Permanent magnet

Abstract

 The invention relates to a method for manufacturing a permanent magnet, wherein particles of the permanent magnet are pressed in a pressure chamber (24) and later bonded together, whereby during the pressing stage a magnetic field is applied to the pressure chamber as to align the magnetic moments of the domains in the particles in a predominant direction (A), characterized in that a magnetic field is applied to the pressure chamber (24) which is converging on at least one side of the pressure chamber (24) in the predominant direction.

Description

[0001] The present invention refers to a Method for manufacturing a permanent magnet comprising a plurality of particles, e.g. grains, powder parts or crystals, which have been baked or bonded together in a connecting process, usually by sintering or other bonding methods. The uppermost part of said particles comprises each at least one magnetic domain whereby the permanent magnet comprises a predominant direction in which the magnetic moments of the domains of the particles are aligned within a certain range. Usually, this alignment of the magnetic moments of the domains is realized by pressing the particles together in a pressing stage whereby a uniaxial magnetic field in a predominant direction is applied to a pressure chamber which leads to an orientation of the magnetic moments of the domains of the particles in the predominant direction. The disadvantage of this method is that the magnetic moments of the domains still deviate within a certain tolerance from the predominant direction which leads to an increase of divergence losses.

[0002] It is therefore object of the present invention to provide a method for manufacturing a permanent magnet which allows the fabrication of permanent magnets with minor divergence losses. It is also object of the invention to provide a permanent magnet with minor divergent losses.

[0003] The object of the invention is solved with a method according to claim 1 and with a permanent magnet according to claim 10. Preferred embodiments of the invention are subject-matter of the corresponding dependent claims.

[0004] Some inventive embodiments are also discussed in the descriptive section of the present application. The inventive content may also consist of several separate inventions, especially if the invention is considered in the light of expressions or implicit subtasks or from the point of view of advantages or categories of advantages achieved. In this case, some of the attributes contained in the claims below may be superfluous from the point of view of separate inventive concepts.

[0005] According to the inventive method, a magnetic field is applied to the pressure chamber which is focused on at least one focus area in the predominant direction. Preferably, this focused magnetic field is focused to a focus area located in a distance of the pressure chamber above/under the center of the permanent magnet. By applying this focussed magnetic field it is achieved that the magnetic moments of the domains in the permanent magnet are slightly inclined to the focus area because they are being aligned along the flux lines which are slightly focused to the focus area in the predominant direction. The focus area may be dependent on the geometry of the permanent magnet a point or a certain plane or three-dimensional area in a distance to the pressure chamber and above/under the center thereof. The area of plane region has anyway to be smaller than the area of the pressure chamber itself to achieve a certain focusing of the magnetic moments of the domains.

[0006] The distance and focus area of the magnetic field is used such that the maximal deviation of the magnetic moments of the domains from the predominant direction via the focusing is between 1 and 10 degrees, particularly between 2 and 5 degrees. This focusing leads to any elimination of outward stray (deviation of the magnetic moments of the domains from the predominant direction pointed outwards, i.e. away from the center of the permanent magnet).

[0007] Accordingly, the magnetic moments of the domains do not any longer stray by a certain tolerance angle outwardly from the predominant direction because being slightly focused. By applying a focused magnetic field during the pressure stage, the magnetic moments of the domains can accordingly either be exactly aligned parallel or even be aligned slightly focused to said focus area which is preferably a center line. The use of such permanent magnets manufactured along this inventive method with a slightly focused orientation of the magnetic moments of the domains is very effective and has minor divergence losses and leads thus to better results in any kind of applications, preferably in permanent magnet motors.

[0008] It is possible to focus the magnetic moments of the domains to a focus area only on one side of the permanent magnet which leads to a kind of conical focus of the magnetic moments of the domains. Of course, it may be preferably to focus the magnetic moments of the domains to opposite focus points with respect to the permanent magnet. In this case, the magnetic flux between these two opposed focus points is barrel type so that the magnetic moments of the domains focus in the direction of both end faces of the permanent magnets in a predominant direction which reduces stray losses of the permanent magnets independent of their orientation in an appliance, e.g. in a permanent magnet motor.

