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Nanothermite of Al nanoparticles and three-dimensionally ordered macroporous CuO: Mechanistic insight into oxidation during thermite reaction
Combustion and Flame 189 (2018) 87–91
Contents lists available at ScienceDirect
Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Brief Communications
Nanothermite of Al nanoparticles and three-dimensionally ordered macroporous CuO: Mechanistic insight into oxidation during thermite reaction Do Joong Shin, Whi Dong Kim, Seokwon Lee, Doh C. Lee∗ Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 July 2017 Revised 16 September 2017 Accepted 11 October 2017
Keywords: Nanothermite Three-dimensionally ordered macroporous (3DOM) Uniform dispersion Al/CuO composite Accumulation factor
a b s t r a c t We report the synthesis and characterization of thermite composites consisting of Al nanoparticles and three dimensionally-ordered macroporous (3DOM) CuO. The pores of 3DOM CuO have size ranging between 190 nm and 320 nm and size dispersion lower than 10%, while 70 nm Al particles we used in this study are dispersed uniformly over the entire composite structures. Both the size uniformity and homogeneous mixing enable quantitative correlation between structures and thermite reaction characteristics. Ignition of the thermite composites in a closed chamber initiates thermite reactions, and the combustion kinetics is recorded in terms of the transient pressure changes. Contrary to a premise that small CuO pores would result in mixing with Al nanoparticles at a smaller length scale and hence higher pressurization rate, 3DOM CuO with pore size smaller than 240 nm exhibits gradually lower pressurization rate as pore size decreases. It turns out that pressurization rate has the highest value when the pore size of CuO is about 240 nm. The size dependence indicates that two different pathways, solid-state and gaseous diffusion, account for oxygen transfer from CuO to Al in the thermite composites. With the pore size of CuO larger than 240 nm, gas-phase diffusion predominates and pressurization rate increases as the size of the pores decreases. On the other hand, at small length scale, i.e., with CuO pore size smaller than 240 nm, condensed-phase diffusion is becoming a visibly more influential factor, reversing the size dependence. The size-dependence of the pressurization rate from thermite composites of Al nanoparticles and geometry-controlled 3DOM CuO reveals that the thermite reaction has the highest combustion rate at the smallest length scale where the gaseous diffusion still surpasses condensed-phase diffusion. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Thermite composites are mixture of metal (e.g. Al) and metal oxide (CuO, Fe2 O3 , Mn2 O3 , etc.) powder [1,2], where oxygen transfers from metal oxide to metal particles which undergo rapid oxidation. Because of the relatively high enthalpy of combustion, thermite reactions have been known to release high energy, putting the composite in the use for welding [3]. The high volumetric energy density of thermites (e.g., 3.9 kcal/cm3 in Al/Fe2 O3 thermite vs 1.0 kcal/cm3 in TNT) creates a vast opportunity space for their use also as explosives and propellants [4]. Recent progress in the synthesis of nanoparticles (NPs) of Al and metal oxide has fueled the study of nanoscale particle composites, in which oxygen transfer is expected to be fast by virtue of the high surface-to-volume ratio.
∗
Corresponding author. E-mail address: [email protected] (D.C. Lee).
Despite the promise of using NPs [5–7], the lack of understanding on the optimal length scale at the nanometer regime has impeded the progress on nanothermite. Several approaches have been proposed in the way of studying the effect of mixing on reactivity [8], with most of them relying on mixing free-floating oxidizer and fuel NPs on various length scales. The approaches include powder mixing [9], arrested reactive milling [10,11], ultrasonication [12], sol–gel [7], bipolar coagulation [13] and electrospray [6] methods. Long-range mixing with controlled homogeneity stands to be one of the most pressing needs in the nanothermites, as NP mixture ends up into aggregation [14,15], again hurting the chance to quantify the oxygen transfer dynamics in the nanometer scale. Three-dimensionally ordered macroporous (3DOM) metal oxide with controlled pore size can be a solution to the problem of homogeneous mixing at controllable length scale. By incorporating metal NPs into the size-controlled and periodic pores of 3DOM metal oxide, one may expect to realize predictable and uniform
https://doi.org/10.1016/j.combustflame.2017.10.018 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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Fig. 1. SEM images of three-dimensionally ordered macroporous copper oxide (3DOM-CuO). The average diameters of pore and internal pore, which is formed by interconnection of PMMA nanospheres, are: (A) 322.7 nm, 128.5 nm; (B) 294.0 nm, 112.3 nm; (C) 275.1 nm, 109.1 nm; (D) 242.1 nm, 100 nm; (E) 232.7 nm, 99.3 nm; and (F) 192.3 nm, 81.9 nm, respectively.
