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Effects of Surface Modificationon the Suspension Stability and Thermal Conductivity of Carbon Nanotubes Nanofluids
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 69 (2015) 699 – 705
International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014
Effects of surface modification on the suspension stability and thermal conductivity of carbon nanotubes nanofluids P. Zhanga, W. Honga, J. F. Wua, G. Z. Liua, J. Xiaoa, Z. B. Chena and H. B. Chenga* a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070, Wuhan, China
Abstract The composite carbon nanotubes (CCNTs) were synthesized by surface modification of the as-synthesized carboxylfunctionalized carbon nanotubes (f-CNTs) with the polyethylene glycol, and the composite carbon nanotubes-based nanofluids (CCNTs-based nanofluids) were prepared with the as-synthesized CCNTs and ethylene glycol. The surface molecular structure, surface morphology of the CCNTs were characterized by Fourier Transform Infrared Spectroscopy, Field Emission Scanning Electron Microscope and Transmission Electron Microscopy. The suspension stability and thermal conductivity enhancement ratio of the CCNTs-based nanofluids were characterized via the thermal conductivity variation method. The results show that suspension stability of the CCNTs-based nanofluids was greatly improved by surface modification of the carbon nanotubes; the thermal conductivity of the CCNTs-based nanofluids was further enhanced and became more stable by surface modification. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG
Keywords: carbon nanotubes nanofluids; surface modification; suspension stability; thermal conductivity.
1. Introduction Carbon nanotubes-based nanofluids (CNTs-based nanofluids) are very promising composite fluids applied in solar thermal power generation to enhance heat transfer capabilities [1], because of their enhanced thermal performance [2], stable chemical property and high temperature stability [3]. However, pristine carbon nanotubes have strong tendency to rapidly aggregate and precipitate to the bottom of the container, which can clog any flow
* Corresponding author. Tel.: +86-13349878089; fax: +86-27-87879468. E-mail address: [email protected]
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.03.080
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channels, seriously decay heat transfer effectiveness, and render practical engineering applications infeasible [4,5]. Therefore, improving suspension and heat transfer enhancement stability of CNTs-based nanofluids is the key challenge to enable successful engineering applications of these CNTs-based nanofluids [6]. To efficiently mitigate these disadvantages, a novel method has been developed to controllably modify the surface characteristics of CNTs through in-situ grafting organic molecular chains on the surface of CNTs, so that composite CNTs (CCNTs) and CCNTs-based nanofluids have been prepared using ethylene glycol carrier fluid. The effects of surface modification on the suspension stability and thermal conductivity enhancement ratio of CCNTs-based nanofluids were investigated. Moreover, the mechanisms that increase the suspension stability, or reduce sedimentation rate, were studied. Nomenclature f nf k nf kf Ƹk=k nf -k f Ƹk/k f Ø T
base fluid nanofluid thermal conductivity of nanofluids thermal conductivity of the base fluid ethylene glycol thermal conductivity enhancement of nanofluids thermal conductivity enhancement ratio volume fraction of CCNTs temperature
2. Experimental procedure 2.1. Preparation of the CCNTs-based nanofluids Firstly, carboxyl-functionalized carbon nanotubes (f-CNTs) were synthesized with carbon nanotubes and a mixed acid composed of nitric and sulfuric acid for 5 hours [7,8]; then the composite carbon nanotubes (CCNTs) were synthesized via esterification reaction between the as-synthesized f-CNTs and polyethylene glycol. Finally, CCNTsbased nanofluids were prepared by suspending CCNTs in ethylene glycol for five different volume fractions (0.1, 0.3, 0.5, 0.7 and 0.9 vol%) by means of mechanical agitation and ultrasonic mixing. 2.2. Characterization of CCNTs-based nanofluids The structure and properties of both CNTs and CCNTs were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscope (FE-SEM), and Transmission Electron Microscopy (TEM). The suspension stability of the CCNTs-based nanofluids was evaluated via the thermal conductivity variation method [9]. Thermal conductivity of nanofluids were measured using a thermal properties analyzer, which complies with ASTM and IEEE Standards [10]. 3. Results and discussion 3.1. The surface molecular structure of CCNTs To judge whether the surface modifier polyethylene glycol was successfully grafted onto the surface of the assynthesized CCNTs, the surface molecular structure of the as-synthesized CCNTs and pristine CNTs were analyzed by FT-IR spectra, because the FT-IR spectra can reflect the changes of the surface molecular structure of the CCNTs as the organic surface modifier polyethylene glycol (PEG) 200 was grafted on the surface [11]. So the FT-IR spectra of as-synthesized CCNTs and pristine CNTs (for comparison) were collected, as shown in Fig. 1. The infrared characteristic absorbing peaks of the CCNTs can be assigned to -COO-stretch (1642.48 cm-1), -OH stretch (1383.49 cm-1), and CH 2 stretch(702.72 cm-1). In contrast, FT-IR spectrum of the pristine CNTs does not exhibit these
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absorption bands. These results show that the surface molecular chain has the characteristic groups of polyethylene glycol: -COO-, -OH, and CH 2 ; that is, the surface modifier polyethylene glycol was successfully grafted onto the surface of composite carbon nanotubes. Thereby, the surface properties of the CCNTs would be modified by the grafted organic molecular chains [12].
