Electrophoretic Fabrication of an Al–Co3O4 Reactive Nanocomposite

Article pubs.acs.org/IECR

Electrophoretic Fabrication of an Al−Co3O4 Reactive Nanocomposite Coating and Its Application in a Microignitor Daixiong Zhang*,† and Qing Xiang‡ †

Department of Chemistry and Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University, Beijing 100084, P. R. China ‡ College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China ABSTRACT: Because of its high energy density, an Al− Co3O4 reactive nanocomposite (Al−Co3O4 RNC) has attracted vast attention. In this study, Al−Co3O4 RNC was successfully prepared as a coating by an electrophoretic deposition (EPD) method. An ethanol−acetylacetone (1:1 in volume) mixture containing 0.25 mM nitric acid was employed as a suitable dispersion medium for EPD. Differential scanning calorimetry revealed that the obtained Al−Co3O4 was a highenergy coating with a maximum heat release of 2638 J·g−1. Moreover, an attempt at assembling an electrophoretic Al− Co3O4 RNC coating onto an electrothermal copper bridge was also made. A successful ignition test indicated that this novel Al−Co3O4 RNC coating has great potential for application in the microignitor field.

1. INTRODUCTION Nanothermites are types of reactive nanocomposites (RNCs) often composed of aluminum nanoparticles and metal oxide nanoparticles including Fe2O3, Co3O4, NiO, MoO3, etc.1 These RNCs have attracted great interest for combustion applications because of their higher energy contents in comparison with conventional RNCs.2−4 The large-particle results show that the combustion performance would be improved by promoting intimate mixing between the fuel and oxidizer via a decrease in the length scale.5−9 Because of high-energy release, fabricating RNC coatings to prepare microignitor devices for the purpose of improving the ignition behavior has been one topic of recent studies.10,11 As well-known coating technologies, magnetron sputtering,10,11 cold spray,12 and thermal evaporation13 have been widely investigated for preparing RNC coatings. Also, assembling RNC coatings onto electrothermal bridges for microignitor applications via these methods exhibited enhanced output energy compared with input energy, which was beneficial to successful ignition. Because of some advantages of simplicity, low cost, and wide adoptability for complex shapes, electrophoretic deposition (EPD) has exhibited great potential in the fast and economical assembly of nanoparticles into corresponding energetic coatings compared with other coating technologies.14−19 Al−Co3O4 is a promising RNC that has a high theoretical heat of reaction of 4232 J g−1 and an adiabatic reaction temperature of 3201 K.20 In this study, we performed the successful fabrication of an Al−Co3O4 RNC coating through a EPD process. The thermodynamic properties of an Al−Co3O4 RNC coating were also studied. Finally, we made an attempt at © XXXX American Chemical Society

assembling Al−Co3O4 RNC coatings onto electrothermal bridges for microignitor devices, and the ignition behavior of the assembled Al−Co3O4 RNC coating was also investigated.

2. EXPERIMENTAL SECTION Nano-cobalt oxide (nano-Co3O4; 30 nm, 99.5%; Aladdin Inc., China) and nano-aluminum (nano-Al; 50 nm, 99.9%; Aladdin Inc., China) were directly used without purification in the study. In energetic reactions, the equivalence ratio is defined as the actual fuel-to-oxidizer ratio divided by the fuel (aluminum)-to-oxidizer (metal oxide) ratio in a stoichiometric reaction: Φ=

(F/O)actual (F/O)stoich

The stoichiometric fuel-to-oxidizer ratio is 8:3 from the balanced reaction 8Al + 3Co3O4 → 4Al 2O3 + 9Co In order to explore the desirable dispersion media, the EPDs of nano-Al and nano-Co3O4 were separately performed using solutions with the same concentration before codeposition of nano-Al and nanoCo3O4. Ethanol−acetylacetone (1:1 in volume) was employed as the solvent for the suspension mixture, and nitric acid was used as an additive. For all EPD processes, the solid loading was 1 g·L−1. For the codeposition process, the equivalence ratio of nano-Al to nano-Co3O4 in a suspension was adjusted in the weighting samples. The suspensions were ultrasonicated for 20 min to break up agglomeration. Received: May 4, 2016 Revised: July 4, 2016 Accepted: July 12, 2016

