Combustion and Flame 161 (2014) 2975–2981

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Pressure loss and compensation in the combustion process of Al–CuOnanoenergetics on a microheater chip

http://dx.doi.org/10.1016/j.combustflame.2014.04.0150010-2180/� 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Institute of Chemical Materials, China Academy ofEngineering Physics, Mianyang 621900, Sichuan, China.

E-mail address: ygcheng@caep.ac.cn (G. Yang).1 These authors contributed equally to this work and should be considered as co-

first authors.

Jinpeng Shen a,b,1, Zhiqiang Qiao a,1, Jun Wang a, Kaili Zhang c, Rui Li a,b, Fude Nie a, Guangcheng Yang a,b,⇑a Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, Chinab Sichuan New Material Research Center, Mianyang 621000, Sichuan, Chinac Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 October 2013Received in revised form 28 January 2014Accepted 24 April 2014Available online 20 May 2014

Keywords:Pressure lossCompensationAl–CuO MICCL-20Microheater chip

This study investigates pressure loss and compensation in the combustion process of Al–CuO metastableintermolecular composite (MIC) on a microheater chip. A ball cell model of pressure change in the com-bustion process is proposed to show the effects of pressure loss on the reaction rate and efficiency ofenergy output at microscale. An effective compensation method for pressure loss is then developed byintegrating Al–CuO MIC with CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane) onto a SiO2/Cr/Pt/Aumicroheater chip. The combustion processes of Al–CuO MIC with different weight percentages of fineCL-20 particles on the microheater chips are observed by high-speed video recording. Results indicatethat the reaction of Al–CuO MIC is a slow combustion process that turns into intense deflagration afteradding fine CL-20 particles to Al–CuO MIC. The pressure–time characteristics indicate higher maximumpressure and pressurization rate for Al–CuO/CL-20 because the pressure loss at microscale is well com-pensated by the addition of fine CL-20. This study proves the importance of pressure loss in the combus-tion process of MIC at microscale and provides an efficient compensation strategy for pressure loss toimprove the reaction rate and efficiency of energy output at microscale environment.

� 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Nanoenergetic materials exhibit better performance in terms ofenergy release rate, ignition, and other properties compared withconventional energetic materials (i.e., propellants, explosives, andpyrotechnics). The integration of nanoenergetic materials with sil-icon-based microelectromechanical systems (MEMS) to achievefunctional nanoenergetics-on-a-chip (NOC) presents a very prom-ising approach [1–5]. NOC is a key factor in the advancement ofmicroscale energy-demanding systems, such as microactuation,microignition, micropropulsion, microfluidics, micropower, andelectro-explosive devices at the microscale and nanoscale [6–12].

MICs, which are mixtures of nano-Al and metal oxides such asCuO [13], Fe2O3 [14,15], MoO3 [16], and Co3O4 [17,18], are themostly used nanoenergetic materials for NOC. The exothermicreactions of MICs have high adiabatic temperature and high energydensity to withstand heat loss and maintain self-sustaining com-

bustion at the microscale. Previous studies [19,20] indicate thatthe critical combustion diameters of MICs can reach micrometerlevel, whereas the critical diameters of traditional monomolecularenergetic materials, such as RDX (cyclotrimethylenetrinitra-mine),TATB (1,3,5-triamino-2,4,6-trinitrobenzene), and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) are only at millimeter level[21–24]. However, the achieved combustion wave speeds (CWS)of MICs are not very high, with a maximum value of 2500 m/sfor Al–Bi2O3 MIC [25]. This finding can be attributed to the fact thatMICs release energy over a long period of time because of theirnanoscale mass transfer lengths [26], which is slower than thatof traditional monomolecular energetic materials with their masstransfer lengths at a molecular level [27]. To increase the CWS ofMICs, RDX was added to Al–Fe2O3 MIC and a very high reactionspeed of 7185 m/s was achieved [28], which mainly resulted fromthe large quantity of gases that were released from RDX during thereaction. A strong influence of pressure on the CWS of MICs wasalso observed [29,30]. High pressure creates a pressure gradientin the reaction zone, which greatly benefits the mass and heattransfer [31–33]. Therefore, the pressure changes are closelyrelated to the CWS of MICs. However, all the aforementioned stud-ies have focused on macroscopic scales, whereas very few studieshave investigated the pressure changes at microscale.

