EP0764955B1 - 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 magnetic material composite with reduced permeability and losses at frequencies below about 100 MHz.

The material is intended to produce in particular cores inductors or transformers.

Electronic systems developments seek to miniaturize power sources. The transition from regulators to linear structure to switching converters was a decisive step for reducing bulk and improving performance power sources. The switching frequency has not stopped increase in order to continue miniaturization. Converters current ones reach and even exceed a megahertz. Architectures low value inductors (some micro-Henrys) should have low total losses (conductor and magnetic circuit losses) under strong induction and low permeability (less than about 200).

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

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

A small number of turns brought to a potential difference high generates high magnetic induction in the nucleus. Like the losses in the nucleus are at least proportional to the square of the induction, they grow very quickly when the number of turns decreases. To get reduced losses, it takes a large number of turns 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 nonmagnetic. 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 are made of ferromagnetic alloys powder dispersed in a dielectric binder. Radiation losses are reduced compared to the nuclei with localized air gap. There is essentially two categories of powders: iron and iron carbonyl powders whose permeability ranges from approximately 5 to 250 and the powders based on iron-nickel alloys whose permeability ranges from about 14 to 550.

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

• 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

Document DE-A-42 14 376 also discloses a material magnetic for power core composed of a homogeneous mixture of ferrite and synthetic material.

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

This composite magnetic material has losses approximately three to five times weaker than that of magnetic materials composites available on the market and permeability around 10 to 100 times weaker than spinel type ferrites, at lower frequencies at around 100 MHz.

More specifically, the composite magnetic material according to the invention comprises magnetic particles dispersed in a binder dielectric, these particles being magnetic ceramic plates polycrystalline 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 in a first then hardening time, such as an epoxy, phenolic resin, polyimide or acrylic based.

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

Platelets belonging to neighboring strata are preferably either staggered or in columns.

Several forms of platelets are possible, including square, the torus or the torus portion. The choice depends on the final form of the magnetic core made 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 pouring slip can be obtained by mixing the ceramic powder, at least one binder, at least one solvent and optionally a deflocculent.

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

They can be placed next to each other, strata by strata.

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

The invention also relates to a core produced with such a material. magnetic composite as well as an inductor or transformer having 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 l'invention 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 wafers in the form of a torus;
• 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 massive, these ferrites have a permeability understood between 500 and 3000.

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

The development of magnetic ceramic plates can be to do it by a classic technique of making ceramics. This technique is used in particular for the manufacture of alumina substrates, of multilayer ceramic boxes or capacitors.

After weighing, the raw materials necessary to obtain magnetic ceramic can be mixed and ground in a jar containing steel balls in aqueous phase. This operation has for purpose of mixing and reducing the grain size of the different constituents so as to make them more responsive. The mixture is then dried and Thames. The powder thus obtained can be pre-sintered in an oven so 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 powder regrind with organic binders, solvents and possibly a deflocculent. This mixture can be done in a jar with beads steel using a mechanical stirrer. The slip after a rest for allow the air bubbles formed during agitation to rise again strip casting on a bench on which a strip of mylar slides, by example, driven at constant speed. The bench is covered with a tunnel to avoid dust deposits and to slow down the evaporation of solvents. A knife held parallel to the mylar strip by screws micrometric forms an opening through which the slip passes. This opening determines the thickness of the cast strip. After evaporation and drying, the casting strip can be peeled off and cut using a cookie cutters. This ease of obtaining complex parts, toroids by example, is very interesting. Machining of massive ferrites is slow and expensive because it requires diamond tools.

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

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

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

In a final step, the plates are oriented and incorporated into a fluid binder, an Araldite type resin (registered trademark) 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.

193,37 g de Fe₂O₃

95,75 g de MnCO₃

17,52 g de ZnO

0,53 g de TiO₂

1000 ppm de CaO

The initial components are weighed:

193.37 g Fe ₂ O ₃

95.75 g MnCO ₃

17.52 g of ZnO

0.53 g TiO ₂

1000 ppm CaO

The grinding is done with steel balls in water deionized.

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 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 constituents are mixed and stirred with steel balls During three hours. A rest of about half an hour precedes the casting.

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 drop 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 applies for larger wafers, in particular the toroids, the portions of torus, the squares of 7 mm x 7 mm.

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

The binder 20 fills the spaces between the different strata.

To improve the performance of such a kernel, it is possible, after having stacked the plates 10 to compress them so as to break into pieces 1. It is preferable, beforehand, to join the plates 10 using for example double-sided adhesive 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 tori.

