EP3039694B1 - Method for producing a monolithic electromagnetic component - Google Patents

Description

The present invention relates to a method of manufacturing monolithic electromagnetic components.

More specifically, the invention relates to a method of manufacturing a monolithic electromagnetic component comprising several elements including a magnetic core of spinel ferrite and at least one planar coil comprising several turns.

Recent research in power electronics has focused on the miniaturization of converters and the electronic components they include, in particular on the size reduction of active and passive components.

In this context, there is a need for monolithic components capable of being integrated as closely as possible to semiconductors and of transferring increasingly large volume powers, that is to say of working at a higher frequency and of '' dissipate heat more efficiently.

In a known manner, certain spinel ferrites are used to manufacture this type of component by conventional sintering at temperatures of the order of 950 ° C. The ferrites obtained then exhibit good performance up to a few hundred megahertz, thanks to a high resistivity.

However, the manufacture of monolithic electronic components from these ferrites via known methods is only possible with coils made of noble metals of the silver or palladium type, which makes the manufacture of large quantities of these power components expensive. . In addition, the known manufacturing processes involve numerous distinct steps carried out in places distinct from each other, and sometimes lead to delaminations, cracks in the materials or material diffusions at the interfaces between the metal and the oxides.

It is also known from the documents “ XXlst Century Ferrites ” by Mazaleyrat F et al et al. JP-A-2010177319 complex manufacturing processes for monolithic electronic components.

One of the objects of the present invention is to provide a method for manufacturing a monolithic electromagnetic component which does not have these drawbacks.

To this end, the invention relates to a method according to claim 1.

According to other embodiments, the method according to the invention comprises one or more of the optional features of claims 2 to 8.

• la Figure 1 est une représentation schématique d'un composant électromagnétique monolithique ;
• la Figure 2 représente des vues en coupe d'un composant électromagnétique monolithique comprenant une unique bobine.
• la Figure 3 représente des vues en coupe d'un composant électromagnétique monolithique comprenant deux bobines.
• la Figure 4 est une illustration schématique d'un procédé selon l'invention de fabrication d'un composant électromagnétique monolithique ;
• la Figure 5 est une illustration schématique d'une étape du procédé de la Figure 4;
• la Figure 6 est une illustration schématique du spectre de perméabilité complexe d'un composant électromagnétique réalisé par un procédé de fabrication selon l'invention ;
• la Figure 7 est une illustration du spectre de perméabilité complexe d'un ferrite d'un composant électromagnétique réalisé par une variante d'un procédé de fabrication ne faisant pas partie de l'invention telle que revendiquée;
• la Figure 8 est une illustration schématique de la micrographie au microscope électronique à balayage, ainsi que l'analyse EDS de l'interface entre une bobine et du ferrite d'un composant électromagnétique monolithique réalisé par un procédé selon l'invention ;
• la Figure 9 est un diagramme de représentation de la mesure de l'inductance et du coefficient de surtension en fonction de la fréquence d'un composant électromagnétique monolithique réalisé par un procédé de fabrication selon l'invention; et
• la Figure 10 est un diagramme de représentation de l'inductance du primaire et du secondaire et du coefficient de surtension d'un composant électromagnétique monolithique réalisé par un procédé selon l'invention.

The invention will be better understood on reading the detailed description which follows, given solely for information and not limiting, and with reference to the appended drawings, in which:

• the Figure 1 is a schematic representation of a monolithic electromagnetic component;
• the Figure 2 shows sectional views of a monolithic electromagnetic component comprising a single coil.
• the Figure 3 shows sectional views of a monolithic electromagnetic component comprising two coils.
• the Figure 4 is a schematic illustration of a method according to the invention for manufacturing a monolithic electromagnetic component;
• the Figure 5 is a schematic illustration of a step in the process of Figure 4 ;
• the Figure 6 is a schematic illustration of the complex permeability spectrum of an electromagnetic component produced by a manufacturing process according to the invention;
• the Figure 7 is an illustration of the complex permeability spectrum of a ferrite of an electromagnetic component produced by a variant of a manufacturing process not forming part of the invention as claimed;
• the Figure 8 is a schematic illustration of the scanning electron microscope micrography, as well as the EDS analysis of the interface between a coil and the ferrite of a monolithic electromagnetic component produced by a method according to the invention;
• the Figure 9 is a diagram showing the measurement of the inductance and of the overvoltage coefficient as a function of the frequency of a monolithic electromagnetic component produced by a manufacturing method according to the invention; and
• the Figure 10 is a diagram showing the inductance of the primary and the secondary and of the overvoltage coefficient of a monolithic electromagnetic component produced by a method according to the invention.

With reference to the figure 1 , a monolithic electromagnetic component of general reference 10 produced by a method according to the invention, hereinafter component 10, comprises a base 12, a coil 14 arranged in the base 12, and an electrically insulating dielectric material 15.

