Structure and properties of 1-amino-2-nitroguanidinium nitrate

Article abstract of DOI:10.1039/c4ra03462g

A nitrogen-rich energetic material 1-amino-2-nitroguanidinium nitrate (ANGN) was synthesized and fully characterized by nuclear magnetic resonance, infrared spectroscopy, mass spectroscopy and elemental analysis. Its thermal behavior and detonation properties were also investigated to give a better understanding of its physical and chemical properties. The results show that the calculated critical temperature of thermal explosion, entropy of activation, enthalpy of activation, free energy of activation, detonation pressure and detonation velocity are 319.8 K, 130.24 J mol^-1 K^-1, 155.45 kJ mol^-1, 106.45 kJ mol^-1 43.0 GPa and 9775 m s^-1, respectively. It is predicted that ANGN possesses excellent energetic properties compared to those of RDX and HMX, and can be considered as a potential energetic material.

Full text of DOI:10.1039/c4ra03462g

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Structure and Properties of 1-Amino-2-Nitroguanidinium
Nitrate
Xinghui Jin, Bingcheng Hu*, Zuliang Liu, Chunxu Lv
(School of Chemical Engineering, Nanjing University of Science and Technology, Jiangsu,
Nanjing 210094)
*Corresponding author. E-mail: hubingcheng210094@163.com
A nitrogen-rich energetic material 1-amino-2-nitroguanidinium nitrate was fully
investigated.

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Structure and Properties of 1-Amino-2-Nitroguanidinium
Nitrate
Xinghui Jin, Bingcheng Hu*, Zuliang Liu, Chunxu Lv
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(School of Chemical Engineering, Nanjing University of Science and Technology, Jiangsu,
Nanjing 210094)
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Abstract: A nitrogen-rich energetic material 1-amino-2-nitroguanidinium nitrate
(ANGN) was synthesized and fully characterized by nuclear magnetic Resonance,
infrared spectroscopy, mass spectroscopy and elemental analysis. Its thermal behavior,
detonation properties and impact sensitivity were also investigated to give a better
understanding of its physical and chemical properties. The results show that the
calculated critical temperature of thermal explosion, entropy of activation, enthalpy of
activation, free energy of activation, detonation pressure and detonation velocity are
319.8 K, 130.24 J mol-1 K-1, 155.45 kJ mol-1, 106.45 kJ mol-1 43.0 GPa and 9775 m s-1,
respectively. It is predicted that ANGN possesses excellent energetic properties than
that of RDX and HMX, and can be considered as a potential energetic material.
Keywords: 1-Amino-2-nitroguanidinium nitrate; Thermal behavior; Detonation
properties; Theoretical study
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18 1 Introduction
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Significant progress has been obtained in the development of novel nitrogen-rich
high-energy density materials (HEDMs) with high performance and decreased
sensitivity as well as environmental compatibility during the last few decades.[1-4]
Unfortunately, in most cases, the requirements of insensitivity (such as oxygen
balance, sensitivities to impact, sensitivities to friction, thermal shock, electrostatic
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discharge) and high detonation properties are often contradictory to each other, which
make the design and synthesis of new HEDMs an interesting and enormous
challenge.[5] To solve these problems, one possible approach is to design and
synthesize energetic salts which were often prepared by neutralization or metathesis
reactions with N-protonated cations and O-deprotonated anions. This is primarily
because salt-based energetic materials often possess advantages over non-ionic
molecules since these salts tend to exhibit lower vapor pressures and higher densities
than their atomically similar non-ionic analogues. Moreover, production of these
energetic salts always involves the combination of different energetic cations and
anions and makes it possible to balance the requirements of low sensitivity and high
detonation properties.[6,7] Thus, nitrogen-rich energetic salts as a family of
environmentally benign high energy density materials (HEDMs), have became one of
the hottest topics in the field of HEDM.[8-10]
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Guanidine, aminoguanidine, diaminoguanidine and triaminoguanidine were often
considered as the nitrogen-rich cations in the previous research.[11] However, little
research was reported on 1-amino-2-nitroguanidine (ANG) which possesses better
energetic properties (such as density, detonation velocity, detonation pressure and so
on) than guanidine and its derivatives.[12,13] Herein, a nitrogen-rich energetic salt
1-amino-3-nitroguanidinium nitrate (ANGN Scheme 1) was synthesized and fully
characterized. Its crystal structure, thermal behavior and detonation properties were
also investigated to give a better understanding of its physical and chemical
properties.
