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Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 1–5

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

xperimental investigation on the thermal transport properties of ethylene glycolased nanofluids containing low volume concentration diamond nanoparticles

ei Yua, Huaqing Xiea,∗, Yang Lia, Lifei Chena, Qiang Wangb

School of Urban Development and Environmental Engineering Shanghai Second Polytechnic University, Shanghai 201209, ChinaQingdao Copton Petrochemical Co., ltd. Qingdao 266100, China

r t i c l e i n f o

rticle history:eceived 14 June 2010eceived in revised form 5 November 2010ccepted 10 November 2010

a b s t r a c t

Homogeneous and stable ethylene glycol based nanofluids containing low volume concentration dia-mond nanoparticles have been prepared. Diamond nanoparticles, purified and surface modified by themixture acid, consist of the highly defective structure and the active functional groups on the surface.

vailable online 18 November 2010

eywords:anofluidiamond nanoparticle

Ultrasound and the alkalinity of solution are beneficial to the deaggregate of soft diamond particle aggre-gation, and the diameters of purified nanodiamond are changed from 30–50 nm to 5–10 nm. The thermalconductivity enhancement decreases with elapsed time for 1.0 vol.% DNP–EG nanofluid at pH = 7.0. Whilefor the stable nanofluids at pH = 8.5, there is no obvious thermal conductivity decrease within 6 months.The thermal conductivity enhancement values are up to 17.23% for the 1.0 vol.% nanofluid at 30 ◦C.

showith th

hermal conductivityiscosity

Viscosity measurementssignificantly decreases w

. Introduction

The thermal conductivity of heat transfer fluids plays a vital rolen the development of energy-efficient heat transfer equipment [1].ver the past decade, great effort has been made to improve the

nherently poor thermal conductivities of traditional heat trans-er fluids, such as water, oil and ethylene glycol. Choi at Argonneational Laboratory of USA proposed the concept of “nanofluid”

2], and nanofluids have attracted considerable interest becausef the reports of great enhancement of heat transfer [3,4], massransfer [5], wetting and spreading [6]. Therefore nanofluids areonsidered as the next generation heat transfer fluids. Most of thehermal conductivities of nanofluids were measured at the vol-me fraction of nanoparticles from 1 to 5 vol.%, while only a fewxperimental data were reported with low volume concentrations≤1 vol.%). For instance, Choi and co-workers firstly studied thenomalous behavior [7], when they reported a 40% enhancementn the thermal conductivity of ethylene glycol with the addition of.30 vol.% Cu nanoparticles, and a 150% enhancement in the thermalonductivity of a synthetic oil with the addition of 1 vol.% carbonanotubes. The toluene-based nanofluids containing Au has the sig-

ificant enhancement of the effective thermal conductivity (by asuch as 14%) at very low concentration of Au nanoparticles in the

.005–0.011 vol.% [8]. The properties of Al2O3–water nanofluids inhe concentration range of 0.01–0.3 vol.% showed that the viscosity

∗ Corresponding author. Tel.: +86 21 50217331; fax: +86 21 50217331.E-mail address: hqxie@eed.sspu.cn (H. Xie).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.11.020

that the nanofluids demonstrate Newtonian behavior, and the viscositye temperature.

© 2010 Elsevier B.V. All rights reserved.

had a nonlinear relation with the concentration [9], and the thermalconductivities increased nearly linearly with the concentration.

Although the potentials for the applications of nanofluids in awide variety of fields are promising, some stumbling blocks seri-ously hinder the practical developments. The outstanding problemis the long-time stability of nanofluids, which we have to face. Thereare few studies on the long-time stability, and the thermal con-ductivity data of long-term placement nanofluids are lack. Manykinds of nanomaterials are used as the additives of nanofluids,such as some metallic nanoparticles [7,10], metallic oxide particles[11], and TiO2 nanotubes [6]. Among these nanomaterials, carbonmaterial, such as carbon nanotubes [12,13], exfoliated graphite andnanofiber [14], is one of the most promising additives. Diamond isone of the carbon materials, and it possesses the highest thermalconductivity among the nature products, and it can be expectedthat the suspensions containing diamond nanoparticles would haveenhanced thermal conductivity.

