Energetic characteristics of the Al/CuO core-shell composite micro-particles fabricated as spherical colloids
Introduction
Metastable intermolecular composites (MICs) can release their stored chemical energy instantaneously upon an external trigger such as a laser, electrical current or mechanical shock. MICs normally compose of fuel (metal) and oxidizer (metal oxide) components which react at their interface. The oxidizer is reduced by the metal fuel and a more stable bond is formed between oxygen and the fuel metal during the thermite reaction, which is shown asM + AO → MO + A + ΔHwhere M is the metal (fuel), AO is the oxidizer, A is the reducing product of AO, and MO is the oxide that generated by this reaction. ΔH is the enthalpy of the reaction. Aluminum (Al) is often used as the fuel constitute of MICs due to its abundance on the earth and high energy release. [1,2] Copper oxide (CuO), molybdenum oxide (MoO3), nickel oxide (NiO) and iron oxide (Fe2O3) are common oxidizing agents. Advanced manufacturing processes such as micro-joining and micro-welding as well as propulsion and thrust are possible applications of MICs, since a large amount of heat release is desired to produce within several milliseconds. With the rapid development of microelectromechanical (MEMS) and micro-chemical devices, miniature and portable energy sources which produce on-demand heat or power are required to be fabricated directly in these devices such as micro-thrusters, micro-ignitors and microturbines. [3] One promising solution is to use micron and nano sized MICs, while both can accelerate the interfacial phenomena. [2,3] Nanoparticles are usually more expensive on a high-volume demand in comparison with micron-sized products. Energy release from Al nanoparticles is less than that of its micron-sized counterpart, due to the existence of the 3-5 nm passivated oxide layer and subsequently a lower active content. On the other hand, micron-sized powders have shown restrained ignition characteristics by impact, heating, friction and electrostatic discharge, due to their inherent heterogeneous microstructure which hinders the direct contact at the interface between these reactive constituents [4]. It is widely accepted that the initiation of the reaction between micron-sized Al and CuO particles usually involves melting of Al and subsequent diffusion of liquid Al to reach CuO via the Al/CuO interface, indicating a high reaction onset temperature. [5]
Many fabrication approaches have been developed to produce high-performance Al/CuO composites with different microstructures, in order to manipulate the characteristics of MICs. [6,7] The required activation energy and the overall energy release from the Al/CuO MIC are predominantly dependent on the mixing or assembly process between Al and CuO constitutes and the packing density, both of which vary with the sample preparation method, in addition to the intrinsic physical and chemical properties of CuO and Al constitutes [8]. A variety of fabrication methods including vacuum filtration [9], ultrasound mixing [10], electrophoretic deposition [11], self-assembly and sol-gel methods [5,12] have been utilized to control the packing density and subsequently the interfacial area between fuel and oxidizers.[7] Physical mixing via ultrasonication is a simple and cost-effective fabrication method, but it has limitations such as a poor controllability and producing larger heterogeneous agglomerates, which increases the onset temperature and reduces the completion degree of the reaction. [13] In order to achieve an intimate contact between fuel and oxidizer and to increase the effective interfacial area, self-assembly and sol-gel methods have been developed. [12] These methods usually result in a porous frame of CuO, while that structure has been found to reduce the overall energy density of the composite. [14,15] Nevertheless, the assembled structures can create a direct contact between Al and CuO, facilitating heat and mass transfer processes and subsequently increasing the rate of energy release [[16], [17], [18]]. Magnetron sputtering and thermal evaporation have been utilized to fabricate heterostructures of Al and CuO. For example, Zheng and co-workers prepared Al/CuO nanowires by coating Al onto CuO nanowires using magnetron sputtering, where the uniformity was greatly improved and the activation energy was reduced compared to those of the physically mixed products. [16] Meanwhile, e-beam evaporation technology was also used to sputter the Al layers with nano- or micro-meter thickness with a high source temperature of up to 1490 K [19]. However, these techniques are very expensive and more suitable for preparing samples at a small production scale.
