WO2015051986A1 - Method for producing a permanent magnet and permanent magnet and electric machine having such a permanent magnet - Google Patents

Abstract

The invention relates to a method for producing a permanent magnet, wherein a powder of a magnetic material is produced and the powder of the magnetic material is processed into a permanent magnet. According to the invention, the producing of the powder of the magnetic material comprises the steps: grinding a suspension of particles of the magnetic material of a first average particle size in liquid nitrogen, obtaining particles of the magnetic material of a second average particle size, which is smaller than the first average particle size, and separating a suspension of the ground magnetic material in liquid nitrogen, wherein particles having a particle size smaller than a predetermined upper particle size are separated.

Description

 description

Process for producing a permanent magnet and permanent magnet and electric

 Machine with such a

The invention relates to a method for producing a permanent magnet, in particular a method for producing a powder of the magnetic starting material for magnet production. The invention further relates to a method produced by the method

Permanent magnet and an electric machine, comprising at least one such permanent magnet.

Demand for high-performance permanent magnets, which are used, for example, in electric machines, is constantly increasing. In particular, permanent magnets are used in electric motors for the traction of motor vehicles, which are gaining in interest in the wake of increasing electric mobility. As magnetic or magnetizable alloys with high coercive forces, rare earth alloys of the type SE-TM-B or SE-TM are predominantly used, SE being a rare-earth element, TM being an iron-group transition metal (Fe, Co, Ni) and B is boron ,

Typical manufacturing processes include the process steps of pulverizing the magnetic starting material, pressing / solidifying the powder into a green part with or without external magnetic field to form a desired shape, sintering the green part for further densification (high temperature treatment), optionally tempering (heat or

Low temperature treatment) for stress relief and microstructure stabilization in the

Magnetic body and magnetization in a magnetic field. Sometimes different process stages are combined with each other and the order is varied.

The pulverization of the magnetic starting material usually comprises several stages. For example, a melt of the alloy is poured into ingots (so-called ingots), mechanically broken and subjected to one or more milling stages. It is also known to process the alloy melt with the method of strip casting to a rapidly cooled band with a polycrystalline structure, which is subsequently further broken and ground. Furthermore, the method of the Hydrogen embrittlement (English: Hydrogen Decrepitation, HD process) is known, in which the material is pressurized with hydrogen, so that it interstitially penetrates into the material and leads to the formation of microcracks in the material during its subsequent release. As a result, the subsequent grinding time can be reduced. (A different method - the so-called HDDR process (for Hydrogenation

Disproportionation desorption recombination) - uses the temporary formation of metal hydrides and their subsequent desorption to the structural phases of the

Magnetic material and improve the magnetic properties.

It is also known to add a lubricant to the rare earth magnet material during the grinding process in order to achieve a stronger orientation of the magnetic domains in the subsequent magnetization step. Thus, EP 1 760 734 A1, a

To grind rare earth magnetomalace in the presence of a lubricant whose particle size is larger than that of the magnetic material. To produce the desired lubricant particle size (e.g., stearate), it is solidified by liquid nitrogen and ground in the frozen state. Particle sizes of the obtained magnetic powder of 2.5 to 10 μm are disclosed.

From DE 1 1 2010 004 576 T5 is a method for producing a sintered

Seltenerdmagneten known in the first of a melt of the

Rare earth magnet material, for example Nd ₂ Fei ₄ B, in a single roll or

A two-roll process is used to produce a rapidly cooled ("quenched") polycrystalline ribbon which, through the cooling process, forms an Nd-rich phase on either or both sides having a lower melting point than Nd ₂ Fe-i ₄ B. The ribbon is pulverized and sintered at low temperature This is to prevent coarsening of the crystallites contained in the polycrystalline phase and having an average particle size of 10 to 200 nm at a temperature corresponding to the melting temperature of the Nd-rich phase.

EP 0 416 595 A2 describes a production method for rare earth magnets in which a solidified melt of the magnetic material is first broken and then ground with the supply of liquid nitrogen in a disc or impact mill, particle sizes of at most 400 μm being formed. This is followed by a hydrogenation of the material and a new grinding in a liquid hydrocarbon to obtain particle sizes of at most 40 μηη, typically 2.7 to 3.5 μηη. The powder thus obtained becomes the purpose its passivation is controlled oxidized, shaped, oriented in a magnetic field, pressed and sintered.

