Preparation of a nanoscale homogeneous energetic lead azides?porous carbon hybrid with high ignition ability by: In situ synthesis

Article abstract of DOI:10.1039/d0ra01317j

The ever-increasing demand for miniaturized explosive systems urgently calls for better performance studies through the synthesis of novel nanoscale materials. In this work, lead azide?porous carbon hybrids (LA?PC) are synthesized by in situ carbonization and azidation of a lead-containing cross-linked gel, in which the nanoscale LA is uniformly distributed on the porous carbon skeleton. The detailed characterization has shown that such outstanding performance stems from the LA nanoscale effect and the excellent conductivity and thermal conductivity of carbon cages. Because of the favorable unique structure, the prepared composite material exhibits excellent ignition performance, and its flame sensitivity can reach 42 cm, which solves the problem of poor ignition capacity of LA on all occasions. In addition, the composite has very low electrostatic sensitivity, further improving the safety of practical application. This work makes it possible for LA to be detonated without using lead styphnate, paving a new way for improving the flame sensitivity of primary explosives.

Full text of DOI:10.1039/d0ra01317j

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Preparation of a nanoscale homogeneous
energetic lead azides@porous carbon hybrid with
Cite this: RSC Adv., 2020, 10, 14347
high ignition ability by in situ synthesis†
*
*
Ji-Min Han, Naimeng Song and Jianchao Liu
Zhenzhan Yan, Li Yang,
The ever-increasing demand for miniaturized explosive systems urgently calls for better performance
studies through the synthesis of novel nanoscale materials. In this work, lead azide@porous carbon
hybrids (LA@PC) are synthesized by in situ carbonization and azidation of a lead-containing cross-linked
gel, in which the nanoscale LA is uniformly distributed on the porous carbon skeleton. The detailed
characterization has shown that such outstanding performance stems from the LA nanoscale effect and
the excellent conductivity and thermal conductivity of carbon cages. Because of the favorable unique
structure, the prepared composite material exhibits excellent ignition performance, and its flame
sensitivity can reach 42 cm, which solves the problem of poor ignition capacity of LA on all occasions. In
addition, the composite has very low electrostatic sensitivity, further improving the safety of practical
application. This work makes it possible for LA to be detonated without using lead styphnate, paving
a new way for improving the flame sensitivity of primary explosives.
Received 11th February 2020
Accepted 2nd March 2020
DOI: 10.1039/d0ra01317j
rsc.li/rsc-advances
(carbon materials), and in situ chemical synthesis of hybrid
explosives with carbon materials. For example, many new lead
Introduction
azide derivatives have been developed by adding dextrin, car-
boxymethyl cellulose, etc. during the preparation of lead
azide.16–20 Zhimin Li et al. doped graphene nanoplates during
the preparation of lead azide and lead styphnate to reduce their
electrostatic sensitivity.21,22 However, the preparation of the
primary explosives by the physical mixing method may have
some problems such as poor uniformity of the sample, high
cost, and high risk in the preparation process. Qianyou Wang
et al. prepared a composite of copper azide (CA) uniformly
distributed on the carbon skeleton by carbonization and in situ
azidation of copper-containing metal organic frameworks
(MOFs) as the precursor, which reduced the electrostatic
sensitivity of copper azide.23 The in situ synthesis method can
avoid problems such as uneven physical mixing of the primary
explosives particles on the carbon skeleton. However, the
application of MOFs is strickly limited, because of the more
complicated requirements for synthesis conditions, including
temperature, mixed solvents, etc. Therefore, there is an urgent
need to develop a cheap and simple synthesis strategy to modify
primary explosives. Fortunately, in our previously reported
work, copper azide@porous carbon hybrids (CA@PC) complex
with nano-scale CA uniformly distributed on the carbon skel-
eton was feasibly prepared from a cheap and readily available
gel.24 This method has provided a new idea to improve the ame
sensitivity of LA and may fundamentally solve the safety
problem of the LA/LS system.
