Magnetorheological fluid and brake device

Abstract

Provided are: a magnetorheological fluid which is capable of achieving both high reliability and high excitation stress even when used at low temperatures or high temperatures; and a brake device which is provided with the magnetorheological fluid. The magnetorheological fluid is characterized by comprising: first metal magnetic particles having an average particle diameter of 5 [mu] m or more; second metal magnetic particles having an average particle diameter of 500 nm or less; and an ionic liquid having at least one cation group selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions, and an anion group, the cation group being one or more ions selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions. The anion group is one or more selected from the group consisting of tetrafluoroboric acid ions, hexafluorophosphoric acid ions, tris (pentafluoroethyl) trifluorophosphoric acid ions, and bis (trifluoromethylsulfonyl) amide ions.

Description

TECHNICAL FIELD

[0001] The present application relates to a magnetorheological fluid and a brake device. BACKGROUND

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

[0003] Research is being conducted on the application of such a magnetorheological fluid to various fields such as control devices such as dampers, brake devices such as brakes or clutches, and the like.

[0004] In these applications, as the temperature range in which the magnetorheological fluid is used, a range of -10°C to 100°C is assumed. Therefore, it is required that the magnetorheological fluid maintain the physical properties not only at normal temperature but also at low temperature or high temperature.

[0005] In Patent Literature 1, a magnetorheological fluid is assumed to be used at a high temperature of 200°C, and as a solution to suppress evaporation of the magnetorheological fluid, an ionic liquid is used in the dispersion medium.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 2010-504635

[0009] From the viewpoint of further expanding the range of use of the magnetorheological fluid, it is required that, even in the case of use at low temperature or high temperature, high reliability is ensured while suppressing the aggregation and sedimentation of the magnetic particles, and a high magnetic excitation stress (excitation stress) is exhibited. SUMMARY

[0010] The magnetorheological fluid according to the application example of the present application is characterized by containing:

[0011] first metal magnetic particles having an average particle diameter of 5 μm or more;

[0012] second metal magnetic particles having an average particle diameter of 500 nm or less; and

[0013] an ionic liquid having a cationic group and an anionic group,

[0014] the cationic group is one or more selected from the group consisting of a quaternary ammonium ion, an imidazolium ion, a pyridinium ion, and a phosphonium ion,

[0015] The anion group is one or more selected from the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, tris(pentafluoroethyl)trifluorophosphate ion, and bis(trifluoromethylsulfonyl)amide ion.

[0016] The application example of the present application relates to a brake device having:

[0017] a fixed portion;

[0018] a movable portion movable with respect to the fixed portion;

[0019] a magnetorheological fluid held between the fixed portion and the movable portion; and

[0020] a magnetic field generating portion that applies a magnetic field to the magnetorheological fluid. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a schematic view showing a magnetorheological fluid according to the embodiment.

[0022] Figure 2 is a longitudinal sectional view showing a brake device according to the embodiment.

[0023] Figure 3 is Table 1 showing a production condition and evaluation results of a magnetorheological fluid.

[0024] Figure 4 is Table 2 showing a production condition and evaluation results of a magnetorheological fluid.

[0025] Figure 5 is Table 3 showing a production condition and evaluation results of a magnetorheological fluid.

[0026] Figure 6 is Table 4 showing a production condition and evaluation results of a magnetorheological fluid.

[0027] Figure 7 is Table 5 showing a production condition and evaluation results of a magnetorheological fluid.

[0028] Figure 8 is Table 6 showing a production condition and evaluation results of a magnetorheological fluid.

[0029] Figure 9 is Table 7 showing a production condition and evaluation results of a magnetorheological fluid.

[0030] Figure 10 is Table 8 showing a production condition and evaluation results of a magnetorheological fluid.

[0031] Figure 11 is Table 9 showing a production condition and evaluation results of a magnetorheological fluid.

[0032] Reference Signs List

[0033] 1: magnetorheological fluid, 2: first metal magnetic particle, 3: second metal magnetic particle, 4: ionic liquid, 100: brake device, 110: fixed disc, 120: movable disc, AX: rotation axis, H: magnetic field. DETAILED DESCRIPTION

[0034] Hereinafter, a magnetorheological fluid and a brake device according to the present application will be described in detail based on an embodiment shown in the drawings.

[0035] 1. Magnetorheological fluid

[0036] First, a magnetorheological fluid according to the embodiment will be described.

[0037] A magnetorheological fluid is a fluid that acts like a liquid when no magnetic field is applied and acts like a semi-solid when a magnetic field is applied. By utilizing such a change in viscosity, it is possible to control the stress of the magnetorheological fluid. Therefore, the magnetorheological fluid can be used in various devices and the like that utilize a change in stress to exert various functions.

[0038] Figure 1 is a schematic view showing a magnetorheological fluid 1 according to the embodiment. Figure 1 The magnetorheological fluid 1 shown includes a first metal magnetic particle 2, a second metal magnetic particle 3, and an ionic liquid 4. The first metal magnetic particle 2 and the second metal magnetic particle 3 are each a dispersed substance. The ionic liquid 4 is a dispersion medium.

[0039] The average particle diameter dl of the first metal magnetic particle 2 is 5 μm or more. On the other hand, the average particle diameter d2 of the second metal magnetic particle 3 is 500 nm or less.

[0040] The ionic liquid 4 has a cationic group and an anionic group.

[0041] Among them, the cationic group is one or more selected from the group consisting of a quaternary ammonium ion, an imidazolium ion, a pyridinium ion, and a phosphonium ion.

[0042] Further, the anionic group is one or more selected from the group consisting of a tetrafluoroborate ion, a hexafluorophosphate ion, a tris(pentafluoroethyl)trifluorophosphate ion, and a bis(trifluoromethylsulfonyl)amide ion.

