110-319-1-PB

Department ofMetallurgical & MaterialsEngineering, IndianInstitute of Technology,Kharagpur 721302, [email protected]

REVIEWS

Synthesis,CharacterizationandApplicationofNanouidAnOverview

Indranil Manna

Abstract | Nanouids are quasi single phase medium containing stable colloidal dispersionof ultrane or nanometric metallic or ceramic particles in a given uid. Despite almost a

negligible concentration (< 1 vol%) of the solid dispersoid, nanouids register an

extraordinarily high level of thermal conductivity, which largely depends on identity

(composition), amount (volume percent), size and shape of the dispersoid and viscosity,

density and related thermo-physical parameters of the base uid. Nanouids possess

immense potential of application to improve heat transfer and energy efciency in several

areas including vehicular cooling in transportation, power generation, defense, nuclear,

space, microelectronics and biomedical devices. In the present contribution, a brief overview

has been presented to provide an update on the historical evolution of this concept,

possible synthesis routes, level of improvements reported, theoretical understanding of the

possible mechanism of heat conduction by nanouid and scopes of application. The

overview is supplemented with a summary of recent results from the authors own group to

highlight certain simple approaches of synthesis and extent of enhancements achieved with

those indigenous efforts. The biggest motivation for exploration and exploitation of

nanouid should come from the fact that the degree of consistently attained enhancement

of thermal conductivity far exceeds the level predicted by the existing theory on the subject.

1. IntroductionNotwithstanding the publicity hype, nanotech-nology has already or is soon likely to usher inseveral revolutionary changes that can signicantlyimprove device performance, communicationtechnology, sensor applications, drug deliveryand several area of practical importance.Nanotechnology is considered to be one of thesignicant forces that could drive the next majorindustrial revolution of the century. The primaryapproach of nanotechnology is to manipulate thestructure at the molecular or atomic aggregate

level with the goal of achieving desired change inproperty with unprecedented precision.

Though exploits of nanotechnology mostlyconcerns engineering solids either for functional(electronic, magnetic, optical, catalytic, etc.)or structural (strength, hardness, wear/abrasionresistance, etc.) applications, the concept ofnanouid is rather new. About a decade back,researchers in Argonne National Laboratory, USAnoticed that the uid used for collecting nanometricalumina (Al2O3) particles synthesized by chemicalvapor deposition showed usually large thermal

Journal of the Indian Institute of Science VOL 89:1 JanMar 2009 journal.library.iisc.ernet.in 21

REVIEW Indranil Manna

conductivity. Successful reproduction of this uidwith ultra ne particle dispersion in very smallquantity and subsequent realization that the degreeof increase in thermal conductivity was far abovethe level expected from the rule of average, createdenough ripples to catch the imagination of scienticcommunity and evoke a wide spread interest insynthesizing new type of nanouids, exploringnew areas of application and proposing plausibletheories to explain the signicant increase in thermalproperties.

Ecient transfer of energy in the form of heat,from one body to another is often required in almostall industries. Thermal and nuclear power plant,refrigeration and air conditioning system, chemicaland processing plants, electronic devices, spaceshuttles and rocket-launching vehicles, satellitesare a few to name where the productivity as well assafety depends on ecient transfer of heat. Often auid is chosen as a medium for transferring heat andaccordingly the mode of heat transfer is convection.The rate of heat transfer in convection is given by anapparently simple looking relationship; popularlyknown as Newtons law of cooling.

q = hAT

where the q is the rate of heat transfer, h is co-ecient of convective heat transfer, A is the surfacearea and T is the temperature dierence acrosswhich the transfer of thermal energy take place. Ithas been always the pursuit of the thermal engineersto maximize q for given T or A. This can bedone by increasing h. However, this is easier saidthan done. Heat transfer coecient is a complexfunction of the uid property, velocity and surfacegeometry. Out of dierent uid properties, thermalconductivity inuences the heat transfer coecientin the most direct way as this is the property thatdetermines the thermal transport at the micro-scalelevel.

It is well known that metals in solid formhave much higher thermal conductivity than thatof uids. Heat transfer by conduction throughsolid is orders of magnitude larger than thatby convection/conduction through a uid. Forexample, the thermal conductivity of copper atroom temperature is about 700 times greater thanthat of water and about 3000 times greater thanthat of engine oil [1]. Therefore, uids containingsuspended solid particles are expected to displaysignicantly enhanced thermal conductivitiesrelative to those of conventional heat transferuids [2]. In fact, numerous studies about theeective thermal conductivity of uids that containsolid particles in suspension have been conducted.

But these studies have been limited to thesuspensions of millimeter or micrometer sizedparticles. However, there is no way to preventsolid particles from settling down from suspensionwhen they are not in use/circulation. Moreover,the suspended particles pose the problem ofmaintenance by causing erosion of the prime moverand fouling of the heat transfer surface. Last butnot least, the penalty of excess pressure drop oftendestroys the benet of heat transfer augmentation. Itis therefore, always benecial to have the suspendedparticles in as small a size as possible. Apart fromthe other points discussed above, the small particleshave a larger surface to volume ratio that favors thetransport of thermal energy more eciently.