In the example of Figure 1 , component 10 is an inductor intended to be used in conjunction with other electronic components, for example for producing power converters or filtering devices. In addition, it is intended to operate in a given frequency band preferably included in the frequency range 100 kHz - 30 GHz. Finally, it is manufactured according to the process according to the invention, as described below.

The base 12 constitutes the bulkier structure of the component 10 and gives it its general appearance.

The base 12 has a generally cylindrical shape with a longitudinal axis X-X ', height h and diameter d.

In the example of Figure 1 , the height h is between 1 and 2 mm, and the diameter d is between 8 and 20 mm.

As a variant, the diameter d is between 5 and 50 mm, and the height h is between 1 and 20 mm.

The base 12 has a high resistivity.

The base 12 is made from a spinel ferrite. Spinels are ferrites of the following general formula (G): AB ₂ - _δ O ₄ , where A is of medium valence 2 and is an element or a combination of elements of the group of cations preferably formed by Mg ²⁺ , Ni ^{2 +} , Co ²⁺ , Zn ²⁺ , V ²⁺ , Ti ²⁺ , Sc ²⁺ , Mn ²⁺ and possibly Fe ²⁺ , where B is of medium valence 3 and is an element or a combination of elements of the group cations preferably formed by Fe ³⁺ and Al ³⁺ , and where δ represents a possible defect in matter. The material defect δ can be intentionally introduced and is for example between 0 and 0.05. In addition, the spinel ferrites have the crystallographic structure of the reference compound MgAl ₂ O ₄ .

Preferably, the spinel ferrite of component 10 has a composition of formula (1) below:
Ni _x Zn _{1-xy-ε + δ} Cu _y Co _ε Fe _2-δ O ₄ , with 0.15 ≤ x ≤ 0.6; 0 <y ≤ 0.2; 0 ≤ ε ≤ 0.1 and 0 ≤ δ ≤ 0.05.

It was thus observed that the components 10 of which the ferrite of the base 12 exhibited the formula (1) exhibited good results in terms of performance. magnetic (low losses) in the frequency band between 300 kHz and 3 MHz in particular and densification during sintering at low temperature (below 1000 ° C).

As will be seen below, the ferrite 12 is obtained by densification of a mixture of nanometric oxides or by successive grinding and calcination of a mixture of nanometric oxides, the calcination being carried out at a temperature between 600 ° C and 1100 ° C.

For the components whose ferrite obeys formula (1), the nanometric oxides are oxides of zinc ZnO, copper CuO, nickel NiO, cobalt Co ₃ O ₄ and iron Fe ₂ O ₃ , the mixture also exhibiting a composition obeying formula (1).

By nanometric is meant that the particle size of the oxides can vary up to a maximum of 5 μm. The particle size is then determined as a function of the frequency at which component 10 is intended to operate.

In the example of Figure 1 , the diameter of the oxides used to produce the base 12 is between 230 and 270 nm, and is substantially equal to 250 nm on average.

The coil 14 is suitable for allowing the good circulation of electric currents through it and for being secured to the ferrite of the base 12 by co-sintering.

Preferably, the coil 14 is made from copper.

As a variant, it is made from a noble metal such as silver Ag or palladium Pd, or from an alloy of Palladium Pd, or from an alloy of Palladium Pd and silver Ag.

The coil 14 is at least partially embedded in the ferrite of the base 12.

Still with reference to the Figure 1 , the coil 14 comprises several turns 16 including an internal turn 161 and an external turn 162.

In the example of Figure 1 , the turns 16 have the general shape of a circular spiral and have a substantially circular section.

As a variant (not shown), the turns have the general shape of a square spiral.

The coil 14 also includes an inner leg 18 and an outer leg 19, which constitute bent ends of the inner turn 161 and the outer turn 162 respectively.

The coil also has a non-zero thickness e, is substantially planar and is orthogonal to the axis X-X ', so that the coil 14 is substantially included in a discoidal slice T of the base 12, orthogonal to the axis XX 'and of thickness e.

The internal 161 and external 162 turns respectively delimit an internal discoidal portion 20 and an external discoidal portion 22 of thickness e of the edge T and of the component 10.

In addition, two successive turns 16 of the coil 14 define a radial gap 24.

The Figures 2a to 2d represent embodiments of a component 10 produced by a method according to the invention comprising a single coil 14.

With reference to the Figure 2b , in the example of this Figure, the interstices 24, as well as the internal 20 and external 22 discoidal portions, are at least partially filled with dielectric material 15.

Only the upper and lower parts of the turns 16 of the coil 14 are in contact with the ferrite.

