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

Abstract

When a slurry s obtained by dispersing a rare-earth-compound powder in a solvent is applied to sintered magnet bodies m, 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 m are warmed or heated before the slurry s is applied. As a result, the rare-earth-compound powder can be efficiently and uniformly applied to the surfaces of the sintered magnet bodies.

Claims

1. A method for producing rare earth magnets, the method comprising the steps of: providing sintered magnet body of an R.sup.1—Fe—B composition, where R.sup.1 is one or more elements selected from the group consisting of Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu, and a slurry comprising a solvent and a powder of one or more compounds selected from oxides, fluorides, oxyfluorides, hydroxides and hydrides of R.sup.2, where R.sup.2 is one or more elements selected from the group consisting of Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu, dispersed in the solvent; warming or heating the sintered magnet body by a near infrared ray of 0.8 to 5 μm wavelength, such that the sintered magnet body is warmed or heated up to a temperature lower by 20° C. than a boiling point of the solvent in the slurry, before applying the slurry to the sintered magnet body; after the warming or heating the sintered magnet body, applying the slurry to surfaces of the sintered magnet body so that the slurry directly covers the surfaces of the sintered magnet body; drying the resulting sintered magnet body by a near infrared ray of 0.8 to 5 μm wavelength to remove the solvent from the slurry and to deposit the powder from the slurry on the surfaces of the sintered magnet body, such that the sintered magnet body, the surfaces of which are coated by the powder, are obtained; and then subjecting the obtained sintered magnet body covered with the powder to heat treatment to cause absorption of R.sup.2 into the sintered magnet body, wherein the slurry is applied on the surfaces of the sintered magnet body by supplying the slurry to a slurry feed tray, dipping a coating roll in the slurry on the slurry feed tray to impregnate the coating roll with the slurry, and moving horizontally the coating roll toward the sintered magnet body and then moving down the coating roll to the sintered magnet body on a transport conveyor, so that the slurry is applied to the sintered magnet body by the coating roll, and the method further comprises a step of transporting the sintered magnet body on the transport conveyor, wherein the steps of warming or heating the sintered magnet body, applying the slurry to the sintered magnet body, and drying the resulting sintered magnet body are conducted while the sintered magnet body is held on the transport conveyor.

2. The production method of claim 1, wherein the solvent in the slurry is water, and in the step of warming or heating, the sintered magnet body is warmed or heated to 40° C. to 80° C.

3. The production method of claim 1, wherein the steps of warming or heating the sintered magnet body, applying the slurry to the sintered magnet body, and drying the resulting sintered magnet body are repeated a plurality of times to conduct recoating.

4. The production method of claim 1, wherein the heat treatment is applied to the sintered magnet body, which has been coated with the powder, in vacuo or in an inert gas at a temperature up to a sintering temperature for the sintered magnet body.

5. The production method of claim 1, further comprising: applying, after the heat treatment, aging treatment at a temperature lower than a temperature of the heat treatment.

6. The production method of claim 1, wherein the solvent in the slurry is at least one solvent selected from the group consisting of water, ethanol, acetone, methanol, and isopropanol.

7. The production method of claim 1, wherein the solvent vaporized from the slurry by the near infrared ray in the step of drying is exhausted by exhaust means from around the sintered magnet body on the transport conveyor.

8. The production method of claim 1, further comprising the steps of: blowing a laminar air flow to the sintered magnet body on the transport conveyor to remove dust from the surfaces of the sintered magnet body and collecting the dust, before applying the slurry to the sintered magnet body.

9. The production method of claim 1, further comprising the steps of: blowing a laminar air flow by an air knife to the sintered magnet body on the transport conveyor to remove dust from the surfaces of the sintered magnet body and collecting the dust by a dust collection duct, before applying the slurry to the sintered magnet body, wherein the step of warming or heating the sintered magnet body is conducted by a preheater to generate the near infrared ray of 0.8 to 5 μm wavelength, and the air knife and the dust collection duct are disposed interposing the preheater, so that the dust blown by the air knife is collated by the dust collection duct.