From US Pat. No. 5,609,695 it is known to pulverize a magnetic alloy by means of the abovementioned HD process to particle sizes of at most 150 μm and then to nitrogenize the material in a nitrogen gas atmosphere, for example to convert Sm ₂ Fe ₇ into Sm ₂ Fe ₇ N _x . Subsequently, the nitrogenated material is ground in liquid nitrogen in a ball mill, wherein particle sizes of 1 to 2 μηη are obtained. It is noted that at the low temperature of liquid nitrogen, the required meal can be reduced due to material embrittlement.

Also, US 5,382,303 describes a method for producing a magnet in which a magnetic material of the Sm-Co type is melted, cast and broken, and

then subjected to a Grobmahlprozess in liquid nitrogen to obtain particle sizes of at most 600 μηη. Finally, a fine grinding step is carried out, in which the powder is milled together with a liquid hydrocarbon in a friction or ball mill up to a maximum particle size of 40 μm, in particular from 3.8 to 4.6 μm. After removal of the hydrocarbon and passivation of the powder, it is compressed in a magnetic field and the green part thus obtained is sintered.

In the pulverization of the starting material as small as possible particle sizes in the finished magnet are basically desirable, ideally in the dimension of magnetic domain, so that the compact magnet is ideally composed of Eindomänenteilenteilchen, whereby particularly high magnetic field strengths are achieved. With the current grinding techniques under inert gas (ball mill, jet mill) particle sizes of 2 to 5 μηη are achieved.

For example, particle sizes of 3 to 5 μm can be achieved with jet mills. By extending the grinding time agglomerates are formed, which in the following

Magnetic manufacture cold weld together and lead to unwanted grain growth. On the other hand, the use of additives during milling, for example of lubricants, may cause contamination of the magnet, which may adversely affect its mechanical and magnetic properties.

The invention is based on the object, a method for producing a

To provide permanent magnet, wherein a magnet having improved magnetic properties, in particular higher coercive force, and an increased mechanical strength is obtained. This object is achieved by a manufacturing method, a permanent magnet and an electric machine having the features of the independent claims.

In the method according to the invention for producing a permanent magnet, a powder of a magnetic material is produced and processed into a permanent magnet. Processing the powder produced into a permanent magnet typically involves molding, densification, solidification, and magnetization. According to the invention, the preparation of the powder of the magnetic material comprises the steps:

 Grinding a suspension of particles of the magnetic material of a first mean particle size in liquid nitrogen to obtain particles of the magnetic material of a second average particle size smaller than the first mean particle size, and

 - Separating a suspension of the ground magnetic material in liquid nitrogen, wherein particles are separated with a particle size below a predetermined upper particle size.

The invention thus comprises a combination of cryogenic grinding (also referred to below as cryogenic grinding) and cryogenic separation (also referred to below as cryogenic separation) of the magnetic material. In both steps, the magnetic material is in the form of a suspension in liquid nitrogen. The cryogenic grinding in liquid nitrogen avoids heating of the millbase due to the low temperature of the liquid nitrogen (77 K). As a result, agglomeration and welding of the particles is prevented and thus enables the production of particularly small particle sizes. The presence of liquid nitrogen thus allows an extension of the grinding time to achieve the desired extremely small particle size. The subsequent step of separating (classifying) also takes place in liquid nitrogen. Thus, also in this step one

Agglomeration of the particles counteracted. Furthermore, the separation allows obtaining a smaller particle size distribution, wherein particles with particle sizes above the

predetermined upper particle size excluded and removed. Due to the small particle size distribution, a denser and more uniform spherical packing is produced

Achieved magnets. As a result, a higher mechanical strength and a higher coercive force of the magnet is obtained.