A

DOI: 10.1021/acs.iecr.6b01702 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Before coating, stainless steel substrates having dimensions of 0.1 × 40 × 85 mm3 were ultrasonicated and cleaned in ethanol for 10 min. Cleaned and dried substrates were then used for EPD of an Al−Co3O4 RNC. The EPD was performed in a beaker with two electrodes vertically immersed in a suspension of 200 mL with a constant voltage of 100 V. Stainless sheet with a deposition area of 55 × 40 mm2 was used as both the anode and cathode. The distance between the anodic and cathodic electrodes was fixed at 1 cm. After deposition for 15 min, the cathode was removed from the suspension. The as-deposited coatings were dried in an oven at 80 °C for 1 h, and the deposit was weighed by an electric balance with an accuracy of 0.0001 g. The deposition weight per area (mg·cm−2) of the deposit was calculated by dividing the increased weight of the cathode after deposition by the deposition area. The phase composition of the coatings was measured with X-ray diffraction (XRD; 6000, Shimadzu, Japan). The morphology and element distribution of the coating were analyzed using focused-ionbeam scanning electron microscopy (FIB/SEM; Auriga, Zeiss, Germany) and energy-dispersive spectroscopy. Because of the insolubility of nano-Co3O4 in acidic media, the equivalence ratio in deposited RNC (ΦS) was measured by the following dissolution− precipitation−weighing method: after nano-Al was completely dissolved in weighted Al−Co3O4 RNC coatings with 5 M nitric acid, the insoluble nano-Co3O4 was dried and weighed, and the weight of nano-Al could be obtained. Therefore, the equivalence ratio in deposited RNC (ΦS) could also be calculated. The exothermic reaction of an Al−Co3O4 RNC coating was investigated by differential scanning calorimetry (DSC; STA449F3, NETZSCH, Germany). The DSC experiment was carried out at a temperature range from 100 to 1000 °C at a heating rate of 20 K· min−1 under a 99.999% argon flow. Al−Co3O4 RNC coatings were also electrophoretically assembled onto custom-made electrothermal copper bridges for microignitor devices. The copper bridge for the test consisted of a ceramic substrate having dimensions of 3.0 × 2.6 × 0.5 mm3 and a copper coating with a thickness of 30 μm. These electrothermal copper bridges with electrophoretic Al−Co3O4 RNC coatings were ignited by a constant current of 0.9 A supplied by direct current power.

Figure 1. Deposition weights of Al and Co3O4 as a function of the concentration of nitric acid (15 min).

EPD of both nano-Al and nano-Co3O4 and was employed for the preparation of the Al−Co3O4 RNC coating in the following work. A smooth Al−Co3O4 RNC coating fabricated with this dispersion media is shown in Figure 2, and the results indicate good coating characteristics.

3. RESULTS AND DISCUSSION 3.1. Optimization of Suspensions for EPD. As is wellknown, successful EPD mainly depends on suitable dispersion media to make particlea charged and electromigrate.21 For EPD of RNCs that are bicomposites, other qualifications for desirable dispersion media were that the surface of both aluminum and metal oxide nanoparticles could be charged with the same sign. First, an ethanol−acetylacetone mixture was employed as the solvent for EPD of nano-Al of 1 g·L−1; the obtained aluminum coating was poor, and even the stainless steel substrate could not be completely covered. Similar results occurred for EPD of nano-Co3O4 in the same solvent. Therefore, various amounts of nitric acid (0.025, 0.25, and 2.5 mM) were added to the suspension to promote EPD behavior for both nano-Al and nano-Co3O4 particles, and the corresponding influence of nitric acid on their deposition rate is shown in Figure 1. For both nano-Al and nano-Co3O4, the deposition weight increased with the nitric acid concentration, which means the surface charging for both nano-Al and nano-Co3O4 could be improved by nitric acid, and their deposition rates were enhanced accordingly. Moreover, when the concentration of nitric acid increased from 0.25 to 2.5 mM, the enhancement in the deposition rate for both nano-Al and nano-Co3O4 was relatively slight, which indicates that 2.5 mM nitric acid was redundant for their sufficient surface charging. Hence, an ethanol−acetylacetone mixture containing 0.25 mM nitric acid might be suitable for

Figure 2. Image of the coating of an Al−Co3O4 RNC prepared using EPD.

3.2. Characterization of an Al−Co3O4 RNC Coating. The XRD pattern of electrophoretic coating was completely indexed as Al (ICDD 85-1327) and Co3O4 (ICDD 42-1467) (Figure 3). The result indicates the successful deposition of an Al−Co3O4 coating, and the acetylacetone−ethanol mixture containing 0.25 mM nitric acid was a suitable dispersion medium for EPD of an Al−Co3O4 coating. A SEM image along with elemental mapping for an electrophoretic Al−Co3O4 RNC coating is shown in Figure 4. Elemental mapping indicates that both nano-Al and nanoCo3O4 were well mixed, except that a few relatively large particles may be formed from slight agglomeration of nano-Al particles. As a result, desirable compositional homogeneity in an Al−Co3O4 RNC coating could be achieved. The higher-magnified SEM images of raw nano-Al particles, nano-Co3O4 particles, and the obtained Al−Co3O4 RNC B