This example advantageously makes it possible to limit the parasitic capacitances which may appear between the turns 16 during the operation of the component 10 via the electrical insulation resulting from the presence of the dielectric material 15.

With reference to the Figure 2c , in the example of this Figure, the interstices 24 as well as the internal discoidal portion 20 are at least partially filled with dielectric material 15, and the external discoidal portion 22 is filled with ferrite.

This example is advantageously used in order to limit the parasitic capacitances which may appear between the turns 16 during the operation of the component 10, while minimizing the quantity of dielectric material 15 used.

With reference to the Figure 2a , in this example, the component 10 is devoid of dielectric material 15, the coil 14 thus being fully embedded in the ferrite of the base 12.

This variant is advantageously used when the frequency at which component 10 is intended to operate is less than 10 MHz. Beyond this value, the addition of dielectric material 15 is preferable.

With reference to the figure 2d , in this example, only the interstices 24 are at least partially filled with dielectric material 15.

The internal 18 and external 19 tabs are suitable for allowing the component 10 to be connected to other elements, for example to an electronic device to which it is integrated.

For this purpose, the inner 18 and outer 19 tabs are bent relative to the inner turn 161 and the outer turn 162 respectively.

The internal tab 18 is oriented along the axis X-X 'and has a length such that it is flush with the upper surface of component 10.

The outer tab 19 is oriented radially and has a length such that it is flush with the lateral surface of the component 10.

As a variant, the two tabs 18, 19 are oriented along the axis X-X 'and are flush with the upper and / or lower surface of component 10.

The tabs 18, 19 are intended to be brought into contact with an electrically conductive cable (not shown), for example directly or via a metal lacquer attached to the component 10 which makes it possible to facilitate the contacting of the cable with the tabs 18 , 19.

The Figures 3a to 3c illustrate three distinct embodiments of a variant of component 10 produced by a method according to the invention, and in which, in addition to the elements already described in the example of Figure 1 , component 10 comprises a second coil 14B.

The second coil 14B is at least partially embedded in the ferrite of the base 12.

The second coil 14B is substantially included in a discoidal slice T _B of the component 10 parallel to the slice T and spaced therefrom, so that the two slices T and T _B define between them a layer C of thickness c of the component 10.

In the example of Figure 3 , this coil 14B is substantially of the same structure and of the same dimensions as the coil 14.

As a variant, the second coil 14B has a number of turns different from the number of turns of the coil 14. This variant is advantageously implemented to modify the behavior of the coils 14, 14B under similar operating conditions.

In the example of Figures 3b and 3c , component 10 is a transformer or a magnetic coupler whose two coils 14, 14B are magnetically coupled and electrically isolated.

In this variant, during the operation of the component 10, the current entering one of the coils 14, 14B results in a current exiting by the other coil and magnetically induced therein.

The value of c is then predetermined as a function of criteria known to those skilled in the art, such as the desired value of the inductance of the coils, of the mutual inductance and of the coupling coefficient between the coils.

Thus, the value of c is between 100 μm and 1 mm.

When component 10 is a transformer, a value of c close to 100 μm is preferable. Conversely, when component 10 is a magnetic coupler, a value of c close to 1 mm is preferable.

In the example of Figures 3b and 3c , layer C is at least partially filled with dielectric material 15.

In the example of Figure 3b , only a portion of the layer C centered on the axis X-X ', of thickness c and of diameter substantially equal to the diameter of the outer turn 162 of the coil 14 is filled with dielectric material 15.

This example is advantageously implemented in order to limit the parasitic capacitances which may appear between the respective turns 16 of the two coils 14, 14B during the operation of the component 10, or when it is desirable to modify the topology of the magnetic field of each of the turns. 161.

In the example of Figure 3c , only a portion of the layer C centered on the axis XX 'and radially delimited on the one hand externally by the position of the outer turn 162 of the coil 14, and on the other hand internally by the position of the internal turn 161 coil 14, is filled with dielectric material 15.

This example is advantageously implemented in order to optimize the coupling between the coils, for example when component 10 is a magnetic coupler, and to limit the leakage fields liable to appear during operation of component 10.

In the example of Figure 3a , the component 10 does not include any dielectric material 15. The two coils 14, 14B are fully embedded in the ferrite of the base 12.

This example is advantageously used when it is desirable not to alter the magnetic field resulting from the flow of current in each of the turns 161.

As a variant (not shown), in addition to the elements already described in the exemplary embodiments of Figures 3b and 3c , the component 10 comprises at least two metallic layers parallel to the coils 14, 14B.

Two successive metal layers are then separated by a layer at least partially filled with dielectric material 15.

The manufacturing process 30 according to an example not forming part of the invention as claimed for the manufacture of a component 10 made from a ferrite of general composition (G) and preferably of composition (1) will now be described with reference to Figure 4 .

