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A nanofluid consists in a liquid suspension of nanometer-sized particles. These fluids may contain (or not) surface-active agents to aid in the suspension of the particles. Nanometer-sized particles have higher thermal conductivity than the base fluids. Oxides, metals, nitrides, and nonmetals, like carbon nanotubes, can be used as nanoparticles in nanofluids. Water, ethylene glycol, oils, and polymer solutions can be used as base fluids. In this chapter, we summarize the recent studies of using CNTs and graphene to improve the thermal conductivity of nanofluids. Moreover, we refer to the studies about the effect of using magnetic fields on enhancing the thermal conductivity of nanofluids. Too much discrepancy about thermal conductivity of nanofluids can be found in the literature. For carbon nanofluids, unfortunately, no significant improvements on thermal conductivity are observed using low concentrations. Different improvement percentages have been reported. This variation in the thermal conductivity can be attributed to many factors, such as particle size temperature, pH, or zeta potential. We believe that more research efforts need to be made in order to, first, improve the thermal conductivity of nanofluids and, second, assess the effect of the different parameters and conditions on the thermal conductivity of nanofluids.
Department of Mechanical and Material Engineering, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates
Amal Al Ghaferi
Department of Mechanical and Material Engineering, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates
Irfan Saadat
Department of Electrical Engineering and Computer Science, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates
Haiping Hong
Department of Material and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD, USA
*Address all correspondence to: hyounes@masdar.ac.ae
1. Introduction
The enhancement of the efficiency of heat transfer fluids has been of great interest to scientists and engineers for the past two decades. Conventional heat transfer fluids, such as mineral oil, water, and ethylene glycol (EG), have poor heat transfer properties, compared to solids. In recent years, nanofluids have been investigated as potential heat transfer fluids [1–11].
The conventional coolants/fluids used in radiators and engines have poor heat transfer properties and contain millimeter- or micrometer-sized particles to improve the heat transfer properties that can clog new cooling technologies that include microchannels. The use of fluids with better heat transfer can allow engines to run at optimal temperatures and allow the use of smaller and lighter radiators, pumps, and other components. Smaller radiators allow a smaller front end of large vehicles, such as trucks. Fluids with better heat transfer can also lead to the production of lighter vehicles with better fuel economy, since less pumping power is needed. Although many factors such as pH, particle size, and zeta potential can affect the thermal conductivity (TC) of nanofluids, they are not promising to improve thermal conductivity values [12–16].
Wensel et al. [17] found that using low concentrations of the nanoparticles do not enhance the TC effectively. For example, using of 4–5 vol.% metal oxides increases the thermal conductivity by 10–20% only, which is not the significant increase that is needed.
In this chapter, two carbon-based nanomaterial nanofluids will be discussed: carbon nanotube (CNT) and graphene-based nanofluids and their thermal conductivity.
2.1. Carbon nanotubes
Carbon nanotubes (CNTs) have attracted significant attention since they were discovered by Iijima in 1991 [18]. The sp2 carbon–carbon bond in the plane of the graphene lattice is the strongest among of all chemical bonds. CNTs can be obtained with different numbers of walls, including single-walled CNTs, double-walled CNTs, and multiwall CNTs. They have very good thermal conductivity along the length of the tubes. It has been shown that the thermal conductivity of CNTs is unusually high (e.g., 3000 W/mK) for MWNTs [19] and even higher for SWCNTs (6000 W/mK). The thermal conductivity of CNTs is more than seven times higher than that of copper, which is well known for having a good thermal conductivity of 385 W/Mk [19–22]. Because of their remarkable thermal, electrical, mechanical properties, and low density, carbon nanotubes are ideal materials for reinforcement in composites [23–27], sensors [28], and many other applications, such as thermal managements [29–31].
