- Email: [email protected]
Fabrication of Al2O3 nanofluid by a plasma arc nanoparticles synthesis system
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of Al2 O3 nanofluid by a plasma arc nanoparticles synthesis system Ho Chang ∗ , Yu-Chun Chang Department of Mechanical Engineering, National Taipei University of Technology, 1 Sec.3, Chung Hsiao E. Road, Taipei 10608, Taiwan, ROC
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper describes the synthesis of Al2 O3 nanofluid with high suspension stability using
Received 10 November 2006
a modified plasma arc system. This system uses high temperature produced by plasma
Received in revised form
arc system to cause the bulk metal to heat and vaporize. The vaporized metallic gas is
28 November 2007
inducted into the collection piping by the induction system. At the same time, it mixes
Accepted 18 December 2007
thoroughly with the pre-condensed deionized water, and the mixture then undergoes a rapid cooling process which helps in grain nucleation and prevents the growth of particle size. Hence, a nanofluid with smaller particle size can be obtained, and this is finally stored in
Keywords:
the collection tank. The suspension stability of the Al2 O3 nanofluid with different pH values
Plasma arc
was analyzed via Zeta potential. The absorption properties of the Al2 O3 nanofluid were
Al2 O3 nanofluid
analyzed using UV–Vis spectrophotometer. Moreover, the fuel calorific test shows that the
Fuel calorific value
combustion efficiency of 92 octane unleaded gas was greater when the weight concentration
UV–Vis absorption
of the Al2 O3 nanofluids was 3%. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Nanomaterials are ultra-fine materials with a granular size of 10−9 m. As early as 1959, Feynman (1959) made a speech at the annual conference of the American Physical Society, presenting many fascinating predictions for the world of nanotechnology. In 1962, the Japanese thermodynamicist Kubo (1962) showed that the electronic level of a fine metallic grain could change according to the difference in the particle size. Gerd Binnig and Heinrich Rohrer of the IBM laboratory in Switzerland developed a new scanning tunneling microscopy (STM) technology in early 1980, developing images of the atomic structure on the surface of objects, which enabled further development for the foundation of nanomaterials. Various techniques have been developed to prepare metal oxide nanoparticles, such as gas-phase chemical reaction, spray pyrolysis, vapor deposition, microwave plasma synthe-
∗
sis, and sol–gel methods (Zhang and Cao, 2003; Chen, 2003; Cao and Deng, 1996; Zheng et al., 2001; Cui et al., 2000). The arc discharge plasma method is a mature and advanced materials processing technique, which has successfully been applied for the production of metal nanoparticles (Ioan, 1999; Wei et al., 2005; Zhiqiang et al., 2006; Hsieh et al., 2006). Chazelas proposed a transferred arc system to produce nanometric particles from the condensation of metallic vapours obtained by controlling the evaporation of the anode material. Chazelas’s experiments show that the heat transfer at the anode precursor strongly depends on the cold boundary layer (CBL) properties close to the anode (Chazelas et al., 2006, 2007). However, the fabrication of nanoscale metal oxides suspended in liquid has been much less thoroughly discussed, and most experimental works on the fabrication of nanoparticle powders have been conducted using plasma arc techniques. This investigation proposes a new process to prepare an Al2 O3
Corresponding author. Tel.: +886 2 27712171x2063; fax: +886 2 27317191. E-mail address: [email protected] (H. Chang). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.070
194
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
nanofluid with high suspension stability using a plasma arc system. Because of its properties of wear-resistance, anti-corrosion, as well as chemical and heat stability, nano aluminum oxide has been widely applied in different commercial applications (Sun et al., 2006; Hesabi et al., 2007). The crystalline phase of Al2 O3 nanoparticles is stable, relatively hard and constant in size, which gives it wide application in products such as plastics, rubber, ceramics and fire-resistant materials (Yatsuya et al., 1973). Gas to particle conversion refers to production of particles from individual atoms or molecules in the gas phase. The method applied in such fabrication is to vaporize the bulk metal with an inert gas such as argon and nitrogen under vacuum condition. When the metal vaporizes into a fog, it will move upward along with the convection of the inert gas and approach the surface of the cooling tube that is filled with liquid nitrogen. As it gets close to the surface of the cooling tube, the metallic vapor firstly form clusters and then nanoparticles. Finally, these nanoparticles accumulate on the surface of the cooling tube, after which they are collected with a scraper to obtain a powder of nanoparticles. The gas condensation method, however, can often result in the loss of various distinctive qualities of nanoparticles because the condensation process itself can cause nanoparticles to aggregate. One of the important challenges to industrial applications of nanoparticles is that, since their sizes are too small, the attractive force between the particles can cause them to aggregate. When commercially available powdered Al2 O3 nanoparticles are poured into deionized water and then dispersed by ultrasonic vibration to become an Al2 O3 nanofluid, aggregation and precipitation in the container commonly be observed within a few minutes. Therefore, nanoparticles need to be dispersed by different methods in order to fully utilize their unique material characteristics. Thus, the one of the main purpose of this paper is to use the proposed nanofluid synthesis system to produce an Al2 O3 nanofluid with good suspension stability, particle dispersion and roundness. Based on the basic theory of gas condensation method, this research presents a system to produce nanoparticles with good suspension stability (Ryogo, 1962; Lieber, 1998). The main equipment of this system includes a plasma arc system, cooling system, vaporized gas induction system and nanofluid collection system. This paper discusses the influence of the working current on the fabricated Al2 O3 nanoparticles. Also, because the Al2 O3 nanofluid can have different pH values, the suspension stability of its Zeta potential is analyzed. Furthermore, the absorption properties of Al2 O3 nanofluid are analyzed by UV–Vis spectrophotometer. Moreover, 92 octane unleaded gas is added into Al2 O3 nanofluids to proceed with a fuel calorific test.