To the best of our knowledge, the thermal transport propertiesof nanofluids containing diamond nanoparticles have not well beeninvestigated [15,16]. In the present work, we propose a method toproduce homogeneous and stable ethylene glycol based nanoflu-ids containing diamond nanoparticles (DNP–EG nanofluids). Theeffects of pH, settlement time, volume fraction and long-term sta-bility on thermal conductivity have been investigated in detail.

2. Experimental

The diamond nanoparticles were pursued from Beijing GrishHitech Co. Ltd., and the density was 3.3 g/cm3. Before dispersed

2 Physicochem. Eng. Aspects 380 (2011) 1–5

imhoawpificon3flIZitshcdwaapstpwtstecmotTcwsSBws

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0 1000 2000 3000 40000

20

40

60

80

100

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1104

1260

1759

2928

Tra

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itta

nce

(%)

3431

cavitation in the liquid medium and it is in favor of the dispersionof diamond nanoparticles in EG.

For some nanofluids, pH values of the suspensions have directeffects on the thermal conductivity enhancement. Xie et al. [4] mea-

-46

-44

-42

-40

-38

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W. Yu et al. / Colloids and Surfaces A:

n ethylene glycol, diamond particles were purified and surfaceodified by the mixture acid of perchloric acid, nitric acid and

ydrochloric acid according to the literature [17]. The preparationf DNP–EG nanofluids was carried out using a sensitive mass bal-nce with an accuracy of 0.1 mg. The volume fraction of the powderas calculated from the weight of dry powder using the densityrovided by supplier and the total volume of suspension. The typ-

cal procedure of preparing DNP–EG nanofluids is as follows: Thexed quality of diamond nanoparticles with different volume con-entrations (�: 0.25–1%) was dispersed in ethylene glycol. The pHf nanofluid was adjusted by NaOH ethylene glycol solution. Theanofluid was stirred and ultrasonic sonicated continuously forh. This ensured uniform dispersion of nanoparticles in the baseuid. FT-IR spectra were recorded with a Bruker Equinox V70 FT-

R spectrometer in dry KBr pullet in the range of 400–4000 cm−1.eta potential measurements were conducted on the Zetasizernstrument of Malvern Instruments. The size and morphology ofhe diamond nanoparticles were examined using field-emissioncanning electron microscopy (Hitachi S4800). A transient shortot-wire (SHW) technique was applied to measure the thermalonductivities of the nanofluids at 30 ◦C [18]. In the thermal con-uctivity measurements, a platinum wire with a diameter of 70 �mas used for the hot wire, and it served both as a heating unit and as

n electrical resistance thermometer. Pt wire was coated with a thinlumina layer for insulation using sputtering apparatus. Initially thelatinum wire immersed in media was kept at equilibrium with theurroundings. When a regulation voltage was supplied to initiatehe measurement, the electrical resistance of the wire changed pro-ortionally with the rise in temperature. The thermal conductivityas calculated from the slope of the rise in the wire’s tempera-

ure against the logarithmic time interval. In addition to hot-wireystem, a temperature-controlled bath was used to maintain theemperature of nanofluids during the measurement process. Thexperimental apparatus was calibrated by measuring the thermalonductivity of ethylene glycol, and the accuracy of these measure-ents was estimated to be around ±1%. The thermal conductivity

f the fluid was measured after the nanofluid was settled for morehan 30 min to ensure the temperature equilibrium of nanofluids.he rheological property of nanofluids was measured by a vis-ometer (LV DV-II+ Brookfield programmable viscometer, America)ith a temperature-controlled bath. Viscosity measurements were

tarted at 60 ◦C, and temperature was gradually reduced to 10 ◦C.pindle SC-18 was used in this viscometer and was calibrated usingrookfield viscosity standard fluids. All the viscosity measurementsere recorded at steady state conditions, and the time of nanofluids

ample in the sample chamber was 10 min.

