WO2016097366A1

WO2016097366A1 - Sintered permanent magnet

Info

Publication number: WO2016097366A1
Application number: PCT/EP2015/080640
Authority: WO
Inventors: Florence Servant; Cyril RADO; Fernand Vial
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives; Vial Consulting
Priority date: 2014-12-18
Filing date: 2015-12-18
Publication date: 2016-06-23
Also published as: FR3030866B1; FR3030866A1

Abstract

A method for producing a magnet comprising the following steps: a) preparing a base powder (a): a1) producing a molten alloy having an R₂M₁₄B magnetic phase T1 and a rare-earth R rich phase, said initial feedstock comprising: - between 28% and 33% of said rare earth R, with Nd > 20%, - the remainder to 100%: metal element M chosen from the group consisting of Fe, Co, Ni and the mixtures of same, with Ni + Co < 5%, - between 0.95% and 1.2% of boron B, - less than 1% of pollutants, with O < 1000 ppm, a2) hydriding; a3) preferably, dehydriding; steps a2) and a3) leading to the transformation of said rare-earth R rich phase into a low melting-point phase, a4) crushing so as to obtain a base powder having a median size of between 1 μm and 7 μm; b) preparing an additive powder (b) comprising - more than 5% of particles of an R'F₃ heavy rare earth R' fluoride, - more than 5% of calcium fluoride CaF₂ particles and/or lithium fluoride LiF particles, c) preparing a feedstock by mixing said powders (a) and (b) and additive; d) shaping and magnetic orientation; e) sintering at a temperature higher than the melting temperature of the low melting-point phase and the melting temperature of the additive powder; f) magnetising.

Description

PERMANENT FRITTE MAGNET

Technical area

The present invention relates to a sintered permanent magnet and a method of manufacturing such a magnet. State of the art

Of the permanent magnets currently known, sintered magnets consisting of magnetic grains Nd2Fei4B, called "NdFeB", are those that combine the highest remanence Br, or energy product (BH) _my x, and the best coercivity Hc (resistance to a demagnetizing field). They are conventionally used at room temperature for VCM (Voice Coil Motor) type applications, for hard disks or for nuclear resonance imaging. In recent years, NdFeB magnets have also been used for motor control for electric power steering (EPS) columns for thermal engine vehicles, for traction motors for electric hybrid vehicles, or for generators for wind turbines. In these applications, the magnets are brought to temperatures of up to 200 ° C. The magnetic properties of the NdFeB magnets (Br and Hc) decrease significantly with temperature. In particular, the magnets must maintain good coercivity to prevent the demagnetizing fields from the entire magnetic system demagnetize irreversibly. For this purpose, it is known to substitute heavy rare earths (atomic number between 64 and 71) with Nd, up to 10% by mass, to increase the coercivity and make possible the use of these magnets beyond 80 ° C.

However, the manufacturing cost of the magnet increases considerably because heavy rare earths are less abundant than light rare earths. In addition, the energy product of the magnet decreases with the increase of the heavy rare earth content.

Recently, several teams have demonstrated that the location of heavy rare earths exclusively in a peripheral region of the NdFeB grains of a magnet makes it possible to improve the temperature resistance by considerably limiting the drop in the energy product.

For this purpose, a paste or powder containing a heavy rare earth is applied to the surface of the sintered magnet. Then a heat treatment between 850 and 960 ° C makes it possible to infiltrate this earth rare heavy in the magnet by the grain boundaries, liquids in this temperature range. The diffusion in the solid grains being slower, it is limited to a fine peripheral region of the grains.

This technique was introduced by Shin Etsu [EP-1830371A1] and is now used industrially by a few NdFeB magnet producers.

This technique, however, is not suitable for thick magnets since the diffusion of heavy rare earths is limited to a few millimeters from the surface of the magnet. Thus, a thick magnet has heterogeneous properties, which degrade from the surface to the center of the magnet.

Other teams have tested the "powder-to-powder mix" technique. According to this technique, especially described in US Pat. No. 6,491,765 B2, US Pat. No. 6,527,874 B2 or US Pat. No. 6,537,385 B2, heavy rare earths are brought to the heart of the green part, before sintering, for example, as described in JP. -A-62 -206802, by means of an additive powder of (Dy, Tb) -M with M selected from V, Nb, Al and Ta. However, the heavy rare earths diffuse too much inside the magnetic grains and are therefore not localized around the magnetic grains, but evenly distributed within the grains. In addition, it is necessary to introduce a large amount of heavy rare earths. The price of the magnet is increased and the energy product greatly decreased.

There is therefore a need for a permanent magnet of great thickness having an energy product as close as possible to that of NdFeB magnets, suitable for applications at temperatures between 80 and 200 ° C and incorporating a small amount of heavy rare earths.

