EP0764955A1 - Composite magnetic material with reduced permeability and losses - Google Patents

Abstract

Composite magnetic material having reduction in the permeability when exposed to a magnetic field of frequencies lower than approximately 100 MHz, consisting of ceramic polycrystalline magnetic plates (5) dispersed in a dielectric binder (6) and orientated so that their faces are parallel to the magnetic field and are in contact with each other. Also claimed are: (i) prodn. of the magnetic material; (ii) a magnetic core consisting of the magnetic material; (iii) inductor comprising the magnetic core; and (iv) a transformer comprising the magnetic core.

Description

The present invention relates to a composite magnetic material with reduced permeability and losses at frequencies below about 100 MHz.

The material is intended to produce in particular inductor or transformer cores.

Developments in electronic systems seek to miniaturize power sources. The transition from regulators with a linear structure to switching converters was a decisive step in reducing the size and improving the performance of power sources. The cutting frequency has continued to increase in order to continue miniaturization. Current converters reach and even exceed one megahertz. Low value inductance architectures (some micro-Henrys) should have low total losses (conductor and magnetic circuit losses) under strong induction and low permeability (less than about 200).

The magnetic materials with reduced permeability currently available on the market exhibit very high losses under strong induction (greater than around 10 mT) which make magnetic components today the most bulky components of converters. For existing magnetic materials, the low permeability and the low losses at high frequency are contradictory characteristics.

An inductance of a few micro-Henrys will include a few turns or a low-permeability core.

A low number of turns brought to a high potential difference generates a high magnetic induction in the nucleus. As the losses in the nucleus are at least proportional to the square of the induction, they increase very quickly when the number of turns decreases. To obtain reduced losses, a large number of turns is required, which imposes a core with low permeability.

There are air inductors with a non-magnetic core. Their permeability is equal to one and the losses in the nucleus are zero. Their size is important because of the permeability equal to one of the core non-magnetic. The "copper" losses dissipated by the winding are significant. The electromagnetic disturbances generated are troublesome for the neighborhood and difficult to eliminate.

There are inductors with a magnetic core in solid ferrite of the spinel type with localized air gap. Ferrite despite its losses of the order of a hundredth or a tenth of W / cm ³ , depending on the induction and the frequency, has permeabilities close to 1000 which is much too high for the application of converters. Ferrites with low permeability such as nickel ferrite, which has a permeability of 10, have losses that are too high for the application of converters.

There are also inductors with a composite magnetic core with distributed air gap. These materials consist of ferromagnetic powder alloys dispersed in a dielectric binder. The radiation losses are reduced compared to the nuclei with localized air gap. There are essentially two categories of powders: iron and iron-carbonyl powders whose permeability ranges from approximately 5 to 250 and powders based on iron-nickel alloys whose permeability ranges from approximately 14 to 550.

The losses in these materials are 15 to 20 times greater than those of massive power ferrites under the same frequency, induction and temperature conditions.

• . fer-carbonyle : pertes supérieures à 1,5 W/cm³
• . fer-nickel : pertes supérieures à 2 W/cm³

For example, the best composite magnetic materials on the market have the following characteristics (supplier catalog data) for toric samples with an average diameter of 10 mm, at room temperature, for an induction of 30 mT at 1 MHz:

• . iron-carbonyl: losses greater than 1.5 W / cm ³
• . iron-nickel: losses greater than 2 W / cm ³

The present invention provides a composite magnetic material which, when subjected to a magnetic field, exhibits both reduced losses and reduced permeability for frequencies below about 100 MHz.

This composite magnetic material exhibits losses approximately three to five times lower than those of the composite magnetic materials available on the market and a permeability approximately 10 to 100 times lower than the spinel type ferrites, at frequencies lower than approximately 100 MHz.

More specifically, the composite magnetic material according to the invention comprises magnetic particles dispersed in a dielectric binder, these particles being polycrystalline magnetic ceramic plates oriented so that their main faces are substantially parallel to the magnetic field.

The polycrystalline magnetic ceramic is advantageously a ferrite of the spinel type corresponding to the formula M _x Zn _y Fe _{2 + ε} O ₄ with x + y + ε = 1 where M is a manganese or nickel ion.

The binder is advantageously a resin, fluid initially hardening thereafter, such as an epoxy, phenolic, polyimide or acrylic-based resin.

The plates are oriented in strata, separated by binder. Each layer may include several plates separated by binder forming an air gap or a single plate.

The plates belonging to neighboring strata are preferably either staggered or in a column.

Several forms of plates can be envisaged, in particular the square, the torus or the torus portion. The choice depends on the final shape of the magnetic core produced with the material thus obtained.

