14. improving the heat transfer of nano fluids and nano lubricants with carbon nanotubes

8/4/2019 14. Improving the Heat Transfer of Nano Fluids and Nano Lubricants With Carbon Nanotubes

1/12JOM December 200532

Carbon NanotubesResearch Summary

Low thermal conductivity is a primary

limitation in the development of energy-

efficient heat transfer fluids required in

many industrial and commercial appli-

cations. To overcome this limitation, a

new class of heat transfer fluids was

developed by suspending nanoparticles

and carbon nanotubes in these fluids.

The resulting heat transfer nanofluids

and nanolubricants possess significantly

higher thermal conductivity compared

to unfilled liquids. Three types of heat

transfer nanofluids and nanolubricants,

each containing controlled fractions of

single-wall carbon nanotubes, multi-

wall carbon nanotubes, vapor grown

carbon fibers, and amorphous carbon

have been developed for multifunctional

applications, based on their enhanced

heat transfer and lubricity properties.

INTRODUCTIONDespite considerable prior R&D

efforts focused on industrial provisions,

major improvements in fluid cooling

capabilities have been held back because

of a fundamental limit in the heat trans-

Improving the Heat Transfer ofNanofluids and Nanolubricants withCarbon Nanotubes

F.D.S. Marquis and L.P.F. Chibante

Table I. Thermal Conductivities ofVarious Solids and Liquids

ThermalConductivity

Material Form (W/m-K)

Carbon Nanotubes 1,8002,000Diamond 2,300Graphite 110190

Fullerenes (film) 0.4Metallic Solids Silver 429

Copper 401Aluminum 237

Nickel 158Non-Metallic Silicon 148

SolidsAlumina 40

Metallic Liquids Sodium 72.3@644K

Others Water 0.613Ethylene Glycol 0.253

Engine Oil 0.145

1997

$2,000

$1,000

$500 $500$400

$200$20

1998 1999 2000

Year

CostperGra

m($)

2,2502,0001,750

1,5001,250

1,000750500

250

02001/2002

2003 2004

fer properties of conventional fluids. This

limitation is associated with the relatively

low thermal conductivity of the heat

transfer fluids (HTFs). However, solid

materials typically have orders-of-mag-

nitude larger thermal conductivity than

commonly used liquids. For example,

the thermal conductivity of carbon nano-

tubes at room temperature is over 3,000

times greater than that of water and over

10,000 times greater than that of engine

oil. This large difference in thermal

conductivity between liquids and carbon

nanotubes is shown in Table I as com-

pared to a variety of materials. Therefore,

fluids containing suspended carbon

nanotubes are expected to exhibit sig-

nificantly higher thermal conductivity

relative to conventional HTFs.

Since Maxwells theoretical work,1

other theoretical and experimental efforts

have been conducted to examine theeffect of particle additions on the thermal

conductivity of HTFs. However, all

previous studies of the thermal conduc-

tivity of suspensions have been confined

to those containing millimeter- or

micrometer-sized particles. Maxwells

model shows that the effective thermal

conductivity of liquid suspensions con-

taining spherical particles increases with

the volume fraction of the solid particles.

In addition, it is also known that the

conductivity of the liquids increases withthe surface area-to-volume ratio of the

added particles.

The application of Hamilton and

Crossers model,2 in addition to recent

calculations,3 predict that for constant

particle size, the thermal conductivity of

a suspension containing large particles

is more than doubled by decreasing the

sphericity of the particles from a value

of 1.0 to 0.3. The sphericity is defined

as the ratio of the surface area of a par-

ticle with a perfectly spherical shape to

that of a non-spherical particle with the

same volume.

These results suggest that a dramatic

improvement in the effective thermal

conductivity is expected by decreasing

the particle size in a solution compared

to the incremental improvement that can

be obtained by altering the shape of large

particles since the surface-area-to-

volume ratio is 1,000 times larger for

particles with a 10 nm diameter than that

of 10 m diameter particles. Conse-

quently, nanofluids are expected to have

superior heat transfer properties com-

pared to conventional fluids and fluids

containing micrometer-sized particle

additions. Also, heat transfer nanofluids

(HTNFs) with carbon nanotubes are

expected to possess even better heat

transfer properties due to the non-

spherical shape of the carbon nanotubes.

The aspect ratio (length/diameter) ofcarbon nanotubes is typically between

103 and 105.

Recent work at Argonne National

Laboratory has demonstrated that nano-

fluids consisting of copper, CuO, or Al2O

3

nanoparticles dispersed in water or eth-

Figure 1. The price evolution of SWNTsdemonstrating a continued decrease inprice that results from increase in SWNTyield and higher volume demand.


2/122005 December JOM 33

EXPERIMENTAL PROCEDURES

Nanomaterials

Single-wall carbon nanotubes (SWNTs) of many grades and characteristics wereproduced by the HiPco facility of Carbon Nanotechnologies, Inc. The metal catalystcontent and the amorphous carbon content varied considerably with the batch used, aspresented under results and discussion. Multi-wall carbon nanotubes (MWNTs) wereproduced by the vacuum-arc method by the NanoTex Corporation (NTC). In addition,NTC produced SWNT soot for this research. Multi-wall carbon nanotubes were alsoproduced by the chemical-vapor deposition (CVD) method at Clemson Universityunder the direction of A. Rao and by the microwave CVD method at the University of

California, San Diego, under the direction of S. Jin.Thermal Measurements of Nanofluids

Hot-disk thermal conductivity testing apparatus was utilized which allowed the uniquefeature of testing fluids with a home-built testing cell. The commercial instrument is atransient plane-source method that is rapid, exhibits good precision, and is typicallywithin 3% of known standards. The essential feature of the apparatus is that the heatsource/probe is sandwiched within the sample and the temporal thermal profile ismonitored, as shown in Figure A. In this study, the sensor was placed in a sealed, insulatedfluid cell. The technique allows the direct measurement of thermal conductivity withoutprevious knowledge of specific heat or density. The Thermal Haake thermal conductivity(TC) measuring system was also used to evaluate the thermal conductivity of a numberof the heat transfer nanofluids (HTNFs). This system is similar to a transient hot wiresystem but works with an embedded probe rather than an exposed wire. The system iseasy to use and the entire sample can, in most cases, be fully recovered. The values areconsistent with those measured by the hot-disc system.

Thermal Gravimetric Analysis

Thermal gravimetric analysis (TGA) of the SWNTs was performed with a TAinstruments SDT-TGA apparatus and opened platinum pans using an air flow at about100 cc using 3 mg of material. The material was weighed in an analytical balancethat used up to four decimal places. This data was necessary to determine the rate ofdecomposition as a function of weight and temperature in an inert gas such as argon andin an oxidizing environment using air. The run in argon yields information on thermaldegradation characteristics, while the air runs provide information on the effect ofoxidation on thermal degradation properties.

