JP6090774B2 - Method for producing nanofluid - Google Patents

Description

  The present invention relates to a method for producing a nanofluid. More specifically, the present invention relates to a method for producing a nanofluid having excellent thermal conductivity.

  Nanoparticles with a particle size in the nanometer range may differ from atoms, molecules, or bulk materials in terms of optical properties, electrical properties, mechanical properties, etc. due to their shape specificity. Utilization in various fields as a material or a catalyst material has been studied and its application has been promoted.

One application of such nanoparticles is, for example, a nanofluid in which metal nanoparticles having a particle size of about 100 nm or less are dispersed in a medium. The nanofluid is, for example, a heat conduction fluid that propagates thermal energy while flowing in a heat transfer channel that has become finer due to miniaturization of various electronic devices in recent years without clogging. Such nanofluids include, for example, various metal nanoparticles having a particle size of about 100 nm or less in a liquid medium serving as a base such as water or ethylene glycol, a surfactant for ensuring dispersibility and stability, etc. It can comprise by disperse | distributing with the protective agent of (for example, refer patent document 1).
As described above, since the metal nanoparticles are dispersed in the nanofluid, it is expected that the heat transfer area is increased and the thermal conductivity is significantly increased. In particular, when ultrafine metal nanoparticles having a particle size of about 2 nm or less are used, the quantum effect is theoretically expressed. Therefore, it is expected that by controlling the interval between the metal nanoparticles in the nanoparticle, a thermal diffusion system similar to that in the solid can be constructed in the liquid, and the thermal conductivity can be increased nonlinearly.

JP 2008-461141 A JP 2008-13810 A JP 2005-68447 A

  Such metal nanoparticles as a heat-conducting material can be more easily produced by recent development of mass synthesis technology and high-speed synthesis technology of nanoparticles. The present inventors also proposed a completely new metal nanoparticle generator capable of generating a large amount and rapidly of metal nanoparticles having a particle size of 500 nm or less by the action of submerged plasma generated in the solution. (For example, refer to Patent Document 2). According to this metal nanoparticle generator, by generating plasma in an aqueous solution containing a metal salt, metal ions in the aqueous solution can be reduced, and metal nanoparticles constituting the metal salt can be generated. It is expected that nanofluids can be produced more easily by applying metal nanoparticles produced using such a metal nanoparticle generator to nanofluids.

  However, according to the technique disclosed in Patent Document 2, it has been difficult to produce metal nanoparticles having a smaller particle size, for example, a particle size of several nanometers. In addition, there remains a problem with producing metal nanoparticles with less variation in particle size. In addition, when metal nanoparticles with a large variation in particle size are applied to nanofluids, the dispersibility of the nanoparticles in the nanofluid is reduced at a microscopic level, so that the thermal conductivity corresponding to the particle size is reduced. There was a possibility that it could not be obtained.

  The present invention has been created in view of the above problems, and forms metal nanoparticles in a form suitable for use as a heat conduction fluid in a liquid medium using liquid plasma, and has excellent thermal conductivity. Another object of the present invention is to provide a method for producing nanofluids.

The present inventors have found that metal nanoparticles can be produced by utilizing the action of plasma generated in an aqueous solution (hereinafter sometimes simply referred to as “liquid plasma”). We have intensively researched to improve the technology into a form more suitable for actual practical application. As a result, the present invention has been completed by finding the following knowledge in constructing a nanofluid by applying metal nanoparticles generated by submerged plasma to a heat transfer fluid.
-By adjusting the environment in which the metal nanoparticles are formed, characteristics such as the particle size and particle size distribution of the formed metal nanoparticles can be controlled in more detail.
-By adjusting the state of the surfactant contained in the aqueous solution, particularly when the metal nanoparticles are formed, the properties related to the thermal conductivity of the nanofluid containing the metal nanoparticles and the aqueous solution containing the metal nanoparticles are adjusted. It can be controlled effectively.

That is, this application provides a method for producing a nanofluid as a solution to the problems of the prior art. This nanofluid manufacturing method reduces Au (III) ions by generating plasma in an aqueous solution containing Au (III) ions and a cationic surfactant, and forms Au nanoparticles in the aqueous solution. To produce nanofluids. And in this manufacturing method, it is characterized by including the following requirements.
(1) the concentration of the cationic surfactant in the aqueous solution prior to generating the plasma, when the critical micelle concentration at 25 ° C. and C _cm, adjusted to be less 1C _cm above 10C _cm.
(2) The pH of the aqueous solution in the aqueous solution before generating plasma is adjusted in the range of 8-10.
(3) Plasma is generated in the aqueous solution while maintaining the temperature of the aqueous solution at 5 ° C. or higher and 15 ° C. or lower.

