRY. Journal de Physique Colloques, 1987, 48 (C4), pp.C4-179-C4-188.
�10.1051/jphyscol:1987412�. �jpa-00226644�
JOURNAL DE PHYSIQUE
Colloque C4, suppl&ment au n a g , Tome
48,
septembre 1987SPECTROSCOPIC STUDIES OF INITIATION AND DETONATION CHEMISTRY
A.M. RENLUND and W.M. TROTT
Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.
Rdsumd
Diverses t e c h n i q u e s d e s p e c t r o s c o p i e o n t d t d u t i l i s d e s pour d t u d i e r l e s matdriaux t r S s e x p l o s i f s (HEs) charges p a r choc e t l e u r d d t o n a t i o n dans l e b u t d l i n t e r p r d t e r l e s r d a c t i o n s chimiques i m p o r t a n t e s dans l ' i n i t i a t i o n des matdriaux dnergdtiques. Par s p e c t r o s c o p i e d'dmission e t photographie u l t r a r a p i d e , on a observd l e s p r o d u i t s formds l o r s d e s i n t e r a c t i o n s avec l a m a t i a r e dans l e s c a v i t e s 3 l a s u r f a c e d e s p a s t i l l e s d e HE en d e t o n a t i o n . CN(B) a d t d form6 i n i t i a l e m e n t dans un d t a t de r o t a t i o n - v i b r a t i o n h o r s d ' d q u i l i b s e , mais r e l a x 6 rapidement pour donner d e s tempdratures d e Bcltzmann > 5000 K. Les experiences photographiques i n d i q u a i e n t que l e s i n t e r a c t i o n s e s s e n t i e l l e s avec l e m a t d r i a u o n t l i e u dans une d c h e l l e d e temps d e l a nanoseconde. On a u t i l i s d l a photographie i n f r a r o u g e r d s o l u e dans l e temps pour d d t e c t e r l e s p r o d u i t s d e d 6 t o n a t i o n en phase gazeuse;
par exemple, l a formation de l ' e a u dans l e RDX en d e t o n a t i o n a p p a r a i t en 5011s.
Dans l e s e x p e r i e n c e s p a r d i f f u s i o n Raman s u r l e TATB choqud
P
d e s p r e s s i o n s s u p d r i e u r e sB
7Q K Bar, nous avons observd u n e n o w e l l e p a r t i c u l a r i t d spec- t r a l es
1120 m-'. Nous avons observd a u s s i d e s ddplacements en frdquences des v i b r a t i o n s C-N e t N-0 q u i s o n t generalement en accord avec l e s r d s u l t a t s obtenus 2 p a r t i r d'expdriences1
h a u t e p r e s s i o n s t a t i q u e . Des d i f f d r e n c e sP
p e i n e p e r c e p r i b l e s , cepenaant, i n d i q n e n t u n e r u p t u r e p l u s f a c i l e du r g s e a u des l i a i s o n s hydrogsne du TATB sous compression dynamique.There is much effort currently directed towards elucidating important microscopic processes in reacting high explosives (HEs). In particular, identification of early chemical steps in initiation is crucial to a better understanding of explosive sensitivity and vulnerability. The intimate coupling of the chemical work required to sustain detonation with the mechanical properties of the material and the resulting wave motion drives experimental studies to focus on realistic cases of initiation
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987412
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PHYSIQUEand steady-state detonation. Extrapolation of results from more homogeneous pressure and temperature domains may provide an inadequate description of the physics and chemistry involved.
In practice the study of molecular properties in the extreme environment of detonation requires fast detection of transient phenomena. Over the past few years, we have applied various optical techniques to studies of molecular mechanisms of
initiation and det~nation.l-~ Our main focus has been to study reactions in compressed granular HEs like those commonly used in weapon components.
We have used four experimental techniques in our studies:
emission spectroscopy, l fast-f raming photography, l time-resolved infrared spectral photography (TRISP),4 and single-pulse Raman spectroscopy.1-3 The details of these methods have been described in previous papers.
Clearly the easiest optical technique is to make use of the inherent light generated in a shock or detonation. We used fast-framing photography to study the spatial and temporal character of emitted light from detonating HEs and have simultaneously recorded spectra of the emission. In these experiments we used small detonators5 to initiate various HE pellets. The resulting two-dimensional wave structure is clearly observed in the photographic records, an example of which is shown in Fig. 1. The light emerged firs@ at the center of the pellet and then subsequently appeared as concentric rings which grew as the curved detonation wavefront interacted with the pellet surface at increasing radial distances. We observed that this light persisted for less than a few ns at any single point on the surface of the pellet. Several experiments were performed to determine the source of this emission. Spectral studies showed that this emission varied somewhat from explosive to explosive; generally fuel-rich explosives such as HNS showed emission which contained prominent atomic and molecular bands, while HEs with a better oxygen balance such as PETN had more intense but unstructured emissi0n.l We further determined that this emission did not arise from reaction with surrounding air but instead arose from the HE pellet.
