CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
EMISSION SPECTROSCOPY APPLIED TO SHOCK TO DETONATION
TRANSITION IN NITROMETHANE
Viviane Bouyer1'2, Gerard Baudin2, Christian Le Gallic2, Philippe Herve1
^aboratoire d'Energetique et d'Economic d'Energie, Paris X University, I Chemin Desvallieres, 92410 Ville
d 'Avray, France, viviane. bouyer@cva. u-parisl O.fr
2
DGA/DCE/Centre d'Etudes de Gramat(CEG), 46500 Gramat, France, [email protected]
Abstract. The objective of this work is to clarify the mechanism of shock to detonation of nitromethane
by time-resolved emission spectroscopy. This paper presents the experiments performed in the spectral
range 0.3-0.85 jim. Experimental results provide several radiance values depending on wavelength and
time that we compared to measurements of pyrometry previously performed. Determination of the
temperature profile is treated from the resolution of the equation of radiative transfer by an inversion
method. The case of the steady state detonation is considered here.
INTRODUCTION
around 3000-4000K and pressure between 1030 GPa complicates the problem.
To improve knowledge of shock to detonation
transition (SDT) of homogeneous high explosives,
especially nitromethane (NM), temperature profiles
are one of the parameters required. Optical
techniques as pyrometry are often used to determine
the NM detonation temperature (1). One of the most
successful techniques today is the six-wavelength
pyrometer developed at CEG (2)(3)(4). These
means are based on the assumption of a surface
emissivity. To obtain more information on the
optical properties of condensed matter, spectroscopy
techniques are widely used (5)(6)(7). This context
and the fact that the different media involved in the
SDT are likely semitransparent brings us to set up a
time-resolved emission spectroscopy technique that
enables a continuous measurement of the radiation
emitted during the detonation of NM with a 1 ns
time resolution. This paper presents the study of the
visible range, where the NM in liquid state is
transparent. From the radiance values obtained
versus time and wavelength, we aim at determining
profiles temperature by resolving the equation of
radiative transfer. The fact that emissive media are
made of gaseous compressed species at temperature
EXPERIMENTAL CONFIGURATION
One interesting property of NM for optical
studies is its semi-transparence. Figure 1 shows the
NM spectrum in the 0.6-3 |tim range. In the visible
range 0.4-0.6 jam, NM is transparent (2).
10080SS
.1
.22
«
6040200
wavelength pm
FIGURE 1. Transmission spectrum of the NM for a thickness of
15 mm measured by a FT-IR Fourier Spectrometer
The experiments consist in plane shock impacts
on explosive targets at 8.6 GPa, under conditions of
one-dimensional strain. A single stage powder gun
1223
propels the projectile on the target at a velocity of
1940 m/s to initiate the detonation (Fig. 2). The NM
is in a polyethylene chamber of 15 to 25 mm depth,
closed by a copper transfer plate. An optical probe
collects the thermal radiation emitted during the
detonation through a lithium fluoride (LiF) window.
This radiation is transmitted to the spectroscopy
system by an optical fiber. A Jobin Yvon Triax 180
spectrometer spectrally disperses the light beam.
The dispersion is 33.5 nm/mm at 565 nm. A
multianode photomultiplier tube (16 channels/
Hamamatsu R5900U-01-L16) is used to detect the
signal in the spectral range [0.3-0.85 jim]. Its
response time is 0.6 ns. Complementary
measurement techniques are used: a polarization
electrode records the shock entrance, the
superdetonation and the detonation, piezo-electric
pins measure the shock and detonation velocities.
Projectile
Opt&al
probe
that propagates until the interaction with the LiF
window at 4.5
1
2
3
4
6
5
time us
FIGURE 3. Results of one experiment at 8.6 GPa on a 25 mm
NM target
RADIANCE PROFILE ANALYSIS
Analysis of one experiment at 8.6 GPa
Optical
fibre
Changes in radiance depending on wavelength
have been studied for different typical moments of
the SDT (Fig. 4). From the formation of the
superdetonation, a discontinuity appears between
0.65 and 0.75 urn It remains until the end of the
propagation of the detonation wave.