. Results and discussion

Diamond nanoparticles were synthesized by the detonationf explosives, and the original product is a mixture of diamondanoparticles, carbon, micrographite, carbon black and metallic

mpurities. The mixture acid of perchloric acid, nitric acid andydrochloric acid was used to purify the diamond nanoparticles.he prepared and purified diamond nanoparticles consist of theighly defective structure and an active surface. The IR spectrumf diamond nanoparticles is shown in Fig. 1. The strongest band,431 cm−1, is mainly the OH stretching vibration of absorbed water.he bond at 2928 cm−1 is assigned to the �sC–H. The band 1759 cm−1

s the typical characteristic of �C O. 1632 cm−1 is the bending mode

f hydroxyl group. 1260 and 1104 cm−1 may be assigned to the �C–On epoxy structure and ıO–H. The IR spectral analysis indicates thathe surface modification method lets diamond nanoparticles possesome functional groups, which may be beneficial to the dispersionn the base liquid.

Wavelength ( cm-1)

Fig. 1. IR spectrum of the purified and modified diamond nanoparticles.

The stability of colloid can be evaluated through the zeta poten-tial. Suspensions that have a measured zeta potential above 30 mVor below −30 mV are considered stable because these particles willpresumably maintain their repulsive forces while dispersed. Fig. 2illustrates the zeta potential (mV) under various pH conditions. Forthe neutral DNP–EG nanofluids, which is not adjusted by NaOH,the zeta potential is −35.7 mV, while the diamond nanoparticlescannot be dispersed in the base fluid well, and the suspension isnot very stable. When pH is above 7.5, the zeta potential is below−42 mV, and the suspension is stable, therefore the alkalinity ofsolution is helpful to the dispersion and stability of the nanofluids.The DNP–EG nanofluids have good long-term stability when pH isabove 8.5, and there is no obvious sedimentation within 6 months,which is vital for the application of nanofluids.

The diameters of purified diamond nanoparticles are 30–50 nm(Fig. 3a), while the diamond nanoparticles dispersed in EG by ultra-sound and adjusting the pH of solution are only 5–10 nm, indicatingthat ultrasound and the alkalinity of solution are beneficial to thedeaggregate of the soft diamond particle aggregation (Fig. 3b andc). The rich carboxyl and hydroxyl groups on the surface of dia-mond nanoparticles let them have good compatibility with polar EGsolution at alkaline environment. Ultrasound will produce acoustic

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5-48

pH

Fig. 2. Zeta potential (mV) under various pH conditions.

W. Yu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 1–5 3

F diamond dispersed in EG at pH = 8.5 (b) and TEM photo of the nanodiamond dispersed inE

sTwocfndetataiditoa8atact1ndKfBi

FD

0 60 120 18014

15

16

17

( kef

f - k

f) /

k f (%

)

ig. 3. SEM photo of the purified nanodiamond particles (a), SEM photo of the nanoG at pH = 8.5 (c).

ured the thermal conductivity of Al2O3 nanoparticle suspensions.he results show that the enhanced thermal conductivity increasesith an increase in the difference between the pH value of aque-

us suspension and the isoelectric point of Al2O3 particle. Wensel’sonclusion [19] shows that the thermal conductivity of heat trans-er nanofluids containing metal oxide nanoparticles and carbonanotubes with very low percentage loading (around 0.02 wt.%)ecreases when the pH is shifted from 7 to 11.45 under the influ-nce of a strong outside magnetic field. Fig. 4 shows the change ofhermal conductivity enhancement of 1.0 vol.% DNP–EG nanofluidst 30 ◦C with different pHs. In this paper, keff and kf represent thehermal conductivity of the nanofluid and base fluid, respectively,nd (keff − kf)/kf is the thermal conductivity enhancement ratio. �s the volume fraction of DNPs. It can be seen that the thermal con-uctivity ratio increases as pH increases from 7.0 to 8.0. When pH

s above 8.0, there is no obvious relationship between pH and thehermal conductivity enhancement. In our opinion, the influencef pH value on thermal conductivity is because that pH value hasdirect effect on the stability of nanofluids. When pH is below