Subtle temporal variation of the emitted light has been
observed. Figure 2 shows a photographic record where the
emission from detonating HNS was imaged through a beamsplitter and the light through different optical filters was recorded simultaneously. The top set of frames shows the emission of the broadband visible light, attenuated by neutral density filters to be of similar intensity with the bottom set of frames. The lower frames shows light imaged through a narrowband filter with a center wavelength of 390 nm, to isolate the strong emission from CN(B-X). Close examination showed that the light at 390 nm lagged the broadband emission by about 1 ns. We also recorded spectra of the emission at various times relative to the arrival of the detonation front at the pellet surface. Just before the detonation wave breaks out on the surface, during the shine- through, we observed non-equilibrium vibrational and rotational distributions of the emitting CN, shown in Fig. 3a. After arrival of the detonation wave at the free surface, the equilibrated spectrum shown in Fig. 3b was obtained.
We also monitored the light from the explosive at different interfaces. The spectral and spatial characters of the light remained similar, but the intensity of the light varied markedly with the interface. A confining window (fused silica, NaC1, PMMA, etc.) increased the light intensity. An interface which wetted the HE, e.g., water or silicone grease, dramatically decreased the intensity of the light. These observations lead us to conclude that the intense light arises from reactions within the void volume, occurring as hot material interacts and/or stagnates against a particle surface. These reactions are therefore at moderate pressures. As the void is collapsed and the pressure increased, the light-generating reactions are quenched. The high temperatures previously observed for the emitting CN(B-X) from HNS and the dependence on the initial porosity are consistent with this d e ~ c r i ~ t i o n . 1 These results are also consistent with the early work of Blackburn and seely6 in which microstructural effects and the role of interstitial gas on detonation light was studied. The temporal lag of the formation of some of the products indicates that the relevant material interaction at the explosive surface takes place on a ns timescale.
Another aspect of our work was to identify early gas-phase
products of detonation. We have used the TRISP' technique to
monitor the formation of water from various detonating HEs. The
C4-182
JOURNAL DE PHYSIQUEexperiment w a s performed using a detonator to initiate a pellet of HE. A n infrared probe beam passed -0.5 mm above the pellet surface. Figure 4 shows an ir absorption spectrum of the detonation products' observed approximately 50 n s after the detonation front reached the pellet surface. The broad
absorption is most likely due to water. We observed this rapid formation of water with HMX and, t o a somewhat lesser extent, w i t h PETN. W e have also monitored absorption in the C-H
stretching frequency range, but the absorption features were generally t o o broad to allow unambiguous assignment.
I n recent experiments w e have used single-pulse Raman scattering t o study changes in shocked TATB. Many ideas have been presented to account for the great stability of TATB, but t h e extensive hydrogen-bonding network is believed to be of critical importance.8 Satija and swansong studied Raman spectra o f TATB under static high pressure conditions. They observed that the Raman spectrum was stable to high densities and a l s o observed substantial coupling between the N O 2 and NH2 motions.
T h e latter conclusion w a s based on the anomalous pressure dependence of the modes associated with the NO2 groups.
Specifically, they observed that, to pressures of 50 kBar, the frequency of VC-N increased with pressure in the ususal manner, while that o f
V N - ~remained nearly unchanged due to increased coupling of the N H 2 and NO2 u p t o this pressure. At higher pressures, the VN-O mode began to. exhibit the normal shift to higher frequency.
Results of our Raman studies on shock-compressed TATB are shown i n Fig. 5. We monitored the frequencies of the
VN-Oand VC-N stretches, initially at 1170 cm-l and 1145 cm-l,
respectively. I n our experiments, a detonator w a s used t o drive a shock through a buffer material of varying thickness into a pellet of TATB pressed t o a density of 1.78 g ~ m - ~ . The front surface of the TATB pellet was confined by a fused silica window. Estimates of the pressures achieved in the TATB were determined from studies of the particle motion at the
TATB/window interface measured by the ORVIS technique.1°
Details of this analysis will be presented in a future paper.
While generally consistent with the results of the static high pressure experiments, our results were different in some of the details from the Satija and Swanson e ~ ~ e r i m e n t s . ~
Especially we noted that the
VN-0mode shifted to higher
frequency even at the lowest dynamic pressure studied. This may indicate that disruption of the hydrogen bonding coupling occurs under dynamic loading. The shift to higher frequency with
increasing pressure was observed for both lines at pressures <60 kBar. At higher pressures we observed that the lines shiftd to lower frequencies and broadened considerably. We attribute this to the effect of temperature in shock compression.
