US20200203069A1

US20200203069A1 - Rare earth permanent magnets and their preparation

Info

Publication number: US20200203069A1
Application number: US16/801,625
Authority: US
Inventors: Hiroaki Nagata; Tadao Nomura; Takehisa Minowa
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 2011-05-02
Filing date: 2020-02-26
Publication date: 2020-06-25
Also published as: TW201308368A; US20120280775A1; US11791093B2; US11482377B2; KR20120124039A; PH12012000096B1; EP2521147A1; MY165562A; KR101855530B1; US10614952B2; CN102768898B; US20200194170A1; EP2521147B1; US20170186534A1; TWI509642B; PH12012000096A1; CN102768898A; MY174972A; US20200194171A1

Abstract

A sintered magnet body (R_aT¹ _bM_cB_d) coated with a powder mixture of an intermetallic compound (R¹ _iM¹ _j, R¹ _xT² _yM¹ _z, R¹ _iM¹ _jH_k), alloy (M¹ _dM² _e) or metal (M¹) powder and a rare earth (R²) oxide is diffusion treated. The R²oxide is partially reduced during the diffusion treatment, so a significant amount of R²can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries. The coercive force is increased while minimizing a decline of remanence.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No. 15/454,433, filed on Mar. 9, 2017, and wherein U.S. application Ser. No. 15/454,433 is a Divisional of U.S. patent application Ser. No. 13/461,043 filed on May 1, 2012, which is a non-provisional application which claims priority under 35 U.S.C. § 119(a) on Japanese Patent Application Nos. 2011-102787 and 2011-102789 filed in Japan on May 2, 2011 and May 2, 2011, respectively, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an R—Fe—B permanent magnet having an enhanced coercive force with a minimal decline of remanence, and a method for preparing the same by coating a sintered magnet body with a mixture of an intermetallic compound, alloy or metal powder and a rare earth oxide and heat treating the coated body for diffusion.

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 Non-Patent Document 1). 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. Specifically, a powder of alloy A consisting of R₂Fe₁₄B primary phase wherein R is mainly Nd and Pr, and a powder of alloy B containing various additive elements including Dy, Tb, Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb are mixed together. This is followed by fine pulverization, molding 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 and Tb are 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 Patent Documents 1 and 2). In this method, however, Dy and Tb diffuse into the interior of primary phase grains during the sintering so that the layer where Dy and Tb are 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, as described in Patent Documents 3 to 5 and Non-Patent Documents 2 and 3. 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 Patent Document 6. With these processes, the elements (e.g., Dy and 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 and 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.
Besides the foregoing methods, Patent Document 6 discloses a method comprising coating a surface of a sintered body with a powdered rare earth inorganic compound such as fluoride or oxide and heat treatment, and Patent Document 8 discloses a method comprising mixing an Al, Cu or Zn powder with a fluoride, coating a magnet with the mixture, and heat treatment. These methods are characterized by a very simple coating step and a high productivity. Specifically, since the coating step is carried out by dispersing a non-metallic inorganic compound powder in water, immersing a magnet in the dispersion and drying, the step is simple as compared with sputtering and evaporation. Even when a heat treatment furnace is packed with a large number of magnet pieces, the magnet pieces are not fused together during heat treatment. This leads to a high productivity. However, since Dy or Tb diffuses through substitution reaction between the powder and the magnet component, it is difficult to introduce a substantial amount of Dy or Tb into the magnet.
Further Patent Document 7 discloses coating of a magnet body with a mixture of an oxide or fluoride of Dy or Tb and calcium or calcium hydride powder, followed by heat treatment. During the heat treatment, once Dy or Tb is reduced utilizing calcium reducing reaction, Dy or Tb is diffused. The method is advantageous for introducing a substantial amount of Dy or Tb into the magnet, but less productive because the calcium or calcium hydride powder needs careful handling.
Patent Documents 9 to 13 disclose coating of the sintered body surface with a metal alloy instead of a rare earth inorganic compound powder such as fluoride or oxide, followed by heat treatment. The method of coating with only metal alloy has the drawback that it is difficult to coat the metal alloy onto the magnet surface in a large and uniform coating weight. In Patent Documents 14 and 15, a metal powder containing Dy and/or Tb is diffused into the mother alloy. The oxygen concentration of the mother alloy is restricted below 0.5% by weight, and the rare earth-containing metal powder is closely contacted with the mother alloy by a barrel painting technique of oscillating impact media within a barrel for agitation. Diffusion takes place under these conditions. However, this method requires many steps as compared with the method of coating a mother alloy magnet with a dispersion of a powder mixture of an intermetallic compound and a rare earth oxide in a solvent. The method is time consuming and is not industrially useful.

CITATION LIST

Patent Document 1: JP 1820677
Patent Document 2: JP 3143156
Patent Document 3: JP-A 2004-296973
Patent Document 4: JP 3897724
Patent Document 5: JP-A 2005-11973
Patent Document 6: JP 4450239
Patent Document 7: JP 4548673
Patent Document 8: JP-A 2007-287874
Patent Document 9: JP 4656323
Patent Document 10: JP 4482769
Patent Document 11: JP-A 2008-263179
Patent Document 12: JP-A 2009-289994
Patent Document 13: JP-A 2010-238712
Patent Document 14: WO 2008/032426
Patent Document 15: WO 2008/139690
Non-Patent Document 1: 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
Non-Patent Document 2: 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 Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000)
Non-Patent Document 3: K. Machida, et al., “Grain Boundary Modification of Nd—Fe—B Sintered Magnet and Magnetic Properties,” Proceedings of 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202