2.1.1. Carbon nanotubes and carbon nanotubes metal oxide-based nanofluids
Due to the high thermal conductivity of CNTs, researchers started to use them to prepare nanofluids in order to improve the heat transfer. CNTs tend to aggregate into groups due to the large surface energy (strong van der Waals attractions between individual tubes) [32]. Therefore, a variety of approaches has been introduced to decrease the nanotubes agglomeration, namely, the modification of their chemistry through noncovalent adsorption using surfactants [33–36], covalent (functionalization) by chemical modification [37–42], and metal coating (like Ni-coated SWNTs). Table 1 summarizes the work carried out so far dealing with the use of CNTs in nanofluids. Xie et al. [52] investigated the functionalized carbon nanotubes and found out that the thermal conductivity enhancement increased with the increase in nanotubes loading. In addition, it was found that the thermal conductivity decreased with the increase in thermal conductivity of the base fluid. Chen et al. [46] studied the effect of MWNTs on the thermal conductivity enhancement of ethylene glycol-based nanofluid and found that the enhancement reached up to 17.5% at 0.01 volume fraction. Ding et al. [49] observed a significant enhancement of the convective heat transfer in comparison with pure water as the working fluid, which depends on the flow condition, CNT concentration, and the pH level (although the latter is small). Grag et al. [44] studied the effect of ultrasonication on the thermal conductivity of MWCNT fluids and found that the thermal conductivity was enhanced by 20% when the ultrasonication was used for 40 min in 1 wt.% MWCNT concentration, in a 130 W, 20 kHz ultrasonication environment.
Current research of carbon nanomaterials-based nanofluids.
PAO: poly (α-olefin) oil.
Phuoc et al. [43] studied the thermal conductivity, viscosity, and stability of nanofluids made of MWCNT dispersed in water. The nanotubes have an outside diameter of 20–30 nm, inside diameter of 5–10 nm, length of 10–30 mm, and average density of 2.1 g/cm3. Different weight ratio of Chitosan surfactant, namely, 0.1, 0.2, and 0.5 wt.%, was used to disperse MWCNTs in water. It was found that Chitosan dispersed and stabilized MWNTs in water efficiently; as only 0.1 wt.% Chitosan surfactant could disperse and stabilize 3 wt.% MWCNTs in water. The thermal conductivity of nanofluids containing 0.5–3 wt.% MWCNTs was enhanced from 2.3 to 13%. However, the enhancement of the thermal conductivity was independent of viscosity of the base fluid, which shows that the particle velocity does not have a significant effect on the thermal conductivity. Figure 1 shows the dispersion behavior of the MWCNTs in deionized (DI) water (DW).
Figure 1.
Nanofluid prepared by dispersing MWCNTs in deionized water (A) shows that MWNTs deposited at the bottom of the container and (B) shows that MWNTs maintain good dispersion: From Ref. [43].
Garg et al. [44] reported that 1 wt.% MWCNT increased the thermal conductivity up to 20% at 35°C. Gum arabic was used as a surfactant to stabilize the nanofluids. The study revealed that the thermal conductivity of CNT nanofluids increased considerably after 24°C. Meng et al. [45] found a 25.4% enhancement of the thermal conductivity of CNT glycol nanofluids with 4.0% mass fraction at room temperature. Figure 2 shows that the transmittances from 200 to 1500 nm, the values drop from 50% to about 10%, indicating that a small amount of CNTs results in significant absorption enhancement of light.
Figure 2.
TEM image of HNO3-treated CNTs (A) and UV–Vis–NIR transmittance spectra (B) of the CNTs glycol nanofluids. From ref. [45].
Chen et al. [46] reported that 1 vol.% of MWCNTs without surfactant, placed in ethylene glycol-based nanofluid, yielded an enhancement of 17.5%. A mechano-chemical reaction method was used to enhance the MWCNTs dispersibility for producing a CNT nanofluid. Figure 3 shows the thermal conductivities of nanofluids as a function of nanotube loadings in two different fluids: deionized water and ethylene glycol. The enhancement using ethylene glycol as fluid and without surfactant at 1 vol.% of MWCNTs is higher than the enhancement using DW as fluid and at 1 vol.% of MWCNTs.
Figure 3.
Thermal conductivities of nanofluids as a function of nanotube loadings. From ref. [47].
