RSC Advances - IIT Madras

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Cite this:RSC Advances, 2013,3, 4199 Received 26th October 2012, Accepted 23rd January 2013

Graphene–multiwalled carbon nanotube-based nanofluids for improved heat dissipation

DOI: 10.1039/c3ra22653k www.rsc.org/advances

S. S. Jyothirmayee Aravind and S. Ramaprabhu*

A solution-free green method using focused solar electromagnetic radiation is used to synthesize graphene and graphene–

multiwalled carbon nanotube (MWNT) composite. Stable nanofluids are prepared by dispersing the nanomaterials in polar base fluids. Thermal conductivity of the nanofluids improves with graphene–MWNT nanocomposites as additives, which could be due to prevention of restacking of graphene sheets by MWNT along with a synergistic effect of intrinsic high thermal conductivity of graphene and MWNT. The advantage of the present synthesis method in particular to nanofluids application is that the presence of oxygen functional groups resulting from pre- functionalized MWNT rules out the need for functionalizing the hybrid composite again, thereby preserving the high thermal properties of graphene. Thermal conductivity enhancement of 9.2% and 10.5% is obtained with graphene and graphene–MWNT nanofluids in de-ionized water at room temperature for 0.04%

volume fraction. The high thermal transport characteristics of graphene–MWNT composite nanofluids is ascribed to the high aspect ratio of MWNT and graphene, which in turn can form tightly bonded clusters and, by suppressing the interface resistance, can become excellent additives to attain high thermal conductivity. Further, an enhancement in heat-transfer coefficient of 193% at Reynolds number 2000 for 0.02% volume fraction of aqueous graphene–MWNT nanofluids suggests the potential application of the present hybrid material-based nanofluids in cooling circuits.

Introduction

Due to rapid development in all sectors, be it infrastructure, industrial, transportation, defense or space, managing high thermal loads has become very critical. For that reason, several technologies devoted to cooling have been researched. From micro to macro industry, everywhere the dissipation of heat is

necessary. Nanofluids are dilute suspensions of nanoparticles established about a decade ago with the specific aim of enhancing the thermal conductivity of heat-transfer fluids, and have now evolved into a promising nanotechnological area.¹ Employing nanostructured materials to generate high surface-to-volume ratios and low dimensionality can lead to improved thermal transport.

Nanofluids with good thermal conductivity with fewer enhancements in electrical conductivity are also under investigation.

Conventionally, fillers that provide increased thermal conductivities with no increase in electrical conductivity are ceramic particles, but they suffer from dispersion or settling problems and are not easily dispersible. Recently, Tijerina et al. reported electrically insulating but thermally conducting nano-oils using 2D hexagonal boron nitride.²Among the several types of nanoparticles which have been used for producing nanofluids, carbon nanomaterials such as carbon nanotubes (CNT) and graphene are special due to their high intrinsic thermal conductivities,³ high aspect ratio, superior strength⁴and low densities compared with metals. The phonon transport along the axial direction in the case of CNT and radial phonon transport in graphene account for their high thermal conductivities. The as-grown carbon nanomaterials, due to high hydrophobicity, cannot be dispersed in polar solvents directly. This problem is resolved by using surfactants or by attaching hydrophilic functional groups on the basal planes of graphene/side walls of CNT.^5,6 However, the selection of the functionalization process must be done judiciously, as delicate morphological organization, uniform dispersion, and ease of processing are essential to the performance of the resulting nanofluids. For instance, the defects introduced during functionalization can inevitably affect the thermal properties of the materials by decreasing the ballistic transport path length and by introducing scattering sites. Recently, hybrid composites of graphene and CNT have gained attention^7–9 and these hybrid nanocomposites can be advantageous for thermal management applications due to the inherent high thermal conductivities of individual counterparts. Hence, in order to exploit fundamental advances in phonon transport of these two high thermally conducting materials, in the present work a composite of CNT

Alternative Energy and Nanotechnology Laboratory (AENL), Nanofunctional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai, India. E-mail: [email protected];

Fax: +91-44-22570509; Tel: +91-44-22574862

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and graphene has been investigated for thermal transport applications.