Modern nanotechnology provides greatopportunities to process and produce materialswith average crystallite size below 50 nm. As alreadystated, the concept of nanouid evolved in 1995 asan oshoot of synthesis of nanoparticles by chemicalvapor deposition [1, 3]. These uids with nanometersized particle suspension in traditional heat transferuid oered signicantly better thermal propertiesrelative to those of conventional heat transfer uidsand liquids with no or micrometer-sized particles.The ratio of surface area to volume is 1000 timesgreater for particles with a 10 nm diameter thanthat for particles with 10 m diameter. The muchlarger surface areas of nanoparticles relative to thoseof micro/macro-sized particles should not onlyimprove heat transfer capabilities, but also increasethe stability of the suspensions. These nanouidshave an unprecedented combination of the twofeatures most highly desired for thermal engineeringapplications: extreme stability and high thermalconductivity.

Thus, nanouid is a new class of heat transferuid that utilizes dispersion of ne scale metallicparticles in a heat transport liquid in appropriatesize and volume fraction to derive a signicantenhancement in the eective heat transfer coecientof the mixture. In comparison to dispersingmicron-size ceramic particles, nanouids consist ofsuspension of ultra-ne or nanometric metallicparticles with much smaller size and volumefraction, and yet oer a remarkably higher eciencyof heat transport.

2. Evolution of NanouidConvective heat transfer can be enhanced passivelyby changing the ow geometry, boundaryconditions, or by enhancing thermal conductivityof the uid. It is obvious from a survey ofthermal properties that all liquid coolants usedtoday as heat transfer uids exhibit rather poorthermal conductivity compared to solid metals.

22 Journal of the Indian Institute of Science VOL 89:1 JanMar 2009 journal.library.iisc.ernet.in

Synthesis, Characterization and Application of Nanouid An Overview REVIEW

Table 1: Thermal conductivities of various solids and liquids.

Material Thermal conductivity (W/m-K)

Metallic solids Copper 401

Aluminum 237

Nonmetallic solids Silicon 148

Alumina (Al2O3) 40

Metallic liquids Sodium (644 K) 72.3

Nonmetallic liquids Water 0.613

Ethylene glycol (EG) 0.253

Engine oil (EO) 0.145

Supplementary eorts to increase heat transfercoecient by agitation, increasing area, or addingsolid dispersoids can achieve limited improvementif the thermal conductivity of the uid itself islow. Thus, it is logical that eorts are made toincrease the thermal conduction behavior of thecooling uid itself. Earlier eorts have been madeto increase the thermal conductivity of base uidsby suspending micro/macro-sized solid particlesin uids since the thermal conductivity of solid istypically 23 orders of magnitude higher than thatof liquids (Table 1). However, adding micrometersize particles cause several problems arising out ofsedimentation, clogging, pressure drop and erosionof channels/pipes/conduits.

That dispersion of solid enhances thermalconductivity or heat transfer coecient of a uidwas known for ages. Maxwell [4] was a pioneerin this area who presented a theoretical basis forcalculating the eective thermal conductivity ofsuspension. His eorts were followed by numeroustheoretical and experimental studies, such asthose by Hamilton and Crosser [5] and Waspet al. [6]. These models work very well in predictingthe thermal conductivity of slurries. However,nanouid evoked particular interest principallybecause the enhancement achieved by dispersingonly 12 vol% particles was far too greater than thatanticipated by the rule of average. Furthermore, thenumber density of particles was insignicant if heatconduction was by physical collision or momentumtransfer.

Modern materials technology provided theopportunity to produce nanometer-sized particleswhich are quite dierent from the parent material(coarse grained) in mechanical, thermal, electrical,and optical properties. Though every uid possessesnanometric molecular chains and hence can becalled nanouid, the real justication of the namenanouid comes from the fact that nanouid

is characterized by stable colloidal dispersion ofultrane or nanometric solids in extremely smallquantity (< 1 vol%) that is present together with thebase uid to form a pseudo-single phase mediumwith phenomenally greater thermal conductivitythan that of the base uid. It must be kept in mindthat biologists have been using the term nanouidfor dierent types of particles, such as DNA, RNA,proteins, or uids contained in nanopores [7].

Nanouids possess a unique combination ofthe two most essential features desired in thermalengineering applications, namely, chemical andphysical stability and high thermal conductivity.The attractive features which made nanoparticlesprobable candidates for suspension in uids are thelarge specic surface area, less particle momentumand high mobility. With respect to conductivityenhancement, starting from copper, one can goup to multi-walled carbon nanotubes, which atroom temperature exhibit 20,000 times greaterconductivity than engine oil [8]. When the particlesare properly dispersed, these features of nanouidsare expected to yield several benets like: higherheat conduction due to large specic surface areaand greater mobility (micro-convection) of tinyparticles. It is already found that: (a) the thermalconductivity of nanouids increases signicantlywith a rise in temperature [9], which may beattributed to the above reasons; (b) greater stabilityagainst sedimentation due to smaller size andweight; (c) more ecient heat transfer in micro-channels; (d) negligible friction and erosion ofconduit surfaces.

The main excitement of using nanouid arisesdue to the following features [10]: (a) Enhancementof thermal conductivity far beyond the levelany theory could predict, (b) Dependence ofthermal conductivity on particle size apart fromconcentration, (c) Greater stability of suspensionusing a stabilizing agent [11], and (d) Retention ofNewtonian behavior at small concentration withoutmuch pressure drop.

The above mentioned potentials providedthe thrust to begin research in nanouids, withthe expectation that these uids will play animportant role in developing the next generationof cooling technology. The result can be a highlyconducting and stable nanouid with exciting newerapplications in the future. Before exploring howmany of these dreams have been attained by theearly results, it is necessary to say that this eldof research is interdisciplinary, with inputs fromchemistry, mechanical and chemical engineering,physics, and material science; hence, it is worthgoing into the details of not only applications butalso synthesis and characterization.



