Patent Application
20160045882
The present invention relates to the field of nanotechnology and more particularly concerns a nanofluid containing nanoparticle-decorated multiwall carbon nanotubes (MWCNTs) and a method of preparation thereof.
A variety of industrial, medical and chemical processes require the use of fluid media (solvents) to absorb and transport energy and mass. Although efficient, many technological developments require more from these solvents. By adding surface-stabilized and well-dispersed nanoparticles (NPs) to different solvents, engineered colloidal suspensions, the so-called nanofluids, are being developed with novel properties. Nanofluids have been shown to have higher mass and heat transfer rates, and high and tunable spectral absorptivity over the UV-Visible-Near Infrared ranges.
When developing nanofluids as optically-active fluids, carbon nanotubes (CNTs), including multiwall carbon nanotubes (MWCNTs), offer great advantages due to their intrinsically high broadband absorbance and thermal conductivity, ease of surface functionalization for stabilization, and high aspect ratio. CNTs uniformly and stably dispersed in the nanofluids can then act as volumetrically-distributed supports for different NPs and other complex molecules chosen specifically to impart particular functions or roles to the desired final application. In the prior art, a few nanofluids containing NP-decorated CNTs have been prepared by the mixing of CNT powder with NPs synthesized by laser ablation in water or by chemical reduction of a metal salt in a solution of CNTs. Unfortunately this method does not produce a nanofluid that is stable, and significant agglomeration is often seen after just a few hours.
The challenges to overcome when designing a new nanofluid include 1) stabilizing the dispersed nanoparticles (CNTs included), that agglomerate due to their high surface area, lack of polar chemical groups (such as NH2, COOH, OH, C═O) and the large van der Waals forces present between the nanoparticles; and 2) using fabrication methods that will not degrade the particles and thus destroy the property which is desired for the nanofluid.
Techniques to stabilize the nanoparticles in a nanofluid have been developed, including, for example, adding ionic surfactants to the solution or wet chemical covalent functionalization. However, both of these approaches require long and tedious preparation steps and/or produce suspensions that are chemically more complex and not stable at high temperatures. Additionally, known techniques used to functionalize CNTs generally involve taking a powder of aggregated CNTs, trying to break apart the aggregates through intense mechanical mixing. However, as a large number of CNTs usually remain aggregated, subsequent treatment leads to selective functionalization of the individual CNT surface area.
There remains a need for a stable nanofluid having improved properties and a simple but robust method of fabricating such a nanofluid.
In accordance with a first aspect of the invention, there is provided a nanofluid comprising a base fluid and multiwall carbon nanotubes (MWCNTs) dispersed in the base fluid. The MWCNTs have an outer surface provided with polar functional groups. The outer surface also has decorated portions covered with nanoparticles and undecorated portions where the polar functional groups are exposed.
In one embodiment, the polar functional groups include oxygen-containing functional groups or nitrogen-containing groups.
In another embodiment, the nanoparticles include metal, semiconductor or polymer nanoparticles.
In a further embodiment, the base fluid includes water, at least one polar organic solvent or a mixture of water and at least one polar organic solvent.
In accordance with another aspect of the invention, there is provided a method of preparation of a nanofluid including the following steps:
In one embodiment, the step of growing the MWCNTs by catalyst-free thermal chemical vapor deposition (t-CVD) includes the following steps:
In another embodiment, the step of functionalizing the outer surface of the MWCNTs is performed by plasma-functionalization.
In another embodiment, the step of depositing the nanoparticles on the outer surface of the MWCNTs is performed by pulsed laser ablation (PLA).
In another embodiment, the step of depositing the nanoparticles on the outer surface of the MWCNTs is performed by exposing a metal, semiconductor, or polymer target facing the MWCNTs' surface to a pulsed laser beam.
In another embodiment, the step of detaching and dispersing the nanoparticle-decorated functionalized MWCNTs in the base fluid is performed by ultrasonication.
In accordance with a further aspect of the invention, there is provided a nanofluid containing functionalized multiwall carbon nanotubes decorated with nanoparticles, wherein the nanofluid is obtained by the method of the invention.
Other features and advantages of the invention will be better understood upon a reading of embodiments thereof with reference to the appended drawings.
