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Microstructured Al/Fe2O3/NitrocelluloseEnergetic Fibers Realized byElectrospinningRui Li a b , Hongmei Xu c , Hailong Hu c , Guangcheng Yang a b , JunWang b & Jinpeng Shen a ba Sichuan Research Center of New Materials , Mianyang , P.R. Chinab Institute of Chemical Materials , China Academy of EngineeringPhysics , Mianyang , P.R. Chinac Southwest University of Science and Technology , Mianyang , P.R.ChinaPublished online: 09 Jul 2013.

To cite this article: Rui Li , Hongmei Xu , Hailong Hu , Guangcheng Yang , Jun Wang & Jinpeng Shen(2014) Microstructured Al/Fe2O3/Nitrocellulose Energetic Fibers Realized by Electrospinning, Journalof Energetic Materials, 32:1, 50-59, DOI: 10.1080/07370652.2012.754515

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Microstructured Al/Fe2O3/Nitrocellulose EnergeticFibers Realized by Electrospinning

RUI LI,1,2 HONGMEI XU,3 HAILONG HU,3

GUANGCHENG YANG,1,2 JUN WANG,2 ANDJINPENG SHEN1,2

1Sichuan Research Center of New Materials, Mianyang, P.R. China2Institute of Chemical Materials, China Academy of Engineering Physics,Mianyang, P.R. China3Southwest University of Science and Technology, Mianyang, P.R. China

At present, metastable intermolecular composites (MICs) have been widely studiedfor their potential in high-density energetic materials and nanotechnology, but therelatively low-pressure discharge in a short period of time and the oxidation of Alpowders have seriously impeded their applications in rocket solid fuels and explo-sives. In this work, the authors successfully fabricated microstructured Al=Fe2O3=nitrocellulose (Al=Fe2O3=NC) fibers via simple electrospinning, introducing nitro-cellulose (NC), a gas generator to MICs. In view of previous reports, wrappingnAl in NC fibers might reduce their further oxidation during storage. In addition,the thermal properties and elastic modulus of NC fibers were measured beforeand after adding Al=Fe2O3.

Keywords mechanical property; metastable intermolecular composites; microfi-bers; thermal analysis

Introduction

Since researchers started to use nanoscale reactants to create thermites, metastableintermolecular composites (MICs) are of growing interest in the energetic communitybecause of enhanced performance compared with their bulk or their micrometer-scalecounterparts [1]. MICs have short heat and mass transfer length, and the reactivitycan be tuned through control of particle size distribution, stoichiometry, and choiceof fuel and oxidizer [2,3]. They exhibit extreme high-energy density, quick energyrelease rates [4], short ignition time, and improved repeatability of response toignition [5,6]. MICs have been synthesized in various morphologies. Tillotson et al.prepared Al=Fe2O3 MICs in the form of powder using a sol-gel synthetic approach[7]. Menon et al. embedded an array of Fe2O3 nanowires inside a thin Al film [8].Zhang et al. made Al=CuO and Al=Co3O4 core=shell structures by integratingnano-Al with one-dimensional CuO nanowires and Co3O4 nanorods [9,10] andcompared the exothermic reaction and ignition properties of the microscale and

Address correspondence to Guangcheng Yang, Sichuan Research Center of NewMaterials, Kechuang Garden Horticulture Street No. 20, Mianyang 621000, P.R. China.E-mail: ygcheng@caep.ac.cn

Journal of Energetic Materials, 32: 50–59, 2014Copyright # Taylor & Francis Group, LLCISSN: 0737-0652 print=1545-8822 onlineDOI: 10.1080/07370652.2012.754515

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nanoscale Al=CuOx [11]. Moreover, they developed a nano initiator by integratingAl=CuO MICs with an Au=Pt=Cr thin-film microheater. Its ignition success ratecan reach 98%, and the energy output was calculated to be around 60 mJ. The ejectedcombustion flame was seen clearly with a potential exceeding 2000�C [12]. Inaddition, substantial efforts were made to prepare and characterize nanofoils ofAl=CuO thermite [13,14]. However, most of these thermites are unable to producegas rapidly during the energetic reaction [15], which limits their applications in rocketsolid fuels and explosives. A typical way to alleviate this shortcoming is to mix a gasgenerator with thermites [16,17]. In addition, aluminum powders are easily oxidized,resulting in poor performance of the MICs.