[0009] Of course it is possible that these two focus areas are equidistant from the permanent magnet. But the opposite focus areas may also be located in different distances from the permanent magnet according to the desired use.

[0010] The focus area may be chosen according to the geometrical size of the permanent magnet. In case the permanent magnet is a cylinder preferably the focus area is an area around or on a center line through the cylinder axis, but at a distance from the permanent magnet. In case the permanent magnet is box-shaped, the focus area may be a smaller rectangular plane above/below the center of the permanent magnet in the predominant direction. Accordingly, different sizes and forms of focus areas are possible according to the design and shape of the permanent magnet. The focus area is always chosen in a way as to focus the magnetic moments of the domains to the center area which extends normal through the permanent magnet in the predominant direction.

[0011] The geometry of the magnetic field in the pressure chamber is advantageously established via the material of the pressure tools, e.g. ferromagnetic/non-ferromagnetic, via the size and thickness of the pressure tools, via the geometry and location of the coils surrounding the pressure chamber and via the thickness of non-ferromagnetic material on both sides of the pressure chamber in pressure direction.

[0012] A further possibility of applying focused fields is to apply at least one concentric tapered coil which leads to the focusing of the corresponding magnetic field in the pressure chamber to one focus area. If a focus in both opposite directions is achieved, at least two concentric tapered coils are used whereby their tapered sections face away from each other. In this case, the flux lines are focused on both opposite ends of the permanent magnets in the predominant direction.

[0013] Preferably, the particles of the permanent magnet are baked together by sintering which is the standard procedure to manufacture NdFeB and other rare earth materials as e.g. SmCo. Accordingly, in a preferred method of the invention, the particles are made of NdFeB resulting in permanent magnets with the highest energy product, preferably for use in permanent magnet motors.

[0014] According to a preferred embodiment of the invention, the focus area is a line, plane or space at a distance of the permanent magnet in the predominant magnetization direction. Via this measure the flux lines of the magnetic moments of the domains in the permanent magnet are convergent around the center line in the predominant direction and not divergent, which may happen, when a unidirectional magnetic field is applied to the pressure chamber during the pressure stage of the permanent magnet.

[0015] A particle of the permanent magnet may house preferably one domain but it also may be several domains.

[0016] After sintering a pulsed magnetic field can be applied to the permanent magnet to increase its field strength, as usually after sintering magnets are demagnetized, as the sintering occurs at a temperatures well above the Curie temperature of permanent magnet.

[0017] A two- or one-sided convergent magnetic orientation of the magnetic moments of the domains, e.g. a barrel formed magnetic flux orientation, can be obtained when on both sides or at least on one side of the pressure chamber a layer of non-magnetic material is located. This leads to a slight diversion of the magnetic field from the predominant direction. This diversion can be used to create one-sided or both-sided converging orientation of the magnetic moments of the domains. Hereby the non-magnetic layers should have a certain minimal thickness, e.g. at least half the thickness of the pressure chamber.

[0018] The invention further relates to a permanent magnet which is manufactured according to the inventive method described above. Of course, such a permanent magnet comprises a plurality of particles which are baked together, preferably by sintering. Anyway, also other methods of baking together may be used as for example gluing or melting or bonding or hot-pressing.

