Method for producing rare-earth magnets, and rare-earth-compound application device

Abstract

When a slurry 41 obtained by dispersing a rare-earth-compound powder in a solvent is applied to sintered magnet bodies 1, and dried to remove the solvent in the slurry and cause the surfaces of the sintered magnet bodies to be coated with the powder, and the sintered magnet bodies coated with the powder are heat treated to cause the rare-earth element to be absorbed by the sintered magnet bodies, the sintered magnet bodies having had the slurry applied thereto are dried by being irradiated with near infrared radiation having a wavelength of 0.8-5 m, to remove the solvent in the slurry, and cause the surfaces of the sintered magnet bodies to be coated with the powder. As a result, the rare-earth-compound powder can be uniformly and efficiently applied to the surfaces of the sintered magnet bodies.

Claims

1. A method for producing a rare-earth sintered magnet, the method comprising: holding a plurality of sintered magnet bodies by a rotatable jig, the sintered magnet bodies composed of a R1-FeB composition (R1 is at least one selected from rare-earth elements including Y and Sc), wherein a bottom part of the rotatable jig is formed of a net formed of metallic wires which are coated with fluororesin: immersing the sintered magnet bodies held by the rotatable jig in a slurry obtained by dispersing a powder containing at least one selected from an oxide, a fluoride, an oxyfluoride, a hydroxide or a hydride of R2 (R2 is at least one selected from rare-earth elements including Y and Sc) in a solvent so that each of the sintered magnet bodies is coated with the slurry; drawing up the slurry-coated sintered magnet bodies held by the rotatable jig from the slurry to allow the slurry to drain from the net of the bottom of the rotatable jig and rotating the slurry-coated sintered magnet bodies together with the jig to remove surplus slurry present on a surface of each of the sintered magnet bodies by a centrifugal force, wherein the sintered magnet bodies held by the rotatable jig are disposed around a rotational axis of the rotatable jig and are inclined such that no part of any of outer surfaces constituting shapes of the sintered magnet bodes is orthogonal to the direction of the centrifugal force acting on each of the sintered magnet bodies; drying the slurry to remove the solvent in the slurry and coat a surface of the sintered magnet bodies with the powder; and heat treating the sintered magnet bodies coated with the powder to cause the R2 to be absorbed into the sintered magnet bodies, wherein in the step of drying, the sintered magnet bodies coated with the slurry held by the rotating jig is dried by irradiation with near infrared radiation of a wavelength of 0.8 to 5 um, while rotating the rotatable jig, to remove the solvent in the slurry while exhausting the solvent evaporated by irradiation with the near infrared radiation from the surroundings of the sintered magnet bodies to the outside of the drying area.

2. The method for producing the rare-earth sintered magnet according to claim 1, wherein the application process of immersing the sintered magnet bodies in the slurry, removing the surplus slurry and drying the slurry-coated sintered magnet bodies is repeated multiple times.

3. The method for producing the rare-earth sintered magnet according to claim 1, wherein the jig is rotated normally and reversely at a low speed of 5 to 20 rpm in a state in which the sintered magnet bodies are immersed in the slurry, thereby to apply the slurry to the sintered magnet bodies.

4. The method for producing the rare-earth sintered magnet according to claim 1, wherein the jig is drawn up from the slurry and rotated normally and reversely at a high speed of 170 to 550 rpm, thereby to remove the surplus slurry present on the surfaces of the sintered magnet bodies.

5. The method for producing the rare-earth sintered magnet according to claim 1, wherein the application of the slurry is conducted by disposing the sintered magnet bodies around a rotational axis of the jig, and holding the sintered magnet bodies in an inclined state such that no part of any of outer surfaces constituting shapes of the sintered magnet bodies is orthogonal to a direction of the centrifugal force acting on each of the sintered magnet bodes.

6. The method for producing the rare-earth sintered magnet according to claim 5, wherein the sintered magnet bodies are in a shape of a tetragonal plate or a tetragonal block, and each of the sintered magnet bodies is held by the jig in a state in which the sintered magnet body is erect with its thickness direction set horizontal and with its length direction or width direction inclined at an angle of more than 0 and less than 45 from the direction of the centrifugal force.

7. The method for producing the rare-earth sintered magnet according to claim 1, wherein the sintered magnet bodies coated with the powder are heat treated in vacuum or an inert gas at temperature of up to sintering temperature of the sintered magnet bodies.

8. The method for producing the rare-earth sintered magnet according to claim 1, wherein after the heat treatment, the sintered magnet bodies coated with the powder are subjected further to an ageing treatment at a temperature lower than temperature of the heat treating the sintered magnet bodies.

