EP1782437B1 - Magneto-rheological materials having a high switch factor and use thereof - Google Patents

Abstract

The invention relates to magnetorheological materials comprising at least one non-magnetisable carrier medium and magnetisable particles contained therein, at least two magnetisable particles fractions being contained as particles and these being formed from non-spherical particles and from spherical particles.

Description

The present invention relates to high switching factor magnetorheological materials, and more particularly to high switching factor magnetorheological fluids (MRF), and their use.

MRF are materials that change their flow behavior under the influence of an external magnetic field. As with their electrorheological analogs, the so-called electrorheological fluids (ERF) are usually non-colloidal suspensions of particles which can be polarized in a magnetic or electric field in a carrier liquid which optionally contains further additives.

The basics of the MRF and the first devices to exploit the magnetorheological effect go back to Jacob Rabinow in 1948 ( Rabinow, J., Magnetic Fluid Clutch, National Bureau of Standards Technical News Bulletin 33 (4) 54-60, 1948 ; U.S. Patent 2,575,360 ). After initially causing a stir, interest in MRF initially ebbed to see a renaissance from the mid-nineties ( Bullough, 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. B. MRF brakes and different 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 Symposium on Smart Structures and Materials, San Diego, CA, March 15, 1998 ). In the following some special properties of MRF and their influenceability are described.

MRF are mostly non-colloidal suspensions of magnetizable particles, from about one micron to one millimeter in size in a carrier liquid. To stabilize the particles against sedimentation and to improve the application properties, the MRF can also additives such. B. dispersants and thickening additives. Without external magnetic field, the particles are ideally homogeneous and isotropically distributed, so that the MRF has a low dynamic basis viscosity η _o [measured in Pa.s] in the magnet-free space. When an external magnetic field H is applied, the magnetizable particles arrange in chain-like structures parallel to the magnetic field lines. This restricts the fluidity of the suspension, which manifests itself macroscopically as an increase in viscosity. As a rule, the field-dependent dynamic viscosity η _H increases monotonically with the applied magnetic field strength H.

definiert.In practice, the dynamic viscosity of an MRF is determined with a rotational viscometer. For this purpose, the shear stress τ [measured in Pa] is measured at different magnetic field strengths and given shear rate D [in s ^-1 ]. In this case, the dynamic viscosity η [in Pa · s] by $$\mathrm{\eta }\mathrm{=}\mathrm{\tau }\mathrm{/}\mathrm{D}$$

Are defined.

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 the properties of the carrier liquid, the additional additives, the applied field, the temperature and other factors. The mutual interrelations of all these parameters are extremely complex, so that individual improvements of an MRF with respect to a specific target size have been the subject of investigations and optimization efforts again and again.

A research focus was the development of MRF with a high switching factor. In equation (2), the switching factor w _D at a fixed shear rate D is defined as the ratio of the shear stress τ _{H of} the MRF in the external magnetic field H to the shear stress τ _O without magnetic field: $${\mathrm{w}}_{\mathrm{D}}\mathrm{=}{\mathrm{\tau }}_{\mathrm{H}}\mathrm{/}{\mathrm{\tau }}_{\mathrm{O}}\mathrm{,}$$

Mit
µ_r: relative Permeabilität des Mediums, dessen magnetische Flussdichte bestimmt werden soll,
µ_o = 4 · π · 10^-7 V·s/A·m : absolute Permeabilität.The external magnetic field strength H [measured in A / m] is correlated with the magnetic flux density B [measured in N / A · m = T] according to equation (3) $$\mathrm{B}\mathrm{=}{\mathrm{\mu }}_{\mathrm{r}}\mathrm{\cdot }{\mathrm{\mu }}_{\mathrm{O}}\mathrm{\cdot }\mathrm{H}\mathrm{,}$$

With
μ _r : relative permeability of the medium whose magnetic flux density is to be determined
μ _o = 4 · π · 10 ^-7 V · s / A · m: absolute permeability.

Mit
τ_B: Schubspannung der MRF im externen Magnetfeld
H mit der magnetischen Flussdichte B.Since it has proven to be useful in practice to specify magnetic characteristics as a function of the magnetic flux density B, the switching factor is subsequently also transformed to this reference system. $${\mathrm{W}}_{\mathrm{D}}\mathrm{=}{\mathrm{\tau }}_{\mathrm{B}}\mathrm{/}{\mathrm{\tau }}_{\mathrm{O}}\mathrm{,}$$

With
τ _B : shear stress of the MRF in the external magnetic field
H with the magnetic flux density B.