Fourier Transform Infrared Spectroscopy

In this study, the various HTNFs were evaluated by Fourier transform infrared

spectroscopy (FTIR). Droplets of the various fluids are placed on filter paper for vacuumevacuation and the residue is removed and placed on test coupons for analysis. Scanswere made of each sample over a similar spectral range. The base fluids are evaluatedas well as the starting nanotubes in solutions that will not produce peaks in the spectralrange of interest. The goals of this study are to identify the surface species on the startingcarbon nanotubes and to identify their interactions and influence on the various heattransfer fluids.

Raman Spectroscopy

Raman spectroscopy was carried out as a characterization method in the study of car-bon nanotubes using a Renishaw Micro Raman spectrometer with a 780 nm laser with aresolution of 2 cm1. The objective used was 50 with a 0.55 m aperture. Several scanswere run for each sample. The various types (SWNT or MWNTs) are easily distin-guished from one another and even differences in the various types of SWNTs, includingeffects from functionalization, can be seen via Raman. Typically used in carbon nanotube

analysis are the radial breathing modes between 200 cm and 300 cm1, the disorder modethat occurs at ~1,300 cm1 giving information about defects, and the tangential stretchingmode indicative of sp2 carbon bonding at approximately 1,590 cm1. From the Raman,one can about learn differences from the purified form, including cross-linking, that

ylene glycol exhibit enhanced thermal

conductivity compared to that of non-

particle-containing fluids.46An increase

in thermal conductivity of ~20% was

observed for 4 vol.% CuO nanoparticles

with an average diameter of 35 nm dis-

persed in ethylene glycol. Similar

behavior was observed by Masuda and

co-workers in another study of Al2O

3

nanoparticles dispersed in water.

7

Largerimprovements in effective thermal con-

ductivity were obtained for a nanofluid

containing smaller-sized and higher-

conductivity copper nanoparticles.6

In recent work Choi and co-workers

demonstrated anomalous enhancements

in the thermal conductivity due to mul-

tiwall nanotube (MWNT) additions to a

synthetic poly (-olefin) oil.8 The ther-

mal conductivity was seen to increase

with MWNT additions at loads up to 1%

in volume. These results exceed those

based on the theoretical predictions of

Hamilton and Crosser, which predict no

effect of particle size and a weak effect

of particle intrinsic conductivity on fluid

effective thermal conductivity. In this

work Choi and co-workers did not con-

sider the fluid properties, such as viscos-

ity, carbon nanotube dispersion, and

nanotube settling time and stability.

Carbon nanotubes have generated

great interest since they were discovered

in 1991.9 The thermal conductivity of

unroped single-wall carbon nanotubes(SWNTs) have been theoretically calcu-

lated to be above 2,000 W/mK and the

thermal conductivity of SWNT ropes

was measured at 200 W/mK, consistent

with the values measured in Buckypa-

per.10,11 This means that the potential for

contribution to the enhancement of the

thermal conductivity of heat transfer

nanofluids is much higher with SWNTs.

However, carbon nanotubes are highly

anisotropic and the transverse thermal

conductivity is expected to be as low asthat of fullerenes (0.4 W/mK).

In principle, carbon fibers, SWNTs,

MWNTs, and vapor-grown carbon fibers

(VGCFs) all have high thermal conduc-

tivity in the axial direction. Note also

that all of them except the carbon fibers

are discontinuous. Based on the thermal

conductivity of carbon fibers and carbon

composites, thermal conductivity of

these materials is expected to be rather

high for loads up to 25%. While the ther-

mal conductivity of individual SWNTs

Figure A. The hot disc heat-ing element/sensorsampleconfiguration.

can occur within the ropes.Functionalizations changethe Raman spectra, intro-ducing a different state forthe nanotubes when com-pared to the pure nanotubecondition.



is expected to be very high, Buckypaper

has been measured to be only 200 W/mK,

consistent with the thermal conductivity

of nanoropes. Buckypaper is a dense film

of SWNT ropes and the thermal conduc-tion is likely limited by poor contact,

the presence of impurities, and poten-

tial mechanisms that lead to scattering

rather than transport. Another possible

explanation has to do with the control

of thermal scattering mechanisms and

this may be one of the most important

issues to engineers so that transport is

optimized and scattering is reduced.

Two of the major challenges that need

to be overcome for the production of

effective heat transfer nanofluids andnanolubricants are carbon nanotube

dispersion and carbon nanotube stabil-

ity. Recently various investigators have

studied methods for carbon nanotube dis-

persion1216 in fluids and other media.

The combination of robust carbon

nanoparticles with superior thermal con-

ductivity and well-established chemical

stability to develop a new class of very

novel, unique, and efficient nanofluids

with much lower additive concentrations

is the focus of this research. Some of the

expected gains from these nanofluids are

cost effectiveness and improved thermal

transfer properties, minimized clogging,

long-term chemical stability, and long-

term effective performance life.

In this article, a series of HTNFs and

lubricants are processed based on water,

water/ethylene glycol, diesel and com-

mercial oils to identify the increase in

thermal conductivity of the fluid withnanotube concentration and mixing

procedure. These nanofluids are com-

pared to the HTNFs produced by Choi

et al. and to similar fluids mixed with

corresponding properties. The fluids

have been characterized based on start-

ing nanotube conditions, conditions of

the nanotubes after mixing, and stability

(settling time and thermal degradation)

of the nanotubes in the oils. In the end,

for the use of these oils and diesel oils

in engines, the HTFs play a number of

important roles that could affect the

thermal and mechanical efficiency of

the engine. The HTFs tend to remove

unwanted heat from the engines while

providing lubricity and temperature

control.

ANTICIPATED BENEFITS

OF HTNFs

The anticipated benefits of these

nanomaterials are several-fold. Since

SWNTs have high thermal conductivity

(1,8002,000 W/m-K) they will enhancefluids for heat transfer use. The addition

of carbon nanotubes together with their

inherent miscibility in hydrocarbons,

such as engine oils, will reduce the

potential for severe clogging in cooling

systems. Because of the relative soft-

ness of nanotubes, the abrasion and

erosion of the cooling circuit are both

expected to be greatly reduced. Further,

nanotubes are expected to possess anti-

oxidant properties. Many degradation

mechanisms involve the generation offree radicals and subsequent propaga-

tion to degrade the fluid. Nanotubes

may be efficient free-radical scavengers

and hence hinder these pathways. This

would increase fluid temperature per-

formance limits and/or prolong HTF

lifetime. Associated with these antioxi-

dant properties, derivatized nanotubes

may have antibacterial and antiviral

properties. Dermatological and animal

studies done to date generally agree

that these nanocarbons are nontoxic.

This is imperative for environmentally

benign use of HTFs. Carbon nanotubes

are oxidatively stable up to 550C and

chemically stable to over 1,000C.

Under excessive thermal conditions, the

nanotubes will not degrade in the fluid

and should help protect the nanofluid.

In addition, the availability of improved

HTFs for automotive cooling will be an

incentive for the auto industry to designsmaller engines by taking advantage

of the improved cooling response of

the engine thermal cycle. This would

have beneficial implications both on the

engine fuel efficiency and reduced size

and operation of cooling components

(i.e., pumps, filters, and lines).