molecular agglomeration. The free energy change G of the generated globe having a radius of r during the process of clustering can be expressed as the following equation (Dai and Wang, 1999; Cui and Hao, 1998): G =
The high energy produced by a plasma arc can vaporize metal. The theory of droplet nuclei generated by vapor can be used to approximate the crystalline nuclei generated by balanced gas phase. A supersaturated vapor would generate clusters of
(1)
where is the interfacial tension and Gv is the free energy change during the generation of unit dimensional liquid by the vapor. When the liquid phase molecules occupy a dimension of v, Gv can be expressed as: Gv = −
kT P ln v Po
(2)
In the position of radius r, G is the maxima G* This can be acquired from dG/dr = 0, and the r and G* are: r∗ =
2 Gv
G∗ =
(3)
16 3 3(Gv )
2
=
16 3 v2 2
3(kT) ln2 P/P0
(4)
In the cluster of r < r* , since the free energy change caused by the molecular adhesion (r increment) is positive, these clusters become unstable and prone to vaporize. On the other hand, in the cluster of r > r* , since the free energy change caused by the molecular adhesion (r increment) is negative, these clusters become stable nuclei. The smaller the barrier height of the generated nucleus G* , the easier it is to generate nuclei. Here, r* is the critical radius, whereas its cluster is the critical nucleus. The abovementioned phenomenon of nucleation temperature can be regarded as the most influential factor. As temperature rises, the concentration of volatile particles will increase, leading to an increase in vapor pressure. According to Eqs. (1)–(4), the high saturation is dependent on the temperature difference between plasma arc and cooling liquid developed by the proposed system. In other words, a high nucleation rate could be obtained if the metal is vaporized at high temperature and condensed at low temperature. Furthermore, the growth rate of nuclei will decrease dramatically if the temperature is diminished. More uniform and smaller nanoparticles can be prepared with the constant pressure control unit and isothermal control unit developed by the proposed system. The working pressure of the proposed system is controlled and maintained at a constant level (12.3 × 10−4 Mpa) along the time frame. Furthermore, the temperature of deionized water can be effectively controlled at 1 ◦ C with variation less than ±0.5 ◦ C.
3. 2. Nucleation theory of nanoparticle by plasma arc
4r2 + 4 3r3Gv
Experimental
In this research a plasma arc discharge system is used as the heat source to produce a nanofluid, together with a vacuum chamber and collection system that was designed and developed by the research team. Fig. 1 is the schematic diagram of the experimental setup (Chang et al., 2004a). This fabrication system is different from the previous dry collection
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
195
Fig. 1 – Schematic diagram of the experimental setup of this research.
method for nanoparticles using a plasma discharge system. In other words, the traditional plasma arc system directly collects metallic particles on the collector, and then scrapes the metallic powder collected therein from the collection surface. The major difference of this experimental equipment is that it induces the metallic nanoparticles directly into deionized water to form a nanofluid. The major equipment used in this experiment includes a plasma arc discharge processing system, nanofluid collection system, cooling circulation system and pressure control system (Chang et al., 2004b; Tsung et al., 2003). The main function of the plasma arc discharge system is to use the high temperature produced by high-energy plasma arc during its instantaneous release to vaporize the metal inside the processed chamber within a short period of time. At the same time, the parameters process of this device can be controlled by varying working current, plasma gas and flow rate of protective gas. The pressure control system uses the pressure differential created between the chamber bodies of the working chamber and collection chamber to induce the particles that become nuclei after vaporization into the collection chamber from the working chamber. The nanofluid collection system and cooling circulation system pre-condenses the deionized water so as to maintain a low temperature during the collection of nanofluid and to further suppress the excess growth and clustering of the particles. First of all, the aluminum bulk inside the crucible is directly heated by plasma arc (15,000 ◦ C) to vaporization temperature, so that the aluminum is vaporized. At this point, the continuous motion of the vacuum pump creates a pressure differential between the collection chamber and the discharge working chamber, inducing the metal that is continuously melting and vaporizing into the collection piping at the instant it leaves the surface of the bulk. At the same time, the valve of the storage tank containing the loading solution (deionized water) is opened, so that the pre-condensed carrying liquid flows into the pipe, mixes with the vaporized metallic vapor and carries the metallic particles away from the discharge working chamber. The low temperate of the carrying liquid (deionized water) will condense the vaporized metal instantly into nanoparticles and finally this fully mixed nanofluid will
be induced into a storage tank to further condense and collect at a low temperature. Thus, the particles suspended in the deionized water will lower its reactivity under a low temperature environment, yielding smaller nanoparticles. Finally, the collected nanofluids are extracted to undergo relative examination of their material properties. The prepared nanofluids were characterized for microstructural properties by a transmission electron microscope (TEM, JEOL JSM-1200EX2) and a field emission scanning electron microscope (FESEM, EM0093). The dry nanoparticle powders were obtained by heating the particle suspension to an appropriate temperature. The crystalline phase was determined by X-ray Diffraction (XRD, MAC-MXP18). All peaks measured by XRD analysis were assigned by comparison with those of the joint committee on powder diffraction standards (JCPDS) data. The zeta potential of Al2 O3 nanofluids was measured using zeta potential analyzer (Zeta Plus, Brookhaven Instruments Co.). The particle size distribution (PSD) of the nanoparticles was measured by dynamic light-scattering measurement (Horiba-LB500). An Ultraviolet–Visible (UV–Vis) spectrophotometer (UV-500, U-2001) is used to analyze the optical property of the nanoparticle. Meanwhile, a fuel calorific value testing machine was used to evaluate the fuel calorific value testing with different particle size and weight concentration of the Al2 O3 nanofluid.
4.
