Method for manufacturing a magnetic core and magnetic core.

Abstract

TITLE: Method for manufacturing a magnetic core and magnetic core. The invention relates to a method for manufacturing a magnetic core comprising the following steps, in order: printing a first layer of paste containing magnetic powder onto a substrate, creating discrete electrical insulation on the previously deposited paste layer, printing another layer of paste containing magnetic powder on top of the previous one, creating discrete electrical insulation on the previously deposited paste layer, repeating steps c) and d) N times, with N a natural number such that N ≥ 1, printing a final layer of paste containing magnetic powder on top of the previous one to obtain a stack of layers with discrete electrical insulation between two successive layers, and sintering the stack thus obtained. Figure 1

Description

Title of the invention: Method for manufacturing a magnetic core and magnetic core. Technical field of the invention

[0001] The invention relates to the manufacture of magnetic cores and to magnetic cores. Such magnetic cores can in particular find application in the field of power electronics, with particular interest in passive components of power converters (USB-C power supplies, data centers, battery chargers, inverters, AC / DC and DC / DC converters). Technical background

[0002] In general, magnetic cores are made of materials capable of acquiring a magnetic polarization J under the effect of a magnetic field H (ferromagnetic or ferrimagnetic).

[0003] The performance of these materials is characterized by three main properties: the saturation polarization (Js in Teslas), the magnetic susceptibility X which relates the polarization J to the applied field, and the coercivity Hc which defines the width of the hysteresis loop. The "soft" ferromagnetic materials preferred for the intended applications ideally possess high values ​​of Js and x (Js > 0.3 T and x > 10) and a low coercivity value Hc < 100 A / m.

[0004] A variety of magnetic materials exist that possess properties suitable for this type of application, such as (i) amorphous, nanocrystalline, or crystalline alloys based on iron (Fe), cobalt (Co), and nickel (Ni), in the form of sheets, ribbons, or powder, (ii) spinel-structured ferrites, hexaferrites, garnets, etc., in polycrystalline form. Electronic components incorporating these magnetic cores are generally subjected to dynamic conditions, and their properties vary according to the operating frequency, which consequently determines the choice of materials.

[0005] In addition to magnetic properties, the materials forming the magnetic cores possess dielectric properties of interest for the intended applications. These include, for example, resistivity and the dielectric constant, which determine the intensity of the currents induced in the magnetic core by varying magnetic fields.

[0006] The performance depends on intrinsic properties (Js) of the materials (themselves driven by the crystalline structure and chemical composition) and extrinsic properties (susceptibility, coercivity, resistivity) adjusted by the microstructure (grain size) or the architecture (composites) of the material or even by the dimensions of the magnetic core.

[0007] On this last point, it is known that the size of the magnetic core can constitute a limitation on its use in terms of frequency due to the phenomenon of dimensional resonance. A dimensional resonance phenomenon occurs when the size of the magnetic core corresponds to the wavelength of propagation of electromagnetic waves in the material. Such resonance leads to an uncontrollable increase in losses and associated heating in the magnetic core.

[0008] Thus, manufacturers of magnetic cores indicate a maximum core size not to be exceeded for a given frequency of use.

[0009] This limiting size is defined by the resonance dimension D s(eq. 1) which is a function of the frequency, resistivity, permittivity and self-permeability of the chosen magnetic material.

[0010] Dr=.......,.........4.............

[0011] with: c, the speed of light in a vacuum (3.108 m / s) so, the vacuum permittivity (8.85.10 12 F / m) sr, the relative permittivity p, the resistivity Hr, the relative magnetic permeability, and f, the frequency.

[0012] As an example for a MnZn ferrite material (rrr = 10000), the resonance dimension is DR = 8.4 mm at f = 0.5 MHz and DR = 1.85 mm at f = 3 MHz. The resonance dimension value is low before reaching high frequencies due to the high relative magnetic permeability.

[0013] This therefore gives a limit in the dimensioning that can be given to a magnetic core, for a given operating frequency.

