Mechanochemical fabrication and properties of CL-20/RDX nano co/mixed crystals

Article abstract of DOI:10.1039/C8RA04122A

By milling 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) together, a nano CL-20/RDX co/mixed crystal explosive with a mean particle size of 141.6?nm is prepared from the raw materials, and the co/mixed crystals are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and thermal-infrared spectrometry online (DSC-IR) technology; furthermore, the impact, friction and thermal sensitivity of the samples are tested. The results show that after milling, the morphology of the co/mixed crystal explosive is near-spherical, and the particle size reveals a normal distribution. The milled sample showed the same molecular structure and surface elements as the raw materials, but the XRD test shows that CL-20/RDX has a new crystal phase and the Raman and IR spectra gave a supplementary confirmation for the existence of a cocrystal phase in the milled sample. The activation energy of the thermal decomposition of CL-20/RDX is 206.49 kJ mol^?1 higher than that of raw RDX. DSC-IR analysis showed that the thermolysis of CL-20/RDX produces a large amount of CO₂ and N₂O and a small amount of H₂O, NO₂ and NO. The mechanical sensitivity of CL-20/RDX is very low. In impact sensitivity tests with a 5 kg hammer, the special height (H₅₀) is 51.43 cm, which is higher than the values of 36.43 cm for raw CL-20 and 9.78 cm for raw RDX. In the friction sensitivity tests, the explosion probability (P) is 56%; however, the thermal sensitivity of CL-20/RDX is higher than that of the raw materials, with its 5 s burst point being only 243.51 °C.

Full text of DOI:10.1039/C8RA04122A

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Mechanochemical fabrication and properties of CL-
2
0/RDX nano co/mixed crystals
Cite this: RSC Adv., 2018, 8, 34126
a
b
a
c
Xiaolan Song, †* Yi Wang,†* Shanshan Zhao and Fengsheng Li
By milling 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and hexahydro-1,3,5-
trinitro-1,3,5-triazine (RDX) together, a nano CL-20/RDX co/mixed crystal explosive with a mean particle
size of 141.6 nm is prepared from the raw materials, and the co/mixed crystals are characterized using
scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD),
Raman spectroscopy, infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), differential
scanning calorimetry (DSC) and thermal-infrared spectrometry online (DSC-IR) technology; furthermore,
the impact, friction and thermal sensitivity of the samples are tested. The results show that after milling,
the morphology of the co/mixed crystal explosive is near-spherical, and the particle size reveals
a normal distribution. The milled sample showed the same molecular structure and surface elements as
the raw materials, but the XRD test shows that CL-20/RDX has a new crystal phase and the Raman and
IR spectra gave a supplementary confirmation for the existence of a cocrystal phase in the milled
ꢀ1
sample. The activation energy of the thermal decomposition of CL-20/RDX is 206.49 kJ mol higher
than that of raw RDX. DSC-IR analysis showed that the thermolysis of CL-20/RDX produces a large
2 2 2 2
amount of CO and N O and a small amount of H O, NO and NO. The mechanical sensitivity of CL-20/
Received 15th May 2018
Accepted 13th July 2018
RDX is very low. In impact sensitivity tests with a 5 kg hammer, the special height (H50) is 51.43 cm,
which is higher than the values of 36.43 cm for raw CL-20 and 9.78 cm for raw RDX. In the friction
sensitivity tests, the explosion probability (P) is 56%; however, the thermal sensitivity of CL-20/RDX is
DOI: 10.1039/c8ra04122a
ꢁ
rsc.li/rsc-advances
higher than that of the raw materials, with its 5 s burst point being only 243.51 C.
nanoexplosives show much lower sensitivities than
microexplosives.
1
Introduction
Energetic materials are usually dened as compounds storing
Moreover, cocrystallization is also a good method to improve the
large amounts of energy which can burn or explode rapidly performance of explosives. As early as in 1978, the 1,3,5,7-tetranitro-
1
,2
under certain external and environmental conditions.
