Investigation of thermal conductivity and rheological properties of nanofluids containing graphene nanoplatelets
© mehrali et al.; licensee Springer. 2014
Received: 18 December 2013
Accepted: 5 January 2014
Published: 13 January 2014
In the present study, stable homogeneous graphene nanoplatelet (GNP) nanofluids were prepared without any surfactant by high-power ultrasonic (probe) dispersion of GNPs in distilled water. The concentrations of nanofluids were maintained at 0.025, 0.05, 0.075, and 0.1 wt.% for three different specific surface areas of 300, 500, and 750 m2/g. Transmission electron microscopy image shows that the suspensions are homogeneous and most of the materials have been well dispersed. The stability of nanofluid was investigated using a UV-visible spectrophotometer in a time span of 600 h, and zeta potential after dispersion had been investigated to elucidate its role on dispersion characteristics. The rheological properties of GNP nanofluids approach Newtonian and non-Newtonian behaviors where viscosity decreases linearly with the rise of temperature. The thermal conductivity results show that the dispersed nanoparticles can always enhance the thermal conductivity of the base fluid, and the highest enhancement was obtained to be 27.64% in the concentration of 0.1 wt.% of GNPs with a specific surface area of 750 m2/g. Electrical conductivity of the GNP nanofluids shows a significant enhancement by dispersion of GNPs in distilled water. This novel type of nanofluids shows outstanding potential for replacements as advanced heat transfer fluids in medium temperature applications including solar collectors and heat exchanger systems.
KeywordsGraphene nanoplatelets Nanofluids Thermal conductivity Rheology Stability
Processes of energy transport have been integrated in a wide range of areas, such as in industry, oil and gas, and electricity. In the past decades, ethylene glycol, water, and oil were used as conventional fluids in heat exchanger systems. However, improvement of these conventional heat transfer fluids, particularly thermal conductivity, has become more and more critical to the performance of energy systems . Choi and Eastman  have introduced the term nanofluids referring to fluids containing dispersed nanosized particles having large thermal conductivity enhancement. In spite of the attention received by this field, uncertainties concerning the fundamental effects of nanoparticles on thermophysical properties of solvent media remain . Thermal conductivity is the property that has catalyzed the attention of the nanofluid research community . A lot of research has been devoted to improve the thermal properties of these fluids by adding a small quantity of a highly thermal conductive solid at concentrations ranging from 0.001 to 50 wt.% of the various nanomaterials including oxide , nitride , metal , diamond , carbon nanotube , carbon fiber , carbon black, graphene oxide , graphene , graphite flake , and hybrid  with different shapes (particle, disk, tube, sheet, fiber, etc.) [4, 15, 16]. Nanofluids have many applications in the industries since materials of nanometer size have unique chemical and physical properties and the thermal conductivity of nanofluids with smaller size of nanoparticles is larger than the those of bigger sizes at specific concentrations .
Recently, a significant number of studies have been conducted on the use of carbon-based nanostructures like carbon nanotubes , single-wall carbon nanotubes , multiwall carbon nanotubes , graphite , graphene oxide , and graphene  to prepare nanofluids. Recent studies reveal that graphene has a very high thermal conductivity, so it is obvious that graphene nanofluid would show a higher thermal conductivity enhancement compared to other nanoparticles. Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, has attracted much attention since its discovery by Novoselov et al. . Graphene nanoplatelets are two-dimensional (2D) with an average thickness of 5 to 10 nm and a specific surface area of 50 to 750 m2/g; they can be produced at different sizes, from 1 to 50 μm. These interesting nanoparticles, including short stacks of platelet-shaped graphene sheets, are identical to those found in the walls of carbon nanotubes but in planar form . Graphene nanoplatelets (GNPs) have drawn a lot of interest due to their excellent electrical conductivity and high mechanical properties; the in-plane thermal conductivity of GNPs is reported to be as high as 3,000 to 5,000 W/m∙K . Further, as this is a 2D material, the heat transfer properties are expected to be much different from the zero-dimensional nanoparticles and one-dimensional carbon nanotubes. Moreover, since GNP itself is an excellent thermal conductor, graphene-based nanofluids are normally expected to display a significant thermal conductivity enhancement . Graphene nanoplatelets are also offered in granular form which could be dispersed in water, organic solvents, and polymers with the right choice of dispersion aids, equipment, and techniques.
In this paper, an attempt is made to prepare aqueous suspensions of stable homogeneous GNP nanofluids by high-power ultrasonication. It is difficult to disperse GNPs for high mass percentages without using any dispersion aids; in this work, stable homogeneous GNP nanofluids were prepared for the first time without the use of any surfactant, and their thermophysical properties were measured at temperatures below 100°C. Stabilization mechanisms of dispersions are analyzed by UV-visible (vis) spectrophotometry and zeta potential measurements to quantitatively characterize the colloidal stability of the GNP dispersions. It is expected that the final results can provide a guideline for selecting ideal dispersants. The present report contains results on thermal conductivity, viscosity, and stability of three different specific surface areas (300, 500, and 750 m2/g) at different concentrations (by weight percentage) of the mixture of GNPs and distilled water as base fluid. Results have been discussed to identify the mechanisms responsible for the observed thermal conductivity and viscosity enhancement in GNPs prepared at different concentrations (0.025, 0.05, 0.075, and 0.1 wt.%) of the mixture of GNPs and distilled water. The feasibility of the GNP nanofluids for use as innovative heat transfer fluids in medium temperature heat transfer systems has been demonstrated.
