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Brief Report Nanometer and Ammonium Prepared with 2D Network Structure via Rapid Freezing Technology

1, , 2, , 3 Yi Wang * † , Xiaolan Song * † and Fengsheng Li 1 School of Materials Science and Engineering, North University of China, Taiyuan 030051, China 2 School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China 3 School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] * Correspondence: [email protected] (Y.W.); [email protected] (X.S.) These authors contributed equally to this work. †  Received: 30 October 2019; Accepted: 7 November 2019; Published: 12 November 2019 

Abstract: Nanometer (nano) (AP) and (AN) were prepared with 2D network structures by the ultra-low temperature spray method. Scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis/infrared spectrometry (TG-IR) were employed to probe the micron structure, crystal phase, and of nano AP and nano AN. SEM images revealed that the sizes of nano AP and AN were in the nanometer scale (<100 nm) in one dimension. XRD patterns showed that the crystal phases of nano AP and AN were in accordance with those of raw AP and raw AN, respectively. DSC traces indicated that the thermal decomposition process of AP depended on its particle size, while the thermolysis of AN was independent of the particle size of AN. TG-IR analyses illustrated that the decomposition products of nano AP were NO2,N2O, HCl and H2O, with a small amount of NOCl, and the main decomposition products of nano AN were N2O and H2O, with a small amount of NH3. The results of mechanical sensitivity tests indicated that nano AP was more sensitive than raw AP and both nano AN and raw AN were very insensitive to impact and friction stimuli.

Keywords: nano AP; nano AN; liquid ; freeze drying; thermolysis

1. Introduction Ammonium perchlorate (AP) and ammonium nitrate (AN) are the most commonly used oxidizers applied in solid [1–3]. In addition to their use in propellants, AP and AN are also employed as oxygen-enriched ingredients in and pyrotechnics [4–6]. Researchers have found that the performance of AP and AN is a function of their particle sizes. For example, the burning rate of a using superfine AP as an oxidizer is obviously higher than that of the propellant using coarse AP as an oxidizer [7]. However, in contrast to AP, the burning rate of propellants that use AN as an oxidizer is independent of the particle size of AN [8]. Interestingly, a decrease in particle size of AN is beneficial as it decreases the combustion pressure exponent of AN-based propellants [9]. The mechanism concerning this is still obscure, of course, and is also not the theme of this work. Kumari et al. prepared nanometer (nano) AP by a precipitation method, and the nano AP was precipitated out in the form of nanoparticles in an HTPB (hydroxy terminated polybutadiene) matrix [10]. The HTPB matrix limited the growth of the particles, resulting in a mean size of 33 nm for the nano AP. This method was very efficient because the yield of nano AP was up to 85% (8.5 g per batch). Frolov et al. prepared nano-sized powders of ammonium nitrate through vacuum deposition on a cooled quartz substrate [11]. The AN crystallites were fairly uniform in size and shape and less

