APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 6 5 FEBRUARY 2001 Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles J. A. Eastmana) Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 S. U. S. Choi Energy Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 S. Lib) Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 W. Yu Energy Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 L. J. Thompson Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 ~Received 21 September 2000; accepted for publication 21 November 2000! It is shown that a ‘‘nanofluid’’ consisting of copper nanometer-sized particles dispersed in ethylene glycol has a much higher effective thermal conductivity than either pure ethylene glycol or ethylene glycol containing the same volume fraction of dispersed oxide nanoparticles. The effective thermal conductivity of ethylene glycol is shown to be increased by up to 40% for a nanofluid consisting of ethylene glycol containing approximately 0.3 vol % Cu nanoparticles of mean diameter ,10 nm. The results are anomalous based on previous theoretical calculations that had predicted a strong effect of particle shape on effective nanofluid thermal conductivity, but no effect of either particle size or particle thermal conductivity. © 2001 American Institute of Physics. @DOI: 10.1063/1.1341218# Heating or cooling fluids are of major importance to many industrial sectors, including transportation, energy supply and production, and electronics. The thermal conductivity of these fluids plays a vital role in the development of energy-efficient heat transfer equipment. However, conventional heat transfer fluids have poor heat transfer properties compared to most solids. Despite considerable previous research and development focusing on industrial heat transfer requirements, major improvements in heat transfer capabilities have been lacking. As a result, a clear need exists to develop new strategies for improving the effective heat transfer behavior of conventional heat transfer fluids. Crystalline solids have thermal conductivities that are typically larger than those of fluids by 1–3 orders of magnitude. Therefore, fluids containing suspended solid particles can reasonably be expected to display significantly enhanced thermal conductivities relative to those of pure fluids. Numerous theoretical and experimental studies of the effective thermal conductivity of liquids containing suspended solid particles have been conducted since Maxwell’s theoretical work1 was first published more than 100 years ago.2–8 However, with very few exceptions, previous studies of the thermal conductivity of suspensions have been confined to those containing millimeter- or micrometer-sized particles. Maxwell’s model1 predicts that the effective thermal conductivity of suspensions containing spherical particles increases with the volume fraction of the solid particles. Because heat transfer takes place at the surface of the particle, it a! Electronic mail: jeastman@anl.gov Present address: Seagate Technology, Bloomington, MN. b! is desirable to use particles with a large surface area-tovolume ratio. Hamilton and Crosser2 focused on the possible effects of increasing particle surface area by controlling particle shapes to be nonspherical and modified Maxwell’s model for spherical particles. However, typically less than an order-of-magnitude improvement in surface area per particle volume is attainable experimentally by this strategy alone. On the basis of this historical background, it was proposed that if nanometer-sized particles could be suspended in traditional heat transfer fluids, a new class of engineered fluids with high thermal conductivity could be produced.9 These so-called ‘‘nanofluids’’ are expected to exhibit superior properties relative to those not only of conventional heat transfer fluids, but also of fluids containing micrometer-sized metallic particles. Since the surface-area-to-volume ratio is 1000 times larger for particles with a 10 nm diameter than for particles with a 10 mm diameter, a much more dramatic improvement in effective thermal conductivity is expected as a result of decreasing the particle size in a suspension than can be obtained by altering the shapes of larger particles. Recently, we demonstrated that nanofluids consisting of CuO or Al2O3 nanoparticles in water or ethylene glycol exhibit enhanced thermal conductivity.10 A maximum increase in thermal conductivity of approximately 20% was observed in that study for 4 vol % CuO nanoparticles with average diameter 35 nm dispersed in ethylene glycol. Similar behavior was observed in another recent study of Al2O3 nanoparticles dispersed in water by Masuda and co-workers.11 The present work demonstrates that significantly larger improvements in effective thermal conductivity are obtained for nanofluids containing smaller sized and higher conductivity 0003-6951/2001/78(6)/718/3/$18.00 718 © 2001 American Institute of Physics Downloaded 25 Feb 2008 to 129.199.129.64. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp Eastman et al. Appl. Phys. Lett., Vol. 78, No. 6, 5 February 2001 719 FIG. 1. Bright-field transmission electron micrograph of Cu nanoparticles produced by direct evaporation into ethylene glycol. Very little agglomeration occurs using this processing method. copper nanoparticles. As will be described, this is a surprising result based on theoretical predictions. A one-step procedure for producing nanofluids containing metallic particles was used to disperse nanocrystalline copper particles into ethylene glycol with little agglomeration. Briefly, this technique, invented by Akoh and co-workers,12 involves the direct condensation of metallic vapor into nanoparticles by contact with a flowing low vapor pressure liquid. A modification of the direct-condensation process developed by Wagener et al.13 was used in the current experiments ~see Ref. 14 for further details!. As seen in Fig. 1, Cu nanoparticles with little agglomeration and an average diameter of less than 10 nm were produced. Loadings of up to approximately 0.5 vol % were produced and tested. Particle loadings were estimated by weighing the resistive evaporation source before and after the nanofluid preparation. For some samples, a small amount of thioglycolic acid ~,1 vol %! was added to the nanofluid to improve the particle dispersion behavior. The transient hot-wire ~THW! method15–17 was used in this study to measure fluid thermal conductivity. A THW system involves a wire ~typically platinum! suspended symmetrically in a liquid in a vertical cylindrical container. Nagasaka and Nagashima’s method,18 in which the wire is coated with a thin electrical insulation layer, was used in the current experiment to avoid problems associated with the measurement of electrically conducting fluids. Briefly, the THW technique works by measuring the temperature/time response of the wire to an abrupt electrical pulse. The wire is used as both heater and thermometer and the thermal conductivity, k is calculated from a derivation of Fourier’s Law k5 SD q t2 , ln 4 p ~ T 2 2T 1 ! t1 ~1! where q is the applied electric power and T 1 and T 2 are the temperatures at times t 1 and t 2 . From the temperature coefficient of the wire’s resistance, the temperature rise of the wire can be determined by the change in its electrical resistance with time. Calibration experiments were performed for ethylene glycol in the temperature range of 290–310 K and at atmospheric pressure. Literature values19 were reproduced with an error of ,1.5%. Figure 2 shows the thermal conductivity of Cu nanoflu- FIG. 2. The effective thermal conductivity of ethylene glycol is seen to be improved by up to 40% through the dispersion of 0.3 vol % Cu nanoparticles. Linear fits to the data are shown as a guide to the eye. The largest increase in conductivity was seen for a nanofluid that also contained a small quantity of thioglycolic acid to improve the stability of the metal particles against settling. A small effect of sample age on thermal conductivity was also observed. Samples without thioglycolic acid tested within two days of preparation are denoted ‘‘fresh,’’ while those stored up to two months prior to testing are denoted ‘‘old.’’ ids as a function of nanoparticle volume fraction. Data are plotted normalized to the conductivity of nonparticlecontaining ethylene glycol. Several important points can be noted. First, very significant increases in thermal conductivity are seen for all measured nanofluids, with conductivity enhancements of up to 40% observed for particle loadings well below one volume per cent. Second, nanofluids containing thioglycolic acid as a stabilizing agent show improved behavior compared to nonacid-containing nanofluids. It should be noted that fluids containing thioglycolic acid, but no particles, showed no improvement in thermal conductivity. Third, fresh nanofluids tested within two days of preparation exhibited slightly higher conductivities than fluids that were stored up to two months prior to measurement. Figure 3 shows comparisons between metallic and oxide nanofluids. All nanofluids containing Cu nanoparticles directly dispersed in ethylene glycol show significantly larger increases in thermal conductivity than nanofluids containing oxide particles produced by a two-step process involving gas-condensation processing of powders followed by dispersion into the fluid.10,11 The conductivity enhancements for both metallic and oxide nanofluids are approximately linear with particle volume %. Compared to previous studies of oxide-containing nanofluids there are two potentially important differences: ~1! the Cu particles used in the current study are expected to have a much higher intrinsic thermal conductivity than the oxide particles used previously, and ~2! the average particle diameters in the present study are more than four times smaller than for the oxide particles. Hamilton and Crosser’s analysis predicts that the conductivity of two-component mixtures can be described by k5k 0 F G k m 1 ~ n21 ! k 0 2 ~ n21 ! a ~ k 0 2k m ! , k m 1 ~ n21 ! k 0 1 a ~ k 0 2k m ! ~2! Downloaded 25 Feb 2008 to 129.199.129.64. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp 720 Eastman et al. Appl. Phys. Lett., Vol. 78, No. 6, 5 February 2001 FIG. 3. Significantly greater enhancements are seen for nanofluids consisting of ,10 nm diameter Cu nanoparticles dispersed into ethylene glycol than for ethylene glycol-based nanofluids containing either CuO or Al2O3 nanoparticles of average diameter 35 nm. This is an unexpected result since theoretical calculations had predicted no effect of either particle diameter or particle conductivity on nanofluid conductivity. where k is the mixture thermal conductivity, k 0 is the liquid thermal conductivity, k m is the thermal conductivity of solid particles, a is the particle volume fraction, and n is an empirical scaling factor that takes into account the effect on thermal conductivity of different particle shapes. Since the nanoparticles produced in this investigation are approximately spherical, n'3 and Eq. ~2! thus predicts very low enhancements in thermal conductivity compared to those observed ~e.g., a conductivity ratio of approximately 1.015 would be predicted for a 0.5 vol % copper particles in ethylene glycol, which is a smaller enhancement than the observed value by well over an order of magnitude!. An obvious shortcoming in Hamilton and Crosser’s theory is the lack of any predicted dependence of conductivity on particle size. While the shape factor in the Hamilton and Crosser analysis takes into account the increased surface area of nonspherical particles, it does not account for the orders-of-magnitude increase in surface-area-to-volume ratio that accompany decreasing particle size into the nanocrystalline regime. A second possible weakness in the Hamilton–Crosser analysis is that the thermal conductivity of the particles has only a weak predicted effect on k in Eq. ~2!. For example, using quantities in Eq. ~2! that correspond to 0.5 vol % particles in ethylene glycol predicts less than 0.1% larger conductivity ratio for Cu particles than for Al2O3 particles. Since in our previous study10 nanofluids containing essentially identically sized and shaped oxide particles of different composition were observed to exhibit conductivity ratios that varied by almost 100%, this weak predicted effect of k m on k appears unreasonable. It was demonstrated that thermal conductivities predicted by theoretical models such as Hamilton and Crosser’s are much lower than the measured data for oxide nanofluids.18 The present study shows the discrepancy is even larger for metallic nanofluids than oxide nanofluids. This suggests that current models, which account only for the volume fraction and shape of the suspended particles and the differences between the thermal conductivity of particles and fluids, are insufficient to explain the energy transfer processes in the nanofluid system. A more comprehensive theory is needed to explain the behavior of nanofluids. Additional studies are required before this theory can be developed and the mechanism~s! responsible for the observed enhancements in fluid thermal conductivity are understood. These include planned experimental studies of samples with varying particle sizes, but the same composition. Additionally, more sophisticated theoretical treatments are needed. Simulation studies have recently been initiated20 that hopefully will provide insight into the atomistic processes controlling the thermal conductivity of nanofluids. In summary, nanofluids consisting of Cu nanoparticles directly dispersed in ethylene glycol have been observed to exhibit significantly improved thermal conductivity enhancements compared to nonparticle-containing fluids or nanofluids containing oxide particles. The large improvement in effective conductivity obtained for nanofluids containing metallic particles holds significant potential for revolutionizing industries that are dependent on the performance of heat transfer fluids. This work was supported by the U.S. Department of Energy, Office of Science and Office of Transportation Technologies, under Contract No. W-31-109-Eng-38. 1 J. C. Maxwell, A Treatise on Electricity and Magnetism, 2nd ed. ~Oxford University Press, Cambridge, 1904!, pp. 435–441. 2 R. L. Hamilton and O. K. Crosser, I & EC Fundamentals 1, 187 ~1962!. 3 Z. Hashin and S. Shtrikman, J. Appl. Phys. 33, 3125 ~1962!. 4 D. J. Jeffrey, Proc. Phys. Soc., London, Sect. A 335, 355 ~1973!. 5 D. J. Jackson, Classical Electrodynamics, 2nd ed. ~Wiley, London, 1975!. 6 R. H. Davis, Int. J. Theor. Phys. 7, 609 ~1986!. 7 R. R. Bonnecaze and J. F. Brady, Proc. Phys. Soc., London, Sect. A 432, 445 ~1991!. 8 S. Lu and H. Lin, J. Appl. Phys. 79, 6761 ~1996!. 9 U. S. Choi, in Developments and Applications of Non-Newtonian Flows, edited by D. A. Siginer and H. P. Wang ~The ASME, New York, 1995!, Vol. 231/MD-Vol. 66, pp. 99–105. 10 S. Lee, U. S. Choi, S. Li, and J. A. Eastman, ASME J. Heat Transfer 121, 280 ~1999!. 11 H. Masuda, A. Ebata, K. Teramae, and N. Hishinuma, Netsu Bussei 4, 227 ~1993!. 12 H. Akoh, Y. Tsukasaki, S. Yatsuya, and A. Tasaki, J. Cryst. Growth 45, 495 ~1978!. 13 M. Wagener, B. S. Murty, and B. Günther, in Nanocrystalline and Nanocomposite Materials II, edited by S. Komarnenl, J. C. Parker, and H. J. Wollenberger ~Materials Research Society, Pittsburgh PA, 1997!, Vol. 457, pp. 149–154. 14 J. A. Eastman, U. S. Choi, S. Li, L. J. Thompson, and S. Lee, in Nanocrystalline and Nanocomposite Materials II, edited by S. Komarnenl, J. C. Parker, and H. J. Wollenberger ~Materials Research Society, Pittsburgh PA, 1997!, Vol. 457, pp. 3–11. 15 J. Kestin and W. A. Wakeham, Physica A 92, 102 ~1978!. 16 H. M. Roder, J. Res. Natl. Bur. Stand. 86, 457 ~1981!. 17 A. I. Johns, A. C. Scott, J. T. R. Watson, and D. Ferguson, Philos. Trans. R. Soc. London, Ser. A 325, 295 ~1988!. 18 Y. Nagasaka and A. Nagashima, J. Phys. E 14, 1435 ~1981!. 19 In Thermal Properties of Matter. The TPRC Data Series, edited by Y. S. Touloukian, and C. Y. Ho ~Plenum, New York, 1970–1977!. 20 P. Keblinski and S. R. Phillpot ~private communication!. Downloaded 25 Feb 2008 to 129.199.129.64. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp