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Physicochemical Properties of Oil-Based Nanofluids Containing Hybrid Structures of Silver Nanoparticles Supported on Silica Subelia S. Botha,* Patrick Ndungu,† and Bernard J. Bladergroen South African Institute of Advanced Materials Chemistry (SAIAMC), Chemistry Department, University of the Western Cape, Bellville, South Africa ABSTRACT: The structural, rheological, thermal, and dielectric properties of transformer oil-based nanofluids containing silica and silver supported on silica were investigated. Thermal conductivity was found to increase with silica concentration. The greatest enhancement was seen with silver nanoparticles supported on silica at very low weight percent of silica.
’ INTRODUCTION Nanofluids are a new kind of cooling medium containing stable nanoparticles that are uniformly distributed in a heat transfer fluid. It is well-known that crystalline solids have a much higher thermal conductivity than conventional heat transfer fluids, such as ethylene glycol, water, and oil.1 For example, the thermal conductivity of copper at room temperature is 700 times greater than that of water, and 3000 times greater than that of engine oil. Suspending thermally conductive nanoparticles, with sizes in the range of 1-100 nm, in heat transfer fluids greatly enhances the thermal conductivity of the resulting nanofluid. Silver nanoparticles are among some of the most widely investigated nanomaterials because they exhibit unusual optical, electronic, and chemical properties, which depend on their size and shape, and make them extremely important for their possibilities in technological applications.2 Although silver nanoparticles exhibit a number of exotic properties, they are particularly characterized by a very high thermal conductivity.3 In contrast, the high thermal and chemical stability of silica also makes it an attractive additive in heat transfer systems. Mineral oil, a byproduct in the distillation of petroleum to produce gasoline, consists mainly of alkanes and cyclic paraffins. Transformer oil is usually a highly refined mineral oil. Some of the properties which make transformer oil unique to other oils and suitable as a heat management fluid include the high stability at high temperatures, and the excellent electrical insulating properties. Since earlier work focused mainly on water and ethylene glycol based nanofluids,4-6 very few reports of the synthesis of oilbased nanofluids have been found.7 In a study by Xuan and Li,8 it was found that oil based nanofluids have superior characteristics compared to water based nanofluids, and that the viscosity of the oil could be crucial for the dispersion and stability of nanofluids. Lee at el. reported that a higher thermal conductivity enhancement can be obtained if a base fluid of lower thermal conductivity is used.9 Therefore, oil-based nanofluids containing carbon nanotubes, TiO2, CuO, Al2O3, AlN and SiO2, for industrial and engineering applications, have attracted some more attention in recent years.10-12 In addition, most of the nanofluids in literature were prepared by a two-step method, where the nanoparticles are r 2011 American Chemical Society
synthesized in the first step, followed by the second step of dispersing the nanoparticles in the base fluid.12,13 The one step method, where the nanoparticles are produced and simultaneously dispersed into the base fluid, is a better method for preparing metallic nanoparticles because agglomeration is greatly reduced.14 Because of the electrical insulation that silica offers, silicacontaining nanofluids can be very important to certain industries where cooling is required for example in high voltage applications. Oil-based nanofluids with varying concentrations of silica and silver nanoparticles were prepared and the thermal conductivity measured. Low concentrations of silver were used due to the poor dissolution of the silver salt in the oil. Surfactants are necessary to prevent particle aggregation; however, in literature15,16 and previous studies,17 it was shown that the surfactant suppressed the thermal conductivity, since they coat the surface of the nanoparticles, resulting in the screening effect on the heat transfer capabilities of the silver nanoparticles. Hence, all the nanofluids were prepared without any surfactant.