Results and discussion
X-ray diffraction (XRD) is used to confirm the crystal phase of the prepared nanoparticles. By comparing the XRD pattern after examination with the standard spectrum of a JCPD card, it can be seen that the crystal structure of the fabricated particles is Al2 O3 , as shown in Fig. 2. In order to investigate the influence of current towards the fabricated nanoparticles by this improved equipment, all the parameters adopted in the experiment such as vacuum pressure (9.2 Torr), temperature of the collector (1 ◦ C), inert gas (Ar) and plasma gas flow rate etc. were fixed values, with the only variant being the current. The current was set at 25 A, 35 A, 50 A, 65 A and 75 A. Fig. 3 shows the FE-SEM images of the Al2 O3 produced when the current was set at 50 A and 75 A. The approximate parti-
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
Fig. 2 – XRD pattern of Al2 O3 nanoparticles prepared by the proposed synthesis system.
cle sizes of the produced Al2 O3 are measured directly from the FE-SEM images by a Midfun Protech 2500 optical measurement system. Fig. 3(a) shows that the particle size is approximately 20 nm and that in Fig. 3(b) is approximately 70 nm. Furthermore, the fabricated nanoparticles shown in Fig. 3 have good roundness and size uniformity. Fig. 4 is the TEM image of the nanoparticle suspension prepared using the current of 35 A. As shown in Fig. 4, the Al2 O3 nanofluids prepared by the proposed synthesis system indicate good nanoparticle dispersion with a mean particle size of 25 nm. For this experiment, the relation of the current towards the nanoparticles can be analyzed by using different currents. Fig. 5 shows the result of the secondary particle size of nanoparticles prepared under various currents of 25 A, 35 A, 50 A, 65 A and 75 A, and it can be seen that the average particle size of aluminum oxide is 57 nm when the current is 50 A. As it can be seen, when the current decreased from 50 A to 25 A, the average particle size increased from 57 nm to 80 nm. The applied current is related to the efficiency of metal evaporation that is generated by plasma arc on the bulk metal. When the current is smaller than the critical current, since the power generated by plasma arc of single period is insufficient, a plasma arc of multiple periods is required to generate sufficient depth of the metal evaporation. But this phenomenon makes particles leaving the evaporation zone larger than those generated by the critical current. In addition, when the current increased from 50 A to 75 A, the average particle size increased from 57 nm to 130 nm. This indicates that 50 A is a critical current value in the existing system setup of the proposed synthesis system. It was found that the particles prepared by the applied current above 75 A easily precipitate at the bottom of the flask. In contrast, the nanofluid is prepared by an applied current equal to or less than 50 A, precipitates do not easily occur. This phenomenon can be attributed to the fact that surface energy of the prepared particles can generate an effect of Brownian motion that is greater than the gravity caused by sedimentation. All the mean particle sizes shown in Fig. 5 are measured by HORIBA LB500 particle size distribution analyzer. Particle size distribution analyzer normally produces much larger readings than the actual size of the particles. Since the analyzer adopts the dynamic light scat-
Fig. 3 – FE-SEM image of Al2 O3 nanoparticles at currents of (a) 50 A and (b) 75 A.
tering principle, the particle size it measures is in fact that of secondary particles, which is far larger than that of primary particles measured by FESEM and TEM images. The pH value of the Al2 O3 nanofluids is adjusted by HNO3 and NaOH solutions and then their zeta potentials are measured by the zeta potential analyzer. As shown by the zeta potential measurement in Fig. 6, the zeta potential is zero when the Al2 O3 nanofluid is at a pH value of 9.7, which is the isoelectric point (i.e.p). When the pH value is greater than 9.7, the particle surfaces begin to have a negative charge. The
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
197
Fig. 6 – Zeta potentials under various pH values.
Fig. 4 – TEM image of Al2 O3 nanoparticles at the current of 35 A.
original pH value of the prepared Al2 O3 nanofluid is five, which is much lower than the pH of i.e.p. The farther the pH value is from the isoelectric point (pH 9.7), the less the Al2 O3 particles tend to aggregate. Since the pH value of the prepared Al2 O3 nanofluid is far from the i.e.p., the suspension is more stable. Furthermore, the zeta potential of the prepared Al2 O3 nanofluid reaches 40 mV. Hence, even before using the acidic solution to adjust the pH value of the solution, the nanofluid already has electrostatic stability. The strong repulsive force among charged particles reduces the probability of coalescing, thus forming more stable suspension in the alkaline media. In addition, the zeta potential of the prepared Al2 O3 nanofluid can be maintained higher than 30 mV when the nanofluid
Fig. 5 – Relation between average particle size and current of Al2 O3 nanoparticles.
Fig. 7 – UV–Vis absorption spectra of the Al2 O3 nanofluid.
are settled for six months. A nanofluid prepared using this method can remain stable for longer than six months. Moreover, the light absorbency of aluminum oxide solution can be found from using a UV spectrophotometer, and the produced nanofluid can be analyzed through its UV–Vis absorption spectra, as shown in Fig. 7. As can be seen, the produced Al2 O3 nanofluid absorbs UV energy when the wavelength is between 200 nm and 300 nm. Aluminum has a high surface reactive energy and good heat conductance. The combustion efficacy of some fuels can be improved by adding aluminum oxide nano powder to the fuel. This experiment used a fuel calorific value testing machine for fuel calorific value testing with 92 octane unleaded gas as the fuel. Before recovery of the powder, the produced nanofluid was dried. A vacuum funnel was used to sort out the particles of the suspension through a filter paper, and place them in a low temperature oven for drying. After adding aluminum oxide powder into 92 octane unleaded gas and mixing it thoroughly, the mixture of unleaded gas and Al2 O3 nanoparticles was placed inside a fuel calorific value testing machine to proceed with the combustion. Basically, the experimental theory is to place and burn a fixed amount of fuel inside the high-pressure bomb. The released combustion heat will be absorbed by the water within the inner container of the high-pressured bomb, causing the temperature to rise. The temperature rise indicates its combustion efficacy. This experiment consists mainly of two aspects. One set of experiments
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
Fig. 8 – Combustion results for different particle sizes.
