Magnetoviscous fluids and braking systems

Abstract

To provide a magnetoviscous fluid that can achieve both high reliability and high excitation stress even when used at low or high temperatures, and a braking device equipped with such a magnetoviscous fluid. [Solution] A magnetorheological fluid comprising metal magnetic particles modified with an ionic liquid modification site containing a first cationic group and a first anionic group, and an ionic liquid dispersion medium containing a second cationic group and a second anionic group, wherein the first cationic group and the second cationic group are of the same type and are selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions and phosphonium ions, and the first anionic group and the second anionic group are selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, bis(trifluoromethanesulfonyl)amide ions, hydrogen phthalate ions and bissulfurylimide ions.

Description

Technical Field

[0001] The present invention relates to a magnetorheological fluid and a braking device.

Background Art

[0002] A magnetorheological fluid (MR fluid) is a fluid obtained by dispersing magnetic particles in a dispersion medium such as mineral oil or silicone oil. When a magnetic field is applied to the magnetorheological fluid, the metal magnetic particles are magnetized and aligned in the magnetic field direction to form chain-like clusters. As a result, the viscosity of the magnetorheological fluid increases and the yield stress becomes higher.

[0003] [[ID=—16]]Such magnetorheological fluids are being considered for use in various fields such as control devices like dampers, and braking devices like brakes and clutches.

[0004] In these applications, the temperature range in which the magnetorheological fluid is used is assumed to be from several tens of degrees below zero to several hundreds of degrees Celsius. Therefore, it is required that the magnetorheological fluid maintain its physical properties not only at normal temperature but also at low temperature and high temperature.

[0005] Patent Document 1 assumes the use of a magnetorheological fluid at a high temperature of 200°C, and discloses that an ionic liquid is used as the dispersion medium as a solution for suppressing the evaporation of the magnetorheological fluid.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] From the perspective of further expanding the range of use of magnetorheological fluids, even when used at low temperatures or high temperatures, it is required to suppress aggregation and sedimentation of magnetic particles while ensuring high reliability and exhibit high excitation stress.

Means for Solving the Problems

[0008] The magnetorheological fluid according to an application example of the present invention is metal magnetic particles whose surfaces are modified at an ionic liquid modification site having a first cation group and a first anion group, and an ionic liquid dispersion medium having a second cation group and a second anion group, and includes the first cation group and the second cation group are the same kind of cations as each other, and are selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions, the first anion group and the second anion group are each selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, tris(pentafluoroethyl)trifluorophosphate ions, bis(trifluoromethanesulfonyl)amide ions, hydrogen phthalate ions, and bis(fluorosulfonyl)imide ions.

[0009] The braking device according to an application example of the present invention is a fixed part, a movable part movable with respect to the fixed part, the magnetorheological fluid according to claim 1 or 2 held between the fixed part and the movable part, and a magnetic field generation part that applies a magnetic field to the magnetorheological fluid, and comprises.

Brief Description of the Drawings

[0010] [Figure 1] It is a schematic diagram showing a magnetorheological fluid according to an embodiment. [Figure 2] It is a longitudinal sectional view showing a braking device according to an embodiment. [Figure 3]Table 1 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 4] Table 2 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 5] Table 3 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 6] Table 4 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 7] Table 5 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 8] Table 6 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Modes for carrying out the invention]

[0011] The magnetorheological fluid and braking device of the present invention will be described in detail below based on the embodiments shown in the accompanying drawings.

[0012] 1. Magnetoviscous fluid First, a magnetorheological fluid according to an embodiment will be described.

[0013] Magnetorheological fluids are fluids that behave like liquids when no magnetic field is applied and like semi-solids when a magnetic field is applied. By utilizing this change in viscosity, the stress of magnetorheological fluids can be controlled. Therefore, magnetorheological fluids can be used in various devices that perform various functions by utilizing changes in stress.

[0014] Figure 1 is a schematic diagram showing a magnetorheological fluid 1 according to an embodiment. The magnetorheological fluid 1 shown in Figure 1 includes metallic magnetic particles 2, non-magnetic particles 3, and an ionic liquid dispersion medium 4. The metallic magnetic particles 2 and non-magnetic particles 3 are dispersed phases dispersed in the ionic liquid dispersion medium 4. The non-magnetic particles 3 may be provided as needed or omitted.

[0015] The surface of the metallic magnetic particle 2 is modified with ionic liquid modification sites having a primary cationic group and a primary anionic group. The ionic liquid dispersion medium 4 has a secondary cationic group and a secondary anionic group.

[0016] The first cationic group of the metallic magnetic particle 2 and the second cationic group of the ionic liquid dispersion medium 4 are of the same type and are selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0017] The first anionic group of the metal magnetic particle 2 and the second anionic group of the ionic liquid dispersion medium 4 are selected from the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, trispentafluoroethyltrifluorophosphate ion, bis(trifluoromethanesulfonyl)amide ion, hydrogen phthalate ion, and bissulfurylimide ion, respectively.

[0018] With this configuration, good affinity between the metal magnetic particles 2 and the ionic liquid dispersion medium 4 can be maintained even at low and high temperatures. In other words, in a magnetorheological fluid 1 in which metal magnetic particles 2 modified in the ionic liquid modification region are dispersed in the ionic liquid dispersion medium 4, sedimentation and uneven distribution of the metal magnetic particles 2 can be suppressed even under harsh environments. As a result, a magnetorheological fluid 1 that exhibits high excitation stress and maintains a good dispersion state of the dispersed phase regardless of the environment can be realized.

[0019] 1.1. Metal magnetic particles The metallic magnetic particle 2 comprises a particle body made of a metallic magnetic material and a surface modification film provided on the surface of the particle body.

[0020] Examples of metallic magnetic materials include Fe-based metallic materials, Ni-based metallic materials, and Co-based metallic materials, and one or more of these composite materials are used. Furthermore, composite materials of these metallic magnetic materials and oxide-based magnetic materials may also be used. Among these, Fe-based metallic materials are preferred from the viewpoint of having high saturation magnetization.

[0021] Fe-based metal materials are metallic materials whose main component is Fe. "Main component" means that the Fe content in the Fe-based metal material is 50% or more in terms of atomic ratio. Such Fe-based metal materials have higher saturation magnetization, toughness, and strength compared to ferrites and other materials. Therefore, Fe-based metal materials are useful as metallic magnetic materials.

[0022] The Fe-based metal material may contain elements that exhibit ferromagnetism on their own, such as Ni or Co, in addition to Fe, and may also contain at least one element selected from the group consisting of Cr, Nb, Cu, Al, Mn, Mo, Si, Sn, B, C, P, Ti, and Zr, depending on the desired properties. Furthermore, the Fe-based metal material may contain unavoidable impurities to the extent that they do not impair the effects of the embodiment. Unavoidable impurities are impurities that are unintentionally mixed in during the raw material or manufacturing process. Examples of unavoidable impurities include all elements other than those mentioned above, and particularly include O, N, S, Na, Mg, K, etc.

