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CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change
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Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change Kyuhyeon Lee a,1, Dahin Kim a,1, Jaewon Shim a, Sangbum Bae a, Do Joong Shin a, Benjamin E. Treml b, Jichang Yoo c, Tobias Hanrath d,∗, Whi Dong Kim a,∗, Doh C. Lee a,∗ a Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea b Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA c The 1st R&D Institute – 6, Agency for Defense Development(ADD), Daejeon, Republic of Korea d School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA
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Article history: Received 17 March 2015 Revised 12 July 2015 Accepted 13 July 2015 Available online xxx Keywords: Nanothermite Al/CuO composites Stoichiometry Convective propagation Heat transfer
a b s t r a c t We analyzed thermite reactions in composites comprised of Al and CuO nanoparticles (NPs) and investigated pressure change and pressurization rate as a function of composition. Although the stoichiometric ratio for the thermite reaction between Al and CuO is 2:3, our experiments show that the maximum pressurization rate occurs in Al-rich thermite composites. Transmission electron microscopy analysis of the reaction products revealed that a considerable number of Al NPs are coated with Cu layer. We attribute the formation of Cu layer on the surface of Al NPs during the reaction to the off-stoichiometry, since the Cu layer renders Al NPs relatively less reactive. Based on our analysis of the experimental data, we propose a mechanism in which gasified Cu grows into Cu layer, as the gas-phase Cu species travels from reaction sites to unreacted Al NP surface. Analysis on characteristic timescale of heat and mass transfer reveals that condensation of Cu on Al NPs occurs faster than the overall thermite reaction. © 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Thermites are mixtures of metal and oxide particles in which transfer of oxygen from the oxide to metal results in a highly exothermic reaction [1]. The high enthalpy of combustion in metal makes thermites energetically dense, which has been the motivation for their use in numerous military and industrial applications, such as propellants, explosives, and pyrotechnics [2]. Recently, nanothermites, mixtures of nanometer sized fuel and oxidizer particles, have received great attention as their large surface area and small interparticle distance help enhance the overall combustion properties [3– 5]. Nanothermites are reported to combust to produce larger pressure change than micron-sized thermite as a result of a decreased ignition delay time and increase in self-propagation rate. Among several types of energetic materials for the thermite reaction, Al–CuO nanothermites have been most intensively investigated,
∗
Corresponding authors. E-mail addresses: [email protected] (T. Hanrath), [email protected] (W.D. Kim), [email protected] (D.C. Lee). 1 These authors contributed equally to this work.
as the combination appears to yield exothermic metallic gas generation [6]. The stoichiometric reaction of Al–CuO composites for ideal combustion is 2Al + 3CuO → Al2 O3 + 3Cu, hence the theoretical Al:CuO molar ratio for complete combustion is 2:3. However, previous studies have reported that the highest propagation speed and pressurization rate occurs in an Al-rich environment even when substantial amount of Al components remain unreacted after combustion [7–9]. Whereas previous studies have alluded to the need for excess Al for a complete combustion, consensus on the mechanism underlying the non-stoichiometric optimum has not yet been established. One explanation for the off-stoichiometry is that the contact area between Al and CuO is not large enough for sufficient transfer of oxygen from CuO to Al, resulting in incomplete combustion of stoichiometric mixtures. An important consideration in Al–CuO nanothermites is that the high-surfae-to-volume ratio in nanometer-sized Al particles facilitates the transfer of oxygen and thus increases combustion rate [10]. However, even in the experiments with varying Al particle sizes, the maximum propagation rate of combustion always occurs when excess Al particles are present. Moreover, other experiments with CuO nanowires providing large interfacial contact area with Al still required Al particles in excess of the stoichiometric ratio [11]. Collectively, these prior reports suggest that the fundamental
http://dx.doi.org/10.1016/j.combustflame.2015.07.019 0010-2180/© 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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reason for the required excess Al (i.e., off-stoichiometry) is due to factors other than the contact area. Answering the fundamental question of “Why are more Al NPs needed in thermites than the stoichiometric amount despite an increase of contact area?” demands a microscopic investigation into what transpires at the surface and interface of Al and CuO particles. In this paper, we examine the evolution of the nano- and microstructure of Al/CuO nanothermites during the combustion and provide new insights into the deactivation of Al NPs relating to the requirement for excess Al. The deactivation accounts for the offstoichiometry at the maximum pressure change into an Al-rich composition. To confirm the effect of the ratio between CuO and Al on the thermite reaction, samples with various ratios from Al-rich to CuOrich were prepared by mixing Al and CuO NPs via sonication, and their combustion was tested in a sealed cell with a pressure change detector. All of the combustion products were analyzed via XRD and TEM, which revealed that thin Cu layers in the form of clusters, products of the thermite reaction, covered the surface of unreacted Al NPs. Microscopic analysis in this study highlights the need to design thermite reaction not only from the compositional point of view, but also in the context of controlling gas-phase intermediates to realize lightweight propellant and explosive. 2. Experimental 2.1. Materials The following chemicals were used without any additional treatment. Aluminum (Al) nanoparticles (NPs) with an average diameter of 95.5 nm and aluminum oxide (Al2 O3 ) shell with a thickness of 4.1 nm were purchased from Nano Technology, Inc. Active Al core comprises about 76.2 wt% with Al2 O3 shell thickness taken into consideration. Copper oxide (CuO) NPs with an average diameter of 37.8 nm were purchased from Sigma-Aldrich. Ethanol (EtOH, 99.5%) was purchased from Samchun Chemicals, Inc. 2.2. Al/CuO sample preparation Al/CuO composites were prepared through the following steps: (i) Al and CuO NPs were dispersed in 20 mL of EtOH; (ii) the suspension was sonicated for 5 min for homogeneous mixing of Al/CuO composites (Fig. S2 shows uniformly mixed Al NPs and CuO NPs in prepared samples.); and (iii) 1 mL of the sonicated mixture was transferred to a substrate and dried in an oven for 1 h at 70 °C. 2.3. Combustion cell test Al/CuO composites were brought into a combustion cell (“closed bomb”), in which a pressure chamber, an electric supplier, a pressure sensor (PCB Piezotronics, Model No. 113B03), a signal amplifier (PCB Piezotronics, Model No. 480C02), and an oscilloscope (Tektronix, TDS 2012B) were equipped. The pressure chamber had a sealed volume of 75 mL and a tungsten coil connected to an electric supplier. 10 mg of a prepared sample in loose-powder form was loaded into the chamber and the tungsten coil was heated by applying a voltage. The loaded samples were then ignited in the chamber, and generated pressure was measured by a pressure sensor. The signal transformed into a voltage output by a signal conditioner. Lastly, the voltage output was recorded by an oscilloscope. 2.4. Characterization Crystallographic information of samples before and after the thermite reaction was characterized using a powder X-ray diffractometer (XRD, Rigaku, D/MAX-2500 (40 kV)). Quantitative analysis from XRD patterns was carried out via the reference intensity ratio (RIR)
Fig. 1. Pressure change and pressurization rate of Al/CuO composites with various Al mass fractions measured by pressure cell chamber equipment when ignition reaction occurred. Dashed line presents stoichiometric ratio.