[0043] According to such a configuration, by including two kinds of dispersoids (the first metal magnetic particles 2 and the second metal magnetic particles 3) that differ in average particle diameter, the relatively small second metal magnetic particles 3 promote the enhancement effect of dispersion of the relatively large first metal magnetic particles 2. Thus, it is possible to maintain a good dispersion state of the dispersoids. As a result, it is possible to suppress the decrease in the magnetic excitation stress, and it is possible to suppress the decrease in the magnetic excitation stress. Further, by using the ionic liquid 4 as the dispersion medium, it is possible to maintain a good affinity of the dispersoids even at low temperatures or high temperatures. That is, the ionic liquid 4 having the cation group and the anion group described above or the like is a liquid even at low temperatures of 0°C or less, and is not easily evaporated even at high temperatures of 250°C or more, and thus becomes a dispersion medium with high reliability. Further, the ionic liquid 4 has a high affinity with the first metal magnetic particles 2 and the second metal magnetic particles 3 compared to mineral oil. Thus, by using the ionic liquid 4 as the dispersion medium, it is not easy to cause separation of the dispersoids from the dispersion medium even at low temperatures or high temperatures. As a result, it is possible to realize a magnetic fluid 1 that maintains a good dispersion state of the dispersoids regardless of the environment, and exhibits a high magnetic excitation stress.

[0044] 1.1. Dispersoids

[0045] The dispersoids of the magnetic fluid 1 include the first metal magnetic particles 2 and the second metal magnetic particles 3.

[0046] As the constituent material of the first metal magnetic particles 2 and the constituent material of the second metal magnetic particles 3, metal-based magnetic materials such as Fe-based metal materials, Ni-based metal materials, and Co-based metal materials can be cited, and a composite material using one or two or more of them can be used. Further, a composite material of these metal-based magnetic materials and oxide-based magnetic materials can also be used. Among them, from the viewpoint of high saturation magnetization, the constituent material of the first metal magnetic particles 2 preferably uses an Fe-based metal material.

[0047] The Fe-based metal material is a metal material in which Fe is the main component. The main component means that the content ratio of Fe in the Fe-based metal material is 50% or more in terms of atomic number ratio. Such an Fe-based metal material has high saturation magnetization, high toughness, and high strength compared to ferrite and the like. Thus, the Fe-based metal material is useful as each of the constituent materials of the first metal magnetic particles 2 and the second metal magnetic particles 3.

[0048] The Fe-based metal material can contain, in addition to Fe, an element that exhibits strong magnetism alone such as Ni or Co, and can contain at least one selected from the group consisting of Cr, Nb, Cu, Al, Mn, Mo, Si, Sn, B, C, P, Ti, and Zr, depending on the characteristics targeted. Furthermore, the Fe-based metal material can contain unavoidable impurities within a range that does not impair the effects of the embodiments. The unavoidable impurities refer to impurities that are unintentionally mixed in during the raw material or during the manufacturing. As the unavoidable impurities, all elements other than the above-described elements can be cited, and particularly, O, N, S, Na, Mg, K, and the like can be cited.

[0049] As such an Fe-based metal material, there is no particular limitation, and for example, in addition to pure iron, carbonyl iron, an Fe-Si-Al-based alloy such as an Fe-Si-Al alloy, an Fe-Ni-based alloy, an Fe-Co-based alloy, an Fe-Ni-Co-based alloy, an Fe-Si-B-based alloy, an Fe-Si-Cr-B-based alloy, an Fe-Si-B-C-based alloy, an Fe-Si-B-Cr-C-based alloy, an Fe-Si-Cr-based alloy, an Fe-B-based alloy, an Fe-P-C-based alloy, an Fe-Co-Si-B-based alloy, an Fe-Si-B-Nb-based alloy, an Fe-Si-B-Nb-Cu-based alloy, an Fe-Zr-B-based alloy, an Fe-Cr-based alloy, an Fe-Cr-Al-based alloy, and the like can be cited.

[0050] Furthermore, each of the constituent materials of the first metal magnetic particle 2 and the second metal magnetic particle 3 can be an amorphous metal material, a crystalline metal material, or a microcrystalline (nanocrystalline) metal material. Among these, at least one of the constituent materials of the first metal magnetic particle 2 and the second metal magnetic particle 3 preferably uses an amorphous metal material or a microcrystalline metal material. These contribute to sufficiently reducing the coercive force of the first metal magnetic particle 2 and the second metal magnetic particle 3 and improving the redispersibility. Furthermore, they have high toughness and strength compared to, for example, a metal oxide, and thus can effectively suppress abrasion, damage, and the like of the first metal magnetic particle 2 and the second metal magnetic particle 3. As a result, a magnetic fluid 1 with stable magnetization stress can be realized. Furthermore, an amorphous metal material and a microcrystalline metal material do not have grain boundaries or are micro, and thus corrosion that starts from a grain boundary is less likely to occur. Therefore, the corrosion resistance of the first metal magnetic particle 2 and the second metal magnetic particle 3 is particularly improved. In addition, the microcrystalline metal material refers to a metal material in which a microcrystal (nanocrystal) with a crystal grain diameter of 100 nm or less is present.

[0051] As the amorphous metal material, for example, Fe-based amorphous alloys of binary or multi-systems such as Fe-Si-B system, Fe-Si-Cr-B system, Fe-Si-B-C system, Fe-Si-B-Cr-C system, Fe-Si-Cr system, Fe-B system, Fe-B-C system, Fe-P-C system, Fe-Co-Si-B system, Fe-Si-B-Nb system, Fe-Zr-B system, and the like, Ni-based amorphous alloys such as Ni-Si-B system, Ni-P-B system, Co-based amorphous alloys such as Co-Si-B system, and the like can be given.

[0052] As the microcrystalline metal material, for example, Fe-based nanocrystalline alloys such as Fe-Si-B-Nb-Cu system, Fe-Zr-B system, Fe-Hf-B system, Fe-Nb-B system, Fe-Zr-B-Co system, Fe-Hf-B-Co system, Fe-Nb-B-Co system, Fe-Si-B-P-Cu system, and the like can be given.

