Temperature-sensitive magnetic fluid and magnetic fluid drive device using the temperature-sensitive magnetic fluid

Abstract

A temperature-sensitive magnetic fluid that realizes a cooling device with higher cooling performance and a magnetic fluid drive device that uses the temperature-sensitive magnetic fluid are provided. [Solution] The thermosensitive magnetic fluid 1 includes a composite 13 formed by combining a thermoresponsive gel 11 and magnetic nanoparticles 12. The composite has a structure in which the magnetic nanoparticles are encapsulated in the thermoresponsive gel or a structure in which the magnetic nanoparticles are adsorbed on the surface of the thermoresponsive gel.

Description

[Technical Field]

[0001] The present disclosure relates to a temperature-sensitive magnetic fluid and a magnetic fluid drive device using the temperature-sensitive magnetic fluid. [Background technology]

[0002] A magnetic fluid drive device using a temperature-sensitive magnetic fluid has been proposed (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2018-41807 Summary of the Invention [Problem to be solved by the invention]

[0004] A thermosensitive magnetic fluid is a fluid in which magnetic nanoparticles (MNPs) with a low Curie temperature are dispersed in water or oil. The magnetic nanoparticles used in thermosensitive magnetic fluids have the property that their magnetization decreases significantly with increasing temperature. As described below, a magnetic field application unit is installed in the circulation flow path through which the thermosensitive magnetic fluid circulates. By heating a part of the magnetic field application unit and creating a temperature difference in the thermosensitive magnetic fluid in the magnetic field application unit, a driving force proportional to the temperature difference is obtained. This driving force can be used to create a flow of thermosensitive magnetic fluid from the low-temperature side to the high-temperature side. Such a device is called a "magnetic fluid drive unit" and can be used as a natural circulation heat transport device. A magnetic fluid drive unit has the advantage of being able to transport heat without the use of mechanical elements such as pumps, and therefore without the need for electrical power.

[0005] However, the heat transport performance of the conventional magnetic fluid drive device described in Patent Document 1, for example, is still insufficient for practical use, and there is room for further improvement.

[0006] The technology according to the present disclosure has been made in view of these circumstances, and its purpose is to realize a magnetic fluid drive device with higher heat transport performance. [Means for solving the problem]

[0007] In order to solve the above problems, a thermosensitive magnetic fluid according to one embodiment of the present disclosure includes a composite of a thermoresponsive gel and magnetic nanoparticles.

[0008] In one embodiment, the composite may have a structure in which magnetic nanoparticles are encapsulated in a thermoresponsive gel.

[0009] In one embodiment, the composite may have a structure in which magnetic nanoparticles are adsorbed onto the surface of a thermoresponsive gel.

[0010] In one embodiment, the complex may have a diameter of 50 nm or more and 500 nm or less at low temperature.

[0011] In one embodiment, the content of magnetic nanoparticles in the composite in the dry state may be 3% by weight or more and 70% by weight or less.

[0012] In certain embodiments, the magnetic nanoparticles may be iron oxide particles, manganese zinc ferrite particles, or cobalt ferrite particles.

[0013] Another aspect of the present disclosure is a magnetic fluid drive device. This device includes a circulation flow path that circulates a temperature-sensitive magnetic fluid, a magnetic field application unit that applies a magnetic field to the temperature-sensitive magnetic fluid in the circulation flow path, and a heating unit that heats a part of the magnetic field application unit to impart a temperature difference to the temperature-sensitive magnetic fluid in the magnetic field application unit. The temperature-sensitive magnetic fluid is any one of the temperature-sensitive magnetic fluids described above.