mixing throughout the three-dimensional space [16,17]. The controllability of the pore size endows the controlled mixing of fuel and oxidizer in a designed dimension, enabling the systematic investigation of structure-reactivity relationship in the nanocomposites. In this study, we synthesize 3DOM CuO with uniform pore size, which ranges from 190 nm to 320 nm. Nanothermite composites are prepared by filling the pores of 3DOM CuO with Al NPs under sonication. Reactivity of thermite mixtures is investigated by pressure cell test as increasing the pore diameter at inert condition. Elemental mapping images of products after combustion provide clue on reaction mechanism of each composites. Furthermore, the mechanism of the thermite reaction was analyzed by the prediction of oxygen molecules by introducing the accumulation factor. This mechanistic insight can propose efficient length scale in nanothermite mixture for better performance.
2. Experimental Polymethylmethacrylate (PMMA) spheres were prepared via previously reported procedures [18]. Cu precursors were introduced into assembled PMMA spheres and underwent double heating treatment at 300 °C, which removes the PMMA template, and at 550 °C, which crystallizes CuO. Each step is maintained for 4 h and the heating rate is 2.5 °C/min. Al NPs and 3DOM CuO (equivalence ratio, φ = 2.1) dispersed in EtOH were mixed via ultrasonication for 20 min. Then, 10 mg of dried mixture of Al NPs and 3DOM CuO was located in the center of the pressure cell as a loose powder form. Before sealing the pressure cell whose inside volume is 75 mL, Ar gas flushed residual air out of the cell for 5 min with a flow rate of 5 L/min. By applying a voltage, the heated tungsten coil induced the combustion of thermite composite. The piezoelectric sensor (PCB Piezotronics, Model No. 113B03) detected generated pressure change in the combustion reaction. This electric signal was amplified by signal amplifier (PCB Piezotronics, Model No. 480C02) and recorded at an oscilloscope (Tektronix, TDS 2012B). Further details are described in supplementary material part 1.
3. Results and discussion Figure 1 shows SEM images of six different 3DOM CuO structures, whose pore diameter and window size have size ranging from 190 nm and 82 nm to 320 nm and 128.5 nm. 3DOM CuO structures were prepared with PMMA microspheres, whose removal leaves the network of pores. Consequently, the size of the pores and internal pores can be tuned by using size-controlled PMMA colloids. The macroporous CuO crystals exhibit long-range ordering of the pores and crystallinity, as shown in Figs. S2 and S3. To examine the mixing between Al and CuO, we prepared three samples: (i) mixture of nanoparticles, (ii) nanoparticles in macropores and (iii) nanoparticles in crushed macroporous structure. Figure 2 shows representative elemental mapping images using energy dispersive X-ray spectroscopy on the samples. The mixture of Al and CuO NPs gives rise to relatively uniform dispersion of Al and CuO; yet, the homogeneity in nanometer length scale could be hampered by immediate aggregation and phase separation between the NPs, as evidenced in previous accounts (Fig. 2A) [19]. On the other hand, the composite of Al NPs and 3DOM CuO shows strikingly uniform distribution of Al and CuO across the mixture (Fig. 2B), indicating that Al NPs diffuse into the macropores of the CuO template, as shown in the Fig. S4. The beauty of the homogeneity in conjunction with controlled size of CuO pores is that the effect of length scale of Al and CuO on the reaction can be investigated under the control which would not be available on the case of NP mixtures. The importance of the macropores in the context of uniform mixing is highlighted in a designed experiment where Al NPs were mixed with 3DOM CuO already collapsed after excessive sonication (Fig. 2C). It turns out that Al NPs and CuO do not exhibit blending in the case of collapsed 3DOM CuO. Conversely, uncollapsed 3DOM CuO provides a channel to uniform dispersion of fuel and oxidizer, which results in controllably tunable distance between Al NPs and 3DOM CuO. Using 3DOM CuO for the composites enables the systematical analysis while minimizing sampleto-sample variation. Since nanothermite composites would show the highest pressurization rate when Al and CuO are mixed in offstoichiometric, Al-rich composition [20], our composites were assembled with equivalence ratio φ = 2.1, where the equivalence ratio is defined as the molar ratio of fuel to oxidizer relative to the
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Fig. 2. Schematic and elemental mapping images of composites of Al NPs and CuO: (A) Al NPs and CuO NPs, (B) Al NPs and 3DOM CuO, (C) Al NPs mixed after 3DOM CuO was sonicated for 5 h. Al and CuO NPs have average diameters of 76.8 nm and 37.8 nm, respectively. 3DOM CuO has 241.7 nm of pores and 100 nm of internal pores on average.