10. The production method of claim 1, further comprising the steps of: allowing the slurry to overflow from a slurry overflow tank to the slurry feed tray and flow from the slurry feed tray to a slurry receiving tank, and returning the slurry from the slurry receiving tank to the slurry overflow tank, so that the slurry recycles from the slurry overflow tank to the slurry receiving tank via the slurry feed tray.

Description

BRIEF DESCRIPTION OF THE DIAGRAMS

(1) FIG. 1 is a schematic plan view illustrating a coating device according to an embodiment of the present invention for coating application of one or more rare earth compounds.

(2) FIG. 2 is a schematic side view illustrating the coating device.

(3) FIG. 3 is a schematic view illustrating slurry coating means that constitutes the coating device.

(4) FIG. 4 is an explanatory diagram representing measurement positions on a rare earth magnet in an Example.

EMBODIMENT FOR CARRYING OUT THE INVENTION

(5) As described above, the production method of the present invention for rare earth magnets produces the rare earth permanent magnets by coating sintered magnet bodies of an R.sup.1—Fe—B composition (R.sup.1 is one or more elements selected from rare earth elements including Y and Sc) with a slurry, in which a powder of one or more compounds selected from oxides, fluorides, oxyfluorides, hydroxides and hydrides of R.sup.2 (R.sup.2 is one or more elements selected from rare earth elements including Y and Sc) is dispersed in a solvent, drying the resulting sintered magnet bodies to deposit the powder on surfaces of the sintered magnet bodies, and subjecting the resulting sintered magnet bodies to heat treatment to cause absorption of R.sup.2 into the sintered magnet bodies.

(6) As the R.sup.1—Fe—B sintered magnet bodies, those which have been obtained by a known method can be used. For example, the R.sup.1—Fe—B sintered magnet bodies can be obtained by subjecting a mother alloy, which contains R.sup.1, Fe and B, to coarse milling, fine pulverizing, forming and sintering in accordance with a usual method. It is to be noted that R.sup.1 is, as described above, one or more elements selected from rare earth elements including Y and Sc, specifically one or more rare earth elements selected from Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu can be mentioned.

(7) In the present invention, the R.sup.1—Fe—B sintered magnet bodies are formed into a predetermined shape by grinding as needed, are coated at the surfaces thereof with the powder of the one or more of the oxides, fluorides, oxyfluorides, hydroxides and hydrides of R.sup.2, and are then subjected to heat treatment to cause absorptive diffusion (grain boundary diffusion) of R.sup.2, whereby rare earth magnets are obtained.

(8) R.sup.2 is, as described above, one or more elements selected from rare earth elements including Y and Sc, and similar to R.sup.1, one or more rare earth elements selected from Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu can be exemplified. Here, R.sup.2 may include, but is not specifically limited to include, preferably at least 10 at %, more preferably at least 20 at %, notably at least 40 at % of Dy or Tb in total as the one or more rare earth elements. It is preferred from the objects of the present invention that at least 10 at % of Dy and/or Tb is included in R.sup.2 as described above and the total concentration of Nd and Pr in R.sup.2 is lower than the total concentration of Nd and Pr in R.sup.1.

(9) In the present invention, the coating of the powder is conducted by preparing a slurry with the powder dispersed in a solvent, coating surfaces of sintered magnet bodies with the slurry, and drying the resulting sintered magnet bodies. Here, the powder is not limited to any particular particle size, and may have a particle size that is common as a powder of one or more rare earth compounds for use in absorptive diffusion (grain boundary diffusion). Specifically, the average particle size may be preferably up to 100 μm, with up to 10 μm being more preferred. No particular limitation is imposed on its lower limit although at least 1 nm is preferred. This average particle size can be determined as a mass average particle size D.sub.50 (specifically, a particle size or median size at 50% cumulative mass), for example, by using a particle size distribution analyzer that relies upon laser diffractometry. The solvent in which the powder is to be dispersed may be water or an organic solvent. As the organic solvent, no particular limitation is imposed, and ethanol, acetone, methanol, isopropanol can be exemplified. Among these, ethanol is suitably used.

