See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/233148417 Stability and thermal conductivity enhancement of carbon nanotube (CNT) nanofluid using gum Arabic ARTICLE in JOURNAL OF EXPERIMENTAL NANOSCIENCE · DECEMBER 2011 Impact Factor: 0.98 · DOI: 10.1080/17458080.2010.487229 CITATIONS 25 READS 265 7 AUTHORS, INCLUDING: Iis Sopyan International Islamic University M… 153 PUBLICATIONS 1,384 CITATIONS SEE PROFILE Mohammad Khalid The University of Nottingham Mal… 34 PUBLICATIONS 221 CITATIONS SEE PROFILE Available from: Rashmi W Retrieved on: 14 January 2016
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Stability and thermal conductivityenhancement of carbon nanotubenanofluid using gum arabicW. Rashmi a b , A.F. Ismail a c , I. Sopyan a d , A.T. Jameel a b , F.Yusof a b , M. Khalid a b & N.M. Mubarak a ba Nanoscience and Nanotechnology Research Group (NANORG),International Islamic University Malaysia, P.O. Box 10, 50728 KualaLumpur, Malaysiab Department of Biotechnology Engineering, International IslamicUniversity Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysiac Department of Mechanical Engineering, International IslamicUniversity Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysiad Department of Manufacturing and Materials Engineering,International Islamic University Malaysia, P.O. Box 10, 50728 KualaLumpur, MalaysiaVersion of record first published: 27 Apr 2011.
To cite this article: W. Rashmi , A.F. Ismail , I. Sopyan , A.T. Jameel , F. Yusof , M. Khalid & N.M.Mubarak (2011): Stability and thermal conductivity enhancement of carbon nanotube nanofluidusing gum arabic, Journal of Experimental Nanoscience, 6:6, 567-579
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Journal of Experimental NanoscienceVol. 6, No. 6, December 2011, 567–579
Stability and thermal conductivity enhancement of carbon nanotube
nanofluid using gum arabic
W. Rashmiab*, A.F. Ismailac, I. Sopyanad, A.T. Jameelab, F. Yusof ab, M. Khalidab
and N.M. Mubarakab
aNanoscience and Nanotechnology Research Group (NANORG), International Islamic UniversityMalaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia; bDepartment of BiotechnologyEngineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur,Malaysia; cDepartment of Mechanical Engineering, International Islamic University Malaysia,P.O. Box 10, 50728 Kuala Lumpur, Malaysia; dDepartment of Manufacturing and MaterialsEngineering, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur,Malaysia
(Received 19 January 2010; final version received 8 April 2010)
This experimental study reports on the stability and thermal conductivityenhancement of carbon nanotubes (CNTs) nanofluids with and without gumarabic (GA). The stability of CNT in the presence of GA dispersant in water issystematically investigated by taking into account the combined effect of variousparameters, such as sonication time, temperature, dispersant and particleconcentration. The concentrations of CNT and GA have been varied from 0.01to 0.1wt% and from 0.25 to 5wt%, respectively, and the sonication time has beenvaried in between 1 and 24 h. The stability of nanofluid is measured in termsof CNT concentration as a function of sediment time using UV-Visspectrophotometer. Thermal conductivity of CNT nanofluids is measuredusing KD-2 prothermal conductivity meter from 25 to 60�C. Optimum GAconcentration is obtained for the entire range of CNT concentration and1–2.5wt% of GA is found to be sufficient to stabilise all CNT range in water.Rapid sedimentation of CNTs is observed at higher GA concentration andsonication time. CNT in aqueous suspensions show strong tendency toaggregation and networking into clusters. Stability and thermal conductivityenhancement of CNT nanofluids have been presented to provide a heat transportmedium capable of achieving high heat conductivity. Increase in CNTconcentrations resulted in the non-linear thermal conductivity enhancement.More than 100–250% enhancement in thermal conductivity is observed forthe range of CNT concentration and temperature.