The potential economic benefits of

commercializing nanofluids include

cost reduction and energy savings due

to the ability to manufacture smaller and

lighter heat exchange systems, reduced

heat transfer fluid inventories, and lower

pumping energies required for existing

heat exchange systems. The impact of

nanotube-laden nanofluids is significant,

considering that improved heat transfer

properties are vital to a number of mul-

tibillion-dollar industries both in the

Figure 2. SWNTs are shown with ~20wt.% SWNTs (1.2 nm average, micrometerlength) with ~20 wt.% amorphous carbon,50 wt.% graphitic particles, 10 wt.%residual catalyst. Degree of entanglementand interconnection is moderate.

500 nm

1 m

Figure 3. MWNTs are shown with 4050wt.% MWNTs (1015 nm diameter,micrometer length), 5060 wt.% graphiticparticles, no residual catalyst. Degree ofroping and interconnection is minimal.

500 nm

Figure 4. HiPco raw fluff: The opennessof the entangled SWNT ropes can beseen. These SWNTs appear highlyinterconnected, probably due to the vander Waals attractions.

500 nm

Figure 5. HiPco purified fluff: ~70 wt.%SWNT (0.81.5 nm diameter, micrometerlength), 30 wt.% residual catalyst (Fe).>95% SWNT,



military and in the domestic sectors.

To further reiterate the potential

economic benefits of carbon nanotube

nanofluid technology, the example of the

ethlyene-glycol-based nanofluids can be

used. In the United States alone, more

than $80 billion are spent annually on

energy for air conditioning and refrigera-

tion equipment. Increasing the efficiency

of this equipment by 25% could result

in annual energy savings of $20 billion.To make this possible, the price of the

carbon nanotubes must be established

below a certain threshold. This price is

interconnected with demand and demand

will depend on how the technologi-

cal challenges are answered. Figure 1

shows the price evolution of SWNTs,

demonstrating a continued decrease in

price that results from increasing SWNT

yield and higher volume demand.

See the sidebar for experimental pro-

cedures.

RESULTS AND DISCUSSION

Carbon NanomaterialsSingle-walled carbon nanotubes,

MWNTs, VGCFs, and carbon black

systems were manufactured for nano-

fluid production and evaluation. These

carbon nanomaterials are characterized

to provide an understanding of therelationships between the type and the

characteristics of these materials and the

performance of the nanofluids containing

them.

Scanning and TransmissionElectron Microscopy

Scanning-electron microscopy (SEM)

and transmission-electron microscopy

(TEM) are some of the best methods

for observing and characterizing carbon

nanotubes. While SEM can see VGCFs,

MWNTs, and SWNT ropes with high

resolution, TEM is one of a few methods

where single unroped SWNTs can be

observed. Carbon nanotubes are con-

ducting and semiconducting materials

but may be coated to enhance certain

features when they occur in low con-

centrations. Figures 2 through 11 show

SEM and TEM micrographs of various

carbon nanomaterials that were used as

nano-additives in this study. Not shown

are SWNT pearls and various other forms

of SWNTs where further preparation wasused. The figures also note the general

composition of the various nanomateri-

als used. Typical types of contamination

include amorphous carbon and metal

catalyst. The degree of entanglement

is a function of the processing mode

and the subsequent preparation by the

manufacturer.

Care has to be taken when coating

CNTs since the coating material only

sees a portion of the nanotubes and

they may curl up during the process andgive a different appearance due to being

coated only on one side. The coatings can

enlarge the diameter and one might esti-

mate the size (rope size) to be larger than

what it actually is. Typically the nano-

materials used in this investigation had

the following characteristics: SWNTs,

unroped, 11.4 nm in diameter; SWNTs,

roped, 1050 nm; MWNTs, typically

1050 nm, unroped; A. Raos material

from Clemson Univeristy, 20300 nm

from the SEM pictures: and VGCFs,

30200 nm and on average 100150

nm. The current data shows that SWNT

lengths are 0.310 m (before cutting),

MWNTs are 1100 m, and VGCFs are

1100 m. Typical results are presented

in Figures 2 to 11.

These results show that VGCFs have

one or two orders of magnitude more

surface area than conventional carbon

fibers, MWNTs and SWNT ropes havethree orders of magnitude more surface

area, and unroped SWNTs have four

orders of magnitude more surface area

than conventional fibers. This means

that VGCFs, MWNTs, and roped

SWNTs in a fluid may produce what is

still considered to be a solid percolated

network while the unroped SWNTs have

a chance to take on similar properties to

the fluid and, therefore, may have thermal

conduction by other means. In addition,

SWNTs are molecular and on a similar

scale to polymers and fluid molecules.

This means that two distinct paths exist

for engineering the nanofluid: produce a

mixture where the nanotubes are added in

order to change the thermal conductivity

via the rule of mixtures starting condi-

tions, and produce a hybrid fluid where

2 m

Figure 6. Multi-walled nanotubes producedby CVD and taken from the sourcesubstrate; open, entangled, and swirlingmorphology; >80 wt.% MWNT (1020 nmdiameter, 10 m length), 15% amorphouscarbon, 5 wt.% catalyst; low defect rate;degree of roping and interconnection ismoderate-high.

500 nm

Figure 7. Multi-walled nanotube taken fromthe source substrate: variation in nanotubesize and the openness to the agglomer-ates (not highly entangled). Some smallparticles can be seen distributed on thenanotubes.

500 nm

0.5 m

Figure 9. An SEM micrograph of as-received MWNTs taken from the wall ofa production vessel. Some nanotubesare very large. Note the small particlesthat are impurities distributed on thenanotubes. The material is not in a purifiedcondition.

Figure 8. A TEM of MWNTs substrate.Note the variation in nanotube size andthe open nature of the network.



the nanotubes are integrated into the

fluid to take on more fluid-like properties

(produce a more fluid-like system). The

second approach is expected to generate

the best performance with the highestdegree of effectiveness.

The SEM was used to characterize the

nanotube rope size, entanglement, type of

entanglement, and rope flaws. The SEM

was also used to look for contamination

type and distribution and for nanotube

ends, but with SWNTs, the ends are

almost never visible (a product of van der

Waals bonding). The ends of MWNTs

and VGCFs can be seen by SEM and

they tend to be open when large enough

to see. During this investigation, SWNTswere found to differ considerably from

each other, based on how they are pro-

duced. These conditions impacted how

well they could be processed and how

thermally conducting they were. One of

the advantages of MWNTs is not being

in rope form, and thus being easier to

disperse. It is likely, though, that they

may tangle again in dynamic use (flowing

conditions) and cause clogging at higher

loads, although this was not observed up

to 0.5% volume load.

Raman Spectroscopy

Several Raman spectra were acquired

for typical samples of the following nano-

materials: amorphous carbon, MWNTs

(wall), MWNTs (substrates), SWNTs in

raw fluff, SWNT purified bucky pearls,

and SWNT functionalized. As the nano-

tubes are functionalized, the ~1,300 cm1

peak changes in height since this D-bandpeak is related to the covalent character

of the nanotubes and therefore covalent

bonding is shown here (this is the disor-

der peak). The peak increase, depending

on the type of functionalization, can be

related to an increase in number of sp3-

hybridized carbons and this can be taken

as a degree of functionalization.