First of all, during an initial step 110, a precursor 32 of the ferrite which will make up the base 12 of component 10 is obtained.

The precursor 32 is a ferrite powder obtained by alternating successive grinding and calcinations of a mixture of nanometric oxides, said calcination being carried out at a temperature substantially between 600 ° C and 1100 ° C, and preferably substantially equal. at 760 ° C.

For a ferrite of composition (1), the precursor 32 is a ferrite powder obtained by alternating grinding and successive calcinations of a mixture of nanometric oxides of zinc ZnO, copper CuO, nickel NiO, cobalt Co ₃ O ₄ and iron Fe ₂ O ₃ , said calcination being carried out at a temperature substantially between 600 ° C and 1100 ° C, and preferably substantially equal to 760 ° C.

The purpose of the grinding is to reduce the diameter of the oxides, and thus to lower the sintering temperature of the ferrite powder obtained.

The purpose of the calcinations is to form the spinel phase of the ferrite, that is to say to transform the mixture of base oxides into a single phase of spinel structure.

The term “phase” is understood to mean crystallographic structure.

During grinding, it can happen that unwanted additions of iron are made via the tools used, such as steel balls.

The initial step 110 then comprises the compensation of these undesirable additions in the mixture obtained, for example by formation of an excess of iron oxide of the order of 5% for example.

In certain examples where the iron defect δ is not zero, the initial step 110 also comprises the removal of the corresponding quantity of iron from the precursor 32. This makes it possible to ensure the absence of Fe ²⁺ which could appear following a slight reduction during sintering (linked to the presence of carbon) or an addition of iron during grinding. Note that the presence of Fe ²⁺ must be avoided because it greatly increases the conductivity of the ferrite, which would produce additional losses by eddy currents during operation of the component. Also, preferably, element A of the general formula for ferrite is not iron or does not contain iron.

At the end of this initial step 110, the precursor 32 obtained is a ferrite powder whose composition obeys the general formula (G), preferably the formula (1), and of which the spinel phase is formed.

During a following preparation step 120, the elements of the component including the coil (s) 14 and other than the ferrite are embedded in a mold 34 in the precursor 32 of the ferrite.

It therefore takes place in a slightly variable manner depending on the structure of the component 10 that is to be obtained.

More specifically, with reference to Figures 2 and 5 , for a component with a single coil 14 and not comprising dielectric material 15, a first layer 36 of precursor 32 is deposited in the mold 34, on which the coil 14 is then deposited. Then a second layer 38 of precursor is deposited. 32 on the coil 14, so as to obtain the desired structure and component dimensions, the elements of component 10 not yet being secured to each other.

For a component with a single coil 14 comprising dielectric material 15, after having deposited the coil 14 on the first layer 36 of precursor 32, the dielectric material 15 is deposited on the coil 14 and the first layer 36, except at least the locations of the turns 16 of the coil 14, so as to form the structure of the desired T wafer ( Figures 2b, 2c and 2d ). Finally, a second layer 38 of precursor 32 is deposited, so as to obtain the general structure of the desired component 10, the elements not yet being secured to each other.

For a component 10 comprising two coils 14, 14B and dielectric material 15, after the first layer 36, a layer of dielectric material 15 is deposited so as to form the structure of the desired wafer T and of the layer C, then is deposited the second coil 14B. A second layer of dielectric material is then deposited with a thickness substantially equal to zero with the exception of at least the locations of the turns 16 of the second coil 14B, so as to form the structure of the desired _{wafer T B.} The second layer 38 of precursor 32 is finally deposited last.

For a component 10 with two coils 14, 14B not comprising any dielectric material, during step 120, the deposition of the layers of dielectric material 15 described above is then replaced by the deposition of layers of precursor 32.

This preparation step 120 is preferably carried out in a controlled environment, for example under a sealed hood, which has the effect of limiting the presence of parasitic particles which may be deposited in the mold and thus reduce the quality of the component 10 obtained.

This step 120 is for example carried out manually, or else automated by any suitable device.

The mold 34 is preferably made from graphite. As a variant, it is made from metal or from a refractory metal alloy, or from electrically conductive ceramic.

Following this preparation step 120, during a co-sintering step 130, the precursor 32 of the ferrite is secured to the other elements of the component 10 by co-sintering under load by pulsed electric current. By "under load" is meant that the elements of the component under subjected to a force, in particular an axial force tending to compress the components 10.

During a compression step 131 of this co-sintering step 130, the mold 34 obtained by the preparation step 120 is placed under neutral gas, and it is subjected to a uniaxial pressure of between 50 and 100 MPa. This pressure is represented by arrows on the Figure 5 . This pressure is maintained until the end of the co-sintering step 130.

As a variant, the mold 34 is placed under vacuum or under oxygen.