Fig. 3. (A) Pressurization rate measured by combusting the mixture of Al NPs and 3DOM CuO plotted against pore diameter of 3DOM CuO. Thermite reaction was carried out in a sealed pressure cell filled with argon. High-angle annular dark field (HAADF) STEM images and EDX elemental mapping images of reaction product using 3DOM CuO of pore size, (B) (blue circle) 192.3 nm and (C) (red circle) 294.0 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
stoichiometry. We analyzed the reactivity of thermite composites by preparing the mixture and Al NPs and 3DOM CuO in a pressure cell and measuring the combustion kinetics after the composite is ignited. Figure 3 shows pressurization rates of combustion from the composites of Al NPs and 3DOM CuO of varying pore diameters. At relatively small pores (180–240 nm), the rate of pressurization increases with increasing pore size, whereas, with 3DOM CuO of large pores (>240 nm), the pressurization rate decreases as the pore gets bigger. The different size-dependence leads us to surmise that the reaction mechanism would differ in the two different size regimes. To highlight the contrast, we took two samples showing similar pressurization rate yet in different size regime. Figure 3B and C shows STEM images of the composites after they undergo the combustion. In the case of smaller CuO pores, Al and Cu are likely to have undergone interdiffusion, a probable outcome of condensed-phase oxidation (Fig. 3B) [21]. On the other hand,
Fig. 3C shows the formation of small Cu particles (∼4 nm), implying that combustion reaction transpired via convective propagation [20]. The striking contrast between the combustion products brings our attention to a question, ‘Where are oxygen molecules located during the thermite reaction?’ Accumulation factor (A) is a dimensionless number indicating whether the generated oxygen molecules remain in the pores [22,23]. The accumulation factor is obtained by comparing the production rate and escape rate of oxygen molecules in the composite:
A = 3.75×
μ R4 τ p × r 4 × P
where μ is viscosity of oxygen gas, τ p is a time scale of gas production, P is a pressure difference, r is a radius of internal pore and R is a radius of pore in 3DOM CuO. Generated oxygen molecules accumulate in the 3DOM CuO pores when A > 1, and oxygen gas flows out of pores when A < 1. When corresponding
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Fig. 4. Schematic illustration of combustion reaction in the 3DOM CuO with different pore size.
values are plugged into the equation, R turns out to be 131 nm for A = 1, which is in relative vicinity of our experimental value of R = 120 nm, the pore radius where pressurization rate is the highest (Fig. 3A) (see supplementary material parts 2 and 3). Figure 4 summarizes the overall process in the reaction of our 3DOM thermite composites. At large pores, i.e., when A > 1, oxygen molecules are produced faster than they diffuse out of the pores, which keep the porous reaction medium relatively oxygenrich. Therefore, in the convective propagation regime, smaller CuO pores result in faster reaction due to higher rate of oxygen production [24,25]. When A < 1, the produced oxygen molecules are more likely to diffuse out than to remain in the pores. As a result, the condensed-phase reaction of Al with Cu2 O predominates. As such, the pressurization rate increases with the increasing pore size because the gaseous oxygen molecules become more readily available at larger pore in this size regime [26]. Differential scanning calorimetry analysis using 3DOM thermite composites in this condensed phase oxidation regime reveals that size dependence of calculated activation energy coincides with that of experimentally measured pressurization rates, interpreted as an evidence that the reactivity is dictated by solid-state diffusion in this regime (Details are described in supplementary material part 4.). It has been widely proclaimed that nanothermite would give rise to uniform mixing of fuel and oxidizer particles and hence improved reactivity. However, our investigation reveals that the uniform dispersion of Al and CuO would lead to efficient combustion only at certain length scale. The dimension of mixing could be controlled by virtue of macroporous CuO templates, enabling us to conclude that thermite reaction is enhanced at a size regime where oxidation occurs via convective propagation as opposed to condensed-phase diffusion. This study suggests that for improving the performance of nanothermite, decreasing length scale of uniform dispersion has a limit hence considers additives in the way of capturing oxygen molecules during the reaction [27,28].
