FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention: DISPERSIONS OF CARBON-CONTAINING NANOP ARTICLES
2. Applicant(s)
NAME NATIONALITY ADDRESS
TATA CONSULTANCY Indian Nirmal Building, 9th Floor, Nariman Point, SERVICES LIMITED Mumbai, Maharashtra 400021, India
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

TECHNICAL FIELD
[0001] The present subject matter relates, in general, to dispersions of carbon-containing
nanoparticles, and in particular, to dispersions of carbon nanotubes in polar solvents.
BACKGROUND
[0002] Dispersion of nanoparticles in fluids is one of the emerging areas of research
nowadays. Generally, a particle having size of the order of 100 nanometer (nm) or less is referred as nanoparticle. The dispersions (referred as nanofluids) are expected to enhance transport and/or structural properties of base fluids. A base fluid may be a solvent which dissolves various substances. The dispersions are typically engineered by suspending nanoparticles, preferably those possessing higher thermal conductivity, such as carbon, metal and metal oxides, with average sizes below 100 nanometer (nm) in the solvent. The dispersions have primarily been of interest due to their reported anomalous enhancement of transport properties of the solvent. In an example, dispersions of carbon-containing nanoparticles may enhance thermal transport properties of the base fluid.
[0003] Various industrial applications require the dispersions of nanoparticles with enhanced properties for effective and efficient working of the applications. Generally, dispersions of carbon nanotubes (CNTs) are used for the various industrial applications, such as heat transfer, mass transfer, lubrications, and composites. The CNTs are made up of graphene sheets that may be rolled in the form of tubes with capped ends. Various conventional techniques are used to prepare dispersions of CNTs, such as milling and ultra-sonication.
[0004] Typically, the dispersions of CNTs obtained from the conventional techniques remain stable for a short duration, say, for several days. For example, the dispersions of CNTs display the enhanced properties only for a limited number of cycles of the industrial applications and the dispersions needs to be replaced for the next cycles of the industrial application.
SUMMARY
[0005] This summary is provided to introduce concepts related to method of preparing
dispersions of carbon-containing nanoparticles and these concepts are further described below in the detailed description. This summary is not intended to identify essential features of the

claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
(0006] In an implementation of the present subject matter, method(s) for preparing
dispersions of carbon-containing particles in polar solvents are described. The preparation of dispersion includes mixing substances, such as the carbon-containing nanoparticles, a milling media, the polar solvent, and a surfactant to obtain the dispersion. Abraded particles of the milling media adheres to the carbon-containing nanoparticles forming a layer and the surfactant is disposed on the layer formed by the particles of the milling media on the carbon-containing nanoparticles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The detailed description is described with reference to the accompanying figures.
In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of the method(s) in accordance with the present subject matter are described, by way of example only, and with reference to the accompanying figures, in which:
[0008] Fig. 1 illustrates a system for preparing dispersions of carbon containing
nanoparticles, in accordance with an embodiment of the present subject matter.
[0009] Figs. 2(a) and 2(b) illustrate images of carbon nanotubes (CNTs) used for
preparation of the dispersions captured through a scanning electron microscope (SEM), in accordance with an embodiment of the present subject matter.
[0010] Figs. 3(a) and 3(b) illustrate images of the CNTs captured through a transmission
electron microscope (TEM), in accordance with another embodiment of the present subject matter.
[0011] Fig. 4 illustrates an absorbance pattern of a dispersion of carbon nanotubes, in
accordance with an embodiment of the present subject matter.
[0012] Fig. 5 illustrates an energy dispersive x-ray spectroscopy (EDAX) analysis of a
post milled CNT dispersion, in accordance with an embodiment of the present subject matter.