Then, during a discharge step 132 of this step 130 and which corresponds to the co-sintering by pulsed electric current itself, an electric current of controlled intensity i and between 1 A and 20,000 is discharged through the mold 34. A, and preferably between 1 A and 1000 A or between 1 and 10 A per square millimeter of component area. This makes it possible to raise the temperature in the mold 34 and to secure the elements of the component 10 to one another. The temperature inside the mold 34 is controlled by controlling the intensity of the current.

The discharge step 132 comprises a co-sintering stage, during which the temperature inside the mold 34 is maintained between 650 ° C and 850 ° C, and preferably between 700 ° C and 800 ° C. The co-sintering stage has a duration of between 1 min and 30 min.

The flow of the discharge step 132 is as follows. The temperature is initially increased at a rate of about 100 ° K per minute, from room temperature, to a value between the above values. The co-sintering stage is then carried out. Then, the temperature inside the mold 34 is quickly lowered by interrupting the current. As previously indicated, the uniaxial pressure resulting from the compressing step is maintained during the discharging step 132.

The average duration of the discharge step 132 is between 10 min and 60 min, and advantageously is substantially 20 minutes.

This discharge step 132 is preferably carried out in an automated manner, via a programmable device suitable for controlling the temperature in the mold 34, so that the temperature in the mold 34 is rapidly brought to a set temperature and maintained at this temperature during sintering bearings.

According to the invention, the precursor 32 obtained at the end of the initial step 110 is a mixture of nanometric oxides corresponding to the general formula (G), preferably to the formula (1) and whose spinel phase is not not formed.

To obtain this precursor 32, during the initial step 110, the various oxides are weighed, they are mixed and then the mixture obtained is ground in order to mix these oxides and decrease their diameter. As before, the iron input due to the grinding tools must then be compensated. No calcination takes place during this step, unlike the embodiment described above.

The following steps of the process 30 remain the same, with the exception of the discharge phase 132 during which a first reaction level is observed. The function of the first reaction stage is to effect the formation of the spinel phase of the precursor 32. This first reaction stage is carried out at a temperature between 400 ° C and 600 ° C. The first reaction stage is prior to the co-sintering stage.

The process according to the invention bears the name of reactive sintering, during which the mixture of ground oxides transforms into a spinel phase during the discharge phase 130, unlike the process described above which bears the name direct sintering and in which the precursor 32 is a crushed and calcined ferrite powder and the spinel phase of which is already formed at the end of the initial step 110.

• il n'est plus nécessaire de réaliser des calcinations lors de la phase initiale 110, de sorte que le procédé 30 selon l'invention est simplifié, la phase spinelle du
  ferrite se formant directement lors de la phase de décharge 132,
• elle permet l'obtention de noyaux magnétiques doux pour hautes fréquences et très hautes fréquences à partir d'un frittage réalisé à une température inférieure à celle des procédés connus.

This method has several advantages:

• it is no longer necessary to carry out calcinations during the initial phase 110, so that the method 30 according to the invention is simplified, the spinel phase of the
  ferrite forming directly during the discharge phase 132,
• it makes it possible to obtain soft magnetic cores for high frequencies and very high frequencies from sintering carried out at a temperature lower than that of the known methods.

As a variant (not shown), and not forming part of the invention as claimed, during the initial step 110, the precursor 32 of general composition (G), preferably of formula (1), is obtained. ), chemically, the initial steps 110 of the direct and reactive sintering processes described above corresponding to so-called solid routes. This variant makes it possible to obtain a ferrite powder of more homogeneous composition and having a particle size distribution that is narrower than by the solid route.

The precursor 32 obtained chemically is then a ferrite powder of general composition (G), the grains of which are particles of mixed spinels. For a ferrite powder of formula (1), the simple spinel particles are for example Fe3O4, NiFe2O4, CoFe2O4 or particles of more complex composition, such as for example of composition (1).

• La synthèse par coprécipitation, qui consiste en la précipitation de solutions aqueuses contenant les ions métalliques à concentration contrôlée pour former le ferrite de composition visée. La cinétique de précipitation est lente et la phase qui précipite est amorphe. La taille des nanoparticules obtenues est comprise entre 5 nm et 7 nm.
• La synthèse par Sol-gel, qui consiste en l'hydrolyse de solutions d'alkoxydes de formule Me(OR)n en milieu alcoolique. On obtient des solutions colloïdales où les nanoparticules sont maintenues en suspension avec une taille de l'ordre de 5 nm, que l'on précipite ensuite.
• La synthèse hydrothermale, qui consiste en la dissolution de composés précurseurs (ou de dérivés intermédiaires) du précurseur 32 lui-même, suivie d'une précipitation des solutions obtenues. La synthèse hydrothermale diffère des autres protocoles par les conditions de température et de pression mises en œuvre, et est réalisée à des températures comprises entre 90 °C et 500 °C dans un réacteur sous une pression de l'ordre de quelques dizaines d'atmosphères. Cette synthèse par voie hydrothermale est avantageuse car elle produit des poudres très fines, faiblement agglomérées, et bien cristallisées. En outre, elle se produit à relativement basse température, les poudres de ferrites peuvent être obtenues à l'état doux, c'est-à-dire présenter une aimantation spécifique à saturation élevée et à champ cœrcitif de faible valeur, les caractéristiques des particules synthétisées sont facilement contrôlables via le contrôle des conditions de la réaction (sa température, sa durée, etc.), et la poudre de ferrite obtenue est adaptée pour être frittée à basse température tout en produisant un matériau massif et dense.

