Enhancing ignition and combustion of micron-sized

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Proceedings of the Combustion Institute 36 (2017) 2317–2324 www.elsevier.com/locate/proci

Enhancing ignition and combustion of micron-sized aluminum by adding porous silicon Venkata Sharat Parimi, Sidi Huang, Xiaolin Zheng∗ Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Received 4 December 2015; accepted 29 June 2016 Available online 25 July 2016

Abstract Micron-sized aluminum (Al), due to its large volumetric energy density, is an important fuel additive for broad propulsion and energetic applications. However, micron-sized Al particles are difficult to ignite and react slowly, leading to problems such as incomplete combustion and product agglomeration. Many pioneering strategies have been investigated to overcome the above challenges, ranging from reducing Al particles to nanoscale, coating them with metallic or polymeric materials, to blending Al with other materials to form composites. On the other hand, porous Si has emerged as a promising energetic material with a volumetric energy density comparable to Al, high reactivity at low temperature, and ultrafast flame propagation speeds. To date, the potential of using porous Si as an additive to enhance ignition and combustion of micron-sized Al has not been explored. Herein, we experimentally investigated the effect of porous Si addition on the ignition and combustion characteristics of micron-sized Al with CuO nanoparticles. We consistently observed that the addition of porous Si facilitates both ignition and combustion of Al/CuO mixtures over a wide range of experimental conditions, ranging from slow heating rate conditions in differential scanning calorimetry, fast heating rate conditions in Xe flash ignition, flame propagation in microchannels, to constant-volume pressure vessel experiments. The enhancement effects are attributed to the easy ignition and fast burning properties of porous Si, which elevate the ambient temperature and/or pressure, and hence enhance the ignition, reaction rate, and combustion efficiency of micron-sized Al particles. This work demonstrates that adding porous Si is another viable strategy toward enhancing the ignition and combustion properties of micron-sized Al particles. © 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Energetic material; Thermite reaction; Porous silicon; Aluminum; Flash ignition

1. Introduction

∗

Corresponding author. Fax: (650) 723-1748. E-mail address: [email protected] (X. Zheng).

The combustion of metals is of great interest to the combustion community because metals have high volumetric and gravimetric energy densities [1–5]. Metals, such as aluminum (Al), have been extensively studied due to their applicability as

http://dx.doi.org/10.1016/j.proci.2016.06.185 1540-7489 © 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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additives for propulsion [6,7] and many other energetic applications [8]. While micron-sized Al particles are typically used in those applications, their relatively high ignition temperature and slow oxidation process lead to problems such as incomplete combustion and product agglomeration [9,10] that results in two-phase flow losses [11]. Extensive efforts have been devoted to overcome those challenges associated with combustion of micron-sized Al particles. One approach reduces the Al particle size down to nanoscale and utilizes the greatly reduced diffusion length for nano-sized Al. Such nano-sized Al indeed exhibits reduced ignition delay times and temperatures [12–14], and significantly increased burning rates [15–18]. However, nano-sized Al particles suffer from high manufacturing cost, high sensitivity [19], and large dead mass [13], which make it hard for them to be employed in practical propulsion applications. Another approach explores fluorinated or metallic coatings (e.g., Ni) that react with the core Al below the Al melting temperature. As such, those coatings modify the ignition and combustion behaviors of Al particles [20] and reduce their agglomeration [11,21]. However, the size of agglomerates still remains one or two orders of magnitude larger than the initial Al particle sizes, which does not alleviate the two-phase flow loss. The third emerging approach blends micron-sized Al particles with dissimilar materials, such as Al/Ni composite particles, polyethylene, nanothermite mixtures, and fluorocarbon additives [22–26]. Of particular interest is the work by Ilunga et al. [26], who demonstrated that the addition of nanoscale Si/Bi2 O3 mixture to micron-Al/nano-CuO thermites results in increased flame propagation speeds and lowered onset temperatures. While these materials are effective in reducing Al agglomeration and/or modifying the reactive properties, they lower the composite volumetric energy density. The potential of using porous silicon (p-Si) as an additive to tailor the ignition and combustion characteristics of micronsized Al particles has been overlooked so far. Porous Si is crystalline Si containing nano-sized pores and hydrogen terminated surfaces [27], and it has been recognized for almost a decade as a promising material for energetic and pyrotechnical applications. Si has a gravimetric energy density (32.43 kJ/g) comparable to Al (31.05 kJ/g), and experimental measurements indicated that pSi/NaClO4 composites are capable of having energy densities as high as 22.5 ± 2.2 kJ/g(Si) [28]. Furthermore, p-Si exhibits reactivity at low temperature [29] and ignitability with Xe flash [30], and p-Si was demonstrated to have flame propagation speeds from 1 m/s up to 3050 m/s [31–33]. In addition, p-Si can be oxidized by diverse oxidizers such as air, sulfur, alkali metal perchlorates, alkali metal nitrates, and metal oxides. Those properties make p-Si a very attractive additive to potentially

