3.1 Chemical composition and morphology analysis of nanocomposite fibers
XRD patterns of Al, RDX, NC fibers and composite fibers are shown in Fig. 2 (a). For NC/Al and NC/Al/RDX composite fibers, all diffraction peaks of nano Al observed at 2θ = 38.5, 44.6, 65.2 and 78.2° can be indexed to (111), (200), (220) and (311) crystal planes of Al, respectively, which corresponds well with the standard pattern of single Al (JCPDS No. 04-0787). All diffraction peaks of recrystallized RDX in NC/RDX and NC/Al/RDX are well consistent with those of raw RDX, indicating the unchanged crystal phase of recrystallized RDX particles during the electrospinning process. A broad protruding peak can be observed in the XRD pattern of NC fibers due to that NC is a type of energetic polymer (Luo et al. 2019). Contrast to the diffraction peaks of NC fibers 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 fibers via electrospinning.
FT-IR spectra of the samples were recorded to explore their surface functional groups, as displayed in Fig. 2 (b). Two strong absorption peaks of NC fibers located at 1629.8 and 1271.0 cm− 1 can be assigned to the symmetric and anti-symmetric stretching vibrations of -ONO2, respectively. The characteristic peaks observed at 821.7, 995–1056 and 670–745 cm− 1 are attributed to the deformation vibration of C-O-NO2, the inter-ring stretching vibration of C-O and the scissoring and rock vibrations of -NO2, respectively. All these absorption peaks match well with the molecular structure of NC (Luo et al. 2020; Ma Angeles et al. 2011). In composite fibers, the FT-IR characteristic absorption peaks of NC are consistent with those of NC fibers, but their intensity changes slightly, indicating that the structure of polymer molecules did not change during the electrospinning process. For the FT-IR spectrum of RDX, the absorption peaks located at 1531.5 and 1570.1 cm− 1 are assigned to the asymmetric stretching vibrations of -ONO2. The peaks located at 1458 and 1421 cm− 1 are related to CH2 deformation vibrations in the ring of RDX molecule. The peaks located at 1384, 1200, 782.5 and 588.9 cm− 1 are ascribed to the symmetric stretching vibrations of -NO2, the stretching vibrations of N-NO2, the deformation vibration and stretching vibration of -NO2, respectively. The absorption peaks of the bending vibrations of the RDX ring can be found at 1037.7 and 908.5 cm− 1 (Chen et al. 2019; Gao et al. 2017; Carlos et al. 2010). 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 significantly weakened due to the low content of RDX in composite fibers. Thus, based on the above analysis, the recrystallized RDX particles were successfully merged in NC fibers.
Optical images of nonwoven nanofiber mats prepared by electrospinning are shown in Fig. 3. For nanocomposite fibers, 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 fiber and NC/RDX composite nanofiber mats present the same color. The nanofiber mats with different composition can be obtained through the appropriate selection of solvent.
Figure 4 shows SEM images of RDX, nano Al and NC before and after electrospinning. RDX particles are ellipsoids with diameters of 4–10 µm. There are obvious cracks on the surface of the particles, which could be unfavorable for improving the sensitivity of RDX crystals. The average diameter of nano Al particles is about 100 nm. Figure 4 (c) and (d) reveal the surface morphology and structural characteristics of NC before and after electrospinning. Before electrospinning, NC presents as short and thick fibers with a diameter of about 20 µm and has a rough and irregular surface morphology. However, after processed, well-defined interconnected networks constructed by NC fibers are obtained, and the insert shows that the NC fibers have smooth surfaces and an average diameter of about 1 µm.
The microstructure and surface morphology of the composite fibers 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 fibers, and the composite fibers 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 fibers gradually become more uniform and smoother, and the beadlike structures in NC/RDX nanofiber mats decrease. The morphology differences between NC/RDX and NC fibers 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 fibers have a significantly effect on the surface morphology and structure of the composite fibers.
Figure 5 (b)-(b") and (c)-(c") show SEM images of NC/Al and NC/Al/RDX via electrospinning. Uniform and well-defined interconnected fibrous networks can be observed in all the six composite samples. It is remarkable that when nano Al content in the composite fibers is high, such as NC/Al with mass ratio of 1:1 in Fig. 5 (b), the obtained NC/Al fibers have rough and irregular surface morphology compared with NC fibers, 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 fibers is caused by the addition of excess nano Al. The composite fibers with low mass loading of Al particles show less obvious aggregation and have relatively smoother surfaces compared with the fibers 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 insignificant differences in fiber 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 influence on the morphology characteristics of composite fibers. Smaller recrystallized RDX particles in NC/Al/RDX fibers 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 fibers with poor morphology.