Graphene prepared via chemical and thermal treatments possesses certain drawbacks such as attenuation in thermal/

electrical conductivities and also the explosive nature of reducing agents used in chemical reduction techniques makes it a process which needs special care.¹⁰ Moreover, most of the chemical methods involve a long time. In this context, we have recently reported a rapid and green synthesis technique capable of producing high-quality few-layer graphene at a large scale.¹¹The method involves the usage of abundant solar radiation for the reduction and exfoliation of graphite oxide (GO). In a similar fashion, highly intense solar radiation was efficiently utilized in the production of multiwalled carbon nanotubes (MWNT)- embedded graphene composite using GO–MWNT as the starting material.¹²The utilization of highly hydrophobic solar exfoliated graphene for tribological applications has been already investigated by our research group by dispersing it in engine oil.¹³ Frictional characteristics, anti-wear and extreme pressure properties of the nanofluids have been evaluated using a four ball tribotester and these properties are found to be improved compared to the base oil. In the present study, the hydrophobic solar graphene (sG) after suitable polar functionalization as well as graphene–MWNT without further treatment have been used for the synthesis of nanofluids. Further, the thermal and electrical transport properties as well as heat-transfer characteristics of the nanofluids have been investigated.

Experimental

Graphene was prepared via the solar exfoliation technique in which graphite oxide was employed as the starting material.¹¹ Initially, GO was prepared from Bay carbon graphite by Hummer’s method.¹⁴ The as-prepared GO was exposed to intense solar radiation wherein a convex lens of 90 mm diameter was employed to focus the radiation towards GO. The as-prepared highly hydrophobic solar graphene was functionalized by refluxing in conc. HNO₃for 1 h (f-sG) in order to render it hydrophilic, which is essential for its dispersion in polar base fluids such as de-ionized (DI) water and ethylene glycol (EG). MWNT were prepared by catalytic chemical vapour deposition technique.¹⁵ As-grown MWNT were purified and functionalized. Air oxidation at 350uC for 2 h and refluxing in conc. HNO₃ for 24 h removed the amorphous carbon and catalytic impurities from MWNT. The functionalization of MWNT (f-MWNT) was performed in a similar way as that of sG. Further, a 1 : 1 ratio of GO and f-MWNT was refluxed in conc. HNO₃for 2 h followed by washing to neutral pH and then dried to get a fine powder of GO–f-MWNT composite.

Solar exfoliation was performed upon this GO–f-MWNT composite by the same technique as described above. The exfoliated product, i.e.sG–f-MWNT, was employed for the synthesis of nanofluids without further chemical treatment. Nanofluids were prepared by a two-step synthesis technique. f-sG and sG–f-MWNT were dispersed in DI water and EG base fluids by ultrasonicating for nearly 30 min. The thermal conductivity of the nanofluids was measured using KD2 Pro thermal property analyzer. Electrical

conductivity was measured using EC-TDS analyzer and forced convective heat-transfer coefficients were measured with an in- house-built heat-transfer set up.¹⁶

Results and discussion

The structure and morphology of graphene and graphene–MWNT composite were determined by both spectroscopic and micro- scopic analyses. The XRD spectrum of f-sG depicted in Fig. 1(a) exhibits a broad peak around 25u, attributed to C(002) hexagonal reflections. The small size of the layers or a relatively short domain order of the stacked sheets resulted in a broadening in the XRD peak of graphene. On the other hand, the spectrum of f-MWNT shows a sharp peak, corresponding to reflections from C(002) and a relatively weak peak at 42u, which is indexed as C(101). It is observed that the XRD of GO–f-MWNT possesses two sharp peaks;

the one with the lower 2hvalue represents the GO while MWNT shows a peak with higher 2hvalue (Fig. 1(b)). Interestingly, the reduction and exfoliation of GO in GO–f-MWNT composite resulted in the disappearance of the GO peak and the resultant composite contains only a single peak corresponding to C(002) reflection.