Nanomaterials 2019, 9, 1605; doi:10.3390/nano9111605 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1605 2 of 8 than 50 nm in diameter. Aside from these two studies, the literature provides a number of other examples on how to prepare superfine AP and AN, which are also of great reference significance. For instance, Ma et al. used as a solvent and ethyl acetate as a non-solvent to prepare superfine AP particles with sizes of 5~10 µm [12]. Wu et al. used an air jet pulverization method to fabricate superfine AP particles with very narrow particle size distribution [13]. Song et al. fabricated spherical superfine AP particles (d 5 µm) via two methods [14]. Specifically, coarse AP particles were 50 ≈ first comminuted to fine particles (d 20 µm) by an air jet pulverization method, and then the 50 ≈ fine particles were further pulverized by a mechanical milling method to obtain superfine particles. The high spheroidization degree of such particles resulted in a high density propellant. Compared with coarse AP, the impact sensitivity and friction sensitivity of the superfine AP deceased by 32% and 22%, respectively. Furthermore, differential scanning calorimetry (DSC) indicated that decomposition of the superfine AP occurred earlier than it did for coarse AP, and that the decomposition activation energy of the superfine AP was lower than that of coarse AP. There are few reports on superfine AN. Chai et al. fabricated superfine AN with a particle size of 2~10 µm by a mechanical milling method [15]. In this case, acetone was used as a grinding medium and, after milling, was coated on the surface of the superfine particle to reduce the hygroscopicity of AN. The abovementioned researches suggest that mechanical milling may be the best method for fabricating nano AP and nano AN. However, so far, this method has been incapable of pulverizing coarse AP and AN into nanoparticles because the inherent texture of AP and AN crystals determine that they cannot be pulverized to nanoparticles by a mechanical method. In fact, many nano explosives have been fabricated using a mechanical milling method, such as nano RDX [14], nano HMX [16,17], nano CL-20 [18,19], nano PETN [19], nano HNS [20,21], nano TATB [20,22], and nano TAGN [23], with the exception of nano AP and nano AN. Thus, the current study aimed to fabricate AP and AN with nanoscale structures via a liquid nitrogen-assisted and vacuum freeze-drying method.

2. Experimental Method

2.1. Materials For this study, ammonium perchlorate (AP) and ammonium nitrate (AN) were purchased from Tianjin Guangfu Chemical Co., Ltd. (Tianjin, China) and liquid nitrogen was purchased from Taiyuan Taineng Co., Ltd. (Taiyuan, China). A mini high pressure atomization pump (ZEKUN, ZK-PW-XT, Shanghai, China), used in the preparation process, was manufactured by Shanghai Zekun Environmental Protection Technology Co., Ltd. (Shanghai, China).

2.2. Fabrication of Nano AP and Nano AN We dissolved 10 g of AP (or 20 g AN) in 190 g (or 180 g for AN) deionized water. The resulting aqueous solution was loaded into the mini high pressure atomization pump. An open container was used to hold about 200 mL of liquid nitrogen. The nozzle of the atomization pump was then aligned with the liquid nitrogen and the pump was started. After all aqueous solutions were sprayed into the liquid nitrogen, the pumped was turned off. As the temperature of the liquid nitrogen was 196 − ◦C and the water and liquid nitrogen were not miscible, all the droplets were frozen to their freezing points instantly. In the preparation process, the liquid nitrogen boiled violently. When all the liquid nitrogen evaporated off, the open container was placed in a freeze dryer and drying commenced. One week later, when all the ice had sublimated, nano AP (or nano AN) was obtained.

2.3. Characterization and Tests Surface morphologies of nano AP and nano AN were investigated using scanning electron microscopy (SEM; JEOL JSM-7500, Tokyo, Japan), while the crystal phases were studied with X-ray diffractometry (XRD; Bruker Advance D8, Karlsruhe, Germany). Further, differential scanning calorimetry (DSC; DSC-100, Nanjing DAZHAN company, Nanjing, China) was employed to investigate Nanomaterials 2019, 9, 1605 3 of 8 the thermal decomposition of samples and thermogravimetric analysis/infrared spectrometry (TG-IR; Mettler Toledo, Zurich, Switzerland) was used to investigate the decomposition products of samples. The impact sensitivity of the samples was tested with an HGZ-1 impact instrument (North University of China, Taiyuan, China). The special height (H50) represents the height at which a 5 kg drop-hammer will cause an event in 50% of the trials. In each determination, 25 drop tests were carried out to calculate the H50. The friction sensitivity of the samples was tested with a WM-1 friction instrument. In each determination, 50 samples were tested, and the explosion probability (P, %) was obtained.

