14. Improving the Heat Transfer of Nano Fluids and Nano Lubricants With Carbon Nanotubes
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Transcript of 14. Improving the Heat Transfer of Nano Fluids and Nano Lubricants With Carbon Nanotubes
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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.
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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.
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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,
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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.
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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.
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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).
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8/4/2019 14. Improving the Heat Transfer of Nano Fluids and Nano Lubricants With Carbon Nanotubes
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4,000 3,500
450
400
350
300
250
200
150
100
0
50
50
3,000 2,500
Wavenumbers (cm-1)
A
bsorbance
2,000 1,500 1,000 500
3,500
Absorbance
35
30
25
20
15
10
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
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8/4/2019 14. Improving the Heat Transfer of Nano Fluids and Nano Lubricants With Carbon Nanotubes
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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
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9/12JOM December 200540
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
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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.
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11/12JOM December 200542
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= +
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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
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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/