JP6470992B2 - Magnetically oriented metal nanowire dispersion fluid - Google Patents

Description

  The present invention relates to a magnetic field-oriented metal nanowire dispersion fluid suitable for a heat transport device, which has a property that metal nanowires are oriented along the magnetic field direction by applying an external magnetic field.

  In this specification, a group of metal wires having a thickness of less than 1000 nm is referred to as “nanowires”. In the case of powder, each wire corresponds to “particles” constituting the powder, and nanowires corresponds to “powder” that is a collection of particles.

  A “magnetic fluid” in which fine magnetic particles are colloidally dispersed in a liquid medium is known. Since the magnetic fluid has the property that the magnetic particles are connected to each other when an external magnetic field is applied, the magnetic fluid exerts a unique function such as being responsible for heat transfer between two separated objects (magnetic poles) only when a magnetic field is applied. Magnetic fluids are attracting attention as working fluids for heat transport devices.

  However, heat flow is difficult to control precisely and actively compared to current. Further, the effect of improving the thermal conductivity due to the continuous magnetic particles cannot be said to be sufficient for many uses of heat transport devices that are expected to increase in the future, and further improvement is desired.

  In Patent Document 1, a magnetic fluid having magnetic particles (spherical nickel having a diameter of about 50 nm) in an ultraviolet curable resin is applied onto a substrate, a magnetic field is applied to form a magnetic particle wire, and the resin is solidified. Is disclosed (paragraph 0021). In this case, since the wire is formed by contact between the magnetic particles, the thermal conductivity of the wire is considered to be smaller than that of the metal wire. In addition, since the wire of magnetic particles is fixed with resin, a heat transport device of a type that changes the state of heat flow (heat flow rate and direction) by ON / OFF of a magnetic field cannot be constructed.

  Patent Document 2 discloses a silver / magnetite composite wire in which magnetite particles are formed on the surface of silver nanowires. This composite wire is useful as a filler capable of forming a network with a small amount of addition in materials such as magnetism, conductivity, thermal conductivity, and electromagnetic shielding (paragraph 0030). However, since the magnetite particles covering the surface are inferior in thermal conductivity to the internal metallic silver, it is impossible to construct a heat transport device utilizing the good thermal conductivity of metallic silver. Moreover, the process for manufacturing a composite wire is required.

JP 2011-190468 A JP 2012-102360 A

  The present invention provides a magnetic functional fluid that has higher thermal conductivity than conventional magnetic fluids and that can control the direction of heat flow and heat flow by the direction of magnetic field, ON / OFF, or strong / weak. For the purpose.

In order to achieve the above object, in the present invention, magnetic particles and metal nanowires are dispersed in a liquid medium, and by applying an external magnetic field, the metal nanowires are oriented along the magnetic field direction. Metal nanowire dispersion fluid is provided.
Here, the external magnetic field means a magnetic field other than the geomagnetism applied from the outside.

More specifically, the magnetic particles and the metal nanowires are dispersed in the liquid medium, the average particle diameter P _M of the magnetic particles is smaller than the average diameter D _{M of the} metal nanowires, and the average length L of the metal nanowires. the average aspect ratio a _M represented by the ratio L _M / D _M of _M and the average diameter D _M is not less than 10, the volume concentration of the magnetic particles is less than 1.0%, is 30ppm or higher volume concentration of metallic nanowires A magnetically oriented metal nanowire dispersion fluid is provided.
Here, the average diameter, average length, and average aspect ratio of the nanowire conform to the following definitions.

[Average diameter]
On a microscopic image (for example, FE-SEM image), in a projection image of a single metal wire, the average value of the diameters when the diameter of the inscribed circle in contact with the contours on both sides in the thickness direction is measured over the entire length of the wire. , Defined as the diameter of the wire. And the value which averaged the diameter of each wire which comprises nanowire (nanowires) is defined as the average diameter of the said nanowire. In order to calculate the average diameter, the total number of wires to be measured is set to 100 or more.

[Average length]
On a microscopic image similar to the above, in a projected image of a single metal wire, the length from one end of the wire to the other end of the line passing through the center of the thickness of the wire (that is, the center of the inscribed circle) This is defined as the length of the wire. And the value which averaged the length of each wire which comprises nanowire (nanowires) is defined as the average length of the said nanowire. In order to calculate the average length, the total number of wires to be measured is set to 100 or more.