augment the ignition and combustion properties of micron-sized Al particles. In the present work, we experimentally investigated the effect of p-Si addition on the ignition and combustion properties of micron-sized Al/nanosized CuO (m-Al/n-CuO) thermite mixtures. For ignition, we uniformly mixed p-Si particles with mAl/n-CuO mixtures. We found that the presence of p-Si effectively lowers the onset temperature of mAl/n-CuO under low heating rate differential scanning calorimetry measurements. The addition of p-Si also lowers the minimum flash ignition energy and enhances the combustion efficiency of mAl/n-CuO under high heating rate Xe flash ignition experiments. As for combustion, we added energetic p-Si filled with sodium perchlorate (NaClO4 ) underneath m-Al/n-CuO without directly mixing them together due to safety concerns. Both flame propagation and constant-volume pressure-vessel experiments show that the rapid combustion of pSi/NaClO4 composites immediately elevate the ambient temperature and/or pressure, which greatly enhances the ignition, and reaction rate of m-Al/nCuO thermite mixtures. These experiments demonstrate for the first time that p-Si is an effective additive to enhance the ignition and combustion properties of micron-sized Al particles. 2. Experimental specifications 2.1. Material synthesis The p-Si film was prepared by the standard electrochemical etching process [27]. Briefly, p-type Si wafers (0.001–0.005 -cm, El-Cat Inc.) were immersed in an etching solution containing equal volumes of ethanol and 48% aqueous hydrofluoric acid (HF) in a Teflon cell. A constant current of 40 mA/cm2 was applied between the Si wafer (anode) and a silver mesh counter electrode for 90 min, which produces a p-Si film about 180 μm thick. Freestanding p-Si films were prepared by electropolishing the p-Si film from the Si wafer in an electrolyte of HF and ethanol (volume ratio: 1:5) at a current density of 50 mA/cm2 for 15 s. The freestanding p-Si films were further dried, ground, and sieved through a mesh to obtain p-Si particles with diameters of 45 μm or less. The specific surface area, average pore diameter, and porosity of those p-Si particles are about 350–400 m2 /g, 5– 9 nm, and 60%, as determined from gas adsorption measurements. Energetic p-Si composites were prepared by impregnating p-Si particles/films with solutions of NaClO4 in methanol, and they were used for the constant-volume pressure vessel and flame propagation experiments to evaluate their impact on the combustion properties of m-Al/n-CuO. The m-Al/n-CuO thermite mixtures studied here were always stoichiometric and were prepared by mixing Al micron particles (3.0–4.5 μm, Alfa Aesar)