DOI: 10.1021/acs.iecr.6b01702 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 3. XRD patterns of nano-Al powder, nano-Co3O4 powder, and Al−Co3O4 RNC coating.

coating examined by FIB/SEM techniques are shown in Figure 5a−c. It can be clearly seen that the particle sizes of both nanoAl and nano-Co3O4 in the deposited coating were nanoscaled, and there was no obvious difference between the raw nanoparticles and nanoparticles in the deposited coating. This result means that the size and micromorphology of the nanoparticles would not be changed by the EPD process, which would greatly benefit the performance of an Al−Co3O4 RNC coating.5 The results of the DSC experiment of an Al−Co3O4 RNC coating with an equivalence ratio (Φs) of 1.0 are shown in Figure 6. The DSC curve shows that the heat release of an exothermic reaction was 2638 J·g−1. The heat energy value was calculated by integration of the area of the exothermic reaction peak in DSC. The considerable heat release of the exothermic reaction of Al−Co3O4 RNC means that the electrophoretic Al− Co3O4 RNC coating has excellent thermodynamic properties and would be of great help in enhancing the energy output in microignitor devices. 3.3. Assembling an Al−Co3O4 RNC Coating on an Electrothermal Bridge for Microignitor Application. With a high heat release of 2638 J·g−1, an Al−Co3O4 RNC coating may be employed for microignitor application for the purpose of improving the ignition behavior. Therefore, the ignition behavior for an electrothermal copper bridge with an electrophoretic Al−Co3O4 RNC coating (Figure 7) was also investigated.

Figure 5. Highly magnified SEM images of (a) a nano-Al powder, (b) a nano-Co3O4 powder, and (c) an Al−Co3O4 RNC coating.

Figure 4. SEM top image of an Al−Co3O4 RNC coating prepared by EPD. Also shown is an image with elemental maps of aluminum, cobalt, and oxygen. The white dotted line in the top view image corresponds to the elemental mapping region. C

DOI: 10.1021/acs.iecr.6b01702 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

coating had great potential for application in the microignitor field.

4. CONCLUSION With a EPD method, a smooth Al−Co3O4 RNC coating was successfully fabricated. The size and micromorphology of both nano-Al and nano-Co3O4 would not be changed by the EPD process, and this bicomposite coating was well mixed. A high heat release of 2638 J·g−1 was obtained for an Al−Co3O4 RNC coating with an equivalence ratio (Φs) of 1.0. The successful ignition for an electrothermal copper bridge assembled with our novel Al−Co3O4 RNC coating indicates great potential for application in the microignitor field.

■

AUTHOR INFORMATION

Corresponding Author

Figure 6. DSC curve of an Al−Co3O4 RNC with Φs of 1.0. The heating rate is 20 K·min−1.

*E-mail: [email protected] or [email protected]. edu.cn. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Wang, L. L.; Munir, Z. A.; Maximov, Y. M. Thermite Reactions: Their Utilization in the Synthesis and Processing of Materials. J. Mater. Sci. 1993, 28, 3693. (2) Zhang, K. L.; Chou, S. K.; Ang, S. S.; Tang, X. S. Multi-Physics System Modeling of a Pneumatic Micro Actuator. Sens. Actuators, A 2005, 122, 113. (3) Morris, C. J.; Mary, B.; Zakar, E.; Barron, S.; Fritz, G.; Knio, O.; Weihs, T. P.; Hodgin, R.; Wilkins, P.; May, C. Rapid Initiation of Reactions in Al/Ni Multilayers with Nanoscale Layering. J. Phys. Chem. Solids 2010, 71, 84. (4) Zhang, K. L.; Rossi, C.; Alphonse, P.; Tenailleau, C.; Cayez, S.; Chane-Ching, J. Y. Integrating Al with NiO Nano Honeycomb to Realize an Energetic Material on Silicon Substrate. Appl. Phys. A: Mater. Sci. Process. 2009, 94, 957. (5) Jian, G. Q.; Feng, J. Y.; Jacob, R. J.; Egan, G. C.; Zachariah, M. R. Super-reactive Nanoenergetic Gas Generators Based on Periodate Salts. Angew. Chem., Int. Ed. 2013, 52, 9743. (6) Milosavljevic, M.; Stojanovic, N.; Perusko, D.; Timotijevic, B.; Toprek, D.; Kovac, J.; Drazic, G.; Jeynes, C. Ion Irradiation Induced Al-Ti Interaction in Nano-scaled Al/Ti Multilayers. Appl. Surf. Sci. 2012, 258, 2043. (7) Milosavljevic, M.; Toprek, D.; Obradovic, M.; Grce, A.; Perusko, D.; Drazic, G.; Kovac, J.; Homewood, K. P. Ion Irradiation Induced

Figure 7. Electrothermal copper ignitors assembled with electrophoretic Al−Co3O4 RNC coatings.