As the energy source for various ignition and detonation
equipment, primary explosives are indispensable in explosive
systems, because they are sensitive to the outside world and can
be detonated by a smaller simple initial energy (ame, impact,
acupuncture, friction, etc.).1–3 At present, although a lot of new
primary explosives have been developed, many factors still need
to be considered and optimized for their practical applications,
such as synthetic difficulties, high costs, and safety issues.4–7
The combination of lead azide (LA) and lead styphnate (LS)
represents one of the most widely used primary explosives.8,9
Among them, LA has a strong detonating ability, but is
vulnerable due to its poor ignition ability. Therefore, LA needs
to be equipped with LS which exhibits a good ignition capacity
and serves as the most commonly used ignition agent. However,
in such an explosive system, the electrostatic sensitivity of the
LS is extremely high, resulting in many terrible explosion acci-
dents ascribable to static electricity.10–15 The safety of the
primary explosives (LS/LA, etc.) becomes a research bottleneck
that needs to be addressed.
Various methods are employed to modify the primary
explosives, including mixing antistatic agents and a crystal form
controlling agents, physically doping conductive materials
State Key Laboratory of Explosion Science and Technology, Beijing Institute of
Technology, 5 South Zhongguancun Street, Beijing, 100081, P. R. China. E-mail:
yanglibit@bit.edu.cn
Herein, we applied the in situ synthesis method for the rst
time to prepare a carbon-based lead azide composite by
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01317j
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carbonization and in situ azidation reaction using a cheap and Samples were tested using the up and down method for each
readily available lead-containing hydrogel template. This in situ condition, and the electrostatic sensitivity (E50) for 50% prob-
synthesis method not only excluded the usage of the sensitive ability of ignition was calculated. Flame sensitivity test: 20 mg of
LS, but also ensured the nano-scale lead azide uniformly the complex was compacted to a copper cap under the press of
attached to the porous carbon skeleton. In such a composite 39.2 MPa and was ignited by black powder pellet. Samples were
system, the size of LA is reduced to the nanometer scale, leading tested using the up and down method for each condition, and
to the smaller decomposition potential energy and the the ame sensitivity (H50) for 50% probability of ignition was
advanced thermal decomposition temperature. Besides high calculated.
specic surface areas, excellent electrical conductivity, and
Preparation of samples synthesis of PAA-Pb hydrogel. 6 ml of
thermal conductivity,23,25,26 the porous carbon skeleton can also PAA was dissolved in 70 ml of distilled water with stirring to
provide binding sites for LA, and can be obtained from inex- ensure uniform stirring. 0.5 acetic acid was then added with
pensive raw materials by the simple carbonization step.27–32 stirring to keep the system acidic. 4.2 g of lead acetate was
Through this strategy, the ame sensitivity of the LA@PC dissolved in 60 ml of distilled water, and the solution was added
material can be increased to 42 cm, while the ignition ability is dropwise to the PAA system to effect gelation. Aer grinding and
comparable to the commonly used ignition powder LS. In stirring with a ball mill, the PPA-Pb hydrogel particles were
addition, the LA@PC composite has a static sensitivity as low as homogenized and centrifuged to collect a solid. The collected
1.25 J and has a higher safety rating than previously reported LA solid hydrogel was added to distilled water for grinding or
modied materials. The synthesis of LA@PC solves the problem stirring to ensure that the system became homogenized again.
of LA ignition difficulties, making it possible to replace LS as The homogel was then centrifuged or allowed to stand, and the
ignition powder.
solid hydrogel was collected. The collected hydrogel is repeated
for the above steps to remove impurity ions such as Na+ and
CH3COOꢀ. The repeatedly washed hydrogel was freeze-dried
and then a white powder was collected.
Experimental
Material and methods
Preparation of in situ PbO nanoparticles doping in porous
Materials. Sodium polyacrylate (PAA) is purchased from carbon frameworks (PbO@PC). The freeze-dried PAA-Pb gel
ꢁ
Aladdin. Lead acetate (Pb(OOCCH3)2$3H2O), acetic acid and powder was carbonized at 800 C for 30 min under a niꢀtr1ogen
ꢁ
Stearic acid were of analytical grade and purchased from Beijing atmosphere. The temperature was ramped at 5 C min and
Chemical Works. Sodium azide was analytical grade and the product was naturally cooled to room temperature. The
purchased from Xiya Reagent. Other chemicals were of analyt- carbonized black loose powder was collected for azide.
ical grade and used without further purication. All solutions
were prepared with distilled water.