The initial step 110 according to the chemical route is then carried out according to one of the following three protocols:

• Synthesis by coprecipitation, which consists of the precipitation of aqueous solutions containing metal ions at a controlled concentration to form ferrite of target composition. The precipitation kinetics are slow and the phase which precipitates is amorphous. The size of the nanoparticles obtained is between 5 nm and 7 nm.
• Synthesis by Sol-gel, which consists of the hydrolysis of solutions of alkoxides of formula Me (OR) n in an alcoholic medium. Colloidal solutions are obtained in which the nanoparticles are kept in suspension with a size of the order of 5 nm, which are then precipitated.
• Hydrothermal synthesis, which consists of the dissolution of precursor compounds (or of intermediate derivatives) of the precursor 32 itself, followed by precipitation of the solutions obtained. Hydrothermal synthesis differs from other protocols by the temperature and pressure conditions implemented, and is carried out at temperatures between 90 ° C and 500 ° C in a reactor under a pressure of the order of a few tens of atmospheres. . This synthesis by the hydrothermal route is advantageous because it produces very fine, weakly agglomerated and well crystallized powders. In addition, it occurs at relatively low temperature, ferrite powders can be obtained in a soft state, that is, have specific magnetization at high saturation and low value coercive field, the characteristics of the particles synthesized are easily controllable via the control of reaction conditions (its temperature, duration, etc.), and the resulting ferrite powder is suitable to be sintered at low temperature while producing a massive and dense material.

Depending on the reaction conditions and the synthesis protocol chosen, the precursor of the precursor 32 obtained at the end of the protocol may not have a formed spinel phase, or have a partially formed spinel phase.

In this case, the initial step 110 comprises an additional calcination phase aimed at forming the spinel phase of the precursor 32, so that the precursor 32 obtained at the end of step 110 has a formed spinel phase.

As a further variant, still not forming part of the invention as claimed, during the initial step 110, the precursor 32 is obtained by the so-called “polyol” route, during which simple acetate compounds are dissolved, nitrate and chloride in liquid polyols, such as 1,2-propanediol, 1,2-ethane diol, and bis (2-hydroxy ethyl) ether. Due to their fairly high dielectric constant which enables them to dissolve inorganic solids, these polyols constitute favorable media for obtaining various inorganic materials: metals, hydroxides and oxides. Complexes are then formed comprising alkoxy groups, from which oxides and hydroxides are obtained by hydrolysis and polymerization.

The competition between these reactions is controllable via the regulation of the hydrolysis rate and the reaction temperature. The control of the stages of germination and growth makes it possible to obtain nanometric, submicron and micron particles exhibiting optimized properties from which the precursor 32 is obtained.

As previously, depending on the conditions for carrying out the initial step 110 by the polyol route, the precursor of the precursor 32 obtained may not have a formed spinel phase, or have a partially formed spinel phase.

In this case, the initial step 110 comprises an additional calcination phase aimed at forming the spinel phase of the precursor 32, so that the precursor 32 obtained at the end of step 110 has a formed spinel phase.

• une poudre de ferrite présentant une phase spinelle formée obtenue par une alternance de broyages et de calcinations successifs d'un mélange d'oxydes nanométriques, et est obtenu par voie solide, variante ne faisant pas partie de l'invention telle que revendiquée, ou
• un mélange d'oxydes nanométriques ne présentant pas de phase spinelle et est obtenu par voie solide selon l'invention, ou
• une poudre de ferrite présentant une phase spinelle formée et est obtenu par voie chimique par synthèse par coprécipitation, par synthèse Sol-gel ou par synthèse hydrothermale, variante ne faisant pas partie de l'invention telle que revendiquée, ou
• une poudre de ferrite présentant une phase spinelle formée et est obtenu par voie polyol, variante ne faisant pas partie de l'invention telle que revendiquée.