[0014] However, to obtain an inductance of high values ​​(for example 200 pH for a filtering inductance) in a magnetic core and / or to avoid saturation in the magnetic core, it is necessary to design magnetic cores with large magnetic sections, therefore large dimensions.

[0015] It is therefore difficult to design a magnetic core capable, at high frequency, of providing a high inductance.

[0016] However, solutions have already been proposed.

[0017] In document DE 10 2018 117 211 A1, the solution for reducing the constraints related to dimensional resonance is based on the assembly of several rings made from the same ferrite material. The ferrite rings are separated by polyurethane polymer rings (non-magnetic), in the form of solid layers.

[0018] However, to achieve this type of geometry, a machining step of ferrite rings is first required, followed by a machining step of polymer rings with the same dimensions as the ferrite rings. Furthermore, an additional constraint lies in the difficulty of producing small parts due to the fragility of the ferrite rings during machining.

[0019] In the article by S. Takahashi et al., “Experimental evaluation of the relationship between dimensional dependencies of MnZn ferrites and filter inductor impedances,” Electr. Eng. Jpn., vol. 214, no. 2, p. 23302, 2021, doi: 10.1002 / eej.23302, the limitation of dimensional resonance is based on a layering process performed by cutting a sintered magnetic core into several slices. The slices are then stacked and assembled. The authors demonstrate an increasing attenuation of the dimensional resonance phenomenon by increasing the number of laminated cores in the stack. With four layers, the authors manage to shift the resonance peak from 750 kHz to 1 MHz.

[0020] There is no electrically insulating layer between the sintered magnetic material slices, which implies fewer manufacturing operations.

[0021] However, the fact remains that the lamination of sintered parts by cutting has significant limitations, such as being restricted to simple geometries and layer thicknesses limited by the capabilities of the cutting techniques used. Thus, machining leads to costly material losses and, in the case of a brittle material, it can generate cracks that may degrade magnetic performance.

[0022] One objective of the invention is to offer an improved solution. Summary of the invention

[0023] To achieve the aforementioned objective, the invention proposes a method for manufacturing a magnetic core comprising the following steps, in order: a) imprint a first layer of a paste containing magnetic powder onto a substrate, a) create a discrete electrical insulation on the previously deposited paste layer, c) print another layer of paste containing magnetic powder on top of the previous one. d) create a discrete electrical insulation on the previously deposited paste layer, e) repeat steps c) and d) N times, with N a natural number such that N > 1, f) print a final layer of paste containing magnetic powder onto the previous layer to obtain a stack of layers with discrete electrical insulation between two successive layers, g) perform sintering of the stack thus obtained

[0024] Thanks to the process according to the invention, the magnetic core can thus be segmented by making layers (the successively deposited paste layers) of thin thickness (a few mm, less than DR), electrically insulated from each other (discrete electrical insulation between the layers) and assembled together (thanks to sintering between the different layers where there is no electrical insulation) in order to obtain a sufficient size (several cm).

[0025] No cutting or machining is carried out.

[0026] Printing also makes it possible to produce complex parts, in particular curved parts, without limitation of geometry and over the entire length of the magnetic circuit.

[0027] The process according to the invention may include at least one of the following additional steps, taken alone or in combination: - to create discrete electrical insulation on the previously deposited paste layer, the substrate is heated to a temperature above ambient temperature; - to create discrete electrical insulation on the previously deposited paste layer, a discrete set of micropores is created by mechanical action on the surface of said layer; - to create discrete electrical insulation on the previously deposited paste layer, electrically insulating particles are sprayed onto the surface of said layer; - prior to step a), a paste containing said magnetic powder is prepared; - the paste containing an organic binder, a debinding is carried out between step f) and step g); - the unbinding is a thermal unbinding; - the debinding is a thermal debinding which takes place at a first temperature stage, between 250°C and 600°C, over a period of between 1h and 6h; - sintering takes place at a second temperature stage, between 900°C and 1200°C, for a duration between 1h and 24h, in particular between 1h and 6h.

