Compatibility and thermal decomposition mechanism of nitrocellulose/Cr2O3 nanoparticles studied using DSC and TG-FTIR

Article abstract of DOI:10.1039/c8ra09632e

Nano metal oxides are common combustion catalysts for enhancing the burning rate of solid propellants. Cr₂O₃ nanoparticles (NPs) are efficient combustion catalysts for the pyrolysis of energetic components. In this study, Cr₂O₃ NPs were synthesized via a modified sol-gel method and further used for studying the thermal decomposition of nitrocellulose (NC). Differential scanning calorimetry (DSC) and thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) analyses indicate that the Cr₂O₃ NPs can be safely used with NC and the mechanism of the reaction between Cr₂O₃/NC and pure NC follows the Avrami-Erofeev equation: f(α) = 3(1 ? α)[?ln(1 ? α)]^1/3/2. The peak temperature and activation energy (E_a) for the thermal decomposition of Cr₂O₃/NC are lower than those of pure NC. NO₂ was detected at a lower temperature after NC was mixed with Cr₂O₃ NPs; this indicated the catalytically accelerated bond cleavage of NC by Cr₂O₃ NPs.

Full text of DOI:10.1039/c8ra09632e

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Compatibility and thermal decomposition
mechanism of nitrocellulose/Cr O nanoparticles
2
3
Cite this: RSC Adv., 2019, 9, 3927
studied using DSC and TG-FTIR
Yu Guo, Ningning Zhao, Ting Zhang, Hujun Gong, Haixia Ma, *a Ting An,c
a
a
a
b
c
c
Fengqi Zhao and Rongzu Hu
Nano metal oxides are common combustion catalysts for enhancing the burning rate of solid propellants.
Cr nanoparticles (NPs) are efficient combustion catalysts for the pyrolysis of energetic components. In
this study, Cr NPs were synthesized via a modified sol–gel method and further used for studying the
thermal decomposition of nitrocellulose (NC). Differential scanning calorimetry (DSC) and
thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) analyses indicate that the Cr
2
O
3
2 3
O
2 3
O
2 3
NPs can be safely used with NC and the mechanism of the reaction between Cr O /NC and pure NC
1/3
Received 23rd November 2018
Accepted 15th January 2019
follows the Avrami–Erofeev equation: f(a) ¼ 3(1 ꢀ a)[ꢀln(1 ꢀ a)] /2. The peak temperature and
activation energy (E
was detected at a lower temperature after NC was mixed with Cr
accelerated bond cleavage of NC by Cr NPs.
a
) for the thermal decomposition of Cr
2
O
3
/NC are lower than those of pure NC. NO
2
DOI: 10.1039/c8ra09632e
2
O
3
NPs; this indicated the catalytically
rsc.li/rsc-advances
2 3
O
in nature, Cr
2 3
O is environmentally compatible, low-cost, less
Introduction
toxic, and naturally abundant and exhibits good catalytic
Solid propellants as the power source of missiles and rocket activity; moreover, it has been widely applied as a burning
engines are signicant. They are composite materials that are catalyst. For instance, annealed Cr nanorods could accel-
O
2 3
made up of an oxidation agent, a burning catalyst, a binder and erate the thermal decomposition process of cyclo-
so on. For solid propellants, the core of the propulsion tech- trimethylenetrinitramine (RDX) and reduce the decomposition
1
ꢁ
9
nology is combustion performance. The combustion perfor- temperature by 10 C. Another study showed that nanosized
mance refers to the burning rate and the pressure index. A Cr NPs decreased the ignition delay time by a factor of 3.5
propellant without a catalyst has a low burning rate, a high- (16 ꢂ 2 vs. 54 ꢂ 4 ms) and accelerated the combustion rate
2 3
O
ꢀ1
pressure index, and unstable combustion. Therefore, (340 ꢂ 10 mm s ) of the Al/Cr
2
O
3
thermite, which was fabri-
a burning-rate catalyst is an indispensable key material in solid cated by Cr
propellants. Nitrocellulose (NC) is a common oxidant in solid (F z 50 nm).
propellants, and its decomposition properties are closely con-
O
micro- or nanoparticles (F z 20 nm) and Al NPs
2
3
10
A number of efforts have been devoted towards the explo-
nected to the combustion performances of propellants. Several ration of the catalytic effects of Cr , and as a burning catalyst,
studies have demonstrated that by adding burning catalysts, the Cr exhibits excellent catalyst performance. However, studies
2
2 3
O
2 3
O
oxidizer decomposition can be signicantly accelerated and the on the compatibility and thermal decomposition mechanism of
3
burning rate of the propellant can be enhanced. In recent nitrocellulose/Cr
2
O
3
NPs have been rarely reported. Compati-
years, nanoscale metal oxides have exhibited high catalytic bility is one of the most important measures for energetic
activity due to their large surface area, high mobility of their materials, which determines whether two components can be
surface atoms, small particle size, and varied lattice defects, used together. Moreover, investigation of the thermal decom-
which have drawn extensive attention in various elds such as position mechanism could help to understand the catalytic
4
5
6
in photocatalysis, electrochemistry, solid-oxide fuel cells,
mechanism and design efficient catalysts.
