Thermal conductivity of polyethylene glycol nanofluids containing
carbon coated metal nanoparticles
Haiyan Zhang, a) Qiguang Wu, Jin Lin, Jin Chen and Zhuowen Xu
Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou 510006,
People’s Republic of China
Abstract: Polyethylene glycol nanofluids containing carbon coated Cu (Al, Fe)
nanoparticles with core-shell structure are prepared in this paper. High efficiency on heat
transfer is achieved by collaborative effect between nanocarbon and metal nanoparticles.
The results show that the thermal conductivity of nanofluid is relevant to the type of filler
and increases with the increasing of filler loading. An 27% enhancement in thermal
conductivity is observed by adding only 0.5wt% of carbon coated Cu nanoparticles in
polyethylene glycol, and 49%, 40% and 30% enhancement in thermal conductivity for
carbon coated Cu, Al and Fe nanoparticles loading of 1.5wt%, respectively. The carbon
coated metal nanofluid exhibit better stability after 14 days and no obvious sedimentation is
observed. Compared with carbon coated Al and Fe nanoparticles, carbon coated Cu
nanoparticles nanofluids have best stability and dispersibility and can efficiently enhance
the thermal conductivity of base fluid. Moreover, nanofluids stabilities are different when
nanofluids were dispersed by three ways. Nanofluids dispersed by ball milling way possess
the best stability, followed by nanofluids dispersed by ultrasonic dispersion way and
magnetic stirring way.
Keywords: Nanofluids, thermal conductivity, carbon coated metal nanoparticles,
polyethylene glycol
Ⅰ. INTRODUCTION
Heat transfer fluids have become more and more important and can be seen in a wide range of industrial
applications such as energy, chemical industry, automobile, construction, microelectronics, and so on.
Microelectronic devices, for example, can work more steadily and reliably even at high temperature if the heat
transfer efficiency is enhanced. In 1995, Choi et al 1 investigated a new class of heat transfer fluids named
nanofluids in which nanometer-sized solid metallic or nonmetallic oxide particles, rods or tubes was added in
traditional heat transfer fluids. Compared with micron-sized particles, nanoparticles have huge advantage in
heat transfer. Firstly, nanoparticles possess relatively larger specific surface area by which can significantly
improve heat transfer capabilities. For instance, the specific surface area of particle with diameter of 10nm is
1000 times larger than that of particles with diameter of 10μm. Secondly, since the nanoparticles are very small
in size, and thus chance of sedimentation is less, which makes nanofluids more stable. Over the last decade,
nanofluids of high thermal conductivity have attracted great interest in the development of energy-efficient heat
transfer domain due to their potential benefits 2-5. Huaqing Xie 6 prepared the nanofluids by dispersing Al2O3
nanoparticles of different size in water, ethylene glycol and engine oil and found a 30% increase in the thermal
conductivity for a filler loading of 5 vol. %. CalvinH.L.7 reported a similar 28% increase in the thermal
conductivity for a loading of 6 vol. % of Al2O3 (36~47nm) nanoparticles in water. Recently, carbon nanotubes
(CNTs) have attracted many researchers because of its super high thermal conductivity of 2000~3000
W/m·K.8-9 The thermal conductivity of carbon nanomaterials based nanofluids is much higher than present
inorganic filler. 5,10 By dispersing MWCNT in polyisobutylene with ultrasonic dispersion method, Ying Yang et
al 11 found a 30% enhancement in thermal conductivity of MWCNTs/polyisobutylene-based nanofluids for
MWCNTs mass fraction of 0.5%. They also observed that the more uniform MWCNTs being dispersed in
nanofluids, the higher thermal conductivity the nanofluids would possess. Huaqing Xie et al 12 prepared
nanofluids by dispersing CNTs, which had hydroxyl groups on the surface by concentrated nitric treatment, in
the water and ethylene glycol. They noticed the thermal conductivity of ethylene glycol nanofluids was
increased by 20% for only a small loading of 1 vol. % CNTs. However, the thermal conductivity of CNTs in
axial direction is much higher than that of horizontal direction and the high theoretical thermal conductivity in
axial direction still hard to be obtained in nanofluids on the whole. The nanofluids loading CNTs of higher
thermal conductivity are obtained depend on strict dispersion conduction. While that heat transfer occurs
through the interface, carbon nanoparticles which are the spherical shape and have larger specific surface area
compared with CNTs may be more favorable for heat transfer and can enhance effectively the thermal
a)Corresponding
author: hyzhang@gdut.edu.cn
conductivity of base fluids.