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Scheme 1 Synthesis of ANGN
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2 Experimental
2.1 Materials
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1-amino-2-nitroguanidine was prepared according to Ref. [14].
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ANGN was prepared as the following procedure: To a vigorously stirred suspension
of 1-amino-2-nitroguanidine (1.19 g, 0.01 mol) in water (10 mL) was added 65%
nitric acid (1.5 mL, 0.02 mol) . The solution was stirred at 60 ˚C for 1 h and then
cooled to the room temperature. The ANGN was crystallized (1.72 g, 94.5%) as white
blocks. m.p. 138-149 ˚C. IR ν/cm−1: 3395, 3290, 1645, 1580, 1470, 1385, 1281, 1215,
1050, 915, 823. δH (d6-DMSO, 500 MHz): 9.49 (s, 1H), 8.31 (s, 5H); δC (d6-DMSO,
125 MHz): 159. m/z(ESI): 62 [M-H]-, 120 [M+H]+. CH6O5N6: calcd/%. C 6.59, H
3.30, N 46.15, found C 6.48, H 3.39, N 46.22.
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2.2 Experimental Equipments and Conditions
Crystallographic data (excluding structure factors) for the structure in this paper
have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12
Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of
charge on quoting the depository number CCDC-989437.
1
BrukerAvance III 500 MHz spectrometer was used for H NMR and 13C NMR
spectra of ANGN; IR spectrum was obtained from a Thermo Nicolte IS10 IR
instrument; ESI-MS results were obtained from a Finnigan TSQ Quantum Mass
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Spectrometer. Elemental analysis was completed on a Perkin-Elmer PE-2400.
TG-DTG-DSC curves were obtained on a NETZSCH STA 409 PC/PG coupling
system with an initial mass of 3.0 mg placed in alumina crucibles (nitrogen
atmosphere with the flow rate of 30 mL min-1). The crystal data were collected with a
Bruker SMART APEX II CCD X-ray diffractometer using graphite-monochromated
Mo-Kα radiation (λ=0.071073 nm).
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3 Results and Discussion
3.1 Crystal Structure
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Single crystal suitable for X-ray measurements were obtained by slow evaporation of
a nitric acid solution of ANGN. A white crystal with dimensions 0.3 mm×0.2 mm×0.2
mm was chosen for X-ray determination. The structure was solved by direct methods
(SHELXTL-97) and refined by full-matrix-block least-squares methods on F2 with
anisotropic thermal parameters for all non-hydrogen atoms. Crystal data and
refinement results were summarized in Table 1. It is also fascinating that the crystal
density measured in our group is 1.85 g cm-1 while the data is 1.91 g cm-1 in Ref.12. It
is certainly due to the different temperatures of analysis, 173 K in Ref. 12 vs. 293 K in
this paper and the current X-ray analysis will make it more useful in predicting
properties for practical applications. This also results in the difference of the
detonation properties directly (P, detonation pressure and D, detonation velocity)
since density is an important data for an energetic material according to
Kamlet-Jacobs equations. The following investigation also proved this fact.
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Table 1. Crystal data and structures refinement details
Chemical formula
CH6N6O5
182.12
Formula weight /g mol-1
Temperature /K
293 (2)
Wavelength /Å
0.71073
Triclinic
P-1
Crystal system
Space group
Crystal colour
White
a/Å
7.1750(14)
7.2020(14)
7.8560(16)
66.12(3)
65.34(3)
68.51(3)
327.69(11)
2
b/Å
c/Å
α/°
β/°
γ/°
Volume/Å3
Z
Dcalc /g cm-3
Absorption coefficient /mm-1
F(000)
1.85
0.180
188.0
θ range /°
2.99-25.39
0≤h≤8, -8≤k≤8, -8≤l≤9
1210
Index ranges
Reflections collected
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Goodness-of-fit on F2
Final R indices [I＞2σ(I)]
R indices (all data)
1.000
R1= 0.0466, wR2= 0.1389
R1= 0.0413, wR2= 0.1320
0.271, -0.245
Largest diff. peak and hole /e·Å-3
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Fig.1 shows the molecular structure and the three-dimensional packing diagram
of ANGN. It is evident that the ANGN structure was made up of one ANG cation and
one nitrate anion. The ANG cation and the nitrate anion distributed in different planes,
and the whole molecule presents an almost perpendicular conformation.