In this study, we prepared Cr NPs using a sol–gel
method and studied the thermal behaviour and non-
isothermal decomposition kinetics of the Cr /NC
7
8
magnetic materials, etc. As a common transition metal oxide
2 3
O
2 3
O
a
School of Chemical Engineering, Northwest University, Shaanxi, Xi'an 710069, P. R.
composite. DSC thermal analysis was applied to evaluate the
compatibility between Cr O NPs and NC. Thermal decom-
China. E-mail: mahx@nwu.edu.cn
2
3
b
State Key Laboratory of Continental Dynamics, Northwest University, Shaanxi, Xi'an
position mechanism of NC under catalysis by Cr O3 was
2
7
10069, P. R. China
c
investigated using DSC and TG-FTIR techniques. The results
Science and Technology on Combustion and Explosion Laboratory, Xi'an Modern
Chemistry Research Institute, Shaanxi, Xi'an 710065, P. R. China
indicate that the Cr
2
O
3
NPs have good compatibility with NC
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and can be used as a potential catalyst in the solid propellant NC (12.6% N, Xi'an Modern Chemistry Research Institute), and
due to their potential catalytic effect on the decomposition of Al powders (z50 nm, Jiaozuo Banlv Nano Material Engineering
the main components of propellants.
Co.). The chemicals were of analytical grade and used without
any further purication. Deionized water was used throughout
the experiment.
Caution! NC is an explosive hazardous material. It is
strongly recommended to wear safety glasses and face shields
while handling these materials in large quantities. Metal
spatulas are strictly forbidden. Fire and static electricity
discharge should be avoided. For the TG-FTIR and DSC anal-
yses, not more than 2 mg of sample should be used to avoid
Experimental
Materials
The chemicals used in the experiment are as follows: chromic
chloride (CrCl
alcohol (C OH, anhydrous, Xilong Chemical Reagent Co.),
propylene oxide (C H O, 99.0%, Kelong Chemical Reagent Co.),
3 2
$6H O, 99.0%, Xilong Chemical Reagent Co.),
2
H
5
3
6
2 3 2 3
Fig. 1 SEM and TEM images of the prepared samples; (a and b) SEM images of Cr O ; (c and d) TEM images of Cr O ; (e and f) SEM images of the
Cr /NC composite and the EDS spectra of Cr NPs, respectively.
2
O
3
2 3
O
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instrument damage. NC was provided by the Xi'an Modern
Chemistry Research Institute.
Preparation of the Cr
Cr nanopowders were prepared by a sol–gel method. In the
synthesis of the Cr gel, CrCl $6H O was used as the chro-
mium source, and propylene oxide was used as the complexing
agent. Typically, 0.66 g of CrCl $6H O was dissolved in 2.0 mL
2 3
O NPs
2 3
O
2
O
3
3
2
3
2
of anhydrous ethanol, and a green transparent solution was
obtained by ultrasonic dispersion at room temperature. Then,
0
.85 mL of propylene oxide was added to form a uniform,
stable dark green wet gel within 10 minutes. Aer aging for
2
6
4 h, the precursors were removed by drying in an oven at
0 C. The resulting xerogel was calcined at 800 C for 2 h.
ꢁ
ꢁ
2 3
Fig. 3 The FTIR spectrum of Cr O NPs.
Preparation of composite materials
The Cr O NPs were evenly mixed with NC to obtain composite
2
3
materials at a 1 : 1 proportion by mass. The products were used
for the DSC experiment to assess the compatibility of Cr NPs
with the main component of the double-base (DB) propellant.
2 3
O
between electrodes. The morphology and size of the as-obtained
products were investigated via transmission electron micros-
copy (TEM) and high-resolution TEM using Libra 200FE (Carl
Zeiss SMT Pte Ltd., Germany). FTIR spectra were obtained using
Samples characterization
Physical phase, composition, morphology and structure of the the Bruker Tensor 27 (KBr) spectrometer. The Brunauer–
raw materials and products were characterized by XRD, SEM- Emmett–Teller (BET) specic surface areas were obtained using
EDS and FTIR spectroscopy. X-ray diffractograms were ob- the Micromeritics Tristar 3000 automated gas adsorption
tained via the D/MAX-3C (Japan) instrument using Cu Ka radi- analyzer.
