EP1053552B1 - Method for the production of a magnetic film - Google Patents

Description

The invention relates to a method for producing a hard magnetic film based on polymer, in particular for Use in electric motors or for sensor applications.

Due to the advancing miniaturization in electronics are increasingly becoming particularly small or flat electric motors needed. For these engines, the usual hard magnetic components in the stator or in the Containing the rotor, naturally also become particularly flat magnetic components second hand. There is also a growing need on particularly flat permanent magnets for miniature relays and sensors that work according to magnetic principles (e.g. speed or position encoder). They should also Magnets should be as flexible as possible in order to Shape into the desired shape after magnetization to let and a break during processing or in Exclude operation.

EP-A-0274034 consists of synthetic resin-bonded magnets made of a polymer and a hard magnetic powder known. The powder used there is an Nd alloy and has a particle size of 80 µm and less. The magnets can be made relatively thin, but are not foils.

According to the for the manufacture of permanent magnets from powdered metallic or non-metallic magnetic Materials commonly used molding processes However, it is difficult to impossible to use flat structures a small thickness of, for example, 100 µm and a high energy density to manufacture in an economical manner.

The object of the present invention is therefore an economical Process for the production of a flexible hard magnetic To provide material of small thickness.

According to the invention, this object is achieved by a manufacturing process solved according to claim 1.

It was found that by using the casting technique it is possible to use carrier-free hard magnetic foils made of a polymer matrix and a hard magnetic powder distributed therein manufacture. The term "strapless" means here that the finished foils are not - as for example as Magnetic tapes or "floppy disks" known films - from one non-magnetic carrier and a magnetizable one or are built on both sides, but from one only continuously magnetic or magnetizable Layer. The hard magnetic powder is useful an average particle size of less than 100 μm, preferably less than 20 µm.

The magnetic foils according to the invention are produced by that (i) a powder made of a hard magnetic material in a solution or dispersion of a polymer material dispersed in a volatile solvent, (ii) so dispersion obtained as a film of defined thickness on a rotating Casting belt cast, (iii) the solvent evaporates and (iv) peeling off the film thus formed from the casting belt becomes. Magnetization of the foil can occur after evaporation the solvent or at a later time (e.g. after assembly), due to the integration the magnetic particle into the polymer matrix is isotropic Magnetic sheet is obtained.

In a preferred embodiment of the invention The orientation of the hard magnetic powder particles takes place in the process by an external magnetic field between the casting process and peeling off the film.

Orientation before solidification is particularly preferred of the cast film.

Particles made of an anisotropic material can form here align in the external magnetic field so that an anisotropic Magnetic sheet is obtained.

The magnetization and optionally alignment can preferably by means of a pulsed magnetic field become. This allows high field strengths with electromagnets can be achieved with low energy consumption. Especially easily orientable hard magnetic powder particles can also oriented in the air gap of a suitable permanent magnet yoke become.

A preferred volatile solvent for soluble polyvinylidene fluoride (Copolymer) is acetone.

The films according to the invention advantageously have a thickness of 50 to 2000 microns, preferably 100 to 500 microns.

The volume fraction of the hard magnetic powder in the invention Magnetic film can be adjusted as required. It is preferably at least 50%, particularly preferably at least 60%. It is possible to keep the polymer content as low to keep the polymer practically just the gaps one fills approximately dense packing of the powder particles.

The hard magnetic powder contained in the invention Foils preferably one or more rare earth alloy (s). However, it is also within the scope of the invention others hard magnetic materials such as Al-Ni-Co or Use Cr-Fe-Co alloys or ferrites.

Rare earth alloys, which can be described by the general formulas SECo ₅ , (SE) ₂ (CO, Fe, Cu, Zr) ₁₇ or (SE) ₂ Fe ₁₄ B, are particularly preferred. Herein SE means one or more elements from the group consisting of yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium or a mixture of several of these elements. The compositions Sm ₂ (Co, Fe, Cu, Zr) ₁₇ and (Pr, Nd, Dy) Fe ₁₄ B are very particularly preferred. Alloys of these types are, for example, under the brands VACOMAX® and VACO-DYM® from Vacuumschmelze GmbH or available under the MAGNEQUENCH® brand from Magnequench Inc.

The polymer matrix can basically be made of any volatile Solvent-soluble or dispersible polymers. But it is also possible to use polymers that from low-viscosity monomers or oligomers to suitable ones Way are available in thin layers. In these cases can optionally be used during manufacture be dispensed with from solvents. Preferably be soluble thermoplastic materials used, in particular soluble polyvinylidene fluoride. However, it is also possible non-thermoplastic materials such as Use one-component polyurethane dispersions.