3. Results and Discussion SEM images of nano AP and nano AN are shown in Figure1. In Figure1a it is clear that there are many spherical-like “particles” with sizes of 5~10 µm. After increasing the magnification of the image, as shown in Figure1b, it can be seen that the “particles” are hollow with rough surfaces. After further increasing the magnification of the image, as shown in Figure1c, it can be observed that the surfaces Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 9 of the “particles” present a 2D network structure with one-dimensional nanometer size (<100 nm). Figure1 d–f reveals that the micron morphology and structure of nano AN is similar to that of nano AP.

FigureFigure 1. 1.Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) imagesimages of samples: (a (a––cc) )nanometer nanometer (nano) (nano) ammonium ammonium perchlorateperchlorate (AP) (AP) and and (d (–df–)f nano) nano ammonium ammonium nitrate nitrate (AN).(AN).

In the fabrication process, the only raw materials were the oxidizer, water, and liquid nitrogen. The water and liquid nitrogen were subsequently evaporated. Thus, it is suggested that no impurity was introduced. However, it was not clear whether the crystal phases of AP and AN transformed in the process. In terms of their practical application, phase transformation is undesired. Consequently, we performed XRD on raw AP, nano AP, raw AN, and nano AN, with the patterns from these analyses illustrated in Figure 2. It is clear that the crystal phases of nano AP and nano AN are consistent with the phases of raw AP and raw AN, respectively. Therefore, we were able to conclude that no phase transformation occurred in the fabrication process.

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In the fabrication process, the only raw materials were the oxidizer, water, and liquid nitrogen. The water and liquid nitrogen were subsequently evaporated. Thus, it is suggested that no impurity was introduced. However, it was not clear whether the crystal phases of AP and AN transformed in the process. In terms of their practical application, phase transformation is undesired. Consequently, we performed XRD on raw AP, nano AP, raw AN, and nano AN, with the patterns from these analyses illustrated in Figure2. It is clear that the crystal phases of nano AP and nano AN are consistent with the phases of raw AP and raw AN, respectively. Therefore, we were able to conclude that no phase Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 9 transformation occurred in the fabrication process.

8000 22000 (a) (b) raw AP 20000 raw AN 7000 nano AN nano AP 18000 6000 16000 5000 14000 12000 4000 10000 3000 8000 6000 Intensity (a.u.)

2000 (a.u.) Intensity 4000 1000 2000 0 0 10 15 20 25 30 35 40 45 50 10 15 20 25 30 35 40 45 50 2θ (degree) 2θ (degree)