2976 J. Shen et al. / Combustion and Flame 161 (2014) 2975–2981

Furthermore, the CWS of MICs significantly reduces to severalmeters per second in thin films or micro-channels, which is verydifferent from the CWS of MICS at macroscopic scales [19]. Someresearchers attributed this phenomenon to heat loss [1], but someresearchers stressed that heat loss could be negligible under partic-ular conditions at microscale [20]. Aside from the heat loss effects,the pressure loss must also be emphasized at microscale applica-tions. The pressure loss at microscale may be significantlyenhanced unlike the pressure loss at macroscopic scale, whichdecreases heat transfer and CWS of MICs. However, the pressureloss and compensation at microscale has never been investigated.

This paper proposes a ball cell model of pressure change in thecombustion process to show the effect of pressure loss on reactionrate and efficiency of energy output at microscale. An effectivecompensation method for pressure loss is also presented,which involves the integration of CL-20 (2,4,6,8,10,12-hexa-nitrohexaazaisowurtzitane) with Al–CuO MIC onto a SiO2/Cr/Pt/Au microheater. The combustion processes of Al–CuO MIC and Al-CuO/CL-20 on the microheater chips are observed by high-speedvideo recording (HSVR) to prove the ability of CL-20 to compensatefor pressure loss.

2. Experimental section

2.1. Preparation of materials

Aluminum (Al) nanoparticles have 30–150 nm particle size (PS),80 wt% active metal content, and 2–5 nm oxide shell layer. Nano-CuO was synthesized by microwave method with 99 wt% purity.First, 50 g CuSO4�5H2O was dissolved in 1000 mL deionized waterto obtain a 0.2 mol L–1 CuSO4 solution. Second, 2 mol L–1 NaOHsolution was dripped at a rate of 10 mL min–1 into the CuSO4 solu-tion and was stirred at a rate of 800 r min–1 at 25 �C. The drippingwas stopped after the pH value reached 13. Third, the reactionsolution was placed in a microwave furnace after a 30-min stirringand was radiated in a microwave for 20 min to obtain the blackpowder of CuO. Fourth, the black powder was repeatedly washedwith deionized water until a pH value of 7 was obtained. Fifth,the powder was freeze dried to obtain the sample.

CL-20 is a highly energetic material with a cage-like structure(see Fig. 1). This material is presently considered as the most pow-erful explosive and displays the most promising applications forincreasing impulse, ballistics, and detonation velocity [34]. 2 gCL-20 explosives were dissolved into 10 g ethyl acetate with stir-ring, and the solution was dripped into distilled water at 25 �C.After washing with distilled water, the fine CL-20 particles werefinally obtained by freeze drying.

Al–CuO MIC has a mass ratio of 22% Al and 78% CuO, which isbased on the optimization results of Shen et al. [32]. Al nanoparti-cles and nano-CuO were mixed in hexane, and the suspensionswere sonicated for 30 min to break the agglomerates and to ensurean even mixing between the fuel and the oxidizer. The Al–CuO/CL-20 mixtures were composed of 0 wt%, 10 wt%, 30 wt%, 50 wt%,70 wt%, and 100 wt% fine CL-20 particles (100 wt%, 90 wt%,

Fig. 1. Chemical structure of CL-20, C6H6N12O12.

70 wt%, 50 wt%, 30 wt%, and 0 wt% Al–CuO MIC, respectively). Afterweighing the Al–CuO and CL-20 components, the mixed powderwas dispersed into hexane and the suspensions were sonicatedfor 30 min to improve dispersibility and homogeneity. To preventthe oxidation of Al nanoparticles at elevated temperatures [35],the mixture was obtained by vacuum drying for 2 h at 50 �C.