Chen and Xie [47] investigated nanofluids made of SWCNTs and DWCNTs in water-based fluids, respectively. Those CNTs were functionalized by a wet-mechano-chemical reaction method in order to obtain a stable dispersion. The study revealed that the thermal conductivity enhancement was 15.6, 14.2, and 12.1% for SWCNT, DWCNT, and MWCNT at a volume fraction of 0.2%, respectively.
Choi et al. [12] reported that the enhancement of thermal conductivity at a 1% volume fraction of MWCNTs in poly (α-olefin) (PAO) oil was 160%, compared to the base fluid at room temperature.
Assael et al. [48] found a 38% enhancement of the thermal conductivity in 0.6 vol.% MWNTs nanofluids in which water was a base fluid and SDS surfactant was used to stabilize the nanofluids. Ding et al. [49] confirmed that 1.0 vol.% of MWCNT dispersed in water, with SDBS surfactant, lead to an enhancement of 79% at 30°C. Shaikh et al. [50] prepared nanofluids by dispersing CNTs, exfoliated graphite (EXG), and heat-treated nanofibers (HTT) in PAO oil. The thermal conductivity for 1% CNTs, EXG, and HTT was enhanced by 161, 131, and 104 %, respectively. Baby and Sundara [51] reported on the synthesis of silver nanoparticle decorated MWCNT-graphene mixture and dispersed the (Ag/(MWNT-HEG)) composite in ethylene glycol without surfactant. They found that only 0.04 vol.% of the nanoparticles enhanced the thermal conductivity by ∼8% at 25°C.
2.2. Graphene
Graphene, the first ever stable two-dimensional (2D) honeycomb lattice of sp2-bonded carbon atoms, has attracted the interest of many researchers and is very promising for a wide range of applications due to its spectacular electronic, mechanical, and optical properties. Since few layers of graphene were isolated for the first time from graphite, using a tape in 2004 [53], the graphene production rate has increased rapidly in order to synthesize large-area and high-quality graphene films, compatible with various applications. Actually, the history of graphene dates back to the 1960s, when single-atom plane of graphite, termed as graphite layer, was isolated. For several decades, the existence of 2D atomic structures remained as an unaccepted concept. Now the term of graphene, the basic block of graphite, is widely used to describe the 2D monolayer of carbon atoms with a honeycomb structure. Graphene lattice consists of repetitive two identical carbon atoms, A and B, that together form one unit cell (Figure 4). The distance between each carbon atom and its neighbors is 1.42 A´ [54]. Figure 5 shows first Brillouin zone and band structure of graphene using tight binding approximation. K and K′ are Dirac points, the transition between the valence and the conduction band.
Figure 4.
Graphene lattice.
Figure 5.
First Brillouin zone and band structure of graphene using tight binding approximation. K and K’ are Dirac points, the transition between the valence and the conduction band. Reproduced from [54].
2.2.1. Graphene and graphene metal oxide-based nanofluids
The high thermal conductivity of graphene encouraged researchers to use it to prepare nanofluids with super high thermal conductivity. Yu et al. [55] made stable nanofluids by dispersing graphene oxide nanosheets in ethylene glycol and found out that the thermal conductivity of the nanofluids was higher than that of the ethylene-glycol base fluid. The thermal conductivity of the nanofluids was enhanced up to 61.0% at 5.0 vol.% of graphene nanosheets (GNs), and almost constant for 7 days, indicating that the hydrophilic surfaces allow graphene oxide nanosheets to have good compatibility with ethylene glycol. Yu et al. [56] used a facial technique to prepare ethylene glycol-based nanofluids containing graphene nanosheets. They found that the thermal conductivity of the graphene nanofluids increased by 86% at 5.0 vol.% graphene, showing that the extensive presence of saturated sp3 bonds and oxygen atoms made graphene oxide nonconducting, hindered the thermal transport, and promoted phonon scattering. Mehrali et al. [57] prepared stable nanofluids by mixing graphene nanoplatelets and water using a tip ultrasonicator. In this study, four concentrations were prepared: 0.025, 0.05, 0.075, and 0.1 wt.% for three different specific surface areas of 300, 500, and 750 m2/g. It was found that the highest thermal conductivity was 27.64% for a concentration of 0.1 wt.