Keywords: carbon nanotubes; gum arabic; nanofluids; stability; enhancedthermal conductivity; temperature
Conventional fluids, such as water and ethylene glycol are commonly used as the heattransfer fluids in many of the industrial applications. The low heat transfer propertiesof such fluids obstruct the performance of the heat transfer equipments. The importantkey parameter to improve the heat transfer of such fluids is to enhance their thermalconductivity. Over the years [1], solid particles of millimetre or micrometre size have beenused to enhance the thermal conductivity of convention fluids. However, it could notbe used in practical application due to sedimentation, erosion, fouling and increasedpressure drop of the flow channel. These problems were further overcome after the recentadvancement in material science and nanotechnology. Conventional base fluids suspendedwith nano-sized particles are known as ‘Nanofluids’. Knowing the fact that solids havehigher thermal conductivity compared to liquids, the suspended particles can changethe transport and thermal properties of the base fluids. Various types of nanoparticles(metals and oxides), such as aluminium oxide (Al2O3), titanium oxide (TiO2), copper oxide(CuO), copper (Cu), silver (Ag), gold (Au), silica nanoparticles and carbon nanotubes(CNT) have been used [1–2] to enhance the thermal characteristics of the base fluids(water, ethylene glycol, propylene glycol, acetone, transformer oil, etc.). Special require-ments for preparation of nanofluids include low sedimentation, even and durablesuspension, stable, low agglomeration and no chemical change [2]. The thermalconductivity of nanofluids containing CNT was measured by Choi et al. [3] and Xieet al. [4] for different base fluids. Choi et al. [3] measured the thermal conductivity ofCNT–oil mixture at room temperature. They found that the thermal conductivityenhancement ratio was more than 2.5 at approximately 1 vol.% of nanotube concentra-tion. Similar results were observed by Xie et al. [4], who dispersed CNT in distilled water,ethylene glycol and decene without the addition of surfactant. They reportedthat nanofluids containing a small amount of CNTs had significantly higher thermalconductivities than their base liquids.
Due to their excellent electrical [5], mechanical [6] and optical properties [7], CNTs(discovered by Ijima [8]) have gained more importance in the enhancement of thermalcharacteristics of fluids. CNTs, however, cause self-aggregation due to attractive van derWaals forces and high surface area. Thus, the dispersion of CNTs has become a challengeto maximise their properties for potential applications. It must be emphasised here thatCNT morphological structure does not have any side (or functional) groups. Therefore,CNTs have a major problem of insolubility in most of the solvents as they cannot interactwith the surrounding solvent to overcome the large intertube van der Waals interactions.The quality of dispersion is, generally, characterised by considering particle size distri-bution, homogeneous spatial distribution and prolonged stability. Exfoliation of CNTsfrom agglomerates into individual tubes depends on the process parameters and thedispersion method chosen. Many attempts [9–14] have been made to solubilise anddisperse CNTs in suitable solvents to broaden the applications of CNTs in the field ofnanotechnology, purification and manipulation. Dispersion methods can be roughlyclassified as chemical and physical methods. Chemical modification involves formation ofcovalent bonds and adding functional groups (–COOH) on the surface of CNT.Functionalisation of CNTs has been carried out by oxidation process under extensivesonication with mixture of sulphuric acid and nitric acid [15]. Physical methods includeaddition of variety of dispersants, surfactants, polymers and aromatic compounds [16–21].
When surfactants are added to CNT dispersions, surfactant molecules adsorb at theinterface followed by self-accumulation into supramolecular structures, which helps CNT
dispersion retain a stable colloidal state. Coulombic or hydrophobic attraction plays a keyrole in achieving stable colloidal systems in ionic or non-ionic surfactants, respectively[22–24]. In general, there are three basic principles for dispersing fine particles in water[25]: (1) repulsion between the particles with their zeta potentials, (2) steric hindrance ofthe adsorption layer and (3) reduction of hydrophobic linkages among dispersed particles.
In the past research, no study has been carried out on the stability of nanofluids for awide range of CNT concentration and dispersant concentration. In this article, aqueous
dispersion of CNTs in the presence of gum arabic (GA) was prepared, which is stableand homogeneous in nature. The concentrations of CNT and GA are varied in the range0.01–0.1wt% and 0.25–5wt%, respectively. Concentration of the CNT is measured asa function of sediment time using UV-Vis spectrophotometer. Effects of CNT
concentration, GA concentration and sonication time is studied to analyse the stabilityof homogeneous dispersion. In addition, the morphology of dispersed CNTs has also beenobserved using scanning electron microscopy (SEM).