As shown in Figure 8a, with a trigger electric current of 0.9 A, the copper bridge without an Al−Co3O4 RNC coating became bright. In contrast, when an electric current was passed through the electrothermal copper bridge assembled with an electrophoretic Al−Co3O4 RNC coating, the heating of the copper bridge inspired a violent thermite reaction of the Al−Co3O4 RNC coating. Consequently, the energy output was significantly enhanced, which caused a bright flash (Figure 8b). This result indicates that the novel electrophoretic Al−Co3O4 RNC

Figure 8. Combustion behavior for an electrothermal copper bridge: (a) without an electrophoretic Al−Co3O4 coating; (b) with an electrophoretic Al−Co3O4 RNC coating. D

DOI: 10.1021/acs.iecr.6b01702 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Solid-State Amorphous Reaction in Ni/Ti Multilayers. Appl. Surf. Sci. 2013, 268, 516. (8) Sarkar, P.; Nicholson, P. S. Electrophoretic Deposition (EPD): Mechanisms, kinetics, and Application to Ceramics. J. Am. Ceram. Soc. 1996, 79, 1987. (9) Boccaccini, A. R.; Zhitomirsky, I. Application of Electrophoretic and Electrolytic Deposition Techniques in Ceramics Processing. Curr. Opin. Solid State Mater. Sci. 2002, 6, 251. (10) Tanaka, S.; Kondo, K.; Habu, H.; Itoh, A.; Watanabe, M.; Hori, K.; Esashi, M. Test of B/Ti Multilayer Reactive Igniters for a Micro Solid Rocket Array Thruster. Sens. Actuators, A 2008, 144, 361. (11) Yang, C.; Hu, Y.; Shen, R. Q.; Ye, Y. H.; Wang, S. X.; Hua, T. L. Fabrication and Performance Characterization of Al/Ni Multilayer Energetic Films. Appl. Phys. A: Mater. Sci. Process. 2014, 114, 459. (12) Bacciochini, A.; Radulescu, M. I.; Yandouzi, M.; Maines, G.; Lee, J. J.; Jodoin, B. Reactive Structural Materials Consolidated by Cold Spray: Al-CuO Thermite. Surf. Coat. Technol. 2013, 226, 60. (13) Zhang, K. L.; Rossi, C.; Ardila Rodriguez, G. A.; Tenailleau, C.; Alphonse, P. Development of a Nano-Al/CuO Based Energetic Material on Silicon Substrate. Appl. Phys. Lett. 2007, 91, 113117. (14) Hamadanian, M.; Sayahi, H.; Zolfaghari, A. R. Modified Multistep Electrophoretic Deposition of TiO2 Nanoparticles to Prepare High Quality Thin Films for Dye-Sensitized Solar Cell. J. Mater. Sci. 2012, 47, 5845. (15) Vazquez, A.; Lopez, I. A.; Gomez, I. Growth Mechanism of One-dimensional Zinc Sulfide Nanostructures through Electrophoretic Deposition. J. Mater. Sci. 2013, 48, 2701. (16) Sullivan, K. T.; Worsley, M. A.; Kuntz, J. D.; Gash, A. E. Electrophoretic Deposition of Binary Energetic Composites. Combust. Flame 2012, 159, 2210. (17) Sullivan, K. T.; Kuntz, J. D.; Gash, A. E. Electrophoretic Deposition and Mechanistic Studies of Nano-Al/CuO Thermites. J. Appl. Phys. 2012, 112, 024316. (18) Sullivan, K. T.; Zhu, C.; Tanaka, D. J.; Kuntz, J. D.; Duoss, E. B.; Gash, A. E. Electrophoretic Deposition of Thermites onto MicroEngineered Electrodes Prepared by Direct-Ink Writing. J. Phys. Chem. B 2013, 117, 1686. (19) Zhang, D. X.; Li, X. M.; Qin, B.; Lai, C.; Guo, X. G. Electrophoretic Deposition and Characterization of Nano-Al/Fe2O3 Thermites. Mater. Lett. 2014, 120, 224. (20) Xu, D. G.; Yang, Y.; Cheng, H.; Li, Y. Y.; Zhang, K. L. Integration of Nano-Al with Co3O4 Nanorods to Realize Highexothermic Core-shell Nanoenergetic Materials on a Silicon Substrate. Combust. Flame 2012, 159, 2202. (21) Zhou, D. X.; Jian, G.; Hu, Y. X.; Zheng, Y. N.; Gong, S. P.; Liu, H. Electrophoretic Deposition of Multiferroic BaTiO3/CoFe2O4 Bilayer Films. Mater. Chem. Phys. 2011, 127, 316.

E

DOI: 10.1021/acs.iecr.6b01702 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX