Method and device for manufacturing a magnetic body with sinusoidal magnetization of the Halbach type

Abstract

The invention relates to a method for manufacturing a magnetic body (C, C') with sinusoidal magnetization of the Halbach type, comprising the following steps: introducing into a conformer (T, T'), of linear or annular shape, a mixture (M) comprising magnetic particles and at least one binder; heating the mixture to a heating temperature between the glass transition temperature of the binder and the Curie temperature of the magnetic particles; aligning the magnetic particles so as to assign them a magnetization vector which extends in a magnetization plane (P, P') and the direction of which varies continuously; cooling the mixture to a cooling temperature below the glass transition temperature of the binder. FIGURE OF THE ABSTRACT: Fig. 4A

Description

Title of the invention: Method and device for manufacturing a magnetic body with sinusoidal magnetization of the Halbach type

[0001] The present invention relates to the manufacture of magnetic parts, and more precisely those having sinusoidal magnetization of the Halbach type.

[0002] BACKGROUND OF THE INVENTION

[0003] As is known per se, a Halbach array is a particular arrangement of permanent magnets which increases the magnetic field on one side of the array while almost completely cancelling the magnetic field on the other side of said array.

[0004] [Fig. 1] and [Fig. 2] respectively illustrate a first Halbach network Ri of linear shape and a second Halbach network R2 of annular shape. Each of the first and second networks Rh R2 here comprises first magnets Au, Ai 2 which have first magnetization vectors Vu, VL2 substantially collinear and which are separated two by two by second magnets A2 b A2 2 having second magnetization vectors V2.i, V2 2 substantially perpendicular to the first magnetization vectors Vu, VL2 of the first magnets Au, AL2.

[0005] It is known to use such magnetic structures for various applications such as magnetic bearings, magnetic refrigeration equipment, magnetic resonance equipment, permanent magnet motors, etc.

[0006] One method for manufacturing such magnetic structures is to use an adhesive to bond the first magnets Au, Ai 2 and the second magnets A21, A22 together. The mutual repulsion between the first magnets Au, Ai 2 and the second magnets A2.b A2 2 makes their positioning difficult and requires a specific mold or tooling to hold them in position and clamp them during bonding, so that such a manufacturing method is not suitable for mass production.

[0007] What is more, the juxtaposition of magnets generates a sudden variation in the magnetic orientation from one magnet to another.

[0008] Another method consists of manufacturing a single magnet using a mold and magnetically orienting it using a complex and expensive magnetization device, including in particular a large number of coils. Such a method is thus only applicable to a given geometry and size of structure, and the power of the magnetic field generated by the structure proves to be limited.

[0009] SUBJECT OF THE INVENTION

[0010] The invention therefore aims to propose a simple and inexpensive manufacturing method for a magnetic body with sinusoidal magnetization of the Halbach type emitting a magnetic field whose direction varies continuously in a plane of magnet- tization of the body. Summary of the invention

[0011] To this end, the invention proposes a method for manufacturing a magnetic body with sinusoidal magnetization of the Halbach type, comprising the following steps: • introduction into a shaper, of linear or annular shape, of a mixture comprising magnetic particles and at least one polymer forming a binder; • heating the mixture to a heating temperature between the glass transition temperature of the binder and the Curie temperature of the magnetic particles; • when the conformer is linear in shape, alignment of the magnetic particles so as to assign them a magnetization vector which extends in a magnetization plane containing a longitudinal axis of the conformer and the direction of which varies continuously as a function of the distance separating the magnetic particles and one end of the conformer; • when the conformer is annular in shape, alignment of the magnetic particles so as to assign them a magnetization vector which extends in a magnetization plane orthogonal to a central axis of the conformer and the direction of which varies continuously as a function of the angular position of the magnetic particles around the central axis; • cooling the mixture to a cooling temperature below the glass transition temperature of the binder.

[0012] In addition to being suitable for mass production, such a process makes it possible to generate a continuous variation in the magnetic orientation of the body.

