CuO thermite films on copper substrates

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Materials and Interfaces

Electrochemical synthesis of Al/CuO thermite films on copper substrates Bin Hu, Wenchao Zhang, Chunpei Yu, Zilong Zheng, Yajie Chen, Jiaxin Wang, Jingping Liu, Kefeng Ma, and Wei Ren Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Industrial & Engineering Chemistry Research

Electrochemical synthesis of Al/CuO thermite films on copper substrates Bin Hu1, Wenchao Zhang1,*, Chunpei Yu1, Zilong Zheng1, Yajie Chen1, Jiaxin Wang1, Jingping Liu1, Kefeng Ma2, Wei Ren3 1School

of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China

2School

of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China

3Shanxi

Applied Physics and Chemistry Research Institute, Xian, 710000, China

Abstract The CuO micro/nano-wires film has been prepared successfully on a Cu substrate using a combination of an electrochemical anodization and an annealing treatment. Its morphology and phase composition have been systemically examined. The vertically aligned CuO micro/nano-wires can form a skeleton to facilitate the embedment of Al nanoparticles so that their interfacial contacts will expand significantly to increase the total energy release. The CuO micro/nano-wires can be used to synthesize the Al/CuO composite thermite film using an electrophoretic deposition of Al nanoparticles. The thermal analyses reveal that the Al/CuO composite thermite film can release a heat output of 2009 J·g-1 at 10 min deposition. This work develops an effective protocol for the synthesis of the Al/CuO composite thermite film, which is highly compatible with microelectromechanical systems to realize functional energetic chips. Keywords: nanothermite; micro/nano-wires structure; electrochemical anodization; electrophoretic deposition Highlights 1. An electrochemical method is developed to prepare Al/CuO composite thermite films. 2. The morphology and exothermic properties can be regulated by an adjustment of deposition time. 3. The technology is completely compatible with microelectromechanical systems. 1. INTRODUCTION Nanoenergetic materials, due to their excellent spatial distribution and great interfacial contact 1

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between fuel and oxidizer, are so highly efficient as to show favorable performances in various ways including energy release, ignition, and other combustion properties.1-5 Nanothermites, as a class of nanoscale metal-based energetic materials, consist of metal fuel (e.g. Al, Mg) and oxidizer (e.g. CuO, Fe2O3, NiO, MnO2, WO3). Most probably benefitted from their rapid energy release rates and low ignition temperatures, nanothermites have been applied broadly in plenty of fields such as propulsions, thermal batteries, material syntheses, metal cuttings as well as blastings. 6-9 Within all kinds of thermites developed so far, the Al/CuO combustion system is of greatest interest, since the reaction between Al and CuO can theoretically provide tremendous heat of 4.06 kJ/g and high adiabatic temperature of 2843 K. Meanwhile, Al and Cu can be usually applied in a microelectronic

process;

it

is

very

convenient

to

achieve

Al/CuO

based

upon

microelectromechanical systems (MEMS).10-13 When the particle size of reactants is reduced to nanoscale and mixing degree close to uniformity between fuel and oxidizer, the average distance of mass transports will be shortened remarkably enough to greatly accelerate energy release rate.14-16 Therefore, in order to achieve the goal of maximizing combustion performances, structural designs are required to increase interfacial contacts. Recently, CuO nanostructures have attracted much attention in different dimensions and morphologies, whose designs lead to a variety of different microstructures such as 3D porous CuO nanostructures,17,18 3D flower- and sheet-like CuO nanostructures,19 2D CuO nanoleaves20 and 1D CuO nanowires (NWs).21 Among these morphologies, CuO NWs are of a high surface-to-volume ratio for a great many active sites to assemble Al nanoparticles (Al-NPs). Furthermore, the vertically well-distribution of NWs make it easy to form a uniform film, which is quite compatible with MEMS. Thus, CuO NWs have a good application prospect in nanothermites. Until now, the synthetic methods of CuO NWs have been extensively reported. One is a wetchemical way to obtain CuO NWs in a relatively facile condition.22,23 After copper compound NWs are formed in a mixed solution containing Cu2+ ions, they are immediately transformed to CuO NWs by annealing. Nevertheless, surfactants or polymers need to be employed to form copper compounds in the shape of NWs, but their introductions will unfortunately result in disorderly piled-up NWs and impurities in products.24 In order to avoid these disadvantages, CuO NWs can be dehydrated from Cu(OH)2 NWs precursors, which will be directly synthesized on Cu substrates as Cu2+ ions 2


