Enhanced Thermal Decomposition and Laser Ignition Properties of Nanostructured NC/Al/RDX Composite Fibers Fabricated by Electrospinning

Nano Al has always been the research hotspot in the eld of energetic materials because of its high energy density and combustion temperature, and has been considered as a fuel to enhance the energy release of various propulsive systems. In this work, nanocomposite bers were fabricated by electrospinning technology, in which nano Al and recrystallized RDX particles were integrated with NC bers. The morphology and chemical components of NC/Al, NC/RDX, and NC/Al/RDX composite bers were characterized by XRD, FT-IR, SEM, TEM and BET. The agglomeration of nano Al particles in bers is signicantly inhibited, and the recrystallized RDX and nano Al particles are uniformly dispersed in NC bers, resulting in the rough surfaces of the composite bers. The thermal analysis shows that nano NC bers have lower thermal decomposition temperature (202.1 ℃ ) and apparent activation energy (149.32 kJ mol -1 ) than raw NC (208.2 ℃ and 218.5 kJ mol -1 ), and NC/Al/RDX exhibits improved thermal decomposition properties compared with NC/RDX and NC/Al. The laser ignition experiments suggest that the uniformly dispersed nano Al particles could obviously promote the combustion and shorten ignition delay time. However, RDX may delay ignition due to its high decomposition temperature, but can signicantly enhance the combustion properties of NC/Al/RDX bers. Among the all samples, the NC/Al/RDX (1:1:0.2) exhibits shortest ignition delay time and most violent combustion ames, which can be attributed to the brous structure and the enhanced heat and mass transfer between the components.


Introduction
Aluminum (Al) powder is one of the most important components for the heterogeneous propellants, and can signi cantly improve the energy performance of solid propellants due to its high enthalpy and combustion temperature (Arkhipov et al. 2012;Dokhan et al. 2002). In addition, the ignition temperature of nano Al powder ( 1000 K) is closer to surface temperature of Al-containing propellants (~ 800 K) compared with that of micro Al powder (~ 2300 K). The burning ame of nano Al-containing propellants is closer to the propellant surfaces compared with that of micro Al-containing propellants, resulting in better heat feedback and higher burning rates (Jian et al. 2013;Young et al. 2015;Pang et al. 2019).
However, the application of nano Al powder in propellants still faces some processing challenges. The nano Al powder with small particle size can increase viscosity of polymer binders and is prone to agglomeration, and its pre-reaction sintering can increase the effective size to a micron scale, which limits its large-scale potential applications in formulations (Lebedeva et al. 2012;Lyu et al. 2019;Galfetti et al 2007). Although many approaches, such as self-assembly, sol-gel synthesis and sputtering, were employed to deal with these issues by controlling structure properties and loading of nanoparticles, the sensitivity of materials to heat and sparks increased, thus decreasing their safety (Petrantoni et  Electrospinning, which is one of the simplest top-down preparation approaches to obtain versatile functional bers from many polymer systems, is usually applied in the elds of optical devices, biomedical and healthcare products, sensors and ltration and puri cation for air and water (Yan et al. 2012;Luo et al. 2012;Ding et al. 2010). Nitrocellulose (NC) and cyclotrimethylenetrinitramine (RDX) are a common energetic binder and a high energy oxidizer for propellants, respectively (Jian et al. 2013). In this work, the energetic NC/Al/RDX nanocomposite bers were prepared by the electrospinning, in which nano Al and recrystallized RDX particles were uniformly dispersed in the NC bers. Morphology, phase composition, thermal decomposition and laser ignition properties of the NC/Al/RDX nanocomposite bers were characterized and analyzed. The effects of nano Al and RDX on the ignition delay and combustion properties were studied. The results will lay the foundation for further investigating the combustion properties of NC/Al/RDX and eventually enlarge the application range of this kind of energetic materials in propellants.