[0023] Such Fe-based metal materials are not particularly limited, but examples include pure iron, carbonyl iron, and Fe-based alloy materials such as Fe-Si-Al alloys like Sendust, Fe-Ni, Fe-Co, Fe-Ni-Co, Fe-Si-B, Fe-Si-Cr-B, Fe-Si-BC, Fe-Si-B-Cr-C, Fe-Si-Cr, Fe-B, Fe-PC, Fe-Co-Si-B, Fe-Si-B-Nb, Fe-Si-B-Nb-Cu, Fe-Zr-B, Fe-Cr, and Fe-Cr-Al.

[0024] Furthermore, the metallic magnetic material may be an amorphous metallic material, a crystalline metallic material, or a microcrystalline (nanocrystalline) metallic material. Of these, amorphous metallic materials or microcrystalline metallic materials are preferred. These contribute to significantly lowering the coercivity of the metallic magnetic particles 2 and improving their redispersibility. In addition, because they have higher toughness and strength compared to, for example, metal oxides, they can effectively suppress wear and chipping of the metallic magnetic particles 2. As a result, a magnetorheological fluid 1 with stable excitation stress can be realized. Moreover, amorphous metallic materials and microcrystalline metallic materials have no or very small grain boundaries, making them less susceptible to corrosion originating from grain boundaries. Therefore, the corrosion resistance of the metallic magnetic particles 2 can be particularly enhanced. A microcrystalline metallic material refers to a metallic material that contains microcrystals (nanocrystals) with a grain size of 100 nm or less.

[0025] Examples of amorphous metallic materials include binary or multi-component Fe-based amorphous alloys such as Fe-Si-B, Fe-Si-Cr-B, Fe-Si-BC, Fe-Si-B-Cr-C, Fe-Si-Cr, Fe-B, Fe-BC, Fe-PC, Fe-Co-Si-B, Fe-Si-B-Nb, and Fe-Zr-B; Ni-based amorphous alloys such as Ni-Si-B and Ni-PB; and Co-based amorphous alloys such as Co-Si-B.

[0026] Examples of microcrystalline metal materials include Fe-based nanocrystalline alloys such as Fe-Si-B-Nb-Cu, Fe-Zr-B, Fe-Hf-B, Fe-Nb-B, Fe-Zr-B-Co, Fe-Hf-B-Co, Fe-Nb-B-Co, and Fe-Si-BP-Cu.

[0027] The metal magnetic particles 2 may be particles manufactured by any method. Examples of the manufacturing method include, for example, various atomization methods such as the water atomization method, the gas atomization method, the rotating water flow atomization method, etc., as well as the pulverization method, the carbonyl method, etc. Among these, according to the atomization method, a main body with a particle shape closer to a perfect sphere can be obtained. Such particles are less likely to aggregate.

[0028] The metal magnetic particles 2 may contain an oxide film provided between the particle main body and the surface modification film. The oxide film enhances the adhesion of the surface modification film to the particle main body. Also, the oxide film can protect the particle main body and suppress aggregation, and can enhance the moisture absorption resistance and rust prevention property of the particle main body. Note that it is preferable that the oxide film covers the entire surface of the particle main body, but it may be provided only on a part of the surface.

[0029] Examples of the constituent material of the oxide film include, for example, silicon oxide, aluminum oxide, titanium oxide, vanadium oxide, niobium oxide, chromium oxide, manganese oxide, tin oxide, zinc oxide, etc., and one or a mixture or composite of two or more of these may be mentioned.

[0030] Among these, silicon oxide is preferably used. Silicon oxide is an oxide represented by the compositional formula SiO x (0 < x ≦ 2), but is preferably SiO2.

[0031] The surface modification film includes an ionic liquid modification site having a first cationic group and a first anionic group. The ionic liquid modification site is bonded to the substrate through a bonding site derived from, for example, a coupling agent, a surfactant, a polymer polymerization film, etc. It is preferable that the surface modification film covers the entire surface of the oxide film or the particle main body, but it may be provided only on a part of the surface. Note that the first cationic group and the first anionic group will be described in detail later.

[0032] A compound derived from a coupling agent is preferably used at the bonding site. By using a coupling agent, an ionic liquid structure can be introduced at high density and stably to the surface of the oxide film or the particle body. This suppresses aggregation of metal magnetic particles 2 even under harsh environments and further improves the dispersibility of metal magnetic particles 2 in the ionic liquid dispersion medium 4. As a result, metal magnetic particles 2 that have excellent responsiveness to changes in magnetic fields and can be uniformly dispersed in the ionic liquid dispersion medium 4 even at high concentrations can be realized.

[0033] Furthermore, the surface modification film contributes to improving the moisture resistance and rust prevention properties of the metal magnetic particles 2. By improving moisture resistance and rust prevention, deterioration due to moisture absorption and rusting of the metal magnetic particles 2 can be suppressed.

[0034] The average particle diameter of the metallic magnetic particles 2 is preferably 0.5 μm to 15 μm, more preferably 2 μm to 12 μm, and even more preferably 4 μm to 10 μm. If the average particle diameter of the metallic magnetic particles 2 is within the above range, the magnetic field responsiveness of the metallic magnetic particles 2 can be sufficiently increased. Furthermore, the settling of the metallic magnetic particles 2 can be suppressed. This allows the excitation stress of the magneto-viscous fluid 1 to be increased. However, if the average particle diameter of the metallic magnetic particles 2 falls below the lower limit, the magnetic field responsiveness of the metallic magnetic particles 2 will decrease, which may reduce the excitation stress of the magneto-viscous fluid 1. On the other hand, if the average particle diameter of the metallic magnetic particles 2 exceeds the upper limit, the metallic magnetic particles 2 may be more prone to settling. If the metallic magnetic particles 2 settle, the necessary excitation stress may not be obtained.

[0035] The average particle size of the metallic magnetic particles 2 can be determined from the volume-based particle size distribution using the laser diffraction-dispersion method. Specifically, the median diameter in the particle size distribution is defined as the average particle size. Examples of equipment used to measure the particle size distribution using the laser diffraction-dispersion method include the MT3300 series from Microtrac-Bell.

[0036] The content of metallic magnetic particles 2 in the magnetorheological fluid 1 is preferably 40% by mass or more and 95% by mass or less, more preferably 50% by mass or more and 90% by mass or less, and even more preferably 60% by mass or more and 85% by mass or less. This allows for optimization of the viscosity of the magnetorheological fluid 1. Furthermore, it allows for a sufficiently large excitation stress in the magnetorheological fluid 1.

[0037] The particle shape of the metallic magnetic particle 2 is not particularly limited and may be a perfect sphere, an ellipsoid, a polyhedron, or any other shape. Examples of other shapes include needle-shaped, fibrous, plate-shaped, flake-shaped, hollow, and so on.