[Average aspect ratio]
The average aspect ratio is calculated by substituting the above average diameter and average length into the following formula (A1).
[Average aspect ratio] = [Average length (nm)] / [Average diameter (nm)] (A1)

As the average particle diameter P _M of the magnetic particles, a volume-based cumulative 50% particle diameter D ₅₀ by a laser diffraction / scattering method can be adopted.

In the magnetic field oriented metal nanowires dispersion fluid, metal nanowires, for example can mean diameter D _M applies those 500nm or less. Examples of the types of metal nanowires include silver nanowires, copper nanowires, and copper-nickel nanowires. Examples of magnetic particles include iron oxide particles composed of one or more of magnetite (Fe ₃ O ₄ ) and maghemite (γ-Fe ₂ O ₃ ).

  According to the present invention, it is possible to construct a heat transport device that exhibits anisotropy of heat conduction by applying a magnetic field. Since heat flow is obtained by direct contact between the metal nanowires oriented in a certain direction, efficient heat transport utilizing the excellent thermal conductivity of the metal nanowires themselves becomes possible. Further, the direction of heat flow and the heat flow rate can be precisely controlled by the direction of the applied magnetic field, ON / OFF, and the like. When an ultraviolet curable resin is used as the liquid medium, for example, an anisotropic magnetic field can be applied with an external magnetic field, and the metal nanowires can be sealed in a metal linear orientation, and exhibits excellent thermal conductivity in only one direction. Conductive heat conductors can be constructed.

The figure which shows typically the behavior of a metal nanowire when an external magnetic field is provided to the magnetic field orientation metal nanowire dispersion fluid according to this invention. The graph which showed the thermal conductivity of each test fluid in a test magnetic field. The figure for demonstrating the measuring method of heat conductivity (cylindrical container which accommodates a test fluid). The figure (probe) for demonstrating the measuring method of thermal conductivity. The figure for demonstrating the measuring method of thermal conductivity (The graph which shows the typical example of the measurement result of the temperature rise with respect to time by the unsteady hot-wire method). The figure for demonstrating the measuring method of thermal conductivity (installation of a Helmholtz coil and a neodymium magnet).

  FIG. 1 schematically shows the behavior of metal nanowires when an external magnetic field is applied to the magnetically oriented metal nanowire dispersion fluid according to the present invention. In the figure, the shape of the metal nanowire is drawn with the length direction being extremely reduced for convenience. In addition, the volume concentration of the metal nanowire is also exaggerated.

  FIG. 1A shows a state where no external magnetic field is applied. Magnetic particles and metal nanowires are randomly dispersed in the liquid medium. In this state, heat flow in a specific direction cannot be obtained by traveling through the metal nanowires.

  FIG. 1B shows a state where an external magnetic field is applied. Even if the metal nanowire is a non-magnetic material such as silver, when a large amount of fine magnetic particles are present in the surrounding area, the individual metal nanowires are aligned along the direction of the magnetic field, and They found that they were connected to each other to form a long metal wire. It is known that nonmagnetic particles placed in a magnetic fluid behave as if they were diamagnetic materials, but nonmagnetic metal nanowires are also considered to exhibit this diamagnetic behavior. Each metal nanowire forms a diamagnetic material whose longitudinal ends are N and S poles, respectively. Wires whose ends are close to each other tend to be connected at the ends when the ends are different from each other, and away from each other by repulsion when the ends are at the same polarity. Therefore, as schematically shown in FIG. 1B, it is presumed that many independent long metal wire wires are formed along the direction of the magnetic field. This long metal wire array exhibits the function of carrying heat in the length direction (direction along the magnetic field). Moreover, since each metal wire array is independent, the heat flow in the direction perpendicular to the magnetic field remains cut off. The magnetic field-oriented metal nanowire dispersion fluid according to the present invention enables the construction of a heat transport device that exhibits remarkable anisotropy in thermal conductivity through such a mechanism.

  The metal nanowire preferably has good thermal conductivity. Applications that can synthesize nanowires, such as copper nanowires, silver nanowires, and copper-nickel nanowires, are applicable. Among them, various synthesis techniques suitable for mass production have been developed for silver nanowires, and are suitable objects in the present invention.