[0053] The first metal magnetic particles 2 and the second metal magnetic particles 3 can also be particles manufactured by any method. As examples of the manufacturing method, in addition to various atomization methods such as water atomization method, gas atomization method, rotating water stream atomization method, and the like, a pulverization method, a carbonyl method, and the like can be given. Among them, according to the atomization method, a particle shape closer to a true sphere is obtained. Such a particle is less likely to be agglomerated.

[0054] The constituent material of the first metal magnetic particles 2 and the constituent material of the second metal magnetic particles 3 can be the same as each other or different from each other.

[0055] The first metal magnetic particles 2 and the second metal magnetic particles 3 can also include an oxide film provided on the surface of the particle main body composed of the metal-based magnetic material described above or the like. The oxide film is interposed between the particle main body and the surface modification film described later, and improves the adhesion of the surface modification film to the particle main body. In addition, the oxide film can suppress agglomeration while protecting the particle main body, and can improve the moisture absorption resistance and rust resistance of the particle main body. In addition, the oxide film preferably covers the entire surface of the particle main body, but can be provided only on a part of the surface.

[0056] As the constituent material of the oxide film, for example, silicon oxide, aluminum oxide, titanium oxide, vanadium oxide, niobium oxide, chromium oxide, manganese oxide, tin oxide, zinc oxide, and the like can be given, and one or two or more kinds of mixture or composite thereof or the like can be given.

[0057] Among them, silicon oxide is preferably used. Silicon oxide is an oxide represented by the composition formula SiO x (0 < x < 2), and is preferably SiO2.

[0058] The surface modification film covers the surface of the particle main body with the oxide film interposed therebetween. Thereby, the dispersibility of the first metal magnetic particles 2 and the second metal magnetic particles 3 in the ionic liquid 4 can be improved. In addition, the surface modification film preferably covers the entire surface of the oxide film or the particle main body, but can be provided only on a part of the surface.

[0059] The material constituting the surface modification film contains an organic compound from a coupling agent, a surfactant, or a polymerization film. The coupling agent is a compound having a functional group and a hydrolytic group. By using the coupling agent, the functional group can be introduced to the surface of the oxide film and the surface of the particle main body. Thereby, the aggregation of the dispersed particles with each other can be suppressed, and the dispersibility into the ionic liquid 4 can be further improved. Thereby, the dispersed particles in which the followability to the change in the magnetic field is excellent and which can be uniformly dispersed even at a high concentration with respect to the ionic liquid 4 can be realized.

[0060] In addition, the surface modification film is also effective in improving the moisture resistance, rust resistance, and the like of the dispersed particles. By improving the moisture resistance and the rust resistance, the deterioration caused by the moisture absorption and rusting of the dispersed particles can be suppressed.

[0061] As the functional group possessed by the coupling agent, for example, a functional group containing an aliphatic hydrocarbon group, a group containing a cyclic structure, a fluoroalkyl group, a fluoroaryl group, a nitro group, an acyl group, a cyano group, and the like can be given, and an aliphatic hydrocarbon group or a group containing a cyclic structure is particularly preferable.

[0062] As described above, the average particle diameter dl of the first metal magnetic particles 2 is 5 μm or more, preferably 6 μm or more and 15 μm or less, and more preferably 7 μm or more and 10 μm or less. If the average particle diameter dl of the first metal magnetic particles 2 is within the range, the magnetic field responsiveness of the first metal magnetic particles 2 can be sufficiently increased. Thereby, the excitation stress of the magnetorheological fluid 1 can be increased. In addition, if the average particle diameter dl of the first metal magnetic particles 2 is less than the lower limit value, the magnetic field responsiveness of the first metal magnetic particles 2 becomes small, and thus the excitation stress of the magnetorheological fluid 1 becomes small. On the other hand, if the average particle diameter dl of the first metal magnetic particles 2 exceeds the upper limit value, the first metal magnetic particles 2 can easily settle even if the improvement effect by the second metal magnetic particles 3 is possessed. If the first metal magnetic particles 2 settle, the desired excitation stress can not be obtained.

[0063] As described above, the average particle diameter d2 of the second metal magnetic particle 3 is 500 nm or less, preferably 10 nm or more and 150 nm or less, and more preferably 15 nm or more and 50 nm or less. If the average particle diameter d2 of the second metal magnetic particle 3 is within the range, the lifting effect due to the Brownian motion of the second metal magnetic particle 3 is sufficiently obtained. That is, the second metal magnetic particle 3 which performs the Brownian motion generates a diffusion force which is larger than the sedimentation force due to gravity, and also contributes to the suppression of the sedimentation of the first metal magnetic particle 2 while floating by itself. In the present specification, such an effect is referred to as a lifting effect. By the lifting effect of the second metal magnetic particle 3, it is possible to maintain a good dispersion state of the dispersed substance. Further, the second metal magnetic particle 3 is small but has magnetism, and thus contributes to the increase in the excitation stress of the magnetorheological fluid 1. In addition, if the average particle diameter d2 of the second metal magnetic particle 3 is less than the lower limit value, the surface area of the second metal magnetic particle 3 becomes small, and thus it can be difficult to obtain a sufficient lifting effect even if the second metal magnetic particle 3 performs the Brownian motion. On the other hand, if the average particle diameter d2 of the second metal magnetic particle 3 exceeds the upper limit value, the Brownian motion of the second metal magnetic particle 3 decreases, and thus the lifting effect becomes small.

[0064] When the ratio of the average particle diameter d2 [μm] of the second metal magnetic particle 3 to the average particle diameter d1 [μm] of the first metal magnetic particle 2 is set as a particle diameter ratio d2 / d1, the 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. Thereby, the balance between the average particle diameter d1 and the average particle diameter d2 becomes good, and it is possible to realize the magnetorheological fluid 1 which can balance the high excitation stress and the good dispersibility of the dispersed substance. In addition, in the case where the particle diameter ratio d2 / d1 is less than the lower limit value or the particle diameter ratio d2 / d1 exceeds the upper limit value, it can be difficult to sufficiently obtain the lifting effect due to the second metal magnetic particle 3, and the dispersibility of the dispersed substance or the excitation stress decreases.

