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 first metallic magnetic particles having an average particle diameter of 5 μm or more, second metallic magnetic particles having an average particle diameter of 500 nm or less, and an ionic liquid having a cationic group and an anionic group, wherein the cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions and phosphonium ions, and the anionic group is one or more selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions and bis(trifluoromethanesulfonyl)amide ions.

Description

[Technical Field]

[0001] This invention relates to magnetorheological fluids and braking devices. [Background technology]

[0002] Magnetorheological fluids (MR fluids) are fluids in which magnetic particles are dispersed in a dispersion medium such as mineral oil or silicone oil. When a magnetic field is applied to a magnetorheological fluid, the metallic magnetic particles become magnetized and align in the direction of the magnetic field, forming chain-like clusters. This increases the viscosity of the magnetorheological fluid and raises its yield stress.

[0003] Such magnetorheological fluids are being explored for use in various fields, including control devices such as dampers and braking systems such as brakes and clutches.

[0004] For these applications, the temperature range in which magnetorheological fluids are used is expected to be from -tens of degrees Celsius to several hundred degrees Celsius. Therefore, magnetorheological fluids are required to maintain their physical properties not only at room temperature but also at low and high temperatures.

[0005] Patent Document 1 discloses a method for suppressing the evaporation of magnetorheological fluids, assuming their use at a high temperature of 200°C, by using an ionic liquid as the dispersion medium. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2010-504635 [Overview of the project] [Problems that the invention aims to solve]

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

Means for Solving the Problems

[0008] The magnetorheological fluid according to an application example of the present invention is first metal magnetic particles having an average particle diameter of 5 μm or more, second metal magnetic particles having an average particle diameter of 500 nm or less, an ionic liquid having a cationic group and an anionic group, and includes The cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions, The anionic group is one or more selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, tris(pentafluoroethyl)trifluorophosphate ions, and bis(trifluoromethanesulfonyl)amide 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 an application example of the present invention held between the fixed part and the movable part, a magnetic field generating part that applies a magnetic field to the magnetorheological fluid, and includes.

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] It is Table 1 showing the production conditions and evaluation results of a magnetorheological fluid. [Figure 4] It is Table 2 showing the production conditions and evaluation results of a magnetorheological fluid. [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. [Figure 9] Table 7 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 10] Table 8 shows the conditions for preparing magnetorheological fluids and the evaluation results. [Figure 11] Table 9 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 magnetoviscous fluid 1 according to an embodiment. The magnetoviscous fluid 1 shown in Figure 1 includes first metallic magnetic particles 2, second metallic magnetic particles 3, and an ionic liquid 4. The first metallic magnetic particles 2 and the second metallic magnetic particles 3 are dispersed phases. The ionic liquid 4 is a dispersion medium.

[0015] The first metallic magnetic particle 2 has an average particle diameter d1 of 5 μm or more. On the other hand, the second metallic magnetic particle 3 has an average particle diameter d2 of 500 nm or less.

[0016] Ionic liquid 4 has both a cationic group and an anionic group. Of these, the cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0017] Furthermore, the anionic group is one or more selected from the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, trispentafluoroethyltrifluorophosphate ion, and bis(trifluoromethanesulfonyl)amide ion.

[0018] With this configuration, by including two types of dispersed particles with different average particle sizes (first metallic magnetic particles 2 and second metallic magnetic particles 3), a lifting effect is obtained in which the relatively smaller second metallic magnetic particles 3 promote the dispersion of the relatively larger first metallic magnetic particles 2. This allows for the maintenance of a good dispersion state of the dispersed particles. As a result, sedimentation of the dispersed particles can be suppressed, and thus the decrease in excitation stress can be suppressed. Furthermore, by using ionic liquid 4 as the dispersion medium, good affinity with the dispersed particles can be maintained even at low and high temperatures. That is, ionic liquid 4, which has cationic and anionic groups as described above, is liquid even at low temperatures below 0°C and does not evaporate easily even at high temperatures above 250°C, making it a highly reliable dispersion medium. In addition, ionic liquid 4 has a higher affinity with the first metallic magnetic particles 2 and second metallic magnetic particles 3 than mineral oil. Therefore, by using ionic liquid 4 as the dispersion medium, separation of the dispersed particles and the dispersion medium becomes less likely even at low and high temperatures. As a result, a magnetoviscous fluid 1 that maintains a good dispersion state of the dispersed particles and exhibits high excitation stress can be realized regardless of the environment.