[0014] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure. [Effects of the Invention]

[0015] According to the present invention, a magnetic fluid drive unit with higher heat transport performance can be realized. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram illustrating the operating principle of a magnetic fluid drive device using a temperature-sensitive magnetic fluid. [Figure 2] 1 is a schematic diagram of a temperature-sensitive magnetic fluid according to an embodiment. [Figure 3] 1 is a schematic diagram of a magnetic fluid drive unit according to an embodiment; [Figure 4] FIG. 3 is a schematic diagram of a thermosensitive magnetic fluid containing a different type of composite from that shown in FIG. 2. [Figure 5] FIG. 1 is a schematic diagram showing the steps of a method for producing a UCST-type gel. [Figure 6] These are photographs of the magnetic nanoparticle / UCST gel composites that were prepared. The four photographs on the left are UCST gels that do not contain magnetic nanoparticles (Sample 1). The four photographs on the right are magnetic nanoparticle / UCST gel composites (Sample 2) prepared using a pre-gel solution with a magnetic nanoparticle content of 1.0 wt%. [Figure 7] FIG. 10 is a diagram showing the results of swelling ratio measurement in an evaluation experiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and redundant explanations will be omitted where appropriate. Furthermore, the embodiments are illustrative and do not limit the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0018] In addition, the dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes, and even if a component A is depicted as being thicker than another component B in the drawings, it is possible that component A is thinner than component B.

[0019] Before describing specific embodiments, the basic knowledge will be explained. Fig. 1 is a schematic diagram showing the operating principle of a magnetic fluid drive device using a temperature-sensitive magnetic fluid. Fig. 1 shows part of the magnetic fluid drive device, which is composed of a circulation flow path extending left and right, a magnetic field application unit (specifically, a magnet) installed near the center of the circulation flow path, and a heat source installed to the left of the center of the magnet. The x-axis is taken to point left along the circulation flow path.

[0020] The magnet applies a magnetic field that is roughly perpendicular to the x-axis to the thermosensitive magnetic fluid in the circulation channel. The magnetic field is strongest near the center of the magnet (H max ) and becomes weaker as it moves away from the center. In other words, a magnetic field gradient is formed in the circulation channel that points toward the center of the magnet.

[0021] The heat source heats a portion of the magnetic field application unit, creating a temperature gradient in the thermosensitive magnetic fluid in the magnetic field application unit. Specifically, the left side of the circulation flow path is hotter due to the heat source located to the left of the center of the magnet. Conversely, the right side of the circulation flow path, farther from the heat source, is cooler.

[0022] The circulation channel is filled with a thermo-sensitive magnetic fluid. The thermo-sensitive magnetic fluid exhibits superparamagnetic properties and behaves as a magnetized fluid when a magnetic field is applied. The magnetization of the thermo-sensitive magnetic fluid decreases as the temperature increases.

[0023] The high-temperature side and the low-temperature side are connected to each other, forming a loop, which forms a power-less heat transport device.

[0024] If the magnetic permeability of a vacuum is μ0, the magnetization of the thermosensitive magnetic fluid is M, and the strength of the applied magnetic field is H, the thermosensitive magnetic fluid has the following magnetic body force F m is working. F m =μ0M·∇H

[0025] The magnetization of the temperature-sensitive magnetic fluid on the right side of the magnet (low temperature side) is M L , the magnetization on the left side (high temperature side) of the magnet is M H Then, as explained above, M L >M H is.

[0026] The magnetic body force acting on the temperature-sensitive magnetic fluid on the right side (low temperature side) of the magnet's center is directed in the positive direction of the x-axis and has a magnitude of μM. L On the other hand, the magnetic body force acting on the temperature-sensitive magnetic fluid on the left side of the center of the magnet (high temperature side) is directed in the negative direction of the x-axis and has a magnitude of μ0M. H ∇H. As a result, the temperature-sensitive magnetic fluid has ΔF m =μ0(M L -M H )·∇H (1) A magnetic body force acts in the positive direction of the x-axis, with a magnitude of . This causes a flow of the thermosensitive magnetic fluid from right to left within the circulation channel, and heat is transported by natural circulation.

[0027] Magnetic fluid drive units using such temperature-sensitive magnetic fluids are expected to be used as heat transport devices, such as cooling systems for electric vehicles (EVs), which require energy savings, and for cooling large outdoor speakers and displays. However, as it stands, the driving force (heat transport performance) is insufficient, and further improvements in driving force are required for practical use.

