ARTICLE pubs.acs.org/JPCC
Investigation of Structural Stability, Dispersion, Viscosity, and Conductive Heat Transfer Properties of Functionalized Carbon Nanotube Based Nanofluids S S Jyothirmayee Aravind,† Prathab Baskar,‡ Tessy Theres Baby,† R Krishna Sabareesh,‡ Sumitesh Das,‡ and S Ramaprabhu†,* †
Alternative Energy and Nanotechnology Laboratory (AENL), Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India ‡ Materials Modeling and Product Design, Research Development & Technology, Tata Steel Limited, Jamshedpur 831 001, India ABSTRACT: The present work investigates the structural stability, dispersion, viscosity, and convective heat transfer properties of nanofluids based on multiwalled carbon nanotubes (MWCNT). Oxidative acid refluxation on MWCNT was examined for different time periods in order to achieve good dispersibility and thermal enhancement. Structural stability and efficient chemical functionalization was investigated by thermogravimetric analysis. Fourier transform infrared spectroscopy was conducted to ascertain the formation of chemical functional groups on covalently modified MWCNT. Raman band study on oxidized MWCNT was analyzed by Raman scattering measurements. Dispersion of functionalized MWCNT was studied by ultravioletvisible spectroscopy. Field emission scanning electron microscopy and transmission electron microscopy have been performed to ascertain the structural durableness of nanomaterials in fluids. Viscosity and thermal conductivity behavior of DI water and ethylene glycol based MWCNT nanofluids were studied with varied volume fractions and temperature (3070 °C). Further, steady-state forced convective heat transfer experiments have been conducted to estimate the cooling capabilities of above-mentioned nanofluids.
1. INTRODUCTION Adding metallic particles into heating/cooling fluids has long been regarded as a promising approach to enhance heat transfer. However, the applications of the particlefluid mixtures are held back due to the poor stability of those suspensions containing microsized particles. In the past decade, a novel kind of particle suspension, nanofluids,1 has been developed and has attracted many researchers due to their intriguing properties such as high thermal conductivity, stability, and prevention of clogging in microchannels. A nanofluid is a kind of new engineering material consisting of nanometer-sized particles dispersed in base fluid. Various nanoparticles, such as carbon nanotubes (CNT), fullerene, copper oxide (CuO), aluminum oxide (Al2O3), and silicon dioxide (SiO2) have been used to produce nanofluids for enhancing thermal conductivity and lubricity. Theoretical and experimental studies have demonstrated that CNT possess very high thermal conductivity24 which advocate CNT as an excellent candidate for preparing thermally conductive nanofluids. Moreover, the heat transfer capabilities of CNT based nanofluids are much enhanced as compared to base fluids. This makes them suitable for use in cooling of electronic equipments, lasers, fuel cells, car radiators, etc. Even though carbon nanotubes have promising applications, fundamental problems still exist: (1) how to remove impurities, such as amorphous carbons and metallic catalysts, and (2) how to obtain uniform dispersions of the carbon nanotubes in dispersing media. r 2011 American Chemical Society
CNT are prone to entangle and aggregate in fluids due to their nonreactive surfaces and very large specific area and aspect ratio.5 Dispersion states of carbon nanotubes involve complicated phenomena, as CNT are produced in bundles or bundle aggregations. The states are affected by at least two competitive interactions: (1) the interactions of van der Waals forces among carbon nanotube threads and (2) the interactions between carbon nanotube threads and dispersion medium. Surfactant addition (noncovalent modification) is an effective way to enhance the dispersibility of CNT.6,7 However, in nanofluid applications, surfactants might cause several problems. First, the addition of surfactants may contaminate the heat transfer media. Second, surfactant addition has a deteriorating effect in heat exchanger systems, which involve continuous heating and cooling cycles, as surfactants tend to produce foam during heating. Furthermore, surfactant molecules attaching on the surfaces of CNT may enlarge the thermal resistance between the CNT and the base fluid, which limits the enhancement of the effective thermal conductivity. To fully utilize the superior thermal performance of CNT and to extend the application fields of CNT containing nanofluids, it is imperative to use a judicious method to prepare nanofluids without adding surfactants. Received: February 20, 2011 Revised: June 7, 2011 Published: July 21, 2011 16737
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Figure 1. Schematic of the convective heat transfer loop.
