Aluminum/copper oxide nanostructured energetic materials prepared by solution chemistry and electrophoretic deposition

Abstract

Nanostructured energetic materials benefit from improved spatial distribution and enhanced interfacial contact between reducing agents and oxidizing agents, and thus they have attracted increasing attention in the past decade. In this study Al/CuO nanostructured energetic materials were prepared by combining a solution chemistry method and electrophoretic deposition. A piece of clean Cu foil was first placed in a solution consisting of NaOH and (NH₄)S₂O₈ to form a layer of copper hydroxide, which was then dehydrated in an oven at 180 °C for 4 h to obtain CuO nanostructures. Nano Al particles were integrated with the CuO nanostructures by electrophoretic deposition to form Al/CuO nanostructured energetic materials. Thermal analysis showed that the prepared nanostructured energetic materials had lower apparent activation energy than that of a randomly mixed counterpart thanks to the structural design.

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1. Introduction

Nanostructured energetic materials feature more uniform distribution and more intimate contact between reducing agents and oxidants compared with those of their randomly mixed counterparts,^1,2 which endow them with even better energetic characteristics and broad potential application areas such as in ignition,³ propulsion,⁴ microfluidics actuation,⁵ and nondestructive welding.⁶ Among various nanostructured energetic materials developed so far, Al/CuO core/shell nanoenergetic arrays have shown promising ignition and energy release characteristics and thus attracted much attention.^7–11 In those studies, physical vapour deposition was usually used to deposit the Cu precursor film and Al thin film; however, the vacuum deposition apparatus is quite expensive. Moreover, high temperature heat treatment (>400 °C) was needed to partly transform the Cu precursor to CuO nanowire arrays; the process is thus energy consuming and can cause compatibility issues with the following procedures.

A new method was developed in this study to prepare Al/CuO nanostructured energetic materials, which avoided the use of vacuum deposition system and reduced the heat treatment temperature to only 180 °C. The core idea was to prepare CuO nanostructures first which functioned as the skeleton and thus contributed to the mixing uniformity and then to deposit nano Al particles into the CuO nanostructures by electrophoretic deposition. The electric field drove the positively charged nano Al particles to the cathode, where they combined with the CuO nanostructures intimately. Oxidizing Cu foil in solution to obtain Cu(OH)₂ or CuO was first studied by Zhang et al.,¹² but its application in nanoenergetic materials has not been reported. Electrophoretic deposition emerged as an efficient method to deposit nanothermites onto conductive substrates in recent years;^13,14 however, its application in preparing nanostructured energetic materials has not been explored. Benefiting from the structural design, the prepared Al/CuO nanostructured energetic materials showed lower apparent activation energy than that of randomly mixing counterpart.

2. Experimental section

2.1. Sample preparation

The schematic diagram of the preparation process is shown in Fig. 1. CuO nanostructures were prepared by a method adapted from that reported by Zhang et al.¹² Cu foil (99.9% purity, 40 mm × 20 mm × 0.1 mm) was immersed in 1 M HCl solution for 2 min to remove the native oxide layer, and then it was rinsed by deionized water and blown dry in air. The foil was then put into 40 mL of alkaline oxidative solution for 30 min under constant agitation. The solution consisted of 3 M NaOH and 0.15 M (NH₄)₂S₂O₈. The reaction occurred at room temperature (around 20 °C). The Cu foil with a layer of hydroxide on top was then put into an oven at the temperature of 180 °C for 4 h to form CuO nanostructures. Nano Al particles used for electrophoretic deposition were bought from Aladdin with the stated diameter of 20–200 nm. Ni plates were used as electrodes, and the Cu foil with CuO nanostructures on top was attached to the cathode for deposition by using conductive tape. The distance between the electrodes was 1 cm and the applied potential was 60 V. Typically 10 mg of nano Al was dispersed into 50 mL of ethanol by using ultrasonic agitation without using dispersants, and 100 μL of 1 M HNO₃ was added to the suspension to enhance the surface charge of nano Al particles. Al/CuO nanostructured energetic materials were obtained after the electrophoretic deposition.

Fig. 1 The schematic diagram of the preparation process.

2.2. Characterization and thermal analysis

The prepared materials were observed directly by using a field-emission scanning electron microscope (FESEM, Hitachi S-4800). X-ray diffraction (XRD, Bruker D8 Advancer) was used to determine the composition of the materials as well as the reaction product after thermal analysis. The XRD was performed at 30 kV using Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB250i) and energy-dispersive X-ray spectroscopy (EDS) were employed to characterize the elemental composition. The XPS measurement was conducted with a monochromatized Al Kα X-ray source (1486.6 eV). The chamber pressure was on the order of 10⁻¹⁰ mbar. The photoelectron takeoff angle (with respect to the sample surface) was 45°. No Ar etching was used before characterization. The thermodynamics and kinetics of the prepared materials were studied by differential thermal analysis (DTA). The materials were scraped off the Cu foils for analysis. N₂ gas was used as protective atmosphere with a flow rate of 100 mL min⁻¹. The data were processed by using TA Universal Analysis 2000 software.

3. Results and discussion

3.1. Morphology and composition of the prepared materials

Fig. 2a shows the top-view FESEM image of the CuO nanostructures on top of Cu foil. The formation mechanism of Cu(OH)₂ nanostructures has been reported in the literature and the overall chemical equation can be summarized as follows:^12,15

Cu (s) + 4NaOH (aq.) + (NH₄)₂S₂O₈ (aq.) → Cu(OH)₂ (s) + 2Na₂SO₄ (aq.) + 2NH₃ (g) + 2H₂O (l) (1)

Fig. 2 Top-view FESEM image of the (a) CuO nanostructures and (b) Al/CuO nanostructured energetic materials.