In accordance with aspects of the invention, there is provided a nanofluid containing NP-decorated multiwall carbon nanotubes (MWCNTs) dispersed in a base fluid, and a method of making such a nanofluid. As will be understood upon a reading of the present description, nanofluids according to embodiments of the invention exhibit long-term stability, heat absorption and other properties suitable for many applications.
Nanofluid Structure and Properties
Generally speaking, a nanofluid consists of a stable colloidal suspension of nanometer-sized particles in a base fluid. In other words, a nanofluid may be defined as a base fluid having nanometer-sized particles uniformly dispersed therein. Nanofluids exhibit enhanced properties with respect to the base fluid, such as higher thermal and mass diffusivities, higher, and possibly selective, gas solubility, and tunable optical absorption coefficient, amongst others. These properties make nanofluids useful in a variety of industrial applications which require the use of fluids to absorb and transport heat and molecules.
Nanometer-sized particles are often defined as particles with at least one dimension below 100 nm. Particles not meeting this threshold, but still of a small enough size to exhibit properties typically associated with nanoparticles, may however still be considered within the scope of the present invention.
The principal nanometer-sized particles dispersed in the base fluid are multiwall carbon nanotubes (MWCNTs). MWCNTs are generally understood to be allotropes of carbon having a cylindrical structure defining an outer surface and an inner surface. MWCNTs have advantageous chemical and physical properties such as high broadband spectral absorptivity, excellent thermal conductivity, ease of surface functionalization, and a large surface over which one can deposit additional sub-entities (e.g. smaller nanoparticles).
The MWCNTs have a broad range of sizes and geometries. They may have structures including straight carbon nanotubes, bamboo-type carbon nanotubes, waved carbon nanotubes, coiled carbon nanotubes and branched carbon nanotubes.
The nanofluids according to some embodiments of the invention contain MWCNTs having a diameter ranging between about 15 and about 100 nm. In another embodiment, the MWCNTs diameter distribution is characterized by a mean diameter ranging from about 30 to about 40 nm. The MWCNTs have a broad length distribution, for example ranging from about 100 nm to about 10 μm, with an average around 1.25 μm.
The term “about”, as used herein before any numerical value, means within an acceptable error range for the particular value as determined by one of ordinary skill in the art. This error range may depend in part on how the value is measured or determined, i.e. the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.
In accordance with embodiments of the invention, the MWCNTs are functionalized so as to include polar functional groups on their outer surface. The polar functional groups generally contain oxygen and/or nitrogen. They can be neutral or ionized groups. The polar groups on the surface of the MWCNTs will enhance their dispersibility into polar solvents such as water, ethanol, methanol, denatured alcohol, glycols etc. . . . . When the polar groups specifically interact with water, they may also be referred to as hydrophilic groups.
In some implementations, the MWCNTs may have been functionalized on their outer surface by plasma functionalization, as will be explained in further detail below. In another embodiment, the polar functional groups are oxygen- or nitrogen-containing functional groups. Oxygenated functional groups include carboxyl, hydroxyl and carbonyl groups. Nitrogen-containing functional groups include amine and amide functional groups. These polar groups are covalently bonded to the outer wall of the MWCNTs.
The presence of polar functions on the surface of the MWCNTs enhances their miscibility with polar solvents such as water or other polar organic chemicals, thus limiting the direct CNT-CNT interactions. Moreover, when the functionalized MWCNTs are placed in a polar-protic solvent, deprotonation of the carboxyl/hydroxyl groups and protonation of the amine/amide groups can lead to a negatively or positively charged MWCNT surface, generating a repulsive force between individual MWCNTs, and thereby limiting agglomeration.
The outer surface of the MWCNTs is also further provided with decorated and undecorated portions. The decorated portions are portions of the MWCNTs' outer surface which are covered with nanoparticles (NPs). The undecorated portions are portions of the MWCNTs' outer surface wherein the polar functional groups resulting from the plasma functionalization are exposed, i.e. the polar groups are in direct contact with the base fluid. The NPs, which may be amorphous or crystalline, confer additional and localized chemical and physical properties to the supporting MWCNTs without destabilizing the suspension.
In an embodiment, the nanoparticles which cover the MWCNTs' outer surface may be metal nanoparticles, such as transition metal nanoparticles. Preferred transition metal nanoparticles include gold (Au), nickel (Ni), iron (Fe), chromium (Cr), cobalt (Co), copper (Cu), silver (Ag), titanium (Ti), and platinum (Pt) nanoparticles. Gold and platinum nanoparticles are preferred nanoparticles. Metal oxide nanoparticles are also encompassed in the group of metal nanoparticles.
