GB2506683A - Anisotropic soft magnetic article and method for its production - Google Patents

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 is arranged such that it has a coercive field strength of less than 100 kA/m. The material may comprise REa(Fe, X)bZc polycrystalline microstructure where RE is one or more lanthenoid elements which includes at least one RE2(Fe, X)14Z phase, X is optional and if present may be one or more of Co, Ti, V, Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu and Z is B and/or C. The coercive field strength of the material may be less than 10 kA/m. When the article is in use a difference in magnetic energy of ΔE between a mag­netic 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 powder particles forming crystal grains which are oriented within a polymer matrix and sintered and processed to form a preferred texture soft magnetic anisotropic material. The magnetic material may have axial, planar or conical anisotropic properties. The article 2 or method may be used in forming a reluctance motor 1 rotor arrangement.

Description

Article and method for producing an article comprising aniso-trcpic soft magnetic properties The present invention relates to an article and a method for producing an article ccmprising anisotrcpic soft magnetic properties.

One type of reluctance motor comprises a rotor and a stator comprising means to prcdnce a variable magnetic field thrcugh 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 proper-ty, aligns with the direction of the magnetic field. The rotor of a reluctance motor typically comprises a soft magnetic ma-terial 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 perpendiou-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 layers. The slits 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 which comprises REa(Fe,X)oZ, a poly-crystalline microstructure, a preferred texture, anisotropic soft magnetic properties and a coercive field strength H. 100 kA/m. RE comprises one or more of the elements of the group consisting of La, Ce, Pr, Nd, Sm, Cd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. X is optional and, if present, comprises one or more of the group consisting of Co, Ti, V, Mo, Nb, Zr, Mn, Ni, Al, Cr, Ca, Si, Ce, In, Sn and Cu. Z is B and or C and 1.7 «= a «= 2.3, 13.5 «= b «= 14.5 and 0.8 «= c «= 1.2. The article com-prises at least one RE2 (Fe,X)-4Z phase.

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 of non-magnetic material or slits or holes within the rotor, an article is provided in which the composition of the materiai forming the article it-self, i.e. RE(Fe,XLZ., provides anisotropic soft magnetic properties. In particular, the article has a coercive field strength of less than 100 kA/m indicating that it is a soft magnet. The article has a polycrystalline microstructure with a preferred texture. Therefore, the article can have a symmet- rical contour and/or a symmetrical cross-section and be with-out non-magnetic layers or slits or holes positioned within it and is more easily magnetised in a first direction than In a second direction which is perpendicular to the first direction due to the anisotropic magnetic properties of the material forming the rotor.

The article comprises a plurality of grains providing a poly-crystalline mlcrostructure. A large proportion of the grains may comprise a phase or composition that has anisotropic soft magnetic properties. These grains are orientated so that they tend to have a common orientation which provides a preferred texture in the article.

Use may be made of a composition in which a single crystal of this composition has anisotropic soft magnetic properties, for example RE2(Fe,X)14Z. However, a single crystal may not be best suited for producing a rotor as it may be difficult or expen-sire to produce a single crystal in a size large enough for practical rotor applications. A plurality of particles of the composition may act similarly to a plurality of single crys- tals. The orientation of the particles results in an arrange- ment in which the particles have the same or a similar orien- tation. This arrangement can be used to produce a larger poly-crystalline article with a preferred texture and anisotropic soft magnetic properties.

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 mutually perpendicular directions to be increased over that achievable through shape anisotropy of the rotor.

The torque producible by a reluctance motor comprising a rotor of a soft magnetic material having anisctrcpic magnetic prop- erties may be increased over that achievable by shape anisot-ropy of the rotor.

In a further embodiment, the coercive field strength is less than 10 kA/m, i.e. H5 < 10 kA/m.

The magnetic anisotropy of the article leads to the article having a magnetic energy difference AF between a magnetic en- ergy F1 in a first direction and a magnetic energy F2 in a se-cond direction which is perpendicular to the first direction, wherein AF = F1-F2 = J1*HJ2*H, J1 is the polarisation in the first direction measured in a magnetic field H and J2 is the polarisation in the second direction measured in a magnet-ic field H. In one particular embodiment, the difference in magnetic energy AF is at least 270 kJ/m3. In further embodi-ments, the difference in magnetic energy AE22 in a magnetic field H of 200 kA/m is greater than 150 kJ/m3 and in a magnet-ic field H of 400 kA/m, hE402, is greater than 240 kJ/m3.

The article may have a magnetic energy difference of at least 270 kJ/m3 at room temperature and/or at the working temperature of a magnetic circuit in which the article is included. For example, the magnetic circuit nay be a motor, in particular, a reluctance motor, and the article may be the rotor of the re-luotance motor. A typical working temperature of a reluctance motor may be 150°C.

The article may comprise a sintered microstructure. The sin-tered structure may be identifIed by optical microscopy or scanning electron microscope analysis of the article. A sin-tered microstructure is characteristically formed by a heat treatment which causes contiguous particles of a body to join one another by mutual diffusion of atoms between the ccntigu- ous particles and the formation of grains. Typically, a sin-tering 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. Tn an embodiment, the article comprises an average grain size of greater than 5 Jim. An increasing grain size is thought to produce a decreasing coercive field strength H. and softer magnetic properties in RE;(FC,X)hZ. -based materials. If the average grain size is too large, this can lead to poorer mechanical properties. In a further embodi-ment, the maximum average grain size is 30 m.