Acknowledgments This work was supported by the Agency for Defense Development of Korea (grant no. 13-70-05-04). We also acknowledge financial support for this research from the National Research Foundation (NRF) grants funded by the Korean government (NRF-20110030256 and NRF-2017R1A2B2011066).
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2017.10. 018. References [1] A.S. Rogachev, A.S. Mukasyan, Combustion of heterogeneous nanostructural systems (review), Combust. Explos. Shock Waves 46 (2010) 243–266. [2] E.L. Dreizin, Metal-based reactive nanomaterials, Prog. Energy Combust. Sci. 35 (2009) 141–167. [3] L.C. Schroeder, D.R. Poirier, The mechanical-properties of thermite welds in premium alloy rails, Mater. Sci. Eng. 63 (1984) 1–21. [4] J.D. Ferguson, K.J. Buechler, A.W. Weimer, S.M. George, SnO2 atomic layer deposition on ZrO2 and Al nanoparticles: pathway to enhanced thermite materials, Powder Technol. 156 (2005) 154–163. [5] K. Park, D. Lee, A. Rai, D. Mukherjee, M.R. Zachariah, Size-resolved kinetic measurements of aluminum nanoparticle oxidation with single particle mass spectrometry, J. Phys. Chem. B 109 (2005) 7290–7299. [6] H.Y. Wang, G.Q. Jian, G.C. Egan, M.R. Zachariah, Assembly and reactive properties of Al/CuO based nanothermite microparticles, Combust. Flame 161 (2014) 2203–2208. [7] T.M. Tillotson, A.E. Gash, R.L. Simpson, L.W. Hrubesh, J.H. Satcher, J.F. Poco, Nanostructured energetic materials using sol-gel methodologies, J. Non-Cryst. Solids 285 (2001) 338–345. [8] I. Monk, M. Schoenitz, R.J. Jacob, E.L. Dreizin, M.R. Zachariah, Combustion characteristics of stoichiometric Al-CuO nanocomposite thermites prepared by different methods, Combust. Sci. Technol. 189 (2017) 555–574. [9] C.D. Park, M. Mileham, L.J. van de Burgt, E.A. Muller, A.E. Stiegman, The effects of stoichiometry and sample density on combustion dynamics and initiation energy of Al/Fe2O3 metastable interstitial composites, J. Phys. Chem. C 114 (2010) 2814–2820. [10] S.M. Umbrajkar, M. Schoenitz, E.L. Dreizin, Control of structural refinement and composition in Al-MoO3 nanocomposites prepared by arrested reactive milling, Propellants Explos. Pyrotech. 31 (2006) 382–389. [11] M. Schoenitz, T.S. Ward, E.L. Dreizin, Fully dense nano-composite energetic powders prepared by arrested reactive milling, Proc. Combust. Inst. 30 (2005) 2071–2078. [12] K.B. Plantier, M.L. Pantoya, A.E. Gash, Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique, Combust. Flame 140 (2005) 299–309. [13] S.H. Kim, M.R. Zachariah, Enhancing the rate of energy release from nanoenergetic materials by electrostatically enhanced assembly, Adv. Mater. 16 (2004) 1821+. [14] S.M. Umbrajkar, M. Schoenitz, E.L. Dreizin, Exothermic reactions in Al-CuO nanocomposites, Thermochim. Acta 451 (2006) 34–43. [15] J.L. Cheng, H.H. Hng, H.Y. Ng, P.C. Soon, Y.W. Lee, Synthesis and characterization of self-assembled nanoenergetic Al-Fe2O3 thermite system, J. Phys. Chem. Solids 71 (2010) 90–94. [16] C. Yu, W. Zhang, R. Shen, X. Xu, J. Cheng, J. Ye, Z. Qin, Y. Chao, 3D ordered macroporous NiO/Al nanothermite film with significantly improved higher heat output, lower ignition temperature and less gas production, Mater. Des. 110 (2016) 304–310.