and CuO nanoparticles (∼50 nm, Sigma Aldrich) in hexane, followed by 30 min sonication, drying and sieving through a 25 μm mesh. For the preparation of mixtures of thermite and p-Si, the same preparation procedures were followed except that p-Si particles were added to the hexane solvent as well. 2.2. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements The thermal characteristics of three p-Si/mAl/n-CuO mixtures (Table 1) were studied by simultaneous TGA/DSC measurement (Setaram Labsys Evo). For typical measurements, samples about 10 mg were placed in alumina crucibles. The TGA/DSC chamber was first flushed with Ar (100 sccm) for 30 min to minimize any residual O2 . Then the samples were heated in an Ar environment (40 sccm) at a heating rate of 10 K/min from 100 °C to 1350 °C. After the samples were cooled down to room temperature, they were heated with the same process again. The second round heat flow traces were used to correct the baseline of the first round heat flow traces following the method described previously [34]. 2.3. Measurement of minimum ignition energy (Emin ) of Xe flash lamp Flash ignition experiments were carried out in ambient air. The samples (loose powders, ∼5 mg) were placed over a 1 mm thick glass slide on top of the Xe flash tube of a commercial flash unit (AlienBeesTM B1600) (Fig. 3) [30]. The samples used for flash ignition experiments are stoichiometric m-Al/n-CuO mixtures with different amounts of p-Si particles added, p-Si, and stoichiometric p-Si/n-CuO mixture for comparison. To determine the minimum ignition energy Emin , the power of the Xe flash tube was gradually increased until ignition occurs. The areal pulse energy of the Xe flash at those ignition conditions, which is defined as Emin here, was determined by measuring the temperature rise of a soot-covered silicon substrate exposed to the same flash intensity using the same method described elsewhere [35]. The samples were replaced upon each exposure to flash to prevent partially Table 1 Compositions of the thermite mixtures used for DSC/TGA analysis. Mixture Al/CuO p-Si/CuO p-Si/Al/CuO

φ

Mass fraction (%) p-Si Al

CuO

– 15.0 8.3

81.6 85.0 83.4

18.4 – 8.3

1 1 1

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reacted samples that may affect the Emin measurement. It should be noted that for both TGA/DSC and flash ignition experiments, we did not add NaClO4 to p-Si since it is difficult to prepare a homogeneous p-Si/NaClO4 /m-Al/n-CuO mixture due to safety concerns. For the combustion experiments below, NaClO4 was added to effectively oxidize p-Si, but the p-Si/NaClO4 composite was not directly mixed with m-Al/n-CuO, but placed underneath. 2.4. Flame propagation in a semi-confined microchannel The qualitative effect of p-Si/NaClO4 addition on the flame propagation of m-Al/n-CuO mixtures was investigated in a semi-confined microchannel. The semi-confined microchannel was made of 1 mm thick glass slides and have overall dimensions of 5 mm (w) × 1 mm (h) × 25 mm (l) (Fig. 5a). Only one end of the microchannel was open, where samples were ignited by an embedded nichrome (NiCr) filament. The other enclosed sides were used to prevent the tested materials from splattering away during the flame propagation. Due to the sensitivity of p-Si/NaClO4 to mechanical impact, the stoichiometric m-Al/n-CuO thermite mixtures were not directly mixed with p-Si/NaClO4 , but they were placed on top (Fig. 5). To be consistent, the thermite mixtures were always placed on top of other comparison substrates, including p-Si film on Si wafers, and SiO2 /Si wafers (Fig. 5). The thickness of the p-Si layer used for the flame propagation experiments was 130 μm. The flame propagation process inside the microchannel was recorded at 90,000 fps using a high-speed camera (Photron FASTCAM SA5) with a 100 mm macro lens. 2.5. Reaction progression measurement in constant-volume pressure vessel The effect of adding p-Si/NaClO4 composite on the combustion behavior of m-Al/n-CuO thermite mixtures was quantified in a constantvolume pressure vessel (volume: 13.5 cm3 ) filled with ambient air for which the schematic setup is shown in Fig. 1. Various control samples (Table 2, Fig. 1b–d) were first placed in a crucible and then mechanically agitated to have a uniform packing density. The thermite samples were ignited by an embedded NiCr filament at the bottom (Fig. 1b). When p-Si/NaClO4 was part of the tested samples, p-Si particles were impregnated with stoichiometric amount of NaClO4 . Since not all NaClO4 will enter into the pores of p-Si, some will remain outside the p-Si particles. We cannot directly measure the pore volume fill percentage due to the highly sensitive nature of NaClO4 filled p-Si particles. We estimate that about 6.5% of the pore volume of p-Si is filled with the oxidizer according to previous gravimetric measurements conducted on NaClO4 filled

