Magnetic fluid particles, magnetic fluid, and method for manufacturing magnetic fluid particles

Abstract

To provide magnetic fluid particles, a magnetic fluid, and a method for manufacturing particles for magnetic fluid, which can reduce the degree of supercooling of a latent heat storage material.SOLUTION: Magnetic fluid particles 1 dispersed in a predetermined dispersion medium have a structure having an outer shell 11 and a core 12 covered by the outer shell 11, and the core 12 contains a latent heat storage material 121 and a plurality of magnetic particles 122 dispersed in the latent heat storage material 121, thereby reducing the degree of supercooling of the latent heat storage material 121 contained in the magnetic fluid particle 1.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to particles for magnetic fluids, magnetic fluids, and methods for producing particles for magnetic fluids. [Background technology]

[0002] Magnetic fluids, in which magnetic particles are dispersed in a dispersing medium such as water or oil, have been known for some time. The particle size of the magnetic particles is typically around 10 nm. When using temperature-sensitive magnetic particles whose magnetic susceptibility changes with temperature, a temperature difference in a magnetic field causes an imbalance in the magnetic force, causing the magnetic fluid to move spontaneously. Therefore, magnetic fluids can be made to flow without the need for an external power source, and in recent years, they have been expected to be used in non-powered cooling devices, waste heat-driven pumps, and other applications.

[0003] When magnetic particles in a liquid approach each other, attractive forces such as magnetic attraction and van der Waals forces act on them. Surfactants are used to generate repulsive forces that resist these attractive forces (see, for example, Patent Document 1). Surfactants are selected that have irreversible adsorption to the particle surface and good compatibility with the dispersion medium. In Patent Document 1, a surfactant containing a mercapto group is adsorbed to the surface of ferromagnetic metal oxide fine particles. The use of a surfactant generates a steric hindrance effect between the adsorbed molecules on the particle surface, creating a repulsive force that resists the attractive force between the particles.

[0004] However, if a surfactant compatible with the dispersion medium does not exist, the dispersion medium cannot be used, which limits the types of dispersion medium that can be used. Another problem is that, because the particle size of the magnetic particles is small, when pressure is applied to the magnetic fluid, it becomes clay-like and easily clogs the flow path.

[0005] To solve these problems, the present inventors have proposed a magnetic fluid in which magnetic fluid particles containing a plurality of magnetic particles are dispersed in a dispersion medium (see Patent Document 2). In the magnetic fluid described in Patent Document 2, the magnetic fluid particles have a skeleton that encapsulates each magnetic particle. This prevents the magnetic particles from agglomerating and improves the flexibility of the dispersion medium selection. Furthermore, because the particle size of the magnetic fluid particles is larger than that of the magnetic particles, the magnetic fluid is less likely to become clay-like even when pressure is applied. Patent Document 2 also describes encapsulating a latent heat storage material in the skeleton to impart latent heat storage functionality to the magnetic fluid particles. In Patent Document 2, the magnetic particles and the latent heat storage material are arranged independently of each other in separate spaces within the skeleton. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-41807 [Patent Document 2] Patent Publication No. 2021-2602 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when a latent heat storage material is used for magnetic fluid particles as in Patent Document 2, the liquid latent heat storage material does not solidify even when cooled past its freezing point, which is known as supercooling.

[0008] The present invention has been made in consideration of the above circumstances, and its purpose is to provide particles for magnetic fluids, magnetic fluids, and methods for manufacturing particles for magnetic fluids that can reduce the degree of supercooling of latent heat storage materials. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention provides: An outer shell portion; a core portion covered with the outer shell portion, The core portion is provided with magnetic fluid particles including a latent heat storage material and a plurality of magnetic particles dispersed in the latent heat storage material.

[0010] According to these magnetic fluid particles, the magnetic particles are dispersed in the latent heat storage material, so that the magnetic particles act as crystal nuclei when the latent heat storage material is cooled, and the degree of supercooling of the latent heat storage material can be reduced compared to when magnetic particles are not dispersed in the latent heat storage material. When a magnetic force acts on the magnetic fluid particles under a magnetic field, the degree of supercooling of the latent heat storage material is even smaller than when no magnetic force acts. The stronger the magnetic force acting on the magnetic fluid particles, the smaller the degree of supercooling of the latent heat storage material.

[0011] In the magnetic fluid particle, the outer shell, the latent heat storage material, and the magnetic particles may each be made of an inorganic material.

