Investigation of thermal conductivity, viscosity, and

Investigation of thermal conductivity, viscosity, and electrical conductivity of graphene based nanofluids Madhusree Kole and T. K. Dey Citation: J. Appl. Phys. 113, 084307 (2013); doi: 10.1063/1.4793581 View online: http://dx.doi.org/10.1063/1.4793581 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i8 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 113, 084307 (2013)

Investigation of thermal conductivity, viscosity, and electrical conductivity of graphene based nanofluids Madhusree Kole and T. K. Deya) Thermophysical Measurements Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

(Received 4 December 2012; accepted 12 February 2013; published online 27 February 2013) Stable and well dispersed functionalized graphene–ethylene glycol (EG) þ distilled water nanofluids having graphene nano-sheets (GnS) volume concentration between 0.041 and 0.395 vol. % are prepared without any surfactant. Graphene nano-sheets are prepared from high purity graphite powder by Hummers method followed by exfoliation and reduction by hydrogen gas. Thus, obtained hydrogen exfoliated graphene (HEG) is then functionalized using acid. The graphene nano-sheets are characterized using XRD, TEM, Raman spectroscopy, and FTIR spectroscopy. Thermal conductivity and viscosity measurements are performed both as a function of graphene loading and temperature between 10 and 70 C. Thermal conductivity enhancement of 15% for a loading of 0.395 vol. % f-HEG is observed at room temperature. The measured nanofluid’s thermal conductivity is explained well in terms of the expression derived by Nan et al. (J. Appl. Phys. 81, 6692 (1997)), which considers matrix-additive interface contact resistance of mis-oriented ellipsoidal particles. The viscosity of the prepared f-HEG nanofluids and the base fluid (EG þ distilled water) displays non-Newtonian behaviour with the appearance of shear thinning and nearly 100% enhancement compared to the base fluid (EG þ DI water) with f-HEG loading of 0.395 vol. %. Known theoretical models for nanofluid’s viscosity fail to explain the observed f-HEG volume concentration dependence of the nanofluid’s viscosity. Temperature dependence of the studied nanofluid between 10 and 70 C is explained well by the correlations proposed earlier for nanofluids with spherical nanoparticles. Electrical conductivity of the f-HEG nanofluids shows significant enhancement of 8620% for 0.395 vol. % loading of f-HEG in a base fluid of 70:30 mixture of EG and distilled C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793581] water. V I. INTRODUCTION

Since the discovery of nanofluids by Choi,1 a large number of investigations have been reported on metallic and metal oxide nanofluids, where the particles are spherical, and also on carbon nanotube (CNT) based nanofluids, where the particles are cylindrical in shape. However, till now, no definitive conclusions could be arrived to suggest a better and stable replacement to the existing coolants. Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, has attracted much attention since its discovery by Novoselov et al.2 in 2004. This two-dimensional (2D) material, graphene, displays many unusual electrical, mechanical, and thermal behaviours, such as very high carrier mobility,2 long-range ballistic transport at room temperature,3 quantum confinement in nanoscale ribbons,4 singlemolecule gas detection sensitivity,5 and high Young’s modulus and fracture strength.6 In view of these unusual properties, graphene is expected to be a potential material for various new applications.7 Graphene is, however, a new entrant in the area of nanofluids. Interestingly, the in-plane thermal conductivity of a suspended single-layer graphene is reported to be as high as 5200 W/mK.8 Further, as graphene is a 2D material, the heat transfer properties are expected to be much different from the zero dimensional nanoparticles a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2013/113(8)/084307/8/$30.00

and one dimensional carbon nano-tube. Moreover, graphene itself being an excellent thermal conductor, the graphene based nanofluids are normally expected to display significant thermal conductivity enhancement. Yu et al.9 first reported the thermal conductivity of graphene oxide-ethylene glycol (EG) nanofluids. They observed an enhancement of 61% for a loading of 5 vol. % of graphene oxide nano-sheets at room temperature. Thermally exfoliated graphene (TEG) based nanofluids with water and with EG has been reported by Baby et al.10 They observed thermal conductivity enhancement of 14% with only 0.056 vol. % of graphene nano-sheets (GnS) dispersed in de-ionized (DI) water at 25 C, which increased to 64% at 50 C. However, a marginal enhancement 4% at 25 C has been obtained for EG based nanofluid with 0.05 vol. % of GnS. The same group later11 reported an enhancement of 16% at 25 C with 0.05 vol. % of functionalized GnS prepared by hydrogen exfoliation of graphite oxide (GO) sheets dispersed DI water. Similar enhancement has also been observed by Gupta et al.12 It may be noted that thermal conductivity of EG based nanofluids did not display any significant enhancement for low volume concentrations. Highest enhancement in thermal conductivity (86%) has been reported so far by Yu et al.13 for 5 vol. % of GnS dispersed in water at 30 C prepared with sodium dodecyl-benzenesulfonate (SDBS) as surfactant. It may be noted that for many applications, EG-water mixture is generally used as the working coolant, while base fluid used in most of the investigations on graphene

