two better than one

The Nanofluids TeamArgonne National Laboratory

TWO ARE BETTER THAN ONE IN NANOFLUIDS

by

Stephen U. S. Choi

Argonne National Laboratory

Energy Technology Division

[email protected]


MULTIDISCIPLINARY NANOFLUIDS TEAMCommitted to pioneer the frontiers of nano thermal engineering

Principal Investigator:

Steve Choi (mech. eng.), ANL

Co-workers:

Wenhua Yu (mech. eng.), ANL John Hull (phys.), ANL

Jeff Eastman (mat. sci.), ANL Simon Phillpot (phys.), ANL

Xianfan Xu (mech. eng.), Purdue Pawel Keblinski (phys.), RPI

George Zhang (chem.) and Fran Lockwood (chem. eng.), Valvoline


WHY ARE NANOSCALE SCIENCE, ENGINEERING, ANDTECHNOLOGY (NSET) IMPORTANT?

Ø Nanoscale structures (< 100 nm) give rise to new properties andphenomena that are basic to macroscopic events.

Ø NSET can change the way almost everything is designed and made.

Ø NSET creates interdisciplinary opportunities, integrating physics,chemistry, biology, materials science, and engineering and coveringdesign, synthesis, assembly, structure, properties,modeling/simulation, and theory.

Ø The National Nanotechnology Initiative (>$500 M/yr) is on the top ofthe U.S. science policy agenda.


NEW IDEAS FOR NEW FRONTIERS OF NSET

Ø Intelligent people are always open to new ideas. In fact, they look forthem. (Proverbs 18:15)

Ø Turning new ideas into reality requires new materials, new tools, andnew science.

NewScience

NewIdeas

NewMaterials

NewInstruments


MINIATURIZATION AND NANOTECHNOLOGY

Ø Since Richard Feynman presented the concept of micromachines(1959), miniaturization has been a major trend in modern science andtechnology.

Ø The scale of miniaturization has dropped from millimeters in the 1950sto the present-day micrometers in micro-electromechanical systems(MEMS).

Ø From virtual obscurity about a decade ago, nanotechnology isemerging as the most exciting technology of the 21st century.

Ø The future will see miniaturization at the nanoscale, and ultimatelyatomic scale.


SIZE GUIDE

Ø The term nano- comes from the ancient Greek for dwarf and is a prefix signifying

one-billionth part (10-9) of any metric unit of measurement.

Ø Molecules and their constituent atoms - of which all living and other matter is

composed - are in the nanometer size.

Ø Water molecule is 1 nm in diameter, which is two gold atoms across.

Ø DNA, < 3 nm in diameter. Proteins, 2-5 nm.

Ø Cell membranes, 8-nm thick.

Ø AIDS virus, ~100 nm in diameter. Red blood cells, ~5000 nm in diameter.

Ø Colloidal particles, 1-1000 nm.

Ø Nanoparticles, 1-100 nm.

Ø Nanocrystals (quantum dots), 1-10 nm.


GOALS OF NANOFLUIDS R&D

Applied Research:

Ø Create nanofluids with stability and ultra-high thermal conductivity.

Ø Develop nanofluids for industrial applications.

Basic Research:

Ø Investigate the properties and performance of nanofluids.

Ø Generate new insights into the fundamentals of energy transport at thenanoscale.


CONCEPT OF NANOFLUIDS

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9

Thermal conductivity of typical materials

Therm

al conductivity (

W/m

-K)

Material

0.15 0.25 0.61

1-Engine Oil2-Ethylene Glycol3-Water4-Alumina5-Silicon6-Aluminum7-Copper8-Silver9-Carbon

Solids have thermal conductivitiesthat are orders of magnitude largerthan those of traditional heattransfer fluids.

Ø Conventional heat transfer fluids haveinherently poor heat transfer propertiescompared to most solids.

Ø Modern nanotechnology has enabledthe production of particles <50 nm.

Ø Nanoparticles have unique propertiesthat can be exploited to develop ultra-high thermal conductivity fluids.

Ø Applying nanotechnology to thermalengineering, Argonne developed theconcept of nanofluids.