Covalent modification of CNT by using oxidizing acid mixtures has been of interest for dispersion enhancement.8,9 Shortening of carbon nanotubes by ultrasonication with oxidizing acid mixtures is frequently used technique to functionalize CNT.10 Carboxylic acid groups are introduced at the tube ends and side walls that can connect nanotubes with organic materials, forming covalent bondings.1113 By creating nanofluid in this way, stable nanofluids resulted without using any property-changing dispersants. This method has much advantage for obtaining nanotube dispersions because the van der Waals forces among nanotubes are eliminated. With the evidence of such functionality, we can employ CNT in various applications and also use consequent chemistry to obtain new functional groups. For chemical reactivity and many industrial applications, it is crucial to understand functionalized CNT more qualitatively and quantitatively.14 Realizing these aspects and part of our continuing research efforts,15 we probed multiwalled carbon nanotubes (MWCNT) with different acid mixtures and functionalization time periods (14 h). Chemical characterization techniques like thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR), Raman, and ultravioletvisible (UVvis) spectroscopy have been utilized to unravel the physics of surface modification and ascertain the optimal reflux period of functionalized MWCNT. Dispersion phenomena of functionalized MWCNT was investigated with DI water and ethylene glycol (EG) for two different volume fractions (0.005 and 0.03 vol %) of the nanomaterial. These nanofluids show great potential in increasing the efficiency of thermal transport. However, the rheology factor hinders the nanofluid stability and flow behavior. This factor could cause numerous side effects in heat transfer systems, especially in cases where nanofluids are used as heat exchange fluids.16 In this sense, experimental analyses were carried out on viscosity and thermal conductivity to elucidate the physical property of MWCNT in DI water and ethylene glycol. Most of the industrial applications involve huge volume flow rates of fluids, during the heat removal process. As a result, it is the convective heat transfer coefficient that is the most sought after parameter, in all thermal engineering applications. From heat transfer theory, for a constant Nusselt number, the convective heat transfer coefficient is directly proportional to the thermal conductivity.17 Also, researchers have given much more attention to the thermal conductivity rather than the heat transfer
characteristics of nanofluids. With this observation, the cooling potentials of nanofluids have been compared and verified by steady-state forced convective heat transfer experiments. An experimental heat transfer setup has been built for this purpose. Physical insight has also been provided to explain the enhanced heat transfer coefficient of functionalized MWCNT-based nanofluids.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemical Functionalization. MWCNT were grown by catalytic decomposition of acetylene over MmNi3 alloy hydride catalysts, using a single-stage furnace thermal chemical vapor deposition technique.18,19 As-grown MWCNT were purified by air oxidation followed by refluxing with concentrated nitric acid for 24 h. The sample was washed with deionized water several times, filtered, and dried at 80 °C for 2 h. In order to oxidize the stable aromatic rings on the surface and attach hydrophilic oxygen containing functional groups (COOH, CdO, and OH) to the surface of MWCNT, the purified MWCNT were refluxed under constant agitation in concentrated acids, viz., H2SO4, HNO3, and H2SO4:HNO3 (3:1), at 60 °C. The sample was then washed several times with copious amount of water until pH 7 followed by filtration through a cellulose membrane of pore size 0.2 μm. Among the three main acid treatments that were tested, dispersion and thermal characterization technique showed that the 3:1 solution of sulfuric and nitric acid was the most effective treatment, in order to modify the MWCNT surfaces and induce the formation of functional groups (COOH, CdO, and OH). The oxidation treatment promoted by H2SO4:HNO3 (3:1) invariably depends on acid exposure time; hence, the reflux period of MWCNT was varied from 1 to 4 h to examine the optimum functionalization time. 2.2. Characterization of MWCNT. TGA was used as a tool to judge efficient MWCNT oxidation and thermal stability of the samples. Thermogravimetric measurements were performed using a Perkin-Elmer TGA 6 analyzer. Samples of approximately 20 mg were heated in air atmosphere from 30 to 900 °C, at a rate of 10 °C/min, and the weight of the sample was recorded as a function of temperature. Identification and characterization of functional groups were carried out using a Perkin-Elmer FT-IR spectrometer in the range 5004000 cm1. Raman scattering 16738
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Figure 2. TGA curves of (a) as-grown MWCNT and (b) 1 h and (c) 4 h f-MWCNT.