2. Experimental procedure
2.1. Materials
Multi-walled carbon nanotubes (MWCNTs) having an average outer diameter of 20 nm,length of 30 mm and purity of495% were obtained from Chinese Academy of Science,China. The transmission electron microscope (TEM) and SEM images of these MWCNTs
are shown in Figures 1 and 2, respectively. GA was obtained from the Gum ArabicCompany, Sudan. Distilled water was used in all studies.
Samples were prepared in the universal bottles. A measured amount of CNT was addedto GA solutions making a total weight of 20 g. The samples were homogenised usinghomogeniser (Fluko, Germany) at 28,000 rpm for 10m. They were further sonicated undermild conditions using water bath sonicator with a bath temperature of 25�C.During sonication, CNTs are gradually exfoliated and distangled from aggregatesand further stabilised by GA. After sonication, the samples were transferred to thecuvette for measurement of concentration using UV-Vis spectrophotometer (Perkin Elmerlambda 35).
2.3. Stability analysis
In this article, the stability characteristics of CNTs (0.01–0.1wt%) in water suspensionwere studied with and without GA. The stability of the dispersion was determinedby measuring the variation of supernatant concentration of the suspensions with thesediment time. UV-Vis spectroscopy is widely used in evaluating the concentration of thesamples. In this, the concentration measurement is based on Beer–Lambert’s law, whichstates that the absorbance is directly proportional to the concentration of the componentin the solution. This spectroscopy method has been used in this study as well. UV-Visabsorbance curve for CNT–water was obtained and UV-Vis spectra appeared at 384 nm asshown in Figure 3. Before proceeding with the measurement of the concentration, a linearcalibration curve was constructed at a wavelength of 384 nm for CNT dispersions withoutGA in the range 0.01–0.1wt%, as shown in Figure 4. To measure the concentration of GAabsorbed in CNT dispersions, the same sample of GA aqueous solution was used asa reference to eliminate the absorbance of GA in the suspensions. Thus, standardcurves were determined for all the concentrations of CNT with and without GA usingUV-Vis spectrophotometer.
Figure 2. Field emission scanning electron microscope micrograph of as-received MWCNT.
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2.4. SEM of CNT adsorbed by GA
In this typical experiment, CNT–GA suspensions of known concentration were dried at100�C in an oven for 24 h and observed under SEM and TEM for their morphology.
2.5. Thermal conductivity analysis
The thermal conductivity of the nanofluids was measured using portable thermalconductivity meter KD2 Pro (Decagon, USA). The thermal conductivity meter was
Figure 4. Linear relationship between light absorption and CNT concentration at 384 nm.
Figure 3. UV-Vis absorption spectrum of CNT in water.
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calibrated using standard glycerine. The transient line source (TLS) uses a sensor(single needle with 60mm in length) to measure the thermal conductivity. A measurementcycle consists of 30 s of each equilibration, heating and cooling time. Temperaturemeasurements are made at the intervals of 1 s during both heating and cooling.Measurements are then fit with exponential integral functions using a non-linear leastsquares procedure. A linear drift term corrects for temperature changes of the sampleduring the measurement, to optimise the accuracy of the readings. To study the effectof temperature, a thermostat bath was used which maintained the temperature ofthe nanofluids. Nearly three readings were taken at each temperature to ensure uncertaintyin measurement with in �5%.
3. Results and discussion
In this section, effect of sonication time and GA concentration on stability of CNTnanofluids is reported as a function of sediment time. Thermal conductivity enhancementof CNT nanofluids is presented at different GA and CNT concentrations.
3.1. Effect of sonication on stability
Figures 5 and 6 show the effect of sonication time on stability of CNT nanofluid for CNTconcentration of 0.01 and 0.1wt% with GA concentration of 1 and 2.5wt%, respectively.
From the above figures, it is evident that there is an optimum sonication time at whichthe CNT nanofluids are found to be more stable. The CNT suspension is observed to bemore stable for 4 h sonication time irrespective of GA concentration. Decrease in stabilitywith sonication time can be explained on the basis that CNT structure gets damagedincluding bending, buckling and dislocations in the carbon structure. However, prolongedsonication increases disorder, reduces nanotube length and ultimately leads to theformation of amorphous carbon and, thus, its properties are not fully utilised [26].
Figure 5. Effect of sonication time on stability of CNT suspension. CNT¼ 0.01wt% andGA¼ 1wt%.