’ EXPERIMENTAL SECTION Materials. All chemicals used in this investigation were reagent grade materials. Silver nitrate (99.8% purity) was purchased from Kimix, transformer oil (d = 0.86 kg/l) was obtained from Eskom Brackenfell and fumed silica (SA 380 m2/g, 99.8%) was purchased from Aldrich. Instrumentation. UV-VIS spectra were recorded with a 1 cm path-length quartz cell using Perkin-Elmer 330 Spectrophotometer. The size and morphology of the silver particles were examined by transmission electron microscopy (TEM). TEM measurements were performed using a Hitachi H 800 instrument operated at an accelerating voltage of 200 kV and a Leo 912 in-column EFTEM with OMEGA spectrometer. A drop of the silver nanofluids in toluene was placed on a carbon-coated copper grid, which was Received: May 13, 2010 Accepted: January 20, 2011 Revised: January 12, 2011 Published: February 04, 2011 3071
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Industrial & Engineering Chemistry Research allowed to dry before being used for observation under the microscope. The average particle size and the size distribution of silver nanoparticles were obtained by image analysis of these micrographs, measuring at least 100 particles. XRD analysis was used to determine crystallinity. XRD spectra were recorded using a Bruker AXS D8 Advance diffractometer. ), on XRD data were taken with CuKR radiation (λ = 1.5418 Å the powder diffractometer operated in the θ/2θ mode primarily in the 30-90° (2θ) range and step-scan of Δ2θ = 0.05°. Liquid samples were prepared using filtration membranes (47 mm Nitrocellulose (Millipore) with pore size of 0.025 μm), where samples were diluted with toluene and filtered through the membranes. Analysis was done on the membranes containing the nanoparticles. All dielectric strength measurements were performed at ESKOM Enterprises, Brackenfell, using a BAUR Dielectric tester DTA. 1-Liter samples were needed for the analysis. Thermal conductivity measurements were performed using the transient hot-wire method.8 Calibrations were done using ethylene glycol and transformer oil before measuring nanofluid samples, and errors in the measurement were found to be approximately (1.0%. Synthesis of Nanofluids. All experiments were done in triplicate. Nanofluids containing silica, with silica concentrations ranging from 0.07 wt % to 4.4 wt %, were prepared by a one-step method where silica was mixed together with the base fluid by means of magnetic stirring and allowed to stir for 2 h at 130 °C. Because of the poor stability of the silica nanofluids in the absence of surfactant, thermal conductivity measurements were performed on freshly prepared samples, before settling occurred. Silver nanoparticles (0.1 wt % to 0.6 wt %) supported on silica were prepared in a similar way by introducing the silver nitrate and silica to the base fluid. Since high temperatures could lead to oxidation of the oil18 and hence the reduction of Agþ ions to Ag by electron transfer reaction, the temperature was increased to 130 °C for 2 h. A typical procedure for preparing a nanofluid suspension containing 0.3 wt % silver on 0.07 wt % silica is as follows: 0.2183 g AgNO3 (1.29 mmol) and 0.0294 g SiO2 (0.49 mmol) were added to 50 mL oil and stirred for 1/2 h. The temperature was then increased to 130 °C and the mixture was allowed to stir at that temperature for 2 h. After cooling to room temperature, the nanofluid was structurally characterized using UV-vis, TEM, and XRD. The rheological properties, thermal conductivity and dielectric strength of the nanofluids were also measured. A summary of the oil-based nanofluids that were synthesized containing silica, with and without silver nanoparticles, is given in Table 1.
’ RESULTS AND DISCUSSION Rheological Properties of Nanofluids. Since nanofluids are used under continuous-flow conditions,12 the rheological properties, such as viscosity, of the nanofluids are of utmost importance. Furthermore, the stability of the nanofluids also plays a crucial role in order to prevent clogging of channels in heattransfer systems. Silica (especially fumed silica), is known for its ability to increase the viscosity of organic media and nonpolar liquids, such as mineral oil and other hydrocarbons, by forming interparticle linkages arising from the net attractive forces between particles. When such linkages are formed extensively and span the sample volume, the result is a colloidal gel (i.e., a three-dimensional network of particles).19-21 Since silica forms
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Table 1. Summary of the Different Oil-Based Nanofluids Synthesized with and without Silver Nanoparticles Ag (wt %)
SiO2 (wt%)
0.00
0.07
0.00 0.00
0.50 1.40
0.00
1.80
0.00
4.40
0.10
0.07
0.30
0.07
0.60
0.07
0.10
0.50
0.30 0.60
0.50 0.50
Figure 1. Viscosity curves for 1.4 wt % silica in oil, 0.60 wt % Ag/1.4 wt % silica, 0.60 wt % Ag/0.07 wt % silica, 0.30 wt % Ag/0.07 wt % silica and oil. The nanofluid suspensions containing silver nanoparticles supported on 0.07 wt % silica showed Newtonian behavior and the two viscosity curves were identical and slightly more viscous than the oil.