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Fig. 1 The molecular structure and packing diagram of ANGN
3.2 Thermal Behavior
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Fig. 2 shows the TG-DTG-DSC curves of title compound with the initial
temperature 30 ˚C up to 450 ˚C at the heating rate of 5 K min-1. From the figure, two
steps were observed by TG curve. The first step, which is the main stage process,
starts from 100 to 150 ˚C with 77.5% weight loss while the second step starts from
150 to 240 ˚C with about 22.5% weight loss. Correspondingly, there appear an evident
sharp peak at about 140 ˚C and a faint peak at around 200 ˚C in the DTG curve, which
also demonstrated that the decomposition reaction was a two stage process. As for the
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DSC curve, there is a faint endothermic peak (140 ˚C) and an evident sharp
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exothermic peak (142 ˚C) which makes it have a number of potential applications in
melt cast explosive, primary explosive, gas generating composition,
low characteristic signal propellant, and so on. The two peaks belong to the melt and
decomposition of ANGN, respectively.
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Fig. 2 TG–DTG–DSC curves of ANGN at the heating rate of 5 K min-1
Figure 3 shows the TG-DSC curves of the decomposition process of ANGN salt at
different heating rates of 5, 10, 15 and 20 K min-1. It was found that, with the increase
of the heating rate, the decomposition temperatures and the exothermic peaks of
ANGN salt shifted to higher temperatures. On the other hand, the endothermic peaks
and the exothermic peaks in DSC curves clearly appear as the heating rate increased.
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Fig.3 TG-DSC curves at different heating rates
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3.3 Non-Isothermal Decomposition Kinetics
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In order to obtain the relative kinetic parameters such as activation energy (Ea),
pre-exponential constant (A), entropy of activation (ꢀS≠), enthalpy of activation (ꢀH≠),
and free energy of activation (ꢀG≠), two thermal analysis kinetic methods were jointly
employed. The Equations were written as follows:
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1) Kissinger Equation:[15]
βi
AK R EK
−
(1)
ln
= ln
Tp2i
EK
RTpi
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where, β is the heating rate; Tpi is the maximum peak temperature which can be
obtained from the DSC curves; R is the gas constant. A straight line was obtained by
2
plotting
vs. reciprocal of the temperature (T-1). The slope of the line is
ln(β
i
/ Tpi
)
−E
K
/ R
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equal to
and the intercept of the line is equal to
. Thus,
ln(AR) / E
K
activation energy Ek can be calculated from the slope and pre-exponential factor A can
be calculated from the intercept of the plot.
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2) Ozawa–Flynn–Wall Equation:[16]
AE0
E0
(2)
logβ
= log
− 2.315− 0.4567
RG(α
)
RT
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where, G(a) is a conversion functional relationship. The degree of conversion is
defined as a= (m0-m)/(m0-mf), where m0, m, mf are the initial, actual and final mass of
the sample respectively; E0 is apparent activation energy; A is the pre-exponential
factor; T is peak temperature; R is gas constant and β is heating rate. On the other
hand, Equation 2 allows evaluating the dependence of the activation energy on the
degree of conversion without the knowledge of the explicit form of G(α) based on
Doyle’s approximation and thus, the degree of conversion (α) at different heating rates
would be at a constant value at the peak temperature. Therefore, equation (2) was
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simplified as follows:[17,18]
E0
(3)
lnβ = const.− 0.4567
RT
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The activation energy and pre-exponential factor can easily be calculated based on the
straight line that was obtained by plotting lnβ vs. reciprocal of the temperature (T-1).
The data β, Tpi and the kinetic parameters obtained by Kissinger and Ozawa
methods were summarized in Table 2. It is obviously seen from the table that the
values of Tpi of the exothermic peak shifted to higher temperatures as the heating rate
increased. On the other hand, the values of E calculated by Kissinger method
(Ek=158.5 kJ mol-1) agrees well with that obtained by Ozawa’s method (Eo=157.6 kJ
mol-1) and both of the linear correlation coefficients are very close to 1. All the data
obtained above indicates that the result is credible.