method [12]. Transmission electron microscopy (TEM, Philips, Tecnai F20 (200 kV)), elemental dispersive X-ray (EDX) mapping analysis, and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM, FEI company, Titan cubed G2 60−300 (200 kV)) were used to investigate the morphology of the ignition products.
3. Results and discussion In Al/CuO composites, one of the most studied thermite material combinations with high combustion enthalpy, CuO oxidizes Al and is reduced to elemental Cu:
2Al + 3CuO → Al2 O3 + 3Cu (H = −1208 kJ) Composites of Al and CuO nanoparticles (NPs), also termed nanothermites or metastable intermolecular composites (MICs), have shown high reaction rates, as large surface area in MICs makes the reaction sites abound [13]. One would expect that the reaction occurs at 2:3 Al:CuO molar ratio. In other words, the most vigorous combustion should occur at about 40% molar fraction of Al in the composites (or 18.4 wt% of Al) with smallest residual reactants. In this study, we prepared composites of Al and CuO NPs with Al mass fraction ranging from 9.9% to 49.9%. Pressure changes recorded upon ignition of the series of samples at different Al:CuO ratios in a sealed chamber are summarized in Fig. 1 and Table 1. Both pressure change and pressurization rate follow a similar trend, with a maximum at 32.4 wt% of Al, which agrees well with results previously reported in the literature [11,14]. Our results clearly show that the maximum pressure rise and pressurization rate occur for Al/CuO nanocomposites with excess Al. This correlation suggests that ‘fuel’ and ‘oxidizer’ need to be mixed at a proper ratio (in our study, Samples 4–6) for optimal exothermic reaction between Al and CuO. On the other hand, samples with excess CuO (Sample 1) or excess Al (Sample 7) show weak thermite reaction due to lack of oxidizer or fuel, respectively. Notably, the best performance of Al/CuO composite among our samples is achieved when more Al (Sample 4) is used than the theoretical stoichiometry dictates. (See Fig. 1) This is consistent with ignition results reported in previous studies [7,15– 17]. These studies conclude that a large amount of gas product is generated at a fuel-rich condition, and propagation is effective because generated gas enhances heat transfer [15–17].
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Table 1 Al mass fraction of prepared samples and resulting pressurization rate.
Al mass fraction (%) Pressurization rate (psi/ms)
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Sample 7
9.9 0.9
12.6 3.1
17.3 14.0
27.6 100.6
32.4 123.5
39.3 70.9
49.9 1.9
Fig. 3. TEM images of Al NPs in reactant (a, b) and in product (c, d). Al/CuO nanothermites with Al mass fraction of 32.4%. Fig. 2. XRD patterns of ignition products from Al/CuO composites with various Al mass fraction. Major peaks from Al, CuO, Cu, Cu2 O, Alx Cuy , and Al2 O3 are indexed.