A significant change in the Raman

spectra occurs when nanotubes are

functionalized by fluorination. For

fluorination, the typical nanotube peaks

at 200263 cm1 and ~1,590 cm1 tend

to decrease with the fluorination while

the ~1,300 cm1 peak and ~1,590 cm1

increases. The spectra obtained show

that when nanotubes get crossed-linked,

the small peak on the side of the largest

peak (~1,590 cm1) tends to go away and

the 1,300 cm1 peak also changes.

In general, the sizes of nanotubes

have a higher average diameter for the

arch grown and laser-ablated than do

the HiPco nanotubes manufactured by

Carbon Nanotechnologies, Inc. TheRaman spectra give information about

functionalization but not rope size. This

is shown in the Raman spectrum for

fluorinated nanotubes.

Carbon Nanotube

Functionalization

Carbon nanotubes are carbon mol-

ecules that have a hollow core and are

relatively defect free. This carbon-carbon

bonding and lack of defects makes

nanotubes inert in many acids and evenin a number of strong acids. The strong

bonding and lack of defects add to the

nanotube having high strength and good

electrical and thermal properties. Much

of the authors research on nanotubes

in fluids is directed toward dispersing

nanotubes at levels where they are soluble

in the fluid, in order that these fluids will

be practically effective. To this end, the

quantities of nanotubes that go in solution

in a fluid have been limited to milligrams

per liter. There are several cases where

the dispersion of nanotubes in fluids

requires higher quantities; these stud-

ies are directed toward achieving high

dispersion of nanotube suspensions in

fluids such as starting fluids for dispers-

ing nanotubes in thermal plastics and

epoxies and nanotube/ceramic powder

slurries for producing nanotube-rein-

forced ceramic nanocomposites. Some

routes seem sophisticated but to producecommercially viable heat transfer nano-

fluids and nanolubricants it is important

to use low-cost processing first. Since the

need for driving up the nanotube content

is so important it has been determined

that one way to accomplish this is by

functionalizion of the nanotubes.

Functionalization is the process of

attaching radicals and functional groups

to the carbon nanotube. Functionaliza-

tion provides an efficient approach to

the unroping of nanotube bundles and

improving their ability to disperse in

organic matrixes and fluids (e.g., motor

oils, coolants, etc.). Sidewall covalent

derivatization of SWNTs with fluorine,

alkyl, and aryl groups and with the

side-chains that are terminated with the

hydroxyl, carboxyl, or ester-terminated

0.5 m

500 nm

Figure 10. VGCFs > 99 wt.% MWNT (50nm-submicrometer diameters, tens ofmicrometers in length), high defect rate.Degree of roping and interconnectionis small.

Figure 11. Carbon black N234 un-pellet-ized: >99% carbon particles (3040 nmprimary particles assembled in ~300 nmaggregates), ppm levels of metals. Degreeof roping and interconnection: minimal.

Figure 12. A TEM of the as-producedHiPCo SWNTs bundled into ropes andamorphous carbon and metal catalystsadhering to the surface of the CNTropes. This has a significant effect ondispersion.

Figure 13. A TEM of purified, HiPCoESD SWNTs showing the multi-ropemorphology. The striations running inthe longitudinal direction of the ropes areindividual SWNTs. The average diameterof the individual tubes is 1.5 nm.



moieties offer new opportunities in the

fabrication of nanotube-based nanofluids

for heat transfer and lubrication man-

agement. By using chemical methods,

functional groups can be tailored and

attached to the nanotubes to enhance the

dispersion of the functionalized SWNTs

in the specific type of HTF. The side-wall

functionalization routes used involve two

major strategies: fluorination of SWNTs

to yield fluoronanotubes (F-SWNTs)

which can be further derivatized when

fluorine is substituted by a number of

nucleophiles, RLi, Grignard reagents,

Li3N followed by quenching, diamines

and aminoacids; and the addition of

organic free radicals generated by ther-

molysis of acyl peroxides.

In some cases the functionalization is

a noncovalent processing that includes

wrapping. The chemistry of the tube is

not changed but the chemistry attached

to the tube changes the nature of thenanotube. A number of approaches to

functionalizing nanotubes were used,

and some of them are of proprietary

nature. Initially, nanotubes were end

functionalized. This occurred when they

were purified and the ends were opened

up during the purification process and the

acids used left COOH groups attached

to the open ends. The open ends had

free carbon bonds and a COOH easily

occurred. Other functionalizations were

directed toward the side walls. Fluorina-tion places fluorine atoms bonded to the

sidewalls of the nanotubes and helps to

separate the individual nanotubes from

the rope (bundle).

This research used fluorination as a first

step in achieving the right functionaliza-

tion for the application. The F-SWNTs

are in smaller bundles and in many cases

are single tubes separated out from the

ropes. Additional functionalization can

occur to the F-SWNTs just like for the

roped unfluorinated nanotubes. Since

oils are hydrocarbon-based, the various

bonding methods mentioned here were

effective for the HTNFs. A quantitative

evaluation is under way of the chem-

istry needed to achieve well-dispersed

nanotubes in the oils and water/ethylene

glycol mixtures. The functionalization

types identified previously are useful but

do not always give individual nanotubes

in the optimal condition. In some casesthe nanotubes can easily be functional-

ized but will remain as functionalized

ropes, which limits their effectiveness.

In other cases, a form of wrapping was

used with or without functionalization.

Fluorination is one of the main

chemical tools in the preparation of the

side-wall functionalized SWNTs that

have already been successfully applied

for the fabrication and manufacturing of

the SWNT-reinforced composites and

ceramics. The second strategy is based

on the use of inexpensive peroxides

applied as polymerization initiators in

industry. The resulting oxidized tubes

were then dispersed in BP166 and min-

eral oils using simple homogenization.

An ashless succinimide dispersant and

oleylamine (a surfactant suitable for

hydrocarbon media) were used to aid

in dispersing the SWNTs in oil. This is

covalent functionalization and is cur-

rently in progress.

In addition, the role of covalent modi-

fication of SWNTs is being explored

to increase dispersion of SWNTs in

oil. Some of the functionalizations are:

attaching carboxylic acids to the ends

of SWNTs via oxidation; fluorinationof oxidized tubes; fluorination of non-

oxidized tubes; carboxylation at the walls

of the SWNTs; hydroxilated SWNTs;

aminated SWNTs; and esterified SWNTs

using two mechanisms. The series of

functionalized SWNTs prepared accord-

ing to these methods is described in a

number of pending patent applications.

This research shows that the preparation

of the functionalized nanotubes can be

easily scaled up to meet the quantity

demand for nanofluid applications.

Cutting of SWNT Ropes and

MWNTs and Exfoliation of SWNTRopes

One of the major obstacles in the

dispersion of SWNTs is the inability to

resolve the entanglements of multi-ropes

200 mm

Figure 14. A TEM of partial exfoliation ofmulti-ropes of SWNTs showing the splitof 7 into 3+3+1.

