Enhanced Thermal Conductivity of Copper Nanofluids: The Effect of

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Enhanced Thermal Conductivity of Copper Nanofluids: The Effect of Filler Geometry Sushrut Bhanushali,†,‡,§ Naveen Noah Jason,§ Prakash Ghosh,‡ Anuradda Ganesh,‡ George P Simon,∥ and Wenlong Cheng*,§,⊥ †

IITB Monash Research Academy and ‡Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India § Department of Chemical Engineering and ∥Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia ⊥ Melbourne Centre for Nanofabrication, Wellington Road, Melbourne, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Nanofluids are colloidal dispersions that exhibit enhanced thermal conductivity at low filler loadings and thus have been proposed for heat transfer applications. Here, we systematically investigate how particle shape determines the thermal conductivity of low-cost copper nanofluids using a range of distinct filler particle shapes: nanospheres, nanocubes, short nanowires, and long nanowires. To exclude the potential effects of surface capping ligands, all the filler particles are kept with uniform surface chemistry. We find that copper nanowires enhanced the thermal conductivity up to 40% at 0.25 vol % loadings; while the thermal conductivity was only 9.3% and 4.2% for the nanosphere- and nanocubebased nanofluids, respectively, at the same filler loading. This is consistent with a percolation mechanism in which a higher aspect ratio is beneficial for thermal conductivity enhancement. To overcome the surface oxidation of the copper nanomaterials and maintain the dispersion stability, we employed polyvinylpyrrolidone (PVP) as a dispersant and ascorbic acid as an antioxidant in the nanofluid formulations. The thermal performance of the optimized fluid formulations could be sustained for multiple heating−cooling cycles while retaining stability over 1000 h. KEYWORDS: nanofluids, thermal conductivity, copper nanowires, copper nanocubes, aspect ratio

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INTRODUCTION Cooling and waste heat management remains one of the top technical challenges in many industries such as microelectronics, automobiles, power generation, precision manufacturing, and high power laser optics. The ever-increasing load of waste heat, high heat fluxes, and local hot spots due to increase in energy consumption poses a significant bottleneck for furthering the advancements. Traditionally, approaches for enhancement of heat transfer have been increasing the flow rates of fluids in heat exchangers and extending heat exchange surface areas, such as fins and microchannels.1,2 However, these approaches lead to an increased cost of materials and energy, as well as increasing the dimensions and complexity of the heat transfer equipment. An alternative approach to achieve an enhancement in heat transfer involves engineering of the heat transfer fluid. Heat transfer fluids such as water, ethylene glycol, propylene glycol, and transformer oils are commonly used for thermal management in a vast range of engineering applications. However, since these materials are liquids, they possess inherently low thermal conductivity, typically 3 to 4 orders of magnitude lower than metals such as silver and copper or carbon-based materials such as graphene and carbon © 2017 American Chemical Society

nanotubes. Adding solid, thermally conductive particles to liquids has been shown to be a potent strategy to increase the thermal conductivity of fluids by Maxwell more than a century ago.3 However, micron-sized particle slurries pose a major issue in terms of significant increase in viscosity, abrasion of equipment, aggregation, and settling. With advances in nanomaterial synthesis over the past two decades, it is now possible to precisely tailor materials in nanodimensions with the desired size, shape, composition, surface chemistry, and morphology, as well as control their dispersion properties.4 Nanomaterials are known to enhance the thermophysical properties of fluids with the potential to address the previously stated issues.5,6 Thermal conductivity enhancement in nanofluids has a strong dependence on particle material, particle loading fraction, and particle size, shape, and degree of agglomeration. The nanofillers reported for nanofluids can be broadly classified into three material groups: metals, metal oxides, and carbon-based materials. By far, most Received: March 8, 2017 Accepted: May 4, 2017 Published: May 4, 2017 18925

