# Patent

## Title
Rare earth permanent magnet and its preparation

## Abstract

A rare earth permanent magnet is prepared by disposing a powdered metal alloy containing at least 70 vol % of an intermetallic compound phase on a sintered body of R—Fe—B system, and heating the sintered body having the powder disposed on its surface below the sintering temperature of the sintered body in vacuum or in an inert gas for diffusion treatment. The advantages include efficient productivity, excellent magnetic performance, a minimal or zero amount of Tb or Dy used, an increased coercive force, and a minimized decline of remanence.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. application Ser. No. 12/049,603, filed on Mar. 17, 2008 which is based upon and claims the benefit of priority under 35 U.S.C. §119(a) on Patent Application Nos. 2007-068803 and 2007-068823 filed in Japan on Mar. 16, 2007 and Mar. 16, 2007, respectively, the entire contents of which are hereby incorporated by reference.


TECHNICAL FIELD
This invention relates to an R—Fe—B permanent magnet in which an intermetallic compound is combined with a sintered magnet body so as to enhance its coercive force while minimizing a decline of its remanence, and a method for preparing the same.
BACKGROUND ART
By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators. It is required to further improve the performance of Nd—Fe—B magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of coercivity enhancing elements such as Al and Ga. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd.
It is believed that the coercivity creating mechanism of Nd—Fe—B magnets is the nucleation type wherein nucleation of reverse magnetic domains at grain boundaries governs a coercive force. In general, a disorder of crystalline structure occurs at the grain boundary or interface. If a disorder of crystalline structure extends several nanometers in a depth direction near the interface of grains of Nd2Fe14B compound which is the primary phase of the magnet, then it incurs a lowering of magnetocrystalline anisotropy and facilitates formation of reverse magnetic domains, reducing a coercive force (see K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75). Substituting Dy or Tb for some Nd in the Nd2Fe14B compound increases the anisotropic magnetic field of the compound phase so that the coercive force is increased. When Dy or Tb is added in an ordinary way, however, a loss of remanence is unavoidable because Dy or Tb substitution occurs not only near the interface of the primary phase, but even in the interior of the primary phase. Another problem arises in that amounts of expensive Tb and Dy must be used.
Besides, a number of attempts have been made for increasing the coercive force of Nd—Fe—B magnets. One exemplary attempt is a two-alloy method of preparing an Nd—Fe—B magnet by mixing two powdered alloys of different composition and sintering the mixture. A powder of alloy A consists of R2Fe14B primary phase wherein R is mainly Nd and Pr. And a powder of alloy B contains various additive elements including Dy, Tb, Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb. Then alloys A and B are mixed together. This is followed by fine pulverization, pressing in a magnetic field, sintering, and aging treatment whereby the Nd—Fe—B magnet is prepared. The sintered magnet thus obtained produces a high coercive force while minimizing a decline of remanence because Dy or Tb is absent at the center of R2Fe14B compound primary phase grains and instead, the additive elements like Dy and Tb are localized near grain boundaries (see JP-B 5-31807 and JP-A 5-21218). In this method, however, Dy or Tb diffuses into the interior of primary phase grains during the sintering so that the layer where Dy or Tb is localized near grain boundaries has a thickness equal to or more than about 1 micrometer, which is substantially greater than the depth where nucleation of reverse magnetic domains occurs. The results are still not fully satisfactory.
Recently, there have been developed several processes of diffusing certain elements from the surface to the interior of a R—Fe—B sintered body for improving magnet properties. In one exemplary process, a rare earth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on the surface of Nd—Fe—B magnet using an evaporation or sputtering technique, followed by heat treatment. See JP-A 2004-296973, JP-A 2004-304038, JP-A 2005-11973; K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida, at al., “Grain Boundary Modification of Nd—Fe—B Sintered Magnet and Magnetic Properties,” Abstracts of Spring Meeting of Japan Society of Powder and Powder Metallurgy, 2004, p. 202. Another exemplary process involves applying a powder of rare earth inorganic compound such as fluoride or oxide onto the surface of a sintered body and heat treatment as described in WO 2006/043348 A1. With these processes, the element (e.g., Dy or Tb) disposed on the sintered body surface pass through grain boundaries in the sintered body structure and diffuse into the interior of the sintered body during the heat treatment. As a consequence, Dy or Tb can be enriched in a very high concentration at grain boundaries or near grain boundaries within sintered body primary phase grains. As compared with the two-alloy method described previously, these processes produce an ideal morphology. Since the magnet properties reflect the morphology, a minimized decline of remanence and an increase of coercive force are accomplished. However, the processes utilizing evaporation or sputtering have many problems associated with units and steps when practiced on a mass scale and suffer from poor productivity.
DISCLOSURE OF THE INVENTION
An object of the invention is to provide an R—Fe—B sintered magnet which is prepared by applying an intermetallic compound-based alloy powder onto a sintered body and effecting diffusion treatment and which magnet features efficient productivity, excellent magnetic performance, a minimal or zero amount of Tb or Dy used, an increased coercive force, and a minimized decline of remanence. Another object is to provide a method for preparing the same.
The inventors have discovered that when an R—Fe—B sintered body is tailored by applying to a surface thereof an alloy powder based on an easily pulverizable intermetallic compound phase and effecting diffusion treatment, the process is improved in productivity over the prior art processes, and constituent elements of the diffusion alloy are enriched near the interface of primary phase grains within the sintered body so that the coercive force is increased while minimizing a decline of remanence. The invention is predicated on this discovery.
The invention provides rare earth permanent magnets and methods for preparing the same, as defined below.
[1] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition R1i-M1j wherein R1 is at least one element selected from rare earth elements inclusive of Y and Sc, M1 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, “i” and “j” indicative of atomic percent are in the range: 15<j≦99 and the balance of i, and containing at least 70% by volume of an intermetallic compound phase, and
heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, for causing at least one element of R1 and M1 in the powder to diffuse to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.
[2] The method of [1] wherein the disposing step includes grinding an alloy having the composition R1i-M1j wherein R1, M1, i and j are as defined above and containing at least 70% by volume of an intermetallic compound phase into a powder having an average particle size of up to 500 μm, dispersing the powder in an organic solvent or water, applying the resulting slurry to the surface of the sintered body, and drying.