An object of the present invention is to respond, at least partially, to this need. Summary of the invention

The present invention provides a method of manufacturing a magnet comprising the following steps:

a) preparing a base powder (a) according to the following operations:

a1) manufacturing a molten alloy having a magnetic phase T1 of type R2M14B and a phase comprising more than 80% by mass of a rare earth R selected from the group consisting of Nd, Pr, Dy, Tb, Ho, Gd and their mixtures, called "rare earth rich phase R", said production being obtained by melting an initial charge so as to obtain a bath of molten material, then cooling so as to obtain said molten alloy, and optionally heat treatment to dissolve the iron of said rare earth-rich phase, said initial charge comprising, for a total of 100%, the percentages being in mass on the basis of the initial charge:

between 28% and 33% of said rare earth R, with Nd> 20%,

- 100% complement: metal element M selected from the group consisting of Fe, Co, Ni and their mixtures, with Ni + Co <5%, the iron optionally being replaced by a replacement element selected from the group consisting of Al , Cu, Ga, Nb, Zr, Ti, Mo, V, Hf, Ta, W, Sn and mixtures thereof,

the replacement element content being between 0 and 3%,

between 0.93% and 1.2%, preferably between 0.95% and 1.2% by weight of Bore B,

less than 1% of constituents other than the rare earth R, the metallic element M, the replacement element and boron B, or "pollutants", with O <1000 ppm,

a2) hydriding said molten alloy;

a3) preferably, partially dehydriding said hydrided molten alloy;

the steps a2) and a3) leading to converting said rich phase in said rare earth R into a phase T2, called "low melting point phase", a4) grinding so as to obtain a base powder having a median size D ₅ o in volume between 1 μηι and 7 μηι, and preferably a ratio D ₉₀ / Di _{0 of} less than 10, preferably less than 5, Di ₀ , D ₅ o and D ₉₀ being defined in volume;

independently of step a), preparing an additive powder (b) comprising, preferably consisting of:

first particles of a heavy rare earth fluoride R 'of type R' F3 on the one hand,

second particles of calcium fluoride CaF ₂ and / or third particles of lithium fluoride LiF on the other hand, the size D ₉₉ of the particles of the additive powder, defined in volume, being preferably less than 30 μηι,

the mass contents of the first particles on the one hand and the sum of the second and third particles on the other hand, in the additive powder, each being greater than 5% of the mass of the additive powder, c) preparation a feedstock by mixing said base powder (a) and additive powder (b), the amount of additive powder being between 0.1% and 2%, in weight percent based on the starting load;

d) shaping of the starting charge and magnetic orientation of the particles, so as to obtain a piece to green;

e) sintering the green part at a sintering temperature higher than the melting temperature of the low melting point phase and the melting temperature of the additive powder, the sintering being preferably carried out after total dehydration of the the room to green,

f) preferably annealing of the sintered part;

g) magnetizing said sintered part so as to obtain a magnet.

Surprisingly, the inventors have found that the distribution of the heavy rare earth R ', for example Dy and / or Tb, in the magnet is particularly homogeneous in the peripheral region of the magnetic grains, whatever the location of the these grains in the volume of the magnet.

Without being limited by this theory, the inventors explain this result because the additive powder is introduced, before sintering, after an intimate mixing with the base powder, which allows the heavy rare earth R 'of the powder additive to be better distributed between the center and the surface of the magnet. In addition, they explain this result because the additive powder is liquid at the temperatures used during sintering, which allows the heavy rare earth R 'to be well distributed at the periphery of the grains.

In addition, the manufacturing process is simplified since it is no longer necessary to diffuse heavy rare earths after sintering.

A method of manufacturing a magnet according to the invention may also comprise one, and preferably several, of the following optional features:

In step a), in the initial charge, said rare earth content R is preferably between 30% and 31.5%; in the initial charge, the total content of light rare earth (accounted for in the rare earth content R) is preferably between 28% and 33%;

in the initial charge, R is preferably Nd;

in the initial charge, the total content of heavy rare earth (accounted for in the rare earth content R) is preferably between 0% and 10%;

in the initial charge, preferably Ni + Co <4%, preferentially Ni + Co <3%, preferably Ni + Co <2%, and preferably Ni + Co> 1%, more preferably Co> 1% ;

- in the initial charge, preferably

- 0, 1% <Al <2%, preferentially 0.15% <Al <0.35%, and / or

0.04% <Cu <0.2%, preferentially 0.08% <Cu <0.12%, and / or

Ga <0.5%, preferentially Ga <0.25%, and / or

Nb <2%, preferentially Nb <0.05%, and / or

Zr <2%, preferentially Zr <0.05%, and / or

Ti <2%, preferentially Ti <0.05%, and / or

Mo <2%, preferentially Mo <0.05%, and / or

V <2%, preferentially V <0.05%, and / or

Hf <2%, preferentially Hf <0.05%, and / or

Ta <2%, preferentially Ta <0.05%, and / or

- W <2%, preferentially W <0.05%, and / or

Sn <1%, preferably Sn <0.2%;

in the initial charge, preferably Pr <10% and / or Dy <10% and / or Tb <10%;

- in the initial charge, preferably

Cr <2000 ppm, preferentially Cr <500 ppm, and / or

O <800 ppm, preferentially O <500 ppm, preferentially O <300 ppm, and / or

- F <4000 ppm, preferentially F <2500 ppm, and / or

C <400 ppm, preferentially C <300 ppm, and / or

- If <3000 ppm, preferably Si <1500 ppm, and / or

- Ca <500 ppm, preferentially Ca <100 ppm, and / or

Li <1000 ppm, preferentially Li <300 ppm, and / or

Zn <1%, preferably Zn <0.05%, and / or

Mn <5000 ppm, preferentially Mn <1000 ppm, and / or N <200 ppm, preferentially N <100 ppm,

