1. Field of the Invention
The present invention relates to an anisotropic rare-earth iron boride/nitride-based magnet, and more particularly to a self-recoverable rare-earth iron-based magnet which is fabricated by taking advantage of a novel self-recoverability so as to have a continuously controlled anisotropy distribution and a (BH)max of 160 kJ/m3 or more.
2. Description of the Related Art
Rare-earth iron-based hard magnetic materials, for example, Nd2Fe14B, αFe/Nd2Fe14B and Fe3B/Nd2Fe14B which are obtained by rapid solidification such as melt spinning, are limited in form to a ribbon, or a flake obtained by milling. For this reason, in order to obtain a bulk magnet for use in a small motor, a technique is necessary by which the form of material is changed, specifically, the ribbon or the powder is solidified into a given bulk form in one way or another. A basic powder fixing means in powder metallurgy is pressureless sintering, but since magnetic properties based on a metastable state must be maintained in the abovementioned ribbon or flake, the pressureless sintering is hardly applicable to the solidification process. For this reason, the ribbon or the flake is consolidated into a specific form mostly by using a binder such as an epoxy resin.
For example, in 1985, R. W. Lee et al. reported that an isotropic Nd2Fe14B-based bonded magnet having a (BH)max of 72 kJ/m3 is obtained when a ribbon having a (BH)max of 111 kJ/m3 is solidified by means of a resin material (refer to Non-Patent Document 1).
In 1986, the present inventors et al. proved in Japanese Patent Application Laid-Open No. S61-38830 that an ring-shaped isotropic Nd2Fe14B magnet which is made of the above described ribbon solidified by using an epoxy resin and which has a (BH)max of up to 72 kJ/m3 is suitable for use in a small motor. Further, for example, in 1990, G. X. Huang et al. clarified that an ring-shaped isotropic magnet is suitable for use in a small motor (refer to Non-Patent Document 2), and such a ring-shaped magnet has been widely used in the 1990's as a magnet for a high-performance small motor which is applied to an electromagnetic drive unit in electric and electronic equipment such as OA (office automation), AV (audio and visual), PC (personal computer), PC peripheral devices, and telecommunication equipment.
On the other hand, a lot of researches on the magnet material formed by a melt spinning method have been actively conducted since the 1980's, wherein an Nd2Fe14B- or Sm2Fe17N3-based material, a nanocomposite material fabricated by taking advantage of an exchange coupling based on a microscopic texture between αFe, Fe3B-based material and the foregoing material, and a material fabricated by fine-controlling a variety of alloy compositions and their texture are known, and also in addition to the materials described above, magnetic powders which are formed by a rapid solidification method other than the melt spinning and have a different powder shape are recently made available (refer to, for example, Non-Patent Documents 3 and 4). Also, Davies et al. reported an isotropic magnet which achieves a (BH)max of as large as 220 kJ/m3 (refer to Non-Patent Document 5). However, it is supposed that a ribbon formed by the rapid solidification method and made industrially available has a (BH)max of up to 134 kJ/m3 and that a ring-shaped isotropic magnet fabricated by using such a ribbon has a (BH)max of about 80 kJ/m3.
Irrespective of the current technical condition described above, a relatively small electromagnetic drive unit to which the present invention relates is always requested to be further miniaturized and to perform with increased output and efficiency in response to the enhancement of the performance of electric and electronic equipment. Thus, it is obvious that just improving the magnetic properties of an isotropic ribbon formed by the rapid solidification method is no longer good enough for catching up with the enhancing performance of electric and electronic equipment. Therefore, it is increasingly required, especially in the field of a small electromagnetic drive unit, to provide a magnet which has a static magnetic field distribution adapted to a magnetic circuit with a core of the motor and at the same time which generates as strong static magnetic field as possible per unit volume.
Sm—Co-based magnetic powder for a rare-earth magnet, even when prepared by milling an ingot, achieves a high coercivity (HcJ). However, Co has problems in terms of securing a stable supply, its resource balance and so on, and therefore it is not appropriate to use Co generally as industrial material. On the other hand, rare-earth iron-based magnetic powder, which is composed mostly of Fe as well as a rare-earth element such as Nd, Pr, Sm or the like, is advantageous in view of a secured stable supply and a resource balance. Such rare-earth magnetic powder, however, achieves a low coercivity (HcJ) even if an ingot or sintered magnet of Nd2Fe14B-based alloy is milled. For this reason, with regard to fabrication of anisotropic Nd2Fe14B magnetic powder, researches based on using a melt spinning material as starting material have been pursued in advance.
In 1989, Tokunaga obtained an anisotropic magnet with a (BH)max of 127 kJ/m3 in such a manner that a bulk prepared by subjecting Nd14Fe80-XB6GaX (X=0.4 to 0.5) to hot upsetting (die-upset) was milled and formed into anisotropic Nd2Fe14B magnetic powder which was then solidified by a resin material (refer to Non-Patent Document 6). Also, in 1991, H. Sakamoto et al. fabricated anisotropic Nd2Fe14B magnetic powder with a coercivity (HcJ) of L30 MA/m by subjecting Nd14Fe79.8B5.2Cu1 to hot rolling (refer to Non-Patent Document 7). As described above, the magnetic powder has been made available which achieves an increased coercivity (HcJ) in such a manner that the hot processing performance is improved with addition of Ga and Cu thereby further miniaturizing the Nd2Fe14B particle size. In 1991, V. Panchanathan et al. introduced an anisotropic magnet with a (BH)max of 150 kJ/m3, which was fabricated by a hot mill method in such a manner that a bulk into which hydrogen was caused to make ingress from a grain boundary was collapsed as Nd2Fe14BHX and dehydrogenated by vacuum heating into HD (hydrogen decrepitation)-Nd2Fe14B magnetic powder which was then solidified by a resin material (refer to Non-Patent Document 8). In 2001, by the same method described above, Iriyama formed Nd13.7Fe7.35Co6.7B5.5Ga0.6 into an anisotropic magnetic powder with a (BH)max of 177 kJ/m3 which was then solidified by an epoxy resin binder and developed into an improved anisotropic magnet having a (MH)max of 177 kJ/m3 (refer to Non-Patent Document 9).
Meanwhile, Takeshita et al. proposed an HDDR (hydrogenation-decomposition-desorption-recombination) method in which an Nd—Fe(Co)—B ingot is heat-treated in hydrogen atmosphere such that: Nd2(Fe, Co)14B phase is hydrogenated (hydrogenation, Nd2(Fe, Co)14BHX); the phase is decomposed at 650 to 1000° C. (decomposition, NdH2+Fe+Fe2B); hydrogen is desorbed (desorption); and recombination is performed (recombination) (refer to Non-Patent Document 10). And, in 1999, an anisotropic magnet with a (BH)max of 193 kJ/m3 was fabricated by solidifying HDDR Nd2Fe14B magnetic powder with an epoxy resin binder (refer to Non-Patent Document 11).
In 2001, Mishima et al. introduced Co-free d-HDDR Nd2Fe14B magnetic powder (refer to Non-Patent Document 12), and N. Hamada et al. fabricated a cubic anisotropic magnet (7 mm cubed) with a density of 6.51 Mg/m3 and a (BH)max of 213 kJ/m3 in such a manner that d-HDDR Nd2Fe14B magnetic powder with a (BH)max of 358 kJ/m3 was compacted together with an epoxy resin binder in the presence of an aligned magnetic field of 2.5 T under a pressure of 0.9 GPa at an elevated temperature of 150° C. (refer to Non-Patent Document 13).
However, such a cubic (or rectangular) magnet as described above is not suitable for an electromagnetic drive unit represented by many of motors to which the present invention relates. Especially, for application in an electromagnetic drive unit represented by a small motor having an output of several ten W or less, a ring-shaped magnet with a thickness of about 1 to 2 mm must be adapted to meet the design concept of the electromagnetic drive unit in terms of reducing diameter or thickness, making thickness uneven, increasing length, and the like. When a magnet is formed directly as a ring-shaped anisotropic magnet, if the ring diameter is reduced (or the length is increased), much of magnetomotive force in the radial magnetic field direction is dissipated as a leakage magnetic flux thus causing the oriented magnetic field to decrease. Consequently, the (BH)max with respect to the radial direction decreases in accordance with reduction in diameter (or increase in length). As a result, for application in an electromagnetic drive unit, a small-diameter ring-shaped anisotropic magnet with one or more magnetic pole pairs has not been widely available as a next generation model after a ring-shaped isotropic magnet having a (BH)max of about 80 kJ/m3.
Japanese Patent No. 2911017 discloses a magnet manufacturing method in which four arc-segments are combined to form a ring-shaped compact, and the compact is sintered under ordinary pressure.
On the other hand, D. Johnson et al. disclosed a “quasi-Halbach array” in which rectangular anisotropic sintered magnets are embedded at respective predetermined positions of a ring-shaped soft magnetic body, instead of a “Halbach array” in which a ring-shaped anisotropic magnet is composed of arc-segments (refer to Non-Patent Document 14).
<<Non-Patent Documents>>
<Non-Patent Document 1> R. W. Lee, E. G Brewer, N. A. Schaffel, “Hot-pressed Neodymium-Iron-Boron magnets” IEEE Trans. Magn., Vol. 21, 1958 (1985)
<Non-Patent Document 2> G. X. Huang, W. M. Gao, S. F. Yu, “Application of melt-spun Nd—Fe—B bonded magnet to the micro-motor”, Proc. of the 11th International Rare-Earth Magnets and Their Applications, Pittsburgh, USA, pp. 583-594 (1990)
<Non-Patent Document 3> B. H. Rabin, B. M. Ma, “Recent developments in Nd—Fe—B power”, 120th Topical Symposium of the Magnetics Society of Japan, pp. 23-23 (2001)
<Non-Patent Document 4> S. Hirasawa, H. Kanekiyo, T. Miyoshi, K. Murakami, Y. Shigemoto, T. Nishiuchi, “Structure and magnetic properties of Nd2Fe14B/FeXB-type nanocomposite permanent magnets prepared by strip casting”, 9th Joint MMM/INTERMAG, CA (2004) FG-05
<Non-Patent Document 5> H. A. Davies, J. I. Betancourt, C. L. Harland, “Nanophase Pr and Nd/Pr based rare-earth-iron-boron alloys”, Proc. of 16th Int. Workshop on Rare-Earth Magnets and Their Applications, Sendai, pp. 485-495 (2000)
<Non-Patent Document 6> G. Tokunaga, “Magnetic Characteristic of Rare-Earth Bond Magnets, Magnetic Powder and Powder Metallurgy”, Vol. 35, pp. 3-7 (1988)
<Non-Patent Document 7> H. Sakamoto, M. Fujikura and T. Mukai, “Fully-dense Nd—Fe—B magnets prepared from hot-rolled anisotropic powders”, Proc. 11th Int. Workshop on Rare-Earth Magnets and Their Applications, Pittsburg, pp. 72-84 (1990)
<Non-Patent Document 8> M. Doser, V. Panchanacthan, and R. K. Mishra, “Pulverizing anisotropic rapidly solidified Nd—Fe—B materials for bonded magnets”, J. Appl. Phys., Vol. 70, pp. 8603-6805 (1991)
<Non-Patent Document 9> T. Iriyama, “Anisotropic bonded NdFeB magnets made from hot-upset powders”, Polymer Bonded Magnet 2002, Chicago (2002)
<Non-Patent Document 10> T. Takeshita, and R. Nakayama, “Magnetic properties and micro-structure of the Nd—Fe—B magnetic powders produced by hydrogen treatment”, Proc. 10th Int. Workshop on Rare-earth Magnets and Their Applications, Kyoto, pp. 551-562 (1989)
<Non-Patent Document 11> K. Morimoto, R. Nakayama, K. Mori, K. Igarashi, Y. Ishii, M. Itakura, N. Kuwano, K. Oki, “Nd2Fe14B-based magnetic powder with high remanence produced by modified HDDR process”, IEEE. Tran. Magn., Vol. 35, pp. 3253-3255 (1999)
<Non-Patent Document 12> C. Mishima, N. Hamada, H. Mitarai, and Y. Honkura, “Development of a Co-free NdFeB anisotropic magnet produced d-HDDR processes powder”, IEEE. Trans. Mang., Vol. 37, pp. 2467-2470 (2001)
<Non-Patent Document 13> N. Hamada, C. Mishima, H. Mitarai and Y. Honkura, “Development of anisotropic bonded magnet with 27 MGOe” IEEE. Trans. Magn., Vol. 39, pp. 2953-2956 (2003)
<Non-Patent Document 14> D. Johnson, P. Pillay and M. Malengre, “High speed PM motor with hybrid magnetic bearing for kinetic energy storage”, IEEE Industry Applications Society Annual Meeting, Chicago 2001
For example, Japanese Patent No. 2911017 discloses the following magnet manufacturing method. A green compact of an arc-shaped segment having an outer diameter of 15.2 mm, an inner diameter of 10.8 mm, a length of 18.0 mm and a volume of 6.47 cm3 is formed such that fine powder of alloy composition Nd14.0Dy1.0Fe77.0Al1.0B7.0 having an average particle size of 3.5 μm is compressed at about 100 MPa, and four of such green compacts are combined and formed under a hydrostatic pressure of 200 MPa into a ring-shaped green compact having an outer diameter of 27A mm, an inner diameter of 19.4 mm, a height of 16.2 mm and a volume of 4.76 cm3. Subsequently, the green compact is sintered for two hours at 1090° C. in a vacuum atmosphere and then subjected to an aging treatment for one hour at 580° C., thus completing a ring-shaped sintered magnet. It is described therein that a 2 mm cube cut out from an arbitrary portion of the ring-shaped sintered magnet fabricated as described above has uniform magnetic properties. That is to say, in the manufacturing method described above, a plurality of arc-segment compacts, each of which has a thickness of 4.4 mm and is brittle, are combined and hydrostatically formed into a ring-shaped compact having a thickness of 4.0 mm, and the ring-shaped compact is subjected to pressureless sintering and thereby rigidified in an integral manner.