(10) No particular limitation is imposed on the amount of the powder dispersed in the slurry. In the present invention, however, it is preferred to prepare a slurry with the amount of the dispersed powder being set at a mass fraction of at least 1%, notably at least 10%, specifically at least 20% for good and efficient deposition of the powder. As an unduly great dispersed amount causes inconvenience such as unavailability of a uniform dispersion, the upper limit may be set at a mass fraction of preferably up to 70%, notably up to 60%, specifically up to 50%.

(11) As the method that coats the slurry on the sintered magnet bodies, no particular limitation is imposed, and a suitable method can be chosen as desired. For example, a dipping method that dips sintered magnet bodies in a slurry, a spray method that sprays and coats a slurry, or a roll coating method that coats a slurry by rolling a coating roll, which has been impregnated with the slurry, on surfaces of sintered magnet bodies can be appropriately adopted. Among these, the roll coating method can easily conduct localized coating compared with the dipping method or spraying method, so that the roll coating method can be appropriately adopted if a part or parts where increased coercivity is required is/are localized. According to the roll coating method, uniform coating of a slurry can be locally conducted only to each necessary location.

(12) In the present invention, the sintered magnet bodies are warmed beforehand by warming or heating the sintered magnet bodies to a predetermined temperature before the slurry coating as mentioned above. Here, the warming or heating temperature for the sintered magnet bodies may normally be, but is not particularly limited to, a temperature lower than the boiling temperature of the solvent used to prepare the slurry, and preferably, may be set at up to a temperature lower by 20° C. than the boiling point of the solvent. If a slurry is prepared using water as a solvent, for example, it is preferred to warm or heat the sintered magnet bodies to a temperature of up to 80° C. It is to be noted that there is no particular lower limit to the warming or heating temperature. If warmed or heated to at least room temperature, the above-mentioned advantageous effects of the present invention can be obtained, but the degrees of these advantageous effects vary depending on the kind of the solvent in the slurry. Under an environment of room temperature ranging of 20° C. to 25° C., for example, the use of water as a solvent can obtain the advantageous effects to a significant level if warmed to 30° C., and to a very substantial level if warmed or heated to at least 40° C. Although not particularly limited, warming or heating to 40° C. to 80° C. is preferred if water is used as a solvent.

(13) The present invention forms coatings of the powder on the surfaces of sintered magnet bodies by conducting the above-mentioned slurry coating on the sintered magnet bodies which have been warmed or heated beforehand as described above, and drying the resulting sintered magnet bodies under heat to remove the solvent from the slurry. Here, although not particularly limited, it is preferred to conduct the warming or heating before the slurry coating and the drying under heat after the slurry coating by radiating infrared rays, especially by radiating near infrared rays of 0.8 to 5 μm wavelength.

(14) As a heater that radiates such near infrared rays, any heater can be used insofar as it can emit near infrared rays of the above-described wavelength, and a commercially-available, infrared heater unit can be used. For example, a twin tube short-wavelength infrared heater unit made of transparent quartz glass (ZKB Series or ZKC Series) available from Heraeus Noblelight GmbH or the like can be used. As warming or heating conditions and drying conditions, the output of the heater, the heating time, the cooling time and the like can be suitably set according to the size and shape of the sintered magnet bodies, the concentration of the slurry, the room temperature, and so on.

(15) Radiation of near infrared rays can heat an object very efficiently. If near infrared rays are used to dry a slurry, however, vapor cannot be carried away. It is, therefore, preferred to remove solvent vapor from around the sintered magnet bodies by suitable exhaust means or the like, whereby more efficient drying can be conducted.

(16) The powder coating steps of warming or heating the sintered magnet bodies beforehand, coating the sintered magnet bodies with the slurry, and drying the resulting sintered magnet bodies can be conducted using, for example, a coating device illustrated in FIGS. 1 to 3.