[0013] Figs. 6(a) and 6(b) illustrate images of the post milled CNT dispersion in water
captured through a scanning electron microscope (SEM), in accordance with an embodiment of the present subject matter.
[0014] Fig. 7 illustrates a flowchart of a method for forming stable CNT dispersions in
polar solvents, in accordance with an embodiment of the present subject matter.
[0015] Fig. 8 illustrates a test apparatus for testing the heat transfer capability of the CNT
dispersion, according to an embodiment of the present matter.
[0016] It should be appreciated by those skilled in the art that any block diagrams herein
represent conceptual views of illustrative systems embodying the principles of the present subject matter.
DETAILED DESCRIPTION
[0017] Method and system for preparing dispersions of carbon-containing nanoparticles
in polar solvents are described. A polar solvent, such as water and ethylene glycol (EG), has molecules whose electric charges are unequally distributed. The polar solvent acts as a base fluid for the dispersion. The carbon-containing nanoparticles, such as carbon nanotubes (CNTs), fullerenes and carbon black, may be used for preparing the dispersions. Further, the dispersions of CNTs prepared from the present subject matter remain stable for a longer period of time and may enhance transport properties of the base fluid. This makes CNT based dispersions a good option for various industrial applications, such as heat transfer applications, mass transfer applications, lubrication, and composites. For example, a heat transfer fluid with improved thermal properties and stability may result in more economical and efficient processes and higher safety margins.
[0018] As known, the CNTs are hydrophobic and therefore cannot be dispersed in any of
the typical polar solvents used commercially in heat and mass transfer applications. Various physical treatments, such as ultrasonication generally do not provide the CNTs dispersions in polar solvents that are stable for even as long as more than a couple of days. Further, various chemical treatments to obtain stable CNTs dispersions may cause extensive damage to structure of the CNTs and destroy properties of the CNTs. The damaged CNTs may be of no use while preparing dispersions. Also, preparation of the dispersion of the CNTs may be a challenging task

as the high aspect ratios in combination with high flexibilities increase the possibilities of formation of aggregates of CNTs that may get entangled. Such entangled aggregates may be difficult to disperse without damaging the nanotubes in different ways.
[0019] Conventional milling techniques, such as dry milling and wet milling, may be
used for preparing stable CNT dispersions. Milling may be understood as the process of mixing and size reduction by attrition. However, cut and open ends of the CNTs may relapse while milling thereby making the CNT dispersion unstable. For example, dry milling may cause the CNTs to rapidly collapse and transform into graphene sheets.
[0020] According to an implementation of the present subject matter, method and system
for preparing stable dispersions of carbon-containing particles in polar solvents, using milling are disclosed. The carbon-containing particles, such as CNTs, fullerenes, and carbon black may be used for preparing dispersions. In the present implementation CNTs are used for preparing the dispersions. Polar solvents, such as water, ethylene glycol (EG), and mixture of water and EG of varying concentrations may be used. The method includes mixing substances, such as the CNTs with milling media, the polar solvent, and a surfactant to obtain a primary mixture. In the present implementation, the milling media may be an oxide milling media, such as Silica, Alumina, and Zirconia.
[0021] In the above implementation of the method, wet milling may be used on the
primary mixture to produce stable dispersions of CNTs coated with metal oxide in various solvents in the presence of surfactants. The wet milling may be performed at a predefined rate such that a stable dispersion is obtained without affecting the CNTs structure. The stable dispersions of the above mentioned substances may be produced in presence of suitable surfactant, such as guar gum, cetyl trimethylammonium bromide (CTAB), and cetyl dimethylammonium bromide (CDAB).
[0022] Accordingly, the wet milling may cause the milling media particles to abrade
slowly. The abraded particles of the milling media may attach to the CNTs in such a manner so as to form a coating on the CNTs. The coating formed on the CNTs may be in patches. The uncoated milling media may be removed from the primary mixture obtained after milling process by using a screen having pores in range of about 4 to 5 millimeters.

[0023] The coating as provided by the milling media may enhance the stability of the
dispersion that is produced. In another implementation, the surface modified CNTs obtained after the wet milling may include three layers. A first layer may include the CNTs, a second layer may include the metal oxide milling media, and a third layer of the surfactant. These surface modified CNTs dispersions are found to be stable for a long duration, say, more than 13 months and even up to 2 years.
[0024] In addition, desired functional groups of surfactant may adsorb at open ended
CNTs to reduce overall energy of a system to make the system thermodynamically stable. The dispersions of the CNTs provide enhanced transport properties, especially heat and mass transfer, the stable CNT dispersion obtained in accordance with the present subject matter may be used in the field of heat transfer, mass transfer, lubrication and composites.
[0025] It should be noted that the description merely illustrates the principles of the
present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended to be only for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art. Moreover, all statements herein reciting principles, aspects, and embodiments of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0026] While aspects of described method for preparing stable CNT dispersions in polar
solvents can be implemented in any number of different production environments, and/or configurations, the embodiments are described in the context of the following environment(s).
[0027] Fig. 1 illustrates a block diagram representation of a system 100 implementing a
method for preparing dispersions of carbon-containing nanoparticles, according to an embodiment of the present subject matter. It will be understood that intermediate mixtures produced at different levels and stages of preparation of the dispersions may be produced in different batches and in various quantities, as would be understood by those skilled in the art.