ation (l ¼ 0.15406 nm) at 40 kV voltage and 40 mA current in the
Thermal behaviors of the samples were determined using
range from 15 to 65 . Scanning electron microscopy (SEM) differential scanning calorimetry (DSC) at the heating rate of
ꢁ
ꢁ
ꢁ
ꢀ1
ꢁ
observations were carried out using the Quanta 400 FESEM (FEI 10 C min from room temperature to 350 C under a N2
ꢀ1
Co., USA) at the acceleration voltage of 30 kV. Energy Dispersive atmosphere at the ow rate of 50 mL min under ambient
Spectroscopy (EDS) were obtained using the INCAIE350 testing atmospheric pressure. To explore the reaction mechanism of
device, obtained from OXFORD Instruments INC (UK), with the the intense exothermic decomposition processes of NC and
discharge voltage of 4–10 kV and a distance of exactly 1 mm Cr O /NC composite energetic materials and obtain the
2
3
2 3
Fig. 4 DSC curves of NC and Cr O /NC obtained at the heating rate of
ꢁ
ꢀ1
Fig. 2 XRD pattern of the Cr
2
O
3
NPs.
10 C min .
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Table 1 Calculated values of the kinetic parameters of the decom- ꢄ100 nm. As determined from the typical TEM image (Fig. 1c),
a
2 3
position reaction for Cr O /NC
thickly dotted pits were found on the surface of the Cr O NPs.
2
3
The cross-sectional HRTEM image of a small part of the Cr O3
ꢁ
ꢀ1
ꢀ1
ꢀ1
2
Eq
b/ C min
E/kJ mol
lg (A/s
)
r
nanoparticle is shown in Fig. 1d. Only one set of clear lattice
MacCallum–Tanner
5.0
172.16
178.83
188.42
191.07
189.04
187.49
170.73
177.03
186.08
188.59
186.67
185.20
171.61
178.12
187.57
190.16
188.10
186.53
175.57
182.14
191.63
194.24
192.20
190.64
190.64
178.12
187.57
190.16
188.10
186.53
184.40
16.57
17.34
18.39
18.68
18.45
18.29
16.45
17.18
18.17
18.45
18.23
18.08
16.56
17.30
18.33
18.62
18.39
18.22
15.34
16.07
17.08
17.36
17.14
16.98
16.98
17.31
18.33
18.62
18.39
18.22
17.64
0.9961 fringes with an interplanar distance of 0.36 nm could be seen,
1
1
2
2
3
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
5.0
0.0
0.9965 which could be indexed to the 012 plane of the Cr
2 3
O structure.
0.9962
Good crystallinity is also conrmed by the corresponding fast
0.9966
0.9964
0.9965
0.9961
Fourier-transform (FFT) image (the inset in Fig. 1d). As dis-
played in Fig. 1e, the Cr
2 3
O /NC composite material shows
ˇ
ˇ
´
a claviform structure with rough and irregular surfaces,
Satava–Sestak
1
1
2
2
3
0.9965 diameter in the range from 25 to 35 mm and length in the range
0.9962
0.9966
0.9964
0.9965
0.9957
from 80 to 200 mm. Elemental analysis (as shown in Fig. 1e)
reveals that the presence of only chromium and oxygen
elements agrees with the chemical composition of Cr O .
2 3
Agrawal
Crystal structure and phase composition of the resulting
1
1
2
2
3
0.9962 sample were characterized using the X-ray powder diffraction
0.9958
(
XRD) technique. Fig. 2 displays the XRD pattern of the as-
0.9963
0.9961
0.9962
prepared Cr O NPs. The reection peaks shown in Fig. 2 are
consistent with those provided in the Joint Committee on
2
3
General integral
Universal integral
0.9959 Powder Diffraction Standards (JCPDS) card 38-1479 of the Cr O3
2
1
1
2
2
3
0.9963 structure. The sharp peaks suggest that the Cr
0.9960
2 3
O NPs are highly
crystalline. These peaks are attributed to the rhombohedral
0.9965
0.9963
0.9963
ꢀ
structure (space group R3c) with the unit cell parameters a ¼ b ¼
˚
˚
ꢁ
ꢁ
4
.9619 A, c ¼ 13.7128 A, a ¼ b ¼90 and g ¼ 120 of the Cr
2 3
O
0.9963 phase.