The article may comprise a polymer matrix and particles com-prising the at least one RE2(Fe,X)11Z phase embedded in the polymer matrix. In this embodiment, the particles may have a preferred orientation within the polymer matrix and provide the article with anisotropic soft magnetic properties. This structure may be useful if a further heat treatment, for exam-plc 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 of a selected crystallographic axis of the grains. In one embodiment, a se-lected crystallographic axis of the RE2(Fe,X)14Z phase lies within 45° of a predetermined direction in at least 80% of the grains.

An effective anisotropy constant, Kef=, is defined as Kef= = HefJ2*Jq, where H;ef= is the effective anisotropic magnetic field strength and J is the saturation polarization. is the magnetic field strength which is required to saturate the article in the less easily magnetisable direction.

The effective anisotropy constant may be IKe=fI > 0.3 MJ/m3 or > 2.0 MJ/mt This value is provided by the anisotropic soft magnetic properties of the article rather than by an aniso-tropic shape or the presence of non-magnetic portions such as non-magnetic material, holes or slits within the article.

Due to the anisotropic soft magnetic properties, the article has a direction which is more easily magnetisable than at least one second direction which is perpendicular to the first direction. The RE2(Fe,X)14Z phase may comprise uniaxially, pla-narly or conically anisotropic soft magnetic properties. A uniaxial, planar or conical soft magnetic anisotropy may be chosen by proper selection of the rare earth element and/or of the proportion of a particular rare earth element.

Uniaxially anisotropic magnetic properties have a single first direction which is more easily magnetisable than two or mo- resecond 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 direotions which are arranged on a cone and which are more easily magnetisable than one or more second directions which are parallel or perpendicular to the axis of the cone.

In order to produce a uniaxial soft magnetic anisotropy, RE may be selected as one or more of the elements of the group consisting of Nd, La, Ce, Cd, Yb, Lu, Y and Pr.

In order to produce a planar soft magnetic anisotropy, RE may be selected as one or more of the group consisting of Sm, Er and Tm. In one particular embodiment, RE comprises Nd, Pr and Sm. In a further particular embodiment, the ratio of Sm in atomic percent to the total rare earth content in atomic per-cent is larger than 35%.

The RE2(Fe,X)-4Z phase may include one or more additional ele-ments of the group denoted X in place of Fe. In one particular embodiment, X is Co. Z is typically B, but may also comprise a proportion of C which is thought to aid in stabilizing the RE2(Fe,X)11Z phase.

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 t4ischmetal includes approximately 50% cerium and 25% lanthanum, with small amounts of neodymium and praseodymium. Cerium Mischmeta 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(Fe,X)14Z phase may be avoided in order to limit the coercive field strength H05 to less than 100 kA/m or less than 10 kA/m. In some embodiments, the total content of the elements Dy, Tb, Mo, Al, Ga and Cu is less than 0.5 wt%.

In some embodiments, the RE2(Fe,X)14Z 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 H.0.

The article may comprise a composition which deviates from the stoichiometric composition RE2(Fe,X)101 which is also known as the Phi phase. In one embodiment, the article comprises a to-tal rare earth content of 1.8 «= a «= 2.2 and a boron content of 0.9 «= c «= 1.1.

The article may further comprise up to 15 Volume percent of one or more of the group consisting of Fe2E, Nd2Fe1, and a-Fe.

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 mo- 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, oarriage or moveable part itself provides two mutu-ally perpendicular directions of differing magnetizability, i.e. a more easily magnetisable direction and a less easily magnetisahle direction.

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.

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 torgue density gen-erated by the motor can be much higher compared to some types of conventional reluctance motors. :0

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 anisctropic soft magnetic properties according to embodiments of the invention may also be used in applica- tions in which a reduction in magnetic flux at higher revolu- tions is desirable, since the magnetization level of the arti- dc 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 inoluding permanent magnets, for example based of RE2TN17Z-phases since the use of elements with a high raw materials cost, such as dysprosium, can be avoided.

A method for produoing an artiole oomprising anisotropio soft magnetic properties is provided which comprises: providing a powder comprising REu(Fe,X)rZ, wherein 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, oomprises one or more of the group consisting of Co, Ti, V. Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu, Z is B and/or C, wherein 1.7 «= a «= 2.3, 13.5 «= h «= 14.5 and 0.8 «= c «= 1.2; orientating particles of the powder to form a preferred texture; compacting the powder and producing a green body; sintering the green body, and producing an arti-cle having a preferred texture, anisotropic soft magnetic properties and a coercive field strength HT of < iOOkA/m.

The preferred texture in the article is produced at least in part due to the orientation of the particles of the powder to produce a preferred texture. The preferred texture of the par-tide is at least partly retained in the article after the sintering. The orientating of the particles may take place be-fore and/or after compaction of the powder.

