TITLE OF THE INVENTION

Organic nanofluids, method and reactor for synthesis thereof

FIELD OF THE INVENTION

[0001] The present invention relates to organic nanofluids. More specifically, the present invention is concerned with organic nanofluids, and a method and a reactor for synthesis thereof.

BACKGROUND OF THE INVENTION

[0002] Nanofluids are two-phase mixtures comprising a continuous phase, consisting of a liquid host, and a dispersed phase of nanoparticles.

[0003] Nanofluids are made by suspending nanoscale particles of materials such as carbon-based structures like fullerenes and carbon nanotubes (CNT), bare metal or metal oxides such as copper and copper oxide respectively in liquids such as oil, water and radiator fluid (a mixture mostly of water and ethylene glycol). Such adding nanoscale particles to the fluids leads to enhanced or new heat transfer, electric, magnetic and/or optical properties.

[0004] In US patent 6,221 ,275, Choi et al. disclose a method for introducing to a fluid particles having thermal conductivities higher than the thermal conductivity of the fluid. In this method, a stabilization step is required, whereby a stabilizing agent is added to the nanofluid obtained.

SUMMARY OF THE INVENTION

[0005] More specifically, in accordance with the present invention, there is provided an integrated vacuum reactor for fabricating organic nanofluids containing

an organic host fluid and nanoparticles that are coated in-situ with a surface layer compatible with the organic host fluid, comprising: a nanoparticle synthesis region including a solid source and an energy source to produce a high-density cloud of vapors from the solid source; a nanoparticle nucleation and growth region, in which a cold inert gas supersaturates the cloud of vapors and transports as-formed nanoparticles away from the nanoparticle synthesis region; a coating region, where a uniform and electrodeless glow discharge plasma is generated and plasma polymerization occurs onto a surface of incoming nanoparticles; and a contact region, comprising a film of the host fluid, receiving a stream of coated nanoparticles and gas, where the coated nanoparticles come into contact with the host fluid, yielding an organic nanofluid containing the organic host fluid and coated nanoparticles.

[0006] There is further provided a method for in-situ synthesis, stabilization and dispersion of nanoparticles in a host fluid comprising the steps of producing a high density cloud of vapors; quenching the vapors with an inert cooling gas, thereby forming nano-sized particles; in-flight coating the nano-sized particles by plasma polymerization with a surface layer compatible with the host fluid; and dispersing the coated nanoparticles in the host fluid into an organic nanofluid.

[0007] There is further provided an organic nanofluid comprising an organic fluid and a suspension of in-situ surface-stabilized nanoparticles, the nanoparticles being coated in-situ with a stabilization layer compatible with the organic host fluid.

[0008] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the appended drawings:

[0010] Figure 1 is a schematic view of a first embodiment of a reactor according to a first aspect of the present invention;

[0011] Figure 2 is a schematic view of a second embodiment of a reactor according to the first aspect of the present invention;

[0012] Figure 3 is a flowchart of a method according to an embodiment of an aspect of the present invention;

[0013] Figure 4 is a FE-SEM image of uncoated copper nanoparticles, collected over a fine polymeric filter, showing significant agglomeration;

[0014] Figure 5 (A) is a FE-SEM image of copper nanoparticles coated with an organic layer, produced by a reactor similar to that of Figure 1 , and collected over a fine polymeric filter, showing a very limited agglomeration; (B) and (C) are FE-SEM images of individual copper nanoparticles coated with an organic layer, the dark core corresponding to copper, the organic layer appearing translucent; and

[0015] Figure 6 is a FTIR spectrum of a collection of copper nanoparticles coated with an organic layer.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0016] According to an aspect of the present invention, there is provided an

integrated vacuum reactor for fabricating organic nanofluids.

[0017] The reactor is capable of operating at reduced pressures, between 1 and 200 torr for example.

[0018] As illustrated in the embodiments of Figures 1 and 2 of the appended drawings, the reactor 20 comprises a nanoparticle synthesis region 22; a nanoparticle nucleation and growth region 24; a nanoparticle surface functionalization and coating region 26; a nanoparticles/ host fluid contact region 28; and, optionally, a nanofluid recirculation circuit 30.

[0019] In the nanoparticle synthesis region 22, a high-density metal vapor cloud is generated by erosion of a cathode 21 under a low-pressure, pulsed or nonpulsed, electric arc.

[0020] The cathode may be a metal cathode or a carbon cathode.