[0028] At this time, there is a limit to how much improvement in driving force can be achieved with the current thermosensitive magnetic fluid that utilizes the temperature dependence of the magnetization of manganese zinc ferrite. However, developing new magnetic nanoparticles with low Curie temperatures is expected to be difficult and costly. Therefore, rather than improving the magnetic nanoparticles or the dispersant contained in the dispersion medium, a new approach to improving driving force is needed.

[0029] In response to this, the inventors realized that by combining a thermosensitive magnetic fluid with a thermoresponsive gel, the number density of magnetic nanoparticles in the circulation channel can be changed, thereby improving the driving force. The technology of the present disclosure will be described below based on typical embodiments.

[0030] [Temperature-sensitive magnetic fluid] 2 is a schematic diagram of a thermosensitive magnetic fluid 1 according to an embodiment. The thermosensitive magnetic fluid 1 includes a composite 13 in which a thermoresponsive gel 11 and magnetic nanoparticles 12 are combined.

[0031] A magnetic fluid is a colloidal solution in which magnetic nanoparticles are dispersed in a liquid (dispersion medium, dispersant) in an extremely stable manner. Iron oxide nanoparticles with a diameter of about 10 nm are generally used as the magnetic nanoparticles. The dispersion medium can be selected from water, solvents, various oils, etc. depending on the application.

[0032] The magnetization of magnetic nanoparticles decreases with increasing temperature. In particular, by mixing magnetic nanoparticles with low Curie temperatures, such as manganese zinc ferrite, into the dispersed particles of a magnetic fluid, a magnetic fluid called a temperature-sensitive magnetic fluid can be obtained. By using magnetic nanoparticles with low Curie temperatures, a temperature-sensitive magnetic fluid exhibits a large change in magnetization in response to temperature changes at room temperature.

[0033] Thermoresponsive gels are gels made by crosslinking thermoresponsive polymers, and have the property of reversibly changing their volume depending on the temperature. There are various types of thermoresponsive polymers, but this specification focuses on the UCST (Upper Critical Solution Temperature) type, which becomes hydrophilic at high temperatures and hydrophobic at low temperatures. Gels made from UCST polymers have the property of swelling at high temperatures and shrinking at low temperatures.

[0034] Examples of gels made from UCST-type thermoresponsive polymers include IPN (interpenetrating polymer network) gels of polyacrylamide and polyacrylic acid, gels of copolymers of acrylamide and acrylonitrile, and IPN gels of poly(N-acryloylglycinamide) and poly(glycidyl methacrylate).

[0035] The thermosensitive magnetic fluid 1 containing the composite 13 shown in FIG. 2 has a structure in which magnetic nanoparticles are dispersed in a UCST-type thermoresponsive gel with a diameter on the order of nm to μm.

[0036] The volume of the composite 13 increases at high temperatures and decreases at low temperatures. This allows a magnetic fluid drive device using the temperature-sensitive magnetic fluid 1 to have higher heat transport performance than a conventional magnetic fluid drive device that uses only magnetic nanoparticles. This notable feature will be explained below using an embodiment of a magnetic fluid drive device.

[0037] [Magnetic fluid drive unit] 3 is a schematic diagram of a magnetic fluid drive unit 100 according to an embodiment. The magnetic fluid drive unit 100 includes a circulation flow path 21 that circulates a temperature-sensitive magnetic fluid, a magnetic field application unit 22 that applies a magnetic field to the temperature-sensitive magnetic fluid in the circulation flow path 21, a heating unit 23 that heats a portion of the magnetic field application unit 22 to create a temperature difference in the temperature-sensitive magnetic fluid in the magnetic field application unit 22, and a radiator 24 that cools the temperature-sensitive magnetic fluid in the circulation flow path 21. The temperature-sensitive magnetic fluid is the temperature-sensitive magnetic fluid 1 described above.

[0038] The configuration of the magnetic fluid drive device 100 is basically the same as that of the magnetic fluid drive device in Fig. 1. The difference is that the temperature-sensitive magnetic fluid circulating inside contains a composite 13 that combines a thermoresponsive gel 11 and magnetic nanoparticles 12.