Fig. 1. FT-IR images of CCNTs and pristine CNTs.
3.2. Surface morphology of CCNTs The surface morphology of the CCNTs was characterized by SEM (as shown in Fig. 2 A), and the surface morphology of the CNTs was characterized by TEM for comparison (as shown in Fig. 2 B). The radial dimension of CCNTs ranged from 25 to 35nm, and the radial dimension of CNTs ranged from 12 to 18 nm. The increment (1317nm) of the radial dimension of CCNTs may be contributed to the grafted organic molecular chains polyethylene glycol. In addition, the intertwining of CCNTs can be clearly observed, which implies that homogeneously dispersing carbon nanotubes in carrier fluid is still a key technique for to prepare stable CCNTs-based nanofluids [13].
Fig. 2. (A) SEM image of CCNTs; (B) TEM image of pristine CNTs.
3.3. The effect of surface modification on the stability of CCNTs-based nanofluids To investigate how surface modification impacts the suspension stability of CCNTs-based nanofluid, 0.3 vol% CCNTs-based nanofluid and 0.3 vol% CNTs-based nanofluid were respectively prepared; and their suspension
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stability was assessed by the thermal conductivity variation method [9]. Thermal conductivities of the base fluids ethylene glycol (k f ) and the CCNTs-based nanofluids (k nf ) were measured by a transient short hot wire method [14]. In order to reduce the error due to the limit of measurement precision, and increase the sensitivity of surface modification on the thermal conductivity of CCNTs-nanofluids, normalization processing was adopted by taking Ƹk/k f (=(k nf -k f )/k f ) describing the thermal conductivity enhancement ratio of CCNTs-based nanofluids. The data are shown in Fig. 3 A and B. The thermal conductivity enhancement ratio of 0.3 vol% CCNTs-based nanofluid (A) was about 12.8 %, and exhibited no measurable decline over a 1000 hours period. In contrast, the thermal conductivity enhancement ratio of 0.3 vol% CNTs-based nanofluid (B) was about 7.9 %, smaller than that of 0.3 vol% CCNTs-based nanofluid by 38.3 %, which shows that surface modification further improved the thermal conductivity enhancement effect of nanotubes. In addition, the thermal conductivity enhancement ratio of the 0.3 vol% CNTs-based nanofluid degraded sharply from 7.9 % to about 0 % after 26 hours. These results clearly demonstrated that the surface modification of CNTs not only increased the thermal conductivity enhancement, but also markedly improved the stability of thermal conductivity enhancement effect [15]. Fig. 3 A inset and Fig. 3B inset are the photo of the 0.3 vol% CCNTs-based nanofluid and the 0.3 vol% CNTsbased nanofluid after standing for 1000 hours and 26 hours, respectively. As shown in Fig. 3 A inset, there was no observable sedimentation in the CCNTs-based nanofluid even after standing for 1000 hours (a). However, rapid sedimentation occurred in the CNTs-based nanofluid after standing for 26 hours (As shown in Fig. 3B inset b), the formation of the mud line separating clarified fluid above the mud line, and fluid with high solids loading fluid below the mud line can be clearly observed. Obviously, the suspension stability of the 0.3 vol% CCNTs-based nanofluid is much higher than that of the 0.3 vol% CNTs-based nanofluid, that is, the stability of CNTs-based nanofluid can be markedly improved by surface modification of CNTs, which in this case was achieved by grafting organic molecular chains polyethylene glycol on the surface of carbon nanotubes. Because grafted organic molecular chains polyethylene glycol has similar molecular structure, based on the principle of the dissolution in the similar material structure, the grafted organic molecular chains polyethylene glycol could stretch into the carrier fluid molecule ethylene glycol to increase the buoyant force of the nanotubes, and surface modification could endow carbon nanotubes with compatibility with the carrier fluid, so that suspension stability has been markedly improved [16]. Thereby, we believe that surface modification is an effective way to improve the stability of nanofluids. The mechanism of surface modification on the thermal conductivity of nanofluids is very complicated, needs to be deeply investigated; according to this experimental results, keeping the CNTs nanoparticles uniformly dispersed in carrier fluid and no sedimentation is of greatest importance.