spectra of the pristine MWCNT and functionalized MWCNT were collected by using a WITec Raman spectrometer, with a Nd:YAG laser having an excitation wavelength of 532 nm. UVvis characterization was carried out using a Leica DMIL microscope. The morphology of as-prepared and functionalized MWCNT samples was analyzed using FEI quanta FEG200 field emission scanning electron microcopy (FESEM) operating at 30 kV. Transmission electron microscopy (TEM) images were taken with a JEOL JEM-2010F operated at 200 keV. 2.3. Preparation and Thermophysical Property Measurement of Nanofluids. Nanofluids were synthesized by taking a calculated amount of functionalized MWCNT in a known amount of DI water and EG base fluids and sonicated for nearly 40 min using a 100 W, 40 kHz ultrasonicator. The ultrasound produces microscopic bubbles in the liquid, which result in shock waves when they collapse, increasing the wetting and ultimately leading to a homogeneous dispersion of MWCNT in base fluid. Viscosity is measured by a Cannon Fenike routine viscometer (CT-500 Series-II). In this study, thermal conductivity of nanofluid with oxidized MWCNT was measured using a Lambda (flucon, Clausthal-Zellerfeld, Germany) instrument based on the transient hot-wire method. 2.4. Estimation of Heat Transfer Coefficient. A convective heat transfer setup, which mimics the heat exchangers in industrial applications, has been built in-house, and a schematic of the setup is shown in Figure 1. It consists of a flow loop, test section, cooling unit, and a flow-measuring unit. The flow loop consists of a reservoir for storing the nanofluid, a pump, and different flow control valves. The test section made of stainless steel tube is wound uniformly by an electrically insulated nichrome wire to provide heat input. Thermocouples are arranged to measure the surface temperature as well as the fluid temperatures at the inlet and outlet of the test section. The shell and tube heat exchanger brings the heated nanofluid back to ambient condition.
3. RESULTS AND DISCUSSION 3.1. Purification/Oxidation. MWCNT was purified using a three-step purification process involving (a) thermal oxidation, (b) acid refluxation, and (c) thermal annealing. The results of the thermogravimetric analysis of the as-grown and functionalized MWCNT (f-MWCNT) are plotted in Figure 2. Minimum and maximum chemical treatment/functionalization curves are illustrated to pronounce the purification process. Figure 2 exhibits the TGA curve of (a) as-grown MWCNT and (b) 1 h and (c) 4 h f-MWCNT. The weight loss occurring above 500 °C in the asgrown material can be attributed to degradation of disordered or amorphous carbon and other metal impurities. The onset of
Figure 3. FTIR spectra of (a) as-grown MWCNT and 1 and 1.5 h f-MWCNT; (b) 2, 2.5, and 3 h f-MWCNT; and (c) 3.5 and 4 h f-MWCNT.
significant (>2%) weight loss is shifted toward temperatures higher than 600 °C for f-MWCNT and corresponds to thermal degradation of the remaining disordered carbon. The purity of the functionalized MWCNT is ∼97%, whereas the TGA curve for the as-grown MWCNT indicates a purity of only 55%. In the case of acid-treated MWCNT, the weight loss below 200 °C can be due to the decomposition of functional groups. 3.2. Infrared Spectroscopy. FT-IR was conducted on nontreated (as-grown MWCNT) and f-MWCNT, and their corresponding spectra are shown in Figure 3. As shown in Figure 3a, for nontreated MWCNT, the IR spectrum shows important absorption bands at 3440 cm1 (attributed to OH stretching), 2926 and 2851 cm1 (asymmetric and symmetric CH2 stretching), and 1627 cm1 (conjugated CdC stretching). Functionalized MWCNT (1, 1.5, 2, 2.5, 3, 3.5, and 4 h) exhibited the same bands with the addition of a band at 1053 cm1 (corresponding to CO stretch in alcohols) and a small intense band at 1714 1726 cm1, which can be assigned to stretching vibrations of carbonyl groups (CdO) present in carboxylic acids (RCOOH). The intensity of the CdO band increased vividly and modestly for 2.5 h and 3 h respectively, with successive oxidative treatment. Subsequent analysis of the 3.5 and 4 h samples shows a drop off of intensity of the CdO and OH groups. The above observations suggest that the oxidation was promoted in all treatments and that the most effective treatments for MWCNT chemical functionalization are those labeled as 2.5 and 3 h. 16739
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Figure 5. UVvis absorption spectra of f-MWCNT solutions.