The hard magnetic powder particles can be random (isotropic) arranged or, if they have an inherent anisotropy, if necessary be aligned. Are preferred aligned parallel or perpendicular to the film surface.

The magnetic remanence of the magnetic foils according to the invention is determined by the type and packing density of the hard magnetic Powder particles determined and is preferably 0.2 to 0.8 Tesla.

The continuous casting belt is preferably made of matt Stainless steel.

A device for producing the magnetic film according to the invention is shown in Figure 1. The real one Casting device comprises a temperature-controllable storage container 1 with stirring device for the casting solution or dispersion, a controllable feed pump 2, a filter device 3 for Separation of agglomerates and the caster 4. The casting solution or dispersion is poured onto an endless casting belt 5, which revolves over rollers 6, 7 and of heating elements 8 indirectly is heated. The casting belt is over one of the rollers, which are provided with a speed-controlled drive 16 is driven. A cooling device 9 cools if necessary the magnetic film 11 before pulling it off the casting belt a removal device 10. To remove remaining The magnetic film can possibly contain solvent residues before winding on a winding mandrel 13 of a drying undergo in a drying section 12, wherein the film is advantageously supported by a carrier web 14 becomes. The carrier web can optionally also be used as a release film serve and wound up together with the magnetic sheet be (not shown). An electromagnet is advantageous for magnetizing the film or permanent magnet yoke 17 attached at a short distance above the casting belt. The entire pouring and drying device is advantageously surrounded by a housing 15, which reduces heat loss and in combination with a suction and filter device prevents the production rooms from being exposed to solvent vapors. Gear pumps or peristaltic pumps, for example, can be used as feed pumps 2 become.

The pourer 4 can be used both as a pressure pourer, in which the casting solution is supplied by the feed pump 2 is fed directly to the casting gap with increased pressure, and also as an open one Scraper, which works only with hydrostatic pressure, be trained. In both In certain cases, the pressure or the filling level kept constant. The film thickness is essentially the width of the The casting gap between the pourer 4 and the casting belt 5 is determined.

The heating devices 8 preferably supply the heat as radiant heat. to Support of the drying process and the removal of solvent vapors advantageously heated air supplied. It is also possible to use the heat, for example transferred to the casting belt via heated rollers or by direct passage of electricity or to be heated inductively. Finally, the cast film can also pass through Microwave energy can be heated.

To increase the strength of the magnetic film before it is removed from the casting belt, it is used advantageously cooled. For this purpose, the casting device can be equipped with one or more Means for cooling 9 may be provided. These can, for example, be cooled Drums or rollers, over which the casting belt 5 is guided, can be formed that the cooling takes place indirectly. On the other hand, there are also facilities for direct The film can be cooled, for example in the form of suitably arranged nozzles for inflation of cold air or other cooling media. Of course there is also a combination both measures possible.

Since the magnetic film according to the invention has a relatively low tensile strength, the Removal device 10 suitably designed so that no excessive pull on the Foil is exerted, which leads to undesirable stretching or even tearing the slide could lead. The removal device advantageously consists of a roller or a pair of rollers, which exerts a controlled tensile stress on the film and preferably is arranged so that a take-off angle of 15 ° to 45 ° results. Instead of the winding device 13, a cutting and stacking device can alternatively also be used be provided to deposit the film as a stack of sheets.

The following examples illustrate the preparation and the properties of the inventive Magnetic foil.

example 1

8.7 parts of soluble polyvinylidene fluoride copolymer (SOLEF® 21508/1001, manufacturer: Solvay Kunststoffe), 1.4 parts of wetting agent (Disperbyk® 180, manufacturer: Byk Chemie) and 89.9 parts of Sm ₂ (Co, Cu , Fe, Zr) ₁₇ magnetic powder (VACOMAX® 240, manufacturer: Vacuumschmelze GmbH) dissolved or dispersed. The magnetic powder was ground in a jet mill under nitrogen and sieved through an 80 µm sieve to remove oversize particles. According to sieve analysis, 60% by mass were particles <25 µm and 1.8% by mass were particles> 40 µm. The average particle size was determined to be 10 µm. The total solids content of the casting solution thus obtained was 78.3% by mass, the volume fraction of the magnetic powder after drying was approx. 63%. With the casting device described above, a film with a thickness of 120-140 μm was produced. The film thus obtained had a density of 2.9-3.3 g / cm ³ . By varying the casting gap width and the magnetic powder content, films with a thickness of 220-230 µm and a thickness of 230-235 µm were further produced with densities of 3.6-3.7 g / cm ³ and 4.0-4.1 g / cm ³ , respectively , The films had a remanence of 0.2-0.29 T with a coercive field strength of 10.6 kOe. The demagnetization curves of the exemplary films are shown in Figure 2.