2.2. Design and fabrication of SiO2/Cr/Pt/Au microheater and NOC

The NOC in this study includes a microheater and CuO–Al MIC.The fabrication process of the thin film SiO2/Cr/Pt/Au microheateris shown schematically in Fig. 2. A Pt-based thin film microheateris used in this study, which has been widely used for differentapplications because of its high reliability [7,36]. The microheaterfabrication began with a 4-in. double-side polished silicon wafer.The Si substrate was cleaned by acetone, rinsed by deionizedwater, and blow dried with nitrogen. To minimize the heat lossin the Si substrate with high thermal conductivity, a 3 lm-thickSiO2 layer with low thermal conductivity was deposited on thesubstrate by low-pressure chemical vapor deposition (LPCVD).Positive photoresist was then spin-coated on the substrate andwas patterned by using photolithography through a designedmask. Metal films of Cr/Pt/Au with thicknesses of 30, 130, and380 nm, respectively, were deposited by e-beam evaporation andpatterned by lift-off process, where the Cr film acts as the adhesionlayer between Pt and SiO2. After cleaning by solvent and DI waterand after blow drying with nitrogen, the substrate with Cr/Pt/Aumetal films was spin-coated with photoresist and was patternedby photolithography using another mask. The substrate was placedinto the Au etchant after removing the developed resist. The Au inthe designed area was removed, and Pt was exposed as the resistor.

Figure 3(a) shows the fabricated SiO2/Cr/Pt/Au microheater chipwith an area of 1.5 mm � 1.5 mm. A drop-casting process was con-ducted to deposit 80 lg Al–CuO MIC or Al–CuO/CL-20 onto theSiO2/Cr/Pt/Au microheater. The Al–CuO MIC and Al–CuO/CL-20were dispersed into ethyl cellulose with the assistance of ultra-sonic to form a thick suspension. A capillary tube that was attachedto a mini-pipet (50 lL) was used to soak up the suspension and todrop the suspension onto the microheater. The capillary tube was100 mm long and had inner and outer diameters of 1.0 and1.2 mm, respectively. After spinning at 400 rpm with a whirlerand after air drying, Au wires were used to connect the contactpad with the conductive substrate by ultrasonic welding (Fig. 3(b)).

2.3. Characterization of combustion process and pressure–timecharacteristics

The morphology and dimensions of the created nanostructureswere determined by using a scanning electron microscope (SEM,S4800). As shown in Fig. 3(c), 0.75A DC direct current was usedto heat the microheater and to ignite the Al–CuO MIC or Al–CuO/CL-20. The combustion processes of Al–CuO MIC and Al–CuO/CL-20 on the SiO2/Cr/Pt/Au microheater chips were captured by HSVR.The reaction was recorded by using Photron, FASTCAM-APX RS2.All experiments were performed in ambient air. The recording ratewas 30,000 frames per second (fps) with a frame size of 256 � 256pixels. The aperture value was shifted to 2.8 to reduce the light intoHSVR. The shape of the flame was determined through the HSVRimages. Pressure–time characteristics for the combustion of Al–CuO MIC and Al–CuO/CL-20 were conducted in a constant-volumeexplosion vessel under atmospheric conditions. The volumes of thevessel and of the tested materials were kept constant at 40 cm3 and1.0000 g, respectively. High-frequency pressure transducers (Xi’anMoshu Instrument Co., Ltd., model MS360) on top of the reactorwere used to measure the pressure change. The samples were

Fig. 2. Schematic fabrication process of the thin film SiO2/Cr/Pt/Au microheater.

Fig. 3. (a) SiO2/Cr/Pt/Au microheater on silicon substrate, (b) microheater chips (samples 2 and 5) and microheater chips with Al–CuO MIC or Al–CuO/CL-20 (samples 1, 3, and4), (c) schematic illustration for igniting the NOC, and (d) NOC after ignition.

J. Shen et al. / Combustion and Flame 161 (2014) 2975–2981 2977

locally ignited by an ignitor (3# nitrocellulose: 0.50 g) that wasinserted into the reactant composites.