As for the size of the metal nanowire, various sizes are allowed as long as the above-described orientation behavior caused by application of an external magnetic field can be expressed. As for the diameter, if it is too thick, the movement in the liquid medium may become slow when a magnetic field is applied. It is preferred in order to obtain a quick orientation average diameter D _M is 500nm or less. For example, it may be adjusted in the range of 10 to 500 nm while considering the balance with the magnetic particle size described later.

On the other hand, the inventors have found that the aspect ratio of the metal nanowire has a great influence on the conductivity improvement effect when the magnetic field is oriented. That is, when the aspect ratio of the metal nanowire increases, the conductivity improving effect tends to increase. Preferably has an average aspect ratio A _M represented by the ratio L _M / D _M the mean diameter D _M and the average length L _M of the metal nanowires is 10 or more, more preferably 40 or more. Since it is difficult to prepare metal nanowires having a very high aspect ratio, the average aspect ratio A _M should normally be in the range of 250 or less.

The length of the metal nanowire may be adjusted in consideration of the aspect ratio. For example it is preferable to adjust the average length L _M in the range of 1.0～30.0Myuemu.

  Various known methods can be applied to the method of synthesizing the metal nanowire. For example, in the case of silver nanowires, various techniques for reducing silver ions in the presence of an organic protective agent using an alcohol-based solvent have been developed and are in practical use. The silver nanowire obtained by this method can obtain what exhibits good dispersibility in a desired aqueous medium or organic liquid medium by appropriately selecting the type of the organic protective agent.

The magnetic particles are preferably spherical. Suitable magnetic materials include iron oxide particles composed of one or more of magnetite (Fe ₃ O ₄ ) and maghemite (γ-Fe ₂ O ₃ ), metallic nickel, and the like. The particle size is preferably such that the average particle diameter P _M is smaller than the average diameter D _{M of the} metal nanowires. If P _M is larger than D _M , it becomes a factor that hinders the movement of the metal nanowire when a magnetic field is applied, which is disadvantageous when rapid orientation is desired. Magnetic fluids commercially available for industrial use can also be used. However, it must be well dispersed in the liquid medium together with the metal nanowires. In accordance with the average diameter D _M of the type and the metal nanowires liquid medium to be used, it is possible to select an appropriate magnetic particles.

  As the liquid medium constituting the magnetic field oriented metal nanowire dispersion fluid according to the present invention, it is important that both the magnetic particles and the metal nanowire are well dispersed. Depending on the application, various aqueous media and organic liquid media can be selected. For example, water and organic solvents, such as kerosene, are mentioned. In the liquid medium, a surfactant, a thickener, a silicon surface treatment agent, and the like are appropriately added as necessary.

  The concentration of the magnetic particles present in the magnetically oriented metal nanowire dispersion fluid according to the present invention is preferably 1.0 vol.% Or more, and more preferably 3.0 vol.% Or more. A high magnetic particle concentration is disadvantageous in maintaining good dispersibility of magnetic particles and metal nanowires. Usually, the volume concentration may be, for example, in the range of 10.0 vol.

  The concentration of the metal nanowires present in the magnetically oriented metal nanowire dispersion fluid is preferably 30 vol. If it is less than that, it may be disadvantageous to sufficiently exhibit good thermal conductivity. When the concentration of the metal nanowire increases, the dispersibility in the liquid tends to decrease. In addition, the wires are entangled with each other, which can prevent rapid orientation. Usually, the volume concentration may be set in the range of, for example, 1000 vol.ppm or less, and may be controlled to 100 vol.ppm or less.

[Metal nanowires]
Three types of silver nanowires (A) to (C) shown in Table 1 were synthesized and stored in an aqueous solvent. The synthesis method will be described below by taking the case of silver nanowires (A) as an example.