However, high-energy explosives oen have problems of low was prepared by a decompression solvent evaporation method in
1,3,5,7-tetrazocane/ammonium perchlorate (HMX/AP) cocrystal
10
safety and high cost, limiting their further development. In which the hydrophobicity of AP was substantially improved.
recent years, improvements in insensitivity without changing Subsequently, researchers prepared 2,4,6,8,10,12-hexanitro-
the performance of explosives have been made with 4 phases 2,4,6,8,10,12-hexaazaisowurtzitane/2,4,6-trinitrotoluene
(CL-20/
11
12–14
(
i.e., surface coating, spheroidization, nanocrystallization and TNT),
CL-20/HMX,
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-
15
cocrystallization). According to current research in this eld, hexaazaisowurtzitane/dinitrobenzene
(CL-20/DNB),
1,3,5-
nanocrystallization and cocrystallization are superior to surface triamino-2,4,6-trinitrobenzene/1,3,5,7-tetranitro-1,3,5,7-tetrazocane
1
6
17–19
coating and spheroidization in terms of the enhancement of the (TATB/HMX), and other cocrystal explosives
and reduced the
3–5
insensitivity of energetic materials. In our previous studies, sensitivity of explosives to a certain extent. For example, the nano
using mechanical milling, we fabricated many kinds of nano- CL-20/HMX explosive was prepared with the spray drying method.
20
6–9
explosives and investigated their properties.
These nano- The micron-scale morphology of the cocrystal was spherical with
explosives were of particle sizes of 100–200 nm. Furthermore, size of 0.5–5 mm, of which the mechanical sensitivities were obvi-
ously lower than those of the raw materials. However, the yield of
traditional cocrystallization technology (solvent evaporation) is very
a
School of Environment and Safety Engineering, North University of China, Taiyuan
21–23
small.
It is difficult to prepare 1 g of ne cocrystal in each pot by
0
30051, China. E-mail: songxiaolan00@126.com
b
the solvent evaporation method. The mechanical milling method is
a novel cocrystallization technology that can be used to solve the
School of Materials Science and Engineering, North University of China, Taiyuan
30051, China. E-mail: wangyi528528@aliyun.com
0
c
School of Chemical Engineering, Nanjing University of Science and Technology, problem of low yield. In fact, the mechanical milling method has
Nanjing 210094, China
24,25
already been used in the fabrication of drug cocrystals.
Aer
†
Authors Yi Wang and Xiaolan Song contributed equally to this work.
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milling, different drug crystals were incorporated with the formed ball mill (YXQM-1L), with a rotation speed set at 350 rpm. Aer
cocrystals. In addition, the mechanical milling method was also 6 hours of milling, the jar was opened, and the material was
used to prepare the explosive. For example, ultrane HMX/TATB removed. Then, the nano CL-20/RDX co/mixed crystal powder
26
fabricated by using this method. Therein, the prepared HMX/ was separated from the zirconia balls using an ultrasonic
TATB particles were sizes of 100–300 nm, and the mechanical cleaner. Aer freeze-drying, the nano CL-20/RDX co/mixed
sensitivity of the prepared particles is lower than that of raw HMX. crystal powder was obtained. This ball mill can hold 4 milling
Overall, if the mechanical milling method was combined jars at a time. Thus, 40 g of nano CL-20/RDX co/mixed crystal
with nanocrystallization and cocrystallization, the sensitivity of powder can be prepared with the existing process.
high-energy explosives would be further reduced i.e. their safety
would further be improved.