UNIQUE PROPERTIES OF NANOPARTICLES

Ø Size-dependent physical properties: Color, Conductivity

Ø Large surface area: The specific surface area of nanoparticles is 3orders of magnitude greater than that of microparticles.

Ø Large number density: For a given mass of material, there are agreater number of particles as their size decreases.

Ø Surface structure: Nanoparticles have ~20% of their atoms near thesurface, allowing them to absorb and transfer heat more efficiently.


WHAT ARE NANOFLUIDS?

Biofluids

Coolants

Emulsions NP

Lubricants

EGH2O

Oil SF

Nanofluids - A Creative Combinationof Solid Nanoparticles and LiquidMolecules

Ø Nanofluids are a new class of fluids

that are engineered by suspending

nanoparticles (NP) in heat transfer

fluids.


WHY USE NANOPARTICLES?

Ø The basic concept of dispersing solid particles in fluids to enhancethermal conductivity is not new - it can be traced back to James ClerkMaxwell.

Ø However, studies of thermal conductivity of suspensions have beenconfined to mm- or µm-sized particles.

Ø The major problem is the rapid settling of these particles in fluids.

Ø The small size of nanoparticles should markedly improve the stability ofthe suspensions.

Ø However, the agglomeration of nanoparticles into larger particles thatare found in liquids is a serious challenge.


SUSPENSION STABILITY

Ø Small spherical particles in the liquid will fall with a constant velocity close toStokes settling velocity

U =29

a2

p − l( )g .

Ø This formula represents a balance of gravity, buoyant force, and frictional force.

Ø To make a stable suspension:

Ø Reduce particle size a (or prevent the particles from agglomeration),

Ø Reduce the density difference between the particle and the fluid ( p l ),

Ø Increase the viscosity of the fluid .

Ø Small particles settle slowly. However, the sedimentation will come to a stop ata critical size ac when Brownian motion kicks in.


POTENTIAL BENEFITS OF NANOFLUIDS

Ø Improve heat transfer.

Ø Reduce pumping power and lower operating costs.

Ø Miniaturize (smaller and lighter) heat exchangers.

Ø Reduce heat transfer fluid inventory.

Ø Reduce emissions.

Ø Suited for applications in microchannel flow passages.

Ø In summary, a better ability to engineer thermal properties translatesinto greater energy efficiency, smaller/lighter systems, lower operatingcosts, and a cleaner environment.


REDUCED PUMPING POWER

1

2

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5

1 2 3 4 5 6 7 8 9 10

k/k0

p/po

Heat

trans

fer Co

efficie

nt rat

io, h/

h0

k/k0 or p/p

0

k = thermal conductivity, p = pumping power

Ø In heat exchangers that use conventional fluids, the pumping power must beincreased by a factor of ≈10 in order to improve the heat transfer by a factor of 2.

Ø If a nanofluid (assuming a thermal conductivity of ≈3 times that of aconventional fluid) were used, the rate of heat transfer would be doubledwithout an increase in pumping power.

Ø The potential savings in pumping power is significant with nanofluids.


NANOFLUIDS FLOWING THROUGHMICROCHANNEL FLOW PASSAGES

Ø Micrometer-sized particles cannot be used in practical heat transferequipment because of severe clogging problems.

Ø However, nanoparticles are ideally suited for applications in whichfluids flow through small passages.


POTENTIAL APPLICATIONS OF NANOFLUIDS

Engineering applications:

Ø Nanofluids can be used for a wide variety of industries, ranging fromtransportation to energy production and supply to electronics.

Ø A nanofluid can be used to cool car engines and welding equipment.

Ø A nanofluid can be used to cool high heat-flux devices, such as high-power microwave tubes and high-power laser diode arrays.

Ø A nanofluid coolant could flow through tiny passages in MEMS.

Medical applications:

Ø Magnetic nanoparticles in biofluids can be used as delivery vehiclesfor drugs or radiation, providing new cancer treatment techniques.


MEDICAL APPLICATIONS

Ø Magnetic nanoparticles absorb much more power than microparticlesat AC magnetic fields tolerable in humans.