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Fig. 3. (A) Thermal conductivity enhancement ratio of 0.3 vol% CCNTs-based nanofluid; (B) 0.3 vol% CNTs-based nanofluid as a function of time. Insets are photos of sedimentation of CCNTs-based nanofluid (a) after standing for 1000 hours, and CNTs-based nanofluid (b) after standing for 26 hours.
3.4. The effect of volume fraction and temperature on the thermal conductivity enhancement ratio of CCNTs-based nanofluids In order to further seek for more suitable CCNTs-based nanofluids with high stability and high thermal conductivity enhancement effect for applied in heat transfer engineering, the effect of volume fraction on the thermal conductivity enhancement ratio was investigated by preparing five CCNTs-based nanofluids with different volume fractions (0.1, 0.3, 0.5, 0.7 and 0.9 vol%), and measuring thermal conductivity enhancement ratio. Thermal conductivities of the base fluids ethylene glycol (k f ) and the CCNTs-based nanofluids (k n f ) were measured by a transient short hot wire method. Ƹk/k f and Ø refer to the thermal conductivity enhancement ratio of CCNTs-based nanofluids ( Ƹ k=k nf -k f ) and the volume fraction of CCNTs, respectively. As shown in Fig. 4, the thermal conductivity enhancement ratio increased with the volume fraction of CCNTs, from 5.9 % up to 21.03 % as the volume faction increased from 0.1 to 0.9 vol%. The results show the effect of the volume fraction on the thermal conductivity enhancement ratio of CCNTs-based nanofluids is in accordance with that reported in the literatures [17,18], so the thermal conductivity of the CCNTs-based nanofluids can be further enhanced by increasing the volume fraction of CCNTs.
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Fig. 4. Thermal conductivity enhancement ratio of CCNTs-based nanofluids as a function of CCNTs volume fraction.
3.5. The effect of temperature on the thermal conductivity enhancement ratio of CCNTs-based nanofluids The nanofluids are different from traditional fluids that the thermal conductivity of nanofluid increases with the temperature, while the thermal conductivity of traditional fluids decreases with the temperature. To expand the applications of the nanofluids at high temperatures, the thermal conductivity enhancement ratio of 0.3% CCNTsbased nanofluid was investigated as a function of temperature. As shown in Fig. 5, within the scope of the test, a linear relationship was obtained, Ƹk/k f =1.065T-13.825, which showed that the thermal conductivity enhancement ratio increased with the temperature, from 12.8 % up to 55.4 % as the temperature increased from 25 to 65 ć. The results show that compared with the thermal conductivity of CuO-based nanofluids at different temperature, reported in the literature [19], the CCNTs-based nanofluids have an advantage on the heat transfer applications under high temperature. So the CCNTs-based nanofluids can be as more promising nanofluids used in heat transfer applications by increasing the temperature of CCNTs to enhance the thermal conductivity.
Fig. 5. Thermal conductivity enhancement ratio of CCNTs-based nanofluid as a function of temperature.