[0065] Each of the average particle diameters d1, d2 of the first metal magnetic particle 2 and the second metal magnetic particle 3 can be found by a laser diffraction / dispersion method from a particle size distribution on a volume basis. As a device for measuring the particle size distribution by the laser diffraction / dispersion method, for example, the MT3300 series manufactured by Microtrac BEL Co., Ltd. or the like can be given.

[0066] The obtained particle size distribution becomes a bimodal distribution including a peak from the first metal magnetic particle 2 and a peak from the second metal magnetic particle 3 in many cases. Further, even in the case where the obtained particle size distribution does not appear to be a bimodal distribution, by providing a fitting process to two normal distributions which are sufficiently separated from the most frequent value (separation of 4 μm or more), it becomes a distribution which can be decomposed into two peaks.

[0067] Therefore, each average particle diameter dl, d2 of the first metal magnetic particles 2 and the second metal magnetic particles 3 is found in the following manner.

[0068] First, the particle size distribution is subjected to fitting processing into two normal distributions of 4 μm or more toward the most frequent value. Next, the large-diameter side of the two normal distributions extracted by the fitting processing is taken as the first normal distribution, and the small-diameter side is taken as the second normal distribution, and the particle diameter corresponding to the peak of each distribution is extracted. Then, the median particle diameter in the first normal distribution is taken as the average particle diameter dl of the first metal magnetic particles 2. Further, the median particle diameter in the second normal distribution is taken as the average particle diameter d2 of the second metal magnetic particles 3.

[0069] The total content ratio of the first metal magnetic particles 2 and the second metal magnetic particles 3 in the magnetorheological fluid 1 is preferably 40 mass% or more and 95 mass% or less, more preferably 50 mass% or more and 90 mass% or less, and further preferably 60 mass% or more and 85 mass% or less. Thereby, the viscosity of the magnetorheological fluid 1 can be optimized. Further, the excitation stress in the magnetorheological fluid 1 can be sufficiently increased.

[0070] When the content ratio of the first metal magnetic particles 2 is cl and the content ratio of the second metal magnetic particles 3 is c2, the contained mass ratio c2 / cl 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 further preferably 1 / 90 or more and 1 / 50 or less (0.011 or more and 0.020 or less). By making the contained mass ratio c2 / cl within the range, the magnetorheological fluid 1 that can balance the high excitation stress and the good dispersibility of the dispersed matter can be realized. In addition, if the contained mass ratio c2 / cl is lower than the lower limit value, the ratio of the content ratio c2 to the content ratio cl becomes too low, and therefore the improvement effect by the second metal magnetic particles 3 cannot be sufficiently obtained, and the dispersibility of the dispersed matter can be reduced. On the other hand, if the contained mass ratio c2 / cl exceeds the upper limit value, the ratio of the content ratio c2 to the content ratio cl becomes too high, and therefore the viscosity of the magnetorheological fluid 1 at the time when no magnetic field is applied can become too high.

[0071] The content ratio c2 of the second metal magnetic particles 3 is particularly preferably 0.01 mass% or more and 5 mass% or less, and more preferably 0.1 mass% or more and 3 mass% or less. Thereby, the magnetorheological fluid 1 that can optimize the viscosity of the magnetorheological fluid 1 at the time when no magnetic field is applied and can balance the high excitation stress and the good dispersibility of the dispersed matter can be realized.

[0072] The particle shape of each of the first metal magnetic particles 2 and the second metal magnetic particles 3 is not particularly limited, and can be a regular sphere, an oval sphere, a polyhedron, or another shape. As another shape, for example, a needle shape, a fiber shape, a plate shape, a scale shape, a hollow shape, or the like can be given.

[0073] The particle shape of each of the first metal magnetic particles 2 and the second metal magnetic particles 3 can be the same as or different from each other. Furthermore, the particle shape of each of the first metal magnetic particles 2 and the second metal magnetic particles 3 can be the same as or different from each other.

[0074] Various additives can also be added to the magnetorheological fluid 1. As the additives, for example, non-magnetic particles, detergents, dispersants, antioxidants, anti-wear agents, extreme pressure agents, friction modifiers, surfactants, thixotropy imparting agents (thickeners), thinning agents, or the like can be given, and one or a mixture of two or more of them can be used.

[0075] As the non-magnetic particles, for example, particles composed of non-magnetic inorganic materials, thermoplastic resins, thermosetting resins, or the like can be given. The content rate of the 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 further preferably 0.5% by mass or more and 2% by mass or less.

[0076] As the dispersants, for example, oleates, naphthenates, sulfonates, phosphates, stearic acid, stearates, glycerol monooleate, sorbitan sesquioleate, lauric acid, fatty acids, fatty alcohols, or the like can be given.

[0077] As the anti-wear agents, for example, organic molybdenum compounds such as molybdenum dialkyldithiocarbamate and molybdenum dialkyldithiophosphate, organic zinc compounds such as zinc dialkyldithiocarbamate and zinc dialkyldithiophosphate, or the like can be given.

[0078] Furthermore, the total content rate of the additives is preferably 10% by mass or less, more preferably 8% by mass or less, and further preferably 6% by mass or less, of the entire magnetorheological fluid 1. Thereby, it is possible to suppress the case where the functions of the first metal magnetic particles 2 and the second metal magnetic particles 3 are hindered by the additives.

[0079] 1.2. Dispersing medium

[0080] The dispersing medium of the magnetorheological fluid 1 contains an ionic liquid 4. The ionic liquid 4 has a cationic group and an anionic group.

[0081] 1.2.1. Cationic group

[0082] The cation group is selected from the group consisting of a quaternary ammonium ion, an imidazolium ion, a pyridinium ion, and a phosphonium ion.

[0083] 1.2.1.1. Quaternary ammonium ion

[0084] As the quaternary ammonium ion, a cation represented by the following formula (A) can be given. The main skeleton of this cation is a nitrogen atom.