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Contents lists available at ScienceDirect
Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Brief Communications
Nanothermite of Al nanoparticles and three-dimensionally ordered macroporous CuO: Mechanistic insight into oxidation during thermite reaction Do Joong Shin, Whi Dong Kim, Seokwon Lee, Doh C. Lee∗ Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 July 2017 Revised 16 September 2017 Accepted 11 October 2017
Keywords: Nanothermite Three-dimensionally ordered macroporous (3DOM) Uniform dispersion Al/CuO composite Accumulation factor
a b s t r a c t We report the synthesis and characterization of thermite composites consisting of Al nanoparticles and three dimensionally-ordered macroporous (3DOM) CuO. The pores of 3DOM CuO have size ranging between 190 nm and 320 nm and size dispersion lower than 10%, while 70 nm Al particles we used in this study are dispersed uniformly over the entire composite structures. Both the size uniformity and homogeneous mixing enable quantitative correlation between structures and thermite reaction characteristics. Ignition of the thermite composites in a closed chamber initiates thermite reactions, and the combustion kinetics is recorded in terms of the transient pressure changes. Contrary to a premise that small CuO pores would result in mixing with Al nanoparticles at a smaller length scale and hence higher pressurization rate, 3DOM CuO with pore size smaller than 240 nm exhibits gradually lower pressurization rate as pore size decreases. It turns out that pressurization rate has the highest value when the pore size of CuO is about 240 nm. The size dependence indicates that two different pathways, solid-state and gaseous diffusion, account for oxygen transfer from CuO to Al in the thermite composites. With the pore size of CuO larger than 240 nm, gas-phase diffusion predominates and pressurization rate increases as the size of the pores decreases. On the other hand, at small length scale, i.e., with CuO pore size smaller than 240 nm, condensed-phase diffusion is becoming a visibly more influential factor, reversing the size dependence. The size-dependence of the pressurization rate from thermite composites of Al nanoparticles and geometry-controlled 3DOM CuO reveals that the thermite reaction has the highest combustion rate at the smallest length scale where the gaseous diffusion still surpasses condensed-phase diffusion. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Thermite composites are mixture of metal (e.g. Al) and metal oxide (CuO, Fe2 O3 , Mn2 O3 , etc.) powder [1,2], where oxygen transfers from metal oxide to metal particles which undergo rapid oxidation. Because of the relatively high enthalpy of combustion, thermite reactions have been known to release high energy, putting the composite in the use for welding [3]. The high volumetric energy density of thermites (e.g., 3.9 kcal/cm3 in Al/Fe2 O3 thermite vs 1.0 kcal/cm3 in TNT) creates a vast opportunity space for their use also as explosives and propellants [4]. Recent progress in the synthesis of nanoparticles (NPs) of Al and metal oxide has fueled the study of nanoscale particle composites, in which oxygen transfer is expected to be fast by virtue of the high surface-to-volume ratio.
∗
Corresponding author. E-mail address: [email protected] (D.C. Lee).
Despite the promise of using NPs [5–7], the lack of understanding on the optimal length scale at the nanometer regime has impeded the progress on nanothermite. Several approaches have been proposed in the way of studying the effect of mixing on reactivity [8], with most of them relying on mixing free-floating oxidizer and fuel NPs on various length scales. The approaches include powder mixing [9], arrested reactive milling [10,11], ultrasonication [12], sol–gel [7], bipolar coagulation [13] and electrospray [6] methods. Long-range mixing with controlled homogeneity stands to be one of the most pressing needs in the nanothermites, as NP mixture ends up into aggregation [14,15], again hurting the chance to quantify the oxygen transfer dynamics in the nanometer scale. Three-dimensionally ordered macroporous (3DOM) metal oxide with controlled pore size can be a solution to the problem of homogeneous mixing at controllable length scale. By incorporating metal NPs into the size-controlled and periodic pores of 3DOM metal oxide, one may expect to realize predictable and uniform
https://doi.org/10.1016/j.combustflame.2017.10.018 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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D.J. Shin et al. / Combustion and Flame 189 (2018) 87–91
Fig. 1. SEM images of three-dimensionally ordered macroporous copper oxide (3DOM-CuO). The average diameters of pore and internal pore, which is formed by interconnection of PMMA nanospheres, are: (A) 322.7 nm, 128.5 nm; (B) 294.0 nm, 112.3 nm; (C) 275.1 nm, 109.1 nm; (D) 242.1 nm, 100 nm; (E) 232.7 nm, 99.3 nm; and (F) 192.3 nm, 81.9 nm, respectively.
mixing throughout the three-dimensional space [16,17]. The controllability of the pore size endows the controlled mixing of fuel and oxidizer in a designed dimension, enabling the systematic investigation of structure-reactivity relationship in the nanocomposites. In this study, we synthesize 3DOM CuO with uniform pore size, which ranges from 190 nm to 320 nm. Nanothermite composites are prepared by filling the pores of 3DOM CuO with Al NPs under sonication. Reactivity of thermite mixtures is investigated by pressure cell test as increasing the pore diameter at inert condition. Elemental mapping images of products after combustion provide clue on reaction mechanism of each composites. Furthermore, the mechanism of the thermite reaction was analyzed by the prediction of oxygen molecules by introducing the accumulation factor. This mechanistic insight can propose efficient length scale in nanothermite mixture for better performance.