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4. Couclusion In this study, CCNTs were synthesized via in-situ grafting polyethylene glycol chains onto the CNTs, and the CNTs-based nanofluids were prepared by suspending CCNTs in ethylene glycol. The stability of suspension and thermal conductivity enhancement ratio of CCNTs-based nanofluids had been investigated. The results show that the stability of thermal conductivity enhancement effect of the CCNTs-based nanofluids was also much better than that of the CNTs-based nanofluids of comparable volume fraction; the thermal conductivity enhancement ratio of the CCNTs-based nanofluids increased with the volume fraction of CCNTs and the temperature, which indicated that the thermal conductivity could be tuned by adjusting the volume fraction and the temperature. Thereby, it can be concluded that surface modification can significantly improve the suspension stability and stabilize thermal conductivity enhancement effect to meet the needs of heat transfer applications in solar thermal power generation. Acknowledgements This work is financial supported by state key development program of basic research of china (2010CB227105) and Natural Science Foundation of China (51372189). References [1] Duong HM, Papavassiliou DV, Mullen KJ, Wardle BL, Maruyama S. A numerical study on the effective thermal conductivity of biological fluids containing single-walled carbon nanotubes, Int. J. Heat Mass Transfer; 2009. 52: 5591-5597. [2] Meyyappan M. Carbon Nanotubes Science and Applications. CRC Press, New York; 2005. [3] Han ZD, Fina A. Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review. Prog. Polym. Sci.; 2011. 36: 914944. [4] Byrne MT, Gun’ko YK. Recent advances in research on carbon nanotube–polymer composites. Adv. Mater.; 2010. 22: 1672-1688. [5] Choi SUS, Zhang ZG, Yu W. Anomalous thermal conductivity enhancement in nanotube suspensions. Appl. Phys. Lett.; 2001. 79: 22522254. [6] Akoh H, Tsukasaki Y, Yatsuya S, Tasaki A. Magnetic properties of ferromagnetic ultrafine particles prepared by vacuum evaporation on running oil substrate. J. Cryst. Growth; 1978. 45: 495–500. [7] Talaei Z, Mahjoub AR, Rashidi AM, Amrollahi A, Meibodi ME. The effect of functionalized group concentration on the stability and thermal conductivity of carbon nanotubefluid as heat transfer media. Int. Commun. Heat Mass; 2011. 38: 513-517. [8] Peng XH, Wong SS. Functional covalent chemistry of carbon nanotube surfaces. Adv. Mater.; 2009. 21: 625-642. [9] Cheng HB, Liu CS, Zhang QJ, Ma WT, Dai YW. Method and apparatus for measuring the sedimentation of a solid-liquid two-phase mixture. 2013, China patent application number CN 201310207325 and 2014, US patent application number 14273651. [10] Sinha K, Kavlicoglu B. A comparative study of thermal behavior of iron and copper nanofluids. J. Appl. Phys.; 2009. 106: 1-7. [11] Bourlinos AB, Georgakilas V, Boukos N, Dallas P, Trapalis C, Giannelis EP. Silicone- functionalized carbon nanotubes for the production of new carbon-based fluids. Carbon; 2007. 45: 1583-1595. [12] Aravind SSJ, Baskar P, Baby TT, Sabareesh RK, Das S, Ramaprabhu S. Investigation of structural stability, disper sion, viscosity, and conductive heat transfer properties of functionalized carbon nanotube based nanofluids. J. Phys. Chem. C; 2011. 115: 16737-16744. [13] Li Q, Dong LJ, Li LB, Su XH, Xie HA, Xiong CX. The effect of the addition of carbon nanotube fluids to a polymeric matrix to produce simultaneous reinforcement and plasticization. Carbon; 2012. 50: 2045-2060. [14] Chen LF, Xie HQ. Silicon oil based multiwalled carbon nanotubes nanofluid with optimized thermal conductivity enhancement. Colloids and Surfaces A: Physicochem. Eng. Aspects; 2009. 352:136-140. [15] Gu BM, Hou B, Lu ZX, Wang ZL, Chen SF. Thermal conductivity of nanofluids containing high aspect ratio fillers. Int. J. Heat Mass Transfer; 2013. 64: 108-114. [16] Singh IV, Tanaka M, Endo M. Effect of interface on the thermal conductivityof carbon nanotube composites. Int. J. Therm. Sci.; 2007. 46: 842-847. [17] Wang XQ, Mujumdar AS. Heat transfer characteristics of nanofluids: a review. Int. J. Therm. Sci.; 2007. 46: 1-19. [18] Meng ZG, Wu DX, Wang LG, Zhu HT, Li QL. Carbon nanotube glycolnanofluids: photo-thermal properties, thermalconductivities and rheological behaviour. Particuology; 2012. 10: 614-618. [19] Li P, Chou ZZ. The measurement and evaluation of thermal conductivities of nanofluids. Energy Conservation; 2005. 4: 13-15.