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Fig. 1. (a) Schematic setup of the constant-volume pressure vessel experiment. The mixtures were placed in an alumina crucible and ignited by NiCr filaments. The dynamic pressure and light emission were collected by a pressure transducer and a photodiode, respectively. (b), (c), and (d) illustrate the sample configurations and the NiCr filament position when testing m-Al/n-CuO, p-Si/NaClO4 , and m-Al/n-CuO over p-Si/NaClO4 , respectively. Table 2 Composition of m-Al/n-CuO and p-Si/NaClO4 samples used for the pressure-vessel experiments. The Al, CuO, and p-Si particle sizes are 3.0–4.5 μm, 50 nm, and < 45 μm, respectively. Sample

Mass (mg) Al CuO

φ (Al /CuO)

Mass (mg) p-Si NaClO4

φ

1 2 3

3.7 – 3.7

1.0 – 1.0

– 1.2 1.2

– 1.0 1.0

16.3 – 16.3

p-Si films [32]. The NiCr filament was always placed on top of the p-Si/NaClO4 mixture to avoid unintentional ignition (Fig. 1c and d). The dynamic pressure and light emission from combustion were recorded by a pressure transducer (603B1, Kistler Inc.) and a photodiode (PDA36A, Thorlabs Inc.), respectively (Fig. 1a). 3. Results and discussion 3.1. Effect of p-Si addition on onset temperature of micron-Al/nano-CuO thermite reaction Figure 2a shows the baseline corrected TGA and DSC traces (normalized by fuel mass) for the stoichiometric m-Al (3.0–4.5 μm)/n-CuO (∼50 nm) and p-Si/CuO (∼50 nm) mixtures (Table 1). Both samples exhibit gradual small mass loss since a small percentage of CuO decomposes and generates gaseous O2 . The m-Al/n-CuO mix-

– 2.6 2.6

(p-Si/CuO)

Total Mass (mg) 20.0 3.8 23.8

ture exhibits two exothermic peaks. One weak exothermic peak is around 580 °C, below the melting temperature of Al (660 °C). The peak position is similar to those of nano-thermites [36] because the Al micron particles are not uniform in size and probably contain small amount of nanoparticles [37]. The dominant exothermic peak for m-Al/n-CuO is at 956 °C, indicating that the onset temperature of micron-sized Al is much higher than that of nano-sized Al. In comparison, the p-Si/CuO mixture also has two major exothermic peaks but both are at much lower temperatures (324 and 670 °C) than that of m-Al/n-CuO. To test if the addition of p-Si can lower the onset temperature for m-Al/n-CuO, the experimentally measured heat flow trace for the stoichiometric fuel (p-Si and Al)/CuO mixture (Table 1) is compared to the estimated one in Fig. 2b. The estimated heat flow trace assumes that there is no interaction between p-Si/CuO and m-Al/n-CuO


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Fig. 2. (a) TGA and DSC traces for stoichiometric m-Al/n-CuO and p-Si/CuO, respectively. (b) The experimentally measured and estimated heat flow trace for an overall stoichiometric p-Si/m-Al/n-CuO mixture. The measured DSC trace shows that p-Si lowers the onset temperature of the m-Al/n-CuO reaction. Sample details are listed in Table 1.

