Article pubs.acs.org/IECR
Al-Nanoparticle-Containing Nanofluid Fuel: Synthesis, Stability, Properties, and Propulsion Performance Xiu-tian-feng E, Lun Pan, Fang Wang, Li Wang, Xiangwen Zhang, and Ji-Jun Zou* Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ABSTRACT: Nanofluid fuels containing energetic nanoparticles (NPs) are a very promising high-density fuel as the presence of NPs can significantly enhance the fuel’s density and energy. Here we reported a comprehensive study on nanofluid fuels through theoretical and experimental methods. Theoretical calculation shows aluminum (Al) is very effective in improving the volumetric specific impulsion and density. Then, a surface-modification method was developed, and oleic acid is the most effective to stabilize Al NPs in fuel. The surface modification inhibits the contact and agglomeration of NPs and makes them stably dispersed in fuel. With the addition of 30% Al NPs, the density and volumetric energy of JP-10 are increased by 20% and 10%, respectively, and the fuel can flow freely. Combustion test shows the combustion efficiency of Al NPs is higher than 95%, with density specific impulsion being increased by 15% through the addition of 16% Al NPs.
1. INTRODUCTION There has always been a consistent ambition to improve the volumetric energy content of fuel to extend the flight distance of aerospace vehicles, and fuel is an effective way to improve the energy content. High-density fuel, with density higher than conventional refined fuel, can provide more propulsion energy and thus extend the flight range and increase the payload.1−3 Until now, impressive progress has been made in the synthesis of high-density liquid fuel by chemical synthesis. For example, the density of JP-10 (with exo-tetrahydrodicyclopentadiene as major component), the widely used jet fuel, can reach 0.93 g/ mL, and its volumetric heat value of 39.1 MJ/L is 10% higher than that of conventional kerosene.4,5 Liquid fuels with density exceeding 1.0 g/mL also have been synthesized, like RJ-5 (with endo-endo-dihydrodi(norbornadiene) as major component), RJ7, and HDF-T1 (with exo-tetrahydrotricyclopentadiene as major component).1,6−8 However, limited by the characteristic of hydrocarbon molecules, it is hard to further promote the density of liquid fuel by chemical synthesis. Alternatively, energetic solid particles have remarkably higher density and volumetric heating value compared with liquid hydrocarbons. So, it is possible to use these energetic particles to promote the energy content of liquid fuels. Additionally, the presence of particles can reduce consumption and consequently less CO2 and NOx emission.9 Actually, some metal, nonmetal, and oxide NPs have been used as additive and catalyst of liquid fuels.10−12 However, the concentration of these NPs is very low, generally at ppm level, where the NPs are fairly stable in liquid.13−16 Nevertheless, for the current topic, the concentration of NPs has to be high enough to make a considerable contribution to the energy © XXXX American Chemical Society
content of fuel. In this case, the nanofluid is very unstable because the NPs with significantly higher density compared to the surrounding liquid are prone to aggregate and precipitate into sediments. Therefore, stabilizing the liquid−NPs suspension becomes a serious challenge for the utilization of energetic NPs as energetic additives in liquid fuel. The stability of particles in the suspension system depends on the balance of repulsive and attractive force existing between the particles. So, in order to obtain stable suspension nanofluids, repulsive force must be larger as compared to attractive force between the particles. Long-chain surfactant molecules can attach to the particle surface to form an absorbed layer. An overlap of the surfactant layer will produce the repulsive force to overcome the universal van der Waals attraction; thus, stability is maintained.9 Therefore, surface modification is the key to make stable nanofluid fuels. Accordingly, we developed a method to improve the stability of suspension fuel containing particles via surface modification. Previously, we demonstrated the idea by adding surfacemodified boron (B) nanoparticles in JP-10 jet fuel.17 Here we introduced a more comprehensive study on this approach. First, we calculated the propulsive performance of fuel with different kinds of energetic particles and determined the desirable particles. Then, we investigated the surface modification of nanoparticles to explore the difference between different modification reagents, especially the dispersion stability of Received: January 5, 2016 Revised: February 17, 2016 Accepted: February 25, 2016
A
DOI: 10.1021/acs.iecr.6b00043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Illustration of nanofluid fuel containing surface-modified nanoparticles.