3. Synthesis and Preparation of NanouidPreparation of nanouids is the rst key step inexperimental studies with nanouids. Nanouidsare not just dispersion of solid particles in a uid.The essential requirements that a nanouid mustfulll are even and stable suspension, adequatedurability, negligible agglomeration of particles,no chemical change of the particles or uid, etc.Nanouids are produced by dispersing nanometer-scale solid particles into base liquids such aswater, ethylene glycol, oil, etc. In the synthesisof nanouids, agglomeration is a major problem.There are mainly two techniques used to producenanouids: the single-step and the two-step method.

3.1. The Single-step ProcessVarious methods have been tried to producedierent kinds of nanoparticles and nano-suspensions. Gleiter [12] provides a good overviewof the synthesis methods. The initial materialstried for nanouids were oxide particles, primarilybecause they were easy to produce and chemicallystable in solution. Various investigators haveproduced Al2O3 and CuO nanopowder by an inert-gas condensation process [3, 13] that produced2200 nm-sized particles. The major problem withthis method is its tendency to form agglomeratesand its unsuitability to produce pure metallicnanopowders. The problem of agglomerationcan be reduced to a good extent by using adirect evaporation condensation method. Thelatter method is a modication of the inert gascondensation process that has been adopted atArgonne National Laboratory, USA [1]. Even thoughthis method has limitations of low vapor-pressureuids and oxidation of pure metals, it providesexcellent control over particle size and producesparticles for stable nanouids without surfactantor electrostatic stabilizers.

The single-step direct evaporation approachwas developed by Akoh et al. [14] and is calledthe Vacuum Evaporation onto a Running OilSubstrate technique. The original idea of thismethod was to produce nanoparticles, but it wasdicult to subsequently separate the particles fromthe uids to produce dry nanoparticles. Eastmanet al. [15] developed amodied vacuum evaporationonto oil technique, in which Cu vapor is directlycondensed into nanoparticles by contact with aowing low-vapor-pressure liquid ethylene glycol.Zhu et al. [16] presented a novel one-step chemicalmethod for preparing copper nanouids by reducingCuSO45H2O with NaH2PO2H2O in ethyleneglycol under microwave irradiation. Results showedthat addition of NaH2PO2H2O and the adoptionof microwave irradiation are two signicant factors

which aect the reaction rate and properties of Cunanouids. Lo et al. [17] developed a vacuum-basedsubmerged arc nanoparticle synthesis system toprepare CuO, Cu2O, and Cu based nanouids withdierent dielectric liquids. The morphologies ofnanoparticles depended on the thermal conductivityof the dielectric liquids. An advantage of the one-step technique is that nanoparticle agglomerationis minimized, while the disadvantage is that onlylow vapor pressure uids are compatible with sucha process.

3.2. The Two Step ProcessThe two-step method is extensively used in thesynthesis of nanouids considering the availablecommercial nano-powders supplied by severalcompanies. In this method, nanoparticles wererst produced and then dispersed in the baseuids. Generally, ultrasonic equipment is usedto intensively disperse the particles and reducethe agglomeration of particles. For example,Eastman et al. [15], Lee et al. [10], and Wanget al. [13] used this method to produce Al2O3nanouids. Also, Murshed et al. [18] preparedTiO2 suspension in water using the two-stepmethod. Other nanoparticles reported in theliterature are gold (Au), silver (Ag), silica andcarbon nanotubes. As compared to the single-stepmethod, the two-step technique works well foroxide nanoparticles, while it is less successful withmetallic particles. Except for the use of ultrasonicequipment, some other techniques such as controlof pH or addition of surface active agents arealso used to attain stability of the suspensionof the nanouids against sedimentation. Thesemethods change the surface properties of thesuspended particles and thus suppress the tendencyto form particle clusters. It should be noted thatthe selection of surfactants should depend mainlyon the properties of the solutions and particles.For instance, salt and oleic acid as dispersant areknown to enhance the stability of transformer oilCu and waterCu nanouids, respectively [11]. Oleicacid and cetyltrimethylammoniumbromide (CTAB)surfactants were used by Murshed et al. [18] toensure better stability and proper dispersion ofTiO2water nanouids. Sodium dodecyl sulfate(SDS) was used by Hwang et al. [19] duringthe preparation of water-based multi wall carbonnanotube dispersed nanouids since the bers areentangled in the aqueous suspension. In general,methods such as change of pH value, additionof dispersant and ultrasonic vibration aim atchanging the surface properties of suspendedparticles and suppressing formation of particles-cluster to obtain stable suspensions. However, theaddition of dispersants can aect the heat transferperformance of the nanouids, especially at hightemperature i.e. in the convective heat transfer andtwo-phase heat transfer regime.



4. Important Experimental Studies withNanouid

The heat transfer resistance of a owing uid is oftenrepresented by the Nusselt number and Prandtlnumber, which take into account the thermalconductivity of the uid directly and indirectly,respectively. Thus, the rst step to assess the heattransfer potential of a nanouid is to measureits thermal conductivity. To date, majority ofthe research eorts concerning nanouid weredevoted to enhancing thermal conductivity of agiven uid than any other aspect of heat transfer,particularly with regards to application of nanouidfor automotive applications. Table 2 provides achronological summary of selected importantstudies on nanouid that reported signicantenhancement of thermal conductivity.