9. The method for producing the rare-earth magnet according to claim 1, wherein the jig is rotated at a speed of 5 to 20 rpm during the step of drying.

Description

BRIEF DESCRIPTION OF THE DIAGRAMS

(1) FIGS. 1 to 5 are illustrations of a rare-earth-compound powder application step in a production method of the present invention that is conducted using an application device according to an embodiment of the present invention, in which FIG. 1 is an illustration of a step of setting sintered magnet bodies on a jig and further setting the jig to rotating means.

(2) FIGS. 1 to 5 are illustrations of the rare-earth-compound powder application step in the production method of the present invention that is conducted using the application device according to the embodiment of the present invention, in which FIG. 2 is an illustration of a step of immersing the jig with the sintered magnet bodies held thereon in a slurry in a slurry tank.

(3) FIGS. 1 to 5 are illustrations of the rare-earth-compound powder application step in the production method of the present invention that is conducted using the application device according to the embodiment of the present invention, in which FIG. 3 is an illustration of a step of drawing up the sintered magnet bodies from the slurry and rotating the sintered magnet bodies to remove surplus slurry.

(4) FIGS. 1 to 5 are illustrations of the rare-earth-compound powder application step in the production method of the present invention that is conducted using the application device according to the embodiment of the present invention, in which FIG. 4 is an illustration of a step of drying the sintered magnet bodies to remove a solvent in the slurry and coat the sintered magnet bodies with a rare-earth-compound powder.

(5) FIGS. 1 to 5 are illustrations of the rare-earth-compound powder application step in the production method of the present invention that is conducted using the application device according to the embodiment of the present invention, in which FIG. 5 is an illustration of a step of detaching the jig from the rotating means and recovering the sintered magnet bodies with the rare-earth-compound powder applied to surfaces thereof.

(6) FIG. 6 is a schematic perspective view of the jig constituting the application device.

(7) FIG. 7 is a schematic perspective view of an arcuate rack constituting an object holding body of the jig.

(8) FIG. 8 is an illustration of the relation between the disposing direction of the sintered magnet bodies held by the jig and the direction of a centrifugal force.

(9) FIG. 9 is a schematic perspective view of an example of the sintered magnet body as an object to be treated in the present invention.

(10) FIG. 10 is an illustration of a measuring position for the rare-earth magnet in Examples.

EMBODIMENT FOR CARRYING OUT THE INVENTION

(11) As above-mentioned, the method of producing a rare-earth magnet of the present invention includes applying a slurry obtained by dispersing a powder containing at least one selected from an oxide, a fluoride, an oxyfluoride, a hydroxide or a hydride of R.sup.2 (R.sup.2 is at least one selected from rare-earth elements including Y and Sc) in a solvent to a sintered magnet body composed of a R.sup.1FeB composition (R.sup.1 is at least one selected from rare-earth elements including Y and Sc), drying the slurry to coat a surface of the sintered magnet body with the powder, and heat treating the powder-coated sintered magnet body to cause the R.sup.2 to be absorbed into the sintered magnet body and to thereby produce a rare-earth permanent magnet.

(12) As the above-mentioned R.sup.1FeB sintered magnet body, those which are obtained by a known method can be used. For example, the R.sup.1FeB sintered magnet body can be obtained by subjecting a mother alloy or alloys containing R.sup.1, Fe and B to milling, pulverization, molding, and sintering by usual methods. Note that as above-mentioned, R.sup.1 is at least one selected from rare-earth elements including Y and Sc, and specific examples thereof include Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu.

(13) In the present invention, the R.sup.1FeB sintered magnet body is formed into a predetermined shape by grinding as required, a powder containing at least one selected from an oxide, a fluoride, an oxyfluoride, a hydroxide and a hydride of R.sup.2 is applied to a surface of the R.sup.1FeB sintered magnet body, and the powder-coated sintered magnet body is heat treated to cause the at least one to be absorbed and diffused (boundary diffusion) into the sintered magnet body to obtain a rare-earth magnet.

(14) As above-mentioned, the R.sup.2 is at least one selected from rare-earth elements including Y and Sc, and, like the above-mentioned R.sup.1, specific examples of the R.sup.2 include Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu. In this case, though not particularly limited, it is preferable that one or more of the R.sup.2 contain Dy or Tb in a total concentration of at least 10 at %, more preferably at least 20 at %, and particularly at least 40 at %. It is more preferable, from the viewpoint of the object of the present invention, that Dy and/or Tb is thus contained in the R.sup.2 in a total concentration of at least 10 at %, and the total concentration of Nd and Pr in the R.sup.2 is lower than the total concentration of Nd and Pr in the R.sup.1.