Ø Nanoparticles are more adhesive to tumor cells than normal cells.

Ø Therefore, magnetic nanoparticles excited by an AC magnetic field ispromising for cancer therapy.

Ø The combined effect of radiation and hyperthermia is due to the heat-induced malfunction of the repair process right after radiation-inducedDNA damage.

Ø We could guide the particles up the bloodstream to a tumor withmagnets to deliver high local doses of drugs or radiation.


IMPACT OF NANOFLUIDS

Ø Impact of nanofluid technology is expected to be significant,considering that heat transfer performance is vital in numerousmultibillion-dollar industries.

Ø For example, transportation industry has strong incentive to reducesize and weight of vehicle thermal management systems.

Ø Nanofluids can increase thermal transport of coolants and lubricants.


METHODS FOR DISPERSING PARTICLES

Ø NPs agglomerate before dispersion and agglomerates settle rapidly.

Ø To keep NPs from agglomeration, they are coated with a surfactant (stericdispersion) or charged to repulse each other in a liquid (electrostaticdispersion).

Ø In practice, these two methods alter the desired properties of nanofluids. Thecharged NPs eventually settle in a week.

Ø The mechanical dispersion method, using a high-speed disperser or anultrasonic probe/bath, has met with limited success.

Ø We have developed a novel process to overcome the van der Waals forcesbetween the NPs. Properly reduced in size and directly dispersed in a liquid,these dispersant-free NPs do not agglomerate or settle in a liquid.

Ø The elusive combination of small particles and high thermal conductivity hadbeen found.


METHODS FOR PRODUCING NANOFLUIDS

Ø Two techniques are used at ANL to make nanofluids:

Cooling System

Liquid

Resistively HeatedCrucible

Production system designed for directevaporation/ condensation of NPs into low-vapor-pressure liquids. The liquid is in the cylinder thatis rotated to continually transport a thin layer ofliquid above a resistively-heated evaporationsource. The cooling system prevents anundesirable increase in vapor pressure due toradiant heating during the evaporation.

Ø In two-step process (Kool-Aid method),nanoparticles are produced by inert-gascondensation, then dispersed in base fluid;works well for oxide nanoparticles.

Ø Unique one-step process simultaneouslymakes and disperses nanoparticlesdirectly into low-vapor-pressure fluid; bestfor metallic nanofluids such as Cunanofluids.


TEM CHARACTERIZATION OF NANOPARTICLES

Al2O3 CuO

0

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20

25

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35

40

0 20 40 60 80 100 120

Median of Particle Sizes=26

Num

ber

of

Par

ticl

es

Particle Sizes of Al2O

3 (nm)

Total Number of Particles =131

0

5

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1 5

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2 5

0 2 0 4 0 6 0 8 0 100

Median of Particle Size =17 nm

Num

ber

of

Par

ticl

es

Particle Sizes of CuO (nm)

Total Number of Particles =107Mean of Particle Sizes = 24 nm


DISPERSION EXPERIMENTS

Stable suspensions of oxide and metallic nanoparticlesin fluids can be achieved.

Deionized water prior to(left) and after (right)dispersion of Al2O3nanoparticles.

Oil prior to (left) andafter (right) evaporationof Cu nanoparticles.


BASE FLUIDS AND NANOPARTICLE MATERIALS

Ø Materials for base fluids and nanoparticles are diverse.

Ø Nanofluids can be engineered with a range of nanoparticles and basefluids to create desired properties.

Ø Therefore, there are many kinds of nanofluids for a wide range ofapplications, e.g., ethylene glycol-based nanofluids in a radiator andoil-based nanofluids in a car engine.