2. Experimental Polymethylmethacrylate (PMMA) spheres were prepared via previously reported procedures [18]. Cu precursors were introduced into assembled PMMA spheres and underwent double heating treatment at 300 °C, which removes the PMMA template, and at 550 °C, which crystallizes CuO. Each step is maintained for 4 h and the heating rate is 2.5 °C/min. Al NPs and 3DOM CuO (equivalence ratio, φ = 2.1) dispersed in EtOH were mixed via ultrasonication for 20 min. Then, 10 mg of dried mixture of Al NPs and 3DOM CuO was located in the center of the pressure cell as a loose powder form. Before sealing the pressure cell whose inside volume is 75 mL, Ar gas flushed residual air out of the cell for 5 min with a flow rate of 5 L/min. By applying a voltage, the heated tungsten coil induced the combustion of thermite composite. The piezoelectric sensor (PCB Piezotronics, Model No. 113B03) detected generated pressure change in the combustion reaction. This electric signal was amplified by signal amplifier (PCB Piezotronics, Model No. 480C02) and recorded at an oscilloscope (Tektronix, TDS 2012B). Further details are described in supplementary material part 1.
3. Results and discussion Figure 1 shows SEM images of six different 3DOM CuO structures, whose pore diameter and window size have size ranging from 190 nm and 82 nm to 320 nm and 128.5 nm. 3DOM CuO structures were prepared with PMMA microspheres, whose removal leaves the network of pores. Consequently, the size of the pores and internal pores can be tuned by using size-controlled PMMA colloids. The macroporous CuO crystals exhibit long-range ordering of the pores and crystallinity, as shown in Figs. S2 and S3. To examine the mixing between Al and CuO, we prepared three samples: (i) mixture of nanoparticles, (ii) nanoparticles in macropores and (iii) nanoparticles in crushed macroporous structure. Figure 2 shows representative elemental mapping images using energy dispersive X-ray spectroscopy on the samples. The mixture of Al and CuO NPs gives rise to relatively uniform dispersion of Al and CuO; yet, the homogeneity in nanometer length scale could be hampered by immediate aggregation and phase separation between the NPs, as evidenced in previous accounts (Fig. 2A) [19]. On the other hand, the composite of Al NPs and 3DOM CuO shows strikingly uniform distribution of Al and CuO across the mixture (Fig. 2B), indicating that Al NPs diffuse into the macropores of the CuO template, as shown in the Fig. S4. The beauty of the homogeneity in conjunction with controlled size of CuO pores is that the effect of length scale of Al and CuO on the reaction can be investigated under the control which would not be available on the case of NP mixtures. The importance of the macropores in the context of uniform mixing is highlighted in a designed experiment where Al NPs were mixed with 3DOM CuO already collapsed after excessive sonication (Fig. 2C). It turns out that Al NPs and CuO do not exhibit blending in the case of collapsed 3DOM CuO. Conversely, uncollapsed 3DOM CuO provides a channel to uniform dispersion of fuel and oxidizer, which results in controllably tunable distance between Al NPs and 3DOM CuO. Using 3DOM CuO for the composites enables the systematical analysis while minimizing sampleto-sample variation. Since nanothermite composites would show the highest pressurization rate when Al and CuO are mixed in offstoichiometric, Al-rich composition [20], our composites were assembled with equivalence ratio φ = 2.1, where the equivalence ratio is defined as the molar ratio of fuel to oxidizer relative to the
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Fig. 2. Schematic and elemental mapping images of composites of Al NPs and CuO: (A) Al NPs and CuO NPs, (B) Al NPs and 3DOM CuO, (C) Al NPs mixed after 3DOM CuO was sonicated for 5 h. Al and CuO NPs have average diameters of 76.8 nm and 37.8 nm, respectively. 3DOM CuO has 241.7 nm of pores and 100 nm of internal pores on average.