On the other hand, D. Johnson et al. disclosed a “quasi-Halbach array” in which rectangular anisotropic sintered magnets are embedded at respective predetermined positions of a ring-shaped soft magnetic body as shown in
In the quasi-Halbach array of D. Johnson et al. shown in FIG. 1B′ in which rectangular anisotropic sintered magnets are embedded at respective predetermined positions of a ring-shaped soft magnetic body, a uniform static magnetic field as seen in the Halbach-array of FIG. 1A′ cannot be achieved in the open space 2 for accommodating a stator. Moreover, the magnetic field line distribution achieved by pressureless sintering as described in Japanese Patent No. 2911017 is a static magnetic field distribution as shown in FIG. 1A′, which prohibits a full control of the anisotropy direction, thus resulting in failure to optimize the static magnetic field distribution according to individual motor structures.
The present invention has been made in view of the circumstances described above, and it is an object of the present invention to provide a rare-earth iron-based magnet in which the direction of anisotropy can be duly controlled.
In order to achieve the object described above, according to an aspect of the present invention, there is provided a rare-earth iron-based magnet with self-recoverability, which includes a plurality of segments, wherein the segments each include a matrix having a microstructure in which rare-earth iron-based aligned magnetic powders of at least one kind are solidified by a cross-linking reaction phase and also in which the cross-liking reaction phase and a viscous deformation phase resulting from a viscosity flow are chemically bound to each other between the magnetic powders, and wherein while the inner and outer circumferential surfaces of the segments are constrained, fracture surfaces of the segments, and also the segments on a needed-basis, are mutually aggregated and rigidified together by taking advantage of self-recovery function based on viscous deformation caused by heat and external force and also on cross-linking reaction.
In the aspect of the present invention, the rare-earth iron-based magnetic powders of at least one kind may have a (BH)max of 250 kJ/m3 or more and a volume fraction of 80 vol. % or more, and further the rare-earth iron-based magnetic powders, the cross-linking phase and the viscous deformation phase may account in total for 97 vol. % or more, and voids may account for 3 vol. % or less in terms of volume fraction.
In the aspect of the present invention, the difference in maximum magnetization Mmax between the segment and a magnet corresponding to the segment may be 0.03 T or less, and the difference in anisotropy dispersion 8 therebetween may be 7% or less.
In the aspect of the present invention, the rare-earth iron-based magnet may have a remanence Mr of 0.95 T or more, a coercivity (HcJ) of 0.95 MA/m or more and a (BH)max of 160 kJ/m3.
In the aspect of the present invention, the rare-earth iron-based magnet may have an annular shape such as arc, circular cylinder and the like, include at least one pole pair, have a permeance coefficient Pc of 3 or more and may constitute a magnetic circuit together with an iron core.
According to the present invention, a rare-earth iron-based magnet with self-recoverability includes a plurality of segments, wherein the segments each include a matrix having a microstructure in which rare-earth iron-based aligned magnetic powders of at least one kind are solidified by a cross-linking reaction phase and also in which the crass-liking reaction phase and a viscous deformation phase resulting from on a slip flow are chemically bound to each other between the magnetic powders, and wherein while the inner and outer circumferential surfaces of the segments are constrained, fracture surfaces of the segments, and also the segments on a needed-basis, are mutually aggregated and rigidified together by means of self-recovery function based on viscous deformation caused by heat and external force and also on cross-linking reaction. Accordingly, the magnet described above for use as a magnet with a thickness of 1 to 2 mm for an electromagnetic drive unit like a small motor is flexible in reduction of diameter and thickness, making thickness uneven, increase in length and like requirements to achieve the design concept of the electromagnetic drive unit. In addition, the self-recovered boundary surfaces of fragmented segment or a plurality of segments are uniform and mechanical defects are not built up heavily in the boundary region. Further, it is configured that only the direction of anisotropy can be controlled without deteriorating the degree of anisotropy of the magnet corresponding to the self-recoverable segment.
Thus, in order to comply with the design concept of an electromagnetic drive unit like a small motor, the self-recoverable rare-earth iron-based magnet can be configured into a Halbach array where a plurality of self-recoverable segments are combined, or into a magnet which has a high (BH)max and in which the anisotropy direction is continuously controlled. Consequently, a strong static magnetic field distribution optimal for individual electromagnetic drive units having respective different structures and operations can be achieved.
In this connection, an ring-shaped self-recoverable iron-based magnet, which is formed such the rare-earth iron-based magnetic powder according to the present invention is highly densely filled and which has a permeance coefficient of 3 or more in a magnetic circuit constituted together with an iron core proves to be advantageous for providing a small-sized, highly reliable, high-output and highly efficient electromagnetic drive unit.