.5, the suspension is not very stable, and diamond nanoparticlesre easy to form aggregation. The alkalinity of solution is helpfulo the dispersion and stability of nanofluid. In order to verify thebove conclusion, the influence of settlement time on the thermalonductivity enhancement is studied (Fig. 5). It is found that thehermal conductivity enhancement decreases with elapsed time for.0 vol.% DNP–EG nanofluid at pH = 7.0. While for the stable DNP–EGanofluids at pH = 8.5, there is no obvious thermal conductivity

ecrease for 6 months (Fig. 6). This is identical to the conclusion ofim’s [3]. He found that the thermal conductivity decreased rapidly

or the instable nanofluids without surfactants after preparation.ut no obvious changes in the thermal conductivity of the nanoflu-

ds with sodium dodecyl sulfate (SDS) as surfactant was observed

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.510

15

20

25

(kef

f - k

f) /

k f (%

)

pH

ig. 4. The influence of pH on the thermal conductivity enhancement of 1.0 vol.%NP–EG nanofluids at 30 ◦C.

Settlement time (minutes)

Fig. 5. The influence of settlement time on the thermal conductivity enhancementfor 1.0 vol.% DNP–EG nanofluid at pH = 7.0.

even after 5 h settlement. It is clear that the thermal conductivityreflects the stability of nanofluid to some extent.

Fig. 7 depicts the thermal conductivity enhancements ofDNP–EG nanofluids as a function of loading at 30 ◦C. The thermalconductivity enhanced ratio is 2.6% for 0.1 vol.% ND–EG nanoflu-ids, and there is a sudden change for the thermal conductivity

enhanced ratio when the concentration is over 0.25 vol.%. Thenanofluids show a nonlinear relation between the thermal conduc-tivity enhanced ratio and the volume fraction of nanoparticles. Itdisplays the similar trend with the Fe based nanofluids [20]. This

0 1 2 3 4 5 612

14

16

18

20

( kef

f - k

f)/k

f(%

)

Settlement time (months)

Fig. 6. The influence of long term settlement on the thermal conductivity enhance-ment for 1.0 vol.% DNP–EG nanofluid at pH = 8.5.

4 W. Yu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 1–5

0.00 0.25 0.50 0.75 1.000

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Fig. 8. Shear stress versus shear rate for 1 vol.% DNP–EG nanofluid at 30 ◦C.

0 10 20 30 40 50 60 700

10

20

30

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1 vol.% ND-EG Pure EG

Vis

cosi

ty (

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Volume fraction (%)

ig. 7. The thermal conductivity enhancements of DNP–EG nanofluids as a functionf loading at 30 ◦C.

onlinearity may be attributed to the increase of cluster size. Thelustering of nanoparticles occurs more actively in a higher volumeraction of nanoparticles, the average cluster size increases rapidlys the concentration increases, which affects the thermal conduc-ivity. The cluster is soft cluster, not hard cluster, so the real volumeraction of diamond nanoparticles is much larger than the solid vol-me fraction. Diamond nanoparticle is a good additive to enhancehe thermal conductivity of the base fluid, and when the loading isnly 1.0 vol.%, the enhancement ratio is up to 17.23%, much largerhan those containing metallic oxide [21]. For the metallic oxideuch as ZnO [22], CuO [23] Al2O3 [24], or Fe2O3 [25], the thermalonductivity enhanced ratios with 1.0 vol.% are not more than 8%.he reason for the high thermal conductivity of DNP–EG nanofluidsies in following factors. First, diamond itself has the high thermalonductivity, and this is the intrinsic reason; second, the size of dia-ond nanoparticle is only 5–10 nm, and it has large surface area;

hird, the rich carboxyl and hydroxyl groups on the surface of dia-ond nanoparticles let them have good compatibility with the base

uid, which will reduce the contact resistance significantly. Theseactors will help improve the effective thermal conductivity of theanofluids.