(15) The application of the powder in the present invention is conducted by preparing a slurry containing the powder dispersed in a solvent, applying the slurry to the surface of the sintered magnet body and drying the slurry. In this case, the particle diameter of the powder is not particularly limited, but can be a particle size generally adopted for a rare-earth-compound powder for use in absorption and diffusion (boundary diffusion); specifically, an average particle diameter is preferably up to 100 m, more preferably up to 10 m. While the lower limit is not particularly restricted, it is preferably at least 1 nm. This average particle diameter can be obtained as mass average value D.sub.50 (namely, the particle diameter or median diameter at a cumulative mass of 50%) by use of a particle size distribution measuring apparatus based on a laser diffraction method, for example. Note that the solvent for dispersing the powder therein may be water or an organic solvent. The organic solvent is not particularly restricted, and examples thereof include ethanol, acetone, methanol, and isopropyl alcohol, among which ethanol is preferably used.

(16) The amount of the powder dispersed in the slurry is not particularly limited. In the present invention, for favorable and efficient coating with the powder, the dispersion amount in the slurry in terms of mass fraction is preferably at least 1%, particularly preferably at least 10%, and further preferably at least 20%. Note that too large a dispersion amount leads to an inconvenient situation such as a situation in which a uniform dispersion cannot be obtained, and, therefore, the upper limit of the mass fraction is preferably up to 70%, particularly preferably up to 60%, and further preferably up to 50%.

(17) In the present invention, when the slurry is applied to the sintered magnet body and dried to coat the surface of the sintered magnet body with the powder, the slurry is dried by irradiation with near infrared radiation of a wavelength of 0.8 to 5 m to remove the solvent in the slurry and form a coating film of the powder on the surface of the sintered magnet body.

(18) A heater for irradiation with such near infrared radiation may be any one that can generate near infrared radiation of the above-mentioned wavelength, and a commercialized infrared heater unit can be used as the heater. For instance, a Twin Tube transparent silica glass-made short-wavelength infrared heater unit (ZKB Series and ZKC Series) made by Heraeus K.K. can be used. As for drying conditions, it is sufficient to appropriately set a heater output, a heating time, and a cooling time according to the size and shape of the sintered magnet body, the number of sintered magnet bodies to be dried at a time, and the concentration of the slurry.

(19) Here, while the irradiation with near infrared radiation can heat an object extremely efficiently, it is impossible, when the irradiation is used for drying of a slurry, to carry away the evaporated portion. Therefore, it is preferable to remove the evaporated portion of the solvent from the surroundings of the sintered magnet bodies by use of appropriate exhaust means, whereby more efficient drying can be performed.

(20) The powder application step from the coating with the slurry to the drying of the slurry, in the present invention, can be carried out, for example, using an application device depicted in FIGS. 1 to 5.

(21) Specifically, FIGS. 1 to 5 are schematic views depicting a rare-earth-compound application device according to an embodiment of the present invention. The application device is for applying the above-mentioned rare-earth-compound powder to a sintered magnet body 1 in the shape of a tetragonal plate or a tetragonal block, as depicted in FIG. 9, by a method in which a plurality of the sintered magnet bodies 1 are held by a jig 2 in the state of being aligned in a circular pattern (FIG. 1), are immersed in the slurry 41 to apply the slurry 41 to each of the sintered magnet bodies 1 (FIG. 2), are drawn up from the slurry 41 and are rotated together with the jig 2 to remove the surplus slurry present on the surface of each of the sintered magnet bodies 1 by a centrifugal force (FIG. 3), and are dried by irradiation with near infrared radiation (FIG. 4), to coat the surfaces of the sintered magnet bodies 1 with the powder, after which the powder-coated sintered magnet bodies 1 are recovered from the jig 2 (FIG. 5).

(22) As depicted in FIG. 6, the above-mentioned jig 2 is composed of a basket 21 formed from metallic wire of stainless steel or the like, and a circular object holding body 22 disposed at a bottom portion of the basket 21. The basket 21 is a hollow cylindrical basket-shaped body in which a plurality (in the figure, five) of ring-shaped frames formed from metallic wire are connected concentrically, with metallic net of stainless steel being arranged over the range of a bottom portion to an intermediate portion in the height direction of a peripheral wall, exclusive of a predetermined range in the center of the bottom portion.