SUMMARY OF INVENTION

An object of the invention is to provide an R—Fe—B sintered magnet which is prepared by coating a sintered magnet body with a powder mixture of an intermetallic compound, alloy or metal powder and a rare earth oxide and effecting diffusion treatment and which magnet features efficient productivity, excellent magnetic performance, a minimal 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.
Regarding the surface coating of an R—Fe—B sintered body with a rare earth oxide which is the best from the aspect of productivity, the inventors attempted to increase the diffusion amount. The inventors have discovered that when a mixture of an oxide containing a rare earth element such as Dy or Tb and an intermetallic compound or metal powder is used for coating, a significant amount of Dy or Tb can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries, as compared with the method of effecting heat treatment after coating with a rare earth inorganic compound powder such as fluoride or oxide, because the oxide is partially reduced during heat treatment. As a consequence, the coercive force of the magnet is increased while minimizing a decline of remanence. Additionally, the process is improved in productivity over the prior art processes. 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 a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, 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, the balance of i, and i+j=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and
heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R¹, R²and M¹in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.
[2] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jH_kwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen, “i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99, 0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and
heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R¹, R², and M¹in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.
[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 magnet body.
[4] The method of any one of [1] to [3] wherein the disposing step includes dispersing the powder mixture in an organic solvent or water, immersing the sintered magnet body in the resulting slurry, taking up the sintered magnet body, and drying for thereby covering the surface of the sintered magnet body with the powder mixture.
[5] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _xT² _yM¹ _zwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, T²is one or both of Fe and Co, M¹is 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, x+z<100, the balance of y, y>0, and x+y+z=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and
heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R¹, R², M and T²in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.
[6] The method of [5] 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 magnet body.
[7] The method of [5] or [6] wherein the disposing step includes dispersing the powder mixture in an organic solvent or water, immersing the sintered magnet body in the resulting slurry, taking up the sintered magnet body, and drying for thereby covering the surface of the sintered magnet body with the powder mixture.
[8] The method of any one of [1] to [7] wherein the sintered magnet 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 a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, 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, the balance of i, and i+j=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein
the elements R¹, R²and M¹in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.
[10] A rare earth permanent magnet, which is prepared by disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jH_kwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen, “i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99, 0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein
the elements R¹, R²and M¹in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.
[11] A rare earth permanent magnet, which is prepared by disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _xT² _yM¹ _zwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, T²is one or both of Fe and Co, M¹is 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, x+z<100, the balance of y, y>0, and x+y+z=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein
the elements R¹, R², M¹and T²in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.
[12] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition M¹ _dM² _ewherein M¹and M²each are at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M¹and M²are different, “d” and “e” indicative of atomic percent are in the range: 0.1≤e≤99.9, the balance of d, and d+e=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and
heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R², M¹and M²in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.
[13] A method for preparing a rare earth permanent magnet, comprising the steps of:
disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an M¹powder wherein M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and
heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R²and M¹in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.
[14] The method of [12] or [13] 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 magnet body.
[15] The method of any one of [12] to [14] wherein the disposing step includes dispersing the powder mixture in an organic solvent or water, immersing the sintered magnet body in the resulting slurry, taking up the sintered magnet body, and drying for thereby covering the surface of the sintered magnet body with the powder mixture.
[16] The method of any one of [12] to [15] wherein the sintered magnet body has a shape including a minimum portion with a dimension equal to or less than 20 mm.
[17] A rare earth permanent magnet, which is prepared by disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition M¹ _dM² _ewherein M¹and M²each are at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M¹and M²are different, “d” and “e” indicative of atomic percent are in the range: 0.1≤e≤99.9, the balance of d, and d+e=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein
the elements R², M¹and M²in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.
[18] A rare earth permanent magnet, which is prepared by disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an M¹powder wherein M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein
the elements R²and M¹in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.

Advantageous Effects of Invention

When a mixture of an oxide containing a rare earth element such as Dy or Tb and an intermetallic compound or metal powder is used for coating, the oxide is partially reduced during subsequent heat treatment. Thus a significant amount of the rare earth element such as Dy or Tb can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries, as compared with the method of effecting heat treatment after coating with a rare earth inorganic compound powder such as fluoride or oxide. As a consequence, the coercive force of the magnet is increased while minimizing a decline of remanence. Additionally, the process is improved in productivity over the prior art processes. The R—Fe—B sintered magnet exhibits excellent magnetic performance, an increased coercive force, and a minimal decline of remanence, despite a minimal amount of Tb or Dy used.

DESCRIPTION OF EMBODIMENTS

Briefly stated, an R—Fe—B sintered magnet is prepared according to the invention by applying a powder mixture of an intermetallic compound-based alloy powder and a rare earth oxide or metal powder onto a sintered magnet body and effecting diffusion treatment. The resultant magnet has advantages including excellent magnetic performance and a minimal amount of Tb or Dy used.
The mother material used herein is a sintered magnet body having the composition R_aT¹ _bM_cB_d, which is sometimes referred to as “mother sintered body.” Herein R is one or more elements selected from rare earth elements inclusive of yttrium (Y) and scandium (Sc), specifically from among Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. The rare earth elements inclusive of Sc and Y account for 12 to 20 atomic percent (at %), and preferably 13 to 18 at % of the sintered magnet body, differently stated, 12≤a≤20, preferably 13≤a≤18. Preferably the majority of R is Nd and/or Pr. Specifically Nd and/or Pr accounts for 50 to 100 at %, more preferably 70 to 100 at % of the rare earth elements. T¹is one or both of iron (Fe) and cobalt (Co). M is one or more elements 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 and accounts for 0 to 10 at %, and preferably 0 to 5 at % of the sintered magnet body, differently stated, 0≤c≤10, preferably 0≤c·5. B is boron and accounts for 4 to 7 at % of the sintered magnet body (4≤d≤7). Particularly when B is 5 to 6 at % (5≤d≤6), a significant improvement in coercive force is achieved by diffusion treatment. The balance consists of T¹. Preferably T¹accounts for 60 to 84 at %, more preferably 70 to 82 at % of the sintered magnet body, differently stated, 60≤b≤84, preferably 70≤b≤82. The subscripts “a,” “b,” “c” and “d” indicative of atomic percent meet a+b+c+d=100.
The alloy for the mother sintered magnet 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 hydrogen decrepitation, with the hydrogen decrepitation being preferred for those alloys as strip cast. The coarse powder is then finely divided 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 magnet block may be machined or worked into a predetermined shape. In the invention, the elements (including R¹, R², M¹, M²and T²) which are to be diffused into the sintered magnet body interior are supplied from the sintered magnet body surface. Thus, if a minimum portion of the sintered magnet 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, a diffusion powder selected from the following powder mixtures (i) to (iv) is disposed on the sintered magnet body before diffusion treatment is carried out.

(i) a powder mixture of an alloy of the composition R¹ _iM¹ _jcontaining at least 70% by volume of a rare earth intermetallic compound phase and an R²oxide
(ii) a powder mixture of an alloy of the composition R¹ _iM¹ _jH_kcontaining at least 70% by volume of a rare earth intermetallic compound phase and an R²oxide
(iii) a powder mixture of an alloy of the composition R¹ _xT² _yM¹ _zcontaining at least 70% by volume of a rare earth intermetallic compound phase and an R²oxide
(iv) a powder mixture of an alloy of the composition M¹ _dM² _econtaining at least 70% by volume of an intermetallic compound phase and an R²oxide
(v) a powder mixture of a metal M¹and an R²oxide