The switching factor w _D can thus be considered as a measure of the feasibility of a magnetic excitation in a rheological state change of the MRF. A "high" switching factor means that a small change in the magnetic flux density B results in a large change in the shear stress τ _B / τ _O or the dynamic viscosity η _B / η _O in the MRF. In the past, there have been numerous attempts to optimize the switching factor by suitable choice of the magnetizable particles with a view to a higher efficiency of the MRF.

As a rule, spherical particles of carbonyl iron are used for MRF. But there are also MRF with other magnetizable substances and mixtures of substances known. That's how it describes WO 02/45102 A1 an MRF with a mixture of high-purity iron particles and ferrite particles, in order to simultaneously optimize the properties of the MRF with and without a magnetic field. No information is given on the particle shape and size. Furthermore, there are numerous patents on specific particle geometries and distributions.

Out US 5,667,715 For example, MRFs containing spherical particles with a bimodal particle size distribution are known, with the ratio of the average particle sizes of the two fractions being between 5 and 10. In addition, the width of the particle size distributions of the two individual fractions must not exceed the value of two-thirds of the respective average particle sizes. In US 5,900,184 and US 6,027,664 are also described MRF with bimodal particle size distributions, wherein the ratio of the average particle sizes of the two fractions is between 3 and 15. In EP 1 283 530 A2 the concentration of magnetisable particles, which in turn are in bimodal size distribution, is given as 86-90% by mass.

US 6,610,404 B2 describes a magnetorheological material of magnetic particles with defined geometric features such as cylinder or prism shapes, among others. The production of such particles is very expensive. For strongly asymmetric particles, a high base viscosity of the MRF is also to be expected. In US 6,395,193 B1 and WO 01/84568 A2 Magnetorheological compositions are described with nonspherical magnetic particles, but these are not combined with spherical magnetic particles.

Out WO 93/21644 a magnetorheological composition is known which contains soft magnetic spherical carbonyl iron particles (1 - 10 microns) and hard magnetic iron oxide or chromium dioxide particles (0.1 - 1 micron).

All mentioned MRF have in common that they are dependent on special particle sizes or particle size distributions and / or defined particle geometries in order to achieve a high switching factor. As a result, their preparation is complex and correspondingly expensive.

On this basis, it is the object of the present invention to propose magnetorheological materials with a high switching factor, in particular MRF with a high switching factor, whose preparation is less complicated and thus cost-effective.

This object is solved by the characterizing features of claim 1. Advantageous developments of magnetorheological materials produced in this way, in particular MRF, are described in the dependent claims. Furthermore, the claims 17 to 19 indicate uses of magnetorheological materials produced in this way.

According to the invention, therefore, magnetorheological materials, in particular MRF, with two types of magnetisable particles are proposed, wherein the first particle fraction p consists of irregularly shaped non-spherical particles and the second fraction q consists of spherical particles. The combination of irregularly shaped non-spherical particles and spherical particles in the carrier medium surprisingly achieves both a low base viscosity without field and a high shear stress in the external magnetic field. That is, the magnetorheological materials of the invention have an exceptionally high switching factor. Besides, the production of the irregular is shaped particle fraction p little expensive and thus extremely inexpensive. Preferably, the average particle size of the fraction p is equal to or greater than that of the fraction q. The use of irregularly shaped, non-spherical particles thus creates a significant cost advantage compared to the production of known materials.

It has been found that e.g. in the case of an MRF, which contains only small spherical particles for comparison, the basic viscosity is markedly increased. In contrast, in another MRF, which contains only the large irregularly shaped particles, significantly lower shear stresses in the magnetic field are detected. The MRF with a combination of large irregularly shaped, non-spherical particles and small spherical particles thus has a significantly improved property profile.

An advantageous embodiment of the magnetorheological materials according to the invention provides that the mean particle size of the fraction p preferably has at least twice the average particle size of the fraction q. Furthermore, it is favorable if the average particle sizes of the fractions p and q are between 0.01 μm and 1000 μm, preferably between 0.1 μm and 100 μm.

A further advantageous embodiment of the magnetorheological materials according to the invention provides that the volume ratio of fractions p and q is between 1:99 and 99: 1, preferably between 10:90 and 90:10.

According to the invention, the magnetizable particles of soft magnetic particles according to the state made of engineering. This means that the magnetisable particles both from the amount of soft magnetic metallic materials such as iron, cobalt, nickel (even 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 amount of soft magnetic oxide ceramic materials such as the cubic ferrites, the perovskites and the garnets of the general formula

MO · Fe ₂ O ₃

with one or more metals from the group M = Mn, Fe, Co, Ni, Cu, Zn, Ti, Cd or Mg.