reactions, and it is calculated by the mass weighted linear addition of individual p-Si/CuO and mAl/n-CuO mixtures (Fig. 2a). It should be noted that the heat release from the Si–Al intermetallic reaction is negligible here [38]. Figure 2b shows that the onset temperature for the exothermic peak related to m-Al and n-CuO reaction is lower for the measured case (804 °C) in comparison to the estimated onset temperature (873 °C). The lowered onset temperature suggests that the addition of pSi facilitates the initiation of the reaction between m-Al and n-CuO particles. 3.2. Effect of p-Si addition on flash ignition of micron-Al/nano-CuO mixtures The above TGA/DSC experiments are normally considered as lower heating rate conditions (< 100 K/min). Many practical applications expose thermites to much higher heating rates, and the heating rate could greatly affect the ignition behavior of thermites. Next, we further investigated the effect of p-Si addition on the ignition properties of m-Al/n-CuO under high heating rate conditions (∼103 –106 K/s) in an Xe flash ignition experiment [30,39,40]. Figure 3 illustrates the flash ignition and burning behaviors of m-Al/n-CuO, p-Si/n-CuO, p-Si, and p-Si/m-Al/n-CuO mixture (20% p-Si by mass) when exposed to the same aerial Xe flash energy intensity (2.15 J/cm2 ). The stoichiometric m-Al/n-CuO mixture cannot be ignited with this flash energy and it was only dispersed a little due to the photothermal heating effect (Fig. 3a). The stoichiometric p-Si/n-CuO mixture can be ignited by the flash. However, the reaction is quite mild (Fig. 3b) as the final product is still black, not red, which indicates that only a small percentage of CuO is converted to Cu. In contrast, when p-Si is placed in ambient air without CuO, it can

be ignited by flash and also burns much more violently (Fig. 3c). The reason is that air is a more effective oxidizer for p-Si than CuO because air can penetrate into the pores of p-Si and provides oxygen more readily than CuO. Finally, when 20% p-Si (by mass) is added to the stoichiometric m-Al/n-CuO mixture, the mixture can be easily ignited by Xe flash with the most vigorous burning (Fig. 3d). In addition, the product is reddish (Fig. 3d) indicating that large amount of CuO is reduced to Cu. Hence, the addition of p-Si has successfully triggered the ignition of m-Al/n-CuO from the heat release from the p-Si burning in air. Next, we further quantify the amount of pSi addition on the minimum flash energy of stoichiometric m-Al/n-CuO mixture. Figure 4 plots the minimal areal ignition energy as a function of the p-Si mass fraction in the p-Si/m-Al/n-CuO mixture. Without p-Si addition, the m-Al/n-CuO mixture cannot be ignited with our current Xe flash tube. The minimal ignition energy (Emin ) quickly decreases when a small amount of p-Si is added. Emin levels off when more than 30% p-Si is added, indicating that p-Si controls the ignition behavior at such high concentration as expected. Those flash ignition results show that the ignition enhancement effect by p-Si addition is also applicable at high heating rate conditions. 3.3. Effect of p-Si addition on the flame propagation of micron-Al/nano-CuO mixture The above DSC and flash ignition results show that p-Si addition effectively facilitates the ignition of m-Al/n-CuO thermite. Next, we investigate the effect of p-Si addition on the flame propagation behavior of m-Al/n-CuO in a microchannel (Fig. 5a). Due to the sensitivity of p-Si/NaClO4 to mechanical mixing, it is not directly mixed with

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Fig. 3. Optical images of (a) m-Al/n-CuO (φ = 1), (b) p-Si/CuO (φ = 1), (c) p-Si, and (d) p-Si/m-Al/n-CuO (p-Si: 20% by mass, m-Al/n-CuO with φ = 1). The total sample mass in each case is about 5 mg.

and its reaction front quickly propagates through underneath m-Al/n-CuO. The heat and hot gas released by p-Si/NaClO4 ignite the above m-Al/nCuO, leading to its abrupt burning. In comparison from the SiO2 section, when the flame propagates to the section of p-Si without NaClO4 , the flame front does not show significant changes (Fig. 5c). Similar behaviors were observed for m-Al/nCuO mixtures of different packing densities. These results suggest that the flame propagation speed of m-Al/n-CuO can be significantly accelerated by adding p-Si/NaClO4 mixture in physical contact since p-Si/NaClO4 is easier to ignite and propagates faster.

