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
USING SIMULTANEOUS TIME-RESOLVED SHG AND XRD
DIAGNOSTICS TO EXAMINE PHASE TRANSITIONS OF HMX
AND TATB*
C. K. Saw, J. M. Zaug, D. L. Farber, B. L. Weeks and C. M. Aracne
University of California, Lawrence Livermore National Laboratory,
7000 East Ave., Livermore, CA 94550
Abstract: Simultaneous SHG (second harmonic generation) and XRD (x-ray diffraction) diagnostics
have been applied to examine the phase behavior of energetic materials, HMX (octahydro-1,3,5,7tetranitro-l,3,5,7-tetrazocine) and TATB (l,3,5-triamino-2,4,6 trinitrobenzene). This unique capability
provides information about both volume and surface effects that occur during the solid-solid
transformation process. This paper reports XRD results for HMX and TATB at elevated temperatures
and on simultaneous SHG and XRD experiments on HMX at fixed temperature. Our results do not
indicate that a solid-solid phase transformation occurs for TATB even at temperatures up to 340°C.
XRD results on HMX held at 165°C and 1 bar, indicate that the (3 to 5 transformation is incomplete
after a period of 4.5 hours which do not temporally correlate with SHG. Overall information indicates
that the observed SHG intensities from surface effects can, in some cases, dominate over volume
generated SHG contributions. Finally, we have run in situ AFM scans of HMX at 180°C and 184°C
that show HMX surface area increases by many orders of magnitude after the 5-phase transformation
is completed.
INTRODUCTION
accurate information concerning the lattice constants
of the entire volume of a powdered sample. The
limitation of our x-ray setup with the sample in the
diamond anvil cell (DAC), is that it requires 5-8
minutes of exposure time to develop an interpretable
pattern. This limitation led us to search for an
accompanying diagnostic tool that could provide a
real-time probe of solid-solid structural transitions.
SHG was first demonstrated as a probe into reaction
kinetics of HMX and TATB phase transitions [1,2].
In order to test the application of SHG to the study
of phase transitions we developed a portable optical
SHG experiment that could be put into the 10-2 xray beamline at the Stanford Synchrotron Linear
Accelerator (SSRL). We can now conduct SHG and
x-ray diffraction experiments simultaneously on
PBX materials contained in DAC's.
After several decades of study there still is not a
universal set of rate laws governing solid-solid
structural phase transitions of polymer bonded
explosive (PBX) materials. The following
parameters all affect the p to 6 transformation
kinetics of HMX at fixed pressure and temperature:
grain size, binder content, impurity content (e.g.,
RDX), and compaction density of the powder. To
date there is not a kinetic rate law that incorporates
these critical rate-limiting parameters. Given the
core mission of the stockpile safety initiative we
have been motivated to develop and refine a series
of experimental diagnostic tools that will allow us to
derive a universal rate law. To do this we first set
out to determine what type or series of diagnostic
tools would be suitable for the task. X-ray
diffraction was the first tool of choice as it provides
856
exposures. The experiment on TATB involved
incrementally increasing the temperature over time.
The experiment on HMX was run at a constant
temperature.
MATERIALS ASPECT
HMX was prepared by the method of Siele et.
al. [3]. This involved the treatment of octahydro1,5-diacetyl-3,7-dinitro-1,3,5,7-tetrazocine (DADN)
with 100% HNO3 and P2O5 at 50°C for 50 minutes,
followed by quenching in ice water. Slow
recrystallization from acetone yielded HMX as
colorless microcrystals. The grain size distribution is
trimodal as shown in Fig. 1. TATB was prepared by
aqueous amination of trichlorotrinitrobenzene
(TCTNB) in a water/nitrobenzene medium. The
grain size distribution has a quasi-Gaussian profile
centered at approximately 75 microns. Both TATB
and HMX powders were introduced into a 500
microns diameter metal gasket, 100 microns thick
that laterally confines samples within the DAC.
Figure 2: Schematic of the SHG/XRD experiment. Pressure
measurement components are not shown as they were not used in
this work.
RESULTS AND DISCUSSION
The x-ray energy is set at 17 KeV. The spectrum
is then obtained by collapsing the two dimensional
image. Figure 3 shows x-ray spectra for TATB with
increasing temperatures at 20°C increments. Listed
in the plot is the JCPDS listing for TATB (43-1708).
Figure 1: Grain size distribution of HMX lot # B-725.
EXPERIMENTAL
The experimental setup consists of building two
separate techniques, XRD and SHG, on an optical
breadboard. Schematically, the setup is shown in
Fig. 2. Both systems are aligned with their beams
(500 jiim diameter) collinear and incident onto the
sample located in the DAC. The laser operates at
1064 nm wavelength, 2-4 jiJ and 20Hz PRF. The
frequency doubled light from DAC is collected
using a Be mirror. The experiments reported in this
paper were conducted at 1 bar constant pressure in a
dual heated hydrothermal DAC. Outputs from the
photo-multiplier tube and photodiode were collected
and recorded using a Tektronix TDS684C
oscilloscope. The x-ray patterns are captured using
image plates (IPs). Both experiments were
performed simultaneously and were interrupted only
to replace IPs after completion of 8-minute
Figure 3: Truncated XRD patterns of TATB from 24° to 340°C.
The JCPDS listed intensities are multiplied by ten.