is used for comparison by adding and mixing aluminum oxide powder having the same weight percentage concentration but different particle size into 92 octane unleaded gas, and then proceeding with combustion. From the experimental results shown in Fig. 8, it can be seen that with smaller particles, the combustion rate increases, indicating that the combustion efficacy is increased (faster increase in temperature within a time unit). Fig. 8 clearly shows its temperature change. Furthermore, from the end temperature and initial temperature of the fuel combustion, the combustion heat value of the fuel can be calculated. Upon calculation, it is found that the heat value of the aluminum oxide powder having the same weight percentage concentration, but different particle size carries no obvious difference when mixing and burning together with gas. The second set of experiments is to mix the aluminum oxide having the same average particle size of 30 nm but different weight percentage concentrations with the unleaded gas mixture. The experimental results in Fig. 9 show that when the weight percentage concentration increases, the fuel heat value will increase, too. But when the concentration reaches a critical point of 3% and after the combustion ends, the final
Fig. 9 – Combustion results for different concentration of Al2 O3 nanofluid.
Fig. 10 – Relation between fuel heat energy and weight concentration.
temperature will stop increasing and start to fall, meaning that the heat value of the fuel combustion also starts to fall. Fig. 9 also clearly shows the relation between the change of fuel heat value with weight percentage concentration. Fig. 10 shows that along with the increase of weight percentage concentration of the aluminum oxide nanopowder, its fuel heat value will also increase. When the weight percentage concentration reaches 3%, the fuel heat value reaches its best performance, and with further increase of the concentration, its fuel heat value decreases due to incomplete combustion.
5.
Conclusions
This study uses a plasma arc system as the heat source supply together with the collection equipment modified and designed by the research team. The following findings are concluded:
(1) The nanofluid prepared by this method can be stable for longer than six months. The particles collected by such wet collection method have a better particle dispersion then the dry powder. (2) Experimental findings show that processing current of 50 A and collection temperature of 1 ◦ C are the best experimental parameters. The secondary particle size of the Al2 O3 nanoparticles is 57 nm, and the fabricated nanoparticles have good roundness. In addition, the FE-SEM image shows that the mean particles size is approximately 20 nm. (3) The zeta potential of the prepared Al2 O3 nanofluid reaches as high as 40 mV, and can remain above 30 mV when the nanofluid has settled for six months. Hence, even before using the solution to adjust the pH value of the nanofluid, the nanofluid already is electrostaticly stable. (4) It is found that aluminum oxide powder with a smaller particle size can create a higher combustion efficacy upon mixing with 92 octane unleaded gas. When the weight percentage concentration is 3%, better combustion performance can be obtained.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
references
Cao, M.S., Deng, Q.G., 1996. J. Inorg. Chem. 12 (1), 88–93. Chang, H., Tsung, T.T., Chen, L.C., Lin, H.M., Lin, C.K., Lo, C.H., Su, H.T., 2004a. Mater. Trans. Jpn. Inst. Met. 45, 1375– 1378. Chang, H., Tsung, T.T., Chen, L.C., Yang, Y.C., Lin, H.M., Han, L.L., Lin, C.K., 2004b. Mater. Trans. Jpn. Inst. Met. 45, 806– 811. Chazelas, C., Coudert, J.F., Jarrige, J., Fauchais, P., 2006. J. Eur. Ceram. Soc. 26, 499–3507. Chazelas, C., Coudert, J.F., Jarrige, J., Fauchais, P., 2007. J. Eur. Ceram. Soc. 27, 947–950. Chen, D.H., 2003. Powder Technol. 133, 247–251. Cui, Z., Hao, Z.Z., 1998. Thin Solid Films 318, 76–83. Cui, Z.L., Dong, L.F., Hao, C.C., 2000. Mater. Sci. Eng. A 286, 205–210. Dai, X.M., Wang, J.L., 1999. Chinese Powder Technol. 5 (6), 28–35. Feynman, R.P., 1959. There’s plenty of room at the bottom. Annual meeting of the American Physical Society.
199
Hesabi, Z.R., Hafizpour, H.R., Simchi, A., 2007. Mater. Sci. Eng. A 454–455, 89–98. Hsieh, W.J., Wang, C.H., Lai, S.H., Wong, J.W., Shih, H.C., Huang, T.S., 2006. Carbon 44, 107–112. Ioan, B., 1999. Mater. Sci. Eng. B 68, 512. Kubo, R., 1962. J. Phys. Soc. Jpn. 17 (6), 975–986. Lieber, C., 1998. Nature 391, 62–66. Ryogo, K., 1962. J. Phys. Soc. Jpn. 17 (16), 876–880. Sun, J., Pantoya, M.L., Simon, S.L., 2006. Thermochim. Acta 444, 117–127. Tsung, T.T., Chang, H., Chen, L.C., Han, L.L., Lo, C.H., Liu, M.K., 2003. Mater. Trans. Jpn. Inst. Met. 44, 1138–1142. Wei, Z.Q., Qiao, H.X., Yan, P.X., 2005. Acta Metall. Sin. 18 (3), 209–215. Yatsuya, S., Kasukabe, S., Uyeda, R., 1973. Jpn. J. Appl. Phys. 12, 1675–1682. Zhang, W.W., Cao, Q.Q., 2003. J. Colloid Interf. Sci. 257, 237–245. Zheng, H.G., Lang, J.H., Zeng, J.H., 2001. Mater. Res. Bull. 36, 947–954. Zhiqiang, W., Pengxun, Y., Wangjun, F., Jianfeng, D., Qing, W., Tiandong, X., 2006. Mater. Charact. 57, 176–181.
journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of Al2 O3 nanofluid by a plasma arc nanoparticles synthesis system Ho Chang ∗ , Yu-Chun Chang Department of Mechanical Engineering, National Taipei University of Technology, 1 Sec.3, Chung Hsiao E. Road, Taipei 10608, Taiwan, ROC
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper describes the synthesis of Al2 O3 nanofluid with high suspension stability using
Received 10 November 2006
a modified plasma arc system. This system uses high temperature produced by plasma
Received in revised form
arc system to cause the bulk metal to heat and vaporize. The vaporized metallic gas is
28 November 2007
inducted into the collection piping by the induction system. At the same time, it mixes
Accepted 18 December 2007
thoroughly with the pre-condensed deionized water, and the mixture then undergoes a rapid cooling process which helps in grain nucleation and prevents the growth of particle size. Hence, a nanofluid with smaller particle size can be obtained, and this is finally stored in
Keywords:
the collection tank. The suspension stability of the Al2 O3 nanofluid with different pH values
Plasma arc
was analyzed via Zeta potential. The absorption properties of the Al2 O3 nanofluid were
Al2 O3 nanofluid
analyzed using UV–Vis spectrophotometer. Moreover, the fuel calorific test shows that the
Fuel calorific value
combustion efficiency of 92 octane unleaded gas was greater when the weight concentration
UV–Vis absorption
of the Al2 O3 nanofluids was 3%. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Nanomaterials are ultra-fine materials with a granular size of 10−9 m. As early as 1959, Feynman (1959) made a speech at the annual conference of the American Physical Society, presenting many fascinating predictions for the world of nanotechnology. In 1962, the Japanese thermodynamicist Kubo (1962) showed that the electronic level of a fine metallic grain could change according to the difference in the particle size. Gerd Binnig and Heinrich Rohrer of the IBM laboratory in Switzerland developed a new scanning tunneling microscopy (STM) technology in early 1980, developing images of the atomic structure on the surface of objects, which enabled further development for the foundation of nanomaterials. Various techniques have been developed to prepare metal oxide nanoparticles, such as gas-phase chemical reaction, spray pyrolysis, vapor deposition, microwave plasma synthe-
∗
sis, and sol–gel methods (Zhang and Cao, 2003; Chen, 2003; Cao and Deng, 1996; Zheng et al., 2001; Cui et al., 2000). The arc discharge plasma method is a mature and advanced materials processing technique, which has successfully been applied for the production of metal nanoparticles (Ioan, 1999; Wei et al., 2005; Zhiqiang et al., 2006; Hsieh et al., 2006). Chazelas proposed a transferred arc system to produce nanometric particles from the condensation of metallic vapours obtained by controlling the evaporation of the anode material. Chazelas’s experiments show that the heat transfer at the anode precursor strongly depends on the cold boundary layer (CBL) properties close to the anode (Chazelas et al., 2006, 2007). However, the fabrication of nanoscale metal oxides suspended in liquid has been much less thoroughly discussed, and most experimental works on the fabrication of nanoparticle powders have been conducted using plasma arc techniques. This investigation proposes a new process to prepare an Al2 O3
Corresponding author. Tel.: +886 2 27712171x2063; fax: +886 2 27317191. E-mail address: [email protected] (H. Chang). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.070
194
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 193–199
nanofluid with high suspension stability using a plasma arc system. Because of its properties of wear-resistance, anti-corrosion, as well as chemical and heat stability, nano aluminum oxide has been widely applied in different commercial applications (Sun et al., 2006; Hesabi et al., 2007). The crystalline phase of Al2 O3 nanoparticles is stable, relatively hard and constant in size, which gives it wide application in products such as plastics, rubber, ceramics and fire-resistant materials (Yatsuya et al., 1973). Gas to particle conversion refers to production of particles from individual atoms or molecules in the gas phase. The method applied in such fabrication is to vaporize the bulk metal with an inert gas such as argon and nitrogen under vacuum condition. When the metal vaporizes into a fog, it will move upward along with the convection of the inert gas and approach the surface of the cooling tube that is filled with liquid nitrogen. As it gets close to the surface of the cooling tube, the metallic vapor firstly form clusters and then nanoparticles. Finally, these nanoparticles accumulate on the surface of the cooling tube, after which they are collected with a scraper to obtain a powder of nanoparticles. The gas condensation method, however, can often result in the loss of various distinctive qualities of nanoparticles because the condensation process itself can cause nanoparticles to aggregate. One of the important challenges to industrial applications of nanoparticles is that, since their sizes are too small, the attractive force between the particles can cause them to aggregate. When commercially available powdered Al2 O3 nanoparticles are poured into deionized water and then dispersed by ultrasonic vibration to become an Al2 O3 nanofluid, aggregation and precipitation in the container commonly be observed within a few minutes. Therefore, nanoparticles need to be dispersed by different methods in order to fully utilize their unique material characteristics. Thus, the one of the main purpose of this paper is to use the proposed nanofluid synthesis system to produce an Al2 O3 nanofluid with good suspension stability, particle dispersion and roundness. Based on the basic theory of gas condensation method, this research presents a system to produce nanoparticles with good suspension stability (Ryogo, 1962; Lieber, 1998). The main equipment of this system includes a plasma arc system, cooling system, vaporized gas induction system and nanofluid collection system. This paper discusses the influence of the working current on the fabricated Al2 O3 nanoparticles. Also, because the Al2 O3 nanofluid can have different pH values, the suspension stability of its Zeta potential is analyzed. Furthermore, the absorption properties of Al2 O3 nanofluid are analyzed by UV–Vis spectrophotometer. Moreover, 92 octane unleaded gas is added into Al2 O3 nanofluids to proceed with a fuel calorific test.