2.3 Characterization and tests
The morphology was observed with eld-emission scanning
electron microscopy (SEM, JEOL JSM-7500) and transmission
electron microscopy (TEM, Philips-Tecnai). X-ray diffraction
2
Experimental
2.1 Materials
(
XRD, Bruker Advance D8) was used to investigate the phase of
Raw CL-20 was purchased from the Beijing Institute of Tech-
nology (Beijing City, P. R. China); raw RDX was purchased from
Gansu Yinguang Chemical Co., Ltd. (Baiyin City, Gan-Su Prov-
ince, P. R. China); ethyl alcohol was purchased from Tianjin
Guangfu Chemical Co., Ltd. (Tianjin City, P. R. China); zirconia
balls (f ¼ 0.3 mm) were purchased from Shandong Zibo Yubang
Ceramics Co., Ltd. (Zibo City, Shandong Province, P. R. China).
the samples. Cu Ka radiation at 40 kV and 30 mA was chosen for
the test. The structure of the samples was analyzed using a JY-
HR800 confocal Raman spectrometer manufactured by Jobin
Yvon, France. IR analysis was performed on an American
Thermo Fisher Scientic Nicolet 6700 infrared spectrometer
using potassium bromide tablets; elemental analysis was per-
formed using X-ray photoelectron spectroscopy (XPS, Ulvac-
PhiPHI-5000, Japan); thermal analysis was conducted with
differential scanning calorimetry (DSC, TA Model Q600) at
2.2 Fabrication
ꢁ
ꢀ1
Raw CL-20 (5 g), raw RDX (5 g), zirconia balls (200 g), ethyl heating rates of 5, 10, 15, and 20 C min ; DSC-IR analysis was
ꢁ
ꢀ1
alcohol (50 mL), and distilled water (50 mL) were put into an carried out at a heating rate of 10 C min using a thermal
aluminum oxide milling jar simultaneously. Aer sealing, the analysis system (TG/DSC, Mettler Toledo) coupled with a Four-
aluminum oxide milling jar was installed in an MITR planetary ier transform infrared spectrometer.
The impact sensitivity of the samples was tested with an
HGZ-1 impact instrument. The special height (H ) represents
5
0
the height from which a 5.0 kg drop-hammer results in an
explosive event in 50% of the trials. In each determination, 25
drop tests were performed to calculate the H50, with each lot
tested three times to obtain a mean value and a standard
deviation (Sdev). The testing conditions are as follows: sample
ꢁ
mass, 35 mg; temperature, 10–35 C; and humidity, #80%. The
friction sensitivity of the samples was tested with a WM-1 fric-
tion instrument. In each determination, 25 samples were tested
and the explosion probability (P, %) was obtained. Each lot was
tested three times to obtain a mean value and a standard
deviation (Sdev). Experimental conditions are shown as follows:
ꢁ
pendulum weight, 1.5 kg; swaying angle, 80 ; pressure,
2.45 MPa; and sample mass, 20 mg. Thermal sensitivity tests
Fig. 1 Optimized structure of CL-20/RDX at the DFT-B3LYP/6-31+G
level.
(i.e., 5 s bursting point tests) were carried out in compliance
with GJB772A-97 method 606.1.
Fig. 2 Molecular electrostatic potential distribution of the structures of explosives: (a) CL-20, (b) RDX, (c) CL-20/RDX.
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Fig. 3 SEM and TEM images and particle size distribution of nano CL-20/RDX co/mixed crystals.