Example 2

The procedure was as described in Example 1, but an NdFeB magnet powder was used instead of the Sm ₂ (Co, Cu, Fe, Zr) ₁₇ magnet powder. The magnetic film thus obtained had a thickness of 315 μm, a density of 4.11 g / cm ³ and a remanence of 0.35 T with a coercive field strength of 11.4 kOe. The demagnetization curve of this film is shown in Figure 3.

Example 3

The procedure was as in Example 2, but an anisotropic NdFeB magnetic powder of the type MAGNEQUENCH® MQP-T was used and the film was exposed to a magnetic field of 2.4-2.9 kOe parallel to the surface after a drying time of 0.5 min, so that could align the powder particles in the not yet solidified film. The finished anisotropic film had a thickness of 333 μm, a density of 4.0 g / cm ³ , a remanence of 0.505 T parallel to the surface and a coercive field strength of 11.5 kOe. The demagnetization curve of this film is shown in Figure 4.

Example 4

The procedure was analogous to Example 1 (magnetic powder: VACOMAX® 240), but the film was exposed after 0.5 minutes of drying time to align the anisotropic powder particles with pulsed external magnetic fields parallel to the surface. The field strength was varied between 15 kOe (12 kA / cm) and 45 kOe (36 kA / cm). The demagnetization curves of the anisotropic magnetic foils thus obtained are shown in Figure 5 together with that of a corresponding isotropic foil. It can be seen that the remanence increases parallel to the surface from 0.26 T for the isotropic film to 0.46 T after alignment at 45 kOe. The corresponding values after alignment at 15 kOe, 20 kOe and 30 kOe are 0.37 T, 0.41 T and 0.43 T. By orienting the powder particles with magnetic field pulses parallel to the film surface, the degree of orientation f _o becomes 0.5 the isotropic magnetic foil improved to 0.95. The coercive field strength decreases due to the improved orientation from 11.5 kOe for the isotropic magnetic foil to approx. 9 kOe for the anisotropic magnetic foil.

Claims (15)

1. Method for the production of a hard-magnetic film consisting of a polymer matrix and a hard-magnetic powder having a mean particle size of less than 100 µm, preferably less than 20 µm, distributed therein, characterised in that it includes at least the steps (i) producing a dispersion of a hard-magnetic powder having a mean particle size of less than 100 µm in a solution or dispersion of a polymer material in a volatile solvent, (ii) casting the dispersion of the hard-magnetic powder in the form of a film having a defined thickness on to a continuous casting belt, (iii) evaporating the solvent and (iv) removing the film formed in this manner from the casting belt.
2. Method according to claim 1, characterised in that the particles of the hard-magnetic powder are magnetised and oriented by an external magnetic field between the casting process (ii) and the removal of the film (iv).
3. Method according to claim 2, characterised in that the magnetisation and orientation are effected before the cast film has solidified.
4. Method according to claim 2 or claim 3, characterised in that the external magnetic field is pulsed.
5. Method according to claim 2 or claim 3, characterised in that the external magnetic field is generated by a permanent-magnet yoke.
6. Method according to one of claims 1 to 5, characterised in that a rare-earth alloy is used as the hard-magnetic powder.
7. Method according to one of claims 1 to 6, characterised in that an alloy of general formula SECo₅, (SE)₂(Co, Fe, Cu, Zr)₁₇ or (SE)₂Fe₁₄B, where SE stands for one or more of the elements Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, is used as the rare-earth alloy.
8. Method according to one of claims 1 to 7, characterised in that soluble polyvinylidene fluoride is used as the polymer material.
9. Method according to claim 8, characterised in that acetone is used as the volatile solvent.
10. Method according to one of the preceding claims, characterised by a film thickness of 50 to 2000 µm.
11. Method according to claim 10, characterised by a film thickness of 100 to 500 µm.
12. Method according to one of the preceding claims, characterised in that the volume content of the hard-magnetic powder is at least 50 %.
13. Method according to one of the preceding claims, characterised in that the hard-magnetic powder particles are aligned parallel or perpendicularly to the film surface.
14. Method according to one of the preceding claims, characterised in that the film displays magnetic remanence of 0.2 to 0.8 T.
15. Method according to one of the preceding claims, characterised in that the mean particle size is less than 20 µm.

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