[3] The method of [1] or [2] wherein the heat treating step includes heat treatment at a temperature from 200° C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts represents the sintering temperature of the sintered body.

[4] The method of [1], [2] or [3] wherein the sintered body has a shape including a minimum portion with a dimension equal to or less than 20 mm.

[5] A method for preparing a rare earth permanent magnet, comprising the steps of:

disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition R1xT2yM1z wherein R1 is at least one element selected from rare earth elements inclusive of Y and Sc, T2 is at least one element selected from Fe and Co, M1 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, x, y and z indicative of atomic percent are in the range: 5≦x≦85, 15<z≦95, and the balance of y which is greater than 0, and containing at least 70% by volume of an intermetallic compound phase, and
heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, for causing at least one element of R1 and M1 in the powder to diffuse to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.
[6] The method of [5] wherein the disposing step includes grinding an alloy having the composition R1xT2yM1z wherein R1, T2, M1, x, y and z are as defined above and containing at least 70% by volume of an intermetallic compound phase into a powder having an average particle size of up to 500 μm, dispersing the powder in an organic solvent or water, applying the resulting slurry to the surface of the sintered body, and drying.

[7] The method of [5] or [6] wherein the heat treating step includes heat treatment at a temperature from 200° C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts represents the sintering temperature of the sintered body.

[8] The method of [5], [6] or [7] wherein the sintered body has a shape including a minimum portion with a dimension equal to or less than 20 mm.

[9] A rare earth permanent magnet, which is prepared by disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition R1i-M1j wherein R1 is at least one element selected from rare earth elements inclusive of Y and Sc, M1 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, “i” and “j” indicative of atomic percent are in the range: 15<j≦99 and the balance of i, and containing at least 70% by volume of an intermetallic compound phase, and heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, wherein

at least one element of R1 and M1 in the powder is diffused to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains so that the coercive force of the magnet is increased over the magnet properties of the original sintered body.
[10] A rare earth permanent magnet, which is prepared by disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition R1xT2yM1z wherein R1 is at least one element selected from rare earth elements inclusive of Y and Sc, T2 is at least one element selected from Fe and Co, M1 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, x, y and z indicative of atomic percent are in the range: 5≦x≦85, 15<z≦95, and the balance of y which is greater than 0, and containing at least 70% by volume of an intermetallic compound phase, and heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, wherein
at least one element of R1 and M1 in the powder is diffused to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains so that the coercive force of the magnet is increased over the magnet properties of the original sintered body.
[11] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition M1d-M2e wherein each of M1 and M2 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, “d” and “e” indicative of atomic percent are in the range: 0.1≦e≦99.9 and the balance of d, and containing at least 70% by volume of an intermetallic compound phase, and
heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, for causing at least one element of M1 and M2 in the powder to diffuse to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.
[12] The method of [11] wherein the disposing step includes grinding an alloy having the composition M1d-M2e wherein M1, M2, d and e are as defined above and containing at least 70% by volume of an intermetallic compound phase into a powder having an average particle size of up to 500 μm, dispersing the powder in an organic solvent or water, applying the resulting slurry to the surface of the sintered body, and drying.

[13] The method of [11] or [12] wherein the heat treating step includes heat treatment at a temperature from 200° C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts represents the sintering temperature of the sintered body.

[14] The method of [11], [12] or [13] wherein the sintered body has a shape including a minimum portion with a dimension equal to or less than 20 mm.

[15] A rare earth permanent magnet, which is prepared by disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition M1d-M2e wherein each of M1 and M2 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, “d” and “e” indicative of atomic percent are in the range: 0.1≦e≦99.9 and the balance of d, and containing at least 70% by volume of an intermetallic compound phase, and heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, wherein

at least one element of M1 and M2 in the powder is diffused to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains so that the coercive force of the magnet is increased over the magnet properties of the original sintered body.
BENEFITS OF THE INVENTION
According to the invention, an R—Fe—B sintered magnet is prepared by applying an alloy powder based on an easily pulverizable intermetallic compound onto a sintered body and effecting diffusion treatment. The advantages of the resultant magnet include efficient productivity, excellent magnetic performance, a minimal or zero amount of Tb or Dy used, an increased coercive force, and a minimized decline of remanence.