[0012] In the magnetic fluid particles, the latent heat storage material may be made of gallium.

[0013] In the magnetic fluid particle, the shell portion may be made of silica.

[0014] In the magnetic fluid particle, the outer shell may be porous.

[0015] In addition, in the present invention, The magnetic fluid particles; and a dispersion medium in which the magnetic fluid particles are dispersed.

[0016] Furthermore, in the present invention, In producing the above magnetic fluid particles, There is provided a method for producing particles for magnetic fluid, in which the outer shell is formed by a sol-gel method.

[0017] In the magnetic fluid particles, the outer shell may be formed using an alkaline sol solution. [Effects of the Invention]

[0018] According to the present invention, it is possible to reduce the degree of supercooling of the latent heat storage material contained in the magnetic fluid particles. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is an explanatory diagram of a magnetic fluid according to an embodiment of the present invention; [Figure 2] FIG. 2 is a schematic cross-sectional view of a particle for magnetic fluid. [Figure 3] FIG. 1 is an explanatory diagram showing a state in which an emulsion is being prepared. [Figure 4] FIG. 10 is an explanatory diagram showing a state in which a sol solution is being prepared before an emulsion is dropped. [Figure 5] FIG. 10 is an explanatory diagram showing a state in which the sol solution into which the emulsion has been dropped is being stirred. [Figure 6] 1 is an SEM image of magnetic particles showing an example. [Figure 7] This is an SEM image of particles for magnetic fluid. [Figure 8] 1 is a graph showing a DSC curve of one cycle of magnetic fluid particles. [Figure 9] 1 is a graph showing the relationship between the stirring speed of a magnetic stirrer and the average particle size of particles for a magnetic fluid. [Figure 10] 1 is a graph showing the relationship between the stirring speed of a magnetic stirrer and the particle size distribution of particles for a magnetic fluid. [Figure 11] 1 is a graph showing the thermal conductivity of water, a slurry in which magnetic fluid particles are dispersed in water, and a slurry in which latent heat material encapsulated particles are dispersed in water. [Figure 12] SEM images of particles for magnetic fluid are shown, where (a) is the emulsion with 0.1 wt% surfactant, (b) is the emulsion with 0.3 wt%, (c) is the emulsion with 1.0 wt%, and (d) is the emulsion with 3.0 wt% surfactant. [Figure 13] 1 is a graph showing the particle size distribution of particles for magnetic fluid. DETAILED DESCRIPTION OF THE INVENTION

[0020] Figures 1 to 5 show one embodiment of the present invention, with Figure 1 being an explanatory diagram of a magnetic fluid, Figure 2 being a schematic cross-sectional explanatory diagram of particles for magnetic fluid, Figure 3 being an explanatory diagram showing the state in which an emulsion is being prepared, Figure 4 being an explanatory diagram showing the state in which a sol solution is being prepared before the emulsion is dropped, and Figure 5 being an explanatory diagram showing the state in which the sol solution into which the emulsion has been dropped is being stirred. Note that the particles for magnetic fluid, magnetic particles, latent heat storage material, outer shell, surfactant, etc. in each figure are merely shown schematically and may differ in actual size, shape, etc.

[0021] As shown in Fig. 1, the magnetic fluid particles 1 have magnetic properties and are used by being dispersed in a dispersion medium 3 of a magnetic fluid 2. Fig. 1 shows a state in which a predetermined flow path 50 is filled with the magnetic fluid 2.

[0022] As shown in FIG. 2, the magnetic fluid particle 1 has a shell 11 and a core 12 covered by the shell 11. The core 12 includes a latent heat storage material 121 and a plurality of magnetic particles 122 dispersed in the latent heat storage material 121. In this embodiment, the magnetic fluid particle 1 includes a surfactant 13 used when forming the shell 11. The shell 11 is made of a non-magnetic material, and in this embodiment, is made of silica. In this embodiment, the shell 11 is formed by a sol-gel method, but may be formed by a method other than the sol-gel method. In addition, in this embodiment, the shell 11 is porous, but it does not have to be porous. In addition, inorganic materials other than silica may be used for the shell 11, or organic materials may also be used. Inorganic materials used for the shell 11 include silica, as well as zeolite, carbon, alumina, and the like. Examples of organic materials used for the outer shell 11 include thermosetting resins such as melamine resin, urea resin, polyurethane, polyurea, polyamide, and polyacrylamide, as well as thermoplastic elastomers such as diene resin, acrylic resin, and olefin thermoplastic elastomer.