As a kind of carbon-metal compound with typical core-shell structure, carbon coated metal nanoparticle
possesses an outer layer of nanocarbon and a core of metal nanoparticle and it combines the good performances
of single-component materials including carbon and copper or aluminum, and can be provided with other
favorable performances such as good corrosion-resistance, oxidative stability, dispersibility and light weight.
While current correlative reports about carbon coated Fe and Ni nanoparticle focus on the magnetic property of
material13-15, very few reports take carbon coated metal nanoparticles as a kind of thermal conductivity filler and
study their thermal properties. In this study, different kinds of stable nanofluids are produced by dispersing
different carbon coated metal nanoparticles into polyethylene glycol, and the affecting factors of stability and
thermal conductivity for these new types of nanofluids are also investigated.
Ⅱ. EXPERIMENT
The analytically pure polyethylene glycol is selected as the base fluid and the analytically pure glycerin is
selected as dispersion agent, while the fillers, carbon coated Cu, Al and Fe nanoparticles are synthesized by
carbon arc discharge method in the lab. In brief, cathode was pure graphite rod, while anode was mixture
powder of different proportion Fe (Cu, Al) powder (micron-size) and graphite powder (micron-size). High
purity argon gas is as protection gas and the pressure was 50 kPa. The arc-discharge was generated by DC
current of 150A and voltage of 22V. After a rapid vaporization at high temperature and the subsequent cooling,
carbon coated metal nanoparticles was produced in the chamber.
Nanofluids are prepared by adding nanoparticles in polyethylene glycol with different proportion and then
dispersed by ball milling, ultrasonic dispersion or magnetic stirring. The thermal conductivity of nanofluids is
test by Hotdisk thermal conductivity analyzer at room temperature. The main methods currently used to
evaluate the stability of disperse system include sedimentation method, size observation method, Zeta potential
method, spectrophotometer method and viscosity method et al. While sedimentation method can be divided into
gravitational sedimentation method and centrifugal sedimentation method, both of them are adopted in our
study to characterize the stability of nanofluids. (1) Gravitational sedimentation: By putting the nanofluids into
test tube (Φ20×200 mm) and regularly recording sedimentation time, the stability of nanofluids can be
evaluated. (2) Centrifugal sedimentation: By rotating the tube containing nanofluids at high speed (10000r/min),
sedimentation would be easier and quicker since the centrifugal force is larger than the gravity, and the stability
of nanofluids can be evaluated. Fig. 1 shows the flow diagram of nanofluids preparation.
FIG. 1. Preparation flow chart of nanofluids using different dispersing methods.
The thermal conductivity of nanofluids is tested by Hotdisk thermal conductivity analyzer at room
temperature. Hotdisk probe was immersed into the nanofluids, output power was 0. 02W and test time was for
10 seconds. Mica probe with 2.001mm in diameter was employed in this experiment. The resistance change of
probe was recorded with the test time, so the relationship between temperature and test time was established
according to the resistance and the thermal conductivity of nanofluids was obtained. The measurement value
was attained by taking the average value of several measurements. Each time interval of measurement was
usually 15 minutes for making the temperature distribution of nanofluids uniform and stable.
Ⅲ. RESULTS AND DISCUSSION
A. TEM image of the filler of carbon coated metal nanoparticles
Fig. 2 shows a TEM image of carbon coated Al nanoparticles prepared with anode containing 60wt% of Al.
Because of the core-shell structure, the outer carbon layer of carbon coated metal nanoparticles not only can
prevent the metal core from being oxidized, but also is well compatible with the polymer and that makes carbon
coated metal nanoparticles could be well dispersed in polymer, which hence enhances the thermal conductivity
of composite materials.
FIG. 2. TEM micrograph of carbon coated Al nanoparticles.