In order to improve the thermal conductivity of a general heat conduction fluid using metal nanoparticles, it is desirable to disperse more metal nanoparticles uniformly without agglomeration. However, nanoparticles having a particle size of 100 nm or less tend to aggregate due to high surface activity, and it is difficult to control the dispersion state over a long period of time. Therefore, use of various protective agents such as surfactants has been proposed in order to ensure the dispersibility of the metal nanoparticles. However, in the production of nanofluids by submerged plasma, the use of a protective agent that effectively improves thermal conductivity has not been studied so far.
According to the present invention, the pH and temperature of the aqueous solution that generates the plasma in liquid are appropriately controlled, so that the particle size is uniform at several nanometers without coarsening the Au nanoparticles to an undesired size, It can be formed without agglomeration in a state where the shape is substantially spherical. Therefore, it is possible to produce a nanofluid that is contained in an aqueous solution in a highly dispersed state of Au nanoparticles. In addition, since the concentration of the surfactant is appropriately controlled, the heat conduction characteristics are improved, and a nanofluid suitable as a heat transport fluid can be manufactured. That is, it is possible to successfully control the particle morphology and improve the dispersibility in the formation process of Au nanoparticles, and to make the resulting nanofluids excellent in heat transport characteristics.

In a preferred embodiment of the nanofluid production method disclosed herein, the concentration of the Au (III) ion in the aqueous solution is adjusted to 0.05 mM to 0.5 mM.
According to this configuration, for example, a nanofluid whose thermal conductivity at 25 ° C. is about 5% higher than that of water (the thermal conductivity of water at 25 ° C. is 0.6075) can be obtained. Such a nanofluid can be suitably used for, for example, constructing an energy transport system that transports heat energy in an intermediate temperature range with low loss and can be used as a heat source.

In a preferred embodiment of the nanofluid production method disclosed herein, Au nanoparticles having an average particle diameter of 3 nm to 5 nm are formed in the aqueous solution.
In a nanofluid in which metal nanoparticles are dispersed in a liquid medium, it is theoretically considered that the thermal conductivity increases as the particle size of the metal nanoparticles decreases. In the production of nanofluid using in-liquid plasma, for example, it is possible to produce nanofluid by forming metal nanoparticles having an average particle size smaller than 3 nm.
However, when nanofluids are produced by submerged plasma under relatively mild conditions as described above, it is not always necessary to form nanoparticle fluidized Au nanoparticles because of the smaller size of Au nanoparticles. The degree is not increased. According to such a configuration, it is possible to produce a nanofluid having excellent heat conduction characteristics and including Au nanoparticles having a particle size of 3 nm to 5 nm, which is suitable as a heat conduction fluid.

In a preferred embodiment of the nanofluid production method disclosed herein, cetyltrimethylammonium bromide (CTAB) or polyvinylpyrrolidone (PVP) is used as the cationic surfactant.
CTAB is known as a surfactant used when producing gold nanorods that are anisotropic nanoparticles by reduction of Au (III) ions (see, for example, Patent Document 3). PVP is a surfactant that is known to be usable in the production of metal nanoparticles by submerged plasma (see, for example, Patent Document 2). These are preferable as the surfactant used when producing the nanofluid among the cationic surfactants. By adjusting the concentration of any of these cationic surfactants to the concentration shown in (1) above and generating plasma in liquid, the nanoparticles have an excellent balance between morphology and dispersibility, and heat conduction characteristics Can be produced.

In a preferred embodiment of the nanofluid production method disclosed herein, a pair of linear electrodes are arranged in the aqueous solution, and the plasma has a pulse width of 0.1 μs to 5 μs between the linear electrodes and a frequency. Is generated by applying a DC pulse voltage of 10 ³ Hz to 10 ⁵ Hz.
According to such a configuration, most of the bubbles generated in the aqueous solution by the Joule heat generated between the linear electrodes can be maintained in a stable state in the aqueous solution without being floated toward the water surface. Plasma can be generated in a stable state. Thereby, a nanofluid can be manufactured in a more efficient and stable state.