[0021] Pulsed electric arcs can be self-triggered using a capacitor charging power supply unit, whereby an arc is initiated every time the voltage across an inter-electrode gap exceeds the breakdown voltage of the gap. Alternatively, an ablation laser may trigger the arcs, the pulses of the laser producing micro- plasmas upon impact with the cathode, when using metal or carbon/metal cathodes. Still alternatively, a powerful laser (pulsed CO ₂ laser) may be used to ablate a solid target and to form a dense cloud of metallic or carbon/metal vapors, or a high-temperature furnace (temperature range between 500 and 650 ⁰ C) may be used to sublimate fullerenes C ₆ o from a fullerenes-rich extract.

[0022] Synthesis of carbon nanotubes can be done by a cathodic arc evaporation process using a carbon cathode containing the metal catalyst particles, such as Ni, Ni/Cu alloys, etc.

[0023] Synthesis of carbon nanotubes can also be done by a combined cathodic arc evaporation/condensation process, whereby a cloud of vapors is generated from a metal catalyst cathode, and carbon nanotubes are synthesized onto the metal nanoparticles with a gaseous hydrocarbon, in an in-flight thermal chemical vapor deposition (CVD) process. In this case, the synthesis region 22 comprises a thermal CVD part where the metal nanoparticles are used as host catalysis sites for the synthesis of the CNT from the hydrocarbon gas. The thermal CVD part comprises heated walls to maintain the desired reaction temperature.

[0024] The cathodic arc evaporation/condensation process for the synthesis of catalyst nanoparticles can be replaced by using a fine resistively heated wire which is also known to lead to the formation of fine nanoparticles by nucleation/condensation of the metal atoms released from its surface.

[0025] In the nanoparticle nucleation and growth region 24, a cold inert gas is injected for supersaturating the vapors, thereby forcing the nucleation process, and transporting the as-formed nanoparticles away from their source.

[0026] In the in-flight coating formation region 26, an uniform and electrodeless glow discharge plasma is generated. This plasma operates with various gases, injected into the in-flight coating formation region 26, including organic vapors, in order to favor a plasma polymerization process onto the surface of the nanoparticles. The plasma can be sustained by capacitive or inductive coupling. Radio-frequency excitation of 13.56 MHz for example has been successfully tested at reactor pressures of about 20 torr or less. Audio frequencies

of a few kHz or less may be used as well, especially at higher reactor pressures.

[0027] The stream of nanoparticles and gas flows through the in-flight coating formation region 26 downstream of the point of injection of the various gases including organic vapors.

[0028] The host organic liquid, injected in the in-flight coating formation region 26, or downstream from this region, forms a stable film flowing on the reactor inside walls, in the case of a vertical reactor as illustrated in Figure 1 , or forms a flowing film on the bottom inner wall of an horizontal reactor as illustrated in Figure 2.

[0029] These inside walls of the vertical reactor or this bottom inner wall of the horizontal reactor provide a contact region 28 between the nanoparticles and the host liquid.

[0030] As it is transported by convection, along the axis of a tubular vertical reactor for example, the stream of coated nanoparticles and gas diffuses by thermophoresis and normal diffusion towards the inner surface of the reactor, where the coated nanoparticles come into contact with the organic host fluid.

[0031] A nanofluid recirculation circuit 30, consisting of a circulation pump, a reservoir, and corresponding tubing for example, may be provided to enrich the nanofluid to a target level, in an iterative process.

[0032] The geometry and dimensions of the reactor can be adapted based on the specific application of interest, and the scale of the production. It is found that a vertically mounted tubular reactor as illustrated in Figure 1 is simple and efficient by providing an increased contacting surface, the entire reactor assembly

may be mounted vertically.

[0033] The reactor may also be mounted so that the contacting surface is horizontal, as illustrated in Figure 2.

[0034] A method according to an embodiment of another aspect of the present invention, as shown in Figure 3, comprises the generation of nanoparticle precursors (step 110); the homogeneous nucleation and growth of nanoparticles (step 120); the functionalization and coating of the surface of the nanoparticles (step 130); and dispersion of the nanoparticles in an organic host fluid (step 140).

[0035] Optionally, the method further comprises recirculating the obtained organic nanofluid (step 150), to enrich the organic nanofluid to a target level.

[0036] In step 110, a range of solid precursors may be used, including metals, in the case when metal-organic liquid nanofluids are desired, and carbon, in the case when fullerenes and CNT-organic liquid nanofluids are desired, for example.