Preparation of in situ lead azides@porous carbon hybrid
(LA@PC). A two-necked ask was connected to an argon inlet
Characterizations. All FT-IR spectra were recorded using FT- valve and a glass tube (containing a cotton-based sample tube,
IR spectrometer (Nicolet 170, SXFT/IR spectrometer, USA) from sample tube opening). A mixture of sodium azide and excess
KBr discs in the range of 4000–400 cmꢀ1. TG-DTG curves: stearic acid was added to the two-necked ask, and the mixture
ꢁ
Thermogravimetric analysis (TG) of PAA-Pb gel powder was was reacted at 120 C to form a hydrazoic acid gas. LA@PC is
determined by STA6000 (PerkinElmer, USA) under a helium prepared by reacting azide gas with a sample (carbonized
ow of 50 ml minꢀ1. The sample (12 mg) was heated from 40 ꢁC product) in a glass tube. Before the reaction, argon gas should
to 600 ꢁC at a heating rate of 10 ꢁC minꢀ1. Differential scanning be introduced to remove the air from the system. The glass tube
calorimetry (DSC) of LA@PC were determined by CDR-4P should be reconnected with NaOH solution as a scrubber for
ꢀ1
ꢁ
(INESA Instrument, China) with 10 C min while heated up unreacted HN3 gas. Aer the completion of the reaction, argon
ꢁ
to 500 C in air atmosphere. The decomposition temperatures gas was again introduced to discharge the unreacted HN3 gas
were given as peak maximum temperature. Scanning electron remaining in the system. Caution! Hydrazoic acid is highly
microscopy (SEM) was performed using S-4800 (HITACHI, toxic, and metal azides are always energetic and especially
Japan) at 15 kV with a point resolution of 1.0 nm. The samples sensitive. The entire system must be set up behind an explosion
were characterized by transmission electron microscopy (TEM) proof shield in a well-ventilated hood. The obtained black uffy
using Tecnai G2 F30 (FEI, USA) at 300 kV with a point resolution powder should be taken lightly (Fig. S6†).
of 0.20 nm. Both SEM and TEM were equipped with an EDX/EDS
system. X-ray photoelectron spectroscopy (XPS) measurements
were performed with a Thermo Scientic Escalab 250Xi using
Results and discussion
the monochromatic Al Ka line (1486.7 eV). Powder X-ray The schematic synthesis process of LA@PC is illustrated in
diffraction patterns were carried out with a Bruker D8 Fig. 1. First, we selected cheap sodium polyacrylate (PAA) as the
˚
Advance diffractometer using Cu Ka radiation (l ¼ 1.5406 A) at precursor material, lead acetate as the crosslinking agent, and
40 kV and 40 mA. Element analysis and ICP: C, N and H contents acetic acid to adjust pH. The sodium polyacrylate lead ion
were measured by EuroEA Elemental Analyser. Cu content was crosslinked gel was then prepared (Fig. 1a and b), in which the
measured by PE optima 7000 with the standard curve method. metal ions lead can be uniformly scattered in the exible
Electrostatic sensitivity test: The test parameters: the charge organic molecular skeleton (Fig. S1c†).33–38 The detailed process
capacitance is 500 pF, the electrode gap length is 0.12 mm. and characterization are shown in ESI (Fig. S1 and S2†). Puried
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intermediate particles (PbO@PC) are both 20–30 nm, which is
sufficiently small, the size of LA@PC can reach nanoscale
(about 110 nm). From the particle size distribution map
(Fig. 2e), D50 of LA@PC is about 120 nm. Compared with the
particle size of LA (about 1 mm),14 which is prepared by the
reaction of sodium azide and lead nitrate, the size of LA@PC
particle is greatly reduced. Elemental mapping (Fig. 2d) indi-
cates that LA@PC is mainly composed of C, O, N, and Pb,
among which N and Pb are dispersed uniformly throughout the
porous carbon framework. In the PXRD analysis (Fig. 2f), the
peaks related to PbO disappeared, while the typical Pb(N3)2
patterns emerged in the LA@PC complex, indicating that PbO is
completely converted into LA. Furthermore, the characteristic
peaks of divalent lead ions and azide anions can be clearly seen
in the XPS spectrum of LA@PC (Fig. 2g, h and S5†). The data
proved that the nal sample is LA@PC composite, while the
impurities such as PbO are not observed. The elemental anal-
ysis (EA) and inductively coupled plasma (ICP) emission spec-
trometer analysis indicate LA and carbon contents are 83.5%
and 16.5%, respectively. The differential scanning calorimeter
(DSC) curve in Fig. 2i exhibits three peaks according to thermal
decomposition of LA at 229 ꢁC, 352 ꢁC, and 431 ꢁC, respectively.
