Synthesis and characterization of self-assembled nanoenergetic Al–Fe2O3 thermite system

Synthesis and characterization of self-assembled nanoenergetic Al–Fe 2O 3thermite system

J.L.Cheng a,n ,H.H.Hng a ,H.Y.Ng b ,P.C.Soon b ,Y.W.Lee b

a School of Materials Science &Engineering,Division of Materials Science,Nanyang Technological University,50Nanyang Avenue,639798Singapore,Singapore b

DSO National Laboratories,20Science Park Drive,Singapore

a r t i c l e i n f o

Article history:

Received 20May 2009Accepted 31August 2009Keywords:

A.Nanostructures A.Metals

B.Chemical synthesis

a b s t r a c t

Nanothermite composites powders have superior exothermic characteristics and possess properties unobtainable by traditional micron-sized predecessors.In this paper,the reaction kinetics of a self-assembled Al–Fe 2O 3thermite system are discussed and compared with conventional physical mixed references.Surfactant self-assembly allows an increase in interfacial contact area between Al and Fe 2O 3,which hastens the thermite reaction process to produce signi?cant enhancement in the reaction kinetics.It was found that intimate mixing is a more important consideration for improving the combustion kinetics of thermite systems rather than the size of the individual components.

1.Introduction

Nanomaterials have allowed the realization of properties that was previously unobtainable in the bulk.In recent years,there has been great interest in nanostructured energetic powders that have improved ignition kinetics compared with the micron-sized pre-decessors.Nanoenergetic materials comprising of at least one of the component in nanometer dimension scale possess properties un-obtainable by traditional energetic materials [1,2].The energy release rate in traditional micro-scale energetic composites was inhibited mainly due to mass transport limited by the diffusion process in the solid state.The most effective method to overcome this is to reduce the scale of the fuel and oxidizer components to achieve higher mass transport.The increase in speci?c surface area at nano-scale allows for more reaction locations and enhances heat transfer rates.

A unique class of energetic material comprising of an oxidizer and a metal fuel will be studied in this work.This type of material is frequently named as superthermites or metastable intermole-cular composites (MIC)[3].A thermite system typically consists of a fuel (e.g.,aluminum)and oxidizer (e.g.,Fe 2O 3)and reaction between the two produces a substantial exothermic heat release.The reaction temperature and heat of combustion can be extremely high and are commonly used for ordinance applica-tions.Such reactions involve oxidation/reduction reactions that provide their own oxygen supply and as such,are self-sustaining and dif?cult to stop.The energy released in thermite systems increases with higher interfacial contact area to result in an enhanced rate of energy release.Many attempts in the energetic research community to increase the interfacial interaction have

been done but there are still rooms for improvement.The efforts made so far in energetic thermite system usually involves physical random mixing of the oxidizer and fuel,which unfortunately leads to inhomogeneous distribution [4–9].

The main focus of this work is concerned with the synthesis and characterization of nanoenergetic superthermites (primarily the Al–Fe 2O 3system)with the goal of attaining distinctive ordered morphologies via surfactant self-assembly,and hence achieving enhanced exothermic energy release rate.The Fe 2O 3/Al mixture is a classical thermite system,which has been used widely for the many applications such as additives to propellants and high explosives,free standing heat sources,airbag ignition materials,hardware destruction devices,welding torches,etc.due to their high adiabatic temperature of 3135K and energy density of 3.71kJ/g [10].In this work,novel Fe 2O 3nanotubes–Al nanoparticles nanothermite has been produced via a surfactant self-assembly using polyvinyl pyridine (PVP).Fe 2O 3nanotubes were chosen because it is able to act as hot spots for localized heating and upon ignition at a point in a heterogeneous energetic material;these regions play important roles in rapid propagation of heat upon collapse,which leads to an accelerated combustion rate [11].The hollow tubular structure also provides a high speci?c surface area for enhanced combustion.A cylindrical morphology is preferred as a relatively larger number of nanoparticles can be assembled as compared with spherical morphology.The Fe 2O 3nanotubes are self-assembled with spherical Al nanoparticles to achieve high contact area to hasten the solid-state diffusion process between the Fe 2O 3oxidizer and Al metal fuel.

2.Exprimental

A schematic diagram illustrating the proposed self-assembly of Fe 2O 3nanotubes–Al system is shown in Fig.1.A solution phase

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Journal of Physics and Chemistry of Solids

?Corresponding author.Tel.:+6567904140;fax:+6567909081.

E-mail address:jlcheng@https://www.360docs.net/doc/162681020.html,.sg (J.L.Cheng).