[0023] The latent heat storage material 121 undergoes a phase change between solid and liquid at a predetermined melting point, and in this embodiment is made of gallium. Note that inorganic materials other than gallium may be used as the latent heat storage material 12, or organic materials may also be used. Examples of inorganic materials used for the latent heat storage material 121 include gallium and metals such as lead, tin, and aluminum. Examples of organic materials used for the latent heat storage material 12 include normal paraffin, paraffin, palmitic acid, stearic acid, hexane, and cyclohexane.

[0024] The magnetic particles 122 are made of a metal compound, and in this embodiment, are made of magnetite. In addition to magnetite, ferrite such as manganese zinc ferrite can be used as the magnetic particles 122. In this embodiment, the particle size of the magnetic particles 122 is 10 nm to 50 nm, and the particle size of the magnetic fluid particles 1 is 1 to 5 μm. The particle sizes of the magnetic particles 122 and the magnetic fluid particles 1 can be arbitrarily changed; for example, the particle size of the magnetic particles 122 can be adjusted appropriately within the range of 800 Å to 50 nm, and the particle size of the magnetic fluid particles 1 can be adjusted appropriately within the range of 0.8 μm to 10 μm. The particle size ratio between the magnetic particles 122 and the magnetic fluid particles 1 can also be arbitrarily changed; for example, the particle size of the magnetic fluid particles 1 can be adjusted to be 100 to 1000 times the particle size of the magnetic particles 122.

[0025] In this embodiment, an ethylene glycol aqueous solution is used as the dispersion medium 3. As will be described later, the use of the magnetic fluid particles 1 of this embodiment improves the freedom of selection of the dispersion medium 3, and the material of the dispersion medium 3 can be changed as desired. Any dispersion medium 3 can be used, and various water-based and oil-based liquids can be used. In addition to the ethylene glycol aqueous solution of this embodiment, for example, ethylene glycol, water, various alcohols, silicon oil, ammonia, an ammonia aqueous solution, silicone oil, propylene glycol, a propylene glycol aqueous solution, a fluorine-based inert liquid, etc. can be used. Furthermore, the magnetic fluid particles 1 may be mixed with a relatively viscous paste-like material, such as a liquid gasket made of a silicone material.

[0026] The magnetic fluid particle 1 of this embodiment can be produced through the following steps. First, composite particles are generated in which a plurality of magnetic particles 122 are dispersed in a latent heat storage material 121. Specifically, each magnetic particle 122 is mixed with a liquid latent heat storage material 121 to create a composite, and the composite is dropped into a predetermined liquid whose temperature is equal to or higher than the melting point of the latent heat storage material 121. Thereafter, the composite in the predetermined liquid is atomized, the predetermined liquid is cooled to solidify the latent heat storage material 121, and the composite particles are separated and collected from the predetermined liquid. In this embodiment, water is used as the predetermined liquid.

[0027] Next, a sol-gel method is used to form a shell 11 on the surface of the composite particles serving as the core 12. First, as shown in FIG. 3, an emulsion 100 is prepared in which the composite particles are dispersed in a predetermined solvent. In this embodiment, the emulsion 100 uses formamide as the solvent and contains CTAB (hexadecyltrimethylammonium bromide) as the surfactant 13. The amount of surfactant 13 is set according to the thickness of the shell 11. The surfactant 13 can be changed as desired. After the composite particles and surfactant 13 are introduced into the solvent, the mixture is stirred for a predetermined time using an ultrasonic stirrer 200 to prepare the emulsion 100.

[0028] As shown in FIG. 4, a sol solution 101 containing the starting material for the outer shell 11 is prepared. In this embodiment, the sol solution 101 uses TEOS (tetraethyl orthosilicate) as the starting material, and anhydrous ethanol and water as the solvent. The pH of the sol solution 101 is adjusted to be an alkaline solution. In this embodiment, the pH of the sol solution 101 is adjusted by adding ammonium hydroxide as a hydrolysis initiator 14 to the solvent. TEOS is added to the solvent, the hydrolysis initiator is added dropwise, and then the mixture is stirred with a magnetic stirrer 201 for a predetermined time to prepare the sol solution 101. After preparing the sol solution 101, the emulsion 100 is added dropwise to the sol solution 101.