[0013] According to a particular characteristic, the method further comprises debinding and sintering of the mixture.

[0014] In particular, the method further comprises a strengthening of the magnetic field emitted by the magnetic particles.

[0015] The invention also relates to a magnetic alignment device for implementing such a method. The device comprises: • a motorized platform arranged to ensure, when the conformer is linear in shape, a translation of said conformer along its longitudinal axis, or, when the conformer is annular in shape, a rotation of said conformer around its central axis; • a heating element fixed relative to the motorized platform and arranged to locally increase the temperature of the mixture contained in the former up to the heating temperature; • a cooling element fixed relative to the motorized platform and arranged to locally reduce the temperature of the mixture contained in the conformer to the cooling temperature; and • a unidirectional flux generator arranged between the heating element and the cooling element, and arranged to locally subject the magnetic particles to a unidirectional magnetic flux which extends in the magnetization plane and the direction of which varies continuously depending on the movement or rotation of the shaper.

[0016] In particular, the heating element comprises a heating collar arranged to extend around a portion of the former.

[0017] In particular, the heating collar is of the resistive type.

[0018] In particular, the cooling element comprises a cooling collar arranged to extend around a portion of the former.

[0019] In particular, the cooling collar is a copper ring cooled by circulation of water or by spraying a decompressed gas.

[0020] In particular, the unidirectional magnetic flux generator comprises at least one electromagnetic coil which is selectively powered by an electric current to generate a magnetic field from which the unidirectional magnetic flux is obtained.

[0021] In particular, the unidirectional magnetic flux generator comprises a first electromagnetic coil and a second electromagnetic coil arranged perpendicular to each other, the first coil and the second coil being respectively powered by a first electric current and a second electric current arranged to generate a first magnetic field and a second magnetic field from which the unidirectional magnetic flux is obtained, and to vary the direction of said unidirectional magnetic flux in the magnetization plane. Brief description of the drawings

[0022] The invention will be better understood in light of the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings, among which:

[0023] [Fig-1] [Fig. 1] is a schematic view of a first Halbach network of the prior art, of linear form;

[0024] [Fig.2] [Fig.2] is a schematic view of a second Halbach network of the art anterior, annular in shape;

[0025] [Fig.3] [Fig.3] illustrates a method of manufacturing a magnetic body with sinusoidal magnetization of the Halbach type, according to the invention;

[0026] [Fig.4A] [Fig.4A] is a schematic view of a magnetic alignment device according to a first embodiment of the invention for implementing the method illustrated in [Fig.3], in which the unidirectional magnetic flux extends in a first direction;

[0027] [Fig.4B] [Fig.4B] is a view identical to [Fig.4A], in which the unidirectional magnetic flux extends in a second direction;

[0028] [Fig.5A] [Fig.5A] is a schematic view of a magnetic alignment device according to a second embodiment of the invention for implementing the manufacturing method illustrated in [Fig.3], in which the magnetic flux extends in a first direction;

[0029] [Fig.5B] [Fig.5B] is a view identical to [Fig.5A], in which the magnetic flux extends in a second direction. DETAILED DESCRIPTION OF THE INVENTION

[0030] With reference to [Fig.3], a method of manufacturing a magnetic body C with sinusoidal magnetization of the Halbach type is described below.

[0031] A tube T of rectangular section is previously filled with a mixture M, also called “feedstock”, comprising magnetic particles and a viscous binder based on polymers. The tube T is made of a non-magnetic material, for example stainless steel (for example 316L, 304L steel or equivalent austenitic) or titanium, and forms a linear conformer of axis X. The magnetic particles are in the form of a powder of permanent magnets such as for example: a powder of particles in an alloy of samarium and cobalt (SmCo); a powder of particles in an alloy of neodymium, iron and boron (NdFeB); a powder of particles in ferrite... The volumetric loading rate of the magnetic particles in the mixture M is here substantially equal to 60%. The binder is composed of thermoplastics, fluidifiers and wetting agents.The mixture M is inserted under pressure into the tube T via an injection press (not shown) which forces the introduction of said mixture M into a mold in which the tube T is previously received.