Figure 2. X-ray diffractometry (XRD) patterns of samples: (a) raw and nano AP and (b) raw and Figure 2. X-ray diffractometry (XRD) patterns of samples: (a) raw and nano AP and (b) raw and nano nano AN. AN. The DSC traces of raw AP, nano AP, raw AN, and nano AN are shown in Figure3, produced using The DSC traces of raw AP, nano AP, raw AN, and nano AN are shown in Figure 3, produced a heating rate of 20 ◦C/min. In Figure3a, each curve has an endothermic peak at 245 ◦C, corresponding tousing the phasea heating transformation rate of 20 of°C/min. AP. For In raw Figure AP, low3a, each temperature curve has decomposition an endothermic occurs peak in the at range245 °C, of corresponding to the phase transformation of AP. For raw AP, low temperature decomposition 310–360 ◦C. However, for nano AP, low temperature decomposition does not occur. Further, the high occurs in the range of 310–360 °C. However, for nano AP, low temperature decomposition does not temperature decomposition of raw AP begins at 446.7 ◦C and the peak point appears at 451 ◦C, but occur. Further, the high temperature decomposition of raw AP begins at 446.7 °C and the peak point the high temperature decomposition of nano AP originates at 387.8 ◦C and the peak point appears at appears at 451 °C, but the high temperature decomposition of nano AP originates at 387.8 °C and the 439.2 ◦C. This means that the thermolysis occurred earlier for nano AP than raw AP. It can be seen from Figurepeak point3b that appears nano AN at 439.2 is quite °C. similar This means to raw ANthat inthe terms thermolysis of DSC trace. occurred For rawearlier AN, for the nano peaks AP at than 60.4 raw AP. It can be seen from Figure 3b that nano AN is quite similar to raw AN in terms of DSC trace. and 135.1 ◦C correspond to a phase transformation of AN. The peak at 170.1 ◦C is related to the melting For raw AN, the peaks at 60.4 and 135.1 °C correspond to a phase transformation of AN. The peak at point of AN. The large endothermic peak at 301.7 ◦C may be ascribed to the thermal decomposition of raw170.1 AN. °C is Upon related careful to the observation, it may of AN. be observedThe large thatendothermic there are peak four smallat 301.7 endothermic °C may be ascribed peaks in to the thermal decomposition of raw AN. Upon careful observation, it may be observed that there are the DSC curve of nano AN. For nano AN, an extra phase transformation occurs at 96.7 ◦C, which does four small endothermic peaks in the DSC curve of nano AN. For nano AN, an extra phase not exist in the DSC curve of raw AN. The melting point of nano AN is 169.9 ◦C, which is roughly the transformation occurs at 96.7 °C, which does not exist in the DSC curve of raw AN. The melting point same as that of raw AN. The thermal decomposition of nano AN begins at 245.9 ◦C, slightly lower of nano AN is 169.9 °C, which is roughly the same as that of raw AN. The thermal decomposition of than raw AN’s 254.2 ◦C. The peak temperature for thermolysis of nano AN (300.2 ◦C) is quite close nano AN begins at 245.9 °C, slightly lower than raw AN’s 254.2 °C. The peak temperature for to that of raw AN (301.7 ◦C), indicating that the decomposition process does not change when the particlethermolysis size ofof ANnano decreases AN (300.2 from °C) the is micronquite close scale to to that the nanoof raw scale. AN (301.7 °C), indicating that the decompositionWith reference process to our does results, not change we noted when that the the part thermalicle size decomposition of AN decreases of AN from was the independent micron scale of itsto the particle nano size, scale. while the thermolysis of AP depended on its particle size. This may be attributed to differencesWith reference in the decomposition to our results, mechanisms we noted that of APthe andthermal AN. decomposition In the first step of of AN their was decomposition, independent of its particle size, while the thermolysis of AP depended on its particle size. This may be attributed AP and AN dissociate to NH3 and HClO4 (for AP) and NH3 and HNO3 (for AN) when the temperature to differences in the decomposition mechanisms of AP and AN. In the first step of their reaches a critical point. NH3 cannot directly react with HClO4 or HNO3 at low temperatures. So, decomposition, AP and AN dissociate to NH3 and HClO4 (for AP) and NH3 and HNO3 (for AN) when further decomposition of HClO4 or HNO3 is essential for sustained thermolysis of AP or AN. However, the temperature reaches a critical point. NH3 cannot directly react with HClO4 or HNO3 at low the decomposition of HClO4 is much easier than that of HNO3, as the decomposition process of HClO4 temperatures. So, further decomposition of HClO4 or HNO3 is essential for sustained thermolysis of is very exothermic and the decomposition process of HNO3 is very endothermic [24,25]. Therefore, AP or AN. However, the decomposition of HClO4 is much easier than that of HNO3, as the the rate limiting step of the decomposition of AP lies in the reaction between NH3 and the pyrolysis decomposition process of HClO4 is very exothermic and the decomposition process of HNO3 is very products of HClO4, while the controlling step of AN’s decomposition is the pyrolysis of HNO3 [26]. Inendothermic fact, the inferior [24,25]. decompositionTherefore, the rate mechanism limiting ofstep AN of accountsthe decomposition for its remarkably of AP lies lowerin the burningreaction between NH3 and the pyrolysis products of HClO4, while the controlling step of AN’s decomposition is the pyrolysis of HNO3 [26]. In fact, the inferior decomposition mechanism of AN accounts for its remarkably lower burning rate compared to AP. The problem for the decomposition of AP lies in that the NH3 and pyrolysis products of HClO4 react with each other only on the surface of the decomposing AP particles. The excessive accumulation of NH3 gas on the surfaces of AP particles forms a thick cage, thereby hindering further decomposition reactions [24]. Thus, increasing the specific surface area of AP particles is conducive to alleviating the negative effects of the ammonia cage, therefore promoting the reaction between NH3 and the pyrolysis products of HClO4. Aside from the case of AP, the reaction between NH3 and the pyrolysis products of HNO3 is instantaneous, which is partly responsible for the detonable property of AN [27]. Hence, thermolysis