[0028] The invention also relates to a magnetic core obtained according to the process of the invention, characterized in that it is formed of several layers of the same magnetic material stacked one on top of the other, said layers being separated from each other by a discrete electrical insulation.

[0029] The discrete electrical insulation separating two layers then occupies a surface representing at least 50% and advantageously at least 70% of the interface between the two layers. Brief description of the figures

[0030] Other objects and features of the invention will become clearer in the following description, made with reference to the accompanying figures, for which:

[0031] Fig. 1 is a diagram representing the different stages of a process according to the invention;

[0032] Fig. 2 shows more precisely an implementation of the first step of the process of Fig. 1 for manufacturing a magnetic core, in this case in the shape of a torus;

[0033] Fig. 3 is a cross-sectional view of the torus-shaped magnetic core obtained after implementing all the steps of the process shown in Fig. 1;

[0034] Fig. 4 is a schematic representation of the cross-sectional view of the torus in Fig. 3

[0035] Figure 5 is a graph showing the evolution of the viscosity of a paste, the paste comprising magnetic powder used to implement the process according to the invention, depending on the shear rate applied to this paste;

[0036] Fig. 6 is a graph representing the evolution of the real and imaginary permeability as a function of frequency, for a magnetic core obtained according to the invention and for a magnetic core obtained with a prior art manufacturing process;

[0037] [Fig.7] is a graph representing the evolution of magnetic losses as a function of frequency, under a fixed magnetic field, for the same magnetic cores as those in [Fig.6];

[0038] The [Fig.8] is a diagram representing a step specific to a second embodiment of the invention;

[0039] The [Fig.9] is a diagram representing a step specific to a third embodiment of the invention. Detailed description of the invention

[0040] In general, the invention relates to a method for manufacturing a magnetic core comprising the following steps, in order: a) imprint a first layer of a paste containing magnetic powder onto a substrate, b) create a discrete electrical insulation on the previously deposited paste layer, c) imprint another layer of paste containing the magnetic powder onto the previous one. d) create a discrete electrical insulation on the previously deposited paste layer, e) repeat steps c) and d) N times), with N a natural number such that N > 1, f) print a final paste layer containing the magnetic powder on the previous one to obtain a stack of layers with a discrete electrical insulation between two successive layers, g) perform a sintering of the stack thus obtained.

[0041] The different stages of this process are shown in [Fig.1].

[0042] Regardless of the embodiment envisaged, the magnetic powder can be a powder of a ferromagnetic or ferrimagnetic material. More generally, one can consider (i) amorphous, nanocrystalline or crystalline alloys based on Iron (Fe), Cobalt (Co) and Nickel (Ni) (for example in the form of sheets, ribbons or powder), (ii) spinel-structured ferrites, hexaferrites, garnets or others in polycrystalline form

[0043] On the other hand, several embodiments of the process are conceivable for creating the discrete electrical insulation between the successive layers of the magnetic core (steps b), d) and their N repetitions).

[0044] In a first embodiment, discrete electrical insulation is obtained by heating the substrate to a temperature above ambient temperature.

[0045] The heat released by the substrate propagates through the deposited paste bead and facilitates the drying of the paste. At a certain temperature, this generates micropores in the interface zones between the successively deposited layers. These micropores are filled with air and are retained during sintering.

[0046] Figures 2 to 5 illustrate this first embodiment, in this case in the manufacture of a magnetic core in the form of a torus.

[0047] Figure 2 shows how step a) of printing a first layer of paste is carried out at room temperature on the SBT substrate, which is heated to a temperature above room temperature. This figure also shows the printing nozzle BS, a bead of paste CP (whose diameter D, measured at the nozzle, corresponds to the internal diameter of the printing nozzle) extruded from the nozzle and deposited at a given speed V and from a certain height H (distance between the tip of the nozzle and the substrate). These parameters are adjustable and characterize the geometry of the successively deposited layers and thus ultimately of the magnetic core. In particular, the height H can be less than the diameter D, so that the nozzle BS itself can mechanically constrain the paste bead to have a width D' greater than the diameter D.