- At the end of the operation a1), the molten alloy has a lamellar structure consisting of T1 phase lamellae of the type R2M14B, representing between 90% and 96% of the mass of the molten alloy, and interlamellar phase rich in said rare earth R;

at the end of step a3), the alloy comprises, in percentage by volume, more than 90% of magnetic phase T1 of type R ₂ Mi ₄ B;

at the end of step a3), the alloy comprises, in percentage by volume, more than 3.5% and / or less than 6% of RH2-type phase,

at the end of step a3), the alloy comprises, in percentage by volume, less than 1% boride phase of type RM ₄ B ₄ ;

at the end of step a3), the alloy comprises, in percentage by volume, less than 3%, preferably less than 2%, preferably less than 1.5% of phases other than the phases T1, RH; ₂ and boride, and in particular nitride phases and / or oxides and / or carbides;

- The low melting point phase has a melting temperature below 900 ° C, preferably below 800 ° C, preferably below 700 ° C, preferably below 650 ° C;

the base powder has a particle size distribution such that the median D50 volume size is preferably between 1 and 5 μηι, preferably less than 4 μηι, preferably less than 3 μηι, preferably less than 2.5 μηι, and or greater than 1 μηι;

in step b), the third particles preferably represent between 44% and 100%, preferably between 50% and 75%, of the total mass of the second and third particles;

the heavy rare earth R 'is preferably chosen from the group formed by Dy, Tb, Ho, Gd and their mixtures;

preferably, the heavy rare earth fluoride is chosen from DyF ₃ and / or TbF ₃ and / or HoF ₃ and / or GdF ₃ ;

- Preferably, the additive powder has a composition adapted to form a eutectic from two R'F ₃ and LiF phases or a mixture of LiF and CaF ₂ ;

the additive powder comprises, in percentages by weight and for a total of 100% by mass,

30 to 90%, preferably 45 to 87%, of first particles R'F ₃ with R '= Dy, Tb, Ho, Gd, and 100% supplement: LiF or mixture of LiF and CaF ₂ , the mixture preferably comprising, for a total of 100%, preferably between 44% and 100% by weight, preferably between 50% and 75% by weight, of LiF and between 0 and 56% by weight, preferably 25 and 50% by weight of CaF ₂ ;

- Preferably, the additive powder has a melting point of less than or equal to 800 ° C, preferably less than or equal to 750 ° C;

preferably, the median volume size of the additive powder is less than or equal to 30 μηι, at 20 μηι, at 10 μηι, at 5 μηι, preferably less than or equal to the median volume volume of the powder of base;

the mass contents of the first particles on the one hand and the second and third particles on the other hand, in the additive powder, are each greater than 10%;

in step c), the amount of additive powder is preferably between 0.2% and 1%, in weight percentage on the basis of the starting charge (consisting of the base powder and the powder additive);

in step e), the sintering temperature is between 850 and 1080 ° C, preferably between 900 ° C and 970 ° C;

the sintered part has a thickness greater than 4 mm, greater than 10 mm, greater than 20 mm, greater than 50 mm.

The invention also relates to an additive powder adapted for implementing a method according to the invention.

The invention also relates to a sintered magnet, in particular manufactured according to a process according to the invention, said magnet comprising magnetic grains, preferably whose magnetic phase is of the R ₂ Mi ₄ B type, more than 80% by number of said magnetic grains each having a center and a peripheral region, the peripheral region containing a heavy rare earth, preferably Dysprosium and / or Terbium, in an amount greater than that at said center of the magnetic grain.

Preferably, a magnet according to the invention also comprises one, and preferably several, of the following optional features:

- said peripheral region extends around the grain considered;

preferably, the magnetic grains represent more than 90% or even more than 95% of the mass of the magnet; the mass content of heavy rare earth (from the additive powder (R ') and possibly from the rare earth R of the base powder) is preferably greater than 0.1% and less than 10%;

preferably, the heavy rare earth is Dysprosium and / or Terbium;

the total mass content of heavy rare earth in the peripheral region of any magnetic grain is preferably greater than the mass content of heavy rare earth in the center of said magnetic grain, the difference between the mass content of heavy rare earth in the peripheral region and the mass content of heavy rare earth in the center of said magnetic grain being preferably greater than 0.1%, 1%, 3%, 5%, and / or preferably less than 10%, preferably less than 7%. %

along any line traversing through any magnetic grain, and in particular along a line passing through the center of the magnetic grain, the total molar content of the rare earths is substantially constant, the maximum relative difference (difference between the maximum value and the minimum value, divided by the minimum value) preferably being less than 1%;

between any two points of any magnetic grain, the relative difference of the total molar content of rare earth elements (difference between the maximum value and the minimum value, divided by the minimum value) is less than 1%;

in the peripheral region, the boron mass content is preferably greater than 0.95%;

the mass composition of the magnet is such that:

Cr <2000 ppm, preferably Cr <500 ppm, and / or

O <4000 ppm, preferably O <2500 ppm, and / or

F <4000 ppm, preferably F <2500 ppm, and / or

- C <2000 ppm, preferably C <800 ppm, and / or

- If <3000 ppm, preferably Si <1500 ppm, and / or

- Ca <500 ppm, preferably Ca <100 ppm, and / or

Li <1000 ppm, preferably Li <300 ppm, and / or

Zn <1% by weight, preferably Zn <0.05%;

preferably, the thickness of the peripheral region, that is to say the depth of diffusion of the heavy rare earth R ', is greater than 2 nm, at 5 nm, at 0.03 μηι, at 0, 05 μηι, and / or less than 2 μηι, preferably 0.6 μηι, preferably 0.5 μηι; the magnet has a thickness greater than 4 mm, greater than 10 mm, greater than 20 mm, greater than 50 mm;

the heavy rare earth R 'is distributed homogeneously in the magnet, in particular between the center and the periphery of the magnet;

the local content of heavy rare earth (R '+ heavy rare earth from R) in any region of the magnet is substantially equal to the average rare earth content of the magnet, said region preferably having a volume greater than 0.02 cm ³ , or greater than 0, 1 cm ³ and less than 1 cm ³ , for example 0.025 cm ³ ;

the ratio (TR _L -TR _L *) / TR _L * is between + 5% and -5%,

TR _L designating the local content of heavy rare earth in any region of

0.025 cm ³ of the magnet,

- TR _L * designating the average content of heavy rare earth of the magnet.

The invention finally relates to the use of a magnet according to the invention or manufactured according to a method according to the invention in an environment at a temperature above 80 ° C. In particular, the invention relates to an apparatus selected from an engine control device for electric power steering columns (EPS) for a motor vehicle and a wind turbine generator comprising a magnet according to the invention or manufactured according to a method according to the invention. invention.

Brief description of the figures

Other characteristics and advantages of the invention will become apparent on reading the detailed description which follows and on examining the drawing, in which

- Figure 1 shows the microstructure of an NdFeB ribbon, after melting and cooling, the light bands corresponding to an interlamellar phase, rich in rare earths and the gray bands corresponding to the magnetic phase T1; - Figure 2 illustrates very schematically an example of evolution of local grades

T light rare earth (TRi) and heavy (TR _L ) at the periphery of a magnetic grain of a magnet according to the invention, along an axis passing in the center of the grain, at the transition between the heart of the magnetic grain C and its peripheral region P. Definitions

- A powder is a dry set of particles unrelated to each other. The percentages relating to a powder are therefore implicitly on the basis of the dry matter.

The size of the particles of a powder is evaluated classically by a particle size distribution characterization performed with a laser granulometer. The laser granulometer may be, for example, a Partica LA-950 from the company HORIBA.

- The percentiles or "percentiles" 10 (A10), 50 (A50), 90 (A90) and 99.5 (Agg.s), and more generally "n" A _n of a property A of a population, by example of a particle population are the values of this property corresponding to the percentages of 10%, 50%, 90%, 99.5% and n%, respectively, on the cumulative distribution curve relating to this property, the relative values to this property being ranked in ascending order. In particular, the percentiles D _n relate to particle sizes of a powder. Percentages are by volume.

For example, 10%, by volume, of the particles of the powder have a size less than D10 and 90% of the particles by volume have a size greater than or equal to D10. Percentiles for particle size can be determined using a particle size distribution using a laser granulometer.

The percentile 50 is classically called the "median" percentile. For example, the percentile D ₅ o is conventionally called "median size".

- "ppm" refers to "parts per million" by mass.

The term "sintering" is used to denote the consolidation by heat treatment of a preform, or "part with green", possibly with a partial or total melting of some of its constituents (but not of all its constituents, so that the preform is not transformed into a liquid mass).

- After sintering, the term "magnetic grains" grains from the magnetic phase T1. A magnetic grain is substantially constituted by a phase having the stoichiometry R2M14B, infiltrated at the periphery of the grain by the heavy rare earth R '.

- The "peripheral region" of a magnetic grain is the region in which the heavy rare earth R 'diffused during the manufacture of the magnet.

- A product is classically described as "melted" when it results from a process involving a melting of raw materials in the form of a liquid mass or "bath of molten material ", and then a solidification of this molten material, for example after casting in a mold or on a rotating wheel.

A phase is said to be "rich" in an element when it comprises more than 80% by weight of said element.

- The melting temperature of an additive powder is the minimum temperature from which all the fluoride particles of said additive powder begin to melt at ambient pressure of 1 bar.

- "containing one", "comprising one" or "comprising one" means "containing at least one", unless otherwise indicated.

- Unless otherwise indicated, "a" rare earth refers to one or more rare earths.

- Unless otherwise indicated, all contents are by mass.

detailed description

A magnet according to the invention may be manufactured according to steps a) to g).

Step a) comprises operations a1) to a4).

In step a1), an initial charge, preferably particulate, preferably consisting of pure metals or alloys, is prepared.

Preferably, the initial charge comprises a super-oichiometric amount of rare earth R, relative to the desired magnetic phase T1. The rare earth superstoichiometry R advantageously makes it possible to obtain a molten alloy comprising, in addition to the magnetic phase T1, a phase rich in rare earth R.

The presence of Ni and Co, in particular Co, improves the remanence of the magnet and its resistance to corrosion. Their contents must however be limited, especially to improve the machinability of the magnet.

The metals are preferably introduced in metallic form.

Preferably, no oxide is voluntarily introduced into the initial charge to ensure an oxygen content of less than 1000 ppm in the initial charge.

The initial charge is heated, preferably under vacuum, at a temperature preferably between 1400 ° C and 1500 ° C, so as to obtain a bath of molten material.

The bath of molten material is then preferably cast on a cooled rotating wheel. The molten material is thus solidified by quenching. The cooling speed is preferably between 100 K / s and 1000 K / s. The melted ribbons obtained preferably have a thickness of 0.12 to 0.5 mm, preferably 0.18 to 0.35 mm.