We also observed a new Raman feature at -1120 cm-l in TATB shocked to above 70 kBar. This feature also shifted with increasing pressure, but was not significantly broadened. We have not yet assigned this feature, but it may be due to a ring- breathing mode of a substituted benzene-like structure. Such features can remain relatively sharp under increasing pressure.
It is possible that this new feature arises from one of the furazan or furoxan derivatives of TATB which have been observed under impact conditions. l 1
Future experiments will be directed towards studying shock and detonation chemistry in more planar geometries, with more well-defined shock inputs. The motivation of this work remains to develop an improved understanding of the microscopic process which control initiation and detonation in secondary HEs.
This work was performed at sandia National Laboratories, Albuquerque, NM, supported by the U. S. Department of Energy under contract number DE-AC04-76DP00789. We wish to acknowledge with gratitude the excellent technical assistance of J. C. Pabst and H.
C.Richardson. We also thank Dr. Basil Swanson of Los Alamos National Laboratory for helpful discussions and for
communicating results prior to publication.
REFERENCES:
1. W. H. Trott and A.
M.Renlund, Proceedings of the Eighth
Symposium (International) on Detonation, Albuquerque, NM,
USA, July 1985, in press.
JOURNAL
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PHYSIQUE2. W. M. Trott and A. M. Renlund, Appl. Optics, 24, 1520 (1985) 3. W. M. Trott, A. M. Renlund and R. G. Jungst, Proceedings of
the Southwest Conference on Optics, SPIE, 540, 368 ( 1 9 8 5 ) 4. A. M. Renlund, S. A. Sheffield and W. M. Trott, Proceedings
of the Fourth American Physical Society Topical Conference o n Shock Waves in Condensed Matter, p. 237, Y. M. Gupta, ed., Spokane, WA, 1985.
5. RP-2 detonators, from Reynolds Industries, Inc.
6. J. H. Blackburn and L. B. Seely, Trans. Faraday Soc., 61,
537 (1965)
7. D. S. Bethune, A. J. Schell-Sorokin, J. R. Lankard, M. M. T.
Loy and P. P. Sorokin, Advances in Laser Spectroscopy, Vol.
2. B.
A.Garetz and
J.R. Lombardi, eds., Wiley and Sons Ltd., New York, 1983, p. 1.
8. J. W. Rogers, Jr., H. C. Peebles, R.
R.Rye,
J. E.oust on and
0.S. Binkley, J. Chem. Phys., 80, 4513 ( 1 9 8 4 )
9. S. K. Satija and B. I. Swanson, private communication.
10. D. D. Bloomquist and S. A. Sheffield, J. Appl. Phys., 54,
1717 (1983)
11. J. Sharma, J. C. Hoffsommer, D. J. Glover, C. S. Coffey, J.
W. Forbes, T. P. Liddiard, W. L. Elban and F. Santiago,
Proceedings of the Eighth Symposium (International) o n
Detonation, Albuguerque, NN, USA, July 1985, in press.
FIG. 1. Fast-framing photograph of detonating
HNS.The top trace
is a record of a strobe-flash illuminated pellet to
illustrate the size of the image. The lower frames are
of the detonating pellet, recorded with 2-11s exposure
and 13-11s interframe time. The curve in the lower right
shows the temporal profile of the emitted light as
viewed by a photomultiplier tube; the numbers shown are
in reference to the photographic frames.
JOURNAL DE PHYSIQUE
FIG. 2. Fast-framing photograph (intensified) of detonating HNS.
Emission was viewed simultaneously through two different
optical filters. The top trace, attenuated through
neutral density filters, shows the broadband emission as
detected by the S-20 response of the electronic camera
tube. The bottom trace shows the emission through a
narrowband (10 nm F W H M ) filter centered at 390 nm to
isolate the CN(B-X, Av-0) transition.
WAVELENGTH (nm)
-
V)- +
Z 3
m P
-
aW 0
t
3-I
:
a
3
WAVELENGTH (nm)
FIG.
3. Emission spectra of CN(B-X) from detonating HNS. The
spectra were recorded using a gatable vidicon detector
with a 20-ns gate width. Emission observed just prior
to arrival of the detonation front (i.e., shine-through)
is shown in spectrum a). Spectrum
b )shows the emission
observed just after the arrival of the detonation front
to the free surface.
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PHYSIQUEWAVELENGTH [MICRONS)
IR PROBE +
w
"DxFIG. 4. Infrared absorption spectrum of gas-phase products from detonating RDX. This spectrum demonstrates formation of H20 within 50 ns of arrival of the detonation front to the free surface of the RDX pellet. The TRISP probe beam used to obtain the spectrum (see text) was at a height 0.5 mm above the pellet surface.
TATB NO2