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nanofluids is either pure water or pure EG. In view of the above, in the present communication, we report our results on the thermal conductivity, viscosity, and electrical conductivity of graphene nanofluids with 70:30 (by volume) mixture of EG and distilled water as the base fluid. It may be mentioned that this is the first report on the viscosity of graphene based nanofluids. Results have been discussed to identify the mechanisms responsible for the observed thermal conductivity and viscosity enhancement in graphene nanofluids prepared with 70:30 mixture of EG and distilled water. II. NANOFLUIDS PREPARATION AND CHARACTERIZATION

Graphite (99.99%, SP-1) is procured from Bay Carbon, Inc., USA. All other materials like sulfuric acid, nitric acid, sodium nitrate, potassium permanganate, hydrogen peroxide, ethanol, and EG are of analytical grade and are procured from M/S Loba Chemicals, India. Distilled water is used throughout the experiment. Graphite oxide is prepared from graphite using Hummers method.14 The dried GO thus obtained is exfoliated and reduced by controlled flow of high purity hydrogen gas11 in the presence of Argon atmosphere at 200 C. The hydrogen exfoliated graphene (HEG) is dark black colour in appearance and is extremely light in weight. The XRD pattern of graphite, GO, and HEG is shown in Fig. 1. The intense crystalline peak (0 0 2) of graphite occurs at 26.65 , which is the characteristic peak of hexagonal graphite with a d-spacing of 0.34 nm. Upon oxidation of graphite into GO, the peak position shifts to 11.14 . The interplanar spacing now increases to 0.79 nm. This increase in d-spacing is due to the intercalation of –OH containing functional groups in between the graphene layers. After exfoliation of GO with hydrogen at 200 C, the 11.14 peak disappears and a broad peak appears, starting from 15 to

FIG. 1. XRD pattern of graphite, graphite oxide, and hydrogen exfoliated graphene.

FIG. 2. Transmission electron micrograph of hydrogen exfoliated graphene nano-sheets.

27 . The inter-planar spacing decreases to 0.39 nm. This indicates a large extent removal of oxygen and water from the interlayer during exfoliation. This broad peak also indicates loss of the long range order in the prepared graphene. The transmission electron micrograph of HEG is shown in Fig. 2, which confirms the sheet-like morphology of the prepared graphene (HEG). The Raman spectra of pristine graphite, GO, and HEG is shown in Fig. 3. For graphite, a highly intense G band occurs at 1586 cm1. This corresponds to the optically allowed E2g phonons at the Brillouin zone centre.15 The absence of D band in graphite suggests that graphite used is defect free. The G band of GO is located at 1611 cm1. The G band of HEG is shifted back to 1591 cm1, which is close the value of pristine graphite indicating the reduction of GO during hydrogen treatment. Moreover, a broadening of G band is observed in GO and HEG and is attributed to an increase in the disorder. The chemical treatment procedures to prepare GO and its exfoliation to get HEG induce defects in the

FIG. 3. Raman spectra of pristine graphite (GR), GO, HEG, and functionalized graphene nano-sheets (f-HEG).