Accordingly, a high-output and highly efficient small electromagnetic drive unit can be provided by using the self-recoverable rare-earth iron-based magnet according to the present invention including a Halbach array with at least one pole pair.
FIGS. 1A and 1A′ are schematic views of a Halbach array, respectively showing a cross section and a static magnetic field distribution, and FIGS. 1B and 1B′ are schematic views of a quasi-Halbach array, respectively showing a cross section and a static magnetic field distribution;
Description will first be made on one or more kinds of anisotropic rare-earth iron boride/nitride magnetic powders according to the present invention.
The anisotropic rare-earth magnetic powder (hereinafter, referred to simply as “magnetic powder” as appropriate) of the present invention is fabricated such that Sm—Fe-based alloy or Sm—(Fe, Co)-based alloy is produced, for example, by a dissolution casting method described in Japanese Patent Application Laid-Open No H2-57663 or by a reduction diffusion method disclosed in Japanese Patent No. 17025441 and Japanese Patent Application Laid-Open No. H9-157803, and such alloy is nitrided and then finely milled. The fine milling process can be performed by a publicly known technique such as a jet mill, a vibration ball mill, a rotation ball mill or the like. The magnetic powder of the present invention is to refer to Sm2Fe17N3-based magnetic powder which is finely milled so as to have a fisher average particle size of 1.5 μm or less, preferably 1.2 μm or less. In this connection, it is preferable if the surface is coated with a slow oxidation film as disclosed, for example, in Japanese Patent Applications Laid-Open Nos. S52-54998, S59-170201, 560-128202, H3-211203, S46-7153, S56-55503, S61-154112, and H3-126801. Also, the magnetic powder of the present invention may be one or more kinds of Sm2Fe17N3 powders subjected to surface treatment conducted by a method which is to form a metallic coating film and which is disclosed in Japanese Patent Applications Laid-Open Nos. H5-230501, H5-234729, H8-143913, and H7-268632, or the Japan Institute of Metals, Lecture Outline (Spring General Assembly of 1996, No. 446 P 184), or by another method which is to form an inorganic coating film and which is described in Japanese Examined Patent Application Publication No. 116-17015 and Japanese Patent Applications Laid-Open Nos. H1-234502, H4-217024, H5-213601, H7-326508, H8-153613, H8-183601.
Further, the magnetic powder of the present invention may be what is called “HDDR-R2Fe14B-based magnetic powder”, “Co-free d-HDDR-R2Fe14B-based magnetic powder”, or their surface-treated powder, which are fabricated such that R2(Fe, Co)14B based alloy (R is Nd, Pr) is hydrogenated (hydrogenation, R2(Fe, Co)14B HX), is phase-decomposed at 650 to 1000° C. (decomposition, RH2+Fe+Fe2B), is hydrogen-desorbed (desorption) and is recombined (recombination), as disclosed, for example, in Japanese Patents Nos. 3092672, 2881409, 3250551, 3410171, 3463911, 3522207, and 3595064.
In addition to the anisotropic rare-earth iron boride/nitride magnetic powders described above, Sm—Co-based, Mn—Al—C-based or Al—No—Co-based non-rare-earth iron-based magnetic powder, or isotropic rare-earth magnetic powder having a remanence (Mr) of as high as 1 T or more may be appropriately used in parallel as needed.
The one or more kinds of anisotropic rare-earth iron boride/nitride magnetic powders according to the present invention preferably have a (BH)max of 250 kJ/m3 or more, because a self-recoverable rare-earth iron-based magnet can easily achieve a (BH)max of 160 kJ/m3 or more if the volume fraction of aligned rare-earth iron-based magnetic powder with a (BH)max of 250 kJ/m3 or more according to the present invention is set to 80 vol. % or more.
Description will now be made, with reference to
Referring to
In order to enhance the pressure transmission, it is necessary to reduce individual coefficients μ of frictions arising from the compaction of the magnetic powder. Accordingly, in the present invention, internal lubricant is added and also the matrix at the time of compacting the magnetic powder is defined by liquid phase, wherein the internal lubricant previously added is eluted so as to produce an internal sliding effect, whereby when the magnetic power is compacted, the entire system is brought in sliding flow condition. As a result, according to the present invention, relative density, including the matrix, can be 97 vol. % or more under a low pressure of 20 to 50 MPa.