In summary, the precursor 32 of general formula (G), preferably of formula (1), obtained at the end of the initial step 110 is:

• a ferrite powder having a formed spinel phase obtained by alternating successive grinding and calcinations of a mixture of nanometric oxides, and is obtained by the solid route, a variant not forming part of the invention as claimed, or
• a mixture of nanometric oxides not exhibiting a spinel phase and is obtained by the solid route according to the invention, or
• a ferrite powder having a formed spinel phase and is obtained chemically by synthesis by coprecipitation, by Sol-gel synthesis or by hydrothermal synthesis, a variant not forming part of the invention as claimed, or
• a ferrite powder having a formed spinel phase and is obtained by the polyol route, a variant not forming part of the invention as claimed.

The Applicant has successfully implemented the method described above and obtained, among other things, an example of component 10 whose ferrite of composition Ni _0.195 Cu _0.2 Zn _0.5999 Co _0.006 Fe ₂ O ₄ was co-sintered with a copper coil. 14 by direct sintering under a uniaxial pressure of 50 MPa, under argon, and at a temperature between 650 ° C and 800 ° C.

Component 10 which was obtained exhibits a magnetic moment at saturation equal to 54 Am ² / kg and a relative density greater than 90%.

The process according to the invention makes it possible to carry out the co-sintering of ferrites with metals other than the noble metals such as silver Ag or palladium Pd. In particular, it allows the production of monolithic components having one or more coils made from copper, which the known methods do not allow.

In fact, conventional sintering methods require prolonged exposure, for periods sometimes up to several days, of the elements of the component at temperatures relatively close to the melting point of copper.

This has the effect of inducing diffusions of copper in the ferrite, which degrades or even renders the components obtained unusable.

The components obtained by the process according to the invention are therefore of lower cost.

In addition, because it contains only a few steps, the method 30 decreases the risks of occurrence of an error in handling of the elements of the material, or of their degradation during their transport between the places where they are. take place respectively, so that the method according to the invention is generally safer and less expensive than the methods of manufacturing this type of known electronic components.

In addition, the process according to the invention does not exhibit any particular susceptibility to the dimensions of the desired components, unlike the processes such as the so-called LTCC process (which comes from the English “Low Temperature Cofired Ceramic) which can only achieve components of small size (maximum 10 mm in diameter and 2 mm in thickness, larger dimensions resulting in delaminations and cracks), so that the only limitations of the process fall within limits intrinsic to the materials used.

The components 10 obtained according to such a process 30 are not subject to any oversizing imposed by any limitations linked to their manufacturing process, and have a compactness of 100%.

Furthermore, the electromagnetic components obtained have a closed magnetic structure which completely confines the magnetic flux and prevents these components from radiating and interfering with neighboring components, so that the integration of the components 10 obtained by the method 30 is facilitated. .

Conversely, a process such as LTCC, which only allows small components to be produced, makes it very difficult to manufacture components in which the magnetic flux is confined, the components obtained proving to be complex to integrate.

With reference to the Figure 6 , which illustrates the complex permeability spectrum as a function of the frequency of an electromagnetic component 10 obtained by the reactive sintering process according to the invention with its real part, µ ', marked on the left scale and its imaginary part , µ ", on the right scale, we see that the initial permeability is close to 120 up to a frequency f _r equal to 10 MHz and decreases beyond that. As regards the imaginary permeability µ", it is less than 0.01 up to 2 MHz and increases beyond this until a resonant frequency f _r equal to 30 MHz. Thus the figure of merit µ '* f _r is equal to 6.6 GHz.

With reference to the Figure 7 , which illustrates complex permeability spectrum as a function of the frequency of a ferrite of a component 10 and produced by the direct sintering process not forming part of the invention as claimed with its actual permeability marked on the scale left and its imaginary permeability marked on the right scale, we see that the initial permeability µ 'is close to 60 up to a frequency equal to 10 MHz, it increases to 67 for a frequency equal to 50 MHz and decreases beyond that. As regards the imaginary permeability µ ", it is less than 0.01 up to 10 MHz and increases beyond this up to a resonant frequency f _r equal to 100 MHz. Thus the figure of merit µ '* f _r is equal to 6 GHz.

With reference to the Figure 8 , whose Figure 8a illustrates the scanning electron microscope (SEM) micrograph of the ferrite / copper interface of a component 10, and whose Figure 8b illustrates the EDS analysis of the interface between a coil 14 and the ferrite of this component 10, it can be seen from the Figure 8a that the mechanical strength after co-sintering is satisfactory. The interfaces are regular and show no delamination or cracks.

The Figure 8b shows that the border between the two elements is perfectly visible. The copper foil remains localized between the two layers of ferrite and is found to a thickness of 100 μm. In view of this Figure 8b , we can therefore conclude that the co-sintering is perfectly successful between the copper and the ferrite of the component 10 obtained.