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rate compared to AP. The problem for the decomposition of AP lies in that the NH3 and pyrolysis products of HClO4 react with each other only on the surface of the decomposing AP particles. The excessive accumulation of NH3 gas on the surfaces of AP particles forms a thick ammonia cage, thereby hindering further decomposition reactions [24]. Thus, increasing the specific surface area of AP particles is conducive to alleviating the negative effects of the ammonia cage, therefore promoting the reaction between NH3 and the pyrolysis products of HClO4. Aside from the case of AP, the reaction between NH3 and the pyrolysis products of HNO3 is instantaneous, which is partly responsible for the Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 9 detonable property of AN [27]. Hence, thermolysis of AN does not have problems like the ammonia cage. Please note that AP is not detonable partly because the reaction between NH and the pyrolysis of AN does not have problems like the ammonia cage. Please note that AP is not3 detonable partly products of HClO4 is not instantaneous. These mechanisms illustrate the discrepancy observed in the because the reaction between NH3 and the pyrolysis products of HClO4 is not instantaneous. These DSC results in Figure3a,b. mechanisms illustrate the discrepancy observed in the DSC results in Figure 3a,b.

25 5 (a) (b) 245.9oC 439.2oC 0 o nano AN o 96.7 C 57.8 C o 20 133.4 C -5 169.9oC melting point phase transformation of AP o -10 15 245 C o nano AP 451.0 C 254.2oC -15 o raw AN 387.8 C o 10 60.4 C 135.1oC -20 o low temperature 170.1oC 300.2 C decomposition -25 melting point 5 -30 Heat flow (mW) Heat flow Heat flow (mW) Heat flow raw AP 0 o 446.7 C -35 o -40 301.7 C -5 50 100 150 200 250 300 350 400 450 500 550 50 100 150 200 250 300 350 400 o o Temperature ( C) Temperature ( C)

Figure 3.3. DiDifferentialfferential scanning calorimetrycalorimetry (DSC)(DSC) tracestraces ofof samples:samples: (a)) rawraw and nano AP and (b) raw andand nanonano AN.AN.