After thermal annealing at 180 °C for 4 h, Cu(OH)₂ dehydrated to form CuO nanostructures. Fig. 2b shows the Al/CuO nanostructured energetic materials on top of Cu foil. The nano Al particles were deposited around CuO nanostructures. The latter formed the skeleton for the nanostructured energetic materials, which improved the mixing uniformity. However, the agglomeration of nano Al particles was still found, indicating that further effort is needed to disperse nano Al particles more effectively and at the same time minimizing the introduced impurities. The electric field drove the charged nano Al particles to the cathode where they contacted the CuO nanostructures and the van der Waals forces generated thereof contributed to the more intimate interfacial contact.

XRD pattern of the Al/CuO nanostructured energetic materials is shown in Fig. 3. The diffraction peaks were indexed to be of CuO (pdf# 65-2309) and Al (pdf# 65-2869), indicating that no apparent impurity was introduced from the wet chemistry method. It should be mentioned that a layer of native Al₂O₃ always exists around the nano Al particles; however, probably due to its amorphous state, there were no corresponding diffraction peaks.

Fig. 3 XRD patterns of Al/CuO nanostructured energetic materials and the reaction products after thermal analysis.

The XPS survey spectrum of the Al/CuO nanostructured energetic materials is shown in Fig. 4a. All the main peaks can be indexed to Al 2p (76.1 eV), Al 2s (123.1 eV), C 1s (285.0 eV), O 1s (531.0 eV), Cu Auger (569.1 and 649.2 eV), and Cu 2p_3/2 (933.1 eV) electrons exclusively, demonstrating the high purity of the materials. The high-resolution spectrum of O 1s is shown in the inset of Fig. 4a. Chemical shifts caused by CuO (529.3 eV) and Al₂O₃ (531.2 eV) can be identified. EDS pattern is shown in Fig. 4b, in which peaks from elements Al, Cu, and O can be seen, corresponding well with the XRD and XPS results. The peak of Au was from the gold coating for SEM imaging.

Fig. 4 (a) XPS survey spectrum (the inset showing O 1s high-resolution spectrum) and (b) EDS pattern of Al/CuO nanostructured energetic materials.

3.2. Heat release characteristics

Fig. 5a shows the DTA results of pure CuO (under the heating rate of 40 K min⁻¹) and the Al/CuO nanostructured energetic materials (under the heating rates of 5 K min⁻¹, 10 K min⁻¹, 20 K min⁻¹, and 40 K min⁻¹, respectively). Pure CuO was stable until the temperature reached around 900 °C where an endothermic peak arose. The melting point of CuO is around 1326 °C, and therefore the endothermic peak was attributed to the decomposition of CuO. The curves of Al/CuO nanostructured energetic materials showed a similar trend. Two exothermic peaks were found, between which was the characteristic endothermic peak of melting Al. The peak temperatures T_p of the first set of exothermic peaks were 545.8 °C, 561.7 °C, 580.9 °C, and 597.6 °C, respectively, corresponding to the increasing heating rates. The endothermic peak temperature was around 657 °C, which is lower than that of bulk Al due to the nano-sized effect. The endothermic peak occurring after 850 °C was probably due to the decomposition of excessive CuO (referring to Fig. 3). Also due to fuel deficiency, the second exothermic peak was not as intense as those reported in literature.^7–9 However, it is not a problem as the stoichiometric ratio can readily be adjusted and the main interest of the current study is the structural design.

Fig. 5 (a) DTA results of pure CuO and Al/CuO nanostructured energetic materials. (b) A plot of ln(β/T_p²) versus T_p for the calculation of apparent activation energy.

The first exothermic peak arose before nano Al melts, which means that it is based on solid state reaction, and thus it can best reflect the effect of structural design on the heat release characteristics. By using a Kissinger analysis, we plotted ln(β/T_p²) versus T_p as shown in Fig. 5b, and the apparent activation energy for this exothermic peak was calculated to be 226.9 kJ mol⁻¹. Compared with that (250 kJ mol⁻¹) reported for randomly mixing counterpart,¹⁰ the apparent activation energy was reduced by about 10%, which is believed to be attributed to the improved mixing uniformity and enhanced interfacial contact resulting from the structural design. It was reported that apart from reduced activation energy, heat of reaction and ignition characteristics were also positively affected by the structural design.^9,11 However, it certainly does not mean that the structural feature prevails over the nano-sized effect in terms of the energetic characteristics of energetic materials. Apparent activation energy as high as 280.8 kJ mol⁻¹ was reported for Al/CuO multilayer foils with the bilayer thickness of 1 μm,¹⁶ although the mixing uniformity and interfacial contact were optimized. Nano-sized effect and structural design are both important, and that is why the study of nanostructured energetic materials is stimulated. It also deserves attention that the overall energetic properties are not completely comparable to those from vacuum deposition; however, this method triumphs in its low cost and simplicity. On the other hand, the method enables more uniform distribution of reactants when compared with physical mixing. Therefore, the method points out a new direction that can be explored more to advance the development of nanostructured energetic materials.

4. Conclusions

Al/CuO nanostructured energetic materials were prepared by the combination of a solution chemistry method and the electrophoretic deposition. The method circumvented the use of expensive vacuum deposition system and reduced the heat treatment temperature to only 180 °C. CuO nanostructures formed a skeleton and thus improved the mixing uniformity with the following electrophoretically deposited nano Al particles, which ultimately led to the reduced apparent activation energy for the solid state exothermic reaction, demonstrating the superior structural design in this study.

Acknowledgements

This work was sponsored by Qing Lan Project and supported by “the Fundamental Research Funds for the Central Universities”, No. 30916011315.

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