(23) The object holding body 22 has a plurality (in the figure, three) of arcuate racks 221 combined and disposed in a circular pattern at a bottom portion inside the basket 21. As depicted in FIG. 7, each of the racks 221 has two arcuately curved sheets 222 and 223 of stainless steel or the like which are disposed vertically overlappingly while spaced by a predetermined spacing and are interconnected by four props 225, with a lower end portion of each of the props 225 protruding downward from a lower surface of the lower-side sheet 223 to form a leg portion. The upper-stage sheet 222 and the intermediate-stage sheet 223 constituting the rack 221 are each formed with a plurality (in this figure, ten) of substantially elongated elliptic through-holes 226 and 227 which are aligned in a row and through which the sintered magnet bodies 1 can be passed. The through-holes 226 in the upper-stage sheet 222 and the through-holes 227 in the lower-stage sheet 223 are formed at vertically aligned positions, and a pair of the upper-stage and lower-stage through-holes 226 and 227 constitute a holding pocket 228 in which to hold the sintered magnet body 1. Besides, as depicted in FIG. 7, the sintered magnet body 1 inserted in the holding pocket 228 is supported by the holding pocket 228 in the state of being placed on the bottom wall of the basket 21, and is held to be erect with its thickness direction T (see FIG. 9) set horizontal.

(24) The through-holes 226 and 227 constituting the holding pocket 228 are each preferably formed so that only four corners of the sintered magnet body 1 inserted therein make contact with both end curved portions thereof, as depicted in FIG. 8. This ensures that the slurry 41 flows reliably into the gaps between the surfaces of the sintered magnet body 1 and the edges of the through-holes 226 and 227, so that the whole surface of the sintered magnet body 1 can be reliably coated with the slurry 41.

(25) As above-mentioned, a plurality (in the figure, three) of the racks 221 are disposed in a circular pattern and are placed on the metallic net at the bottom surface inside the basket 21 in a state in which each rack 221 is in contact with the metallic net at the circumferential wall surface of the basket 21, whereby the circular ring-shaped object holding body 22 is configured.

(26) The jig 2 is fixed to a chuck section 31 of rotating means 3 which will be described later, and is rotated about a rotational axis 231 (in this example, a rotational axis along the vertical direction). The object holding body 22 is in the state of being disposed in a circular form around the rotational axis 231, and the plurality of sintered magnet bodies 1 held in the holding pocket 228 of the object holding body 22 are in the state of being disposed in a circular pattern around the center of rotation by the rotational axis 231.

(27) The holding pocket 228 is formed in the substantially elongate elliptic shape, as above-mentioned. As depicted in FIG. 8, the holding pocket 228 is formed along a direction 233 inclined at a predetermined angle r relative to a direction 232 of the centrifugal force with the rotational axis 231 as a center. Each sintered magnet body 1 held in the holding pocket 228 is held in the state of being erect with its thickness direction T set horizontal and with its width direction W inclined at a predetermined angle r from the direction 232 of the centrifugal force. Note that while an example in which the sintered magnet body 1 is held to be erect with its length direction L (see FIG. 9) set vertical has been depicted in this example, the sintered magnet body 1 may be held to be erect with its width direction W (see FIG. 9) set vertical in some cases; in that case, the sintered magnet body 1 is held with its length direction L inclined at a predetermined angle r from the direction 232 of the centrifugal force.

(28) With such a setting that the sintered magnet body 1 is thus held in the state of being inclined at the predetermined angle r relative to the direction 232 of the centrifugal force, it is ensured that no surface of the sintered magnet body 1 being in the shape of a tetragonal plate or a tetragonal block is orthogonal to the direction 232 of the centrifugal force, and the centrifugal force is exerted on the surplus slurry present on the surfaces of the sintered magnet body 1 in a state in which all the surfaces of the sintered magnet body 1 are inclined at the predetermined angle r relative to the centrifugal force, without facing perpendicularly to the centrifugal force, so that the surplus slurry on the surfaces can be removed without stagnation and that uniform coating with the slurry can be achieved. The inclination angle r is appropriately set according to the shape and size of the sintered magnet body 1 and rotational speed, and is not particularly limited. However the inclination angle r is preferably set appropriately in the range of 0 to less than 45, more preferably in the range of 5 to 40, and more preferably in the range of 10 to 30.