B. Influence of the filler and its concentration on stability of polyethylene glycol based nanofluids
It is the precondition for nanofluids practical application that nanofluids should be stable and no
sedimentation in a certain period of time. Sedimentation properties of polyethylene glycol based nanofluids
containing carbon coated Cu, Al and Fe with filler loadings of 0.1~1.5% are respectively showed in Fig. 3. It
can be seen from the three photographs that carbon coated metal nanofluid exhibit better stability after 14 days
and no obvious sedimentation is observed. As for carbon coated Al nanofluid, when at low concentration (i.e.
0.1~1%) of nanoparticles, it also has good stability, only when the concentration of carbon coated Al
nanoparticles reaches 1.5%, few nanoparticle agglomeration appears at the tube bottom, so its stability is not
better than that of carbon coated Cu nanofluid and carbon coated Fe nanofluid.
FIG. 3. Sedimentation properties after 14 days of carbon coated Cu nanofluids (a), carbon coated Fe nanofluids (b)
and carbon coated Al nanofluids (c), each with filler loading of 0.1%, 0.5%, 1%, 1.5%, respectively.
Fig. 4 shows the centrifugal sedimentation time of three kinds of nanofluids with different loadings of
carbon coated Cu, Al and Fe nanoparticles. All three curves indicate that centrifugal sedimentation time of
nanofluids is shortened with the increasing of filler nanoparticles. The centrifugal sedimentation time of carbon
coated Al nanofluid and carbon coated Fe nanofluid have the same value of 70 min when both at filler loading
of 0.1%. As the loading of carbon coated Al nanoparticles increases to 1.5%, sedimentation time is shortened to
40min. While the sedimentation time of carbon coated Fe nanofluid remain the same of 50min for the filler
concentration changes from 1% to 1.5%. It can also be concluded from Fig. 4 that carbon coated Cu nanofluid
possesses the best stability in our experiments, with sedimentation time of 80min for filler loading of 0.1% and
60min for 1.5%, respectively.
FIG. 4. Centrifugal sedimentation time of carbon coated Cu nanofluid (a), carbon coated Fe nanofluid (b) and
carbon coated Al nanofluid (c).
C. Influence of dispersion agent on stability of polyethylene glycol based nanofluids
To improve the stability of nanofluids, glycerin of different proportions i.e. 0.5%, 1%, 1.5% and 2%, used
as dispersion agent in our study, is added into nanofluids which contain 1% of carbon coated Cu, Fe and Al,
respectively. The relationship between centrifugal sedimentation time and its correlative glycerin concentration
is showed in Fig. 5. It can be seen from the figure that centrifugal sedimentation time increases when dispersion
agent increases in loading from 0 to 1%. At dispersion agent loading of 1%, the centrifugal sedimentation time
of carbon coated Cu, Al and Fe nanofluids increase from 60min to 85min, 52min to 80min and 45min to 60min,
respectively. When dispersion agent loading increase from 1% to 2%, centrifugal sedimentation time show a
decline trend and finally reduce to 50min, 65min and 70min for carbon coated Fe, Al and Cu nanofluids at
dispersion agent loading of 2%, respectively. Since the dispersion agent could limit the agglomeration of
nanoparticles and thus make nanofluids hard to agglomerate and sedimentate.16 But too much dispersion agent
will influence nanofluid stability. The experimental result indicates an optimal value for dispersion agent to
improve the nanofluid stability and the best stability can be obtained when the concentration of dispersion agent
and filler are almost the same.
FIG. 5. Effect of different contents of dispersion agent on the stability of carbon coated Cu nanofluid (a), carbon
coated Fe nanofluid (b) and carbon coated Al nanofluid (c).
D. Influence of dispersing way on stability of polyethylene glycol based nanofluids
The nanofluids with filler loading of 1% are prepared by different dispersing ways such as magnetic stirring,
ultrasonic dispersion and ball milling. Magnetic stirring and ultrasonic dispersion were carried out for 30min at
room temperature, while ball milling was carried out for 4 h at the speed of 300 r/min. It can be seen from the
Fig. 6 that nanofluid dispersing by magnetic stirring and ultrasonic dispersion method shows poor stability and
a certain amount of sedimentation appear at the test tube bottom after 14 days. However the ball milling
nanofluid exhibits fine stability and shows no obvious sedimentation.
