Construct a 3D microsphere of HMX/B/Al/PTFE to obtain the high energy and combustion reactivity

Jin Wng , Jie Chen , Yofeng Mo , Yongjun Deng , Wei Co , Fude Nie ,**,Jun Wng ,*

a Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621999, PR China

b Shock and Vibration of Engineering Materials and Structures Key Lab of Sichuan Province, Mianyang 621010, PR China

Keywords:HMX/B/Al/PTFE 3D microspheres Surface etching Reaction mechanism

ABSTRACT Metal (aluminum and boron) based energetic materials have been wildly applied in various fields including aerospace, explosives and micro-devices due to their high energy density.Unfortunately, the low combustion efficiency and reactivity of metal fuels,especially boron(B),severely limit their practical applications.Herein, multi-component 3D microspheres of HMX/B/Al/PTFE (HBA) have been designed and successfully prepared by emulsion and solvent evaporation method to achieve superior energy and combustion reactivity.The reactivity and energy output of HBA are systematically measured by ignitionburning test, constant-volume explosion vessel system and bomb calorimetry.Due to the increased interfacial contact and reaction area,HBA shows higher flame propagation rate,faster pressurization rate and larger combustion heat of 29.95 cm/s,1077 kPa/s,and 6164.43 J/g,which is 1.5 times,3.5 times,and 1.03 times of the physical mixed counterpart (HBA-P).Meanwhile, HBA also shows enhanced energy output and reactivity than 3D microspheres of HMX/B/PTFE(HB)resulting from the high reactivity of Al.The reaction mechanism of 3D microspheres is comprehensively investigated through combustion emission spectral and thermal analysis(TG-DSC-MS).The superior reactivity and energy of HBA originate from the surface etching of fluorine to the inert shell(Al2O3 and B2O3)and the initiation effect of Al to B.This work offers a promising approach to design and prepare high-performance energetic materials for the practical applications.

1.Introduction

With the development of aerospace, space exploration and micro-devices, deeper demands are placed on the energy density and combustion reactivity of energetic materials(EMs).Metal fuels,such as Aluminum (Al) and Boron (B) can excellently improve the energy density of EMs due to their high calorific values(58 kJ/g of B and 31 kJ/g of Al)[1-3].However,the inert layer(Al2O3and B2O3)on the surface of metal fuels leads to long ignition delay time and low combustion efficiency, which extremely limits the energy release and combustion reactivity of metal-based EMs [4-7].Especially for B,it has much higher calorific values compared to Al[8].Nevertheless,it is hard to ignited and combustion,resulting in low energy output and combustion reactivity.Therefore,it is still a great challenge to design and prepare EMs with high energy density and combustion reactivity.

Recent studies show that removing the inert layer (Al2O3and B2O3)from the metal particle surface by surface etching techniques is an efficient approach to improve combustion reactivity[6,9-11].The fluorine (F) with the greatest electronegativity can rob oxide layers(Al2O3and B2O3)to generate volatile metal fluoride(AlF3and BF3), which can form some holes on the particle surface to accelerate the ignition and combustion process of metal fuels [12].Meanwhile,when F is employed as an oxidant the combustion heat could be further improved(105 kJ/g of B/F,56 kJ/g of Al/F)[13].Our previous work has confirmed that fluorine coating (perfluoro dodecanoic acid [14], fluorographene [15,16], polytetrafluoroethylene[17])can enhance their ignition behavior,and fluorinated oxidants (polytetrafluoroethylene [18], graphite fluoride [15,19])can improve their combustion heat.Furthermore, Al can be easier and faster to ignite but its calorific values are finite, while B owns higher calorific values but its ignition and combustion performance is poor.Many studies have confirmed that the combustion reaction of B can be initiated by the fast ignition reaction of Al [16,20,21].Thus,based on the high reaction heat and reaction kinetics derived from the fluorination reaction, and the initiation characteristics of Al to B, energy and combustion reactivity can be improved simultaneously when Al, B, and fluorine are compounded in EMs.

3D microsphere, as a high-energy structural unit, with rich microporous structure, high packing density, and high specific surface area can significantly increase the reaction area and shorten mass and heat transfer distance [22].Moreover, the existence of rich porous structures in 3D microspheres can help to maintain a high-pressure state to accelerate the mass and heat transfer during the burning process[23].Thus,the 3D microsphere structure offers great potential in designing structures and improving the combustion performance of EMs[24,25].For example,Hu[26]prepared Al/PTFE/AP spherical composite to exhibit higher combustion heat release, shorter ignition delay time, and burning time than physically mixed ones.Yan [27] fabricated nano-Al@RDX@Viton hollow microspheres to obtain a shorter laser ignition delay and a more vigorous combustion flame than the physical mixture.The emulsion and solvent evaporation method is an efficient,easy, safe and fast way to prepare the 3D microsphere structure.Compared with other methods,including electrospray[28],mechanical ball milling[29], electrospinning [30], self-assembly [31,32], etc., it can better balance safety issues and production efficiency [33].