• la réalisation d'une poudre magnétique céramique ;
• la réalisation, à partir de la poudre magnétique céramique d'une barbotine de coulage ;
• la découpe des plaquettes dans une pellicule de la barbotine séchée ;
• le frittage des plaquettes ;
• l'élaboration du matériau magnétique composite à partir des plaquettes frittées, dispersées dans le liant et dont les faces principales sont orientées par rapport au champ magnétique.

The present invention also relates to a method for producing such a composite magnetic material. This process includes the following steps:

• the production of a ceramic magnetic powder;
• the production, from ceramic magnetic powder, of a casting slip;
• cutting the wafers from a film of the dried slip;
• sintering of platelets;
• the development of the composite magnetic material from sintered wafers, dispersed in the binder and whose main faces are oriented relative to the magnetic field.

The casting slip can be obtained by mixing the ceramic powder, at least one binder, at least one solvent and optionally a deflocculant.

The orientation of the plates can be manual. You can stack the pads on top of each other and then compress them to break them.

They can be placed one next to the other, strata by strata.

Orientation can also be done by vibration or by a magnetic field.

The invention also relates to a core produced with such a composite magnetic material as well as an inductor or transformer comprising such a core.

• la figure 1 illustre l'évolution du pourcentage en oxygène de l'atmosphère pendant la phase de refroidissement du frittage des plaquettes ;
• les figures 2a, 2b, 2c, 2d deux exemples d'un noyau selon rinvention en vue de dessus et en coupe réalisé à partir de plaquettes en forme de tore ;
• les figures 3a, 3b un autre exemple d'un noyau selon l'invention en vue de dessus et de face ;
• les figures 4a, 4b, encore un exemple d'un noyau selon l'invention en vue de dessus et de face ;
• les figures 5a, 5b l'évolution des pertes totales d'un noyau selon l'invention en fonction de la température et de l'induction respectivement à 300 kHz et à 1 MHz ; (mesures réalisées en laboratoire)
• les figures 6a, 6b respectivement une inductance et un transformateur selon l'invention.

The invention will be better understood and other advantages will appear on reading the following description given by way of non-limiting example and the appended figures which represent:

• FIG. 1 illustrates the evolution of the oxygen percentage of the atmosphere during the cooling phase of the sintering of the wafers;
• Figures 2a, 2b, 2c, 2d two examples of a core according to the invention in top view and in section made from toroidal plates;
• Figures 3a, 3b another example of a core according to the invention in top view and front view;
• Figures 4a, 4b, yet another example of a core according to the invention in top view and front view;
• FIGS. 5a, 5b the evolution of the total losses of a core according to the invention as a function of the temperature and of the induction at 300 kHz and 1 MHz respectively; (laboratory measurements)
• Figures 6a, 6b respectively an inductor and a transformer according to the invention.

The composite magnetic material according to the invention comprises polycrystalline magnetic ceramic plates dispersed in a binder. The main faces of the plates are oriented substantially parallel to the magnetic field.

The magnetic ceramic can be a spinel type ferrite corresponding to the formula M _x Zn _y Fe _{2 + ε} O ₄ with x + y + ε = 1 where M is a manganese or nickel ion.

When they are massive, these ferrites have a permeability of between 500 and 3000.

The process for preparing the composite magnetic material, according to the invention, makes it possible to control the shape of the plates and their positioning in the composite so as to control its permeability and its losses.

The development of magnetic ceramic plates can be done by a conventional technique of developing ceramics. This technique is used in particular for the manufacture of alumina substrates, of casing or of multilayer ceramic capacitors.

After weighing, the raw materials necessary to obtain the magnetic ceramic can be mixed and ground in a jar containing steel balls in the aqueous phase. The purpose of this operation is to mix and reduce the size of the grains of the various constituents so as to make them more reactive. The mixture is then dried and then sieved. The powder thus obtained can be pre-sintered in an oven so as to obtain the desired crystalline phase. This operation is often called chamotte.

A second grinding can follow the chamotte to reduce the grains which have grown during the chamotte operation. This second grinding can be done under the same conditions as the first grinding.

A casting slip can be obtained by mixing the regrind powder with organic binders, solvents and optionally a deflocculant. This mixing can be done in a jar with steel balls using a mechanical stirrer. The slip after resting to allow the air bubbles formed during the agitation time to rise is cast in a strip on a bench on which slides a strip of mylar, for example, driven at constant speed. The bench is covered with a tunnel to avoid depositing dust and to slow down the evaporation of solvents. A knife held parallel to the mylar strip by micrometric screws forms an opening through which the slip passes. This opening determines the thickness of the cast strip. After evaporation and drying, the cast strip can be peeled off and cut using a cookie cutter. This ease of obtaining complex parts, toroids by example, is very interesting. Machining solid ferrites is slow and expensive because it requires diamond tools.