DESCRIPTION OF THE PREFERRED EMBODIMENT
Briefly stated, an R—Fe—B sintered magnet is prepared according to the invention by applying an intermetallic compound-based alloy powder onto a sintered body and effecting diffusion treatment. The resultant magnet has advantages including excellent magnetic performance and a minimal amount of Tb or Dy used or the absence of Tb or Dy.
The mother material used in the invention is a sintered body of the composition Ra-T1b-Bc, which is often referred to as “mother sintered body.” Herein R is at least one element selected from rare earth elements inclusive of scandium (Sc) and yttrium (Y), specifically from among Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. Preferably the majority of R is Nd and/or Pr. Preferably the rare earth elements inclusive of Sc and Y account for 12 to 20 atomic percents (at %), and more preferably 14 to 18 at % of the entire sintered body. T1 is at least one element selected from iron (Fe) and cobalt (Co). B is boron, and preferably accounts for 4 to 7 at % of the entire sintered body. Particularly when B is 5 to 6 at %, a significant improvement in coercive force is achieved by diffusion treatment. The balance consists of T1.
The alloy for the mother sintered body is prepared by melting metal or alloy feeds in vacuum or an inert gas atmosphere, preferably argon atmosphere, and casting the melt into a flat mold or book mold or strip casting. A possible alternative is a so-called two-alloy process involving separately preparing an alloy approximate to the R2Fe14B compound composition constituting the primary phase of the relevant alloy and a rare earth-rich alloy serving as a liquid phase aid at the sintering temperature, crushing, then weighing and mixing them. Notably, the alloy approximate to the primary phase composition is subjected to homogenizing treatment, if necessary, for the purpose of increasing the amount of the R2Fe14B compound phase, since primary crystal α-Fe is likely to be left depending on the cooling rate during casting and the alloy composition. The homogenizing treatment is a heat treatment at 700 to 1,200° C. for at least one hour in vacuum or in an Ar atmosphere. Alternatively, the alloy approximate to the primary phase composition may be prepared by the strip casting technique. To the rare earth-rich alloy serving as a liquid phase aid, the melt quenching and strip casting techniques are applicable as well as the above-described casting technique.
The alloy is generally crushed or coarsely ground to a size of 0.05 to 3 mm, especially 0.05 to 1.5 mm. The crushing step uses a Brown mill or hydriding pulverization, with the hydriding pulverization being preferred for those alloys as strip cast. The coarse powder is then finely pulverized to an average particle size of 0.2 to 30 μm, especially 0.5 to 20 μm, for example, on a jet mill using high-pressure nitrogen.
The fine powder is compacted on a compression molding machine under a magnetic field. The green compact is then placed in a sintering furnace where it is sintered in vacuum or in an inert gas atmosphere usually at a temperature of 900 to 1,250° C., preferably 1,000 to 1,100° C. The sintered block thus obtained contains 60 to 99% by volume, preferably 80 to 98% by volume of the tetragonal R2Fe14B compound as the primary phase, with the balance being 0.5 to 20% by volume of a rare earth-rich phase and 0.1 to 10% by volume of at least one compound selected from among rare earth oxides, and carbides, nitrides and hydroxides of incidental impurities, and mixtures or composites thereof.
The resulting sintered block may be machined or worked into a predetermined shape. In the invention, R1 and/or M1 and T2, or M1 and/or M2 which are to be diffused into the sintered body interior are supplied from the sintered body surface. Thus, if a minimum portion of the sintered body has too large a dimension, the objects of the invention are not achievable. For this reason, the shape includes a minimum portion having a dimension equal to or less than 20 mm, and preferably equal to or less than 10 mm, with the lower limit being equal to or more than 0.1 mm. The sintered body includes a maximum portion whose dimension is not particularly limited, with the maximum portion dimension being desirably equal to or less than 200 mm.
According to the invention, an alloy powder is disposed on the sintered body and subjected to diffusion treatment. It is a powdered alloy having the composition: R1i-M1j or R1xT2yM1z or M1d-M2e. This alloy is often referred to as “diffusion alloy.” Herein R1 is at least one element selected from rare earth elements inclusive of Y and Sc, and preferably the majority of R1 is Nd and Pr. M1 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi. In the alloy M1d-M2e, M1 and M2 are different from each other and selected from the group consisting of the foregoing elements. T2 is Fe and/or Co. In the alloy R1i-M1j, M1 accounts for 15 to 99 at % (i.e., j=15 to 99), with the balance being R1. In the alloy R1xT2yM1z, M1 accounts for 15 to 95 at % (i.e., z=15 to 95) and R1 accounts for 5 to 85 at % (i.e., x=5 to 85), with the balance being T2. That is, y>0, and T2 is preferably 0.5 to 75 at %. In the alloy M1d-M2e, M2 accounts for 0.1 to 99.9 at %, that is, e is in the range: 0.1≦e≦99.9. M1 is the remainder after removal of M2, that is, d is the balance.
The diffusion alloy may contain incidental impurities such as nitrogen (N) and oxygen (O), with an acceptable total amount of such impurities being equal to or less than 4 at %.
The invention is characterized in that the diffusion alloy material contains at least 70% by volume of an intermetallic compound phase in its structure. If the diffusion material is composed of a single metal or eutectic alloy, it is unsusceptible to pulverization and requires a special technique such as atomizing for a fine powder. By contrast, the intermetallic compound phase is generally hard and brittle in nature. When an alloy based on such an intermetallic compound phase is used as the diffusion material, a fine powder is readily obtained simply by applying the alloy preparation or pulverization means used in the manufacture of R—Fe—B sintered magnets. This is quite advantageous from the productivity aspect. Since the diffusion alloy material is advantageously readily pulverizable, it preferably contains at least 70% by volume and more preferably at least 90% by volume of an intermetallic compound phase. It is understood that the term “% by volume” is interchangeable with a percent by area of an intermetallic compound phase in a cross-section of the alloy structure.
The diffusion alloy containing at least 70% by volume of the intermetallic compound phase represented by R1i-M1j, R1xT2yM1z or M1d-M2e may be prepared, like the alloy for the mother sintered body, by melting metal or alloy feeds in vacuum or an inert gas atmosphere, preferably argon atmosphere, and casting the melt into a flat mold or book mold. An arc melting or strip casting method is also acceptable. The alloy is then crushed or coarsely ground to a size of about 0.05 to 3 mm, especially about 0.05 to 1.5 mm by means of a Brown mill or hydriding pulverization. The coarse powder is then finely pulverized, for example, by a ball mill, vibration mill or jet mill using high-pressure nitrogen. The smaller the powder particle size, the higher becomes the diffusion efficiency. The diffusion alloy containing the intermetallic compound phase represented by R1i-M1j, R1xT2yM1z or M1d-M2e, when powdered, preferably has an average particle size equal to or less than 500 μm, more preferably equal to or less than 300 μm, and even more preferably equal to or less than 100 μm. However, if the particle size is too small, then the influence of surface oxidation becomes noticeable, and handling is dangerous. Thus the lower limit of average particle size is preferably equal to or more than 1 μm. As used herein, the “average particle size” may be determined as a weight average diameter D50 (particle diameter at 50% by weight cumulative, or median diameter) using, for example, a particle size distribution measuring instrument relying on laser diffractometry or the like.
After the powder of diffusion alloy is disposed on the surface of the mother sintered body, the mother sintered body and the diffusion alloy powder are heat treated in vacuum or in an atmosphere of an inert gas such as argon (Ar) or helium (He) at a temperature equal to or below the sintering temperature (designated Ts in ° C.) of the sintered body. This heat treatment is referred to as “diffusion treatment.” By the diffusion treatment, R1, M1 or M2 in the diffusion alloy is diffused to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.
The diffusion alloy powder is disposed on the surface of the mother sintered body, for example, by dispersing the powder in water or an organic solvent to form a slurry, immersing the sintered body in the slurry, and drying the immersed sintered body by air drying, hot air drying or in vacuum. Spray coating is also possible. The slurry may contain 1 to 90% by weight, and preferably 5 to 70% by weight of the powder.
Better results are obtained when the filling factor of the elements from the applied diffusion alloy is at least 1% by volume, preferably at least 10% by volume, calculated as an average value in a sintered body-surrounding space extending outward from the sintered body surface to a distance equal to or less than 1 mm. The upper limit of filling factor is generally equal to or less than 95% by volume, and preferably equal to or less than 90% by volume, though not critical.
The conditions of diffusion treatment vary with the type and composition of the diffusion alloy and are preferably selected such that R1 and/or M1 and/or M2 is enriched at grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains. The temperature of diffusion treatment is equal to or below the sintering temperature (designated Ts in ° C.) of the sintered body. If diffusion treatment is effected above Ts, there arise problems that (1) the structure of the sintered body can be altered to degrade magnetic properties, and (2) the machined dimensions cannot be maintained due to thermal deformation. For this reason, the temperature of diffusion treatment is equal to or below Ts° C. of the sintered body, and preferably equal to or below (Ts-10)° C. The lower limit of temperature may be selected as appropriate though it is typically at least 200° C., and preferably at least 350° C. The time of diffusion treatment is typically from 1 minute to 30 hours. Within less than 1 minute, the diffusion treatment is not complete. If the treatment time is over 30 hours, the structure of the sintered body can be altered, oxidation or evaporation of components inevitably occurs to degrade magnetic properties, or M1 or M2 is not only enriched at grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains, but also diffused into the interior of primary phase grains. The preferred time of diffusion treatment is from 1 minute to 10 hours, and more preferably from 10 minutes to 6 hours.
Through appropriate diffusion treatment, the constituent element R1, M1 or M2 of the diffusion alloy disposed on the surface of the sintered body is diffused into the sintered body while traveling mainly along grain boundaries in the sintered body structure. This results in the structure in which R1, M1 or M2 is enriched at grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.
The permanent magnet thus obtained is improved in coercivity in that the diffusion of R1, M1 or M2 modifies the morphology near the primary phase grain boundaries within the structure so as to suppress a decline of magnetocrystalline anisotropy at primary phase grain boundaries or to create a new phase at grain boundaries. Since the diffusion alloy elements have not diffused into the interior of primary phase grains, a decline of remanence is restrained. The magnet is a high performance permanent magnet.
After the diffusion treatment, the magnet may be further subjected to aging treatment at a temperature of 200 to 900° C. for augmenting the coercivity enhancement.
EXAMPLE
Examples are given below for further illustrating the invention although the invention is not limited thereto.
Example 1 and Comparative Example 1
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5.2 μm. The fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 4×4×2 mm. It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd16.0FebalCo1.0B5.3.
By using Nd and Al metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, a diffusion alloy having the composition Nd33Al67 and composed mainly of an intermetallic compound phase NdAl2 was prepared. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 7.8 μm. On electron probe microanalysis (EPMA), the alloy contained 94% by volume of the intermetallic compound phase NdAl2.
The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to form a slurry, in which the mother sintered body was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered body covered with the diffusion alloy powder was subjected to diffusion treatment in vacuum at 800° C. for one hour, yielding a magnet of Example 1. In the absence of the diffusion alloy powder, the sintered body alone was subjected to heat treatment in vacuum at 800° C. for one hour, yielding a magnet of Comparative Example 1.
Table 1 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compound in the diffusion alloy, the temperature and time of diffusion treatment in Example 1 and Comparative Example 1. Table 2 shows the magnetic properties of the magnets of Example 1 and Comparative Example 1. It is seen that the coercive force (Hcj) of the magnet of Example 1 is greater by 1300 kAm−1 than that of Comparative Example 1 while a decline of remanence (Br) is only 15 mT.