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Table 2 Calculated values of the kinetic parameters for the major exothermic
decomposition reaction of ANGN
β(˚C min-1) Tpi(˚C)
Kissinger method
Ozawa method
Ek (kJ mol-1) logAk (s-1)
rk
Eo (kJ mol-1)
157.6
ro
5
142.0
148.3
151.9
154.3
158.6
19.7
0.999
0.999
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15
20
14
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The corresponding parameters such as the entropy of activation (ꢀS≠), the
enthalpy of activation (ꢀH≠), and the free energy of activation (ꢀG≠) could be
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calculated by equations (4)-(6) based on the calculated values of Ea and A. [19,20]
kBT
h
ꢀS≠
R
(4)
A = (
)exp(
)
ꢀH ≠ = E
a
− RT
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4
5
6
7
8
9
(5)
(6)
ꢀG≠ = ꢀH ≠ −TꢀS≠
where T=Tp0, the peak temperature (Tpi) corresponding to β→0; Ea=Ek, calculated by
Kissinger’s method; A=Ak, calculated by Kissinger’s method; kB, the Boltzmann
constant, 1.3807×10-23J K-1; h, the Plank constant, 6.626×10-34J s-1.
In order to get the values of ꢀS≠, ꢀH≠ and ꢀG≠, the value of Tp0 must be known
firstly according to equations (4)-(6). Then the next important work is to calculate the
value of Tp0 which could be calculated by Eq. (7).[21]
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Tp = Tp0 + bβi + cβi2 + dβi3
(7)
i
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where b, c and d are coefficients.
Based on the above-described equations, the value of Tp0, ꢀS≠, ꢀH≠, and ꢀG≠
were calculated as 378.45 K, 130.24 J mol-1 K-1, 155.45 kJ mol-1 and 106.45 kJ mol-1 ,
respectively.
3.4 Critical ignition temperature
The critical temperature is another important parameter since it is necessary to give a
better understanding of the thermal stability and to insure the safe storage involving
explosives, propellants and pyrotechnics. It can be defined as follows: the lowest
temperature to which a specific charge may be heated without undergoing thermal
runaway.[22] The value of Tb of 318.8 K was calculated by the following equation:[23]
E0 − E02 − 4E0RTp0
Tb =
2R
22
(8)
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where Eo is the apparent activation energy obtained by Ozawa’s method; R is the gas
constant, Tp0 is the peak temperature corresponding to β→0.
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3.5 Energetic properties
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Computations were performed with the Gaussian 03 suite of programs.[24] The
geometric optimization of the structures were based on single crystal structures, where
available, and frequency analyses are carried out using B3LYP functional with
6-31+G(d,p) basis set, and single energy points were calculated at the
MP2(full)/6-311++G(d,p) level. All of the optimized structures were characterized to
be true local energy minima on the potential energy surface without imaginary
frequencies.
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Detonation velocity (D) and detonation pressure (P) are two important
parameters when an energetic material was designed or synthesized. Previous studies
have proved that the Kamlet-Jacobs equations can be considered as the most reliable
approach to estimate the detonation properties of an energetic material.[25]
1/2
1/2 1/2
15
(9)
D = 1.01(N M
Q
)
(1+1.3ρ)
1/2
2
1/2
16
17
P = 1.558ρ N M
Q
(10)
where each term in the Eqs. (9) and (10) is defined as follows: ρ, the loaded density of
explosives (g cm-3); D, the detonation velocity (km s-1); P, the detonation pressure
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(GPa); N, the moles of detonation gases per gram explosive;
M
, the average
molecular weight of these gases; Q, the heat of detonation (cal g-1). Values of N,
,
M
21
22
and Q could be obtained using the following equations that summarized in Table 3.