[0029] When the emulsion 100 is added to the sol solution 101, the shell 11 is formed on the surface of the composite particle due to hydrolysis of the starting material. As shown in FIG. 5 , after the emulsion 100 is dropped, the sol solution 101 is stirred for a predetermined time using a magnetic stirrer 201. The stirring speed is set according to the thickness of the shell 11. After the predetermined stirring time has elapsed, the magnetic fluid particles 1 are separated and recovered from the sol solution. Thereafter, the shell 11 is made porous as necessary. In this embodiment, the dried magnetic fluid particles 1 are fired to make the shell 11 porous. The firing conditions are arbitrary, but for example, air, nitrogen, and carbon dioxide can be used as firing gases, the firing temperature can be 700°C to 800°C, and the firing time can be 3 to 5 hours.

[0030] According to the magnetic fluid particle 1 configured as described above, each magnetic particle 122 is covered with the outer shell 11, which improves the freedom of dispersion medium selection compared to dispersing each magnetic particle directly in a dispersion medium. Also, because the particle size of the magnetic fluid particle 1 is larger than that of each magnetic particle 122, the magnetic fluid is less likely to become clay-like even when pressure is applied to it.

[0031] Furthermore, because the magnetic particles 13 are dispersed in the latent heat storage material 12, the magnetic particles 13 act as crystal nuclei when the latent heat storage material 12 is cooled, and the degree of supercooling of the latent heat storage material 12 can be reduced compared to when the magnetic particles 13 are not dispersed in the latent heat storage material 12. When a magnetic force acts on the magnetic fluid particles 1 in a magnetic field, the degree of supercooling of the latent heat storage material 12 is even smaller than when no magnetic force acts. The stronger the magnetic force acting on the magnetic fluid particles 1, the smaller the degree of supercooling of the latent heat storage material 12.

[0032] Furthermore, since the latent heat storage material 12 is covered by the outer shell portion 11, oxidation of the latent heat storage material 12 can be suppressed. Furthermore, by using an alkaline solution as the sol solution 101, oxidation of the latent heat storage material 12 can also be suppressed. Gallium is known to have a very large degree of supercooling, and the increase in the degree of supercooling is thought to be caused by the formation of an oxide film on the gallium. In this embodiment, oxidation of gallium, which is the latent heat storage material 12, is suppressed, so the degree of supercooling of gallium can be dramatically reduced, for example, to 20°C or less.

[0033] Furthermore, since the outer shell 11, the latent heat storage material 121, and the magnetic particles 122 are each made of an inorganic material, the thermal conductivity of the magnetic fluid particles 1 can be increased compared to when an organic material is used for any of them, thereby increasing the thermal conductivity of the magnetic fluid 2 in which the magnetic fluid particles 1 are dispersed.

[0034] Furthermore, according to the particle 1 for magnetic fluid of this embodiment, since the outer shell 11 is porous, any function can be imparted to the particle 1 for magnetic fluid by utilizing the pores formed in the outer shell 11. For example, by adding a dye to the pores of the outer shell 11, it becomes possible to visualize the flow field of the magnetic fluid 2.

[0035] The magnetic fluid particles 1 of this embodiment were actually produced, and the particle size, properties, etc. were confirmed. Note that no porosity was made in the following examples. To prepare magnetic fluid particles, magnetite particles were first synthesized by coprecipitation. Specifically, 0.60 g of iron(II) chloride tetrahydrate (FeCl2·4H2O) and 1.35 g of iron(III) chloride hexahydrate (FeCl3·6H2O) were added to 100 ml of water at 50°C and stirred until the solution became clear. Next, ammonium hydroxide was added dropwise to form an alkaline solution, and CTAB was added as a surfactant and stirred for 30 minutes. The black precipitate thus formed was separated from the solution using an external magnetic field, washed with water, and then dried to obtain magnetite particles. Figure 6 shows an SEM image of the resulting magnetite particles. As shown in Figure 6, it was confirmed that magnetite particles with a particle diameter of approximately 20–30 nm were synthesized.

[0036] Next, composite particles were prepared in which magnetite particles were dispersed in gallium as a latent heat storage material. Specifically, the obtained magnetite particles were mixed with liquid gallium in the same mass ratio to prepare a composite, and the composite was then dropped into water at a temperature above the melting point of gallium. Next, the composite was atomized using an ultrasonic homogenizer, and the water was cooled to solidify the gallium. After this, the composite particles were separated from the water using a centrifuge and collected.