[0028] In one implementation, carbon-containing nanoparticles 102, a polar solvents 104,
a milling media 106, and a surfactant 108 are mixed together to form a primary mixture 110. The carbon-containing nanoparticles 102, such as carbon nanotubes (CNTs), fullerenes, and carbon black may be used for preparing dispersions. In one example, the various substances of the dispersion, i.e., the CNTs 102 in range of 0.1 to 3.5 gram (gm), the milling media 106 in range of 350 to 450 gm, the polar solvent 104 in range of 140 to 180 gm, and the surfactant 108 in range of 0.035 to 1.6 gm are used to form the primary mixture HO.In the present implementation, CNTs 102 are used for preparing the dispersions. The CNTs 102 are of two types, namely, single wall and multi-wall. The CNTs 102 having about 1 to about 8 layers of graphene may be referred as single walled carbon nanotubes (SWCNTs). The SWCNTs are considered to be difficult to synthesize. Further, the CNTs 102 having about 8 to about 50 layers of graphene may be referred as multi-wall carbon nanotubes (MWCNTs). Conventional methods of synthesis of the CNTs 102 may include arc-discharge, catalytic decomposition of hydrocarbons, and chemical vapor deposition. Further, the CNTs 102 are observed to possess high mechanical and electrical properties due to typically high aspect ratios of the CNTs 102, Although the description herein is with reference to the CNTs 102, it will be understood to a person skilled in the art that the present subject matter may be implemented for other carbon-containing nanoparticles.
[0029] Figs. 2(a) and 2(b) illustrate images 102-1 and 102-2 CNTs 102 used for
preparation of the dispersions captured through a scanning electron microscope (SEM), in accordance with an embodiment of the present subject matter. Further, Figs. 3(a) and 3(b) illustrate images 102-3 and 102-4 of the CNTs 102 captured through a transmission electron microscope (TEM), in accordance with another embodiment of the present subject matter. The images captured through various techniques as mentioned above may confirm presence of the CNTs 102 in test samples before milling.
[0030] Referring, back to Fig. 1, the polar solvent 104 may be selected from Water, EG,
mixture of Water-EG and other based aqueous mixtures of polar solvents 104 of varying concentrations. The polar solvents 104 have molecules whose electric charges are unequally distributed, leaving one end of each molecule more positive than the other. The milling media 106 may be an oxide milling media, such as Silica, Alumina, and Zirconia. Further, the milling media 106 has average particle size in the range of about 6-10 millimeters. The surfactants 108