F i g u r e 1 5 .The intensity-weighted nano-part ic le s izedistribution for0.05% MWNTsin water /eth-y lene g lyco l(50/50).

F i g u r e 1 6 .The intensity-weighted par-ticle size dis-t r ibut ion for0 . 0 1 % E S DCNI SWNTs inwater/ethyleneglycol (50/50).



4,000 3,500

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bsorbance

2,000 1,500 1,000 500

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35

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5

0

4,000 3,000 2,500 2,000 Wavenumbers (cm-1)

1,500 1,000 500

3,5004,000

100

90

80

70

60

50

40

30

20

10

0

3,000 2,500 2,000

Wavenumbers (cm-1)

Absorba

nce

1,500 3,000 500

into single ropes and the inability to exfo-

liate the single ropes into single SWNTs.

The entanglement of the multi-ropes is

often magnified during purification of

the as-produced SWNTs as shown in

Figures 4 and 5. One of the most promis-

ing methods for exfoliating these ropes is

cutting. Cut (shortened) nanotubes, both

bare and functionalized, are of further

interest for these applications.This process can both contribute to

the exfoliation of the ropes and help

retard the process of re-agglomeration

by shortening the nanotubes. The ropes

and the nanotubes were cut either physi-

cally or chemically, leaving them open on

the ends for end-cap functionalization.

Chemical and physical means are used

to cut the multi-ropes and the nanotubes

at the ends, shorten them over time,

and exfoliate the ropes by penetration

between individual tubes in bundles. One

process of exfoliation is achieved through

complex and proprietary processing

used in conjunction with sonication

to shorten the nanotubes from the end

caps inward. These sites at the end of

nanotubes, once opened up, are inhabited

by a COOH group until replaced with

a final group used to functionalize the

nanotubes, leading to higher solubility

in a particular solvent. This is confirmed

by the data presented in Figures 12 to

14. This data is in very good agreement

with the in-situ particle size distributionobserved in typical nanofluids as shown

in Figures 15 and 16.

Figures 15 and 16 show that the

agglomerate size for the same base

nanofluid is much smaller for MWNTs

than for SWNTs. This is consistent with

a higher dispersibility of MWNTs due

to their unroped condition.

Design and Manufacture ofNanomaterials

Three types of advanced nanomateri-als have been developed, manufactured,

and tested. They consisted of both fluid

and grease materials based on water,

water/ethylene glycol mixtures, anti-

freeze, and mineral and synthetic oils.

The first type of nanomaterials, Type

1, is heat transfer nanofluids or nanocool-

ants based on three base fluids: water,

water/ethylene glycol mixtures, and

water/antifreeze mixtures. The primary

capability of these nanomaterials is to

transfer heat due to their higher thermal

Figure 17. The FTIR spectrum of a 15W-40 oil in the as-received condition. Samples thatwere sonicated show similar FTIR results.

Figure 18. The FTIR spectrum of a sample with 1% raw HiPco SWNTs. Notice changes at~2,300 cm-1 and



conductivity, modified heat transfer coef-

ficient, and heat capacity, which are the

main design parameters. Typical applica-

tions of this type of nanomaterials are

radiator coolants for all types of vehicles

both military and domestic, air condition-

ing systems, cutting fluids, quenching

fluids, and many others based on heat

transfer capabilities. In radiator coolants,

an anticipated benefit is the downsizingof the entire coolant system.

Type 2 nanomaterialsarenanolubri-

cant fluids based on three types of base

fluids: commercial 15W-40 oil, BP

Amoco DS-166 Durecene oil (synthetic

poly-- olefin oil), and military and U.S.

Department of Defense specification-

based fluids. The primary capabilities

of this type of nanomaterials are: heat

transfer due to the control of the design

parameters described previously and

higher lubricicity due to a lower fric-

tion coefficient. Typical applications are

engine coolants for all types of engines

both military and domestic. The antici-

pated benefits are: downsizing engine

blocks, redesign of engine systems,

higher durability of engine components,

shorter engine downtime, and lower

operation costs.

Type 3 nanomaterials arenanolubri-

cant greases based on fluids and greases:

BP 166 Durecene oil and military speci-

fications-based greases. The primary

capabilities of these nanomaterialsare also twofold, as described previ-

ously. However, in this case the carbon

nanotube load is much higher typically,

between 1 vol.% and 6 vol.%. The carbon

nanotube load in the previous two types

of nanomaterials is typically between

0.05 vol.% and 0.5 vol.%. Applications

of the nanolubricant greases include

high-stress contact gears for rotary craft.

In this case, the anticipated benefits

are higher torque transmitted, less heat

generated, more heat dissipated, longerflight duration, and longer life of the

metallic components.

This article presents results from the

first generation of nanomaterials (heat

transfer nanofluids and nanolubricants)

only. Proprietary data including pending

patent applications is not included.

Production of Nanofluids in

Synthetic and Mineral Oils

Nanofluids were produced in two

oil systems: a commercial diesel oil

0.01.00

1.10

1.20

1.30

1.40

1.50

0.2 0.4 0.6 Nanotube (vol. %)

TCRatio

0.8 1.0 1.2

0.21.00

1.10

1.20

1.30

1.40

1.50

0 0.4 0.6 NT Loading (vol.%)

y = 0.1167x2+ 0.1288x+ 0.9976

R2= 0.9979

y = 0.3446x2+ 0.1022x+ 0.9919

R2= 0.9934

TCRatio

0.8 1 1.2

Poly. (Vortexed) Poly. (Non Vortexed)

Non Vortexed Vortexed

Figure 20. The hot-disc thermal conductivityratio of SWNTs in 15W-40 oil as a functionof loading.

Figure 21. The effect of architecture of

HTNFs on the thermal conductivity of thefirst generation of HTNFs.

(Shell Rotella 15W-40) and synthetic

poly--olefin oil with 5 wt.% succin-

imide dispersant. The commercial oil

was initially tested to determine the

direct applicability of nanotube addi-

tives, with the synthetic oil providing

a model system to gain a fundamental

understanding of nanofluids. The mixing

was performed by various schemes of

high-shear homogenization with sonica-tion assistance. Enhanced dispersion by

preliminary dispersal in volatile solvents

such as toluene or chloroform, which act

as compatibilizers for nanotubes with the

oil, was also carried out. Various proto-

cols were evaluated as well as sample

preparation of the nanotubes (drying,

degassing, pretreatment with dispersant,

and various addition sequence methods).

The total mixing energy was always

monitored.

As a screening tool to help evaluate

architectural thermal property changes

as a function of nanotube type, mixing

energy, loading, and various processing

methods, the thermal conductivity test-

ing was used. More than 100 fluids were

designed and manufactured in laboratory

amounts of approximately 100 mL each.

In addition, bulk quantities of selected

HTNFs were manufactured for advanced

testing.