a colloidal gel at high concentrations, a yield value is expected. This is due to intermolecular forces (Van-der-Waals), which includes dipole-dipole interactions among the particles and between the particles and the surrounding base fluid.19,14,21 In general, samples which display yield points tend to flow inhomogeneously and only begin to flow when the external force acting on the material is larger than the internal structural forces.22 By measuring the viscosity of the oil based silica nanofluids, the yield point can be determined, as well as the maximum concentration of silica that can be integrated into oil based nanofluids before inhomogeneous flow and colloidal gelation become factors. Figure 1 shows the viscosity curves for different nanofluid systems. Both suspensions containing 1.4 wt % silica showed shear-thinning behavior at low shear rates, after which idealviscous behavior was observed (figure 1). Shearing probably caused the three-dimensional network to disintegrate, resulting in a decrease in the interaction forces among the particles and hence a lowering in the flow resistance. A similar observation was noted by Chen et al. with oil based suspensions of fumed silica.21 In addition, the suspension containing Ag supported on silica (0.60 wt % Ag/1.4 wt % silica) showed a lower viscosity compared to the nanofluid containing silica without Ag. This is a unique feature of the Ag-silica system and could be the result of the immobilization of silver nanoparticles on the silica, decreasing the available active surface area of the silica particles that is 3072
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Figure 3. UV-vis spectra obtained for silica (without silver nanoparticles) and Ag nanoparticles (0.60 wt %) supported on silica (0.07 wt %). Figure 2. Flow curves for 1.4 wt % silica in oil, 0.60 wt % Ag/1.4 wt % silica, 0.60 wt % Ag/0.07 wt % silica and 0.30 wt % Ag/0.07 wt % silica.
involved in the development of the three-dimensional network that it is known to form.19 However, when a lower concentration of silica was used, a different observation was made in the viscosity curves. Both suspensions containing different concentrations of silver nanoparticles supported on 0.07 wt % silica showed Newtonian behavior (figure 1). This is possibly due to the much lower concentration of silica, resulting in a relatively large separation distance between the particles and hence no significant interaction forces between the particles, which allowed it to have a negligible effect on the viscosity. Figure 2 shows the behavior of silica-based nanofluids in terms of the applied shear stress and resultant shear rate. The trend line of the correlation was calculated and the equation of the relation was investigated. The flow curves revealed that both suspensions containing 1.4 wt % silica had yield values and therefore followed the Bingham flow model22 (Figure 2). Figure 2 also showed the flow behavior of both 0.60 wt % Ag and 0.30 wt % Ag supported on 0.07 wt % silica. The Newtonian behavior (Figure 1) was confirmed in the flow curve (Figure 2), which revealed no yield value and therefore followed the ideal Newtonian flow model. The two suspensions, although different concentrations of silver were used, had identical flow behavior. Since the silver nanoparticles are fixed on the silica, the resulting flow behavior could be due to the silica and not the silver nanoparticles. Newtonian behavior is only seen at low wt% of silica, thus Ag/silica samples were made using 0.07 wt % silica. UV-vis Spectroscopy of Silver Nanoparticles Supported on Silica Dispersed in Oil. UV-vis spectroscopy could be used to predict nanoparticle sizes prior to TEM analysis. It is noted that silica has no absorption peak in the UV-vis spectrum (Figure 3). The Ag/silica nanofluid however, gave rise to two absorption peaks at 384 and 443 nm, which was also very broad. Hence, a broad particle size distribution is expected because of the presence of smaller and larger particles. The exact position of this plasmon band is extremely sensitive to particle size and shape and to the optical and electronic properties of the medium surrounding the particles.23 Silica is electronically inert (it does not exchange charge with the silver particles), but its refractive index is different from that of silver. Hence, since all the particles appeared to be spherical and monodispersed on the silica support, the observed spectrum could be as a result of the electronic properties of the surrounding medium.