may be a suitable substance selected based on the polar solvent 104 and may include, but is not limited to, guar gum, cetyl trimethylammonium bromide (CTAB), and cetyl dimethylammoniurn bromide (CDAB), and the like or combinations thereof.
[0031] In an implementation, the primary mixture 110 may be prepared by mixing the
CNTs 102, the polar solvent 104, the milling media 106, and the surfactant 108 in a predefined proportion as mentioned above. In an example, the amount of the milling media 106 and the CNTs 102 need to be in proportion with each other. For example, if the CNTs 102 are more in proportion than the milling media 106, a stable dispersion would not be achieved. In the present implementation, the mixing ratio (by weight) of the CNTs 102 and the milling media 106 may be in the range of about 1:75 to 1:125. Accordingly, in one example, one gram (gm) of the CNTs 102 may be mixed with 75 gm of the milling media 106. The mixing ratio (by weight) between the CNTs 102 and the polar solvent 104 is in range of 1:60 to 1:90. For example, 1 gm of the CNTs 102 may be mixed with 60 gm of the polar solvent 104. The mixing ratio (by weight) between the surfactant ] 08 and CNTs 102 may be in range of about 1:3 to 1:5, where 1 mg of the surfactant 108 is mixed with 3 mg of the CNTs 102. In the present example, concentration of the surfactant 108 in the polar solvent 104 is also adjusted so that concentration of the surfactant 108 lies below its critical micelle concentration (CMC). The CMC may be defined as the concentration above which micelles may be formed and almost all additional surfactant 108 molecules may become a part of the micelles. The micelles may be understood as aggregates of surfactant molecules that may be dispersed in a liquid colloid.
[0032] In the above implementation, a mill 112 may be used for milling the primary
mixture 110. The mill 112 may perform a wet milling on the primary mixture 110 to produce stable dispersions of CNTs 102. The mill 112 may be a planetary mill; however, it will be evident that the mill 112 may perform wet milling in other types of mills, such as a ball mill and a rod mill. When the mill 112 performs the wet milling, the abraded particles of the milling media 106 may get coated on the surface of the CNTs 102 in the polar solvents 104 in the presence of surfactants 108 (also referred as dispersants).The surfactant 108 may help in preparing the stable dispersions of the above mentioned substances. The surfactants 108 may be selected based on the polar solvents 104. The surfactant 108, such as guar gum, cetyl trimethylammonium bromide (CTAB), and cetyl dimethylammoniurn bromide (CDAB) may be

added to promote uniform suspension of particles and prevent agglomeration. When the substances are milled by the mill 112, slurry may be formed that may be diluted by use of a mixture of the surfactant 108 and the polar solvent 104.
{0033] As mentioned above, the mill 112 may perform wet milling on the primary
mixture 110 to modify surface of the CNTs 102 with the metal oxide milling media so to as to produce stable dispersions in the polar solvents 104. The mill 112 may mill the primary mixture 110 at a predefined rate and for a predefined amount of time. For example, in the present implementation, the primary mixture 110 may be milled mildly for about an hour at a rate of about 125-250 revolutions per minute (RPM). It will be understood that the predefined rate, i.e., RPM may be chosen for milling the primary mixture 110 such that the surface of the CNTs 102 are not affected. In other words, the RPM may be high enough to produce the dispersion and cause self-grinding of the milling media 106, but at the same time the RPM may be kept low enough for protecting the CNTs 102 from being broken. Similarly, the time taken by the mill 112 for milling the components may need to be balanced such that a stable dispersion is formed.
[0034] Accordingly, the wet milling performed by the mill 112 may cause the milling
media 106 particles to abrade slowly. The abraded particles of the milling media 106 may attach to the CNTs 102 in such a manner so as to form a coating on the CNTs 102. In an example, the milling media 106 particles may get deposited on the CNTs 102. It will be understood that the milling media 106 may form a non uniform coating on the CNTs 102. In other words, the coating may be in patches on the CNTs 102.
[0035] The dispersion of CNTs 102 obtained in accordance with the abovementioned
embodiment of the milling process, are stable over a long duration of time. The same may be demonstrated by an absorbance pattern of the obtained dispersion. For example, Fig. 4 illustrates an absorbance pattern 400 of dispersion of the CNTs 102, in accordance with an embodiment of the present subject matter. A curve 402 depicts absorbance of a freshly obtained CNTs 102 dispersion having a peak at 253 nm that may signify presence of dispersed CNTs 102 in the dispersion and a curve 404 depicts absorbance of a 7 months old CNTs 102 dispersion having a peak around 253 nm. The peak in curve 402 confirms the presence of dispersed CNTs 102 in the 7 months old dispersion.