ScienceDirect Energy Procedia 69 (2015) 699 – 705
International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014
Effects of surface modification on the suspension stability and thermal conductivity of carbon nanotubes nanofluids P. Zhanga, W. Honga, J. F. Wua, G. Z. Liua, J. Xiaoa, Z. B. Chena and H. B. Chenga* a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070, Wuhan, China
Abstract The composite carbon nanotubes (CCNTs) were synthesized by surface modification of the as-synthesized carboxylfunctionalized carbon nanotubes (f-CNTs) with the polyethylene glycol, and the composite carbon nanotubes-based nanofluids (CCNTs-based nanofluids) were prepared with the as-synthesized CCNTs and ethylene glycol. The surface molecular structure, surface morphology of the CCNTs were characterized by Fourier Transform Infrared Spectroscopy, Field Emission Scanning Electron Microscope and Transmission Electron Microscopy. The suspension stability and thermal conductivity enhancement ratio of the CCNTs-based nanofluids were characterized via the thermal conductivity variation method. The results show that suspension stability of the CCNTs-based nanofluids was greatly improved by surface modification of the carbon nanotubes; the thermal conductivity of the CCNTs-based nanofluids was further enhanced and became more stable by surface modification. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG
Keywords: carbon nanotubes nanofluids; surface modification; suspension stability; thermal conductivity.
1. Introduction Carbon nanotubes-based nanofluids (CNTs-based nanofluids) are very promising composite fluids applied in solar thermal power generation to enhance heat transfer capabilities [1], because of their enhanced thermal performance [2], stable chemical property and high temperature stability [3]. However, pristine carbon nanotubes have strong tendency to rapidly aggregate and precipitate to the bottom of the container, which can clog any flow
* Corresponding author. Tel.: +86-13349878089; fax: +86-27-87879468. E-mail address: [email protected]
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.03.080
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channels, seriously decay heat transfer effectiveness, and render practical engineering applications infeasible [4,5]. Therefore, improving suspension and heat transfer enhancement stability of CNTs-based nanofluids is the key challenge to enable successful engineering applications of these CNTs-based nanofluids [6]. To efficiently mitigate these disadvantages, a novel method has been developed to controllably modify the surface characteristics of CNTs through in-situ grafting organic molecular chains on the surface of CNTs, so that composite CNTs (CCNTs) and CCNTs-based nanofluids have been prepared using ethylene glycol carrier fluid. The effects of surface modification on the suspension stability and thermal conductivity enhancement ratio of CCNTs-based nanofluids were investigated. Moreover, the mechanisms that increase the suspension stability, or reduce sedimentation rate, were studied. Nomenclature f nf k nf kf Ƹk=k nf -k f Ƹk/k f Ø T
base fluid nanofluid thermal conductivity of nanofluids thermal conductivity of the base fluid ethylene glycol thermal conductivity enhancement of nanofluids thermal conductivity enhancement ratio volume fraction of CCNTs temperature
2. Experimental procedure 2.1. Preparation of the CCNTs-based nanofluids Firstly, carboxyl-functionalized carbon nanotubes (f-CNTs) were synthesized with carbon nanotubes and a mixed acid composed of nitric and sulfuric acid for 5 hours [7,8]; then the composite carbon nanotubes (CCNTs) were synthesized via esterification reaction between the as-synthesized f-CNTs and polyethylene glycol. Finally, CCNTsbased nanofluids were prepared by suspending CCNTs in ethylene glycol for five different volume fractions (0.1, 0.3, 0.5, 0.7 and 0.9 vol%) by means of mechanical agitation and ultrasonic mixing. 2.2. Characterization of CCNTs-based nanofluids The structure and properties of both CNTs and CCNTs were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscope (FE-SEM), and Transmission Electron Microscopy (TEM). The suspension stability of the CCNTs-based nanofluids was evaluated via the thermal conductivity variation method [9]. Thermal conductivity of nanofluids were measured using a thermal properties analyzer, which complies with ASTM and IEEE Standards [10]. 3. Results and discussion 3.1. The surface molecular structure of CCNTs To judge whether the surface modifier polyethylene glycol was successfully grafted onto the surface of the assynthesized CCNTs, the surface molecular structure of the as-synthesized CCNTs and pristine CNTs were analyzed by FT-IR spectra, because the FT-IR spectra can reflect the changes of the surface molecular structure of the CCNTs as the organic surface modifier polyethylene glycol (PEG) 200 was grafted on the surface [11]. So the FT-IR spectra of as-synthesized CCNTs and pristine CNTs (for comparison) were collected, as shown in Fig. 1. The infrared characteristic absorbing peaks of the CCNTs can be assigned to -COO-stretch (1642.48 cm-1), -OH stretch (1383.49 cm-1), and CH 2 stretch(702.72 cm-1). In contrast, FT-IR spectrum of the pristine CNTs does not exhibit these
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absorption bands. These results show that the surface molecular chain has the characteristic groups of polyethylene glycol: -COO-, -OH, and CH 2 ; that is, the surface modifier polyethylene glycol was successfully grafted onto the surface of composite carbon nanotubes. Thereby, the surface properties of the CCNTs would be modified by the grafted organic molecular chains [12].