0.2 to 0.4 g/cm3
2.0 to 2.25 g/cm3
Specific surface area
300, 500, and 750 m2/g
Peak in UV–vis spectrophotometer
265 to 270 nm
Parallel to surface
Perpendicular to surface
Parallel to surface
Perpendicular to surface
Detailed microstructures were further examined under a transmission electron microscope (TEM; TEM-LIBRA 120, Carl Zeiss, Oberkochen, Germany). The rheological behavior of the nanofluids of different weight percentage of graphene nanoplatelets was measured using an Anton Paar rheometer (Physica MCR 301, Anton Paar GmbH, Graz, Austria), which had recorded the viscosity and shear rate for different temperatures. Electrical conductivity and zeta potential of the nanofluids were measured using Zetasizer Nano (Malvern Instruments Ltd., Malvern, UK). A transient heated needle (KD2 Pro, Decagon Devices, Inc., Pullman, WA, USA) was used to measure the thermal conductivity with 5% accuracy at constant temperature. The thermal conductivity measurements were repeated ten times, and the average values were reported. Light transmission of all samples was measured with a Shimadzu UV spectrometer (UV-1800, Shimadzu Corporation, Kyoto, Japan) operating between 190 and 1,100 nm. The nanofluid solution was diluted with distilled water to allow sufficient transmission while each measurement was repeated three times to achieve a better accuracy.
Results and discussion
Morphology of GNP dispersions
Stability investigation with UV–vis spectroscopy
The rate of sedimentation after 600 h is different among these 12 samples as different concentrations and specific surface areas are imposed. This rate is changing as the lowest precipitation rate appears from 1% by GNP 750 (0.025 wt.%) to the highest of 24% by GNP 300 (0.1 wt.%). These results show that different concentrations and specific surface areas affect the rate of sedimentation as well as properties, which agree well with the results of previous studies .
Stability investigation with zeta potential
Rheological behavior of GNPs
The results indicate that prepared nanofluids are suitable to use at elevated temperatures. By increasing the temperature, thermal movement of molecules and Brownian motion intensify and intramolecular interactions become weakened. In addition, rheological test on nanofluids revealed that higher concentration increases the viscosity; however, other investigated parameters such as temperature and specific surface areas have an important influence on the viscosity behavior of nanofluids.
Thermal conductivity enhancement of recent nanofluids in literature
Based on the results in Table 2, it is outstandingly evident that GNP nanofluids provide a significant thermal conductivity enhancement compared to those of other works when they have higher concentrations of nanoparticles. From these results, it can be seen that the use of low concentration of GNPs can achieve acceptable thermal conductivity enhancement for medium-temperature applications including solar collectors and heat exchanger systems.
Electrical conductivity analysis
Stability and thermophysical properties of GNP nanofluids have been studied systematically, and the following conclusions could be drawn from the results. The nanofluids of GNPs prepared by ultrasonication were stable for a long period of time. Detailed measurements were carried out to determine the effect of particle mass concentration, specific surface area, and temperature on the thermophysical properties of GNP nanofluid. The rate of increase in thermal conductivity of nanofluids is found to be very significant at higher specific surface area of GNPs due to factors like stability, homogeneity, and rate of agglomeration. The maximum percentage of enhancement in thermal conductivity was obtained at 27.64% for the loading of 0.1 wt.% of GNP 750 at 35°C. The shear rate of nanofluids increased when higher specific surface areas and concentration of GNPs were used. It can be inferred that GNP nanofluids could be a useful and cost-effective material for heat transfer applications along with the development of a facile approach to a large-scale production of aqueous GNP dispersions without any surfactant stabilizers.
b optical path (cm)
c molar concentration (mol/dm3)
GNPs graphene nanoplatelets
I transmitted light intensity
I i incident light intensity
k bf thermal conductivity of base fluid
k nf thermal conductivity of nanofluids
k p thermal conductivity of the particle
TEM transmission electron microscopy; wt.% weight percentage
ϕ particle volumetric fraction
ϵ molar absorptivity, L (mol−1 cm−1)
This research work has been financially supported by High Impact Research (MOHE-HIR) grant UM.C/625/1/HIR/MOHE/ENG/45, IPPP grant PV113/2011A, and Malaysian FRGS national grant FP007/2013A. The author wishes to thank the Bright Sparks unit (University of Malaya) for the additional financial support.
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