[0019] 1.1.Dispersion The dispersed phase of the magnetorheological fluid 1 includes first metallic magnetic particles 2 and second metallic magnetic particles 3.

[0020] Examples of constituent materials for the first metallic magnetic particle 2 and the second metallic magnetic particle 3 include metallic magnetic materials such as Fe-based metal materials, Ni-based metal materials, and Co-based metal materials, respectively, and one or more composite materials of these types are used. Alternatively, composite materials of these metallic magnetic materials and oxide-based magnetic materials may also be used. Of these, Fe-based metal materials are preferred as the constituent material for the first metallic magnetic particle 2 from the viewpoint of 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 ferrite and other materials. Therefore, Fe-based metal materials are useful as constituent materials for the first metallic magnetic particles 2 and the second metallic magnetic particles 3.

[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 constituent materials of the first metallic magnetic particles 2 and the second metallic magnetic particles 3 may be amorphous metallic materials, crystalline metallic materials, or microcrystalline (nanocrystalline) metallic materials. Of these, at least one of the constituent materials of the first metallic magnetic particles 2 and the second metallic magnetic particles 3 is preferably an amorphous metallic material or a microcrystalline metallic material. These contribute to significantly lowering the coercivity of the first metallic magnetic particles 2 and the second metallic magnetic particles 3, thereby improving redispersibility. In addition, because these materials have higher toughness and strength compared to, for example, metal oxides, wear and chipping of the first metallic magnetic particles 2 and the second metallic magnetic particles 3 can be effectively suppressed. As a result, a magnetorheological fluid 1 with stable excitation stress can be realized. Furthermore, 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 first metallic magnetic particles 2 and the second metallic magnetic particles 3 can be particularly enhanced. Microcrystalline metal materials refer to metal materials that contain 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 first metal magnetic particles 2 and the second metal magnetic particles 3 may be particles manufactured by any method. Examples of the manufacturing method include, for example, various atomization methods such as water atomization method, gas atomization method, rotating water flow atomization method, etc., as well as pulverization method, 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 constituent materials of the first metal magnetic particles 2 and the constituent materials of the second metal magnetic particles 3 may be the same as each other or different from each other.

[0029] The first metal magnetic particles 2 and the second metal magnetic particles 3 may include an oxide film provided on the surface of the particle main body composed of the above metal-based magnetic material. The oxide film is interposed between the particle main body and the surface modification film described later, and enhances the adhesion of the surface modification film to the particle main body. In addition, 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 the oxide film preferably covers the entire surface of the particle main body, but may be provided only on a part of the surface.

[0030] Examples of the constituent material of the oxide film include silicon oxide, aluminum oxide, titanium oxide, vanadium oxide, niobium oxide, chromium oxide, manganese oxide, tin oxide, zinc oxide, etc., and one or more mixtures or composites of these may be mentioned.

[0031] Among these, silicon oxide is preferably used. Silicon oxide is an oxide represented by the compositional formula SiO x (0 < x ≦ 2), but preferably SiO2.

[0032] The surface modification film covers the surface of the particle main body through the oxide film. Thereby, the dispersibility of the first metal magnetic particles 2 and the second metal magnetic particles 3 in the ionic liquid 4 can be enhanced. Note that the surface modification film preferably covers the entire surface of the oxide film or the particle main body, but may be provided only on a part of the surface.

[0033] The constituent materials of the surface modification film include a coupling agent, a surfactant, or an organic compound derived from a polymer polymerization film. The coupling agent is a compound having functional groups and hydrolyzable groups. By using a coupling agent, functional groups can be introduced to the surface of the oxide film or the surface of the particle body. This suppresses aggregation between particles of the dispersed phase and further improves the dispersibility in the ionic liquid 4. As a result, a dispersed phase can be realized that is highly responsive to changes in magnetic field and can be uniformly dispersed in the ionic liquid 4 even at high concentrations.