1
1
2
2
3
0.9962
0.9958
0.9963
0.9961
0.9962
The FTIR spectrum of Cr O NPs (Fig. 3) in the 4000–
2 3
ꢀ1
4
1
00 cm region displayed the characteristic bands at 3448,
ꢀ1
ꢀ1
636, 649, and 582 cm . The band at 3448 cm is due to the
O–H stretching of the surface hydroxyls groups and undisso-
1
1
ꢀ1
Mean
Flynn–Wall–Ozawa
Kissinger
ciated water molecules. Absorption at 1636 cm
1.0000 relatively assigned to the bending modes of undissociated
1.0000
can be
189.16 (Epo)
190.86 (E
179.14
ꢀ1
K
)
18.87
water molecules. Strong bands at 649 and 582 cm
observed and indicated the presence of crystalline a-Cr
were
O ;
2 3
Mean (Eeo, Epo, E
K
)
1
2
a
E with the subscript eo and po is the apparent activation energy this was consistent with the XRD analysis results. Further-
) and the peak temperature
), respectively, by the Ozawa's method, E with the subscript K is the
obtained from the onset temperature (T
e
more, the absorption peaks of a-Cr O NPs have an obvious
2
3
(
T
p
p
apparent activation energy obtained from the peak temperature (T )
by the Kissinger's method.
corresponding kinetic parameters [apparent activation energy
ꢀ
1
ꢀ1
a
(E /kJ mol ) and the pre-exponential constant (A/s )] and the
most probable kinetic model function, the DSC curves were
obtained at the heating rates of 5.0, 10.0, 15.0, 20.0, 25.0 and
ꢁ
ꢀ1
3
0.0 C min . Thermal decomposition studies of NC and
Cr O /NC were also performed using the TG-FTIR simultaneous
2
3
analysis device (Netzsch STA 409, Brucker V70) under a nitrogen
ꢁ
ꢀ1
atmosphere at the heating rate of 10 C min
.
Results and discussion
The morphology and size of the nanometer oxides and
composite materials were investigated by SEM and TEM.
Fig. 1a and b show the SEM images of Cr O NPs obtained with
2
3
ꢃ50 000 and ꢃ100 000 magnication, respectively. In these
images, a loosely agglomerated structure can be observed. The
Fig. 5
a 2 3
E vs. a curves of NC and Cr O /NC obtained by the Flynn–
2 3
Cr O
NPs are spherical in shape with an average diameter of Wall–Ozawa's methods.
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blue-shi via the quantum connement effect relative to those
of the bulk crystal.
E
a
–a (a, the extent of the reaction) relation (Fig. 5). To compare
1
3
the thermal behaviors of NC and the composite material, the
Ea–a relation of NC was obtained by the same method as
shown in Fig. 5. The Cr O NPs had an obvious catalytic effect
2
3
Thermal behavior of NC and composite materials
on NC due to their lower apparent activation energy. From the
curve of Cr /NC shown in Fig. 5, it could be seen that the
values of Ea in the range of a ¼ 0.25–0.675 were in good
agreement with the calculated values of E obtained by the
Kissinger's method and Ozawa's method. The E values calcu-
lated using the Flynn–Wall–Ozawa equation were used to
check the validity of the activation energy values determined
by other methods.
2 3
O
Fig. 4 shows the thermal decomposition behavior of NC and
ꢁ
ꢀ1
Cr O /NC at the heating rate of 10 C min . It can be seen that
2
3
a
the entire decomposition processes of NC and Cr
2
O
3
/NC are
similar, and the peak temperature of Cr
that of pure NC at 0.14 C. To obtain the kinetic parameters
2
O
3
/NC is lower than
1
4
ꢁ
[
apparent activation energy (E
of the exothermic main decomposition reaction for Cr
and nd out the catalytic effect of Cr , DSC curves were ob-
tained at the heating rates of 5.0, 10.0, 15.0, 20.0, 25.0 and
a
) and pre-exponential factor (A)]
2 3
O
/NC
ˇ
ˇ
´
The integral equations (MacCallum–Tanner, Satava–Sestak,
Agrawal, general integral, universal integral, and Flynn–Wall–
Ozawa) have been cited to obtain the values of E, A and the most
probable kinetic model function G(a) from a single non-
2 3
O
ꢁ
ꢀ1
3
0.0 C min . Moreover, two model-free isoconversional
14
methods, the Kissinger equation and the Flynn–Wall–Ozawa
(
16
15
isothermal DSC curve.
F–W–O) equation, were employed for calculations.
Herein, forty-one types of kinetic model functions reported
in the ref. 17 and basic data were put into the integral equa-
tions and the Kissinger equation for calculation, respectively.
The kinetic parameters and the probable kinetic model func-
tion were selected by the logical choice method, satisfying the
ordinary range of the thermal decomposition kinetic
As listed in Table 1, the E
a
obtained by the Kissinger
14
ꢀ1
method is determined to be 190.86 kJ mol , and the pre-
1
8.87 ꢀ1
exponential constant (A) is 10
s
. The linear correlation
obtained by the Ozawa's
coefficient (r
k
) is 1.0000. The value of E
a
15
ꢀ1
method is 189.16 kJ mol , and the value of r is 1.0000.