To furtherfurther investigateinvestigate thethe thermalthermal decompositiondecomposition ofof nanonano APAP andand nanonano AN,AN, TG-IRTG-IR analysisanalysis was conductedconducted atat aa heatingheating rate rate of of 10 10◦ C°C/min,/min, the the results result ofs of which which are are shown shown in in Figure Figure4. As4. As observable observable in Figurein Figure4a, 4a, which which shows shows the the TG TG curve curve of of nano nano AP, AP, decomposition decomposition begins begins at at 360 360◦ °CC andand endsends atat 434434 ◦°C.C. This result is slightly didifferentfferent from thethe DSCDSC tracetrace (Figure(Figure3 3a)a)due dueto tothe thedi differentfferent heatingheating ratesrates employedemployed inin thethe DSCDSC andand TG-IRTG-IR analyses.analyses. The IR spectra at 400, 414,414, 436,436, andand 450450 ◦°CC were extractedextracted andand are shown in in Figure Figure 44b.b. From From the the figure, figure, it it is is obvious obvious that that the the main main gas gas products products of ofnano nano AP AP are are NO ,N O, HCl, and H O, while some NOCl is also detected. For nano AN, its decomposition NO2, N22O, 2HCl, and H2O,2 while some NOCl is also detected. For nano AN, its decomposition products are N O and a massive amount of H O (steam). It should be pointed out that some NH products are N22O and a massive amount of H22O (steam). It should be pointed out that some NH3 is3 isproduced produced in inthe the case case of of nano nano AN, AN, but but it itis is not not present present in in the the decomposition decomposition products products of of nano nano AP. AP. NH is produced as a product of the dissociation of AN [28]. In fact, most ammonia is instantaneously NH33 is produced as a product of the dissociation of AN [28]. In fact, most ammonia is instantaneously oxidized by the NO produced from the pyrolysis of HNO , which gives rise to the formation of large oxidized by the NO22 produced from the pyrolysis of HNO33, which gives rise to the formation of large amounts of N O and H O[29]. Nevertheless, it is hard for NH to react with N O at low temperatures. amounts of N22O and H22O [29]. Nevertheless, it is hard for NH3 to react with N22O at low temperatures. So, when the concentration of N O is low, a small amount of NH remains. In fact, TG-IR analysis of So, when the concentration of N22O is low, a small amount of NH33 remains. In fact, TG-IR analysis of thethe decompositiondecomposition ofof micronmicron APAP andand ANAN waswas conductedconducted inin ourour otherother papers,papers, andand thethe paperspapers ofof other authorsauthors reportedreported similar similar results. results. For example,For example, in Ref. in [30 Ref.], Liu [30], et al. Liu found et thatal. found micron that AP decomposedmicron AP to NO ,N O, HCl, H O, and NOCl, which is in accordance with the results of this work. In Ref. [26], decomposed2 2 to NO2, 2N2O, HCl, H2O, and NOCl, which is in accordance with the results of this work. the micron AN decomposed to N O and H O, which is also consistent with the results of this paper. In Ref. [26], the micron AN decomposed2 to 2N2O and H2O, which is also consistent with the results of Therefore,this paper. on Therefore, the aspect ofon final the decomposition aspect of final products, decomposition it is believed products, that the it decomposition is believed ofthat AP the or ANdecomposition happens at theof AP molecular or AN happens level, independent at the molecular of the particlelevel, independent sizes of AP of and the AN. particle sizes of AP and AN.

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100 NO (a) (b)0.20 2 90 nano AP 360 400 o N O 80 400 C 2 0.16 o 70 414 414 C o 60 430 C 0.12 450oC 50 H O

Abs 2 40 430 NOCl 0.08 H O N O 2 HCl 2 30

Weight loss (%) 20 450 0.04 10 nano AP 434 0 0.00 50 100 150 200 250 300 350 400 450 500 4000 3500 3000 2500 2000 1500 1000 500 Temperature (oC) -1 Wavenumber (cm )

110 (c) H O nano AN 100 0.12 H O 2 (d) 2 (massive) (massive) 250 90 250 N O 268 220 2 80 0.10 285 70 300 268 0.08 60 NH 3 50 285 0.06 N O

40 Abs 2 30 0.04 20

Weight loss (%)Weight loss 300 10 0.02 nano AN 0 293 -10 0.00 50 100 150 200 250 300 350 400 4000 3500 3000 2500 2000 1500 1000 500 Temperature (oC) -1 Wavenumber (cm )