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are yielded from the substrate surface under constant agitations.25-27 However, this preparation procedure is quite complicated and time-consuming. In contrast to wet-chemical methods, thermal oxidations are adopted more commonly to directly grow CuO NWs on Cu substrates at high temperature under air or O2 atmosphere.28,29 It is desirable that aligned CuO NWs can be formed in a simple way. However, such reaction requires high temperature and a long time. Furthermore, it is rather hard to alleviate the cracking or even peeling of CuO NWs from substrates over the temperature of 400 ℃.30 All these problems will awfully affect the properties and practical applications of CuO NWs. It is widely agreed that an electrochemical reaction is effective to synthesize well-defined nanostructures in a facile and controllable fashion.31,32 Wu et al.33 acquired dense and highly-ordered 1D Cu(OH)2 or CuO nanostructures using an electrochemical anodization of a Cu foil in an alkaline aqueous solution. CuO nanostructures, which are synthesized in this method, have shown potential applications in biosensors,34 super-hydrophobic surfaces35 and metal electrocatalysts.36 However, their application in nanothermite has been rarely reported. In addition, electrophoretic deposition (EPD) can be utilized to easily assemble nanoparticles (NPs) on an electrode with a controlled thickness in one-step under an applied electric field. It also exhibits several advantages including low-cost, binder-free, multicomponent deposition and wide adoptability for complex shapes.37,38 Therefore, EPD has gained wide attentions in the preparation of an energetic film of nanothermite. In this study, an Al/CuO composite thermite film has been formed directly on a Cu substrate by means of an electrochemical anodization after Al-NPs is subsequently loaded by EPD. All the process mainly depends on the role of electricity, so that it could be expected to provide advantages of simplicity, mild reaction conditions, low-cost, binder-free, time-save, and mass production. With respect to the influence of molar ratios on the morphology and heat release, the Al/CuO composite thermite films have been designed using three different deposition times. Finally, the prepared samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and preliminary laser ignition tests. 2. EXPERIMENTAL SECTION 2.1. Materials The chemical reagents of acetylacetone (≥99.0%), KOH (≥90.0%), anhydrous ethanol (≥99.7%), 3



and H2SO4 (95.0-98.0%), all of which were of analytical grade, were purchased from Sinopharm Chemical Reagent Co.Ltd. Platinum (Pt) electrode (≥99.9 %, 2×2 cm2) was obtained from Xuzhou Zhenghao Electronic Technology Co.Ltd. CuO nanoparticles (CuO-NPs, 100 nm – 200 nm, ≥99.9 %) and Al-NPs (50 nm, 78%, with the other 22% being the native Al2O3 shell) were supplied by Aladdin Industrial Corporation. Cu foils were used as substrates from Sinopharm Chemical Reagent Co.Ltd. 2.2. Preparations of a CuO micro/nano-wires film The Cu foil was precut into coupons (2×4 cm2). These Cu coupons were ultrasonically cleaned to remove surface impurities in acidic solution and ethanol, respectively, before they were dried in air. Anodization was carried out in a double-electrode system with a Pt electrode (2×2 cm2) as the counter electrode. The distance is set to be 2 cm between the Cu anode and Pt cathode. Secondly, the pretreated Cu foil, half of which was immersed into a 3 M KOH solution for a reaction area of 2×2 cm2, was electrochemically anodized at a constant current density of 3 mA · cm-2 at room temperature. After anodizing for 20 minutes, a blue film was formed on the Cu foil surface. The Cu substrate was then taken out from the solution, washed with deionized water, and dried in an oven at 50 ℃. Finally, a dark brown layer of CuO micro/nano-wires was obtained on the surface of Cu foil after the Cu(OH)2 film was heated up to 200 ℃, maintained for 3 h, and cooled to room temperature. 2.3. Preparations of an Al/CuO composite thermite film Al-NPs were deposited onto the surface of CuO micro/nano-wires by EPD. Typically, 20 mg of nano Al was added into a 50 mL mixed solution of ethanol and acetylacetone (1:1 in volume ratio), followed by 30 minutes’ ultrasonication. Graphite plate was used as the anode, while the Cu foil with CuO micro/nano-wires on top was used as the cathode. The distance between the two electrodes was fixed by 2 cm. EPD was performed as the two electrodes were vertically immersed in the suspension at a constant voltage of 60 V, before the cathode was removed out from the suspension. Finally, the Al/CuO composite thermite film was dried in an oven at 50 ℃. In order to not only explore an appropriate molar ratio of Al to CuO but also acquire the best morphology and heat release of Al/CuO composite thermite film, three different deposition times were designed to be 5 min, 10 min, 15 min, respectively. All the experiments were repeated at least three times to check reproducibility. 4