To theoretically investigate the intermolecular interactions CL-20 interacts with the H atom of C–H in RDX. Obviously,
of CL-20/RDX, the structure of CL-20/RDX was optimized at the intermolecular interactions primarily occur due to C–H/O
2
7
28
˚
˚
DFT-B3LYP/6-31+G using Gaussian 09. Multiwfn was used to bonds. The H/O bond lengths are 2.561 A and 2.488 A,
analyze the molecular surface electrostatic potential. respectively. These values are within the length scope of the
29
hydrogen bonds. To obtain the molecular electrostatic potential
distribution of structures of CL-20, RDX and their cocrystals,
these structures were analyzed by the Multiwfn program. The
results are shown in Fig. 2 from which it can be seen that the blue
3
Results and discussion
3.1 Theoretical calculation
The optimized structure of CL-20/RDX at the DFT-B3LYP/6-31+G portion refers to the negative electrostatic region, the red portion
level is shown Fig. 1. It can be found that the O atom of N–O in refers to the positive electrostatic region, and the white portion
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morphology and size agree with the SEM images. Approximately
500 particles were counted in Fig. 3b by Nano Measurer 1.2
soware. Then, the frequency distribution curve and the
volume distribution curve were obtained and are shown in
Fig. 3e and f. It is clear that the particle size distribution
generally exhibits a normal distribution with a mean size of
1
41.6 nm. The volume distribution curve reveals that a majority
of the milled particles have diameters less than 197.6 nm (d90
97.6 nm), with a median diameter of 123.8 nm (d50 ¼ 123.8
nm).
¼
1
To investigate the sample phases of raw CL-20, raw RDX, and
CL-20/RDX aer milling, XRD analysis was performed, and the
patterns are shown in Fig. 4. Compared with the patterns of raw
CL-20 and raw RDX, new diffraction peaks appeared on the
ꢁ
ꢁ
ꢁ
patterns of CL-20/RDX, such as 12.0 , 24.3 and 34.8 . Addi-
tionally, some peak positions of CL-20/RDX did not move, but
the peak intensities changed. These changes illustrate that the
as-prepared samples did not result in simple mixing of CL-20
and RDX, but they interacted to form a new state of co/mixed
crystals. To further verify this conclusion, raw RDX and raw
CL-20 were also milled respectively and the sample “milled
RDX”, “milled CL-20”, and “mixture of milled [CL-20 + RDX]”
were analyzed by XRD analysis. The sample “mixture of milled
Fig. 4 XRD pattern of samples.
[CL-20 + RDX]” was prepared with simply blending the “milled
CL-20” and the “milled RDX”. The results were also illustrated
in Fig. 4. It can be found that XRD patterns of milled CL-20 and
milled RDX are same to the patterns of raw CL-20 and raw RDX.
Compared with the XRD patterns of “milled CL-20” and “milled
RDX”, there are no new diffraction peak presenting in XRD
pattern of “mixture of milled [CL-20 + RDX]”. However, con-
trasting the XRD patterns of “CL-20/RDX co/mixed crystal” and
“mixture of milled [CL-20 + RDX]”, it is clear that new diffraction
peaks exist in the pattern of “CL-20/RDX co/mixed crystal”. The
result further conrms the formation of some new phase(s) in
the sample of “CL-20/RDX co/mixed crystal”.
Fig. 5 Raman spectra of the samples.
Raman tests of the samples are used to explore the cause of
co/mixed crystal formation, as shown in Fig. 5. Table 1 lists the
assigned major bands of the Raman spectra of the samples. It
can be seen that most of the Raman peaks of CL-20 and RDX
refers to the neutral region. For the CL-20 and RDX molecules,
negative electrostatic regions are mainly distributed on two O
2
atoms in the –NO group, and positive electrostatic regions are
mainly distributed on –CH –, –CH and –H. However, there are
2
3
shied in the co/mixed crystal Raman spectrum. The asym-
different phenomena in the CL-20/RDX cocrystal. At the junction
of the molecules, the blue area disappears into white, meaning
that the charge is changed and tends to be neutral. This is
because the surface of electrostatic potential of the O atoms in
ꢀ1
2
metric –NO stretching vibration in CL-20 is at 1628.2 cm
ꢀ1
ꢀ1
and 1562.4 cm , but they shied to 1624.6 cm
and
ꢀ1
1
558.4 cm
in CL-20/RDX. A similar phenomenon was
ꢀ1
observed with the peak at 3000.7 cm
symmetric the –CH
, representing
stretching vibrations in RDX, which
2
the –NO group in the CL-20 molecule coincides with the surface
2
ꢀ1
of the electrostatic potential of C–H in the RDX molecule and the
positive and negative electrostatic regions overlap, resulting in
a change in electrostatic potential at the junction. This indicates
that there are intermolecular forces in the CL-20/RDX cocrystal,
which is consistent with the structural analysis results.
shied to 2997.7 cm . Interestingly, some peaks disappeared.
