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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
ELECTROMAGNETIC PROPERTIES OF PREDETONATING EXPLOSIVES
G. P. Chambers1, R. J. Lee1, T. J. Oxby2, and W. F. Perger2
1
Energetic Materials Research and Technology Department, NA VSEA Indian Head Division,
101 Strauss Ave, Indian Head MD 20640-5035
2
Department of Electrical Engineering, Michigan Technological University,
Houghton, MI 49331
ABSTRACT: Current theories of reaction processes suggest that changes in electronic band
structure and radiation producing dipole oscillations occur during shock loading of an energetic
crystal prior to detonation. To test these theories, a broadband antenna, capable of measuring
polarization, was employed to observe shock-induced electromagnetic radiation from a
crystalline explosive, RDX. The frequency spectra from these experiments were analyzed
using time/frequency Fourier methods. Changes in conductivity resulting from this shock
loading were also measured at the opposite end of the crystal from the shock source. A fourpoint-probe arrangement was used to eliminate errors involving lead resistance. This
arrangement uses two leads and a fast discharge circuit to pass current through the crystal
interface at the time conductivity begins to change in conjunction with the arrival of the shock
wave. Also reported are corresponding light (observed with a high-speed electronic camera)
and sub-microwave emission observed during the passing of the shock wave in the RDX crystal
prior to detonation.
experiments to measure the frequency spectrum
through
the
radio-frequency
band
of
electromagnetic emissions, as well as the
conductance of energetic materials near the onset
of shock-induced reaction.
INTRODUCTION
There exists motivation from both theoretical
considerations and previous experiments to pursue
both the measurement of electromagnetic fields
radiated from and the conductance of energetic
materials under shock. Recently, work by B.
Kunz's group ] has indicated that shock energy is
converted to lattice vibrations, which in turn give
rise to molecular vibrations, electronic excitations
and collective excitations. As the electronic energy
gap of an energetic crystal destabilizes during
shock induced deformation, metallization and
reaction onset occur, a model advocated by J.
Oilman2. We have therefore devised a series of
EXPERIMENTAL ARRANGEMENT
The experimental arrangement consisted of an
RDX crystal (cube, nominally 9-mm on each edge)
sandwiched in a Teflon holder with a 10-mm wide
groove down the middle to allow back-lit
photography of the crystal. A 19-mm diameter
Pentolite pellet initiated by an RP-80 detonator
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initiation of reaction at the crystal/nylon interface
(>10.5 us). Other control shots were fired with the
detonator and pentolite donor into water, to
determine the radio pickup onto the antennae as
well.
provided a planar shockwave into the crystal
through a variable PMMA gap. CTH3 simulations
were used to confirm the planarity of the
shockwave with respect to the front face of the
crystal.
Four conducting pins were placed in a line
against the crystal opposite from the donor charge
to record voltage and current in a four-point probe
arrangement. Each pin was 0.75-mm in diameter
and set 1.5-mm between centers. The crystal
rested on a nylon disk from which the pins
protruded to make contact with the crystal surface.
Silver paint was used to provide an ohmic contact
between the pins and the crystal.
The circuit is similar to that used by Weir et al.4,
consisting of a 100-nF capacitor charged by 205Volt battery. The capacitor was connected across
the two outside-pins to deliver current when the
crystal interface began to conduct. No current
flowed through the circuit until the RDX sample
changed from an insulator to a conductor upon
arrival of the shock wave. The capacitor was
placed just behind the nylon disk to minimize
inductance and thereby insure a fast rise time for
the current. Voltage was measured across the two
inner pins. Both measurements were made using a
LeCroy oscilloscope at 500 ps/point. The circuit
was calibrated using thin wafers of copper (a good
conductor) and n-doped Silicon (a typical
semiconductor).
RF radiation was measured with two dipole
antennae positioned nominally 38 mm from the
central axis of the crystal. They were mounted on
a single substrate and aligned vertically and
horizontally with respect to the shock propagation,
in order to measure longitudinal and transverse
electromagnetic fields. Signals were recorded using
separate LeCroy oscilloscopes at 125 ps/point.
For shots involving the detection of RF
radiation, water was used to surround the sample
and thereby prevent signals from air ionization.
The detonator was fired into both Teflon and
Lexan to determine the sensitivity of the antenna to
noise during its operation alone.
While the antennae were found to be sensitive to
the currents generated by the firing pulser, signals
did not appear in the window between onset of the
shockwave into the crystal (-7.5 us) and the
RESULTS
FIGURE 1. High-speed photograph of shocked RDX crystal
exhibiting shock-induced luminescence.
Fig 1 shows a high-speed camera record of a
125-kbar shock wave from a pentolite pellet
entering an RDX crystal. Frame 1 starts at 7.5 |is
after firing of the detonator pulse. The interframe
time is 400 ns, with a duration of 40 ns per frame.
Light emission can be seen at 8.7 (frame 4) and
again at 9.5 us (frame 6). The light emission
appears to be the result of passage of the
shockwave through the sample and is occurring
prior to detonation. By 8.7 us, light from the
pentolite detonation has died out, so reflection of
this light through the crystal is not the source of the
light emission.
Concurrent with light emission, we observed RF
signals on the antenna temporally correlated with
the onset of light emission. The raw voltage data
from this measurement can be seen in Fig. 2.
Signals at 8.7 and 9.6 us are temporally correlated
with the light emission observed in Fig. 1.