5. Mechanism of Heat Conduction byNanouids

Besides carrying out experimental studies on theextent of increase in thermal conductivity ofnanouid possible with dierent combination ofbase uid, dispersoids and surfactant, it is equallyimportant to develop theories on mechanism ofheat conduction and genesis of high enhancementof thermal properties of nanouid by appropriatemodeling and simulation exercises.

The conventional understanding of the eectivethermal conductivity of mixtures originates fromcontinuum formulations which typically involveonly the particle size/shape and volume fraction,and assume diusive heat transfer both in uidand solid phases. This method can give a goodprediction for micrometer or lager-size solid/uidsystems (say with macroscopic distribution),but fails to explain the unusual heat transfercharacteristics of nanouids. To explain thereasons for the unusual or anomalous increaseof thermal conductivity in nanouids, Keblinskiet al. [27] and Eastman et al. [28] proposed fourpossible mechanisms e.g. Brownian motion of thenanoparticles, molecular-level layering of the liquidat the liquid/particle interface, anomalous natureof heat transport among the nanoparticles, andnanoparticle clustering. The possible mechanismsare schematically shown in Fig. 1. They postulatedthat the eect of Brownian motion can be ignoredsince contribution of thermal diusion is muchgreater than Brownian diusion. However, they onlyexamined the cases of stationary nanouids.

Wang et al. [13] argued that the thermalconductivities of nanouids should be dependenton the microscopic motion (Brownian motion andinter-particle forces) and particle structure. Xuanand Li [11] also discussed several possible reasons

for the improved eective thermal conductivityof nanouids: the increased surface area due tosuspended nanoparticles, the increased thermalconductivity of the uid, the interaction andcollision among particles, the intensied mixinguctuation and turbulence of the uid, and thedispersion of nanoparticles. Many researchers usedthe concept of liquid/solid interfacial layer toexplain the anomalous improvement of the thermalconductivity in nanouids.

Yu and Choi [29, 30] suggested models basedon conventional theories which consider a liquidmolecular layer around the nanoparticles. However,a study by Xue et al. [31] using molecular dynamicsimulation showed that simple monoatomic liquidshad no eect on the heat transfer characteristicsboth normal and parallel to the surface. Thismeans that thermal transport in layered liquid maynot be adequate to explain the increased thermalconductivity of suspensions of nanoparticles.Khaled and Vafai [32] investigated the eect ofthermal dispersion on heat transfer enhancement ofnanouids. These results showed that the presenceof the dispersive elements in the core region didnot aect the heat transfer rate. However, thecorresponding dispersive elements resulted in 21%improvement of Nusselt number for a uniform tubesupplied by a xed heat ux as compared to theuniform distribution for the dispersive elements.These results provide a possible explanation forthe increased thermal conductivity of nanouidswhich may determined partially by the dispersiveproperties.

Wen and Ding [33, 34] studied the eect ofparticle migration on heat transfer characteristicsin nanouids owing through mini-channels(D = 1 mm) theoretically. They studied the eectof shear-induced and viscosity-gradient-inducedparticle migration and the self-diusion dueto Brownian motion. Their results indicated asignicant non-uniformity in particle concentrationand thermal conductivity over the tube cross-section due to particle migration. As comparedto the uniform distribution of thermal conductivity,the non-uniform distribution caused by particlemigration induced a higher Nusselt number. Kooand Kleinstreuer [35] discussed the eects ofBrownian, thermo-phoretic, and osmo-phoreticmotions on the eective thermal conductivities anddemonstrated that the role of Brownian motion wasmuch more important than the thermo-phoreticand osmo-phoretic motions. Furthermore, theparticle interaction can be neglected when thenanouid concentration is low (< 0.5%). It wasargued that liquid layering, phonon transport, andagglomeration could play more signicant role in



Figure 1: Schematic diagrams of several possible mechanisms (after [27]): (a) Enhancement of k due to formation of highlyconductive layer-liquid structure at liquid/particle interface; (b) Ballistic and diffusive phonon transport in a solid particle;(c) Enhancement of k due to increased effective of highly conducting clusters.

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iviii

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heat transport by nanouid rather than Brownianmotion. However, these ndings have not beenvalidated by experiment yet.

Recently, Evans et al. [36] suggested that thecontribution of Brownian motion to the thermalconductivity of the nanouid is very small andcannot be responsible for the unusual thermaltransport properties of nanouids. They alsosupported their argument using the moleculardynamic simulations and the eective mediumtheory. However, they just limited their discussionto stationary uids, which weakens their results.Lee et al. [37] experimentally investigated theeect of surface charge state of the nanoparticlein suspension on the thermal conductivity. Theyshowed that the pH value of the nanouid stronglyaected the thermal performance of the uid. Withfurther diverged pH value from the iso-electricpoint of the particles, the nanoparticles in thesuspension got more stable so as to change thethermal conductivity. That may partially explain thedisparities between dierent experimental data sincemany researchers used surfactants in nanouids,but with insucient descriptions.

Hence, there is no established mechanismto universally explain the strange behavior ofnanouids including the highly improved eectivethermal conductivity, although many possiblefactors have been considered, including Brownianmotion, liquidsolid interface layer, ballisticphonon transport, and surface charge state.Besides these microscale theories, there are stillsome other possible macro-scale explanationssuch as heat conduction, particle-driven naturalconvection, convection induced by electrophoresis,thermophoresis, etc.