Viscosity is related to molecular momentum transport, andetermining the viscosity of the nanofluid is necessary to calculatehe pumping power. Therefore it is necessary to study the rheo-ogical behavior of nanofluids. While there are some debates about

hether the nanofluids are Newtonian or non-Newtonian fluids,he analysis of Kabelac and Kuhnke [26] shows that the viscositiesf Al2O3 nanoparticle suspensions decrease with the shear rates.n the contrary, Prasher’s results demonstrate that the viscositiesf nanofluids of Al2O3–propylene glycol are independent of shearates, indicating that nanofluids are Newtonian fluids in nature [27].n order to investigate the rheological behavior, whether DNP–EGanofluid is Newtonian or non-Newtonian fluid should be verifiedrstly. The equation governing Newtonian behavior of a fluid isiven by

= �� (1)

here � is the shear stress, � is the coefficient of viscosity, andis the shear strain rate. The shear stress versus shear rate for

.0 vol.% ND–EG nanofluid at 30 ◦C is shown in Fig. 8. The linearelation between shear stress and shear rate shows that ND–EGanofluids demonstrate Newtonian behavior. The viscosity of EGan be measured by the Brookfield programmable viscometer withigh precision. The measured viscosity of EG at 25 ◦C (16.1 cp) is

T ( C)

Fig. 9. Viscosity of DNP–EG nanofluids as a function of temperature for 1 vol.%DNP–EG nanofluid.

consistent with the literature value [28]. Fig. 9 shows the viscosityof 1.0 vol.% ND–EG nanofluids as a function of temperature. Withthe increase of temperature, the viscosity of nanofluids decreasesrapidly. The reason of viscosity’s decrease with the increase intemperature is the weakening effect on the inner-particle/inter-molecular forces [29].

4. Conclusions

The thermal transport properties of ethylene glycol basednanofluids containing low volume concentration diamondnanoparticles were investigated. In order to obtain homogeneousand stable DNP–EG nanofluids, diamond nanoparticles should bepurified and surface modified by the mixture acid, which let thempossess the rich carboxyl and hydroxyl groups on the surface ofdiamond nanoparticles, and diamond nanoparticles have goodcompatibility with polar EG solution in alkaline environment. TheDNP–EG nanofluids have good long-term stability when pH isabove 8.5, and there is no obvious sedimentation within 6 months,

which is vital for the application of nanofluids. Ultrasound andthe alkalinity of solution are beneficial to the deaggregate of thesoft diamond particle aggregation, and the diameters of purifieddiamond nanoparticles are changed from 30–50 nm to 5–10 nm.

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W. Yu et al. / Colloids and Surfaces A:

he thermal conductivity enhancement is correlative with the pHalues of the suspensions, because pH value has a direct effect onhe stability of nanofluids. The thermal conductivity ratio increasess pH increases from 7.0 to 8.0. When pH is above 8.0, there is nobvious relationship between pH and the thermal conductivitynhancement. It is clear that the thermal conductivity reflects thetability of nanofluid to some extent. The thermal conductivitynhancement values 1.0 vol.% DNP–EG nanofluids is up to 17.23%t 30 ◦C, much larger than those containing metallic oxide. Vis-osity measurements show that ND–EG nanofluids demonstrateewtonian behavior, and the viscosity significantly decreases with

emperature.

cknowledgements

The work was supported by Program for New Century Excellentalents in University (NECT-10-883) and the Program for Professorf Special Appointment (Eastern Scholar) at Shanghai Institutionsf Higher Learning.

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