[0036] Further, Fig. 5 illustrates an energy dispersive x-ray spectroscopy (EDAX)
analysis 500 of post milled CNTs 102 dispersion, in accordance with an embodiment of the present subject matter. The EDAX analysis 500 of the post milled samples may indicate the presence of the oxides in the obtained CNTs 102 dispersion. For example, in this case, where silicon dioxide particles were used as the milling media 106, the EDAX analysis 500 shows the presence of silicon and oxygen along with carbon. Three peaks 502, 504 and 506 in the Fig. 5 represents the carbon, oxygen, and silicon respectively which confirms the presence of the CNTs 102, the milling media 106, and the surfactant 108 in the dispersion obtained after milling of the primary mixture 110.
[0037] Referring to Fig. 1 once again, in an implementation, a screen 114 may be used
for removing the uncoated milling media 106 from the dispersion after the milling process performed by the mill 112. The screen 114 may include pores having size in range of about 4 to 5 millimeters. The screen 114 may facilitate filtration of the uncoated milling media 106 from the milled primary mixture 310 in an efficient manner to obtain a dispersion 116. The dispersion 116 obtained may not require ultrasonication and is found to be stable even after diluting to over 5 L. Further, the coating as provided by the milling media 106 may enhance the stability of the dispersion 116 that may be produced. Figs. 6(a) and 6(b) illustrate images 600-1 and 600-2 of the dispersion 116 in water captured through scanning electron microscope (SEM), in accordance with an embodiment of the present subject matter. Accordingly, images 600-1 and 600-2 illustrate structure of the finally obtained stable dispersion 116 at a magnification of 101.38 Kx and 223.43 Kx respectively.
[0038] In another implementation, the surface modified CNTs 102 that may be obtained
after the wet milling may include three layers. A first layer may include the CNTs 102, a second layer may include the coating of the milling media 106, and a third layer of the surfactant 108. These surface modified CNTs 102 dispersions may remain stable for a long duration, say, more than about 13 months and even up to 2 years. The desired functional groups of surfactant 108 may get adsorb at open ended the CNTs 102 to reduce overall energy of a system to make the system thermodynamically stable. As the dispersions of the CNTs 102 provide enhanced transport properties, especially heat and mass transfer, the stable CNTs 102 dispersion obtained

in accordance with the present subject matter may be used in. the field of heat transfer, mass transfer, lubrication, and composites.
[0039] Fig. 7 illustrates a flowchart of a method 700 for forming stable CNT dispersions
in polar solvents, in accordance with an embodiment of the present subject matter. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or an alternative method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein.
[0040] At block 702, substances, such as carbon-containing nanoparticles 102, a polar
solvent 104, a milling media 106, and surfactant 108 may be mixed together to obtain a primary mixture 110. In one implementation, the carbon-containing nanoparticles, such as carbon nanotubes (CNTs) 102 are used for obtaining the primary mixture 110. The polar solvent 104 may be a solvent, such as water, ethylene glycol (EG), and mixture of water and EG in varying concentration. Further, the miffing media 106 may include metaf oxides, such as Silica, Alumina, and Zirconia. The surfactant 108, such as guar gum, CTAIij, and CDAB may be used for preparing the primary mixture 110.
[0041] At block 704, the primary mixture 110 may be milled by a mill 112 to form a
coating of the milling media 106 and the surfactant 108 on the carbon-containing nanoparticles. In one implementation, the mill 112, such as a planetary mill may be used to perform a wet milling on the primary mixture 110 at a predefined rate and for a predefine time. The wet milling process may result in abrasion of the milling media 106 particles. The abraded milling media particles may get adhered to the CNTs 102. For example, the milling media 106 may entirely cover or may get attached to the CNTs 102 at a few patches to form a coating on the CNTs 102.
[0042] At block 706, uncoated milling media 106 may be removed from the primary
mixture 110 obtained after milling, using a screen 114 to obtain a dispersion 116. The adherence of the milling media particles may result in enhancing the stability of the dispersion 116. In one implementation, the milling media 106 may form a layer on the CNTs 102 and the layer is further covered with the surfactant 108. The uncoated milling media may be removed using the screen 114. The screen 114 pore may have size in range of 4 to 5 run. The dispersion 116