The most important information about the attachment of functional groups can be obtained from the FTIR spectral analysis.

The FTIR spectrum of f-sG shows oxygen-containing functional groups resulting from the functionalization process (Fig. 2). It is observed that due to the presence of functional groups in f-MWNT, the as-grown sG–f-MWNT also contains the same compared to pure sG obtained by solar exfoliation technique.¹¹

Fig. 1XRD spectra of (a) f-sG, f-MWNT and (b) GO–f-MWNT, sG–f-MWNT. The broad peak of f-sG compared to sharp peaks of MWNT samples indicates short- range order in f-sG. Also, the disappearance of GO peak in the case of sG–f-MWNT confirms the complete reduction of GO to sG.

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Hence, it is concluded that these functional groups are from f-MWNT, as is evident from the FTIR spectrum. The major groups present are: OH group at 3530 cm²¹, CH2vibrations at 2700 cm²¹, a CLC band at 1700 cm²¹, CLO band at 1050 cm²¹.¹⁷ The presence of these hydrophilic functional groups helps the efficient dispersion of sG–f-MWNT in aqueous solvents without any further chemical treatment.

The introduction of defects or, in other words, the effect of functionalization can also be analysed by recording Raman spectra. The defect-induced band (D-band) intensity, as shown in Fig. 3, increases after treating sG in concentrated acid as compared with pure sG. This is due to the introduction of functional groups over the graphene layers, corroborating the FTIR analysis. The increase in the ratio of amplitudes of D and G bands (ID/IG) of functionalized solar graphene (0.44) from that of as-grown solar graphene (0.20)¹¹validates the creation of defects during functionalization. The high value ofI_D/I_G of the hybrid composite can be due to defects present in f-MWNT.

The morphology analysis clearly reveals the transformation of materials from one form to another. The thick flakes of GO and the thin, transparent, separated sheets of graphene obtained from this GO by focusing solar radiation (Fig. 4) clearly reveal a drastic change in morphology in a short time after irradiating with sunlight. Further, MWNT can be seen embedded throughout the GO flakes in GO–f-MWNT composite, whereas after the exfoliation process graphene sheets are taken apart by the MWNT thereby ensuring high surface area and non-agglomeration of graphene.

HRTEM analyses (not shown here) revealed the number of graphene layers obtained by solar exfoliation technique as 2–3 and the layer number of MWNT is found to bey40. The inner and outer diameters of MWNT are respectively 6.2 nm and 19.2 nm.

The average lateral dimension of solar exfoliated graphene isy1 mm.¹¹

The thermal conductivity (k) of solar graphene nanofluids in DI water and EG with respect to temperature and volume fraction is measured and is depicted in Fig. 5. The nanofluids clearly display non-linear behaviour in the thermal conductivity–temperature

Fig. 2FTIR spectra of f-sG and sG–f-MWNT composite demonstrating the attachment of functional groups.

Fig. 3Raman spectra of f-sG and sG–f-MWNT composite, showing the vibrational characteristics of the carbon nanomaterials.

Fig. 4FESEM images of (a) GO (b) sG (c) GO–f-MWNT and (d) sG–f-MWNT depicting clearly the morphology difference between GO and sG after exfoliation with focused solar radiation.

Fig. 5Thermal conductivity plots of f-sG in (a) DI water and (b) EG as a function of temperature and volume fraction of nanoparticles, demonstrating an increase in thermal conductivity with temperature and volume fraction of nanoparticles.

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relationship. Furthermore, the thermal conductivity of the nanofluids increased with increase in volume fraction of nanoparticles. The highest volume fraction,i.e. 0.04% aqueous f-sG nanofluids, exhibits an enhancement in thermal conductivity of 9.2% and 73% at 25uC and 50uC, respectively. On the other hand, f-sG/EG nanofluids with same volume fraction possess an enhancement in thermal conductivity of 6.9% and 20% at 25uC and 50uC respectively.