[0048] Figure 3 shows a cross-sectional view of a magnetic core, in this case in the form of a torus (a non-limiting form for implementing the invention), after carrying out all the steps of the process. It is understood Step e) was repeated N = 2 times to ultimately obtain 5 layers. The MCP micropores are located at the various interfaces between successive layers and are found across the entire width of these layers, namely between the inner and outer diameters of the torus. This is advantageous for use in the magnetic core since the different layers are separated in the direction of the magnetic flux intended to flow through the torus (this direction is that of the longitudinal axis of the torus, perpendicular to the diameter): this generates a segmentation that helps to attenuate dimensional resonance.

[0049] Finally, [Fig.4] is a schematic representation of [Fig.3] allowing for a better visualization of the discrete electrical insulation ZON zones between two successive layers.

[0050] It is understood that printing makes it possible to obtain any desired geometries, without any particular limitations. The magnetic core is segmented into several layers along its thickness, with discrete electrical insulation, which allows operation at high frequencies without encountering dimensional resonance problems and at the same time, its total thickness remains adjustable with the number N of layers deposited, which makes it possible in use to design cores with high inductance.

[0051] An example of an embodiment according to this first embodiment is given below.

[0052] We are interested here in the fabrication of a magnetic core based on MnZn ferrite. The The paste is made from a liquid solution (deionized water) containing the following additives: polyvinyl acid (PVA, Acros Organics, USA - chemical formulation: [-CH2CH(OH)-]n), Pluronic® F-127 (Sigma Aldrich, France - a PEO-PPO triblock copolymer with the chemical formula (C3H60.C2H40)x), and Disperbyk® 111 (BYK, Germany - a mixture of acids), to which MnZn ferrite powder, namely (MnZn)Fe2O4, with a particle size between 10 and 130 microns, is added. Disperbyk® 111 is an additive that disperses the ferrite powder in the liquid solution. Pluronic® F-127 is an organic binder that transforms into a paste at a temperature of 18°C. A paste can thus be made from a liquid solution by controlling the temperature of the medium. Furthermore, the paste can be agitated, which allows it to be sheared in a controlled manner and ultimately to give it a certain viscosity. On the [Fig.[5] The evolution of the viscosity (on the y-axis) of a paste (temperature of 25°C) obtained from a solution as described above, with a ferrite powder mass content of 82%, is also shown as a function of the shear rate (on the x-axis). This figure demonstrates that the paste exhibits shear-thinning behavior. Furthermore, a regular shape can be achieved in the filaments extruded by the printing nozzle at room temperature (typically between 20°C and 25°C).

[0053] The diameter of the printing nozzle is fixed at 1mm. This nozzle diameter defines the diameter of the (cylindrical) paste bead intended to be extracted from the nozzle.

[0054] The printing height (distance from nozzle outlet to substrate or distance from nozzle outlet to previously deposited paste layer) is between 0.2 mm and 0.3 mm. Note that this height is less than the diameter of the filament bead, and therefore the nozzle, through mechanical action, flattens the filament to create a relatively flat layer.

[0055] The printing speed is set at 2mm / s. This speed allows control of the deformations of the deposited paste bead, with regard to the viscosity of the paste.

[0056] The substrate is heated to 30°C. This heating allows control of the paste's viscosity and therefore, in conjunction with the printing speed, contributes to the correct definition of the geometry of the deposited paste bead. The heating also ensures the paste dries. Moreover, once completely dry, the deposited bead is rigid because Pluronic® F-127 is rigid at 30°C. The choice of this temperature also allows for the creation of discrete pores (electrical insulation) on the surface of the newly deposited paste layer. Indeed, the drying process induces a loss of wettability and adhesion to the surface (upper, opposite the substrate) of the newly deposited layer, which generates microporosity.

