GB2506681A

GB2506681A - Soft anisotropic magnetic material article and method for its production

Info

Publication number: GB2506681A
Application number: GB201217979A
Authority: GB
Inventors: Matthias Katter; Christoph Brombacher; Axel Regnat; Kaan A Stuner
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 2012-10-08
Filing date: 2012-10-08
Publication date: 2014-04-09
Also published as: GB201217979D0

Abstract

An article 2, or a method of making an article, comprises a polycrystalline soft magnetic material including grains with a common preferred orientation and a magnetic anisotropy. The said article 2 may be arranged such that during its use a difference in magnetic energy of Î”E between a magÂnetic energy El in a first, easy magnetisation, direction 10 and a magnetic energy E2 in a second, more difficult magnetisation, direction 11 is at least 150 to 270 kJ/m3. The method may provide particles which are oriented to form a preferred texture soft magnetic anisotropic material or provide an amorphous, or nanocrystalline, material which is processed to induce a preferred texture soft magnetic anisotropic material. The article 2 or method may include sintering, a polymer matrix. The magnetic material may have axial and/or planar anisotropic properties. The magnetic material may include rare earth elements, particles or alloys. The article 2 or method may be used in forming a reluctance motor 1 rotor arrangement.

Description

Article and method for producing an article The present invention relates to an article and a method for producing an article, in particular an article and a method for producing an article comprising anisotropic soft magnetic properties.

One type of reluctance motor comprises a rotor and a stator comprising means to produce a variable magnetic field through the rotor. The stator may comprise one or more windings through which current can flow to produce a changeable magnet-

ic field through the rotor.

The reluctance motor is based on the principle that a body with a directionally dependent magnetic reluctance is caused to move to a position within a magnetic field such that the direction, in which the body has its minimum magnetic reluc-tance, aligns with the direction of the magnetic field. The rotor of a reluctance motor typically comprises a soft magnet-ic material and has an anisotropic cross-section, for example an elliptical cross-section, so that the rotor is more easily magnetised in a first direction, e.g. the longest dimension of the ellipse, than in a second direction, e.g. the shortest di- mension of the ellipse, the second direction being perpendicu-lar to the first direction.

DE 39 31 484 Al discloses a reluctance motor comprising a ro-tor which is made up of soft magnetic layers interleaved with non-magnetic layers so that the rotor is more easily magnet-ised in directions parallel to the soft magnetic layers than in directions perpendicular to the soft magnetic layers. In other embodiments, the rotor may comprise slits in place of the non-magnetic material, which also produce a rotor that is more easily magnetisable in directions parallel to the slits than in directions perpendicular to the slits.

The maximum torque or revolutions per minute which can be pro-duced by a reluctance motor depends, among other parameters, on the difference between the reluctance in the direction in which the reluctance is maximum and the reluctance in the di-rection in which the reluctance is a minimum. The torque which can be generated increases as the difference in reluctance be-tween the two direction increases.

Therefore, it is desirable to produce a rotor with a reluc- tance which is as directionally anisotropic as possible in or-der to increase the torque producible by the reluctance motor.

An article is provided that comprises polycrystalline soft magnetic material comprising a preferred texture and magnetic anisotropy. A difference in magnetic energy AE between a mag-netic energy E-in a first direction and a magnetic energy E2 in a second direction is at least 270 kJ/mT, wherein E = - E21 = IJl*H -J2*H, J1 is the polarisation in the first direc-tion measured in a magnetic field H and J2 is the polarisation in the second direction measured in a magnetic field H. In contrast to a rotor of a reluctance motor in which aniso- tropic soft magnetic properties are produced by shape anisot-ropy, for example by an anisotropic contour or cross-section of the rotor itself or by positioning non-magnetic material or slits or holes within the rotor, an article is provided in which the composition of the material forming the article it- self provides anisotropio soft magnetic properties. The aniso-tropic soft magnetic properties result in the article having a directionally dependent magnetic energy difference AB of at least 270 kJ/m3.

The article may have a magnetic energy difference AF of at least 270 kJ/m3 at room temperature and/or at the working tem- perature of a magnetic circuit in which the article is includ- ed. For example, the magnetic circuit may be a motor, in par-ticular, a reluctance motor, and the article may be the rotor of the reluctance motor. A typical working temperature of a reluctance motor may be 150°C.

The article has a polycrystalline microstructure with a pre-ferred texture. The article may comprise a plurality of grains providing the polycrystalline microstructure. A large propor-tion of the grains may comprise a phase or composition that has anisotropic soft magnetic properties. The grains are ori-entated so that they tend to have a common orientation which provides a preferred texture in the article.

Use may be made of a composition for the article for which a single crystal of this composition has anisotropic soft mag- netic properties. An example of a magnetic material with ani- sotropic properties is Nd2Fe-1B. A single crystal of a particu-lar composition may not be best suited for producing a rotor as it may be difficult or expensive to produce a single crys-tal in a size large enough for practical rotor applications. A plurality of particles of the composition may act similarly to a plurality of single crystals. By arranging a plurality of particles so that the particles have the same or a similar orientation, an article can be produced which has a polycrys-talline microstructure with a preferred texture. The article has anisotropic soft magnetic properties due to the preferred texture of a plurality of partioles or grains of a oomposition having anisotropic magnetic properties.

The article may have a symmetrical contour and/or a symmet-rical cross-section and be without non-magnetic layers or slits or holes positioned within it. The article is more easi-ly magnetised in a first direction than in a second direotion due to the anisotropic magnetic properties of the material forming the article and the preferred texture of the particles of the material.