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graphitic structure. As a result, a broad D band with an intensity comparable to that of the G band is obtained in GO and HEG as well. The D and G bands denote the presence of sp3 and sp2 hybridized carbon atoms, respectively, in the sample. Due to their hydrophobic nature, the as-prepared HEG does not disperse in water and EG and requires functionalization, which is done by treating HEG with (3:1) concentrated H2SO4 and HNO3. The acid-HEG mixture is ultrasonicated for 2 h at room temperature and is then washed several times with distilled water until pH becomes neutral. The solution is then centrifuged and dried in a vacuum oven to get functionalized graphene. Figure 3 also includes the Raman spectra of f-HEG. In HEG, the G and D bands occur at 1591 cm1 and 1355 cm1, respectively. In the case of f-HEG, both the band positions are shifted to higher wave number side and also broadened with respect to HEG. The G band has a broad peak centred around 1596 cm1 and D band has a peak centred around 1366 cm1. The ratio of the D band intensity to G-band intensity in f-HEG is higher than that of HEG. This increase in the relative intensity of the disordered mode is attributed to the increased number of structural defects.9,16,17 After acid functionalization, hexagonal carbon order gets disrupted, i.e., the carbon atoms get sp3 hybridized. The effect of acid treatment and attachment of functional groups of the synthesized f-HEG are further confirmed from FTIR studies (Fig. 4). It may be observed that for f-HEG, the peaks at around 3446 and 1620 cm1 are due to -OH functional groups. A small doublet peak of -CH2 (2933 and 2866 cm1) and -CH at 1363 cm1 is present both in HEG and f-HEG. The peaks at 1710 and 1380 cm1 can be assigned to the C¼O and C-O stretching vibrations of -COOH. These functional groups help the graphene sheets to disperse in waterEG mixture. Functionalized graphene–EG þ distilled water (70:30) nanofluids with various volume concentrations of GnS (viz., 0.041, 0.124, 0.207, and 0.395 vol. %) are prepared by intense ultrasonication for 45 min. No surfactant is used in the preparation of the present nanofluids. It may be noted that constancy in thermal conductivity of the nanofluid with time reflects the stability of the nanofluids.9 Therefore, the suspension stability of the prepared nanofluids is verified by measuring periodically for several months, the thermal

FIG. 4. FTIR spectra of HEG and f-HEG.

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conductivity of the stationary nanofluid maintained at 30 C. The result (Fig. 5) shows that the thermal conductivity of the nanofluids does not display any worthwhile degradation and is constant for 150 days (5 months). The constant thermal conductivity clearly reflects high stability of the garphene(EG-DI water) nanofluids. III. EXPERIMENTAL DETAILS A. Thermal conductivity

A transient hot-wire (THW) method is used for thermal conductivity measurements. Since the THW measurement lasts for a period of the order of seconds, the problems associated with convection can be eliminated. According to the principle presented by Nagasaka and Nagashima,18 the thermal conductivities of measured fluids ðkÞ can be determined from q dT : (1) k¼ 4p d ln t As the thermal conductivity of the liquid is inversely proportional to the slope of the temperature–time response of the wire, it is calculated by measuring the slope of the straight line represented by Eq. (1). The experimental uncertainty of thermal conductivity measurements is estimated to be within 61%. The hot-wire cell is calibrated using ethylene glycol and distilled water. The measured thermal conductivity of ethylene glycol and water is found to be within 1% of the reported values. B. Viscosity

The viscosity of the nanofluids is measured by a Brookfield programmable viscometer (model: LV DV-II-Pro) appropriately connected to a PC controlled Julabo temperature controlled bath to vary the fluid temperature between 10 and 70 C. Basically, the viscometer drives a spindle immersed in nanofluids. Due to rotation of the spindle, a viscous drag of the fluid against the spindle is developed, which is measured by the deflection of the calibrated spring. The temperature of the fluid is measured by a Pt-100 temperature

FIG. 5. Thermal conductivity versus number of days of the prepared nanofluids (containing 0.395 vol. % f-HEG) confirming the stability of the suspension.

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sensor. By choosing the most appropriate spindle, data are taken when the applied torque is between 10% and 100%. The operation of the viscometer and data collection (namely, viscosity, shear stress, shear strain rate, RPM, torque, and temperature) is done using the WINGATHERV software. All the measurements are performed under steady-state conditions and the calibration of the viscometer was checked with the standard fluid provided by Brookfield Engineering Laboratories. The estimated maximum uncertainty in the viscosity measurement is 3%. R