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TATB is triclinic with a= 9.01 b=9.028 and
c=6.812 (A), a=108.59, p=91.82 and y=l 19.97. A
few less intense unidentified lines can be observed,
are perhaps from the background. The data do not
substantiate the existence of a solid-solid phase
transformation at these temperatures. However, a
shift of the (002) peak position to higher d-spacings
is observed with increasing temperature as shown in
Fig. 4. This result suggests the opening of the
intermolecular distances with temperature. The
linear coefficient of thermal expansion ac, calculated
from a linear fit of the data in fig. 4 is found to be
225 x 10'6/°C. The lack of significant change in
(hkO) peaks, which are related to intra-molecular
arrangement, indicates no major changes in
molecular structure even up to 340 °C. Hence, most
of the volume expansion in TATB occurs along the
c-axis due to weak inter-planar Van der Waals
interactions. The (002) line shift and/or the increase
in (004) peak intensity, as observed in the literature
[2] cannot be interpreted as a phase transformation
but merely a reorganization of the triclinic phase.
Figure 5: Truncated XRD patterns for HMX at 165°C versus
time with the peak listing for the calculated powder patterns from
published single crystal data.
§3.30-
0
50
100
150
200
250
300
Figure 6 shows the normalized SHG signal and
four
HMX
diffraction
peak
intensities.
Normalization was carried out by simply dividing
the data set by the highest amplitude point. The
structure in the SHG growth curve is real and is
most likely related to surface energy and grain scale
effects. The SHG data suggests that 80% of the
HMX has converted to the delta phase after -8000
seconds while the XRD data suggests that only
-15% of the sample volume has converted. Also
note that the SHG intensity increases at the onset of
the experiment even though there is no evidence of 5
phase.
350
temperature C
Figure 4: Changes in d-spacing (A) for the (002) reflection of
TATB.
Figure 5 shows the diffraction patterns for HMX
held at 165°C as a function of time as indicated on
the right side of the plot. These patterns are
compared to the calculated powder pattern from
single crystal results [4,5] for both (3 and 6 phases
using LAZY-PULVERIC programs. Clearly, at the
start of the experiment, all the lines can be
accounted for by P HMX as indicated and the 5
phase emerges at -7000 seconds into the
experiment, which ran for 4.5 hours.
We have conducted in situ high-temperature AFM
experiments on HMX single crystals [6]. The data
show that the total surface area of 8-HMX is on the
order of 103 to 105 times higher than the starting PHMX material. Grain size effects and the
corresponding increase in surface area can explain
the incongruent contrast between our SHG and XRD
858
data on HMX. We have conducted numerous survey
XRD and SHG tests that confirm that larger grain
HMX single crystals (50-200 micron length) phase
convert at near instantaneous times at a given
temperature whereas smaller crystals (0.1-20
microns) can take hours to convert at the same
rates previously derived for HMX and TATB using
only the SHG diagnostic. Studying uniform grain
sized HE materials (sample lot # issues should be
studied too) may give SHG a foothold concerning
the rigorous determination of kinetic rate
determinations for polymer blended explosive
materials.
1.0
ACKNOWLEDGEMENTS
0.9
0.8
f
0.7
The authors would like to thank D. M. Hoffinan
for the grain size distribution measurements and
C.O. Boro, and D.G. Ruddle for assisting with the
experimental setup at SSRL We thank P. Pagoria
for HMX samples and F. Foltz for TATB samples.
>65 % S-phase
\ 0.6
| 0.4
I 0.3
*This work performed under the auspices of the U.S.
Department of Energy by the Lawrence Livermore National
Laboratory under contract number W-7405-Eng-48.
- SHG intensity
O (100) 5-phase intensity
0.2
A (001) p-phase intensity
(113) 5-phase intensity
J (-122) P-phase intensity
0.1
U
0
REFERENCES
2000 4000 6000 80001000012000140001600018000
Time (sec)
1. Henson, B.F., Asay, B.W., Sander, R.K., Son,
S.F., Robinson, J.M. and Dickson, P.M., Phys.
Rev. Lett. 82, 1213-1216 (1999).
Figure 6: Normalized SHG intensity and XRD peak intensity
ratios versus time at 165°C.
temperature. It is the dramatic increase in surface
area that accounts for the primary contribution in
SHG intensity observed.
For example SHG
intensity was enhanced by a factor of ~104 on a
roughened silver surface [7]. Indeed the SHG
surface effect has been used for over ten years to
track protein conformational changes by generating
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2. Son, S.F., Asay, B.W., Henson, B.F., Sander,
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Hutchinson, R.W., Motto, R, Gilbert, E.E,
Benzinger, T.M., Corburn, M.D., Rohwer, R.K.,
Davey, R.K., Propell. and Explosiv., 6, 67-73
(1981).
CONCLUSIONS
High temperature XRD and SHG experiments
have been performed. For TATB, our results do not
indicate any solid-solid phase transformation
occurrence, even up to 340°C, which directly
conflicts with the results of Son et al. [2]. Changes
in XRD peak intensity are merely due to molecular
re-arrangement and annealing effects. The molecular
stacking distance relating to the c- lattice parameter
increases with increasing temperature. No major
peak changes in the (hkO) reflections are observed
suggesting that there is no change in molecular
conformation. Our simultaneous SHG and XRD
experiments on HMX show that SHG can give
misleading results, which bring into question the
4. Cobbledick, R.J. and Small, R.W.H., Acta.
Cryst. B30, 1918-1922 (1982).
5. Choi, C.S. and Boutin, H., Acta Cryst. B26,
1235-1240(1970).
6. Paper submitted to Ultramicroscopy (August
2001).
7.
C. K. Chen, A. R. B. de Castro, and Y. R.
Shen, Phys. Rev. Lett. 46, 145-148 (1981).
8. Campagnola, P.J., Wei, M., Lewis, A. and Loew,
L.M., J. Biophys. 77, 3341-3349 (1999).
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