molecular agglomeration. The free energy change G of the generated globe having a radius of r during the process of clustering can be expressed as the following equation (Dai and Wang, 1999; Cui and Hao, 1998): G =
The high energy produced by a plasma arc can vaporize metal. The theory of droplet nuclei generated by vapor can be used to approximate the crystalline nuclei generated by balanced gas phase. A supersaturated vapor would generate clusters of
(1)
where is the interfacial tension and Gv is the free energy change during the generation of unit dimensional liquid by the vapor. When the liquid phase molecules occupy a dimension of v, Gv can be expressed as: Gv = −
kT P ln v Po
(2)
In the position of radius r, G is the maxima G* This can be acquired from dG/dr = 0, and the r and G* are: r∗ =
2 Gv
G∗ =
(3)
16 3 3(Gv )
2
=
16 3 v2 2
3(kT) ln2 P/P0
(4)
In the cluster of r < r* , since the free energy change caused by the molecular adhesion (r increment) is positive, these clusters become unstable and prone to vaporize. On the other hand, in the cluster of r > r* , since the free energy change caused by the molecular adhesion (r increment) is negative, these clusters become stable nuclei. The smaller the barrier height of the generated nucleus G* , the easier it is to generate nuclei. Here, r* is the critical radius, whereas its cluster is the critical nucleus. The abovementioned phenomenon of nucleation temperature can be regarded as the most influential factor. As temperature rises, the concentration of volatile particles will increase, leading to an increase in vapor pressure. According to Eqs. (1)–(4), the high saturation is dependent on the temperature difference between plasma arc and cooling liquid developed by the proposed system. In other words, a high nucleation rate could be obtained if the metal is vaporized at high temperature and condensed at low temperature. Furthermore, the growth rate of nuclei will decrease dramatically if the temperature is diminished. More uniform and smaller nanoparticles can be prepared with the constant pressure control unit and isothermal control unit developed by the proposed system. The working pressure of the proposed system is controlled and maintained at a constant level (12.3 × 10−4 Mpa) along the time frame. Furthermore, the temperature of deionized water can be effectively controlled at 1 ◦ C with variation less than ±0.5 ◦ C.
3. 2. Nucleation theory of nanoparticle by plasma arc
4r2 + 4 3r3Gv
Experimental
In this research a plasma arc discharge system is used as the heat source to produce a nanofluid, together with a vacuum chamber and collection system that was designed and developed by the research team. Fig. 1 is the schematic diagram of the experimental setup (Chang et al., 2004a). This fabrication system is different from the previous dry collection
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Fig. 1 – Schematic diagram of the experimental setup of this research.
method for nanoparticles using a plasma discharge system. In other words, the traditional plasma arc system directly collects metallic particles on the collector, and then scrapes the metallic powder collected therein from the collection surface. The major difference of this experimental equipment is that it induces the metallic nanoparticles directly into deionized water to form a nanofluid. The major equipment used in this experiment includes a plasma arc discharge processing system, nanofluid collection system, cooling circulation system and pressure control system (Chang et al., 2004b; Tsung et al., 2003). The main function of the plasma arc discharge system is to use the high temperature produced by high-energy plasma arc during its instantaneous release to vaporize the metal inside the processed chamber within a short period of time. At the same time, the parameters process of this device can be controlled by varying working current, plasma gas and flow rate of protective gas. The pressure control system uses the pressure differential created between the chamber bodies of the working chamber and collection chamber to induce the particles that become nuclei after vaporization into the collection chamber from the working chamber. The nanofluid collection system and cooling circulation system pre-condenses the deionized water so as to maintain a low temperature during the collection of nanofluid and to further suppress the excess growth and clustering of the particles. First of all, the aluminum bulk inside the crucible is directly heated by plasma arc (15,000 ◦ C) to vaporization temperature, so that the aluminum is vaporized. At this point, the continuous motion of the vacuum pump creates a pressure differential between the collection chamber and the discharge working chamber, inducing the metal that is continuously melting and vaporizing into the collection piping at the instant it leaves the surface of the bulk. At the same time, the valve of the storage tank containing the loading solution (deionized water) is opened, so that the pre-condensed carrying liquid flows into the pipe, mixes with the vaporized metallic vapor and carries the metallic particles away from the discharge working chamber. The low temperate of the carrying liquid (deionized water) will condense the vaporized metal instantly into nanoparticles and finally this fully mixed nanofluid will
be induced into a storage tank to further condense and collect at a low temperature. Thus, the particles suspended in the deionized water will lower its reactivity under a low temperature environment, yielding smaller nanoparticles. Finally, the collected nanofluids are extracted to undergo relative examination of their material properties. The prepared nanofluids were characterized for microstructural properties by a transmission electron microscope (TEM, JEOL JSM-1200EX2) and a field emission scanning electron microscope (FESEM, EM0093). The dry nanoparticle powders were obtained by heating the particle suspension to an appropriate temperature. The crystalline phase was determined by X-ray Diffraction (XRD, MAC-MXP18). All peaks measured by XRD analysis were assigned by comparison with those of the joint committee on powder diffraction standards (JCPDS) data. The zeta potential of Al2 O3 nanofluids was measured using zeta potential analyzer (Zeta Plus, Brookhaven Instruments Co.). The particle size distribution (PSD) of the nanoparticles was measured by dynamic light-scattering measurement (Horiba-LB500). An Ultraviolet–Visible (UV–Vis) spectrophotometer (UV-500, U-2001) is used to analyze the optical property of the nanoparticle. Meanwhile, a fuel calorific value testing machine was used to evaluate the fuel calorific value testing with different particle size and weight concentration of the Al2 O3 nanofluid.
4.