TEM and XRD Characterization of Silver Nanoparticles Supported on Silica Dispersed in Oil. Particle size and mor-
phology was studied using TEM. Figure 4A shows the TEM micrograph of commercially obtained silica in oil. Since no surfactant was used, the silica particles appeared to have agglomerated, as evidenced by the TEM image in Figure 4A. Silver nanoparticles of varying concentrations were then synthesized and supported on 0.07 wt % silica in one single step. Figure 4 B-C shows the TEM micrograph of 0.60 wt % silver nanoparticles supported on 0.07 wt % silica, with corresponding particle size distribution. Well-dispersed silver nanoparticles were successfully deposited on the silica surface. Particles are spherical in shape with a particle size distribution of 5.5 ( 2.4 nm (Figure 4B-C). The broad size distribution was therefore successfully predicted from the two peaks observed in the UV-vis spectrum (Figure 3). The Ag/silica dispersion however, was not very stable, as expected, since no surfactant was used. Hence, particles started settling within 1 h. Therefore, all analysis were done before settling occurred. The TEM images (Figure 4A and B) confirmed that, although silver nanoparticles were well dispersed on the support, the silica particles agglomerated. The viscosity of the oil therefore did not offer enough support toward forming stable dispersions of the Ag/silica particles in oil. After 4 months, the same scenario of well-dispersed silver particles supported on silica was observed (Figure 4D). No free particles were visible in the TEM micrograph and it would appear that all the silver nanoparticles stayed intact on the surface of silica. Particles are mainly spherical in shape with a particle size distribution of 4.5 ( 1.6 nm (Figure 4E). The Ag nanoparticle sizes did not change much after 4 months. Most of the bigger and hence heavier Ag/silica particles settled after 4 months, leaving behind the lighter (smaller) and well dispersed Ag nanoparticles on silica, suspended in oil. The XRD pattern obtained for silver nanoparticles supported on silica consisted of many sharp peaks, which revealed the crystalline nature of the material (Figure 5). The crystallite sizes were determined by means of the Scherrer formula. Considering the [111] direction in the XRD spectrum, a value of d = 9.52 nm was found. The value obtained from TEM micrographs was much lower (Figure 4). The disagreement between the crystal size obtained from TEM and XRD data is due to the fact that in the X-ray pattern, mainly the large nanocrystals contribute to the Bragg peaks. In addition, the Scherrer formula is based on size limited bulk structure. Hence, relating particle size to the peak width cannot be used with great accuracy for particles of very small size.24 3073
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Figure 4. (A) TEM micrograph showing commercially obtained silica dispersed in oil, (B) TEM micrograph showing the well dispersed silver particles (0.60 wt %) supported on the silica (0.07 wt %) with (C) corresponding particle size distribution graph, and (D) TEM micrograph showing the well dispersed silver particles (0.60 wt %) supported on the silica (0.07 wt %) after 4 months with (E) corresponding particle size distribution graph.
Figure 5. XRD pattern obtained for 0.60 wt % Ag supported on 0.07 wt % silica.
Thermal Conductivity of Nanofluids. Figure 6 shows the thermal conductivity of silica with concentrations ranging from 0.07 wt % to 4.4 wt %. A 1.7% increase in thermal conductivity was observed with 0.5 wt % silica and a 3.5% increase for 1.8 wt % silica, without silver nanoparticles, in oil. The highest concentration of silica under investigation was 4.4 wt % and resulted in a 5.2% increase in thermal conductivity (Figure 6). However, the higher the concentration of silica, the more gel-like the suspensions. Recent work by Shalkevich et al. has shown such gel like suspensions have a higher thermal conductivity because of particle-particle contact.25 The maximum enhancement seen with our silica in oil samples is with the gel-like suspensions, which does indicate that the main mechanism for heat transfer is via particle-particle contact, and not Brownian motion. Thus, low wt% samples will only show small increases in thermal conductivity when compared to the higher wt% samples. The nanofluid containing high concentration of silica showed shear-thinning behavior with a yield value (figure 1 and figure 2). Therefore, a silica concentration of 0.07 wt % was chosen to ensure a homogeneous suspension for the silver nanoparticles/
Figure 6. Thermal conductivity of silica with concentrations ranging from 0.07 wt % to 4.4 wt %.