internal diameter was used for the propulsion test. No external cooling system was adopted, and the duration of combustion was limited to ensure the safety of the combustor. Oxygen was supplied from a high-pressure vessel, and fuel stored in a cylinder tank was pressurized by nitrogen. The chamber pressure was 2.8−3.0 MPa, and the total mass flow rate of the propellant was 122 g/s with O/F (oxygen to fuel) ratio of 1.7, 1.8, and 1.9. The specific impulse (Isp) was obtained by
particles in various liquid fuels. Basic physical and chemical properties of the nanofluid fuel are measured to show its potential in practical applications. Finally, an engine combustion test was conducted to confirm the improved specific impulse of nanofluid fuel (shown in Figure 1).
2. METHOD AND EXPERIMENT 2.1. Thermodynamic Calculation on Propulsion of Nanofluid Fuels. The theoretical specific impulse (Is) of nanofluid fuels in a rocket engine was calculated as shown below: Is =
Isp =
T mt
where mt is the mass flow rate of fuel and gas including oxygen, fuel, water, and nitrogen used to protect the igniter.
⎡ ⎛ pe ⎞(k − 1)/ k ⎤ 2k R 0 ⎢ ⎥ Tf 1 − ⎜ ⎟ ⎥ k − 1 m ⎢⎣ ⎝ Pc ⎠ ⎦
3. RESULTS AND DISCUSSION 3.1. Theoretical Screen of Energetic Particles. In this work, three energetic particles, including boron, aluminum, and magnesim (Mg), were considered, because they are commonly used in solid propellant as energy additives. As shown in Table 1, the combustion heat of B is very high, but its density is
Here Tf (the constant pressure combustion temperature), m (the average molecular weight of the combustion gas), and k (the specific heat of combustion gas) were calculated using a chemical minimum free energy method.18 The calculation was conducted under the following scenarios: pressure of combustion chamber as 70.924 × 105 Pa, environmental pressure as 1.013 × 105 Pa, fully expanded nozzle, adiabatic combustion, and gaseous combustion products. 2.2. Surface Modification of Al NPs and Structure Characterization. Typically, under argon atmosphere, 10 g of Al NPs (averaged size of 50 nm) was added in 200 g of JP-10 containing a defined amount of surface modification ligand. (Caution! Unmodif ied Al NPs are pyrophoric and should be stored and handled in inert gas atmosphere with extreme care.) The mixture was heated to reflux temperature (ca. 186 °C) under magnetic stirring and maintained for 6 h. Then the particles were purified by centrifugation/wash/redispersion in JP-10 and finally dried at 80 °C for 24 h. The modified NPs were subjected to several characterizations. X-ray diffraction (XRD) patterns were collected on a Rigaku D-max 2500 V/PC X-ray diffractometer (Rigaku Corporation) using Cu Kα radiation source (40 kV, 200 mA). Infrared spectra were recorded on a Vertex 70 FT-IR spectrometer. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F20 field-emission transmitting electron microscope (Philips) operating at 30 kV. Surface composition and chemical states were analyzed with a PHI-1600 X-ray photoelectron spectroscope (XPS) equipped with Al Kα radiation, and the binding energy was calibrated by the C 1s peak (284.6 eV) of contamination carbon. Thermogravimetric analysis (TGA) was conducted on a TGA-50 spectrometer under argon atmosphere. Particle size distribution of the nanofluid was recorded using a Zetasizernano (Malvern, Nano ZS-ZEN3500) instrument. 2.3. Propulsion Test in Combustor. A small scale rocket combustor with chamber of 207 mm length and 52 mm
Table 1. Physical Properties of Energetic Particles and Fuels Involved in This Work fuel aluminum boron magnesium JP-10 HDF-T1 70% HDF-T1 + 30% JP-10
density g/cm3 2.70 2.34 1.74 0.93 1.03 1.01
viscosity mPa s
combustion heat MJ/kg
oxygen consumption kg/kg
3.2 29 13.5
31.1 58.5 25.2 42.1 42.8 42.6
0.88 2.22 0.66 3.08 3.25 3.19