[0032] Once filled with the mixture M, the tube T is ready to be inserted into a magnetic alignment device 1 making it possible to orient the magnetic particles of the mixture M according to a magnetization vector which extends in a plane P containing the longitudinal axis X of the tube T, and the direction of which varies continuously as a function of the distance separating the magnetic particles from one end of the tube T.

[0033] Referring to Figures 4A-4B, the magnetic alignment device 1 comprises a motorized platform 2, a heating element 3, a cooling element 4 and a unidirectional magnetic flux generator 5.

[0034] The motorized platform 2 is mounted movably on a frame and arranged to ensure a translation of the tube T along its longitudinal axis X. The translation speed of the tube T is controllable. The motorized platform 2 comprises, for example, a carriage which is driven in translation along a rail by a motor and which is provided with means for clamping one end of the tube T.

[0035] The heating element 3 is fixed relative to the frame and is arranged to locally increase the temperature of the mixture M contained in the tube T up to a predetermined heating temperature. The heating temperature is between the glass transition temperature (Tg) of the binder below which said binder is said to be glassy and exhibits the behavior of an elastic solid body, and the Curie temperature of the magnetic particles of the powder above which said magnetic particles lose their ferromagnetic properties. The heating temperature is chosen so that at this temperature, the binder used is both sufficiently liquid to allow orientation of the magnetic particles via the unidirectional magnetic flux generator 5, and sufficiently solid to prevent any movement of the magnetic particles due to the displacement of the tube T via the motorized platform 2.The heating element 3 here comprises a heating collar arranged to extend around a section of the tube T and to travel the entire length of the tube T as said tube T moves along its axis X via the motorized platform 2. The heating collar is for example of the resistive type.

[0036] The cooling element 4 is fixed relative to the frame and is arranged to locally reduce the temperature of the mixture M heated by the heating element 3, down to a predetermined cooling temperature. The cooling temperature is lower than the glass transition temperature (Tg) of the binder and is chosen so that at said cooling temperature, the binder used is sufficiently solid or viscous to prevent any movement of the magnetic particles. The cooling element 4 here comprises a cooling collar arranged to extend around a section of the tube T and to travel along the entire tube T as said tube T moves along its axis X via the motorized platform 2. The cooling collar 4 is for example a copper ring cooled by circulating water or by spraying a decompressed gas.

[0037] The unidirectional magnetic flux generator 5 is arranged between the heating element 3 and the cooling element 4, and is arranged to locally subject the mixture M heated by the heating element 3 to a unidirectional magnetic flux F. This magnetic flux F extends in the magnetization plane P comprising the longitudinal axis X of the tube T and has a direction varying continuously as a function of the movement of the tube T via the motorized platform 2 by pivoting around a fixed point O called the magnetic alignment point. This magnetic alignment point O belongs to the longitudinal axis X of the tube T and is here arranged substantially at an equal distance from the heating element 3 and the cooling element 4. The magnetic flux F has sufficient intensity to orient, in the direction of the magnetic flux F, the magnetic particles of the mixture M heated by the heating element 3.

[0038] The unidirectional magnetic flux generator 5 comprises an electromagnet comprising here a first electromagnetic coil 5.1 of axis Xi and a second electromagnetic coil 5.2 of axis X2. The first coil 5.1 and the second coil 5.2 are selectively powered by a first electric current and a second electric current respectively. The axes Xb X2 of the first and second coils 5.1, 5.2 are perpendicular and extend in the magnetization plane P.

[0039] The electromagnet formed by the first coil 5.1 and the second coil 5.2 is here carried by a motorized plate 5.3 which is mounted movably on the frame to ensure rotation of the electromagnet in the magnetization plane P, substantially around the magnetic alignment point O.

[0040] The magnetic flux F is obtained from the magnetic fields generated by the first and second coils 5.1, 5.2. The variation of the direction of the magnetic flux F is obtained by the rotation of the electromagnet via the plate 5.3 ([Fig.4A]), and / or by the modulation of the intensity of the first current and / or the second electric current passing through the first and second coils 5.1, 5.2 ([Fig.4B]).