Reagents and materials
All the chemical reagents are commercially available and were directly used as received without further puri cation. Aluminum nanoparticles (Al NPs) with an average diameter of ~ 100 nm were purchased from Beijing Innochem Technology Co., Ltd. The active aluminum content was determined to be ~ 82 wt.% by thermogravimetry/differential scanning calorimetry (TG/DSC) analysis. NC with the nitrogen content of 11.8-12.1% and RDX were provided by Xi'an Modern Chemistry Research Institute.

Precursor preparation
Ethanol and acetone were mixed at a volume ratio of 1:1 to form a homogeneous mixture solution. NC was added into the mixture solution at a mass ratio of 1:10 (NC/mixture solution) in each experimental formulation. Typically, for NC/Al/RDX composite bers, NC was rst dissolved in the mixture solution and vigorously stirred for 0.5 h to obtain a NC solution. Then, Al NPs were uniformly dispersed in the NC solution through ultrasonic treatment for 0.5 h. Finally, RDX was added into the above Al /NC solution and magnetically stirred for 24 h at room temperature. The preparation processes of other precursor solutions for NC/Al, NC/RDX or NC were similar to those depicted above.

Electrospinning process
The fabrication schematic diagram of composite bers is shown in Fig. 1. As illustrated, the precursor solution was loaded in a syringe, fed by a syringe pump at a rate of 0.02 mm s − 1 . A working voltage of 18 kV was attached to the stainless nozzle to create a high electrostatic eld. The working distance between nozzle-tip and collector substrate was kept at 22 cm. The products were collected from the aluminum foil which covers the collector substrate at -2 kV static. For safety purposes, all operations were carefully processed in case of electric shock, and the whole experimental processes were carried out in the fume hood.

Morphology and composition characterization
The surface morphology and microstructure properties of the samples were observed by Field emission scanning electron microscopy (FESEM, Carl Zeiss SIGMA HV) with a working distance of ~ 6 mm at an accelerating voltage of 5 kV. The nanocomposite bers were adhered to a carbon-tape on an SEM stage and then sputtered with gold using ion sputtering device for the FESEM observation. Transmission electron microscopy (TEM, FEI Tecnai G2 microscope) was employed to characterize the distribution of Al NPs inside the NC bers at 200 kV. The composition and crystal phase of the nanocomposite bers were determined by powder X-ray diffraction (XRD, Rigaku Mini Flex 600, Japan) using Cu Kα radiation (λ = 1.5406 Å) at 40 kV voltage and 40 mA current with a scanning rate of 5 ° min − 1 in the 2θ range from 5 °t o 80 °. Infrared spectra were recorded on a Fourier transform infrared spectrophotometry (FT-IR, Shimadozu IRA nity-1S WL, Japan) over the range of 400-4000 cm − 1 . The speci c surface areas of nanocomposite bers were measured using nitrogen adsorption/desorption instrument (Micromeritics ASAP 2460) and calculated by a multipoint Brunauer-Emmett-Teller (BET) method. The samples were degassed at 120 ℃ for 12 h before Nitrogen adsorption.

Thermal behavior analysis
Differential scanning calorimetry (DSC) was employed to investigate the thermal decomposition properties of NC, RDX and nanocomposite bers prepared by electrospinning, and performed on a DSC Instrument (Netzsch, 200F3) in a high-purity Nitrogen atmosphere with a ow rate of 20 mL min − 1 under ambient pressure. About 1 mg samples was used and placed in an alumina crucible for thermal analysis at a heating rate of 10 ℃ min − 1 from room temperature to 350 ℃.

Laser ignition tests
The laser ignition properties of the samples were investigated by a carbon dioxide laser ignition system (SLC 110) in air atmosphere. The output power was corrected by an optical power meter before each ignition experiment. The optical emission indicates an ignition event, which could be used to measure the ignition delay time (Dai et al. 2018b). The ignition delay time was de ned by the time difference from the triggering of the laser to the initial photo signal of igniting ame. The combustion processes were recorded by a high-speed digital camera to capture the ame images at a frame rate of 1000 frames per second. For reducing the experimental error, each sample was performed at least three times under the same laser power density, and then the average values were taken to investigate the ignition properties of the samples. Contrast to the diffraction peaks of NC bers and RDX, the intensity of diffraction peaks for NC and RDX in NC/Al and NC/Al/RDX is relatively weak, which is caused by the strong intensity of the diffraction peaks of nano Al. The XRD results suggest that RDX and nano Al particles can be successfully incorporated into NC bers via electrospinning.