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 layer by layer next to each other flat in a crown leaving a space 6 or between them. Two pads neighboring strata are staggered.

Figures 4a, 4b again show a variant of a core toroid according to the invention. The plates 7 are eighths of a torus. They are arranged layer by layer next to each other flat, in crown by leaving a space or gap between them. Platelets 7 of two neighboring strata coincide, they form columns. They could also have been staggered as in the figures 3a, 3b.

Instead of manually orienting the platelets, it is possible to do it by vibration using a vibrating spatula by example. This method usable for an industrial application is suitable to smaller platelets.

Another method usable for industrial application and suitable for small wafers is the magnetic orientation. She leads to better accuracy than orientation by vibration.

The plates are placed in a closed transparent container by a holey plug. The container is placed in the air gap of a electro magnet. A magnetic field is created in the air gap. Doing turn the container on itself in the magnetic field, the plates are regularly arranged in several strata and visual control is easy. The position of the pads can be fixed by pushing in the plug to keep the strata in contact.

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

According to the frequencies of use and the apparent permeability desired magnetic composite material, we choose a solid ferrite optimized in frequency and the dimensions of the air gaps between platelets.

Total loss measurements per unit volume of a nucleus according to the invention in composite magnetic material depending on the temperature and induction are recorded on the diagrams of Figures 5a and 5b. These measurements are made for a ferrite plate toroid MnZn at a frequency of 300 kHz for Figure 5a and at a frequency of 1MHz for Figure 5b. In both cases, the volume loading rate of magnetic wafers is 42%.

Very low losses are observed over a wide range of temperature, and these losses are compatible with the majority of converter applications. The magnetic cores present in plus high 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 a inductor and a transformer according to the invention.

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

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 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 H .

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

Claims (18)

1. Composite magnetic material having reduced losses and a reduced permeability when it is subjected to a magnetic field at frequencies below approximately 100 MHz, characterized in that it comprises several strata (2) formed from one or more wafers (10) made of a polycrystalline magnetic ceramic powder, the wafers being dispersed in a binder (20) and oriented so that their main faces are substantially parallel to the magnetic field and not in contact with one another.
2. Magnetic material according to Claim 1, characterized in that the polycrystalline magnetic ceramic is a spinel-type ferrite satisfying the formula M_xZn_yFe_2+εO₄ with x+y+ε = 1, where M is a manganese or nickel ion.
3. Magnetic material according to either of Claims 1 and 2, characterized in that the binder is an epoxy, phenolic, polyimide or acrylic-based resin.
4. Magnetic material according to one of Claims 1 to 3, characterized in that, when a stratum comprises several wafers (5), they are separated by a binder constituting an airgap (6).
5. Magnetic material according to one of Claims 1 to 4, characterized in that the wafers (5) belonging to neighbouring strata are in a staggered configuration.
6. Magnetic material according to one of Claims 1 to 5, characterized in that the wafers (7) belonging to neighbouring strata are in a column configuration.
7. Magnetic material according to one of Claims 1 to 6, characterized in that the wafers are squares, tori or portions of a torus.
8. Magnetic material according to one of Claims 1 to 7, characterized in that the wafers (10) are broken into pieces (1).
9. Process for producing the composite magnetic material according to one of Claims 1 to 8, characterized in that it comprises the following steps:

   the production of a ceramic magnetic powder;

   the production of a casting slip from the ceramic magnetic powder;

   the cutting-out of the wafers from a film of the dried slip;

   the sintering of the wafers;

   the formation of the composite magnetic material from the sintered wafers which are dispersed in the binder and the main faces of which are oriented with respect to the magnetic field.
10. Process according to Claim 9, characterized in that the casting slip is obtained by mixing together the ceramic powder, at least one binder, at least one solvent and optionally a deflocculant.
11. Process according to either of Claims 9 and 10, characterized in that the wafers are oriented manually.
12. Process according to one of Claims 9 to 11, characterized in that the wafers are stacked on one another and then compressed in order to break them.
13. Process according to one of Claims 9 to 12, characterized in that the wafers are deposited one beside another, strata by strata.
14. Process according to either of Claims 9 and 10, characterized in that the wafers are oriented by vibration.
15. Process according to either of Claims 9 and 10, characterized in that the wafers are oriented by a magnetic field.
16. Magnetic core, characterized in that it is made of a magnetic material according to one of Claims 1 to 8.
17. Inductor, characterized in that it includes a core according to Claim 16.
18. Transformer, characterized in that it includes a core according to Claim 16.

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