We now turn to describe the underlying mechanism to explain why excess Al is needed in the most effective nanothermite reaction. One possible interpretation is that Al reacts with ambient air, so that excess Al is required to fully utilize atmospheric oxygen and solid CuO to obtain the largest pressure generation. To test this interpretation, we performed a control experiment with identical thermite reactions in a pressure chamber filled with Ar gas (See Fig. S3). Compared to experiments carried out in air, the Al–CuO ratio that yields the highest pressurization rate shifted slightly to lower Al mass fraction. The fact that the combustion gives rise to the maximum pressurization rate at Al-rich mixture even in Ar indicates that gaseous oxidizer is not a primary reason for the off-stoichiometry for the maximum pressurization rate. The results from these control experiments are consistent with literature reports, which showed that the rate of propagation of Al–CuO thermite reaction exhibits marginal increase even at a drastically higher oxygen concentration [18]. Moreover, Asay et al. reported that air does not affect combustion reaction of nanothermite materials with tube experiments at rough vacuum (3.3 Pa) [19]. Another interpretation of the off-stoichimetry requirement for Al/CuO nanothermites involves the incomplete mixing of fuel and oxidizer. Even though some literature reports suggest that sufficient mixing causes effective thermite reaction, the actual ratio of Al:CuO at which the highest pressurization rate is observed still remains to be on the Al-rich side [10,11]. From these studies, it is implied that insufficient mixing is not a comprehensive reason for the offstoichiometric ratio of Al and CuO for the maximum pressurization rate. To gain a deeper understanding in the role of micro- and nanostructure on the combustion of Al-rich nanocomposites, we analyzed XRD patterns of the products after ignition of the Al/CuO composites. XRD reveals the presence of several compounds including Al, CuO, Cu, Cu2 O, Al2 O3 and Alx Cuy alloys in the ignition products of Al/CuO composites (Fig. 2). Since imbalance of Al and CuO gives rise to unreacted species during the ignition, Al and CuO seem to be the major components in the products from the Al-rich (Sample 7) and CuO-rich (Samples 1 and 2) samples, respectively. In other words, large amount of remaining reactant after combustion results from insufficient Al oxidation due to imbalance between Al and CuO. We hypothesized that
the lower combustion performance of stoichiometric Al/CuO nanothermites results from ‘deactivated’ Al nanoparticles in the composite. To understand how the Al nanoparticles may be precluded from participating in the combustion, we must consider critical chemical and physical processes accompanying the reaction. Cu, a product resulting from reduction of CuO in Al/CuO thermite reaction, is a major product from the combustion reaction of the samples have relatively balanced ratio of Al:CuO. This indicates that the maximum pressure change and pressurization rate of Sample 5 result from almost complete thermite reaction with only a small fraction of unreacted species. Cu2 O is an intermediate product formed during decomposition of CuO with O2 release. Then, Cu2 O is decomposed into Cu and O2 completely under higher temperature [20]. Not only unreacted reactants, but also Cu2 O are the results of low-pressure generation ability in CuO-rich samples. We detected Alx Cuy alloys, Al2 Cu and Al4 Cu9 , in XRD patterns. Although the stoichiometric thermite reaction does not involve the formation of such alloys, it has been reported that Alx Cuy may form via solid-state diffusion of Al and Cu at a temperature ranging between 550 and 1000 K [21] and at an elevated temperature in the case of Al/CuO composites [6,7]. Despite these observations, the effect of Alx Cuy alloys on thermite reaction has been underexplored. Dreizen and co-workers reported that Alx Cuy alloys, which were formed during a ball milling step and during the cooling of combustion products, suppress the thermite reaction [7]. From this argument, we infer that the formation of Alx Cuy alloys during the thermite reaction requires more Al in samples than the stoichiometric ratio. However, in quantitative XRD analysis, Alx Cuy alloys accounted for only 6.4 wt% of the total combustion product in the case of sample 3 (Fig. S4). Since substantive portion of Al is about 1 wt% in 6.4 wt% of Al4 Cu9 alloy, the hypothesis that more Al fraction than stoichiometric ratio is needed because of the Al consumption for alloy formation cannot fully explain that why the best performance is observed at an Al-rich case. To better understand how Alx Cuy alloys affect the thermite reaction, we investigated the products of thermite reaction from TEM images. As shown in Fig. 3, Al NPs have rough surface capped with small particles, while surface is observed to be smooth before the ignition. The average diameter of attached particles (5.5 nm) is significantly smaller than that of starting materials, such as CuO NPs (∼37.8 nm) and Al NPs (∼95.5 nm). This disparity in particle size indicates that
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Fig. 4. (a) HAADF STEM and (b,c) elemental mapping images of Al NP in product from Al/CuO nanothermites with 32.4% of Al mass fraction which presents highest pressurization rate in combustion reaction. Scale bars are 50 nm. (d) Elemental line-scan as a function of position across the arrow line marked in (a).
that small particles are created during the reaction. We identified the small particles to be crystalline Cu by fast Fourier transform (FFT) analysis of TEM images with atomic-resolution, as their lattice spacings correspond to (111) and (200) plane of a cubic Cu crystal [22] (Fig. S6). We estimated the amount of produced Cu necessary to cover all of the unreacted Al NPs. From the calculation [23], the width of Cu layer (3.3 nm) estimated from TEM analysis is in striking agreement with that (3.5 nm) estimated from our XRD analysis (See Fig. S4). This result indicates that Al covered by Cu is a major product from the thermite reaction. We sought to understand the detailed composition of nanostructured reaction products and measured the elemental distribution using energy dispersive X-ray spectroscopy (EDX). EDX results further confirmed the formation of Cu layer on Al NPs. Figure 4 shows HAADF (Fig. 4a) and scanning EDX elemental mapping images (Fig. 4b and c) of a Al NP in the product.In addition, Fig. 4d shows an elemental linescan as a function of position across the arrow line marked in Fig. 4a. The respective colors in Fig. 4b and c represent Al and Cu elements. Al exists mainly at the Al core region, whereas most Cu signals are detected from the surface of Al NP. Now that both Al and Cu are observed at the outside of Al NPs, it is believed that Alx Cuy alloys exist at the outer layer. The linescan elemental mapping result clearly shows that substantial amount of Cu