Separating (also referred to as classifying) is understood to mean a process in which a particulate starting material having a certain particle size distribution (Usually, according to a Gaussian distribution) a fraction is obtained, which has a smaller (narrower) particle size distribution than the starting material. In other words, a particle fraction is separated and precipitated at the upper and / or lower end of the original particle size distribution. In the context of the present invention, the separation comprises at least one separation of a particle fraction with particle sizes above the predetermined maximum particle size so that the target fraction contains exclusively particles whose particle sizes are smaller than or equal to the maximum particle size. In this case, the term "particle size" refers to the so-called equivalent diameter, which takes into account the fact that the particles generally do not have an exactly spherical shape, for example, a particle which, regardless of its geometric shape, just has a square hole in a sieve with an edge length of 1 μηη can happen, an equivalent diameter ("particle size") of 1 μηη on.

In principle, in the step of cryogenic separation, any lower or upper

Particle sizes are separated, for example, particle sizes <4 μηη. For the production of permanent magnets, however, smaller particle sizes, especially in the nanometer range, are desirable in order to obtain better magnetic properties. In a

According to an embodiment of the invention, in the step of cryo-separation, particles are separated which have a predetermined maximum particle size of <500 nm, in particular of <400 nm, preferably of <350 nm and more preferably of <300 nm. This is, for example, due to the use of sieves with corresponding mesh sizes of 500 nm, 400 nm, 350 nm and 300 nm, respectively. For example, particles separated with a sieve of 350 nm mesh become 100 mass%

Particle size of <350 nm. Due to the small particle size of at most 500 nm, the particles have sizes in the range of magnetic domains, that is to say they are so-called single-domain particles. The limitation of particle sizes on the size

magnetic domains leads to permanent magnets with a particularly high coercive force.

According to a further embodiment of the invention, in addition to the upper particle size, the lower particle size of the separated particles is limited in the step of cryomilling, so that, for example, particles with a particle size in the range of> 2 to 4 μm are separated. In a preferred embodiment for permanent magnets, particles having a particle size in the range of> 100 nm to <500 nm, in particular in the range of> 100 nm to <400 nm, preferably in the range of> 150 nm to <350 nm, and in the separation step particularly preferably separated in the range of> 200 nm to <300 nm. The low

Grain size distribution also leads to a high packing density and especially regular packing of the particles in the finished permanent magnet, whereby particularly high mechanical strength and high coercive field strengths are achieved. The representation of particle fractions with defined upper and lower particle sizes can be done in a simple manner by successive use of two (or more) sieves. Around

For example, to separate a particle fraction whose particle size is 100% by mass in the range of> 200 nm to <300 nm, first carried out a screening with a sieve with a mesh size of 300 nm, with particles> 300 nm retained on the screen and be separated. Subsequently, the fraction passed through the first sieve with a

Particle size of <300 nm through a second sieve with a mesh size of 200 nm sieved, with particles <200 nm pass through the second sieve. The one from this sieve

retained fraction has only particle sizes in the range of> 200 nm to <300 nm.

According to a preferred embodiment of the invention, the particles retained in the separation step with a particle size above the predetermined upper and / or lower particle size are returned to the preceding milling step. As a result, the valuable magnetic material is virtually lossless processed and a high

Material yield ensured.

The particle sizes obtained in the step of cryomilling are freely adjustable via the process parameters, in particular by the selected milling time. It is preferably provided that at least 50% by mass, in particular at least 70% by mass and particularly preferably at least 80% by mass, of the particles obtained by the milling have a particle size of at most 500 nm, in particular of at most 400 nm, preferably of at most 350 nm and preferably at most 300 nm. This has the advantage that a large part of the material used in the separation step already has the desired maximum particle size and passes through the subsequent separation step. The desired

Particle sizes can be adjusted but also other grinding parameters.

In a particularly preferred embodiment of the invention, it is provided that the steps of grinding and separating are linked together in such a way that the suspension of the magnetic material subjected to the grinding step is fed to the separation step. In other words, the suspension of ground magnetic material and liquid nitrogen obtained from the milling step is passed without further material preparation into a device used for the separation step. By giving up one Changing or removing the liquid medium (liquid nitrogen) shortens the process time and reduces material and energy costs.

Preferably, the cryogenic grinding is carried out in a ball mill. The balls of the mill provide a high surface through which efficient heat removal occurs, thereby further suppressing the agglomeration of the particles. However, other grinding devices in which suspensions can be processed are also usable within the scope of the invention.

The separation preferably takes place in a vibrating screen device.

