Thermal conductivity of polyethylene glycolnanofluidscontaining carboncoatedmetalnanoparticles

Haiyan Zhang, a)Qiguang Wu, Jin Lin, Jin Chen and Zhuowen Xu

Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou510006, 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. An27% 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 havebest stability and dispersibility and can efficiently enhance the thermal conductivity of base fluid. Moreover,nanofluids stabilities are different whennanofluids were dispersedby 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 athigh temperature if the heat transfer efficiency is enhanced.In 1995, Choi et al 1investigated a new class of heat transfer fluids named nanofluidsin whichnanometer-sized solid metallic or nonmetallic oxide particles, rods or tubes wasadded 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 5vol. %. CalvinH.L.7 reported a similar 28% increase in the thermal conductivity for a loading of 6vol. % of Al2O3 (36~47nm) nanoparticles in water. Recently, carbon nanotubes (CNTs) have attracted many researchers because of its super high thermal conductivity of2000~3000W/m·K.8-9 Thethermal conductivity of carbon nanomaterials based nanofluids is much higher than present inorganic filler. 5,10By dispersing MWCNT in polyisobutylene with ultrasonic dispersion method, Ying Yang et al11 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 obtainedin nanofluids on the whole. The nanofluids loading CNTs of higher thermal conductivity are obtained depend on strict dispersionconduction. Whilethat heat transfer occurs through the interface,carbon nanoparticleswhich are the spherical shape and have larger specific surface area compared with CNTsmay be more favorable for heat transfer and can enhance effectively the thermal 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 nanoparticlefocus on the magnetic property ofmaterial13-15, 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 andthe analytically pure glycerin is selected as dispersion agent, while the fillers, carbon coated Cu, Al and Fe nanoparticles aresynthesized 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 dispersedby ball milling, ultrasonic dispersion ormagnetic 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 flowchart of nanofluids usingdifferent 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 10seconds. Mica probe with2.001mmin 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 ofnanofluids was obtained. The measurement value was attained by taking the average value of several measurements. Each time interval of measurement was usually 15minutes for making the temperature distribution of nanofluids uniform and stable.

Ⅲ. RESULTS AND DISCUSSION

  1. TEMimageof the fillerof 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 henceenhances the thermal conductivity of composite materials.

FIG. 2. TEM micrograph of carbon coated Al nanoparticles.

  1. 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 loadingsof 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 daysandno 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 14daysof 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 isshortened with the increasing of fillernanoparticles. 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 inour 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) andcarbon coated Al nanofluid (c).

  1. 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 contain1% 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 thefigure that centrifugal sedimentation time increases when dispersion agent increases in loading from 0 to1%. 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 loadingincreasefrom 1% to 2%, centrifugal sedimentation time show adecline trend and finally reduce to 50min, 65min and 70min for carbon coated Fe, Al and Cu nanofluids at dispersion agentloading of 2%, respectively. Since the dispersion agent could limit the agglomeration of nanoparticles and thus make nanofluids hard to agglomerate and sedimentate.16But 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 almostthe same.

FIG.5. Effect ofdifferent contents of dispersion agent on the stability of carbon coated Cu nanofluid (a), carbon coated Fe nanofluid (b) and carbon coated Al nanofluid (c).

  1. 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 outfor 30min at room temperature, whileball 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 showsno obvious sedimentation.

FIG.6. Stabilities of nanofluidsprepared 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 byball 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.

  1. 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 nanoparticlesare showed in Fig.7. The curve (a) represents carbon coated Cu nanofluid without glycerindispersion agent and curve (b) represents carbon coated Cu nanofluid with glycerin dispersion agent loading of 1%.Whileλ0 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 nanofluidswith0%~ 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 inmicroscale heat transfer and micro convection between particles and base fluid.17The 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 agenthas 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.18By adding small amount of dispersion agent in nanofluidsthe 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

  1. 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 5vol.% in water, ethylene glycol and engine oil nanofluids prepared by Huaqing Xie et al.6

FIG. 8. Thermal conductivities of carbon coated Al nanofluidswith 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 Alnanofluid with 1% loading of glycerin dispersion agent.

  1. 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 glycerindispersion 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.Whilethe thermal conductivity of nanofluid increases only slightly by adding dispersion agent in fluid becauseCu(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 andthe stabilityand thermal conductivity of nanofluidare 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 wrappingaroundmetal 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. A27% 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 sinceCu(Al,Fe) nanoparticles coated by carbon already have better dispersibilityin 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.