Results and discussion
X-ray diffraction (XRD) is used to confirm the crystal phase of the prepared nanoparticles. By comparing the XRD pattern after examination with the standard spectrum of a JCPD card, it can be seen that the crystal structure of the fabricated particles is Al2 O3 , as shown in Fig. 2. In order to investigate the influence of current towards the fabricated nanoparticles by this improved equipment, all the parameters adopted in the experiment such as vacuum pressure (9.2 Torr), temperature of the collector (1 ◦ C), inert gas (Ar) and plasma gas flow rate etc. were fixed values, with the only variant being the current. The current was set at 25 A, 35 A, 50 A, 65 A and 75 A. Fig. 3 shows the FE-SEM images of the Al2 O3 produced when the current was set at 50 A and 75 A. The approximate parti-
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Fig. 2 – XRD pattern of Al2 O3 nanoparticles prepared by the proposed synthesis system.
cle sizes of the produced Al2 O3 are measured directly from the FE-SEM images by a Midfun Protech 2500 optical measurement system. Fig. 3(a) shows that the particle size is approximately 20 nm and that in Fig. 3(b) is approximately 70 nm. Furthermore, the fabricated nanoparticles shown in Fig. 3 have good roundness and size uniformity. Fig. 4 is the TEM image of the nanoparticle suspension prepared using the current of 35 A. As shown in Fig. 4, the Al2 O3 nanofluids prepared by the proposed synthesis system indicate good nanoparticle dispersion with a mean particle size of 25 nm. For this experiment, the relation of the current towards the nanoparticles can be analyzed by using different currents. Fig. 5 shows the result of the secondary particle size of nanoparticles prepared under various currents of 25 A, 35 A, 50 A, 65 A and 75 A, and it can be seen that the average particle size of aluminum oxide is 57 nm when the current is 50 A. As it can be seen, when the current decreased from 50 A to 25 A, the average particle size increased from 57 nm to 80 nm. The applied current is related to the efficiency of metal evaporation that is generated by plasma arc on the bulk metal. When the current is smaller than the critical current, since the power generated by plasma arc of single period is insufficient, a plasma arc of multiple periods is required to generate sufficient depth of the metal evaporation. But this phenomenon makes particles leaving the evaporation zone larger than those generated by the critical current. In addition, when the current increased from 50 A to 75 A, the average particle size increased from 57 nm to 130 nm. This indicates that 50 A is a critical current value in the existing system setup of the proposed synthesis system. It was found that the particles prepared by the applied current above 75 A easily precipitate at the bottom of the flask. In contrast, the nanofluid is prepared by an applied current equal to or less than 50 A, precipitates do not easily occur. This phenomenon can be attributed to the fact that surface energy of the prepared particles can generate an effect of Brownian motion that is greater than the gravity caused by sedimentation. All the mean particle sizes shown in Fig. 5 are measured by HORIBA LB500 particle size distribution analyzer. Particle size distribution analyzer normally produces much larger readings than the actual size of the particles. Since the analyzer adopts the dynamic light scat-
Fig. 3 – FE-SEM image of Al2 O3 nanoparticles at currents of (a) 50 A and (b) 75 A.
tering principle, the particle size it measures is in fact that of secondary particles, which is far larger than that of primary particles measured by FESEM and TEM images. The pH value of the Al2 O3 nanofluids is adjusted by HNO3 and NaOH solutions and then their zeta potentials are measured by the zeta potential analyzer. As shown by the zeta potential measurement in Fig. 6, the zeta potential is zero when the Al2 O3 nanofluid is at a pH value of 9.7, which is the isoelectric point (i.e.p). When the pH value is greater than 9.7, the particle surfaces begin to have a negative charge. The
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Fig. 6 – Zeta potentials under various pH values.
Fig. 4 – TEM image of Al2 O3 nanoparticles at the current of 35 A.
original pH value of the prepared Al2 O3 nanofluid is five, which is much lower than the pH of i.e.p. The farther the pH value is from the isoelectric point (pH 9.7), the less the Al2 O3 particles tend to aggregate. Since the pH value of the prepared Al2 O3 nanofluid is far from the i.e.p., the suspension is more stable. Furthermore, the zeta potential of the prepared Al2 O3 nanofluid reaches 40 mV. Hence, even before using the acidic solution to adjust the pH value of the solution, the nanofluid already has electrostatic stability. The strong repulsive force among charged particles reduces the probability of coalescing, thus forming more stable suspension in the alkaline media. In addition, the zeta potential of the prepared Al2 O3 nanofluid can be maintained higher than 30 mV when the nanofluid
Fig. 5 – Relation between average particle size and current of Al2 O3 nanoparticles.
Fig. 7 – UV–Vis absorption spectra of the Al2 O3 nanofluid.
are settled for six months. A nanofluid prepared using this method can remain stable for longer than six months. Moreover, the light absorbency of aluminum oxide solution can be found from using a UV spectrophotometer, and the produced nanofluid can be analyzed through its UV–Vis absorption spectra, as shown in Fig. 7. As can be seen, the produced Al2 O3 nanofluid absorbs UV energy when the wavelength is between 200 nm and 300 nm. Aluminum has a high surface reactive energy and good heat conductance. The combustion efficacy of some fuels can be improved by adding aluminum oxide nano powder to the fuel. This experiment used a fuel calorific value testing machine for fuel calorific value testing with 92 octane unleaded gas as the fuel. Before recovery of the powder, the produced nanofluid was dried. A vacuum funnel was used to sort out the particles of the suspension through a filter paper, and place them in a low temperature oven for drying. After adding aluminum oxide powder into 92 octane unleaded gas and mixing it thoroughly, the mixture of unleaded gas and Al2 O3 nanoparticles was placed inside a fuel calorific value testing machine to proceed with the combustion. Basically, the experimental theory is to place and burn a fixed amount of fuel inside the high-pressure bomb. The released combustion heat will be absorbed by the water within the inner container of the high-pressured bomb, causing the temperature to rise. The temperature rise indicates its combustion efficacy. This experiment consists mainly of two aspects. One set of experiments
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Fig. 8 – Combustion results for different particle sizes.