 Schwingsiebvorrichtungen comprise at least one horizontally arranged sieve, which is set on a swing axis in vibration. Vibrating screen devices are particularly well suited for the processing of suspensions.

Preferably, the separation comprises the use of several serially connected screening stages with screens which have smaller mesh sizes. In this way, particles with comparatively large particle sizes are removed in the upstream screening stages, and smaller particle diameters are sorted out by the downstream screening stages. The use of several screening stages of different mesh size, especially in

continuous process, allows a shortening of the sieving time. The

Connecting several screening stages in series is particularly easy in one

Realize vibrating screen device.

The invention further relates to a produced by the method according to the invention

Permanent magnets. This is characterized by the structure of particles of the magnetic material having a particle size of at most 500 nm, in particular of at most 400 nm, preferably of at most 300 nm, and a defined particle size distribution. The permanent magnet according to the invention therefore has a high packing density, high mechanical stability and high coercive force.

Finally, the present invention relates to an electrical machine comprising at least one permanent magnet according to the invention, in particular a plurality of such. In a particular embodiment, the electric machine is designed as an electric motor, in which the permanent magnets are typically part of the rotor, for example, embedded in a laminated core of the rotor or are mounted on the surface thereof. Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

The invention is described below in embodiments with reference to the associated

Drawings explained. Show it:

Figure 1 Process for preparing a powder of a magnetic material according to a first embodiment of the invention;

Figure 2 illustrates the HD process;

FIG. 3 process step of cryogenic milling in a ball mill;

FIG. 4 shows a process step of cryogenic separation in a vibrating screen device;

Figure 5 Process for preparing a powder of a magnetic material according to a second embodiment of the invention and

Figure 6 further process flow for the production of a permanent magnet.

FIG. 1 shows a flow chart for visualizing a method sequence for producing a powder of a magnetic material according to a first embodiment of the invention.

Starting material of the process is a magnetic material (hereinafter also

Called magnetic material) having at least one element of rare earths (also called rare earths) and at least one element of the iron group (Fe, Co, Ni). In particular, it is a rare earth alloy of the type SE-TM-A or SE-TM type, wherein SE is a rare earth element, TM is an iron group transition metal (Fe, Co, Ni) and A is an element of III. Main group of the Periodic Table of the Elements, in particular boron (B). For example, an Nd-Fe-B alloy or an Sm-Co alloy is used.

In step S1 of FIG. 1, first, a melt of the magnetic material is processed into a thin strip by a tape casting method. In this case, the melt is poured onto a rotating cooled roller, the melt solidifies abruptly. The obtained thin tape of the magnetic material has a polycrystalline nano-structure. This is shown on the left side of FIG. 2 using the example of the rare earth alloy Nd ₂ Fei ₄ B. It is It can be seen that the crystals of this alloy are enclosed by a neodymium-rich alloy phase, which is formed by the quenching of the alloy.

In order to further increase the brittleness of the starting material, in step S2 of the process with the known method of hydrogen embrittlement (HD process for hydrogen decrepitation), the material obtained by the strip casting is treated. For this purpose, the polycrystalline magnetic material is exposed to a hydrogen atmosphere under high pressure, wherein the hydrogen is absorbed by the alloy material. For example, an alloy of the Nd-Fe-B type absorbs about 2.5% hydrogen. Subsequently, the pressure under which the material is suddenly released, whereby the hydrogen escapes. This process is illustrated on the right side of FIG. In particular, it can be seen that in the neodymium-rich phase surrounding the Nd ₂ Fe ₁₄ B crystals,

Microcracks develop which embrittle the material. There are average particle sizes of, for example, 5 to 10 μηη.

Subsequent to the HD process, the cryogenic grinding of the brittle magnet material according to the invention takes place in step S3 of the method. The process of cryogenic milling is shown in a ball mill in FIG. The ball mill 10 has a double-walled container 1 1, in the interior of which a stirrer 12 is rotatably arranged. Inside the vessel 1 1 are also balls 13 made of a hard metal, such as stainless steel or Zr ₂ o- The embrittled magnetic material from step S2, which has a first average particle size is in the form of a suspension 20 in liquid nitrogen in the Vessel 1 1 of the ball mill 10 given. The liquid nitrogen has a temperature of about 77 K.