silica nanofluids. Additionally, it should be noted that the silica will not inhibit the heat transfer process, as evidenced by the thermal conductivity results obtained for silica in oil. When Ag was supported on silica, the thermal conductivity was found to increase with an increase in silver concentration (Figure 7). A thermal conductivity increase of 15% was obtained when only 0.60 wt % Ag was supported on 0.07 wt % silica. It would appear that the silver nanoparticles need to be close enough for thermal transport to take place between them, and supporting the particles on a suitable support provides good grounds for a stable heat transfer system. When silver nanoparticles are fixed on a support (Figure 8, scenario B), the probability of a phonon reaching another particle is far greater. Hence, an enhancement in the heat transfer process is observed as opposed to free particles in a suspension at varying distances from each other (Figure 8, scenario A). In addition, free particles in a suspension are in constant motion because of Brownian motion. Therefore, the probability of particles colliding to form bigger agglomerates is much greater than when a 3074
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stable system is created by supporting the particles on a suitable support. Silica is not only a good support for silver nanoparticles but has a slightly higher thermal conductivity (1.4 W/m-K) than transformer oil (0.110 W/m-K). Therefore, nanofluids containing silica can enhance the heat transfer properties of transformer oil.
With silica in oil, the increase in thermal conductivity with increasing wt % and viscosity can be attributed to increase in particle-particle contact. At 0.07 wt % silica, there is negligible enhancement with the thermal conductivity, and the Ag on the silica results in little change in the rheological properties of the nanofluid when compared to the base fluid. Thus the Ag/silica nanofluid enhancement is unlikely because of particleparticle contact. The smaller Ag nanoparticles on silica have a large surface area and are more effective in transferring heat to and from the base fluid.13 The Ag nanoparticles are fixed on large silica particles, thus enhancement due to Brownian motion seems unlikely. Simply accounting for the individual contribution of each component in the nanofluid does not account for the observed enhancement, that is, if we apply the effective medium theory26,27 (eq 1), to the Ag nanoparticles present in the nanofluid, the theoretical increase is much lower than that observed (Figure 9). This does suggest there is some kind of synergistic effect between the Ag nanoparticles and the silica support, and not a simple additive effect. 3ðKp =Kf -1Þφ Keff ð1Þ ¼ 1þ Kf ðKp =Kf þ2Þ-ðKp =Kf -1Þφ
Figure 7. Thermal conductivity increase as a function of Ag concentration, supported on 0.07 wt % silica. Because of the poor dissolution of silver salt in oil, a maximum concentration of 0.60 wt % silver was used.
Recently, Chen et al demonstrated that nanoparticle clustering can account for the enhanced thermal conductivity observed with nanofluids.28 This does support the simplified scenario presented in figure 8; and thus the current system of supported Ag nanoparticles on silica, which can be viewed as a similar clustered aggregate of nanoparticles, seems to favor a similar mechanism for the enhanced thermal conductivity demonstrated in this work. Dielectric Strength of Nanofluids. As mentioned previously, the stability and viscosity of the nanofluids play an important role in the development of nanofluids. In oil-based nanofluids, where transformer oil is used as the base fluid, the dielectric strength of the resulting nanofluids needs to be investigated. The dielectric strength is the most important electrical property of transformer oils. These oils are designed to provide electrical insulation under high electric fields. If the maximum electric field strength is reached, breakdown occurs and the oil would therefore experience failure of its insulating properties. Therefore, a high dielectric strength is a prerequisite for good quality transformer oil. The dielectric strength of the transformer oil-based nanofluids containing silica and Ag/silica, with Newtonian flow behavior, were investigated. The results are summarized in Table 2 below. The difference in the dielectric strength observed after the introduction of silica to the oil could possibly be due to water, which is one of the main factors that reduce the breakdown voltage. Since silica is known to absorb moisture, it could possibly be responsible for the increase in water content observed (Table 2). Some other factors such as the presence of oil-degradation byproduct and high temperatures, may possibly have aided in the
Figure 8. Schematic diagram showing (scenario A) the nanofluid system containing unsupported silver nanoparticles at varying distances apart and (scenario B) silver nanoparticles fixed at short distances from each other on the silica support.