[0057] In the present example, we observe the presence of discrete MCP micropores ([Fig.3]) with dimensions between 8.4 and 15.6 microns (12 ± 3.6 microns).

[0058] Debinding allows the degradation of additives still present after printing. In this case, it is carried out by a heat treatment. For this purpose, the toroidal stack is placed in an oven. The oven temperature is maintained at a plateau of 400°C for 4 hours, under air. The oven can be brought to this plateau temperature from ambient temperature up to 400°C with a ramp rate of 0.5°C / min. This debinding process is relatively slow to avoid any cracking of the part being treated. Indeed, debinding releases gases resulting from the degradation of the organic additives (PVA and Pluronic® F-127).

[0059] Once the organic additives have been removed, sintering is carried out. This can be done, in particular, following debinding, in the same furnace, by applying a temperature ramp from a plateau of 400°C to a plateau, here set at 1160°C, and maintaining this temperature for 4 hours, under air but with the addition of 0.7% dioxygen (partial pressure). The temperature ramp for moving from one plateau to the other can have a slope of 10°C every 3 minutes. Sintering consolidates and densifies the part in order to increase the mass density and reduce intragranular and intergranular porosity. In addition, the presence of dioxygen during this sintering allows the formation of the spinel magnetic phase of the MnZn ferrite.

[0060] Debinding (thermal) and sintering has no effect on the micropores which remain present after manufacturing.

[0061] Figures 6 and 7 provide results relating to the magnetic behavior of a magnetic core obtained according to the process according to the invention (namely with discrete electrical insulation) and according to the previous Part (solid magnetic core).

[0062] Thus, [Fig. 6] is a graph representing the evolution of the real (p') and imaginary (p'') permeability as a function of frequency, for a magnetic core obtained according to the example considered (C'inv curve as a solid line for the real part and C''inv as dashed lines for the imaginary part) and for a magnetic core obtained with a prior art manufacturing process (C'ref curve for the real part and C''ref for the imaginary part: solid core). The magnetic permeability curves show the appearance of a resonance frequency for the part obtained with the process according to the invention that is higher than that of the part according to the prior art.

[0063] Figure 7 is a graph showing the evolution of magnetic losses as a function of frequency, under a fixed magnetic field (50 mT), for the same magnetic cores as those in Figure 6. It can be seen that the magnetic losses are greater in the solid magnetic core (Cref) than in the magnetic core obtained according to the invention (Cinv) when the frequencies become higher. This also demonstrates the advantage of implementing the invention.

[0064] In a second embodiment, to create a discrete electrical insulation between two successive layers of the magnetic core, a discrete set of micropores is created by mechanical action after each deposition of a layer of paste on the surface of said layer.

[0065] The mechanical action can, in particular, by way of non-limiting example, be carried out by a PGN comb-shaped tool whose teeth indent (or punch) the surface of the deposited paste (bead) to create hollows which will be covered later by another bead, thus forming closed cavities which have the same characteristics as micropores.

[0066] Such a step is illustrated in [Fig.8].

[0067] The magnetic core obtained at the end of the manufacturing process then exhibits micropores which are distributed regularly, unlike the magnetic core obtained with the first embodiment, in which the micropores are distributed more randomly. This regular distribution reduces the inhomogeneity of the magnetic flux generated within the magnetic core during operation.

[0068] Furthermore, the pore size can be controlled according to the characteristics of the tool (dimensions) and the depth of penetration of the tool into the paste. One can Typically, micropores with a size between 10 and 20 microns are used, although more commonly, the micropore size can range from 10 to 500 microns. For the magnetic core obtained with the first embodiment, the micropore size exhibits some variation (in the example given previously, the micropore size was 12 ± 3.6 microns). Controlling the pore size allows for control of the magnetic core's performance.

[0069] It should be noted that, unlike the first embodiment, the substrate remains at ambient temperature during the implementation of the manufacturing process according to the second embodiment.