element is detected at the interface, compared to relatively small amount of Al outside the Al2 O3 shell. This implies that Al NPs are covered by Cu layer and small amount of Alx Cuy is formed at the interface between Al NPs and Cu layer by intermetallic diffusion of Al and Cu [21]. Also, the Alx Cuy alloy layer could slow down the outward diffusion of Al. From additional TEM analysis (Fig. S7), we confirmed that most of the Al exists inside the confine of the Cu shell. Therefore, we believe that deposited Cu layer effectively prevents not only Al2 O3 shell from spallation, but Al diffusion through the oxide shell and Cu layer, despite the very thin Cu layer (3.3 nm). Since crystalline Cu results from reduction-oxidation between Al and CuO, we expect that reduced Cu would be formed near the oxidized Al2 O3 as a consequence of the thermite reaction. However, as clearly illustrated in the results discussed above, the Cu layer is located on unreacted spherical Al NPs like Al/Cu core/shell structure. To elucidate how Cu layer is formed on unreacted Al NPs, we associate the propagation mechanism of nanothermite with temperature profile, which is previously studied [24–26]. We provide a detailed description of the temperature profile in the supplementary material. We schematically illustrate the proposed mechanism for the ‘passivation’ of Al NPs by Cu condensation in Fig. 5 which shows reaction steps and corresponding temperature profile near the reaction zones derived from heat equations. At the initial stage, Al/CuO
composites start to explode when an external heat source heats the sample above the ignition temperature. Then, the combustion front propagates to the neighboring area by heat transfer from the ignition point (reaction zone in Fig. 5). Almost concurrently, pre-heated area, which is heated to a temperature below ignition point, forms at a forward region by heat transfer. In the case of bulk or microthermite reaction, heat conduction is the predominant mechanism for propagation [16,17]. By contrast, convective mode is predominant rather than conduction and radiation according to several studies of nanothermites [17,19,27–30]. Since propagation of nano-thermite is preceded by mass transfer of gasified thermite materials, the mass transfer can also help dissipate heat via convection mode [30]. At the ignition point, we speculate that generated heat induces the gasification of reduced Cu. Experimental and theoretical studies have investigated the gasification process [15,31,32]. The gasified Cu is likely to travel toward unreacted neighboring area (e.g., pre-heated zone in Fig. Fig. 5). At this time, it is likely that hetero-nucleation of Cu gas rapidly occurred by heat exchange with unreacted Al NPs or surrounding air molecule [24]. Since Cu gas acts as a convection medium, we speculate that condensation of Cu occurs before the reaction propagation (See Fig. 5). Our hypothesis concerning the formation time to condense a Cu film is corroborated by TEM images shown in Fig. 6. In the case of hollow Al2 O3 shell, the surface is smooth and there is no significant trace of Cu species on its surface. Cu layer would be observed regardless of product types, if condensation of Cu occurred after the combustion. In our TEM analysis, Cu layer is observed only on the Al filled NPs. This contrast suggests that condensation of Cu occurs prior to propagation and then inhibits Al oxidation. Figure 7 schematically illustrates the Al–CuO NP reactions that involve gasified Cu species. Based on the XRD and TEM analysis and study of propagation mechanism, we conclude that Cu layer on Al NPs is formed before combustion propagation. TEM analysis reveals that hollow Al2 O3 shells result from thermal expansion of Al via either rupture of Al2 O3 shell [33–35] or diffusion of Al [36,37]. In the case of Al covered by Cu, we believe that the Cu layer not only suppresses the fracture of alumina shell, but also acts as buffer layer of Al diffusion by formation of alloys as shown in Fig. 4 and Fig. S7. Furthermore, exothermic reaction is not preceded between Cu layer and CuO or Al. Therefore, we suggest that formation of Cu layer on Al NPs can be one of the main reasons for efficient combustion at off-stoichiometry ratio of Al and CuO due to the wasted Al NPs by Cu layer. Notably, only a small fraction of Cu appears to be able to deactivate Al NPs, which is a key to the Al-richness at the peak combustion. Further investigation is underway to test possibility of shifting the peak position in Fig. 1 by controlling convection and condensation of gasified Cu species.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Fig. 5. Reaction steps and corresponding temperature profile near reaction zones. Preheating zone features temperature below ignition point, leading to condensation of gasified Cu species. Ti and Tc represent temperature of initiation and condensation.
Fig. 6. TEM images of hollow Al2 O3 shell with clean surface in product. Scale bars are 20 nm.
Fig. 7. Schematic illustration of Al/CuO thermite reaction steps. Condensation of gasified Cu species on Al NPs hampers reaction of Al NPs with neighboring CuO NPs.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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4. Conclusions In summary, we studied the thermite reaction in Al/CuO nanoparticle composites with varying Al and CuO ratios. Maximum pressure change and pressurization rate occurred in composites in which Al was present in excess compared to the stoichiometric ratio. The optimum was achieved with Al/CuO composites with excess Al (32.4 wt%) in reference to theoretical stoichiometry (18.4 wt%). TEM analysis of the reaction products revealed that unreacted Al NPs were covered by Cu layer. We interpret the need for excess Al and the Cu coating as an indication that the deposition of a thin film of Cu on the surface of Al renders the particle inactive in the reaction, which eventually decreases the maximum pressure and pressurization rate. Because of their high surface atom population, transformation and solid-state reaction in NPs often take different forms [38,39] and it is important to analyze how the reactions of nanoscale affect ensemble properties of composite materials. In particular, understanding the role of gasified intermediates in thermite reactions will help design the materials and reactors for propellants and explosives. Acknowledgments This work was supported by the Agency for Defense Development of Korea (Grant no. 13-70-05-04). Supplementary Materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2015.07.019. References [1] S. Bless, T.R. Rodney, K.A. Schroder, D.L. Willauer, D.E. Wilson, in: Google Patents: 2007. [2] E.L. Dreizin, Prog. Energy Combust. 35 (2009) 141–167. [3] M.L. Pantoya, J.J. Granier, J. Therm. Anal. Calorim. 85 (2006) 37–43. [4] M.L. Pantoya, V.I. Levitas, J.J. Granier, J.B. Henderson, J. Propul. Power 25 (2009) 465–470. [5] K.B. Plantier, M.L. Pantoya, A.E. Gash, Combust. Flame 140 (2005) 299–309.