It is worth noting that the three exothermic peaks in the DSC of
LA@PC have different degrees of advancement compared with
the conventional LA,14 which may be related to the particle size
reduction and the good thermal conductivity of the carbon
skeleton.
Since LA's ignition ability is poor, in practical applications,
LA must be used together with LS as the ignition powder. The
high static sensitivity of LS brings great security risks to LS/LA.
In order to compare the ignition capability and safety perfor-
mance between the nano-sized LA@PC thus prepared and the
rational LS/LA system, the same methods and equipment re-
ported in our recent work are used to test the ame sensitivity
and electrostatic sensitivity.24 The value of H50 (ame sensi-
tivity) corresponds to the height from standard black powder
pellets to sample for 50% probability of ignition, revealing the
ignition ability of the primary explosives. From Fig. 3a, the pure
LA ame sensitivity is only 8 cm. The H50 values of LS/LA at 1 : 2
and 1 : 3 are 27 cm and 21 cm, respectively. Although the ame
sensitivity of LS/LA at 2 : 1 and 3 : 1 reaches 40 cm and 42 cm
respectively, since LS is the main component, the detonation
ability and safety performance are greatly reduced. Proudly, the
H50 can reach 42 cm LA@PC (carbon content only 16.5%) with
excellent ignition capability, comparable to LS. To further
compare the safety performance of the LA@PC and LS/LA
systems, the electrostatic sensitivity of LA@PC was tested by
a quantitative electrostatic stimulus, shown as E50 (the energy
for 50% probability of ignition, mJ). The schematic diagram of
the electrostatic sensitivity test principle is shown in Fig. 3b. As
shown in Fig. 3c, the values of the LS and LA mixtures at
different ratios of 3 : 1, 2 : 1 and 1 : 1 were 0.23, 0.61 and 1.15
mJ, respectively, the high electrostatic sensitivity in the LS/LA
system is due to the presence of LS. Surprisingly, compared to
pure LA and LS/LA system, the electrostatic sensitivity of
LA@PC reached 1.25 J, which means the new LA@PC variety
exhibits higher safety performance. Similarly, it can be seen
Fig. 1 Schematics of the synthesis process (a) the molecular structure
of polyacrylic acid (PAA), inset is the digital image of PAA solution; (b)
ionic-linked hydrogel prepared from PAA and Pb2+ (PAA-Pb), inset is
the digital image of the formed hydrogel; (c) PbO nano-particles well-
distributed in porous carbon frameworks (named as PbO@PC); (d) LA
nano-crystals adhering to the porous carbon frameworks (named as
LA@PC).
by repeated washing, the prepared crosslinked gel of polymers
and lead ions exhibited the anti-swelling and self-healing
properties. Through the strong hydrogen bonding with the
hydrophilic –COOꢀ groups, a large number of water molecules
were locked in the three-dimensional polymer chains network
to generate the desired hydrogel.39,40 The porous dry gel
framework was obtained by removing water aer vacuum
freeze-drying. Aer the ice crystals are sublimated, cavities were
le at the position originally occupied by the water molecules in
voids between polymer chains, and the metal ions are evenly
distributed between organic molecular chains. The freeze-dried
gel with rich pores provides a prerequisite for the smooth
progress of the azide reaction, and it can also be stored under
normal conditions for a long time without deterioration.
Second, aer the process of freeze-drying, powder of PAA-Pb gel
was pyrolyzed at 800 ꢁC for 30 min in nitrogen atmosphere. The
resulting decomposition products were nanometer scale (about
30 nm) lead oxide uniformly distributed on the carbon skeleton.
