Kinetic study of thermal- and impact-initiated reactions in Al–Fe2O3 nanothermite
Introduction
Nanoenergetic materials have superior exothermic characteristics and possess properties unattainable with traditional micron-sized materials. Metal-based energetic materials consisting of a mixture of nano-sized aluminum and metal-oxide powders are frequently referred to as a superthermite or a metastable intermolecular composite [1]. The energy released by the redox reaction in such thermite materials depends largely on the arrangement of the oxidizer and fuel. The reaction involves a solid-state diffusion process that is dependent on the interfacial contact area between the oxidizer and the fuel. Hence, an increase in interfacial contact area results in enhanced rate of energy release. A number of methods, such as physical mixing [2], [3], arrested ball milling [4], [5], [6] and sol–gel synthesis [7], have been investigated to prepare thermite systems with increased interfacial contact between the oxidizer and the fuel. However, inherent problems exist with some of these processing methods. Physical methods, such as ultrasonic mixing and arrested ball milling produce thermites with a random distribution of the oxidizer and fuel particles. The unstructured nature of the mixed composite causes a large and almost uncontrollable combustion behavior. The sol–gel approach offers better control over the composition of the solid at the nanometer scale which is difficult to achieve by physical mixing techniques. However, one key disadvantage of the sol–gel method is the necessary dilution of the thermite with inert oxides such as Al2O3 (from dissolved AlCl3 salt) or SiO2 (from added silicon alkoxide). This leads to a material that is not as energetic as the mixture of pure iron (III) oxide and Al due to the dead weight that can amount up to 10 wt.% [2], [3]. In light of these limitations in processing of nanothermites, we have developed an approach to produce nanostructured superthermites (Al metal fuel and Fe2O3 oxidizer) by a surfactant self-assembly method to achieve molecular-level mixing whereby the fuel nanoparticles are arranged in an orderly and controlled manner around the oxidizer and vice versa. The orderly arranged composition can result in maximum interfacial contact area between the fuel and oxidizer, and hence provide a higher rate of energy release.
Ignition of energetic materials has been investigated by various techniques, including thermal, chemical, optical, or mechanical impulse, each of which involves a unique reaction mechanism, and therefore, different material behavior. Impact-initiated chemical reactions have been characterized as those occurring due to solid state mechanochemical effects leading to shock-induced reactions occurring in the time scale of pressure equilibration (typically microsecond duration) [8], or shock-assisted reactions occurring in the time scale of temperature equilibration (typically tens to hundreds of microseconds), with heating rates in excess of 106–108 K/s [9]. Initiation for impact-induced chemical reactions is linked to the degree of short-time-scale mixing and has been demonstrated to rely primarily on the intrinsic mechanical properties of the energetic material constituents, whereby the material first undergoes a deformation and flow or fracture and dispersion prior to ignition [8]. The onset and/or the extent of impact-initiated reactions can be correlated with the impact energy or stress. Thermally-initiated reactions using thermal analysis equipments, on the other hand, involve a thermochemical mechanism. This includes liquid-phase reactions based on evidence of observation of localized melt regions (hot-spots) in recovered partially-reacted materials [10], [11], [12] all of which conducted under static configuration. Heating rates normally fall in the range of 10–103 K/s which are several orders of magnitude slower than impact-induced reactions. This often allows diffusion-driven reactions to precede melting and reaction ignition under a static configuration. However, reaction initiation mechanisms that capture the effects of reactant size and contact area of energetic materials are still not well understood.
In this work, we report on reaction kinetics studies of Al + Fe2O3 nanothermite based on thermally-initiated and impact-induced reactions. Comparative studies are performed with intimately mixed self-assembled and random solvent-mixed reactants. Particle size is also varied to study the influence of size effects on reaction kinetics. The goal of this study is to understand the influence of different mixing methods and reactant size effects on the design, ignition mechanisms, and performance of Al + Fe2O3 superthemite based energetic material system.
Section snippets
Experimental
Motivated by previous reported self-assembly approaches to prepare reactive nanocomposite materials using nano-sized aluminum powder and nanorods oxidisers [13], [14], intimately mixed self-assembled nanothermite consisting of novel Fe2O3 nanotubes and Al nanoparticles have been achieved in our prior work [15]. The self-assembly process consists of mainly three steps: coating of Fe2O3 nanotubes with P4VP (polyvinyl pyridine from sigma aldrich; a surfactant) by ultrasonicating in 2-propanol for 4
Results and discussion
The morphology and microstructure of the as-synthesized samples were first characterized using SEM and TEM. Images of the self-assembled Al–Fe2O3 system in Fig. 1a–d clearly show that high interfacial contacts between the Fe2O3 nanotubes and Al nanoparticles have been achieved. The inset in Fig. 1b shows the ∼3 to 4 nm thick alumina shell around the commercially purchased nano-Al powder used in this study. It is also noted that the average particle size distribution of the purchased nano-Al is
Conclusion
Upon thermal initiation, Al + Fe2O3 nanothermite system prepared by self-assembly has been shown to exhibit significant improvements in reaction kinetics as compared to nano- and micro-scale thermite samples prepared by simple physical solvent-mixing. By increasing the intimacy between the fuel and oxidizer by self-assembly, solid-state diffusion between the reactants is significantly enhanced. It is also deduced that interfacial contact area is more important than the reactant size, as revealed
Acknowledgments
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. J.L. Cheng would also like to acknowledge the Ian Ferguson Postgraduate Fellowship award for his attachment at Georgia Institute of Technology. The impact experiments at Georgia Institute of Technology were performed with support from DTRA Grant No. HDTRA1-07-1-0018.
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