relatively lower; it needs very high oxygen consumption. Al has moderate combustion heat compared with B, but it has higher density and very low oxygen consumption. In contrast, the density, combustion heat, and oxygen consumption of Mg are the lowest. Using JP-10 as the benchmark fuel, we first evaluated the effect of particles on the specific impulsion. Figure 2a shows that the presence of solid particles in JP-10 has a negative effect on the Isp. When the concentration of particles in the fuel reaches 60%, the Isp of Mg-, B-, and Al-containing fuel declines 8.5%, 6.0%, and 5.3% compared to pure JP-10, but Al has the smallest effect on the Isp. As shown in Figure 2b, one advantage of particle addition is to reduce the oxygen demand for combustion. When the ratio of NPs content increases to 0.6, the theoretical oxygen demand of Mg-, B-, and Al-containing fuel decreases to 1.71, 2.65, and 1.85 kg, respectively. Note that the oxygen demand of pure JP-10 is 3.08 kg. It means that, with B
DOI: 10.1021/acs.iecr.6b00043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Theoretical calculated (a) Isp, (b) theoretical oxygen demand, (c) density, and (d) volumetric Isp of nanofluid fuels.
these particles as fuel additives, a rocket can load less oxygen, or an air-breathing vehicle can work well in high altitude where the oxygen concentration is relatively low. Moreover, the density of fuel is greatly improved when particles are added in the fuel, as shown in Figure 2c. Therefore, the volumetric Isp (Isp multiplied by density), which is the key to evaluate the performance of the rocket, is promoted by the presence of particles. As shown in Figure 2d, the volumetric propulsion is increased by 1.4, 1.8, and 2.0 times when 60% Mg, B, and Al NPs are added in the fuel, and reaches 3703, 4773, and 5392 N s/L, respectively. Overall, the Al particle is the most effective in promoting the density specific impulse and reducing the oxygen demand. Thus, here we focus on the stabilization of aluminum nanoparticles in liquid fuel, the properties of obtained nanofluid fuels, and its propulsion. 3.2. Surface Modification of Al NPs. To make Al NPs stably dispersed in liquid fuel with high concentration, the type of amphiphilic ligand is very important. Previously, we found trioctylphosphine oxide (TOPO) can be chelated on the B surface via the B−P interaction and can make B NPs stably dispersed in jet fuel.17 However, the surface chemistry of Al NPs is different from B NPs, and thus the bonding between the organic protecting layer and Al may be different. Nine ligands with a similar long nonpolar carbon chain but different polar groups were tested to screen the best ligand for Al NPs. In this case, the mass ratio of ligand to Al NPs is 1 in the medication reaction. As shown in Figure 3a,b, pristine Al NPs without any modification settle down in the bottle within 1 h when
dispersed in JP-10. After surface modification, the dispersion stability of NPs is improved to some degree. The stability of the suspension was further examined by optical absorption at 600 nm and was recorded with a Hitachi U-3010 spectrometer, using the pristine NPs as reference. The modified NPs show improved stability compared with pristine particles, and it is clearly seen that oleic acid is the most effective among the ligands tested (Figure 3c), with only 8% settlement rate compared to the 17.8% of TOPO modified ones. Therefore, in the following work we focus on oleic-acidmodified Al NPs. 3.3. Structure of Oleic-Acid-Modified Al NPs. XRD patterns in Figure 4a prove that the oleic-acid-modified NPs are pure metallic Al (JCPDS no. 04-0787). The diffraction peaks at 38.5°, 44.7°, 65.1°, 78.2°, and 82.4° are indexed to the {111}, {200}, {220}, {311}, and {222} planes of metallic Al, respectively. This result suggests the ligand only modifies the surface of NPs and has no effect in the bulk crystal phase. Meanwhile, no diffractive patterns of alumina oxide are found, indicating the ligand cladding on the surface of NPs effectively prevents the oxidation of Al NPs. Note that pristine Al NPs are very active, and the surface will be oxidized very quickly when exposed to air.19 According to the Scherrer equation, the average grain size of Al NPs is ca. 50 nm. In the TEM image shown in Figure 4b, the size distribution of oleic-acid-modified Al NPs is around 50 nm, which is in good agreement with the XRD measurement. It is worth mentioning that an amorphous organic layer with thickness of about 4 nm is clearly seen in the high-resolution TEM image. C
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Figure 3. Photographs of modified Al NPs (1%) in JP-10 after standing (a) 1 h and (b) 2 weeks. (c) Relative settlement rate of Al NPs after standing 72 h.