For dynamic flow conditions and

in-situ engine testing, increased sample

distribution, and as deliverables, largervolumes (500 mL to 1 L) of preferred

nanofluids in synthetic oil base were

prepared. One of the preferred sets was

designed under the following specifica-

tions:

High thermal conductivity (TC)

SWNT nanofluid which was still

fluid at room temperature (RT)

High TC SWNT nanogrease

which was paste-like at RT

High TC MWNT nanofluid

which was still fluid at RT High TC MWNT nanogrease

which was paste-like at RT

Based on these specifications the

following sample set was manufactured

with the following compositions:

SWNT nanofluid1.0 vol.%

HiPco purified

SWNT nanogrease2.0 vol.%

HiPco raw

MWNT nanofluid3.0 vol.%

VGCF annealed

High TC MWNT nanogrease

6.6 vol.% VGCF annealed

Single-wall nanotube nanofluids were

prepared by first pre-dispersing oven-

dried nanotubes in chloroform using 15

min. of homogenization and 5 min. of

sonication. This was followed by adding

the appropriate amount of poly--olefin

oil+5 wt.% dispersant to achieve final

volume percent. The chloroform was

removed by closed distillation withmechanical stirring. Final drying was

done under heated dynamic vacuum

until the odor of chloroform could

no longer be detected. Multi-walled

nanotube nanofluids were prepared by

the slow addition of oven-dried VGCF

with homogenization. An additional 15

min. of homogenization was performed.

For higher loading, the nanotubes were

added in 1 vol.% increments with a 3

min. homogenization until the desired

nanogrease consistency was achieved.

Physical Properties of

Nanofluids

The thermal decomposition properties

of base heat transfer fluids and developed

heat transfer nanofluids based on of the

Shell SAE 15W-40 diesel oil and the

BP Amoco DS-166 oil in argon and air

were measured by thermal gravimetric

analysis (TGA) and compared with

each other. The 15W-40 oil began to

decompose around 200C with total



Table II. Hot Disk Thermal Conductivity of Nanotubes Loaded in 15W-40Oil Data Summary

Sample ID NT vol.% Th. Cond. TC/TC_o Th. Diff. Sp. Ht. Probe Depth

15W-40-0 0.000 0.147 1.000 0.111 1.331 3.57915W-40-.25 0.250 0.161 1.098 0.120 1.340 3.725

15W-40-.50 0.500 0.172 1.170 0.121 1.424 3.826

15W-40-1.00 1.000 0.215 1.461 0.173 1.257 3.871

Table III. Thermal Conductivity of Bulk Nanofluids

Nano Material Composition TC Ratio

SWNT Nanofluid 1.0 vol.% HiPco purified 1.53

SWNT Nanogrease 2.0 vol.% HiPco raw 1.85

MWNT Nanofluid 3.0 vol.% VGCF annealed 1.97

High TC MWNT Nanogrease 6.6 vol.% VGCF annealed 3.32

Nanolubricant 1 0.175 HiPco ESD grade, B&S SAE 30 1.22

Nanolubricant 2 0.175 HiPco ESD grade, B&S BP 166 1.35

decomposition occurring at about

500C. Again, a triplicate measurement

was made using 1020 mg of sample

to ensure accuracy of the results. The

TGA data of SWNTs, diesel oil, and 1

vol.% solutions of SWNTs in diesel oil

showed some of their properties. Two

conditions were used: the inert argon

atmosphere and the oxidizing environ-

ment of air in order to examine thermaldegradation characteristics and the effect

of oxidation on thermal degradation. The

SWNTs showed enhanced thermal prop-

erties. This is due to their decomposition

temperature occurring at a much higher

temperature when compared to diesel oil

and the nanofluid. As was expected, the

diesel oil began to decompose at around

200C. The nanofluids, however, showed

significant increases in their thermal

decomposition when compared to the

base oils.

The TGA results show that the pres-

ence of nanotubes in oil does not lead

to very large changes in the degradation

temperature of the oil, but that these

changes are significant. This research

also shows that appropriate methods

for processing the nanotubes in the oils

can be developed in order to provide for

increased thermal conductivity without

degrading the oil (these results do not

include the use of functionalization at

this time).

Fourier Transform Infrared

Spectroscopy

A number of the nanofluids were

analyzed by Fourier transform infrared

spectroscopy (FTIR). The study of some

of the key FTIR spectra and their com-

edly to obtain precision statistics. The

results are summarized in Table II and

depicted in Figure 20. These are averaged

data values from four to six measure-

ments of each suspension under similar

conditions. The thermal conductivity

statistical standard deviation averaged

0.004. The TC increase is somewhat

non-linear with nanotube loading with

a substantial increase of >45% with only

1 vol.% nanotube additive.A series of experiments addressing

the effects of various types of nanotubes,

loading, effect of intense mixing on base

oil, and dispersion on thermal conductiv-

ity were performed. The nanofluid inven-

tory developed by the authors provides a

detailed description of the samples that

were prepared and evaluated.

The images of the raw materials used

show that the challenge for SWNTs is

the adequate breakdown of the intercon-

nected network of nanoropes into awell-dispersed system, followed by

further exfoliation of individual nano-

tubes from the nanoropes. The nanoropes

may contain up to 100 individual nano-

tubes. Detailed research in the solution

properties of SWNT indicate that dis-

persed nanotubes can readily aggregate

(super ropes) depending on the fluid

system. In this study, without the use of

additional dispersant or functionalization

of the nanotubes to maintain their sepa-

ration, a similar phenomenon was

parison with those of the base oils show

that the nanotubes lead to alterations

in the spectra, and that specific bond-

ing groups can be identified. Typical

results are shown in Figures 17, 18, and

19. There are spectral changes in three

regions: >3,500, ~2,300, and



0 5102

101

1

10

102

103

104

10 15 20 25 30 35

Shear Rate (Pa)

0.5% Acid Treat SWNT 0.5% Rao SWNT 0.5% MER SWNT

Viscosity(Pa)

0 5 10 102

1

10

102

103

105

104

101

15 20

Shear Rate (Pa)

1% CNI ESD SWNT 1% Carbolex SWNT BP166

Viscosity(Pa)

25 30 35 40

observed and its effect on thermal con-

ductivity. The data in Figure 21 shows

a SWNT nanofluid system where high-

purity HiPco was pre-dispersed in

chloroform, added to 15W-40 oil, and

the solvent removed by low heated

evaporation. Upon sitting, the sample

showed a 25% decrease in TC with 1

vol.% loading along with the same non-

linear behavior. Upon re-dispersion by

a desktop vortexer or test-tube shaker,

the same sample then showed a substan-

tial increase to 45% TC improvement.

Significant loose agglomeration prior to

initial testing is most likely to have

occurred.

The continued use of commercial oil(15W-40) nanofluids led to concerns of

complications of the many additives used

in the oil formulations. This was espe-

cially noted in viscosity, as well as visual

and odor changes in the nature of the

nanofluids with intensive sonication. The

base oil typically comprises only about

25

wt.% additives such as wear/corrosion

inhibitors (zinc thioalkylphosphates),

detergents, viscosity modifiers, and

thermal stabilizers.To focus on the contributions of nano-

tubes and separate interferences, a new

set of fluids was used based on a synthetic

poly--olefin (BP Amoco DS -166) oil

with a single dispersant. Several disper-

sion factors were identified. It was

observed with the commercial diesel oil

that the state of nanotubes before mixing

affected the thermal conductivity. In

addition, the following observations were

made:

Nanotubes that had agglomerated

during aqueous processing and

purification were more difficult to

disperse. They produced lower

thermally conductive suspensions

for the same mixing energy.