The alloy which is often referred to as “diffusion alloy” is in powder form having an average particle size of less than or equal to 500 m. The R²oxide wherein R²is one or more elements selected from rare earth elements inclusive of Y and Sc is in powder form having an average particle size of less than or equal to 100 μm. The powder mixture consists of the diffusion alloy and at least 10% by weight of the R²oxide. The powder mixture is disposed on the surface of the sintered magnet body. The sintered magnet body having the powder mixture disposed on its surface is heat treated at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, whereby the oxide in admixture with the (rare earth) intermetallic compound is partially reduced. During the heat treatment, the elements R¹, R², M¹, M²and T²in the powder mixture (selected depending on a particular diffusion powder used) can be diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains, in a more amount than achievable by the prior art methods.
Herein R¹is one or more elements selected from rare earth elements inclusive of Y and Sc. Preferably the majority of R¹is Nd and/or Pr. Specifically Nd and/or Pr accounts for 1 to 100 at %, more preferably 20 to 100 at % of R¹. M¹is one or more elements 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. T²is Fe and/or Co.
In the alloy R¹ _iM¹ _j, M¹accounts for 15 to 99 at %, preferably 20 to 90 at %, differently stated, j=15 to 99, preferably j=20 to 90, with the balance of R¹(meaning i+j=100).
In the alloy R¹ _iM¹ _jH_k, M¹accounts for 15 to 99 at %, preferably 20 to 90 at %, differently stated, j=15 to 99, preferably j=20 to 90. Hydrogen (H) is present in an amount of 0<k≤(i×2.5) at %, preferably at least 0.1 at % (k≤0.1). The balance consists of R¹(meaning i+j+k=100), and R¹is preferably present in an amount of 20 to 90 at %, namely i=20 to 90.
In the alloy R¹ _xT² _yM¹ _z, M¹accounts for 15 to 95 at %, preferably 20 to 90 at %, differently stated, z=15 to 90, preferably z=20 to 90. R¹accounts for 5 to 85 at %, preferably 10 to 80 at %, differently stated, x=5 to 85, preferably x=10 to 80. The sum of M¹and R²is less than 100 at % (x+z<100), preferably 25 to 99.5 at % (x+y=25 to 99.5). The balance consists of T²which is Fe and/or Co (meaning x+y+z=100), and y>0. Typically T²accounts for 0.5 to 75 at %, preferably 1 to 60 at %, differently stated, y=0.5 to 75, preferably y=1 to 60.
In the alloy M¹ _dM² _e, M¹and M²are different from each other and each is one or more elements selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi. The subscripts d and e indicative of atomic percent are in the range: 0.1≤e≤99.9, preferably 10≤e≤90, and more preferably 20≤e≤80, with the balance of d.
In the M¹metal powder, M¹is one or more elements selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi.
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 %, preferably equal to or less than 2 at %, and more preferably equal to or less than 1 at %.
The diffusion alloy containing at least 70% by volume of the intermetallic compound phase may be prepared, like the alloy for the mother sintered magnet 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. A high-frequency melting method and a strip casting method may also be employed. 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 hydrogen decrepitation. The coarse powder is then finely divided, 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, 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.
The M¹metal powder may be prepared by crushing or coarsely grinding a metal mass to a size of 0.05 to 3 mm, especially 0.05 to 1.5 mm on a suitable grinding machine such as a jaw crusher or Brown mill. The coarse powder is then finely divided, for example, by a ball mill, vibration mill or jet mill using high-pressure nitrogen. Alternatively, fine division may be achieved by an atomizing method of ejecting a metal melt through small nozzles under high-pressure gas as mist. The M¹metal powder 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.
The other component of the powder mixture is an R²oxide which may be any of oxides of rare earth elements inclusive of Y and Sc, preferably oxides containing Dy or Tb. The R²oxide powder has an average particle size equal to or less than 100 μm, more preferably equal to or less than 50 μm, and even more preferably equal to or less than 20 μm. The R²oxide is present in an amount of at least 10% by weight, preferably at least 20% by weight, and more preferably at least 30% by weight of the powder mixture. Less than 10% by weight of the R²oxide is too small for the rare earth oxide to exert its mixing effect. The upper limit of the amount of the R²oxide is up to 99% by weight, especially up to 90% by weight.
After the powder mixture of the diffusion alloy powder or M¹metal powder and the R²oxide powder is disposed on the surface of the mother sintered magnet body, the mother sintered magnet body coated with the powder mixture is 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 magnet body. This heat treatment is referred to as “diffusion treatment.” The diffusion treatment causes the rare earth oxide in admixture with the intermetallic compound to be partially reduced, whereby elements R¹, R², M¹, M²and T²in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within sintered magnet body primary phase grains in more amounts than achievable in the prior art.
The powder mixture of the diffusion alloy powder or M¹metal powder and the R²oxide powder is disposed on the surface of the mother sintered magnet body, for example, by dispersing the powder mixture in water or an organic solvent to form a slurry, immersing the magnet body in the slurry, taking up the magnet body, and drying the magnet body by hot air drying or in vacuum or in air. Spray coating is also possible. The slurry may contain 1 to 90% by weight, and preferably 5 to 70% by weight of the powder mixture.
The conditions of diffusion treatment vary with the type and composition of the powder mixture (including the type and composition of two components) and are preferably selected such that elements R¹, R², M¹, M²and T²in the diffusion powder are enriched at grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within sintered magnet body primary phase grains. The temperature of diffusion treatment is equal to or below the sintering temperature (designated Ts in ° C.) of the sintered magnet body. If diffusion treatment is effected above Ts, there arise problems that (1) the structure of the sintered magnet 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 magnet body, and preferably equal to or below (Ts−10)° C. The lower limit of temperature may be selected as appropriate though the temperature is typically at least 200° C., preferably at least 350° C., and more preferably at least 600° 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 exceeds 30 hours, the structure of the sintered magnet body can be altered, oxidation or evaporation of components inevitably occurs to degrade magnetic properties, or R¹, R², M¹, M²and T²are not only enriched near grain boundaries in the interior of the sintered body and/or 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 elements R¹, R², M¹, M²and T²in the powder mixture disposed on the surface of the sintered magnet body are diffused into the sintered magnet body while traveling mainly along grain boundaries in the sintered magnet body structure. This results in the structure in which R¹, R², M¹, M²and T²are enriched near grain boundaries in the interior of the sintered magnet body and/or grain boundaries within sintered magnet body primary phase grains.
The permanent magnet thus obtained is improved in coercivity because the diffusion of R¹, R², M¹, M²and T²modifies 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 elements in the powder mixture 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 Examples 1 and 2