below 100 °C is associated with the evaporation of physical adsorption molecules like residual solvent. The mass loss that occurs from 100 to 450 °C is attributed to the decomposition of oleic acid bound on NP surfaces. According to the amount of weight loss in this range, we calculated the amount of ligand on NP surfaces. With the increase of oleic acid used (x from 0.05 to 1) in the modification reaction, the amount of ligand on NPs is increased gradually. At x = 1, the amount of oleic acid weighs 3.5% of the resulting NPs. However, when further increasing the dosage of oleic acid (x = 1.5), the amount of ligand anchored on NPs begins to decrease. This suggests that the concentration of ligand affects the modification reaction, although the reason is still not clear. As mentioned before, sufficient ligand should be bound on the NP surface to ensure good dispersibility in fuel. So, in the following, the Al NPs were modified using x = 1. 3.4. Dispensability of Al NPs in Fuel. The particle size distribution in JP-10 was measured using Zetasizer. Each sample was tested three times to get an averaged value. As illustrated in Figure 7, pristine Al NPs show two size distribution peaks, with about 70% of NPs bigger than 300 nm. This indicates that most of the NPs agglomerate into big particles that will inevitably settle down during the long-term test. However, for oleic-acid-modified Al NPs, the particle size distribution is 50−100 nm and is in accordance with the original size distribution of particles. We further centrifuged the suspension at 8000 r/min for 10 min, but no obvious
Figure 5a shows the IR spectra of oleic acid and oleic-acidmodified Al NPs. Oleic acid shows CH2 flexural vibration and stretching vibration in the regions of 720 and 2990−3100 cm−1, CO and CO stretching vibration at 1210−1320 and 1710−1780 cm−1, and O−H flexural vibration at 1395−1440 cm−1. Additionally, there are several signals in the “fingerprint region” below 1400 cm−1 corresponding to various lowfrequency modes of the molecule. The modified NPs also exhibit a −CH2 stretching vibration band, confirming that oleic acid is attached on the surface. Compared with pure oleic acid, the CO, CO, and OH peaks of modified Al NPs are weakened relative to other groups, suggesting the COOH groups may interact with Al atoms. XPS characterization was used to further detect the bonding between the organic ligand and Al NPs. The Al 2p spectrum in Figure 5b shows a signal at ∼73.52 eV referring to AlxOy and another at ∼72.65 eV corresponding to Al metal.19 The C 1s vicinity in Figure 5c presents a peak centered at ∼283.5 eV that is assigned to aliphatic carbon and a small shoulder at ∼288.1 eV which may be due to intact carboxylates. No Al−C signals, centered at about 282.5 eV,19 are found in the C 1s spectra. In O 1s region in Figure 5d, there is a strong signal at 531 eV, where AlxOy and Al−O−C signals have been observed previously.20,21 So it can be concluded that oleic acid is attached on the Al NP surface via Al−O−C bonds. TG analysis was conducted to quantify the amount of ligand chelated on Al NPs. In Figure 6, the weight loss at temperature D
DOI: 10.1021/acs.iecr.6b00043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. (a) XRD patterns and (b) TEM and (c) high-resolution TEM images of oleic-acid-modified Al NPs.