These plates can be cut into squares, for example, 2 mm x 2 mm or 4 mm x 4 mm or 7 mm x 7 mm. Thin toroids or portions of thin tori (eighth, quarter, half) can also be cut.

After cutting, the plates are sintered to ensure the cohesion of the powder grains.

Sintering is carried out, in particular for Mn-Zn ferrites under controlled partial pressure of oxygen in order to fix the level of divalent iron in the platelets.

In a final step, the wafers are oriented and incorporated into a fluid binder, an Araldite type resin for example, which ensures the mechanical cohesion of the composite material after hardening.

Example of realization

Production of a composite magnetic material according to the invention intended to operate at frequencies below about 100 MHz.

$${\text{95,75 g de MnCO}}_{\text{3}}$$

$$\text{17,52 g de ZnO}$$

$${\text{0,53 g de TiO}}_{\text{2}}$$

$$\text{1000 ppm de CaO}$$

The initial components are weighed: $${\text{193.37 g Fe}}_{\text{2}}{\text{<figure-callout id="3" label="O" filenames="imgf0002.png" state="{{state}}">O</figure-callout>}}_{\text{3}}$$

$${\text{95.75 <figure-callout id="3" label="g MnCO" filenames="imgf0002.png" state="{{state}}">g MnCO</figure-callout>}}_{\text{3}}$$

$$\text{17.52 g of ZnO}$$

$${\text{0.53 <figure-callout id="2" label="g TiO" filenames="imgf0001.png,imgf0002.png" state="{{state}}">g TiO</figure-callout>}}_{\text{2}}$$

$$\text{1000 ppm CaO}$$

The grinding is done with steel balls in deionized water.

After grinding, the mixture is dried in an oven and sieved through a 400 µm opening sieve.

Chamotte is done at 1100 ° C with a bearing time in air of 3 hours.

The new grinding is carried out under the same conditions as the first. It is followed by a new drying and sieving.

• la poudre précédemment obtenue ;
• deux solvants : éthanol et trichloréthylène ;
• des liants organiques : polyéthylène-glycol, diéthyl-hexylephtalate et polyvinyl-butyral ;
• un défloculent éventuel.

The pouring slip is prepared with:

• the powder previously obtained;
• two solvents: ethanol and trichlorethylene;
• organic binders: polyethylene glycol, diethylhexylephthalate and polyvinyl butyral;
• a possible deflocculent.

These components are mixed and stirred with steel balls for three hours. A rest of about half an hour before pouring.

After drying, the wafers are cut and then sintered.

• une montée en température à 600°C en 12 heures sous air ;
• une montée en température de 600°C à 1220°C en 6 heures ;
• un palier à 1220°C pendant 1 heure 30 ;
• une descente en température de 1220°C à 1200°C avec ajustement du pourcentage d'oxygène à 2,6 % dans l'atmosphère en 15 minutes ;
• un palier à 1200°C pendant 15 minutes avec le même pourcentage d'oxygène;
• un refroidissement de 100°C par heure avec baisse du pourcentage d'oxygène suivant la loi Log(PO₂) = f(1/T) représentée sur la figure 1. PO₂ est le pourcentage d'oxygène et T la température.

Sintering is carried out according to the following cycle:

• a temperature rise to 600 ° C in 12 hours in air;
• a temperature rise from 600 ° C to 1220 ° C in 6 hours;
• a plateau at 1220 ° C for 1 hour 30 minutes;
• a temperature drop from 1220 ° C to 1200 ° C with adjustment of the percentage of oxygen to 2.6% in the atmosphere in 15 minutes;
• a plateau at 1200 ° C for 15 minutes with the same percentage of oxygen;
• cooling to 100 ° C. per hour with a decrease in the percentage of oxygen according to the Log law (PO ₂ ) = f (1 / T) represented in FIG. 1. PO ₂ is the percentage of oxygen and T the temperature.

After sintering, the thickness of the plates varies between 100 µm and 130 µm.

The resin is poured before or after orientation, it depends on the orientation method used.

Orientation can be manual. This method is applicable for larger wafers, in particular toroids, torus portions, 7 mm x 7 mm squares.

Figures 2a, 2b show an O-ring made of magnetic material according to the invention. It is made from plates 10 in the form of a torus. We stack several on top of each other in strata. The stack is placed in a mold and the binder 20, resin of the epoxy, phenolic, polyimide or acrylic-based type, for example, is poured.