In a preferred aspect of the method for producing a nanofluid disclosed herein, the plasma is a glow discharge plasma.
The plasma generated in the liquid can be in the form of spark discharge, corona discharge, glow discharge, arc discharge. Especially, glow discharge plasma can be utilized for manufacture of nanofluid as a more preferable form of plasma in liquid. Thereby, non-equilibrium low-temperature plasma can be generated, and nanofluid can be stably produced with less energy.

It is a schematic diagram which shows an example of a structure of the in-liquid plasma generator used for manufacture of nanofluid. It is the schematic diagram which illustrated the structure of the reaction field by the plasma in a liquid utilized for manufacture of nanofluid. It is a transmission electron microscope (TEM) image which illustrated the mode of Au nanoparticle in the nanofluid manufactured by making surfactant concentration into 0.44 mM in one Embodiment. It is a transmission electron microscope (TEM) image which illustrated the mode of the Au nanoparticle in the nanofluid manufactured by making surfactant concentration into 0.88 mM in one Embodiment. It is a transmission electron microscope (TEM) image which illustrated the mode of the Au nanoparticle in the nanofluid manufactured by making surfactant concentration into 1.32 mM in one Embodiment.

Hereinafter, the manufacturing method of the nanofluid of this invention is demonstrated. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and the drawings and common general technical knowledge in the field.
The nanofluid production method disclosed herein essentially reduces this Au (III) ion by generating a plasma in an aqueous solution containing Au (III) ions and a cationic surfactant. It forms Au nanoparticles inside. In this method, the following (1) to (3) are included as essential requirements.

(1) the concentration of the cationic surfactant in the aqueous solution before generating a plasma, when the critical micelle concentration at 25 ° C. and C _cm, adjusted to be less 1C _cm above 10C _cm.
(2) The pH of the aqueous solution in the aqueous solution before generating plasma is adjusted to the range of 8-10.
(3) Plasma is generated in the aqueous solution while keeping the temperature of the aqueous solution at 5 ° C. or higher and 15 ° C. or lower.

Gold (Au) constituting the Au nanoparticles has a thermal conductivity at 20 ° C. of 295 W / mk, which is a relatively high value among various metals. Therefore, the nanofluid containing Au nanoparticles can be expected to have high thermal conductivity. As a raw material for such Au nanoparticles, Au (III) ions are preferably used. There is no particular limitation on the preparation of the aqueous solution containing Au (III) ions as the raw material. For example, an aqueous solution containing Au (III) ions in the form of various complexes may be used. Such a complex may be, for example, a trivalent gold halogeno complex, and specifically may be in the form of tetrachlorogold (III) acid ion: [AuCl ₄ ] ⁻ or the like.
It is preferable that Au (III) ions are contained in an aqueous solution before generating plasma at a concentration of 0.05 mM to 0.5 mM. When the concentration is less than 0.05 mM, the amount of Au nanoparticles formed is reduced. Therefore, the thermal conductivity of the nanofluid obtained is not sufficiently high, and the thermal conductivity characteristics are poor for use as a heat conduction fluid. If the concentration exceeds 0.5 mM, it is not preferable because Au nanoparticles may partially start to aggregate in the nanofluid. The concentration of Au (III) ions is more preferably in the range of about 0.07 mM to 0.3 mM.

  Various types of cationic surfactants can be used. For example, quaternary ammonium salt type or alkylamine salt type surfactants can be considered. Specific examples include polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), octyltrimethylammonium bromide (OTAB), benzalkonium salt, alkylamine oxide and the like. Among them, examples of cationic surfactants that are more suitable for producing nanofluids by generating liquid plasma include polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB). When these surfactants are used, Au nanoparticles can be formed with better morphology and dispersibility, and the thermal conductivity of the resulting nanofluid can be further enhanced.