6. Experimental Studies on NanometricAl-alloy and Oxide DispersedNanouids at IIT Kharagpur

It has recently been shown that two stagedevelopment of nanouid by synthesizingnanometric Al-alloy (Al2Cu and Ag2Al) powderparticles by mechanical alloying and subsequentlydispersing the same in water/ethylene glycol couldbe a exible method of producing nanouidwith greater scope of scaling up the processof synthesis [3843]. The performance of thisclass of nanouids critically depends upon thesize and distribution of dispersoids and theirability to remain suspended and chemically un-reacted in the uid. It is suggested that theuniformity and stability of suspension can beensured by maintaining appropriate pH, usingsurface activators or surfactant and employingultrasonic vibration. It may be pointed out that themaximum level of enhancement in conductivityratio achieved by these nano-Al alloy dispersednanouids surpasses the earlier reported degree ofimprovement using other nanoparticles (Cu, CuO,Al2O3) in identical medium. The synthesis routineand relevant results of this development and theirsignicance have been briey discussed below.

Al-30 at % Cu and Al-30 at % Ag dispersednanouids were prepared by a two-step method,in which Al-30 at % Cu and Al-30 at % Ag alloyedparticles were rst produced by mechanical alloyingof elemental powder in appropriate proportionusing a high energy planetary ball mill withWC media at 10:1 ball to powder weight ratio,followed by dispersing these submicron solidparticles into ethylene glycol to produce nanouidswith dierent amount of particle dispersion. The



Table 2: Summary of Landmark Experimental Studies in Nanouid.

Year Nanouid Used Studies Conducted Reference

1993 Al2O3, SiO2 and TiO2 dispersed waterbased nanouid

Nanouid prepared by two step method and temperature effectstudied. 26%, 7% and 11% enhancement in thermal conductivityof water based nanouid by dispersing 4.3 vol% Al2O3, SiO2 and TiO2nanoparticles

Masuda et al. [20]

1999 CuO and Al2O3 nanoparticles dispersedin water and ethylene glycol

20% enhancement in thermal conductivity of ethylene glycol bydispersing 4 vol% CuO nanoparticles

Lee et al. [10]

1999 CuO and Al2O3 nanoparticles dispersed inwater, ethylene glycol and vacuum pumpoil

20% enhancement in thermal conductivity of water by dispersing 3vol% Al2O3 nanoparticles

Wang et al. [13]

2000 Cu nanoparticles dispersed in water Thermal conductivity ratio varies from 1.24 to 1.78 if volume percentof Cu nanoparticles increase from 2.5 to 7.5%

Xuan and Li [11]

2001 Cu nanoparticles dispersed in ethyleneglycol

Effective thermal conductivity of ethylene glycol improved by up to40% through the dispersion on 0.3% Cu nanoparticles

Eastman et al. [1]

2003 Ag and Au nanoparticles dispersed inwater and toluene

0.011 vol% of Au nanoparticles dispersed toluene nanouid showsenhancement in thermal conductivity 7% at 30C and 14% at 60C.

Patel et al. [21]

2003 CuO and Al2O3 nanoparticles dispersedin water (effect of temperature)

4 vol% Al2O3 dispersed water nanouids thermal conductivity raise9.4% to 24.3% with increase in Temperature from 21 to 51 C

Das et al. [9]

2001 Carbon nanotube dispersed in oil Thermal conductivity ratio exceeded 2.5 at 1 volume % nanotube Choi et al. [2]

2003 Carbon nanotube dispersed in distilledwater and ethylene glycol

At 1 volume %, the thermal conductivity enhancements are 12.7%and 7.0% for TCNT in ethylene glycol and distilled water, respectively

Xie et al. [22]

2004 MWCNT (sodium dodecyl sulfate)dispersed water based nanouid

By dispersing 0.6 vol.% carbon nanotube in water 38% thermalconductivity enhancement. Also shown sonication and surfactant effecton thermal conductivity of nanouid.

Assael et al. [23]

2005 Al2O3 nanoparticles dispersed in water(effect of temperature)

Dispersing 1 vol.% Al2O3 , 29% enhancement in water based nanouidat 71C

Chang et al. [24]

2005 Nano-Fe dispersed ethylene glycol basednanouid

Shown sonication effect. 10% enhancement by dispersing 0.55 vol.Fe nanoparticles in ethylene glycol

Hong et al. [25]

2005 TiO2(cetyl trimethyl ammonium bromide)nanoparticle in water

By dispersing 15 nm TiO2sphere (5 vol%) 30% and 1540 nm TiO2 rod(5 vol.%) 33% enhancement in thermal conductivity

Murshed et al. [18]

2006 CuO-water based nanouid Dispersing 1 vol.% CuO, 5% enhancement in water based nanouid Hwang et al. [19]SiO2-water based nanouid Dispersing 1 vol.% SiO2, 3% enhancement in water based nanouid

MWCNT-water based nanouid Dispersing 1 vol.% carbon nanotube, 7% enhancement in water basednanouid

CuO-ethylene glycol based nanouid Dispersing 1 vol.% CuO, 9% enhancement in ethylene glycol basednanouid

MWCNT-mineral oil based nanouid 5 vol% carbon nanotube dispersed mineral oil based nanouid haveshown 9% enhancement in thermal conductivity

2006 A12O3 in water at 27.5C to 37.7C 2 and 10 vol.% of A12O3 in water showed 822% enhancement inthermal conductivity

Li and Peterson [26]