o
The values of E obtained by the Ozawa's method from the
a
ꢀ1
parameters for energetic materials (E ¼ 80–250 kJ mol
,
isoconversional DSC curves at the heating rates of 5.0, 10.0,
ꢀ
1
ꢁ
ꢀ1
log A ¼ 7–30 s ). These data together with the appropriate
1
5.0, 20.0, 25.0 and 30.0 C min were used to draw the
a
2 3
Table 2 Calculated values of the kinetic parameters of the decomposition reaction for NC and Cr O /NC
ꢀ1
ꢁ
ꢁ
ꢁ
ꢁ
s
ꢀ1 ꢀ1
s
ꢀ1
s
ꢀ1
Sample
NC
E
a
log(A/s
)
T
e0/ C
T
p0/ C
T
be0/ C
T
bp0/ C
DS J mol
k
DH kJ mol
DG kJ mol
207.48
184.40
20.22
17.64
181.76
180.74
197.00
193.50
191.42
192.18
206.69
203.49
138.40
112.61
199.68
190.86
134.61
138.31
Cr O /NC
2 3
a
e p
The values (Te0 and Tp0) of the onset temperature (T ) and peak temperature (T ), the thermal ignition temperature (Tbe0), and the critical
temperatures of thermal explosion (Tbp0).
2 3
Fig. 6 TG-DTG curves of NC (a) and Cr O /NC (b).
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values for the linear correlation coefficient (r), standard mean
e
The values Te0 and Tp0 of the onset temperature (T ) and the
square deviation (S) and believable factor (d, where d ¼ (1 ꢀ r) peak temperature (T ) corresponding to b / 0 obtained by eqn
p
ꢁ
ꢁ
S) are presented in Table 1. The values of E are very close to (2) are 180.74 C and 193.50 C, respectively.
each other. The values of E and A obtained from a single non-
2
3
T
e or p ¼ Te0 or p0 + ab
i
+ bb
i
+ cb
i
i ¼ 1–6
(2)
isothermal DSC curve are in good agreement with the calcu-
lated values obtained by the Kissinger's method and Ozawa's
method. Therefore, we concluded that the main exothermic
where a, b and c are coefficients.
The corresponding critical temperatures of thermal explo-
sion (T ) obtained from eqn (3) taken from the ref. 17 is
2 3
decomposition reaction mechanism of Cr O /NC could be
b
ꢁ
classied as the nucleation and growth mechanism, and the
mechanism followed the Avrami–Erofeev equation, where n ¼
2
03.49 C.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
8
2
/3. The reaction mechanism of the exothermic main
2
E
O
ꢀ
E
O
ꢀ 4E
O
RTPO
T
b
¼
(3)
decomposition process of the compound is classied as f(a) ¼
2
R
1
/3
2/3
3
(1 ꢀ a)[ꢀln(1 ꢀ a)] /2, G(a) ¼ [ꢀln(1 ꢀ a)] . f(a) was
ꢀ
1
ꢀ1
1/3
where R is the gas constant (8.314 J mol
value of E obtained by the Ozawa's method.
K
) and E
O
is the
substituted with 3(1 ꢀ a)[ꢀln(1 ꢀ a)] /2, E was substituted
ꢀ
1
17.64 ꢀ1
with 184.40 kJ mol and A was substituted with 10
s
in
s
The entropy of activation (DS ), enthalpy of activation
eqn (1).
s
s
(
DH ) and free energy of activation (DG ) corresponding to T ¼
A
da=dT ¼ b f ðaÞeꢀE=RT
T
PO
, E ¼ E and A ¼ A (obtained by eqn (4)–(6)) are 112.61 J
(1)
a
k
k
ꢀ1
ꢀ1
ꢀ1
ꢀ1
mol
K , 190.86 kJ mol and 138.31 kJ mol , respectively.
where f(a) and da/dT are the differential model function and the
rate of conversion, respectively.
The kinetic equation for the exothermic decomposition
reaction can be described as
k
B
T
s
DS =R
A ¼
e
(4)
(5)
h
ꢃ
ꢄ
ꢃ
ꢄ
k
B
T
DSs
DHs
A expð ꢀE
a
=RTÞ ¼
exp
exp
ꢀ
h
R
RT
1
7:82
da
dT
10
ꢀ
ꢁ ꢂ
1=3
4
¼
ð1 ꢀ aÞ½ꢀlnð1 ꢀ aÞꢅ
exp ꢀ2:22 ꢃ 10
T
b
s
s
s
DG ¼ DH ꢀ TDS
(6)
Fig. 7 IR spectra at some temperature before decomposition (T
x
), the initial temperature (T ), the peak
i
), the extrapolated onset temperature (T
e
temperature (T ), the extrapolated end temperature (T ) and the final temperature (T
L
c
f
2 3
) evolved from the degradation of NC (a) and Cr O /NC (b).