C. Electrical conductivity

Electrical conductivity of the GnS–(EG þ distilled water) nanofluids both as functions of f-HEG loading and fluid temperature between 10 and 70 C are measured using a M/S EUTECH Instruments, Bench Conductivity/TDS meter (model CON510). The conductivity meter has a measuring range between 0.2 and 200 000 lS/cm and a resolution of 0.1%. Prior to the measurements, the meter is calibrated using the buffer solutions of known electrical conductivities. Measurements are performed using 40 ml of the nanofluid sample in a cylindrical glass tube, with the conductivity probe immersed in it. This entire assembly is placed in a temperature controlled bath (60.01 C). At each temperature, the measurements are repeated for 5 times, and the average value is taken. The estimated uncertainty in the measurement of electrical conductivity of the nanofluid is within 65%. IV. RESULTS AND DISCUSSION A. Thermal conductivity

Experiments are performed both as a function of functionalized GnS concentration (0.041-0.395 vol. %) and fluid temperature between 10 and 70 C. The thermal conductivity ratio ðknf =kbf Þ of the prepared nanofluids measured at 30 C is shown in Fig. 6 as a function of f-HEG concentration. Thermal conductivity of the prepared nanofluids increases nearly linear with graphene concentration. Thermal conductivity shows a maximum enhancement of 15% for a loading of 0.395 vol. % f-HEG at 30 C. Figure 6 also includes

FIG. 6. Thermal conductivity ratio ðknf =kbf Þ of the prepared nanofluids as a function of nano-sheet concentration at room temperature. Comparison with the thermal conductivity results on graphene nanofluids of others.11,12

the thermal conductivity enhancement of pure water and pure EG based graphene nanofluids reported by others.11,12 It may be seen that our results are close to that reported by Gupta et al.12 for chemically reduced graphene-water nanofluids. It is important to note from Fig. 6 that the thermal conductivity enhancement obtained in the present case falls in between, which reported for f-HEG dispersed in pure DI water and pure EG, respectively.11 This is expected in view of the fact that the base fluid in the present case is 70:30 (by volume) mixture of EG and distilled water. Hamilton and Crosser19 stated that the particle shape has a substantial effect on the effective thermal conductivity of the suspension when the particle-to-liquid thermal conductivity ratio is above 100. Gao et al.20 theoretically explained the large enhancement of the effective thermal conductivity of nanofluids by adjusting the shape of nanoparticles. Graphene is a two-dimensional solid and has a very high aspect ratio. In addition, graphene has the largest surface area compared to nano-tubes and other nanoparticles; consequently, graphene nano-sheets will have significantly larger contact area/interface with the base fluid. Therefore, the thermal contact resistance (Kapitza resistance) at the graphene–fluid interface is reduced profoundly. This should help to improve the effective thermal conductivity of the nanofluid. A benchmark study21 performed by researchers from over 30 organizations worldwide on the thermal conductivity of nanofluids demonstrated that the experimental data were in good agreement with the effective medium theory developed for dispersed particles by Maxwell22 and generalized by Nan et al.23 considering the matrix-additive interface contact resistance. The effective thermal conductivity expression as suggested by Nan et al.23 for composite with completely mis-oriented ellipsoidal particles is knf ¼ kbf

3 þ /½2b11 ð1 L11 Þ þ b33 ð1 L33 Þ ; 3 /ð2b11 L11 þ b33 L33 Þ

(2)

where Lii is the geometrical factor. bii is defined as bii ¼

kp kbf kbf þ Lii ðkp kbf Þ

(3)

with kp : the thermal conductivity of the ellipsoid particles. For graphene, the aspect ratio is very high, so, L11 ¼ 0 and L33 ¼ 1.13 It may be seen from Fig. 7 that the effective thermal conductivity proposed by Nan et al.23 fairly well explains the measured data. Using least square fitting of the experiment data, the in-plane thermal conductivity of functionalized HEG (f-HEG) is obtained as: 10.9 6 1.08 W/mK. It is seen that the thermal conductivity of graphene estimated from the effective-medium approximation is much lower than the intrinsic thermal conductivity of suspended graphene sheet.8 This is possible because of the fact that the hydrogen exfoliated graphene contains various structural defects caused by strong oxidization of graphite. Using Nan et al.23 model, Yu et al.13 also estimated the in-plane thermal conductivity of chemically reduced graphene to be 6.8 6 0.8 W/mK. Similarly, the thermal conductivity of reduced graphene

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FIG. 7. Estimation of thermal conductivity of the prepared nanofluids by Nan et al.23 model.