The soft magnetic properties of the article are confirmed by a coercive field strength HJ of < 100 kA/m or < 10 kA/m. The anisotropy of the soft magnetic properties results from the preferred texture of the grains of the article and, in partic-ular, from the preferred orientation of the magnetic domains of the grains of the article.

In an embodiment, the orientating the particles of the powder to form a preferred texture oonprises applying a magnetio

field to the powder.

The orientating the particles may comprise applying a rotating magnetic field to the particles. The powder may be compacted in a compaction directiou 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.

The method may further comprise quenching to room temperature after the sintering. Quenching to room temperature may assist in producing a coercive field strength of cc 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 of < 100 kA/m.

In an embodiment, the method further comprises adding lubri-cant to the powder. This may be performed before orientating and compacting the particles. Lubricant may assist achieving a higher density of the green body and may also lead to an im-provement in the alignment of the particles. 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 powder comprises an average particle size (D5r1) of greater than 5 tm or greater than 10 F.tm. An in-creasing average particle size may lead to an increasing grain size after sintering. If the average particle size is too large, the density of the green body and of the article may be reduced.

In some embodiments, the sintering the green body comprises heating the green body at a temperature T 900°C < I «= 1200°C for a time t 0.5 < t «= 50 hours. The temperature and time re-quired to produce an article with a good density and suitable coercive field strength may depend on the composition of the powder, for example on the rare earth element or elements.

The temperature and time may also be selected to produce a de- sired average grain size in the article. Increasing the aver-age grain size may assist the production of a lower coercive field strength. In some embodiments, the average grain size of the article is greater than 5 pm and, optionally, less than 30 A mean grain size of greater than 30 m may result in poorer mechanical strength of the article.

In some embodiments, the powder comprises at least one RE2(Fe,X)14Z phase. The RE2(Fe,X)14Z phase typically has aniso-tropic magnetic properties. In this case, particles comprising the RE2(Fe,X)-4Z phase may be orientated by applying a magnetic field to produce a preferred texture. The powder may be essen-tially free of the elements Dy, Tb, Ho, Al, Ga and Cu. These elements are known to Increase the coercive field strength.

Consequently, omitting or reducing the amounts of these ele-ments may be used to assist in limiting the coercive field strength to less than 100 k/m.

If the article is to comprise one or more of the elements Sm, Er and Tm, these elements may be introduced into the article after the sintering treatment, for example, by diffusion. In some embodiments, the method further comprises diffusing one or more of the group consisting of Sm, Er and Tm into the ar-tide.

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 which produces a polycrystalline article with anisotropic soft magnetic proper- ties. Tn 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.

Embodiments and examples will now be described with reference to the drawings and Tables.

Figure 1 illustrates a schemat±c diagram of a reluctance mo-tor.

Figure 2 illustrates the composition of samples on a portion of the Nd-Fe-B phase diagram of the Nd2(Fe,X)1LB phase.

Figure 3 illustrates a graph of density for samples of differ-ing composition.

IS

Figure 4 illustrates a hysteresis curve of sample Ndl.

Figure 5 illustrates a hysteresis curve for sample Nd2.

Figure 6 illustrates a hysteresis curve for sample Nd4.

Figure 7 illustrates a hysteresis curve measured for sample Ndl measured using a permagraph.

Figure 8 illustrates a hysteresis curve measured for sample Nd4 measured using a permagraph.

Figure 9 illustrates a hysteresis curve for sample Nd5 meas-ured using a permagraph.

Figure 10 illustrates a hysteresis curve measured for sample Nd8 measured using a permagraph.

Figure ii illustrates the composition of six CeMJl4-containing samples on a portion of the RE-Fe-B phase diagram of the R52(Fe,X)14B phase.

Figure 12 illustrates the density of six CeL4M containing sam-ples.

Figure 13 illustrates the density of samarium containing sam-ples sintered at differing temperatures.

Figure 14 illustrates hysteresis curves for a sample with a low samarium content.

Figure 15 illustrates hysteresis curves for a sample including a higher samarium content.

Figure 16 illustrates an SEN micrcgraph.

Figure 17 illustrates a bright field light micrograph.

Figure 18 illustrates a micrograph ci a sample taken under po-larised light.

Figure 19 illustrates a micrograph of a sample taken under po-larised light.

Figure 20 illustrates an SEN micrograph of a sample.

Figure 21 illustrates a micrograph of a sample taken under pa-larised light.

Table 1 illustrates the compositions of four neodymium-containing powders in weight percent with the remainder of RE being Nd, remainder Fe.

Table 2 illustrates the compositions of 12 neodymium-containing samples in weight percent, remainder Fe.

Table 3 illustrates magnetic properties of neodymium- containing samples measured in a permagraph at room tempera-ture.

Table 4 illustrates magnetic properties of neodymium- containing samples measured in a permagraph at room tmpera-ture.

Table 5 illustrates magnetic properties of neodymium-containing samples measured in a permagraph at 15000.

Table 6 illustrates magnetic properties of neodymium- containing samples measured with a yoke technique at room tem-perature.

Table 7 illustrates the compos±ticn of cerium t4ischmetal-containing powders in weight peronet, reainder Fe.