Fig. 1. FT-IR images of CCNTs and pristine CNTs.
3.2. Surface morphology of CCNTs The surface morphology of the CCNTs was characterized by SEM (as shown in Fig. 2 A), and the surface morphology of the CNTs was characterized by TEM for comparison (as shown in Fig. 2 B). The radial dimension of CCNTs ranged from 25 to 35nm, and the radial dimension of CNTs ranged from 12 to 18 nm. The increment (1317nm) of the radial dimension of CCNTs may be contributed to the grafted organic molecular chains polyethylene glycol. In addition, the intertwining of CCNTs can be clearly observed, which implies that homogeneously dispersing carbon nanotubes in carrier fluid is still a key technique for to prepare stable CCNTs-based nanofluids [13].
Fig. 2. (A) SEM image of CCNTs; (B) TEM image of pristine CNTs.
3.3. The effect of surface modification on the stability of CCNTs-based nanofluids To investigate how surface modification impacts the suspension stability of CCNTs-based nanofluid, 0.3 vol% CCNTs-based nanofluid and 0.3 vol% CNTs-based nanofluid were respectively prepared; and their suspension
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stability was assessed by the thermal conductivity variation method [9]. Thermal conductivities of the base fluids ethylene glycol (k f ) and the CCNTs-based nanofluids (k nf ) were measured by a transient short hot wire method [14]. In order to reduce the error due to the limit of measurement precision, and increase the sensitivity of surface modification on the thermal conductivity of CCNTs-nanofluids, normalization processing was adopted by taking Ƹk/k f (=(k nf -k f )/k f ) describing the thermal conductivity enhancement ratio of CCNTs-based nanofluids. The data are shown in Fig. 3 A and B. The thermal conductivity enhancement ratio of 0.3 vol% CCNTs-based nanofluid (A) was about 12.8 %, and exhibited no measurable decline over a 1000 hours period. In contrast, the thermal conductivity enhancement ratio of 0.3 vol% CNTs-based nanofluid (B) was about 7.9 %, smaller than that of 0.3 vol% CCNTs-based nanofluid by 38.3 %, which shows that surface modification further improved the thermal conductivity enhancement effect of nanotubes. In addition, the thermal conductivity enhancement ratio of the 0.3 vol% CNTs-based nanofluid degraded sharply from 7.9 % to about 0 % after 26 hours. These results clearly demonstrated that the surface modification of CNTs not only increased the thermal conductivity enhancement, but also markedly improved the stability of thermal conductivity enhancement effect [15]. Fig. 3 A inset and Fig. 3B inset are the photo of the 0.3 vol% CCNTs-based nanofluid and the 0.3 vol% CNTsbased nanofluid after standing for 1000 hours and 26 hours, respectively. As shown in Fig. 3 A inset, there was no observable sedimentation in the CCNTs-based nanofluid even after standing for 1000 hours (a). However, rapid sedimentation occurred in the CNTs-based nanofluid after standing for 26 hours (As shown in Fig. 3B inset b), the formation of the mud line separating clarified fluid above the mud line, and fluid with high solids loading fluid below the mud line can be clearly observed. Obviously, the suspension stability of the 0.3 vol% CCNTs-based nanofluid is much higher than that of the 0.3 vol% CNTs-based nanofluid, that is, the stability of CNTs-based nanofluid can be markedly improved by surface modification of CNTs, which in this case was achieved by grafting organic molecular chains polyethylene glycol on the surface of carbon nanotubes. Because grafted organic molecular chains polyethylene glycol has similar molecular structure, based on the principle of the dissolution in the similar material structure, the grafted organic molecular chains polyethylene glycol could stretch into the carrier fluid molecule ethylene glycol to increase the buoyant force of the nanotubes, and surface modification could endow carbon nanotubes with compatibility with the carrier fluid, so that suspension stability has been markedly improved [16]. Thereby, we believe that surface modification is an effective way to improve the stability of nanofluids. The mechanism of surface modification on the thermal conductivity of nanofluids is very complicated, needs to be deeply investigated; according to this experimental results, keeping the CNTs nanoparticles uniformly dispersed in carrier fluid and no sedimentation is of greatest importance.