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FIG. 9. Viscosity vs. shear strain rate of EG-distilled water mixture at different temperatures.

oxide measured by Schwamb et al.24 was reported to be between 0.14 and 2.87 W/mK. Such a low value for thermal conductivity of graphene was thought to be due to significant amount of lattice defects introduced during the chemical oxidation of graphite to GO. Moreover, theoretical calculation25 demonstrated that the thermal conductivity of graphene is also dependent on the size, the edge roughness, and the defect density. Figure 8 shows the temperature dependence of the thermal conductivity of the base fluid (70 vol. % EG þ 30 vol. % distilled water), as well as that of nanofluids containing various volume concentrations of f-HEG. Temperature variation of the thermal conductivity of both the base fluid and the prepared nanofluids is qualitatively similar and increases linearly with increase in temperature. The enhancement ratios are 4.3% and 13.4% for the nanofluids with f-HEG concentrations 0.041 and 0.395 vol. % at 10 C, respectively. However, the values are enlarged up to only 5.6% and 17%, respectively, when the temperature is raised to 70 C. The above results demonstrate that temperature does not have a strong influence on the thermal conductivity enhancement of the present nanofluids. It may be noted that for graphenepure EG nanofluids nearly temperature independent behaviour of thermal conductivity enhancement has been reported earlier.10,11,13 On the contrary, the thermal conductivity

In the absence of any prior data on the viscosity of both base fluid (70 vol. % EG þ 30 vol. % distilled water) and the functionalized graphene nanofluids, it is important to confirm whether they display Newtonian or non-Newtonian behaviour as a function of both fluid temperature and graphene concentration. The viscosity of the base fluid measured as a function of shear strain rate between 10 and 70 C is shown in Fig. 9. It may be seen that over the measured temperature range, viscosity of the base fluid decreases with the shear strain rate, indicating non-Newtonian behaviour. With increasing GnS loading in the base fluid, this non-Newtonian feature of the base fluid becomes more prominent (Fig. 10). Shear stress vs. shear strain rate plot for both base fluid and the prepared nanofluids at 30 C are shown in Fig. 11. It is seen that for all the studied nanofluids, the shear stress varies

FIG. 8. Temperature dependence of thermal conductivity of both base fluid and the prepared nanofluids.

FIG. 10. Viscosity vs. shear strain rate of the prepared nanofluid containing 0.395 vol. % f-HEG within the measured temperature range (10–70 C).

enhancement of graphene-water nanofluids displays significant temperature dependence. In view of the above, for the present (70:30) mixture of EG and distilled water based graphene nanofluids, the observed marginal influence of temperature on the thermal conductivity enhancement may be expected. B. Viscosity

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FIG. 11. Shear stress vs. shear strain rate plots of various GnS-EG þ distilled water nanofluids at room temperature.

nonlinearly with shear strain rate, indicating the “shear thinning” behaviour, which is consistent with the observed decrease in viscosity with increasing shear strain rate for all the studied nanofluids. Relative viscosity ðlnf =lbf Þ of GnS– (EG þdistilled water) nanofluids increases with increasing f-HEG loading at 30 C as is shown in Fig. 12. Viscosity of the nanofluid enhances nearly by 100% that of the base fluid, at a loading of 0.395 vol. % of f-HEG. It may be noted that though the percentage enhancement in viscosity is large, the absolute value of the nanofluid viscosity is very nominal (12.1 cP) and is almost similar to that of ethylene glycol at room temperature. Volume concentration dependence of viscosity of the f-HEG-EG þ distilled water nanofluids at room temperature are estimated on the basis of the several existing and widely used models26–31 and is compared with the measured data (Fig. 12). We confirm that none of these models viz., Einstein,26 Brinkman,27 Krieger and Dougherty,28 Batchelor,29 Neilson,30 and Kitano et al.31 could provide an acceptable estimate of the measured viscosity of the graphene nanofluids. Temperature dependence of viscosity of the nanofluids containing different volume concentrations of f-HEG is shown in Fig. 13. The viscosity decreases rapidly with rise in temperature and displays an asymptotic

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FIG. 13. Temperature dependence of the viscosity of the prepared nanofluids between 10 and 70 C for various f-HEG volume concentrations.