[0085] [Chemical Formula 1]

[0086]

[0087] [In formula (A), R 11 ~R 14 are each independently a linear or branched alkyl group having 1 or more and 20 or less carbon atoms. R 11 ~R 14 may also be bonded to each other to form a ring.]

[0088] The quaternary ammonium ion in the case where R 11 ~R 14 are each a linear or branched alkyl group is also particularly referred to as an aliphatic quaternary ammonium ion. Furthermore, the quaternary ammonium ion in the case where R 11 ~R 14 are bonded to each other to form a ring is also particularly referred to as an alicyclic quaternary ammonium ion.

[0089] As the linear alkyl group of R 11 ~R 14 , for example, an alkyl group having 1 or more and 20 or less carbon atoms can be given. Specifically, a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, a undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an icosyl group, and the like can be given.

[0090] As the branched alkyl group of R 11 ~R 14 , for example, an alkyl group having 3 or more and 20 or less carbon atoms can be given. Specifically, a 1-methylethyl group, a 1-methylpropyl group, a 2-methylpropyl group, a 1-methylbutyl group, a 2-methylbutyl group, a 3-methylbutyl group, a 1-ethylbutyl group, a 2-ethylbutyl group, a 1-methylpentyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 4-methylpentyl group, and the like can be given.

[0091] In the case where R 11 ~R 14 are bonded to each other to form a ring, as the formed ring, for example, a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, a cyclodecyl group, a cyclododecyl group, and the like can be given.

[0092] Specific examples of the quaternary ammonium ion include tetraethylammonium, tetramethylammonium, tetrapropylammonium, tetrabutylammonium, and tetrapentylammonium.

[0093] As the quaternary ammonium ion, a cation represented by (A)-1, (A)-2, or (A)-3 below is preferable. The ionic liquid 4 having such a cation is liquid even at -20°C or less and is not easily evaporated even at 350°C or more. Thus, the ionic liquid 4 having such a cation is particularly effective in achieving the magnetorheological fluid 1 with high reliability. In addition, (A)-1 is an aliphatic quaternary ammonium ion, and (A)-2 and (A)-3 are alicyclic quaternary ammonium ions.

[0094] [Chemical Formula 2]

[0095]

[0096] 1.2.1.2. Imidazolium Ion

[0097] As the imidazolium ion, a cation represented by the following formula (B) is exemplified. The main skeleton of this cation is a five-membered ring structure composed of carbon atoms and nitrogen atoms.

[0098] [Chemical Formula 3]

[0099]

[0100] [In formula (B), R 20 is a linear or branched alkyl group having 1 or more and 20 or less carbon atoms.]

[0101] The description of the alkyl group of R 20 in formula (B) is the same as the description of the alkyl group of R 11 to R 14 in formula (A) above.

[0102] R 20 is preferably an alkyl group having 1 or more and 10 or less carbon atoms, and more preferably an alkyl group having 2 or more and 8 or less carbon atoms.

[0103] As the imidazolium ion, a cation represented by the following formula (B)-1 is more preferable. The ionic liquid 4 having such a cation is liquid even at -20°C or less and is not easily evaporated even at 350°C or more. Thus, the ionic liquid 4 having such a cation is particularly effective in achieving the magnetorheological fluid 1 with high reliability.

[0104] [Chemical Formula 4]

[0105]

[0106] [In formula (B)-1, R 21It is an alkyl group with 4 or more but less than 8 carbon atoms.

[0107] As R 21 Examples include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, etc.

[0108] 1.2.1.3. Pyridium ion

[0109] Examples of pyridinium ions include cations represented by the following formula (C). The backbone of this cation is a six-membered ring structure composed of carbon and nitrogen atoms.

[0110] [Chemical Formula 5]

[0111]

[0112] In formula (C), R 30 It is a straight-chain or branched alkyl group having 1 or more but less than 20 carbon atoms.

[0113] Regarding R in equation (C) 30 The description of the alkyl group and the R in the above formula (A) are related. 11 ~R 14 The description of alkyl groups is the same.

[0114] As R 30 Preferably, the alkyl group has 1 or more and 10 or less carbon atoms, and more preferably, it has 2 or more and 8 or less carbon atoms.

[0115] The preferred cation for the pyridinium ion is one represented by the following formula (C)-1. The ionic liquid 4 containing this cation remains liquid even below -20°C and does not readily evaporate even above 350°C. Therefore, the ionic liquid 4 containing this cation is particularly helpful in realizing a highly reliable magnetorheological fluid 1.

[0116] [Chemical Formula 6]

[0117]

[0118] In equation (C)-1, R 31 It is an alkyl group with 4 or more but fewer than 6 carbon atoms.

[0119] As R 31 Examples include n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, and hexyl.

[0120] 1.2.1.4. Phosphorus ions

[0121] Examples of phosphonium ions include cations represented by the following formula (D). The main framework of this cation is phosphorus.

[0122] [Chemical Formula 7]

[0123]

[0124] (In formula (D), R 41 ~R 44 are each independently a linear or branched alkyl group having 1 to 20 carbon atoms. R 41 ~R 44 may also be bonded to each other to form a ring.

[0125] The alkyl group of R 41 ~R 44 in formula (D) is the same as the alkyl group of R 11 ~R 14 in formula (A) described above.

[0126] As R 41 ~R 44 , an alkyl group having 2 to 18 carbon atoms is preferred, and an alkyl group having 4 to 16 carbon atoms is more preferred.

[0127] As the phosphonium ion, specific examples include tetrabutylphosphonium, tetrapropylphosphonium, tetraethylphosphonium, tetramethylphosphonium, hexadecyltributylphosphonium, and the like.

[0128] As the phosphonium ion, a cation represented by the following (D)-1 is preferred. The ionic liquid 4 having this cation is a liquid even at -20°C or lower, and is not easily evaporated even at 350°C or higher. Therefore, the ionic liquid 4 having this cation is particularly useful in achieving a magnetorheological fluid 1 with high reliability.