3. Results and discussion

3.1. Ball cell model of pressure change in the combustion process

A ball cell model of pressure change in the combustion processis proposed to show the effect of pressure loss on the reaction rateand on the efficiency of energy output at microscale (Fig. 4). Theboundary expands outward under the effect of pressure gradient.Pressure loss (5P) occurs in the boundary of the ball cell whenpressure increment and heat transfer are neglected in the combus-tion process. The following equation can be obtained in accordancewith the equation of state of ideal gas:

P �rPð ÞT�rT

� 4p3

Rþ m � rtð Þ3 ¼ PT� 4p

3R3; ð1Þ

where P is initial pressure, 5P is pressure loss, T is initial tempera-ture, 5T is temperature change from adiabatic expansion, R is

radius of the ball cell, and m and 5t are movement speed and timeof the boundary, respectively. When further simplification is possi-ble, the following equation for the combustion ball cell can bederived:

rPP¼ 1� T �rT

T� 1

1þrt � m=R

� �3

: ð2Þ

For different-sized combustion ball cells, the movement speedof the boundary (m) and its variations are only affected by the dif-ference between the pressure outside and inside of the boundary. Ifm is regarded as constant, the ratio of pressure loss (rP

P ) becomes afunction of R and T. At the macroscopic scale where R is large, rP

P isdetermined by the temperature variables, but 5T can be ignoredwithin a short time (T�rT

T ¼ 1). Therefore, rPP is close to zero accord-

ing to Eq. (2) and pressure loss (5P) can be neglected. However, rPP

cannot be ignored for the combustion of MICs on a microheaterchip because R is at microscale. According to Vielle’s Law, the com-bustion speeds of the propellant increase exponentially with pres-sure [30,37]. Therefore, the non-negligible rP

P can significantly

Fig. 4. Ball cell model of pressure change in the combustion process.

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reduce the reaction rate and efficiency of energy output for thesmall-scale combustion of MICs on a microheater chip. In this case,the pressure loss during the combustion of MICs on a microheaterchip and the compensation for such loss must be determinedbefore conducting practical applications. Pressure loss can be com-pensated for by adding gas-generating materials into MICs, whichwill be described in the latter parts of this paper.

3.2. Microstructure of Al–CuO/CL-20 on the microheater chip

The microstructure of Al–CuO/CL-20 on the SiO2/Cr/Pt/Aumicroheater substrate was characterized by SEM. The CL-20 inFig. 5(a) had a size of 10–20 lm with a prismatic microstructure.Figure 5(b) shows the morphology of Al–CuO MIC after adding50 wt% fine CL-20 particles. The iconograph, which is the magnifi-cation of Al–CuO MIC, showed that the average particle diameter ofAl ranged from 30 nm to 100 nm. Nano–CuO was synthesized bymicrowave method, which indicated a lamellar structure. Theaverage length, width, and thickness of each CuO particle were

Fig. 5. SEM images of (a) fine CL-20, (b) Al–CuO MIC, (c) Al–CuO MIC with 50 wt% finerespectively, (d) top view and (e) cross-section view of Al–CuO/CL-20 composite on the

measured at 2 lm, 200 nm, and 30 nm, respectively. SupportingInformation (Fig. S1) shows the detailed SEM images of nano–Aland nano–CuO.

Al–CuO MIC or CuO–Al/CL-20 mixture particles are depositedonto the SiO2/Cr/Pt/Au microheater substrate by drop casting(Fig. 3(b), samples 1, 3, and 4). Figure 5(c) and (d) show the topand cross-section views of the Al–CuO/CL-20 composite film,respectively. The film of Al–CuO/CL-20 has acceptable uniformityas seen from the top view, and the composite film has a thicknessof 60 lm as seen from the cross-section view. Moreover, the fineCL-20 particles are randomly scattered in the Al–CuO MIC withan even mixing.

3.3. Combustion processes of Al–CuO/CL-20 composite films on themicroheater chip

The combustion behaviors of Al–CuO/CL-20 composite filmswith different mass fractions of CL-20 on the SiO2/Cr/Pt/Au micro-heater chips were investigated by HSVR. Figure 6(a) and (b) showsthe HSVR images of the combustion processes of Al–CuO/CL-20composite films with 100 wt% and 50 wt% Al–CuO MIC on themicroheater chips, respectively. Tables S1–S5 show the combus-tion propagation versus time and flame areas of Al–CuO/CL-20composite films (with CL-20 weight percentages ranging from0 wt% to 100 wt%) on the microheater chips.