In an alternative embodiment, the nanoparticles which cover the MWCNTs' outer surface may be semi-conductor nanoparticles such as, for example, cadmium selenide (CdSe) nanoparticles.
Other nanoparticles may be polymer nanoparticles, for example polystyrene or polypyrrole.
In an embodiment, the nanoparticle size ranges between about 1 and about 60 nm. In another embodiment, most of the NPs have a diameter which is less than about 5 nm. NPs with small size can be advantageous since they offer the largest specific surface area and minimize the use of the MWCNT surface.
The MWCNTs decoration level, which can be defined as the fraction of the total surface area of the MWCNTs which is covered by NPs, ranges from a minimum decoration level to a maximum decoration level for which the nanofluid can perform to its designed intent while avoiding destabilization. This destabilization of the nanofluid will occur when too large a fraction of the stabilizing polar functions loses contact with the base fluid.
In another embodiment, the decorated and undecorated portions are distributed on the outer surface of the MWCNTs in such a way that the NPs and exposed polar groups are available over the entire surface.
The nanofluid requires a base fluid in which the functionalized MWCNTs can be dispersed. A polar base fluid provides a better dispersibility. Preferably, the base fluid is a polar solvent chosen from water and polar organic solvents such as ethylene glycol, propylene glycol, isopropanol, ethanol, methanol or denatured alcohol, to name a few. The base fluid may also be a mixture of polar organic solvents or a mixture of water with at least one polar organic solvent. Deionized or reverse osmosis water may preferably be used as the base fluid for biological applications.
The concentration of NP-decorated MWCNTs in the nanofluid may be up to about 1.0 g/L. In an embodiment, the concentration of NP-decorated MWCNTs in the nanofluid may be up to about 0.150 g/L. In another embodiment, the nanofluid contains from about 0.005 to about 0.01 g/L NP-decorated MWCNT.
In the nanofluid, polar functional groups remain accessible at the surface of the MWCNTs despite the co-existence of the NP-decorated portions. Thus, once the MWCNTs are added to the base fluid, the polar groups can be in direct contact with the base fluid, thus increasing dispersibility of the MWCNTs in the nanofluid. When the dispersibility of the MWCNTs is optimal, one can say that the MWCNTs are “homogeneously” dispersed in the base fluid. One can also refer to a mono-disperse nanofluid. The NP-decorated functionalized MWCNTs do not settle out of the fluid (they do not aggregate) over an extended period of time at the required working temperature and the nanofluid can be qualified as being “stable”.
In the context of the invention, the “stability” of the nanofluid thus refers to the period of time during which the MWCNTs remain dispersed in the base fluid at the required working temperature, i.e. the period of time during which the MWCNTs do not settle out of the fluid. The stability of the nanofluid will depend on the decoration level at the surface of the MWCNTs, but also on the nature of the base fluid. At the lowest decoration level, stability over time will be increased due to the availability of the polar groups at the surface of the MWCNTs. Moreover, stability over time can be increased with solvents such as ethylene glycol, polyethylene glycol or denatured alcohol compared to water due to various fluid properties such as viscosity, pH, polarity etc.
The required stability of the nanofluid will depend on its intended application. For some applications the nanofluid will only need to be stable for a few weeks while it can be preferable that the nanofluid be stable for months or even years in other applications. For the context of this invention, a nanofluid is classified as stable if the absorptivity of the non-agitated nanofluid, as quantitatively determined by absorption spectroscopy, decreases by less than about 15% over the required period of time at the required working temperature, although specific stability requirements will vary with the application.
Nanofluids are best known for heat transfer enhancements. They can be used in many applications including, for example, forced convection in pipes, pool boiling and heat pipes. Nanofluids according to embodiments of the invention may be useful for known applications of such substances, and may provide improved performances in some cases. Furthermore, the advantageous properties of nanofluids according to embodiments of the invention can open the door to new applications.