In this work, to achieve high energy and high combustion reactivity EMs,Al,B,and PTFE(with the highest fluorine content of 76 wt%)are compounded in HMX to form a high-energy structural unit.HMX was chosen due to its relatively high energy density and wide range of applications.To further optimize combustion performance and achieve tunable structures and components, the 3D microsphere structure is successfully constructed by the emulsion and solvent evaporation method.Meanwhile, the combustion reaction process was explored through combustion emission spectrum analysis and thermal analysis.This work provides a new way for the design and preparation of high-energy structural EMs.

2.Experimental section

2.1.Materials and preparation

Aluminum particles(Al)were purchased from China New Metal Materials Technology Co., Ltd with a size ranging from 50 to 200 nm.Boron particles(B)were purchased from Zhong Nuo New Materials with a size of～5 μm.PTFE nanoparticles and ethyl acetate(AR grade) were purchased from Aladdin (Shanghai, China).The size of PTFE nanoparticles was～100 nm.HMX with an average size of ～60 μm was obtained from the Institute of Chemical Materials,CAEP (Mianyang, China).Poly (vinyl alcohol) (PVA-1788, average degree of polymerization 1700, degree of hydrolysis 88%) was provided by Sinopec Sichuan Vinylon Works (Chongqing, China).

3D microspheres were prepared by the emulsion and solvent evaporation method.The formulation of 3D microspheres is designed based on the traditional fluor aluminum explosives.First,an oil phase(O) preparation: 2 g solid mixture composed of HMX,PTFE,binder,and metal fuels(nano-Al or boron)was mixed in 12 g ethyl acetate.The mixture was firstly sonicated for 1 h, and continually stirred for 2 h to form a homogeneous suspension.Then, the water phase (W) was formed by blending 0.5 wt%emulsifier (PVA) and deionized water.These two phases were mixed and homogenized at a stirring rate of 1000 rpm for 30 min.In this step, the oil phase can uniformly distribute in the water phase to form some O/W emulsion droplets containing solid components,which is identified as the high-energy structural unit.Next, the stirring speed and the temperature were turned to 500 rpm and 35°C, respectively, to start the solvent evaporation processes.Three hours later, the resultant 3D microspheres were filtered and washed repeatedly by utilizing deionized water and ethanol, respectively.Finally, the 3D microspheres were dried at 40°C for 12 h.For comparison, the physically mixed samples were prepared by resonance acoustic mixing methods[15,16].The detail information of sample is listed in Table 1.

2.2.Morphology and structure

The morphology of sample was imaged by field emission scanning electron microscopy (FE-SEM, Sigma HD, ZIESS) at an acceleration voltage of 3 kV.The surface element analysis of samples was characterized by an energy dispersive x-ray spectroscopy (EDS).The size distribution of 3D microspheres could be measured by using a laser particle size analyzer(LPSA,MICROTRAC SDC,China).Furthermore, the structure and bulk composition were characterized by X-ray diffraction (2θ: 10°-80°).

2.3.Energy and combustion performance

The flame structures were investigated by the ignition-burning test.Briefly,25 mg sample was put on the platform and Ni-Cr wire(diameter is 0.4 mm, length is 10 cm) was put under the samples.The loaded wire was rapidly heated to be the melting point of Ni-Cr wire by a homemade DC current.The whole ignition process was recorded by a high-speed camera(UX50,Japan)taken at 2000 frames per second.

The pressure traces of samples as a function of time were measured using a constant-volume explosion vessel system.The system consisted of a pressure cell with a constant volume of 330 mL,a pressure sensor(PC290-ACAEFA1A,GAILIN),oscilloscope,and power supply.Approximately 0.5 g samples were put into a 5 mL crucible installed inside the pressure cell.The samples were ignited by the Ni-Cr wire(diameter is 0.4 mm,length is 10 cm)and the pressure was automatically measured using the pressure sensor connected to the pressure cell.