%, with a specific surface area of 750 m2/g. Aravind and Ramaprabhu [58] adopted a simple thermal treatment method to prepare graphene nanosheets (GNs)-based nanofluids. The procedure was carried out without surfactant and harsh chemical treatments, which reduced the alkaline pH of graphite oxide suspension in ethylene glycol (EG) and deionized (DI) water-based fluids. It was found that the thermal conductivity at 25°C and 0.14% volume fraction of GN in EG and DI water is enhanced by 6.5 and 13.6%, respectively. Ijam et al. [59] prepared a glycerol-water-based nanofluid containing graphene oxide nanosheets. The mixture was stable for up to 5 months. The thermal conductivity of the prepared nanofluid with different temperatures (25–45°C) and weight fractions (0.02–0.1 wt.%) revealed an enhancement of thermal conductivity of 4.5% at 25°C for a weight fraction of 0.02 and 11.7% for a weight fraction of 0.1 wt.% and 45°C, respectively. Kole et al. [60] studied the thermal and electrical conductivity of stable and well dispersed functionalized graphene-ethylene glycol. High purity graphite powder was used to make Graphene nanosheets. Then the Hummers’ method [61] was followed, and exfoliation and reduction by hydrogen gas were used to obtain hydrogen-exfoliated graphene (HEG), which was functionalized with acid. The material was dispersed in distilled water nanofluids with volume concentration between 0.041 and 0.395 vol.%. It was found that the thermal conductivity at 0.395 vol.% was enhanced by 15%, and the thermal conductivity of both the base fluid and the prepared nanofluid increased linearly with temperature. However, the thermal conductivity ratios (TC of the nanofluids over the TC of base fluids) were nearly independent of temperature. The study also showed that the electrical conductivity of the f-HEG nanofluids had a significant enhancement of 8620% at 0.395 vol.% loading of f-HEG, in a base fluid of 70:30 mixture of EG and distilled water. Hajjar et al. [62] studied the thermal conductivity of graphene oxide nanofluids, which comprised of graphene oxide obtained by the Hummers’ method and water. They found that 0.25 wt.% of graphene oxide could enhance the thermal conductivity by 33.9% at 20°C and up to 47.5% at 40°C. Ghozatloo et al. [63] investigated the stability and thermal conductivity behavior of the nanofluids made of graphene- and water-based fluids. They also analyzed the influence of time and temperature on the effective thermal conductivities for different concentrations of graphene that was functionalized in mild conditions by potassium persulfate. The thermal conductivity for 0.05 wt.% of the functionalized graphene is enhanced around 14.1% at 25°C and 17% at 50°C compared with water. Liu et al. [64] studied the thermodynamic properties (including thermal conductivity, viscosity, specific heat, and density) of graphene-dispersed nanofluids for temperatures ranging from room temperature to around 200°C. The studied nanofluids were made out of graphene and the ionic liquid 1-hexyl-3-methylimidazolium tetra fluoroborate ((HMIM)BF4). The work showed that the thermal conductivity of the nanofluid containing 0.06 wt.% graphene increased by 15.2–22.9% at the tested temperature range (25–200°C). Viscosity, specific heat, and density decreased, respectively, by 4.6–13.1, 3, and 2.8%, compared to (HMIM)BF4.
3. Effect of the alignment of the carbon nanostructures on the thermal conductivity of nanofluids
Many researchers have studied the effect of the alignment of iron oxide nanoparticles and carbon nanomaterials on the thermal, electrical, and mechanical properties of fluids and polymers. Table 2 summarizes most work about the effect of the alignment of iron oxide nanoparticles and carbon nanomaterials on the thermal conductivity of nanofluids.
Current research of the effect of the alignment of iron oxide nanoparticles and carbon nanomaterials on the thermal conductivity of nanofluids.
Hong et al. [65] studied the effect of the alignment on the thermal conductivity of nanofluids under an external magnetic field and obtained the thermal conductivity and microscopic images taken with and without magnetic field, shown in Figures 6 and 7.