The temperature and volume fraction dependence of the thermal conductivity of sG–f-MWNT hybrid nanocomposite in DI water and EG is depicted in Fig. 6. The enhancement in thermal conductivity is 10.5% and 87.9% at 25uC and 50uC, respectively, for 0.04% volume fraction of aqueous sG–f-MWNT nanofluids.

The sG–f-MWNT/EG nanofluids also exhibit the same trend of enhancement in thermal conductivity and the values are 13.7%

and 24% at 25uC and 50uC, respectively. sG–f-MWNT nanofluid has a higher thermal conductivity enhancement than f-sG nanofluids. From this result, it is obvious that the thermal conductivity enhancement of nanofluids depends on that of the suspended particles. Many previous studies show similar results.

The mechanism behind augmentation in thermal conductivity with respect to temperature and volume fraction is now considered. The increase in thermal conductivity of the nanofluids with temperature may be due to the decrease in interfacial thermal resistance between base fluid and solid nanoparticles at high temperature.¹⁸ It is well known that the thermal conductivity enhancement is due to the heat conduction within the solid, and hence high thermal conductivity additives can give better thermal conductivity.¹⁸ Hence, the increment in thermal conductivity of the present graphene nanofluids can be due to the high inherent thermal conductivity of graphene. The superior thermal conduc-

tivity enhancement with graphene–MWNT composite nanofluids can be attributed to the synergistic effect of high thermal conductivity of both 1D MWNT and 2D graphene. The anom- alously high enhancement in thermal conductivity of CNT was explained in literature by percolation model.¹⁹According to this model, CNT forms long chains of interconnected networks, which act as conducting paths. The geometry of nanotubes with high aspect ratio is exemplary to develop such a network. As the percolation path increases, the effective thermal conductivity also enhances. This in turn points to the fact that CNT with higher aspect ratio would result in a longer percolation path and hence greater thermal conductivity enhancement. In a similar way, a mixture containing the high-aspect-ratio MWNT and graphene networks can act as excellent conducting paths for heat conduction and hence better enhancements in thermal transport.

The superior performance of graphene–f-MWNT nanofluids over pure graphene nanofluids can be understood as follows: the as-grown graphene is highly hydrophobic and the interlayer cohesive energy (van der Waals interaction) increases sharply with increasing graphene sheet size.²⁰This increase usually results in irreversible aggregation without electrostatic or steric protection, which may be detrimental to the stability of graphene nanofluids.²¹Hence graphene has been functionalized using concentrated acid to make it soluble in polar base fluids. But the introduction of insulating functional groups may deteriorate the intrinsic high thermal conductivity of pure graphene. The addition of MWNT on the other hand may increase the basal spacing between graphene sheets, thereby efficiently preventing their agglomeration.²⁰The preparation of graphene–CNT composites by post-mixing of graphene and CNT by refluxing in acid has been reported.²² In the present case, even though we have prepared GO–f-MWNT composite by functionalization of GO and MWNT in acid for 2 h, graphene–f-MWNT composite has been prepared by a one-step green synthesis method wherein the exfoliation of GO into graphene in the presence of MWNT occurred and which could efficiently separate graphene with MWNT. The morphology analysis corroborates this assumption since insertion of MWNT between the graphene layers as well as the transparent nature of graphene can be seen from the FESEM images. The advantage of the present synthesis method in particular to nanofluids application is that the oxygen functional groups present in the f-MWNT in the hybrid composite ruled out the need for functionalizing the composite again, thereby preserving the high thermal properties of graphene in the composite. Morphology analysis reveals the connected network of graphene–f-MWNT composite, favouring the possible improvement in thermal conductivity due to low interfacial resistance. It is reported that particulates with high aspect ratios such as carbon nanotubes and graphene, by forming tightly bonded clusters and suppressing the interface resistance, can become excellent additives to attain high thermal conductivity.¹⁹ The incorporation of both graphene and MWNT into a hybrid composite can effectively make use of the excellent thermal properties of both species and can give a better result, as has been seen from the present study. Hence, the present thermal transport studies shed light on the fact that the sG–f-MWNT material prepared byin situsolar reduction of GO–f-MWNT can be used as

Fig. 6Thermal conductivity plots of sG–f-MWNT in (a) DI water and (b) EG as a function of temperature and volume fraction of nanoparticles.