(17) Specifically, FIGS. 1 to 3 are schematic views illustrating the coating device according to an embodiment of the present invention for coating application of the one or more rare earth compounds. This coating device applies the above-described slurry onto only one sides of rectangular block, sintered magnet bodies by roll coating. In these figures, numeral 1 designates a transport conveyor, which transports the above-described sintered magnet bodies m placed thereon. The transport conveyor is intermittently driven by an unillustrated drive source so that the sintered magnet bodies m placed on an upper surface thereof are intermittently and horizontally transported. The sintered magnet bodies m are fed to an upstream-side end portion (the right-side end portion in FIGS. 1 and 2) of the transport conveyor 1, and are transported. In the course of this transport, the sintered magnet bodies are warmed or heated, coated with the slurry, and dried to coat the powder of the rare earth compound or compounds. The sintered magnet bodies m coated with the powder are then collected from a downstream-side end portion (the right-side end portion in FIGS. 1 and 2) of the transport conveyor 1.

(18) In the figures, numeral 2 indicates slurry coating means, which exists at an intermediate part of the transport conveyor 1 as viewed in the direction of the transport and coats the slurry onto upper sides of the sintered magnet bodies m placed on the transport conveyor 1. The slurry coating means 2 is provided with a coating roll 21, and a slurry feed unit 22 that impregnates the coating roll 21 with the slurry as needed.

(19) The coating roll 21 is suspended from a horizontal shaft 211 and a vertical shaft 212, and as indicated by arrows in the figures, is movable in a horizontal direction and a vertical direction above the transport conveyor 1 at the intermediate part thereof as viewed in the direction of the transport.

(20) The slurry feed unit 22 includes a slurry overflow tank 221 and a slurry receiving tank 222 connected each other via a shallow slurry feed tray 223, exists at a position where the coating roll 21 is disposed, and is disposed close to one side of the transport conveyor 1. The plane of a top opening of the slurry overflow tank 221 is arranged at a position higher than the plane of a top opening of the slurry receiving tank 222. A slurry s which has overflowed from the slurry overflow tank 221 flows into the slurry receiving tank 222 via the slurry feed tray 223, and the slurry s is returned from the slurry receiving tank 222 to the slurry overflow tank 221 by a pump 224 and a return pipe 225. The slurry feed unit 22 is, therefore, configured to circulate the slurry s. At this time, a slurry pool that is slowly flowing in a laminar form is formed in the slurry feed tray 223.

(21) Through horizontally and vertically movements of the coating roll 21, the roll portion is dipped in the slurry feed tray 223 to impregnate the coating roll 21 with the slurry s. The coating roll 21 then moves again horizontally and vertically to return to above the transport conveyor 1, and coats some of the sintered magnet bodies m on the transport conveyor 1 with the slurry by roll coating. Numeral 23 in the figures designates an ultrasonic cleaner, which exists at the position where the coating roll 21 is disposed and is disposed close to the other side of the transport conveyor 1. The coating roll 21 is cleaned by the ultrasonic cleaner as needed, thereby avoiding uneven slurry coating that would otherwise occur by deposition of the powder. This cleaning of the roll is generally conducted during a pause of the coating operation.

(22) No particular limitation is imposed on the coating roll 21, and the coating roll 21 can be chosen from known rolls such as so-called coating rolls with various bristles, hair, wire or the like planted thereon, sponge rolls, rubber rolls, resin rolls and metal rolls. In this embodiment, a sponge roll is adopted for its readily impregnation with the slurry and its easy periodical cleaning. The width of the roll may be set as desired according to the size and shape of the sintered magnet bodies m. To further ensure uniform slurry coating, the width of the roll may be set preferably at 10 to 300 mm, more preferably 30 to 100 mm.

(23) In the figures, numeral 3 designates preheater means, which exists more upstream than the slurry coating means 2 as viewed in the direction of the transport and is disposed over the transport conveyor 1. The preheater means 3 radiates infrared rays to the sintered magnet bodies m on the transport conveyor 1 by an infrared heater 31 to warm or heat the sintered magnet bodies m to the above-mentioned predetermined temperature.

(24) In a downstream-side proximity of the preheater means 3, an air knife 41 is disposed, and in a downstream-side proximity of the preheater means 3, a dust collecting duct 42 is disposed. The air knife 41 blows a laminar air flow against the sintered magnet bodies m transported under the preheater means 3, and removes dust and the like stuck on surfaces of the sintered magnet bodies m. The dust collecting duct 42 draws an air flow containing the dust and the like so removed, and remove them from the upper surface of the transport conveyor 1. With these air knife 41 and dust collecting duct 42, cleaning means 4 that cleans the surfaces of the sintered magnet bodies m is configured.