NANOPARTICLE CHARACTERISTICS

Surfacecharge

Size

Shape Materials

Loadings

• MaterialsOxides – Al2O3, CuO

Ceramics Carbides - SiCNon-oxides

Nitrides - AlN, SiN

Metals – Al, Cu

Nonmetals – Graphite, Carbon nanotubes

Layered – Al + Al2O3, Cu + C

Phase change materials – Solid/Solid

Functionalized nanoparticles

• Size1-40 nm

• Shape

• Loadings0.1 – 1.0 vol.%

1.0 – 10.0 vol.%

>10.0 vol.%

Spherical Single particles

Cylindrical

Agglomerated particles

• Surface ChargePositive

Negative

Neutral


TRANSIENT HOT-WIRE APPARATUS

Fig.1 Schematic diagram of transient hot-wire apparatus for measuring thermal conductivities of nanofluids (A/D = analog-to-digital)

A/Dconverter

DC power supply

Switch

Nanofluids inmass cylinder

R1

R2 R3

Rw

R4

Wheatstone bridgeStabilizer

Hot wire PC for data storage

Ø Briefly apply power to Pt wire.

Ø Determine wire T rise by measuring resistance.

Ø Calculate k from k = q*ln(t1/t2)/(4 π dT).

Ø An advantage over steady-state techniques is in the elimination of problemsassociated with convection.

ASME Trans. J. Heat Transfer, 121, 280-289, 1999.


THERMAL CONDUCTIVITY MEASUREMENTS

1

1.1

1.2

1.3

1.4

1.5

0 0.01 0.02 0.03 0.04 0.05 0.06

Ethylene glycol + Al2O3Ethylene glycol + CuO

Ethylene glycol + Cu (old)

Ethylene glycol + Cu (fresh)Ethylene glycol + Cu + Acid

Th

erm

al

co

nd

uc

tiv

ity

ra

tio

Volume fraction

Ø Metallic (Cu) nanofluids show much moredramatic enhancements than oxidenanofluids.

Ø Vol.% is reduced by one order ofmagnitude at comparable k enhancement.

Ø The largest increase in conductivity (up to40% at 0.3 vol.% Cu nanoparticles) wasseen for a nanofluid that contained acid.

Ø A German group has also used metalnanoparticles in fluids, but these NPssettled. Our innovation was putting stablemetal nanoparticles into fluids.

Appl. Phys. Lett. 78, 718-720, 2001.


CARBON NANOTUBE NANOFLUIDS

Scanning electron photomicrographshows that the metallic MWNTs of ~30annular layers have a mean dia. of 25nm and a length of 50 µm.Scale bar is 800 nm.

Produced nanotube-in-oil suspensions by a2-step method:

Ø Produced multiwalled carbon nanotubes(MWNTs) in a chemical vapor depositionreactor, with xylene as the primary carbonsource and ferrocene as the iron catalyst.

Ø Dispersed MWNTs in a host fluid such assynthetic polyalpha-olefin oil.


THERMAL CONDUCTIVITY MEASUREMENTS

1.0

1.5

2.0

2.5

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Th

erm

al

co

nd

uc

tiv

ity

ra

tio

Volume fraction (%)

1.001.021.041.061.08

0.0 0.4 0.8 1.2

A

BC

Ø Dispersion of a small amount of nanotubesproduces a remarkable change in thermalconductivity of the base fluid (up to 250%at 1.0 vol.%).

Ø Thermal conductivity of nanotubesuspensions (solid circles) is one order ofmagnitude greater than predicted by theexisting models (dotted lines).

Ø Thermal conductivity of nanotubesuspensions is nonlinear with nanotubevolume fraction, while theoreticalpredictions show a linear relationship(inset).

Appl. Phys. Lett. 79, 2252-2254, 2001.


FUNDAMENTAL QUESTIONS

Ø What allows formation of stable suspensions? Why do the molecules of abase fluid keep NPs suspended so well, when they are still dramatically largerand heavier than liquid molecules?

Ø Why did we get so intriguing results (the big conductivity gap between our dataand predictions)? What are the mechanisms for increased thermalconductivity?

Ø How does particle size affect nanofluid thermal conductivity? Thermalconductivity of nanomaterials has been shown to be less than that of bulkmaterial. Why is thermal conductivity of nanofluids significantly increased overthat of base fluid?

Ø What is the importance of nanoparticle-fluid interactions and nanoparticle-nanoparticle interactions in the heat transfer process?