As for them, the Figure 8c presents the micrograph of the BaTiO ₃ / Cu interface observed by SEM and the Figure 8d represents the EDS analysis of this interface.

Good mechanical strength is observed for the co-sintered part and a regular interface between the different materials. The copper remains well confined between the dielectric and ferrite layers. In addition, we do not find any of the elements of the dielectric in the copper layer and conversely, we do not find copper in the dielectric. This indicates that there was no diffusion between the different elements of each layer at the micron scale.

With reference to the Figure 9 , which presents as a function of the frequency the series inductor L _s in thick lines and in thin lines the overvoltage factor Q of an integrated monolithic inductor produced by the method according to the invention at 800 ° C for five minutes, under a uni-axial pressure of 50 MPa and under argon, it can be seen that the series inductance value L _s of this component 10 according to the invention is equal to 3.4 μH up to 10 MHz, the Q factor being greater at 35 to 1 MHz and canceling out at 10 MHz. The Figure 10 presents the measurements of the primary and secondary inductance of a transformer 10 without dielectric material 15 and operating from 100 kHz to 10 MHz as a function of frequency. This transformer 10 is produced by the manufacturing process not forming part of the invention as claimed during which the ferrite material NiZnCuFe ₂ O ₄ is co-sintered with a copper coil 14 of circular spiral shape by direct co-sintering at 800 ° C for five minutes under a uniaxial pressure of 50 MPa and under argon. The value of the primary and secondary inductance of this transformer 10 is marked on the left scale (in µH) and is close to 1.8 and 2.2 µH up to 10 MHz, the overvoltage coefficient being marked on the scale of right and being greater than 25 at 1 MHz and canceling out at 40 MHz.

A component 10 produced by a method according to the invention comprising a single coil 14 is for example an inductor intended to be used in a filtering device.

A component 10 produced by a method according to the invention comprising two coils 14, 14B is for example a transformer or a magnetic coupler.

Claims (8)

1. A method for producing a monolithic electromagnetic component (10) comprising several elements including a base (12) made from spinel ferrite and at least one planar coil (14, 14B) comprising several turns (16), one of the elements of the monolithic electromagnetic component (10) being a dielectric material,
   characterized in that it comprises the following steps:

   - during an initial step (110), a precursor (32) of the ferrite is obtained, the precursor (32) of the ferrite being a mixture of oxides which do no exhibit a spinel formed phase, the granulometry of the oxides being at most 5 µm,

   - during a preparation step (120), in a mold (34), the elements of the monolithic electromagnetic component (10), including said at least one coil (14, 14B) and other than the ferrites, are submerged in the precursor (32), and

   - during a co-sintering step (130), said precursor (32) is secured with the other elements of the monolithic electromagnetic component (10), including said at least one coil (14, 14B), by co-sintering under a load by pulsed electric current.
2. The method according to claim 1, characterized in that the or each coil (14, 14B) is made from copper.
3. The method according to claim 1 or 2, characterized in that the ferrite has a composition with formula Ni_xZn_1-x-y-ε+δCu_yCo_εFe_2-δO₄, with: $$0.15\le \mathrm{x}\le 0.6;$$

   

   $$0<\mathrm{y}\le 0.2;$$

   

   $$0\le \mathrm{\epsilon }\le 0.1;$$

   

   and $$0\le \mathrm{\delta }\le 0.05.$$

   
4. The method according to any one of the preceding claims, characterized in that the turns (16) of the or each coil (14, 14B) have a general circular spiral or square spiral shape.
5. The method according to any one of the preceding claims, characterized in that during the preparation step (120), a first precursor (32) layer (36) of the ferrite is deposited in the mold (34), then the other elements of the monolithic electromagnetic component are arranged, including the or each coil (14, 14B), then a second precursor (32) layer (38) is deposited.
6. The method according to any one of the preceding claims, characterized in that the co-sintering step (130) comprises the following steps:

   - a compression step (131), during which the mold (34) is subjected to a uniaxial pressure comprised between 50 and 100 MPa, and

   - a discharge step (132), during which an electric current with an intensity i comprised between 1 A and 20000 A, and preferably between 1 A and 1,000 A or between 1 and 10 A per square millimeter of component surface, is delivered through the mold (34), such that the temperature in the mold (34) rises and the elements of the monolithic electromagnetic component (10) become secured to one another.
7. The method according to claim 6, characterized in that the discharge step (132) comprises a co-sintering plateau during which the temperature inside the mold (34) is kept between 650°C and 850°C, preferably between 700°C and 800°C, for a duration comprised between 1 min and 30 min.
8. The method according to claim 6 or 7, characterized in that the discharge step (132) also comprises a first reaction plateau during which the temperature in the mold (34) is comprised between 400°C and 600°C, and during which the spinel phase of the precursor (32) forms.

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