[0129] [Chemical Formula 8]

[0130]

[0131] 1.2.2. Anion group

[0132] The anion group is selected from the group consisting of a tetrafluoroborate ion, a hexafluorophosphate ion, a tris(pentafluoroethyl)trifluorophosphate ion, and a bis(trifluoromethylsulfonyl)amide ion.

[0133] The cations listed and the anions listed are all bulky ions. Therefore, as the cation group and the anion group possessed by the ionic liquid 4, by using the listed ions respectively, an ionic liquid 4 that can disperse the dispersed substance well even at low temperatures or high temperatures can be realized. That is, such an ionic liquid 4 is liquid even at low temperatures of 0°C or less, and does not easily evaporate even at high temperatures of 250°C or more, and thus becomes a dispersion medium with high reliability. Furthermore, the ionic liquid 4 has high affinity with the first metal magnetic particles 2 and the second metal magnetic particles 3 compared to mineral oil, and thus by being used as a dispersion medium, separation of the dispersed substance from the dispersion medium is not easily caused. As a result, a magneto-rheological fluid 1 that maintains good dispersibility of the dispersion medium regardless of the environment, and exhibits high magnetic excitation stress, can be realized.

[0134] In addition, the dispersion medium is preferably composed of the ionic liquid 4, but within a range that does not impair the effects of the ionic liquid 4, the dispersion medium can also include an additive other than the ionic liquid 4. As such an additive, for example, thixotropic agents, surfactants, viscoplastic media, water-in-oil emulsions, and the like can be given.

[0135] The content ratio of the ionic liquid 4 in the magneto-rheological 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 further preferably 10% by mass or more and 30% by mass or less. Thereby, the dispersed state of the dispersed substance can be well maintained. Furthermore, the viscosity of the magneto-rheological fluid 1 can be optimized.

[0136] 1.3. Physical properties of the magneto-rheological fluid

[0137] The magnetic excitation stress of the magneto-rheological fluid 1 can be evaluated as the yield stress when a magnetic field of magnetic flux density 1.0 [T] is applied. The magnetic excitation stress at a temperature of 250°C is preferably 15 [kPa] or more, and more preferably 20 [kPa] or more. Thereby, a magneto-rheological fluid 1 that exhibits sufficient magnetic excitation stress is obtained. Such a magneto-rheological fluid 1 is useful in various applications.

[0138] The magnetic excitation stress of the magneto-rheological fluid 1 is measured in the following manner.

[0139] First, at a prescribed temperature, a magnetic field of magnetic flux density 1.0 [T] is applied to the magneto-rheological fluid 1. Next, in this state, a shear rate of 333 [ / s] is provided, and the shear stress is measured. In the measurement of the shear stress, for example, a rheometer MCR102 manufactured by Anton Paar Co., Ltd., or the like can be given. Furthermore, the measured shear stress is taken as the yield stress.

[0140] The boiling point of the magnetorheological fluid 1 is preferably 250°C or higher, more preferably 300°C or higher, and further preferably 350°C or higher. Thus, a magnetorheological fluid 1 having sufficient heat resistance is obtained.

[0141] The boiling point of the magnetorheological fluid 1 is measured in the following manner.

[0142] First, 2 mL of the magnetorheological fluid 1 is heated with a hot plate. Next, the temperature of the magnetorheological fluid 1 at which white smoke is generated is measured, and the measured value is taken as the boiling point.

[0143] The freezing point of the magnetorheological fluid 1 is preferably 0°C or lower, more preferably -10°C or lower, and further preferably -20°C or lower. Thus, a magnetorheological fluid 1 having sufficient cold resistance is obtained.

[0144] The freezing point of the magnetorheological fluid 1 is measured in the following manner.

[0145] First, 2 mL of the magnetorheological fluid 1 is cooled with a freezer. Next, the temperature after the magnetorheological fluid 1 is solidified is measured, and the measured value is taken as the freezing point.

[0146] 1.4. Examples of use of the magnetorheological fluid

[0147] As the use of the magnetorheological fluid 1, various devices, apparatuses, and the like that utilize the difference in excitation stress at the time of switching the applied magnetic field can be given. As the above devices and apparatuses, for example, a vibration damping device such as a linear damper, a rotary damper, or a shock absorber, a braking device such as a brake, a power transmission device such as a clutch, a muscle portion of a robot, an end effector, a valve for liquid flow control, a haptic presentation device, an acoustic device, a medical / health care manipulator, a nursing hand, a personal mobility device, and the like can be given.

[0148] 1.5. Method for producing the magnetorheological fluid

[0149] The method for producing the magnetorheological fluid 1 first mixes and stirs the raw materials of the above-described magnetorheological fluid 1. As the stirring method, for example, stirring with a scraper, a vortex mixer, a high-shear mixer, a low-frequency acoustic resonance mixer, and the like can be given. The stirring time is appropriately set depending on the stirring method, but is preferably 5 minutes or longer and 4 hours or shorter. The stirring temperature is appropriately set depending on the stirring method, but is preferably 15°C or higher and 70°C or lower.

[0150] 2. Braking device

[0151] Next, the braking device according to the embodiment will be described.

[0152] Figure 2 is a longitudinal sectional view showing a braking device 100 according to the embodiment.

[0153] Figure 2 The illustrated brake device 100 is provided with a fixed disk 110 (fixed portion), a movable disk 120 (movable portion), and a magnetorheological fluid 1. The movable disk 120 is rotatable (movable) with respect to the fixed disk 110 about the rotation axis AX. The magnetorheological fluid 1 is held between the fixed disk 110 and the movable disk 120. Further, the brake device 100 is provided with a magnetic field generating portion, not illustrated. The magnetic field generating portion applies a magnetic field to the magnetorheological fluid 1.

[0154] In the brake device 100, by switching the magnetic field applied to the magnetorheological fluid 1, the excitation stress of the magnetorheological fluid can be changed. Thereby, the resistance to the rotation of the movable disk 120 with respect to the fixed disk 110 can be changed. As a result, in the brake device 100, the resistance is used as a braking force for the braking of a vehicle or the like.