MOLECULAR DYNAMICS SIMULATION (MDS) OF NANOFLUIDS

RPI is conducting MDS of nanofluids to gain insights into the nature of heatconduction in nanoparticles and to study the effect of particle size on thermaltransport. Long-term goal – To calculate the thermal conductivity of nanofluidsby MDS.

Heat current autocorrelation functionsfor liquid (dashed line) and particle(solid line), showing monotonic decayof correlations in liquid and oscillatorydecay in solid, a signature of ballisticphonons moving back and forthinside the particle.

1

1.5

2

2.5

3

0 5 10 15 20 25 30

0.5 nm1 nm1.5 nm2 nm

κ

d (nm)

d d+2h

Excess thermal-conductivityenhancement κ due to formation ofhighly conductive layered-liquidstructure at liquid/particle interface forseveral values of layer thickness h asa function of particle diameter d.

Int. J. Heat Mass Trans. 45, 855-863, 2002.


WHAT’S NEW?

Ø We have discovered the fundamental limits of conventional heat conductionmodels for solid/liquid suspensions, which are rooted in macroscopic transportlaws, such as the Fourier law of diffusion heat conduction.

Ø For nearly 200 years heat conduction in liquids at rest was thought to bediffusive. In contrast, we propose ballistic/diffusive conduction as the heattransfer mode in nanofluids.

Ø Heat conduction in solid nanostructures such as thin films/superlattices is wellknown to be ballistic. However, we are the first to suggest ballistic conductionas the heat transfer mode in solid/liquid systems in order to explain whynanoparticles have drastically enhanced the thermal conductivity of baseliquids.


FUTURE COLLABORATIVE RESEARCH

PRODUCTION (SYNTHESIS AND DISPERSION)

Ø Identify dominant forces in nanofluids as a function of particle size.

Ø Develop methods for producing nonagglomerating NPs <10 nm.

Ø Address and resolve scale-up issues to bring costs down. Nanofluids areexpensive, because the equipment used to manufacture them is one-of-a-kind.

MACROSCALE EXPERIMENTS

Ø Measure the transport properties (k, µ) of nanofluids and develop a propertiesdatabase.

Ø Measure the flow and heat transfer behavior of nanofluids as a function of NPtype, size, and conc. and develop a database of nanofluid performance.


FUTURE COLLABORATIVE RESEARCH (cont’d)

NANOSCALE EXPERIMENTS

Ø Determine the nanoscale structure and dynamics of nanofluids using X-ray/neutron scattering.

Ø Measure the thermal conductivity of two NPs suspended in liquids to exploretheir thermal interactions at the nanoscale.

COMPUTER MODELING AND SIMULATION

Ø Model and simulate the thermal conductivity of nanofluids.

Ø Extend studies on heat conduction in solid nanostructures to nanofluids.

THEORY

Ø Develop fundamental understanding of heat conduction mechanisms innanofluids.

Ø Create accurate models of the thermal behavior of nanofluids.


SUMMARY AND CONCLUDING REMARKS

Ø Applying nanotech to thermal engineering, ANL has developed the novelconcept of nanofluids.

Ø Compared to larger particles, nanoparticles have better suspension behavior,very large surface area and number density, and lower kinetic energy.

Ø ANL has produced stable nanofluids by one- and two-step processes,measured their effective thermal conductivity, and concluded that:

Ø The measured thermal conductivity is substantially greater than theoreticalpredictions.

Ø This anomalous behavior shows the fundamental limits of conventional heatconduction models for two-phase systems.

Ø At present, we do not understand the fundamental mechanisms of thermal-conductivity enhancement of nanofluids.


SUMMARY AND CONCLUDING REMARKS

(cont’d)

Ø We have postulated physical concepts for explaining the anomalous thermalbehavior of nanofluids.

Ø The suggested research would lead to major breakthroughs in making newsolid/liquid materials for engineering/medical applications.

Ø Invention of nanofluids has opened the door for thermal engineers to thenanoworld.

Ø There's plenty of room for creative, collaborative research on nano thermalengineering.

Ø Since thermal properties are important ingredients of NSET, thermalengineers have a major role to play.

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two better than one