In addition, however, it is also possible to use mixed ferrites such as MnZn, NiZn, NiCo, NiCuCo, NiMg or CuMg ferrites.

However, the magnetizable particles can also consist of iron carbide or iron nitride particles and of alloys of vanadium, tungsten, copper and manganese as well as of mixtures of the mentioned particle materials or of mixtures of different magnetizable types of solids. In this case, the soft magnetic materials may also be present all or partially in contaminated form.

As a carrier medium in the context of the invention carrier liquids and fats, gels or elastomers are considered. As carrier liquids, the liquids known from 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 silicones, organically modified silicones and copolymers thereof or mixtures of these liquids.

In an advantageous embodiment of the magnetorheological materials according to the invention, inorganic particles such as SiO ₂ , TiO ₂ , iron oxides, layered silicates or organic additives and combinations thereof may be added to the suspension to reduce sedimentation.

A further advantageous embodiment of the magnetorheological materials according to the invention provides that the inorganic particles are at least partially organically modified.

Further particular embodiments of the magnetorheological materials envisage that the suspension contains particulate additives such as graphite, perfluoroethylene or molybdenum compounds such as molybdenum disulfite and combinations thereof in order to reduce abrasion phenomena. It is also possible that the suspension for use in the surface treatment of workpieces may contain special abrasive and / or chemical caustic additives, e.g. Corundum, cerium oxides, silicon carbide or diamond contains.

Overall, it has proven to be advantageous if 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, and the proportion of nonmagnetizable additives is between 0.001 and 20% by mass, preferably between 0.01 and 15 mass% (based on the magnetizable solids), is.

The invention further relates to the use of the materials described in more detail above.

An advantageous embodiment of the magnetorheological materials according to the invention provides for their use in adaptive shock and vibration dampers, controllable brakes, clutches and in sports or training equipment. Special materials can also be used for the surface treatment of workpieces.

Finally, 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, for simulating viscous, elastic and / or viscoelastic properties or the consistency distribution of an object, in particular for training purposes. and / or research and / or for medical applications.

An example of the production of the magnetorheological materials according to the invention, in particular the production of a magnetorheological fluid (MRF), is described below.

example 1

• Polyalphaolefin mit einer Dichte von 0,83 g/cm³ bei 15° C und einer kinematischen Viskosität von 48,5 mm/s² bei 40° C,
• unregelmäßig geformte Eisenpartikel (p) mit einer mittleren Teilchengröße von 41 µm, gemessen in Isopropanol mittels Laserbeugung mit Hilfe eines Mastersizers S der Firma Malvern Instruments,
• kugelförmige Eisenpartikel (q) mit einer mittleren Teilchengröße von 4,7 µm, gemessen in Isopropanol mittels Laserbeugung mit Hilfe eines Mastersizers S der Firma Malvern Instruments.

Used educts:

• Polyalphaolefin having a density of 0.83 g / cm ³ at 15 ° C and a kinematic viscosity of 48.5 mm / s ² at 40 ° C,
• irregularly shaped iron particles (p) with a mean particle size of 41 μm, measured in isopropanol by means of laser diffraction with the aid of a Mastersizer S from Malvern Instruments,
• spherical iron particles (q) with an average particle size of 4.7 μm, measured in isopropanol by means of laser diffraction with the aid of a Mastersizer S from Malvern Instruments.

• 43,16 g Polyalphaolefin werden in einem Stahlbehälter von 250 ml Inhalt auf 0,001 g Einwaagegenauigkeit eingewogen. Unter ständigem Rühren werden dann zuerst 146,96 g des unregelmäßig geformten Eisenpulvers (p) langsam eingestreut, anschließend erfolgt in gleicher Weise die Zugabe von 73,45 g der kugelförmigen Eisenpartikel (q). Die Dispergierung des Eisenpulvers im Öl erfolgt mit Hilfe eines Dispermat der Firma VMA-Getzmann GmbH mittels einer Dissolverscheibe mit einem Durchmesser von 30 mm, wobei ein Abstand zwischen der Dissolverscheibe und dem Behälterboden von 1 mm besteht. Die Behandlungsdauer beträgt 3 min bei ca. 6500 Umdrehungen pro Minute. Die Rührgeschwindigkeit ist der Viskosität des Ansatzes dann optimal angepasst, wenn die Drehscheibe unter Bildung einer Trombe von oben deutlich sichtbar ist.