Table 8 illustrates the compos±ticn of cerium Mischmetal-containing samples in weight percent, with the remainder of RE being Nd, remainder Fe.

Table 9 illustrates magnetic properties of cerium Mischmetal-containing samples.

Table 10 illustrates magnetic properties of cerium Mischmetal-containing samples.

Table 11 illustrates the composition of samarium-containing samples in weight percent.

Table 12 illustrates magnetic properties of samarium-containing samples.

Table 13 illustrates magnetic properties of samarium-containing samples.

Table 14 illustrates samples examined using microscopy.

Table 15 illustrates calculated effective anisotropic con-stants for various materials.

Figure 1 illustrates a schematic diagram of a reluctance motor 1 according to an embodiment of the invention. The reluctance motor 1 comprises a rotor 2 wh±ch 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 windirigs 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 arrow 7. The rotor 2 is able to rotate around an axis 8, as indicated by 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 s perpendicular to the more easily magnetisable direction 10. Upon application of the mag- netic field 7 to the rotor 2, the more easily rnagnetisable di-rection 10 tries to align with the direction of the applied magnetic field. This causes the rotor 2 to rotate around the axis B 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-metrica' 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 form.

IS

In one particular embodiment, the rotor 2 comprises REa(Fe,X)bZc, a coercive field strength, of less than 100 kA/m, indicating that it is a soft magnet, a poiycrystalline microstructure 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 oonsisting of Cc, 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 having a composition of RE(Fe,X)14Z.

The preferred texture creates the anisotropic magnetic proper-ties of the rotor as a whole since the individual grains of the RE2(Fe,X)4Z-based phase of the polycrystalline microstruc-ture have anisotropic properties and, due to the preferred texture, these anisotropic magnetic properties are provided for the rotor 2 on a larger scale suitable for application 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)1LB-based phase.

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 exampe, 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 coeroive field strength.

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

In a group of embodiments, neodymium, Nd, is used as the rare earth element in the RE2(Ee,X)14E-based phase. In the following embodiments, the RE2(Fe,X)143-based phase is 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 RE2 (Fe, X) 14B-based magnets.

Two melts of differing boron content are fabricated having compositions of 30 weight percent neodymium, 0.91 weight per-cent boron, remainder iron and of 30 weight percent neodymium, 1.23 weight percent boron, remainder iron. The solidified blocks are homogenised at 1050°C for 52 hours to reduce the alpha-iron content to less than 1 weight percent. After the homogenizing treatment, the ingots are crushed, milled to an average particle size of less than 450 m, mixed with Fe-powder and/or NdHX powder and milled to a fine powder having a D50 value of around 7 pm. D50 is the mean diameter of the par-ticles.

The compositions of the powders produced from the two melts are given in Table 1 in weight percent, remainder Fe. The re-mainder of RE is Nd. The powders are mixed together to produce boron rich, boron poor, neodymium-rich or neodymium-poor com-positions in relation to the stoiohiometrio composition of 2RE:l4Fe:1B which is denoted as Phi in Figure 2. The composi- tions are illustrated on a section of the Nd-Fe-B phase dia-gram in Figure 2 and the compositions are listed in Table 2 in weight percent, wherein the remainder is Fe. Three series of samples are listed: Series 1 includes samples Ndl to Nd4, Se- ries 2 includes samples NdS to Nd8 and Series 3 includes sam-ples Nd9 to Nd12.

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

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

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 108000 to 1140°C. The density of the samples is given in Figure 3 and the magnetic properties in Tables 3, 4 and 5.

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 3 to 5 list the sinter temperature, density, maximum polarisation, J,<, coercive field strength, Hrj, permeability, p, in the easily magnetisable direction, permeability in the less easily magnetisable direction, the ratio of two permea-bilities, the alignment coefficient, 00, measured from the hysteresis curve, H;eff, the effective anisotropy field strength, the effective anisotropy constant, wherein Kf = Hacff/2*J, and J. is the saturation polarization, the polari- sation, J, in the easily and less easily magnetisable direc- tion, the difference in the poThrization between the polariza- tion measured in the easily and less easily magnetisable di-rection, the ratio of the polarization measured in the easily and less easily magnetisabie direction measured in a magnetic field of 200 kA/m and 400 kA/m, AE2 which is the difference in magnetic energy fE between a magnetic energy 5-in a first direction and a magnetic energy 2 in a second direction which is perpendicular to the first direction, AE2 = 51-E2 I = J1*H_J2*H, J1 is the polarisation in the first direction measured in a magnetic field H of 200 kA/m and J2 is the po-larisation in the second direction measured in a magnetic field I-I of 200 kA/m and AE40 is the difference in magnetic en-

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

Tables 3 and 4 illustrate magnetic properties measured in a permagraph at room temperature and Table 5 illustrates magnet-ic properties measured in a permagraph at 15000.

Most of the samples have a coercive field strength H of less than 10 kA/m. The coercive field strength is lower for the samples sintered at higher temperatures. The permeability in the less easily magnetisable direction decreases for increas-ing sinter temperatures, whereas no trend is observed for the permeability measured in the more easily magnetisable direc-tion.