FIG. 6. Stabilities of nanofluids prepared by magnetic stirring (a), ultrasonic dispersion (b) and ball milling (c).
So it can be concluded from the experimental result that ball milling nanofluid possesses the best stability,
followed by ultrasonic dispersion nanofluid and magnetic stirring nanofluid. By using the whirlpool results
from magnetic rotor circumrotating in fluid to disperse the nanoparticles, the force acting on particles is
relatively weak as compared with the force produced by ball milling. The most stable nanofluid can be prepared
by ball milling way is mainly due to the high shear force produced by random movement of grinding ball.
E. Thermal conductivity of polyethylene glycol based nanofluid
1. Thermal conductivity of carbon coated Cu nanofluid
Thermal conductivities of carbon coated Cu nanofluids with different concentration of carbon coated Cu
nanoparticles are showed in Fig. 7. The curve (a) represents carbon coated Cu nanofluid without glycerin
dispersion agent and curve (b) represents carbon coated Cu nanofluid with glycerin dispersion agent loading of
1%. While λ0 represents the thermal conductivity of pure polyethylene glycol and λ represents the thermal
conductivity of carbon coated Cu - polyethylene glycol based nanofluid.
FIG. 7. Thermal conductivities of carbon coated Cu nanofluids with 0%~ 1.5% concentrations of carbon coated
Cu nanoparticles. The curve (a) represents carbon coated Cu nanofluid without dispersion agent and curve (b)
represents carbon coated Cu nanofluid with 1% loading of glycerin dispersion agent.
As we can see from the curve (a) that the thermal conductivity of carbon coated Cu - polyethylene glycol
based nanofluid increases with the increasing of filler loading. 23% and 46% enhancement in thermal
conductivity are observed for carbon coated Cu concentration of 0.5% and 1.5%, respectively. The
improvement in thermal conductivity with the increasing of filler attributes to the enhancement in microscale
heat transfer and micro convection between particles and base fluid.17 The main carrier for heat transfer in
nanofluid is nanoparticles. Heat transfer occurs between particle and particle, as well as particles and fluid
which are mainly by conduction. Heat transfer ability of nanofluids are the results both the heat conduction and
convection. The higher the concentration of particles, the greater the probability of collision between particle
and particle, the faster energy transfer. Therefore, the thermal conductivity of nanofluids increases with the
increasing of particle concentration. It can be seen from the curve (b) that at a certain loading of carbon coated
Cu nanoparticles, nanofluid containing dispersion agent has a higher thermal conductivity than that without
dispersion agent. For instance, at 0.5% loading of carbon coated Cu nanoparticles, the thermal conductivities of
nanofluids with and without dispersion agent are increased by 23% and 27%, respectively. Nanofluid with
dispersion agent shows a 4% increase in thermal conductivity when compared with the nanofluid without
dispersion agent. The enhancement in thermal conductivity of nanofluid with dispersion agent even reaches
49% for 1.5% loading of carbon coated Cu nanoparticles. While CuO/ ethylene glycol nanofluids showed only
enhancement of 20% at 4 vol.% of nanoparticles reported by Lee et al.18 By adding small amount of dispersion
agent in nanofluids the filler particles can be distributed more uniformly in nanofluids and the aggregation
between filler particles is limited, which help nanoparticles to do Brownian motion in nanofluids, and is
beneficial to heat energy transferred by dispersive nanoparticles as carriers.19
2. Thermal conductivity of carbon coated Al nanofluid
The thermal conductivities of carbon coated Al nanofluids with different concentration of carbon coated Al
nanoparticles are showed in Fig. 8, where the curve (a) represents nanofluid without dispersion agent and (b)
represents nanofluid with glycerin loading of 1%. The curve A indicates that thermal conductivity of carbon
coated Al nanofluid increases with the increasing of filler loading, which is similar to that of carbon coated Cu
nanofluid. At filler loading of 0.5% and 1.5%, the thermal conductivities of nanofluids are increased by 23%
and 37% respectively, which are slightly lower than that of carbon coated Cu nanofluids. Also, as we can see
from curve (b) that adding dispersion agent in fluid can improve the thermal conductivity of nanofluid and a
40% increase in thermal conductivity can be observed for filler loading of 1.5%. It is better comparing with
30% increase in the thermal conductivity for Al2O3 filler loading of 5 vol.% in water, ethylene glycol and
engine oil nanofluids prepared by Huaqing Xie et al.6
FIG. 8. Thermal conductivities of carbon coated Al nanofluids with 0%~1.5% concentration of carbon coated Al
nanoparticles. The curve (a) represents carbon coated Al nanofluid without dispersion agent and curve (b)
represents carbon coated Al nanofluid with 1% loading of glycerin dispersion agent.