Additionally, the use of the anisotropic soft magnetic proper-ties of the material forming the article itself enables the difference between the maximum reluctance and the minimum re-luctance in differing directions to be increased over that achievable through shape anisotrcpy of the rotor. The torgue producible by a reluctance motor comprising a rotor of a soft magnetic material having anisotropic magnetic properties may be increased over that achievable by shape anisotropy of the rotor.

A soft magnet differs from a permanent magnet in that the co-ercive field strength Hr.!T for a soft magnet is less than that for a permanent magnet. In an embodiment, the article has a coercive field strength of less than 100 kA/m indicating that it is a soft magnet. In a further embodiment, the coercive

field strength H!:!i is less than 10 kA/m.

The soft magnetic material of the article is pclycrystalline and has a preferred texture. The polycrystalline microstruc- ture and the preferred texture may be provided by various ar-rangements.

In an embodiment, the artiole may oomprise a sintered mioro-structure. A sintered structure may be identified by optical microscopy or scanning electron microscope analysis of the ar-ticle. A sintered microstructure is characteristically formed by a heat treatment whioh causes contiguous particles of a body to join one another by mutual diffusion of atoms between the contiguous particles which results in the formation of grains. Typically, a sintering heat treatment increases the density and mechanical strength of the body.

The sintering treatment typically results in an average grain size which is larger than the average particle size from which the grains are formed. In an embodiment, the article comprises an average grain size of greater than 5 jim. For some soft ma-terials, for example Nd2Fe-4B -based materials, an increasing grain size is thought to produce a decreasing coercive field strength Hj and softer magnetic properties.

A very coarse microstructure may result in poor mechanical properties so that, in a further embodiment, the average grain size of the polycrystalline soft magnetic material is less than 30 jim.

In an embodiment, the polycrystalline soft magnetic material is embedded in a polymer matrix and may be provided in the form of a plurality of particles embedded in a polymer matrix.

The particles may have a preferred orientation within the ma-trix and provide the article with anisotropic soft magnetic properties. This structure may be useful if a further heat treatment, for example a sinter heat treatment, is to be avoided.

The preferred texture of the article may be determined by ob- servation of the grains and/or by observation a selected crys- tallographic axis of the grains or particles. In one embodi-ment, a selected crystallographic axis of at least 80 volume percent of the polycrystalline soft magnetic material lies within ± 45° of a predetermined direction.

Alternatively or in addition, the preferred texture of the ar-ticle may be determined by observation of the magnetic domain orientation of the grains or particles. In one embodiment, a selected magnetic domain orientation of at least 80 volume percent of the polycrystailine soft magnetic material lies within ± 45° of a predetermined direction.

Due to the anisotropic soft magnetic properties, the article has a first direction which is more easily magnetisable than at least one second direction. In a particular embodiment, the second direction is perpendicular to the first direction.

The polycrystalline soft magnetic material may comprise uniax- ially, planarly or conically anisotropic soft magnetic proper-ties. tJniaxialiy anisotropic magnetic properties have a single first direction which is more easily magnetisable than two or moresecond directions which are perpendicular to the first di-rection. Planarly anisotropic properties have two or more first directions which are arranged in a plane and are more easily magnetisable than a second direction, which is perpen- dicular to the first directions. Conically anisotropic proper-ties have two or more first directions which are arranged on a cone and which are more easily magnetisable than two or more second directions which are parallel or perpendicular tc the axis of the cone.

tjniaxial, planar or conical soft magnetic anisctropy may be chosen by proper selection of the soft magnetic material and/or the composition of the soft magnetic material.

Various soft magnetic materials may be used to fabricate an article that comprises a preferred texture and magnetic ani- sotropy with a directionally dependent magnetic energy differ- ence of at least 270 kJ/m). For example, the soft magnetic ma-terial of the article may comprise one or more of the group of alloy systems consisting of RE2(TM)1C, RE2(TMh;C, RE2(TM)7, RE(TF4)7, (RE,Zr) (TM)3, RE(TM)12, and RE3(TM)29, wherein RE com-prises one or more of the elements from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and TM is one or more of the group consisting of Fe, Go, Mn and Ni.

The polycrystalline soft magnetic material may also comprise the alloy system RE2(TM)4Z. Z is B and/or C. In the RE2 (TM) 14Z phase, RE may comprise one or more of the elements from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V. TM may be Fe and may further comprise one or more of the group consisting of Go, Ti, V, Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu.

RE2(TM)34Z is commonly considered to be a permanent magnet, but can be produced with soft magnetic properties with a coercive field strength H3 of less than 100 kA/m or less than 10 kA/m by selection of the composition and of the production condi-tions, such as the sinter conditions and by control of the grain size.

The article may include a total composition which deviates from the stoichiometric 2RE:14TM:1B, whereby if RE3 (TM)IB, then 1.7 «= a «= 2.3, 13.5 «= b «= 14.5 and 0.8 «= c «= 1.2 In order to produce a uniaxial soft magnetic anisotropy in the RE2 (TM) 11Z phase, RE may be selected as one or more of the dc-ments of the group consisting of Nd, La, Ce, Gd, Yb, Lu, Y and Pr.

In order to produce a planar soft magnetic anisotropy in the RE2 (TMhZ phase, RE may be selected as one of more of the group consisting of Sm, Er and Tm. In one particular embodi- ment, RE comprises Nd, Pr and Sm. In a further particular em-bodiment, the ratio of Sm in atomic percent to the total rare earth content in atomic percent is larger than 35%.