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efficient coolant additive due to the high thermal conductivity of this material resulting from the novel preparation technique.

Now, the electrical transport property, i.e. the electrical conductivity (s) of the nanofluids, which is technologically and scientifically important due to its application in electrical and electronic circuits, has been recorded as a function of both concentration and temperature. The electrical conductivities of the base fluids,i.e.DI water and EG, were measured to be 3.2mS cm²¹ and 0.89 mS cm²¹, respectively, at 25 uC. Fig. 7 shows the measured electrical conductivity of f-sG in DI water and EG. It is observed that the electrical conductivity of the nanofluids increases with volume fraction and with temperature. The effective electrical conductivity of colloidal nano-suspensions in a liquid depends on several parameters such as the physical properties of the fluid, conductivity of the particles, electric double layer (EDL) characteristics, volume fraction, ionic concentrations and other physicochemical properties.²³ Furthermore, it is also proposed that the homogeneously dispersed nanoparticles may exhibit reduced equivalent particulate masses resulting in high electro- phoretic mobility, which in turn enhances the effective electrical conductivity of the nanofluids.²⁴Graphene and MWNT are well known for their high electrical conductivities.^25,26In the present case, with an increase in particle volume fraction of f-sG the availability of conducting pathways in the solution increases, which in turn enhances the overall electrical conductivity of the solution. Also, the increased Brownian motion may enhance the electrical conductivity of nanofluids when temperature increases.

Graphene–f-MWNT composite-based nanofluids also exhibit the same trend of enhancement in electrical conductivity with temperature and volume fraction (Fig. 8). This can be due to the

good electrical conductivities of both graphene and MWNT as well as the good transport of electrons through high-aspect-ratio tight- binding network of graphene and MWNT.

While an increase in effective thermal conductivity is an indication of improved heat-transfer behaviour of nanofluids, the net benefit of nanofluids as heat-transfer fluids is evaluated through the heat-transfer coefficient. Many experimental and numerical investigations have been carried out to determine the convective heat-transfer characteristics of nanofluids based on metallic/metal oxide nanoparticles as well as CNT. The main parameters investigated in these studies are size of nanoparticles,²⁷ concentration of nanoparticles²⁸ and thermal conductivity.²⁹Hence, the forced convective heat-transfer coefficients (h) of f-sG and sG–f-MWNT based nanofluids flowing through a horizontal tube has been investigated in the present study using an in-house-developed heat-transfer set up. The reliability and accuracy of the experimental set up has been checked with de- ionized water as working fluid and fitting the experimental results to the well-known Shah correlation³⁰for laminar fluid flow and Dittus-Boelter correlation³¹for turbulent fluid flow. The convective heat-transfer coefficient has been calculated using the equation:

h~ q

T_w(x){T_f(x)

whereqis the heat flux (W m²²),xis the axial distance from the entrance of the test section (cm),Tw(x) andTf(x) correspond to the temperature on the wall of the test section and the fluid (uC), respectively.T_f(x) has been calculated from the following energy balance equation:

Fig. 7Electrical conductivity of sG in (a) DI water and (b) EG base fluids, depicting a positive effect with respect to temperature and volume fraction.

Fig. 8Electrical conductivity plots of sG–f-MWNT in (a) DI water and (b) EG as a function of temperature and volume fraction of nanoparticles.

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T_fð Þx~T_inz qSx rCpuA

whereTinis the inlet temperature of the fluid,Sis the perimeter of the tube,ris the density of the fluid,Ais the cross sectional area, C_pis the heat capacity andu is the average fluid velocity. This equation is based on the assumption that no heat is lost through the insulation layer.