precipitate appears on the bottom of the centrifuge tube, which excludes the possibility of big aggregates in the suspension. Moreover, the supernatant still shows a similar size distribution in the range 15−80 nm. These results confirm that surface modification can inhibit the contact and agglomeration of NPs and thus greatly improve the dispensability in fuel. Then, we tested the possibility of stabilizing a large amount of Al NPs in liquid fuel for a prolonged period. Herein, three fuels were considered, as shown in Table 1. JP-10 is a widely used high-density fuel, while HDF-T1 is a newly synthesized fuel with higher density.4−8 Since the viscosity of HDF-T1 is relatively high, a blending with 30% JP-10 was also considered to balance the density and viscosity. So the density and viscosity of the three fuels is pure HDF-T1 > blended fuel > JP-10. 30% of NPs were added in the three fuels, mixed thoroughly by ultrasonication and then stored. During this period, some sediments appear on the bottom due to the formation of big aggregates. The concentration of NPs in the upper suspension was measured to calculate the percentage of NPs dispersed in the fuel. As shown in Figure 8, the settling of particles is obvious in the first 3 days, and the number of NPs in the suspension quickly declines, because unstable particles such as big particles will precipitate. However, it is noted that, after 2 weeks, the nanofluids are significantly stable. After 6 weeks, the amount of NPs suspended in the three fuels is very close, namely, 46.1% in JP-10, 51.8% in HDF-T1, and 48.5% in blended fuel, i.e. about 15% of NPs stably suspend in the fuels.
It is also noted that the stability of NPs in the three fuels is HDF-T1 > blended fuel > JP-10, suggesting that the stability of nanofluid fuel is correlated to the density and/or viscosity of liquid fuel used. 3.5. Basic Properties of Al-NP-Containing Nanofluid Fuels. The density and energy are critical parameters to evaluate the effect of energetic NP addition in liquid fuel. As shown in Figure 9a, the density of all the three fuels increases with the increase of NPs concentration. It is important to see that the fuel density increases almost linearly with the NP concentration, very close to the theoretical density calculated in Figure 2. When 30% NPs are added in JP-10, HDF-T1, and blended fuel, the density rises from 0.93 to 1.12 g/mL, from 1.03 to 1.23 g/mL, and from 1.01 to 1.18 g/mL, respectively. We further calculated the volumetric energy of the three nanofluid fuels according to the energy of Al and fuels provided in Table 1. As exhibited in Figure 9b, the volumetric energy increases with the addition of Al NPs. When 30% NPs are added, the volumetric energy is increased by 10% for JP-10, 9.8% for blended fuel, and 9.5% for HDF-T1, and reaches 43.5, 47.2, and 48.3 MJ/L, respectively. Viscosity is another important parameter for liquid fuel. As shown in Figure 10a, the fuel becomes more viscous when NPs are added, especially when the concentration of NPs exceeds 20%. When the concentration of Al NPs reaches 30%, the viscosity of JP-10 is 62.5 mPa s. The viscosity of Al NPs/HDFT1 and Al NPs/blended fuel increases more obviously. When E
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Figure 5. (a) IR spectra and (b, c, d) XPS binding energy of oleic-acid-modified Al NPs.
Figure 6. TG pattern of Al NPs modified using different dosages of oleic acid.
the concentration of Al NPs reaches 30%, the viscosity is 2000 and 400 mPa s. Even though, the HDF-T1 containing 30% NPs can still flow through a glass slope-like liquid, see Figure 10b. 3.6. Combustion Result. A combustion test was conducted to demonstrate the role of Al NPs in improving the specific impulse, using JP-10 containing 16% Al NPs. It has
Figure 7. Particle distribution of (a) pristine Al NPs, (b) oleic acidmodified Al NPs, and (c) upper suspension after centrifugation at 8000 r/min. F
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Figure 10. (a) Dynamic viscosity of nanofluid fuels with different amount of Al NPs. (b) Time series of 50 mg HDF-T1 containing 30% Al NPs flowing down a 30° and 7 cm glass slope.
Figure 8. Mass concentration of modified Al NPs dispersed in fuels during long-term storage.
Figure 11. Density specific impulse of 16% Al-containing JP-10 tested at different O/F ratio.
and reaches 274 kg s/L. The combustion efficiency of Al NPs is estimated to be 95.6−97.9%. This indicates that the Al NPs are combusted efficiently, and a full combustion is expected when the combustor is further optimized.