Coneave
j Pholomuitiplier
^
3.0x10 -
&
2.5x1 06-
e
*
1
o
0
« f
«** 1.5x106-
FIGURE 2. Experimental device.
fo
^ cf
^ 2.0x1 0 6 E
Data acquisition system
4
o
m
o' 9
» i
I
S- 1.0x106-
^
s
| 5.0x1 0 5 -
EXPERIMENTAL RESULTS
1
1
ii 8 0
S wi
« 0* ^S •o*
*
0
0
• 1»
0.0-
We analyzed one of our experiments carried out
at 8.6 GPa on a 25 mm thick NM target. Figure 3
represents the radiance signals for different
wavelengths versus time. Only 6 measurements
among 16 are represented. The spectral resolution is
28 nm and time resolution is 20 ns after filtering
through noise. These measurements, with the piezoelectric pins and electrode signals clearly show the
different phases of the SDT as described by
Chaiken (8): a first jump at 1.65 jis characterizes the
formation of the superdetonation, which overtakes
the initial shock wave at a second jump at 2.15 u.s.
A strong detonation is then formed and gradually
decays into an overdriven steady state detonation
O
'•
;:,
i a 2- A" *&^' ^ A
0.4
0.5
°
^ * *
0.6
*'
0.7
*'
^
A
0.8
wavelength pm
FIGURE 4. Radiance profiles at different times: a) after shock
entrance (0.3 us), b) before the superdetonation formation (1.2
us), c) superdetonation formation (1.65 us), d) 1.9 us, e) catch up
of the shock wave (2.15 us), f) strong detonation (2.45 us),
g) overdriven steady state detonation (3.8 to 4.4 us). Radiance
values were averaged around the given time value.
One interpretation for this discontinuity may be
gaseous H2O produced during the SDT: the H2O
spectrum shows absorption lines between 0.65 and
0.75 jim. Another interpretation could be given from
Gruzdkov and Gupta works (6). They measured the
emission spectrum between 0.4 and 0.75 jim of NM
1224
shocked at 16.7 GPa under a stepwise loading
process, and a peak appeared at 0.65 um. They
explained it as luminescence from reaction products,
maybe NO2.
Determination of the temperature from the
equation of radiative transfer
By using inversion methods, it should be possible
to determine the temperature profiles during the
SDT from the equation of radiative transfer (ERT).
In this paragraph, we will only present the ERT and
compare the detonation temperatures obtained from
pyrometry and spectroscopy results.
Comparison with previous pyrometry
measurements
In 1998, a time-resolved six-wavelength
pyrometer was developed at CEG (4). The same
experiments on NM had been carried out with this
device so we can compare our results to those
obtained by pyrometry (Fig. 5). However, the
comparison is limited to the overdriven detonation
phase because we do not obtain the same time of
apparition of the superdetonation and detonation.
Radiative transfer for a semitransparent medium
Given a semitransparent medium at a temperature
T, with an absorption coefficient K^ and a thickness
(x*-x0), the ERT gives the spectral intensity
transmitted and emitted by the medium along the
detonation axis as:
2,5x106^ 2,0x106-
(1)
(0
I 1,5x106-
x*
\K,(x')L°,(T)exp\
^ 1,0x1065
5,0x10 1
0,0-
X0
x*
\-K,(x)dx\dx'
\Xr
)
Applying (1) to the SDT, different media are
involved: neat NM, shocked NM, reaction products
and detonation products. The two parameters T and
KX are unknown except for neat NM. The difficulty
lies in the knowledge of the media involved. If
thermochemical codes give the main present
chemical species, a model giving their absorption
properties at the pressure and temperature
encountered during the SDT does not exist.
Therefore, the resolution of (1) by mathematical
methods will require suitable absorption models.
6 wavelength pyrometer measurements
emission spectroscopy measurements
Planck curve at 3570 K
0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6
wavelength um
FIGURE 5. Radiance profiles obtained by pyrometry and
spectroscopy during the steady state detonation, x error bars
represent the spectral resolution.