A macromolecule-chain is named as a preferred viscous deformation phase to constitute the matrix according to the present invention, and, for example, a polyamide-12 having a number average molecular weight (Mw) of 4000 to 12000, or its copolymer, is given as an example. Further, the internal lubricant, which is used as an additive agent as needed, is preferably constituted by an organic compound which has a melting point of 50° C. and which contains, in one molecule, at least one each of the followings: hydrophilic functional group to accelerate elution from melted chain molecule away outside the system when the magnetic powder is compacted; and long-chain alkyl group to enhance the internal sliding effect when the magnetic powder is compacted. As a concrete example, an organic compound may be named which contains, in one molecule, one hydroxyl group (—OH) and also three hexadecyl groups of a carbon number 17 (—(CH2)17—CH3).
In the meantime, further, in the present invention, when Sm2Fe17N3-based magnetic powder having an average particle size of about 3 μm is used as rare-earth iron-based magnetic powder in order to improve the pressure transmission in the process of compacting the magnetic powder, for example, Nd2Fe14B-based magnetic powder having an average particle size of 100 to 150 μm is used together, which allows a constant k shown in
When magnetic powder aligned by an external magnetic field Hex as shown in
When chain molecules are melted, their particle chains can be represented as tangling thread-like lines (melted chain particles) 24 as shown in
Description will now be made, with reference to
When the self-recoverable segment cross section Oa-Ob-B-A according to the present invention is deformed by an external force into a cross section Oa-Ob-C-B and further into a cross section Oa-Ob-D-C, the rare-earth iron-based magnetic powder 31 as a minute rigid body located at the center of the diagonal line Oa-B is relocated respectively at the center of a diagonal line Oa-C and further at the center of a diagonal line Oa-D while generating respective tensile forces F1 and F2 and causing respective rotations with angles α and β. Then, the angle Mθ to indicate the anisotropy direction is rotated by the angles α and β, respectively, with respect to the tangent line of a self-recoverable segment surface B-C-D. Thus, when the cross-linking reaction phase 22 is of an imperfect network structure containing chain particles, non-recoverable deformation is retained when the external force is released. The non-recoverable deformation occurs solely when plastic substances such as clay are deformed, where generally a slide occurs between the chain particles. Shearing is caused by the elongation and rotation of a minute portion, and in the present invention, the C-axis, that is to say, the anisotropy direction is controlled by the rotation of a rigid body of a specific minute portion solidified.
When the tensile forces F and F′ as well as heat are applied to the self-recoverable segment while the inner and outer circumference surfaces of the segment are constrained as shown in
Now, when the angle Cθ to the wall surfaces 35a and 35b is 30 degrees as shown in
Further, when the angle Cθ with regard to the wall surfaces 35a and 35b is 90 degrees as shown in
As described above, also a ring-shaped configuration, in which the anisotropy direction alone is arbitrarily controlled continuously from the plane perpendicular direction to the in-plane, can be achieved from the Halbach array without lowering the degree of anisotropy of the aligned magnetic powders 31 by the action of the slip surface formation and the viscous deformation caused due to the heat and the external force while constraining the inner and outer circumferential surfaces of the self-recoverable segment according to the present invention having a microstructure in which a three-dimensional network and chain particles are cross-linked to each other.
The difference of maximum magnetization Mmax between the self-recoverable segment according to the present invention and the magnet located corresponding to the segment is preferably 0.03 or less, and the difference of anisotropy dispersion 8 therebetween is preferably 7% or less. Also, it can be configured that only the anisotropy dispersion is different between the self-recoverable segment and the magnet located corresponding to the segment, or configured that no difference is present therebetween.
Also, the self-recoverable segment according to the present invention is preferably composed such that the volume fraction of a rare-earth iron-based magnetic powder is set at 80 vol. % or more, the remanence (Mr) is set at 0.95 T or more with respect to the anisotropy direction, the coercivity (HcJ) is set at 0.95 MA/m or more, and the (BH) value is set at 160 kJ/m3 or more.
Description will be made, with reference to
A preferred system including the microstructure shown in
The cross-linking phase is mainly composed of, for example, o-cresol novolak epoxy oligomer having an epoxy equivalent of 205 to 220 g/eq and a melting point of 70 to 76° C. An imidazole adduct (2-phenyl-4,5-dihydroxymethylimidazole) having a decomposition temperature of 230° C. is used as a cross-linking agent. A linear polyamide which contains amino active hydrogen in molecular chain adapted to bind chemically to an oxazolidone ring of the aforementioned epoxy oligomer and which has an average molecular weight Mw of 4000 to 12000 is used as a chain molecule of the viscous deformation phase. And, a partial ester compound which is formed between pentaerythritol and higher fatty acid and which has a melting point of about 52° C. can, for instance, be used as an internal lubricant acting effectively as the additive agent on a needed basis, because the partial ester compound includes, in one molecule, one hydroxyl group (—OH) and three hexadecyl groups of a carbon number 17 (—(CH2)17—CH3) wherein the polar group has compatibility with melted chain molecule and the hexadecyl group has a lubricating action resulting from slip flow phenomenon.