TABLE 1









Diffusion alloy











Main
Diffusion treatment













Sintered body
Composition
intermetallic compound
Temperature
Time
















Example 1
Nd16.0FebalCo1.0B5.3
Nd33Al67
NdAl2
800° C.
1 hr


Comparative
Nd16.0FebalCo1.0B5.3
—
—
800° C.
1 hr


Example 1




















TABLE 2







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 1
1.310
1970
332



Comparative
1.325
670
318



Example 1










Example 2 and Comparative Example 2
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5.2 μm. The fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 4×4×2 mm. It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd16.0FebalCo1.0B5.3.
By using Nd, Fe, Co and Al metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, a diffusion alloy having the composition Nd35Fe25Co20Al20 was prepared. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 7.8 μm. On EPMA analysis, the alloy contained intermetallic compound phases Nd(FeCoAl)2, Nd2(FeCoAl) and Nd2(FeCoAl)17 and the like, with the total of intermetallic compound phases being 87% by volume.
The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to form a slurry, in which the mother sintered body was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered body covered with the diffusion alloy powder was subjected to diffusion treatment in vacuum at 800° C. for one hour, yielding a magnet of Example 2. In the absence of the powdered diffusion alloy, the sintered body alone was subjected to heat treatment in vacuum at 800° C. for one hour, yielding a magnet of Comparative Example 2.
Table 3 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compounds in the diffusion alloy, the temperature and time of diffusion treatment in Example 2 and Comparative Example 2. Table 4 shows the magnetic properties of the magnets of Example 2 and Comparative Example 2. It is seen that the coercive force of the magnet of Example 2 is greater by 1150 kAm−1 than that of Comparative Example 2 while a decline of remanence is only 18 mT.










TABLE 3









Diffusion alloy











Main




intermetallic
Diffusion treatment













Sintered body
Composition
compound
Temperature
Time





Example 2
Nd16.0FebalCo1.0B5.3
Nd35Fe25Co20Al20
Nd(FeCoAl)2
800° C.
1 hr





Nd2(FeCoAl)





Nd2(FeCoAl)17


Comparative
Nd16.0FebalCo1.0B5.3
—
—
800° C.
1 hr


Example 2




















TABLE 4







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 2
1.307
1820
330



Comparative
1.325
670
318



Example 2










Example 3
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5.2 μm. The fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 50×50×15 mm (Example 3-1) or a shape having dimensions of 50×50×25 mm (Example 3-2). It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd16.0FebalCo1.0B5.3.
By using Nd and Al metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, a diffusion alloy having the composition Nd33Al67 and composed mainly of an intermetallic compound phase NdAl2 was prepared. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 7.8 μm. On EPMA analysis, the alloy contained 93% by volume of the intermetallic compound phase NdAl2.
The diffusion alloy powder, 30 g, was mixed with 90 g of ethanol to form a slurry, in which each mother sintered body of Examples 3-1 and 3-2 was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered bodies covered with the diffusion alloy powder were subjected to diffusion treatment in vacuum at 850° C. for 6 hours, yielding magnets of Example 3-1 and 3-2.
Table 5 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compound in the diffusion alloy, the temperature and time of diffusion treatment, and the dimension of sintered body minimum portion in Examples 3-1 and 3-2. Table 6 shows the magnetic properties of the magnets of Examples 3-1 and 3-2. It is seen that in Example 3-1 where the sintered body minimum portion had a dimension of 15 mm, the diffusion treatment exerted a greater effect as demonstrated by a coercive force of 1584 kAm−1. In contrast, where the sintered body minimum portion had a dimension in excess of 20 mm, for example, a dimension of 25 mm in Example 3-2, the diffusion treatment exerted a less effect.