The alignment coefficient 00 is estimated from the hysteresis curve by the point at which a linear regression of the curve from 2 k0e to 7 hOe in the first quadrant crosses the y axis at H = 0. These intersection points are named r,ca and :iard The alignment coefficient is calculated by 00 = cos(arctan(2*Jr,harj/Jr,raOv)) All of the samples have an alignment coefficient 00 of at least 95%.

Each of the samples has an anisotropy constant K0=f of 2.3 to 3.5 MJ/m3 and is higher than that of a typical elliptical ro-tor of a reluctance motor made from iron foils which has a maximum anisotropy constant of around 0.3 MJ/m.

The magnetic measurements illustrated in Tables 3 to 5 were taken using a permagraph. Measurements in a permagraph suffer from parasitic air gaps which may lead in particular to an un-derestimation of the permeability in the easily magnetisable direction. In a further embodiment, samples Nd2, Nd4 and Nd8 sintered at a temperature of 11400 C were measured using a yoke measuring technique. The hysteresis curves are illustrat-ed in Figures 4 to 6.

Table 6 illustrates magnetic properties using a yoke technique at room temperature. A comparison of the magnetic properties measured with the yoke technique with those given in Tables 3 and 4 illustrates a variation in the values measured, e.g. the values of the coercive field strength being somewhat lower when measured in the yoke measuring technique. The permeabil-ity measured with the yoke technique is 448 to 1544 which is higher than the values measured using the permagraph and demonstrates the good soft magnetic properties of the aniso-tropic soft magnets.

Test samples were taken from the samples Ndl, Nd4, Nd5 and Nd8. A first slice is taken in which the orientation 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 7 to 10. In each case, the hysteresis curves show that the soft magnetic properties are different depending on the direction of the measured surface with respect to the orientation direc- tion and confirm that the samples have anisotropic soft mag-netic properties.

An estimate of the alignment coefficient OG was made from the hysteresis curves by taking a linear regression of the curves in the first quadrant between 2 kOe and 7 kOe. The point at which this line crosses the y-axis at H = 0 kA/m for the easi- ly magnetisable direction and the less easily magnetisable di-rection are used to calculate the alignment coefficient 0G.

The values all lie above 95% as summarised in Table 3. The samples with an alignment coefficient of 100% may arise due to the presence of very large grains in the sample.

Regions of the phase diagram around the stoichiometric Phi phase may be useful for the fabrication of magnets and, in particular, soft magnets. Samples Nd6 and Nd7 were found to be difficult to sinter to produce a high density. These composi-tions may be less suitable for use as a rotor than the other compositions for which a high density is more easily achieva-ble. The further samples have good density and have a coercive field strength of less than 100 kA/m and, when heated at 1140°C, of less than 10 kA/m indicating that they are good soft magnets.

The preferred texture is thought to be largely created during tapping of particles comprising Nd2Fe4B 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 as a rotor in a re-luctance motor.

In further set of embodiments, cerium Mischmetai is used as the rare earth element in the RE2(Fe,X)143-based phase.

Cerium Mischmetal is an alloy of rare earth elements in van-ous naturally occurring proportions and is useful in that it is less expensive than other rare earth metals, such as neo-dymium. A typical composition of cerium Mischmetal includes approximately 50% cerium and 25% lanthanum, with smail amounts of neodymium and praseodymium.

In these embodiments, cerium Nischmetai having a composition of 0.02 wt% Al, 0.31 wt% Fe, 5.32 wt% Pr, 0.04 wt%Si, 27.23 wt% La, 0.03 wt% Nn, 16.48 wt% Nd, 0.035 wt% C, 0.1 wt% Sm, 0.1 wt% Y, remainder Ce is used. wt% denotes weight percent.

Two melts of differing boron content are fabricated having compositions of 29.5 weight percent cerium Mischmetal, 0.91 weight percent boron, remainder iron and 29.5 weight percent cerium Mischmetal, 1.23 weight percent boron, remainder iron, homogenised at 1050°C for 52 hours to reduce the alpha-iron content to less than 1 weight percent. The soiidified plates are crushed, milled to an average particle size of less than 450 pm and milled to a fine powder having a D50 value of 6.6 pm.

The compositions of the cerium Mischmetal-containing powders are given in Table 7 in weight percent, wherein the remainder is Fe. The powders are mixed together, along with further neo-dymium iron boron -containing powders to produce six powders of differing composition. The compositions are illustrated in relation to a section of the RE:Fe:B phase diagram Figure 11 and listed in Table 8 in weight percent, wherein the remainder is Fe. The remainder of RE is Nd.

Each of the powders is mixed with 0.5 wt% isopropanoi, and tapped in the external magnetic field to cause reorientation of particles and produce a preferred texture. The particles are then isostaticaily pressed to produce the green body. The green bodies were packed in iron foil under argon and heated for 4 hours at temperatures in the range of 1060°C to 1090°C.

The density of the samples is given in Figure 12 and the mag-netic properties are listed in Tables 9 and 10. Samples Cel to Ce3 are rare earth rich and samples Ce4 to Ce6 are rare earth poor in relation to the stoichiometric Phi phase.