The binder 20 fills the spaces between the different strata.

To improve the performance of such a core, it is possible, after stacking the plates 10, to compress them so as to break them into pieces 1. It is preferable, beforehand, to secure the plates 10 using for example double sided tape. The binder is then added. Figure 2c is a top view of an O-ring obtained with this method and Figure 2d is a section. The different strata bear the reference 2. The binder fills the spaces on the one hand between the broken pieces 1 of the same torus and on the other hand between the different strata 2 of toruses.

The pieces 1 are then separated by air gaps 3 in resin. Two layers 2 are also separated by a layer 4 of resin.

The binder is fluid at first, and then hardens.

Figures 3a, 3b show a variant of an O-ring according to the invention. It is obtained from square plates 5. They are arranged stratum by stratum one next to the other flat in a crown, leaving a space 6 or gap between them. The plates of two neighboring strata are staggered.

Figures 4a, 4b also show a variant of an O-ring according to the invention. The plates 7 are eighths of a torus. They are laid out stratum by stratum one next to the other flat, in a crown, leaving a space or gap between them. The plates 7 of two neighboring strata coincide, they form columns. They could also have been staggered as in Figures 3a, 3b.

Instead of carrying out the orientation of the plates manually, it is possible to do it by vibration using a vibrating spatula for example. This method, which can be used for industrial application, is suitable for smaller wafers.

Another method which can be used for industrial application and which is suitable for small wafers is magnetic orientation. It leads to better precision than orientation by vibration.

The plates are placed in a transparent container closed by a perforated stopper. The container is placed in the air gap of an electromagnet. A magnetic field is created in the air gap. By rotating the container on itself in the magnetic field, the platelets are regularly arranged in several strata and visual control is easy. The position of the plates can be fixed by pushing the plug to keep the strata in contact.

In the latter two methods, the binder can be added before or after orientation.

Depending on the frequencies of use and the desired apparent permeability of the composite magnetic material, a solid ferrite optimized in frequency is chosen and the dimensions of the air gaps between plates are determined.

Total loss measurements per unit volume of a core according to the invention in composite magnetic material as a function of temperature and induction are recorded in the diagrams of FIGS. 5a and 5b. These measurements are made for a torus with MnZn ferrite plates at a frequency of 300 kHz for FIG. 5a and at a frequency of 1 MHz for FIG. 5b. In both cases, the volume charge rate of the magnetic plates is 42%.

Very low losses are observed over a wide temperature range, and these losses are compatible with the majority of converter applications. The magnetic cores also have great temperature stability as illustrated in Figures 5a, 5b.

Under the same conditions, at 1 MHz, the losses of an iron-carbonyl composite toroid at 30 mT amount to at least 2.5 W / cm ³ at 80 ° C.

A core according to the invention has losses equal to 0.5 W / cm ³ as illustrated in FIG. 5b, hence a gain of a factor of 5.

• plaquettes 4 mm x 4 mm en quinconce
  • * taux de charge volumique compris entre 21 et 29 %
  • * perméabilité 17
    
• plaquettes 4 mm x 4 mm en colonnes
  • * taux de charge volumique compris entre 18 et 25 %
  • * perméabilité 17
  • * T = 30°C
    
• plaquettes 7 mm x 7 mm en quinconce
  • * taux de charge compris entre 28 et 40 %
  • * perméabilité 60
  • * T = 60°C
    
• plaquettes 2 mm x 2 mm orientées sous champ magnétique
  • * taux de charge volumique compris entre 30 et 42 %
  • * perméabilité 40
  • * T = 60°C
    
• plaquettes en 1/8 de tore, 8 strates
  • * taux de charge volumique compris entre 39 et 55 %
  • * perméabilité 60
  • * T = 60°C
    
• plaquettes en tore 12 strates
  • * taux de charge volumique compris entre 59 et 83 %
  • * perméabilité 60
  • * T = 60°C
    
• Plaquettes en tore empilées, cassées, imprégnées
  • * taux de charge volumique compris entre 40 et 56 %
  • * perméabilité 60
  • * T = 60°C
    

The following tables record the values of the total losses per unit of volume (W / cm ³ ) as a function of the induction B, at 1 MHz for different variants of toric cores according to the invention.