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Fig. 3. (A) Thermal conductivity enhancement ratio of 0.3 vol% CCNTs-based nanofluid; (B) 0.3 vol% CNTs-based nanofluid as a function of time. Insets are photos of sedimentation of CCNTs-based nanofluid (a) after standing for 1000 hours, and CNTs-based nanofluid (b) after standing for 26 hours.
3.4. The effect of volume fraction and temperature on the thermal conductivity enhancement ratio of CCNTs-based nanofluids In order to further seek for more suitable CCNTs-based nanofluids with high stability and high thermal conductivity enhancement effect for applied in heat transfer engineering, the effect of volume fraction on the thermal conductivity enhancement ratio was investigated by preparing five CCNTs-based nanofluids with different volume fractions (0.1, 0.3, 0.5, 0.7 and 0.9 vol%), and measuring thermal conductivity enhancement ratio. Thermal conductivities of the base fluids ethylene glycol (k f ) and the CCNTs-based nanofluids (k n f ) were measured by a transient short hot wire method. Ƹk/k f and Ø refer to the thermal conductivity enhancement ratio of CCNTs-based nanofluids ( Ƹ k=k nf -k f ) and the volume fraction of CCNTs, respectively. As shown in Fig. 4, the thermal conductivity enhancement ratio increased with the volume fraction of CCNTs, from 5.9 % up to 21.03 % as the volume faction increased from 0.1 to 0.9 vol%. The results show the effect of the volume fraction on the thermal conductivity enhancement ratio of CCNTs-based nanofluids is in accordance with that reported in the literatures [17,18], so the thermal conductivity of the CCNTs-based nanofluids can be further enhanced by increasing the volume fraction of CCNTs.
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Fig. 4. Thermal conductivity enhancement ratio of CCNTs-based nanofluids as a function of CCNTs volume fraction.
3.5. The effect of temperature on the thermal conductivity enhancement ratio of CCNTs-based nanofluids The nanofluids are different from traditional fluids that the thermal conductivity of nanofluid increases with the temperature, while the thermal conductivity of traditional fluids decreases with the temperature. To expand the applications of the nanofluids at high temperatures, the thermal conductivity enhancement ratio of 0.3% CCNTsbased nanofluid was investigated as a function of temperature. As shown in Fig. 5, within the scope of the test, a linear relationship was obtained, Ƹk/k f =1.065T-13.825, which showed that the thermal conductivity enhancement ratio increased with the temperature, from 12.8 % up to 55.4 % as the temperature increased from 25 to 65 ć. The results show that compared with the thermal conductivity of CuO-based nanofluids at different temperature, reported in the literature [19], the CCNTs-based nanofluids have an advantage on the heat transfer applications under high temperature. So the CCNTs-based nanofluids can be as more promising nanofluids used in heat transfer applications by increasing the temperature of CCNTs to enhance the thermal conductivity.
Fig. 5. Thermal conductivity enhancement ratio of CCNTs-based nanofluid as a function of temperature.