[0037] For the synthesis of nanofluids containing coated metal nanoparticles or CNT, in step 110 a cloud of metal vapors is generated from the erosion of a cathode 21 by an electric arc, in the nanoparticle synthesis region 22 of a reactor. Operating the arc at chamber pressures in a range between about 1 and about 10 torr (range between about 133 and about 1333 Pa) ensures compatibility with the subsequent step of surface functionalization of the nanoparticles (step 130), and favors high cathode erosion rates. The erosion rates and consequently, the flux of metal vapors leaving the cathode surface, are controlled through the arc current.

[0038] A pulsed-arc configuration 23 can be used in order to minimize the thermal load to the cathode and to maximize the instantaneous current, i.e. erosion rate. The electric arc can be initiated by gaseous breakdown with a self-triggering capacitor circuit or by laser triggering.

[0039] For CNT production according to cathodic arc evaporation process described hereinabove, a graphite cathode hosting metal inclusions may be used for example, formed metal nanoparticles providing catalytic surfaces for the growth of CNT.

[0040] Alternatively, an ablation laser such as a pulsed CO2 laser may be used to produce the cloud of vapors.

[0041] Alternatively, a resistively heated metal filament may be used to produce the cloud of vapors.

[0042] The high-density cloud of vapors thus generated is transported away from the solid source thereof and cooled by a stream of inert gas flowing in the inter-electrode gap defined by the cathode 21 and anode 25. The erosion/evaporation products are transported downstream of the source, nano- sized particles being formed by the rapid quenching of the vapors, as a result of the supersaturation and homogeneous nucleation phenomena.

[0043] In the case of CNT synthesis by the cathodic arc process, the inert cooling gas is preferably helium and the formed metal nanoparticles act as host catalyst sites for the growth of the CNT.

[0044] In the case of CNT synthesis by the combined cathodic arc (or hot filament) and thermal CVD process discussed hereinabove, an inert gas such as

argon can be used for the supersaturation of the metal vapor clouds and a gaseous hydrocarbon can be used for precursor for the growth by thermal CVD of CNT onto the metal nanoparticles.

[0045] The formed nanoparticles or CNT are then transported into a coating region of the reactor for in-flight coating, where a radio frequency (RF) capacitively coupled glow discharge plasma is maintained under reduced pressure conditions, between about 1 and about 50 torr for example. This low pressure sustains a stable, large volume uniform glow discharge plasma medium allowing efficient plasma polymerization.

[0046] Other types of electrical energy transfer to the polymerization plasma may be used, such as inductive coupling, as well as higher reactor pressures, providing that the plasma remains diffuse and fills the available volume.

[0047] The monomer gas used in the plasma polymerization process may originate from two sources: either from the vapor of the organic host liquid present in the reactor, or from the injection of a foreign gas that mixes with the already present organic gas.

[0048] The method allows an in-situ stabilization of the nanoparticles, whereby the nanoparticles are coated, in-situ, by plasma polymerization with a surface layer compatible with the organic host fluid. Such coating forms a solid- liquid interlayer acting as a stabilizing agent for the suspension of nanoparticles in the host fluid, thus eliminating the need of a further step of stabilization by using a stabilizing foreign chemical which properties are likely to degrade under real operating conditions and/or to affect the performance of the resulting nanofluid.

[0049] The method allows a control of the size of the nanoparticles, of the chemical composition and thickness of the coating, and the weight content of

nanoparticles in the nanofluid.

[0050] The reactor pressure, the inert gas flow rate and the length of quenching zone may be adjusted to control the size of the metal nanoparticles.

[0051] The composition of the host fluid or foreign gas may be selected to control the composition of the coating.

[0052] The RF plasma power, length of plasma polymerization zone, and content in plasma polymerization gas are used to control the thickness of the coating.

[0053] The weight content of thenanoparticles into the host fluid is impacted by the flow rate of the recirculation loop and processing time.

[0054] The method may be performed in continuous, and allows scalability while yielding high throughput.

[0055] The method may be used to synthesize organic nanofluids containing surface-coated metal nanoparticles and also fullerenes- and CNT- based nanofluids.

[0056] When using a vertical reactor as illustrated in Figure 1 of the appended drawings, the organic host fluid is injected through a flange separating the nanoparticle synthesis region 22 from the in-flight coating region 26, and forms a stable film falling on the reactor inside wall. The organic vapor liberated from the organic host fluid, and the foreign gas in the case of the injection of a foreign gas as discussed hereinabove, reach the glow discharge plasma area of the reactor, where they are excited and dissociated by electron-impact collisions. Such process

leads to the formation of active chemical fragments that deposit onto the surface of the nanoparticles. This plasma polymerization process on the surface of the nanoparticles leads to the formation of a dense organic coating. Such coating stabilizes the nanoparticles by reducing their surface free energy and enhances the compatibility of the nanoparticles with the organic host fluid.