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a
Table 3 Gaseous products generated during the decomposition processes of NC and Cr
2
3
O /NC at different temperatures
NC
Temperature/ C
Cr
2 3
O /NC
ꢁ
ꢁ
Gaseous products
Temperature/ C
Gaseous products
1
1
2
2
2
2
64.92 (T
78.81 (T
00.16 (T
09.43 (T
21.70 (T
47.45 (T
x
)
)
H
H
H
H
H
H
2
2
2
2
2
2
O, CO
O, CO
O, CO
O, CO
O, CO
O, CO
2
2
2
2
2
2
159.96 (T
175.93 (T
195.32 (T
209.79 (T
226.18 (T
269.02 (T
x
)
)
H
H
H
H
H
H
2
2
2
2
2
2
O, CO
O, CO
O, CO
O, CO
O, CO
O, CO
2
2
2
2
2
2
i
, NO
, NO
, NO
, NO, HCOOH
, NO, HCOOH
2
2
2
i
, NO
, NO
, CO, NO
, CO, NO
2
2
e
)
, NO, CO
, NO, N O, HCHO, HCOOH
e
)
, NO, CO
L
)
2
L
)
2
, NO, N
, NO, N
2
2
O, HCHO, HCOOH
O, HCHO, HCOOH
c
)
c
)
)
2
f
)
f
, NO, NO
2
, HCOOH
a
T
x
, some temperature below the initial decomposition temperature; T
i
, the initial decomposition temperature; T
, the nal temperature.
e
, the extrapolated onset
temperature; T
L
, the peak temperature; T , the extrapolated end temperature; and T
c
f
ꢀ
23
where k is the Boltzmann constant (k ¼ 1.3806505 ꢃ 10
ꢀ34
J
The compatibility analysis
B
B
ꢀ1
K
) and h is the Planck's constant (h ¼ 6.6260693 ꢃ 10
J s).
Compatibility of two or more materials refers to their ability to
maintain their original physical and chemical properties when
mixed together. Poor compatibility may result in changes in
the formulation quality, activity and efficiency. It can also lead
to variations in thermal safety. Analytical techniques such as
The thermal behavior of NC was analyzed using the same
method. The results show that the reaction mechanism of the
intense exothermic decomposition process can be classied as the
1/3
2/3
reaction order f(a) ¼ 3(1 ꢀ a)[ꢀln(1 ꢀa)] /2, G(a) ¼ [ꢀln(1 ꢀ a)]
.
The calculated values of the kinetic parameters of the decompo-
sition reaction for NC and Cr O /NC are listed in Table 2.
differential scanning calorimetry (DSC), pressure differential
2
3
20,21
scanning
calorimetry
(PDSC),
thermogravimetry/
According to the ref. 19, the higher the values of T , T , T , E
a
e0
p
b
differential thermal analysis (TG/DTA), isothermal storage
tests (IST), pressure isothermal storage tests (PIST), mano-
metric vacuum stability tests (MVST), pressure vacuum
s
and DH , the better the heat resistant feature of the materials.
This means that materials with low values of Te0, T , T , E and
DH easily decompose when heated. Compared to NC, the
Cr /NC composite will easily decompose based on the
calculated thermal parameters.
p
b
a
s
22
stability tests (PVST), and isothermal microcalorimetry
2
O
3
23
(
IMC) have been used for compatibility studies. The DSC
24
thermal analysis method is widely applied to evaluate the
Fig. 8 The density distribution of the gas phase decomposition products of NC (a) and Cr
2
O
3
/NC (b). The temperature refers to the peak
temperature.
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chemical compatibility between explosives and the contacted in Fig. 7 and Table 3. For pure NC, the IR absorption peaks of
ꢀ
1
ꢀ1
materials at high temperatures due to its features such as H O (3600–3740 cm ), CO2 (2300–2380, 660–670 cm ) and
2
ꢀ1
26,27
ꢁ
are detected at 178.81 C (T ), which
i
time-saving ability, reduced sample usage and operation NO (1593–1635 cm
)
2
convenience. Herein, the DSC thermal analysis method was is the initial temperature of NC. The IR spectrum acquired at
ꢁ
applied to evaluate the compatibility of Cr
2
O
3
NPs with NC. 164.92 C was obtained (Fig. 7a (T
x
)) to determine whether H
2
O
ꢁ
The experimental data show that the decomposition peak and CO
2
detected at 178.81 C originated from the decompo-
ꢁ
temperature of Cr
2
O
3
/NC is lower than that of NC at 0.14 C, sition of NC. The intensities of the IR absorption of H
2
ꢁ
O and
which is within the experimental and instrumental error. The CO
result indicates that the compatibility of the Cr powder 178.81 C; this indicated that H
with NC is good. Therefore, the Cr O /NC composite can be degradation products of NC. Therefore, NO was considered to
2
remained basically unchanged at both 164.92 C and
ꢁ
2
O
3
2 2
O and CO were not the initial
2
5
2
3
2
used as a component in the preparation of propellants and be the initial degradation product; this meant that the breakage
explosives.
of the O–NO bond was deemed to be the rst decomposition
2
28–32
step as reported in previous studies.
increased (T , T , T ), the gaseous products such as NO (1762–
2
965 cm ), CO (2150–2194 cm ), N O (2200–2300 cm ),
When the temperature
e
p
c
Thermal decomposition mechanism
ꢀ1
ꢀ1
ꢀ1
1
The TG-FTIR simultaneous analysis technique was employed to
investigate the thermal degradation process of NC and Cr
NC at the heating rate of 10 C min . It was found from the TG-
DTG curves of NC and Cr O /NC (Fig. 6) that only one stage of
the total mass loss occurred. The onset decomposition
temperature (T
than that of NC.