DOI: 10.1021/acsami.7b03339 ACS Appl. Mater. Interfaces 2017, 9, 18925−18935

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ACS Applied Materials & Interfaces of the reports have focused on metal oxides like alumina,7,8 copper oxide,9 and titania10,11 based fluids, owing to their low costs, excellent chemical stability, and versatility with regard to surface modification. 12 However, the reported thermal conductivity enhancements for oxide nanofluids typically occur at high particle loading fractions, due to their spherical or similar morphologies.13 For example, Wang et al. reported a 1.18- to 1.54-fold enhancement in thermal conductivity for 5− 16 vol % CuO particle loadings.8 Peterson et al. reported 9% to 22% enhancements for 6−10 vol % Al2O3 loadings.14 The high particle loadings lead to a significant increase in viscosity, which results in an increase in pumping power, negating the advantage of thermal conductivity gain. As far as carbon-based nanomaterials are concerned, carbon nanotubes (CNTs) have been the primary focus due to their inherent high thermal conductivity and one-dimensional morphology.15,16 As indicated by the advantageous anisotropy of CNTs, there has been particular interest recently in developing nanofluids with nonspherically shaped fillers.17−20 A primary reason for this is the ability of these high aspect ratio nanofillers to exhibit an enhancement in thermal conductivity at much lower filler loadings as compared to spherical nanofillers, due to percolation dominant thermal transport. Although CNTs are fibrillary with a high aspect ratio, they are also hydrophobic and tend to form bundles due to van der Waals interactions, particularly π−π stacking. These bundles are difficult to disperse, which in turn is detrimental to the nanofluid stability and consequently their thermal properties.21 To obtain a good dispersion, for example, the CNTs need to be functionalized by oxidation using strong acids, which leads to defects in the continuous sp2-bonded carbon structure, causing a drastic reduction of their thermal conductivity due to scattering of phonons, which are the dominant heat carriers in CNTs. Functionalization of ∼1% carbon atoms in the nanotube structure can cause a loss of more than 50% of the inherent CNT thermal conductivity.22 The thermal transport in metallic nanomaterials, in contrast, is dominated by electrons.23 Metals represent another class of material selection for nanofluids with enhanced thermal conductivity at low particle concentrations.24 In recent times there have been significant advances in synthesis of metallic nanomaterials which allow for precise tuning of the particle size, shape, composition, and even surface chemistrya capability difficult to achieve with metal oxides or carbon materials. Metal nanoparticles can be produced by scalable wet chemical methods under mild conditions of temperature and pressure,25 in contrast to methods employed in the production of CNTs which require high temperatures, vacuum, and specialized equipment affecting the production cost and scalability.26 Metallic nanofluids have been found to enhance the thermal conductivity at very low filler loadings (10 and reaching a maximum for aspect ratios of ca. 500 for a system with zero interfacial resistance. For a system with the Kapitza resistance value of 10−8 m2 K/W, the thermal conductivity enhancements are suppressed at lower aspect ratios, but the same asymptotic maximum thermal conductivity value is achieved for a nanoparticle network with interfacial resistance at higher aspect ratios (∼4000) which is comparable to a network with zero interfacial thermal resistance.49 The effect of interfacial resistance can thus be diminished by using fillers with very high aspect ratios. The experimental data for the copper nanofluids exhibits a trend which is consistent with the effective medium theory calculations. However, there is still a difference between the experimental results and predicted thermal conductivity at high aspect ratios, which could be due to the CuNWs being flexible, leading to bending and curvature reducing the effective aspect ratio of the fillers. Also, the diameter of the nanowires is in the range of the mean free path of electrons, which are dominant heat carriers in the case of metals. The thermal conductivity is thus reduced due to boundary scattering of electrons, as compared to the assumed bulk thermal conductivity values. Thus, tailoring suitable

enhancements with respect to temperature for the fluids with the four distinctly shaped fillers. The thermal conductivity of CuNW-based fluids shows minimal change with increasing temperature. The weak dependence on temperature of the enhancements is substantiated by the percolation dominant mode of heat conduction, where the temperature does not play a significant role in network formation. A similar temperature independent thermal conductivity enhancement behavior was observed in the case of high aspect ratio carbon nanotubes39 and graphene oxide nanosheets.19 By contrast, the CuNC and CuNS based fluids show a temperature-dependent enhancement of thermal conductivity ranging from 3.3% and 7.6% at 20 °C to 6.8% and 13.9% at 70 °C, respectively, for 0.25 vol % loadings. The temperature dependent enhancement in thermal conductivity could originate from the translational Brownian motion of the particles as well as the associated free convection at elevated temperatures.50−52 The model proposed by Prasher predicts that convection due to Brownian motion of the particles dominates the heat transport for small particles and shows a strong dependence on temperature, while the same model reduces to pure conduction based transport for large particles such as CuNWs, where the Brownian motion of the fillers is found to be negligible.53 The Brownian motion of the particles increases with temperature, leading to an increased probability of the formation of such aggregates leading to an increase in thermal conductivity at higher fluid temperatures.54 This trend of increased thermal conductivity with temperature 18930