They represent the symmetric –NO stretching vibration in CL-
0 at 1280.9 cm to 1248.6 cm . This is due to the formation
of C–H/O hydrogen bonds between –NO (CL-20) and –CH
RDX).
IR analysis was employed to identify the molecular structure
of the nano CL-20/RDX co/mixed crystal, and the results are
The morphology of as-prepared samples was probed by SEM displayed in Fig. 6. In the IR spectrum of the nano CL-20/RDX
2
ꢀ1
ꢀ1
2
2
2
(
3.2 Morphology and structure
ꢀ1
and TEM analysis, with the results illustrated in Fig. 3. In Fig. 3a co/mixed crystal, the peak at 3023 cm reects the stretching
ꢀ
1
ꢀ1
and b, the ne particles obtained are near-spherical and of vibrations of C–H. The peaks at 1610 cm and 1331 cm
uniform size. Fig. 3c and d show that the TEM images of as- represent the asymmetric stretching vibrations of –NO and the
prepared samples and the TEM images of the particle symmetric stretching vibrations of –NO , respectively. The peak
2
2
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Table 1 Assignment of the major bands of the Raman spectra of the samples
Assignment
Raw CL-20
Co/mixed crystal
Raw RDX
Assignment
3
3
073.8
062.0
3076.8
3065.0
C–H stretching vibration
–CH stretching vibration
2
–CH
2
stretching vibration
3050.4
3050.9
3030.2
3031.7
2997.7
2942.3
2903.9
3000.7
2948.2
2906.9
C–H stretching vibration
Asymmetric –NO
2
stretching vibration
1628.2
1624.6
1609.9
1591.4
1569.3
1558.4
1608.2
1598.4
1579.0
1562.4
1589.9
1571.6
–NO
2
2
stretching vibration
1
1
461.2
422.9
1454.4
1430.6
1383.8
1345.2
1310.2
–CH
deformation vibration
C–H and –CH
2
deformation vibration
1389.4
1380.8
1351.1
1307.3
1
1
342.7
309.2
–CH
2
and N–N deformation vibration
Symmetric –NO
2
stretching vibration
1280.9
248.6
1
1
209.4
1210.0
1026.4
C–N stretching vibration
Asymmetric C–H stretching vibration
N–N stretching vibration
1129.4
1059.6
1109.3
1055.5
1029.3
C–N stretching vibration
C–N stretching vibration
C–C stretching vibration
914.4
912.0
8
837.8
819.5
789.3
80.0
881.2
844.0
Ring stretching vibration
Ring deformation vibration
Symmetric –NO stretching vibration
2
840.8
823.9
793.7
788.8
782.7
2
–NO deformation vibration
XPS spectra of the nano CL-20/RDX co/mixed crystal are illus-
trated in Fig. 7. In both spectra, the peak in Fig. 7a is associated
with O 1s, C 1s and N 1s, and the peak in Fig. 7b is associated
with N 1s. The excitation of the N 1s electron results in the two
curves, i.e., ammonia nitrogen (N–N or N–C) and nitrate
nitrogen (N–O). Because of the different electronegativity of C, N
and O elements (the electronegativity order is O > N > C), the
corresponding two kinds of nitrogen atoms have different
chemical shis. The O 1s peak allows for the determination of
the effect of the nitrate nitrogen (N–O) on the electron excitation
of O 1s. The C 1s peak illustrates the inuence of the hydrogen
atoms in –CH group and the ammonium nitrogen on electron
2
excitation of C 1s. No other elements were detected in the
spectrum, an indication that the sample was not polluted.
3.3 Thermal analysis
Fig. 6 IR spectra of samples.