The alignment coefficient OG estimated from the hysteresis curve is 90% or greater indicating that the samples have a preferred grain orientation. Each of the samples has an ani-sotropy constant, of 1.6 to 1.8 MJ/m2 and is higher than that of a typical rotor of a reluctance motor made from iron foils which has an anisotropy constant of around 0.3 MJ/mt Table 10 lists the polarisation, J, in the easily and less easily magnetisable direction, the difference in the polariza-tion between the polarization measured In the easily and less easily magnetisable direction, the ratio of the polarization measured In the easily and less easily magnetisable direction and the magnetic energy difference tIE measured in a magnetic field of 200 kA/m and 400 kA/m. AE43 is greater than 330 kJ/m7 for all of the samples.

In further embodiments, samples including samarium as a rare earth element are provided. The samples include samarium, neo- dymium and praseodymium as rare earth elements and further in-clude cobalt. The samples are produced by mixing SmCo5 powder with powder comprising (Nd,Pr)2Fe-4B. The composition of the samples in weight percent is listed in Table 11 along with the ratio of samarium to the total rare earth content.

Each of the powders is mixed with 0.5 wt% isopropanol, and tapped in an external magnetic field to produce a preferred texture. The particles are then isostatically pressed to pro-duce a green body. The green bodies were packed in iron foil under argon and heated for 3 hours in a vacuum and 1 hour in Argon at temperatures in the range of 1052°C and 1110°C. Sam- ples were cooled guickly to room temperature from the sinter-ing temperature by forced air cooling. No further annealing heat treatment is carried out.

The effect of the sintering temperature on the density of the samples is illustrated in the graph of Figure 13. Figure 13 illustrates that, for all the samarium-containing composi-tions, the density increases for increasing temperatures and increases to a greater degree over the temperature range of 1052°C and 1073°C increases more slowly for higher sintering temperatures.

The hysteresis curves of samples Sm1 and Sm5, which have the lowest and highest samarium content of 20% and 40%, respec- tively, are illustrated in Figures 14 and 15. Figure 14 illus-trates that the hysteresis curve of sample Sml has a higher polarisation in the direction parallel to the preferred tex-ture compared to perpendicular to the preferred texture of samples. In contrast, Figure 15 illustrates that the sample Sm5 is more easily polarised in directions perpendicular to the preferred texture and less easily polarised in!directions parallel to the preferred texture. This reversed behaviour may be a result of the tact that SinS represents an article with planar soft magnetic anisotropy whereas Smi has uniaxial soft magnetic anisotropy.

The magnetic properties of samples Sml and Sm5 are summarised in Tables 12 and 13. Sample SmI has an effective anisotropy constant K=f cf 1 NJ/rn3 and sample Srn5 has an effective ani-sotropy constant KL of -0.7 NJ/rn3. IKclLIis in both cases greater than the value of 0.3 NJ/rn3, which is achieved by a reluctance motor with an elliptical iron rotor. Sarnple Srnl has an alignment coefficient OG estirnated frorn the hysteresis curve of 97%, whereas the value of the alignment coefficient OG estimated from the hysteresis curve for sarnple Srn5 is lower at 70%.

In Table 13, AF = AJ*HI. Sample Smi also has an energy dif-ference of 204 kJ/m3 in a magnetic field of 200kA/m and of 344 kJ/rn1 in a magnetic field of 400 kA/rn, which is higher than the respective values of a classical reluctance rnotor of 144 kJ/m3 and 232 kJ/m3. Sample 5rn5 has lower energy difference values AE233 and AS733, which may be attributable to the lower alignment coefficient of this sample.

The microstructure of samples comprising Nd as the rare earth of differing compositions which deviate slightly from the stoichiometric cornposition of Nd2Fe-4B are illustrated in Fig-ures 16 to 21.

Table 14 lists the cornposition of the samples and heat treat-ment conditions examined using light microscopy in bright field and in polarised light and scanning electron microscopy.

Samples parallel to the orientation direction and perpendicu-lar to the orientation direction are illustrated. The even numbered samples were taken from regions parallel to the alignment direction and the samples given an odd number cut perpendicular to the alignment direction.

The average grain size is measured using the circle method and an average taken of 6 independent measurements. The domain structure is visible under polarised light in the light micro- soope for the samples taken parallel to the orientation direo-tion.

Samples Nd13 and Nd14 were sintered at 1080°C for 4 hours have a fine-grained microstructure having an average grain size of 7.1 pm. Figure 16 illustrates sample Nd13 and illustrates ho- ride inclusions which appear dark grey and neodymium oxide in-clusions which appear light within the matrix. In the regions examined, the magnetic domain structure of these samples ap-pears to be well orientated apart from in the large grains, which sometimes contain misorientated magnetic domain struc-tures.

Samples Ndl5 and Ndl6 were sintered at a higher temperature of 1140°C for 4 hours than samples 13 and 14. Figure 17 illus-trates that sample Nd15 has a larger average grain size of around 350 pm. In contrast to the samples Nd13 and Nd14, bo- ride inciusions appear at the grain boundaries, whereas neo-dymium oxide inclusions are present mainly within the grains.