[0038] 1.2.Nonmagnetic particles Non-magnetic particles 3 are composed of non-magnetic materials and have a smaller average particle diameter than the metallic magnetic particles 2. Such non-magnetic particles 3 are dispersed in the ionic liquid dispersion medium 4 by random Brownian motion. The non-magnetic particles 3 undergoing Brownian motion generate a diffusion force greater than the settling force due to gravity, allowing them to float on their own and also contributing to suppressing the settling of the metallic magnetic particles 2. Therefore, by using non-magnetic particles 3, a good dispersion state of the metallic magnetic particles 2 can be maintained. Preferably, the average particle diameter of the non-magnetic particles is about 0.1% to 1% of the average particle diameter of the metallic magnetic particles 2.

[0039] The average particle diameter of the non-magnetic particles 3 is preferably 10 nm to 800 nm, more preferably 12 nm to 600 nm, and even more preferably 14 nm to 550 nm. If the average particle diameter of the non-magnetic particles 3 is within the above range, a sufficient lifting effect due to the Brownian motion of the non-magnetic particles 3 can be obtained. However, if the average particle diameter of the non-magnetic particles 3 falls below the lower limit, the surface area of ​​the non-magnetic particles 3 becomes small, and even if the non-magnetic particles 3 are undergoing Brownian motion, a sufficient lifting effect may not be obtained. On the other hand, if the average particle diameter of the non-magnetic particles 3 exceeds the upper limit, the Brownian motion of the non-magnetic particles 3 decreases, and a lifting effect may be reduced.

[0040] The average particle size of non-magnetic particles 3 can be determined from the volume-based particle size distribution using the laser diffraction-dispersion method. Specifically, the median diameter in the particle size distribution is defined as the average particle size. Examples of devices used to measure particle size distribution using the laser diffraction-dispersion method include the MT3300 series from Microtrac-Bell.

[0041] Examples of constituent materials for the non-magnetic particles 3 include non-magnetic inorganic materials, thermoplastic resins, and thermosetting resins.

[0042] Examples of non-magnetic inorganic materials include non-magnetic metals such as gold, silver, copper, palladium, and platinum; ceramics such as metal oxides, metal nitrides, metal carbides, metal carbonates, metal halides, metal phosphates, and metal sulfides; and carbon-based materials such as carbon black and graphite. By using such inorganic materials, the durability of the non-magnetic particles 3 can be easily increased.

[0043] Examples of metal oxides include alumina, silica, titanium oxide, zinc oxide, calcium oxide, magnesium oxide, tin dioxide, silicon dioxide, nonmagnetic chromium oxide, cerium oxide, and nonmagnetic iron oxide. Examples of metal nitrides include boron nitride and silicon nitride. Examples of metal carbides include silicon carbide, molybdenum carbide, boron carbide, tungsten carbide, and titanium carbide. Examples of metal carbonates include magnesium carbonate and calcium carbonate. Examples of metal halides include calcium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, and lithium chloride. Examples of metal sulfides include barium sulfate and calcium sulfate.

[0044] Examples of thermoplastic resins include acrylic resins, polystyrene resins, vinyl resins such as polyvinyl acetate resins and polyvinyl chloride resins, polyester resins, ABS resins, and AS resins.

[0045] Examples of thermosetting resins include phenolic resins, epoxy resins, melamine resins, urea resins, unsaturated polyester resins, and alkyd resins.

[0046] The content of non-magnetic particles 3 in the magnetorheological fluid 1 is preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.1% by mass or more and 3% by mass or less, and even more preferably 0.5% by mass or more and 2% by mass or less. This allows for good suppression of the sedimentation of metallic magnetic particles 2 by the lifting effect while maintaining a good dispersion state of the non-magnetic particles 3. If the content of non-magnetic particles 3 falls below the lower limit, the lifting effect may be reduced. On the other hand, if the content of non-magnetic particles 3 exceeds the upper limit, aggregation of non-magnetic particles 3 is more likely to occur, and the dispersion state of non-magnetic particles 3 may become poor.

[0047] Various additives may be added to the magnetorheological fluid 1. Examples of additives include detergents, dispersants, antioxidants, anti-wear agents, extreme pressure agents, friction modifiers, surfactants, thixotropy imparters (thickeners), and viscosity reducers, and one or more of these may be used in mixtures of two or more.

[0048] Examples of dispersants include oleates, naphthenates, sulfonates, phosphate esters, stearic acid, stearates, glycerol monooleate, sorbitan sesquioleate, lauric acid, fatty acids, and fatty alcohols.

[0049] Examples of wear inhibitors include organic molybdenum compounds such as molybdenum dialkyldithiocarbamate and molybdenum dialkyldithiophosphate, and organic zinc compounds such as zinc dialkyldithiocarbamate and zinc dialkyldithiophosphate.

[0050] Furthermore, the total content of the additives is preferably 10% by mass or less of the total magnetorheological fluid 1, more preferably 8% by mass or less, and even more preferably 6% by mass or less. This prevents the additives from inhibiting the functions of the metallic magnetic particles 2 and non-magnetic particles 3.

[0051] 1.3. Ionic liquid dispersion medium The ionic liquid dispersion medium 4 has a secondary cationic group and a secondary anionic group. The secondary cationic group and the secondary anionic group will be described in detail later.

[0052] The content of the ionic liquid dispersion medium 4 in the magnetorheological fluid 1 is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and even more preferably 10% by mass or more and 30% by mass or less. This allows for good maintenance of the dispersion state of the dispersed phase. Furthermore, the viscosity of the magnetorheological fluid 1 can be optimized.

[0053] Furthermore, the magnetorheological fluid 1 may contain liquid components other than the ionic liquid dispersion medium 4 (dispersion medium), or additives, as long as they do not impair the effects of the ionic liquid dispersion medium 4. Examples of such additives include thixotropic agents, surfactants, viscoplastic media, and water-in-oil emulsions.

[0054] 1.4. Cationic and Anionic Groups As mentioned above, the ionic liquid modification sites on the metal magnetic particles 2 have a first cationic group and a first anionic group. The ionic liquid dispersion medium 4 also has a second cationic group and a second anionic group.

[0055] The first and second cationic groups are cations of the same type and are selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0056] The first and second anionic groups are selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, bis(trifluoromethanesulfonyl)amide ions, hydrogen phthalates, and bissulfurylimide ions, respectively.

[0057] The specific cations and anions mentioned above are both bulky ions with relatively high ionic strength. By using ions with high ionic strength as the first cationic group and the first anionic group, the ionic liquid modification site can maintain good affinity to the ionic liquid dispersion medium 4 even under harsh conditions. Furthermore, by using ions with high ionic strength as the second cationic group and the second anionic group, an ionic liquid dispersion medium 4 can be obtained that is liquid even at low temperatures below 0°C and does not evaporate easily even at high temperatures above 250°C. As a result, the ionic liquid dispersion medium 4 can exist without solidification or evaporation at low or high temperatures, and can maintain good affinity to the ionic liquid modification site even under harsh conditions.