Figure 9. Comparison of theoretical values calculated using Maxwell equation, and experimental values obtained.
Table 2. Dielectric Strength Results Obtained for Transformer Oil-Based Nanofluid Containing Silica and Silver Supported on Silica sample transformer oil (heat treated) transformer oil-based nanofluid containing silica
dielectric strength (kV)
water (ppm)
acidity (mg/KOH/g oil)
56 34
30 36
0 0.01
transformer oil-based nanofluid containing dried silica
30
34
0.01
transformer oil-based nanofluid containing silver nanoparticles supported on silica
22
64
0.02
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Industrial & Engineering Chemistry Research reduction of the dielectric strength. Furthermore, high temperatures could also lead to oxidation of oil (and hence the reduction of silver) which could increase the acidity of the resulting nanofluid due to the presence of carboxylic acids. The acidity and the moisture content of the nanofluids were determined and the results are given in Table 2. From Table 2, the acidity of the nanofluids containing silica seems to be within limits compared with the acidity obtained for the pure oil. However, an increase in moisture content was found in the nanofluids containing silica, which could possibly explain the reduced dielectric strength. The highest moisture content was observed in the nanofluid containing silver supported on silica. The reduction in the dielectric strength observed when silver nanoparticles were supported on the silica could be due to both the amount of moisture present and the presence of electrically conductive silver nanoparticles.
’ CONCLUSIONS Oil-based nanofluids containing silica have been successfully prepared using a high temperature pathway. The viscosity of the silica nanofluids were found to be shear-thinning and followed the Bingham flow model at high concentrations, which could be due to the 3-dimensional network silica is known to form. However, at lower silica concentrations, the nanofluids were found to be Newtonian, probably due to insignificant interaction between particles. Thermal conductivity was found to increase with silica concentration. Oil-based nanofluids containing silver nanoparticles with particle size distribution of 5.5 ( 2.4 nm supported on silica have been successfully prepared using a high temperature pathway. Small silver particles were deposited uniformly on the silica support. Nanofluids containing Ag supported on silica showed a lower viscosity compared to the nanofluid containing unsupported silica. This could be due to the immobilization of silver nanoparticles on the silica, preventing the formation of the threedimensional network that silica is known to form. An enhancement in thermal conductivity of 15% was observed when 0.60 wt % silver was supported on 0.07 wt % silica. The enhancement in thermal conductivity could be due to the fixed distances of the nanoparticles on the silica support, allowing the phonons to travel shorter distances from particle to particle as compared to free particles at random distances from each other, and does seem to support the nanoparticle clustering mechanism for thermal enhancement. Thermal conductivity was found to increase with an increase in silver concentration. The dielectric strength measurements showed much reduced electrical insulating properties of the oil, due to the introduction of electrically conductive silver nanoparticles, but requires more intensive research in this particular field before use in transformer applications. The current composite system of silver nanoparticles supported on silica, and dispersed in transformer oil without the use of surfactant was found to be stable for approximately one hour. This limitation may hamper its use in real world applications; however, further study in a continuously flowing system (which may provide the necessary agitation to maintain the dispersion) to test its stability, and thermal conductivity would be needed to conclusively ascertain its usefulness in real world applications. ’ AUTHOR INFORMATION Corresponding Author
*Address: Electron Microscope Unit, University of the Western Cape, South Africa. E-mail:
[email protected]. Fax: þ27 21 959 9378.
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Present Addresses †
School of Chemistry, University of Kwa-Zulu Natal, Durban, South Africa.
’ ACKNOWLEDGMENT The authors would like to acknowledge Eskom Holdings, Ltd., and the National Research Foundation for financial assistance. Nazeem George, Remi Bucher, and Mohamed Jaffer are acknowledged for their assistance in viscosity, XRD, and TEM measurements, respectively. ’ REFERENCES (1) Eastman, J. A.; Phillpot, S. R.; Choi, S. U. S.; Keblinski, P. Thermal transport in nanofluids. Annu. Rev. Mater. Res. 2004, 34, 219. valos(2) Slistan-Grijalva, A.; Herrera-Urbina, R.; Rivas-Silva, J. F.; A Borja, M.; Castillo n-Barraza, F. F.; Posada-Amarillas, A. Assessment of growth of silver Nanoparticles synthesized from an ethylene glycol-silver nitrate-polyvinylpyrrolidone solution. Phys. E 2005, 25, 438. (3) Patel, H. E.; Das, S. K.; Sundararajan, T.; Nair, A. S.; George, B.; Pradeep, T. Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects. Appl. Phys. Lett. 2003, 83, 2931. (4) Murshed, S. M. S.; Leong, K. C.; Yang, C. Enhanced thermal conductivity of TiO2-water-based nanofluids. Int. J. Therm. Sci. 2005, 44 (4), 367. (5) Murshed, S. M. S.; Leong, K. C.; Yang, C. Investigations of thermal conductivity and viscosity of nanofluids. Int. J. Therm. Sci. 2008, 47, 560. (6) Eastman, J. A.; Choi, S. U. S.; Li, S.; Yu, W.; Thompson, L. J. Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles. Appl. Phys. Lett. 2001, 78 (6), 718. (7) Choi, C.; Yoo, H. S.; Oh, J. M. Preparation and heat transfer properties of nanoparticle-in-transformer oil dispersions as advanced energy-efficient coolants. Curr. Appl Phys. 2008, 8, 710. (8) Xuan, Y.; Li, Q. Heat transfer enhancement of nanofluids. Int. J. Heat Fluid Flow. 2000, 21, 158. (9) Hwang, Y.; Lee, J. K.; Lee, C. H.; Jung, Y. M.; Cheong, S. I.; Lee, C. G.; Ku, B. C.; Jang, S. P. Stability and thermal conductivity characteristics of nanofluids. Thermochim. Acta 2007, 455, 70. (10) Chen, L.; Xie, H. Silicon oil based multiwalled carbon nanotubes nanofluid with optimized thermal conductivity enhancement. Colloids Surf., A. 2009, 352 (1-3), 136. (11) Wong, K. V.; De Leon, O. Review Article: Applications of Nanofluids: Current and Future. Adv. Mech. Eng. 2010, 1, No. 519659. (12) Murshed, S. M. S.; Tan, S.-H.; Nguyen, N.-T. Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics. J. Phys. D: Appl. Phys. 2008, 41 (8), 085502. (13) Li, D.; Hong, B.; Fang, W.; Guo, Y.; Lin, R. Preparation of welldispersed silver nanoparticles for oil-based nanofluids. Ind. Eng. Chem. Res. 2010, 49, 1697. (14) Nsofor, E. C. Recent patents on nanofluids (nanoparticles in liquids) heat transfer. Recent Pat. Mech. Eng. 2008, 1, 190. (15) Kim, S. H.; Choi, S. R.; Kim, D. Thermal conductivity of metal-oxide nanofluids: Particle size dependence and effect of laser irradiation. Trans. ASME 2007, 129, 298. (16) Liu, M.-S.; Lin, M. C-C.; Tsai, C. Y.; Wang, C.-C. Enhancement of thermal conductivity with Cu for nanofluids using chemical reduction method. Int. J. Heat Mass Transfer 2006, 49 (17-18), 3028. (17) Botha, S. S. Synthesis and characterization of a nanofluids for cooling applications. PhD Dissertation, University of the Western Cape, Bellville, RSA, October 2007. (18) Robinson, N. Transformer oil analysis. Tech. Bull.—Wearcheck, Africa 2006, 36, 1. (19) Raghavan, S. R.; Walls, H. J.; Khan, S. A. Rheology of silica dispersions in organic liquids: New evidence for solvation forces dictated by hydrogen-bonding. Langmuir 2000, 16, 7920. 3076
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