Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/44—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of magnetic liquids, e.g. ferrofluids
  • H01F1/447—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of magnetic liquids, e.g. ferrofluids characterised by magnetoviscosity, e.g. magnetorheological, magnetothixotropic, magnetodilatant liquids

Definitions

• the present invention relates to magne ⁇ torheological materials with magnetic and non-magnetic inorganic additives, in particular magnetorheological fluids (MRF) with magnetic and non-magnetic inorganic additives, and their use.
• MRF magnetorheological fluids
• MRF are materials that change their flow behavior under the influence of an external magnetic field.
• electrorheological fluids they usually consist of non-colloidal suspensions of particles which are polarizable in a magnetic or electric field in a carrier liquid which optionally contains further additives.
• the basics of MRF and first devices for exploiting the x-magnetorheological effect date back to Jacob Rabinow in 1948 (Rabinow, J., Magnetic Fluid Clutch, National Bureau of Standards Technical News Bulletin 33 (4) 54-60, 1948; US Pat Tent 2,575,360). After initially causing a stir, interest in MRF initially faded to a renaissance in the mid-nineties (William, WA (Editor), Proceedings of the 5th International Conference on Electro-Rheological Fluids).
• Magneto-Rheological Suspensions and Associated Technology (1st), Singapore, New Jersey, London, Hong Kong: World Scientific Publishing, 1996). Meanwhile, numerous magnetorheological fluids and systems are commercially available such. MRF brakes and various vibration and shock absorbers (Mark R. Jolly, Jonathan W. Bender, and J. David Carlson, Properties and Applications of Commercial Magnetorheological Fluids, SPIE 5th Annual Int Sympo- sium on Smart Structures and Materials, San Diego, CA, March 15, 1998). Some special properties of MRF and their influenceability are described below.
• MRF are usually non-colloidal suspensions of magnetizable particles of about one micron to one millimeter in size in a carrier liquid.
• the MRF can also contain additives, such as, for example, B. Dispergiertoskar and thickening additives contain.
• the particles are ideally homogeneous and distributed isotropically, so that the MRF has a low basic viscosity in the magnet-free space.
• the magnetisable particles arrange themselves in ketones. tenartigen structures parallel to the magnetic field lines.
• the fluidity of the suspension is limited, which manifests itself macroscopically as an increase in viscosity.
• the viscosity increases monotonically with the applied magnetic field strength.
• the changes in the flow behavior of the MRF depend on the concentration and type of magnetizable particles, their shape, size and size distribution; but also on the properties of the carrier liquid, the additional additives, the applied field, the temperature and other factors.
• the mutual interactions of all these parameters are extremely complex, so that individual improvements of an MRF with respect to a specific target variable have always been the subject of investigations and optimization efforts.
• One focus of research is the development of MRF with a low propensity for sedimentation.
• MRFs tend to segregate due to the different masses of their constituents in gravitational and centrifugal fields, ie, previously homogeneous mixed phases separate into a pure liquid phase and a sediment-rich sediment over time. This effect is undesirable since it primarily concerns the magnetizable particles and thus impairs the functioning of the MRF and the systems constructed therewith.
• a development goal is therefore the provision of MRF with the lowest possible sedimentation tendency.
• Another aim, which is directly related to this, is the simplest possible redispersibility. Since sedimentation can never be completely ruled out, it is intended that the segregated MRF at least be such that you can easily, ie with minimal effort, again converted into a homogeneous mixture. Furthermore, it is desirable for the materials to have the lowest possible base viscosity without an external magnetic field.
• MRFs usually contain additives for stabilizing the magnetisable particles in relation to sedimentation.
• various organic additives are known.
• inorganic additives for stabilizing the MRF are also mentioned. These include oxidic particles such as silicon dioxide, in particular as nanoparticles in the form of fumed silica, and also phyllosilicates, in some cases organically modified.