[0041] The tube T containing the mixture M is thus inserted into the magnetic alignment device 1. While the motorized platform 2 causes a translation of the tube T along its longitudinal axis X, each section of said tube T is successively subjected to: • to the heating element 3 in order to locally heat the mixture M until it reaches the heating temperature and thus allow a modification of the orientation of the magnetic particles contained in said mixture M; • to the unidirectional magnetic flux generator 5 in order to orient the magnetic particles in a direction which extends in a plane containing the longitudinal axis X of the tube T and which varies continuously as a function of the distance separating the magnetic particles and one end of the tube T; and • to the cooling element 4 in order to locally cool the heated mixture M until it reaches the cooling temperature and thus freeze the orientation of the magnetic particles bound by the binder.

[0042] Thus, at the output of the magnetic alignment device 1, a tube T is obtained containing a powder of linked magnets emitting a sinusoidal magnetic field like a Halbach network. The variation in the orientation of the magnetic particles in the magnetization plane P depends on: • the translation speed to which the tube T is subjected via the motorized platform 2, • the speed at which the direction of the magnetic flux F varies via plate 5.3, and • the speed at which the direction of the magnetic flux F varies via the relative modulation of the intensities of the first and second currents passing through the first and second coils 5.1, 5.2.

[0043] The linked magnets thus form a magnetic body C of linear shape with sinusoidal magnetization of the Halbach type.

[0044] In order to strengthen the magnetic field emitted by the permanent magnets, it is possible to insert the tube T again into the magnetic alignment device 1 in order to subject said permanent magnets once again to the unidirectional magnetic flux F emitted by the unidirectional magnetic flux generator 5, but by deactivating the heating element 3 and the cooling element 4.

[0045] For the same purpose, it is also possible to increase the relative density of permanent magnets inside the tube T by performing debinding and then densification by sintering the mixture M. Debinding, in other words the elimination of the binder present inside the tube T, may for example be carried out by placing the tube T vertically in a furnace under partial argon pressure, having previously screwed a plug onto the lower end of the tube or pinched said lower end. Such debinding makes it possible to obtain a relative density of permanent magnets greater than 90%. If a relative density equal to 100% is necessary, the end of the tube T that remains open will in turn be closed and then the tube T will be inserted into a HIP (Hot Isostatic Pressing) enclosure to undergo hot isostatic compaction.Once the sintering is carried out, we obtain a tube T containing a massive magnet emitting a sinusoidal Halbach-type magnetic field.

[0046] It will be noted that the mixture M must have properties compatible with the implementation of the manufacturing process of the body C, but also the application in which said body C is used. It must thus have a rheology adapted to the injection operations, but also, if necessary, to the debinding and sintering operations (low residual impurity level of the permanent magnet powder, in particular the residual carbon level): polymer mixtures composed of polyethylene (PE), stearic acid (SA), polyethylene glycol (PEG), paraffin (PW), and / or polymethyl methacrylate (PMMA) can be used as binders.

[0047] It will also be noted that such a manufacturing method and such a magnetic alignment device 1 allow the manufacturing of a Halbach network having a length ranging from ten millimeters to several meters, and this without modification of said magnetic alignment device 1.

[0048] It will also be noted that the tube T can be cut into sections.

[0049] Figures 5A-5B illustrate a magnetic alignment device 1' which is none other than that a variant of the magnetic alignment device 1 for the manufacture of a magnetic body C' of annular shape with sinusoidal magnetization of the Halbach type.

[0050] A hollow ring T' of rectangular section is previously filled with the mixture M. The ring T' is made of a non-magnetic material, for example stainless steel (for example 316L, 304L steel or equivalent austenitic) or titanium, and forms an annular conformer of axis X'. The mixture M is inserted under pressure into the ring T' via an injection press (not shown) which makes it possible to force the introduction of said mixture M into a mold in which the ring T' is previously received.