Fig. 3. (A) Pressurization rate measured by combusting the mixture of Al NPs and 3DOM CuO plotted against pore diameter of 3DOM CuO. Thermite reaction was carried out in a sealed pressure cell filled with argon. High-angle annular dark field (HAADF) STEM images and EDX elemental mapping images of reaction product using 3DOM CuO of pore size, (B) (blue circle) 192.3 nm and (C) (red circle) 294.0 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
stoichiometry. We analyzed the reactivity of thermite composites by preparing the mixture and Al NPs and 3DOM CuO in a pressure cell and measuring the combustion kinetics after the composite is ignited. Figure 3 shows pressurization rates of combustion from the composites of Al NPs and 3DOM CuO of varying pore diameters. At relatively small pores (180–240 nm), the rate of pressurization increases with increasing pore size, whereas, with 3DOM CuO of large pores (>240 nm), the pressurization rate decreases as the pore gets bigger. The different size-dependence leads us to surmise that the reaction mechanism would differ in the two different size regimes. To highlight the contrast, we took two samples showing similar pressurization rate yet in different size regime. Figure 3B and C shows STEM images of the composites after they undergo the combustion. In the case of smaller CuO pores, Al and Cu are likely to have undergone interdiffusion, a probable outcome of condensed-phase oxidation (Fig. 3B) [21]. On the other hand,
Fig. 3C shows the formation of small Cu particles (∼4 nm), implying that combustion reaction transpired via convective propagation [20]. The striking contrast between the combustion products brings our attention to a question, ‘Where are oxygen molecules located during the thermite reaction?’ Accumulation factor (A) is a dimensionless number indicating whether the generated oxygen molecules remain in the pores [22,23]. The accumulation factor is obtained by comparing the production rate and escape rate of oxygen molecules in the composite:
A = 3.75×
μ R4 τ p × r 4 × P
where μ is viscosity of oxygen gas, τ p is a time scale of gas production, P is a pressure difference, r is a radius of internal pore and R is a radius of pore in 3DOM CuO. Generated oxygen molecules accumulate in the 3DOM CuO pores when A > 1, and oxygen gas flows out of pores when A < 1. When corresponding
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Fig. 4. Schematic illustration of combustion reaction in the 3DOM CuO with different pore size.
values are plugged into the equation, R turns out to be 131 nm for A = 1, which is in relative vicinity of our experimental value of R = 120 nm, the pore radius where pressurization rate is the highest (Fig. 3A) (see supplementary material parts 2 and 3). Figure 4 summarizes the overall process in the reaction of our 3DOM thermite composites. At large pores, i.e., when A > 1, oxygen molecules are produced faster than they diffuse out of the pores, which keep the porous reaction medium relatively oxygenrich. Therefore, in the convective propagation regime, smaller CuO pores result in faster reaction due to higher rate of oxygen production [24,25]. When A < 1, the produced oxygen molecules are more likely to diffuse out than to remain in the pores. As a result, the condensed-phase reaction of Al with Cu2 O predominates. As such, the pressurization rate increases with the increasing pore size because the gaseous oxygen molecules become more readily available at larger pore in this size regime [26]. Differential scanning calorimetry analysis using 3DOM thermite composites in this condensed phase oxidation regime reveals that size dependence of calculated activation energy coincides with that of experimentally measured pressurization rates, interpreted as an evidence that the reactivity is dictated by solid-state diffusion in this regime (Details are described in supplementary material part 4.). It has been widely proclaimed that nanothermite would give rise to uniform mixing of fuel and oxidizer particles and hence improved reactivity. However, our investigation reveals that the uniform dispersion of Al and CuO would lead to efficient combustion only at certain length scale. The dimension of mixing could be controlled by virtue of macroporous CuO templates, enabling us to conclude that thermite reaction is enhanced at a size regime where oxidation occurs via convective propagation as opposed to condensed-phase diffusion. This study suggests that for improving the performance of nanothermite, decreasing length scale of uniform dispersion has a limit hence considers additives in the way of capturing oxygen molecules during the reaction [27,28].
Acknowledgments This work was supported by the Agency for Defense Development of Korea (grant no. 13-70-05-04). We also acknowledge financial support for this research from the National Research Foundation (NRF) grants funded by the Korean government (NRF-20110030256 and NRF-2017R1A2B2011066).
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