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For comparison purpose, CuO-NPs/Al-NPs thermite were also prepared by classic ultrasonication mixing. Stoichiometric mixtures of CuO-NPs and Al-NPs were added into hexane for continuing ultrasonic dispersion for 30 min. Then the suspension was dried in a vacuum oven at 70 ℃ under 1.5 × 104 Pa in order to obtain CuO-NPs/Al-NPs thermite. 2.4. Characterizations and thermal analyses The crystal structures of all the samples were analyzed by powder X-ray diffractometer (Bruker, D8 Advance) using Cu Kα radiation and nickel filter (λ = 0.15406). The morphologies were characterized using a field emission scanning electron microscope (Quanta 250F). Simultaneously, the molar ratios of Al to CuO were analyzed by energy dispersive X-ray spectrum (EDS) according to different deposition times. The differential scanning calorimetry (Mettler Toledo) was used to obtain the reaction heat of the Al/CuO composite thermite film from 100 to 900 ℃ at a heating rate of 10 ℃·min-1 under a 30 mL·min-1 Ar flow. To investigate the ignition performance, the Al/CuO composite thermite film on Cu substrates was directly ignited by a pulsed laser (Beamtech, DAWA350). The wavelength, pulse width and incident laser energy were 1064 nm, 100 ns and 167 mJ, respectively. The whole ignition process was recorded by a high-speed camera (Redlake Motion Xtra, HG-100K), which runs at 50000 frames per second. 3. RESULTS AND DISCUSSIONS 3.1. Phase characterizations The crystal structures and phase components of Cu(OH)2 film, CuO film and Al/CuO composite thermite film are identified by XRD characterizations (Figure 1). Three intense peaks, as marked by pentagram, are attributed to Cu substrates (JCPDS 04-0836). As shown in Figure 1b, the XRD pattern of Cu(OH)2 film exhibits seven characteristic peaks at 16.71°, 23.84°, 34.06°, 35.89°, 38.20°, 39.80°, 53.21°, which correspond to planes (020), (021), (002), (111), (022), (130), (150) of Cu(OH)2 (JCPDS 13-0420), respectively. After a thermal annealing, the characteristic peaks of CuO (JCPDS 44-0706) are identified at 35.24° and 38.47° in Figure 1c, which can be assigned to planes of (002) and (111), respectively. The formation of CuO micro/nano-wires can be simply represented by the following electrochemical and dehydration reactions:33,35 Cu(s) → Cu2 + +2e ― 5


(1)


Cu2 + +2OH ― (aq) → Cu(OH)2(s) Cu(OH)2(s) → CuO(s) + H2O

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(2) (3)

From Figure 1d, the diffraction peaks of Al are observed at 38.47° and 44.83°, which are resulted from planes of (111) and (200), respectively. Meanwhile, the characteristic peaks of CuO remain exactly in the same places without a peak from any impurity. It reveals that no reaction between Al and CuO has happened at all. All these evidences indicate the presence of both Al and CuO on the surface of Cu foil, that is to say, a successful preparation of an Al/CuO composite thermite film.