Thermal analysis was conducted, and the DSC traces are shown
in Fig. 8. The DSC traces of raw CL-20 have a small endothermic
ꢀ
1
ꢁ
at 1271 cm refers to the stretching vibrations of N–N. The peak at 160 C, corresponding to the phase transformation of
ꢀ
1
ꢀ1
ꢁ
peaks at 879 cm and 752 cm correspond to the stretching CL-20. When the temperature exceeds 230 C, a distinct
vibrations of C–N and C–C, respectively. These results mean exothermic peak appearing on the DSC traces reects the
that the functional groups in the as-prepared sample structures thermal decomposition of CL-20. Furthermore, there was
are consistent with those in the raw material. Overall, the a normal phenomenon presented in the DSC traces of raw CL-
molecular structures of CL-20/RDX were not changed aer the 20, i.e., the decomposition heat increased as the heating rate
milling process.
increased. The DSC traces of raw RDX reveal that there were
In addition, XPS analysis was conducted to conrm whether neither exothermic nor endothermic processes before
ꢁ
the milling process introduces impurities into the sample. The a temperature of 200 C. When the temperature reached
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Fig. 7 XPS spectrum of the nano CL-20/RDX co/mixed crystal.
ꢀ
ꢁ
ꢀ
ꢁ
approximately 203 C, there were small endothermic peaks
EK
k ¼ A
K
exp ꢀ T
(2)
followed by the emergence of large exothermic peaks, indi-
cating the melting of RDX. Fig. 8c shows the DSC traces of nano
CL-20/RDX co/mixed crystal, and it is obvious that CL-20/RDX
was very stable before a temperature of 200 C. A small endo-
thermic peak appears at a temperature of approximately 203 C,
p
R
ꢀ
ꢁ
ꢁ
EK
K T
B
p
DGs
A exp
ꢀ
¼
exp
ꢀ
(3)
ꢁ
T R
p
h
T R
p
ꢁ
referring to the melting process of the raw RDX. Subsequently,
two exothermic peaks appeared, representing the thermal
decomposition of CL-20/RDX and RDX, respectively. In fact,
there is still a small amount of free RDX in co/mixed crystal
s
DH ¼ EK ꢀ RTp
(4)
(5)
s
s
s
DG ¼ DH ꢀ T
p
DS
explosives. However, it can be seen that the thermal decompo- where T is the peak temperature in the DSC trace with a heat-
p
ꢁ
ꢀ1
sition heat of CL-20/RDX decreased, which indicates that two ing rate of 15 C min ; K and h are Boltzmann's (K ¼ 1.381 ꢂ
B
B
ꢀ
23
ꢀ1
ꢀ34
ꢀ1
explosive molecules are not simply mixed, and most of the RDX 10
J K ) and Planck's constants (h ¼ 6.626 ꢂ 10
J s ),
and CL-20 forms a co/mixed crystal explosive. Probing the respectively; b is the heating rate; k is thermal decomposition
kinetic and thermodynamic parameters is very important for rate constant; EK and AK are the activation energy and pre-
s
mastering the thermolysis properties of energetic materials. exponential factor calculated by the Kissinger equation; DH
Herein, in order to further study the thermal decomposition is the thermal decomposition activation enthalpy, J mol ; DG
ꢀ
1
s
ꢀ1
process of nano CL-20/RDX co/mixed crystal explosives, the is the thermal decomposition activation free energy, J mol
;
s
thermodynamic and kinetic parameters of thermal decompo- and DS is the thermal decomposition activation entropy, J
ꢀ
1
ꢀ1
sition were calculated by using eqn (1)–(5). The results are mol
K .
shown in Fig. 8d and Table 2.