Results And
FT-IR spectra of the samples were recorded to explore their surface functional groups, as displayed in In the spectra of both NC/RDX and NC/Al/RDX, the positions of characteristic absorption peaks of the recrystallized RDX particles are roughly same with those of RDX, but their intensity is signi cantly weakened due to the low content of RDX in composite bers. Thus, based on the above analysis, the recrystallized RDX particles were successfully merged in NC bers.
Optical images of nonwoven nano ber mats prepared by electrospinning are shown in Fig. 3. For nanocomposite bers, the nonwoven mats with different composition show different colors, and the colors change from white for NC/RDX to gray for NC/Al/RDX and black for NC/Al, because of the addition of nano Al particles. The pure NC ber and NC/RDX composite nano ber mats present the same color. The nano ber mats with different composition can be obtained through the appropriate selection of solvent. The microstructure and surface morphology of the composite bers with different composition were characterized by SEM and shown in Fig. 5. In Fig. 5 (a)-(a"), the recrystallized RDX particles during the electrospinning process are tightly adhered to the surfaces of NC bers, and the composite bers present a rough surfaces and uneven diameter distribution. It should be noted that with the decrease of RDX, the diameters and surfaces of NC/RDX bers gradually become more uniform and smoother, and the beadlike structures in NC/RDX nano ber mats decrease. The morphology differences between NC/RDX and NC bers are mainly caused by two factors. One is that the addition of RDX nanoparticles leads to the inhomogeneity of the precursor solution. The other is that partial recrystallized RDX particles agglomerate to form the beadlike structures during the electrospinning process. SEM images indicate that RDX particles tightly loading on NC bers have a signi cantly effect on the surface morphology and structure of the composite bers.  Fig. 5 (b), the obtained NC/Al bers have rough and irregular surface morphology compared with NC bers, which may be accused by the increased viscosity of the precursor solutions due to the addition of excess nano Al power and the agglomeration of nano Al particles during the electrospinning process. Moreover, the inhomogeneous of precursor polymer solution of the composite bers is caused by the addition of excess nano Al. The composite bers with low mass loading of Al particles show less obvious aggregation and have relatively smoother surfaces compared with the bers in Figs. 5 (b) and (c"). By comparing NC/Al and NC/Al/RDX, it can be found that NC/Al and NC/Al/RDX present insigni cant differences in ber surface morphology and diameters, which is because the addition of nano Al particles increases the electrical conductivity of the precursor solution. Thus, compared with RDX, nano Al presents predominant in uence on the morphology characteristics of composite bers. Smaller recrystallized RDX particles in NC/Al/RDX bers can also be observed in Fig. 5 (a)-(a"). Moreover, the excess addition of RDX can mess the texture of precursor solution, resulting in the composite bers with poor morphology.
TEM images of nano Al particles and NC/Al composite bers with different mass ratio of NC to Al are shown in Fig. 6. Nano Al particles with smooth surfaces and an average diameter of about 100 nm are covered by oxide caps with a thickness of about 5.8 nm, as shown in Fig. 6 (a). Figure 6 (b-d) shows that NC/Al composite bers loaded by nano Al with different contents have quite clear boundaries, and nano Al particles are uniformly incorporated inside the NC bers and well dispersed, especially for NC/Al with a low nano Al content. When the content of nano Al particles is high in NC/Al [ Fig. 6 (b)], nano Al particles display more vigorous aggregations inside the bers. Thus, the surfaces of the composite bers with a high nano Al content is rougher than those with a low nano Al content, which is consistent with the above SEM results.
BET analysis was performed to further investigate speci c surface areas and porous nature of the nano ber mats prepared by electrospinning. The nitrogen adsorption-desorption isotherms of NC, NC/Al, NC/RDX and NC/Al/RDX composite bers are shown in Fig. 7. The insert in Fig. 7 presents the pore size distribution of the corresponding samples. The NC bers have a relatively small speci c surface area of about 6.22 m 2 g − 1 . However, after nano Al and RDX particles are incorporated into NC bers, the speci c surface areas of NC/Al, NC/RDX and NC/Al/RDX increase enormously and reach 30.42, 18.32 and 23.26 m 2 g − 1 , respectively. In addition, compared with NC bers, the pore size distribution of the composite bers is narrower than that of NC bers as shown in the inset. The larger speci c surface areas and narrower pore size distribution of NC/Al, NC/RDX and NC/Al/RDX can be attributed to the uniform distribution of nano Al and recrystallized RDX particles in the NC bers and their rough surfaces.