Method of Preparation of a Nanofluid
In accordance with another aspect of the invention, there is provided a method of preparation of a nanofluid containing nanoparticle-decorated functionalized multiwall carbon nanotubes (MWCNTs). The method includes the following steps:
A particular feature of this method is that the functionalization step and nanoparticles deposition step occur while the grown MWCNTs are still anchored to the substrates used for the thermal chemical vapor deposition. The MWCNTs are only detached from the substrate in the last step of the process of making the nanofluid. As explained in more details below, this feature allows obtaining individual MWCNTs with a highly functionalized surface and a better distribution of the nanoparticles over the MWCNTs' surface.
The first step for preparing the nanofluid consists of the synthesis of the MWCNTs by thermal chemical vapor deposition.
In an embodiment, the MWCNTs are synthesized by catalyst-free thermal chemical vapor deposition (t-CVD) using acetylene gas (C2H2) as the carbon source. Stainless steel substrates, such as stainless steel meshes or flat sheets, are positioned in a quartz tube which is placed in a furnace wherein the substrates are brought to about 650 to about 800° C., preferably about 700° C. Then, the substrates are exposed to acetylene and the MWCNTs are allowed to grow on the substrate. The acetylene concentration and injection time are adjusted to allow growth of the MWCNTs on the substrate. In an embodiment, MWCNTs can be grown by injecting acetylene at a flow rate of about 68 standard cubic centimeters per minute (sccm) for about 1 to 5 minutes. MWCNTs covering the entire mesh are obtained at the end of this first step.
The stainless steel substrates used to grow the MWCNTs are preferably stainless steel meshes of the 400 series, such as for example stainless steel 304 or 316 square meshes (4 cm2, approximate composition: 67 wt % Fe, 17 wt % Cr, 12 wt % Ni, 2 wt % Mo, 2 wt % others). During the heating process, chromium migrates to the surface of the substrate causing the roughness of the surface to increase drastically. This surface break-up generates small particle-like sites of exposed iron, from which the MWCNTs can precipitate once the meshes are exposed to carbon.
As previously mentioned, other stainless steel substrates, such as flat sheets can also be used to grow MWCNTs. With such stainless steel flat sheets, an etching pre-treatment is preferably required to remove a thick oxide layer prior to growth.
The acetylene flow rate and stainless steel mesh size may be adjusted to the size of the quartz tube. A person skilled in the art would readily appreciate the specific impact of the acetylene flow on mesh size and the resulting density of MWCNTs growth. For instance, an acetylene flow rate of 68 standard cubic centimeters per minute (sccm) and a mesh size of 4 cm2 will be efficient for a quartz tube being 5.5 cm in diameter.
The so-produced MWCNTs protrude radially outwards from the grid bars of the mesh and are typically from about 15 nm to about 100 nm in diameter and from about 1 μm to about 10 μm, preferably from about 4 μm to about 6 μm, in length. No additional purification step is required as no amorphous carbon or catalytic particles are produced. The graphitic walls of the MWCNTs which are obtained may present different shapes, including for example straight carbon nanotubes, bamboo-type carbon nanotubes, waved carbon nanotubes, coiled carbon nanotubes, branched carbon nanotubes, candy-cane like bent tips, as shown in
The MWCNTs obtained at the end of the first step exhibit hydrophobic characteristics. Hence, such pure CNTs will not be well-dispersed in a polar solvent as they will rather tend to agglomerate and settle.
The second step of the method of preparation of the nanofluid aims to change the surface chemistry of the MWCNTs to allow them to be efficiently dispersed in the polar base fluid upon mixing. For this purpose, the outer surface of the MWCNTs is functionalized with polar functions.
The person skilled in the art would know the methods for functionalizing MWCNTs with polar functions, such as dry and wet functionalization techniques.
In an embodiment, the outer surface of the MWCNTs is plasma-functionalized by exposure to a radio-frequency glow discharge plasma using a Ar/C2H6/O2 mixture, thus allowing the formation of covalently-bonded polar functional groups (e.g., carboxyl, hydroxyl, carbonyl groups) to the outer surface of the MWCNTs.
This plasma functionalization can be performed by a radio-frequency (RF, 13.56 MHz) glow discharge plasma at 20 W in a vacuum chamber set at 2 Torr and filled with oxygen (5 sccm), ethane (1 sccm) and argon (500 sccm). The treatment time can vary between about 1 to about 20 minutes. In some cases a treatment time as short as about 1 minute may be sufficient to obtain the desired functional groups on the MWCNTs' outer surface. The vacuum chamber may be a quartz tube which can be the same as, or different than, the tube used for growing the CNTs. A person skilled in the art will be able to adapt the plasma conditions (power, flowrates, pressure, treatment time) to obtain a high degree of surface functionalization in various types of glow discharge plasma.