  At room temperature, 2.5 g of polyvinylpyrrolidone (PVP) was added to 60 g of ethylene glycol, and the temperature was raised to 135 ° C. over 10 min while stirring at 500 rpm. After that, stirring was continued and maintained at 135 ° C. After 10 minutes from the time when the temperature reached 135 ° C., 0.006 g (0.1 mmol) of sodium chloride previously dissolved in 0.6 g of ethylene glycol in another container was added. After 3 minutes had elapsed since the addition of sodium chloride, 0.85 g (5.0 mmol) of silver nitrate previously dissolved in 7.65 g in ethylene glycol in another container was added. After the addition of silver nitrate, the stirring speed was changed to 100 rpm, the heating was terminated while maintaining 3.0 hours at 135 ° C., and the mixture was naturally cooled to 80 ° C. or less as it was. After the temperature became 80 ° C. or lower, a part of the solution (slurry after reaction) was taken into a centrifuge tube, washed by adding distilled water, and centrifuged at 3000 rpm for 5 min. After removing the supernatant after centrifugation, methanol was added to wash the precipitate, and the methanol dispersion was centrifuged at 2500 rpm for 5 minutes. After removing the supernatant after centrifugation, methanol was added again to wash the precipitate, and the methanol dispersion was centrifuged at 1500 rpm for 10 minutes. After removing the supernatant after the centrifugation, the precipitate was dispersed in water and stored in a screw tube bottle. This precipitate is an aggregate of silver nanowires. Thus, the silver nanowire (A) shown in Table 1 was obtained.

  Similarly, silver nanowires (B) and (C) were obtained. However, the silver nanowire (B) was synthesized under the condition in which metal nitrate ions were further added. Silver nanowires (C) were synthesized under conditions using tetrabutylammonium chloride in place of sodium chloride as a chlorine source.

[Magnetic fluid]
Here, a commercially available magnetic fluid shown in Table 2 (manufactured by FeroTec, MSG-W11) was used. This is a mixture of magnetite (Fe ₃ O ₄ ) particles and maghemite (γ-Fe ₂ O ₃ ) particles dispersed in an aqueous liquid medium, and the average particle size is 10 nm.

[Magnetic fluid containing metal nanowires]
The fluid sample which mixed said magnetic fluid and any 1 type of silver nanowire (A)-(C) was obtained. For comparison, a fluid sample containing only magnetic fluid was also prepared. These four types of fluid samples were used for the later-described thermal conductivity measurement as test fluids. Table 3 shows the silver nanowire average aspect ratio and composition of each test fluid. The sample symbol “MF” in Table 3 represents the magnetic fluid, and (A) to (C) represent the silver nanowires.

[Measurement of thermal conductivity of fluid]
The thermal conductivity of the test fluid was measured by the Transient Hot Wire Method. The thermal conductivity without applying a magnetic field (Zero field) and the thermal conductivity in the direction perpendicular to the magnetic field direction when the magnetic field was applied (Perpendicular field) and the parallel direction (Parallel field) were determined. The specific measurement method is shown as an appendix at the end of the specification.

The results are shown in FIG. The thermal conductivity without applying a magnetic field (Zero field) and the thermal conductivity in the perpendicular direction of the magnetic field (Perpendicular field) were almost the same in any of the test fluids.
Looking at the thermal conductivity in the magnetic field parallel direction (Parallel field), in the test fluid (MF) that does not contain silver nanowires, the improvement in thermal conductivity was about + 3% compared to when no magnetic field was applied. An improvement of +9 to + 15% was observed in the test fluid containing. It was confirmed that the orientation of silver nanowires by applying a magnetic field functions effectively for heat transport.

  Moreover, the tendency for the thermal conductivity improvement effect of a magnetic field parallel direction to increase was seen, so that the aspect ratio of the silver nanowire to be used was large. For example, when comparing the test fluid MF + (A) with a large average aspect ratio of silver nanowires of 177 and 51 with a small test fluid MF + (C), the former has a much smaller silver nanowire volume concentration than the latter. The former is more effective in improving the thermal conductivity in the direction parallel to the magnetic field. From this, it can be said that the aspect ratio is more dominant in the effect of improving the thermal conductivity in the direction parallel to the magnetic field than the volume concentration of the wire.

  As described above, the thermal conductivity in the direction parallel to the magnetic field can be remarkably improved only by adding a small amount of metal nanowires having a volume concentration on the order of several tens of ppm to the magnetic fluid.