Journal of Physics and Chemistry of Solids 71(2010)90–94

approach adopted from the work reported by Jia et al.[12]was used to synthesize the iron oxide nanotubes.Subsequently,the self-assembly process consists of mainly three steps:(1)coating of Fe2O3nanotubes with P4VP,(2)mixing and ultra-sonicating with Al nanoparticles,and(3)removal of excess surfactant by heating to obtain the?nal product.In the?rst step,0.12g of Fe2O3 nanotubes were sonicated for4h in200ml of propan-2-ol containing0.1%(w/v)P4VP(Sigma Aldrich,Mw60000).The coated nanotubes were subsequently separated from the solution by centrifugation and dried at601C for6h.The second step mainly involved dispersing0.1g of coated oxidizer nanotubes with0.034g of Al nanoparticles(purchased from American Elements,APS18nm,99.7%purity)in 1.5ml of propan-2-ol using a sonic bath for1h.Finally,the particles were separated by repeated process of centrifugation with propanol and dried at 601C for6h to obtain the?nal material.Physically solvent-mixed Fe2O3–Al thermite samples were also prepared for comparison purpose.The samples were prepared by simple physical mixing of the fuel and oxidizer components for10min in hexane solvent. The powders were then separated and dried at651C for6h prior to analysis.All the samples were prepared with a stoichiometric ratio of Fe2O3:Al(74wt%:26wt%)based on the following equation:

Fe2O3+2Al-2Fe(s)+Al2O3(s)(1) The?eld-emission scanning electron microscope(JEOL JSM-6340F)and high-resolution transmission electron microscope (JEOL JEM2100F)were used to image the size and morphology of the powders.X-ray diffraction(XRD)patterns of the powders were obtained with a Shimadzu6000X-ray diffractometer using a copper target(l Cu-k a=1.5405?A).Ignition of the prepared powders was studied using an electrically heated wire ignition test.This technique was adopted from Dreizin’s group and has been described extensively[13,14]and was done at a well-controlled heating rate of80001C/s.Dynamic pressure measurements were conducted in a constant volume explosion vessel(Parr Instrument 1108oxygen bomb and a Kistler211B2pressure sensor)under oxygen atmosphere of500psi for the combustion of different charges of thermite powders.3.Results and discussion

XRD analysis(Fig.2(a))con?rmed the hematite phase of the product.All the diffraction peaks were indexed to the pure corundum structure of hematite Fe2O3(JCPDS:33-0664)phase. The morphology of the as-synthesized Fe2O3nanotubes was studied by TEM,and the results are shown in Fig.2(b).The product consists entirely of nanotubes that are completely hollow as observed in the TEM image.The dimensions of the tubes consist of inner diameters of50–55nm,outer diameters of65–70nm and lengths of150–250nm.The average particle size measured using the Malvern Zetasizer is found to be192.4nm and the result compares well with the TEM observation.A polydispersity value of0.126was measured and relates well to having a fairly monodisperse distribution.It is noted that some of the tubes are opened on one end and closed on the other.

The phase purity of the assembled nanothermite was exam-ined by XRD analysis.The XRD pattern(Fig.3(a))reveals no unknown crystalline phases and impurities.All the peaks correspond to the pure aluminum and haemitite Fe2O3phases. Images of the self-assembled system(Fig.3(b))show distinct coupling of the Fe2O3nanotubes and the Al nanoparticles.In comparison,segregated agglomeration of Al clusters were observed in the physically solvent-mixed reference sample (Fig.4).There are negligible interfacial contacts between the oxidizer and fuel particles in the physically mixed samples.

Preliminary results based on ignition wire tests and dynamic pressure measurements(Fig.5)are promising.Signi?cant enhancement in the combustion kinetics as compared with the reference samples prepared by physical mixing is observed and the results are summarized in Table1.The ignition temperature is registered as a sudden spike in voltage measured by the photodiode during the ignition wire test as shown in Fig.5(a).A bare wire reference test was done to distinguish the point where the graph starts to deviate positively from the reference.The point where voltage starts to deviate is registered as the point of ignition attributed by the powdered sample alone and the ignition delay time(t delay)was introduced and de?ned as the time taken to reach that point.When compared with the solvent-mixed Fe2O3–Al micron-scale thermites(comprise of325mesh99.8%

granular

PVP

coating

Fe2O3 nanotubes

synthesized by

hydrothermal

method

PVP

coated

Fe2O3

nanotubes

Al nanoparticles

Removal of

excess

surfactant

200nm

Fig.1.A schematic diagram illustrating the self-assembly of Fe2O3nanotubes–Al nanoparticles system.