CuO in water at 28.9C to 33.4C 2 and 6 vol% CuO in water showed 3551% enhancement in thermalconductivity

particle and grain sizes of the milled product werevaried by milling up to dierent hours. Millingwas carried out in wet condition (using about50 ml of toluene) to prevent undue oxidation,agglomeration of powders, changing the millingdynamics by smearing and coating the vials andball surfaces with Al/Cu/Ag-powders, and ensuringsucient output (alloyed powder) from the millingoperation. Nano-ceramic dispersed nanouids werealso prepared by suspending spherical 3 mol.%Y2O3 partially-stabilized nanometric ZrO2,Al2O3and TiO2 nanoparticles (Inframat AdvancedMaterials, USA) in water/ethylene glycol. Identity

and particle/crystallite size of the phases weredetermined by X-ray diraction (XRD). Averagecrystallite size and lattice strain were measured byseparating the X-ray peak broadening contributionsdue to the Gaussian and Cauchian factors aftersubtracting the broadening due to strain andinstrumental errors. Bright eld transmissionelectron microscopy (TEM) studies were carriedout to ascertain the size, distributions andmorphology of powder particles in nanouid. Solidparticles were deagglomerated and homogenizedin nanouids by adding an appropriate surfactant (1vol% oleic acid/tetramethyl ammonium hydroxide



Figure 2: (a) Thermal conductivity ratio (ke/kf ) as a function ofnanoparticle volume percent of Al70Cu30 dispersed water and ethyleneglycol based nanouid [4041]; (b) TEM micrograph of mechanicallyalloyed (for 30 h) Al70Cu30 nanoparticles dispersed in ethylene glycol[4041].

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((CH3)4NOH, TMAH) and intensive ultrasonicvibration and magnetic stirring, respectively. Tomeasure the thermal conductivity of nanouids, weused an indigenously developed and improvisedthermal comparator, based on the original conceptof Powell [38]. Pool boiling heat transfer study withnanouid has been conducted using a custom madepool boiling experimental facility [39].

6.1. Al(Cu)-Nanoparticle Dispersed NanouidFig. 2a shows the variation of normalized thermalconductivity ratio of nanouid (to that of the baseuid) as a function of Al70Cu30(Al2Cu) nanoparticlevolume percent. The results indicate that thenanouids containing small volume percent ofnanoparticles have signicantly higher thermalconductivity (up to 2.4 times) than that of the base

liquid without nanoparticles. Fig. 2b shows the sizeand distribution of Al70Cu30(Al2Cu) nanoparticlesin ethylene glycol, as observed in the bright eldTEM image of the powders shed on a polymercoated Cu-grid. It is evident that the particlesare truly nanometric, isolated (not agglomerated)and uniformly distributed. The overall variationis sigmoidal with a nearly linear portion in theintermediate (0.75 to 1.5 vol%) concentration ofparticles. The increase in conductivity ratio is over100% with merely 1.5 vol% particles present inthe liquid. Though conductivity ratio increasesfurther with greater amount of particles, stability ofnanouid is adversely aected due to sedimentationand inhomogeneity beyond 1.5 vol% of particles.Fig 2a also shows that the experimental results aresignicantly greater than the thermal conductivitypredicted by the existing models as a functionof nanoparticle amount [40]. It may be pointedout that synthesis of Al-based nano-dispersoidsby mechanical alloying and dispersing the samein ethylene glycol to prepare nanouid have beenreported in the literature by our group for therst time. Furthermore, the maximum level ofenhancement in conductivity ratio achieved in thisstudy surpasses the earlier reported data in theliterature [40, 41].

6.2. Al(Ag)-Nanoparticle Dispersed NanouidThe same two-stage method has been applied toprepare nanouid comprising a small amount(< 2 vol%) of nanocrystalline Ag2Al nano-particlesdispersed as a stable colloid in ethylene glycol andwater. Thermal conductivity measurements, usingthe same indigenously designed thermal comparatordevice, show that the conductivity of this nanouidis signicantly greater (1.22.4 times) than that ofthe base uid as well as that predicted by the existingmodels. Though conductivity ratio is proportionalto the amount of nanoparticle concentration,loading above 2 vol% seems to adversely aect thestability and performance of nanouid. In general,the increase in conductivity ratio is a function ofidentity/composition, size, volume percent, andthermal property of the nanoparticles or solidsuspension [41, 42]. Ag2Al-water nano-uids showdierent performance and phenomena comparedto pure water in terms of natural convection andnucleate boiling. The addition of Ag2Al nano-particles causes a decrease in nucleate boiling heattransfer. The heat transfer coecient decreases withincrease in particle concentration. However, thepresent results deserve attention from the scienticcommunity both as a source of new data andnew trend that can be useful for modeling andcomparison.



Figure 3: TEM image of (a) uniformly distributed, (b) chain distribution of mechanically alloyed (for 30and 40 h) Al70Ag30 (Ag2Al) nanoparticle dispersed ethylene glycol based nanouid [4041].

(a) (b)

Fig. 3(a) and 3(b) show the size and distributionof nanoparticles in the ethylene glycol, as obtained inbright eld TEM of powder particles collected/shedon carbon coated Cu grid, respectively. It is evidentthat the particles are truly nanometric (2080 nm).