• No. 5,985,168 describes the stabilization of the magnetizable particles in the MRF by a combination of small particles, in particular silicon dioxide, and a bridging polymer. Both together form a gel that envelops the magnetizable particles as a layer.
• US Pat. No. 6,592,772 B2 shows an MRF whose carrier liquid consists of various components and in which different organically modified sheet silicates, so-called “organoclays" are contained for stabilizing the magnetizable particles, each of which has the specific properties of the components of the carrier liquid are coordinated.
• colloidal metal oxides such as, for example, for stabilizing the MRF.
• fumed silica which has been rendered hydrophobic by surface modification, as well as hydrophilic silicone oligomers and copolymeric organosilicon oligomers.
• US Pat. No. 6,203,717 B1 describes an MRF which contains a hydrophobic organoclay for stabilizing the magnetizable particles, which is obtained from bentonite. In the MRF, a low hardness of the se ⁇ dimentes is found.
• No. 6,132,633 describes a water-based MRF as a carrier liquid using bentonites and hectorites.
• EP 1 283 530 A2 and EP 1 283 531 A2 disclose the use of pyrogenic silica for stabilizing an MRF having a bimodal particle size distribution based on a hydrocarbon-based carrier liquid, in the latter patent with the addition of a molybdenum-amine complex. Also in WO 03/021611 pyrogenic silica is used.
• WO 93/21644 A1 describes the composition of an MRF which, in addition to the magnetizable soft magnetic particles, also contains hard magnetic particles, preferably iron oxide or chromium dioxide with particle sizes between 0.1 and 1 ⁇ m.
• the hard magnetic particles are adsorbed on the surface of the soft magnetic particles.
• magnetorheological materials in particular MRF, which are a combination of magnetic and non-magnetic inorganic materials and / or composite particles thereof.
• the combination of magnetic and non-magnetic inorganic materials in the sense of the invention is understood as meaning all magnetic and non-magnetic inorganic materials which interact with one another. It may be interactions such as e.g. van der Waals have interactions or electromagnetic interactions, which can lead to a cladding of the core.
• nonmagnetic inorganic materials in particular those of anisometric particles, such as platelets or rods, are preferred. Examples of these are platelet-shaped phyllosilicates, such as, for example, Mica.
• the magnetic materials may be any of the magnetic materials known from the prior art, in particular in the form of inorganic particles. An example of this is magnetite.
• the average particle size of the non-magnetic materials may be between 0.001 and 1000 ⁇ m, preferably between 0.01 and 200 ⁇ m.
• the volume ratio of the magnetic and non-magnetic materials to one another is between 1:99 and 99: 1, preferably 10:90 and 90:10.
• Composite particles within the meaning of the invention are discrete particles which consist of both magnetic and nonmagnetic materials.
• a further advantageous embodiment of the magnetorheological materials according to the invention provides that the inorganic particles are at least partially organically modified.
• a magnetorheological carrier material with such additives of magnetic and non-magnetic inorganic materials has a very high stability compared to the sedimentation of the magnetizable particles and at the same time a particularly low base viscosity.
• an extremely easy redispersibility is observed. This manifests itself in the fact that the sediment formed after a long time with a stirring tool with the application of only a small amount of force again in the carrier medium, for example. can be distributed in the liquid phase of the MRF.
• the sediment usually has a firmer consistency and thus requires a higher expenditure of force for redispersing the magnetisable particles.
• a slight redispersibility offers a great advantage for the technical application, since the magnetorheological materials can be more easily homogenized again in the case of use after a prolonged operation-free state. Otherwise, their performance would be limited by property changes.
• a further advantage of the materials according to the invention which comprise the additives according to the invention is that the use of inorganic additives results in a substantial insensitivity to temperature fluctuations.
• At- Organic additives have a higher temperature stability than the organic additives used in commercial materials.
• a lower temperature dependence of the stabilizing effect of inorganic additives is to be expected in comparison with organic additives, since organic stabilizers of polymers can form temperature-variable structures in the carrier medium.