[0070] In a third embodiment, to create discrete electrical insulation between two successive layers of the magnetic core, electrically insulating microparticles MP are sprayed onto the surface of said layer, for example with a PST gun. These particles then become embedded in the paste on the surface of each layer before being covered by another layer of CP paste. They naturally remain in place after the debinding and sintering steps.

[0071] Such a step is illustrated in [Fig.9].

[0072] Electrically insulating microparticles can have an average size of between 10 and 500 microns, in particular between 10 and 20 microns.

[0073] Electrically insulating microparticles can in particular be metal oxides. By way of non-limiting examples, alumina (Al2O3), titanium dioxide (TiO2), copper(I) oxide (CuO2), copper(II) oxide (CuO), zinc oxide (ZnO) or a mixture of these different elements can be chosen.

[0074] Unlike the first two embodiments, the magnetic core thus obtained does not have micropores (filled with air), but microparticles which nevertheless fulfill the same function.

[0075] The microparticles may exhibit a somewhat dispersed (and therefore non-regular) distribution, but this is not random due to the use of a spray. As for the dimensions of the microparticles, they may also exhibit some dispersion, which will depend on their manufacturing process.

[0076] Finally, whatever the embodiment envisaged, the manufacturing process results in a magnetic core formed of several layers of the same magnetic material stacked one on top of the other, said layers being separated from each other by a discrete electrical insulation.

[0077] Discrete electrical insulation can be in the form of microporosity (air), or microparticles in an electrically insulating material.

[0078] Moreover, the discrete electrical insulation separating two layers occupies a surface area advantageously representing at least 50% of the interface between the two layers. This provides a noticeable electrical insulation effect. Even more advantageously, the discrete electrical insulation separating two layers occupies a surface area representing at least 70% of the interface between the two layers. This results in a very significant electrical insulation effect.

Claims

Demands

1. A method for manufacturing a magnetic core comprising the following steps, in order: a. printing a first layer of paste containing magnetic powder onto a substrate, b. creating a discrete electrical insulation on the previously deposited paste layer, c. printing another layer of paste containing magnetic powder on the previous one, d. creating a discrete electrical insulation on the previously deposited paste layer, e. repeating steps c) and d) N times), with N a natural number such that N > 1, f. printing a final layer of paste containing magnetic powder on the previous one to obtain a stack of layers with a discrete electrical insulation between two successive layers, g. performing a sintering of the stack thus obtained.

2. A method for manufacturing a magnetic core according to claim 1, wherein, in order to create the discrete electrical insulation on the previously deposited paste layer, the substrate is heated to a temperature above ambient temperature.

3. A method for manufacturing a magnetic core according to claim 1, wherein, in order to create discrete electrical insulation on the previously deposited paste layer, a discrete set of micropores is created by mechanical action on the surface of said layer.

4. A method for manufacturing a magnetic core according to claim 1, wherein, in order to create discrete electrical insulation on the previously deposited paste layer, electrically insulating particles are sprayed onto the surface of said layer.

5. A method for manufacturing a magnetic core according to any one of the preceding claims, wherein, prior to step a), a paste comprising said magnetic powder is prepared.

6. A manufacturing process according to any one of the preceding claims, wherein the paste comprising an organic binder is debinded between step f) and step g).

7. A method according to the preceding claim, wherein the unbinding is a thermal unbinding.

8. A method for manufacturing a magnetic core according to the preceding claim, wherein the debinding is a thermal debinding which takes place at a first temperature stage, between 250°C and 600°C, over a period of between 1h and 6h.

9. A method for manufacturing a magnetic core according to any one of the preceding claims, wherein the sintering is carried out at a second temperature step, between 900°C and 1200°C, for a duration between 1h and 24h, in particular between 1h and AK

10. UH. Magnetic core obtained according to the process of any one of the preceding claims, characterized in that it is formed of several layers of the same magnetic material stacked one on top of the other, said layers being separated from each other by a discrete electrical insulation.

11. Magnetic core according to the preceding claim, in which the discrete electrical insulation separating two layers occupies an area representing at least 50% and advantageously at least 70% of the interface between the two layers.

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