Thermal properties analysis
The thermal behaviors of the samples were investigated by DSC in Fig. 8. The kinetic parameters of thermal decomposition were calculated by Kissinger and Ozawa methods with the DSC data at different heating rates (5, 10, 15 and 20 ℃ min − 1 ), as listed in Table 1 (Ozawa 1970;Zhang et al. 2020). The thermal decomposition temperature and apparent activation energy of NC bers are 202.1 ℃ and 149.32 kJ mol − 1 , respectively, and are signi cantly lower than those of raw NC (208.2 ℃ and 218.5 kJ mol − 1 ), indicating that nanoization of NC can promote its thermal decomposition. For NC/Al [ Fig. 8 (b)], especially to NC/Al with the mass ratio of 1:1 (208.5 ℃ and 319.42 kJ mol − 1 ), their thermal decomposition temperature and apparent activation energy are higher than those of NC bers. The reason is that nano Al particles act as the energy absorber to absorb part of the heat during the heating process, thus delaying the decomposition of NC bers. The thermal decomposition of NC/RDX exhibits two-stage exothermic processes in Fig. 8 (c), corresponding to two obvious exothermic peaks located at about 206 and 237 ℃ from the exothermic decomposition of NC and RDX, respectively. Compared with RDX, no melting peak of RDX exists at about 205 ℃ on DSC curves of NC/RDX, which may be because the strongly exothermic decomposition peak of NC covers the weakly melting endothermic peak of RDX. The DSC results indicate that RDX particles are successfully incorporated into the NC bers with unchanged exothermic behaviors, and the NC decomposes prior to the decomposition of RDX without any intermediate reaction between NC and RDX. Moreover, as shown in Table 1, the thermal decomposition kinetic calculations show that with the increase of NC content in NC/RDX, the apparent activation energy of NC decreases, which is bene cial to promote the ignition of NC/RDX. Figure 8 (d) shows that the thermal decomposition peaks of NC and RDX appear simultaneously in NC/Al/RDX, and NC in NC/Al/RDX has lower decomposition temperature than that in NC/RDX and NC/Al. In addition, DSC curve of NC/Al/RDX (1:1:0.2) is similar to that of NC/Al (1:1) due to the low content of RDX. Meanwhile, the apparent activation energy of NC and RDX in NC/Al/RDX (3:1:1) (242.32 and 141.76 kJ mol − 1 ) is higher than those in the NC/Al/RDX (5:1:1) (202.57 and 133.08 kJ mol − 1 ). Thus, NC/Al/RDX (5:1:1) exhibits the enhanced thermal decomposition performances, which is consistent with the short ignition delay time of NC/Al/RDX (5:1:1) during the laser ignition experiments.