behaviour. The decrease in nanofluid viscosity with increase in temperature is expected due to the weakening of the interparticle and inter-molecular adhesion forces and similar trends have also been observed in almost all other varieties of nanofluids.32–34 Unfortunately, theoretical formulations to predict the temperature dependence of viscosity of nanofluids are practically absent. A few empirical correlations for temperature dependence of nanofluid viscosity were, however, suggested by a few authors32,34–36 mainly to explain their own set of viscosity data. These correlations were tested to fit the present set of data on the temperature dependence viscosity of GnS–(EG þ distilled water) nanofluids. It may be noted that all the four correlations referred above follow equally well the measured temperature dependence of viscosity of the graphene–EG þ distilled water nanofluids with average R2 value between 0.97728 and 0.98763. C. Electrical conductivity

Though important, the electrical conductivity of nanofluids has not yet been widely studied compared to their thermal conductivity. The electrical conductivity of a suspension can either increase or decrease depending on the

l

FIG. 12. Relative viscosity ðlnf Þ of GnS-EG þ distilled water nanofluids at bf 30 C as a function of functionalized GnS concentration. The solid lines are the relative viscosity predicted by various classical models.26–31

FIG. 14. Electrical conductivity as a function of f-HEG concentration at room temperature. Line is a guide to the eye.

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containing 0.395 vol. % f-HEG. Though the percentage enhancement in viscosity is large, its absolute value is, however, very nominal (12.1 cP), nearly same as EG at room temperature. None of the classical models succeed in explaining the observed viscosity enhancement with f-HEG loading at room temperature. Temperature dependence of viscosity of the prepared nanofluids is well explained by some of the existing empirical correlations. Electrical conductivity of the nanofluids increases linearly both with f-HEG concentration and the base fluid temperature. Enhancement of electrical conductivity by 87 times that of the base fluid is obtained for nanofluid with 0.395 vol. % of f-HEG at 30 C. The above results indicate that the graphene–(EG þ water) nanofluids could possibly be a useful candidate for coolant applications. FIG. 15. Electrical conductivity as a function of fluid temperature for base fluid and all the prepared nanofluids. All lines are guide to the eye.

background electrolyte, the particle size,37,38 the particle loading,39 and the charge of the particle.40 An analytical model was proposed38 to explain the particle size and concentration effects on nanofluid electrical conductivity. Electrical conductivity of the present f-HEG–EG þ distilled water nanofluids increases linearly with volume concentration of functionalized HEG as shown in Fig. 14. The electrical conductivity of the base fluid increases from 1 lS/cm to 87.2 lS/cm for a loading of 0.395 vol. % f-HEG at 30 C, which corresponds to an anomalous enhancement of 8620%. Fig. 15 shows the temperature dependence of electrical conductivity of both the base fluid and prepared nanofluids. It is seen that the electrical conductivity increases linearly with temperature for all cases; however, the rate of enhancement is higher for nanofluids with higher loading of functionalized graphene. Baby et al.11 observed an enhancement in electrical conductivity by 1400% for f-HEG loading of 0.03 vol. % in DI water at 25 C. But in case of pure EG as base fluid, the increment was only 220%. In both cases, the electrical conductivity increased with increasing volume concentration as well as increasing fluid temperature. V. CONCLUSIONS

Summarizing, hydrogen exfoliated graphene nano-sheets are prepared and functionalized. Surfactant free graphene– EG þ distilled water nanofluid prepared by ultrasonication are found to be stable for greater than 5 months. Thermal conductivity, viscosity, and electrical conductivity of the nanofluids are measured both as a function of f-HEG concentration and fluid temperature between 10 and 70 C. Thermal conductivity results show a maximum enhancement of 15% for a loading of 0.395 vol. % f-HEG at 30 C, and the thermal conductivity of f-HEG is estimated to be 10.9 6 1.08 W/mK. Thermal conductivity of both the base fluid and the prepared nanofluids increases linearly with temperature. However, the thermal conductivity ratios are nearly temperature independent. Viscosity measurements confirm that both the base fluid and prepared nanofluids are non-Newtonian in nature throughout the measured temperature range. Viscosity enhances by 100% that of base fluid at room temperature for nanofluid

ACKNOWLEDGMENTS

One of the authors (T.K.D.) acknowledges the financial help received from Department of Science & Technology (DST), New Delhi, in the form of a research Grant. Award of a CSIR Senior Research Fellowship is also gratefully acknowledged by Miss Madhusree Kole. 1

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