Heat release in air of samples was measured by bomb calorimetry(ZDHW-8E,China)with 0.5 g at an air pressure of 0.1 MPa atm(ambient).The sample state for both 3D microspheres and the physical mixed one is loose deposits.For each test,ignition voltage was 20 V and ignition wire was Ni-Cr wire with a diameter of 0.4 mm and a length of ～10 cm.Measurements for each sample were repeated three to four times to obtain an average.

2.4.Mechanism analysis

Combustion emission spectral were performed using a CO2laser ignition test system, which has been successfully employed in investigations of metal fuel [21,34].Approximately 10 mg of samplewas placed onto a tungsten metal sheet,which was then placed in a pressurized combustion chamber and sealed.The sample was ignited by a 0.5 s continuous CO2laser with 240 W power.Then,characteristic spectral signals were recorded with a fiber optic spectrometer.The test was repeated 3 times for each sample.

The thermal behavior of samples was performed by a simultaneous thermal analysis (TG/DSC, NETZSCH 449F5).In detail, samples (0.5-1 mg) were placed in an alumina crucible and heated from 50°C to 800°C at a constant rate of 10°C/min.All of the thermal tests were performed under a N2atmosphere with a flow rate of 60 mL/min.The thermal decomposition of samples was measured using an online evolved gas analysis via a mass spectrometer(MS)(PFEIFFER,OMNI star,Germany).Experiments were conducted on 6 mg samples heated to 800°C at a heating rate of 10°C/min under a flow of N2atmosphere.

3.Results and discussions

3.1.Morphology and structure of 3D microspheres

The morphology and structure of 3D microspheres were characterized.Al and PTFE particles show spherical shapes with sizes ranging from 50 nm to 200 nm (Fig.S1(a)) and an average size of～200 nm(Fig.S1(c)),respectively.Boron particles are in an irregular sheet structure shape with a size of 1-5 μm (Fig.S1(b)).HMX particle has octahedral morphologies with a size of ～70 μm(Fig.S1(d)).Fig.1(a)shows 3D microspheres that present apparent mono-dispersity and highly spherical morphology with a size distribution ranging from 250 μm to 450 μm.From the high magnification SEM of the surface of 3D microspheres in Fig.1(b), metal fuels (both B and Al) and PTFE was uniformly distributed around the HMX crystal, and the binder plays the role of connection between the metal fuels, PTFE, and the HMX crystal.This structure can efficiently short interfacial heat and mass transport distance of metal fuel.Meanwhile, a large amount of gaseous is produced to throw the B and Al particles when HMX decomposed, which is beneficial to reduce the agglomeration effect.The cross-section SEM images in Fig.1(c) and 1(d) are used to illustrate the internal structure of 3D microspheres.The images clearly reveal the presence of porous structure in the interior.The porosity is caused by the volatilization of solvent during the evaporation process,which can maintain a high-pressure state and accelerate the heat and mass transfer during the combustion process to enhance energy and combustion behavior [23].The elemental mapping of 3D microspheres was carried out to verify the distribution of Al, B,PTFE,and HMX.Fig.1(e) indicates that uniform distribution and improved interaction among reactive components are achieved for 3D microspheres.The uniform distribution means a higher reaction area for 3D microspheres, which could be beneficial to enhance energy output and combustion reactivity.

From the size distribution of 3D microspheres in Fig.S2(a), the D90of HBA is 450 μm,while that of HB is 235.2 μm.The particle size of HBA is larger than HB, which can influent the combustion properties.Thus, maintaining the same particle size is necessary.Due to the particle size distribution both HB and HBA are the normal distribution and have some overlapping particle size distribution range,all samples can be sieved through a graded sieve to guarantee the particle size is the same.The XRD patterns of 3D microspheres illustrate that (Fig.S2(b)) the content and crystal structures of each component are not changed through the emulsion and solvent evaporation method.It should be mentioned that boron is not detected due to its amorphous structure.