4. CONCLUSIONS We developed a simple strategy to synthesize nanofluid fuels containing energetic nanoparticles. Al NPs were selected as the optimal particle species via theoretical calculation of propulsive performance. The surface of Al NPs was modified by oleic acid through the interaction between Al atoms and O−C groups under reflux treatment. The modified NPs are well-dispersed in jet fuels because the contact, agglomeration, and settlement of NPs are prevented. Adding high-concentration Al NPs in fuels can significantly increase the density and volumetric energy. Engine combustion test using JP-10 containing 16% Al NPs proved that the addition of Al NPs could significantly improve specific impulse by 15%. By this work, we showed that the promising potential of nanofluid fuels may be used for aerospace vehicles in the future.
Figure 9. (a) Density and (b) volumetric energy of nanofluid fuels with different amount of Al NPs.
been reported that the combustion of heterogeneous metalcontaining compositions may occur agglomeration, causing incomplete combustion of metal powders. However, nanosized NPs were thought to improve combustion performance in liquid fuels.22,23 As shown in Figure 11, the addition of Al NPs significantly improved specific impulse. At O/F ratio of 1.9, the density Isp is increased by 15% as compared to that of JP-10, G
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geneous catalytic cracking of JP-10 jet fuel. Ind. Eng. Chem. Res. 2014, 53, 12312. (15) E, X.-T.-F.; Zhang, Y.; Zou, J.-J.; Zhang, X.; Wang, L. Shape evolution in Brust-Schiffrin synthesis of Au nanoparticles. Mater. Lett. 2014, 118, 196. (16) Van Devener, B.; Anderson, S. L. Breakdown and combustion of JP-10 fuel catalyzed by nanoparticulate CeO2 and Fe2O3. Energy Fuels 2006, 20, 1886. (17) E, X.-T.-F.; Zhi, X.; Zhang, Y.; Li, C.; Zou, J.-J.; Zhang, X.; Wang, L. Jet fuel containing ligand-protecting energetic nanoparticles: A case study of boron in JP-10. Chem. Eng. Sci. 2015, 129, 9. (18) Bonnie, J. M.; Stanford, G. Computer program for calculation of complex chemical equilibrium compositions and applications-users manual and program description. NASA Reference Publication 1996, 1311. (19) Lewis, W. K.; Rosenberger, A. T.; Gord, J. R.; Crouse, C. A.; Harruff, B. A.; Shiral Fernando, K. A.; Smith, M. J.; Phelps, D. K.; Spowart, J. E.; Guliants, E. A.; Bunker, C. E. Multispectroscopic (FTIR, XPS, and TOFMS−TPD) investigation of the core-shell bonding in sonochemically prepared aluminum nanoparticles capped with oleic acid. J. Phys. Chem. C 2010, 114, 6377. (20) Underhill, R.; Timsit, R. S. Interaction of aliphatic acids and alcohols with aluminum surfaces. J. Vac. Sci. Technol., A 1992, 10, 2767. (21) Bournel, F.; Laffon, C.; Parent, P.; Tourillon, G. Adsorption of acrylic acid on aluminum at 300 K: a multi-spectroscopic study. Surf. Sci. 1996, 352−354, 228. (22) Sambamurthi, J. K.; Price, E. W.; Sigmant, R. K. Aluminum agglomeration in solid-propellant combustion. AIAA J. 1984, 22, 1132. (23) Sippel, T. R.; Son, S. F.; Groven, L. J. Aluminum agglomeration reduction in a composite propellant using tailored Al/PTFE particles. Combust. Flame 2014, 161, 311.
AUTHOR INFORMATION
Corresponding Author
*Tel and fax: 86-22-27892340. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate the supports from the National Natural Science Foundation of China (U1462119, 21476168). NOMENCLATURE Is = theoretical specific impulse Tf = the constant pressure combustion temperature k = the specific heat of combustion gas m = the average molecular weight of the combustion gas pe = the outlet pressure of the nozzle Pc = the pressure of combustion chamber Isp = the specific impulse T = the thrust measured with thrust frame mt = the total mass flow rate of fluid and gas O/F = oxygen to fuel TOPO = trioctylphosphine oxide TOP = trioctylphosphine TPP = triphenylphosphine
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