[0057] As shown in Figure 1 , the host fluid may be injected downstream of the plasma polymerization region of the reactor (see arrow A), instead of upstream thereof (see arrow B), in which case direct contact of the polymerization plasma with the host fluid is avoided thus limiting the extent of plasma-induced chemical reactions in the organic host fluid.

[0058] Further downstream in a vertical reactor, a falling film of the organic liquid is exposed to the stream of coated nanoparticle and gas. In the case of a horizontal contact zone, a film of the organic liquid flowing on the inner bottom wall of the reactor is exposed to the stream of coated nanoparticles and gas. In both cases, this leads to the dispersion of the nanoparticles into the organic fluid.

[0059] The nanofluid thus produced may be re-circulated in a recirculation circuit until a target loading of nanoparticles is reached.

[0060] As mentioned hereinbefore, the vapor of the organic host fluid alone may be used as the precursor gas for the organic film formation. The expected range of temperatures for the falling liquid film is comprised in a range between about 25 and about 8O ⁰ C. As an indicator of the vapor pressures involved, ethylene glycol (C ₂ H ₆ O ₂ ), a well-known organic heat transfer fluid, has a vapor pressure of about 0.06 torr at 25 ⁰ C while it reaches about 0.7 torr at 5O ⁰ C. Such vapor pressure range gives rise to dilution ratios with the inert gas of the order of a few %, which is adequate for the plasma polymerization process. Using organic fluids such as methanol and ethanol for example may be contemplated, and

involves operating the reactor at significantly higher pressures due to the higher vapor pressures of these organic fluids.

[0061] In the case where the host fluid is a polar organic solvent, additional gaseous chemical compounds known to produce polar functionalities by plasma polymerization might be contemplated as precursors.

[0062] According to an embodiment of still a further aspect of the present invention, there is provided an organic nanofluid comprising a low-vapor pressure organic fluid, such as ethylene glycol for example, and a suspension of metal nanoparticles, carbon nanotubes or fullerenes, coated in-situ with a thin stabilization layer compatible with the organic host fluid. Such a coating is likely to maintain its stabilizing properties under real use conditions, thereby avoiding the need of a surfactant.

[0063] Fine metal nanoparticles show strong optical activity, in particular in terms of absorptivity, in the UV and visible ranges. Such property can be used for the development of liquid-based optical filters for solar-panel applications and windows, increasingly powerful dye lasers, and random lasers (lasers which do not require an optical cavity for the light amplification) for example. Furthermore, nanofluids containing magnetic nanoparticles, such as iron for example, could be pumped with magnetic fields (no need for mechanical pumps) and used as tracer fluids.

[0064] As shown with nanoparticles coated with a dense organic layer, in

Figures 4 and 5, the coating on the nanoparticles limits or eliminates the agglomeration of the nanoparticles. In Figure 6, peaks at 2953 cm ^"1 , 2924 cm ^"1 and 2855 crrf ¹ of the FT-IR spectrum are associated to saturated C-H stretch bonds, a peak at 1454 cm ^"1 to a C-H bending bond and a peak at 1361 cm ^"1 to the C-H bending bond of the CH ₃ group. No unsaturated C-H and C-C bonds are

observed in the spectrum. The coating therefore presents a macromolecular organic structure, in contrast to a graphite-like structure.

[0065] The present nanofluids containing surface-stabilized nanoparticles,

C ₆₀ fullerenes or CNT having superior properties under real operating conditions may be used in the field of heat transfer for example, and allows reductions in heat exchanger size, storage needs, pumping capacity, devices' volume and mass. Due to the increasing demand on engines and microelectronic devices, heat transfer fluids with considerably enhanced heat conductivity are required.

[0066] Such nanofluids may also find uses in optics, as filters (UV-VIS) and lasing media (random lasers) for example. Magnetic nanofluids (ex. with Fe nanoparticles) may be used with electromagnetic pumps and as biofluids.

[0067] As people in the art will appreciate, the present nanofluids may therefore find a range of applications as heat transfer fluids (ex. automotive and chemical processing industries, microelectronic and microfluidic devices, magnetically-controllable fluids (ferrofluids), optics (ex. filters and absorbers, random lasers), biomedical engineering (ex. tracer fluids and fluid vectors).

[0068] Although the present invention has been described hereinabove by way of specific embodiments thereof, it can be modified, without departing from the nature and teachings of the subject invention as described herein.