Fig. 9 shows the effect of concentration of f-sG in f-sG nanofluids on the local heat-transfer coefficients at various axial distances from the entrance of the test section corresponding to different Reynolds numbers. The results of pure DI water at similar Reynolds numbers are also shown. It is observed that the addition of f-sG increases the convective heat-transfer coefficient significantly, in particular at the entrance of the test section. The heat-transfer enhancement improves with increasing the particle volume concentration as does the thermal conductivity (discussed previously). A decrease in the heat-transfer enhancement with increasing distance from the entrance section is also seen. For instance, with 0.01% volume fraction of aqueous f-sG nanofluids, the local heat-transfer coefficients atx/D= 19 andx/D= 53 are 44%

and 40% respectively atRe = 2000 in comparison with pure DI water. For 0.02% volume fraction of aqueous f-sG nanofluids, the enhancement decreases from 69% atx/D= 19 to 64% atx/D= 53 at Re= 2000. f-sG/EG nanofluids also exhibit similar behaviour of increase in heat transfer with increase in Reynolds number and particle volume fraction. For 0.01% volume fraction of f-sG/EG nanofluids, the local heat-transfer coefficients atx/D= 19 andx/D

= 53 are 104% and 95% respectively atRe= 200 in comparison with pure EG.

Now, a comparison of thermal conductivity plot and heat- transfer characteristics of f-sG nanofluids indicates that the enhancement of the convective heat-transfer coefficient is much more rapid than that entirely due to the enhancement of effective thermal conductivity alone. When the heat-transfer enhancement is as high as 69% (at Re = 2000) in Fig. 9(a), the thermal conductivity enhancement is nearly 3% (Fig. 5(a)) at 0.02% volume fraction of f-sG/DI water nanofluids. This indicates that the presence of nanoparticles in the flow influences the heat transfer beyond what would be expected from increased thermal conductivity alone. This has been attributed to particle–fluid interactions by some researchers. Similar studies have been reported in literature. For example, an enhancement in heat- transfer coefficient of 60% for an aqueous nanofluid containing 2% Cu nanoparticles by volume has been reported where the effective thermal conductivity of the nanofluid was only 12.5%

higher than that of the base liquid.³² Aqueous c-alumina nanofluids exhibit a 47% increase in the convective heat-transfer coefficient for 1.6% nanoparticles loading andRe= 1600, which is much greater than that due to the enhancement of thermal conduction (y10%).³³The studies on the convective heat transfer of CNT-based nanofluids shows that the maximum convective heat-transfer coefficient enhancement reaches over 350% at laminar flow and constant wall heat flux conditions and also that the convective heat-transfer enhancement depends on the flow condition and CNT concentration.³⁴Garg et al.have reported a shear-thinning non-Newtonian behaviour in the case of CNT nanofluids.³⁵

The enhancement in heat-transfer coefficient with nanofluids is now considered: the heat-transfer coefficient is a macroscopic parameter and can be approximately given byk/dtwithdtis the thickness of thermal boundary layer.³⁴At the entrance of the test section (x/D= 0), since the theoretical boundary layer thickness is zero, the heat-transfer coefficient approaches infinity. With increase in axial distance, the boundary layer increases until fully developed, after which the boundary layer thickness and hence the convective heat-transfer coefficient is constant. In other words, convective heat-transfer coefficient can be improved by either increasing k or decreasing dt. But from the obtained thermal conductivity values, it is clear that the enhancement of the convective heat-transfer coefficient is much greater than that due to the increase in the thermal conductivity, especially at high Reynolds numbers. Hence, the heat-transfer enhancement can be largely ascribed to a decline in the thermal boundary layer thickness. The possible reasons reported for this reduction in boundary layer thickness are particle migration in nanofluids due to shear action, viscosity gradient, and Brownian motion in the cross-section of the tube.³⁶The increase of the thermal conduction under dynamic conditions could be another decisive factor for heat transfer improvement.³² This is due to the fact that while effective thermal conductivities have been acquired under static conditions, significant shear exists under the conditions of convective heat-transfer experiments.