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Another possibility is that the sonication not only helps in dispersing the CNTs in water,but it also decomposes the dispersant to form new species, structure or complex and thusincreasing the sedimentation rate.
3.2. Effect of GA
Figures 7–11 show the effect of GA concentration on the stability of CNT dispersions.GA concentration is varied from 0.25 to 5wt%. The results are plotted for the optimumsonication time of 4 h at which time CNTs are more stable (refer to Figures 5 and 6).
Figures (7–11) demonstrate that in the presence of GA, CNTs are well-dispersed inwater and a slight sedimentation is observed. Optimum values of GA concentration(Table 1) were obtained for all the CNT concentrations used. Further, as the GAconcentration increased beyond the optimum level, a decrease in stability was observeddue to the self-aggregation effect of GA molecules.
It is further observed that the suspension stability also decreases with increasing CNTconcentration, which is due to the formation of dense solution, as it is a well-known factthat dense particles sediment faster compared to less dense particles.
4. Thermal conductivity enhancement of CNT nanofluids
The roles of CNT and GA concentrations on the thermal conductivity enhancement ofCNT nanofluids are reported in this section. Effective thermal conductivity is the thermalconductivity of the nanofluid, and the percentage enhancement is calculated as follows:
% Enhancement ¼knanofluid � kbasefluid
kbasefluid� 100: ð1Þ
Figure 6. Effect of sonication time on stability of CNT suspension. CNT¼ 0.1wt% andGA¼ 2.5wt%.
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4.1. Effect of CNT concentration
Effects of CNT concentration and temperature have been studied on effective thermalconductivity of CNT nanofluid.
Figure 12 shows that effective thermal conductivity increases with increasing CNTconcentration and temperature. Our results show much higher values of effective thermal
Figure 7. Effect of GA concentration on stability of CNT¼ 0.01wt%.
Figure 8. Effect of GA concentration on stability of CNT¼ 0.02wt%.
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conductivity compared to the literature values, due to the formation of stable homoge-
neous suspensions. There is a gradual increase in the effective thermal conductivity from
CNT concentration of 0.01–0.08wt% and further a sudden increase is observed at CNT
concentration of 0.1wt%. However, the exact mechanism is not yet known for this
non-linear variation of effective thermal conductivity with CNT concentration. Percentage
enhancement in effective thermal conductivity is observed in Figure 13. More than
Figure 10. Effect of GA concentration on stability of CNT¼ 0.08wt%.
Figure 9. Effect of GA concentration on stability of CNT¼ 0.04wt%.
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100–250% enhancement is observed for the CNT concentration and temperature rangestudied.
4.2. Characterisation
Figure 14 shows the SEM monographs of CNT-GA dispersion. The structures of CNT areshown for 0%, 1% and 5% GA, respectively. The figures were taken after 24 h sonicationof the samples. With an addition of GA, CNTs are further adsorbed with GA, whichhinders the CNTs to cluster and agglomerate and make them remain suspended in thesolution. Further addition of GA to 5wt% covers the entire CNTs and the large GAmolecules interact with each other. The large molecular weight makes the CNTs to settlealong with GA and this explains the decrease in stability of suspension with increase in GAconcentration from 1 to 5wt%, respectively.
Figure 11. Effect of GA concentration on stability of CNT¼ 0.1wt%.
Table 1. Optimum values of GA concentration.
CNT concentration(wt%)
OptimumGA concentration
(wt%)
0.01 1.00.02 1.00.04 1.50.08 2.50.1 2.5
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5. Conclusions
Stability and thermal conductivity analysis of CNT nanofluids using GA has beenreported in this article. The nanotubes dispersed in water show a strong tendency toaggregation and networking into clusters in the absence of GA. Optimum GAconcentration ranges from 1 to 2.5wt% and 4 h sonication. UV-Vis spectrophotometerwas successfully used to study the sedimentation of CNT nanofluid. The effective thermalconductivity of aqueous CNT nanofluid was measured accurately at different CNTs andGA concentrations, respectively. The results show an increase in effective thermalconductivity with the increase in CNT concentration and there was no effect of GA onthermal conductivity enhancement of CNT nanofluid.
Figure 12. Effect of CNT concentration on effective thermal conductivity on CNT nanofluid (atoptimum GA values) as a function of temperature.
Figure 13. Percentage enhancement in effective thermal conductivity as a function of temperature.
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