[0034] Furthermore, the surface modification film also contributes to improving the moisture resistance and rust prevention properties of the dispersed phase. By improving moisture resistance and rust prevention, deterioration due to moisture absorption and rusting of the dispersed phase can be suppressed.

[0035] Examples of functional groups that coupling agents possess include aliphatic hydrocarbon groups, cyclic structure-containing groups, fluoroalkyl groups, fluoroaryl groups, nitro groups, acyl groups, cyano groups, and the like, with aliphatic hydrocarbon groups or cyclic structure-containing groups being particularly preferred.

[0036] As mentioned above, the average particle diameter d1 of the first metallic magnetic particles 2 is 5 μm or more, preferably 6 μm to 15 μm, and more preferably 7 μm to 10 μm. If the average particle diameter d1 of the first metallic magnetic particles 2 is within the above range, the magnetic field responsiveness of the first metallic magnetic particles 2 can be sufficiently increased. This allows the excitation stress of the magneto-viscous fluid 1 to be increased. If the average particle diameter d1 of the first metallic magnetic particles 2 falls below the lower limit, the magnetic field responsiveness of the first metallic magnetic particles 2 decreases, and therefore the excitation stress of the magneto-viscous fluid 1 decreases. On the other hand, if the average particle diameter d1 of the first metallic magnetic particles 2 exceeds the upper limit, the first metallic magnetic particles 2 may be more prone to settling, even with the lifting effect of the second metallic magnetic particles 3. If the first metallic magnetic particles 2 settle, the necessary excitation stress may not be obtained.

[0037] As mentioned above, the average particle diameter d2 of the second metallic magnetic particles 3 is 500 nm or less, preferably 10 nm to 150 nm, and more preferably 15 nm to 50 nm. If the average particle diameter d2 of the second metallic magnetic particles 3 is within the above range, a sufficient lifting effect due to the Brownian motion of the second metallic magnetic particles 3 can be obtained. That is, the second metallic magnetic particles 3 undergoing Brownian motion generate a diffusion force greater than the settling force due to gravity, causing them to float on their own and also contributing to the suppression of the settling of the first metallic magnetic particles 2. In this specification, this effect is called the lifting effect. The lifting effect of the second metallic magnetic particles 3 allows for the maintenance of a good dispersion state of the dispersed phase. In addition, because the second metallic magnetic particles 3 are small but magnetic, they contribute to an increase in the excitation stress of the magnetorheological fluid 1. Furthermore, if the average particle diameter d2 of the second metallic magnetic particles 3 falls below the lower limit, the surface area of ​​the second metallic magnetic particles 3 decreases, and even if the second metallic magnetic particles 3 undergo Brownian motion, a sufficient lifting effect may not be obtained. On the other hand, if the average particle diameter d2 of the second metallic magnetic particles 3 exceeds the upper limit, the Brownian motion of the second metallic magnetic particles 3 decreases, resulting in a reduced lifting effect.

[0038] When the ratio of the average particle diameter d2 [μm] of the second metallic magnetic particle 3 to the average particle diameter d1 [μm] of the first metallic magnetic particle 2 is defined as the particle diameter ratio d2 / d1, this particle diameter ratio d2 / d1 is preferably 0.002 or more and 0.100 or less, and more preferably 0.010 or more and 0.080 or less. This ensures a good balance between the average particle diameter d1 and the average particle diameter d2, enabling the realization of a magnetorheological fluid 1 that can achieve both high excitation stress and good dispersibility of the dispersed phase. If the particle diameter ratio d2 / d1 falls below the lower limit or exceeds the upper limit, the lifting effect by the second metallic magnetic particle 3 may not be sufficiently obtained, potentially leading to decreased dispersibility of the dispersed phase or low excitation stress.