TABLE 5









Diffusion alloy

Sintered












Sintered
Main
Diffusion
body



body
intermetallic
treatment
minimum














composition
Composition
compound
Temperature
Time
portion

















Example 3-1
Nd16.0FebalCo1.0B5.3
Nd33Al67
NdAl2
850° C.
6 hr
15 mm


Example 3-2
Nd16.0FebalCo1.0B5.3
Nd33Al67
NdAl2
850° C.
6 hr
25 mm




















TABLE 6







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 3-1
1.305
1584
329



Example 3-2
1.305
653
308










Examples 4 to 52
As in Example 1, various mother sintered bodies were coated with various diffusion alloys and subjected to diffusion treatment at certain temperatures for certain times. Tables 7 and 8 summarize the composition of the mother sintered body and the diffusion alloy, the type and amount of main intermetallic compound in the diffusion alloy, the temperature and time of diffusion treatment. Tables 9 and 10 show the magnetic properties of the magnets. It is noted that the amount of intermetallic compound in the diffusion alloy was determined by EPMA analysis.










TABLE 7









Diffusion alloy













Amount of
Diffusion



Main
intermetallic
treatment
















intermetallic
compound
Temperature




Sintered body
Composition
compound
(vol %)
(° C.)
Time



















Example
4
Nd16.0FebalCo1.0B5.4
Nd35Fe20Co15Al30
Nd(FeCoAl)2
85
780
1
hr






Nd2(FeCoAl)



5
Nd16.0FebalCo1.0B5.4
Nd35Fe25Co20Si20
Nd(FeCoSi)2
92
880
1
hr






Nd2(FeCoSi)



6
Nd16.0FebalCo1.0B5.4
Nd33Fe20Co27Al15Si5
Nd(FeCoAlSi)2
88
820
50
min






Nd2(FeCoAlSi)



7
Nd11.0Dy3.0Tb2.0FebalCo1.0B5.5
Nd28Pr5Al67
(NdPr)Al2
84
800
2
hr



8
Nd18.0FebalCo1.5B6.2
Y21Mn28Cr1
Y6(MnCr)23
74
920
6
hr



9
Nd13.0Pr2.5FebalCo2.8B4.8
La33Cu60Co4Ni3
La(CuCoNi)2
73
820
2
hr






La(CuCoNi)