Nitrocellulose (NC) is widely used as chief ingredient in single-base and double-base gun and rocket propellants [18]. However, the poor burning and mechanicalproperties of NC limit its widespread use. At present, the addition of nanoparticles,such as Al, MgO, NiO, and Fe2O3, in solid propellants has been shown to significantlyenhance the burning rates and mechanical properties of the propellant [19–21]. Theadvantages of these metals or metal oxides are clear, but they have relatively lowenergy densities. In 2011, An et al. prepared Al=PbO, Al=CuO, and Al=Bi2O3 andfound that double-base propellants containing these superthermites showed excellentperformance of combustion [22]. This result indicated that MICs have tremendouspotential superiorities in application of propellant.

Electrospinning is a rather simple and highly versatile technique to generate mul-tifunctional ultrathin fibers from various polymers, polymer blends, and polymers=nanoparticles [23–25]. In this study, Al=Fe2O3=NC energetic fibers were synthesizedusing an electrospinning method, which combined the properties of NC fibers andMICs. Not only was a gas generator introduced to the MIC, but it also might providea way to prevent further oxidation of Al powders during storage. Moreover, anumber of papers [26–29] have demonstrated that the polymer-nanofiller fibers havebetter mechanical properties. Thus, it is expected that MICs have the same effect onNC fibers. We hope this work can provide a reference for future research andapplication of MIC fibers.

Experimental

Materials

Nano-aluminum (Al, 40 nm, 98% purity) was obtained from Xuzhou Hongwu Nano-materials Co., Ltd. (Xuzhou, China). Its active aluminum content varied fromapproximately 50 to 80wt%, which was estimated by determining the thickness ofthe alumina layer combined with data on the particle size distribution. The alpha-ironoxide (Fe2O3) particles were supplied by Aladdin (Shanghai, China) and had a nom-inal particle size of 30 nm. Both the aluminum and iron oxide particles were approxi-mately spherical. Particle size and shape information was provided by the suppliers.NC (11.9–12.4%, N) was purchased from Luzhou North Chemical Industries Co.,Ltd. (Luzhou, China) Acetone and N,N-dimethyl acetamide were purchased fromChengDu Kelong Chemical Co., Ltd. (Chengdu, China).

Preparation of Al/Fe2O3 MICs

Al=Fe2O3 MICs were prepared by first weighing out 40 nm Al and 30 nm Fe2O3, andthen approximately 20mL of hexane was added. Next, the mixture was sonicated to

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ensure intimate mixing of the fuel and oxidizer particles. After 30min, the sampleswere placed in a vacuum oven at 50�C to drive off the hexane. A loose powder wasobtained.

Preparation of the Spinning Solution

The solution was prepared by dissolving NC in a mixture of acetone and N,N-dimethyl acetamide (V:V¼ 2:1), and then the Al=Fe2O3 MICs powders were added.The NC concentration was 10% (w=v, with respect to solvent) and the Al=Fe2O3

MICs concentration was 30% (w=w, with respect to NC). The suspension wasconstantly stirred and sonicated for several hours to obtain a homogeneous solution.The as-prepared solution was referred to the spinning solution. Moreover, part ofthis solution was dried and denoted as Al=Fe2O3=NC powders.

Electrospinning

The spinning solution was immediately loaded into a plastic syringe with a needleinner diameter of 0.9mm, and the syringe was fixed horizontally on a 56JX47G syr-inge pump (Beijing Concent Technology Co., Ltd., Beijing, China). In the process ofelectrospinning, 25 kV potential was attached to the needle using a high-voltagepower supply (Dongwen High Voltage, Tianjin, China). The flow was set at 8mL=h.h. The Al=Fe2O3=NC fibers were collected on a rotating wire drum device situated at20 cm from the needle. The rotation speed was set at 400 rpm.