Figure 4. Raman spectra of (a) as-grown MWCNT and 1 and 1.5 h f-MWCNT; (b) 2 h, 2.5, and 3 h f-MWCNT; and (c) 3.5 and 4 h f-MWCNT.
3.3. Raman Spectroscopy. Carbon nanomaterials display two important Raman absorption peaks. The disorder induced mode, the D band (observed at 1340 cm1), derived from the disordered carbon and defects of MWCNT, is a feature common to all sp2 hybridized disordered carbon materials. The peak at 1589 cm1 is due to the Raman-active E2g mode analogous to that of graphite, the G band. Figure 4 represents Raman spectra of as-grown MWCNT and f-MWCNT. The samples exposed to acid treatment for 14 h result in the upshift of D and G band regions. The band shift of 2.5 and 3 h exhibit higher upshifts, in particular, the D band (36 cm1) and G band (27 cm1) shifts of 2.5 h are enormous. The gradual upshift of the peaks’ positions is explained by the intercalation of acid molecules in the interstitial channels of the MWCNT.20 Another aspect that should be considered is the oxidative modification of f-MWCNT upon the acid treatment. The ratio ID/IG represents the relative degree of defect formation present in the material. Upon acid treatment, ID/IG ratio increases with increase of D (defect) band intensity, which indirectly expounds the covalent modification/ purity of f-MWCNT samples. 3.4. UVVis Spectra and Stability Characterization of MWCNT Solution. Individual CNT are active in the UVvis region and exhibit characteristic bands corresponding to additional absorption due to 1D van Hove singularities. 21 Further, it is possible to establish a relationship amid the amounts of MWCNT individually dispersed in solution and the intensity of the corresponding absorption spectrum.22 Moreover, UVvis spectroscopy can be used to monitor the dynamics of the
dispersion process of CNT. After refluxation, the absorbance of MWCNT solutions show a maximum between 200 and 300 nm and gradually decreases from UV to near-IR, which is partly due to scattering, especially in the lower wavelength range. Initially, MWCNT exist as big aggregates and bundles in solution that are strongly entangled, and no absorption is evident in the UVvis spectrum (Figure 5). During oxidative treatment, the provided mechanical energy can indeed overcome the van der Waals interactions in the MWCNT bundles and lead to their disentanglement and dispersion. The increasing amount of dispersed f-MWCNT results in an increasing area below the spectrum lines representing the absorbance (Figure 5, 14 h). Also, the maximum absorbance of 2.5 h (Figure 5) replicates the effective oxidative modification of 2.5 h f-MWCNT nanofluid. On the basis of FT-IR, Raman, and UVvis spectral inference, it has been found that maximum oxidative/acid exposure occurs at 2.5 h functionalization, and hence, stability, thermo-physical, and heat transfer measurements have been evaluated for 2.5 h f-MWCNT nanofluids. 3.5. Electron Microscopy. Field emission scanning electron microscopy (FESEM) was used to investigate possible MWCNT fragmentation that occurred during oxidative treatment. Figure 6 shows typical FESEM images of (a) as-grown and collected samples of 2.5 h f-MWCNT in (b) DI-water- and (c) EG-based nanofluids. Images of other acid treatments showed no distinctive features and thus they are not shown. In spite of our numerous efforts (by FESEM and AFM), except for a few nanotubes, the edges of a single MWCNT were difficult to determine precisely, so a reliable statistical length distribution was not possible to obtain. In spite of this limitation, a general but conspicuous assessment can be made on the basis of image analysis. Analysis of FESEM images with permitted reliable length measurements revealed that the length of functionalized MWCNT was between 1 and 3 μm, similar to the nontreated material. SEM evidence of MWCNT damage due to harsh acid treatment has been reported by the reduction of the pristine micrometer-length nanotubes into small fragments.23 In our case, no drastic fragmentation is observed after the chemical treatments and nanofluid dispersed sample (see Figure 6b,c), although a slight statistical reduction in the MWCNT lengths cannot be totally discarded. A similar observation is drawn from TEM analysis of oxidative and nanofluid dispersed samples of 2.5 h f-MWCNT (see Figure 7a,b). 3.6. Thermophysical Property Estimation. 3.6.1. Viscosity. The viscosities of DI-water- and EG-based f-MWCNT nanofluids were measured at temperatures ranging from 30 to 70 °C. The rheological behavior of any nanofluid follows two important aspects: (a) an increase in viscosity with nanomaterial concentration (as particle concentration increases, the internal viscous 16740
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Figure 7. TEM images of (a) DI-water- and (b) EG-based 2.5 f-MWCNT nanofluids.