[0039] The average particle diameters d1 and d2 of the first metallic magnetic particle 2 and the second metallic magnetic particle 3 can be determined from the volume-based particle size distribution using the laser diffraction-dispersion method. Examples of devices for measuring particle size distribution using the laser diffraction-dispersion method include the MT3300 series manufactured by Microtrac-Bell.

[0040] The resulting particle size distribution is often a bimodal distribution, containing a peak originating from the first metallic magnetic particle 2 and a peak originating from the second metallic magnetic particle 3. Furthermore, even if the resulting particle size distribution does not appear bimodal at first glance, it can be resolved into two peaks by being fitted to two normal distributions whose modes are sufficiently far apart (more than 4 μm apart).

[0041] Therefore, the average particle diameters d1 and d2 of the first metallic magnetic particle 2 and the second metallic magnetic particle 3 are determined as follows.

[0042] First, the particle size distribution is fitted to two normal distributions whose modes are separated by 4 μm or more. Next, of the two normal distributions extracted through the fitting process, the one with the larger diameter is designated as the first normal distribution, and the one with the smaller diameter is designated as the second normal distribution. The particle sizes corresponding to the peak values ​​of each distribution are then extracted. The median diameter in the first normal distribution is then considered as the average particle size d1 of the first metallic magnetic particle 2. The median diameter in the second normal distribution is then considered as the average particle size d2 of the second metallic magnetic particle 3.

[0043] The total content of the first metallic magnetic particles 2 and the second metallic magnetic particles 3 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.

[0044] When the content of the first metallic magnetic particles 2 is c1 and the content of the second metallic magnetic particles 3 is c2, the content mass ratio c2 / c1 is preferably 1 / 150 or more and 1 / 4 or less (0.007 or more and 0.250 or less), more preferably 1 / 120 or more and 1 / 10 or less (0.008 or more and 0.100 or less), and even more preferably 1 / 90 or more and 1 / 50 or less (0.011 or more and 0.020 or less). By keeping the content mass ratio c2 / c1 within the above range, a magnetorheological fluid 1 that can achieve both high excitation stress and good dispersibility of the dispersed phase can be realized. However, if the content mass ratio c2 / c1 falls below the lower limit, the ratio of content c2 to content c1 becomes too low, so the lifting effect by the second metallic magnetic particles 3 cannot be sufficiently obtained, and the dispersibility of the dispersed phase may decrease. On the other hand, if the mass ratio of content c2 / c1 exceeds the upper limit, the ratio of content c2 to content c1 becomes too high, which may cause the viscosity of the magnetoviscous fluid 1 to become too high when no magnetic field is applied.

[0045] The content c2 of the second metallic magnetic particles 3 is preferably 0.01% by mass or more and 5% by mass or less, and more preferably 0.1% by mass or more and 3% by mass or less. This makes it possible to realize a magnetoviscous fluid 1 that can achieve both high excitation stress and good dispersibility of the dispersed phase while optimizing the viscosity of the magnetoviscous fluid 1 when no magnetic field is applied.

[0046] The particle shapes of the first metallic magnetic particle 2 and the second metallic magnetic particle 3 are not particularly limited and may be spheres, ellipsoids, polyhedra, or other shapes. Examples of other shapes include needle-like, fibrous, plate-like, flake-like, and hollow shapes.

[0047] The particle shapes of the first metallic magnetic particle 2 and the second metallic magnetic particle 3 may be the same as or different from each other. Furthermore, the particle shapes of the first metallic magnetic particles 2 and the second metallic magnetic particles 3 may be the same as or different from each other.

[0048] Various additives may be added to the magnetorheological fluid 1. Examples of additives include non-magnetic particles, 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.

[0049] Examples of non-magnetic particles include particles composed of non-magnetic inorganic materials, thermoplastic resins, thermosetting resins, and the like. The content of non-magnetic particles 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.

[0050] Examples of dispersants include oleates, naphthenates, sulfonates, phosphate esters, stearic acid, stearates, glycerol monooleate, sorbitan sesquioleate, lauric acid, fatty acids, and fatty alcohols.

[0051] 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.

[0052] 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 first metal magnetic particles 2 and the second metal magnetic particles 3.