TEM images of nano Al particles and NC/Al composite fibers 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 fibers loaded by nano Al with different contents have quite clear boundaries, and nano Al particles are uniformly incorporated inside the NC fibers 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 fibers. Thus, the surfaces of the composite fibers 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 specific surface areas and porous nature of the nanofiber mats prepared by electrospinning. The nitrogen adsorption-desorption isotherms of NC, NC/Al, NC/RDX and NC/Al/RDX composite fibers are shown in Fig. 7. The insert in Fig. 7 presents the pore size distribution of the corresponding samples. The NC fibers have a relatively small specific surface area of about 6.22 m2 g− 1. However, after nano Al and RDX particles are incorporated into NC fibers, the specific surface areas of NC/Al, NC/RDX and NC/Al/RDX increase enormously and reach 30.42, 18.32 and 23.26 m2 g− 1, respectively. In addition, compared with NC fibers, the pore size distribution of the composite fibers is narrower than that of NC fibers as shown in the inset. The larger specific 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 fibers and their rough surfaces.
3.2 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 fibers are 202.1 ℃ and 149.32 kJ mol− 1, respectively, and are significantly 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 fibers. 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 fibers. 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 fibers 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 beneficial 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.
Table 1
The kinetic parameters of NC and RDX in different samples obtained by Kissinger and Ozawa Methods.
Samples | Component | Kissinger method | Ozawa method |
Ek (kJ mol− 1) | logA (s− 1) | r | Eo (kJ mol− 1) | r |
Raw NC | NC | 218.5 | 21.99 | 0.9913 | 215.65 | 0.9919 |
NC-Fibers | 149.32 | 14.47 | 0.9861 | 149.54 | 0.9874 |
Raw RDX | RDX | 177.14 | 16.11 | 0.9898 | 176.56 | 0.9906 |
NC/Al (1:1) | NC | 319.42 | 33.42 | 0.9997 | 311.3 | 0.9997 |
NC/Al (3:1) | 226.91 | 22.97 | 0.9844 | 223.37 | 0.9854 |
NC/Al (5:1) | 174.24 | 17.04 | 0.997 | 173.32 | 0.9972 |
NC/RDX (1:1) | NC | 188.04 | 18.67 | 0.9948 | 186.4 | 0.9952 |
RDX | 149.98 | 13.46 | 0.9978 | 150.66 | 0.9980 |
NC/RDX (3:1) | NC | 183.72 | 18.24 | 0.9908 | 182.26 | 0.9915 |
RDX | 158.32 | 14.30 | 0.9984 | 158.61 | 0.9986 |
NC/RDX (5:1) | NC | 175.33 | 17.27 | 0.9936 | 174.31 | 0.9941 |
NC/Al/RDX (3:1:1) | NC | 242.32 | 24.87 | 0.9949 | 237.98 | 0.9952 |
RDX | 141.76 | 12.42 | 0.9891 | 142.96 | 0.9902 |
NC/Al/RDX (5:1:1) | NC | 202.57 | 20.39 | 0.9803 | 200.18 | 0.9818 |
RDX | 133.08 | 11.43 | 0.9771 | 134.71 | 0.9797 |
NC/Al/RDX (1:1:0.2) | NC | 154.71 | 14.9 | 0.9983 | 154.75 | 0.9985 |
3.3 Laser ignition properties of energetic nanocomposite fibers
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 fibers [Fig. 9 (a)-(c)]. The comparison between different composite fibers is presented in Fig. 9 (d). For all composite fibers, the ignition delay time gradually decreases with the increase of laser power density at low laser power density (≤ 200 W cm− 2). However, when the laser power density exceeds 200 W cm− 2, the ignition delay time almost remains constant and reaches the minimum. For NC/Al, NC/RDX and NC/Al/RDX, the ignition delay time decreases from about 55, 140, and 80 ms at 17.38 W cm− 2 to about 23, 30, and 24 ms at 250 W cm− 2, respectively. The ignition delay time 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 fibers 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 fibers, 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 (Yan et al. 2020). After incorporating nano Al into NC fibers, the ignition delay time of NC/Al is significantly 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 fibers, and the agglomeration between the particles is significantly 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 fibers, 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 efficiency 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 fibers 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 fibers, 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 coefficient than that of RDX (Yan et al. 2020). 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 fibers 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 fibers ༞ 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 fibers, which further indicates that RDX has a retarding effect on the laser ignition process of the composite fibers.
The combustion processes of nano Al, NC/Al (1:1), and NC/Al/RDX (1:1:0.2) were captured with a high-speed 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 flame 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 flame for NC/Al (1:1) and NC/Al/RDX (1:1:0.2) also present significant differences. When NC/Al (1:1) and NC/Al/RDX (1:1:0.2) were ignited, an obvious flame with a great number of twinkling sparks appeared, which then steadily propagated in the radial and axial directions. These bright flames 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 fibers, 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 flames 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 flames 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 flames 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 flames 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) deflagrate rapidly after the ignition. This is because the nano Al particles dispersed in the fibers act as the main laser energy absorber and firstly 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 flames. In summary, the results of the laser ignition experiments further show that the fibrous 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 fibers.