Figure 1. The XRD patterns of (a) Cu substrates (b) Cu(OH)2 film, (c) CuO film, (d) Al/CuO composite thermite film. 3.2. Morphology characterizations The morphologies of Cu(OH)2 film, CuO film and Al/CuO composite thermite film are examined by FE-SEM (Figure 2). In Figure 2a, the surface of the Cu foil is densely covered with a large-area of grass-like uniform Cu(OH)2 micro/nano-wires. Their magnified views (Figure 2b and c) show that these Cu(OH)2 micro/nano-wires are of radially protruding shapes with the average diameter of 450 nm. In combination with the results of XRD patterns, it indicates that Cu(OH)2 micro/nanowires can successfully grow in a crowded well-distribution on the Cu substrate by an 6


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electrochemical anodization of the Cu foil. After a thermal annealing, the transformation has been evidenced from Cu(OH)2 to CuO by the contrast of their XRD patterns. The SEM images present the CuO micro/nano-wires at various magnifications from Figure 2d to 2f. The micromorphology of CuO micro/nano-wires is bent to some extent, and their average diameter gets decreased up to 360 nm. This is due to the departure of H2O vapor that caused breakage of the Cu(OH)2 micro/nanowires to transform into smaller CuO micro/nano-wires with the preservation of the overall 1D morphology.39 What’s more important, it can be clearly seen that the CuO micro/nano-wires are still able to maintain their structural integrity and consistency without observed collapses. As seen from their corresponding cross-section view (Figure 2g), these CuO micro/nano-wires are arrayed approximately uprightly on the Cu substrate with an average height of 10 μm. The disorderly piledup micro/nano-wires form reticular structures, which will be taken as a skeleton to assemble AlNPs for an effective improvement in a microscopic homogeneity. The coverage density of Al can be easily controlled by an adjustment of the deposition time using EPD. The SEM images (Figure 3) of Al/CuO composite thermite films, which are prepared for three different deposition times, are observed from the surface view. In comparison with the SEM image of a CuO micro/nano-wires film (Figure 2d), it is quite obvious that their morphology and structure still remain as before after an EPD process. However, the agglomeration of Al-NPs cannot be avoid. With the deposition time increasing, the quantity of Al-NPs is also growing. When the deposition time extends up to 15 min, the micro/nano-wires are coated with Al so completely that it is hard to distinguish the micro/nano-wires apparently. As a result, Al has been filled with the CuO skeleton tightly, which would be of great benefits to both reinforce their interfacial contacts and improve reactivity. For a comparison, Figure 3d shows the SEM image of CuO-NPs/Al-NPs thermite. Due to their high surface energy, CuO-NPs are easy to agglomerate and the sizes of CuO-NPs are from nanoscales to macroscales. The spherical Al-NPs are randomly distributed among CuO-NPs. It reveals that the classic ultrasonication mixing cannot mix Al and CuO with a large degree of spatial homogeneity. Moreover, the inhomogeneity distribution causes poor interfacial contacts between CuO-NPs and Al-NPs.

7



Figure 2. The SEM images of (a, b, c) Cu(OH)2 film, (d, e, f) CuO film and (g) cross-section view of CuO film.

Figure 3. The SEM images of Al/CuO composite thermite film at different deposition times: (a) 5 min; (b) 10 min; (c) 15 min; (d) a typical CuO-NPs/Al-NPs thermite. 3.3. Elemental analyses 8


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As seen from the EDS patterns of Al/CuO composite thermite films with three different deposition times (Figure 4), the major elements are Al, Cu, and O. The impurity peak of Pt, which has been deposited onto samples to increase conductivity, could be ignored. The atomic percentages of Al at different deposition times of 5 min, 8 min and 11 min are 15.3%, 28.1%, 37.7%, respectively. Similarly, the atomic percentages of Cu are 43.6%, 37.7%, 32.7%. Only Al and Cu are taken into account for their molar ratios, as summarized in Table 1. With increasing deposition time, the molar ratio rises in linearity. The calculation results also show that the molar ratio at 10 min deposition is a little larger than the stoichiometry for a balanced reaction between Al to CuO (0.66), so Al is in excess already. Table 1. Elemental compositions of the samples for different deposition times Deposition time