The apparent variation in the IR characteristic absorption
peaks of the gaseous decomposition products of NC and Cr O /
2 3
NC formed due to the thermal decomposition process at typical
temperatures, including the temperature before decomposition
ꢀ1
ꢀ1
HCHO (2700–2900 cm , 1720–1740 cm ), and HCOOH (1080–
2
O
3
/
ꢀ1
ꢀ1
27,28
ꢀ1
1
1
128 cm ) were detected.
769 cm are attributed to the modes of the C–H and C]O
The peaks at 2852 cm and
ꢁ
ꢀ1
2
3
28
stretching bands of HCHO, respectively, which are generated
by the –CH ONO group. Evolution of the HCOOH gas is due
33
2
2
ꢁ
ꢁ
i
) of Cr
2
O
3
/NC is 175.93 C, which is 2.88 C lower
31–33
to the secondary autocatalytic reactions of NC.
the decomposition process (T ), the IR absorption peaks of H
are still easily identiable in Fig. 7a
). The species and the intensities of the gas products ob-
At the end of
f
2
O,
26,27
2
CO , NO and HCOOH
(T
f
tained via the degradation of NC are also shown in the 3D FTIR
spectra (Fig. 9a).
(
T ), the initial temperature (T ), the extrapolated onset
x
i
From the gaseous product data obtained at T and T of
x
i
temperature (T
e
), the peak temperature (T
p
), the extrapolated
Cr O /NC listed in Table 3, we can see that H O and CO
2
2
3
2
end temperature (T ), and the nal temperature (T ), are shown
c
f
ꢁ
ꢀ1
.
Fig. 9 3D-IR spectra of the gas products of NC (a) and Cr
2
O
3
/NC (b) at the heating rate of 10 C min
Fig. 10 (a) SEM image of the Cr
2
O
3
/NC reaction residue, (b) XRD pattern of the Cr
2 3 2 3
O /NC reaction residue, and (c) FTIR spectra of the NC, Cr O /
ꢀ
l
NC and Cr /NC residues, inset: FTIR spectrum of the Cr
O
2 3
O
2 3
/NC residue in the range of 1700–1000 cm .
3934 | RSC Adv., 2019, 9, 3927–3937
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a
Table 4 Assignments of the IR characteristic absorption peaks of NC, NC residue, Cr
2
O
3
/NC and Cr
2
3
O /NC residue
NC
NC residue
Frequency/cm
3429
Cr
2
O
3
/NC
Cr
2
O
3
/NC residue
ꢀ1
ꢀ1
ꢀ1
ꢀ1
Frequency/cm
Assignments
Assignments Frequency/cm
Assignments
Frequency/cm
3435
Assignments
3
2
2
1
1
1
1
1
1
1
9
8
7
6
565
978
919
654
376
279
159
122
067
029
96
y(O–H)
y(O–H)
(–CH)
d(O–H)
d(–CH)
3467
2973
2908
1637
1382
1279
1164
1122
1062
1025
996
y(O–H)
y(O–H)
y
y
y
as(–CH
(–CH)
as(–NO
2
)
y
y
y
as(–CH
(–CH)
as(–NO
2
)
s
2929
1621
1403
y
s
s
2922
1627
1399
s
y (–CH)
d(O–H)
d(–CH)
2
); d(O–H)
2
); d(O–H)
d(–CH)
d(–CH)
y
y
y
s
(–NO
as(oxygen bridges)
as(ring)
2
)
y
y
y
s 2
(–NO )
as(oxygen bridges)
as(ring)
1111
1040
yas(ring)
y(interannular C–O)
y(C–O)
y(C–O)
y(interannular C–O)
y(C–O)
y(C–O)
y(–NO )
2
2
s(O–NO )
y(C–O)
1045
y(C–O)
33
41
81
y(O–NO
s(O–NO
s(O–NO
2
2
2
)
)
)
822
747
6
5
49
73
y(Cr–O)
y(Cr–O)
649
578
y(Cr–O)
y(Cr–O)
a
s
y is the stretching vibration, y is the symmetrical stretching vibration, yas is the asymmetrical stretching vibration, d is the bending vibration, and
s is the twisting vibration.