[0019] In such a permanent magnet, the magnetic moments of the domains of the particles are aligned exactly parallel or are even aligned such that the magnetic flux lines of the magnetic moments of the domains are focused to at least one side of the permanent magnet in the predominant direction to a focus area. By applying the focused magnetic field during the pressing stage, the magnetic moments of the domains are accordingly at least parallel or even slightly focused to the focus area. Thereby, the focus can be provided on one side of the permanent magnets or even on two opposite sides of the permanent magnet in the predominant direction. In the first case, the magnetic flux lines are kind of tapered to the focus area and in case of two focus areas on opposite sides, the flux lines of the magnetic moments of the domains in the permanent magnets are either barrel formed or correspond to two cones connected with their bottom face concentrically. Such kind of permanent magnet has minimum stray from the predominant direction outwardly. Accordingly, used in preferred appliances as permanent magnet motors, these permanent magnets lead to a permanent magnet motor with a clearly reduced divergence loss. Accordingly, the efficiency of such permanent magnet motor can be increased essentially or the use of permanent magnet material can be reduced.

[0020] Preferably, the magnetic moments of the domains of the particles in the permanent magnet deviate from the predominant direction by a maximum angle of 1 to 10 degrees, preferably 3 to 9 degrees in the direction of the focus area.

[0021] Further advantages of the inventive permanent magnets are apparent from the above description of the inventive method.

[0022] It shall be clear that the different embodiments described above may be combined with each other as long as this is technically feasible.

[0023] The invention is now described schematically with the aid of the enclosed drawings. In these drawings

Fig. 1

shows a longitudinal cross-section of a pressure arrangement having non-magnetic layers on both sides of the pressure chamber,

Fig. 2

shows a cross-sectional view of a second embodiment of a pressure arrangement similar to Fig. 1 but with a tapered coil and one non-magnetic die, and

Fig. 3

shows a cross-sectional view of a third embodiment of a pressure arrangement similar to Fig. 1, with two ferromagnetic dies of a different size.

[0024] Fig. 1 shows a pressure arrangement 10 having two dies 12, 14 made of a ferromagnetic material as e.g. iron, which dies are movable with respect to each other by a pressure drive along the pressure axis A. An isolating layer 16, 18 of non-magnetic material is arranged at the ends of the dies facing each other. Between the isolating layers 16, 18, a pressure chamber 24 is formed, which is surrounded by side walls 22 of a non-magnetic material. In the pressure chamber 24 powder, particles or grains of a magnetic material 20, e.g. NdFeB, are arranged to be bonded by pressure appliance on the dies 12, 14 , eventually before further bonding treatments, e.g. heat treatment, e.g. sintering or bonding.

[0025] One of the pressure dies 12, 14 (ore even both) are surrounded by a circular coil 26 creating a magnetic field in the ferromagnetic dies 12, 14 parallel to the predominant direction (and pressure direction) A.

[0026] As between the dies 12, 14 layers 16, 18 of a non-ferromagnetic material are positioned the magnetic flux slightly diverges in the area of the pressure chamber. Hereby it is to be noted that the non-magnetic layers should have a certain thickness, e.g. at least half the thickness of the pressure chamber.

[0027] Fig. 2 shows a similar pressure arrangement 30 which is nearly identical to the pressure arrangement 10 of Fig. 1. Identical parts or functional corresponding parts are designated with the same reference numbers. In contrast to Fig. 1, in Fig. 2 the upper die 32 is of a non-magnetic material whereas the lower die 34 is made of a ferromagnetic material. A coil 36 with a conical shape is arranged around the pressure chamber 24, which leads to a conical or one-sided converging orientation of the magnetic moments in the domains of the magnetic material 20 with respect to the center axis C of the pressure chamber, which can be seen via the flux lines 29, the direction being marked with arrows.

[0028] Finally, Fig. 3 shows a pressure arrangement 40 having two ferromagnetic dies 42, 44 of a different size. The upper die is slightly larger than the lower die. This leads to a convergent orientation of the magnetic moments of the domains in the magnetic material 20. The course and direction of the magnetic flux lines are indicated with lines 29 and arrows.

[0029] The different embodiments of the invention can be combined with each other as long as technically feasible.

[0030] Instead of one coil, several separate coil parts may be used. The coils 26, 36 are preferably arranged to surround the pressure chamber completely. Anyway, they may surround the pressure chamber 24 only partially and may extend only along a part of the axial length of the pressure chamber 24.