(29) Here, while the sintered magnet body 1 in the shape of a tetragonal plate or a tetragonal block with a thickness T, a length L and a width W which are different as depicted in FIG. 9 is used in this example, such a sintered magnet body 1 is not restrictive, and two or three of the dimensions including the thickness T, the width W and the length L may be equal or substantially equal. In the case where two of the dimensions are equal or substantially equal, the direction of the smaller dimension may be the thickness direction T, and either of the other directions may be the width W or the length L. In the case where three of the dimensions are equal or substantially equal, the thickness T, the width W or the length L may be in any of the directions. Further, the sintered magnet body 1 may be in other shape than the shape of the tetragonal plate or the tetragonal block; for example, various shapes such as a semicircular shape and a roofing tile-like shape can be adopted. In that case, it is sufficient that the sintered magnet body 1 is disposed in the state of being inclined at an appropriate angle such that no part of any of the outer surfaces constituting the shape of the sintered magnet body 1 is orthogonal to the direction 232 of the centrifugal force.

(30) Note that since the basket 21 and the object holding body 22 are immersed in the slurry 41 together with the sintered magnet bodies 1 and coated with the slurry, if the metal such as stainless steel forming them has not been subjected to any treatment, the rare-earth-compound powder may be deposited on them to increase the wire diameter of the net or frames of the basket 21, or to change the dimensions of the holding pockets 228, possibly causing inconveniences in coating the sintered magnet bodies 1 with the slurry. Therefore, though not particularly limited, it is preferable to apply coating to the metal such as stainless steel forming the basket 21 and the object holding body 22 so that the slurry is hardly adhered to them. The kind of the coating is not particularly restricted, and coating with a fluororesin such as polytetrafluoroethylene (Teflon (registered trademark)) is preferred from the viewpoint of excellent abrasion resistance and water repellency.

(31) Numeral 3 in FIGS. 1 to 5 denotes the rotating means having the chuck section 31 for holding the jig 2, and the jig 2 can be rotated normally and reversely at a controllable speed by the rotating means 3. Note that in this example, the jig 2 is rotated about the rotational axis 231 set along the vertical direction.

(32) Numeral 4 in FIGS. 1 to 5 denotes a slurry tank, the slurry 41 is contained in the slurry tank 4, and the sintered magnet bodies 1 held by the jig 2 is immersed in the slurry 41, whereby the slurry 41 is applied to the surfaces of the sintered magnet bodies 1. The slurry tank 4 is held on a lift 42 (lifting means), and is vertically moved by the lift 42 (lifting means).

(33) Numerals 51 in FIGS. 1 to 5 denote two heaters which are disposed at positions deviated by 180 from each other, in the surroundings of the jig 2 held by the chuck section 31 of the rotating means 3. The sintered magnet bodies 1 are dried by the heaters 51 to remove the solvent in the slurry applied to the sintered magnet bodies 1. On the upper side of the heaters 51 are disposed exhaust hoods 52, by which the evaporated solvent from the slurry is removed from the surroundings of the sintered magnet bodies 1, to achieve effective drying. The heaters 51 and the exhaust hoods 52 constitute drying means 5.

(34) Here, the heaters 51 are for drying the sintered magnet bodies 1 held in the jig 2 by irradiating the sintered magnet bodies 1 with near infrared radiation of a wavelength of 0.8 to 5 m. In the device of this example, three Twin Tube transparent silica glass-made short-wavelength infrared heater units (ZKB 1500/200G, with cooling fan, output 1,500 W, heating length 200 mm) made by Heraeus K.K. are incorporated in each of the heaters 51.

(35) This heater for irradiation with infrared radiation of a short wavelength of 0.8 to 5 m is fast in build up, can start effective heating in one to two seconds, can heat up to 100 C. in ten seconds, and can complete drying in an extremely short time. Further, the heater can be configured inexpensively, and is advantageous in regard to power consumption, as compared to the case of performing induction heating. In addition, according to the radiational heating by irradiation with the near infrared radiation, the near infrared radiation is transmitted and absorbed into the inside of the slurry coating film, whereby heating and drying can be achieved. Therefore, generation of cracking due to drying being started from the outside of the coating film, as in the case of drying by blowing hot air from the exterior, for example, can be prevented as securely as possible, and a uniform and dense coating film of powder can be formed. Further, the heater tube for generating the near infrared radiation of a short wavelength is comparatively small in size, so that the application device can be made smaller in size.

(36) At the time of applying a powder containing at least one selected from an oxide, a fluoride, an oxyfluoride, a hydroxide or a hydride of R.sup.2 (R.sup.2 is at least one selected from rare-earth elements including Y and Sc) (rare-earth-compound powder) to the surfaces of the sintered magnet bodies 1 by use of this application device, as depicted in FIG. 1, first, the slurry 41 obtained by dissolving the powder in a solvent is contained in the slurry tank 4, the slurry tank 4 is filled with the slurry 41 up to an intermediate portion in the height direction of the slurry tank 4, and, simultaneously, a predetermined space where the slurry 41 is absent is secured at an upper portion inside the slurry tank 4.