[0058] Furthermore, by selecting cations of the same type as the first and second cationic groups, the metal magnetic particles 2 having ionic liquid modification sites can maintain a good dispersion state in the ionic liquid dispersion medium 4. As a result, a magnetoviscous fluid 1 that exhibits high excitation stress can be realized regardless of the environment.

[0059] The term "identical cation" refers to a concept where the main skeletons of the cations are the same. In other words, in this specification, two cations that have the same main skeleton but different other molecular structures—for example, two cations with different numbers of carbon atoms in the alkyl groups attached to their main skeletons—are considered to be identical cations.

[0060] Among the alkyl groups bonded to the main skeleton of the first cation group, the one with the largest number of carbon atoms is defined as the first long-chain alkyl group. Among the alkyl groups bonded to the main skeleton of the second cation group, the one with the largest number of carbon atoms is defined as the second long-chain alkyl group. At this time, it is preferable that the number of carbon atoms in the first long-chain alkyl group is the same as the number of carbon atoms in the second long-chain alkyl group. That is, the first cation group and the second cation group are preferably the same type of cations as each other, and the number of carbon atoms in the first long-chain alkyl group and the second long-chain alkyl group is also preferably the same as each other. Thereby, the dispersibility of the metal magnetic particles 2 in the ionic liquid dispersion medium 4 can be particularly enhanced. As a result, a magnetorheological fluid 1 that exhibits a higher excitation stress can be realized even under harsh environments.

[0061] 1.4.1. Cation group The first cation group and the second cation group used in this embodiment are each selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0062] 1.4.1.1. Quaternary ammonium ion Examples of the quaternary ammonium ion include a cation represented by the following formula (A). The main skeleton of this cation is a nitrogen atom.

[0063] [Chemical formula] [In formula (A), R 11 ~R 14 are each independently a linear or branched alkyl group having 1 to 20 carbon atoms. R 11 ~R 14 may be bonded to each other to form a ring.]

[0064] R 11 ~R 14 When R 11 ~R 14Quaternary ammonium ions, in which these atoms are bonded to each other to form a ring, are also specifically called alicyclic quaternary ammonium ions.

[0065] R 11 ~R 14 Examples of linear alkyl groups include those with 1 to 20 carbon atoms. Specifically, these include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decanyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, and eicosyl groups.

[0066] R 11 ~R 14 Examples of branched alkyl groups include those with 3 to 20 carbon atoms. Specifically, examples include 1-methylethyl group, 1-methylpropyl group, 2-methylpropyl group, 1-methylbutyl group, 2-methylbutyl group, 3-methylbutyl group, 1-ethylbutyl group, 2-ethylbutyl group, 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, and 4-methylpentyl group.

[0067] R 11 ~R 14 When these groups are bonded to each other to form a ring, examples of the rings that can be formed include cyclopropyl groups, cyclobutyl groups, cyclopentyl groups, cyclohexyl groups, cycloheptyl groups, cyclooctyl groups, cyclodecyl groups, and cyclododecyl groups.

[0068] Specific examples of quaternary ammonium ions include tetraethylammonium, tetramethylammonium, tetrapropylammonium, tetrabutylammonium, and tetrapentylammonium.

[0069] As the quaternary ammonium ion, the cation represented by (A)-1 or (A)-2 below is more preferred.

[0070] [ka] [In formula (A)-1, R 15 R is an alkyl group having 1 to 2 carbon atoms, 16 is an alkyl group having 1 to 8 carbon atoms. Also, in formula (A)-2, R 17 R is an alkyl group having 1 to 2 carbon atoms, 18 This is an alkyl group having 1 to 8 carbon atoms.

[0071] As the quaternary ammonium ion, the cation represented by (A)-3 or (A)-4 below is even more preferred. This makes it possible to realize a magnetorheological fluid 1 that can maintain a good dispersion state of the metal magnetic particles 2 even at high temperatures of 300°C or higher. In this case, the tetrafluoroborate ion is particularly preferred as the anionic group.

[0072] [ka] [In formula (A)-3, R 51 R is an alkyl group with 2 carbon atoms, 52 is an alkyl group having 1 to 2 carbon atoms. Also, in formula (A)-4, R 53 R is an alkyl group with 2 carbon atoms, 54 This is an alkyl group having 1 to 2 carbon atoms.

[0073] 1.4.1.2. Imidazolium ion Examples of imidazolium ions include the cation represented by the following formula (B). The main skeleton of this cation is a five-membered ring structure composed of carbon atoms and nitrogen atoms.

[0074] [ka] [In formula (B), R 21 and R 22 Each of these is independently a linear or branched alkyl group having 1 to 20 carbon atoms.

[0075] R in equation (B) 21 and R 22 The explanation for the alkyl group is as follows: 11 ~R 14 This is similar to the explanation regarding alkyl groups.

[0076] R 21 and R 22 Preferably, the alkyl group has 1 to 10 carbon atoms, and more preferably, an alkyl group has 1 to 8 carbon atoms.

[0077] R 21 and R 22 Examples of such groups include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, and octyl groups.

[0078] As the imidazolium ion, the cation represented by the following formula (B)-1 is more preferred. This makes it possible to realize a magnetorheological fluid 1 that can maintain a good dispersion state of metal magnetic particles 2 even at high temperatures of 300°C or higher. In this case, tetrafluoroborate ions are particularly preferred as the anionic group.

[0079] [ka] [In formula (B)-1, R 23 R is an alkyl group with 2 carbon atoms, 24 This is an alkyl group having 1 to 2 carbon atoms.

[0080] 1.4.1.3. Pyridinium ions Examples of pyridinium ions include the cation represented by the following formula (C). The main skeleton of this cation is a six-membered ring structure composed of carbon atoms and nitrogen atoms.

[0081] [ka] [In formula (C), R30 This is a linear or branched alkyl group having 1 to 20 carbon atoms.

[0082] R in equation (C) 30 The explanation for the alkyl group is as follows: 11 ~R 14 This is similar to the explanation regarding alkyl groups.

[0083] R 30 Preferably, the alkyl group has 1 to 10 carbon atoms, and more preferably, an alkyl group has 1 to 8 carbon atoms.

[0084] R 30 Examples of such groups include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, and hexyl groups.

[0085] As the pyridinium ion, the cation represented by the following formula (C)-1 is more preferred. This makes it possible to realize a magnetorheological fluid 1 that can maintain a good dispersion state of metal magnetic particles 2 even at high temperatures of 300°C or higher. In this case, the tetrafluoroborate ion is particularly preferred as the anionic group.

[0086] [ka] [In formula (C)-1, R 31 This is an alkyl group having 1 to 2 carbon atoms.