It is already noted that ethylene glycol basednanouids containing small volume percent (< 2vol%) of Ag2Al nanoparticles induce signicantlyhigher thermal conductivity than that of the baseliquid without nanoparticles, (Fig. 4(a) and 4(b)).Fig. 4(a) suggests that enhancement in thermalconductivity follows a linear relationship both

with the particle size and volume percent withinthe present range of investigation. 36 nm Ag2Aldispersed water based nanouid show 59 and 102%enhancement in thermal conductivity for 1 and 2vol% of particles loading, respectively.

6.3. Nano-ceramic Dispersed NanouidA systematic eort has been made to characterizecommercially available (Y2O3 stabilized) ZrO2,TiO2 and Al2O3 nano particles, dispersed in ethyleneglycol and water in very low volume percentfollowing a special routine to prepare a new

Figure 4: Variation of thermal conductivity ratio (ke/kf ) as a function of nanoparticle size. (a) Ag2Al dispersed water basednanouid (b) Ag2Al dispersed ethylene glycol based nanouid [42].

20 40 60 80 100 120

Water+Al 70Ag30 dispersed NF (with different particle size) 1.0 vol % 1.5 vol % 1.8 vol % 2.0 vol %

Ethylene glycol+A l70Ag30 dispersed NF (with different particle size)

1.0 vol %1.5 vol %1.8 vol %2.0 vol %

Ther

mal

con

duct

ivity

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io (

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/kf)

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1.6

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ivity

Rat

io (

k eff

/kf)

2.2

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Particle Size (nm)20 40 60 80 100 120

Particle Size (nm)



type of nanouid and carry out characterizationand thermal conductivity measurement of thisnanouid using an indigenously developed thermalcomparator device [43]. Use of suitable surfactantand stirring routine ensured uniformity and stabilityof dispersion. Nucleate pool boiling of ZrO2 basedaqueous nanouid has also been studied. Thoughenhancement in nucleate boiling heat transfer hasbeen observed at low volume percent of soliddispersion, the rate of heat transfer falls with theincrease in solid concentration and eventuallybecomes inferior even to pure water [39]. Whilesurfactants increase the rate of boiling heat transfer,addition of surfactant to the nanouid shows adrastic deterioration in nucleate boiling heat transfer.Furthermore, the boiling of nanouid rendersthe heating surface smoother. Repeated runs ofexperiments with the same surface give a continuousdecrease in the rate of boiling heat transfer.

Figure 5: (a) Boiling curves of pure water and nano ZrO2 dispersedwater based nanouid without addition of surfactant [39]; (b) TEMmicrographs showing 2530 nm ZrO2 particles dispersed in water [39,43].

1800

1600

1400

1200

1000

800

600

400

200

Hea

t flu

x (kW

/m2 )

0 5 10 15 20 25 30 35 40 45Degree of superheat (C)

WaterWater+ZrO2 (0.005%)Water+ZrO2 (0.01%)Water+ZrO2 (0.02%)Water+ZrO2 (0.05%)Water+ZrO2 (0.07%)Water+ZrO2 (0.15%)

(CHF)

(a)

(b)

Fig. 5a depicts the boiling curves for pure waterand nanouid having dierent vol% of particleconcentration in nucleate boiling regime. It isinteresting to note that at the lowest concentrationof nanoparticle (0.005 vol%) the coecient ofboiling heat transfer in the nucleate boiling regionis substantially higher compared to that observedin pure water. However, there is a steady decreasein the heat transfer coecient with the increasein ZrO2 nanoparticle concentration further. At aconcentration of 0.15 vol% of ZrO2 the boilingcurve falls much below that recorded for purewater. Fig. 5b shows the TEM image of ZrO2 (2030nm, spherical) particles dispersed in water, afterultrasonic vibration and magnetic stirring.

6.4. Summary of AchievementsWe have been able to prepare nanouid by dispersingsmall amount (< 2 vol%) of nanocrystalline Al-30 at. %Cu, Al-30at. %Ag, partially stabilizednanometric ZrO2 (with 3 mol.% Y2O3), TiO2and Al2O3 in deionized water and ethyleneglycol. Use of 12 vol% of oleic acid andtetraethyl ammonium hydroxide as surfactantand appropriate stirring routine preventedagglomeration and ensured uniform dispersionof nanoparticles. Thermal conductivity of nanouid,measured using an indigenously developed thermalcomparator, recorded up to 1.22.0 times (20100%) enhancement over that for concerned baseuid under comparable conditions. In general,thermal conductivity of nanouids increases withvolume percent of nanoparticles (within thepresent limit of 2 vol%). Besides amount, bothspecic size (surface area to volume) and shape(aspect ratio) signicantly inuence the thermalconductivity enhancement of nanouids. The resultsfor TiO2 nanoparticle dispersed nanouid showgood agreement with Murshed et al. [18] datafor TiO2-water based nanouids which indirectlysupport that measurement of thermal conductivityby thermal comparator is reliable and thermalcomparator can serve as an alternate techniquefor transient hot wire method. Nucleating boilingheat transfer of aqueous nanouids with dierentcombinations of nanoparticles has been investigatedcovering a wide range of surface superheat. Boilingcurve has also been constructed for nanouidsstabilized with appropriate surfactant. Some ofobserved results are interesting and are not intotal conformity with those reported earlier. Ingeneral, the results appear quite promising fordevelopment of nanouid for advanced thermalengineering applications. The level of thermalconductivity enhancement is signicantly higherthan that predicted by the existing models andexperimental results reported in the literature.