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Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change Kyuhyeon Lee a,1, Dahin Kim a,1, Jaewon Shim a, Sangbum Bae a, Do Joong Shin a, Benjamin E. Treml b, Jichang Yoo c, Tobias Hanrath d,∗, Whi Dong Kim a,∗, Doh C. Lee a,∗ a Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea b Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA c The 1st R&D Institute – 6, Agency for Defense Development(ADD), Daejeon, Republic of Korea d School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA
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Article history: Received 17 March 2015 Revised 12 July 2015 Accepted 13 July 2015 Available online xxx Keywords: Nanothermite Al/CuO composites Stoichiometry Convective propagation Heat transfer
a b s t r a c t We analyzed thermite reactions in composites comprised of Al and CuO nanoparticles (NPs) and investigated pressure change and pressurization rate as a function of composition. Although the stoichiometric ratio for the thermite reaction between Al and CuO is 2:3, our experiments show that the maximum pressurization rate occurs in Al-rich thermite composites. Transmission electron microscopy analysis of the reaction products revealed that a considerable number of Al NPs are coated with Cu layer. We attribute the formation of Cu layer on the surface of Al NPs during the reaction to the off-stoichiometry, since the Cu layer renders Al NPs relatively less reactive. Based on our analysis of the experimental data, we propose a mechanism in which gasified Cu grows into Cu layer, as the gas-phase Cu species travels from reaction sites to unreacted Al NP surface. Analysis on characteristic timescale of heat and mass transfer reveals that condensation of Cu on Al NPs occurs faster than the overall thermite reaction. © 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Thermites are mixtures of metal and oxide particles in which transfer of oxygen from the oxide to metal results in a highly exothermic reaction [1]. The high enthalpy of combustion in metal makes thermites energetically dense, which has been the motivation for their use in numerous military and industrial applications, such as propellants, explosives, and pyrotechnics [2]. Recently, nanothermites, mixtures of nanometer sized fuel and oxidizer particles, have received great attention as their large surface area and small interparticle distance help enhance the overall combustion properties [3– 5]. Nanothermites are reported to combust to produce larger pressure change than micron-sized thermite as a result of a decreased ignition delay time and increase in self-propagation rate. Among several types of energetic materials for the thermite reaction, Al–CuO nanothermites have been most intensively investigated,
∗
Corresponding authors. E-mail addresses: [email protected] (T. Hanrath), [email protected] (W.D. Kim), [email protected] (D.C. Lee). 1 These authors contributed equally to this work.
as the combination appears to yield exothermic metallic gas generation [6]. The stoichiometric reaction of Al–CuO composites for ideal combustion is 2Al + 3CuO → Al2 O3 + 3Cu, hence the theoretical Al:CuO molar ratio for complete combustion is 2:3. However, previous studies have reported that the highest propagation speed and pressurization rate occurs in an Al-rich environment even when substantial amount of Al components remain unreacted after combustion [7–9]. Whereas previous studies have alluded to the need for excess Al for a complete combustion, consensus on the mechanism underlying the non-stoichiometric optimum has not yet been established. One explanation for the off-stoichiometry is that the contact area between Al and CuO is not large enough for sufficient transfer of oxygen from CuO to Al, resulting in incomplete combustion of stoichiometric mixtures. An important consideration in Al–CuO nanothermites is that the high-surfae-to-volume ratio in nanometer-sized Al particles facilitates the transfer of oxygen and thus increases combustion rate [10]. However, even in the experiments with varying Al particle sizes, the maximum propagation rate of combustion always occurs when excess Al particles are present. Moreover, other experiments with CuO nanowires providing large interfacial contact area with Al still required Al particles in excess of the stoichiometric ratio [11]. Collectively, these prior reports suggest that the fundamental
http://dx.doi.org/10.1016/j.combustflame.2015.07.019 0010-2180/© 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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reason for the required excess Al (i.e., off-stoichiometry) is due to factors other than the contact area. Answering the fundamental question of “Why are more Al NPs needed in thermites than the stoichiometric amount despite an increase of contact area?” demands a microscopic investigation into what transpires at the surface and interface of Al and CuO particles. In this paper, we examine the evolution of the nano- and microstructure of Al/CuO nanothermites during the combustion and provide new insights into the deactivation of Al NPs relating to the requirement for excess Al. The deactivation accounts for the offstoichiometry at the maximum pressure change into an Al-rich composition. To confirm the effect of the ratio between CuO and Al on the thermite reaction, samples with various ratios from Al-rich to CuOrich were prepared by mixing Al and CuO NPs via sonication, and their combustion was tested in a sealed cell with a pressure change detector. All of the combustion products were analyzed via XRD and TEM, which revealed that thin Cu layers in the form of clusters, products of the thermite reaction, covered the surface of unreacted Al NPs. Microscopic analysis in this study highlights the need to design thermite reaction not only from the compositional point of view, but also in the context of controlling gas-phase intermediates to realize lightweight propellant and explosive. 2. Experimental 2.1. Materials The following chemicals were used without any additional treatment. Aluminum (Al) nanoparticles (NPs) with an average diameter of 95.5 nm and aluminum oxide (Al2 O3 ) shell with a thickness of 4.1 nm were purchased from Nano Technology, Inc. Active Al core comprises about 76.2 wt% with Al2 O3 shell thickness taken into consideration. Copper oxide (CuO) NPs with an average diameter of 37.8 nm were purchased from Sigma-Aldrich. Ethanol (EtOH, 99.5%) was purchased from Samchun Chemicals, Inc. 2.2. Al/CuO sample preparation Al/CuO composites were prepared through the following steps: (i) Al and CuO NPs were dispersed in 20 mL of EtOH; (ii) the suspension was sonicated for 5 min for homogeneous mixing of Al/CuO composites (Fig. S2 shows uniformly mixed Al NPs and CuO NPs in prepared samples.); and (iii) 1 mL of the sonicated mixture was transferred to a substrate and dried in an oven for 1 h at 70 °C. 2.3. Combustion cell test Al/CuO composites were brought into a combustion cell (“closed bomb”), in which a pressure chamber, an electric supplier, a pressure sensor (PCB Piezotronics, Model No. 113B03), a signal amplifier (PCB Piezotronics, Model No. 480C02), and an oscilloscope (Tektronix, TDS 2012B) were equipped. The pressure chamber had a sealed volume of 75 mL and a tungsten coil connected to an electric supplier. 10 mg of a prepared sample in loose-powder form was loaded into the chamber and the tungsten coil was heated by applying a voltage. The loaded samples were then ignited in the chamber, and generated pressure was measured by a pressure sensor. The signal transformed into a voltage output by a signal conditioner. Lastly, the voltage output was recorded by an oscilloscope. 2.4. Characterization Crystallographic information of samples before and after the thermite reaction was characterized using a powder X-ray diffractometer (XRD, Rigaku, D/MAX-2500 (40 kV)). Quantitative analysis from XRD patterns was carried out via the reference intensity ratio (RIR)
Fig. 1. Pressure change and pressurization rate of Al/CuO composites with various Al mass fractions measured by pressure cell chamber equipment when ignition reaction occurred. Dashed line presents stoichiometric ratio.