Claims

1. Method for manufacturing a permanent magnet, wherein particles of the permanent magnet are pressed in a pressure chamber (24) and later bonded together, whereby during the pressing stage a magnetic field is applied to the pressure chamber as to align the magnetic moments of the domains in the particles in a predominant direction (A), characterized in that a magnetic field is applied to the pressure chamber (24) which is converging on at least one side of the pressure chamber (24) in the predominant direction.

2. Method according to claim 1, wherein the applied magnetic field is arranged to converge at two opposite sides with respect to the pressure chamber (24).

3. Method according to claim 1 or 2, wherein the magnetic field is applied by at least one coil (26; 36) or coil parts with a conical cross-section in axial direction.

4. Method according to claim 3, wherein ferromagnetic dies (42, 44) with different sizes are used on both sides of the pressure chamber (24).

5. Method according to one of the preceding claims, wherein on both sides of the pressure chamber a layer (16, 18) of non-magnetic material is located, preferably having at least a thickness of half the thickness of the pressure chamber .

6. Method according to one of the preceding claims, wherein the particles (20) are bonded together by sintering.

7. Method according to one of the preceding claims, wherein the particles (20) are NdFeB or magnetic rare earth particles .

8. Method according to one of the preceding claims, wherein the applied magnetic field is focussed to a focus area (C) on at least one side of the pressure chamber (24).

9. Method according to one of the preceding claims, wherein the orientation of the applied magnetic field is established via the material and/or geometry of the dies and/or via the location and/or geometry of the coil(s) (26; 36) creating the magnetic field.

10. Permanent magnet comprising a plurality of particles (20) which is manufactured according to a method of one of the preceding claims, characterized in that the magnetic moments of the domains of the particles are aligned exactly parallel or are aligned such that the magnetic flux lines (29) of the magnetic moments of the domains are converging to at least one side of the permanent magnet in the predominant direction (A).

11. Permanent magnet according to claim 10, wherein the field (29) of the magnetic moments of the domains of the particles deviate from the predominant direction (A) by an angle of 1 to 10 degrees, preferably 3 to 9 degrees.

12. Permanent magnet according to claim 10 or 11, being a NdFeB-magnet.

13. Permanent magnet according to one of claims 10 to 12, wherein the magnetic moments of the domains in the permanent magnet are focussed to a focus area, which is a center point above/below or a center line (C) through the permanent magnet.

14. Permanent magnet according to one of claims 10 to 13, wherein the particles (20) are sintered together.

15. Permanent magnet according to one of claims 10 to 14, wherein the magnetic moments of the domains of the particles (20) are aligned along flux lines (29) between two focus areas in opposite direction with respect to the permanent magnet.

16. Permanent magnet motor having permanent magnets according to one of claims 10 to 15.

Amended claims

Amended claims in accordance with Rule 137(2) EPC.

1. Permanent magnet motor having permanent NdFeB magnets comprising a plurality of particles (20) which are pressed in a pressure chamber (24) and later bonded together, whereby during the pressing stage a magnetic field is applied to the pressure chamber as to align the magnetic moments of the domains in the particles in a predominant direction (A), which magnetic field is converging on at least one side of the pressure chamber (24) in a predominant direction, in which magnets the magnetic moments of the domains of the particles are aligned exactly parallel or are aligned such that the magnetic flux lines (29) of the magnetic moments of the domains are converging to at least one side of the permanent magnet in the predominant direction (A), wherein the field (29) of the magnetic moments of the domains of the particles deviate from the predominant direction (A) by an angle of 1 to 10 degrees, preferably 3 to 9 degrees, wherein the magnetic moments of the domains in the permanent magnet are focused to a focus area, which is a center point above/below or a center line (C) through the permanent magnet and wherein the particles (20) are sintered together.

2. Permanent magnet according to claim 1, wherein the magnetic moments of the domains of the particles (20) are aligned along flux lines (29) between two focus areas in opposite direction with respect to the permanent magnet.

Drawing