In certain embodiments, the rare earth element RE is Nd or Ce-rium Mischmetal. Cerium Mischmetal is an alloy of rare earth elements in various naturally occurring proportions. A typical composition of cerium Mischmetal includes approximately 50% cerium and 25% lanthanum, with small amounts of neodymium and praseodymium. Cerium Mischmetal may be useful as the raw mate-rials cost is less than that of other rare earth metals.

Elements which are known to increase the coercive field strength of the RE2(TN)14Z phase may be avoided in order to limit the coercive field strength H!25 to less than 100 kA/m or less than 10 kA/m. In some embodiments, the total content of the elements Dy, Tb, Ho, Al, Ga and Cu is less than 0.5 wt%.

In some embodiments, the RE2(TM)Z phase is essentially free of the elements Dy, Tb, Ho, Al, Ga and Cu. Essentially free is used to denote amounts of these elements which are sufficient-ly small that there is no measurable influence of the element

or elements on the coercive field strength Hr.

The soft magnetic material of the article may comprise the al- loy system RE(Co)5. This alloy system may further comprise ad-ditions comprising one or more of the elements of the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Fe, Ni, Mn, Cu, B and C. In a further embodiment, the soft magnetic material comprises alloy system RE2(Co)11. This alloy system may further comprise additions comprising one or more of the elements of the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Mn, Zr, Hf, Cr, V, Ti, Nb, Mo, Fe, Ni, Cu, B and C. in further embodiments, the alloy system RE2 (Fe,Co)11 is essen-tially free of Cu and Zr and Hf.

In a further embodiment, the soft magnetic material comprises a (Fe,Co,Ni,Mn)-rich intermetallic material which may be one of the alloy systems Mn(Ga,Ge,Al), c"Fe16N2 and Fe2C.

In a further embodiment, the soft magnetic material comprises a metastable RE2 (TM) 17N alloy, where RE is one of more of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and TM is one or more of Mn, Fe, Co and Ni.

The magnetic anisotropy of the article may also be demonstrat-ed by an effective anisotropy constant, Ke=t, which is defined as Ke = Haef/2*J, wherein Haeff is the effective anisotropic magnetic field strength and J is the saturation polarization and may be K1.-f > 0.3 IC/m or > 2.0 MJ/m. This value is provided by the anisotropic soft magnetic properties of the article rather than by an anisotropic shape or the presence of non-magnetic portions such as non-magnetic material, holes or slits within the article.

Hae is the magnetic field strength which is required to satu-rate the article in the less easily magnetisable direction.

The article according to one of the above embodiments may be used as a part in a magnetic circuit, the part being movable in response to an application of a magnetic field external to the part. The part may be a rotor in a rotative reluctance mc- tor, a carriage of a linear motor or movable part of an actua-tor.

The rotor, carriage or moveable part may be solid and without non-magnetic material, holes or empty spaces inside it that produces anisotropic soft magnetic properties sufficient to cause the rotor, carriage or movable part to move, since the anisotropic magnetic properties of the soft magnet material of the rotor, carriage or moveable part itself provides two di- rections of differing magnetizability, i.e. a more easily mag-netisable direction and a less easily magnetisable direction.

The two directions of differing magnetizability may be mutual-ly perpendicular.

A magnetic circuit is also provided which comprises a movable part and a magnetic field source external to the movable part, the movable part comprising an article according to one of the above embodiments.

A reluctance motor is provided that comprises a rotor compris-ing an article according to one of the above embodiments and a stator comprising an electrically conductive winding. In one embodiment, the rotor is solid and without non-magnetic mate-rial, holes or empty spaces inside it producing anisotropic soft magnetic properties sufficient to cause the rotor to ro-tate.

The reluctance motor may be a synchronous reluctance motor or a switched reluctance motor. The stator and/or the rotor of the reluctance motor may comprise a plurality of poles, for example 2, 4, 6 or 8 poles. The stator may comprise an odd number of poles or an even number of poles.

An article including a material with anisotropic soft magnetic properties is capable of providing a larger soft magnetic ani- sotropy than that achievable by increasing the shape anisotro-py, for example shape anisotropy of an elliptical iron rotor in a reluctance motor. In addition, the whole volume of the rotor can be built up by these materials for these anisotropic soft magnetic materials. As a result, the torque density gen-erated by the motor can be much higher compared to some types of conventional reluctance motors.

The article may also be useful in applications such as motors which conventionally include permanent magnets, for example permanent-magnet synchronous machines. Due to the fixed mag- netic flux provided by the permanent magnets in these applica-tions, it is difficult to operate above base speed where the back electromotive force equals the supplied limit voltage.

Flux weakening methods may be used to increase the speed of a permanent-magnet synchronous machine.

An article with anisotropic soft magnetic prcperties acccrding to embodiments of the invention may also be used in applica- tions in which a reduction in magnetic flux at higher revolu- ticns is desirable, since the magnetization level of the arti- cle is proportional to the driving magnetic field of the mo-tor. At high speeds, the magnetic field strength can easily be reduced which leads to a reduction of the magnetisation level of the anisotropic soft magnetic materials. This will lower the back electromotive force and the motor can accelerate to higher speeds.

Additionally, the permanent magnets of permanent-magnet syn-chronous machines may be subjected to high demagnetising field strengths and/or high temperatures under short circuit of overload conditions such that the permanent magnets may be permanently demagnetised and the motor no longer functions.

The article according to embodiments of the invention may be used to avoid a total failure in that, if the article is de- magnetized, it will be automatically remagnetised by the driv- ing magnetic field, since the article comprises a soft magnet-ic material.