is used for comparison by adding and mixing aluminum oxide powder having the same weight percentage concentration but different particle size into 92 octane unleaded gas, and then proceeding with combustion. From the experimental results shown in Fig. 8, it can be seen that with smaller particles, the combustion rate increases, indicating that the combustion efficacy is increased (faster increase in temperature within a time unit). Fig. 8 clearly shows its temperature change. Furthermore, from the end temperature and initial temperature of the fuel combustion, the combustion heat value of the fuel can be calculated. Upon calculation, it is found that the heat value of the aluminum oxide powder having the same weight percentage concentration, but different particle size carries no obvious difference when mixing and burning together with gas. The second set of experiments is to mix the aluminum oxide having the same average particle size of 30 nm but different weight percentage concentrations with the unleaded gas mixture. The experimental results in Fig. 9 show that when the weight percentage concentration increases, the fuel heat value will increase, too. But when the concentration reaches a critical point of 3% and after the combustion ends, the final
Fig. 9 – Combustion results for different concentration of Al2 O3 nanofluid.
Fig. 10 – Relation between fuel heat energy and weight concentration.
temperature will stop increasing and start to fall, meaning that the heat value of the fuel combustion also starts to fall. Fig. 9 also clearly shows the relation between the change of fuel heat value with weight percentage concentration. Fig. 10 shows that along with the increase of weight percentage concentration of the aluminum oxide nanopowder, its fuel heat value will also increase. When the weight percentage concentration reaches 3%, the fuel heat value reaches its best performance, and with further increase of the concentration, its fuel heat value decreases due to incomplete combustion.
5.
Conclusions
This study uses a plasma arc system as the heat source supply together with the collection equipment modified and designed by the research team. The following findings are concluded:
(1) The nanofluid prepared by this method can be stable for longer than six months. The particles collected by such wet collection method have a better particle dispersion then the dry powder. (2) Experimental findings show that processing current of 50 A and collection temperature of 1 ◦ C are the best experimental parameters. The secondary particle size of the Al2 O3 nanoparticles is 57 nm, and the fabricated nanoparticles have good roundness. In addition, the FE-SEM image shows that the mean particles size is approximately 20 nm. (3) The zeta potential of the prepared Al2 O3 nanofluid reaches as high as 40 mV, and can remain above 30 mV when the nanofluid has settled for six months. Hence, even before using the solution to adjust the pH value of the nanofluid, the nanofluid already is electrostaticly stable. (4) It is found that aluminum oxide powder with a smaller particle size can create a higher combustion efficacy upon mixing with 92 octane unleaded gas. When the weight percentage concentration is 3%, better combustion performance can be obtained.
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references
Cao, M.S., Deng, Q.G., 1996. J. Inorg. Chem. 12 (1), 88–93. Chang, H., Tsung, T.T., Chen, L.C., Lin, H.M., Lin, C.K., Lo, C.H., Su, H.T., 2004a. Mater. Trans. Jpn. Inst. Met. 45, 1375– 1378. Chang, H., Tsung, T.T., Chen, L.C., Yang, Y.C., Lin, H.M., Han, L.L., Lin, C.K., 2004b. Mater. Trans. Jpn. Inst. Met. 45, 806– 811. Chazelas, C., Coudert, J.F., Jarrige, J., Fauchais, P., 2006. J. Eur. Ceram. Soc. 26, 499–3507. Chazelas, C., Coudert, J.F., Jarrige, J., Fauchais, P., 2007. J. Eur. Ceram. Soc. 27, 947–950. Chen, D.H., 2003. Powder Technol. 133, 247–251. Cui, Z., Hao, Z.Z., 1998. Thin Solid Films 318, 76–83. Cui, Z.L., Dong, L.F., Hao, C.C., 2000. Mater. Sci. Eng. A 286, 205–210. Dai, X.M., Wang, J.L., 1999. Chinese Powder Technol. 5 (6), 28–35. Feynman, R.P., 1959. There’s plenty of room at the bottom. Annual meeting of the American Physical Society.
199
Hesabi, Z.R., Hafizpour, H.R., Simchi, A., 2007. Mater. Sci. Eng. A 454–455, 89–98. Hsieh, W.J., Wang, C.H., Lai, S.H., Wong, J.W., Shih, H.C., Huang, T.S., 2006. Carbon 44, 107–112. Ioan, B., 1999. Mater. Sci. Eng. B 68, 512. Kubo, R., 1962. J. Phys. Soc. Jpn. 17 (6), 975–986. Lieber, C., 1998. Nature 391, 62–66. Ryogo, K., 1962. J. Phys. Soc. Jpn. 17 (16), 876–880. Sun, J., Pantoya, M.L., Simon, S.L., 2006. Thermochim. Acta 444, 117–127. Tsung, T.T., Chang, H., Chen, L.C., Han, L.L., Lo, C.H., Liu, M.K., 2003. Mater. Trans. Jpn. Inst. Met. 44, 1138–1142. Wei, Z.Q., Qiao, H.X., Yan, P.X., 2005. Acta Metall. Sin. 18 (3), 209–215. Yatsuya, S., Kasukabe, S., Uyeda, R., 1973. Jpn. J. Appl. Phys. 12, 1675–1682. Zhang, W.W., Cao, Q.Q., 2003. J. Colloid Interf. Sci. 257, 237–245. Zheng, H.G., Lang, J.H., Zeng, J.H., 2001. Mater. Res. Bull. 36, 947–954. Zhiqiang, W., Pengxun, Y., Wangjun, F., Jianfeng, D., Qing, W., Tiandong, X., 2006. Mater. Charact. 57, 176–181.