Fig. 9Convective heat-transfer coefficients of f-sG in (a) DI water and (b) EG. The square, inverted triangle and star symbols correspond to DI water, 0.01%

f-sGnanofluids and 0.02% f-sG nanofluids, respectively. The pink, green, and purple colored lines correspond to Reynolds numbers 2000, 5000 and 10 000 respectively.

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The convective heat-transfer coefficient of sG–f-MWNT nanofluids has also been evaluated with increasing Reynolds number (flow rate) as well as nanoparticle volume fraction and is depicted in Fig. 10. The enhancement in heat-transfer coefficient is nearly 140% and 193% atRe= 2000 for, respectively, 0.01% and 0.02%

volume fraction of sG–f-MWNT/DI water nanofluids at the entrance of the test section. With sG–f-MWNT/EG nanofluids, the enhancement is 148% and 282% for, respectively, 0.01% and 0.02% volume fraction at Re = 200. Hence, it is obvious that, similar to thermal and electrical conductivities, the forced convective heat-transfer coefficient is also higher in the hybrid composite-based nanofluids than in pure graphene nanofluids.

Also, the nanofluids were stable for more than two months. Hence it can be concluded that graphene–MWNT composite nanofluids can be better heat exchangers in cooling applications.

Conclusions

In summary, graphene and graphene–f-MWNT composite prepared byin situreduction of graphite oxide–f-MWNT composite are investigated for thermal-transport applications. The green synthesis method for preparation of hybrid carbon nanomaterial involves the focusing of solar electromagnetic radiation, in comparison with traditional approaches of post-treatment of MWNT and graphene in acids. The hybrid composite-based nanofluids show better thermal transport characteristics as compared to pure graphene nanofluids, ascribed to the synergistic effect of high thermally conducting individual counterparts. The enhancement in thermal conductivity with 0.04% volume fraction

of aqueous graphene–f-MWNT nanofluids is 10.5%, whereas the improvement in convective heat-transfer coefficient is found to be 193% at 0.02% volume fraction and for Reynolds number 2000.

This points to the potential application of the present graphene–f- MWNT composite nanofluids for industrial thermal management.

Further, a dramatic enhancement in heat-transfer coefficient of sG–f-MWNT nanofluids suggests that heat-transfer coefficient is also affected by thinning of the thermal boundary layer, in addition to the improvement in thermal conductivity.

Acknowledgements

The financial support from Indian Institute of Technology Madras, India, is gratefully acknowledged. The authors thank SAIF, IIT Madras, for FTIR measurements.

Notes and references

1 S. U. S. Choi,Developments and Applications of Non-Newtonian Flows, ASME, New York, 1995.

2 J. Tijerina, T. N. Narayanan, G. Gao, M. Rohde, D.

A. Tsentalovich, M. Pasquali and P. M. Ajayan, ACS Nano, 2012, 6, 1214.

3 A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao and C. N. Lau,Nano Lett., 2008, 8, 902.

4 Y. Chandra, F. Scarpa, R. Chowdhury, S. Adhikari and J. Sienz, Composites, Part A, 2013, 46, 147.

5 H. Xie and L. Chen,J. Chem. Eng. Data, 2011, 56, 1030.

6 S. S. Gupta, V. M. Siva, S. Krishnan, T. S. Sreeprasad, P.

K. Singh, T. Pradeep and S. K. Das,J. Appl. Phys., 2011, 110, 084302.

7 V. C. Tung, L.-M. Chen, M. J. Allen, J. K. Wassei, K. Nelson, R.

B. Kaner and Y. Yang,Nano Lett., 2009, 9, 1949.

8 E. Yoo, J. Kim, E. Hosono, H.-s. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277.

9 Q. Su, Y. Liang, X. Feng and K. Mullen,Chem. Commun., 2010, 46, 8279.

10 S. Park and R. S. Ruoff,Nat. Nanotechnol., 2009, 4, 217.

11 V. Eswaraiah, S. S. Jyothirmayee Aravind and S. Ramaprabhu,J.

Mater. Chem., 2011, 21, 6800.