Such cationic surfactants are, when the critical micelle concentration at 25 ° C. and _{C cm,} adjusts the aqueous solution to be equal to or less than _{1C cm} above _{10C cm.}
Here, the critical micelle concentration (C _cm ) is the concentration of the surfactant when the surfactant starts to produce micelles in the aqueous solution. Below this concentration, the surfactant is present in solution without forming micelles. Therefore, in a solution containing a surfactant, the physical properties may vary greatly depending on whether the concentration of the surfactant is lower than the critical micelle concentration or higher than the critical micelle concentration. In this specification, the critical micelle concentration is based on the critical micelle concentration at 25 ° C. which is generally adopted. It is known that the critical micelle concentration is lowered when the temperature is lowered. When the temperature of the aqueous solution in the present invention is 5 ° C. or more and 15 ° C. or less, the cationic surface activity is considered as described above in consideration of this point. The concentration range of the agent is determined.
In the present invention, in-liquid plasma is generated in a state where the cationic surfactant reliably forms micelles (that is, a critical micelle concentration of C _cm or higher). Thereby, the heat conduction characteristic of the obtained nanofluid is effectively improved by the action of plasma in liquid. Incidentally, if it is the concentration of cationic surfactant is less than the critical micelle concentration C _cm, for example particle size of the Au nanoparticles formed by, for example, smaller than about 3 nm, the thermal conduction properties effectively improved It is hard to be done. In addition, when the concentration of the surfactant is greatly increased from the critical micelle concentration, the surfactant is adsorbed on the specific crystal plane of the Au nanoparticles formed in the liquid, and the Au nanoparticles are only in a specific orientation. It can grow into a rod shape. Therefore, the upper limit of the concentration of the surfactant is suitably set to about 10C _cm. The critical micelle concentration in the aqueous solution before generating plasma, that is, the initial aqueous solution is preferably 1 C _cm or more and 10 C _cm or less, more preferably 1 C _cm or more and 5 C _cm or less, and further preferably 1 C _cm or more and 2 C _cm or less. _cm or less.

  The above critical micelle concentration can be measured by various known methods. For example, the measurement is performed by an electric conduction method, a surface tension method, an absorbance method, a dynamic light scattering method, or the like. Specifically, the critical micelle concentration is measured, for example, by adding a surfactant to 100 ml of distilled water kept at 25 ° C. to prepare an aqueous solution of a surfactant having a predetermined concentration. Measure physical properties such as tension, absorbance, and scattering. In the relationship between the surfactant concentration and any one of these physical property values, the point at which the relationship between the two rapidly changes can be obtained as the critical micelle concentration.

  Further, the pH of the aqueous solution is adjusted in the range of 8-10. By making the pH of the aqueous solution a weak alkali of about 8 to 10, Au nanoparticles having little variation in particle size, a sharp particle size distribution curve, and a substantially spherical particle shape are formed by in-liquid plasma. be able to. In the nanofluid, it is preferable that each nanoparticle with such a small anisotropy realizes a dispersed state in which short-range order is maintained. On the other hand, when the pH of the aqueous solution is less than 8, it is not preferable because Au particles having a relatively large particle size, a wide particle size distribution curve, and a variation in particle size tend to be formed. In addition, when the pH of the aqueous solution exceeds 10, it is easy to obtain Au nanoparticles having a finer particle size and uniform size, but the particle size tends to be too small or the shape of the obtained particles tends to be distorted. It is not preferable.

  And in this invention, after adjusting the state of a solution as mentioned above, plasma is generated in aqueous solution, keeping the temperature of aqueous solution in the range of 5 to 15 degreeC. Here, in the nanofluid manufacturing method, in-liquid plasma generated in the aqueous solution is used as a field for reducing Au (III) ions. That is, by the action of active species such as positive and negative ions, electrons, and radicals constituting the plasma, reduction of Au (III) ions contained in the aqueous solution and accompanying formation of nanoparticles composed of zero-valent Au. Realized. Such active species can typically be hydrogen ions, hydroxide ions, oxygen ions, hydrogen radicals, oxygen radicals, hydroxy radicals, and the like that are generated by the decomposition of water molecules in an aqueous solution.

  Here, the in-liquid plasma serving as the reaction field described above is a partial or thorough arrangement of molecules constituting the gas in a gas (gas phase) generated by applying microwaves or high-frequency waves between electrodes in the liquid. It can be formed by completely ionizing. In other words, in the plasma in liquid, the gas phase surrounding the plasma phase is further surrounded by the liquid phase, and the active species such as ions, electrons and radicals constituting the plasma freely move in the limited gas phase. This is a possible state. Therefore, it exhibits physical and chemical properties that are different from gas phase plasma (typically atmospheric pressure plasma, low pressure plasma, etc.) generated in the released gas phase.

  For example, gas phase plasma is generated when neutral molecules constituting the gas are ionized and turned into plasma when the temperature of the gas is raised. At this time, unlike the phase transition between solid, liquid, and gas, the transition from gas to plasma occurs gradually, so that even a very small amount of ionization of constituent molecules can be sufficiently plasma. . On the other hand, the plasma in liquid is typically formed by first vaporizing the liquid by Joule heating by discharge in the liquid to form a gas phase, and further generating plasma in the gas phase. In other words, in the plasma in liquid, a high energy state called plasma is confined in the liquid (that is, the condensed phase), which realizes a closed system physics and forms a high-density plasma reaction field that is not released. .