By the rotation of the stirrer 12 of the ball mill, the balls 13 are set in motion. The right side of Figure 3 shows an enlarged view of two balls 13, between which particles 21 of the magnetic starting material are crushed by the forces prevailing between two balls 13 forces. This results in particles of the magnetic material having a second average particle size, which is smaller than the first average particle size of the material fed to the ball mill.

The liquid nitrogen wets the powder particles 21 of the magnetic material during milling and dissipates the resulting heat. This prevents agglomeration of the particles. Furthermore, the liquid nitrogen protects the powder surface Impurities and prevents in-contact of the pyrophoric magnetic material with atmospheric oxygen.

The grinding process is preferably carried out until at least 90% by mass of the powder material has a particle size of at most 500 nm, preferably of at most 300 nm. The milling process parameters are chosen so that the desired particle size is obtained. They depend on the system used and the size of the system. For example, a speed of the stirrer is set from 150 to 1000 rpm and a grinding time of 1 to 12 hours.

According to FIG. 1, the cryogenic separation (classification) takes place in step S4 following the cryogenic grinding. This process is shown in FIG.

The separation device 30 shown here has a double-walled housing 31, which is charged with the suspension 20 obtained from the cryogenic grinding, consisting of the powder of the magnetic material and liquid nitrogen. In the housing 31 a plurality of vibrating screens 33 are arranged horizontally one above the other. It takes the

Mesh size of the sieves 33 from top to bottom. The sieves 33 are mechanically connected to a vertical swing axle 32. The oscillating axis 32 is offset by a drive, not shown, in a vertical vibration, which is thus transmitted to the vibrating screens 33. For example, screens may be used which have sintered metallic wire nets. Also suitable are so-called MEMS sieves (for micro-electro-mechanical structure), which are produced by wet or dry etching processes.

The powder of the magnetic material fed in the form of the suspension 20 first reaches the uppermost, coarsest sieve on which particles above the corresponding one

Mesh size of the screen, for example above 500 nm, are retained.

Particles with particle sizes <500 nm reach the next below sieve arranged, which has a slightly smaller mesh size than the top sieve, for example of 400 nm. On the second sieve thus particles in the range of> 400 nm to <500 nm are retained. This process continues to the bottom sieve, which defines the desired maximum particle size, for example of 300 nm.

Via a valve 34 is thus the filtered and separated suspension 20, which only

Particle sizes preferably <300 nm contains, drained and collected. The fractions retained on the screens 33 become the above process step S3 of FIG returned cryogenic milling. Optionally, a cooling stage can be interposed.

During the separation in step S4, the generated frictional heat is dissipated by the liquid nitrogen, thereby preventing agglomeration of the magnetic particles. Furthermore, the use in the form of the suspension in liquid N ₂ allows the particle fractions retained on the sieves 33 to be returned to the step of cryogenic grinding without media breakage.

FIG. 5 shows a flow chart of a method sequence for producing the magnetic powder according to a second embodiment of the invention. The method differs from the method shown in Figure 1 only in the first two steps, while the inventive steps S3 and S4 are the same and will not be explained again.

According to FIG. 5, in the first step S1 'the alloy melt is first poured into small-sized castings and solidified. These are then broken mechanically under protective gas, with particles having an average particle size of, for example, 500 μm.

In the subsequent step S2 ', a further mechanical pulverization of the material takes place by conventional grinding in a protective gas atmosphere, for example in gaseous nitrogen or argon. The coarse grinding step S2 'can take place, for example, in a jet mill or in a ball mill. Particle sizes of, for example, 3 to 5 μm are obtained. These are further pulverized in the already explained steps S3 and S4 in order to obtain particle sizes of preferably at most 300 nm.

FIG. 6 shows by way of example a further process sequence in which the powder obtained in step S4 (from FIG. 1 or 5) is further processed into a permanent magnet.

In step S5, compacting and shaping of the powder of the magnetic material takes place, for example, by pressing. The pressing may be anisostatic in a mechanical pressing tool, with mechanical pressure being applied to the compact from one or two opposite spatial directions. Alternatively, the pressing may be carried out isostatically by subjecting the powder to a high pressure of an inert gas atmosphere. In both cases, the pressing can be done in an external magnetic field, so that a magnetically anisotropic compact arises. Result of the Compaction step S5 is a (magnetically isotropic or anisotropic) compact, which is also referred to as a green part.