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complex in the initial phase of the synthesis, while CTAB cannot participate in the formation of Cu complex, owing to the positively charged quaternary ammonium moiety.56 The nature of the nanoparticle boundary is thus affected due to this distinction, despite similarities in their molecular dimension. The thermal stability could not be retained for the CuNSs nanofluids, as the nanoparticles aggregated and settled and could not be redispersed even with sonication, indicating that the CuNSs clusters are not stable as indicated by the SEM studies performed after first, second, and third heating−cooling cycles of the sample (Supporting Information Figure S6). The protocol used here for CuNSs synthesis is thus not suitable for nanofluid applications. Obtaining a stable dispersion of nanoparticles is an important concern for utilizing the enhanced thermal conductivity of nanofluids for applications. Since CuNWs were found to be the best candidate for conductivity enhancement as well as showing sustained performance, further studies were limited to CuNW based nanofluids. We found that the centrifugation speed and washing conditions significantly influenced the stability of the fluids. Centrifugation speeds of 8000 rpm and above adversely affected the redispensability of the CuNWs, while limiting the centrifugation speed up to 5000 rpm retained their excellent redispersibility. Also, washing the CuNWs with deionized water at 40−45 °C (close to the melting point of HDA) removed the excess capping agent more effectively than washing the CuNWs with water at room temperature (∼20 °C). Washing the centrifuged residue up to two times with warm deionized water retained excellent redispersibility, while bundles of CuNWs started to form after excessive washing due to the removal of the weakly binding alkylamine capping agent from the nanowire surface. Figure 7 shows the long-term stability of the thermal performance of the CuNW fluids with respect to time. As can be seen, post-synthesis processing significantly affects the performance and stability of the fluid. For fluids that were prepared after only one washing cycle, the thermal conductivity enhancements were significantly low (∼10%), but the fluid was stable and retained the magnitude of its thermal conductivity for many weeks. A single washing cycle was found to be inadequate to remove the excess HDA (see Supporting Information Figure S7), the signatures of which were observed in the SEM analysis (Figure 7A). For CuNWs treated with two successive washing cycles with deionized water at 40−45 °C, thermal conductivity significantly improved by 39.6%, which can be attributed to the removal of the excess capping agent with washing with warm water, which arises because it is close to the melting point of HDA (46−48 °C). With respect to time, the thermal conductivity showed a gradually decreasing trend, from 39.8% on day 0 to 28.5% on day 42 (∼1000 h). Copper, though an inexpensive material with excellent thermal conductivity, has limitations in applicability due to its susceptibility to oxidation.57 The oxidation kinetics is further enhanced at elevated temperatures, challenging the applicability of copper nanofluids in thermal applications.58 The surface chemistry of the nanomaterials plays a critical role in stability toward oxidation. Copper nanofluids with enhanced thermal conductivity of up to 40% at 0.3 vol % loadings were reported by Eastman et al.28 However, the enhancement deteriorated to less than 10% within 48 h. The thermal conductivity could be improved by using thioglycolic acid which was responsible for removal of surface oxides. Recently, copper nanowires capped with ethylenediammine (EDA) were found to oxidize within a

for CuNSs and CuNCs is consistent with the Brownian motion enhanced aggregation phenomena, while the thermal conductivity for the high aspect ratio CuNWs stays nearly constant with temperature, where the Brownian motion of the fillers is greatly diminished. While the direct contribution of Brownian motion to the enhancement of thermal conductivity is negligible,55 it contributes indirectly by increasing the probability of particle cluster formation with increased temperature and decrease in the viscosity of the fluid.40 Due to enhanced free convection at higher temperatures, it is difficult to precisely assign the contribution of the particles in enhancing the thermal conductivity. In real world applications, nanofluids would undergo repeated heating and cooling cycles; thus it is important to assess their thermal properties with respect to thermal cycling. Many studies report parametric thermal conductivity profiles of nanofluids. However, the stability and performance over repeated thermal cycling have seldom been reported. Extending the temperature dependent performance, we evaluated the thermal conductivity of the copper nanofluids with repeated heating and cooling cycles. The nanofluid sample−sensor assembly was placed inside a thermostatic oven, and the thermal conductivity was measured at preset temperatures. Figure 6 shows the thermal conductivity of the CuNW and

Figure 6. Thermal cycling of the copper nanowire and nanosphere nanofluids with repeated heating and cooling cycles.