Laser ignition properties of energetic nanocomposite bers
The ignition delay time was measured as a function of the laser power density for NC/Al (a), NC/RDX (b) and NC/Al /RDX (c) composite bers [ Fig. 9 (a)-(c)]. The comparison between different composite bers is presented in Fig. 9 (d). of NC/Al and NC/Al /RDX is shorter than that of NC/RDX due to the incorporation of nano Al, which indicates that nano Al can shorten the ignition delay time.
The insert in Fig. 9 (a) shows that both nano-Al and NC bers have longer ignition delay time compared to NC/Al at low laser power density, and the ignition delay time of nano Al is larger than that of NC bers, which may be related to the high melting temperature (~ 2070 ℃) of the oxide cap on the surfaces and the aggregation of nano Al particles. Meanwhile, the larger heat loss rate relative to the energy release rate is also detrimental to the ignition of nano Al ). After incorporating nano Al into NC bers, the ignition delay time of NC/Al is signi cantly shortened. Meanwhile, in the same laser power density, the higher content of NC results in shorter the ignition delay time, which is most likely due to the following two factors. First, when the content of nano Al is low, the nano Al particles are evenly dispersed in the NC bers, and the agglomeration between the particles is signi cantly restrained by NC, which can be proved by SEM and TEM images of NC/Al. During the laser ignition process, nano Al particles uniformly dispersed in NC act as the major laser energy absorber, which can accelerate the heat diffusion rate from nano Al to NC, thereby shortening the ignition delay time. Second, at the low nano Al content, nano Al particles are mainly dispersed inside the NC bers, forming NC/Al with structure similar to nested core-shell structure. The NC coating on the surfaces of NC/Al can effectively improve the energy absorption e ciency and prevent the heat diffusion [4] . Figure 9 (b) shows that NC/RDX composites present longer ignition delay time than NC, and with the increase of RDX content, the ignition delay time of the composite bers increases. The characteristic that the ignition delay time of NC/RDX varies with the RDX content may be attributed to the higher thermal decomposition temperature of RDX (~ 242 ℃) than that of NC (~ 208 ℃), resulting in the more energy absorbed by NC/RDX during the laser ignition, so RDX particles have the effect of delaying ignition on NC/RDX. In addition, the recrystallized RDX particles have fewer surface defects, reducing the generation of hot spots, which may also have a certain effect on the ignition delay time of NC/RDX. It can be seen from Fig. 9 (c) that when the laser power density is low, the ignition delay time of NC/Al/RDX (3:1:1) is lightly longer than that of NC, while the ignition delay time of NC/Al/RDX (5:1:1) with a high NC content is below that of NC. The ignition delay time of both NC/Al/RDX (3:1:1) and NC/Al/RDX (5:1:1) are lower than that of NC/RDX composite bers, which suggests that when nano Al and RDX co-exist in NC/Al/RDX, the promotion effect of nano Al on the ignition delay time of NC is more dominant than the inhibition effect of RDX. This may be related to the high dispersion properties of nano Al and the larger laser energy absorption coe cient than that of RDX ). The characteristic curve of the ignition delay time for NC/Al/RDX (1:1:0.2) also proves the above results, that is, the low content of RDX can shorten the ignition delay. For the composite bers with different components [ Fig. 9 (d)], the order of ignition delay time is nano Al NC/ RDX (3:1) NC/Al/RDX (3:1:1) NC bers NC/Al (3:1) at low laser power density. When the laser power density increases, the ignition delay time of NC/RDX (3:1) is still larger than that of other bers, which further indicates that RDX has a retarding effect on the laser ignition process of the composite bers.
The combustion processes of nano Al, NC/Al (1:1), and NC/Al/RDX (1:1:0.2) were captured with a highspeed camera during the laser ignition experiments at the laser power density of 200 W cm − 2 , and the corresponding snapshots are shown in Fig. 10. The burning snapshots of nano Al show that the ame size and shape remain small and constant during the whole burning process, the combustion residues remain as an intact structure. In addition, nano Al has the longer ignition delay time and combustion duration compared with NC/Al (1:1) and NC/Al/RDX (1:1:0.2). The time-consuming of the entire burning process for nano Al is about 7-fold of NC/Al (1:1) and NC/Al/RDX (1:1:0.2). These may be attributed to the high melting temperature (about 2070 ℃) of the oxide cap on the nano Al surfaces and the aggregation of Al nanoparticles, which is consistent with the previous SEM and TEM analysis.