Atomic percentage of the elements (%)

Molar ratio of Al to

(min)

Al

Cu

CuO

5

15.3

43.6

0.35

10

28.1

37.7

0.74

15

37.7

32.7

1.15

9



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Figure 4. The EDS spectra of Al/CuO composite thermite films with different deposition times, (a) 5 min; (b) 10 min; (c) 15 min. 3.4. Thermal analyses The thermite reaction of the Al/CuO system can be described in the following equation (4): 2Al + 3CuO → Al2O3 +3Cu ∆Hr = 4.06kJ ∙ g ―1

(4)

DSC technology is used to investigate the heat-release characteristics of the Al/CuO composite thermite film. Each DSC measurement on the samples was repeated for three times. The DSC 10


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patterns (Figure 5) of Al/CuO composite thermite film, which have been obtained for three different deposition times, display similar curves with two exothermic peaks. One small peak ranges from 200 ℃ to 300 ℃, while the other peak lies between 550 ℃ to 700 ℃. The reaction heats of the small exothermic peaks are 191 J·g-1, 233 J·g-1 and 225 J·g-1 for the deposition times of 5 min, 10 min and 15 min, respectively. The similar low-temperature exothermic peaks have been found for the Al/CuO nanothermite prepared by arrested reaction milling.40 Moreover, based on the Cabrera-Mott mechanism, a reaction model has been created to describe the early stages of redox reaction.41 This phenomenon was also found for the core-shell Al/CuO nanothermite prepared by combining a solution chemistry method and magnetron sputtering process.42 However, the reaction mechanism is still unclear in the low-temperature process. After that, a broad exothermic peak comes into being with a heat output of 1178 J·g-1 from the sample at 5 min deposition (Figure 5a), thus indicating insufficiency of Al, which is consistent with the deficient molar ratio of 0.35 between Al to CuO from EDS. When the deposition time is added to 10 min, it is noted that a small endothermic peak appears in the broad exothermic peak at 621 ℃ (Figure 5b) as a result of the melting of Al. The molar ratio of Al to CuO, which is calculated to be 0.75 by EDS, is larger than the theoretical value of 0.66. Since Al is easily oxidized during sample preparation, the oxidized part needs to be deducted from the reactants. The heat of reaction of this broad exothermic peak are 1776 J·g-1 with the onset temperature of 563 ℃. With the deposition time extending more to 15 min, the melting peak of Al becomes more obvious in Figure 5c. Its endothermic peak is of a lower temperature (590 ℃) and a smaller heat output (1311 J·g-1) than the one of 10 min deposition, since Al is considerably excess. The total reaction heats of the samples are tabulated in Table 2. The thermite reaction between Al and CuO at the low-temperature stage deserves a further study. The reaction for the primary exothermic peak can be divided into two steps, one before and the other after the melting of Al, which can be attributed to a solid-solid diffusion mechanism and a liquid-solid diffusion mechanism, respectively. Such behaviors are quite similar to the widely-reported Al/CuO system.43,44 In order to make a comparison, the DSC curve (Figure 5d) of the CuO-NPs/Al-NPs thermite has been obtained under the same test condition. One primary exothermic peak rises with a small endothermic peak at 635 ℃ indicating the melting of Al. The heat output of the CuO-NPs/Al-NPs 11



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thermite (904 J·g-1) is much lower than that of the Al/CuO composite thermite films at 10 min deposition, in spite of CuO-NPs and Al-NPs that were mixed in the stoichiometric ratio. Moreover, as shown in Table 2, the largest heat output of Al/CuO composite thermite film is 2009 J·g-1, which is much higher than those reported in the literature for Al/CuO systems.45-47 This further demonstrates that the excellent energy output performance of the Al/CuO composite thermite film could be ascribed to their micro/nano-wires structure, which may provide more space for Al-NPs integration and enable uniform distribution of reactants. This structure thus guarantees the compact interfacial contact and mass transmission between fuel and oxidizer.