Figure 6.
Experimental setup for taking optical microscopy images using the optical microscope, Leica Z16 APO under application of a magnetic field. From Ref. [70].
Figure 7.
Experimental setup for measuring TC under application of a magnetic field. From Ref. [70].
Figure 8 shows that the nanotubes align very well in the magnetic field direction.
Figure 8.
SEM picture of 0.01 wt.% carbon nanotube + 0.02 wt.% Fe2O3 with magnetic field. From Ref. [65].
The heat transfer of nanofluids containing carbon nanotubes and magnetic-field-sensitive nanoparticles of Fe2O3 was enhanced when a magnetic field was applied for some time. The enhancement was attributed to the formation of aligned chains of Fe2O3 under the applied magnetic field and to the fact that chains help to connect the nanotubes. Wensel and Wright [17, 66] found that the thermal conductivity of nanofluids containing 0.02 wt.% was enhanced by 10% under a magnetic field. That was attributed to the aggregation of metal oxide particles on the surface of nanotubes by electrostatic attraction, which forms the aggregation chain along the nanotubes. Horton et al. [67] investigated the thermal conductivity of nanofluids containing magnetic-metal-coated carbon nanotubes (Ni-coated SWCNTs), which significantly enhanced by 60% under a magnetic field. Figure 9 shows a color online microscope image of 0.05 wt.% Ni-coated SWCNT in DI water, before (A) and after the magnetic field (B) was applied (H = 0.62 kG).
Figure 9.
Color online microscope image of 0.05 wt.% Ni-coated SWCNT in DI water. (A) Before magnetic field and (B) after magnetic field (H = 0.62 kG). From Ref. [66].
Figure 10 shows TEM images of carbon nanotubes. (A) As received, uncoated CNTs, the image shows that the wall of one CNT is clear and nothing adhere to it. (B) Ni-coated CNTs, the image shows that the wall of the CNTs has been modified with some Ni adhered to the surface of the CNTs and (C) shows that the wall of the Ni-coated CNTs after applying the magnetic field experiment. It can be seen that the density of the Ni coated has decreased due to the effect of sonication and the magnetic field experiment.
Figure 10.
TEM images of carbon nanotubes. (A) As received, uncoated. (B) Ni-coated nanotube before the magnetic field experiment, and (C) after the experiment. From Ref [67].
Hong et al. [68] studied the thermal conductivity of nanofluids containing CNTs and magnetic-field-sensitive nanoparticles of Fe2O3 with NaDDBS surfactant and found that an electrostatic attraction among the nanotubes, the surfactant, and the metal oxide caused aggregation. NaDDBS surfactant with a negative charge attracted Fe2O3, which had positive zeta potential charge. When a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), replaced the anionic surfactant NaDDBS, no alignment was found because both the surfactant and Fe2O3 have the same positive charge. Figure 11 shows that the nanotubes align very well in the direction of the magnetic field, either in scale bar 100 μm (A) or 10 μm (B).
Figure 11.
Electron SEM images of 0.017 wt.% SWCNT, 0.017 wt.% Fe2O3, and 0.17 wt.% NaDSSB with magnetic field. (A) Scale bar 100 m, and (B) scale bar 10 m. From Ref. [68].
Younes et al. [27, 70] studied the alignment of different functionalized SWCNTs using Fe2O3 nanoparticles under an external magnetic field. These authors found that, even in the absence of NaDDBS as surfactant, some functionalized SWNTs, i.e., SWNT-SO2OH and SWNT-COOH, could disperse well in water and showed a clear alignment under an external magnetic field. Figure 12 shows microscopy image of 0.016 wt.% SWNT-COOH and 0.016 wt.% Fe2O3 in DI water. This microscope image of 0.016 wt.% SWNT-COOH and 0.016 wt.% γ-Fe2O3 in DI water was obtained using a high-speed microscope video system. Magnetic field (H = 0.14 kG) was applied with an internal reference of 30 μm. As shown in Figure 12, it is clearly seen that the randomly dispersed SWNT-COOH/γ-Fe2O3 forms large and long lines, indicating that SWNT-COOH/γ-Fe2O3 nanoparticle mixture aligns under the external magnetic field.