As-produced plasma-arc materials

(containing about 2530%

SWNTs) gave higher TC fluids

than purified (>90%) perhaps due

to the agglomeration issues.

The agglomeration of the purified

SWNTs can be overcome by pre-

dispersing them in a polar solvent

(such as CHCl3) and then blending

the suspension into the oil. With

higher concentrations, some re-

agglomeration may occur.

Poly--Olefin (BP Amoco DS -166) Oil

Batches of test fluid of 5 wt.% suc-

cinimide (Chevron Oronite 1107) in

synthetic poly--olefin oil (BP Amoco

DS-166) were prepared and used

throughout all further blending. Suspen-

sions were prepared from a master batch

of 1 vol.% of MWNT, purified SWNTs(HiPco), raw HiPco, and as-produced

SWNTs (plasma). Nanofluids reproduc-

ing the work of Choi et al. were prepared

using a similar source of MWNTs (A.

Rao at Clemson University) and the same

oil/additive ingredients. Suspensions of

1.0 vol.%, 0.5 vol.%, and 0.25 vol.%

were prepared as described in 5 wt.%

succinimide/poly-alpha-olefin.

Results are shown in Figure 22, which

shows for comparison the original pub-

lication result by Choi and co-workers.

At 1% volume, even higher TC improve-

ment (175% vs. 160%) is achieved than

formerly reported. After an investigation

of the MWNTs supplied by Rao and

those used by Choi, it was learned that

Type 2 were the MWNTs most similar

to that used by Choi and co-workers. No

nanofluids were produced at lower load-

ings to accurately relate the non-linear

behavior.

The thermal conductivity of bulk

quantities of nanofluids, designed and

manufactured for advanced testing, was

Figure 24. The viscosityversus shear rate with 0.5%MER SWNT, 0.5% acid-treated CNI ESD SWNT,and 0.5% Rao MWNT inBP166.

Figure 25. The viscosityversus shear rate withBP166, 1% CNI ESD SWNT,and 1% Carbolex SWNT inBP166.

3.00

2.752.502.252.001.751.50

1.251.000.75

Processing/NT

TCRatio

BaseOil

SWNT,Mixed

SWNT,Mixed +Homog

SWNT,Mixed +Homog+ Sonic

SWNT,Pre-Disp.

InToluene

SWHiPcoRaw

VacuumDrip

SWNTRaw,Mixed

Multi-WallRao

Type II

Multi-WallRao

CarbonBlack

Figure 23. The effect of processing parameters and nanotube type on the thermal conductivityof HTNFs.



Table IV. Shape Factor A for CommonFiller Types

Filler Type Aspect Ratio Ratio A

Cubes 1 2Spheres 1 1.50Random Fibers 2 1.58

Random Fibers 4 2.08Random Fibers 6 2.80Random Fibers 10 4.93

Random Fibers 15 8.38Uniaxially Oriented 2 L/D (a)

FibersUniaxially Oriented 0.5(b)

Fibers

(a) in fiber axis, (b) transverse to fiber axis

Table V. Maximum Packing Fraction ofSelected Fillers

Particle PackingShape Order m

Spheres Hexagonal close 0.7405Spheres Face-centered cubic 0.7405Spheres Body-centered cubic 0.60

Spheres Simple cubic 0.524Spheres Random loose 0.601

Spheres Random close 0.637Irregular Random close ~0.637Fibers 3-D random 0.52

Fibers Uniaxial hexagonal close 0.907Fibers Uniaxial simple cubic 0.785Fibers Uniaxial random 0.82

also carried out. The results are shown

in Table III.

Effect of Processing Variables

In the first generation of HTNFs with

SWNT nanofluids, the largest TC

improvements were achieved with raw

HiPco materials (60% at 1 vol.%), though

considerably lower than MWNT results

described. This was not expected, as the

SWNTs have lower defects and thus,

higher inherent thermal conductivity,

which should impart a greater contribu-

tion to the overall TC. Furthermore, with

the raw HiPco having very high intercon-

nectivity, it was difficult to readily dis-

perse them, often resulting in thick

pastes, not the desired free-flowing

fluids. Based on the characteristics of

the nanotubes used, it was surmised that

the following issues are at play: Strong van der Waals attractions

among the more finely nanostruc-

tured carbons is not allowing ade-

quate dispersion, but rather form-

ing a loose intertwined template

that adsorbs the oil and signifi-

cantly increases the viscosity.

Minimal exfoliation of the indi-

vidual nanotubes from the nano-

ropes does not take advantage of

the smaller dimensions, and hence

colloidal stability.

There is a strong propensity to re-

agglomerate, creating large vol-

umes of non-filled, thermal insu-

lative domains, thereby reducingthe overall TC.

The very high surface area of

SWNTs may have a significant

amount of adsorbed water and

gases providing a thermal barrier

interface and minimizing thermal

transfer from the matrix to the

conductive tubes.

Aggregation and bundling also re-

duce the interaction contact sur-

face area with the oil matrix, drop-

ping the effective thermal transfer

rate.

As this study was originally focused

on the manufacture of SWNT nanofluids,

a series of experiments was performed

to address issues that concentrated on

nanotube sample preparation and pro-

cessing. Typical results are summarized

in Figure 23. Typical levels of the thermal

conductivity increments are: nanomate-

rials Type 1, 60%; Type 2, 175%; and

Type 3, 243%.

RheologyThe effect of the shear rate on the

viscosity of HTNFs and nanolubricants

based in the BP166 at room temperature

(gap = 150) was investigated. Typical

results show that the viscosity decreases

sharply with increasing shear rate as

presented in Figures 24 and 25. This

confirms the lubricating potential of these

nanolubricants. These properties are

currently being measured at room tem-

perature and at two elevated tempera-

tures, 100C and 150C, respectively, in

order to understand the performance/

architecture relationships at appropriate

working conditions for each specific

nanolubricant.

Modeling of Heat Transfer

Nanofluids Performance

This research confirms that the rule

of mixtures is not adequate to describe

the carbon nanotube contributions to

thermal conductivity. Thus, a new

approach was developed based on the

morphologies of the agglomerates of the

carbon nanotubes observed and on the

Nielsen Model for conductive fillers ina matrix. These observed morphologies

are represented schematically in Figure

26. It is assumed that nanotubes in sus-

pension would not remain straight and

rigid, but would be in a dynamic folded

state, thereby providing an effective

agglomerate of various aspect ratio (L/D)

shapes. This model is based on Einsteins

viscosity model and can be represented

by the following equation:

The physical meaning of these param-

eters is:

K = thermal conductivity of the binary

system;

A quantifies the particle shape;

k1

= thermal conductivity of the base

fluid;

k2

= thermal conductivity of carbon

nanotubes;

2 = volume loading of carbon nano-

tubes.