An alloy was prepared by weighing amounts of Nd, Co, Al and Fe 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 on a single roll of copper in an argon atmosphere, that is, strip casting into a strip of alloy. The alloy consisted of 12.8 at % of Nd, 1.0 at % of Co, 0.5 at % of Al, 6.0 at % of B, and the balance of Fe. This is designated alloy A. Alloy A was then subjected to hydrogen decrepitation by causing the alloy to absorb hydrogen, vacuum evacuating and heating up to 500° C. for desorbing part of hydrogen. In this way, alloy A was pulverized into a coarse powder under 30 mesh.
Another alloy was prepared by weighing amounts of Nd, Dy, Fe, Co, Al and Cu 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. The alloy consisted of 23 at % of Nd, 12 at % of Dy, 25 at % of Fe, 6 at % of B, 0.5 at % of Al, 2 at % of Cu, and the balance of Co. This is designated alloy B. Alloy B was ground on a Brown mill in a nitrogen atmosphere into a coarse powder under 30 mesh.
Next, 94 wt % of alloy A powder and 6 wt % of alloy B powder were mixed in a nitrogen-purged V-blender for 30 minutes. The powder mixture was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 4.1 μm. The fine powder was compacted in a nitrogen atmosphere under a pressure of about 1 ton/cm²while being oriented in a magnetic field of 15 kOe. The green compact was then placed in a sintering furnace where it was sintered in an argon atmosphere at 1,060° C. for 2 hours, obtaining a magnet block of 10 mm×20 mm×15 mm (thick). Using a diamond grinding tool, the magnet block was machined on all the surfaces into a shape having dimensions of 4 mm×4 mm×2 mm (magnetic anisotropy direction). The machined magnet body was washed in sequence with alkaline solution, deionized water, acid solution, and deionized water, and dried, obtaining a mother sintered magnet body which had the composition: Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0.
Tb and Al metals having a purity of at least 99% by weight were used and high-frequency melted in an argon atmosphere to form a diffusion alloy having the composition Tb₃₃Al₆₇and composed mainly of an intermetallic compound phase TbAl₂. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 8.6 μm. On electron probe microanalysis (EPMA), the alloy contained 94% by volume of the intermetallic compound phase TbAl₂.
The diffusion alloy Tb₃₃Al₆₇powder was mixed with terbium oxide (Tb₄O₇) having an average particle size of 1 μm in a weight ratio of 1:1. The powder mixture was combined with deionized water in a weight fraction of 50% to form a slurry, in which the mother sintered magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The magnet body covered with the powder mixture was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Example 1.
Separately, the diffusion alloy Tb₃₃Al₆₇powder having a mass median particle diameter of 8.6 μm alone was combined with deionized water in a weight fraction of 50% to form a slurry, in which the magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The magnet body covered with the diffusion alloy powder was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Comparative Example 1. In the absence of the diffusion powder, only the mother sintered magnet body was similarly heated treated in vacuum at 900° C. for 8 hours, yielding a magnet of Comparative Example 2.
Table 1 summarizes the composition of the mother sintered magnet body, diffusion rare earth alloy and diffusion rare earth oxide, and a mixing ratio (by weight) of the diffusion powder in Example 1 and Comparative Examples 1 and 2. Table 2 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the magnets. It is seen that the magnet of Example 1 has a coercive force (Hcj) which is greater by 90 kAm⁻¹than that of Comparative Example 1 and a remanence (Br) which is higher by 8 mT than that of Comparative Example 1. The coercive force (Hcj) of the magnet of Example 1 is greater by 1,090 kAm⁻¹than that of Comparative Example 2 while a decline of remanence (Br) is only 5 mT.

	TABLE 1

	Diffusion powder mixture

Mother sintered	Rare earth	Rare earth	Mixing ratio
magnet body	alloy	oxide	(by weight)

Example 1	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	Tb₃₃Al₆₇	Tb₄O₇	50:50
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	Tb₃₃Al₆₇	—	Tb₃₃Al₆₇alone
Example 1
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	—	—	—
Example 2

	TABLE 2

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)_max
(° C.)	(hr)	(T)	(kAm⁻¹)	(kJ/m³)

Example 1	900	8	1.415	2,130	390
Comparative	900	8	1.407	2,040	386
Example 1
Comparative	900	8	1.420	1,040	380
Example 2

Example 2 and Comparative Example 3

As in Example 1, a mother sintered magnet body having the composition: Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0was prepared.
Tb, Co, Fe and Al metals having a purity of at least 99% by weight were used and high-frequency melted in an argon atmosphere to form a diffusion alloy having the composition Tb₃₅Fe₂₁Co₂₄Al₂₀. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 8.9 μm. On EPMA analysis, the alloy contained intermetallic compound phases Tb(FeCoAl)₂, Tb₂(FeCoAl) and Tb₂(FeCoAl)₁₇, which summed to 87% by volume.
The diffusion alloy Tb₃₅Fe₂₁Co₂₄Al₂₀powder was mixed with Tb₄O₇having an average particle size of 1 μm in a weight ratio of 1:1. The powder mixture was combined with deionized water in a weight fraction of 50% to form a slurry, in which the mother sintered magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The magnet body covered with the powder mixture was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Example 2.
In the absence of the diffusion powder, only the mother sintered magnet body was similarly heat treated in vacuum at 900° C. for 8 hours, yielding a magnet of Comparative Example 3.
Table 3 summarizes the composition of the mother sintered magnet body, diffusion rare earth alloy and diffusion rare earth oxide, and a mixing ratio (by weight) of the diffusion powder in Example 2 and Comparative Example 3. Table 4 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the magnets. It is seen that the coercive force (Hcj) of the magnet of Example 2 is greater by 1,020 kAm⁻¹than that of Comparative Example 3 while a decline of remanence (Br) is only 4 mT.

	TABLE 3

	Diffusion powder mixture

Example 2	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	Tb₃₅Fe₂₁Co₂₄Al₂₀	Tb₄O₇	50:50
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	—	—	—
Example 3

	TABLE 4

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)_max
(° C.)	(hr)	(T)	(kAm⁻¹)	(kJ/m³)

Example 2	900	8	1.416	2,060	390
Comparative	900	8	1.420	1,040	380
Example 3

Examples 3 to 55

As in Example 1, a series of mother sintered magnet bodies were coated with a different powder mixture of diffusion alloy and rare earth oxide and diffusion treated at a selected temperature for a selected time. Table 5 summarizes the composition of the mother sintered magnet body, diffusion rare earth alloy and rare earth oxide, and a mixing ratio (by weight) of the diffusion powder. Table 6 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the resulting magnets. All the diffusion alloys contained at least 70% by volume of intermetallic compounds.