12 S. S. Jyothirmayee Aravind, R. Imran Jafri, N. Rajalakshmi and S. Ramaprabhu,J. Mater. Chem., 2011, 21, 18199.

13 V. Eswaraiah, V. Sankaranarayanan and S. Ramaprabhu,ACS Appl. Mater. Interfaces, 2011, 3, 4221.

14 W. S. Hummers and R. E. Offeman,J. Am. Chem. Soc., 1958, 80, 1339.

15 A. L. M. Reddy, M. M. Shaijumon and S. Ramaprabhu, Nanotechnology, 2006, 17, 5299.

16 S. S. J. Aravind, P. Baskar, T. T. Baby, R. K. Sabareesh, S. Das and S. Ramaprabhu,J. Phys. Chem. C, 2011, 115, 16737.

17 U. J. Kim, C. A. Furtado, X. Liu, G. Chen and P. C. Eklund,J. Am.

Chem. Soc., 2005, 127, 15437.

18 J. J. Wang, R. T. Zheng, J. W. Gao and G. Chen,Nano Today, 2012, 7, 124.

19 N. N. Venkata Sastry, A. Bhunia, T. Sundarajan and S. K. Das, Nanotechnol., 2008, 19, 055704.

20 Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian and F. Wei,Adv. Mater., 2010, 22, 3723.

21 H. Liu, J. Gao, M. Xue, N. Zhu, M. Zhang and T. Cao,Langmuir, 2009, 25, 12006.

Fig. 10Convective heat-transfer coefficients of sG–f-MWNT in (a) DI water and (b) EG. The square, inverted triangle, and star symbols correspond to DI water, 0.01%

f-sG nanofluids and 0.02% f-sG nanofluids, respectively. The pink, green, and purple colored lines correspond to Reynolds numbers 2000, 5000 and 10 000, respectively.

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22 R. I. Jafri, T. Arockiados, N. Rajalakshmi and S. Ramaprabhu,J.

Electrochem. Soc., 2010, 157, B874.

23 R. J. Hunter, Zeta potential in colloid science, Principles and Applications, Academic Press, London, 1981.

24 S. Ganguly, S. Sikdar and S. Basu,Powder Technol., 2009, 196, 326.

25 Z.-S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang and H.-M. Cheng,ACS Nano, 2009, 3, 411.

26 T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H.

F. Ghaemi and T. Thio,Nature, 1996, 382, 54.

27 C. H. Li and G. P. Peterson,J. Appl. Phys., 2007, 101, 044312.

28 J.-H. Lee, K. S. Hwang, S. P. Jang, B. H. Lee, J. H. Kim, S. U.

S. Choi and C. J. Choi,Int. J. Heat Mass Transfer, 2008, 51, 2651.

29 X. Zhang, H. Gu and M. Fujii,Exp. Therm. Fluid Sci., 2007, 31, 593.

30 R. K. Shah,Proceedings of 3rd National Heat and Mass Transfer Conference, 1975, vol. 1, HMT-11.

31 F. W. Dittus and L. M. K.Boelter,Heat Transfer in Automobile Radiators of Tubular Type. University of California Press, Berkeley, 1930, vol. 12, p. 13.

32 Y. Xuan and Q. Li,J. Heat Transfer, 2003, 125, 151.

33 D. Wen and Y. Ding,Int. J. Heat Mass Transfer, 2004, 47, 5181.

34 Y. Ding, H. Alias, D. Wen and R. A. Williams,Int. J. Heat Mass Transfer, 2006, 49, 240.

35 P. Garg, J. L. Alvarado, C. Marsh, T. A. Carlson, D. A. Kessler and K. Annamalai,Int. J. Heat Mass Transfer, 2009, 52, 5090.

36 R. J. Phillips, R. C. Armstrong, R. A. Brown, A. L. Graham and J.

R. Abbott,Phys. Fluids A, 1992, 4, 30.