7. Potential Scope of Application ofNanouid

Nanouids can be used to improve heat transferand energy eciency in a variety of thermalsystems, including the important application ofvehicle cooling or transportation, microelectronics,nuclear power generation, and rocket launchingvehicles. Several studies conducted in academicinstitutions and national laboratories are now readyfor eld trials before commercial exploitation andimplementation could be realized.

8. Concluding RemarksNanouids are engineered colloids made ofa base uid and nanoparticles (1100 nm)that can oer signicant advantage in thermalmanagements of both large installation likeheat exchanger, evaporator, radiators as wellas miniature or micro electronics devices. Theperformance of nanouid critically depends uponthe size, quantity (volume percentage), shapeand distribution of dispersoids, and their abilityto remain suspended and chemically un-reactedin the uid. Despite the exciting opportunities,lack of agreement among experimental results,poor characterization of suspension, inadequatetheoretical understanding of the mechanisms ofheat transfer by nanouid are serious impedimentsagainst large scale commercial exploitation ofnanouids for thermal management in importantelds such as electronics, transportation andthermal engineering. In summary, the nextsteps in the nanouid research cycle are toconcentrate on heat transfer enhancement anddetermine its physical mechanisms, taking intoconsideration such items as the optimum particlesize and shape, particle volume concentration, uidadditive, particle coating, and base uid. Precisemeasurement and documentation of the degreeand scope of enhancement of thermal propertiesis extremely important. Better characterizationof nanouids is also important for developingengineering designs based on the work of multipleresearch groups, and fundamental theory to guidethis eort should be improved. Important featuresfor commercialization must be addressed, includingparticle settling, particle agglomeration, surfaceerosion, and large-scale nanouid production atacceptable cost. Finally, it is pertinent to suggestthat nanouid research warrants a genuinely multi-disciplinary approach with complementary eortsfrommaterial scientists (regarding synthesis andcharacterization), thermal engineers (for measuringthermal conductivity and heat transfer coecientunder various regimes and conditions), chemists (tostudy the agglomeration behavior and stability ofthe dispersoid and liquid) and physicists (modelingthe mechanism and interpretation of results).

AcknowledgementThe author is indebted to his students and colleaguesassociated with nanouid research in his group(Prof P K Das, Mr Manoj Chopkar, Ms GayatriPaul, Mr S Sudarshan, Mr V Vaikunthanathan).Most of the experimental results reported hereare from the PhD thesis of Mr Manoj Chopkar.Partial nancial support from ISAC-ISRO, IGCAR-Kalpakkam and DST-NSTI (Grant no. SR/S5/NM-04/2005) is gratefully acknowledged.

Received 02 October 2008; revised 09 November 2008.

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Professor Indranil Manna (born on22.01.1961) received his Ph. D degreefrom the Indian Institute of Technology(IIT), Kharagpur in 1990. Earlier, heobtained his bachelors (B. E.) andmasters (M. Tech.) degree from CalcuttaUniversity and IIT-Kanpur in 1983 and1984, respectively. Prof. Manna workedat the Mishra Dhatu Nigam, Hyderabad(an integrated superalloys plant) before

joining IIT-Kharagpur in 1985. Since then, he has been at thesame Institute as Lecturer (19851990), Assistant Professor (19901997), Associate Professor (19972003) and a Full Professor(2003present). In between, he worked as guest scientist indierent renowned Institutions and Universities abroad likeMax Planck Institute at Stuttgart, Technical University ofClausthal, Nanyang Technological University, Liverpool University,University of Ulm for periods up to one year (on leavefrom IIT-Kharagpur). Prof. Manna teaches courses on phasetransformation, surface engineering, X-ray diraction, etc. Hisresearch interest concerns structure-property correlation inengineering materials including synthesis/application of nano-materials, surface coating/engineering, phase transition, oxidefuel cell, bainitic steel and modeling. His current activities onamorphous/nanocrystalline Al-alloys, nano-uid and laser/plasmaassisted surface engineering have evoked wide interest in thescientic community. He has published about 200 peer-reviewedpapers, supervised 9 Ph.D, 25 M. Tech. and 35 B. Tech. theses,carried out about 30 sponsored projects worth over Rs. 100million and recently obtained a patent on amorphous AlCuTi alloy.Prof. Manna won several prizes and awards in India and abroadincluding INSA Medal for Young Scientists, Young Metallurgistand Metallurgist of the Year Awards, MRSI Medal, AICTE careeraward and DAAD and Humboldt Fellowships, Acta Materialia Best



Referee Award (1999 and 2003). Currently, he is the Coordinator,Mission Project on Nano Science and Technology, and theChairman, Central Research Facility, IIT, Kharagpur. Prof. Mannahas been elected a Fellow (FNAE) of the Indian National Academyof Engineering (INAE), New Delhi, a Fellow (FNASc) of TheNational Academy of Sciences of India, Allahabad, and a Fellow(FASc) of the Indian Academy of Sciences, Bangalore. Recently,

the German Academic Exchange Service (DAAD), Bonn hasappointed him an Honorary DAAD Advisor for three years,INAE has awarded him INAE-AICTE Distinguished IndustryProfessorship for 2007-08, and Indian Institute of Metals (IIM)has conferred the GD Birla Gold Medal (2008) on him.

Prof. Manna is married (to Mrs Snigdha Manna) and has twochildren aged 18 (Oindrila, daughter), and 14 (Rudroneel, son).


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