  In addition, Au (III) ions, which can be said to be a precursor of Au nanoparticles, are supplied via a liquid phase in plasma in liquid. That is, in the present invention, Au (III) ions are efficiently supplied to the reaction field at a relatively high density. Therefore, in the production method of the present invention, Au (III) ions can be reduced with high efficiency, and Au nanoparticles can be formed with high productivity.

  The above-mentioned plasma in liquid is classified into spark discharge such as lightning, corona discharge, glow discharge, arc discharge, etc., depending on the difference in potential applied between the electrodes. When the spark discharge continuously flows, it becomes glow discharge or arc discharge. Here, glow discharge plasma (hereinafter also referred to as solution plasma) generated in the liquid has different characteristics from other liquid plasmas. For example, the arc discharge plasma is a thermal plasma having a high particle density and a local thermal equilibrium state in which the temperature of ions and neutral particles is substantially equal to the electron temperature. In contrast, glow discharge plasma is a low-temperature plasma in a non-equilibrium state in which the electron temperature is high but the temperature of ions and neutral particles is low. In addition, it is difficult to generate a continuous plasma by corona discharge, and there is a feature that a relatively large amount of oxidizing hydroxy radicals are formed together with hydrogen radicals by decomposition of water. On the other hand, glow discharge plasma has high plasma energy, and oxidizing hydroxy radicals are further decomposed to generate many reducing hydrogen radicals. That is, according to glow discharge plasma, the reduction of Au (III) ions is performed more efficiently. Therefore, in the present invention, it is preferable to generate glow discharge plasma as submerged plasma.

  Such glow discharge plasma can be generated relatively stably by applying a voltage with a pulse width of submicroseconds at a high repetition frequency. Therefore, a stable state can be maintained for a long time (for example, 2 hours or more) while the expansion / compression motion of the liquid surrounding the plasma phase and the plasma phase are linked. Therefore, for example, in the solution plasma, a part of the gas phase generated between the electrodes may rise from the electrodes due to buoyancy and reach the liquid surface, but most of the gas phase is constant between the electrodes. It is constantly maintained as a large gas phase. Therefore, in the solution plasma, the plasma generation state can be constantly controlled. In the method for producing a nanofluid of the present invention, it is preferable to use such controlled plasma, and the nanofluid can be produced more efficiently. Whether or not the generated plasma is glow discharge plasma can be confirmed by, for example, the Townsend second coefficient obtained by plasma emission spectroscopic analysis or the like being in the range of 0.0005 to 0.005.

  Here, it is important to keep the temperature of the aqueous solution at 5 ° C. or more and 15 ° C. or less during the generation of plasma. When the temperature of the aqueous solution exceeds 15 ° C., Au nanoparticles tend to be coarsened, and the particle size tends to vary or aggregate. For example, by maintaining the temperature of the aqueous solution at 15 ° C. or less, a nanofluid in which Au nanoparticles of 5 nm or less are dispersed can be suitably obtained. In addition, when the temperature of the aqueous solution is less than 5 ° C., the coarsening of the Au nanoparticles can be prevented, but when the aqueous solution prepared as described above is used, it is difficult to obtain an effect commensurate with the cost. Practically, the temperature of the aqueous solution is shown as a more preferable example in the range of, for example, 10 ° C. or more and 15 ° C. or less. Here, there is no restriction | limiting in particular as a means to keep aqueous solution in the said temperature range, Various temperature control mechanisms (typically cooling mechanism) can be utilized. Examples of such a cooling mechanism include stirring means such as a stirrer and fins, a thermostat such as a thermostatic water tank, a reflux cooling means for circulating a cooling gas, cooling water, and the like. You may make it use these cooling mechanisms in combination of 2 or more types.

  Although the above-mentioned plasma in liquid cannot be generally described because it depends on the amount of aqueous solution, detailed plasma generation conditions, and the like, it can be generated, for example, for several minutes or more, typically 10 minutes or more. In order to shorten the processing time, it is possible to devise to improve the contact efficiency between the plasma in liquid and Au (III) ions in the aqueous solution.