<< Appendix >> Method for measuring the thermal conductivity of the test fluid by the unsteady hot wire method The thermal conductivity of the test fluid was measured by the Transient Hot Wire Method. The apparatuses are TPSYS02 and TP08 manufactured by CLIMATEC. In this method, a 180 ml sample is placed in a cylindrical container (made of glass), and a measurement probe is installed in a vertical direction on a cylindrical shaft. A schematic diagram related to heat input from the probe is shown in FIG.

  As shown in FIG. 3, heat is input from the probe to the cylindrical sample in the radial direction in a step function. By heating, the temperature rises according to the physical properties of the test fluid. At this time, the temperature is measured by a thermocouple installed in the probe. A schematic diagram of the probe is shown in FIG. The probe has two temperature sensors built in, and the temperature of the test fluid is increased by the thermocouple of the needle section using the temperature measured by the platinum resistance thermometer in the base in Fig. 4 as the reference temperature. Measure from time to time.

The unsteady hot wire method can be measured according to the unsteady heat conduction equation. The governing equations for the cylindrical coordinate system are shown below.
r is the position from the cylindrical axis. Here, the initial temperature is T ₀ and the temperature difference is θ = T−T _0.
far. At this time, initial conditions and boundary conditions are as follows.
Where r ₀ is the radius of the thin line. From the above conditions, the solution of the heat conduction equation can be derived as follows.
Therefore, equation (3) is
It becomes. Furthermore, if the equation for thermal conductivity λ is arranged,
It becomes. From the above, the thermal conductivity can be measured by obtaining the slope of the temperature difference θ with respect to the natural logarithm of time t by the equation (5).

  The experiment is performed in a constant temperature room where the temperature is constant at 25 ° C. In order to reduce the measurement error due to the convection phenomenon of the test fluid, wait approximately 15 minutes until the test solution temperature at the probe temperature measurement point becomes constant (the temperature change within 1 minute is within 0.1 ° C). do an experiment. The amount of heat input is 5 W / m.

  A typical example of the measurement result of the temperature rise with respect to time by the unsteady hot wire method is shown in FIG. In section (a), there is a first-order lag in the amount of heat generated with respect to the input of current. Further, in the section (c), there is an influence of natural convection caused by a large temperature difference. Therefore, the thermal conductivity is obtained by the equation (5) using the gradient of the temperature difference θ with respect to the natural logarithm of the time t in the section (b).

  In the measuring device, heating is performed in the radial direction (horizontal direction) in the cylindrical container. There are two types of magnetic field application: a vertical magnetic field is applied by a Helmholtz coil, and a horizontal magnetic field is applied by a neodymium magnet. FIG. 6 schematically shows a method for installing a Helmholtz coil and a neodymium magnet. When measuring the thermal conductivity in the vertical direction of the magnetic field (Perpendicular field), the cylindrical container containing the test fluid is set between the upper and lower Helmholtz coils, and the heat conduction in the parallel direction of the magnetic field (Parallel field) When measuring the rate, set between the left and right neodymium magnets.

  The Helmholtz coil has a uniform magnetic field distribution. On the other hand, it becomes non-uniform between neodymium magnets. The table shown in FIG. 6 shows values obtained by measuring the magnetic flux density in the direction between the neodymium magnets with the temperature measurement point as the origin in the container dimensions. In the unsteady hot wire method, since the temperature measurement region is very small, it is considered to be uniform in that region.

Claims (4)

Magnetic particles and metal nanowires are dispersed in a liquid medium, the average particle diameter P _M of the magnetic particles is smaller than the average diameter D _{M of the} metal nanowires, and the average length L _M and the average diameter D _{M of the} metal nanowires. the ratio L _M / D _M average aspect ratio a _M represented by is not less than 10, the volume concentration of the magnetic particles is less than 1.0%, a magnetic field oriented metal nanowires dispersion volume concentration of the metal nanowires is 30ppm or more fluid. Metal nanowires have an average diameter D _M is the magnetic field oriented metal nanowires dispersion fluid according to claim 1 is of 500nm or less. The magnetically oriented metal nanowire dispersion fluid according to claim 1 or 2 , wherein the magnetic particles are iron oxide particles. Magnetic field oriented metal nanowires dispersion fluid according to any one of claims 1 to third metal nanowires are silver nanowires.