80 ml of a suspension containing 35.00% by volume of iron powder, of which 23.33% by volume of irregularly shaped particles (p) and 11.66% by volume of spherical particles (q) in polyalphaolefin are prepared as follows:

• 43.16 g of polyalphaolefin are weighed in a steel container of 250 ml content to 0.001 g weighing accuracy. With constant stirring, 146.96 g of the irregularly shaped iron powder (p) are then slowly interspersed first, followed by the addition of 73.45 g of the spherical iron particles (q) in the same manner. The dispersion of the iron powder in the oil takes place with the aid of a Dispermat from VMA-Getzmann GmbH by means of a dissolver disk with a diameter of 30 mm, whereby there is a distance between the dissolver disk and the container bottom of 1 mm. The treatment time is 3 min at about 6500 revolutions per minute. The stirring speed is the viscosity of the approach then optimally adapted when the turntable is clearly visible to form a Trombe from above.

• MRF 1 anstelle der Partikelmischung (p) + (q), 35 Vol.-% der reinen kugelförmigen Eisenpartikel (q) in Polyalphaolefin und
• MRF 2 anstelle der Partikelmischung (p) + (q), 35 Vol.-% der reinen unregelmäßig geformten Eisenpartikel (p) in Polyalphaolefin.

The thus prepared magnetorheological fluid MRF 3 with the iron particle mixture (p) + (q) was then in terms of their properties characterized and compared with two other magnetorheological fluids prepared accordingly. It contained

• MRF 1 instead of the particle mixture (p) + (q), 35 vol.% Of the pure spherical iron particles (q) in polyalphaolefin and
• MRF 2 instead of the particle mixture (p) + (q), 35 vol.% Of the pure irregularly shaped iron particles (p) in polyalphaolefin.

The rheological and magnetorheological measurements were carried out in a Searle Systems MCR 300 from Paar Physica. The rheological properties were carried out without applied magnetic field in a measuring system with coaxial cylinder geometry, while the measurements were carried out in the magnetic field in a plate-plate arrangement perpendicular to the field lines.

The results of this study are summarized in Figures 1 to 3.

illustration 1 shows the shear stress τ _O as a function of the shear rate D for the inventive MRF 3 and the two comparative approaches MRF 1 and MRF 2 without applied magnetic field. It can be seen that the flow curve of the MRF 3 according to the invention is below that of MRF 1 and MRF 2 at all shear rates outside the quasistatic range (D> 1 s ^-1 ). This means that the MRF 3 according to the invention has the lowest dynamic basic viscosity η _O in magnetic-field-free space at a fixed shear rate D in comparison with the other approaches (compare equation (1) of the description).

Figure 2 shows the shear stress τ _B as a function of the magnetic flux density B for the inventive MRF 3 and the two comparative approaches MRF 1 and MRF 2 in the quasistatic range (D = 1 s ^-1 ). It can be seen that the MRF 3 according to the invention has higher shear stresses τ _{B over} the entire measuring range than the comparative batch MRF 2, which contains only irregularly shaped iron particles (p). Furthermore, it can be seen that the shear stress τ _{B of} the MRF 3 according to the invention extends congruently with that of MRF 1 up to a shear rate of D = 400 s ^-1 , but then still exceeds its values. This means that the MRF 3 according to the invention has identical or higher shear stresses τ _B in the magnetic field than MRF 1, which contains only small spherical iron particles (q).

In summary, it can thus be said that the MRF 3 according to the invention has the highest shear stresses τ _B overall in the magnetic field in comparison with the lugs MRF 1 and MRF 2 without particle mixtures.

Figure 3 shows the switching factor w _D as a function of the magnetic flux density B for the inventive MRF 3 and the two comparative approaches MRF 1 and MRF 2 at a constant shear rate of D = 100 s ^-1 . It can be seen that the switching factor W _{D of} the MRF 3 according to the invention exceeds those of the MRF 1 and MRF 2 approaches over the entire measuring range. For example, at a flux density of B = 500 mT, an increase in the switching factor W _{D can be} determined by a factor of 3 in relation to MRF 1 or by a factor of 5 in relation to MRF 2.

Overall, it should be noted that the inventive MRF 3 with the particle mixture of large irregular formed iron particles and small spherical iron particles both the lowest dynamic basis viscosity η _o in field-free space and the largest switching factor w _D in the magnetic field in relation to the comparison approaches MRF 1 and MRF 2 has.