(37) On the other hand, as depicted in FIG. 1, the sintered magnet body 1 is inserted and held in each holding pocket 228 provided in the object holding body 22 (see FIG. 6) in the jig 2, whereby the plurality of sintered magnet bodies 1 are disposed in a circular pattern around the rotational axis 231 and are held to be erect with the thickness direction T thereof set horizontal and with the width direction W (233) thereof inclined at the predetermined angle r from the direction 232 of the centrifugal force, as depicted in FIGS. 6 to 8. The jig 2 is mounted to the chuck section 31 of the rotating means 3, and is set on the upper side of the slurry tank 4.

(38) In this condition, the slurry tank 4 is lifted up to an uppermost stage by the lift (lifting means) 42, whereby the sintered magnet bodies 1 held in the jig 2 are immersed in the slurry 41 in the slurry tank 4, as depicted in FIG. 2, and the slurry 41 is applied to the sintered magnet bodies 1. In this instance, though not particularly limited, the jig 2 may be rotated normally and reversely at a low speed of approximately 5 to 20 rpm by the rotating means 3, whereby the slurry 41 can be favorably distributed and applied to the whole surface of each of the sintered magnet bodies 1 held in the holding pockets 228 of the object holding body 22.

(39) Next, as depicted in FIG. 3, the slurry tank 4 is lowered to an intermediate stage by the lift (lifting means) 42, whereby the sintered magnet bodies 1 are drawn up from the slurry 41, and are held at an upper portion inside the slurry tank 4. In this condition, the jig 2 is rotated normally and reversely at a high speed by the rotating means 3, whereby surplus slurry present on the surfaces of the sintered magnet bodies 1 are removed by the centrifugal force. The surplus slurry thus removed is returned to a slurry reservoir in the slurry tank 4.

(40) In this instance, the rotational speed of the jig 2 is appropriately set at such a rotational speed as to enable favorable removal of residual slurry drops, according to the concentration of the slurry 41, the shape and size of the sintered magnet body 1, and the number of the sintered magnet bodies 1, and is not particularly limited. Normally, the rotational speed is set at a rotational speed of 170 to 550 rpm such that a centrifugal force of 5 to 50 G is exerted on each of the sintered magnet bodies 1. By such a setting, collection of the liquid on the surfaces of the sintered magnet bodies 1 can be avoided, and a coating amount can be made uniform.

(41) After the removal of the surplus slurry is conducted, the slurry tank 4 is further lowered to a lowermost position by the lift (lifting means) 42, as depicted in FIG. 4, whereby the jig 2 is taken out completely upward from the slurry tank 4. In this condition, the sintered magnet bodies 1 are heated and dried by irradiation with near infrared radiation of a wavelength of 0.8 to 5 m by the drying means 5, to remove the solvent in the slurry applied to the surfaces of the sintered magnet bodies 1 and to cause the powder to be applied to the surfaces of the sintered magnet bodies 1, thereby forming coating films of the powder on the surfaces. In this instance, as aforementioned, the heaters 51 of the drying means 5 swiftly build up in one to two seconds to speedily start effective heating, and can heat up to at least 100 C. in a few seconds and can complete drying in an extremely short time. In addition, the near infrared radiation is transmitted and absorbed into the inside of the slurry coating films, whereby heating and drying is conducted, and uniform coating films of powder can be formed without causing cracking. Note that at the time of the drying, the drying may be conducted while rotating the jig 2 (the sintered magnet bodies 1) at a low speed (approximately 5 to 20 rpm) by the rotating means 3, and the rotation may be conducted either in one direction or in both normal and reverse directions.

(42) After the drying, the jig 2 is detached from the rotating means 3, as depicted in FIG. 5, and the sintered magnet bodies 1 coated with the powder are recovered from the jig 2. Then, in the present invention, the sintered magnet bodies are heat treated to cause the R.sup.2 in the powder (the rare-earth compound) to be absorbed and diffused into the sintered magnet bodies, thereby obtaining rare-earth permanent magnets. Note that the heat treatment for causing the rare-earth element represented by the R.sup.2 to be absorbed and diffused may be performed according to a known method, and, if necessary, a known post-treatment such as an aging treatment in appropriate conditions or further grinding to a shape for practical use can be conducted after the heat treatment.