The article according to embodiments of the invention may also be useful in the construction of lower cost motors in compari-son with motors including permanent magnets, for example based of RE21M171Z-phases since the use of elements with a high raw materials cost, such as dysprosium, can be avoided.

A method for producing an article is provided that comprises: providing particles comprising a total composition capable of providing a soft magnetio material with magnetioally aniso- tropic properties; orientating the particles to provide a pre- ferred texture; forming the particles, and producing an arti-ole oomprising polyorystalline soft magnetio material having a preferred texture and anisotropic magnetic properties.

The anisotropy of the soft magnetic properties of the article results from the preferred texture of the grains of the poly- crystalline soft magnetic material of the article and, in par- ticular, from the preferred orientation of the magnetic do- mains of the grains of the article. The soft magnetic proper-ties of the article may be confirmed by a coercive field strength of < 100 kA/m or < 10 kA/m. The preferred texture may be produced by orientating the particles using various methods and may be produced before and/or after forming the particles and producing the article. The article may be suita- ble for use in a magnetic circuit or as a rotor in a reluc-tance motor for example.

In an embodiment, the orientating the particles of the powder to form a preferred texture comprises applying an external

magnetic field to the particles.

The orientating the particles may comprise applying a rotating magnetic field to the particles. The particles may be compact-ed in a compaction direction and the rotating magnetic field may be applied in directions perpendicular to the direction of compaction in order to produce a preferred texture before and/or after compaction.

Alternatively, the orientating the particles may comprise ro-tating the particles in a static magnetic field in order to produce a preferred texture. For example, the particles may be placed in a container that is rotated in a static magnetic field to re-orientate the particles and produced a preferred texture.

The particles may be formed to produce a massive or solid ar-ticle using various methods. In an embodiment, forming the particles comprises compacting and/or sintering the particles.

If the particles are sintered, the sintering may take place at a temperature T and for a time t, wherein 900°C «= T «= 1300°C and 0.5h «= t «= 50h. The sintering conditions may be selected to produce a desired density in order that the article has a sufficient mechanical integrity for use as a rotor or moving part and/or to produce the desired soft magnetic properties.

In some materials, for example RE2TM14Z, the coercive field strength H0 decreases for increasing grain size. For such ma- terials, a larger grain size may be desirable in order to im-prove the soft magnetic properties.

In an embodiment, the article comprises an average grain size of greater than 5 tm after sintering. The grains may also have an average diameter of less than 30 jim.

The method may further comprise quenching to room temperature after the sintering. Quenching to room temperature may assist in producing a coercive field strength H3 of < 100 kA/m. Fur- ther annealing treatments at temperatures lower than the sin-tering temperature may also be avoided to assist in producing

a coercive field strength H!Di of < 100 kA/m.

In an embodiment, the method further comprises adding lubri-cant to the powder. This may be performed before orientating the particles. Lubricant may assist achieving a higher density of the green body if the particles are compacted to produce a green body. The lubricant may be isopropanol, zinc-stearate and/or isostearic acid and may be added in amounts of around 0.02 to 2 weight percent, for example.

In some embodiments, the particles are orientated by mechani- cal deformation. The mechanical deformation may be applied af-ter forming the particles to produce an article. Mechanical deformation may be used if the particles have an anisotropic shape, for example, to produce a preferred texture. The me- chanical deformation is produced by one or more of the pro- cesses of rolling, swaging, drawing, up-setting, backward ex-trusion or extrusion.

In some embodiments, a preferred texture is induced by direc-tionally solidifying the article.

After forming the article, the method may further comprise in- troducing one or more of the elements from the group consist-ing of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, Yb and Y into the article by diffusion. This embodiment may be useful for articles comprising a RE2(TM)14Z phase. This diffusion treatment may help to reduce the coercive field strength to less than 100 kA/m or less than 10 kA/m.

In further embodiments, the powder is mixed with a polymer be- fore orientating the particles. The polymer may provide a ma-trix of the article in which particles comprising anisotropic particles are embedded. The particles have anisotropic soft magnetic properties and a preferred texture whioh produces a polycrystalline article with anisotropic soft magnetic proper- ties. In embodiments, in which a polymer is added to the pow- der, a heat treatment may be used to cure the polymer and pro-duce a solid article.

The previously described embodiments of methods for producing an article for use as a rotor in a reluctance motor include the use of particles comprising a total composition capable of providing a soft magnetic material with magnetically aniso- tropic properties. The particles may also comprise a soft mag-netic material with magnetically anisotropic properties.

In further embodiments, a soft magnetic material is provided in a non-particulate, i.e. bulk form. The soft magnetic mate- rial may be provided as a block, a cylinder, or a foil for ex- ample. This bulk soft magnetic material may then be mechani-cally deformed to produce magnetic anisotropy.

In an embodiment, a method for producing an article is provid-ed that comprises: providing a soft magnetic material that is amorphous or nanocrystalline; mechanically deforming the soft magnetic material and producing a preferred texture, heat treating the soft magnetic material, and producing an article comprising polycrystalline soft magnetic material having a preferred texture and anisotropic magnetic properties.

In these embodiments, the preferred texture is produced by me- chanical deformation of a solid rather than orientating parti- des and producing a solid from the previously orientated par- ticles. These embodiments may be used for soft magnetic mate-rial which displays a preferred crystallographic orientation when subjected to mechanical deformation, for example a roll-ing texture and an annealing texture.

The mechanically deforming the soft magnetic material may com- prise one or more of the processes of roiling, swaging, draw- ing, up-setting, backward extrusion or extrusion and/or sub- jecting the soft magnetic material to hot mechanical defor-mation.