method [12]. Transmission electron microscopy (TEM, Philips, Tecnai F20 (200 kV)), elemental dispersive X-ray (EDX) mapping analysis, and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM, FEI company, Titan cubed G2 60−300 (200 kV)) were used to investigate the morphology of the ignition products.
3. Results and discussion In Al/CuO composites, one of the most studied thermite material combinations with high combustion enthalpy, CuO oxidizes Al and is reduced to elemental Cu:
2Al + 3CuO → Al2 O3 + 3Cu (H = −1208 kJ) Composites of Al and CuO nanoparticles (NPs), also termed nanothermites or metastable intermolecular composites (MICs), have shown high reaction rates, as large surface area in MICs makes the reaction sites abound [13]. One would expect that the reaction occurs at 2:3 Al:CuO molar ratio. In other words, the most vigorous combustion should occur at about 40% molar fraction of Al in the composites (or 18.4 wt% of Al) with smallest residual reactants. In this study, we prepared composites of Al and CuO NPs with Al mass fraction ranging from 9.9% to 49.9%. Pressure changes recorded upon ignition of the series of samples at different Al:CuO ratios in a sealed chamber are summarized in Fig. 1 and Table 1. Both pressure change and pressurization rate follow a similar trend, with a maximum at 32.4 wt% of Al, which agrees well with results previously reported in the literature [11,14]. Our results clearly show that the maximum pressure rise and pressurization rate occur for Al/CuO nanocomposites with excess Al. This correlation suggests that ‘fuel’ and ‘oxidizer’ need to be mixed at a proper ratio (in our study, Samples 4–6) for optimal exothermic reaction between Al and CuO. On the other hand, samples with excess CuO (Sample 1) or excess Al (Sample 7) show weak thermite reaction due to lack of oxidizer or fuel, respectively. Notably, the best performance of Al/CuO composite among our samples is achieved when more Al (Sample 4) is used than the theoretical stoichiometry dictates. (See Fig. 1) This is consistent with ignition results reported in previous studies [7,15– 17]. These studies conclude that a large amount of gas product is generated at a fuel-rich condition, and propagation is effective because generated gas enhances heat transfer [15–17].
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Table 1 Al mass fraction of prepared samples and resulting pressurization rate.
Al mass fraction (%) Pressurization rate (psi/ms)
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Sample 7
9.9 0.9
12.6 3.1
17.3 14.0
27.6 100.6
32.4 123.5
39.3 70.9
49.9 1.9
Fig. 3. TEM images of Al NPs in reactant (a, b) and in product (c, d). Al/CuO nanothermites with Al mass fraction of 32.4%. Fig. 2. XRD patterns of ignition products from Al/CuO composites with various Al mass fraction. Major peaks from Al, CuO, Cu, Cu2 O, Alx Cuy , and Al2 O3 are indexed.