Figure 26. A schematicof typical morphologiesobserved in the disper-sion of SWNT agglom-erates.

Kk A BB1

2

211= +


12/12

m

=maximum packing efficiency

The constants A and m

are calculated

in Tables IV and V, respectively.

This model can be used to explorevarious factors that can affect thermal

conductivity of nanofluids such as L/D

of the nanotube agglomerate, decoupled

orientation effects (along fiber axis, Kl,

transverse to fiber Ktrans

), along with

overall volume loading. An Excel pro-

gram was written to investigate these

relations. An example of the output is

shown in Figure 27. It is interesting to

note that there are non-linear portions to

the model, which may assemble to link

to general non-linear behavior observed

by Choi and co-workers. Further model

development synergistic with experi-

mental nanofluid production is in prog-

ress.

The degree of dispersion achieved in

all three types of nanomaterials has been

very good. However, microstructural

observations by optical microscopy,

SEM, and TEM show that most of the

carbon nanotubes are present in the form

of agglomerates and not as single indi-

vidual nanotubes. This is consistent with

the values obtained by particle sizeanalysis and is in excellent agreement

with the results predicted by the

model.

The degree of stability achieved has

been very good in Type 1 and Type 3,

and good in Type 2 of the nanomaterials

described. It is expected that the over-

coming of some of the most important

challenges as described will result in

further considerable advances in both

the degree of dispersion and degree of

stability and thus the effectiveness of

these HTNFs and nanolubricants.CONCLUSIONS

Carbon nanotubes, both SWNTs and

MWNTs, and VGCFs have been found

to considerably increase the thermal

conductivity of many heat transfer fluids

such as mineral and synthetic oils, water,

water/ethylene glycol mixtures, and

other commercial heat transfer fluids

such as antifreeze. Prior functionaliza-

tion and mixing in appropriate solvents,

followed by homogenization and sonica-

tion were some of the methods used to

achieve various levels of dispersion of

carbon nanotubes in these fluids. A 175%

increase in the thermal conductivity was

obtained for 1 vol.% load. Increments

of 243% have been achieved at 6% loads

with considerable increase in the viscos-

ity. The thermal conductivity levels

obtained varied considerably with the

type of carbon nanotube used, the load,

and the processing route. More aggres-

sive mixing protocols lead to the altera-

tion of the additives in the commercialheat transfer fluids. Carbon nanotubes

are believed to contribute to the thermal

conductivity of HTNFs through Brown-

ian motion and through a tridimensional

network formation within the fluid. The

preliminary evaluation of the viscosity

of heat transfer nanofluids and other

dynamic properties show that these fluids

have the potential to find a place in many

applications such as engine cooling

systems, oil coolers, and heat pumps to

significantly improve their thermal andlubricating performance.

ACKNOWLEDGEMENTS

This work is supported by the U.S.

Army Research Laboratory, Aberdeen

Proving Ground, under Cooperative

Agreement number DAAD19-02-2-0011.

The authors acknowledge the collabora-

tion of Professor E.V. Barrera of Rice

University, and the support from Carbon

Nanotechnology, Inc., and Professors A.

Rao, Clemson University, and S. Jin,

University of California at San Diego

for providing some of the carbon nano-

tubes for this project. The authors also

want to acknowledge B. Rostro, D.

Rebsom, and H. Hong for doing some

of the measurements.References

1. J.C. Maxwell, A Treatise on Electricity andMagnetism, 2nd edition (Wotton-under-Edge, U.K.:Clarendon Press, 1881).2. R.L. Hamilton and O.K. Crosser, I and ECFundamentals, 1 (3) (1962), p. 187.3. U.S. Choi, Developments and Applications of Non-Newtonian Flows, eds. D.A. Siginer and H.P. Wang, Vol.231/MD-Vol.66 (New York: American Society ofMechanical Engineers, 1995), p. 99.4. J.A. Eastman et al., Enhanced Thermal ConductivityThrough the Development of Nanofluids, Mater.Research. Soc. Symp. Proc. 457 (Warrendale, PA:MRS, 1997), pp. 311.

5. S. Lee et al., Measuring Thermal Conductivity ofFluids Containing Oxide Nanoparticles, Journal ofHeat Transfer, 121 (1999), pp. 280289.6. J.A. Eastman et al., Anomalously IncreasedEffective Thermal Conductivities of Ethylene Glycol-Based Nanofluids Containing Copper Nanoparticles,Appl. Phys. Lett., 78 (6) (2001), pp. 718720.7. H. Masuda et al., Netsu Bussei (Japan), 4 (1993),pp. 227233.8. S.U.S. Choi et al., Anomalous Thermal ConductivityEnhancement in Nanotube Suspensions,Appl. Phys.Lett., 79 (14) (2001), pp. 22522254.9. S. Iijima, Nature(London), 354 (1991), p. 56.10. J. Horne, M. Whitney, and A. Zettl, Synth. Met., 103(1999), p. 2498.11. S. Berber, Y.K. Kwon, and D. Tomanek, Phys. Rev.

Lett., 84 (2000), p. 4613.12. J. Hilding et al., Dispersion of Carbon Nanotubesin Liquids,Dispersion Science and Technology, 24 (1)(2003), pp. 141.13. Y. Sabba and E.L. Thomas, High-ConcentrationDispersion of Single-Wall Carbon Nanotubes,Macromolecules, 37 (2004), pp. 48154820.14. C. Richard et al., Supramolecular Self Assembly ofLipid Derivatives on Carbon Nanotubes,Science, 300(2003), pp. 775778.15. S. Niyogi et al., Ultrasonic Dispersions of Single-walled Carbon Nanotubes, J. Phys. Chem. B, 107(2003), pp. 87998804.16. M.F. Islam et al., High Weight Fraction SurfactantSolubilization of Single-Wall Carbon Nanotubes inWater,Nanoletters, 3 (2) (2003), pp. 269273.

F.D.S. Marquis is with the Department of Materials

and Metallurgical Engineering at the South Dakota

School of Mines and Technology in Rapid City,

South Dakota. L.P.F. Chibante is with NanoTex

Corporation in Houston, Texas.

For more information, contact F.D.S. Marquis, SouthDakota School of Mines and Technology, Departmentof Materials and Metallurgical Engineering, RapidCity, SD 57701; (605) 394-1283; fax (605) 394-3369;e-mail [email protected].

Figure 27. The effect of loading andarchitectural parameters (aspect ratio) onthe thermal conductivity of nanomaterialswith carbon nanotube agglomerates.

0.000.0

0.51.0

1.5

2.0

2.5

3.0

3.5

4.0

0.02

Volume Loading

L/D = 100 L/D = 50 L/D = 10

TCRatio

0.04 0.06 0.08 0.10 0.12

= 1+ 1- m

2

m

2

Bk k

k k A=

+

2 1

2 1 2

1/

14. improving the heat transfer of nano fluids and nano lubricants with carbon nanotubes

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