In the present invention, a compound can be preferably exemplified which is prepared in the following manner: a composition, which is composed of rare-earth iron-based magnetic powder coated with epoxy oligomer having a thickness of 40 to 50 nm as a main component of the cross-linking phase, linear oligomer as the sliding phase, and additive agent used as needed, and which does not contain a cross-linking agent, is melted and kneaded together by using, for example, a mixing roll heated to 140 to 150° C. into a kneaded mixture; the kneaded mixture is cooled at room temperature, milled to a size of, for example, 710 μm or smaller and classified; and the milled substance is dry-mixed with a cross-linking agent and formed into a granule.
As shown in
After the anisotropy direction control is performed as needed in an arbitrary manner as described above, fracture surfaces or also segments can be mutually aggregated by the viscous deformation and the cross-linking reaction.
Further, according to the present invention, the self-recoverable segment fragments and also the segments are mutually aggregated and then rigidified together by increasing cross-linking density with heat treatment, whereby environment resistance, such as mechanical strength and dimensional stability required for a magnet, can be ensured.
In addition, in the self-recoverable rare-earth iron-based magnet according to the present invention, it is preferably arranged that the sum of the volume fraction (that is the relative density) of the rare-earth iron-based magnetic powder, the cross-linking reaction and the viscous deformation phase accounts for 97 vol. % or more and the void ratio accounts for 3 vol. % or less. The reason for the arrangement described above is because when the components described above, after re-aggregation due to the self-recovery, are rigidified together by heat, the magnetic properties are advantageously suppressed from deteriorating due to oxidation reaction by heat treatment in the air.
An electromagnetic drive unit using the recoverable rare-earth iron-based magnet as described above according to the present invention is preferred so that a magnetic circuit structure, in which the magnet has a pole pair number of 1 or more and a permeance coefficient Pc of 3 or more, ensures demagnetization resistance against the reversed magnetic field generated from the iron core side (exciting winding) of the magnet.
Thus, an anisotropic magnet including a Halbach array with a pole pair number of 1 or more, as well as a high-output and high-efficiency small electromagnetic drive unit incorporating such a magnet can be provided.
The present invention will be described in more details with reference to invention examples. It should be, however, noted that the present invention is by no means limited to the examples.
<Adjustment of self-recoverable segment> The sum of the volume fraction of Sm2Fe17N3 (Mr=1.22 T, HcJ=0.91 MA/m, (BH)max=240 kJ/m3) having a particle size of 3 to 5 μm and Nd2Fe14B (Mr=1.34 T, HcJ=1.15 MA/m, (BH)max=316 kJ/m3) having a particle size of 38 to 150 μm is set to 80.8 vol. %, and the rest of 19.2 vol. % is composed of 6.5 vol. % o-cresol novolak epoxy oligomer having an epoxy equivalent of 205 to 220 g/eq and a melting point of 70 to 76° C., and functioning as a cross-linking reaction phase to solidify the magnetic powders; 1.8 vol. % imidazole derivative (2-phenyl-4,5-dihydroxymethylimidazole) having a decomposition temperature of 230° C.; 9.1 vol. % linear polyamide having an average molecular weight Mw of 4000 to 12000, containing amino active hydrogen atoms in molecular chain to bind chemically to an oxazolidone ring of the aforementioned epoxy oligomer, and functioning as a chain molecule of the viscous deformation phase; and 1.8 vol. % partial ester compound of pentaerythritol and higher fatty acid, functioning as an internal lubricant. In the above composition, one hydroxyl group (—OH) and three hexadecyl groups of a carbon number 17 (—(CH2)17—CH3) are included in one molecule, so that the polar group works to improve compatibility with melted chain molecule, and the hexadecyl group works to improve self-recoverability resulting from slip flow.
First, the composition components according to the present invention excluding the cross-linking agent were melted and kneaded together by using a mixing roll whose front and back roll temperatures are set to 140° C. and 150° C., respectively. The melting and kneading process for eliminating the voids is conducted in order to ensure the low-pressure compressibility and to suppress the degradation of the squareness characteristic of demagnetization curve attributable to the surface oxidation of the rare-earth iron-based magnetic power.
Subsequently, the above kneaded mixture was milled to a size of 710 μm or smaller and classified at room temperature, the classified substance was dry-mixed with a cross-linking agent having an average particle size of 3 μm, and a granule compound was fabricated.