We now turn to describe the underlying mechanism to explain why excess Al is needed in the most effective nanothermite reaction. One possible interpretation is that Al reacts with ambient air, so that excess Al is required to fully utilize atmospheric oxygen and solid CuO to obtain the largest pressure generation. To test this interpretation, we performed a control experiment with identical thermite reactions in a pressure chamber filled with Ar gas (See Fig. S3). Compared to experiments carried out in air, the Al–CuO ratio that yields the highest pressurization rate shifted slightly to lower Al mass fraction. The fact that the combustion gives rise to the maximum pressurization rate at Al-rich mixture even in Ar indicates that gaseous oxidizer is not a primary reason for the off-stoichiometry for the maximum pressurization rate. The results from these control experiments are consistent with literature reports, which showed that the rate of propagation of Al–CuO thermite reaction exhibits marginal increase even at a drastically higher oxygen concentration [18]. Moreover, Asay et al. reported that air does not affect combustion reaction of nanothermite materials with tube experiments at rough vacuum (3.3 Pa) [19]. Another interpretation of the off-stoichimetry requirement for Al/CuO nanothermites involves the incomplete mixing of fuel and oxidizer. Even though some literature reports suggest that sufficient mixing causes effective thermite reaction, the actual ratio of Al:CuO at which the highest pressurization rate is observed still remains to be on the Al-rich side [10,11]. From these studies, it is implied that insufficient mixing is not a comprehensive reason for the offstoichiometric ratio of Al and CuO for the maximum pressurization rate. To gain a deeper understanding in the role of micro- and nanostructure on the combustion of Al-rich nanocomposites, we analyzed XRD patterns of the products after ignition of the Al/CuO composites. XRD reveals the presence of several compounds including Al, CuO, Cu, Cu2 O, Al2 O3 and Alx Cuy alloys in the ignition products of Al/CuO composites (Fig. 2). Since imbalance of Al and CuO gives rise to unreacted species during the ignition, Al and CuO seem to be the major components in the products from the Al-rich (Sample 7) and CuO-rich (Samples 1 and 2) samples, respectively. In other words, large amount of remaining reactant after combustion results from insufficient Al oxidation due to imbalance between Al and CuO. We hypothesized that
the lower combustion performance of stoichiometric Al/CuO nanothermites results from ‘deactivated’ Al nanoparticles in the composite. To understand how the Al nanoparticles may be precluded from participating in the combustion, we must consider critical chemical and physical processes accompanying the reaction. Cu, a product resulting from reduction of CuO in Al/CuO thermite reaction, is a major product from the combustion reaction of the samples have relatively balanced ratio of Al:CuO. This indicates that the maximum pressure change and pressurization rate of Sample 5 result from almost complete thermite reaction with only a small fraction of unreacted species. Cu2 O is an intermediate product formed during decomposition of CuO with O2 release. Then, Cu2 O is decomposed into Cu and O2 completely under higher temperature [20]. Not only unreacted reactants, but also Cu2 O are the results of low-pressure generation ability in CuO-rich samples. We detected Alx Cuy alloys, Al2 Cu and Al4 Cu9 , in XRD patterns. Although the stoichiometric thermite reaction does not involve the formation of such alloys, it has been reported that Alx Cuy may form via solid-state diffusion of Al and Cu at a temperature ranging between 550 and 1000 K [21] and at an elevated temperature in the case of Al/CuO composites [6,7]. Despite these observations, the effect of Alx Cuy alloys on thermite reaction has been underexplored. Dreizen and co-workers reported that Alx Cuy alloys, which were formed during a ball milling step and during the cooling of combustion products, suppress the thermite reaction [7]. From this argument, we infer that the formation of Alx Cuy alloys during the thermite reaction requires more Al in samples than the stoichiometric ratio. However, in quantitative XRD analysis, Alx Cuy alloys accounted for only 6.4 wt% of the total combustion product in the case of sample 3 (Fig. S4). Since substantive portion of Al is about 1 wt% in 6.4 wt% of Al4 Cu9 alloy, the hypothesis that more Al fraction than stoichiometric ratio is needed because of the Al consumption for alloy formation cannot fully explain that why the best performance is observed at an Al-rich case. To better understand how Alx Cuy alloys affect the thermite reaction, we investigated the products of thermite reaction from TEM images. As shown in Fig. 3, Al NPs have rough surface capped with small particles, while surface is observed to be smooth before the ignition. The average diameter of attached particles (5.5 nm) is significantly smaller than that of starting materials, such as CuO NPs (∼37.8 nm) and Al NPs (∼95.5 nm). This disparity in particle size indicates that
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Fig. 4. (a) HAADF STEM and (b,c) elemental mapping images of Al NP in product from Al/CuO nanothermites with 32.4% of Al mass fraction which presents highest pressurization rate in combustion reaction. Scale bars are 50 nm. (d) Elemental line-scan as a function of position across the arrow line marked in (a).
that small particles are created during the reaction. We identified the small particles to be crystalline Cu by fast Fourier transform (FFT) analysis of TEM images with atomic-resolution, as their lattice spacings correspond to (111) and (200) plane of a cubic Cu crystal [22] (Fig. S6). We estimated the amount of produced Cu necessary to cover all of the unreacted Al NPs. From the calculation [23], the width of Cu layer (3.3 nm) estimated from TEM analysis is in striking agreement with that (3.5 nm) estimated from our XRD analysis (See Fig. S4). This result indicates that Al covered by Cu is a major product from the thermite reaction. We sought to understand the detailed composition of nanostructured reaction products and measured the elemental distribution using energy dispersive X-ray spectroscopy (EDX). EDX results further confirmed the formation of Cu layer on Al NPs. Figure 4 shows HAADF (Fig. 4a) and scanning EDX elemental mapping images (Fig. 4b and c) of a Al NP in the product.In addition, Fig. 4d shows an elemental linescan as a function of position across the arrow line marked in Fig. 4a. The respective colors in Fig. 4b and c represent Al and Cu elements. Al exists mainly at the Al core region, whereas most Cu signals are detected from the surface of Al NP. Now that both Al and Cu are observed at the outside of Al NPs, it is believed that Alx Cuy alloys exist at the outer layer. The linescan elemental mapping result clearly shows that substantial amount of Cu element is detected at the interface, compared to relatively small amount of Al outside the Al2 O3 shell. This implies that Al NPs are covered by Cu layer and small amount of Alx Cuy is formed at the interface between Al