The amorphous or nanocrystailine soft magnetic material may be produced by a rapid solidification process.

Rapid solidification followed by hot mechanical deformation may be used for RE2 (TMh4Z compositions, for example.

Hot mechanical deformation may be useful for an article com-prising RE2TM-4Z which is produced in the form of an amorphous and/or nanocrystalline foil by a rapid solidification process.

In an embodiment, RE comprises Sm, TM comprises Fe and Z com-prises B. Embodiments and examples will now be described with reference to the drawings and Tables.

Figure 1 illustrates a schematic diagram of a reluctance mo-tor.

Figure 2 illustrates a SEM micrograph of a polycrystalline soft magnetic material.

Figure 3 illustrates a hysteresis curve illustrating aniso-tropic soft magnetic properties.

Figure 4 illustrates a hysteresis curve illustrating aniso-tropic soft magnetic properties.

Figure 5 illustrates a hysteresis curve illustrating aniso-tropic soft magnetic properties.

Figure 6 illustrates a hysteresis curve illustrating aniso-tropic soft magnetic properties.

Table 1 illustrates compositions of twelve samples in weight percent with the remainder being iron.

Table 2 illustrates magnetic properties of the samples meas-ured in a permagraph at room temperature.

Table 3 illustrates magnetic properties of the samples samples measured in a permagraph at room temperature.

Table 4 illustrates magnetic properties of the samples meas-ured in a permagraph at 15000.

Table 5 illustrates calculated magnetic properties of compari- son material and material according to embodiments of the in-ventiori.

Figure 1 illustrates a schematic diagram of a reluctance motor 1 according to an embodiment of the invention. The reluctance motor comprises a rotor 2 which has a circular cross-section and a generally tubular-shaped stator 3 which surrounds the rotor 2. The stator 3 includes three windings 4, 4', 5, 5' and 6, 6' . Current is supplied to the windings 4, 4' , 5, 5' and 6, 6' to generate a magnetic field through the rotor 2. The mag-netic field is schematically illustrated in Figure 1 for the winding 5, 5' by the arrow 7. The rotor 2 is able to rotate around the axis 8, as indicated by the arrow 9.

According to the invention, the rotor 2 comprises a soft mag-netic material with anisotropic soft magnetic properties. The anisotropic soft magnetic properties are illustrated schemati-cally in figure 1 by a first arrow 10 which indicates a more easily magnetisable direction and a second arrow 11, which in- dicates a more difficult magnetisable direction. The less eas-ily magnetisable direction 11 is perpendicular to the more easily magnetisable direction 10. Upon application of a mag- netic field 7 to the rotor 2, the more easily magnetisable di-rection 10 tries to align with the direction of the applied magnetic field. This causes the rotor 2 to rotate around the axis 8 and the reluctance motor 1 generates torque.

Since the rotor 2 is made up of a soft magnetic material with anisotropic soft magnetic properties, the rotor 2 has a sym-metrical outer contour and is free of slits or holes which may themselves produce anisotropic soft magnetic properties by virtue of an asymmetrical outer contour or shape.

The soft magnetic material of the rotor comprises a coercive field strength FLJ, wherein H(:J < 100 kA/m indicating that it is a soft magnet rather than a permanent magnet.

Embodiments and specific examples of alloys suitable for use as the rotor in a reluctance motor will now be described.

Suitable alloy systems includes RE>(TM)1C, RE2(TM)i1C, RE2(TM)7, (RE,Zr) (TM)3, RE(TM)2, and RE2(TM)29, RE(Co)5, RE(TM)7 RE2(Fe,00)17, a (Fe,Co,Ni,Mn)-rich intermetallic material, for example Mn3(Ga,Ge,Al), a"Fe12N and FeC and a metastable RE2 (TM) l7N alloy, wherein RE is one or more of the group con-sisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and TM is one or more of the group consisting of Fe, Co, Mn and Ni.

In one particular embodiment, the rotor 2 comprises RE(Fe,X)Z, a coercive field strength, H-, of less than 100 kA/m, indicating that it is soft magnet, a polycrystalline mi-crostructure and a preferred texture. RE comprises one or more of the elements of the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, X is optional and, if present, comprises one or more of the group consisting of Go, Ti, V, Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu, Z is B and or C. The article comprises at least one phase hav-ing a composition of RE2 (Fe,X)14Z.

The preferred texture creates the anisotropic magnetic proper-ties of the rotor, since the individual grains of the RE2 (Fe,X)-4Zbased phase of the polycrystalline microstructure have anisotropic soft magnetic properties and, due to the pre-ferred texture, these anisotropic soft magnetic properties are provided for the rotor 2 on a larger scale suitable for appli-cation in a reluctance motor.

The rotor may also be essentially free of elements such as Dy, Tb, Pr, Al, Ga and Cu which are known to increase the coercive field strength when included in the RE2 (Fe,X)4B-based magnets.

The overall composition of the rotor may deviate from stiochi- ometric ratio of 2RE: 14 (Fe,X) : lB in order to further tai-lor the coercive field strength and the anisotropy of the soft magnetic properties. For example, the overall composition may lie within the following ranges: 1.7 «= a «= 2.3, 13.5 «= b «= 14.5 and 0.8 «= c «= 1.2.

Additionally, the average grain size of the rotor may be ad-justed by adjusting the sintering conditions used to fabricate the rotor. An increased grain size may encourage the formation of a suitably low value of the coercive field strength.

Table 1 illustrates the compositions of twelve samples having compositions that deviate slightly from the stoichiometric Nd2Fe1,B phase. Series 1 includes samples Ndl to Nd4, Series 2 includes samples Nd5 to Nd8, and Series 3 includes samples Nd9 to Nd12.