[0155] The fixed disk 110 is connected to the vehicle body side of an automobile, for example, and the movable disk 120 is connected to the wheel side of an automobile, for example.

[0156] 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.

[0157] When a magnetic field H is applied to the magnetorheological fluid 1 (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 becomes large, and a braking force is generated.

[0158] 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 brake device 100 according to the embodiment has high reliability and does not cause a decrease in braking force or the like even in a severe environment.

[0159] In addition, the structure of the brake device 100 is not limited to the above-described structure. For example, the brake device of the present application can be provided with three or more disks.

[0160] 3. Effects of the Embodiment

[0161] As described above, the magnetorheological fluid 1 according to the embodiment contains the first metal magnetic particles 2, the second metal magnetic particles 3, and the ionic liquid 4. The average particle diameter dl of the first metal magnetic particles 2 is 5 μm or more. The average particle diameter d2 of the second metal magnetic particles 3 is 500 nm or less. The ionic liquid 4 has a cationic group and an anionic group.

[0162] The cationic group is one or more selected from the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions.

[0163] The anionic group is one or more selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, tris(pentafluoroethyl)trifluorophosphate ions, and bis(trifluoromethylsulfonyl)amide ions.

[0164] According to such a configuration, a magnetorheological fluid 1 that can balance high reliability and high magnetic excitation stress even when used at low temperatures or high temperatures is obtained.

[0165] In the magnetorheological fluid 1 according to the embodiment, the cationic group can also include a cation represented by Formula (A)-1, Formula (A)-2, Formula (B)-1, Formula (C)-1, or Formula (D)-1.

[0166] [Chemical Formula 9]

[0167]

[0168] According to such a configuration, the liquid is maintained even at lower temperatures, and evaporation is less likely even at higher temperatures, and thus a magnetorheological fluid 1 with particularly high reliability is obtained.

[0169] In the magnetorheological fluid 1 according to the embodiment, the average particle diameter d2 of the second metal magnetic particles 3 is preferably 15 nm or more and 50 nm or less.

[0170] According to such a configuration, the boosting effect resulting from the Brownian motion of the second metal magnetic particles 3 is more sufficiently obtained. Furthermore, according to such a configuration, the second metal magnetic particles 3 contribute to further increase in the magnetic excitation stress of the magnetorheological fluid 1.

[0171] In the magnetorheological fluid 1 according to the embodiment, the content ratio of the second metal magnetic particles 3 is preferably 0.01 mass% or more and 5 mass% or less.

[0172] According to such a configuration, a magnetorheological fluid 1 that can optimize the viscosity of the magnetorheological fluid 1 when no magnetic field is applied and that can balance high magnetic excitation stress and good dispersibility of the dispersed phase is realized.

[0173] In the magnetorheological fluid 1 according to the embodiment, when the content ratio of the first metal magnetic particles 2 is denoted as c1 and the content ratio of the second metal magnetic particles 3 is denoted as c2, the content mass ratio c2 / c1 is preferably 1 / 150 or more and 1 / 4 or less.

[0174] According to such a configuration, a magnetorheological fluid 1 that can balance high magnetic excitation stress and good dispersibility of the dispersed phase is realized.

[0175] In the magnetorheological fluid 1 according to the embodiment, when the average particle diameter of the first metal magnetic particle 2 is d1 [μm] and the average particle diameter of the second metal magnetic particle 3 is d2 [μm], the particle diameter ratio d2 / d1 is preferably 0.002 or more and 0.100 or less.

[0176] According to such a configuration, it is possible to realize a magnetorheological fluid 1 that can balance high magnetic excitation stress and good dispersibility of the dispersoid.

[0177] In the magnetorheological fluid 1 according to the embodiment, at least one of the constituent material of the first metal magnetic particle 2 and the constituent material of the second metal magnetic particle 3 can also be an amorphous metal material or a microcrystalline metal material.

[0178] According to such a configuration, it is helpful to make the coercive force of the first metal magnetic particle 2 and the second metal magnetic particle 3 sufficiently low, and to improve the redispersibility. In addition, they have high toughness and strength compared to, for example, metal oxides, and thus it is possible to effectively suppress abrasion, damage, and the like of the first metal magnetic particle 2 and the second metal magnetic particle 3.

[0179] The brake device 100 according to the embodiment includes a fixed disc 110 (fixed portion), a movable disc 120 (movable portion), the magnetorheological fluid 1 according to the embodiment, and a magnetic field generating portion. The movable disc 120 is movable with respect to the fixed disc 110. The magnetorheological fluid 1 is held between the fixed disc 110 and the movable disc 120. The magnetic field generating portion applies a magnetic field to the magnetorheological fluid 1.

[0180] According to such a configuration, even in a severe environment, a decrease in braking force or the like does not occur, and it is possible to realize a brake device 100 with high reliability.

[0181] The magnetorheological fluid and the brake device according to the present application have been described above based on the preferred embodiments, but the present application is not limited thereto.

[0182] For example, the magnetorheological fluid and the brake device according to the present application can be additionally provided with any structure in the embodiments. In addition, the structure of each portion of the brake device according to the embodiments can be replaced by a structure having the same function as described above.

[0183] Example

[0184] Next, a specific example of the present application will be described.

[0185] 4. Production of magnetorheological fluid

[0186] Figures 3 to 11 Tables 1 to 9 are tables showing the production conditions and evaluation results of the magnetorheological fluid.

[0187] Magnetorheological fluids are fabricated in the following manner.

[0188] First, Table 1 ( Figure 3 Table 9 Figure 11 The first metallic magnetic particles, the second metallic magnetic particles, and the ionic liquid were mixed as shown in Tables 1 to 9. The respective contents of the first and second metallic magnetic particles are shown in Tables 1 to 9. The remaining portion of the contents was the ionic liquid. Next, the resulting mixture was stirred. A high-shear mixer (Silverson, L5M-A) was used as the stirring apparatus. The stirring conditions were 3000 rpm and 30 minutes. Thus, magnetorheological fluids of Examples 1 to 36 and Comparative Examples 1 to 8 were prepared.