(25) In the figures, numeral 5 indicates dryer means, which exists on a side downstream of the slurry coating means 2 as viewed in the direction of the transport, is disposed over the transport conveyor 1, and is configured of an infrared heater 51 and exhaust ducts 52 disposed on upstream and downstream sides of the infrared heater 51. The dryer means 5 radiates infrared rays from the infrared heater 51 to the sintered magnet bodies m on the transport conveyer 1 to heat the sintered magnet bodies m, so that the solvent is evaporated and removed from the slurry coated on the sintered magnet bodies m to deposit the powder of the rare earth compound or compounds. At this time, the evaporated solvent is evacuated through the exhaust ducts 52, whereby the vaporized solvent is removed from around the sintered magnet bodies m to effectively conduct drying.

(26) Here, the infrared heaters 31 and 51 that constitute the preheater means 3 and dryer means 5 may preferably be, but are not limited to, those which radiate near infrared rays of 0.8 to 5 μm wavelength. In the device of this embodiment, twin tube short-wavelength infrared heater units made of transparent quartz glass (ZKB 1500/200G, with cooling fan, output: 1,500 W, heated length: 200 mm) available from Heraeus Noblelight GmbH are used as both the infrared heaters 31 and 51.

(27) These heaters can radiate short-wavelength infrared rays of 0.8 to 5 μm wavelength, have a fast temperature rise, can begin effective heating in one to two seconds, can heat to 100° C. in ten seconds, and can warm or heat the sintered magnet bodies in an extremely short time. In addition, these heaters can configure at lower cost than conducting induction heating, and are also advantageous from the standpoint of power consumption. Furthermore, according to the radiation heating by the radiation of near infrared rays, the near infrared rays, upon drying under heat, can be also transmitted and absorbed into the coatings of the slurry to conduct drying under heat. It is, therefore, possible to avoid, as much as possible, development of cracking which would otherwise occur because of the beginning of drying from the outer sides of the coatings as in a case that drying is conducted, for example, by blowing hot air from the outside, and to form uniform and dense coatings of the powder. In addition, the above-described heater tubes that emit near infrared rays of short wavelength are relatively small, and therefore can also contribute to the downsizing of the coating device.

(28) When coating the slurry, in which the powder of the one or more rare earth compounds selected from the oxides, fluorides, oxyfluorides, hydroxides and hydrides of R.sup.2 (R.sup.2 is one or more elements selected from rare earth elements including Y and Sc) (the powder of the one or more rare earth compounds) is dispersed in the solvent, to the surfaces of the sintered magnet bodies m by using the coating device, the sintered magnet bodies m are fed to the upstream-side end portion of the transport conveyor 1, and are intermittently transported in a horizontal direction by the transport conveyor 1.

(29) To the sintered magnet bodies m which are placed on the transport conveyor 1 and are being transported intermittently and horizontally, infrared rays are radiated from the infrared ray heater 31 of the preheater means 3 to warm and heat them to the above-mentioned predetermined temperature when they are intermittently stopped under the preheater means 3. At this time, dust and the like on the surfaces of the sintered magnet bodies m are removed by the cleaning means 4 as mentioned above. Therefore, the sintered magnet bodies m are warmed and heated, and at the same time their surfaces are cleaned.

(30) When the sintered magnet bodies m warmed or heated beforehand to the predetermined temperature by the preheater means 3 have then intermittently been moved to and stopped under the coating roll 21 of the slurry coating means 2, the slurry s is coated onto the surfaces of the sintered magnet bodies m by vertically and horizontally movements of the coat roll 21. At this time, the coating roll 21 is fed and impregnated with the slurry s through the above-mentioned procedures by the slurry feed unit 22 as needed, so that the slurry s is ensured to be coated in a constant amount every time.