Hereinafter, the nanofluid production method of the present invention will be described in more detail by taking as an example the production of a nanofluid using solution plasma as a reaction field as a preferred embodiment of the present invention.
FIG. 1 is a diagram showing an outline of a solution plasma generator 10 for generating a solution plasma 4 in an aqueous solution 2. In this embodiment, the aqueous solution 2 containing Au (III) ions and a cationic surfactant is placed in a container 5 such as a glass beaker. A pair of electrodes 6 for generating plasma 4 is disposed in the aqueous solution 2 at a predetermined interval (for example, about 0.1 mm to 5 mm), and is held by the container 5 via an insulating member 9. The electrode 6 is connected to an external power supply 8, and a pulse voltage of a predetermined condition is applied from the external power supply 8. As a result, the solution plasma 4 can be constantly generated between the pair of electrodes 6.

  The electrode 6 may be in various forms such as a plate electrode, a rod electrode, and a combination thereof, and the material thereof is not particularly limited. For example, it may be an electrode of iron (Fe), gold (Au), tungsten (W), platinum (Pt), or the like. In this embodiment, a linear (needle-like) electrode 6 made of tungsten and having a diameter of about several millimeters (for example, about 0.5 to 3 mm) capable of locally concentrating an electric field is used. In order to suppress an excessive current that hinders electric field concentration, the electrode 6 is preferably exposed only at the tip (for example, about 0.1 to 2 mm) and insulated by the insulating member 9 or the like. . For example, the insulating member 9 is made of rubber or resin (for example, fluororesin). In this embodiment, the insulating member 9 has a configuration in which the electrode 6 is fixed to the container 5 and also serves as a stopper for keeping the electrode 6 and the container 5 watertight. In such an apparatus 10, the application conditions of the pulse voltage for generating the solution plasma depend on conditions such as the type and concentration of the raw material compound contained in the aqueous solution 2, and further the configuration conditions of the apparatus 10, but an example As an example, a pulse current having a pulse width of about 1 to 5 μs and a repetition frequency of about 10 to 30 kHz are applied so that the voltage (secondary voltage) is about 1000 to 2000 V. .

Here, the aqueous solution 2 is preferably adjusted to have an electric conductivity of about 300 μS · cm ^{−1 to} 2500 μS · cm ⁻¹ as a guide in order to enable generation of stable solution plasma. When it is necessary to adjust the electrical conductivity, for example, an electrolyte such as potassium chloride (KCl), potassium hydroxide (KOH), or sodium hydroxide (NaOH) may be dissolved in the aqueous solution 2. In the present invention, it is shown as a suitable example that alkali salt such as potassium hydroxide or sodium hydroxide, more preferably potassium hydroxide is used also for adjusting the pH of the aqueous solution. If the electric conductivity is less than 300 μS · cm ⁻¹ , a large amount of electric power is required to generate the solution plasma, and it is difficult to generate the solution plasma suitably. In addition, when the electric conductivity exceeds 2500 μS · cm ⁻¹ , the electric power input between the electrodes for plasma generation is consumed as an ionic current, and it tends to be difficult to generate plasma constantly. It is not preferable because there is. Electrical conductivity is preferably set to 500μS ^{· cm} -1 ~2300μS ^· cm about ^-1, and more preferably set to 1000μS ^{· cm} -1 ~2000μS ^· cm approximately ^-1.

  Then, the solution plasma 4 is formed by applying the pulse voltage to the aqueous solution 2 by the solution plasma generator 10. The plasma reaction field generated by the solution plasma generator 10 has a configuration as shown in FIG. 2, for example. That is, a gas phase 3 is formed in the aqueous solution (liquid phase) 2, and a solution plasma (plasma phase) 4 is formed in the gas phase 3. This plasma reaction field is constantly maintained between the electrodes 6. In such a plasma reaction field, active species such as electrons, ions and radicals having high energy are supplied from the plasma phase 4 toward the liquid phase 2. On the other hand, from the liquid phase 2 toward the gas phase 3 and the plasma phase 4, water constituting the liquid phase 2 or Au (III) ions dissolved in the water is supplied. These contact (collision) mainly at the interface between the liquid phase 2 and the gas phase 3. In particular, hydrogen radicals, hydrogen ions, hydroxy radicals, and the like generated from water are highly reactive. In particular, when the hydrogen radicals come into contact with Au (III) ions contained in the liquid phase 2, such Au (III) ions It is thought that the reduction action of is shown. For the sake of easy understanding, FIG. 2 shows a state in which each interface between the liquid phase 2 and the gas phase 3 and between the gas phase 3 and the plasma phase 4 is clearly formed in a substantially spherical shape. The interface is not necessarily limited to being clearly formed. For example, there is no critical interface between the gas phase 3 and the plasma phase 4, and this interface may have a spatial extent.