[0051] Once filled with the mixture M, the tube T' is ready to be inserted into the magnetic alignment device 1' making it possible to orient the magnetic particles of the mixture M according to a magnetization vector which extends in a plane P' orthogonal to the central axis X' of the ring T', and the direction of which varies continuously as a function of the angular position of said magnetic particles around said central X'. The plane P' here contains a generatrix of the ring T'.

[0052] The magnetic alignment device 1' differs from the magnetic alignment device 1 in that the motorized platform 2 is replaced by a motorized platform 2' mounted movably on the frame to ensure rotation of the ring T' around its central axis X'. The speed of rotation of the ring T' is controllable. The motorized platform 2' comprises, for example, a carriage which is driven in rotation along a rail by a motor and which is provided with means for clamping one or more sections of the ring T'.

[0053] The heating element 3 is arranged to locally increase the temperature of the mixture M contained in the ring T' up to the predetermined heating temperature.

[0054] The cooling element 4 is arranged to locally reduce the temperature of the mixture M heated by the heating element 3, down to the predetermined cooling temperature.

[0055] The magnetic flux F delivered by the unidirectional magnetic flux generator 5 extends in the magnetization plane P' orthogonal to the central axis X of the ring T' and has a direction varying continuously as a function of the rotation of the ring T' via the motorized platform 2' by pivoting around a fixed point O' called the magnetic alignment point. This magnetic alignment point O' belongs to the generatrix of the ring T' and is arranged substantially equidistant from the heating element 3 and the cooling element 4.

[0056] The electromagnet of the unidirectional magnetic flux generator 5 is carried by the motorized plate 5.3' which is mounted movably on the frame to ensure rotation of the electromagnet in the magnetization plane P', substantially around the magnetic alignment point O'.

[0057] The ring T' containing the mixture M is thus inserted into the alignment device

[0058]

[0059]

[0060]

[0061]

[0062] magnetic 1'. While the motorized platform 2' causes a rotation of the ring T' around its central axis X', each sector of said ring T' is successively subjected to: • to the heating element 3 in order to locally heat the mixture M until it reaches the heating temperature and thus allow a modification of the orientation of the magnetic particles contained in said mixture M; • to the unidirectional magnetic flux generator 5 in order to orient the magnetic particles in a direction which extends in the plane P' orthogonal to the central axis X' of the ring T' and which varies continuously as a function of the angular position of said magnetic particles around said central X'; • to the cooling element 4 in order to locally cool the heated mixture M until it reaches the cooling temperature and thus freeze the orientation of the magnetic particles bound by the binder. We thus obtain, at the output of the magnetic alignment device 1', a ring T' containing a powder of linked magnets emitting a sinusoidal magnetic field like a Halbach network. The variation of the orientation of the magnetic particles in the plane P' of magnetization depends on: • the rotation speed to which the ring T' is subjected via the motorized platform 2', • the speed at which the direction of the magnetic flux F varies via plate 5.3, • the speed at which the direction of the magnetic flux F varies via the relative modulation of the intensities of the first and second currents passing through the first and second coils 5.1, 5.2. The linked magnets thus form a magnetic body C' of annular shape with sinusoidal magnetization of the Halbach type. In order to strengthen the magnetic field emitted by the permanent magnets, it is possible to insert the ring T' again into the magnetic alignment device 1' in order to subject said permanent magnets once again to the unidirectional magnetic flux F emitted by the unidirectional magnetic flux generator 5, but by deactivating the heating element 3 and the cooling element 4. For the same purpose, it is also possible to increase the relative density of permanent magnets inside the ring T' by carrying out debinding and then densification by sintering of the mixture M. It should be noted that the mixture M must have properties compatible with the implementation of the manufacturing process of the body C', but also the application in which said body C' is used. It must therefore have a rheology adapted to the operations injection, but also, if necessary, to debinding and sintering operations.

[0063] It will also be noted that such a manufacturing method and such a magnetic alignment device 1' allow the manufacturing of a Halbach network having a diameter ranging substantially from several centimeters to several tens of centimeters, and this without modification of said magnetic alignment device 1'.