Figure 6. FESEM images of (a) as-grown MWCNT and (b) DI-waterand (c) EG-based 2.5 f-MWCNT nanofluids.
shear stress increases, which results in higher fluid viscosity) and (b) a decrease in viscosity with increasing temperature (increasing the temperature weakens the intermolecular forces of the particles and fluid itself, hence decreasing viscosity). In this work, increments in viscosity are nearly negligible with nanomaterial concentration, which augments the efficacy of oxidative/chemical treatment. Conversely, viscosity decreases with increase in temperature, which complies with Ko et al.24 In order to elucidate the accuracy of viscosity measurement, we have plotted the viscosity of pure DI water and EG along with f-MWCNT added base fluids (see Figure 8a,b). Figure 8 approves the close coordination between pure and nanodoped samples, which implies the fluidity and dispersivity of chemically treated f-MWCNT nanofluids. 3.6.2. Thermal Conductivity of Nanofluids. The thermal conductivity enhancements of DI-water- and EG-based nanofluids containing f-MWCNT showed augmentation with respect to temperature. Also, fluid temperature may play an important role in enhancing the thermal conductivity of nanofluids. Therefore, we investigated the temperature effect on the thermal conductivity of nanofluids. Figure 9a,b shows the thermal conductivity plot of f-MWCNT-based nanofluids as a function of temperature. Two different volume fractions (0.005% and 0.03%) were examined, and thermal conductivity variations with temperature have been explored. Our samples show larger thermal conductivity enhancement at much lower volume fraction of f-MWCNT due to the homogeneous dispersion in base fluids and formation of more hydrophilic phase by attachment of oxygen-containing functional 16741
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Figure 10. Heat transfer coefficients of DI water at different flow rates.
(i.e., nanolayer), thereby enhancing the local ordering of the liquid layer at the interface region. The liquid layer at the interface would reasonably have a higher thermal conductivity than the bulk liquid. Thus, the nanolayer is considered as an important factor enhancing the thermal conductivity of f-MWCNT-based nanofluids.25 3.7. Heat Transfer Coefficient of Nanofluids. 3.7.1. Calculation of Convective Heat Transfer Coefficient. The local convective heat transfer coefficient, h(x) is calculated as follows hðxÞ ¼ q00 =ðTw ðxÞ Tf ðxÞÞ
Figure 8. Viscosity profiles of 2.5 h f-MWNT nanofluids in (a) DI water and (b) EG.
ð1Þ
where q00 represents the heat flux and x represents the axial distance from the entrance of the test section. Tw and Tf denotes the wall and fluid temperature. The fluid temperature is determined using the following energy balance equation. Tf ðxÞ ¼ Tin þ ðq00 πDxÞ=mCp
Figure 9. Thermal conductivity plots of 2.5 h f-MWNT nanofluids in (a) DI water and (b) EG.