Core-shell microstructures can enlarge the contact area between the reactive components and effectively decrease the diffusion distance of reactive ions during the reaction. Meanwhile, individual core-shell particles are transportable in small devices such as micro-channels and can be easily delivered to a target location on a MEMS component for energy or gas generation. Challenges exist for fabricating the individual Al/CuO core-shell particles and little information can be found in literature about how to fabricate isolated core-shell spherical Al/CuO particles. Part of the difficulty is due to the complexity of the formation of porous Cu-containing intermediates on the shell of Al particles, which can lead to an unstable oxidizer shell and affect the reactivity at the core-shell interface. Levitas et al. studied the effects of the pre-treatment of micro-scale aluminum/alumina core-shell particles on their reactivity in air. [20] Zheng and co-workers demonstrated two simple synthesis methods, i.e., the precipitation (PC) and displacement (DP) methods to prepare micron-sized Al/CuO thermites with shortened diffusion ion distances and improved dispersion at the reactive interface. [21] They found the DP-thermite possessed a dense shell of small CuO particles coated on the surface of the micron-sized Al particles. Both Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) showed a limited coverage of CuO particles on the Al particle. Meanwhile, these coated particles were highly agglomerated. According to a recent review on fabrication methods of MIC systems, there is no report on the synthesis of isolated, individual, spherical Al/CuO core-shell structures [22]. Most recently a very interesting fabrication technique was developed by Rossi and coworkers who utilized DNA to assemble Al and CuO nanoparticles into micron-sized particles of an Al/CuO nanocomposite. [23,24] The product showed an exquisite energetic performance in comparison with physically mixed powders. The energy release and onset temperature from assembled particles using 80 nm Al nanoparticles were measured as 1.8 kJ/g and 410 °C, respectively. That process, which demonstrated the feasibility of using DNA as a structural material to assemble Al/Al, CuO/CuO and Al/CuO composite materials, is being improved for reaching a greater hybridization efficiency (currently around ∼5% and ∼10% of single-stranded oligonucleotides grafted onto the CuO and Al surfaces, respectively) [23].
A new wet-chemistry synthesis method is developed in this work to fabricate spherical, Al/CuO micron-sized particles with a core-shell geometry. Energetic properties such as the onset temperature and energy release of the derived core-shell particles are measured and compared to the powder composites with similar particle sizes. A new reaction mechanism is proposed to understand the solid-state processes occurring at the core-shell interface which precede the exothermic reaction. Solution based synthesis methods are generally low-cost and can facilitate large-scale production. Core-shell micron-sized energetic particles, in contrast to other shapes such as films and arrays, are attractive in applications involving mobile energy-storage particles. [25]
Section snippets
Fabrication of the core-shell Al/CuO colloids
Al powders with an average diameter of 1 μm used in this work were purchased from US Research Nanomaterials, Inc. The elemental contents were 97% of Al by weight and 3% of Al2O3. Acetone, ethanol, ammonia hydroxide solution, and cupric nitrate (Cu(NO3)2 2.5H2O) were used as received from Sigma Aldrich. The procedure for synthesizing of Al/CuO colloids, as a separate reaction system which consists of a CuO shell and an Al core, is illustrated in Fig. 1. The synthesis in principle included two
Results and discussion
Samples 1, 2 and 3, which have the nominal equivalence ratio of 5, 3 and 1 respectively, were analyzed by means of XRD, SEM, DSC, TEM and XPS. The results were used to elaborate the influence of the equivalence ratio on microstructures and reaction properties. Since the equivalence ratio was calculated based on the mass of chemicals used for chemical synthesis, it has been labeled as the nominal equivalence ratio in this study (see the explanation earlier).
Conclusions
In this work, a novel procedure to synthesize and characterize core-shell Al/CuO metastable micro-sized particles was reported. The shell of CuO was fabricated on micron-sized Al particles by a wet-chemistry approach followed by annealing, which could be easily scaled up to the industrial level. The structural and thermochemical properties of these as-synthesized core-shell composites were examined and compared to two reference powder mixtures. The micron-sized Al and CuO core-shell composite
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.
CRediT authorship contribution statement
Yiqi Zhang: Data curation, Formal analysis, Investigation, Methodology, Validation, Writing - original draft. Hongtao Sui: Data curation, Formal analysis, Validation, Visualization. Yuning Li: Supervision, Conceptualization, Writing - review & editing, Funding acquisition. John Z. Wen: Supervision, Resources, Conceptualization, Methodology, Writing - review & editing, Funding acquisition.
Acknowledgement
This research had been supported by Natural Sciences and Engineering Research Council of Canada (NSERC) through Discovery grants. Zhang thanks Dr. Pei Zhao for useful discussions about the reaction mechanism.
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