  According to the above configuration, for example, Au (III) ions are reduced by the action of solution plasma, and Au nanoparticles are formed in the solution. The nanoparticles are not agglomerated, and are generated, for example, as very fine particles (for example, an average particle size of about 3 μm to 5 μm) in a highly dispersed state in an aqueous solution, thereby forming a nanofluid. Become.

  As mentioned above, although the manufacturing method of the nanofluid was demonstrated based on suitable embodiment, this manufacturing method is not limited to this example, It can carry out by changing an aspect suitably. For example, when generating the solution plasma, it is not always necessary to use a needle-like electrode made of tungsten. For example, the solution plasma may be generated by a low inductance induction coil. Furthermore, the plasma in liquid is not limited to that by solution plasma (glow discharge plasma), and for example, arc plasma in liquid may be used. Needless to say, the conditions for generating the solution plasma can be adjusted as appropriate according to the conditions of the aqueous solution, the apparatus, and the like.

As described above, the nanofluid production method of the present invention is a novel nanofluid production method using an unprecedented plasma reaction field, and produces a nanofluid having a possibility of further development of functions. Can be a thing.
Next, although the Example regarding this invention is described, it is not intending limiting this invention to what is shown to this Example.

A 0.1 mM HAuCl ₄ solution was prepared using gold (III) chloride trihydrate. In this solution, commercially available cetyltrimethylammonium bromide (CTAB) as a cationic surfactant has three concentrations (a) 0.44 mM, (b) 0.88 mM, and (c) 1.32 mM. As such, potassium hydroxide was prepared and added to a concentration of 0.4 mM. The pH of this solution was about 8. The critical micelle concentration of the CTAB measured by the electric conduction method was 0.88 mM.
300 ml of HAuCl ₄ solution adjusted to the above concentrations (a) to (c) was prepared, and solution plasma was generated in each solution using the apparatus shown in FIG. FIG. 1 is a diagram schematically illustrating a solution plasma generator 10 for generating plasma in an aqueous solution. In this embodiment, the solutions 2 prepared in the above (a) to (c) are put in a container 5 made of a glass beaker and stirred by a stirring device 7 made of a magnetic stirrer. Moreover, the container 5 is accommodated in the thermostat 11 set so that 15 degreeC may be maintained so that the solution 2 may not be heated by the heat_generation | fever during plasma generation. A pair of electrodes 6 for generating plasma are disposed in the solution 2 at a predetermined interval, and are held in the container 5 via an insulating member 9. In this embodiment, the needle-like electrode 6 capable of concentrating the electric field locally is used. The electrode 6 is composed of a tungsten wire (manufactured by Nilaco) having a diameter of 1.0 mm, the distance between the opposing electrodes is set to 0.3 mm, and the tip (in order to suppress an excessive current that hinders electric field concentration) For example, only a few mm) was exposed, and the rear portion was insulated with an insulating member 9 made of a fluororesin. In this embodiment, the insulating member 9 is configured to fix the electrode 6 to the container 5 and also serve as a stopper for keeping the water-tightness between the electrode 6 and the container 5. The electrode 6 is connected to an external power supply 8, and a pulse voltage of a predetermined condition is applied from the external power supply 8. As the external power source 8, a bipolar pulse power source (manufactured by Kurita Manufacturing Co., Ltd., MPS-R06K02C-WP1F) was used.

In this embodiment, the application conditions of the pulse voltage for generating the solution plasma are as follows: voltage: 1700 V, pulse width: 1 μs, repetition frequency: 25 kHz. Under these conditions, the solution plasma is 900 seconds (15 minutes) in each solution 2. Generated. It is confirmed that the plasma generated under such conditions has a Townsend second coefficient determined by plasma emission spectroscopy or the like in the range of 0.0005 to 0.005.
All of the solutions (a) to (c) were colorless and transparent, but immediately after the generation of the solution plasma, the color of the solution gradually changed to light red while remaining transparent. This discoloration indicates that trivalent gold ions have been reduced to zero. The solutions (a) to (c) obtained after performing the treatment with the solution plasma for 900 seconds are referred to as nanofluids (a) to (c), respectively. Hereinafter, the following tests were performed on nanofluids (a) to (c).