	TABLE 5

	Diffusion powder mixture

Example 3	Nd_15.0Fe_balCo_1.0B_5.4	Nd₃₅Fe₂₀Co₁₅Al₃₀	Tb₄O₇	30:70
Example 4	Nd_15.0Fe_balCo_1.0B_5.4	Nd₃₅Fe₂₅Co₂₀Si₂₀	Dy₂O₃	60:40
Example 5	Nd_15.0Fe_balCo_1.0B_5.4	Nd₃₃Fe₂₀Co₂₇Al₁₅Si₅	Nd₂O₃	10:90
Example 6	Nd_11.0Dy_2.0Tb_2.0Fe_balCo_1.0B_5.5	Nd₂₈Pr₅Al₆₇	Pr₂O₃	90:10
Example 7	Nd_16.5Fe_balCo_1.5B_6.2	Y₂₁Mn₇₈Cr₁	Dy₂O₃	50:50
Example 8	Nd_13.0Pr_2.5Fe_balCo_2.8B_4.8	La₃₃Cu₆₀Co₄Ni₃	Tb₂O₃	50:50
Example 9	Nd_13.0Pr_2.5Fe_balCo_2.8B_4.8	La₅₀Ni₄₉V₁	CeO₂	70:30
Example 10	Nd_13.0Dy_1.5Fe_balCo_1.0B_5.9	La₃₃Cu_66.5Nb_0.5	La₂O₃	30:70
Example 11	Nd_16.5Fe_balCo_3.0B_4.7	Ce₂₂Ni₁₄Co₅₈Zn₆	Tb₄O₇	80:20
Example 12	Nd_16.5Fe_balCo_3.0B_4.7	Ce₁₇Ni₈₃	CeO₂	50:50
Example 13	Nd_17.3Fe_balCo_3.5B_6.3	Ce₁₁Zn₈₉	Gd₂O₃	50:50
Example 14	Nd_16.0Dy_1.5Fe_balCo_4.5B_5.1	Pr₃₃Ge₆₇	Y₂O₃	50:50
Example 15	Nd_12.2Pr_2.5Fe_balCo_1.0B_5.3	Tb₃₃Al₆₀H₇	Dy₂O₃	50:50
Example 16	Nd_14.5Pr_2.5Fe_balCo_3.5B_5.6	Pr₃₃Al₆₆Zr₁	Tb₄O₇	75:25
Example 17	Nd_13.0Tb_1.5Fe_balB_5.5	Gd₃₂Mn₃₀Fe₃₁Nb₇	Dy₂O₃	50:50
Example 18	Nd_12.0Fe_balCo_1.0B_4.8	Gd₃₇Mn₄₀Co₂₀Mo₃	Tb₄O₇	25:75
Example 19	Nd_13.0Tb_1.5Fe_balB_5.5	Gd₂₁Mn₇₈Mo₁	Dy₂O₃	40:60
Example 20	Nd_12.0Fe_balCo_1.0B_4.8	Gd₃₃Mn₆₆Ta₁	Tb₄O₇	50:50
Example 21	Nd_12.0Pr_2.7Fe_balCo_2.5B_5.2	Tb₂₉Fe₄₅Ni₂₀Ag₆	Yb₂O₃	50:50
Example 22	Nd_13.0Pr_2.0Fe_balCo_2.5B_5.2	Tb₅₀Ag₅₀	Tb₄O₇	60:40
Example 23	Nd_12.5Dy_3.0Fe_balCo_0.7B_5.9	Tb₅₀In₅₀	Dy₂O₃	50:50
Example 24	Nd_12.5Pr_2.5Tb_0.5Fe_balCo_0.5B_5.0	Dy₃₁Ni₈Cu₅₅Sn₆	Tb₄O₇	50:50
Example 25	Nd_10.0Pr_2.5Dy_2.5Fe_balCo_0.6B_5.7	Dy₃₃Cu_66.5Hf_0.5	Pr₂O₃	50:50
Example 26	Nd_13.0Pr_2.2Fe_balCo_1.0B_5.3	Dy₃₃Fe₆₇	Dy₂O₃	50:50
Example 27	Nd_12.8Pr_2.5Tb_0.2Fe_balCo_1.0B_4.5	Er₃₃Mn₃₀Co₃₅Ta₂	Tb₄O₇	50:50
Example 28	Nd_13.2Pr_2.5Dy_0.5Fe_balCo_3.0B_6.3	Er₂₁Mn_78.6W_0.4	Er₂O₃	50:50
Example 29	Nd_12.0Tb_3.5Fe_balCo_3.5B_6.2	Yb₂₄Co₅Ni₆₉Bi₂	Tb₄O₇	50:50
Example 30	Nd_13.0Dy_3.0Fe_balCo_2.0B_4.8	Yb₅₀Cu₄₉Ti₁	Pr₂O₃	50:50
Example 31	Nd_11.0Tb_3.5Fe_balCo_3.5B_6.2	Yb₂₅Ni_74.5Sb_0.5	Yb₂O₃	50:50
Example 32	Nd_15.5Fe_balCo_1.0B_5.3	Nd₃₃Al₆₇	Tb₄O₇	90:10
Example 33	Nd_15.1Fe_balCo_1.0B_5.4	Nd₅₀Si₅₀	Dy₂O₃	80:20
Example 34	Nd_14.8Fe_balCo_1.0B_5.3	Nd₃₃Al₃₇Si₃₀	Dy₂O₃	20:80
Example 35	Nd_11.8Pr_3.0Fe_balCo_1.0B_5.3	Nd₃₄Al₆₁H₅	Tb₄O₇	50:50
Example 36	Nd_12.3Dy_2.5Fe_balCo_3.5B_5.4	Nd₂₇Pr₆Al₆₇	Tb₄O₇	50:50
Example 37	Nd_15.1Fe_balCo_1.0B_5.3	Dy₃₃Al₆₇	Dy₂O₃	75:25
Example 38	Nd_13.6Tb_1.5Fe_balCo_3.5B_5.2	Dy₃₃Ga₆₇	Tb₄O₇	50:50
Example 39	Nd_15.1Fe_balCo_1.0B_5.3	Tb₃₃Al₆₇	Dy₂O₃	80:20
Example 40	Nd_13.5Pr_2.0Dy_2.0Fe_balCo_2.5B_5.3	Tb₂₂Mn₇₈	Tb₄O₇	50:50
Example 41	Nd_12.5Pr_2.5Fe_balCo_1.0B_5.3	Tb₃₃Co₆₇	Dy₂O₃	50:50
Example 42	Nd_19.0Fe_balCo_3.0B_5.4	Y₁₀Co₁₅Zn₇₅	Y₂O₃	70:30
Example 43	Nd_18.0Fe_balCo_2.5B_6.6	Y₆₈Fe₂In₃₀	Tb₄O₇	50:50
Example 44	Nd_18.0Fe_balCo_3.0B_5.4	Y₁₁Zn₈₉	Dy₂O₃	80:20
Example 45	Nd_13.5Pr_1.5Dy_0.8Fe_balCo_2.5B_4.5	La₃₂Co₄Cu₆₄	Tb₄O₇	50:50
Example 46	Nd_13.5Pr_1.5Dy_0.8Fe_balCo_2.5B_4.5	La₃₃Cu₆₇	Pr₂O₃	50:50
Example 47	Nd_20.0Fe_balCo_5.5B_4.1	Ce₂₆Pb₇₄	Tb₄O₇	40:60
Example 48	Nd_15.2Fe_balCo_1.0B_5.3	Ce₅₆Sn₄₄	CeO₂	50:50
Example 49	Nd_15.5Dy_2.5Tb_0.5Fe_balCo_2.6B_4.4	Pr₃₃Fe₃C₆₄	Dy₂O₃	50:50
Example 50	Nd_12.5Dy_2.0Tb_0.5Fe_balCo_3.8B_6.2	Pr₅₀P₅₀	Nd₂O₃	50:50
Example 51	Nd_12.7Pr_2.5Dy_0.6Fe_balCo_1.4B_5.6	Gd₅₂Ni₄₈	Tb₄O₇	70:30
Example 52	Nd_13.1Pr_1.5Tb_0.5Fe_balCo_2.8B_6.3	Gd₃₇Ga₆₃	Dy₂O₃	60:40
Example 53	Nd_15.3Dy_0.6Fe_balCo_1.0B_4.9	Er₃₂Mn₆₇Ta₁	Nd₂O₃	50:50
Example 54	Nd_14.5Pr_1.0Dy_0.5Fe_balCo_2.8B_4.6	Yb₆₈Pb₃₂	Tb₄O₇	50:50
Example 55	Nd_12.0Pr_1.5Dy_0.5Fe_balCo_4.2B_5.3	Yb₆₉Sn₂₉Bi₂	Yb₂O₃	80:20