NPs and Cu layer by intermetallic diffusion of Al and Cu [21]. Also, the Alx Cuy alloy layer could slow down the outward diffusion of Al. From additional TEM analysis (Fig. S7), we confirmed that most of the Al exists inside the confine of the Cu shell. Therefore, we believe that deposited Cu layer effectively prevents not only Al2 O3 shell from spallation, but Al diffusion through the oxide shell and Cu layer, despite the very thin Cu layer (3.3 nm). Since crystalline Cu results from reduction-oxidation between Al and CuO, we expect that reduced Cu would be formed near the oxidized Al2 O3 as a consequence of the thermite reaction. However, as clearly illustrated in the results discussed above, the Cu layer is located on unreacted spherical Al NPs like Al/Cu core/shell structure. To elucidate how Cu layer is formed on unreacted Al NPs, we associate the propagation mechanism of nanothermite with temperature profile, which is previously studied [24–26]. We provide a detailed description of the temperature profile in the supplementary material. We schematically illustrate the proposed mechanism for the ‘passivation’ of Al NPs by Cu condensation in Fig. 5 which shows reaction steps and corresponding temperature profile near the reaction zones derived from heat equations. At the initial stage, Al/CuO
composites start to explode when an external heat source heats the sample above the ignition temperature. Then, the combustion front propagates to the neighboring area by heat transfer from the ignition point (reaction zone in Fig. 5). Almost concurrently, pre-heated area, which is heated to a temperature below ignition point, forms at a forward region by heat transfer. In the case of bulk or microthermite reaction, heat conduction is the predominant mechanism for propagation [16,17]. By contrast, convective mode is predominant rather than conduction and radiation according to several studies of nanothermites [17,19,27–30]. Since propagation of nano-thermite is preceded by mass transfer of gasified thermite materials, the mass transfer can also help dissipate heat via convection mode [30]. At the ignition point, we speculate that generated heat induces the gasification of reduced Cu. Experimental and theoretical studies have investigated the gasification process [15,31,32]. The gasified Cu is likely to travel toward unreacted neighboring area (e.g., pre-heated zone in Fig. Fig. 5). At this time, it is likely that hetero-nucleation of Cu gas rapidly occurred by heat exchange with unreacted Al NPs or surrounding air molecule [24]. Since Cu gas acts as a convection medium, we speculate that condensation of Cu occurs before the reaction propagation (See Fig. 5). Our hypothesis concerning the formation time to condense a Cu film is corroborated by TEM images shown in Fig. 6. In the case of hollow Al2 O3 shell, the surface is smooth and there is no significant trace of Cu species on its surface. Cu layer would be observed regardless of product types, if condensation of Cu occurred after the combustion. In our TEM analysis, Cu layer is observed only on the Al filled NPs. This contrast suggests that condensation of Cu occurs prior to propagation and then inhibits Al oxidation. Figure 7 schematically illustrates the Al–CuO NP reactions that involve gasified Cu species. Based on the XRD and TEM analysis and study of propagation mechanism, we conclude that Cu layer on Al NPs is formed before combustion propagation. TEM analysis reveals that hollow Al2 O3 shells result from thermal expansion of Al via either rupture of Al2 O3 shell [33–35] or diffusion of Al [36,37]. In the case of Al covered by Cu, we believe that the Cu layer not only suppresses the fracture of alumina shell, but also acts as buffer layer of Al diffusion by formation of alloys as shown in Fig. 4 and Fig. S7. Furthermore, exothermic reaction is not preceded between Cu layer and CuO or Al. Therefore, we suggest that formation of Cu layer on Al NPs can be one of the main reasons for efficient combustion at off-stoichiometry ratio of Al and CuO due to the wasted Al NPs by Cu layer. Notably, only a small fraction of Cu appears to be able to deactivate Al NPs, which is a key to the Al-richness at the peak combustion. Further investigation is underway to test possibility of shifting the peak position in Fig. 1 by controlling convection and condensation of gasified Cu species.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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Fig. 5. Reaction steps and corresponding temperature profile near reaction zones. Preheating zone features temperature below ignition point, leading to condensation of gasified Cu species. Ti and Tc represent temperature of initiation and condensation.
Fig. 6. TEM images of hollow Al2 O3 shell with clean surface in product. Scale bars are 20 nm.
Fig. 7. Schematic illustration of Al/CuO thermite reaction steps. Condensation of gasified Cu species on Al NPs hampers reaction of Al NPs with neighboring CuO NPs.
Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019
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4. Conclusions In summary, we studied the thermite reaction in Al/CuO nanoparticle composites with varying Al and CuO ratios. Maximum pressure change and pressurization rate occurred in composites in which Al was present in excess compared to the stoichiometric ratio. The optimum was achieved with Al/CuO composites with excess Al (32.4 wt%) in reference to theoretical stoichiometry (18.4 wt%). TEM analysis of the reaction products revealed that unreacted Al NPs were covered by Cu layer. We interpret the need for excess Al and the Cu coating as an indication that the deposition of a thin film of Cu on the surface of Al renders the particle inactive in the reaction, which eventually decreases the maximum pressure and pressurization rate. Because of their high surface atom population, transformation and solid-state reaction in NPs often take different forms [38,39] and it is important to analyze how the reactions of nanoscale affect ensemble properties of composite materials. In particular, understanding the role of gasified intermediates in thermite reactions will help design the materials and reactors for propellants and explosives. Acknowledgments This work was supported by the Agency for Defense Development of Korea (Grant no. 13-70-05-04). Supplementary Materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2015.07.019. References [1] S. Bless, T.R. Rodney, K.A. Schroder, D.L. Willauer, D.E. Wilson, in: Google Patents: 2007. [2] E.L. Dreizin, Prog. Energy Combust. 35 (2009) 141–167. [3] M.L. Pantoya, J.J. Granier, J. Therm. Anal. Calorim. 85 (2006) 37–43. [4] M.L. Pantoya, V.I. Levitas, J.J. Granier, J.B. Henderson, J. Propul. Power 25 (2009) 465–470. [5] K.B. Plantier, M.L. Pantoya, A.E. Gash, Combust. Flame 140 (2005) 299–309.
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Please cite this article as: K. Lee et al., Formation of Cu layer on Al nanoparticles during thermite reaction in Al/CuO nanoparticle composites: Investigation of off-stoichiometry ratio of Al and CuO nanoparticles for maximum pressure change, Combustion and Flame (2015), http://dx.doi.org/10.1016/j.combustflame.2015.07.019