The spectroscopy technique gives more
information than the pyrometry technique because
of the greater number of detectors and of the better
spectral resolution (28 nm against 40 to 100 nm).
Thus, the discontinuity can not be recorded by
pyrometry. Consequently, the exploitation of the
only six radiance values can give approached
results, in particular for the temperature. Moreover,
the black or grey body assumption often used when
determining the temperature of the overdriven
detonation may be questioned from these results. As
shown on Fig. 5, emission spectroscopy
measurements do not match with the Planck curve.
Further to this, we analyze the influence of the
number and kind of radiance values on the steady
state detonation temperature calculation.
Case of the overdriven steady state detonation
To get onto the resolution problem and to
improve the comparison between the pyrometry and
the spectroscopy techniques, it could be interesting
to approach the case of the determination of the
temperature of the overdriven steady state
detonation. T is uniform (according to the Jouguet
model) and equal to the semitransparent detonation
products temperature Tj. K^J is only depending on
the wavelength A,. Then, as the detonation wave
interacts with the window, the intensity is (I is the
depth of the cell):
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choice of the pyrometer wavelength is critical.
Afterwards, emission spectroscopy will be
performed in the infrared range 0.8-5 jum to
characterize the functional groups of gaseous
species.
The determination of temperature profiles by an
inversion method is not so obvious because of the
absorption coefficient. Chemical species produced,
N2, H2O, CO2, CO and C(s), are in such a
thermodynamic
state
(high pressure and
temperature) that it is not represented by existing
absorption models (like Hitemp or Hitran data
bases). Thermochemical calculations, existing
detonation models will be then required to initiate
the ETR resolution. But first, it is necessary to
resolve the direct problem to analyze the sensitivity
of the solution L(^,t) to the parameters of the chosen
absorption model.
(2)
No accurate model for K^ is available yet that
could consider the absorption window. Except for
this discontinuity, calculation results can point out
relevant remarks. Table 1 represents calculations of
Tj and of the transmissivity r A = exp^-K^ -I) from
the pyrometry measurements and from 5 and 7
values of spectroscopy measurements.
TABLE 1. Determination of temperature Tj
pyrometry
5 measurements
of spectroscopy
7 measurements
of spectroscopy
Tj(K)
3570±16
3533±21
3604±36
^=0.5
TA=06
0.01
0.66
0.02
0.38
TA=08
0.06
0.04
0.7
0.47
0.14
The three calculations give similar values.
However, transmissivity values obtained from
pyrometry measurements justify the hypothesis of a
surface emissivity, close to black body's, whereas
spectroscopy results don't confirm it. Consequently,
the choice of the wavelength affects the estimation
of the optical characteristic of the studied medium.
ACKNOWLEDGMENTS
The work described here was carried out with
financial support from DGA/SPNuc, for the interest
of CEA. Each impact experience was performed at
Physics Explosive Laboratory of CEG with the
assistance of ARES and Metrology staff.
CONCLUSION
REFERENCES
Time-resolved emission spectroscopy performed
during the detonation of NM gives radiance
measurements versus time between 0.3 and 0.85
jim, with a 28 nm spectral resolution. This
technique brings out more information than timeresolved six-wavelength pyrometry, which was
previously used to study the SDT of the NM at
CEG. Indeed, our results showed a discontinuity
between 0.65 and 0.75 jam in the radiance profile,
appearing
from
the
formation
of
the
superdetonation. We still have to explain the
presence of this discontinuity. It affects the
determination of the temperature; however, we
resolved the equation of radiative transfer outside
the discontinuity in the case of an overdriven
detonation. Detonation temperatures calculated are
close to the temperature obtained by pyrometry but
there is a significant difference between the
transmissivity. Therefore, black or grey body
assumption commonly used seems doubtful and the
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nitromethane and some solid high explosives, in 8th
Symposium on detonation, Albuquerque, NM, 1985,
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Poitiers, France, 1998.
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9333-9340(1997).
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2322-2331 (1998).
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