J.L.Cheng et al./Journal of Physics and Chemistry of Solids71(2010)90–9491

aluminum (Alfa Aesar)mixed with 325mesh 99.9%granular Fe 2O 3(Alfa Aesar)),the ignition temperature were signi?cantly lowered by half and the ignition delay time was decreased by more than 2folds.The ignition temperature ranges from 686to

10301C with the self-assembled Fe 2O 3

nanotubes–Al nanoparticles having the lowest ignition temperature at 685.81C.

An important ?nding was deduced from the ignition wire test results.Physically mixed sample (m in Fig.5(a))and self-assembled sample (’in Fig.5(a))showed totally different ignition behavior despite having the same composition.An extremely sharp increase in ignition ?ash intensity (d V /d t )was observed for the self-assembled sample with the shortest t delay of 0.026s.Physically mixed nanosample (m in Fig.5(a))only perform slightly better than the physically mixed micron sample (K in Fig.5(a)).This indicates the intimate mixing is a more important consideration than size effect when designing superthermite systems.

The dynamic pressure results of the various samples are shown in Fig.5(b).For the self-assembled thermite sample,the maximum pressure rise was increased by more than 4times with

Al nanoparticles

Fe 2O 3 nanotubes

I n t e n s i t y / A r b i t u a r y u n i t s

20304050607080

2 Theta/ Deg

Hematite (Fe 2O 3) peaks Aluminum peaks

Fig.3.(a)XRD pattern

and (b)TEM image of the self-assembled Fe 2O 3–Al nanothermite.

Al agglomerate

Little or no interfacial contacts

Fig.4.(a)SEI and (b)

TEM images of physically solvent-mixed Fe 2O 3nanotubes–Al nanoparticles sample.

I n t e n s i t y / A r b i t r a r y u n i t s

(012)

(104)

(110)

(113)

(024)

(116)(018)

(214)

(300)(208)

(1010)(220)

(306)

2θ/ °

203040

50607080

Fig.2.(a)XRD pattern and (b)TEM image of the as-synthesized Fe 2O 3nanotubes.

J.L.Cheng et al./Journal of Physics and Chemistry of Solids 71(2010)90–94

the rate of energy release also markedly improved by nearly 38folds.The self-assembled thermite is observed to achieve the largest P max generated in the shortest time.This further supports the superior reaction kinetics of adopting self-assembly in improving the reaction kinetics of nanoenergetics upon thermal initiation.

Self-assembly approach to produce superthermites has been shown to be of great signi?cance since the relative positions of the fuel and oxidizer determine the burning characteristics of the particular material.The results obtained have clearly showed that the arrangement of oxidizer and fuel plays an important role in the reaction kinetics of the overall thermite system.Surfactant

self-assembly in particular,using polymer allows the organization of the particulate entities to be achieved without the use of external ?elds.This is an added advantage for adopting such method in the ?eld of reactive materials providing a simple and general route for fabrication of nanostructured energetic materi-als with hierarchical order.

4.Conclusion

Novel Fe 2O 3nanotubes–Al nanoparticles nanothermites have been successfully produced by surfactant self-assembly method.

024*******

time/sec

P o t e n t i a l / V

0Time (msec)

N o r m a l i z e d P r e s s u r e (k P a )

0.010.020.030.040.050.060.070.080.090.10.110.120.13

500

1000150020002500

Fig.5.(a)Ignition wire test and (b)dynamic pressure measurement pro?les.

Table 1

Tabulation of results from ignition wire test and dynamic pressure measurements.Sample

Fe 2O 3

Method

Dynamic pressure results Ignition wire test results (d P /d t )max (kPa/ms)

t max (s)

P max (kPa)

Ignition Temp./1C Ignition delay (t delay )time/s A Spherical nanopowder (25nm)

Nanotubes (200nm;aspect ratio=3:1)Self-assembled 6.4030.207180685.80.026B Spherical nanopowder (25nm)

Nanotubes (200nm;aspect ratio=3:1)

Solvent mixed 2.500.3381071002.20.060C

Granular micron powder (40m m)

Solvent mixed

0.166

1.05

1026.2

0.067

J.L.Cheng et al./Journal of Physics and Chemistry of Solids 71(2010)90–94

Directed assembly between the fuel and oxidizer particles has been achieved and high interfacial contact area between the two components was achieved.This signi?cantly hastens the solid-state diffusion process between the two and was proven by the signi?cant improvement in the reaction kinetics as compared with the physically solvent-mixed reference samples.The results from this work suggest that interfacial contact area between the fuel and oxidizer is a more important consideration than the size effect when designing superthermite system. Acknowledgements

The authors would like to thank the Defence Science and Technology Agency(Singapore)and DSO National Laboratories for funding and support given to this project.

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