[0039] 1, in the magnetic fluid drive unit 100, the magnetovolume force acting on the temperature-sensitive magnetic fluid to the right of the center of the magnet (low temperature side) is greater than the magnetovolume force acting on the temperature-sensitive magnetic fluid to the left of the center of the magnet (high temperature side), so a total magnetovolume force acts to the left. This causes the temperature-sensitive magnetic fluid to circulate counterclockwise within the circulation flow path 21.

[0040] Furthermore, in the magnetic fluid drive device 100, an effect occurs due to the difference in number density caused by the change in volume of the composite 13 due to temperature, as will be described below.

[0041] As described above, the volume of the composite 13 increases at high temperatures and decreases at low temperatures. Therefore, the number density of the composites 13 (the number of composites 13 per unit volume) in the circulation flow channel 21 is higher on the low temperature side than on the high temperature side. In other words, the number density of the composites 13 on the low temperature side is d L , the number density on the hot side of the complex 13 is d H Then, d L >d H Due to this difference in number density, the magnetic body force acting on the thermosensitive magnetic fluid per unit volume is stronger on the low temperature side than on the high temperature side. Specifically, the thermosensitive magnetic fluid has the following characteristics: ΔF m =μ0(M L d L -M H d H )·∇H (2) A magnetic body force of magnitude ≈ 1 acts in the positive direction of the x-axis. Needless to say, the magnetic body force in equation (2) is greater than the magnetic body force in equation (1).

[0042] As described above, according to this embodiment, the thermosensitive magnetic fluid contains a composite 13 that combines a thermoresponsive gel 11 and magnetic nanoparticles 12, thereby achieving higher heat transport performance than conventional methods.

[0043] The magnetic fluid drive unit in FIG. 3 is provided with a radiator 24 for the purpose of improving cooling efficiency, but the radiator 24 may be omitted if sufficient natural cooling is possible.

[0044] [Example 1 of a complex] The composite 13 shown in Figure 2 has a structure in which magnetic nanoparticles 12 are encapsulated in a thermoresponsive gel 11. This type of structure has the advantages of being relatively easy to fabricate and of being prone to swelling and shrinkage due to temperature, since the thermoresponsive gel 11 is exposed in the thermosensitive magnetic fluid.

[0045] [Compound example 2] Figure 4 is a schematic diagram of a thermosensitive magnetic fluid 2 containing a different type of composite 14 from that shown in Figure 2. The composite 14 has a structure in which magnetic nanoparticles 12 are adsorbed onto the surface of a thermoresponsive gel 11. This type of structure has the advantage of large volumetric changes due to swelling and shrinkage caused by temperature, since the magnetic nanoparticles 12 are not contained inside the thermoresponsive gel 11.

[0046] [Complex size] Preferably, the diameter of the composite 13 at low temperatures is 50 nm or more and 500 μm or less. If the composite 13 is smaller than this range, sufficient swelling and shrinkage may not occur. Conversely, if the composite 13 is larger than this range, the composite 13 may not be stably dispersed in a liquid (dispersion medium, dispersant).

[0047] [Magnetic nanoparticle content] Preferably, the content of magnetic nanoparticles in the composite 13 in a dry state is 3% by weight or more and 70% by weight or less. If the content of magnetic nanoparticles is less than this range, they may not be attracted to the magnet, and sufficient driving force may not be obtained. Conversely, if the content of magnetic nanoparticles is more than this range, a gel may not be formed, or swelling and shrinkage may not occur.

[0048] [Magnetic nanoparticle materials] Preferably, the magnetic nanoparticles are iron oxide particles, manganese zinc ferrite particles, or cobalt ferrite particles. By using such materials for the magnetic nanoparticles, a magnetic fluid drive device with higher heat transport performance can be realized.

[0049] [Method for preparing UCST gel and magnetic nanoparticle / UCST gel composite] An example of a method for producing UCST gel and a magnetic nanoparticle / UCST gel composite is shown below.

[0050] An interpenetrating polymer network (IPN) hydrogel of polyacrylamide and polyacrylic acid is used as the UCST-type gel for fabricating the magnetic nanoparticle / UCST-type gel composite. An IPN gel is a gel with a structure in which two or more polymers exist independently and are entangled with each other without forming a crosslinked network through chemical bonds. IPN gels have the following characteristics: they swell in solvents but do not release polymers, and they are free of creep (the phenomenon in which deformation progresses over time under a constant stress) and fluidity.