3. Thermal conductivity of carbon coated Fe nanofluid
Fig. 9 shows the thermal conductivities of carbon coated Fe nanofluids with different concentration of
carbon coated Fe nanoparticles. With a filler loading of 1.5%, the thermal conductivity of carbon coated Fe
nanofluid is increased by 28% and will be further increased up to 30% by adding glycerin dispersion agent in
nanofluid. We can notice from Fig. 7 to Fig. 9 that there is a similar trend in high enhancement of thermal
conductivity by adding filler particles in fluid. While the thermal conductivity of nanofluid increases only
slightly by adding dispersion agent in fluid because Cu (Al, Fe) nanoparticles coated by carbon already have
better dispersibility before adding dispersion agent. Heat transfer occurs in the particle interface, carbon coated
metal nanoparticles possess the interaction between nano-carbon and nano-metal in which carbon layer can
better consistent with not only polymer fluid but also metal. These result a high increase in thermal conductivity
of nanofluid. Meanwhile, the thermal conductivity of Cu is higher than that of Al and Fe. Therefore, thermal
conductivity of carbon coated Cu nanofluid is higher than that of carbon coated Al and Fe nanofluids.
FIG. 9. Thermal conductivities of carbon coated Fe nanofluids with 0%~ 1.5% concentrations of carbon coated
Fe nanoparticles. The curve (a) represents carbon coated Fe nanofluid without dispersion agent and curve (b)
represents carbon coated Fe nanofluid with 1% loading of glycerin dispersion agent.
Ⅳ. CONCLUSION
Polyethylene glycol based nanofluids containing carbon coated Cu (Al, Fe) nanoparticles with core-shell
structure are prepared and the stability and thermal conductivity of nanofluid are investigated. The stabilities of
nanofluids produced by different dispersing ways are different from each other. Nanofluid dispersed by ball
milling way exhibits the best stability, followed by nanofluids dispersed by ultrasonic dispersion way and
magnetic stirring way. Carbon coated metal nanoparticles possess unique nanostructure with nanocarbon
wrapping around metal nanoparticles surface. By the collaborative optimization effect between nanocarbon and
metal nanoparticles, these composite particles combine good performances of single-component materials, thus
the high efficiency heat transfer can be achieved. The thermal conductivity of nanofluid is relevant to the type
of filler particles and increases with the increasing of filler loading. A 27% enhancement in thermal
conductivity is observed by dispersing only 0.5wt% of carbon coated Cu nanoparticles in polyethylene glycol,
and high increase of 49%, 40% and 30% in thermal conductivity are achieved with 1.5wt% loading of carbon
coated Cu, Al and Fe nanoparticles, respectively. By adding dispersion agent in the nanofluid, the thermal
conductivity could slightly increase since Cu (Al, Fe) nanoparticles coated by carbon already have better
dispersibility in fluid even without any dispersion agent. Compared with carbon coated Al and Fe nanoparticles,
carbon coated Cu nanoparticles seem to be more compatible with polyethylene glycol in terms of stability and
dispersibility and can efficiently enhance the thermal conductivity of base fluids.
ACKNOWLEDGEMENTS
This work is supported by the National Natural Science Foundation of China (Grant No. 20971027), by the
Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.
20094420110005), by the Guangdong Provincial Natural Science Foundation of China (Grant No.
9251009001000006), and by the Science and Technology Program of Guangdong Province of China (Grant No.
2009A030301008, No.2009B090300017).
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