originate from gases in the environment, and NO
detected at 175.93 C. Moreover, the noticeable IR peaks of CO rate increased, and the activation energy of Cr O /NC
and NO can be found at a lower temperature of 195.32 C (T , decreased under the direct control of the secondary reaction
2
can be rst increased during the secondary reaction. Then, the reaction
ꢁ
2
3
ꢁ
e
Fig. 7b) than those obtained for pure NC. Aer comparing the including the reaction between the condensed phase and the
gaseous species with pure NC, we found that the Cr
composite released more gaseous products at T and T . This is words, the Cr
additional evidence of the catalytic effect of Cr NPs on the position of NC.
thermal decomposition of NC, which may be due to the The decomposition residue of the Cr
released NO or radical stagnate in the polymer skeleton, characterized by SEM and XRD measurements, as shown in
2
O
3
/NC gaseous products or among the gaseous products. In other
c
f
2 3
O
NPs could accelerate the thermal decom-
2 3
O
2 3
O /NC composite was
2
which then reacts with the other radicals or degradation Fig. 10a and b, respectively. Fig. 10a shows that there exists
products to produce several small molecular products. The obvious aggregation in the reaction residue. The particle size
density distributions of the gas-phase decomposition products (about 100 nm) of Cr O showed no obvious change before and
2
3
of NC and Cr
NO is the initial released gas during the thermal decompo- spherical in shape, except for a few particles sticking together in
sition process of NC and Cr /NC. Other products were clumps at elevated temperatures. Fig. 10b shows that the
released aerward. Notably, due to the catalytic effect of Cr diffraction peaks in the pattern are indexed to the rhombohe-
NO, CO, N O, HCHO and HCOOH gases were gradually dral phase of Cr (JCPDS: 38-1479), which reveals that the vast
released and reached their highest densities at lower temper- majority of the Cr O /NC residues obtained aer the thermal
2 3
O /NC are shown in Fig. 8. It can be seen that aer the decomposition reaction. Most of the particles are
2
2 3
O
2 3
O ,
2
2 3
O
2
3
atures. Upon comprehensive analysis of the density distribu- treatment are Cr O powders.
2
3
tion and IR spectra of two samples, we can conclude that Cr O3
Fig. 10c displays the FTIR spectra of NC, NC residue, Cr O /
2 3
2
2 3
can accelerate the bond cleavage of NC and the secondary NC and Cr O /NC residue. The characteristic absorption peaks
reactions of the condensed phases to form numerous gaseous and the corresponding assignments are listed in Table 4. It can
products. be seen that the quantities and varieties of the absorption peaks
The Brunauer–Emmett–Teller (BET) specic surface area of NC and Cr /NC are consistent in the range of 4000–
NPs is 11.2 m g . The specic surface area of NPs is 400 cm . The Cr–O vibrational absorption bands are found in
2 3
O
2
ꢀ1
ꢀ1
of Cr
2 3
O
ꢀ
1
the dominant factor for adsorption capacity. Generally, the the FTIR spectrum of both Cr O /NC (649 and 573 cm ) and
larger the surface areas that the NPs possess, the higher the the Cr O /NC residue (649 and 578 cm ). Furthermore, the
2
3
ꢀ
1
12
2
3
adsorption capacity and the better the catalytic ability that FTIR spectrum of the Cr O /NC residue shows the characteristic
2
3
the NPs exhibit. The decomposition reaction is controlled by peaks of the –CH and C–O groups, indicating that the NC
the condensed phase, the gaseous products or the reaction residues (Fig. 10c) exist in the Cr
between the condensed phase and the gaseous products. treatment.
2 3
O NPs aer thermal
34
With the adsorption effect of Cr
such as NO do not escape easily from the condensed phase. proposed thermal decomposition mechanism of NC and Cr
Therefore, the concentration of intermediate reactants NC composite has been suggested as follows (Scheme 1):
2
O
3
NPs, gaseous products
Based on the TG-FTIR analyses and literature results, the
2
2 3
O /
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2 3
Scheme 1 The proposed thermal decomposition mechanism of NC and the Cr O –NC composite.
Conclusion
Acknowledgements
Herein, Cr
a shortened synthesis time when compared with the case of the Natural Science Foundation of China (No. 21673179) and the
reported method. The prepared Cr NPs were subsequently Opening Foundation of the State Key Laboratory of Continental
2 3
O NPs were synthesized by a sol–gel method with This work was supported by the program for the National
2 3
O
combined with NC to evaluate their catalytic effect. The non- Dynamics (Northwest University, No. 12LCD07).
isothermal decomposition kinetic studies carried out on NC
and Cr O /NC showed that the Cr O NPs could decrease the
2
3
2 3
activation energy and the decomposition temperature of the
thermal explosion of NC. Cr O NPs were found to have good
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