As shown in FIG. 1, the typical microstructure has T1-phase lamellae and an interlamellar I phase rich in rare earth R. The rare earth-rich phase is easily identifiable by those skilled in the art.

The composition of the initial charge is preferably adapted so that the ribbons have R2M14B phase lamellae separated by an interlamellar phase rich in rare earth R residual, the spacing of the lamellae being preferably between 1, 5 and 7 μηι, of preferably between 1, 5 and 5 μηι.

In operations a2) and a3), the ribbons undergo a hydriding-dehydriding treatment to obtain a coarse powder.

Such a hydriding-dehydriding treatment is known to those skilled in the art, and has already been proposed for producing high magnetic property powders, for example in the documents EP 0 173 588 and EP 0 538 320.

The dehydriding operation is optional, but preferred.

In general, the hydriding-dehydriding treatment is conventionally carried out in a sealed reactor.

It consists, first, in putting the molten alloy obtained in the operation a1), in contact with hydrogen (hydriding operation), typically by injecting hydrogen at room temperature into the reactor. Preferably, the hydrogen pressure is between 1 and 3 bar absolute. Without wishing to be bound by theory, the R2M14B phase, in contact with hydrogen, will undergo swelling leading to embrittlement of the material. The hydriding operation of this phase can be represented by the following schematic reaction: 2 R2M14B + xH ₂ -> 2 R2M14BHX with x between 0 and 4. In case of rare earth supersecretion R, the residual rare earth R of the rare earth rich phase R is hydrided according to the following reaction R + 1, 5.H ₂ -> RH ₃ .

In a second step, the hydrogen of the reactor is removed and can be advantageously recycled. A vacuum heat treatment, for example between 450 and 550 ° C, allows to dehydride one or more phase (s) previously hydrolyzed (s). Hydrogen released during this last operation can be captured and recycled. The lamellar phase is dehydrided, preferably substantially completely, by the following schematic reaction, which leads to new to the magnetic phase T1: 2 R2M14B HX -> 2 R ₂ Mi ₄ B + xH ₂ . The dehydration of the phase rich in said rare earth R hydrurée is partial and leads to a phase called "low melting point", in particular by the partial dehydruration of RH ₃ following the following schematic reaction: RH ₃ -> RH ₂ + 0 5. H ₂ .

This phase is called a "low melting point" phase to improve the clarity of the description.

The composition of the low melting point phase is however different from that of the rare earth rich phase R because the partial dehydridation of RH ₃ .

The hydriding-dehydriding treatment also leads to a fragmentation of the ribbons in the form of a coarse powder, which facilitates the grinding operation a4).

In step a4), the coarse powder is preferably mixed with a lubricant, for example zinc stearate, in a content of preferably between 0.035% and 0.07% by weight based on the coarse powder. The coarse powder is preferably milled, preferably dry, preferably in a neutral gas jet mill such as Nitrogen, Helium or Argon, to produce the base powder.

Step b) can be performed independently of step a), that is to say before, after or during step a).

The sources of fluoride particles are metered to obtain an additive powder having the desired composition. Preferably, substantially pure sources are used.

Preferably, the median size of the additive powder is less than the median size of the base powder, preferably less than 2 μηι, preferably less than 1 μηι. The homogeneity of the feedstock is improved.

Advantageously, the inventors have found that, during sintering, the diffusion of the heavy rare earth R 'resulting from the additive powder within the magnetic grains is limited.

Preferably, prior to step c), the fluoride particles are degassed, preferably at more than 120 ° C. and preferably at less than 400 ° C., preferably under vacuum and preferably for more than 10 hours, preferably more than 24h, preferably more than 48h.

In step c), the additive powder is mixed with the base powder to form a feedstock. The starting charge preferably has a median size D ₅ o in volume between 1 μηι and 4 μηι, preferably less than 3 μηι, preferably less than 2 μηι, and / or preferably greater than 1 μηι.

Preferably, a lubricant, preferably temporary, that is to say which is removed during sintering, whose content is preferably between 200 and 2000 ppm, preferably between 500 and 1200 ppm can be added.

The lubricant content is preferably less than 1%, 0.5%, 0.3%, 0.2%, by mass percentage based on the mass of the particles to be mixed. The person skilled in the art knows how to determine the nature and the contents of the possible lubricants.

The mixing is preferably carried out for more than one hour, so as to obtain a substantially homogeneous starting charge.

In step d), the starting charge is preferably conventionally poured into a mold, then subjected to a magnetic field to orient the particles. Preferably, this magnetic field is greater than 3 Tesla, preferably greater than 5 Tesla, preferably greater than 7 Tesla.

The magnetic field may be a static axial or transverse magnetic field greater than 1.5 Tesla, preferably greater than 2 Tesla or a pulsed field greater than 3T, preferably greater than 5T, preferably greater than 7 Tesla.

More preferably, the feedstock is then compacted, preferably using a transverse compaction, axial, RIP ("Rubber Isostatic Pressing") or isostatic cold, so as to obtain a green part having a density preferably between 50% and 70% of the theoretical density of the material of the mixture of additive powders and base of the starting charge (completely dense).

In step e), the green parts are preferably dehydrided and then sintered.