Figure 18 illustrates sample Nd16 and illustrates that, in the regions examined, the orientation of the magnetic domains de-viates from an optimal orientation in some of the large grains.

Samples Nd17 and Nd18 have an average grain size of 8.5 pm af- ter a sinter heat treatment at 114000 for 4 hours. The inclu- sions comprise alpha-iron, Fe2B and neodymium oxide. Some de-viation from the desired orientation is observed in a few grains.

Samples Ndl9 and Nd20 have an average grain size of 7.1 pm af-ter sintering at 112000 for 4 hours. The small inclusions were observed to comprise neodymium iron oxide. The magnetic domain orientation appears good in the region examined.

Samples Nd21 and Nd22 comprise a very large grain size after a double sintering treatment of.140°C for 4 hours. The inclu- sions are observed to comprise neodymium oxide. Figure 19 il-lustrates sample Nd22 and illustrates that the magnetic domain structure is generally well orientated in the region examined.

The magnetic domain structure is inclined at an angle in some grains in the region examined.

Samples Nd23 and Nd24 have an average gain size of around 8.7 pm after a double sintering heat treatment of 1140°C for 4 hours. Figure 20 illustrates sample Nd23 and Figure 21 illus-trates sample Nd24 and illustrates that the magnetic domain orientation is guite good with some magnetic domain structures inclined at an angle in the region examined.

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% 0 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 NJ of around 7.5 koe 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 anisotropy which can be achieved using an asymmetric shape for the rotor.

This is illustrated by the following calculated examples.

Known values of the saturation polarization J. and the anisop-tropy constants K1 and K2 for various materials are listed in Table 15. The ratio c/a indicates the length of the rotor in two mutually perpendicular directions and represents an ellip-tical rotor. No and Na are the demagnetization factor in the c and a directions, respectively, and = (Nu-N(.)/2J2/tc /(c/a) (Nd,Dy)FeBl and (Nd,Dy)Fe32 are permanent magnets. For an el- liptical rotor made of iron with c/a of 1.5, the effective an- isotropy constant K is around 0.25 MJ/m3. The anisotropy con- stant IC is increased for increasing shape anisotropy, for ex- ample, c/a = 9, but the effective anisotropy constant is de-creased to around 0.17 MJ/m.This results in an increase in the size of the motor as a large proportion of the space re-guired to accommodate the rotor is empty.

The further materials Nd2Fe-4B, Pr2Fe1LB, La2Fe1LB, 0e2Fe14B, CeJ4N2Fe-4B and Srn2Fe14B have anisotropic soft magnetic proper- ties so that the rotor is symmetrioal 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 materials, the whole vol- ume of the rotor can be built up by these materials. As a re-sult, the torque density can be hiqher than that of some types of conventional reluctance motor.

Claims (45)