	TABLE 6

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)max
(° C.)	(hr or min)	(T)	(kAm⁻1)	(kJ/m³)

Example 3	780	8	h	1.404	2,032	385
Example 4	880	8	h	1.419	1,992	390
Example 5	820	6	h	1.416	2,036	389
Example 6	750	5	h	1.411	1,987	388
Example 7	930	10	h	1.343	1,008	343
Example 8	780	5	h	1.367	1,225	354
Example 9	890	7	h	1.388	1,219	363
Example 10	820	8	h	1.432	1,052	396
Example 11	450	12	h	1.348	920	349
Example 12	840	6	h	1.353	940	343
Example 13	400	5	h	1.327	1,052	340
Example 14	830	5	h	1.328	1,890	341
Example 15	820	8	h	1.412	2,130	385
Example 16	850	8	h	1.371	2,048	363
Example 17	960	10	h	1.410	1,785	376
Example 18	940	6	h	1.454	1,620	398
Example 19	920	5	h	1.411	1,615	381
Example 20	860	5	h	1.452	1,748	396
Example 21	920	10	h	1.414	1,672	379
Example 22	920	6	h	1.412	1,910	384
Example 23	940	12	h	1.405	1,955	381
Example 24	870	12	h	1.404	1,930	382
Example 25	860	10	h	1.409	1,870	383
Example 26	850	8	h	1.408	2,060	382
Example 27	1,020	8	h	1.376	1,610	362
Example 28	980	12	h	1.368	1,521	363
Example 29	320	15	min	1.397	1,580	370
Example 30	380	25	min	1.351	1,430	354
Example 31	410	40	min	1.430	1,243	390
Example 32	790	8	h	1.404	2,070	382
Example 33	820	10	h	1.421	2,034	388
Example 34	910	5	h	1.416	2,095	386
Example 35	760	8	h	1.417	2,100	386
Example 36	770	8	h	1.421	2,120	387
Example 37	830	8	h	1.410	2,130	384
Example 38	760	3	h	1.414	2,140	386
Example 39	880	8	h	1.416	2,170	389
Example 40	660	20	h	1.353	1,860	354
Example 41	860	8	h	1.414	2,110	386
Example 42	450	12	h	1.317	1,290	326
Example 43	1,030	2	h	1.286	1,346	309
Example 44	450	8	h	1.332	1,211	334
Example 45	660	14	h	1.350	1,407	347
Example 46	620	12	h	1.347	1,314	344
Example 47	520	10	h	1.203	1,305	276
Example 48	460	14	h	1.361	1,120	350
Example 49	860	30	h	1.278	1,258	312
Example 50	360	40	min	1.412	1,185	368
Example 51	960	2	h	1.390	1,545	366
Example 52	850	30	min	1.415	1,410	382
Example 53	700	10	h	1.373	1,099	355
Example 54	750	12	h	1.351	1,460	346
Example 55	420	10	h	1.448	1,020	396

Example 56 and Comparative Example 4

An alloy was prepared by weighing amounts of Nd, Co, Al and Fe 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 on a single roll of copper in an argon atmosphere, that is, strip casting into a strip of alloy. The alloy consisted of 12.8 at % of Nd, 1.0 at % of Co, 0.5 at % of Al, 6.0 at % of B, and the balance of Fe. This is designated alloy A. Alloy A was then subjected to hydrogen decrepitation by causing the alloy to absorb hydrogen, vacuum evacuating and heating up to 500° C. for desorbing part of hydrogen. In this way, alloy A was pulverized into a coarse powder under 30 mesh.
Another alloy was prepared by weighing amounts of Nd, Dy, Fe, Co, Al and Cu 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. The alloy consisted of 23 at % of Nd, 12 at % of Dy, 25 at % of Fe, 6 at % of B, 0.5 at % of Al, 2 at % of Cu, and the balance of Co. This is designated alloy B. Alloy B was ground on a Brown mill in a nitrogen atmosphere into a coarse powder under 30 mesh.
Next, 94 wt % of alloy A powder and 6 wt % of alloy B powder were mixed in a nitrogen-purged V-blender for 30 minutes. The powder mixture was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 4 μm. The fine powder was compacted in a nitrogen atmosphere under a pressure of about 1 ton/cm²while being oriented in a magnetic field of 15 kOe. The green compact was then placed in a sintering furnace where it was sintered in an argon atmosphere at 1,060° C. for 2 hours, obtaining a magnet block of 10 mm×20 mm×15 mm (thick). Using a diamond grinding tool, the magnet block was machined on all the surfaces into a shape having dimensions of 4 mm×4 mm×2 mm (magnetic anisotropy direction). The machined magnet body was washed in sequence with alkaline solution, deionized water, acid solution, and deionized water, and dried, obtaining a mother sintered magnet body which had the composition: Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0.
Al and Co metals having a purity of at least 99% by weight were used and high-frequency melted in an argon atmosphere to form a diffusion alloy having the composition Al₅₀Co₅₀and composed mainly of an intermetallic compound phase AlCo. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 8.9 μm. On EPMA analysis, the alloy contained 94% by volume of the intermetallic compound phase AlCo.
The diffusion alloy Al₅₀Co₅₀powder was mixed with terbium oxide (Tb₄O₇) having an average particle size of 1 μm in a weight ratio of 1:1. The powder mixture was combined with deionized water in a weight fraction of 50% to form a slurry, in which the mother sintered magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The magnet body covered with the powder mixture was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Example 56.
Separately, terbium oxide having an average particle size of 1 μm alone was combined with deionized water in a weight fraction of 50% to form a slurry, in which the magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The coated magnet body was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Comparative Example 4.
Table 7 summarizes the composition of the mother sintered magnet body, diffusion alloy and diffusion rare earth oxide, and a mixing ratio (by weight) of the diffusion powder mixture in Example 56 and Comparative Example 4. Table 8 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the magnets. It is seen that the coercive force (Hcj) of the magnet of Example 56 is greater by 90 kAm⁻¹than that of Comparative Example 4 while a decline of remanence (Br) is only 3 mT. The coercive force (Hcj) of the magnet of Example 56 is greater by 1,040 kAm⁻¹than that of previous Comparative Example 2 while a decline of remanence (Br) is only 4 mT.