Figure 12.
Microscopy image of 0.016 wt.% SWNT-COOH, 0.016 wt.% Fe2O3 in DI water. At 8 min, the internal reference is 30 μm. From Ref. [70].
Younes et al. [30] studied the effect of the alignment of carbon nanofibers in water and epoxy under an external magnetic field on the thermal conductivity of nanofluids. They found that the magnetic field aligned the nanofibers, not only in water but also in epoxy, as clearly seen in Figure 13. Adding 0.02 wt.% of carbon nanofibers to water enhanced the thermal conductivity by 10%. The addition of an external magnetic field enhanced the thermal conductivity around 20%.
Figure 13.
Alignment of CNFs in epoxy. (A) Optical microscopy images for 0.002 wt.% CNFs, 0.002 wt.% Fe2O3, and 0.02 wt.% NaDDBS in epoxy after 48 h. (B) Digital camera image of 0.02 wt.% CNFs, 0.02 wt.% Fe2O3, and 0.2 wt.% NaDDBS in epoxy after 2 h. Magnetic field (H = 1.2 kG) was applied. From Ref. [30].
Christensen et al. [26] studied the effect of nonionic surfactant on the dispersion and alignment of carbon nanotubes in the presence of Fe2O3. Gum arabic and Triton X-100 were used as nonionic surfactants. Gum arabic did not allow good dispersion or alignment in aqueous fluid. Triton X-100 allowed dispersion and alignment of both Fe2O3 and SWNT in an aqueous base fluid. Due to the partial negative charge of Triton X-100, alignment did occur, but the alignment was slower, when compared to surfactants or functional groups containing stronger charges.
Figure 14.
(a) Thermal conductivity ratio k/kf and the corresponding percentage of enhancement as a function of applied magnetic field for 6.3 vol.% of Fe3O4. (B)–(D) Micrographs of ferrofluid emulsion of 200 nm, (B) without, (C) with low, and (D) with high magnetic field. (E)–(G) Schematic alignment of particles under the above three cases. From Ref. [71].
Glover et al. [72] prepared a nanofluids that consisted of water and 0–0.5 wt.% of sulfonated carbon nanotubes. The electrical conductivity of the prepared nanofluids was measured. The study found that the electrical conductivity increased by 13 times when only 0.5 wt.% of functional sulfonated carbon nanotubes was used. Philip et al. [71] observed a dramatic enhancement of thermal conductivity in a nanofluid containing magnetic particles of Fe3O4 (of 6.7 nm diameter) under a magnetic field (see Figure 14). The maximum enhancement in thermal conductivity was 300% (k/kf = 4.0) for a particle loading of 6.3 vol.%.
The main focus of this chapter is to summarize the recent studies using CNTs and graphene to improve the thermal conductivity of nanofluids. Moreover, this chapter summarizes the investigation about the effect of using magnetic fields on enhancing the thermal conductivity of nanofluids. Too much discrepancy about thermal conductivity of nanofluids can be found in the literature. For carbon nanofluids, unfortunately, no significant improvements on thermal conductivity are observed using low concentrations. The thermal conductivity increased by approximately 10–20% when 1 vol.% SWCNT was used. However, at these high concentrations, the viscosity is greatly increased and the nanofluid becomes “mud-like,” which are the difficulties reported in the literature. This variation in the thermal conductivity can be attributed to many factors, such as particle size, temperature, pH, or zeta potential. Finally, we believe that more efforts need to be done in order to improve the thermal conductivity of nanofluids and study the effect of the different parameters in thermal conductivity. In addition, more investigation about the use of magnetic field to align magnetic sensitive nanoparticles is required.
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Written By
Hammad Younes, Amal Al Ghaferi, Irfan Saadat and Haiping Hong
Submitted: 27 February 2016Reviewed: 08 June 2016Published: 05 October 2016
Open access peer-reviewed chapter