In RE-TM-alloys, oxygen, carbon and nitrogen bind a proportion of rare earth elements in non-magnetic phases. RErt depicts the remaining effective part of rare earth elements related to the metallic part of the alloy and B depicts the effective boron content related to the metallic part of the alloy. REm and B:[j given in Table 1 are defined as follows.

RE11, = ( RE -ARE) x f B11 = B x f ARE = 5,993 x 0 + 16,05 x C + 10,30 x N f = 100 / (100 -ARE -C -C -N) RE, B, 0, C and N is the amount of rare earth elements, boron, oxygen, carbon and nitrogen in wt%, ARE is the amount of rare earth elements bound in non-magnetic phases and f is a stand-ardization factor.

The alloys are milled to a mean particle size of 5 gm to 10 m. Each of the powders is mixed with 0.5 wt% of a lubricant in the form of isopropanol, and tapped in an external magnetic field to produce a preferred texture in an intermediate prod- uct. The intermediate product is isostatically pressed to pro-duce a green body. The green bodies are packed in iron foil under argon and heated for 4 hours at temperatures in the range of 1080°C to 1140°C.

The microstructure of a sample Ndl comprising Nd as the rare earth of differing compositions which deviate slightly from the stoichiometric composition of Nd2Fe-4B is illustrated in the SEM micrograph of Figure 2.

Figure 2 illustrates that sample Ndl has a polycrystalline fi-ne-grained microstructure having an average grain size of 7.1 pm after sintering at 1080°C for 4 hours. Figure 2 illustrates that sample Ndl includes boride inclusions which appear dark grey and neodymium oxide inclusions which appear light within the matrix. Tn the regions examined, the magnetic domain structure of these samples appears to be well orientated apart from in the large grains, which sometimes contain misorientat-ed magnetic domain structures.

The magnetic properties of the samples are given in Tables 2, 3 and 4. Tables 2 to 4 list the sinter temperature, density, maximum polarisation, max, coercive field strength, Ho!i, per- meability, u, in the easily magnetisable direction, permeabil-ity in the less easily magnetisable direction, the ratio of two permeabilitles, the alignment coefficient, 0G. measured from the hysteresis curve, Heft, the effective anisotropy field strength, the effective anisotropy constant, Kef, where-in K0 = Hcrff/2*Jn, and J is the saturation polarization, the polarisation, J, in the easily and less easily magnetisable direction, the difference in the polarization between the po-larization measured in the easily and less easily magnetisable direction, the ratio of the polarization measured in the easi- ly and less easily magnetisable direction measured in a mag- netic field of 200 kA/m and 400 kA/m, AE29c which is the dif-ference in magnetic energy AE between a magnetic energy E-in a first direction and a magnetic energy F2 in a second direc-tion which is perpendicular to the first direction, AF2 = = IJi*H_J2*HI, 111 is the polarisation in the first di-rection measured in a magnetic field H of 200 kA/m and J2 is the polarisation in the second direction measured in a magnet-ic field H of 200 kA/m and AE4191: is the difference in magnetic

energy measured in a magnetic field H of 400 kA/m.

In the figures and tables, the more easily magnetisable direc-tion is denoted as the "easy" direction and the less easily magnetisable direction is denoted as the "hard" direction.

Tables 2 and 3 illustrate magnetic properties measured in a permagraph at room temperature and Table 4 illustrates magnet-ic properties measured in a permagraph at 150°C.

Most of the samples have a coercive field strength H!j of less than 10 kA/m indicating that the Nd2.Fe143-based phases have soft magnetic prcperties. The coercive field strength is lower for the samples sintered at higher temperatures. These Nd2Fe1B-based phases also have anisotropic soft magnetic prop-erties, i.e. have soft magnetic properties which are different in differing directions.

The permeability in the less easily magnetisable direction de-creases for increasing sinter temperatures, whereas no trend is observed for the permeability measured in the more easily magnetisable direction.

The alignment coefficient OG is estimated from the hysteresis curve by the point at which a linear regression of the curves from 2 kOe to 7 kOe in the first quadrant crosses the y axis at H = 0. These intersection points are named J,e38y and hard-The alignment coefficient is calculated by OG = cos(arctan(2*Jriia/Jr,e4) All of the samples have an alignment coefficient OS of at least 95% indicating that the samples have directionally de-pendent magnetic properties.

Each of the samples also has an anisotropy constant Keff of 2.8 to 3.5 MJ/m3. This value is higher than that of a typical el-liptical rotor of a reluctance motor made from iron foils which has a maximum anisotropy constant of around 0.3 NJ/m2.

Therefore, the samples are suitable for use as a rotor in a reluctance motor and may also be used to provide a reluctance motor which can produce a higher torque than some types of conventional reluctance motor.

Test samples were taken from the samples Ndl, Nd4, NdS and Nd8. A first slice is taken in which the crientation direction is perpendicular to the major surface of the slice. A second slice is taken in which the orientation direction lies paral-lel to the major surface of the slice. Hysteresis curves of these samples were measured and are illustrated in Figures 3 to 6.

In each case, the hysteresis curves show that the soft magnet-ic properties are different depending on the direction of the measured surface with respect to the orientation direction and confirm that the samples have anisotropic soft magnetic prop-erties.

The preferred texture is thought to be largely created during tapping of particles comprising Nd2Fe14B in the external mag- netic field. The preferred texture is retained after the sin- tering treatment so that a polycrystalline sample with a pre- ferred grain orientation and anisotropic soft magnetic proper- ties is produced which is suitable for use in a rotor in a re-luctance motor.