Figure 6(a) and Table S1 (see video 1 in Supporting Information,certain parts of the whole combustion process) show that the maincombustion of Al–CuO MIC on the microheater chip lasts for 64 ms,small amounts of bright spots expand outward in the neighboringregions, and the flame area increases at first but decreases after-wards. The maximum flame area is measured at 12 mm2. Thesehot spots do not have a bright color, which indicates a low averagecombustion temperature. However, relatively bright spots areappearing randomly, which indicates a local high combustion tem-perature [38,39]. On the contrary, the main combustion of Al–CuO/CL-20 composite films with 50 wt% Al–CuO MIC on the microheat-er chip lasts for only 4.1 ms (see video 2 Supporting Information,the whole combustion process) with a maximum flame area of120 mm2. Unlike that of Al–CuO/CL-20 composite films with otherweight percentages of Al–CuO MIC, the combustion of Al–CuO/CL-20 composite films with 50 wt% Al–CuO MIC produces strong lightspots, with the largest spot measuring 1.1 cm in diameter (Fig. 6B).

CL-20 particles, where A and B are the fine CL-20 particles and the Al–CuO MIC,SiO2/Cr/Pt/Au microheater substrate (see also Fig. 3(b), samples 1, 3, and 4).

Fig. 6. HSVR images of NOCs with Al–CuO MIC (A) and Al–CuO/CL-20 (B) composite films during their combustion processes.

J. Shen et al. / Combustion and Flame 161 (2014) 2975–2981 2979

Moreover, a large number of hot spots spray violently around themicroheater and may create plasma. Most of the combustion prod-ucts on the microheater chip are ejected from the microheater(Fig. 3(d)). Table 1 summarizes the combustion properties ofAl–CuO/CL-20 composites with different weight percentages ofAl–CuO MIC or CL-20, including their reaction duration, durationto maximum flame, and maximum flame area. The combustiondurations of Al–CuO/CL-20 composites with 0 wt%, 10 wt%,30 wt%, 50 wt%, and 70% fine CL-20 particles are recorded at63.966, 9.665, 4.995, 4.067, and 29.537 ms, respectively, and theirduration to maximum flame are recorded at 1.033, 0.867, 0.700,0.633, and 2.733 ms, respectively. The HSVR images show that noflame is observed when the CL-20 is left alone on the microheater,except for a plume of white smoke with more than 100 ms dura-tion time. Therefore, the addition of 50 wt% fine CL-20 particlesinto Al–CuO MIC results in a short reaction duration, a relativelyshort maximum flame duration, and a large maximum flame area.

In accordance with these observations, the combustion perfor-mance of Al–CuO MIC on the microheater chip improves signifi-cantly after adding a suitable amount of fine CL-20 particles.Compared with the Al–CuO/CL-20 composites, the combustionreaction degree and reaction rate of a single Al–CuO MIC orCL-20 obviously decrease on the microscale chip. The average life-time of the light spots in the Al–CuO MIC reaction is at least a mag-nitude longer than those in the Al–CuO/CL-20 composites with50 wt% CL-20 reactions. Therefore, Al–CuO MIC with a suitable

Table 1Combustion properties of Al–CuO/CL-20 composites with different weight percentages of

Sample Al–CuO MIC (wt%) CL-20 (wt%) Reaction duration (m

1 100% 0% 63.9662 90% 10% 9.6653 70% 30% 4.9954 50% 50% 4.0675 30% 70% 29.5376 0% 100% –

amount of CL-20 is more reactive than a single Al–CuO MIC orCL-20 on the microscale chip. This finding can be attributed tothe fact that only a small amount of gas has been produced duringthe reaction of Al–CuO MIC [28]. However, the thermal decompo-sition of CL-20 during the combustion process of Al–CuO/CL-20composites with 50 wt% releases a large amount of gas [34,40],which greatly increases the pressure. Moreover, the Al–CuO/CL-20 composites with 50 wt% CL-20 demonstrate the best combus-tion properties by combining the high heat from Al–CuO MIC withthe high pressure from CL-20. The generated pressure can consid-erably strengthen the transfer of mass and heat onto the micro-heater chip, which leads to a transition from burning todeflagration.