The Kissinger equation (eqn (1)) and Arrhenius equation
eqn (2)) were enlisted to calculate the activation energy (EK),
pre-exponential factor (A ), and rate constant (k). The results
indicate that the activation energies of raw CL-20 and raw RDX
(
b
RA
¼ ln E
K
K
E 1
K
ln
ꢀ
(1)
2
T
p
K
R T
p
ꢀ1
ꢀ1
are 274.00 kJ mol and 164.59 kJ mol , respectively. The
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Fig. 8 DSC traces of the samples.
ꢀ
1
activation energy of CL-20/RDX is 206.49 kJ mol , which is decomposition of the explosives cannot occur spontaneously.
ꢀ
1
4
6
1.9 kJ mol
higher than the E
K
of the raw RDX but Therefore, the energetic materials were in a very stable state
lower than that of raw CL-20. For the rate unless sufficient energy was introduced.
Investigation of decomposition products is of importance to
ꢀ
1
7.51 kJ mol
constant k, CL-20/RDX is larger than raw RDX and smaller than
CL-20. To gain a deeper insight into their thermal decomposi- provide deeper insight into thermolytic properties of energetics.
tion, eqn (3)–(5) were used to obtain the activation enthalpy Via DSC-IR analysis, the products of the thermal decomposition
s
s
(
(
DH ), activation entropy (DS ), and activation free energy of CL-20/RDX were detected, and the results are shown in Fig. 9.
s
s
DG ). The physical meaning of DH is the energy that the In Fig. 9a, there are two relatively sharp absorption peaks
molecules absorb to change from the common state to the between 1004.8 s and 1188.1 s, recording the decomposition of
s
activated state, and thus the value of DH is much closer to that CL-20/RDX. By analyzing the IR spectra in Fig. 9b, the main gas
s
K
of E for each sample. In this paper, we found that DH of CL- products were identied as follows. Strong peaks in the range
ꢀ1
2
0/RDX is more than that of raw RDX and less than that of raw from 2310–2360 cm revealed the existence of a large amount
CL-20, meaning activation of CL-20/RDX needs more energy of CO . Then, nitrogen oxides were also detected, in which the
2
s
ꢀ1
than raw RDX. DG is the change in Gibbs energy that reaches peaks located at 2200–2240 cm correspond to a large amount
the activated state of the explosive molecules. For all samples, of N O gas, and the peak at 1630 cm indicate NO gas. It is
2 2
the values of DG were positive numbers, meaning that thermal generally believed that the chief reason for this phenomenon in
ꢀ
1
s
Table 2 Thermodynamic and kinetics parameters derived from the DSC traces
Thermodynamics
Kinetics
(kJ mol
s
ꢀ1
s
ꢀ1
s
ꢀ1 ꢀ1
ꢀ1
ꢀ1
Samples
T
p
(K)
DH (kJ mol
)
DG (kJ mol
)
DS (J mol
K
)
E
K
)
ln A
K
k (s )
Raw CL-20
Raw RDX
CL-20/RDX
511.95
520.15
499.45
269.74
160.26
202.33
125.00
129.03
122.52
282.72
60.04
159.80
274.00
164.59
206.49
65.00
38.24
50.19
1.88
1.20
1.60
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Fig. 9 DSC-IR spectra of CL-20/RDX: (a) the DSC curve in DSC-IR analysis; (b) IR spectra of gas products intercepted at different times.
Table 3 Results of sensitivity tests for the samples
Impact sensitivity
50 (cm)
Friction sensitivity
Thermal sensitivity
ꢁ
Samples
H
S
dev
P (%)
S
dev
T
5s ( C)
Raw CL-20
Raw RDX
CL-20/RDX blend
CL-20/RDX co/mixed crystal
15.00
41.65
30.18
51.43
3.86
7.11
4.25
5.71
100%
74%
100%
56%
0
3.34
0
283.57
301.20
—
2.85
243.51
the IR spectra is that the split radical of $NO
2
(from N–NO
2
)
However, the thermal sensitivity test results are contrary to the
reacted with the –CH– fragment (from the carbon chain). The mechanical sensitivity test results. The 5 s burst point (T5s) of
ꢀ
1
ꢁ
peak at 1749 cm reected the fragment of CH
6
2
O. The peak at CL-20/RDX is only 243 C, which means that it has the highest
ꢀ1
69 cm implies the existence of HCN resulting from the thermal sensitivity compared to those of the raw materials. CL-
rupture of the N–N bond in nitroamine molecules. Actually, the 20/RDX is not suitable for use as a heat-resistant explosive.