Fig. 4. Minimum areal flash ignition energy for p-Si/mAl/n-CuO mixtures (∼5 mg) as a function of p-Si particle mass fraction.

m-Al/n-CuO but placed underneath for flame propagation study. For consistency, m-Al/n-CuO mixtures are also placed on top of other control substrates (e.g., SiO2 and p-Si) for comparison (Fig. 5). The flame propagation speed of thermites is known to be sensitive to the powder packing density [41]. Since the packing density of m-Al/n-CuO is nearly constant on the same substrate, we only compare its flame propagation behavior on the same substrate with two different sections: SiO2 /p-Si filled with NaClO4 (Fig. 5b) and SiO2 /p-Si (Fig. 5c). For both cases, the m-Al/n-CuO mixtures were ignited over the SiO2 section. While the flame is propagating on SiO2 , there is a clear flame front with faster speed in the center of the channel. The burning of the thermite is mild and steady. When the flame propagates onto the p-Si filled with NaClO4 section, the flame abruptly sets the remaining thermite to react instantaneously (Fig. 5b, 4 ms). It is likely because the flame ignites the underneath p-Si/NaClO4

3.4. Effect of p-Si/NaClO4 addition on combustion of micron-Al/nano-CuO mixtures The effect of adding p-Si/NaClO4 on the combustion properties of m-Al/n-CuO is quantitatively analyzed in the constant-volume pressure vessel experiment (Fig. 1). The detailed sample mass and compositions are listed in Table 2 and their respective dynamic pressure and light emission are shown in Fig. 6. Time zero is set as the time when a small pressure increase (∼0.3 atm) is observed for the chamber pressure to remove the impact of different ignition delays. The P-t trace (Fig. 6a) for such a constant-volume vessel shows the accumulative effect of heat release, and the light emission trace (Fig. 6b) can be regarded as proportional to the instantaneous reaction rate. First, the pressure of p-Si/NaClO4 peaks immediately after ignition and its light emission has a narrow and sharp peak for the first 0.4 ms. In comparison, the pressure of m-Al/n-CuO rises much slower and its light emission shows much broader and milder peaks. This confirms that p-Si/ NaClO4 reacts much faster than m-Al/n-CuO. Second, we study the effect of p-Si/NaClO4 addition on the combustion of m-Al/n-CuO by placing p-Si/NaClO4 underneath (Fig. 1d). The measured


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Finally, we quantify the acceleration effect of pSi/NaClO4 addition by defining a reaction progress parameter α(t), α(t ) =

∫t0 Vphotodiode (t )dt , ∫∞ 0 Vphotodiode (t )dt

(1)

where Vphotodiode (t) is the photodiode signal measured at time t. The definition of α(t) assumes that the photodiode response is proportional to the instantaneous reaction rate, and hence α(t) represents the relative degree of reaction completion. The pressure increase for the thermite mixtures is caused by both heat release from thermite reactions and decomposition of n-CuO particles [42], so only the photodiode signal is used to evaluate the reaction. As shown in Fig. 6c, the time to achieve about 80% of reaction completion (α = 0.8) for pSi/NaClO4 and m-Al/n-CuO are 0.9 and 3.1 ms, respectively, confirming that p-Si/NaClO4 reacts much faster. The measured time for α = 0.8 for the m-Al/n-CuO + p-Si/NaClO4 mixture is 2.1 ms, which is shorted than the estimated 3.1 ms using the estimated photodiode trace in Fig. 6b. In addition, the α(t) curve for the m-Al/n-CuO + p-Si/NaClO4 mixture shows a two-stage behavior with a sharp increase before 0.2 ms followed by a much slower increase afterward. This behavior suggests that pSi/NaClO4 rapidly reacts first and releases heat that accelerates the reaction of m-Al/n-CuO. In another words, m-Al/n-CuO burns faster because it burns at elevated ambient temperature and pressure due to the reaction of p-Si/NaClO4 . 4. Conclusions

Fig. 5. (a) Schematic of the semi-confined glass microchannel for the flame propagation study. The m-Al/nCuO is ignited at the open end and flame propagates downstream. High speed images of flame propagation of m-Al/n-CuO on segmented substrates: (b) SiO2 /p-Si with NaClO4 ; and (c) SiO2 /p-Si. The flame speed for the mAl/n-CuO mixture is about 6.7 m/s over the SiO2 section and ∼150 m/s over the p-Si/NaClO4 section. The flame propagation of p-Si/NaClO4 without the m-Al/n-CuO mixture on top is about 4.1–4.3 m/s.