Table 3 Formulas for calculating the values of N, M, and Q for an explosive
23
CaHbOcNd
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paramet
ers
Stoichiometric ratio
2a+b/2 >c≥ b/2
c≥2 a+b /2
(b+2c+2d )/4M
4M/(b+2c+2d)
b/2 > c
N
(b+2c+2d)/4M
(b+d)/2M
(2b+28d+32c)/(
b+d)
(56d+88c−8b)/(b+2c+2d)
M
Q*10-3
(28.9b+94.05a+0.239ꢀ [28.9b+94.05(c/2−b/4)+0.239 (57.8c+0.239ꢀH
Hf)/M ꢀHf]/M f)/M
1
2
3
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aa, b, c stand for the number of C, H, O and N atoms in the explosive molecule
respectively
bM in the formula is the molecular weight of the explosive (g mol-1); ꢀHf is the heat of
formation of the explosive (kJ mol-1).
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According to the above-described equations, the heat of formation is an essential
parameter to calculate the detonation properties of an energetic material. Then the
next important task is to determine the heat of formation ( 0 ) of ANGN using
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ꢀHf
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Born–Haber energy cycle (Figure 4) and equations (11) – (13):[26]
H0 (ionic salt，298 K) =
ꢀ
H0 (cation, 298 K)+
ꢀ
H0f (anion, 298 K)−
ꢀ
HL
(11)
ꢀ
∑
∑
f
f
10
where
ꢀ
HL is the lattice energy of the salt.
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Fig. 4 Born–Haber cycle for the formation of energetic salts. a, b, c, d are the number
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of moles of the respective products
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The value of
ꢀ
H
L
could be predicted by the formula suggested by Jenkins et al.:
(12)
ꢀ
H
L
= UPOT + [ p(n
M
/ 2 − 2) + q(n / 2 − 2)]RT
X
3
4
5
6
7
where UPOT is the lattice potential energy and nM and nX depend on the nature of the
ions
p+ and q-, respectively, and are equal to three for mono-atomic ions, five for
M
X
linear polyatomic ions, and six for nonlinear polyatomic ions. The equation for the
lattice potential energy UPOT can be written as follows:
(13)
U
POT (kJ mol-1)= /M )1/3
γ (ρ +δ
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where ρ is the density (g cm-3),
M is the chemical formula mass of the ionic material
(g mol-1), and the coefficients γ (kJ mol-1 cm) and δ (kJ mol-1) are assigned literature
10
values.
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12
Isodesmic reactions (Scheme 2) and protonation reactions were employed
(Scheme 3) in this paper to obtain the heat of formation ( 0 ) of the cations, anions
ꢀHf
13
and the parent ions (heat of formation of H+ is 1530 kJ mol-1).[27]
NO2
NH
N
+
+ NH2NO2 NH2NH3
+ 2 NH3
H2N NH2
H2N HN NH3
NH NH +
H
NH2NH3
2
2
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15
Scheme 2 Isodesmic and protonation reactions for calculations of heat of formation
16
Then the enthalpy of the isodesmic reaction ( 0 ) is obtained by combining the
ꢀHf
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18
19
20
MP2/6-311++G** energy difference for the reaction, the zero-point energies
(B3LYP/6-31+G**), and other thermal factors(B3LYP/6-31+G**). Thus, the
detonation properties of the title salt (Table 4) can be calculated based on the
above-described equations and schemes. For a comparison, detonation properties of
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two well-known energetic material (RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and
HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) were also listed in Table 4. It is
seen that detonation properties of the title energetic salt ANGN is superior to that of
RDX and HMX.
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Table 4 Detonation properties of ANGN
d
e
f
g
b
c
da
Tm
Td
cation
Ph
Di
OBj
0
ꢀ
H0f
ꢀ
H 0f
ꢀ
H 0f
ꢀ
H 0f
anion
lattice
salt
Compd.
ANGN
RDX[28]
HMX[28]
1.85
1.82
1.91
140
—
142
230
287
862
-308
533
22
—
—
43.0
35.2
39.6
9775
8997
9320
—
—
—
—
—
—
0
—
0
a
b
c
d
6
Density (g cm-3). Melting point (˚C). Thermal degradation (˚C). Calculated molar enthalpy of formation of
e
f
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the cation (kJ mol-1). Calculated molar enthalpy of formation of the anion (kJ mol-1). Calculated molar lattice
energy (kJ mol-1). Calculated molar enthalpy of formation of the salt (kJ mol-1). Detonation pressure (GPa).
g
h
i
9
Detonation velocity (m s-1). j Oxygen balance (OB), for the compound with molecular formula CaHbNcOd (without
crystal water), OB (%) =1600 ( d -2a- b/2)/ Mw (%).