In the following, comparison examples will be described.

A comparison sample comprising 29.3wt% Nd, 0.13 wt% Dy, 0.4 wt% Pr, 0.95 wt% B, 0.02 wt% Al, 0.5 wt% Co, 0.05 wt% Cu, 0.052 wt% C, 0.14 wt% C and 0.025 wt% N are sintered at tem-peratures between 1090 and 1100°C for 4 hours in vacuum and 1 hour in argon and annealed at temperatures in the range of 490°C to 510°C.

In comparison to the embodiments and examples according to the invention, the comparison sample is subjected to an additional annealing heat treatment after sintering and includes 0.13 wt% Dy.

This comparison sample has an alignment coefficient of around 98% which is similar to that of the examples according to em- bodiments of the invention. The comparison sample has a coer-cive field strength HC! of around 7.5 kUe or around 600 kA/m which is much higher than 100 kA/m. The comparison sample is a permanent magnet rather than a soft magnet.

As discussed above, the anisotropy of the soft magnetic prop- erties achieved by use of a material which itself has aniso-tropic magnetic properties is greater than the anisotrcpy which can be achieved using an asymmetric shape for a rotor.

This is illustrated by the following calculated examples.

Known values of the saturation polarization J9 and the anisop-tropy constants K1 and K2 for various materials are listed in Table 5. The ratio c/a indicates the length of the rotor in two mutually perpendicular directions and represents an ellip-tical rotor. Nc and Na are the demagnetization factor in the c and a directions, respectively, and Kef = (N0N() /2J2// (c/a) (Nd,Dy)FeB1 and (Nd, Dy)FeB2 are permanent magnets. For an el- liptical rotor made of iron with c/a of 1.5, the effective an-isotropy constant Kef is around 0.25 MJ/m. The anisotropy constant K1 is increased for increasing shape anisotropy, for example, c/a = 9, but the effective anisotropy constant is de-creased to around 0.17 MJ/m3. This results in an increase in the size of the motor as a large proportion of the space re-quired to accommodate the rotor is empty.

The further materials Nd2FeyB, Pr2Fe11B, La2FeB, Ce2FeB, CeMM2Fe1tB and Sm2Fe-4B have anisotropic soft magnetic proper- ties so that the rotor is symmetrical and c/a = 1. These mate-rials are capable of providing a larger anisotropy constant K than that achievable by increasing the shape anisotropy of an elliptical iron rotor. For these anisotropic soft magnetic ma-terials the whole volume of the rotor can be built up by these materials. As a result, the torque density can be much higher compared to some types of conventional reluctance motors.