[0064] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0065] Although the unidirectional flux generator 5 here comprises two coils 5.1, 5.2, it may comprise only one. The orientation of the unidirectional magnetic flux F in the magnetization plane P, P' will then depend solely on the angular position of the plate 5.3.

[0066] To carry out the debinding and sintering operations, it may be considered to remove the shaper T, T' and / or to install a new one.

[0067] The dimensions and shape of the conformer T, T' may be different from those described.

[0068] Although the binder here comprises a mixture of polymers composed of thermoplastics, plasticizers and wetting agents, it can also comprise a mixture composed of a single polymer.

[0069] The final viscosity of the mixture M will be adapted according to the binder, the size and geometry of the magnetic particles, the geometry of the conformer, etc.

Claims

Claims

1. Method for manufacturing a magnetic body (C, C') with sinusoidal magnetization of the Halbach type, comprising the following steps: • introducing into a conformer (T, T'), of linear or annular shape, a mixture (M) comprising magnetic particles and at least one polymer forming a binder; • heating the mixture to a heating temperature between the glass transition temperature of the binder and the Curie temperature of the magnetic particles; • when the conformer (T) is of linear shape, aligning the magnetic particles so as to assign them a magnetization vector which extends in a magnetization plane (P) containing a longitudinal axis (X) of the conformer and the direction of which varies continuously as a function of the distance separating the magnetic particles and one end of the conformer;• when the conformer (T') is annular in shape, alignment of the magnetic particles so as to assign them a magnetization vector which extends in a magnetization plane (P') orthogonal to a central axis (X') of the conformer and the direction of which varies continuously as a function of the angular position of the magnetic particles around the central axis; • cooling of the mixture to a cooling temperature lower than the glass transition temperature of the binder.;

2. The method of claim 1, further comprising debinding and sintering the mixture.

3. A method according to any preceding claim, further comprising strengthening the magnetic field emitted by the magnetic particles.

4. Magnetic alignment device (1, 1') for implementing the method according to any one of claims 1 to 3, comprising: • a motorized platform (2, 2') arranged to ensure, when the shaper (T) is of linear shape, a translation of said conformer along its longitudinal axis (X), or, when the conformer (T') is annular in shape, a rotation of said conformer around its central axis (X'); • a heating element (3) fixed relative to the motorized platform and arranged to locally increase the temperature of the mixture contained in the conformer up to the heating temperature; • a cooling element (4) fixed relative to the motorized platform and arranged to locally decrease the temperature of the mixture contained in the conformer down to the cooling temperature; and • a unidirectional flux generator (5) arranged between the heating element and the cooling element, and arranged to locally subject the magnetic particles to a unidirectional magnetic flux (F) which extends in the magnetization plane (P, P') and the direction of which varies continuously depending on the movement or rotation of the conformer.

5. A device according to claim 4, wherein the heating element (3) comprises a heating collar arranged to extend around a portion of the former.

6. Device according to claim 5, in which the heating collar (3) is of the resistive type.

7. A device according to claim 4, wherein the cooling element (4) comprises a cooling collar arranged to extend around a portion of the former.

8. Device according to claim 7, in which the cooling collar (4) is a copper ring cooled by circulation of water or by spraying of a decompressing gas.

9. Device according to claim 4, wherein the unidirectional magnetic flux generator (5) comprises at least one electromagnetic coil (5.1, 5.2) which is selectively powered by an electric current to generate a magnetic field from which the unidirectional magnetic flux (F) is obtained.

10. Device according to claim 9, wherein the unidirectional magnetic flux generator (5) comprises a first electro- magnetic (5.1) and a second electromagnetic coil (5.2) arranged perpendicular to each other, the first coil and the second coil being respectively supplied with a first electric current and a second electric current arranged to generate a first magnetic field and a second magnetic field from which the unidirectional magnetic flux (F) is obtained, and to vary the direction of said unidirectional magnetic flux in the magnetization plane (P, P').

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