A dehydriding stage of green parts is preferably conventionally carried out, preferably at a temperature of between 600 and 800 ° C., preferably under a secondary vacuum, so as to avoid the demixing of the NdFeB phase in the presence of H ₂ .

Those skilled in the art are familiar with the conditions for sintering.

Preferably, the sintering is carried out in an environment containing substantially no oxygen, water or hydrogen, preferably under vacuum so that the measured pressure in the chamber of the sintering furnace is less than 10 ^{~ 3} mbar, preferably less than 5.10 ^" ⁵ mbar, preferably at a temperature greater than or equal to 850 ° C and / or less than or equal to 960 ° C, and preferably for 3 to 15 hours.

Under the effect of sintering, the low-melting phase and the additive powder turn into a binder phase which includes the magnetic grains and allows diffusion of the heavy rare earth R 'into the magnetic grains. Then, the sintered parts are subjected to rapid cooling, preferably greater than 20 ° C / min, preferably about 30 ° C / min, between the sintering temperature and 50 ° C.

The density of the sintered parts is preferably greater than 95% of the theoretical density of the material which constitutes them.

In step f), the sintered parts are preferably annealed, or "tempered".

Annealing advantageously increases the resistance to demagnetization of the magnet.

Those skilled in the art are familiar with the conditions for annealing.

The annealing is preferably carried out according to the following cycle:

heating at 50 ° C. to a temperature of between 800 and 900 ° C., for example at 820 ° C., at 5 ° C./min;

- Bearing between 800 and 900 ° C, for example at 820 ° C, for 2 hours;

cooling from the bearing temperature to 50 ° C at 20 ° C / min;

heating from 50 ° C to (460-650 ° C) at 5 ° C / min;

- temperature bearing (460-650 ° C);

- cooling of the temperature (460-650 ° C) at room temperature to 30 ° C / min.

The annealed sintered part can then be machined and / or surface treated, for example polishing or applying a coating to prevent oxidation and corrosion.

In step g), the annealed sintered piece is subjected to a complementary magnetization, preferably by application of a magnetic field tap greater than 2.6 Tesla, preferably greater than 4 Tesla, preferably greater than 5 Tesla.

At the end of step g), a magnet according to the invention is obtained.

Remarkably, the distribution of heavy rare earths is particularly homogeneous in the magnet, regardless of the thickness of the magnet. The magnetic grains may all have the same composition, or have different compositions. In a preferred embodiment, more than 90%, more than 95%, more than 98%, more than 99% or even substantially 100% by mass of the magnetic grains have the same composition, and preferably consist of Nd2Fei4B.

The size of the magnetic grains is preferably greater than 1.0 μηι and / or less than 8.0 μηι.

The binder phase results from the melting of the additive powder and the low melting point phase in the base powder.

The binder phase binds the magnetic grains together leaving a porosity. Preferably, the porosity of the magnet is less than 3%, preferably less than 1%.

Preferably, the magnetic grains and the binder phase together represent more than 95%, more than 98%, more than 99%, more than 99.5%, preferably substantially 100% of the mass of the magnet.

The melting of the additive powder and the low melting point phase in the base powder results in a diffusion of the heavy rare earth R 'into the magnetic grains. The heavy rare earth concentration R 'in the peripheral region of a magnetic grain is advantageously homogeneous, regardless of the magnetic grain considered. Finally, the diffusion of the heavy rare earth R 'into the magnetic grain is limited to a shallow depth from the outer surface of the magnetic grain.

The inventors have found that the total molar content of the rare earths is substantially constant regardless of the magnetic grain considered. In particular, along any line crossing a magnetic grain, the maximum relative difference (difference between the maximum value for this content and the minimum value for this content, divided by the said minimum value) is typically less than 1%. Examples

The following examples are provided for illustrative purposes and do not limit the invention.

A particulate initial charge is prepared so as to obtain, after the steps a1) to a4), a base powder having the following composition, in percentages by weight:

Nd: 20%

Pr: 12% B: 1, 03%

Co: 0.5%

Fe: 100% complement

Ti: 0.1%

Al: 0.8%

Cu: 0.12%

Ga <0.25%

Nb + Zr + Ti + Mo + V + Hf + Ta + W <0.05%

Sn <0.2%

Cr + O + N + C + Si + F + Li + Ca + Zn <1%, with

Cr <500 ppm,

O <300 ppm,

F <200 ppm,

C <300 ppm,

If <1500 ppm,

Ca <100 ppm,

Li <300 ppm, and

Zn <0.05%.

More precisely, the melting is carried out under vacuum in an alumina crucible at a maximum temperature of 1450 ° C. The bath of molten material is cast on a copper-based wheel cooled with water and having a rotational speed to obtain crystallized ribbons of a thickness of about 100 μηι ("Strip Casting") .

The ribbons are then cooled to room temperature on a turntable or on a conduction and convection cooled cart. The spacing between two lamellae of phase Nd2Fi ₄ B measured on the section of a ribbon is 1. 7 μηι on average, as represented in FIG.

The ribbons are then subjected to a pure hydrogen environment at 1 bar in a sealed chamber of an oven at room temperature, for a time sufficient to hydride the entire material. Then, the enclosure is evacuated to evacuate the hydrogen, then filled with argon at 0.7 bar, and finally heated at a temperature of 550 ° C for 2 hours. The coarse powder resulting from this treatment is then cooled to room temperature. The coarse powder is then homogenized in a mixer in which 0.04% by weight of zinc stearate is introduced. The mixing is carried out for 1 h 30.