[0087] 1.4.1.4. Phosphonium Ion An example of a phosphonium ion is the cation shown in the following formula (D). The main skeleton of this cation is a phosphorus atom.

[0088] [ka] [In formula (D), R 41 ~R 44Each of these is independently a linear or branched alkyl group having 1 to 20 carbon atoms. 41 ~R 44 These elements may be joined to each other to form a ring.

[0089] R in equation (D) 41 ~R 44 The explanation for the alkyl group is as follows: 11 ~R 14 This is similar to the explanation regarding alkyl groups.

[0090] R 41 ~R 44 Preferably, the alkyl group has 2 to 18 carbon atoms, and more preferably, an alkyl group has 4 to 16 carbon atoms.

[0091] Specific examples of phosphonium ions include tetrabutylphosphonium, tetrapropylphosphonium, tetraethylphosphonium, tetramethylphosphonium, and hexadecyltributylphosphonium.

[0092] 1.4.2. Anionic Groups The first and second anionic groups used in this embodiment are selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, bis(trifluoromethanesulfonyl)amide ions, hydrogen phthalates, and bissulfurylimide ions, respectively.

[0093] These anions are all bulky ions. Therefore, by using the listed anions as the first and second anionic groups, a magnetorheological fluid 1 can be realized in which the dispersed phase can maintain a good dispersion state even at low and high temperatures.

[0094] Furthermore, it is preferable that the first and second anionic groups are anions selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, and bis(trifluoromethanesulfonyl)amide ions, respectively. This makes the above effects more pronounced.

[0095] Furthermore, it is more preferable that the first and second anionic groups are tetrafluoroborate ions, respectively. This makes it possible to realize a magnetorheological fluid 1 that can maintain a good dispersion state of the metal magnetic particles 2 even after heating at a high temperature of 300°C.

[0096] Furthermore, while different anions may be selected for the first and second anion groups, it is preferable that the same type of anion is selected for each. This allows the metal magnetic particles 2 having ionic liquid modification sites to maintain a better dispersion state in the ionic liquid dispersion medium 4. As a result, a magnetoviscous fluid 1 that exhibits high excitation stress regardless of the environment can be realized.

[0097] 1.4.3. Ionic Strength Let μ1 be the ionic strength of the ionic liquid modification site having a first cationic group and a first anionic group. Let μ2 be the ionic strength of the ionic liquid dispersion medium 4 having a second cationic group and a second anionic group. In this case, the ionic strength of the entire magnetorheological fluid 1 can be evaluated using the average value μA of the ionic strengths μ1 and μ2. The average value μA of the ionic strength is preferably between 5.00 mol / L and 14.00 mol / L, and more preferably between 10.00 mol / L and 13.00 mol / L. In this case, ionic liquids with relatively high ionic strengths will interact with each other. Therefore, even at high temperatures, the metal magnetic particles 2 can maintain a better dispersion state in the ionic liquid dispersion medium 4.

[0098] Furthermore, if the average value μA falls below the lower limit, the ionic bonds between molecules in the ionic liquid may weaken. In this case, the dispersibility of the metal magnetic particles 2 at high temperatures may decrease. On the other hand, if the average value μA exceeds the upper limit, it may become difficult to select ions.

[0099] Furthermore, the ionic strength μ1 of the ionic liquid modification site is preferably 5.00 mol / L or more and 14.00 mol / L or less, and more preferably 10.00 mol / L or more and 13.00 mol / L or less. In this case, the ionic bonding of the ionic liquid modification site becomes sufficiently strong.

[0100] Furthermore, the ionic strength μ2 of the ionic liquid dispersion medium 4 is preferably 5.00 mol / L or more and 14.00 mol / L or less, and more preferably 10.00 mol / L or more and 13.00 mol / L or less. In this case, the ionic bonding of the ionic liquid dispersion medium 4 becomes sufficiently strong.

[0101] Ionic strength is calculated by summing the products of the molar concentration of an ion and the square of the ion's valence for all ion species, and then multiplying the result by 1 / 2.

[0102] 1.3. Physical Properties of Magnetorheological Fluids The excitation stress of the magneto-viscous fluid 1 can be evaluated as the yield stress when a magnetic field with a magnetic flux density of 1.0 [T] is applied. This excitation stress is preferably 10 [kPa] or more, and more preferably 15 [kPa] or more. This allows for the production of a magneto-viscous fluid 1 exhibiting sufficient excitation stress. Such a magneto-viscous fluid 1 is useful in various applications.

[0103] The excitation stress of magnetorheological fluid 1 is measured as follows: First, a magnetic field with a magnetic flux density of 1.0 [T] is applied to the magnetorheological fluid 1 at a predetermined temperature. Next, a shear rate of 333 [ / s] is applied under these conditions, and the shear stress is measured. For measuring the shear stress, for example, an Anton Paar rheometer MCR102 can be used. The measured shear stress is then defined as the yield stress.

[0104] Furthermore, it is preferable that the excitation stress described above remains within the above range regardless of temperature. For example, the excitation stress of the magneto-viscous fluid 1 after being left in an environment at 300°C for 1 hour is preferably 10 kPa or more, and more preferably 15 kPa or more. This makes it possible to obtain a magneto-viscous fluid 1 that exhibits sufficient excitation stress even at high temperatures. Such a magneto-viscous fluid 1 can maintain the same properties as at room temperature even in harsh temperature environments.

[0105] The boiling point of the magnetorheological fluid 1 is preferably 250°C or higher, more preferably 300°C or higher, and even more preferably 350°C or higher. This provides a magnetorheological fluid 1 with sufficient heat resistance.

[0106] The boiling point of magnetorheological fluid 1 is measured as follows: First, 2 mL of magnetorheological fluid 1 is heated on a hot plate. Next, the temperature of magnetorheological fluid 1 when white smoke is generated is measured, and this measured value is taken as the boiling point.

[0107] The freezing point of the magnetorheological fluid 1 is preferably 0°C or lower, more preferably -10°C or lower, and even more preferably -20°C or lower. This provides a magnetorheological fluid 1 with sufficient cold resistance.

[0108] The freezing point of magnetorheological fluid 1 is measured as follows: First, 2 mL of magnetorheological fluid 1 is cooled in a refrigerator. Next, the temperature at which the magnetorheological fluid 1 solidifies is measured, and this measured value is defined as the freezing point.

[0109] 1.4. Examples of applications of magnetorheological fluids Applications of magnetorheological fluid 1 include various devices and equipment that utilize the difference in excitation stress when the applied magnetic field is switched. Examples of such devices and equipment include vibration damping devices such as linear dampers, rotary dampers, and shock absorbers; braking devices such as brakes; power transmission devices such as clutches; robot muscle parts and end effectors; liquid flow control valves; tactile presentation devices; acoustic devices; medical and welfare robot hands; caregiving hands; and personal mobility devices.