CuNS based nanofluids for repeated thermal cycles between 20 and 70 °C. The fluid containing 0.25 vol % CuNWs showed a drop of thermal conductivity enhancement by 2.2% over the first two cycles, past which the nanofluid was found to be stable in the temperature range over ten cycles of heating and cooling. We limited the heating up to 60 °C from the third cycle onward to prevent errors due to sample evaporation and subsequent change in concentration. A similar investigation of the CuNSs, however, showed a rapidly reducing thermal conductivity enhancement, which was negligible from the third heating cycle onward. The CuNSs are capped with CTAB, a molecule similar in dimensions to HDA with a nitrogen polar group, with the difference being that HDA has the nitrogen as a primary amine; while in CTAB, the nitrogen moiety is present as a quarternary ammonium salt. These long chain alkyl ligands have the role of capping agent but also play a significant role in the formation process of the nanoparticle. HDA forms a Cu-alkylamine 18931


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oxidation resistance. By fine-tuning the post-synthesis processing, we could attain stable fluids with only a marginal loss in their thermal performance. The fluids were found to be stable for at least three months without aggregation. As discussed, apart from thermal conductivity, the addition of solid particles to the base fluids also affects the density, specific heat,63 and viscosity64 of the nanofluids. These parameters need to be taken into consideration when designing and considering the thermal applications of the nanofluids. The effect on density and specific heat capacity is minimal for very low particle loadings and is dominated by the base fluid. The specific heat of nanofluids decreases with increasing filler fractions since the heat capacity of copper (390 kJ/kgK) is much lower than that of water (4183 kJ/kg K). The addition of 10 vol % of copper nanoparticles would drastically reduce the specific heat to ∼2300 kJ/kg K, which would be detrimental to the heat transfer capabilities of the fluid.63 Thus, enhancing thermal conductivity while keeping the filler fraction to a minimum is critical for the improvement of thermal transport capabilities of the nanofluids. The interaction between the base fluid and the solid fillers plays a significant role in the rheological behavior of nanofluids. Further, the viscosity enhancements are more dramatic for nonspherical fillers, owing to the increased filler−matrix and filler−filler interactions.65 An increase in viscosity would increase the pressure drop and consequently have a penalty on pumping power, jeopardizing the applicability of nanofluids.66,67 Figure 8 shows measured viscosity of the copper-

Figure 7. Stability of copper nanowire-based nanofluids with respect to time for 0.25% loadings, showing the effect of post-processing protocols. Open symbols denote CuNWs redispersed in deionized water after washing, solid symbols denote CuNWs redispersed in deionized water with 2.5 mg/mL PVP and 1 mg/mL ascorbic acid. Corresponding SEM micrographs of CuNWs after (A) 1 washing cycle, (B) 2 washing cycles, and (C) 3+ washing cycles with deionized water at 45 °C.

week, forming a mace-like structure.59 The oxidation stability was found to improve with using an alkylamine ligand (HDA) with longer carbon chain length.60 Moon et al. devised a facile protocol to improve the dispersion stability by exchanging the HDA surface ligands with polyvinylpyrrolidone (PVP).61 In order to retain the thermal conductivity of the copper nanofluid formulation, we employed polyvinylpyrrolidone (PVP-K30, Sigma-Aldrich) as a secondary dispersant to redisperse the CuNWs, which is known to bind well to copper nanoparticles. Ascorbic acid (vitamin C) is a nontoxic, watersoluble radical scavenger, which sacrificially retards the oxidation process of copper nanomaterials.62 We first functionalized the nanowires with polyvinylpyrrolidone by redispersing the washed CuNWs in an aqueous solution containing 2.5 mg/ mL of PVP K-30, followed by stirring the solution for 60 min at 250 rpm. Ascorbic acid (1 mg/mL) was added to the PVP functionalized copper nanofluid formulation. The long-term stability was found to be improved, and the thermal conductivity was retained for more than 42 days (∼1000 h). However, when the nanowires were subjected to excessive washing, the thermal conductivity deteriorated rapidly, despite the use of PVP and ascorbic acid, and the brick red solution turned yellowish-brown, indicating oxidation of the copper (see Figure S8 in the Supporting Information). Figure 7C shows the SEM of the copper nanowires after excessive washing that have formed bundles, which destabilized the dispersion, leading to loss of thermal conductivity of the fluid. Excess washing and removal of the capping agent from the nanowire surface are known to cause the formation of bundles and aggregates as also surface oxidation of copper nanowires, severely affecting their thermal performance.59 The PVP functionalization was found to be pivotal for achieving dispersion stability as well as the

Figure 8. Viscosity of copper nanofluids with respect to temperature for long CuNWs, short CuNWs, CuNSs, and CuNCs for 0.25 vol % loading.

based nanofluids with respect to temperature for filler fraction of 0.25 vol %. As expected, CuNWs show a larger enhancement in viscosity, as compared to the CuNCs and CuNSs, owing to the high aspect ratio and entanglement of the wires as a result of the transition from dilute to the semidilute regime. Thus, there exists a critical link between thermal conductivity and viscosity enhancements in nanofluids, where both parameters are sensitive to the interparticle interaction.68 The viscosity drops sharply with temperature, and the gaps between viscosities of CuNWs, CuNCs, and CuNSs nanofluids were significantly reduced at elevated temperatures. At 23 °C, the viscosity was 2.24 mPas, 1.75 mPas, 1.52 mPas, and 1.4 mPas 18932