[0051] Here, we describe a method for fabricating UCST-type gels consisting of polyacrylamide and polyacrylic acid. The reagents used were acrylamide (AAM) and acrylic acid (AAC) as the main gel materials, N,N'-methylenebisacrylamide (MBAAm) as the crosslinker, and azobisisobutyronitrile (AIBN) as the radical polymerization initiator. Ultrapure water and tetrahydrofuran (THF) were used as solvents.

[0052] Figure 5 is a schematic diagram showing the steps in the method for producing UCST-type gels. First, an initial gel containing AAM as the main component is produced. Next, an aqueous solution containing AAC and MBAAm is infiltrated into the initial gel and polymerized to produce an IPN gel. This IPN gel exhibits UCST properties. The details of the production method are as follows:

[0053] Preparation of initial gel (1) Mixture of water, AAM, MBAAm, AIBN, and THF Mix 4.5 mL of water, 670 mg of AAM, 10 mg of MBAAm, 2.35 mg of AIBN, and 0.5 mL of THF in a 13.5 mL screw cap bottle and stir. To prepare a magnetic nanoparticle / UCST gel, mix the magnetic nanoparticles into this pregel solution.

[0054] (2) Removal of dissolved oxygen from the mixed solution To remove dissolved oxygen from the mixed solution prepared in (1), Ar bubbling was performed for 10 minutes. Dissolved oxygen removal was performed because the UCST gel to be prepared is made by polymerizing and crosslinking AAM and AAC by radical polymerization, and if oxygen molecules with radicals exist in the solution, the progress of the polymerization and crosslinking reactions would be hindered.

[0055] (3) Encapsulation into a gel mold The mixed solution from which dissolved oxygen was removed in (2) is poured into a 12.5mm x 12.5mm x 45mm gel mold, wrapped in plastic wrap, and left to polymerize in a thermostatic chamber set at 68°C for 8 hours.

[0056] (4) Washing and drying the gel After leaving it for 8 hours, the initial gel is removed from the gel mold and washed several times with ultrapure water. After washing, the gel is cut and air-dried in a fume hood for 24 hours, and then dried in a vacuum at 40°C.

[0057] Preparation of UCST-type gel (5) Mixture of water, AAC, MBAAm, and AIBN 30 mL of water, 2.07 mL of AAC, 60 mg of MBAAm, and 14.1 mg of AIBN were placed in a 50 mL screw tube bottle, mixed and stirred, and then subjected to Ar bubbling for 10 minutes.

[0058] (6) Gel immersion The dried initial gel prepared in (4) is immersed in the solution (AAC solution) prepared in (5) and left for 24 hours. This operation is carried out in an N2-purged glove box.

[0059] (7) Gel polymerization After leaving it for 24 hours, discard the remaining AAC solution and seal the 50 mL screw cap. This procedure is performed in a glove box purged with N2. Then, leave it in a thermostatic bath set at 68°C for 8 hours to polymerize again. After 8 hours, wash the gel several times with ultrapure water.

[0060] [Evaluation experiment] The inventors conducted evaluation experiments using the magnetic nanoparticle / UCST gel composite they prepared. Figure 6 shows photographs of the magnetic nanoparticle / UCST gel composite they prepared. The four photographs on the left are UCST gels that do not contain magnetic nanoparticles (Sample 1). The four photographs on the right are magnetic nanoparticle / UCST gel composites (Sample 2) prepared using a pre-gel solution containing 1.0 wt% magnetic nanoparticles.

[0061] As can be seen from Figure 6, both Sample 1 and Sample 2 show swelling behavior with increasing temperature. This indicates that the volume change of UCST gels can be utilized even when magnetic nanoparticles are added to the UCST gels.

[0062] For both Sample 1 and Sample 2, the mass was measured in a 2°C shrunk state, a 50°C swollen state, and a dry state. The swelling ratio at each temperature was calculated using the following formula (3), and the volume change with temperature was evaluated. Q T =(m T -m D ) / m D (3) Here, Q T is the swelling ratio of the composite at temperature T, mT is the mass at temperature T, m D is the dry mass.