• 4 mm x 4 mm staggered plates
  • * volume loading rate between 21 and 29%
  • * permeability 17
    
• 4 mm x 4 mm plates in columns
  • * volume loading rate between 18 and 25%
  • * permeability 17
  • * T = 30 ° C
    
• 7 mm x 7 mm staggered plates
  • * charge rate between 28 and 40%
  • * permeability 60
  • * T = 60 ° C
    
• 2 mm x 2 mm plates oriented under magnetic field
  • * volume loading rate between 30 and 42%
  • * permeability 40
  • * T = 60 ° C
    
• 1/8 torus plates, 8 strata
  • * volume loading rate between 39 and 55%
  • * permeability 60
  • * T = 60 ° C
    
• torus plates 12 strata
  • * volume loading rate between 59 and 83%
  • * permeability 60
  • * T = 60 ° C
    
• Stacked, broken, impregnated torus plates
  • * volume loading rate between 40 and 56%
  • * permeability 60
  • * T = 60 ° C
    

Figures 6a, 6b schematically show an inductor and a transformer according to the invention.

s'établissant dans le noyau est matérialisé par le cercle en traits pointillés.The inductance of FIG. 6a comprises an O-ring core made of composite magnetic material according to the invention. This core is formed of plates 70 in quarter torus dispersed in the dielectric binder. There are several layers separated by the binder and each layer has four plates 70 separated by an air gap 71. Around the core is a coil 72. The magnetic field $\overrightarrow{\text{H}}$

establishing itself in the nucleus is materialized by the circle in dotted lines.

s'établissant dans le noyau est matérialisé par les pointillés. Les faces principales des plaquettes sont sensiblement parallèles au champ magnétique $\overrightarrow{\text{H}}$

.The transformer of Figure 6b has an E-shaped core with rectangular legs including a central 760 and two ends 761, made of composite magnetic material according to the invention. This core comprises square plates 73 embedded in the binder. Two windings 74, 75 around the extreme legs 761 contribute to forming the primary and the secondary of the transformer. The two coils could have been around the central leg 760. The magnetic field $\overrightarrow{\text{H}}$

establishing itself in the nucleus is materialized by the dotted lines. The main faces of the plates are substantially parallel to the magnetic field $\overrightarrow{\text{H}}$

.

The cores according to the invention have been shown in torus or in E but the invention is not limited to these types. It applies to other U-shaped types, pots etc.

Claims (19)

Composite magnetic material with reduced losses and permeability when subjected to a magnetic field at frequencies below about 100 MHz, comprising polycrystalline magnetic ceramic plates (5) dispersed in a dielectric binder (6), oriented their main faces are substantially parallel to the magnetic field, characterized in that the plates are in contact with each other. Magnetic material according to claim 1, characterized in that the polycrystalline magnetic ceramic is a spinel type ferrite corresponding to the formula M _x Zn _y Fe _{2 + ε} O ₄ with x + y + ε = 1, where M is a manganese ion or nickel. Magnetic material according to one of claims 1 or 2, characterized in that the binder is an epoxy, phenolic, polyimide or acrylic-based resin. Magnetic material according to one of claims 1 to 3, characterized in that it comprises several layers (2) formed of one or more plates (10), these layers (2) being separated by binder (20). Magnetic material according to claim 4, characterized in that, when a layer comprises several plates (5), they are separated by binder constituting an air gap (6). Magnetic material according to one of claims 4 or 5, characterized in that the plates (5) belonging to neighboring strata are staggered. Magnetic material according to one of claims 4 or 5, characterized in that the plates (7) belonging to neighboring strata are in columns. Magnetic material according to one of claims 1 to 7, characterized in that the plates are squares, toroids or torus portions. Magnetic material according to claim 8, characterized in that the plates (10) are broken into pieces (1). Process for the preparation of the composite magnetic material according to one of Claims 1 to 9, characterized in that it comprises the following stages: - the production of a ceramic magnetic powder; - The production, from ceramic magnetic powder, of a casting slip; - cutting the wafers from a film of the dried slip; - sintering of platelets; - The development of the composite magnetic material from sintered plates dispersed in the binder and whose main faces are oriented relative to the magnetic field. Method according to claim 10, characterized in that the casting slip is obtained by mixing the ceramic powder, at least one binder, at least one solvent and optionally a deflocculant. Method according to one of claims 10 or 11, characterized in that the orientation of the plates is manual. Method according to claim 12, characterized in that the plates are stacked on top of each other and then compressed to break them. Method according to claim 12, characterized in that the plates are deposited one next to the other, strata by strata. Method according to one of claims 10 or 11, characterized in that the plates are oriented by vibration. Method according to one of claims 10 or 11, characterized in that the plates are oriented by a magnetic field. Magnetic core characterized in that it is made of a magnetic material according to one of claims 1 to 9. Inductor characterized in that it comprises a core according to claim 17. Transformer characterized in that it comprises a core according to claim 17.

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