• the surprisingly high stabilizing effect of the com ⁇ ponent particles compared to the sedimentation of the magnetisable particles in the materials according to the invention is attributed to the formation of particular structures in the carrier medium.
• a possible Er ⁇ explanation is the formation of web-like connections between the magnetizable particles on the Kom ⁇ positpitate.
• the composite particles thus create bridges between the magnetizable particles and keep them in suspension.
• the attachment of the composite particles to the magnetizable particles is attributed to weak magnetic interactions of the magnetic shell of the composite particles due to low remanence. During the shearing of the materials according to the invention without a magnetic field, the weak bridges are broken up with relatively little force and can regress again after the end of the shearing. This means that the Bais sis viscosity is relatively low.
• the magnetic inorganic particles at least partially envelop the nonmagnetic particles and in this way form "composite particles" of both species, which in turn are stable Build structures between the magnetizable particles.
• the discrete composite particles are preferably produced by prior coating of the non-magnetic inorganic particles with magnetic material.
• the coating can be produced by the attachment of smaller magnetic particles to larger nonmagnetic inorganic substrate particles.
• the coating can also be formed by the separate addition of larger Vietnamesemagneti ⁇ 's inorganic particles and smaller Magneti ⁇ 's particles in the support medium, so that thereby composite particles formed.
• a preferred form of the core is an anisometric mold such. As platelets or rods.
• An example is formed by platy sheet silicates, such as e.g. Mica.
• the smaller magnetic particles, e.g. Magnetites cover the surface of the non-magnetic inorganic particles.
• a further advantageous embodiment of the magnetorheological materials according to the invention in relation to the composite particles provides that the average particle size of the composite particles is between 0.005 and 1000 ⁇ m, preferably between 0.01 ⁇ m and 200 ⁇ m. It has further been shown that it is favorable if the volume ratio of the magnetic and nonmagnetic inorganic components of the composite particle is between 1:99 and 99: 1, preferably between 10:90 and 90:10.
• the magnetisable particles can be formed from soft magnetic particles according to the prior art. This means that the magnetizable particles particles both from the amount of soft magnetic metallic materials such as iron, cobalt, nickel (also in non-pure form) and alloys thereof such as iron-cobalt, iron-nickel; magnetic steel; Iron-silicon can be selected as well as from the soft magnetic metallic materials such as iron, cobalt, nickel (also in non-pure form) and alloys thereof such as iron-cobalt, iron-nickel; magnetic steel; Iron-silicon can be selected as well as from the soft magnetic metallic materials such as iron, cobalt, nickel (also in non-pure form) and alloys thereof such as iron-cobalt, iron-nickel; magnetic steel; Iron-silicon can be selected as well as from the
• ferrites such as MnZn, NiZn, NiCo, NiCuCo, NiMg, and , CuMg ferrites are used.
• the magnetizable particles can also consist of iron carbide or iron nitride particles or of alloys of vanadium, tungsten, copper and manganese or of mixtures of the mentioned particle materials or of mixtures of different magnetizable types of solids.
• the soft magnetic materials may also be present all or partially in contaminated form.
• the carrier medium of the magnetorheological materials can be prepared from carrier liquids of the prior art such as water, mineral oils, synthetic oils such as polyalphaolefins, hydrocarbons, silicone oils, esters, polyethers, fluorinated polyethers, polyglycols, fluorinated hydrocarbons, halogenated hydrocarbons, fluorinated Silicone, orga nisch modified silicones and copolymers thereof or consist of liquid mixtures.
• carrier liquids of the prior art such as water, mineral oils, synthetic oils such as polyalphaolefins, hydrocarbons, silicone oils, esters, polyethers, fluorinated polyethers, polyglycols, fluorinated hydrocarbons, halogenated hydrocarbons, fluorinated Silicone, orga nisch modified silicones and copolymers thereof or consist of liquid mixtures.
• the carrier medium of the magnetorheological materials consists of fats or gels or of elastomers.