groups. The thermal conductivity vs temperature enhancement profile of f-MWCNT-doped DI water and EG up to 70 °C corroborates the stability of f-MWCNT even at higher temperatures. The intrinsic heat transfer capacity of the f-MWCNT should be the main factor for thermal enhancement. For nanofluid containing MWCNT with chemical functionalization, in addition to the Brownian motion of MWCNT, the chemical surface effects of MWCNT dominate the extent of energy transfer in nanofluids. Liquid molecules form layers around the f-MWCNT
ð2Þ
where D is the diameter of the pipe, m represents the mass flow rate, and Cp represents the specific heat capacity of the nanofluid. The accuracy of the experimental setup was elucidated by carrying out experiments using DI water and by comparing the convective heat transfer coefficient measured by the instrument with the standard heat transfer correlations. The following sections demonstrate the effect of flow rate and nanoparticle concentration on the heat transfer coefficient. 3.7.2. Effect of Flow Rate. It is a known fact that the convective heat transfer coefficient varies along the axis of the pipe, with the maximum value close to the entrance of the pipe. When the flow rate increases, it is expected that the heat transfer coefficient also increases. Figure 10 shows the effect of flow rate on the convective heat transfer coefficient of DI water. From the result, we can conclude that, for effective heat removal, it is always imperative to keep the most critical heat dissipating elements closer to the entrance of the pipe. 3.7.3. Heat Transfer Coefficients of f-MWCNT-Based Nanofluids. An increase in the heat transfer coefficient is anticipated with an increase in the volume fraction of nanotubes. The behavior is expected to be different for different base fluids. Heat transfer studies have been experimented with 2.5 h f-MWCNT dispersed base fluids in DI water and EG. Figure 11a,b shows the comparison of convective heat transfer coefficients of f-MWCNT added to DI water for two different volume fractions (0.005% and 0.03%) and three different volume flow rates. The heat transfer coefficient profiles of f-MWCNT dispersed in EG showed almost the same trend as that of MWCNT dispersed in DI water. For the sake of brevity, the heat transfer coefficient profile at a flow rate of 56 mL/s for two different volume fractions alone is shown in Figure 12. An observation noteworthy in Figures 11 and 12 is that the enhancements in heat transfer 16742
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of MWCNT-based nanofluids, investigated by TGA, FTIR, Raman, UVvis, FESEM, and thermal measurements, indicate that the stability of MWCNT increases significantly with chemical treatment. We presented direct observation of homogeneous coverage of hydrophilic oxygen containing functionalities on the outer walls of MWCNT. The viscosity study affirms the superior dispersion phenomenon of acid-treated f-MWCNT samples, which is instrumental for thermal enhancement and their good stability in DI water and EG. Further, the thermal conductivity and heat transfer enhancements at lower volume fraction of f-MWCNT-based nanofluids have been confirmed, which are attributed to the thinning of the thermal boundary layer by MWCNT, which lowers the resistance to heat removal. The promising features of no surfactant, good fluidity, long-term stability, and high thermal conductivity (or heat transfer) would enable the f-MWCNT nanofluids to be used as advanced coolants in thermal energy/heat exchange engineering.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (O) + 91 44 22574862. Fax: (O) + 91 44 22570509. E-mail:
[email protected]. Figure 11. Heat transfer coefficients of f-MWCNT-based nanofluids of volume fractions (a) 0.005% and (b) 0.03%.
’ ACKNOWLEDGMENT The authors are thankful to Indian Institute of Technology Madras and Research Development & Technology, Tata Steel Ltd. for the support of this work. ’ REFERENCES
Figure 12. Heat transfer coefficient of f-MWCNT dispersed in ethylene glycol.
coefficient of nanofluids greatly exceed the thermal conductivity enhancements for both the base fluids. The maximum heat transfer coefficient enhancements for MWCNT added to DI water and EG are respectively 65% and 180% for a volume fraction of 0.03% at 56 mL/s, whereas the maximum enhancements in thermal conductivity are respectively 33% and 40%. This behavior can be qualitatively explained by a simple scale analysis. Heat transfer coefficient can be approximately modeled as k/δt, where δt represents the thermal boundary layer thickness. So for increasing the heat transfer coefficient, either k should be increased or δt should be decreased or both. Our studies suggest that the carbon nanomaterials tend to play a role in decreasing the thermal boundary layer thickness.26 More studies, typically flow visualization studies, should be conducted to get a clearer picture of the heat transfer enhancement mechanism.
4. CONCLUSION The structural aspects, dispersion phenomena, thermal conductivity enhancement, and convective heat transfer coefficients
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’ NOTE ADDED AFTER ASAP PUBLICATION This manuscript was originally published on the Web on August 8, 2011, with and error to Figure 3. The corrected version was reposted with the Issue on August 25, 2011.
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