pressure and light emission traces are compared to the estimated ones in Fig. 6a and b. The estimation traces were calculated by PAl/CuO (t) + Pp-Si/NaClO4 (t) – Pbaseline and VAl/CuO (t) + Vp-Si/NaClO4 (t) – Vbaseline , respectively, for which it assumes no interaction between p-Si/NaClO4 and m-Al/n-CuO. Clearly, the measured pressure and light emission traces for the mixture shows much earlier, higher and sharper pressure rise and light emission than those of the estimated ones. This suggests that the presence of p-Si/NaClO4 renders m-Al/n-CuO react much faster and more efficiently.

In summary, this work is the first to investigate the effect of p-Si addition on the ignition and combustion characteristics of m-Al/n-CuO thermites. We consistently observed that the addition of pSi facilitates both ignition and combustion of mAl/n-CuO thermites over a wide range of experimental conditions, ranging from slow heating rate conditions in TGA/DSC, large heating rate conditions in Xe flash ignition, flame propagation in microchannels, to constant-volume pressure vessel experiments. Those enhancement effects are attributed to the easy ignition and fast burning properties of p-Si, which elevates the ambient temperature and/or pressure and hence accelerates the ignition and combustion of m-Al/n-CuO. We believe that this work provides a viable pathway toward enhancing the ignition and combustion properties of micron-sized Al particles by adding p-Si. Acknowledgments This work was supported by the Office of Naval Research under agreement number N00014-15-12028 and Army Research Office under agreement number W911NF-14-1-0271.

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Fig. 6. (a) Pressure versus time, (b) light emission versus time, and (c) reaction progress parameter α versus time for m-Al/nCuO, p-Si/NaClO4 , and m-Al/n-CuO with p-Si/NaClO4 mixtures. The estimated mixture traces are obtained by summing the responses from m-Al/n-CuO and p-Si/NaClO4 samples.

References [1] E.L. Dreizin, Prog. Energy Combust. Sci. 35 (2009) 141–167. [2] R.A. Yetter, G.A. Risha, S.F. Son, Proc. Combust. Inst. 32 (2009) 1819–1838. [3] M.W. Beckstead, Combust. Explos. Shock Waves 42 (2006) 623–641. [4] S.H. Fischer, M.C. Grubelich, Theoretical Energy Release of Thermites, Intermetallics, and Combustible Metals, Sandia National Labs., Albuquerque, NM (US), 1998. [5] X. Zhou, M. Torabi, J. Lu, R. Shen, K. Zhang, ACS Appl. Mater. Interfaces 6 (2014) 3058–3074. [6] A. Ingenito, C. Bruno, J. Propul. Power 20 (2004) 1056–1063. [7] K.K. Kuo, G.A. Risha, B.J. Evans, E. Boyer, in: Symp. AA – Synth. Charact. Prop. Energy Nanomater., 2003. [8] C. Rossi, K. Zhang, D. Esteve, P. Alphonse, P. Tailhades, C. Vahlas, J. Microelectromech. Syst. 16 (2007) 919–931. [9] E.W. Price, J. Propul. Power 11 (1995) 717–729. [10] V.A. Babuk, V.A. Vasilyev, J. Propul. Power 18 (2002) 814–823. [11] T.R. Sippel, S.F. Son, L.J. Groven, Combust. Flame 161 (2014) 311–321. [12] M.L. Pantoya, J.J. Granier, J. Therm. Anal. Calorim. 85 (2006) 37–43. [13] J.J. Granier, M.L. Pantoya, Combust. Flame 138 (2004) 373–383. [14] K. Brandstadt, D.L. Frost, J.A. Kozinski, Proc. Combust. Inst. 32 (2009) 1913–1919. [15] M.R. Weismiller, J.Y. Malchi, J.G. Lee, R.A. Yetter, T.J. Foley, Proc. Combust. Inst. 33 (2011) 1989–1996. [16] B.S. Bockmon, M.L. Pantoya, S.F. Son, B.W. Asay, J.T. Mang, J. Appl. Phys. 98 (2005) 64903. [17] S.F. Son, B.W. Asay, T.J. Foley, R.A. Yetter, M.H. Wu, G.A. Risha, J. Propul. Power 23 (2007) 715–721. [18] K.S. Martirosyan, L. Wang, A. Vicent, D. Luss, Nanotechnology 20 (2009) 405609. [19] D.G. Piercey, T.M. Klapötke, Cent. Eur. J. Energy Mater. 7 (2010) 115–129. [20] J.D.E. White, R.V. Reeves, S.F. Son, A.S. Mukasyan, J. Phys. Chem. A 113 (2009) 13541–13547.