(43) Here, the rare-earth-compound applying operation using the application device may be repeated multiple times to apply the rare-earth-compound powder repeatedly, whereby thicker coating films can be obtained and the uniformity of the coating films can be enhanced. The repetition of the applying operation may be conducted by repeating plural times the powder applying process from the slurry application to drying as depicted in FIGS. 2 to 4. As a result, it is possible, by repeated thin coating, to obtain a coating film with a desired thickness and to favorably control the coating amount of powder. In addition, by the repeated thin coating, it is possible to shorten the drying time and to enhance time efficiency.

(44) In this way, according to the production method of the present invention in which application of a rare-earth-compound powder is conducted using the application device, drying is performed by irradiation with infrared radiation (near infrared radiation) of a wavelength of 0.8 to 5 m, so that the drying can be completed in an extremely short time, and, further, an inexpensive configuration can be adopted and an advantage in regard to power consumption can be obtained as compared to the case of induction heating. Therefore, the powder can be applied through inexpensive and efficient drying of the slurry. In addition, since the near infrared radiation is transmitted and absorbed into the inside of the slurry coating films and heating and drying can be thereby conducted, generation of cracking due to drying being started from the outside of coating films, as in the case of drying by blowing hot air from the exterior, for example, can be prevented as securely as possible, and uniform and dense coating films of powder can be formed. Further, since the heater tube for generating the near infrared radiation of a short wavelength is comparatively small in size, the dryer and the application device can be made smaller in size, and rare-earth magnets can be produced efficiently with small-scale equipment. Therefore, the coating amount can be controlled accurately, uniform and dense coating films of the rare-earth-compound powder can be efficiently formed on the surfaces of the sintered magnet bodies, and the application device for carrying out the application process can be made smaller in size.

(45) Note that the application device of the present invention is not limited to the device depicted in FIGS. 1 to 8. For example, the lifting means may lift the jig 2 up and down together with the rotating means 3, instead of lifting the slurry tank 4 up and down. Further, the shape and holding mode (holding angle) of the sintered magnet bodies 1 and other configurations of the jig 2, the rotating means 3, and the drying means 5 may be appropriately modified without departing from the gist of the present invention.

EXAMPLE

(46) A more specific mode of the present invention will be described in detail below in terms of Examples, but the invention is not to be limited to Examples.

Example 1

(47) An alloy in thin plate form was prepared by a strip casting technique, specifically by weighing Nd, Al, Fe and Cu metals having a purity of at least 99 wt %, Si having a purity of 99.99 wt %, and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt on a copper single roll. The alloy consisted of 14.5 at % of Nd, 0.2 at % of Cu, 6.2 at % of B, 1.0 at % of Al, 1.0 at % of Si, and the balance of Fe. Hydrogen decrepitation was carried out by exposing the alloy to 0.11 MPa of hydrogen at room temperature to occlude hydrogen and then heating at 500 C. for partial dehydriding while evacuating to vacuum. The decrepitated alloy was cooled and sieved, yielding a coarse powder under 50 mesh.

(48) The coarse powder was finely pulverized, by a jet mill using a high-pressure nitrogen gas, into a powder with a weight median particle diameter of 5 m. The mixed fine powder thus obtained was formed under a pressure of approximately 1 ton/cm.sup.2 into a block shape, while being oriented in a magnetic field of 15 kOe in a nitrogen atmosphere. The formed body was put into a sintering furnace in an Ar atmosphere, and sintered at 1,060 C. for two hours, to obtain a magnet block. The magnet block was subjected to grinding of the whole surfaces by use of a diamond cutter, followed by cleaning sequentially with an alkaline solution, pure water, nitric acid and pure water in this order and drying, to obtain a block-shaped magnet body measuring 20 mm (W)45 mm (L)5 mm (T: direction of giving magnetic anisotropy) similar to the one depicted in FIG. 9.

(49) Next, a powder of dysprosium fluoride was mixed with water at a mass fraction of 40%, and the powder of dysprosium fluoride was well dispersed to prepare a slurry. The slurry was applied to the magnet bodies by use of the application device depicted in FIGS. 1 to 8, and dried to cause the dysprosium fluoride powder to be applied to the magnet bodies. In this case, the inclination angle r depicted in FIG. 8 was set at 30. This applying operation was repeated five times to form coating films of the dysprosium fluoride powder on the surfaces of the magnet bodies. Note that the applying conditions were set as follows.