<TEM observation>
For the nanofluids (a) to (c) obtained above, the morphology of Au nanoparticles formed in the fluid was observed using a nanoanalytical electron microscope (JEM-2500SE, manufactured by JEOL Ltd.). . 3A to 3C show TEM images obtained by TEM observation of nanofluids (a) to (c), respectively. Moreover, from TEM observation, the particle diameter was measured about 100 or more Au particles formed in each solution, and the average particle diameter was examined. The results are shown in Table 1 below.

<Thermal conductivity>
Further, the thermal conductivities of the nanofluids (a) to (c) were measured by an unsteady hot wire method using a thermal conductivity measuring device (manufactured by Rigaku Corporation, CTi). The results are also shown in Table 1.

As is clear from FIGS. 3A to 3C, it was confirmed that the Au nanoparticles were almost uniformly dispersed in the nanofluids (a) to (c) without aggregation. That is, it was confirmed that even when the surfactant concentration was 0.44 mM, which was approximately half the critical micelle concentration, as in the nanofluid (a), the effect of sufficiently suppressing the aggregation of Au nanoparticles was confirmed. Further, in nanofluids (a) to (c), the shape of Au nanoparticles formed in nanofluid (c) having the highest surfactant concentration is the most smooth and nearly spherical, and there is no variation in shape. all right.
Regarding the particle size of the Au nanoparticles, the nanofluid (a) in which the surfactant concentration was less than the critical micelle concentration was the smallest. In comparison between nanofluids (b) and (c), the particle diameters of Au nanoparticles formed when the amount of surfactant was larger than the critical micelle concentration were almost the same.

The thermal conductivity of the nanofluids (b) and (c) with the surfactant concentration equal to or higher than the critical micelle concentration is significantly higher than 0.6075 which is the thermal conductivity of water at 25 ° C. In particular, it was confirmed that the nanofluid (c) having a higher surfactant concentration than the critical micelle concentration had the highest value. On the other hand, the nanofluid (a) with the smallest Au nanoparticles had the lowest thermal conductivity, which was significantly lower than the thermal conductivity of water. In other words, in nanofluids produced by generating solution plasma, reducing the particle size of Au nanoparticles contained in the fluid and increasing the specific surface area does not necessarily contribute to the improvement of heat conduction characteristics. I understand that there is no.
The nanofluids (b) and (c) prepared in this way are both excellent in dispersibility and stability. For example, even when 4 months have passed since the production, It was confirmed that there was no change in physical properties such as dispersibility. Although not specifically exemplified in this specification, the thermal conductivity of the obtained nanofluid can be further enhanced by increasing the concentration of Au (III) ions in the aqueous solution to about 0.5 mM. It has been confirmed.
Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The invention disclosed herein can include various modifications and alterations of the specific examples described above.

2 Aqueous solution (liquid phase)
3 Gas phase 4 Solution plasma (plasma phase)
5 Container 6 Electrode 7 Stirrer 8 External Power Supply 9 Insulating Member 10 Solution Plasma Generator 11 Constant Temperature Bath

Claims (6)

A method for producing a nanofluid by reducing Au (III) ions by generating plasma in an aqueous solution containing Au (III) ions and a cationic surfactant to form Au nanoparticles in the aqueous solution. Because
The concentration of the cationic surfactant in the aqueous solution prior to generating the plasma, the critical micelle concentration when the C _cm, adjusted to less 1C _cm above 10C _cm at 25 ° C.,
The pH of the aqueous solution in the aqueous solution prior to generating the plasma was adjusted to a range of 8-10,
Generating plasma in the aqueous solution while maintaining the temperature of the aqueous solution at 5 ° C. or higher and 15 ° C. or lower;
A method of producing a nanofluid characterized by the above.   The method for producing a nanofluid according to claim 1, wherein the concentration of the Au (III) ion in the aqueous solution is adjusted to 0.05 mM to 0.5 mM.   The method for producing a nanofluid according to claim 1 or 2, wherein Au nanoparticles having an average particle diameter of 3 nm to 5 nm are formed in the aqueous solution.   The method for producing a nanofluid according to any one of claims 1 to 3, wherein cetyltrimethylammonium bromide or polyvinylpyrrolidone is used as the cationic surfactant. A pair of linear electrodes are disposed in the aqueous solution,
5. The plasma according to claim 1, wherein the plasma is generated by applying a DC pulse voltage having a pulse width of 0.1 μs to ⁵ μs and a frequency of 10 ³ Hz to 10 ⁵ Hz between the linear electrodes. A method for producing a nanofluid according to Item.   The method of producing a nanofluid according to claim 1, wherein the plasma is glow discharge plasma.