3.4. Pressure–time characteristics of Al–CuO MIC and Al–CuO/CL-20

The pressure–time characteristics of Al–CuO MIC and Al–CuOMIC with 50 wt% fine CL-20 particles are measured to understandbetter the excellent combustion properties of Al–CuO/CL-20. Figure7(a) shows that the pressure for Al–CuO MIC increases rapidly dur-ing the reaction process and reaches the maximum pressure (Pmax)at 4.5 MPa, but gradually decreases afterwards. However, a higherPmax of 135 MPa is obtained after adding 50 wt% fine CL-20 parti-cles into Al–CuO MIC (Fig. 7(b)). This can be attributed to therelease of high amounts of gas from the CL-20 particles duringthe combustion process. The duration to Pmax for Al–CuO MIC

Al–CuO MIC or CL-20 on the microheater.

s) Time to maximum flame (ms) Maximum flame area (mm2)

1.033 120.867 700.700 1000.633 1202.733 50– No flame (white smoke)

Fig. 7. Pressure–time plots of (a) Al–CuO MIC and (b) Al–CuO MIC with 50 wt% fine CL-20 particles.

2980 J. Shen et al. / Combustion and Flame 161 (2014) 2975–2981

and Al–CuO/CL-20 are recorded at 35 ms (Fig. 7(a)) and 0.13 ms(Fig. 7(b)), respectively, while their pressurization rates are mea-sured at 0.13 and 1038 MPa ms–1, respectively. CL-20 helps pro-mote combustion and enhance the reaction propagationcharacteristics by generating a large amount of gas, which leadsto a significantly improved combustion performance. Therefore,the addition of CL-20 can justly compensate for the pressure lossduring the combustion of Al–CuO MIC on a microscale chip.

4. Conclusions

A ball cell model of pressure change in the combustion processhas been proposed in this paper to show the effects of pressure losson reaction rate and on efficiency of energy output at microscale.The addition of fine CL-20 particles into Al–CuO MIC has been pro-ven as an effective compensation for pressure loss during the com-bustion of the MIC on a 1.5 mm � 1.5 mm SiO2/Cr/Pt/Aumicroheater chip. The combustion of Al–CuO MIC with 50 wt% fineCL-20 particles on the microscale chip produces a much strongerlight spot compared with that produced by the combustion ofAl–CuO MIC. The combustion duration of Al–CuO MIC and Al–CuO/CL-20 composites with 50 wt% CL-20 are recorded at 64 and4.1 ms, respectively, and their pressurization rates are recordedat 0.13 and 1038 MPa ms–1, respectively. These results indicatethat the combustion of Al–CuO/CL-20 is intense deflagration, andthat the pressure losses during the combustion at microscale arejustly compensated by the fine CL-20 particles by releasing a largeamount of gas, which substantially improves the mass and heatdiffusion and subsequently improves the reactivity and combus-tion performance. The Al–CuO/CL-20 composites with 50 wt% CL-20 demonstrate the best combustion properties by combiningthe high heat release from Al–CuO MIC and the high pressure fromCL-20. These findings can be extended to other gas-generatingmaterials (i.e., HMX and RDX) and MICs (i.e., Al–Fe2O3, Al–MoO3,and Al–Co3O4). Therefore, this study provides a general strategyfor pressure loss compensation during the combustion of MICson microscale chips, which is very significant for their practicalapplications.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (No. 11002128, 11272292, 11372288, 11172275and 11172276), Science Foundation for Young Scientist of SichuanProvince (2012JQ0038), and Young Talent Foundation of Instituteof Chemical Materials (QNRC 2012-1, QNRC 2013-10).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.combustflame.2014.04.015.

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