[0053] 1.2. Dispersion medium The dispersion medium of the magnetorheological fluid 1 includes an ionic liquid 4. The ionic liquid 4 has both cationic and anionic groups.

[0054] 1.2.1. Cationic groups The cationic group is selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions and phosphonium ions.

[0055] 1.2.1.1. Quaternary ammonium ions 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.

[0056] [[ID=拾]]

Chemical formula

[0057] R 11 ~R 14 When each of R 11 ~R 14 is a linear or branched alkyl group, the quaternary ammonium ion is particularly also referred to as an aliphatic quaternary ammonium ion. Also, when R 11 ~R 14 are bonded to each other to form a ring, the quaternary ammonium ion is particularly also referred to as an alicyclic quaternary ammonium ion.

[0058] R 11 ~R 14 Examples of the linear alkyl group of R 11 ~R 14 include those having 1 to 20 carbon atoms. Specifically, methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decanyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, nonadecyl group, icosyl group and the like can be mentioned.

[0059] R 11 ~R 14Examples 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.

[0060] 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.

[0061] Specific examples of quaternary ammonium ions include tetraethylammonium, tetramethylammonium, tetrapropylammonium, tetrabutylammonium, and tetrapentylammonium.

[0062] As the quaternary ammonium ion, the cations represented by (A)-1, (A)-2, or (A)-3 below are preferred. The ionic liquid 4 having these cations is liquid even below -20°C and does not evaporate easily even above 350°C. For this reason, the ionic liquid 4 having these cations contributes to the realization of a particularly reliable magnetorheological fluid 1. Note that (A)-1 is an aliphatic quaternary ammonium ion, and (A)-2 and (A)-3 are alicyclic quaternary ammonium ions.

[0063] [ka]

[0064] 1.2.1.2. 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.

[0065] [ka] [In formula (B), R 20 This is a linear or branched alkyl group having 1 to 20 carbon atoms.

[0066] R in equation (B) 20 The explanation for the alkyl group is as follows: 11 ~R 14 This is similar to the explanation regarding alkyl groups.

[0067] R 20 Preferably, the alkyl group has 1 to 10 carbon atoms, and more preferably, an alkyl group has 2 to 8 carbon atoms.

[0068] As the imidazolium ion, the cation represented by the following formula (B)-1 is more preferred. The ionic liquid 4 having this cation remains liquid even below -20°C and does not evaporate easily even above 350°C. For this reason, the ionic liquid 4 having this cation contributes to the realization of a particularly reliable magnetorheological fluid 1.

[0069] [ka] [In formula (B)-1, R 21 This is an alkyl group having 4 to 8 carbon atoms.

[0070] R 21 Examples of such groups include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, and octyl groups.

[0071] 1.2.1.3. 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.

[0072] [ka] [In formula (C), R 30 This is a linear or branched alkyl group having 1 to 20 carbon atoms.

[0073] 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.

[0074] R 30 Preferably, the alkyl group has 1 to 10 carbon atoms, and more preferably, an alkyl group has 2 to 8 carbon atoms.

[0075] As the pyridinium ion, the cation represented by the following formula (C)-1 is preferred. The ionic liquid 4 having this cation remains liquid even below -20°C and does not evaporate easily even above 350°C. For this reason, the ionic liquid 4 having this cation contributes to the realization of a particularly reliable magnetorheological fluid 1.

[0076] [ka] [In formula (C)-1, R 31 This is an alkyl group having 4 to 6 carbon atoms.

[0077] R 31 Examples of such groups include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, and hexyl groups.

[0078] 1.2.1.4. 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.

[0079] [ka] [In formula (D), R 41 ~R 44 Each 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.

[0080] 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.

[0081] 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.

[0082] Specific examples of phosphonium ions include tetrabutylphosphonium, tetrapropylphosphonium, tetraethylphosphonium, tetramethylphosphonium, and hexadecyltributylphosphonium.

[0083] As the phosphonium ion, the cation shown in (D)-1 below is preferred. The ionic liquid 4 having this cation remains liquid even below -20°C and does not evaporate easily even above 350°C. For this reason, the ionic liquid 4 having this cation contributes to the realization of a particularly reliable magnetorheological fluid 1.