3.2.Combustion and energy performance

Fig.2 shows high-speed images of samples ignited by Ni-Cr wire in the air.The green flame is attributed to the intermediate combustion product BO2formed during the burning process[21,35].All the flame of samples can be divided into three stages.The first stage is the sample ignition step with the flame rise.The second stage is a steady burning process with the highest flame height.The third step is the decay burning process with flame height decline.In the three stages, the flame of 3D microspheres has a stable propagation process with a flame front structure of circular,while that of the physical mixed counterpart is an unstable propagation with a sparkling flame front.Furthermore, a large amount of agglomeration burning metal fuel particles can be observed in the burning process of HB-P and HBA-P.These phenomena indicated that the 3D microsphere achieved more outstanding combustion performance than the physical mixed samples.Compared the HB,the HBA shows a faster sample ignition stage and stronger steady burning stage, indicating that Al can dramatically improve the reactivity of B and enhance the combustion performance of 3D microspheres.It should be noticed that the flame color of HBA transforms from yellow to green with the combustion process continuing in Fig.2(b).The detailed initial flame changes of the microsphere are shown in Fig.2(d).The yellow flame color is detected in the intermediates of Al burning(AlO and AlO2),while the green flame color is related to the intermediates of B burning (BO2).With the heating of the Ni-Cr wire, aerosol products with oxidizing gases are produced rapidly due to HMX decomposition.Then,those aerosol products ignite the Al particles to produce bright yellow flames and splattered particles.Because of the combustion of Al,the burning temperature of 3D microspheres is further improved.Therefore, the liquid oxide layer (B2O3) is accelerated to evaporate from the surface of B.Finally,the ignition and the combustion of B occur and the flame color turns to green.Thus, the ignition and combustion reaction of boron can be accelerated by the initiation effect of Al to B.

The time-dependent flame height curves can be calculated according to the measuring scale (Fig.3(a)).It can be directly observed from the curves that the HBA owns the maximum flame height in the steady burning process compared with HB,HB-P,and HBA-P.Besides, the average flame rise rate of HBA in the sample ignition stage is 29.95 cm/s,which is 1.5 times of HBA-P(20.02 cm/s),2.9 times of HB(10.24 cm/s),and 3.1 times of HB-P(9.56 cm/s),respectively.The corresponding flame rise rate also can be obtained by derivation to the time-dependent flame height curves(Fig.3(b)).Similarly,the HBA also has the maximum instantaneous flame rise rate in all samples.The time as the instantaneous flame rise rate turn to zero is identified as the time of maximum flame height(Time of Hmax).The HBA also has the shortest time of Hmax, which means the HBA owns a faster reaction rate compared HB,HB-P,and HBA-P.All of those results indicate that 3D microspheres own higher combustion reactivity than physically mixed counterpart and Al can enhance the combustion performance of B.

Fig.1.(a) and (b) SEM images of 3D microspheres (take HBA as example); (c) and (d) Cross-section SEM images of 3D microspheres; (e) Elemental mapping of 3D microspheres.

Pressure output performance is an important index to assess the capacity of work and reactivity for EMs.Fig.4 shows the pressure trace and pressurization rate curve of samples.Pmaxrepresents the maximum peak pressure.The time of instantaneous pressurization rate becomes zero is identified as the time of Pmax.From Fig.5(a),the HBA not only has the maximum Pmaxbut also achieves the fastest average pressurization rate with 1077 kPa/s, which is 5.6 times, 6.1 times, and 3.5 times higher than HB (191 kPa/s), HB-P(177 kPa/s), and HBA-P (kPa/s), respectively.Furthermore, the instantaneous pressurization rate and time of Pmaxof HBA is also larger and shorter than that of HB,HB-P and HBA-P from Fig.4(b).These results further prove that the microspheres can achieve higher combustion reactivity compared with their physically mixed counterparts, and Al with higher reactivity can accelerate the ignition and combustion of boron by initiation effect, which is completely consistent with the ignition-burning test.In addition,we found that the pressurization rate curve had a change of double peak turn to single peak with Al added.Due to the long ignition delay time of B, the combustion of B is left far behind at the decomposition of HMX, the double peak appeared in the pressurization rate curve of HB.As we discussed above, the Al can fast occur ignition and combustion to release energy and initiated B.Thus, the combustion process of B can be accelerated and the double peak gradually turns into a single peak in the pressurization rate curve of HBA.

As shown in Fig.5,heat release,as an important parameter was investigated for three samples.Compared with HB (5724.99 J/g),the heat release in air of HBA with Al compounded owns higher caloric (6164.43 J/g).The increased caloric reflects that the combustion efficiency of metal fuel in HBA is higher than the HB.This maybe originates from the initiation effect of Al to B and the decreased oxygen consumption.For the physical mixed one, the combustion heat release of 5573.01 J/g and 5994.5 J/g is lower than 5724.99 J/g and 6164.43 J/g of 3D microsphere.These results indicate that the energy output can be enhanced by compounding Al and fabricating the functional structure.