[0037] Next, silica was formed as an outer shell on the surface of the composite particles using a sol-gel method. Specifically, CTAB (surfactant 14) and the resulting composite particles were added to 50 ml of formamide at 50°C and stirred for 20 minutes with an ultrasonic stirrer to prepare emulsion 100. Sol solution 101 was prepared by adding TEOS to 30 ml of absolute ethanol and 10 ml of water, and adjusting the pH of the solution to 11.5 by adding ammonium hydroxide as a hydrolysis initiator. The mixture was then stirred for 30 minutes with a magnetic stirrer to prepare sol solution 101.

[0038] The emulsion was added dropwise to the sol solution prepared in this way, and the mixture was stirred for 2 hours using a magnetic stirrer. After this, the magnetic fluid particles were separated from the sol solution using a centrifuge and collected. The collected magnetic fluid particles were washed multiple times with water and then dried at room temperature.

[0039] Figure 7 shows an SEM image of the resulting magnetic fluid particles. As shown in Figure 7, we confirmed that magnetic fluid particles with a particle diameter of approximately 2 μm or less were synthesized. When a magnetic field was applied to the resulting aggregate of magnetic fluid particles, the magnetic fluid particles, which were uniformly dispersed before the magnetic field was applied, began to aggregate. Five minutes after the magnetic field was applied, clusters were formed in the direction of the magnetic flux. Since the magnetic fluid particles are magnetic, it is believed that the resulting magnetic fluid particles contain magnetic particles. Furthermore, when the resulting magnetic fluid particles were observed using energy dispersive X-ray spectroscopy (EDS), iron and gallium elements were detected in the areas where the particles were present. This confirmed that the magnetic fluid particles contain gallium as a latent heat storage material and magnetite particles as magnetic particles.

[0040] Furthermore, when the coercive force of the obtained particles for magnetic fluid was measured, it was confirmed to be almost the same as that of magnetite. Furthermore, the value of the particles for magnetic fluid (residual magnetization mr / saturation magnetization mm) was also almost the same as that of magnetite, confirming that the ferromagnetic properties of the magnetite particles were maintained.

[0041] Furthermore, the magnetic fluid particles were subjected to 50 phase changes, but no significant changes were observed in their appearance. This confirmed that the gallium latent heat storage material had not eluted from the silica outer shell, and that the outer shell was not damaged.

[0042] Differential scanning calorimetry (DSC) was also performed on the resulting magnetic fluid particles. Figure 8 is a graph showing the DSC curve of the magnetic fluid particles, including their behavior during supercooling. The measurement was performed by heating the magnetic fluid particles from -20°C to 40°C at 1°C per minute, holding the temperature for 10 minutes, and then cooling from 40°C to -20°C at 1°C per minute. As a result, as shown in Figure 8, the gallium in the magnetic fluid particles melted at 31.5°C, with solidification peaks occurring at 17.3°C and 13.1°C. In other words, supercooling of 14.2°C and 18.4°C was confirmed for the gallium in magnetic fluid particle 1. As mentioned above, gallium is known to have a very large degree of supercooling. For example, there are documents stating that the supercooling degree is 43.9°C to 49.2°C for micro- to submicron-sized gallium particles, and 114.1°C for 35 nm gallium particles. Furthermore, there are documents stating that no supercooling behavior was observed even at -183°C for 3 to 15 nm gallium particles. In these conventional examples, the supercooling degree is thought to have increased due to the formation of an oxide coating on the gallium. In this example, the supercooling degree of gallium is thought to have been dramatically reduced by (1) dispersing magnetite particles in gallium, (2) using an alkaline solution during the synthesis of magnetite particles, (3) using an alkaline solution instead of a sol solution, and (4) coating the composite particles with silica. It is believed that the supercooling degree of gallium can be reduced by any one of the above methods (2) to (4) without dispersing magnetite particles in gallium.