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4. Couclusion In this study, CCNTs were synthesized via in-situ grafting polyethylene glycol chains onto the CNTs, and the CNTs-based nanofluids were prepared by suspending CCNTs in ethylene glycol. The stability of suspension and thermal conductivity enhancement ratio of CCNTs-based nanofluids had been investigated. The results show that the stability of thermal conductivity enhancement effect of the CCNTs-based nanofluids was also much better than that of the CNTs-based nanofluids of comparable volume fraction; the thermal conductivity enhancement ratio of the CCNTs-based nanofluids increased with the volume fraction of CCNTs and the temperature, which indicated that the thermal conductivity could be tuned by adjusting the volume fraction and the temperature. Thereby, it can be concluded that surface modification can significantly improve the suspension stability and stabilize thermal conductivity enhancement effect to meet the needs of heat transfer applications in solar thermal power generation. Acknowledgements This work is financial supported by state key development program of basic research of china (2010CB227105) and Natural Science Foundation of China (51372189). References [1] Duong HM, Papavassiliou DV, Mullen KJ, Wardle BL, Maruyama S. A numerical study on the effective thermal conductivity of biological fluids containing single-walled carbon nanotubes, Int. J. Heat Mass Transfer; 2009. 52: 5591-5597. [2] Meyyappan M. Carbon Nanotubes Science and Applications. CRC Press, New York; 2005. [3] Han ZD, Fina A. Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review. Prog. Polym. Sci.; 2011. 36: 914944. [4] Byrne MT, Gun’ko YK. Recent advances in research on carbon nanotube–polymer composites. Adv. Mater.; 2010. 22: 1672-1688. [5] Choi SUS, Zhang ZG, Yu W. Anomalous thermal conductivity enhancement in nanotube suspensions. Appl. Phys. Lett.; 2001. 79: 22522254. [6] Akoh H, Tsukasaki Y, Yatsuya S, Tasaki A. Magnetic properties of ferromagnetic ultrafine particles prepared by vacuum evaporation on running oil substrate. J. Cryst. Growth; 1978. 45: 495–500. [7] Talaei Z, Mahjoub AR, Rashidi AM, Amrollahi A, Meibodi ME. The effect of functionalized group concentration on the stability and thermal conductivity of carbon nanotubefluid as heat transfer media. Int. Commun. Heat Mass; 2011. 38: 513-517. [8] Peng XH, Wong SS. Functional covalent chemistry of carbon nanotube surfaces. Adv. Mater.; 2009. 21: 625-642. [9] Cheng HB, Liu CS, Zhang QJ, Ma WT, Dai YW. Method and apparatus for measuring the sedimentation of a solid-liquid two-phase mixture. 2013, China patent application number CN 201310207325 and 2014, US patent application number 14273651. [10] Sinha K, Kavlicoglu B. A comparative study of thermal behavior of iron and copper nanofluids. J. Appl. Phys.; 2009. 106: 1-7. [11] Bourlinos AB, Georgakilas V, Boukos N, Dallas P, Trapalis C, Giannelis EP. Silicone- functionalized carbon nanotubes for the production of new carbon-based fluids. Carbon; 2007. 45: 1583-1595. [12] Aravind SSJ, Baskar P, Baby TT, Sabareesh RK, Das S, Ramaprabhu S. Investigation of structural stability, disper sion, viscosity, and conductive heat transfer properties of functionalized carbon nanotube based nanofluids. J. Phys. Chem. C; 2011. 115: 16737-16744. [13] Li Q, Dong LJ, Li LB, Su XH, Xie HA, Xiong CX. The effect of the addition of carbon nanotube fluids to a polymeric matrix to produce simultaneous reinforcement and plasticization. Carbon; 2012. 50: 2045-2060. [14] Chen LF, Xie HQ. Silicon oil based multiwalled carbon nanotubes nanofluid with optimized thermal conductivity enhancement. Colloids and Surfaces A: Physicochem. Eng. Aspects; 2009. 352:136-140. [15] Gu BM, Hou B, Lu ZX, Wang ZL, Chen SF. Thermal conductivity of nanofluids containing high aspect ratio fillers. Int. J. Heat Mass Transfer; 2013. 64: 108-114. [16] Singh IV, Tanaka M, Endo M. Effect of interface on the thermal conductivityof carbon nanotube composites. Int. J. Therm. Sci.; 2007. 46: 842-847. [17] Wang XQ, Mujumdar AS. Heat transfer characteristics of nanofluids: a review. Int. J. Therm. Sci.; 2007. 46: 1-19. [18] Meng ZG, Wu DX, Wang LG, Zhu HT, Li QL. Carbon nanotube glycolnanofluids: photo-thermal properties, thermalconductivities and rheological behaviour. Particuology; 2012. 10: 614-618. [19] Li P, Chou ZZ. The measurement and evaluation of thermal conductivities of nanofluids. Energy Conservation; 2005. 4: 13-15.