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heating and cooling cycles. The filler loading fractions were well below 1 vol %, keeping the consequential increase in viscosity to a minimum. To the best of our knowledge, this is the first report on nanofluids with metallic nanomaterials with a very large aspect ratio (>3000). The developments clearly show a trend of evolution of nanofluids toward low filler fraction, high aspect ratio nanofluid formulations for enhanced thermal transport. Our results indicate the potential of engineered copper nanofluids in real-world industrial applications.

for long CuNWs, short CuNWs, CuNSs, and CuNCs based fluids, respectively, at 0.25 vol % loadings (Supporting Information Figure S9). The viscosity dropped sharply with temperature to 0.78 mPas, 0.63 mPas, 0.55 mPas, and 0.51 mPas, respectively, at 70 °C for the same fluids, as shown in Figure 8. Thus, for low filler fractions, the viscosity trends are dominated by the base fluid. A similar particle shape based study was reported for alumina nanofluids with filler loadings up to 10 vol % for four distinct shapes of particles.69 The highest thermal conductivity enhancements of 1.29-fold were observed at 8.5 vol % loading for one-dimensional nanorod fillers. However, at such high filler loadings, the viscosity dramatically increased by 8-fold (800%). Viscosity enhancement is detrimental for heat transport using nanofluids as it increases the pumping power, thereby nullifying the advantage of the enhanced thermal conductivity of the fluids for thermal applications. It should be noted that the alumina particle loadings were some 2 orders of magnitude greater than our copper nanofluids, while the aspect ratios of the alumina nanorods were rather small, in the range of 6−10 against the aspect ratios of 300−5000 for our CuNW-based nanofluids. The increase in thermal conductivity of the nanofluid formulation with temperature, along with the drastic drop in viscosity, makes the nanofluids particularly attractive for elevated temperature applications such as cooling. The viscosity jumps are not dramatic for these copper nanofluids owing to the low volume fraction of particles, as compared to oxide nanofluids such as alumina described earlier, where loadings are about 2 orders of magnitude higher than our copper based fluids. Thus, carefully choosing the filler material, particle geometry, and loading fractions is necessary to take full advantage of the one-dimensional nanofillers for the nanofluids.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03339. Images of synthesis procedure, fluid formulations, measurement techniques; additional characterization data: UV−vis spectra, SEM micrographs, viscosity measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 3 990 53147. ORCID

Wenlong Cheng: 0000-0002-2346-4970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S. B. would like to thank the Prime Minister’s Fellowship Scheme for Doctoral Researcha public-private partnership between Science and Engineering Research Board (SERB) and Confederation of Indian Industry (CII)for the funding and support. The academic partner is IITB-Monash Research Academy and the industry partner for this research is Thermax Ltd. We also acknowledge financial supports from ARC discovery projects DP140100052 and DP150103750. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).

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CONCLUSIONS We have investigated for the first time thermal conductivity of copper nanofluids with four distinct particle shapes. The very high aspect ratio of the CuNWs and trends in thermal conductivity suggests a percolation dominant mechanism of enhancements for high aspect ratio CuNW nanofluids; though a distinct percolation threshold is not observed. The CuNCs and CuNSs, on the other hand, show linear and marginal enhancements in conductivity, owing to a large number of thermal interfaces encountered and not being able to achieve thermal percolation at low filler loadings. The CuNSs showed better enhancements than CuNCs due to formation of loose aggregates, increasing their effective filler fraction; although the degree of agglomeration and cluster morphology cannot be precisely controlled. The thermal conductivity values lie well within the bounds of effective medium theory when the nanoparticle system is allowed to form clusters and percolating structures, thus sufficient to account for the thermal conductivity enhancements, without resorting to exotic mechanisms such as ordered surface layer of matrix at the nanoparticle boundaries. The detrimental effect of interfacial resistance can be attenuated by increasing the aspect ratio of the fillers. The effect of particle shape is thus an important parameter toward realizing the suitability of nanofluids for the proposed applications. The stability of the nanofluids was improved by optimizing the washing−redispersing protocol and using PVP as a dispersant and ascorbic acid as an antioxidant in the fluid formulations. These nanofluid formulations were able to retain the thermal conductivity over a period of 1000 h over repeated

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Enhanced Thermal Conductivity of Copper Nanofluids: The Effect of

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