10
Nd13.0Pr2.5FebalCo2.8B4.8
La50Ni49V1
La(NiV)
71
800
2
hr



11
Nd13.0Dy2.5FebalCo1.0B5.9
La33Cu66.5Nb0.5
La(CuNb)2
75
830
8
hr



12
Nd17.0FebalCo3.0B4.7
Ce22Ni14Co58Zn6
Ce2(NiCoZn)7
76
460
10
hr






Ce(NiCoZn)5



13
Nd17.0FebalCo3.0B4.7
Ce17Ni87
Ce2Ni5
72
420
10
hr



14
Nd19.0FebalCo3.5B6.3
Ce11Zn89
Ce2Zn17
77
580
3
hr



15
Nd17.5Dy1.5FebalCo4.5B5.1
Pr33Ge67
PrGe2
84
860
40
min



16
Nd15.5Pr2.5FebalCo3.5B5.6
Pr33Al66Zr1
Pr(AlZr)2
87
880
50
min



17
Nd15.0Tb1.5FebalB5.5
Gd32Mn30Fe31Nb7
Gd(MnFeNb)2
87
980
3
hr






Gd(MnFeNb)3



18
Nd12.0FebalCo1.0B4.8
Gd37Mn40Co20Mo3
Gd(MnCoMo)2
88
970
2
hr






Gd6(MnCoMo)23



19
Nd15.0Tb1.5FebalB5.5
Gd21Mn78Mo1
Gd6(MnMo)23
85
960
3
hr



20
Nd12.0FebalCo1.0B4.8
Gd33Mn66Ta1
Gd(MnTa)2
86
940
2
hr



21
Nd13.0Pr3.0FebalCo2.5B5.2
Tb29Fe45Ni20Ag6
Tb(FeNiAg)2
79
820
3
hr






Tb2(FeNiAg)17



22
Nd13.0Pr3.0FebalCo2.5B5.2
Tb50Ag50
TbAg
82
850
3
hr



23
Nd12.5Dy3.0FebalCo0.7B5.9
Tb50In50
TbIn
81
870
4
hr



24
Nd12.5Pr2.5Tb0.5FebalCo0.5B5.0
Dy31Ni8Cu55Sn6
Dy(NiCuSn)2
84
860
3
hr






Dy2(NiCuSn)7



25
Nd12.0Pr2.5Dy2.5FebalCo0.6B5.7
Dy33Cu66.5Hf0.5
Dy(CuHf)2
86
940
2
hr



26
Nd12.8Pr2.5Tb0.2FebalCo1.0B4.5
Er28Mn30Co35Ta2
Er(MnCoTa)2
78
1030
3
hr






Er6(MnCoTa)23



27
Nd13.2Pr3.5Dy0.5FebalCo3.0B6.3
Er21Mn78.6W0.4
Er6(MnW)23
81
980
6
hr



28
Nd12.0Tb3.5FebalCo3.5B6.2
Yb24Co5Ni69Bi2
Yb(CoNiBi)3
72
230
10
min






Yb(CoNiBi)5



29
Nd12.5Dy4.0FebalCo2.0B4.8
Yb50Cu49Ti1
Yb(CuTi)
73
280
5
min



30
Nd12.0Tb3.5FebalCo3.5B6.2
Yb25Ni74.5Sb0.5
Yb(NiSb)3
74
260
10
min



















TABLE 8









Diffusion alloy













Amount of
Diffusion



Main
intermetallic
treatment
















intermetallic
compound
Temperature




Sintered body
Composition
compound
(vol %)
(° C.)
Time



















Example
31
Nd16.0FebalCo1.0B5.3
Nd33Al67
NdAl2
94
780
3
hr



32
Nd16.0FebalCo1.0B5.4
Nd50Si50
NdSi
92
940
4
hr






Nd5Si4



33
Nd16.0FebalCo1.0B5.3
Nd33Al37Si30
Nd(AlSi)2
93
830
3
hr



34
Nd13.5Dy2.0FebalCo3.5B5.4
Nd27Pr6Al67
(NdPr)Al2
94
750
2
hr



35
Nd16.0FebalCo1.0B5.3
Dy33Al67
DyAl2
93
820
4
hr



36
Nd14.0Tb1.5FebalCo3.5B5.2
Dy33Ga67
DyGa2
91
780
40
min



37
Nd16.0FebalCo1.0B5.3
Tb33Al67
TbAl2
93
840
3
hr



38
Nd13.5Pr2.5Dy2.0FebalCo2.5B5.3
Tb22Mn78
Tb6Mn23
87
640
10
hr






TbMn2



39
Nd20.0FebalCo3.0B5.4
Y10Co15Zn75
Y2(CoZn)17
75
450
5
hr






Y(CoZn)5



40
Nd18.0FebalCo2.5B6.6
Y68Fe2In30
Y2(FeIn)
72
1020
30
min






Y5(FeIn)3



41
Nd20.0FebalCo3.0B5.4
Y11Zn89
Y2Zn17
73
420
5
hr



42
Nd13.5Pr1.5Dy0.8FebalCo2.5B4.5
La32Co4Cu64
La(CoCu)2
81
670
4
hr






La(CoCu)5



43
Nd13.5Pr1.5Dy0.5FebalCo2.5B4.5
La33Cu67
LaCu2
79
630
4
hr



44
Nd20.0FebalCo5.5B4.1
Ce26Pb74
CePb3
76
520
3
hr



45
Nd15.2FebalCo3.5B6.9
Ce56Sn44
Ce5Sn4
78
480
6
hr



46
Nd15.5Dy2.5Tb0.5FebalCo2.6B4.4
Pr33Fe3C64
PrC2
73
830
30
hr



47
Nd12.5Dy2.5Tb0.5FebalCo3.8B6.2
Pr50P50
PrP
70
350
20
min



48
Nd14.8Pr1.8Dy0.6FebalCo1.4B5.6
Gd52Ni48
GdNi
82
980
30
min



49
Nd13.6Pr1.5Tb0.5FebalCo2.8B6.3
Gd37Ga63
GdGa2
76
870
20
min



50
Nd16.0Dy0.6FebalCo1.0B4.9
Er32Mn67Ta1
Er(MnTa)2
76
680
6
hr






Er6(MnTa)23



51
Nd14.5Pr1.5Dy0.5FebalCo2.8B4.6
Yb68Pb32
Yb2Pb
73
750
5
hr



52
Nd12.0Pr1.5Dy0.5FebalCo4.2B5.8
Yb69Sn29Bi2
Yb2(SnBi)
71
420
4
hr






Yb5(SnBi)3




















TABLE 9







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 4
1.300
1871
327



Example 5
1.315
1831
333



Example 6
1.310
1879
331



Example 7
1.305
1966
329



Example 8
1.240
844
286



Example 9
1.260
1059
297



Example 10
1.280
892
304



Example 11
1.335
1059
339



Example 12
1.252
756
292



Example 13
1.245
780
288



Example 14
1.225
892
283



Example 15
1.220
1855
282



Example 16
1.265
1887
305



Example 17
1.306
1528
318



Example 18
1.351
1250
341



Example 19
1.305
1457
323



Example 20
1.348
1297
338



Example 21
1.311
1520
322



Example 22
1.308
1719
326



Example 23
1.298
1767
322



Example 24
1.304
1695
316



Example 25
1.306
1703
325



Example 26
1.273
1306
304



Example 27
1.265
1361
305



Example 28
1.292
1106
312



Example 29
1.254
1258
291



Example 30
1.325
1083
332





















TABLE 10







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 31
1.300
1910
324



Example 32
1.315
1871
329



Example 33
1.310
1934
328



Example 34
1.318
1958
330



Example 35
1.305
1966
326



Example 36
1.314
1974
328



Example 37
1.311
2006
330



Example 38
1.263
1528
297



Example 39
1.220
1130
269



Example 40
1.180
1186
251



Example 41
1.235
1051
278



Example 42
1.245
1146
289



Example 43
1.242
1154
286



Example 44
1.104
971
221



Example 45
1.262
1043
293



Example 46
1.173
1098
255



Example 47
1.307
971
311



Example 48
1.285
1178
309



Example 49
1.311
1226
325



Example 50
1.268
939
298



Example 51
1.252
1003
290



Example 52
1.352
860
341










Example 53
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5.2 μm. The fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 4×4×2 mm. It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd16.0FebalCo1.0B5.3.
By using Al and Co metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, a diffusion alloy having the composition Al50Co50 (in atom %) and composed mainly of an intermetallic compound phase AlCo was prepared. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 8.5 μm. On EPMA analysis, the alloy contained 93% by volume of the intermetallic compound phase AlCo.
The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to form a slurry, in which the mother sintered body was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered body covered with the diffusion alloy powder was subjected to diffusion treatment in vacuum at 800° C. for one hour, yielding a magnet of Example 53.
Table 11 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compound in the diffusion alloy, the temperature and time of diffusion treatment in Example 53. Table 12 shows the magnetic properties of the magnet of Example 53. It is seen that the coercive force of the magnet of Example 53 is greater by 1170 kAm−1 than that of the preceding Comparative Example 1 while a decline of remanence is only 20 mT.