The sample used for the measurement of magnetic characteristics is a 7 mm cube with a density of 6.0 to 6.2 Mg/m3 which was compacted in an orthogonal magnetic field of 1.4 MA/m under a pressure of 50 MPa temperature of 110 to 160 b° C. In this connection, Sm2Fe17N3/Nd2Fe14B magnet obtained by compacting under a high pressure of 1.5 Gpa has a problem of magnetic characteristic deterioration resulting from generation of new surfaces or damage of surfaces due to the fracture of Nd2Fe14B (K. Noguchi, K. Machida, G. Adachi, “Preparation and characterization of composite-type bonded magnets of Sm2Fe17NX and Nd—Fe—B HDDR powders”, Proc. 16th Int. Workshop on Rate Earth Magnets and Their Applications, pp. 845-854, 2000). However, according to the example of the present invention, the rare-earth iron-based magnetic powders are isolated from one another by the cross-linking phase and the viscous deformation phase, and the relative density of the composition exceeds 97 vol. % under a slight pressure of 50 MPa in the heat and in the magnetic field. Consequently, the magnetic characteristic deterioration due to the generation of new surfaces and the damage of surfaces in Nd2Fe14B can be suppressed.
Referring to
A magnet which is fabricated by compacting Sm2Fe17N3 magnetic powder and at the same time which has a density of 5 Mg/m3 or more has not been available. For example, a magnet, which is fabricated by compacting Sm2Fe17N3 magnetic powder together with liquid saturated polyester resin composition at room temperature, has a density of 4.79 Mg/m3, a relative density of 62.5% calculated based on the true density of 7.67 Mg/m3, and a (BH)max of 94.7 kJ/m3 (K. Ohmori, S. Hayashi, S. Yoshizawa, “Injection molded Sm—Fe—N anisotropic magnets using unsaturated polyester resin”, Proc. Rare-Earth's 04 in Nara, (2004) J0-02).
Based on the intersection points between the operating line having a permeance coefficient Pc of 3 and the demagnetization curves in the second quadrant of the room temperature M—H loops in
<Aggregation of segments based on self-recoverability> A self-recoverable segment Seg. 61 with a cross section shown in
Next, four (Seg. 61-1 to Seg. 61-4) of the above recoverable segment Seg. 61 were arranged, as shown in
The present example shows a ring-shaped magnet which is made of self-recoverable segments of so-called “parallel orientation” and which has two pole pairs. That is to say, the pole center (the angle Mθ1) in
On the other hand, referring to
It is self-explanatory that the Halbach array made of eight segments as shown in
<Anisotropy direction control of self-recoverable segments> Description will now be made, with reference to an example, about an anisotropy direction control which is performed utilizing the self-recovery function as a principle that operates such that the fracture surface formation and the viscous deformation are caused due to the heat and the external force while the inner and outer circumferential surfaces of the self-recoverable segment according to the present invention are constrained as shown in
Referring to
In
On the other hand, the degree of anisotropy was evaluated in terms of anisotropy dispersion δ. The anisotropy dispersion δ, or the anisotropy (C-axis) distribution of aligned rare-earth iron-based magnetic powders, was analyzed in such a manner that in an expression: a total energy E in rotational magnetization=Ku·sin2λ−Ms·H·cos(λ−λo), firstly, λ was determined by the solution that minimizes the total energy E of the circular cylindrical magnet, that is: (δE/δλ)=Ku·sin2λ−Ms·H·sin(λ−λo)=0, then M—H loop that maximizes M was measured from an expression: M=Ms cos(λo−λ) by a vibrating sample magnetometer (VSM), and further that X was found from: Ku·sin2λ−Ms·H·sin(λo−λ)=0, and the entire orientation state, that is the anisotropy dispersion δ, was found by applying the probability distribution of λ. In the above expressions, λo is an angle of the external magnetic field, λ is an angle of the rotation of Ms, Ms is a spontaneous magnetic moment, Ku is a magnetic anisotropy constant, and E is a total energy. The analysis shows that the angles at which the magnetization Ms is the largest with respect to all the directions in the samples 11, 12 and 13, and the samples 21, 22 and 23 (the angles are, namely, the Hθ and the Mθ) are substantially equal to each other as shown in Table 1, wherein the largest of the differences in the anisotropy dispersion 6 respectively between the samples 11, 12 and 13 and the samples 21, 22 and 23 located corresponding respectively to the samples 11, 12 and 13 is seen between the samples 13 and 23, that is when the direction of the anisotropy was controlled from the plane perpendicular direction to the in-plane direction, but the largest difference is less than 7%. The difference can be treated as an equivalent level in consideration of measurement deviation, and accordingly it is indicated that the direction of anisotropy can be duly controlled by taking advantage of the self-recovery function which is generated such that the fracture surface formation and the viscous deformation are caused due to the heat and the external force while the inner and outer circumferential surfaces of the gelated self-recoverable segment according to the present invention are constrained and which works so that only the direction of anisotropy is changed without deteriorating the degree of anisotropy thereby.
Number | Date | Country | Kind |
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2009-044970 | Feb 2009 | JP | national |