Results and Discussion

SEM Analysis

Electrospun NC and Al=Fe2O3=NC fibers were coated with a thin gold layer using ionsputtering (SBC-12, KYKY, Beijing, China) and their morphologies were observed byscanning electron microscopy (SEM, TM-1000, Hitachi High Technologies Co., Tokyo,Japan) at an accelerating voltage of 15kV. As seen in Fig. 1, the surface morphology ofNC fibers was smooth (Fig. 1a), but that of the Al=Fe2O3=NC fibers was much rougher(Fig. 1b). Similar results have been observed in electrospun multiwalled nanotu-be=poly(methyl methacrylate) nanofibers [30]. The average diameter of fiber we obtainedwas approximately 320nm. Further, based on the observation of single electrospunfibers at high magnification, we could state that most MICs were wrapped by NC,although some MICs protruded out of a fiber surface (as shown in Fig. 1d). The aggre-gated MICs observed in Al=Fe2O3=NC fibers were probably generated in the initialspinning solution, becauseMICs could not be fully dispersed within a thick NC solution.

X-ray Diffraction Analysis

X-ray diffraction (XRD) patterns of the fibers were recorded on an X-ray diffract-ometer (X’Pert PRO, PANalytical B.V., the Netherlands) at a voltage of 40 kV anda current of 40mA using Cu Ka radiation (k¼ 0.154 nm). The scanning rate was8�=min. As shown in Fig. 2, several new diffraction peaks, such as (012), (104),(111), and (220), appeared in the XRD pattern of Al=Fe2O3=NC fibers compared

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to NC fibers, which corresponded to the phases of Al and Fe2O3. These results indi-cated that Al=Fe2O3=NC microfibers were composed of Al=Fe2O3 MICs and NC.

X-ray Photoelectron Spectroscopy Analysis

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic techniquethat measures the elemental composition of the surface (usually the top 1–10 nm).

Figure 2. XRD patterns of the electrospun NC and Al=Fe2O3=NC fibers.

Figure 1. Low- and high-magnification SEM images of (a), (c) NC fibers and (b), (d)Al=Fe2O3=NC fibers.

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Thus, to investigate whether the aluminum particles were wrapped by NC, theAl=Fe2O3=NC fibers obtained were measured using an ESCALAB 250 XPS (ThermoElectron Corp., Waltham, MA, USA). As shown in Fig. 3, the characteristic peaks atthe binding energies of 287.38, 407.94, and 534.14 eV correspond to C1 s, N1 s, andO1 s, respectively. The Al2p peak located at 74.9 eV could hardly be detected in theXPS spectrum. These results confirmed that the aluminum particles were wrappedby NC in the composite fibers. In light of previous woks [31,32], this might providea way to increase the stability of aluminum powders in air during the storage period,leading to higher reactivity of Al powders for application in energetic systems.

Thermal Analysis

The thermal analyses were performed using a Netzsch STA 449F3 (Selb, Germany).In each scan, 2–6mg of reactive material was heated from 25 to 1000�C at 10�Cmin�1

in an atmosphere of pure argon flowing at 30mL min�1. As shown in Fig. 4, therewere two distinct peaks at 582 and 654�C in the DSC curve of Al=Fe2O3 MICs, whichcorresponded to the thermite reaction and melting of redundant Al powders, respect-ively [33]. For NC fibers, the sharp exothermic peak at 198�C could be attributed tothe decomposition of NC [34]. In the DSC curve of Al=NC fibers, an obvious peak at654�C (the melting of Al) could be observed, suggesting that there was not sufficientoxygen in the NC to completely oxidize the nAl.

Moreover, to study the reaction characteristics between Al=Fe2O3MICs and NC,the DSC curves of Al=Fe2O3=NC fibers and Al=Fe2O3=NC powders were also plottedin Fig. 4. It was found that, after adding Al or Al=Fe2O3 MICs, the decompositiontemperature of nitrocellulose was shifted to lower temperatures compared to pureNC fibers (NC fibers: 198�C; Al=NC fibers: 193�C; Al=Fe2O3=NC fibers: 189�C;Al=Fe2O3=NC powders: 173�C). These shifts in peak temperature were attributedto the catalysis of Al and Fe2O3. The metals (Al) had high specific surface area (highreactivity) and potential ability to store energy in surfaces [35]. The metal oxides

Figure 3. XPS pattern of the electrospun Al=Fe2O3=NC fibers.

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(Fe2O3) can accelerate the fission of oxygen–nitrogen bond during the thermaldecomposition of NC [20], which was the first and rate-determining step [36]. There-fore, the thermal decomposition of NC was accelerated.