TABLE 11









Diffusion alloy
Diffusion










Inter-
treatment














Com-
metallic
Tem-




Sintered body
position
compound
perature
Time
















Example
Nd16.0FebalCo1.0B5.3
Al50CO50
AlCo
800° C.
1 hr


53




















TABLE 12







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 53
1.305
1840
329










Example 54 and Comparative Example 3
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5.2 μm. The fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 50×50×15 mm (Example 54) or a shape having dimensions of 50×50×25 mm (Comparative Example 3). It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd16.0FebalCo1.0B5.3.
By using Al and Co metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, a diffusion alloy having the composition Al50 Co50 (in atom %) and composed mainly of an intermetallic compound phase AlCo was prepared. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 8.5 μm. On EPMA analysis, the alloy contained 92% by volume of the intermetallic compound phase AlCo.
The diffusion alloy powder, 30 g, was mixed with 90 g of ethanol to form a slurry, in which each mother sintered body of Example 54 and Comparative Example 3 was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered bodies covered with the diffusion alloy powder were subjected to diffusion treatment in vacuum at 850° C. for 6 hours, yielding magnets of Example 54 and Comparative Example 3.
Table 13 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compound in the diffusion alloy, the temperature and time of diffusion treatment, and the dimension of sintered body minimum portion in Example 54 and Comparative Example 3. Table 14 shows the magnetic properties of the magnets of Example 54 and Comparative Example 3. It is seen that in Example 54 where the sintered body minimum portion had a dimension of 15 mm, the diffusion treatment exerted a greater effect as demonstrated by a coercive force of 1504 kAm−1. In contrast, where the sintered body minimum portion had a dimension in excess of 20 mm, for example, a dimension of 25 mm in Comparative Example 3, the diffusion treatment exerted little effect as demonstrated by little increase of coercive force.












TABLE 13












Sintered



Sintered
Diffusion alloy
Diffusion
body












body
Intermetallic
treatment
minimum














composition
Composition
compound
Temperature
Time
portion

















Example 54
Nd16.0FebalCo1.0B5.3
Al50Co50
AlCo
850° C.
6 hr
15 mm


Comparative
Nd16.0FebalCo1.0B5.3
Al50Co50
AlCo
850° C.
6 hr
25 mm


Example 3




















TABLE 14







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 54
1.306
1504
328



Comparative
1.306
710
309



Example 3










Examples 55 to 84
As in Example 53, various mother sintered bodies were coated with various diffusion alloy powder and subjected to diffusion treatment at certain temperatures for certain times. Table 15 summarizes the composition of the mother sintered body and the diffusion alloy, the type and amount of main intermetallic compound phase in the diffusion alloy, the temperature and time of diffusion treatment. Table 16 shows the magnetic properties of the magnets. It is noted that the amount of intermetallic compound phase in the diffusion alloy was determined by EPMA analysis.