(31) The sintered magnet bodies m coated with the slurry s are then intermittently transported to below the dryer means 5 and stopped there. Infrared rays are radiated from the infrared heater 51 of the dryer means 5 to heat and dry the coated sintered magnet bodies m. The solvent is evaporated from the slurry s to deposit the powder, so that coatings of the powder are formed on the surfaces of the sintered magnet bodies m. The solvent evaporated and vaporized at this time is evacuated through the exhaust ducts 52 and is removed from around the sintered magnet bodies m, and therefore the above-described drying processing is efficiently conducted.

(32) After the drying, the sintered magnet bodies m are horizontally transported further, and are collected by a worker, a robot arm or the like at the downstream-side end portion of the transport conveyor 1.

(33) Here, by repeating a plurality of times the operation of feeding the sintered magnet bodies m, which have been collected from the downstream-side end portion of the transport conveyor 1, back to the upstream-side end portion of the transport conveyor 1 and coating them with the one or more rare earth compounds, the powder of the rare earth compound or compounds can be coated repeatedly. As a consequence, thicker coatings can be obtained with further improved uniformity. For the repetition of the coating operation, the coating processing may be repeated a plurality of times by using the same coating device, or plural coating devices may be arranged one after another to repeat the coating operation. In this manner, recoating can be conducted to obtain coatings of a desired thickness, and therefore the coat weight of the powder can be well adjusted. The repeated coating of thin layers makes it possible to shorten the drying time and hence to improve the time efficiency. In repeating the coating operation as described above, it is not absolutely necessary to conduct the preheating treatment every time. After the preheating treatment is conducted, the slurry coating and drying operation may be repeated a plurality of times.

(34) When desired to conduct the coating of the powder to both the front and back sides of each sintered magnet body m, each sintered magnet body m collected at the downstream-side end portion of the transport conveyor 1 may be turned upside down by a worker, a robot arm or the like, may be fed back to the upstream-side end portion of the transport conveyor 1, and may then be coated similarly. Also in this case, the coating processing of both the front and back sides may be conducted using the same coating device, or a coating device for front sides and a coating device for back sides may be arranged one after another to conduct the coating operations of both the front and back sides. Needless to say, the above-described recoating may be applied to each of the front and back sides.

(35) As described above, according to the production method of the present invention in which the coating of the powder of the rare earth compound or compounds is conducted using the above-described coating device, the sintered magnet bodies m are warmed or heated to the predetermined temperature before coating them with the slurry, the warmed or heated sintered magnet bodies m are coated with the slurry s, and the coated, sintered magnet bodies m are then dried to form coatings of the powder of the rare earth compound or compounds. By warming the sintered magnet bodies m beforehand, drying can be completed in an extremely short time upon drying under heat after the slurry coating. With the dryer means 5 included in the device of this embodiment and relying upon radiation of infrared rays, the solvent in the slurry can be evaporated and dried almost instantaneously, so that uniform coatings can be efficiently and surely formed without dripping of the slurry s to side surfaces where increased coercivity is not needed.

(36) Described specifically, the device of this embodiment applies the slurry s by roll coating, so that a coating can be locally formed at each necessary location on the surface of each sintered magnet body m by locally coating the slurry to the necessary location only. It is, therefore, possible to effectively reduce the treatment amount of the powder of the valuable rare earth compound or compounds. According to the present invention, the drying after the slurry coating can be completed in an extremely short time as described above. Therefore, the present invention can avoid, as much as possible, dripping of the slurry to side surfaces and like where increased coercivity is not needed, can avoid wasteful consumption of the powder of the valuable rare earth compound or compounds, and can extremely efficiently achieve an increase in coercivity.

(37) In this embodiment, the preheating (prewarming) before the slurry coating and the drying under heat after the slurry coating are conducted by radiation heating that radiates short-wavelength, near infrared rays of 0.8 to 5 μm wavelength. It is, therefore, possible to efficiently conduct the preheating (prewarming) and the drying under heat in a short time, to surely obtain uniform coatings from the powder without developing cracking or the like, and moreover to achieve downsizing of the coating device.