	TABLE 7

	Diffusion powder mixture

Mother sintered	Diffusion	Rare earth	Mixing ratio
magnet body	alloy	oxide	(by weight)

Example 56	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	Al₅₀Co₅₀	Tb₄O₇	50:50
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	—	Tb₄O₇	Tb₄O₇alone
Example 4
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	—	—	—
Example 2

	TABLE 8

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)_max
(° C.)	(hr)	(T)	(kAm⁻¹)	(kJ/m³)

Example 56	900	8	1.416	2,080	390
Comparative	900	8	1.419	1,990	393
Example 4
Comparative	900	8	1.420	1,040	380
Example 2

Example 57 and Comparative Example 5

As in Example 56, a mother sintered magnet body having the composition: Nd_13.3DY_0.5Fe_balCo_2.4CU_0.1Al_0.5B_6.0was prepared.
Ni and Al metals having a purity of at least 99% by weight were used and high-frequency melted in an argon atmosphere to form a diffusion alloy having the composition Ni₂₅Al₇₅and composed mainly of an intermetallic compound phase NiAl₃. The alloy was finely pulverized on a ball mill using an organic solvent into a fine powder having a mass median particle diameter of 9.3 μm. On EPMA analysis, the alloy contained 94% by volume of the intermetallic compound phase NiAl₃.
The diffusion alloy Ni₂₅Al₇₅powder was mixed with terbium oxide (Tb₄O₇) having an average particle size of 1 μm in a weight ratio of 1:1. The powder mixture was combined with deionized water in a weight fraction of 50% to form a slurry, in which the mother sintered magnet body was immersed for 30 seconds under ultrasonic agitation. The magnet body was pulled up and immediately dried with hot air. The magnet body covered with the powder mixture was diffusion treated in an argon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet of Example 57. In the absence of the diffusion powder mixture, the sintered magnet body alone was heat treated in vacuum at 900° C. for 8 hours, yielding a magnet of Comparative Example 5.
Table 9 summarizes the composition of the mother sintered magnet body, diffusion alloy and diffusion rare earth oxide, and a mixing ratio (by weight) of the diffusion powder mixture in Example 57 and Comparative Example 5. Table 10 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the magnets. It is seen that the coercive force (Hcj) of the magnet of Example 57 is greater by 1,010 kAm⁻¹than that of Comparative Example 5 while a decline of remanence (Br) is only 4 mT.

	TABLE 9

	Diffusion powder mixture

Example 57	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	Ni₂₅Al₇₅	Tb₄O₇	50:50
Comparative	Nd_13.3Dy_0.5Fe_balCo_2.4Cu_0.1Al_0.5B_6.0	—	—	—
Example 5

	TABLE 10

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)_max
(° C.)	(hr)	(T)	(kAm⁻¹)	(kJ/m³)

Example 57	900	8	1.416	2,050	390
Comparative	900	8	1.420	1,040	380
Example 5

Examples 58 to 96

As in Example 56, a series of mother sintered magnet bodies were coated with a different powder mixture of diffusion alloy (or metal) and rare earth oxide and diffusion treated at a selected temperature for a selected time. Table 11 summarizes the composition of the mother sintered magnet body, diffusion alloy and rare earth oxide, and a mixing ratio (by weight) of the diffusion powder mixture. Table 12 shows the temperature (° C.) and time (hr) of diffusion treatment and the magnetic properties of the resulting magnets. All the diffusion alloys contained at least 70% by volume of intermetallic compounds.

	TABLE 11

	Diffusion powder mixture

Mother sintered	Diffusion	Rare earth	Mixing ratio
magnet body	alloy or metal	oxide	(by weight)