Claims (19)

1. Magnetorheological materials comprising at least one non-magnetisable carrier medium and soft magnetic magnetisable particles contained therein,
   characterised in that
   at least two magnetisable particle fractions p and q are contained as particles,
   p being formed from non-spherical particles and q from spherical particles, the average particle size of p being greater than q.
2. Magnetorheological materials according to claim 1, characterised in that the average particle size of the fraction p has preferably at least twice the value of the average particle size of the fraction q.
3. Magnetorheological materials according to one of the preceding claims, characterised in that the average particle sizes of the fractions p and q are between 0.01 µm and 1000 µm, preferably between 0.1 µm and 100 µm.
4. Magnetorheological materials according to one or more of the preceding claims, characterised in that the volume ratio of the fractions p and q is between 1 : 99 and 99 : 1, preferably between 10 : 90 and 90 : 10.
5. Magnetorheological materials according to one of the claims 1 - 4, characterised in that the magnetisable particles are selected from soft magnetic metallic materials, in particular from iron, cobalt, nickel (also in non-pure form) and alloys thereof, such as iron-cobalt, iron-nickel; magnetic steel; iron-silicon and/or mixtures thereof.
6. Magnetorheological materials according to one of the claims 1 - 4, characterised in that the magnetisable particles are selected from soft magnetic oxide-ceramic materials, in particular from cubic ferrites, perovskites and garnets of the general formula
   
           MO · Fe₂O₃
   
   with one or more metals from the group M = Mn, Fe, Co, Ni, Cu, Zn, Ti, Cd or Mg and/or mixtures thereof.
7. Magnetorheological materials according to one of the claims 1 - 4, characterised in that the magnetisable particles are selected from mixed ferrites, such as MnZn-, NiZn-, NiCo-, NiCuCo-, NiMg-, CuMg-ferrites and/or mixtures thereof.
8. Magnetorheological materials according to one of the claims 1 - 4, characterised in that the magnetisable particles are selected from iron carbide or iron nitride and also alloys of vanadium, tungsten, copper and manganese and/or mixtures thereof.
9. Magnetorheological materials according to one of the claims 1 - 4, characterised in that the magnetisable particles are present in pure and/or impure form.
10. Magnetorheological materials according to one or more of the preceding claims, characterised in that the carrier medium is selected from

    - carrier fluids such as water, mineral oils, synthetic oils, such as polyalphaolefins, hydrocarbons, silicone oils, esters, polyethers, fluorinated polyethers, polyglycols, fluorinated hydrocarbons, halogenated hydrocarbons, fluorinated silicones, organically modified silicones and also copolymers thereof or fluid mixtures thereof,

    - fats or gels or

    - from elastomers.
11. Magnetorheological materials according to one or more of the preceding claims, characterised in that they contain dispersion agents, antioxidants, defoamers and/or anti-abrasion agents as additives.
12. Magnetorheological materials according to one or more of the preceding claims, characterised in that they contain inorganic particles such as SiO₂, TiO₂, iron oxides, phyllosilicates or organic supplements and also combinations thereof as further additives in order to reduce sedimentation.
13. Magnetorheological materials according to claim 12, characterised in that the inorganic particles are at least in part organically modified.
14. Magnetorheological materials according to one or more of the preceding claims, characterised in that they contain particulate supplements, such as graphite, perfluoroethylene or molybdenum compounds such as molybdenum disulphite and combinations thereof as further additives in order to reduce abrasion phenomena.
15. Magnetorheological materials according to one or more of the preceding claims, characterised in that they contain abrasively acting and/or chemically etching supplements, such as e.g. corundum, cerium oxides, silicon carbide and/or diamond as further additives for use in the surface treatment of workpieces.
16. Magnetorheological materials according to one or more of the preceding claims, characterised in that

    - the proportion of magnetisable particles is between 10 and 70% by volume, preferably between 20 and 60% by volume,

    - the proportion of the carrier medium is between 20 and 90% by volume, preferably between 30 and 80% by volume,

    - the proportion of additives is between 0.001 and 20% by mass, preferably between 0.01 and 15% by mass (relative to the magnetisable solids).
17. Use of the magnetorheological materials according to one or more of the claims 1 to 16 in adaptive shock and vibration dampers, controllable brakes, clutches and also in sports or training appliances.
18. Use of the magnetorheological materials according to one or more of the claims 1 to 16 for surface machining of workpieces.
19. Use of the magnetorheological materials according to one or more of the claims 1 to 16 in order to generate and/or display haptic information, such as characters, computer-simulated objects, sensor signals or images; for simulation of viscous, elastic and/or visco-elastic properties or the consistency distribution of an object, in particular for training and/or research purposes and/or for medical applications.

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