[0084] [ka]

[0085] 1.2.2. Anionic Groups The anionic group is selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, and bis(trifluoromethanesulfonyl)amide ions.

[0086] The listed cations and anions are all bulky ions. Therefore, by using the listed ions as the cationic and anionic groups of the ionic liquid 4, it is possible to realize an ionic liquid 4 that can disperse the dispersed phase well even at low and high temperatures. In other words, such an ionic liquid 4 is liquid even at low temperatures below 0°C and does not evaporate easily even at high temperatures above 250°C, making it a highly reliable dispersion medium. Furthermore, because the ionic liquid 4 has a higher affinity for the first metal magnetic particles 2 and the second metal magnetic particles 3 than mineral oil, separation of the dispersed phase and the dispersion medium is less likely to occur when used as a dispersion medium. As a result, it is possible to realize a magnetoviscous fluid 1 that maintains good dispersibility of the dispersion medium and exhibits high excitation stress, regardless of the environment.

[0087] While the dispersion medium is preferably composed of ionic liquid 4, the dispersion medium may also contain additives other than ionic liquid 4, as long as the effects of ionic liquid 4 are not impaired. Examples of such additives include thixotropic agents, surfactants, viscoplastic media, and water-in-oil emulsions.

[0088] The content of the ionic liquid 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.

[0089] 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 15 [kPa] or higher, and more preferably 20 [kPa] or higher, at a temperature of 250°C. This yields a magneto-viscous fluid 1 that exhibits sufficient excitation stress. Such a magneto-viscous fluid 1 is useful in various applications.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 1.5. Method for manufacturing magnetorheological fluid The method for producing the magnetorheological fluid 1 involves first mixing and stirring the raw materials for the magnetorheological fluid 1 as described above. Examples of stirring methods include stirring with a spatula, using a vortex mixer, a high-shear mixer, or 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.

[0097] 2. Braking device Next, a braking device according to an embodiment will be described.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] When no magnetic field H is applied to the magnetorheological fluid 1 (H = 0), in the magnetorheological fluid 1, the first metal magnetic particles 2 and the second metal magnetic particles 3 are in a dispersed state. In this case, the excitation stress of the magnetorheological fluid 1 is sufficiently small, and almost no braking force is generated.

[0102] When a magnetic field H is applied to the magnetorheological fluid (0 < H), in the magnetorheological fluid 1, the first metal magnetic particles 2 and the second metal magnetic particles 3 form a cluster structure. In this case, the excitation stress of the magnetorheological fluid 1 increases, and a braking force is generated.

[0103] 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. For this reason, 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.

[0104] 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.

[0105] 3. Effects achieved by the above embodiment As described above, the magnetorheological fluid 1 according to the above embodiment includes the first metal magnetic particles 2, the second metal magnetic particles 3, and the ionic liquid 4. The first metal magnetic particles 2 have an average particle diameter d1 of 5 μm or more. The second metal magnetic particles 3 have an average particle diameter d2 of 500 nm or less. The ionic liquid 4 has a cationic group and an anionic group.

[0106] The cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0107] The anionic group is one or more selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, tris(pentafluoroethyl)trifluorophosphate ions, and bis(trifluoromethanesulfonyl)amide ions.

[0108] With this configuration, a magnetorheological fluid 1 can be obtained that achieves both high reliability and high excitation stress, even when used at low or high temperatures.

[0109] In the magnetorheological fluid 1 according to the above embodiment, the cationic group may include a cation represented by the following formulas (A)-1, (A)-2, (B)-1, (C)-1, or (D)-1.

[0110] [ka]

[0111] With this configuration, a magnetorheological fluid 1 with particularly high reliability can be obtained, as it remains liquid even at lower temperatures and is less prone to evaporation even at higher temperatures.

[0112] In the magnetorheological fluid 1 according to the above embodiment, it is preferable that the average particle diameter d2 of the second metallic magnetic particles 3 is 15 nm or more and 50 nm or less.