1. Claims 1. An article, comprising REa(Fe,X)hZc, a polycrystalline mi- crostructure, a preferred texture, anisotropic soft mag-netic properties and a coercive field strength < 100 kA/m, wherein 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 elements of the group con-sisting of Cc, Ti, V, Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu, Z is B and or C, wherein 1.7 «= a «= 2.3, 13.5 «= b «= 14.5 and 0.8 «= c «= 1.2 and the article compris-es at least one RE2 (Fe,X)-4Z phase.
2. 2. The article according to claim 1, wherein the coercivefield strength H5 < 10 kA/m.
3. 3. The article according to claim 1 or claim 2, wherein the article comprises a magnetic energy difference AE between a magnetic energy F-in a first direction and a magnetic energy E2 in a second direction which is perpendicular to the first direction, the difference being at least 270 kJ/m1, wherein AN = I E1-E2 I = J1*H_J2*H I, J1 is polarisa-tion in the first direction measured in a magnetic field H and J2 is polarisation in the second direction measured ina magnetic field H.
4. 4. The article according to one of claims 1 to 3, wherein AE22r) in a magnetic field H of 200 kA/m is greater than 150 kJ/m3.
5. 5. The article according to one of claims 1 to 4, wherein AE4 in a magnetic field H of 400 kA/m is greater than 240 kJ/m1.
6. 6. The article according to one of claims 1 to 5, wherein the article comprises a sintered microstructure.
7. 7. The article according to one of claims 1 to 6, wherein the article comprises an average grain size of greater than 5 m and/or less than 30 tm.
8. 8. The article according to one of claims 1 to 5, wherein the article comprises a polymer matrix and particles compris-ing the at least one RE2(Fe,X)14Z-based phase embedded in the matrix.
9. 9. The article according to one of claims 1 to 8, wherein a selected crystallographic axis of the at least one RE7(Fe,X)/Z-based phase of 80% of the grains lies within 45° of a predetermined direction.
10. 10.The article according to one of claims 1 to 9, wherein I KCL > 0.3 MJ/m3, wherein K11 = Hw:r/2*J, K( is effective anisotropy constant, Hae=f is effective anisotropy field strength and J is saturation polarization.
11. 11.The article according to one of claims 1 to 10, wherein the at least one RE2 (Fe,X) 1L7-based phase comprises uniaxi-ally anisotropic soft magnetic properties.
12. 12.The article according to claim 11, wherein RE is one or more of the elements of the group consisting of Nd, La, Ce, Gd, Yb, Lu, Y and Pr.
13. 13.The article according to one of claims 1 to 10, wherein the at least one RE2 (Fe,X)1LZ-based phase comprises pla-narly anisotropic soft magnetic properties.
14. 14.Ihe article according to claim 13, wherein RE comprises one or more of the group consisting of Sm, Er and Tm.
15. 15.Ihe article according to claim 13 or claim 14, wherein RE comprises La, Ce, Nd, Pr and Sm.
16. 16.Ihe article according to claim 14 or claim 15, wherein a ratio of Sm in atomic percent to total rare earth content in atomic percent is larger than 35%.
17. 17.Ihe article according to one of claims 13 to 16, wherein X is Co.
18. 18.Ihe article according to one of claims 1 to 11, wherein RE is Nd or Cerium Mischmetal.
19. 19.Ihe article according to one of claims 1 to 18, wherein the at least one RE2 (Fe,X) 1Z-based phase is essentially free of the elements Dy, Tb, Ho, Al, Ga and Cu.
20. 20.Ihe article according to one of claims 1 to 19, wherein the article comprises a total rare earth content 1.8 «= a 2.2 and a boron content 0.9 «= c «= 1.1.
21. 21.The article according to one of claims 1 to 20, further comprising up to 15 Volume percent of one or more of the group consisting of FC2B, Nd2Fe-7 and a-Fe.
22. 22.Use of the article according to one of claims 1 to 21 as a part in a magnetic circuit.
23. 23.The use according to claim 22, wherein the part is movable in response to an application of a magnetic field external to the part.
24. 24.Use according to claim 22 or claim 23, wherein the part is a rotor in a rotative reluctance motor, a linear motor or an actuator.
25. 25.Use according to one of claims 22 to 24, 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.
26. 26.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 21.
27. 27.A reluctance motor, comprising a rotor comprising an arti- cle according to one of claims 1 to 21 and a stator com-prising an electrically conductive winding.
28. 28.The reluctance motor according to claim 27, 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.
29. 29.The reluctance motor according to claim 27 or 28, wherein the reluctance motor is a synchronous reluctance motor or a switched reluctance motor.
30. 30.The reluctance motor according to one of claims 27 to 29, wherein the stator comprises a plurality of poles.
31. 31.The reluctance motor according to one of claims 27 to 30, wherein the rotor comprises a plurality of poles.
32. 32.A method for producing an article comprising anisotropic soft magnetic properties, comprising: providing a powder comprising REa(Fe,X)hZo, wherein RE comprises one or more of the elements of the group con-sisting of La, Ce, Pr, Nd, Sm, Cd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, X is optional and, if present, comprises one or more of the elements of the group consisting of Cc, Ti, V. Mo, Nb, Zr, Mn, Ni, Al, Cr, Ga, Si, Ge, In, Sn and Cu, Z is B and or C, wherein 1.7 f a «= 2.3, 13.5 «= b «= 14.5 and 0.8 «= c «= 1.2, orientating particles of the powder to form a preferred texture, compacting the powder and producing a green body, sintering the green body, and producing an article having a preferred texture, aniso-tropic soft magnetic properties and a coercive field strength H. of < lOOkA/m.
33. 33.The method according to claim 32, wherein the orientating particles of the powder to form the preferred texture com-prises applying a magnetic field to the powder.
34. 34.The method aocording to claim 32 or ciaim 33, further com-prising quenching to room temperature after the sintering.
35. 35.The method according to one of ciaims 32 to 34, further comprising adding lubricant to the powder.
36. 36.The method according to claim 35, wherein the iubricant is iscprcpanoi, zinc-stearate and/cr iscstearic acid.
37. 37.The methcd according to one of claims 32 to 36, wherein the powder comprises an average particie size (D1) of greater than 5 m or greater than 10 j..Lm.
38. 38.The method according tc one of claims 32 to 37, wherein the sintering the green body comprises heating the green body at a temperature I 900°C < I «= 1200°C for a time t 0.5 < t «= 50 hours.
39. 39.The method according to one of claims 32 to 38, wherein the average grain size of the article is greater than 5 pm or less than 30 jIm.
40. 40.The method according to one of claims 32 to 39, wherein the powder comprises at least one RE2(Fe,X)14Z phase.
41. 41.Ihe method according to one of claims 32 to 40, wherein the powder is essenti ally free of the elements Dy, Tb, Rn, Al, Ga and Cu.
42. 42.Ihe method according to one of claims 32 to 41, wherein the powder is compacted in a compaction direction and on- entating the particles comprises applying a rotating mag-netic field to the particles.
43. 43.The method according to claim 42, wherein the rotating magnetic field is applied in directions perpendicular to the direction of compaction.
44. 44.The method according to one of claims 32 to 41, wherein the orientating the powder comprises rotating the powderin a static magnetic field.
45. 45.The method according to one of claims 32 to 44, further comprising diffusing one or more of the group consisting of Sm Er, and Tm into the article.