In the following step S6, a sintering of the green part takes place. In this case, the green part is solidified at a temperature which is lower or the melting temperature of the magnetic material. For example, for alloys of the type NdFeB, for example Nd ₂ Fei ₄ B,

Temperatures in the range of 1000 to 1 150 ° C applied. Through the process of sintering, the particles of the powder are further compressed and solidified, and enter into mechanical, in some cases materially bonded, bonds. After sintering, a non-magnetized (isotropic or anisotropic, depending on whether or not pressed in magnetic field) body is present since the Curie remains during sintering.

In the subsequent step S7, an optional tempering process follows, in which the magnet is subjected to a further thermal treatment (low-temperature treatment). The aim of annealing is the reduction of residual stresses in the crystal structure.

In a further optional step S8, a shaping treatment and / or surface treatment of the magnet takes place in order to give it a desired shape and dimension. In particular, cutting techniques are used, such as grinding,

Cutting, milling, etc. Preferably, however, the final shape of the magnet is already determined in the compression step S5, for example, a corresponding pressing tool, so that dispenses with a machining or this can be at least reduced. Furthermore, the magnet may be coated with a surface coating, such as a

Epoxy resin or a metallic coating, be provided.

Finally, in step S9, the magnet is magnetized in an external magnetic field in which the magnetic dipoles are magnetized, that is to say aligned.

The permanent magnet produced by the method according to the invention is characterized by small particle sizes on the order of magnetic domains, that is, it consists of so-called single-domain particles. He also has a uniform

Particle size distribution preferably in the range of 200 to 300 nm. As a result, the magnet is characterized by a high coercive force and high temperature stability. Its good magnetic properties are not impaired by increased levels of rare earths, for example in the form of Dy or Tb, which are introduced by conventional processing steps (eg GBDP for grain boundary diffusion process). The fact that the sensitive steps of the powder preparation are carried out in liquid nitrogen, other impurities in the form of carbon or oxygen are reduced. Disturbing effects, such as recrystallization, thermal stress and

Agglomeration is eliminated in the cryogenic milling. The method is also characterized by a high level of safety, since the liquid nitrogen insulates the tendency of the magnet powder to ignite spontaneously.

LIST OF REFERENCE NUMBERS

ball mill

vessel

stirrer

roll

suspension

particle

separating

casing

swing axle

vibrating screens

Valve

Claims

claims Process for producing a permanent magnet, wherein a powder of a magnetic material is produced and the powder of the magnetic material is processed into a permanent magnet, characterized in that the Producing the powder of the magnetic material comprising the steps of:  Grinding a suspension of particles of the magnetic material of a first mean particle size in liquid nitrogen to obtain particles of the magnetic material of a second average particle size smaller than the first mean particle size, and  - Separating a suspension of the ground magnetic material in liquid nitrogen, wherein particles are separated with a particle size below a predetermined upper particle size. A method according to claim 1, characterized in that in the separation step particles having a particle size of <500 nm, in particular of <400 nm, preferably of <350 nm and more preferably of <300 nm, are separated. Method according to one of the preceding claims, characterized in that in the separating step particles having a particle size in the range of> 100 nm to <500 nm, in particular in the range of> 100 nm to <400 nm, preferably in the range of> 150 nm to <350 nm and particularly preferably in the range of> 200 nm to  <300 nm, to be separated. Method according to one of the preceding claims, characterized in that particles having a particle size above the predetermined upper particle size from the separation step are returned to the milling step. Method according to one of the preceding claims, characterized in that the steps of grinding and separating are concatenated with each other such that the suspension of the magnetic material subjected to the grinding step is supplied to the separating step. 6. The method according to any one of the preceding claims, characterized in that the grinding is carried out in a ball mill. 7. The method according to any one of the preceding claims, characterized in that the separation takes place by means of a Schwingsiebvorrichtung. 8. The method according to any one of the preceding claims, characterized in that the separation comprises the application of a plurality of successive screening stages with decreasing mesh sizes. 9. permanent magnet, produced by a method according to one of claims 1 to 8. 10. Electrical machine comprising at least one permanent magnet according to claim 9.