[0063] Figure 7 shows the results of the swelling ratio calculations. Here, the swelling ratio of the magnetic nanoparticle / UCST gel composite at each temperature was calculated based on the mass in the dry state. These results confirm that the swelling ratio increases with increasing temperature, even for the magnetic nanoparticle / UCST gel composite prepared using a pre-gel solution with a magnetic nanoparticle content of 1.0 wt%. However, it can be seen that the change in swelling ratio of the UCST gel decreases with the addition of magnetic nanoparticles. This is thought to be due to the hydroxyl groups on the surface of the magnetic nanoparticles strengthening the hydrogen bonds between polymers, which are related to UCST behavior.

[0064] The inventors also conducted a simple experiment to confirm the magnetic response of the magnetic nanoparticle / UCST gel composite. As a result, it was confirmed that the magnetic nanoparticle / UCST gel composite prepared using a pre-gel solution containing 1.0 wt% magnetic nanoparticles exhibited a behavior of being attracted to a magnet, and thus possessed ferromagnetic properties.

[0065] The above experimental results confirmed that a material was obtained that had both ferromagnetism and an increase in volume with increasing temperature, demonstrating the effectiveness of the technology disclosed herein.

[0066] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and treatment processes, and that such modifications are also within the scope of the present invention.

[0067] Any combination of the above-described embodiments and modifications is also useful as an embodiment of the present disclosure. A new embodiment resulting from the combination has the combined effects of the combined embodiments and modifications.

[0068] When understanding the abstract technical ideas of the embodiments, the technical ideas should not be interpreted as being limited to the contents of the embodiments. The above-described embodiments and variations are merely illustrative examples, and many design modifications, such as changes, additions, and deletions of components, are possible. In the embodiments, the contents in which such design modifications are possible are emphasized by adding the notation "embodiment." However, design modifications are also permitted even in contents without such notation. [Industrial Applicability]

[0069] The temperature-sensitive magnetic fluid and magnetic fluid drive device disclosed herein can be widely used as heat transport devices, such as cooling devices for electric vehicles (EVs) that require power saving, and cooling devices for large speakers and displays used outdoors. [Explanation of symbols]

[0070] 1...temperature-sensitive magnetic fluid, 2...temperature-sensitive magnetic fluid, 11··Thermoresponsive gel, 12··Magnetic nanoparticles, 13··complex, 14··complex, 21··Circulation flow path, 22 Magnetic field application unit, 23... Heating section, 24··Radiator, 100··Magnetic fluid drive unit.

Claims

1. A temperature-sensitive magnetic fluid characterized by containing a composite material comprising a thermoresponsive gel and magnetic nanoparticles.

2. The thermosensitive magnetic fluid according to claim 1, characterized in that the composite has a structure in which the magnetic nanoparticles are encapsulated in the thermoresponsive gel.

3. The thermosensitive magnetic fluid according to claim 1, characterized in that the composite has a structure in which the magnetic nanoparticles are adsorbed on the surface of the thermoresponsive gel.

4. The temperature-sensitive magnetic fluid according to claim 1, characterized in that the content of magnetic nanoparticles in the composite in a dry state is 3% by weight or more and 70% by weight or less.

5. The temperature-sensitive magnetic fluid according to claim 1, characterized in that the size of the composite at low temperatures is 50 nm or more and 500 nm in diameter or less.

6. 2. The temperature-sensitive magnetic fluid according to claim 1, wherein the magnetic nanoparticles are iron oxide particles, manganese zinc ferrite particles, or cobalt ferrite particles.

7. A magnetic fluid drive device comprising: a circulation flow path for circulating a temperature-sensitive magnetic fluid; a magnetic field application unit for applying a magnetic field to the temperature-sensitive magnetic fluid in the circulation flow path; and a heating unit for heating a part of the magnetic field application unit to impart a temperature difference to the temperature-sensitive magnetic fluid in the magnetic field application unit, wherein the temperature-sensitive magnetic fluid is the temperature-sensitive magnetic fluid described in any one of claims 1 to 6.

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