This plasma treatment generates carboxyl functional groups (COON), hydroxyl groups (OH) and carbonyl groups (C═O) on the MWCNTs' outer surface.
In another embodiment, the MWCNTs can be plasma-functionalized in the presence of NH3 instead of O2 to generate amine and amide functional groups on the MWCNTs' outer surface.
During the functionalization step, the MWCNTs are still anchored to the growth surface (grid bars of the stainless steel mesh) and separated enough to allow a functionalization of the entire surface. Moreover, the grown MWCNTs present defects on their surface that are prime sites to add functional groups. Functionalization of the MWCNTs while they are still anchored to the growth surface allows a uniform functionalization of the entire surface of the MWCNTs compared to functionalization of a powder of aggregated MWCNTs. This results in a nanofluid with improved stability, as well as improved absorption and heat treat transfer properties, as the nanofluid contains suspended individual CNTs instead of agglomerates.
The next step for the preparation of the nanofluid involves the deposition of NPs on the outer surface of the plasma-functionalized MWCNTs. This step can also be named “decoration step”. This allows obtaining NP-decorated functionalized MWCNTs which will be dispersed in the last step for forming the nanofluid.
As previously mentioned, the NPs which decorate the outer surface of the MWCNTs can be metal nanoparticles such as transition metal nanoparticles, including gold (Au), nickel (Ni), iron (Fe), chromium (Cr), cobalt (Co), copper (Cu), silver (Ag), titanium (Ti) and platinum (Pt) nanoparticles. Gold and platinum nanoparticles are preferred noble metal nanoparticles.
In an alternative embodiment, the nanoparticles that cover the MWCNTs' outer surface may be composed of a semiconductor material, such as CdSe. Other nanoparticles may be polymer nanoparticles, for example polystyrene or polypyrrole nanoparticles.
Decoration of the functionalized MWCNTs with metal nanoparticles can be performed by pulsed laser ablation or thermal evaporation/inert gas condensation. Deposition of semiconductor nanoparticles and polymer nanoparticles is generally performed by pulsed laser ablation (PLA).
When deposition of NPs is performed by pulsed laser ablation, a pulsed laser beam is focused on a target (metal, semiconductor, or polymer) causing immediate vaporization of the material and formation of a high-density vapor plasma plume in rapid expansion. In an embodiment, the laser pulse duration is about 5 ns and the laser fluence is about 1 J/cm2. Supersaturation of the material vapor plume upon cooling in the buffer background gas leads to homogeneous nucleation followed by nanoparticle formation. The buffer gas pressure is used to control the nanoparticle size. In an embodiment, the buffer gas pressure can be readily adjusted to control the mean nanoparticle size between about 5 and about 60 nm.
In an embodiment, gold NPs are deposited on the MWCNTs' outer surface. The deposition of gold NPs is performed by ablation of a gold target, preferably 100% pure gold target, using a focused high energy pulsed Nd:YAG laser beam in a vacuum chamber at a set pressure of about 1 mTorr to about 4 Torr, preferably about 1 mTorr (laser wavelength: 355 nm, laser fluence: 1.2 J/cm2).
The deposition treatment is carried out during a predetermined time allowing the formation of the NPs on portions of the outer surface of the MWCNTs, while some other portions of the MWCNTs' surface contain accessible polar functional groups. The skilled person in the art will be able to determine the required time for depositing the desired amount of NPs at the MWCNTs outer surface. In the embodiment where the MWCNTs are decorated with gold NPs, gold deposition is preferably carried out for about 180 to about 600 seconds. However, the time during which NPs deposition is carried out will depend on the laser power and repetition rate. The skilled person in the art will be able to determine the time deposition to obtain the required decoration level.
The chamber pressure at which deposition is performed may affect the size of the NPs formed. Hence, the skilled person in the art will be able to determine the pressure to be applied for obtaining the desired NPs size. Small size NPs may be obtained at low pressure. In the embodiment where the MWCNTs are decorated with gold NPs, the gold deposition is carried out at a pressure of 1 mTorr and the gold NPs produced may have a nominal size as little as about 1 nm.