The powder obtained is then introduced into a fluidized bed gas jet mill to obtain a base powder. The gas used is helium. The grinding pressure, the diameter of the nozzles and the selector speed are adjusted to obtain a median particle size of 1.7 μηι, measured by a laser granulometer.

At the same time, LiF powders (Sigma-Aldrich, D ₅ ≤20 μηι, purity> 99.98%) and DyF ₃ (Dysprosium fluoride (III), purity of 99.99%) are dried at 400 ° C. for 24 hours, so as to avoid the introduction of moisture into the base powder.

Then the powders of LiF ₃ and DyF ₃ are dosed and mixed in a weight ratio of 26% / 74% by weight, for 2 hours under argon, so as to obtain the additive powder.

0.6% by weight of additive powder, 0.1% of liquid lubricant (decanoate) and 99.3% of base powder are mixed, under a neutral atmosphere (Argon), for 1.5 hours.

Magnetic orientation consists in orienting the crystals by applying a magnetic field.

The mixture is then introduced into a rubber mold. It is subjected to a magnetic field of 7 Tesla to guide the particles, then cold isostatic compaction at 1500 bar to obtain ten cylinders 15 mm in diameter and 20 mm long. Three blocks of 46 * 78 * 35 mm ³ are made from the same starting charge, and under the same conditions. The magnetic field allows an orientation of the particles according to the thickness of the block (35 mm).

For each of the samples, a secondary vacuum sintering (1, 5.10 ^{~ 5} mbar at the end of the sintering stage) is carried out according to the following thermal profile:

- Heating 2 ° C / min to 350 ° C and then bearing for 1 hour;

- Heating 2 ° C / min up to 550 ° C and then plateau for 1 hour;

- heating 2 ° C / min to 750 ° C and then bearing for 2 hours;

- Heating 1 ° C / min to 915 ° C then plateau for 4 hours.

Cooling is then carried out under argon, at an absolute pressure of 2 bar, up to room temperature, to obtain a cooling rate greater than 30 ° C./min. The samples are then annealed under secondary vacuum according to the following thermal profile:

- heating at 5 ° C / min from 50 ° C to 820 ° C and then plateau for 2 hours;

- cooling at 20 ° C / min from 820 ° C to 50 ° C;

heating at 5 ° C / min from 50 ° C to (460-560 ° C), in particular 510 ° C, then plateau for 3 hours;

cooling at 30 ° C / min to room temperature.

The cylinders are machined using a centerless grinding machine and a planar grinding machine equipped with a diamond grinding wheel to remove the oxide layer and obtain parallel surfaces to press them to the poles of the hysteresis.

The samples are then magnetized in a Bitter coil under a magnetic field of 5 Tesla.

At the core of each block, four cylinders of 15 mm +/- 0.02 mm in diameter and 20 mm +/- 0.02 mm in length are extracted by electroerosion and a parallelism of +/- 0.02 mm between them. two bases of the cylinders. These cylinders are called "extracted cylinders" to distinguish cylinders already preformed in cylindrical form before sintering, called "original cylinders".

A magnetometer delivering a 2.7 Tesla magnetic field is used to measure the intrinsic properties (remanence and coercive field) in the second quadrant of the hysteresis cycle (characteristics of the "hardness" of magnetic materials).

Magnetic characteristics of the magnets are measured at 20 ° C and summarized in the tables below:

Table 1: original cylinders (average values on the 10 original cylinders)

Table 2: Extracted cylinders (average values on the 12 cylinders extracted)

Br HcJ (BH) max (BH) max + HcJ kG kOe MGOe Magic number

13.33 28.5 44> 70 Remarkably, the properties are substantially the same despite the fact that the thickness of the blocks was greater than the diameter of the original rolls.

Thick magnets manufactured according to a process according to the invention therefore have the same magnetic properties as fine magnets.

In comparison, the magnetic properties are almost identical to those obtained with 2 mm thick magnets having the same composition but made by the diffusion technique after sintering Dysprosium at the grain boundaries.

However, a conventional method (without heavy rare earth diffusion at the grain boundaries) using a mixture of a heavy rare earth type powder T1 and a powder of the type R 2M14B with R '= heavy rare earth, leads to at a lower coercivity, less than 22 kOe, for magnets having the same dimensions as said blocks. The results obtained are therefore remarkable, this conventional process being so far considered to provide the best coercivity results without resorting to a heavy rare earth diffusion process.

In addition, the original cylinders and extracts are observed by means of a scanning electron microscope (SEM-FEG) equipped with an EDX probe to analyze the different phases.

The observation shows the presence of a very fine peripheral region enriched with Dysprosium on the contour of the magnetic grains. The thickness of this peripheral region is 0.3 μηι on average. Unlike magnets manufactured according to the prior art, Dysprosium is substantially homogeneously distributed around the magnetic grains, but is substantially absent in the core of the magnetic grains if the base powder does not contain heavy rare earth. The effect of the heavy rare earth is thus preserved.

The size of the magnetic grains is less than 2.5 μηι on average and the rich Nd phase surrounds the grains continuously.

As now clearly apparent, the invention provides a method for producing a large magnet which has a high energy product, with greater coercivity than that obtained by a homogeneous addition of heavy rare earth in the magnetic grains. In particular, according to the invention, the increase in coercivity can be about 7 kOe for an average addition of 0.5% by mass of dysprosium whereas this increase of coercitivity is only about 1, 2 kOe for the same addition of dysprosium in the case where this element is distributed homogeneously in the grains of the magnetic phase.

Of course, the invention is not limited to the embodiments described, provided by way of illustration and not limitation.

Claims

1. A method of manufacturing a magnet comprising the following steps:

a) preparing a base powder (a) according to the following operations:

a1) manufacturing a molten alloy having a magnetic phase T1 of type R2M14B and a phase comprising more than 80% by mass of a rare earth R selected from the group consisting of Nd, Pr, Dy, Tb, Ho, Gd and their mixtures, called "rare earth rich phase R", said manufacture being obtained by melting an initial charge so as to obtain a bath of molten material, then cooling to obtain said molten alloy, said initial charge comprising, for a total of 100%, the percentages being in mass on the basis of the initial charge:

between 28% and 33% of said rare earth R, with Nd> 20%,

the replacement element content being between 0 and 3%,

between 0.93% and 1.2% by weight of Bore B,

a2) hydriding said molten alloy;

a3) preferably, partially dehydriding said hydrided molten alloy;

the steps a2) and a3) leading to converting said rich phase in said rare earth R into a so-called "low melting point phase" a4) grinding so as to obtain a base powder having a median size D ₅ o in volume between 1 μηι and 7 μηι;

b) independently of step a), preparing an additive powder (b) comprising

first particles of a heavy rare earth fluoride R 'of type R' F3 on the one hand, second particles of calcium fluoride CaF ₂ and / or third particles of lithium fluoride LiF, on the other hand, the mass contents of the first particles on the one hand and the sum of the second and third particles on the other hand, in the additive powder, each being greater than 5% of the mass of the additive powder, c) preparing a feedstock by mixing said base powder (a) and additive powder (b), the amount of additive powder being between 0.1% and 2%, in weight percent based on the feedstock;

e) sintering the green part at a sintering temperature higher than the melting temperature of the low melting point phase and the melting temperature of the additive powder;

f) preferably annealing of the sintered part;

g) magnetizing said sintered part so as to obtain a magnet.

2. Method according to the preceding claim, wherein said initial charge comprises between 0.95% and 1, 2% by weight of Bore B.

3. Process according to any one of the preceding claims, in which, in the initial charge, in percentages by weight:

the rare earth content R is between 30 and 31.5% and / or

- 1% <Ni + Co <4%; and or

- Pr <10% and / or Dy <10% and / or Tb <10%, and / or

the heavy rare earth content is between 0% and 10%.

4. Method according to any one of the preceding claims, wherein in step b), the third particles represent between 44% and 100%, preferably between 50% and 75%, of the total mass of the second and third particles.

5. A method according to any one of the preceding claims, wherein in step b) the heavy rare earth R 'is selected from the group consisting of Dy, Tb, Ho, Gd and mixtures thereof.

6. Method according to any one of the preceding claims, wherein the additive powder comprises, in percentages by weight and for a total of 100% by mass,

30 to 90%, preferably 45 to 87%, of first particles R'F ₃ with R '= Dy, Tb, Ho, Gd, and

100% complement: LiF or mixture of LiF and CaF ₂ .

The method of any one of the preceding claims, wherein the median volume size of the additive powder is less than or equal to the median volume size of the base powder.

The process of any of the preceding claims, wherein the additive powder has a melting temperature of less than 800 ° C.

9. Process according to any one of the preceding claims, wherein the sintered part has a thickness greater than 4 mm.

Sintered magnet having a thickness greater than 4 mm and having magnetic grains, preferably having a magnetic phase of the R2M14B type, greater than 80% by number of said magnetic grains, each having a peripheral region containing a heavy rare earth, preferably Dysprosium and / or Terbium, in an amount greater than that at the center of the corresponding magnetic grain.

A magnet according to the immediately preceding claim, wherein the ratio (TR _L - TR _L *) / TR _L * is between + 5% and -5%,

TR _L denoting the local content of heavy rare earth in any region of 0.025 cm ³ of the magnet,

- TR _L * designating the average content of heavy rare earth of the magnet.

12. Magnet according to any one of the two immediately preceding claims, wherein the thickness of the peripheral region is less than 2 μηι.

13. Magnet according to the immediately preceding claim, wherein the thickness of the peripheral region is less than 0.5 μηι.

14. Magnet according to any one of the four immediately preceding claims, having a thickness greater than 50 mm.

A magnet according to any of the five immediately preceding claims, wherein the total mass content of heavy rare earth in the peripheral region of any magnetic grain is greater than the mass content of heavy rare earth in the center of said magnetic grain.

16. Magnet according to the immediately preceding claim, the difference between the mass content of said heavy rare earth in the peripheral region and the center of said magnetic grain being greater than 1%.

17. Magnet according to any of the seven immediately preceding claims, wherein, between any two points of any magnetic grain, the relative difference in the total molar content of rare earths is less than 1%.

18. Magnet according to any one of the eight immediately preceding claims, manufactured according to a process according to any one of claims 1 to 9.

19. Use of a magnet according to any one of the nine immediately preceding claims in an environment at a temperature above 80 ° C.

20. Apparatus selected from the group consisting of an engine control device for

electric power steering columns for a motor vehicle, and a wind turbine generator, said apparatus comprising a magnet according to any one of claims 10 to 18.