[0113] With this configuration, the lifting effect due to the Brownian motion of the second metallic magnetic particle 3 can be obtained more sufficiently. Furthermore, with this configuration, the second metallic magnetic particle 3 contributes to a further increase in the excitation stress of the magnetoviscous fluid 1.

[0114] In the magnetorheological fluid 1 according to the above embodiment, it is preferable that the content of the second metallic magnetic particles 3 is 0.01% by mass or more and 5% by mass or less.

[0115] With this configuration, it is possible to realize a magnetoviscous fluid 1 that optimizes the viscosity of the magnetoviscous fluid 1 when no magnetic field is applied, while simultaneously achieving high excitation stress and good dispersion of the dispersed phase.

[0116] In the magnetorheological fluid 1 according to the above embodiment, when the content of the first metallic magnetic particles 2 is c1 and the content of the second metallic magnetic particles 3 is c2, it is preferable that the content mass ratio c2 / c1 is 1 / 150 or more and 1 / 4 or less.

[0117] With this configuration, a magnetorheological fluid 1 can be realized that can achieve both high excitation stress and good dispersion of the dispersed phase.

[0118] In the magnetorheological fluid 1 according to the above embodiment, when the average particle diameter of the first metallic magnetic particle 2 is d1 [μm] and the average particle diameter of the second metallic magnetic particle 3 is d2 [μm], it is preferable that the particle diameter ratio d2 / d1 is 0.002 or more and 0.100 or less.

[0119] With this configuration, a magnetorheological fluid 1 can be realized that can achieve both high excitation stress and good dispersion of the dispersed phase.

[0120] In the magnetorheological fluid 1 according to the above embodiment, at least one of the constituent materials of the first metallic magnetic particle 2 and the second metallic magnetic particle 3 may be an amorphous metallic material or a microcrystalline metallic material.

[0121] This configuration helps to significantly reduce the coercivity of the first and second metallic magnetic particles 2 and 3, thereby improving their redispersibility. Furthermore, because these particles have higher toughness and strength compared to, for example, metal oxides, wear and chipping of the first and second metallic magnetic particles 2 and 3 can be effectively suppressed.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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]

[0126] Next, specific embodiments of the present invention will be described. 4. Preparation of magnetorheological fluid Figures 3 to 11 are Tables 1 to 9, which show the conditions for preparing magnetorheological fluids and the evaluation results.

[0127] The magnetorheological fluid was prepared as follows: First, the first metallic magnetic particles, the second metallic magnetic particles, and the ionic liquid shown in Tables 1 (Figure 3) to 9 (Figure 11) were mixed. The respective content of the first metallic magnetic particles and the second metallic magnetic particles are as shown in Tables 1 to 9. The remainder of the content was the ionic liquid. 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 to 36 and Comparative Examples 1 to 8.

[0128] In Tables 1 through 9, "C chain of the cation group" refers to the alkyl group with the largest number of carbon atoms bonded to the main skeleton of the cation group.

[0129] 5. Evaluation of magnetoviscous fluids For each example and comparative example, the boiling point, freezing point, excitation stress, and viscosity in the absence of a magnetic field were evaluated for the magnetorheological fluid.

[0130] 5.1. Boiling point The boiling point of the magnetorheological fluids in each example and comparative example was measured using the method described above. The measured boiling points were then evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. Note that, among the evaluation criteria, if the evaluation result is A or B, it can be evaluated as "the magnetorheological fluid has a high boiling point (good heat resistance)."

[0131] A: Its boiling point is 350°C or higher. B: Boiling point is between 250°C and 350°C C: Boiling point is less than 250°C

[0132] 5.2. Freezing point The freezing point of the magnetorheological fluids in each example and comparative example was measured using the method described above. The measured freezing points were then evaluated against the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. Note that, among the evaluation criteria, if the evaluation result is A or B, it can be evaluated as "the magnetorheological fluid has a low freezing point (good cold resistance)."