The decoration step (deposition of NPs on the MWCNTs' surface) is also performed while the MWCNTs are still anchored to the growth surface (grid bars of the stainless steel mesh). The incoming stream of nanoparticles produced by pulsed laser ablation is generally directed so as to treat one side of the mesh. Since the MWCNTs are not agglomerated and are separated enough from each other, this allows depositing the NPs substantially all over the surface of the MWCNTs facing the incoming stream of nanoparticles.
The last step of the method is to obtain the nanofluid itself by dispersing the functionalized NP-decorated MWCNTs resulting from the previous step in a base fluid. In this step, the NP-decorated functionalized MWCNTs which are still attached to the stainless steel mesh substrate where the CNTs have grown are also detached there from.
The base fluid is usually a polar base fluid to provide a better dispersibility of the MWCNTs. Preferably, the base fluid is a polar solvent chosen from water and polar organic solvents such as ethylene glycol, propylene glycol, isopropanol, ethanol, methanol or denatured alcohol, to name a few. However, the base fluid can also be a mixture of polar solvent or a mixture of water with at least one polar organic solvent.
This step thus involves placing the substrate in the base fluid under ultrasonication, for example using an ultrasonic bath or probe. The MWCNTs are broken off near their base (they are not uprooted) and dispersed in the fluid. During ultrasonication, some of the NP-decorated functionalized MWCNTs are also broken into fragments having a smaller length than the initial grown MWCNTs (before being detached from the substrate). In the suspension, the NP-decorated functionalized MWCNTs have a broad length distribution ranging from about 100 nm to about 10 μm. This length distribution is substantially Gaussian in nature as can be seen in
As previously mentioned, the diameter of the MWCNTs may range from about 15 to about 100 nm. As can be seen in
The concentration of the NP-decorated functionalized MWCNTs in the nanofluid may be up to about 1.0 g/L. In other words, it is possible to obtain a mono-disperse nanofluid with a concentration of MWCNTs up to about 1.0 g/L of the fluid. The concentration of the MWCNTs in the nanofluid will be adapted depending on the intended application.
In accordance with another aspect of the invention, there is provided a nanofluid containing functionalized multiwall carbon nanotubes (MWCNTs) decorated with nanoparticles, obtained by the above-described method.
The resulting nanofluid is stable for an extended period of time and towards heating at high temperatures. Thanks to the accessible polar functional groups on the surface of the decorated MWCNTs, it is possible to obtain a dispersion which remains homogeneous during a required period of time. With these properties, the nanofluid may find applications in a variety of industrial processes which require the use of fluids to absorb and transport energy and/or molecules. Alternatively, the nanofluid of the invention may be used as a starting material to prepare other nanofluids wherein the MWCNTs can be further modified for specific applications or to prepare bulk solids and films of polymer-based nanocomposite materials.
Plasma Functionalization
Gold Nanoparticle Synthesis and Deposition
Making Nanofluid
The nanofluid's particle morphology, composition, absorbance and stability were analysed by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV/Vis/NIR absorbance spectroscopy and zeta potential measurements.
SEM, TEM, STEM and EDS
The MWCNTs were imaged both while attached to the substrate and after removal. In the case of post removal analysis, the nanofluid was either deposited onto a silicon substrate (SEM) or a TEM grid. In the case of
XPS
X-ray photoelectron spectroscopy was performed on the gold-decorated oxygen-functionalized MWCNTs (gold decoration level 300 sec). The XPS spectrum of
UV-Vis Spectroscopy
UV/visible/near infrared absorbance spectra of water, Au—F-MWCNTs (absorption due to water removed) and Au—F-MWCNTs nanofluid (with water) were recorded. The spectra show the typical broadband optical absorbance of MWCNTs, which peaks in the UV and gradually decreases towards the infrared (see
Zeta Potential
To evaluate further the stability of the system, zeta potential measurements were taken for the Au—F-MWCNT aqueous nanofluid (gold decoration level 300 sec). Values around 38 mV (±2) were found, which indicates good stability of the MWCNTs and Au-decorated MWCNTs in the suspension.
Of course, numerous modifications could be made to the embodiments described above without departing from the scope of the present invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2014/050276 | 3/17/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61791314 | Mar 2013 | US | |
| 61904264 | Nov 2013 | US |