Claims

Claims 1. An article, comprising polycrystalline soft magnetic mate- rial comprising a preferred texture and magnetic anisotrc-py, wherein a difference in magnetic energy AE between a magnetic energy E1 in a first direction and a magnetic en- ergy E2 in a second direction is at least 270 kJ/m, where- in AE = I1 -= - wherein J1 is the polari-sation in the first direction measured in a magnetic field H and J2 is the polarisation in the second direction meas-ured in a magnetic field H.
2. The article according to claim 1, wherein the polycrystal-line soft magnetic material has a sintered microstructure.
3. The article according to claim 1, wherein the polycrystal- line soft magnetic material is embedded in a polymer ma-trix.
4. The article according to one of claims 1 to 3, wherein a selected crystallographic axis of at least 80 volume per-cent of the polycrystalline soft magnetic material lies within ± 45° of a predetermined direction.
5. The article according to one of claims 1 to 4, wherein a predetermined magnetic domain orientation of at least 80 volume percent of the polycrystalline soft magnetic mate-rial lies within ± 45° of a predetermined direction.
6. The article according to one of claims 1 to 5, wherein the second direction is perpendicular to the first direction.
7. The article according to one of claims 1 to 6, wherein the polycrystailine soft magnetic material comprises uniaxial-ly anisotropic soft magnetic properties.
8. The article according to one of claims 1 to 6, wherein the polycrystalline soft magnetic material comprises planarly anisotropic soft magnetic properties.
9. The article according to one of claims 1 to 8, wherein the soft magnetic material comprises one of more of the group of alloy systems consisting of RE2(TMhLC, RE2(TMh1C, RE2(TM)7, RE(TM)7, (RE,Zr) (TM), RE(TM)12, and REs(TM)29, wherein TM is one of more of the group consisting of Fe, Co, Mn and Ni and RE is one or more of the group consist-ing of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y.
10.The article according to one of claims 1 to 8, wherein the soft magnetic material comprises the alloy system RE2(TM)4Z, wherein RE is one or more of the group consit-ing to La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, TM is one of more of the group consisting of Fe, Cc, Mn, Ti, V, Mo, Nb, Zr, Al, Cr, Cu, Ga, Si, Ge, In, Sn and Ni and Z is B and/or C.
11.The article according to claim 10, wherein the alloy sys-tem is essentially free of the elements Dy, Tb, Ho, Al, Ga and Cu.
12.The article according to claim 10 or claim 11, wherein RE comprises one or more of the elements from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Er, Tm, Yb, Lu and Y.
13.The article acccrding to claim 10 cr claim 11, wherein RE is Nd or Cerium Mischmetal.
14.The article according to one of claims 1 to 8, wherein the soft magnetic material comprises alloy the system RE(Co).
15.The article according to claim 14, wherein the alloy sys- tem comprises additions comprising one or more of the ele-ments of the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Lu, 1, Fe, Ni, Mn, Cu, B and C.
16.The article according to one of claims 1 to 8, wherein the soft magnetic material comprises alloy system RE2 (Fe,Co)-7.
17.The article according to claim 16, wherein the alloy sys- tem comprises additions comprising one or more of the ele-ments of the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Lu, Y, Mn, Zr, Hf, Cr, V, Ti, Nb, Mo, Fe, Ni, Cu, B and C.
18.The article according to claim 17, wherein the alloy sys-tem is essentially free of Cu and Zr and Hf.
19.The article according to one of claims 1 to 8, wherein the soft magnetic material comprises a (Fe,Co,Ni,Mn)-rich in-termetallic material.
20.The article according to claim 19, wherein the (Fe,Cc,Ni,Mn)-rich intermetallic material is one of 14n (Ga,Ge,Al), c"FelbN2 and Fe30.
21.The article according to one of claims 1 to 8, wherein the soft magnetic material comprises a metastable RE2 (Fe,Co)-7N alloy.
22.The article according to one of claims 1 to 21, wherein the soft magnetic material comprises a coercive field strength Ej wherein ft-< 100 kA/m.
23.The article according to claim 22, wherein H!j < 10 kA/m.
24.The article according to 1 of claims 1 to 23, wherein AE at a magnetic field of 200 kA/m is greater than 150 kJ/m3.
25.The article according to claim 24, wherein AE at a magnet-ic field of 400 kA/m is greater than 240 kJ/m3.
26.The article according to one of claims 1 to 25, further comprising an effective anisotropy constant K2, wherein I K > 0.3 F4J/m, Kf = Haeff/2*J, Haef is the effective anisotropic magnetio field strength, J is the saturation polarisation.
27.Use of the article according to one of claims 1 to 26 as a part in a magnetic oircuit.
28.The use according to claim 27, wherein the part is movable in response to an application of a magnetic field external to the part.
29.The use according to claim 28, wherein the part is a rotor in a rotative reluctance motor, a linear motor or an actu-ator.
30.LJse according to one of claims 27 to 29, wherein the rotor is solid and without non-magnetic material, holes or empty spaces inside it producing anisotropic soft magnetic prop-erties sufficient to cause the rotor to rotate.
31.A magnetic circuit comprising a movable part and a magnet-ic field source external to the movable part, the movable part comprising an article according to one of claims 1 to 26.
32.A reluctance motor, comprising a rotor comprising an arti- cle according to one of claims 1 to 26 and a stator com-prising an electrically conductive winding.
33.The reluctance motor according to claim 32, wherein the rotor is solid and without non-magnetic material, holes or empty spaces inside it producing anisotropic soft magnetic properties sufficient to cause the rotor to move.
34.The reluctance motor according to claim 32 or 33, wherein the reluctance motor is a synchronous reluctance motor or a switched reluctance motor.
35.The reluctance motor according to one of claims 32 to 34, wherein the stator comprises a plurality of poles.
36.The reluctance motor according to one of claims 32 to 35, wherein the rotor comprises a plurality of poles.
37.A method for producing an article, comprising: providing particles comprising a total composition ca- pable of providing a soft magnetic material with magneti-cally anisotropic properties, orientating the particles to provide a preferred tex-ture, forming the partioles, and producing an article comprising polycrystalline soft magnetic material having a preferred texture and aniso-tropic magnetic properties.
38.The method according to claim 37, wherein orientating the particles comprises applying an external magnetic field.
39.The method according to claim 37 or claim 38, wherein af-ter orientating the particles, the orientated particles are formed to produce an article.
40.The method according to one of claims 37 to 39, wherein orientating the particles comprises applying a rotatingmagnetic field.
41.The method according to one of claims 36 to 39, wherein the particles are formed by compaction in a compaction di-rection.
42.The method according to claim 41 wherein the rotating mag-netic field is applied perpendicularly to the compaction direction.
43.The method according to one of claims 37 to 39, wherein the particles are orientated by rotating the particles ina static magnetic field.
44.The method according to one of claims 37 to 43, wherein forming the particles comprises sintering the particles.
45.The method according to claim 44, wherein the article is sintered at a temperature T and for a time t, wherein 900°C «= I «= 1300°C and 0.5h «= t «= 50h.
46.The method according to claim 44 or claim 45, wherein after sintering, the article comprises an average grain size of greater than 5 pun.
47.The method according to one of claims 44 to 46, further comprising quenching the article after sintering.
48.The method according to one of claims 37 to 47, wherein the grains have an average diameter of less than 30 Lm.
49.The method according to one of claims 37 to 48, wherein after forming the particles to produce an article, the particles are orientated by mechanical deformation.
50.The method according to claim 49, wherein the mechanical deformation is produced by one or more of the processes of rolling, swaging, drawing, up-setting, backward extrusion or extrusion.
51.The method according to one of claims 37 to 48, wherein the article is directionally solidified and a preferred texture is induced.
52.The method according to one of claims 37 to 51, further comprising introducing one or more of the elements from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, ho, Er, Tm, Lu, Yb and Y into the article by diffusion.
53.A method for producing an article, comprising: providing a soft magnetic material that is amorphous or nanocrystalline, mechanically deforming the soft magnetic material and inducing a preferred texture, heat treating the soft magnetic material, and producing an article comprising polycrystalline soft magnetic material having a preferred texture and aniso-tropic magnetic properties.
54.The method according to claim 53, wherein the mechanically deforming the soft magnetic material comprises subjecting the soft magnetic material to hot mechanical deformation.
55.The method according to claim 53 or claim 54, further 0cm-prising producing the amorphous or nanoorystalline soft magnetic material by a rapid solidification process.