Figure 5. DSC curve of Al/CuO composite thermite film at different deposition times, (a) 5 min; (b) 10 min; (c) 15 min; (d) DSC curve of CuO-NPs/Al-NPs thermite.

Table 2. The heat releases of Al/CuO systems reported in the literature and obtained in this work. Heat releases among different temperature ranges (J

(45)

Total heat releases (J·

·g-1)

Al/CuO systems 200-300 ℃

550-700 ℃

0

1454.5

12


g-1) 1454.5

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(46)

0

1186

1186

(47)

0

1140±50.6

1140±50.6

CuO-NPs/Al-NPs

0

904±50

904±50

191±10

1178±50

1369±60

233±15

1776±40

2009±55

225±5

1311±60

1536±65

Al/CuO (5 min deposition) Al/CuO (10 min deposition) Al/CuO (15 min deposition)

3.5. Laser ignition To evaluate its ignition performance, the Al/CuO composite thermite film at 10 min deposition time was selected to be tested by a Nd:YAG laser device, as recorded by a HG-1000K high-speed camera. The images of its laser ignition process are shown at the time interval of 40 μs (Figure 6). After the laser pulse excites the Al/CuO composite thermite film, a twinkling white cone-shaped flash, which is encircled by a pale blue halo, bursts out immediately. Then, the splattered products continue to burn in the form of a yellow flame. The maximum height of the flame is 5 mm at 120 μs and the whole ignition process lasts about 480 μs. All these data demonstrate that the Al/CuO composite film is ready to be successfully ignited by a laser pulse.

Figure 6. High-speed camera observations during a laser ignition test for the Al/CuO composite 13



thermite film at 10 min deposition. 4. CONCLUSIONS In summary, the Al/CuO composite thermite film has been synthesized successfully. The CuO film was prepared by an electrochemical anodization, followed by EPD to obtain the Al/CuO composite thermite film. The deposition time can be used to adjust for a quantity regulation of nano Al-NPs, which are coated onto the CuO micro/nano-wires. The maximum heat output of the nanothermite film is 2009 J·g-1 at the optimal deposition time of 10 min. The main purpose of this work is to provide a new method for the synthesis of the Al/CuO composite thermite film by means of an electrochemical method. Based upon its relatively mild preparation process and rapid preparation of high-density nanothermites, it can be convinced that this technique is beneficial to the integration of nanothermite films with MEMS to make functional energetic chips. AUTHOR INFORMATION Corresponding Author *Tel/Fax: +86-25-84315515; E-mail: [email protected]. ORCID Wenchao Zhang: 0000-0002-8752-2690 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was financially supported by the National Natural Science Foundation of China (Grant 51576101, 51676100), the Fundamental Research Funds for the Central Universities (Grant 30918015102) and Qing Lan Project. REFERENCES (1) Rossi, C.; Zhang, K.; Esteve, D.; Alphonse, P.; Tailhades, P.; Vahlas, C. Nanoenergetic materials for MEMS: a review. J. Microelectromech. Syst. 2007, 16, 919-931. (2) Rossi, C. Two decades of research on nano-energetic materials. Propellants, Explos., Pyrotech. 2014, 39, 323-327. (3) Dreizin, E. L. Metal-based reactive nanomaterials. Prog. Energy Combust. Sci. 2009, 35, 141-167. (4) Martirosyan, K. S. Nanoenergetic gas-generators: principles and applications. J. Mater. Chem. 2011, 21, 9400-9405. (5) Dai, J.; Wang, F.; Ru, C.; Xu, J.; Wang, C.; Zhang, W.; Ye, Y.; Shen, R. Ammonium perchlorate as an effective additive for enhancing the combustion and propulsion performance of Al/CuO 14


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CuO thermite films on copper substrates

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