[0133] A: The freezing point is below -20°C. B: The freezing point is greater than -20°C and less than or equal to 0°C. C: Freezing point is above 0°C

[0134] 5.3. Excitation Stress The excitation stress was measured for each example and comparative example of the magnetorheological fluid using the method described above. The measured excitation stress was then evaluated against the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. Note that, among the evaluation criteria, if the evaluation result is A or B, it can be evaluated as "high excitation stress of the magnetorheological fluid."

[0135] A: The excitation stress is 20 kPa or more. B: The excitation stress is 15 kPa or more and less than 20 kPa. C: Excitation stress is less than 15 [kPa]

[0136] 5.4 Viscosity in a non-magnetic field The viscosity of the magnetorheological fluids in 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], and the temperature of the magnetorheological fluid was set to 25°C. The measured viscosity was then evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 1 to 9 as "viscosity in the absence of a magnetic field." In the evaluation criteria, an evaluation result of A indicates that "the viscosity of the magnetorheological fluid in the absence of a magnetic field is good."

[0137] A: The viscosity is less than 1000 mPa·s. B: The viscosity is 1000 mPa·s or higher.

[0138] As is clear from Tables 1 to 9, the boiling point, freezing point, and excitation stress evaluation results for the magnetorheological fluids in each example were all A or B, indicating favorable results.

[0139] On the other hand, the magnetoviscous fluid of Comparative Example 1, which did not use an ionic liquid as the dispersion medium, showed poor results in terms of boiling point and excitation stress evaluation.

[0140] Furthermore, the magnetoviscous fluid of Comparative Example 2, which did not contain the second metallic magnetic particles, showed poor results in the evaluation of excitation stress.

[0141] Furthermore, although ionic liquids were used as dispersion media, the magnetorheological fluids of Comparative Examples 3 to 6, which used ionic liquids that did not contain specific ions, exhibited poor boiling or freezing points.

[0142] Furthermore, in Comparative Examples 7 and 8, the magnetoviscous fluids in which the average particle diameter d1 of the first metallic magnetic particles or the average particle diameter d2 of the second metallic magnetic particles fell outside the predetermined range showed poor evaluation results for excitation stress.

[0143] 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]

[0144] 1...Magnetorheological fluid, 2...First metallic magnetic particle, 3...Second metallic magnetic particle, 4...Ionic liquid, 100...Brake device, 110...Fixed disk, 120...Movable disk, AX...Rotation axis, H...Magnetic field

Claims

1. A first metallic magnetic particle having an average particle diameter of 5 μm or more, Second metallic magnetic particles having an average particle diameter of 500 nm or less, An ionic liquid having a cationic group and an anionic group, Includes, The aforementioned cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions. The magnetorheological fluid is characterized in that the anionic group is one or more selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, trispentafluoroethyltrifluorophosphate ions, and bis(trifluoromethanesulfonyl)amide ions.

2. The magnetorheological fluid according to claim 1, wherein the cationic group comprises a cation represented by the following formula (A)-1, formula (A)-2, formula (B)-1, formula (C)-1, or formula (D)-1. 【Chemistry 1】

3. The magnetorheological fluid according to claim 1 or 2, wherein the average particle diameter of the second metallic magnetic particles is 15 nm or more and 50 nm or less.

4. The magnetorheological fluid according to claim 1 or 2, wherein the content of the second metallic magnetic particles is 0.01% by mass or more and 5% by mass or less.

5. The magnetorheological fluid according to claim 1 or 2, wherein when the content of the first metallic magnetic particles is c1 and the content of the second metallic magnetic particles is c2, the content mass ratio c2 / c1 is 1 / 150 or more and 1 / 4 or less.

6. The magnetorheological fluid according to claim 1 or 2, wherein when the average particle diameter of the first metallic magnetic particles is d1 [μm] and the average particle diameter of the second metallic magnetic particles is d2 [μm], the particle diameter ratio d2 / d1 is 0.002 or more and 0.100 or less.

7. The magnetorheological fluid according to claim 1 or 2, wherein at least one of the constituent materials of the first metallic magnetic particle and the constituent material of the second metallic magnetic particle is an amorphous metallic material or a microcrystalline metallic material.

8. 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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