Detail characterization and morphology of PbO@PC are shown
in support information (Fig. S3†). The performance of obtained
composite material was compared with the raw material lead
pyrolysis product under the air atmosphere (Fig. S4†). The
decomposition product in the air atmosphere has a particle
diameter of about 1 mm, probably because of the lead oxide
agglomeration aer the oxidization of the carbon skeleton,
which is not suitable as a raw material for preparing nano-scale
lead azide. Thirdly, through the course of the azidation reac-
tion, lead azides@porous carbon hybrid (LA@PC) was prepared,
in which nano-scale LA is evenly distributed in the carbon
skeleton.
It can be seen from the TEM image in Fig. 2a–c that the
particle size of the precursor material PAA-Pb, PbO@PC, and
LA@PC are 20 nm, 30 nm, and 110 nm, respectively. Since the
copper-containing gel particles (PAA-Pb) and the carbonized
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Fig. 2 (a) TEM image of PAA-Pb; (b) TEM image of PbO@PC; (c) and (d) TEM image and energy dispersive X-ray spectroscopy (EDS) mapping of
LA@PC; (e) particle size distribution curves of LA@PC; (f) P-XRD pattern (JCPDS card no. 14-0629); (g) and (h) XPS spectra (Pb and N) curves of
LA@PC; (i) DSC curve of LA@PC.
prepared LA@PC sample, the LA is uniformly dispersed at the
nanometer-scaled carbon frameworks. As the LA size becomes
smaller, the number of atoms on the surface and the specic
surface area increases. In addition to the high thermal
conductivity of carbon material, the energy barrier to be over-
come by each LA particle decomposition becomes smaller and
Fig. 3 (a) Flame sensitivities of LA@PC and the mixtures of LS/LA,
respectively; (b) schematic diagram of electrostatic sensitivity test
mechanism; (c) electrostatics sensitivities of LA@PC and mixtures of LA
and LS, respectively; (d) the whole sensitivities comparison of LA@PC,
LA, LS, LA/GNP, mixtures of LS/LA.
from Fig. 3d that LA@PC has the most excellent ignition
capability and safety performance, compared with the ignition
and safety performance of LA, LA/GNP, LS, LA and LS hybrid
systems, which is of great signicance in practical applications.
Comparing and analyzing the differences in scale, structure
and sensitivity performance of primary explosives such as LA,
LS/LA, and LA/GNP, we proposed the plausible mechanism of
LA@PC ame sensitivity and electrostatic sensitivity reduction.
Fig. 4a is a schematic diagram of the mechanism of the
increase in ame sensitivity. As we know, the thermal conduc-
tivity of lead azide is poor. If the ame contacts the bulky lead
Fig. 4 (a) Schematic diagram of the mechanism of rising flame
azide aggregates, the heat cannot be dispersed in time, result-
sensitivity; (b) schematics illustration of the mechanism of electrostatic
sensitivity for LA.
ing in poor ignition capability of the lead azide. However, for the
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¨
the heat transfer efficiency is increased, which makes a small
amount of heat sufficient for the LA to be rapidly decomposed
in a short time, resulting in an increase in the ame sensitivity
of LA@PC. Therefore, the thermal conductivity of the carbon
skeleton and nanoscale LA is the key to the increase in ame
sensitivity. Moreover, the schematic diagram of the mechanism
of electrostatic sensitivity reduction of LA@PC is shown in
Fig. 4b. Since the large particles of LA are gathered together, the
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In summary, we rst prepared LA@PC by in situ azidation and
carbonization of low-cost PAA-Pb hydrogel, in which nanoscale
LA was uniformly distributed on the oxygen-rich porous carbon
skeleton. The prepared LA@PC composite has lower electro-
static sensitivity and high ignition capability, and the ame
sensitivity can reach 42 cm, which makes it possible for lead
azide to be used without the ignition powder LS. The synthesis
of LA@PC solves the problem that the pure lead azide ame has
low ame sensitivity and poor ignition ability, and has high
research value and application prospect.
19 J. Liu, Y. Jiang, W. Tong, T. Zhang and L. Yang, Propellants,
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20 G. W. C. Taylor, W. Abbey and S. E. Napier, Lead Styphnate
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There are no conicts to declare.
Acknowledgements
We gratefully acknowledge nancial support from the National
Natural Science Foundation of China (No. 11672040), the State
Key Laboratory of Explosion Science and Technology (No.
YB2016-17), and Beijing Institute of Technology Research Fund
Program for Young Scholars.
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