obtained may not require ultrasonication and may remain stable for a long duration, say, more than two years.
[0043] Although embodiments for stable CNTs dispersions in polar solvents have been
described in language specific to structural features and/or methods, it is to be understood that the present subject matter is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as exemplary implementations for the stable CNTs dispersions in polar solvents.
VALIDATION AND RESULTS
[0044] The results of methods for preparing stable dispersion in polar solvents have been
validated using following examples. It will be understood that the examples discussed herein are only for the purpose of explanation and not to limit the scope of the present subject matter. Further, the test results are shown for a specific example of the dispersions 116 and should in no way be construed as the only stable dispersion 116 that can be formed through the described method.
[0045] The dispersion 116 produced with the above described method was tested for heat
transfer efficiency under convection using a test apparatus as illustrated in Fig. 8. The test apparatus was a shell and tube type of heat exchanger. The test apparatus included a customized double pipe heat exchanger 805, which was fabricated using material, such as glass, steel, copper, and combination thereof. For the purpose of testing, the length of a heat transfer region may be about 0.52 meters. Further, the inner diameters of an inner tube 805-1 and an outer tube 805-2 may be about 6 millimeters and 25.4 millimeters respectively. Further, the inner tube 805-l and the outer tube 805-2 may have a thickness of about 1 millimeter. It would be understood by those skilled in the art that the inner tube can either be a straight tube, or can form a coil. In the present example, the inner tube 805-1 used may be in coil form having around 23 to 40 coils with a pitch between two coils of around 10 to 12 mm. Further, the outer pipe 805-2 may be a straight tube with a diameter in the range of 25 mm to 100 mm which houses the inner coiled tube.
[0046] To provide insulation from the atmosphere, the outer surface of the double pipe
805 was insulated with Polyurethane Foam (PUF) and glass wool. To test the heat transfer

capability of the CNTs dispersion produced using the above described method. In the validation, the dispersion 116 obtained is referred to as a nanofluid 810. The nanofluid 810 was passed through the inner tube 805-1 and heated water 815 was passed through the outer tube 805-2 to transfer heat to the dispersion 116. In another implementation of the present subject matter, instead of the heated water 815, steam was used for the purpose of heat transfer. The temperature at the inlet and the outlet of the inner tube 805-1 was measured using thermocouples and recorded through a data acquisition system 825 as a function of time. Based on the recordings, a time-temperature relation, of both the water flowing through the outer tube 805-2 and the nanofluid 810 flowing through the inner tube 805-1 was measured on a continuous basis. The nanofluid 810 was flown in the inner tube 805-1 is heated by hot water that was received from a constant temperature water bath, flows through the outer tube 805-2, Heat transfer took place through the stainless steel heat transfer contact surface. The fluids were flown in counter current mode. Further, the nanofluid 810 after passing through douhle pipe heat exchanger 805 was collected in a collector 820.
[0047] To analyze the data obtained in the form of inlet and outlet temperatures of the
two fluids at steady state, Log Mean Temperature Difference (LMTD) and Overall Heat Transfer Coefficient (U) were calculated. As would be known to a person skilled in the art, 'U' reflects the effectiveness of the heat exchanger. In other words, higher the value of 'U', more effective is the heat exchanger. To compare the nanofluids 810 (MWCNT in Water with dispersant) with water (base fluid), the ratio of the 'U' for nanofluid to 'U' for water was calculated.
[0048] The outer fluid flow rate was set at about 180 L/hr, the test fluid flow rate was set
at about 49.8-50 L/hr and the constant bath temperature was set at about 80 degree Celsius, the enhancement with surface modified dispersion 116 in water as the heat transfer fluid was found to be in the range of about 1.2% to 10% over water as the heat transfer fluid. This results in an equivalent percentage of decrease in the heat transfer area and. hence corresponding amount of decrease in the material used for the construction of the heat exchanger for same performance. In another example, the dispersion 116 was tested for cyclic heat transfer tests. The dispersion 116 produced with alumina balls was tested for performance when subjected to cyclic heating and cooling cycles. The enhancement for about 0.35% (wt%) was found to result in the range of about 5% to about 10% enhancement in the overall heat transfer coefficient over the number of