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12
13
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17
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Finally, it should be pointed out that we use isodesmic reactions (Scheme 2) and
protonation reactions to calculate the heat of formation of the title compound in this
paper instead of the atomization energy method that cited in Ref.12. However, the
calculated results from the two methods show that the heat of formation of the cation,
anion and the salt are 862 kJ mol-1(Ref.12, 877 kJ mol-1), -308 kJ mol-1(Ref.12, -313.6
kJ mol-1) and 22 kJ mol-1(Ref.12, 22.2 kJ mol-1), respectively. The values are
approximately the same which indicates that the isodesmic reaction and protonation
reactions are of a feasible way to evaluate the heat of formation of an energetic salt.
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The detonation properties (
P
, detonation pressure and D, detonation velocity)
calculated in this paper (P, 43.0 GPa; D, 9775 km s-1) also shows some difference
from Ref.2 (P, 42.7 GPa; D, 9551 km s-1). This may be due to the different density
that calculated from the crystal structure.
3
4
5
3.6 NBO analysis
6
In order to give a better understanding of the chemical and physical properties of
ANGN, the molecular orbital and the natural bond orbital (NBO) analyses based on
the optimized structure (B3LYP/6-311+G(d,p)) were carried out by using Gaussian 03
program. As were shown in Fig. 5, NBO analysis indicated that the positive charge is
delocalised over the C and H atoms in 1-amino-2-nitroguanidinium group (ranges
from 0.388 e-0.610 e) and the negative charge is mostly delocalised over the N and O
atoms except N10 and N18 (ranges from 0.388 e-0.610 e). This may be caused by the
7
8
9
10
11
12
13
-
effect of the O atoms in the —NO2 group and NO3 anion.
14
15
Fig. 5 Charge distribution in ANGN at B3LYP/6-31+G(d,p) level
16
17
The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) of ANGN were shown in Fig. 6. According to the
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3
4
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6
molecule structure and calculated data, there are 95 molecule orbits in this system
including 48 unoccupied ones. It also can be seen from Fig. 6 that the electron
-
density of HOMO is mainly focus on the NO3 anion, while that of LUMO is mainly
on the —NO2 group of the1-amino-2-nitroguanidium. The results indicate that the
group of —NO2 and —NH2 make an important difference to some properties of the
title compound.
7
8
a
b
9
10
11
12
13
14
15
16
17
Fig. 6 The highest occupied molecular orbital (HOMO, a) and the lowest unoccupied
molecular orbital (LUMO, b) of ANGN
Since the gap energy of the highest occupied molecular orbitals and the lowest
unoccupied molecular orbitals (△E) is an important parameter to measure the stability
of the energetic material, the molecular energy (Etotal), the highest occupied molecular
orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO) and
their gaps (△E) were calculated. They were obtained as -745.8411766, -0.21153,
-0.11045 and 0.10108 Hartree, respectively. It indicates that ANGN has a better
thermal stability.
18 4 Conclusions
19
20
A nitrogen-rich energetic material 1-amino-2-nitroguanidinium nitrate (ANGN) was
synthesized. The following conclusions about ANGN can be drawn:
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(1) The single-crystal X-ray diffraction indicates that ANGN was crystallized in a
triclinic system, space group -1.
P
3
(2) The thermal behavior of ANGN presents one main decomposition stage in the
temperature range 100–250 ˚C with about 95% mass loss.
4
5
(3) The calculated thermal dynamic parameters of ANGN were obtained as follows:
6
S H
Ek, 158.6 kJ mol-1; Eo, 157.6 kJ mol-1; ꢀ ≠, 130.24 J mol-1 K-1; ꢀ ≠, 155.45 kJ mol-1
7
and ꢀG
≠106.45 kJ mol-1.
8
(4) The calculated detonation velocity and detonation pressure of ANGN are 9775 m
s-1) and 43.0 GPa respectively. It indicates that ANGN has superior detonation
properties than that of RDX and HMX, and can be considered as a potential energetic
material.
9
10
11
12
13
Acknowledgements
This study was supported by the National Defense Advanced Research Projects
14
15
(No. J-KY-2012-1317) and the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD).
16
17
18
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