• inventive magnetorheological materials of the invention further inorganic particles such as SiO 2 / TiO 2, iron oxides, silicates such.
• inorganic particles such as SiO 2 / TiO 2, iron oxides, silicates such.
• particulate additives such as graphite, perfluoroethylene or molybdenum compounds such as molybdenum disulfite and combinations thereof to the magnetorheological materials in order to reduce abrasion phenomena.
• the magnetorheological materials further provide that the suspension for use for the surface treatment of workpieces has special abrasives and / or chemically etching additives, such as, for example, As alumina, cerium oxide, silicon carbide or diamond contains.
• the proportion of magnetizable particles between 10 and 70 vol .-%, preferably between 20 and 60 vol .-%, is; the proportion of the carrier medium is between 20 and 90% by volume / preferably between 30 and 80% by volume, the total proportion of the combination of the magnetic and non-magnetic additives and / or of composite particles is between 0.1 and 20% by mass. , preferably between 0.2 and 15% by mass, and the proportion of non-magnetisable additives is between 0.001 and 20% by mass, preferably between 0.01 and 15% by mass (in each case based on the magnetizable solids).
• the invention further relates to the use of the materials according to the invention.
• magnetorheological materials according to the invention provides for their use in adaptive shock and vibration dampers, controllable brakes, clutches and in sports or exercise equipment. Special materials can also be used for the surface treatment of workpieces.
• the magnetorheological materials can also be used to generate and / or display haptic information such as characters, computer-simulated objects, sensor signals or images, in haptic form, to simulate viscous, elastic and / or visco-elastic properties or the consistency distribution of an object, in particular Training and / or research purposes and / or used for medical applications.
• haptic information such as characters, computer-simulated objects, sensor signals or images, in haptic form, to simulate viscous, elastic and / or visco-elastic properties or the consistency distribution of an object, in particular Training and / or research purposes and / or used for medical applications.
• the sedimentation analysis was carried out in glass tubes (total height 160 mm, internal diameter 14.1 mm, wall thickness 0.8 mm) at 25 ° C.
• the phase boundary between the sediment and the supernatant was recorded visually at defined time intervals. In doing so, following the height of the settled solid referred to the total height of the MRF sample referred to as "sediment height" [%].
• the results are shown in Figure 1.
• the two MRF 3 and MRF 4 according to the invention have an extremely low phase separation within the first observation days and then remain stable for 60 days without the sedimentation progressing. Even after 60 days, the sediment level is still> 97%.
• the two comparative suspensions MRF 1 and MRF 2 sediment much more strongly according to the prior art and after only a few days only have sediment heights of about 73 and 90%, respectively.
• the two MRF 3 and MRF 4 according to the invention provide significantly better results than the two comparative suspensions MRF 1 and MRF 2 both in terms of the sedimentation level and in terms of the redispersing behavior.
• the two suspensions MRF 3 and MRF 4 according to the invention thus have an outstanding property profile which predestines them for use as magneto-rheological fluids.
• the two MRF 3 and MRF 4 according to the invention have a significantly higher shear stress than the two magnetorheological fluids MRF 1 and MRF 2 according to the state of the art from a magnetic flux density of approximately 200 mT.
• high shear stresses in the applied magnetic field are desirable because they cause an effective conversion of a magnetic excitation into a rheological change in the MRF.
• the two MRF 3 and MRF 4 according to the invention have a further advantageous property for use as magneto-rheological fluids.
• MRF 3 and MRF 4 follow a lower temperature dependence the two MRF according to the invention over the prior art, which can be evaluated as a further advantage ge.
• inventive MRF 3 and MRF 4 with magnetic and non-magnetic inorganic additives compared to prior art magnetorheological fluids with respect to the property combination sedimentation stability, redispersibility, Basis ⁇ viscosity, shear stress in the magnetic field and Viskosi - Depend on temperature / temperature dependence key advantages.

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