test cycles that was conducted to test the efficacy of the nanofluid 810. This may result in minimal decrease on the performance of the dispersion 116 that was produced by the method as described above.
[0049] In yet another example, effect of concentration of the CNTs 102 on heat transfer
properties was tested. Concentrations ranging from about 0.0021% to about 0.35% (in weight) were produced by the method described above. These concentrations may then be tested by the technique as described in the last example. It was found that, as the concentrations increase, the enhancement produced also increases. In a scenario, 0.07 % of the dispersion 116 may show enhancement of about 1.22% and 0.35 % (in weight) of the dispersion 116 may show about 8.20% enhancement in the heat transfer properties.
[0050] Further, the stable dispersions 116 produced in accordance with the present
subject matter were found to show very little the change in concentration in 7 months old dispersion 116 when compared to a freshly prepared dispersion 116 of CNTs 102. In a test it was found that the stable dispersions 116 prepared by the method proposed in the present subject matter could be diluted in about 5 liters of the polar solvent 104, such as water. In an example, 0.0021 wt% of the multi-walled CNTs 102 were found to form a stable dispersion even after diluting the dispersion with about 5 liters of water. In another example, 0.07 wt% of the multi-walled CNTs were found to form a stable dispersion 116 in about 5 liters of water.
[0051] Although implementations for preparation of the stable dispersions 116 has been
described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as examples and implementations for producing stable CNTs dispersions.

I/We claim:
1. A dispersion comprising:
carbon-containing nanoparticles (102);
a polar solvent (104);
particles of a milling media (106), wherein the particles of the milling media (106) adheres to the carbon-containing nanoparticles (102) forming a layer; and
a surfactant (108), wherein the surfactant (108) is disposed on the layer formed by the milling media (106) on the carbon-containing nanoparticles (102).
2. The dispersion as claimed in claim 1, wherein the carbon-containing nanoparticles (102) are at least one of single wall carbon nanotubes, multi-wall carbon nanotubes, fullerenes, and carbon black.
3. The dispersion as claimed in claim 1, wherein the polar solvent (104) is at least one of water, ethylene glycol (EG), and a mixture of water and EG in varying concentration.
4. The dispersion as claimed in claim 1, wherein the milling media (106) includes an oxide of at least one of alumina, silicon, and zirconia, having an average particle size in the range of about 6 mm to 10 mm.
5. The dispersion as claimed in claim 1, wherein the surfactant (108) is selected based on the polar solvent (104).
6. The dispersion as claimed in claim 1, wherein the carbon-containing nanoparticles (102) and the milling media (106) have a weight ratio in range of 1:75 to 1:125.
7. The dispersion as claimed in claim 1, wherein the carbon-containg nanoparticles (102) and the polar solvent (104) have a weight ratio in range of 1:60 to 1:90.
8. The dispersion as claimed in claim 1, wherein the surfactant (108) and the carbon-containing nanoparticles (102) have a weight ratio based on a critical micelle concentration (CMC).
9. A method for preparing a dispersion of carbon-containing nanoparticles, the method comprising:
mixing the carbon-containing nanoparticles, a surfactant, a polar solvent, and a milling media to form a primary mixture, wherein the carbon-containing nanoparticles and the milling media have a weight ratio in range of 1:75 to 1:125;

milling the primary mixture to form a coating of abraded particles of the milling media and the surfactant on the carbon-containing nanoparticles; and
removing remaining milling media from the primary mixture using a screen to obtain the dispersion.
10. The method as claimed in claim 9, wherein the polar solvent is at least one of water, a ethylene glycol (EG), and mixture of water and EG in varying concentration, and wherein the surfactant is selected based on the polar solvent.
11. The method as claimed in claim 9, wherein the milling media includes an oxide of at least one of alumina, silicon, and zirconia, having an average particle size ranging between 6 mm to 10 mm.
12. The method as claimed in claim 9, wherein the milling is a wet milling process.
13. The method as claimed in claim 9, wherein the milling is done at speed ranging between 150 rpm to 250 rpm.
14. The method as claimed in claim 9, wherein the milling is done for about an hour.
15. The method as claimed in claim 9, wherein the screen has pores in range of 4mm to 5 mm.
16. A stable dispersion of carbon-containing nanoparticles produced by a method comprising:
mixing 3 parts of a carbon-containing nanoparticles by weight, 1 part of a surfactant by weight, 180 parts of a polar solvent by weight, and 400 parts of a milling media by weight to form a primary mixture;
milling the primary mixture to form a coating of the milling media and the surfactant on the carbon-containing nanoparticles; and
removing uncoated milling media from the primary mixture using a screen to obtain the dispersion of carbon-containing nanoparticles.