[0189] In addition, the “C chain of the cationic group” in Tables 1 to 9 refers to the alkyl group with the most carbon atoms bonded to the main skeleton of the cationic group.

[0190] 5. Evaluation of magnetorheological fluids

[0191] For the magnetorheological fluids of each embodiment and each comparative example, the boiling point, freezing point, magnetic stress, and viscosity under non-magnetic field conditions were evaluated.

[0192] 5.1. Boiling point

[0193] For the magnetorheological fluids of each embodiment and each comparative example, the boiling point was determined by the method described above. Then, the determined boiling point was evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. In addition, if the evaluation result in the evaluation criteria is A or B, it can be evaluated as "the magnetorheological fluid has a high boiling point (good heat resistance)".

[0194] A: Boiling point above 350℃

[0195] B: Boiling point above 250℃ and below 350℃

[0196] C: Boiling point less than 250℃

[0197] 5.2. Freezing point

[0198] For the magnetorheological fluids of each embodiment and each comparative example, the freezing point was determined by the method described above. Then, the measured freezing point was evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. In addition, if the evaluation result in the evaluation criteria is A or B, it can be evaluated as "the magnetorheological fluid has a low freezing point (good cold resistance)".

[0199] A: Freezing point is below -20℃

[0200] B: Freezing point above -20℃ and below 0℃

[0201] C: freezing point is more than 0°C

[0202] 5.3. Excitation stress

[0203] For each of the examples and each of the comparative examples, the excitation stress of the magnetorheological fluid was measured by the above method. Then, the measured excitation stress was evaluated in accordance with the following evaluation criteria. The evaluation results are shown in Tables 1 to 9. In addition, in the case where the evaluation result in the evaluation criteria is A or B, it can be evaluated as "the excitation stress of the magnetorheological fluid is high".

[0204] A: excitation stress is 20 [kPa] or more

[0205] B: excitation stress is 15 [kPa] or more and less than 20 [kPa]

[0206] C: excitation stress is less than 15 [kPa]

[0207] 5.4. Viscosity under no magnetic field

[0208] For each of the examples and each of the comparative examples, the viscosity under no magnetic field (at a magnetic flux density of 0 [T]) was measured. In the measurement of the viscosity, a rheometer MCR102 manufactured by Anton Paar was used. Further, the shear rate at the time of measurement was set to 0.033 [ / s], and the temperature of the magnetorheological fluid at the time of measurement was set to 25°C. Then, the measured viscosity was evaluated in accordance with the following evaluation criteria. The evaluation results are shown as "viscosity under no magnetic field" in Tables 1 to 9. In addition, in the case where the evaluation result in the evaluation criteria is A, it can be evaluated as "the viscosity under no magnetic field of the magnetorheological fluid is good".

[0209] A: viscosity is less than 1000 mPa-s

[0210] B: viscosity is 1000 mPa-s or more

[0211] As is clear from Tables 1 to 9, in the magnetorheological fluids of each of the examples, each of the evaluation results of the boiling point, the freezing point, and the excitation stress was A or B, which is a good result.

[0212] On the other hand, each of the evaluation results of the boiling point and the excitation stress of the magnetorheological fluid of Comparative Example 1, in which no ionic liquid was used in the dispersion medium, was poor.

[0213] Further, the evaluation result of the excitation stress of the magnetorheological fluid of Comparative Example 2, which did not contain the second metal magnetic particles, was poor.

[0214] Furthermore, although an ionic liquid was used as the dispersion medium, the boiling point or the freezing point of the magnetorheological fluids of Comparative Examples 3 to 6, which used an ionic liquid that did not have a specific ion, was poor.

[0215] Further, the evaluation results of the excitation stress of the magnetic fluid of Comparative Examples 7 and 8 in which the average particle diameter dl of the first metal magnetic particles or the average particle diameter d2 of the second metal magnetic particles deviated from the prescribed range were poor.

[0216] From the above results, it was demonstrated that, according to the present application, a magnetic fluid in which high reliability and high excitation stress can be achieved even when used at low or high temperatures can be realized.

Claims

1. A magnetorheological fluid, characterized in that, Include: The first metallic magnetic particles have an average particle size of 5 μm or more. The second metallic magnetic particles have an average particle size of less than 500 nm; and Ionic liquids possess both cationic and anionic groups. The cation group is selected from one or more of the group consisting of quaternary ammonium ions, imidazolium ions, pyridinium ions, and phosphonium ions. The anionic group is selected from one or more of the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, tris(pentafluoroethyl)trifluorophosphate ion, and bis(trifluoromethanesulfonyl)amide ion.

2. The magnetorheological fluid according to claim 1, wherein, The cationic group comprises a cation represented by formula (A)-1, formula (A)-2, formula (B)-1, formula (C)-1, or formula (D)-1. 。 3. The magnetorheological fluid according to claim 1 or 2, wherein, The average particle size of the second metallic magnetic particles is greater than 15 nm and less than 50 nm.

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 set as c1 and the content of the second metallic magnetic particles is set as c2, the mass ratio c2 / c1 is more than 1 / 150 and less than 1 / 4.

6. The magnetorheological fluid according to claim 1 or 2, wherein, When the average particle size of the first metallic magnetic particle is set as d1 and the average particle size of the second metallic magnetic particle is set as d2, the particle size ratio d2 / d1 is greater than 0.002 and less than 0.100, and the units of d1 and d2 are μm.

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 second metallic magnetic particle is an amorphous metallic material or a microcrystalline metallic material.

8. A braking device, characterized in that, have: Fixing part; The movable part is movable relative to the fixed part; The magnetorheological fluid of claim 1 or 2 is held between the fixed part and the movable part; and The magnetic field generating unit applies a magnetic field to the magnetorheological fluid.

Images