[0043] Next, we investigated the relationship between the stirring speed of the magnetic stirrer after dripping the emulsion into the sol solution and the particle size of the resulting magnetic fluid particles. The stirring speed of the magnetic stirrer was varied between 500 rpm, 750 rpm, 1000 rpm, and 1500 rpm, while all other conditions were kept constant. Figure 9 shows the relationship between the stirring speed of the magnetic stirrer and the average particle size of the magnetic fluid particles. As shown in Figure 9, the average particle size decreased with increasing stirring speed. Because the composite particles contained in the emulsions were prepared using the same method in all studies, it is likely that the thickness of the silica shell varied with the stirring speed. Therefore, the thickness of the shell, i.e., the particle size of the magnetic fluid particles, can be controlled by the stirring speed. Figure 10 also shows the relationship between the stirring speed of the magnetic stirrer and the particle size distribution of the magnetic fluid particles. As shown in Figure 10, the particle size distribution also decreased with increasing stirring speed.

[0044] Next, the thermal conductivity of the resulting slurry of magnetic fluid particles dispersed in water was measured. For comparison, latent heat material-encapsulated particles were prepared using the same procedure as the magnetic fluid particles, but without the magnetite particles, using only gallium as the core. Thermal conductivity measurements were performed on water, a slurry of magnetic fluid particles dispersed in water, and a slurry of latent heat material-encapsulated particles dispersed in water. The slurries were prepared by dispersing magnetic fluid particles or latent heat material-encapsulated particles at a concentration of 1 wt%. Figure 11 is a graph showing the thermal conductivity of water, a slurry of magnetic fluid particles 1 dispersed in water, and a slurry of latent heat material-encapsulated particles dispersed in water. In Figure 11, "water" represents water, "Ga+Fe" represents the slurry of magnetic fluid particles dispersed in water, and "Ga" represents the slurry of latent heat material-encapsulated particles dispersed in water. As shown in Figure 11, it was confirmed that the thermal conductivity of the slurry in which magnetic fluid particles 1 were dispersed in water and the slurry in which latent heat material encapsulated particles were dispersed in water was improved by 9.5% and 15.3%, respectively, compared to water. The reason why the thermal conductivity is higher when the core is made of gallium alone is thought to be because the thermal conductivity of magnetite is lower than that of gallium. In other words, it is thought that the thermal conductivity of the slurry depends on the thermal conductivity of the core encapsulated inside.

[0045] We also investigated the relationship between the amount of surfactant in the emulsion and the particle size of the resulting magnetic fluid particles. The amount of CTAB was varied from 0.1 wt%, 0.3 wt%, 1.0 wt%, and 3.0 wt%, while keeping all other conditions the same. Figure 12 shows SEM images of magnetic fluid particles: (a) 0.1 wt%, (b) 0.3 wt%, (c) 1.0 wt%, and (d) 3.0 wt%. As shown in Figure 12, the outer shell was properly formed when the surfactant amount was 0.3 wt% and 1.0 wt%, but not when the surfactant amount was 0.1 wt% and 3.0 wt%. This confirmed that the outer shell was properly formed when the surfactant amount was between 0.3 wt% and 1.0 wt%. Figure 13 is a graph showing the particle size distribution of magnetic fluid particles. As shown in FIG. 13, it was confirmed that the particle size increased as the amount of surfactant increased within the range in which the outer shell portion was properly formed. [Explanation of symbols]

[0046] 1 Particles for magnetic fluids 2 Magnetic fluid 3 Dispersion medium 11 Outer shell 12 Core 13 Surfactants 14 Hydrolysis initiator 50 flow paths 100 emulsion 101 Sol solution 121 Latent heat storage material 122 Magnetic particles 200 Ultrasonic Stirrer 201 Magnetic Stirrer

Claims

1. An outer shell portion; a core portion covered with the outer shell portion, The core portion includes a latent heat storage material and a plurality of magnetic particles dispersed in the latent heat storage material.

2. 2. The magnetic fluid particle according to claim 1, wherein the outer shell, the latent heat storage material, and the magnetic particles are each made of an inorganic material.

3. 3. The magnetic fluid particle according to claim 2, wherein the latent heat storage material is made of gallium.

4. 4. The magnetic fluid particle according to claim 3, wherein the shell portion is made of silica.

5. The magnetic fluid particle according to claim 2 , wherein the outer shell is porous.

6. A magnetic fluid particle according to any one of claims 1 to 5, a dispersion medium in which the magnetic fluid particles are dispersed.

7. In producing the magnetic fluid particles according to claim 2, The method for producing particles for magnetic fluids comprises forming the outer shell portion by a sol-gel method.

8. The method for producing particles for magnetic fluid according to claim 7, wherein the outer shell is formed using an alkaline sol solution.

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