[0110] 1.5. Method for manufacturing magnetorheological fluid An example of a method for producing magnetorheological fluid 1 is described below. In the method for producing magnetorheological fluid 1, first, a surface modification film is formed on the surface of the particle body of the metallic magnetic material. For example, a coupling agent having an ionic liquid modification site as a functional group (ionic liquid coupling agent) is used to form the surface modification film. The ionic liquid coupling agent is a compound having an ionic liquid modification site and a hydrolyzable group, and is preferably used because it introduces the ionic liquid modification site to the particle surface or oxide film surface of the metallic magnetic material by hydrolysis and polycondensation reactions. Specifically, the ionic liquid coupling agent is dissolved in a solvent to prepare a solution. The obtained solution is subjected to ultrasonic treatment and manual stirring. This causes the ionic liquid coupling agent to react with the surface of the particle body. The treated particle body is recovered by magnetic separation and subjected to heat treatment. Examples of heating conditions include a heating temperature of 50°C to 150°C and a heating time of 30 minutes to 180 minutes. This forms a surface modification film, and metallic magnetic particles 2 are obtained.

[0111] Furthermore, after the formation of the surface modification film, the formation state of the surface modification film can be confirmed by surface elemental analysis. For example, ESCA (X-ray photoelectron spectroscopy) is used for surface elemental analysis. By checking for the presence or absence of elements specific to cations (such as nitrogen) and elements specific to anions (such as boron, fluorine, and sulfur), the formation state of the surface modification film can be determined.

[0112] Next, the metallic magnetic particles 2, non-magnetic particles 3, and ionic liquid dispersion medium 4 are mixed and stirred. Examples of stirring methods include stirring with a spatula, a vortex mixer, a high shear mixer, and a low-frequency acoustic resonance mixer. The stirring time is set appropriately according to the stirring method, but is preferably between 5 minutes and 4 hours. The stirring temperature is set appropriately according to the stirring method, but is preferably between 15°C and 70°C.

[0113] 2. Braking device Next, a braking device according to an embodiment will be described.

[0114] Figure 2 is a longitudinal cross-sectional view showing the braking device 100 according to an embodiment. The braking device 100 shown in Figure 2 comprises a fixed disk 110 (fixed part), a movable disk 120 (movable part), and a magnetorheological fluid 1. The movable disk 120 is rotatable (movable) around the rotation axis AX relative to the fixed disk 110. The magnetorheological fluid 1 is held between the fixed disk 110 and the movable disk 120. The braking device 100 also includes a magnetic field generating unit (not shown). This magnetic field generating unit applies a magnetic field to the magnetorheological fluid 1.

[0115] In the braking device 100, the excitation stress of the magnetoviscous fluid can be changed by switching the magnetic field applied to the magnetoviscous fluid 1. This makes it possible to change the resistance force of the movable disk 120 to the rotation of the fixed disk 110. As a result, the braking device 100 utilizes this resistance force as braking force for braking vehicles and the like.

[0116] The fixed disk 110 is connected, for example, to the vehicle body side, and the movable disk 120 is connected, for example, to the vehicle wheel side.

[0117] When no magnetic field H is applied to the magneto-viscous fluid 1 (H=0), the metallic magnetic particles 2 and non-magnetic particles 3 are dispersed in the magneto-viscous fluid 1. In this case, the excitation stress of the magneto-viscous fluid 1 is sufficiently small, and almost no braking force is generated.

[0118] When a magnetic field H is applied to the magnetorheological fluid 1 (0 < H), in the magnetorheological fluid 1, the ferromagnetic particles 2 form a cluster structure. In this case, the excitation stress of the magnetorheological fluid 1 increases, and a braking force is generated.

[0119] The magnetorheological fluid 1 according to the above-described embodiment can achieve both high reliability and high excitation stress even when used at low temperatures or high temperatures. Therefore, the braking device 100 according to the embodiment has high reliability without causing a decrease in braking force or the like even in a harsh environment.

[0120] Note that the configuration of the braking device 100 is not limited to the above. For example, the braking device of the present invention may include three or more disks.

[0121] 3. Effects achieved by the above embodiment As described above, the magnetorheological fluid 1 according to the above embodiment includes the ferromagnetic particles 2 and the ionic liquid dispersion medium 4. The surface of the ferromagnetic particles 2 is modified at the ionic liquid modification site having the first cation group and the first anion group. The ionic liquid dispersion medium 4 has a second cation group and a second anion group.

[0122] The first cation group and the second cation group are the same type of cations as each other, and are selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions. The first anion group and the second anion group are each selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, tris(pentafluoroethyl)trifluorophosphate ions, bis(trifluoromethanesulfonyl)amide ions, hydrogen phthalate ions, and bis(fluorosulfonyl)imide ions.

[0123] According to such a configuration, a magnetorheological fluid 1 that can achieve both high reliability and high excitation stress can be obtained even when used at low temperatures or high temperatures.

[0124] In the magnetorheological fluid 1 according to the above embodiment, the first cationic group and the second cationic group are preferably cations represented by the following formulas (A)-3, (A)-4, (B)-1, and (C)-1.

[0125] [ka] [In formula (A)-3, R 51 R is an alkyl group with 2 carbon atoms, 52 R is an alkyl group having 1 to 2 carbon atoms. In formula (A)-4, R 53 R is an alkyl group with 2 carbon atoms, 54 R is an alkyl group having 1 to 2 carbon atoms. In formula (B)-1, R 23 R is an alkyl group with 2 carbon atoms, 24 R is an alkyl group having 1 to 2 carbon atoms. In formula (C)-1, R 31 This is an alkyl group having 1 to 2 carbon atoms.

[0126] With this configuration, a magnetorheological fluid 1 can be realized that can maintain a good dispersion state of metallic magnetic particles 2 even at high temperatures of 300°C or higher.

[0127] In the magnetorheological fluid 1 according to the above embodiment, the first cationic group and the second cationic group may each have an alkyl group. The alkyl group with the most carbon atoms among those of the first cationic group is designated as the first long-chain alkyl group, and the alkyl group with the most carbon atoms among those of the second cationic group is designated as the second long-chain alkyl group. In this case, the number of carbon atoms of the first long-chain alkyl group and the number of carbon atoms of the second long-chain alkyl group may be the same.

[0128] This configuration significantly enhances the dispersibility of the metal magnetic particles 2 in the ionic liquid dispersion medium 4. As a result, a magnetorheological fluid 1 that exhibits even higher excitation stress can be realized, even under harsh conditions.

[0129] In the magnetorheological fluid 1 according to the above embodiment, the first anionic group and the second anionic group may be tetrafluoroborate ions.

[0130] With this configuration, it is possible to realize a magnetorheological fluid 1 that can maintain a good dispersion state of the metallic magnetic particles 2 even after heating at a high temperature of 300°C.