(50) Applying Conditions

(51) Applying time in slurry: three seconds (without rotation)

(52) Rotating condition at the time of removal of surplus slurry: normal rotation at 400 rpm for ten seconds, reverse rotation at 400 rpm for ten seconds; 20 seconds in total

(53) Drying: heating with near infrared radiation for seven seconds while rotating in one direction slowly at a rotational speed of 10 rpm

(54) After the formation of the coating films of the dysprosium fluoride powder, the coating amount (g/mm.sup.2) was measured for a central portion and nine end portions of the magnet body as depicted in FIG. 10 by use of an X-ray fluorescent analysis thickness meter. The ratios of coating amount per unit area when the coating amount at which a coercivity increasing effect reached a peak was taken as 1.00 are set forth in Table 1.

(55) The magnet body formed on its surfaces with the thin film of the dysprosium fluoride powder was heat treated at 900 C. in an Ar atmosphere for five hours, thereby performing an absorption treatment, and was further subjected to an ageing treatment at 500 C. for one hour, followed by rapid cooling, to obtain a rare-earth magnet. Magnet bodies measuring 2 mm2 mm2 mm were cut out from the central portion and the nine end portions of the magnet as depicted in FIG. 10, and the magnet bodies were each subjected to measurement of coercivity, determine an increase in coercivity. The results are set forth in Table 2.

Example 2

(56) In the same manner as in Example 1, block-shaped magnet bodies measuring 20 mm45 mm5 mm (the direction of giving magnetic anisotropy) were prepared. In addition, dysprosium fluoride having an average powder particle diameter of 0.2 m was mixed with ethanol in a mass fraction of 40%, and well dispersed to prepare a slurry, then coating films of the dysprosium fluoride powder were formed in the same manner as in Example 1, and measurement of coating amount (g/mm.sup.2) was conducted in the same manner as above. The ratios of coating amount per unit area when the coating amount at which the coercivity increasing effect reached a peak was taken as 1.00 are set forth in Table 1.

(57) In addition, in the same manner as in Example 1, a heat treatment was conducted to perform an absorption treatment, and an ageing treatment was conducted, followed by rapid cooling, to obtain rare-earth magnets. In the same manner as in Example 1, magnet bodies were cut out, and were each subjected to measurement of coercivity, to determine an increase in coercivity. The results are set forth in Table 2.

(58) TABLE-US-00001 TABLE 1 Ratios of coating amount on measurement point basis 1 2 3 4 5 6 7 8 9 Exam- 1.03 1.01 1.00 1.05 1.05 1.03 1.04 1.04 1.04 ple 1 Exam- 1.00 1.01 0.98 0.99 0.99 1.05 1.01 1.02 1.01 ple 2

(59) TABLE-US-00002 TABLE 2 Increase in coercivity (unit: kA/m) 1 2 3 4 5 6 7 8 9 Exam- 480 475 460 480 480 470 480 480 480 ple 1 Exam- 470 470 460 460 470 470 475 470 475 ple 2

Examples 3 and 4

(60) The formation of coating films of dysprosium fluoride on sintered magnet bodies and the measurement of coating amount (g/mm.sup.2) were conducted in the same manner as in Example 1, except that the inclination angle r depicted in FIG. 8 was changed to 15 (Example 3) or 30 (Example 4). The ratios of coating amount per unit area when the coating amount at which the coercivity increasing effect reached a peak was taken as 1.00 are set forth in Table 3.

(61) TABLE-US-00003 TABLE 3 Ratios of coating amount on measurement point basis 1 2 3 4 5 6 7 8 9 Exam- 1.04 1.02 1.06 1.01 1.02 1.02 1.02 1.03 1.03 ple 3 Exam- 1.09 1.05 1.08 1.02 1.04 1.03 1.03 1.04 1.04 ple 4

(62) As seen from Tables 1 to 3, uniform coating films of powder is formed by the drying treatment by heating for only seven seconds. Besides, as seen from Table 2, coercivity can be uniformly increased, by the absorption treatment by heating the powder-coated sintered magnet bodies.

REFERENCE SIGNS LIST

(63) 1 sintered magnet body 2 jig 21 basket 22 object holding body 221 rack 222 upper-stage sheet 223 lower-stage sheet 225 prop 226, 227 through-hole 228 holding pocket 231 rotational axis (rotational center) 232 direction of centrifugal force 233 formation direction of holding pockets (width direction of sintered magnet body) 3 rotating means 31 chuck section 4 slurry tank 41 slurry 42 lift (lifting means) 5 drying means 51 heater 52 exhaust hood r inclination angle T thickness direction L length direction W width direction