Example 58	Nd_15.0Fe_balCo_1.0B_5.4	Mn₂₇Al₇₃	Tb₄O₇	30:70
Example 59	Nd_12.0Pr_3.0Fe_balCo_3.0B_5.2	Ni₂₅Al₇₅	Dy₂O₃	90:10
Example 60	Nd_13.3Dy_0.5Fe_balCo_2.0B_6.0	Al	Tb₄O₇	50:50
Example 61	Nd_14.3Dy_1.2Fe_balCo_2.0B_5.3	Cr_12.5Al_87.5	Nd₂O₃	20:80
Example 62	Nd_13.8Tb_0.7Fe_balCo_1.0B_5.5	Co₃₃Si₆₇	Pr₂O₃	70:30
Example 63	Nd_15.8Fe_balCo_1.5B_5.3	Mn₂₅Al₂₅Cu₅₀	Tb₄O₇	50:50
Example 64	Nd_14.4Dy_0.8Tb_0.3Fe_balCo_1.0B_5.4	Fe₅₀Si₅₀	CeO₂	60:40
Example 65	Nd_18.2Fe_balCo_4.0B_5.3	Fe_49.9C_0.1Si₅₀	La₂O₃	30:70
Example 66	Nd_13.3Dy_0.5Fe_balCo_2.0B_6.0	Si	Tb₄O₇	50:50
Example 67	Nd_17.6Fe_balCo_3.5B_4.2	Cr_12.5Al_87.5	Tb₄O₇	50:50
Example 68	Nd_15.6Fe_balCo_1.0B_6.8	Mn₆₇P₃₃	Dy₂O₃	50:50
Example 69	Nd_12.0Fe_balCo_2.0B_6.0	Ti₅₀Cu₅₀	Gd₂O₃	50:50
Example 70	Nd_12.9Dy_1.0Fe_balCo_2.0B_6.0	Cu	Dy₂O₃	50:50
Example 71	Nd_15.2Fe_balCo_1.0B_5.5	V₇₅Sn₂₅	Tb₄O₇	75:25
Example 72	Nd_14.3Fe_balB_6.1	Cr₆₇Ta₃₃	Dy₂O₃	50:50
Example 73	Nd_14.8Fe_balCo_3.0B_5.4	Cu₇₅Sn₂₅	Y₂O₃	50:50
Example 74	Pr_15.0Fe_balCo_6.5B_5.3	Cu₇₀Zn₅Sn₂₅	Er₂O₃	60:40
Example 75	Nd_13.8Dy_0.8Fe_balCo_2.0B_6.2	Zn	Dy₂O₃	50:50
Example 76	Nd_15.8Pr_1.5Fe_balCo_2.5B_5.2	Ga₄₀Zr₆₀	Tb₄O₇	60:40
Example 77	Nd_13.5Dy_1.0Fe_balCo_2.0B_6.0	Ga	Tb₄O₇	50:50
Example 78	Nd_15.2Fe_balCo_3.0B_5.3	Cr₇₅Ge₂₅	Yb₂O₃	50:50
Example 79	Nd_14.0Dy_0.8Fe_balCo_3.0B_6.0	Ge	Dy₂O₃	50:50
Example 80	Nd_14.6Pr_2.0Dy_0.8Fe_balCo_2.0B_5.3	Nb₃₃Si₆₇	Dy₂O₃	50:50
Example 81	Pr_13.7Dy_1.0Fe_balCo_1.0B_5.4	Al₇₃Mo₂₇	Pr₂O₃	40:60
Example 82	Nd_15.0Fe_balCo_1.0B_6.4	Ti₅₀Ag₅₀	Nd₂O₃	60:40
Example 83	Nd_13.8Dy_1.0Fe_balCo_1.0B_5.8	Ag	Tb₄O₇	50:50
Example 84	Nd_14.3Fe_balCo_1.0B_5.3	In₂₅Mn₇₅	Tb₄O₇	50:50
Example 85	Nd_13.9Fe_balB_5.6	Hf₃₃Cr₆₇	Dy₂O₃	70:30
Example 86	Nd_15.2Fe_balCo_1.0B_5.6	Cr₂₅Fe₅₅W₂₀	Tb₄O₇	50:50
Example 87	Nd_15.1Yb_0.2Fe_balCo_1.0B_4.8	Ni₅₀Sb₅₀	Er₂O₃	50:50
Example 88	Nd_15.7Fe_balCo_5.0B_6.9	Ti₈₀Pb₂₀	Tb₄O₇	60:40
Example 89	Nd_14.6Fe_balCo_1.0B_5.3	Mn₂₅Co₅₀Sn₂₅	La₂O₃	70:30
Example 90	Nd_14.9Fe_balCo_0.7B_5.3	Co₆₀Sn₄₀	Tb₄O₇	50:50
Example 91	Nd_14.6Fe_balCo_1.5B_5.5	V₇₅Sn₂₅	Er₂O₃	30:70
Example 92	Nd_12.8Pr_2.0Fe_balCo_3.0B_5.6	Sn	Tb₄O₇	50:50
Example 93	Nd_14.2Fe_balCo_0.5B_5.6	Cr₂₁Fe₆₂Mo₁₇	Tb₄O₇	50:50
Example 94	Nd_15.0Dy_0.6Fe_balCo_0.1B_4.1	Bi₄₀Zr₆₀	Dy₂O₃	40:60
Example 95	Nd_15.2Fe_balCo_3.5B_6.4	Ni₅₀Bi₅₀	Yb₂O₃	50:50
Example 96	Nd_12.0Pr_3.0Fe_balCo_2.0B_6.1	Bi	Dy₂O₃	50:50

	TABLE 12

	Diffusion treatment

Temperature	Time	Br	Hcj	(BH)_max
(° C.)	(hr or min)	(T)	(kAm⁻¹)	(kJ/m³)

Example 58

790

3

h

1.413

2,087

387

Example 59

810

3 h 30 min

1.407

2,187

384

Example 60	850	8	h	1.414	1,980	388
Example 61	760	1	h	1.380	1,928	368

Example 62

820

2 h 30 min

1.423

2,042

394

Example 63	770	5	h	1.394	2,223	373
Example 64	820	4	h	1.402	1,861	383
Example 65	940	12	h	1.298	1,904	328
Example 66	870	8	h	1.415	1,930	389
Example 67	1,060	28	h	1.284	1,713	319
Example 68	380	15	min	1.358	1,512	353
Example 69	680	8	h	1.476	1,498	409
Example 70	820	8	h	1.417	1,820	390
Example 71	940	5	h	1.414	1,816	387
Example 72	1,020	10	h	1.426	1,896	393
Example 73	650	8	h	1.420	1,641	387
Example 74	600	10	h	1.406	1,689	379
Example 75	760	8	h	1.403	1,760	379
Example 76	840	5	h	1.355	1,940	351
Example 77	870	8	h	1.415	1,950	389
Example 78	850	7	h	1.420	1,816	390
Example 79	880	8	h	1.411	1,890	387
Example 80	1,000	10	h	1.358	1,896	355
Example 81	770	1	h	1.417	2,085	386
Example 82	760	4	h	1.404	1,530	380
Example 83	920	8	h	1.413	1,910	386
Example 84	630	13	h	1.446	1,780	401
Example 85	960	7	h	1.433	1,620	394
Example 86	920	15	h	1.413	1,940	385
Example 87	750	6	h	1.381	1,537	363
Example 88	920	5	h	1.369	1,338	355
Example 89	640	6	h	1.424	1,418	391
Example 90	880	40	min	1.414	2,040	383
Example 91	1,020	10	h	1.420	1,450	387
Example 92	730	5	h	1.408	1,820	383
Example 93	880	15	h	1.454	1,800	406
Example 94	510	20	h	1.346	1,430	343
Example 95	360	5	min	1.392	1,211	362
Example 96	420	15	min	1.382	1,510	358

Japanese Patent Application Nos. 2011-102787 and 2011-102789 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

What is claimed is:

1. A method for preparing a rare earth permanent magnet, comprising the steps of:

disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “2c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jH_kwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen, “i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99, 0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and

heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, for causing the elements R¹, R², and M¹in the powder mixture to diffuse to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains.

2. 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 magnet body.

3. The method of claim 1 wherein the disposing step includes dispersing the powder mixture in an organic solvent or water, immersing the sintered magnet body in the resulting slurry, taking up the sintered magnet body, and drying for thereby covering the surface of the sintered magnet body with the powder mixture.

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

5. A rare earth permanent magnet, which is prepared by disposing a powder mixture on a surface of a sintered magnet body having the composition R_aT¹ _bM_cB_dwherein R is at least one element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M 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, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixture comprising an alloy powder having the composition R¹ _iM¹ _jH_kwherein R¹is at least one element selected from rare earth elements inclusive of Y and Sc, M¹is at least one element selected from the group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen, “i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99, 0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% by volume of an intermetallic compound phase, and having an average particle size of up to 500 μm, and at least 10% by weight of an R²oxide wherein R²is at least one element selected from rare earth elements inclusive of Y and Sc, having an average particle size of up to 100 μm, and heat treating the sintered magnet body having the powder mixture disposed on its surface at a temperature lower than or equal to the sintering temperature of the sintered magnet body in vacuum or in an inert gas, wherein

the elements R¹, R²and M¹in the powder mixture are diffused to grain boundaries in the interior of the sintered magnet body and/or near grain boundaries within the sintered magnet body primary phase grains so that the coercive force of the rare earth permanent magnet is increased over the original sintered magnet body.