[0131] In the magnetorheological fluid 1 according to the above embodiment, the average particle diameter of the metallic magnetic particles 2 may be 0.5 μm or more and 15 μm or less.

[0132] This configuration allows for a sufficiently large magnetic field response of the metallic magnetic particles 2. Furthermore, it suppresses the sedimentation of the metallic magnetic particles 2. This, in turn, allows for a large excitation stress of the magnetorheological fluid 1.

[0133] In the magnetorheological fluid 1 according to the above embodiment, the average value μA of the ionic strength μ1 of the ionic liquid modification site and the ionic strength μ2 of the ionic liquid dispersion medium may be 5.00 mol / L or more and 14.00 mol / L or less.

[0134] With this configuration, ionic liquids with relatively high ionic strengths interact with each other. As a result, even at high temperatures, the metal magnetic particles 2 can maintain a better dispersion state in the ionic liquid dispersion medium 4.

[0135] In the magnetorheological fluid 1 according to the above embodiment, non-magnetic particles 3 having an average particle diameter smaller than that of metallic magnetic particles 2 may be included.

[0136] In this configuration, the non-magnetic particles 3 undergo Brownian motion, generating a diffusion force greater than the settling force due to gravity, allowing them to float on their own and also contributing to suppressing the settling of the metallic magnetic particles 2. Therefore, by using the non-magnetic particles 3, a good dispersion state of the metallic magnetic particles 2 can be maintained.

[0137] In the magnetorheological fluid 1 according to the above embodiment, the constituent material of the non-magnetic particles 3 may be an inorganic material. With this configuration, the durability of the non-magnetic particles 3 can be easily increased.

[0138] In the magnetorheological fluid 1 according to the above embodiment, the content of non-magnetic particles 3 may be 0.01% by mass or more and 5% by mass or less.

[0139] With this configuration, the dispersion state of the non-magnetic particles 3 can be well maintained, while the sedimentation of the metallic magnetic particles 2 can be effectively suppressed by the lifting effect.

[0140] In the magnetorheological fluid 1 according to the above embodiment, it is preferable that the yield stress when a magnetic field of 1.0 [T] is applied after being left in an environment of 300°C for 1 hour is 15 [kPa] or more.

[0141] With this configuration, a magnetorheological fluid 1 that exhibits sufficient excitation stress even at high temperatures can be obtained. Such a magnetorheological fluid 1 can maintain the same properties as at room temperature even in harsh temperature environments.

[0142] In the magnetorheological fluid 1 according to the above embodiment, the first anionic group and the second anionic group may be the same type of anion.

[0143] With this configuration, the metal magnetic particles 2 having ionic liquid modification sites can maintain a better dispersion state in the ionic liquid dispersion medium 4. As a result, a magnetoviscous fluid 1 that exhibits high excitation stress regardless of the environment can be realized.

[0144] The braking device 100 according to the above embodiment comprises a fixed disk 110 (fixed part), a movable disk 120 (movable part), a magnetoviscous fluid 1 according to the above embodiment, and a magnetic field generating unit. The movable disk 120 is movable relative to the fixed disk 110. The magnetoviscous fluid 1 is held between the fixed disk 110 and the movable disk 120. The magnetic field generating unit applies a magnetic field to the magnetoviscous fluid 1.

[0145] With this configuration, a highly reliable braking system 100 can be realized without causing a decrease in braking force or other issues even under harsh environments.

[0146] Although the magnetorheological fluid and braking device of the present invention have been described above based on preferred embodiments, the present invention is not limited thereto.

[0147] For example, the magnetorheological fluid and braking device of the present invention may have any configuration added to the above embodiment. Furthermore, the configuration of each part of the braking device according to the above embodiment may be replaced with components having the same function as described above. [Examples]

[0148] Next, specific embodiments of the present invention will be described. 4. Preparation of magnetorheological fluid Figures 3 to 8 are Tables 1 to 6, which show the conditions for preparing magnetorheological fluids and the evaluation results.

[0149] The magnetorheological fluid was prepared as follows: First, the metallic magnetic particles, non-magnetic particles, and ionic liquid dispersion medium shown in Tables 1 (Figure 3) to 6 (Figure 8) were mixed. The average particle size of the metallic magnetic particles was 8 μm, and the average particle size of the non-magnetic particles was 40 nm. The respective content percentages of the metallic magnetic particles and non-magnetic particles, as well as the ionic strength (unit: mol / L) of the ionic liquid, are shown in Tables 1 to 6. The remainder of the content percentage was the ionic liquid dispersion medium.

[0150] Next, the resulting mixture was stirred. A high-shear mixer (Silverson, L5M-A) was used as the stirring device. The stirring conditions were a rotation speed of 3000 rpm and a stirring time of 30 minutes. This produced the magnetorheological fluids of Examples 1-15 and Comparative Examples 1-7.

[0151] The comparative examples are magnetoviscous fluids as follows: Comparative Example 1: Using mineral oil as the dispersion medium Comparative Example 2: Surface modification by ionic liquid modification sites is omitted. Comparative Example 3: The first and second cationic groups are heterogeneous. Comparative Example 4: The first and second anionic groups are not specific ions. Comparative Example 5: The first and second anionic groups are not specific ions. Comparative Example 6: The first and second anionic groups are not specific ions. Comparative Example 7: The first and second anionic groups are not specific ions.

[0152] In Tables 1 through 6, "C chain of the cationic group" refers to the alkyl group bonded to the main skeleton of the cationic group.

[0153] 5. Evaluation of magnetoviscous fluids For each example and comparative example, the viscosity, dispersion stability, and excitation stress of the magnetorheological fluid were evaluated at room temperature and high temperature, respectively.

[0154] 5.1.Viscosity The viscosity of the magnetorheological fluids of each example and comparative example was measured in the absence of a magnetic field (magnetic flux density 0 [T]). Anton Paar's MCR102 rheometer was used for viscosity measurement. The shear rate during measurement was set to 0.033 [ / s].

[0155] The viscosity of a magnetorheological fluid left at 25°C for one day was measured under the above measurement conditions, and the measured value was defined as "Viscosity (room temperature, left for one day)". Similarly, the viscosity of a magnetorheological fluid heated at 300°C for one hour was measured, and the measured value was defined as "Viscosity (300°C, left for one hour)".

[0156] The obtained viscosity was then evaluated against the following evaluation criteria. The evaluation results are shown in Tables 1 to 6.

[0157] A: The viscosity is less than 10,000 mPa·s. B: Viscosity is between 10,000 mPa·s and 100,000 mPa·s. C: Viscosity is 100,000 mPa·s or higher.

[0158] 6.2.Dispersion stability The dispersion stability of the magnetorheological fluids in each example and comparative example was evaluated using the following method.