TABLE 15









Diffusion alloy











Amount of
Diffusion



intermetallic
treatment
















Intermetallic
compound
Temperature




Sintered body
Composition
compound
(vol %)
(° C.)
Time



















Example
55
Nd16.0FebalCo1.0B5.4
Mn27Al73
Al11Mn4
95
770
1
hr



56
Nd13.0Pr3.0FebalCo3.0B5.2
Ni25Al75
NiAl3
93
780
50
min



57
Nd15.3Dy1.2FebalCo2.0B5.3
Cr12.5Al87.5
Al7Cr
91
750
45
min



58
Nd15.0Tb0.7FebalCo1.0B5.5
Co33Si67
CoSi2
94
840
2
hr



59
Nd17.0FebalCo1.5B5.3
Mn25Al25Cu50
Cu2MnAl
87
750
3
hr



60
Nd15.2Dy0.8Tb0.3FebalCo1.0B5.4
Fe50Si50
FeSi
92
870
4
hr



61
Nd20.0FebalCo4.0B5.3
Fe49.9C0.1Si50
FeSi
86
920
10
hr



62
Nd18.0FebalCo3.5B4.2
Ti50C50
TiC
85
1040
28
hr



63
Nd16.0FebalCo1.0B6.8
Mn67P33
Mn2P
71
350
5
min



64
Nd12.0FebalCo2.0B6.0
Ti50Cu50
TiCu
82
640
5
hr



65
Nd16.0FebalCo1.0B5.5
V75Sn25
V3Sn
79
920
2
hr



66
Nd16.0FebalB6.1
Cr67Ta33
Cr2Ta
76
980
5
hr



67
Nd15.5FebalCo3.0B5.4
Cu75Sn25
Cu3Sn
84
580
3
hr



68
Pr16.0FebalCo6.5B5.3
Cu70Zn5Sn25
(Cu,Zn)3Sn
73
520
5
hr



69
Nd17.0Pr1.5FebalCo2.5B5.2
Ga40Zr60
Ga2Zr3
83
800
2
hr



70
Nd16.0FebalCo3.0B5.3
Cr75Ge25
Cr3Ge
84
820
4
hr



71
Nd14.6Pr3.0Dy0.8FebalCo2.0B5.3
Nb33Si67
NbSi2
89
950
5
hr



72
Pr14.6Dy1.0FebalCo1.0B5.4
Al73Mo27
Al8Mo3
86
780
50
min



73
Nd16.0FebalCo1.0B6.4
Ti50Ag50
TiAg
85
740
2
hr



74
Nd15.2FebalCo1.0B5.3
In25Mn75
InMn3
75
570
8
hr



75
Nd15.4FebalB5.6
Hf33Cr67
HfCr2
85
940
4
hr



76
Nd16.3FebalCo1.0B5.6
Cr20Fe55W20
Cr5Fe11W4
74
830
8
hr



77
Nd15.6Yb0.2FebalCo1.0B4.8
Ni50Sb50
NiSb
78
680
2
hr



78
Nd16.4FebalCo5.0B6.9
Ti80Pb20
Ti4Pb
79
710
3
hr



79
Nd15.5FebalCo1.0B5.3
Mn25Co50Sn25
Co2MnSn
77
650
6
hr



80
Nd16.2FebalCo0.7B5.3
Co60Sn40
Co3Sn2
78
870
30
min



81
Nd15.7FebalCo1.5B5.5
V75Sn25
V3Sn
82
970
6
hr



82
Nd14.5FebalCo0.5B5.6
Cr21Fe62Mo17
Cr6Fe18Mo5
73
850
10
hr



83
Nd15.0Dy0.6FebalCo0.1B4.1
Bi40Zr60
Bi2Zr3
78
440
15
hr



84
Nd16.6FebalCo3.5B6.4
Ni50Bi50
NiBi
70
210
1
min




















TABLE 16







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 55
1.303
1815
327



Example 56
1.295
1847
320



Example 57
1.290
1982
319



Example 58
1.315
1902
334



Example 59
1.282
1688
310



Example 60
1.297
1815
324



Example 61
1.190
1664
268



Example 62
1.173
1258
260



Example 63
1.246
1186
290



Example 64
1.370
1473
350



Example 65
1.305
1528
327



Example 66
1.313
1401
329



Example 67
1.312
1656
325



Example 68
1.296
1449
317



Example 69
1.236
1640
288



Example 70
1.312
1576
330



Example 71
1.247
1656
295



Example 72
1.309
1775
320



Example 73
1.295
1369
323



Example 74
1.335
1290
340



Example 75
1.331
1242
337



Example 76
1.301
1178
322



Example 77
1.263
1297
295



Example 78
1.258
1098
292



Example 79
1.314
1616
330



Example 80
1.303
1703
322



Example 81
1.311
1560
326



Example 82
1.342
1210
342



Example 83
1.227
1043
280



Example 84
1.290
971
314










Examples 85 to 92 and Comparative Example 4
A magnet alloy was prepared by using Nd, Fe and Co metals having a purity of at least 99% by weight and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt in a copper mold. The alloy was ground on a Brown mill into a coarse powder with a particle size of up to 1 mm.
Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 4.2 μm. The atmosphere was changes to an inert gas so that the oxidation of the fine powder is inhibited. Then, the fine powder was compacted under a pressure of about 300 kg/cm2 while being oriented in a magnetic field of 1592 kAm−1. The green compact was then placed in a vacuum sintering furnace where it was sintered at 1,060° C. for 1.5 hours, obtaining a sintered block. Using a diamond grinding tool, the sintered block was machined on all the surfaces into a shape having dimensions of 4×4×2 mm It was washed in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried, obtaining a mother sintered body which had the composition Nd13.8FebalCo1.0B6.0.
By using Dy, Tb, Nd, Pr, Co, Ni and Al metals having a purity of at least 99% by weight and arc melting in an argon atmosphere, diffusion alloys having various compositions (in atom %) as shown in Table 17 were prepared. Each alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 7.9 μm. On EPMA analysis, each alloy contained 94% by volume of the intermetallic compound phase shown in Table 17.
The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to form a slurry, in which each mother sintered body was immersed for 30 seconds under ultrasonic agitation. The sintered body was pulled up and immediately dried with hot air.
The sintered bodies covered with the diffusion alloy powder were subjected to diffusion treatment in vacuum at 840° C. for 10 hours, yielding magnets of Examples 85 to 92. A magnet of Comparative Example 4 was also obtained by repeating the above procedure except the diffusion alloy powder was not used.
Table 17 summarizes the composition of the mother sintered body and the diffusion alloy, the main intermetallic compound in the diffusion alloy, and the temperature and time of diffusion treatment in Examples 85 to 92 and Comparative Example 4. Table 18 shows the magnetic properties of the magnets of Examples 85 to 92 and Comparative Example 4. It is seen that the coercive force of the magnets of Examples 85 to 92 is considerably greater than that of Comparative Example 4, while a decline of remanence is only about 10 mT.











TABLE 17









Sintered
Diffusion alloy












body
Intermetallic
Diffusion treatment













composition
Composition
compound
Temperature
Time
















Example 85
Nd13.8FebalCo1.0B6.0
Dy34Co33Al33
Dy(CoAl)2
840° C.
10 hr


Example 86
Nd13.8FebalCo1.0B6.0
Dy34Ni33Al33
Dy(NiAl)2
840° C.
10 hr


Example 87
Nd13.8FebalCo1.0B6.0
Tb33Co50Al17
Tb(CoAl)2
840° C.
10 hr


Example 88
Nd13.8FebalCo1.0B6.0
Tb33Ni17Al50
Tb(NiAl)2
840° C.
10 hr


Example 89
Nd13.8FebalCo1.0B6.0
Nd34Co33Al33
Nd(CoAl)2
840° C.
10 hr


Example 90
Nd13.8FebalCo1.0B6.0
Nd34Ni33Al33
Nd(NiAl)2
840° C.
10 hr


Example 91
Nd13.8FebalCo1.0B6.0
Pr33Co17Al50
Pr(CoAl)2
840° C.
10 hr


Example 92
Nd13.8FebalCo1.0B6.0
Pr33Ni50Al17
Pr(NiAl)2
840° C.
10 hr


Comparative
Nd13.8FebalCo1.0B6.0
—
—
840° C.
10 hr


Example 4




















TABLE 18







Br (T)
Hcj (kAm−1)
(BH)max (kJ/m3)





















Example 85
1.411
1720
386



Example 86
1.409
1740
384



Example 87
1.412
1880
388



Example 88
1.410
1890
385



Example 89
1.414
1570
387



Example 90
1.413
1580
386



Example 91
1.409
1640
384



Example 92
1.408
1660
382



Comparative
1.422
890
377



Example 4










Japanese Patent Application Nos. 2007-068803 and 2007-068823 are incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

## Claims

1. A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing an alloy powder on a surface of a sintered body of the composition Ra-T1b-Bc wherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T1 is at least one element selected from Fe and Co, B is boron, “a,” “b” and “c” indicative of atomic percent are in the range: 12≦a≦20, 4.0≦c≦7.0, and the balance of b, said alloy powder having the composition M1d-M2e wherein each of M1 and M2 is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, “d” and “e” indicative of atomic percent are in the range: 0.1≦e≦99.9 and the balance of d, and containing at least 70% by volume of an intermetallic compound phase, and
heat treating the sintered body having the powder disposed on its surface at a temperature equal to or below the sintering temperature of the sintered body in vacuum or in an inert gas, for causing at least one element of M1 and M2 in the powder to diffuse to grain boundaries in the interior of the sintered body and/or near grain boundaries within sintered body primary phase grains.

2. The method of claim 1 wherein the disposing step includes grinding an alloy having the composition M1d-M2e wherein M1, M2, d and e are as defined above and containing at least 70% by volume of an intermetallic compound phase into a powder having an average particle size of up to 500 μm, dispersing the powder in an organic solvent or water, applying the resulting slurry to the surface of the sintered body, and drying.

3. The method of claim 1 wherein the heat treating step includes heat treatment at a temperature from 200° C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts represents the sintering temperature of the sintered body.

4. The method of claim 1 wherein the sintered body has a shape including a minimum portion with a dimension equal to or less than 20 mm.