The effect of the NC on the thermite reaction was also investigated. Compared tosingle MICs, the DSC curve of Al=Fe2O3=NC fibers showed a broader exothermicpeak in the temperature range of 250–750�C, whereas in the DSC curve of Al=Fe2O3=NC powders, no peak could be observed in the same temperature range. In addition,the heats of reaction for as-prepared samples were investigated by estimating the exo-thermic peak area. It was noted that the energy released at about 180�Cwas decreasedin the following order: Al=Fe2O3=NC powders>NC fibers>Al=Fe2O3=NC fibers.Based on these results, one could conclude that the heat released from the NCdecomposition in Al=Fe2O3=NC composites was sufficient to trigger the reactionbetween Al and Fe2O3. Therefore, we observed a combined decomposition=thermiteermite reaction process in the DSC curve of Al=Fe2O3=NC powders and a lower onsettemperature of thermite reaction in the Al=Fe2O3=NC fibers. The difference betweenAl=Fe2O3=NC powders and Al=Fe2O3=NC fibers was ascribed to the reduction inthe rate of heat transfer that is enabled by the high thermal resistances atthe fiber–fiber interfaces, which were not present in the continuous powders [37].

Nanomechanical Testing

The elastic modulus of NC and Al=Fe2O3=NC fibers was obtained by performing ananoscale three-point bending test on a single fiber suspended over the etched groove

Figure 4. DSC curves of Al=Fe2O3 MICs, NC fibers, Al=NC fibers, Al=Fe2O3=NC fibers, andAl=Fe2O3=NC powders.

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in a silicon wafer. A schematic of the measurement of fiber modulus using atomicforce microscopy (AFM) is shown in Fig. 5 (not to scale), and the single fibersuspended on the grooved silicon wafer is shown in Fig. 6.

Generally, the deflections of beams under a certain force are the sum of thedeformations of bending and shear. But the contribution of shear can be negligiblewhen the length-to-diameter ratio of fibers is more than 20 [38], so that the elasticmodulus (E) can be determined using Eq. (1) as the pure bending formula [39,40]:

Figure 6. AFM image of a single fiber on grooved silicon wafer. (color figure available online.)

Figure 5. Schematic of the measurement of mechanical properties using AFM.

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E ¼ FL3

192dI; ð1Þ

where F is the concentrated force loaded in the midpoint of a beam, L is the span, d isthe fiber deflection, I is the second moment of area of the beam (I ¼ pD4=64 for afilled cylinder with diameter of D). The fiber deflection (d) is obtained using d¼(Z�Z0) �bcantilever, where Z is the absolute z-piezo extension, Z0 is the initial z-piezoextension upon probe contact, and bcantilever is the bending of the cantilever at Z.

The content of catalysts in propellants is usually less than 10% [19–21], so theelastic modulus of NC fibers and NC with 5wt% and 10wt% MICs was measuredin our work using an SPA 300-HV AFM (Seiko, Japan). The results in Table 1 showthat the elastic modulus of NC fibers could be improved by adding 5.0wt% Al=Fe2O3

MICs. This stiffening could be attributed to the high stiffness of MICs and stresstransfer from NC to MIC particles. However, relatively poor fiber mechanicalproperties were found when the MIC content in NC increased to 10wt%, resultingfrom agglomeration of the MIC particles.

Conclusions

In this work, micrometer-size Al=Fe2O3=NC fibers were successfully prepared byelectrospinning techniques, which introduced a gas generator toMICs and might pro-vide a way to prevent further oxidation of Al powders during the storage period. Theresult of the thermal analysis revealed that the reaction process of Al=Fe2O3=NCfibers was different from that in Al=Fe2O3=NC powders due to the lower rate of massdiffusion and the rate of thermal diffusion in fibers. In addition, an increase in theelastic modulus of NC fibers with the addition of 5wt% MICs was observed.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(Grant Nos. 11002128, 11172276, and 11172275), the Science Foundation for YoungScientist of Sichuan Province (2012JQ0038), and the Open Project of State KeyLaboratory Cultivation Base for Nonmetal Composites and Functional Materials(No. 11zxfk22) from the Southwest University of Science and Technology.

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