[0159] First, 1 mL of magnetorheological fluid was placed in a 1.5 mL sample bottle. After standing for 24 hours at 25°C, the thickness tA of the phase-separated layer containing metal magnetic particles (sedimentary layer) and the thickness tB of the ionic liquid dispersion layer (supernatant layer) were measured. The thickness of the layer containing metal magnetic particles is the thickness of the layer composed of precipitated metal magnetic particles, and the thickness of the ionic liquid dispersion layer is the thickness from the top of the layer containing metal magnetic particles to the liquid surface.

[0160] Next, the supernatant ratio tB / (tA+tB) was calculated from the thicknesses tA and tB. Then, using the calculation method described above, the supernatant of the magnetorheological fluid left at 25°C for 30 days was calculated, and the result was defined as "dispersion stability (left at room temperature for 30 days)". Furthermore, using the calculation method described above, the supernatant of the magnetorheological fluid heated at 300°C for 1 hour was calculated, and the result was defined as "dispersion stability (left at 300°C for 1 hour)".

[0161] Next, the dispersion stability of the magnetorheological fluid was evaluated by comparing the calculated supernatant ratio against the following evaluation criteria.

[0162] The evaluation results are shown in Tables 1 to 6. Note that a smaller supernatant ratio indicates higher dispersion stability of magnetic metal particles in a magnetorheological fluid.

[0163] AA: The supernatant is less than 10%. A: The supernatant is between 10% and 30%. B: The supernatant is 30% or more. C: Due to gelation, the supernatant cannot be calculated.

[0164] 5.3. Excitation Stress The excitation stress was measured for the magnetorheological fluids of each example and comparative example using the method described above.

[0165] The excitation stress of a magnetorheological fluid left at 25°C for one day was measured, and the measurement result was defined as "Excitation Stress (Room Temperature, 1 Day)". Additionally, the excitation stress of a magnetorheological fluid heated at 300°C for one hour was measured, and the measurement result was defined as "Excitation Stress (300°C, 1 Hour)".

[0166] The measured excitation stress was then evaluated against the following evaluation criteria. The evaluation results are shown in Tables 1 to 6.

[0167] AA: The excitation stress is 20 [kPa] or higher. A: The excitation stress is 15 kPa or more and less than 20 kPa. B: The excitation stress is 10 kPa or more and less than 15 kPa. C: The excitation stress is less than 10 [kPa].

[0168] As is clear from Tables 1 to 6, the magnetorheological fluids of each example all received evaluation results of B or higher for viscosity, dispersion stability, and excitation stress, indicating favorable results. Furthermore, the boiling points of the magnetorheological fluids of each example were all 250°C or higher, and the freezing points were all 0°C or lower. On the other hand, the magnetoviscous fluids in each comparative example all showed unsatisfactory evaluation results.

[0169] Furthermore, when the average particle size of the metallic magnetic particles was changed to 5 μm, 12 μm, and 15 μm, and the same experiment as above was performed for each, evaluation results showing the same trend as above were obtained.

[0170] Furthermore, when a magnetoviscous fluid was prepared separately by omitting non-magnetic particles from the magnetoviscous fluid of Example 9, and the same experiment as above was performed, the evaluation results were the same as those of Example 9, except that the evaluation results for dispersion stability and excitation stress were both A.

[0171] The results above confirm that, according to the present invention, it is possible to realize a magnetoviscous fluid that can achieve both high reliability and high excitation stress, even when used at low or high temperatures. [Explanation of Symbols]

[0172] 1...Magnetorheological fluid, 2...Metallic magnetic particles, 3...Non-magnetic particles, 4...Ionic liquid dispersion medium, 100...Brake device, 110...Fixed disk, 120...Movable disk, AX...Rotation axis, H...Magnetic field

Claims

1. Metal magnetic particles whose surfaces are modified with ionic liquid modification sites having a primary cationic group and a primary anionic group, An ionic liquid dispersion medium having a secondary cationic group and a secondary anionic group, Includes, The first and second cationic groups are cations of the same type, selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions. A magnetorheological fluid characterized in that the first anionic group and the second anionic group are each selected from the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, trispentafluoroethyltrifluorophosphate ion, bis(trifluoromethanesulfonyl)amide ion, hydrogen phthalate ion, and bissulfurylimide ion.

2. The magnetorheological fluid according to claim 1, wherein the first cationic group and the second cationic group are cations represented by the following formulas (A)-3, (A)-4, (B)-1, and (C)-1. [In formula (A)-3, R ５１ R is an alkyl group with 2 carbon atoms. ５２ R is an alkyl group having 1 to 2 carbon atoms. In formula (A)-4, R ５３ R is an alkyl group with 2 carbon atoms. ５４ R is an alkyl group having 1 to 2 carbon atoms. In formula (B)-1, R ２３ R is an alkyl group with 2 carbon atoms. ２４ R is an alkyl group having 1 to 2 carbon atoms. In formula (C)-1, R ３１ This is an alkyl group having 1 to 2 carbon atoms.

3. The first cationic group and the second cationic group each have an alkyl group bonded to their respective main skeletons. Among the alkyl groups bonded to the main skeleton of the first cation group, the one with the most carbon atoms is defined as the first long-chain alkyl group. When the alkyl group with the most carbon atoms among those bonded to the main skeleton of the second cation group is defined as the second long-chain alkyl group, The magnetorheological fluid according to claim 1 or 2, wherein the number of carbon atoms in the first long-chain alkyl group and the number of carbon atoms in the second long-chain alkyl group are the same.

4. The magnetorheological fluid according to claim 1 or 2, wherein the first anionic group and the second anionic group are tetrafluoroborate ions.

5. The magnetorheological fluid according to claim 1 or 2, wherein the average particle diameter of the metal magnetic particles is 0.5 μm or more and 15 μm or less.

6. The magnetorheological fluid according to claim 1 or 2, wherein the average value of the ionic strength of the ionic liquid modification site and the ionic strength of the ionic liquid dispersion medium is 5.00 mol / L or more and 14.00 mol / L or less.

7. The magnetorheological fluid according to claim 1 or 2, comprising non-magnetic particles having an average particle diameter smaller than that of the metallic magnetic particles.

8. The magnetorheological fluid according to claim 7, wherein the constituent material of the non-magnetic particles is an inorganic material.

9. The magnetorheological fluid according to claim 7, wherein the content of the non-magnetic particles is 0.01% by mass or more and 5% by mass or less.

10. The magnetorheological fluid according to claim 1 or 2, wherein the yield stress when a magnetic field of 1.0 [T] is applied after being left in an environment at a temperature of 300°C for 1 hour is 15 [kPa] or more.

11. The magnetorheological fluid according to claim 1 or 2, wherein the first anionic group and the second anionic group are anions of the same type.

12. The fixing part, A movable part that is movable relative to the fixed part, The magnetorheological fluid according to claim 1 or 2 is held between the fixed portion and the movable portion, A magnetic field generating unit that applies a magnetic field to the magnetoviscous fluid, A braking device characterized by comprising the following:

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