3.3.Mechanism of 3D microspheres

In order to investigate the reaction mechanism of 3D microspheres,the three-dimensional emission spectra were obtained by fiber optic spectroscopy.By collating the three-dimensional data,a two-dimensional curve of the characteristic spectral intensity with time or wavelength for different conditions can be obtained [21].Fig.6 shows a two-dimensional curve of the characteristic spectral intensity wavelength.From curves,a total of 14 characteristic peaks belonging to three intermediates were observed in the emission spectra and their corresponding wavelengths are listed in Table S1.In order to distinguish the characteristic peaks, the local enlarged picture is employed to identify it(Fig.S3).For both of HB and HBA,the BO2intermediates were observed, which are derived from the direct reaction between boron and oxygen (R1) [35].Differenced from HB,AlO and AlO2are observed in the emission spectral of HBA.AlO comes mainly from the direct reaction between aluminum and oxygen(R2)[36].As for AlO2,the main source is the further reaction between AlO and oxygen(R3)[37].These results verified the flame color changers are origin from intermediate combustion products of Al and B in the ignition combustion test.

Fig.2.The burning process and flame structure of samples ignited in air:(a)3D microspheres of HB;(b)Physical mixed of HB-P;(c)3D microspheres of HBA;(d)Physical mixed of HBA-P; (e) Detailed initial flame color transform process of HBA.

Fig.3.(a) Time-dependent flame height of samples; (b) Flame rise rate of samples.

Fig.4.(a) Time-dependent pressure evolution of samples; (b) The corresponding pressurization rate curve of samples.

Based on the full-time spectral intensity curve of the four samples, the appearance and disappearance of the characteristic peak represent the beginning and end of combustion.The start time of the laser is set to time 0.The tirespects as the laser irradiation time(500 ms).As shown in Fig.7,four parameters are used to characterize the profile of the intensity of the emission spectrum at 579 nm with time.The duration between time 0 and the start of time is defined as ignition delay time (tid).The duration between the start and the end of the burn is designated as the burn time of the sample (tc), which conclude the laser irradiation burning time(tic) and self-sustained burning time (tsc).The peak of the spectral intensity count is defined as the maximum characteristic spectral intensity (Imax), which is related to the combustion temperature[38].The time difference between the start of the burn and the appearance of the maximum characteristic spectral intensity is time of Imax.The detailed information of these four parameters is listed in Table 2.It is an essential result from the emission spectrum that HBA has a shorter tidand tcthan HB.This phenomenon is consistent with the results of the above ignition burning test and constant-volume explosion test.Furthermore, due to the time of Imaxbeing shorter and the combustion heat being larger, HBA achieves higher Imax, that is, the higher flame temperature.Although the calorific values of B are far higher than Al, the low combustion efficiency and reactivity limit its energy release.As discussed in subsection 3.2,Al with higher reactivity than B can be ignited first to release additional energy to accelerate the ignition and combustion of B.Thus, the ignition delay time, combustion time and flame temperature of HBA are significantly enhanced compared to HB.

Fig.5.Heat release of samples in air (atmosphere).

Fig.6.Maximum emission spectral intensity curves of 3D microspheres (full wave).

Fig.7.Full-time spectral intensity curves of 3D microspheres at a wavelength of 579 nm.

Table 2 The detailed information of these four parameters of the intensity of the emission spectrum at 579 nm with time.

Fig.8(a) and 8(b) shows the thermal analysis results of 3D microspheres in the temperature range of 50-800°C under N2atmosphere.Three loss steps in TG curves accompanying three heat release peaks in DSC curves indicated that the thermal reaction process of 3D microspheres can be divided into three steps.In the thermogravimetric analysis, the total mass loss of HBA is 91.1%,which is lower than that of HB of 94.7% (Table S2).This phenomenon may be caused by the different reaction products state of B and Al (BF3is almost gas, while AlF3is almost solid when the temperature is low than 800°C).In the DSC results, the total heat release reached 4733.0 J/g for HBA, which is larger than HB of 4286.8 J/g(Table S3).As we discussed in the combustion heat test,the initiation effect of Al to B and the decreased oxygen consumption could be enhanced to release more heat when Al was compounded.