FigureFigure 4.4. ThermogravimetricThermogravimetric analysisanalysis/infrared/infrared spectrometryspectrometry (TG-IR)(TG-IR) spectraspectra ofof samples:samples: ((aa,,cc)) TGTG b d curvescurves andand ((b,,d)) IRIR spectraspectra forfor decomposition decomposition products products of of nano nano AP AP and and nano nano AN. AN. Impact and friction sensitivities of raw AP, nano AP, raw AN, and nano AN were also tested and Impact and friction sensitivities of raw AP, nano AP, raw AN, and nano AN were also tested the results are presented in Table1. The results show that when the particle size of AP or AN was and the results are presented in Table 1. The results show that when the particle size of AP or AN reduced to the nanometer scale, their sensitivities change significantly. The impact sensitivity of nano was reduced to the nanometer scale, their sensitivities change significantly. The impact sensitivity of AP was slightly higher than that of raw AP, and the friction sensitivity of nano AP was noticeable nano AP was slightly higher than that of raw AP, and the friction sensitivity of nano AP was higher than that of raw AP. However, for AN, the increase in sensitivity was not obvious. The impact noticeable higher than that of raw AP. However, for AN, the increase in sensitivity was not obvious. sensitivity of nano AN was similar to that of raw AN, and the friction sensitivity of nano AN was The impact sensitivity of nano AN was similar to that of raw AN, and the friction sensitivity of nano somewhat higher than that of raw AN. This is because AN itself is a very insensitive, energetic material. AN was somewhat higher than that of raw AN. This is because AN itself is a very insensitive, Of course, these results are not consistent with the results reported by Luo or Dobrynin [31,32]. In Luo’s energetic material. Of course, these results are not consistent with the results reported by Luo or work, the addition of nano LLM-105 was beneficial as it decreased the impact sensitivity of NC/GAP Dobrynin [31,32]. In Luo’s work, the addition of nano LLM-105 was beneficial as it decreased the (nitrocellulose/glycidyl azide polymer) fibers, and in Dobrynin’s work nano NC presented lower impact sensitivity of NC/GAP (nitrocellulose/glycidyl azide polymer) fibers, and in Dobrynin’s work friction sensitivity than micron NC. Now, it cannot yet be elucidated why nano AP is more sensitive nano NC presented lower friction sensitivity than micron NC. Now, it cannot yet be elucidated why than raw AP, but nano NC is less sensitive than raw NC. However, at this time, this discrepancy may nano AP is more sensitive than raw AP, but nano NC is less sensitive than raw NC. However, at this simply be attributed to the different decomposition mechanisms of AP and NC (NC is nitrocellulose as time, this discrepancy may simply be attributed to the different decomposition mechanisms of AP a nitrate ester) [33]. The decomposition of AP begins when it dissociates to NH and HClO , while and NC (NC is nitrocellulose as a nitrate ester) [33]. The decomposition of 3AP begins 4when it decomposition of NC originates from the rupture of O–NO2 bonds. Further research on this topic will dissociates to NH3 and HClO4, while decomposition of NC originates from the rupture of O–NO2 be continued in our next work. bonds. Further research on this topic will be continued in our next work. Table 1. Impact and friction sensitivities of samples.

Samples Impact Sensitivity (H50, cm) Friction Sensitivity (P, %) raw AP 95 26 nano AP 83 94 raw AN >120 0 nano AN >120 8

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4. Conclusions Nano AP and nano AN, with structures that were in the nanometer scale in one dimension, were prepared with an ultra-low temperature spray method. The micron morphologies of nano AP and nano AN were 2D network structures. XRD analysis confirmed that crystal phase transformation did not occur in the fabrication process. For DSC curves, compared with raw AP, the peak temperature of nano AP was larger by 12 ◦C. The thermal decomposition of AN was found to be independent of its particle size; that is, the DSC peak point of nano AN was roughly the same as that of raw AN. The decomposition products of nano AP were NO2,N2O, HCl, and H2O, as well as a small amount of NOCl, while the main decomposition products of nano AN were N2O and H2O, with a small amount of NH3. The impact sensitivity of nano AP was somewhat higher than that of raw AP and the friction sensitivity of nano AP was noticeably higher than that of raw AP. But, in the case of AN, the increase of sensitivity was not obvious.

Author Contributions: Conceptualization, Y.W. and X.S.; methodology, Y.W.; validation, Y.W., X.S. and F.L.; formal analysis, Y.W. and X.S.; investigation, Y.W. and X.S.; resources, Y.W.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, X.S.; F.L. analyzed the sensitivity experiments. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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