Furthermore, the strength of the signals tends to
correlate with the amount of light emission
occurring at the same time. The RF signal at 8.7 jus
appears weaker than the signal at 9.6 us, consistent
with the degree of light emission in the high speed
photograph, which appears stronger at the later
time, 9.5 jis. This agreement tends to support the
view that the signals are correlated to some
physical phenomena.
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PSO of puts* at 8.5 its In VMtfc* channel ol RDX trial In w
. Li J
FIGURE 2. Voltage trace observed directly on the antennae as
a function of time.
-0.8
-0.6
4U
-0.2
(a)
Fig. 3 shows spectrally analyzed signals from
the RF, generated through time/frequency Fourier
analysis of the antennae signals. The signal at 9.6us spectrally resolves into a series of discrete
frequencies at 0.5, 0.43, and 0.37-GHz. At 8.7-us,
lines appear at 0.1 and 0.25-GHz.
These
frequencies are in the sub-microwave regime, and
hence are not likely due to molecular rotational
spectra which occur at microwave frequencies, in
the tens or hundreds of GHz range.
A conductivity measurement is shown in Fig. 4
using the four-point probe test. This figure
compares the conductivity of Cu and an n-doped Si
wafer with changes in conductivity as a function of
time of an RDX crystals shock loaded at 12.5 GPa.
This figure shows that the current flowing in the
shocked RDX lies between that obtained by
applying disks of n-doped Si and Cu across the
electrodes. Hence, the conductivity associated
with the shock loaded RDX crystal is comparable
to that of a semiconductor. Also of interest is the
timing of this event. The RDX began to conduct
around 8.4 us as a result of the shockwave arrival
at the interface. (The current trace was shifted in
Figure 4 to allow comparison with the two
calibration trials.) Current decays quickly in each
case due to the small value of capacitance used in
these experiments. It is interesting to note that no
reaction was observed prior to 9 us in the camera
record, i.e., 0.6 us after the arrival of the shock
front.
; It ik
A
,
i AJ Ml JuLaJl^J
(b)
FIGURE 3. Comparison of spectrally analyzed antennae
signals occurring at 8.7 us (a) and at 9.6 us (b).
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time (ps)
FIGURE 4. Change in RDX resistivity as a function of time
for an incident shock wave.
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directly supports theoretical work by Zwitter et al
while conductivity changes towards metallization
support models of Oilman for intermolecular
processes.
DISCUSSION
In previous work by Dick , luminescent
emissions from a shocked PETN crystal were
interpreted as being due to shock induced
decomposition products NO2 and possibly NO<|.
Visible light is known to be produced by electronic
transitions, however, without spectroscopy, we
cannot distinguish between the possibility that it is
correlated with excited states of shock
decomposition products or due to radiative
emissions from the RDX molecule as a whole.
One possible explanation for the source of RF
emission is excitation of plasma oscillations within
the crystal6. Such plasma modes do not exist in
insulators, but can be excited in metals or
semiconductors. Since the shock wave transforms
RDX into a semiconductor during its passage
through the crystal, it is possible to excite such
plasmon oscillations within the crystal. These
oscillations may be the source of the RF emissions
detected with the antennae.
The correlation of RF emissions with
luminescent emissions, and the correlation of the
latter with sensitivity, suggests a relationship
between the RF field and reaction initiation in
RDX. This mechanism may need to be considered
in simulations if an accurate prediction of
sensitivity is to be obtained.
Lastly, conductivity changes have been observed
as a result of the passage of the shockwave through
the crystal, prior to onset of reaction. This was
predicted by Oilman2. This represents a several
order of magnitude change in conductivity of a
molecular RDX crystal as a result of the incident
shockwave. Such changes have previously been
measured in the ionic crystal KC1 at 140-kbar
shock pressures7. These changes ultimately push
the organic crystal towards metallization.
A
metallized organic crystal therefore has electrons
available to participate in intermolecular reactions,
which may either facilitate or be a necessary
condition for detonation chemistry to occur.
Thus, our results may provide support for both
models of reaction onset. The detection of RF
emissions from the crystal during shock loading
SUMMARY AND CONCLUSIONS
Shock-induced light emission from an RDX
crystal prior to detonation has been observed. This
luminescence correlated with shock-induced
electromagnetic radiation. A change in crystal
conductivity as a function of time was also
observed prior to and during the detonation
process. Future parametric studies, using the
techniques developed here, will help broaden our
understanding of the roles these phenomena play in
initiation.
ACKNOWLEDGMENTS
Discussions with R. Doherty, R. Chau and
funding from the IHD Internal Research Program
are gratefully acknowledged.
REFERENCES
1. Zwitter, D. E., Kuklja, M. M., and Kunz, A. B.,
Shock Compression in Condensed Matter,
Snowbird Utah, June 1999
2. J. Oilman, Shock Compression in Condensed
Matter, Amherst MA, July 1997
3. R.L. Bell et al. "CTH User's Manual and Input
Instructions" version 2.00 Sandia National
Laboratories (Sept 1995)
4. W.J. Nellis, ST. Weir, C Mitchell Physical
Review B, Vol. 59. P. 3436
5. JJ. Dick, R.N. Mulford, W.J. Spencer, D.R.
Pettit, E. Garcia, D.C. Shaw, J Appl. Phys. 70
p. 3572 (1991)
6. JJ. Oilman, Philos. Mag. B, 1999, Vol 79, No.
4, 643-54
7. N.K. Bourne and D. Townsend. Shock
Compression of Condensed Matter, 1999
ed. M.D. Furnish, p. 109
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