Comparing with nano Al, in addition to the obviously shortened ignition delay and burning time, the shape and size of ame for NC/Al (1:1) and NC/Al/RDX (1:1:0.2) also present signi cant differences. When NC/Al (1:1) and NC/Al/RDX (1:1:0.2) were ignited, an obvious ame with a great number of twinkling sparks appeared, which then steadily propagated in the radial and axial directions. These bright ames can be roughly divided into central high temperature zone and outer low temperature zone, as distinguished by the brightness. The central high temperature zone is mainly caused by the solid burning residues ejected from the combustion of nano Al dispersed in the composite bers, while the outer low temperature zone is principally caused by the gas products from the high temperature decomposition of NC and RDX particles. In addition, although the ignition delay time of NC/Al/RDX (1:1:0.2) at the same laser power density (200 W cm − 2 ) is slightly longer than that of NC/Al (1:1), the combustion ames of NC/Al/RDX (1:1:0.2) are brighter and more violent with more sparks splashing out to the ambient and stronger light emission. The ames of NC/Al/RDX (1:1:0.2) consist of stochastic bright spots which were emanated by combustion residues of the nano Al particles, which are much larger and brighter than those of NC/Al (1:1). The larger and brighter ames imply enhanced heat convection and feedback from nano Al particles to ambient gases, which may contribute to the improvement of combustion reactions.
Among nano Al, NC/Al (1:1), and NC/Al/RDX (1:1:0.2), the combustion ames of NC/Al/RDX (1:1:0.2) are the most violent, which are mainly due to the addition of a small amount of RDX. Introducing a few RDX into NC/Al makes NC/Al/RDX (1:1:0.2) de agrate rapidly after the ignition. This is because the nano Al particles dispersed in the bers act as the main laser energy absorber and rstly react with NC to release gases and heat, which promotes the ignition of RDX. As a good gas generator, the recrystallized RDX particles rapidly decompose and further release a large quantity of gases and heat. The enormous gaseous products from RDX and NC in the system involved in the quickly combustion process can further retard sintering of the nano Al, which promotes the heat transfer dominated by convection to a certain extent, so NC/Al/RDX (1:1:0.2) shows bright and intense burning ames. In summary, the results of the laser ignition experiments further show that the brous NC/Al/RDX composites enhance the interface contact of the components and improve heat feedback to ignition during ultra-fast laser irradiation, thus resulting in the enhancement heat release of the composite bers.

Conclusion
In summary, NC/Al, NC/RDX and NC/Al/RDX nanocomposite bers were successfully fabricated by electrospinning. The investigations on morphology and chemical components indicate that nano Al particles and recrystallized RDX particles are well dispersed in NC bers, and the agglomeration of nano Al particles is obviously inhibited, which is bene cial to enhance the heat feedback and improve the laser ignition properties of the composite bers. The ignition delay time of the composite bers is related to the activation energy of decomposition for NC in the composite bers. For the composite bers with the same composition and different NC contents, the smaller activation energy for NC is, the shorter ignition delay of composite bers becomes. Meanwhile, the composite bers containing nano Al particles have shorter ignition delay than that of the bers without nano Al particles, due to the high dispersion and absorption coe cient of nano Al particles, the enhanced interfacial contact extent and the shortened diffusion distance. But RDX retards ignition due to its higher decomposition temperature than that of NC.
However, RDX can signi cantly enhance the combustion of NC/Al/RDX by inhibiting the agglomeration of nano Al particles. In brief, this research is conductive to further understand the combustion process of NC/Al/RDX composite bers, and electrospinning would be extended to other nanocomposite bers for eventually enlarging the application range of this class of energetic materials in propellants.