[21] O.G. Glotov, D.A. Yagodnikov, V.S. Vorob’ev, V.E. Zarko, V.N. Simonenko, Combust. Explos. Shock Waves 43 (2007) 320–333. [22] T.R. Sippel, S.F. Son, L.J. Groven, Propellants Explos. Pyrotech. 38 (2013) 286–295. [23] T.R. Sippel, S.F. Son, L.J. Groven, Propellants Explos. Pyrotech. 38 (2013) 321–326. [24] Y. Aly, M. Schoenitz, E.L. Dreizin, Combust. Sci. Technol. 183 (2011) 1107–1132. [25] M. Schoenitz, T.S. Ward, E.L. Dreizin, Proc. Combust. Inst. 30 (2005) 2071–2078. [26] K. Ilunga, O. del Fabbro, L. Yapi, W.W. Focke, Powder Technol 205 (2011) 97–102. [27] L. Canham, Properties of Porous Silicon, INSPEC, 1997. [28] N.W. Piekiel, C.J. Morris, W.A. Churaman, M.E. Cunningham, D.M. Lunking, L.J. Currano, Propellants Explos. Pyrotech. 40 (2015) 16–26. [29] C.R. Becker, L.J. Currano, W.A. Churaman, C.R. Stoldt, ACS Appl. Mater. Interfaces 2 (2010) 2998–3003. [30] Y. Ohkura, J.M. Weisse, L. Cai, X. Zheng, Nano Lett. 13 (2013) 5528–5533. [31] C.R. Becker, S. Apperson, C.J. Morris, S. Gangopadhyay, L.J. Currano, W.A. Churaman, C.R. Stoldt, Nano Lett. 11 (2011) 803–807. [32] V.S. Parimi, A. Bermúdez Lozda, S.A. Tadigadapa, R.A. Yetter, Combust. Flame 161 (2014) 2991–2999. [33] A. Plummer, V. Kuznetsov, T. Joyner, J. Shapter, N.H. Voelcker, Small 7 (2011) 3392–3398. [34] D. Stamatis, Z. Jiang, V.K. Hoffmann, M. Schoenitz, E.L. Dreizin, Combust. Sci. Technol. 181 (2008) 97–116. [35] H.K. Aslin, Rev. Sci. Instrum. 38 (1967) 377–381. [36] Y. Ohkura, S.-Y. Liu, P.M. Rao, X. Zheng, Proc. Combust. Inst. 33 (2011) 1909–1915. [37] V.I. Levitas, J. McCollum, M. Pantoya, Sci. Rep. 5 (2015) 7879. [38] J.L. Murray, A.J. McAlister, Bull. Alloy Phase Diagr. 5 (1984) 74–84. [39] Y. Ohkura, P.M. Rao, X. Zheng, Combust. Flame 158 (2011) 2544–2548. [40] Y. Ohkura, P.M. Rao, I.S. Cho, X. Zheng, Appl. Phys. Lett. 102 (2013) 43108. [41] M.L. Pantoya, V.I. Levitas, J.J. Granier, J.B. Henderson, J. Propul. Power 25 (2009) 465–470. [42] K. Sullivan, M. Zachariah, J. Propul. Power 26 (2010) 467–472.