thermal decomposition of explosives is related to the cracking
of the molecule, independent of the crystal phase, and it does
4
Conclusions
not alter the structure of the molecule whether it is a cocrystal or
a mixed crystal. As a rule, the thermolysis of nitroxide and The mechanical milling method is used to prepare nano CL-20/
nitroamine explosives begins with the cracking of C–NO
or N– RDX co/mixed crystal explosives with a mean particle size of
NO
bonds in the molecule. In this paper, we nd that a large 141.6 nm. Before and aer milling, the surface elements and
2
2
amount of CO gas rather than CO appeared on the IR spectra, molecular structures of CL-20/RDX do not change compared to
2
indicating that at relatively low temperatures, decomposition those of raw CL-20 and raw RDX. However, new crystal phases
proceeded nearly to completion and fully released its chemical were presented in the XRD pattern. Raman spectra imply that
energy.
the formation of CL-20/RDX cocrystals originates from C–H/O
hydrogen bonds between –NO and –CH . The DSC analysis is
2
2
employed to probe the thermolytic properties of the samples.
CL-20/RDX has a high activation energy and activation enthalpy
3.4 Sensitivities
To evaluate the safety of CL-20/RDX, tests on the impact, fric- of thermal decomposition. DSC-IR is performed to detect
tion and thermal sensitivities were performed, with the results decomposition products of CL-20/RDX. The results show that
presented in Table 3. Raw CL-20 and raw RDX are of the mean the decomposition products are mainly gases, such as CO2,
size of 59.21 mm and 88.03 mm, respectively. The sample “CL-20/ N O, and NO . Finally, the impact sensitivity, friction sensitivity
2
2
RDX blend” is a simple mixture of raw CL-20 and raw RDX. Aer and thermal sensitivity of the samples were tested. The tests
milling, the impact and friction sensitivities of the nano CL-20/ revealed that the mechanical sensitivity of CL-20/RDX is much
RDX co/mixed crystal signicantly decreased. As seen in Table lower than those of the raw materials and their blends and that
3, the special heights (H50) of raw CL-20 and raw RDX were the thermal sensitivity of CL-20/RDX does not decrease but
15.0 cm and 41.65 cm, respectively, while the special height of increases.
the nano co/mixed crystal explosive was 51.43 cm, which was
higher than the 36.43 cm value of raw CL-20 and 9.78 cm of raw
RDX. In addition, the explosion probability of friction (P) was
Conflicts of interest
56% lower than that of raw CL-20, raw RDX and their blends. There are no conicts to declare.
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Paper
1
4 Q. Q. He, Y. C. Liu and L. W. Yan, Preparation of high purity
CL-20/HMX cocrystal explosive by suspension method, Chin.
J. Explos. Propellants, 2018, 41, 82–85.
Acknowledgements
This research was supported by the Weapon Equipment Pre-
Research Fund of China (Grant No.: 6140656020201; Recip- 15 Y. P. Wang, Z. W. Yang, H. Z. Li, X. Q. Zhou, Q. Zhang,
ient: Yi Wang).
J. H. Wang and Y. C. Liu, A Novel Cocrystal Explosive of
HNIW with Good Comprehensive Properties, Propellants,
Explos., Pyrotech., 2014, 39, 590–596.
1
6 C. H. Hou, Z. Q. Liu and Y. P. Zhang, Study on preparation
and properties of TATB/HMX cocrystal explosive, Chin. J.
Explos. Propellants, 2018, 40, 44–49.
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