Combined with the Real-time monitoring of the MS technique(Figs.8(c) and 8(d)), the reaction process of 3D microspheres was further investigated.The main evolved gas appears at three temperatures, ～300°C, ～450°C and ～580°C, which is consisted of the three mass loss steps in TG curves and the three exothermic peaks in DSC curves.In the first step, the gaseous products can be identified as H2O(m/z=18),N2O/CO2(m/z=44),HCNO(m/z=43)and NO2(m/z = 46) according to the MS results.These gaseous come from the decomposition of HMX(R4).Because HMX decomposed,a mass loss and a sharp exothermic peak appeared in TG-DSC results.In fact, there are produced a few fluorine-containing species (C2F4(m/z = 100), C3F6(m/z = 150), HF (m/z = 20)) and metal oxide species(AlF3(m/z=84),BF3(m/z=68),BO2(m/z=43)and BOF(m/z = 46)) in the step.This is originally from the sample room temperature higher than the temperature programming due to the HMX decomposition, leading to some reaction of fluoropolymer and metal fuel.Because of this, the mass loss in this step is also inconsistent with the HMX content.

in the second step, the lost weight is ～5%, corresponding to the binder content in 3D microspheres.From MS results, the main gaseous species are fluorine-containing species(HF(m/z=20))and metal fluoride(AlF3(m/z=84),BF3(m/z=68)).Thus,the fluorinecontaining species belongs to the decomposition of the binder(R5).Meanwhile, a small heat release peak is observed in DSC curves.This relates to the PIR (Pre-ignition reaction) reaction between Al2O3/B2O3and HF(R6).The PIR reaction can help to form a series of holes to promote the ignition and combustion of metal fuels(shellbreaking effect of F).The condensed combustion products (CCPs)are investigated by SEM and EDS (Fig.S4).According to our previous work[14,15,19],the cube and the flakes are identified to be AlF3and B2O3, respectively.Meanwhile, the Al2O3empty shell is found in the CCPs.

Fig.8.(a) TG curves of 3D microspheres; (b) DSC curves 3D microspheres; (c) MS results of 3D microspheres of HB; (d) MS results of 3D microspheres of HBA.

in the third step, accompanying a mass loss step, a number of fluorine-containing species,C2F4(m/z=100)and C3F6(m/z=150)were produced,which comes from the decomposition of PTFE(R7).The holes produced by PIR reaction on the surface of metal fuel can provide reaction channels for active core and fluorine species(R8).Thus,some metal fluoride(AlF3(m/z=84),BF3(m/z=68))and an exothermal peak were identified in MS and DSC results.

Fig.9.The schematic illustration of combustion reaction mechanism and the synergistic effects of 3D microspheres.

When Al was added to 3D microspheres (HBA), the AlF3fragment emerges in the whole decomposition process, indicating the different reaction mechanism with the addition of Al.Another special phenomenon is that the relative content of BF3is different in the three decomposition steps between HB and HBA.In the metal fuel reaction step (the second and the third decomposition step),the relative content of BF3in HBA(84.28%)is much higher than HB(79.85%), further demonstrating that Al can promote the ignition and combustion of boron when metal fuel occurs reaction.

For 3D microsphere of HBA,the energy output and combustion reactivity are mainly dependent on the combustion of metal fuel(Al and B).From the chemical reaction, the enhanced combustion properties originate from two aspects (as shown in Fig.9).Firstly,the fluorine-containing species can react with the native oxide layer(Al2O3and B2O3)by shell-breaking effect to expose the active core,leading to a fast heat release reaction.Secondly,the burned Al can accelerate the ignition and combustion of B by the initiation effect.Then,the burning of B can release more heat to improve the combustion heat of 3D microspheres.In the other hands, the role played by the physical process in improving the energy and combustion reactivity is of the same importance as the chemical reaction.The more uniform distribution of 3D microsphere of reaction components makes the reaction distance reduced and reaction area increased.Moreover,compared with the physically mixed samples,the inner porous structures can maintain high-pressure state in the burning process.Thus,the decomposition of HMX and combustion of metal fuel were accelerated,leading to a higher heat release and faster energy release rate than the physically mixed samples.

4.Conclusions

In summary, 3D microspheres of HMX/B/Al/PTFE (HBA) are successfully designed and prepared by the emulsion and solvent evaporation method.HBA show higher energy output and combustion performance than their physically mixed counterparts(HBA-P) because of the increased interfacial contact and reaction area.Furthermore, the energy and reactivity of HBA are also enhanced compared to 3D microspheres of HMX/B/PTFE (HB) due to the initiation effect of Al to B.Thus, HBA has the highest combustion heat,the highest flame height and flame propagation rate,the largest peak pressure and pressurization rate.The combustion spectrum analysis and thermal analysis indicate that the initiation effect of Al and the shell-breaking effect of F help to achieve superior energy and pressure output.This work provides an effective way to simultaneously achieve high energy density and high reactivity energetic materials.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant Nos.T2222027,12202416 and 12272359).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2023.07.016.