(38) Described specifically, the infrared heaters 31 and 51 that radiate short-wavelength, near infrared rays have a fast temperature rise, and can complete heating or warming in an extremely short time. Further, it can be configured at lower cost than conducting induction heating, and it is also advantageous from the standpoint of power consumption. It is hence possible to conduct the coating of the powder by efficiently warming or heating the sintered magnet bodies m and drying the slurry s at lower cost. Furthermore, according to the drying processing through radiation heating by the radiation of near infrared rays, the near infrared rays can be also transmitted and absorbed into the coatings of the slurry to conduct heating or warming. It is, therefore, possible to avoid, as much as possible, development of cracking which would otherwise occur because of the beginning of drying from the outer sides of the coatings as in a case that warming/heating and drying are conducted, for example, by blowing hot air from the outside, and to form uniform and dense coatings of the powder. In addition, the above-described heater tubes that emit near infrared rays of short wavelength are relatively small, and therefore can downsize the dryer or the coating device. Therefore, rare earth magnets can be efficiently produced by small-scale facilities.

(39) It is to be noted that the coating device according to the present invention is not limited to the above-described device of FIGS. 1 to 3. For example, a belt conveyor is illustrated as the transport conveyor 1 in the figures, but a roller conveyor may also be used. Further, as illustrated by dot-dash lines in FIG. 2, a reflection sheet 32 may be arranged on a back side of the conveyor to reflect infrared rays, so that the sintered magnet bodies m can be warmed or heated more efficiently. Furthermore, the device of FIGS. 1 to 3 is configured to conduct roll coating with the coating roll 21. In some cases, however, the device may be configured to conduct spray coating or dip coating. Concerning other to elements such as the preheater means 3, dryer means 5 and slurry feed unit 22, modifications may also be applied as needed within a scope not departing from the spirit of the present invention.

(40) According to the production method of the present invention, the sintered magnet bodies which have been coated with the powder as described above are subjected to heat treatment to cause absorptive diffusion of the rare earth element or elements represented by R.sup.2 and contained in the powder. The heat treatment, which causes absorptive diffusion of the above-described rare earth element or elements represented by R.sup.2, can be conducted by a known method. After the above-described heat treatment, known post-treatment can be applied as needed, for example, aging treatment can be applied under appropriate conditions, and further the resulting rare earth magnets can be ground into a practical shape.

EXAMPLES

(41) About more specific aspects of the present invention, a detailed description will hereinafter be made based on Examples. It should, however, be noted that the present invention shall not be limited to the Examples.

Examples 1 to 4 and Comparative Example

(42) 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.

(43) The coarse powder was finely pulverized into a powder having a weight median particle size of 5 μm in a jet mill that used high-pressure nitrogen gas. The resulting mixed fine powder was formed into block-shaped compacts under a pressure of approximately 1 ton/cm.sup.2 while allowing its particles to orient in a magnetic field of 15 kOe under a nitrogen gas atmosphere. The compacts were placed in a sintering furnace under an Ar atmosphere, and were sintered at 1,060° C. for two hours to obtain magnet blocks. After the magnet blocks were subjected to full-surface grinding with a diamond cutter, the resulting magnet blocks were cleaned with an alkaline solution, deionized water, nitric acid, and deionized water in this order, followed by drying to obtain block-shaped magnet bodies of 20 mm×45 mm×5 mm (in the direction of magnetic anisotropy).

(44) Next, a powder of dysprosium fluoride was mixed at a mass fraction of 40% in water, followed by thorough dispersion of the powder of dysprosium fluoride to prepare a slurry. Using the above-described coating device illustrated in FIGS. 1 to 3, the above-described magnet bodies were coated with the slurry, and the resulting magnet bodies were dried to deposit the powder of dysprosium fluoride. At this time, the powder coating processing was conducted by varying the temperature of the preheating (prewarming) in the preheater means 3 as indicated in FIG. 1 (Examples 1 to 4). In addition, as a Comparative Example, similar powder coating processing was conducted without any preheating (prewarming) in the preheater means 3 (Comparative Example). In all the Examples and Comparative Example, collected sintered magnet bodies were subjected again to coating processing, whereby recoating was conducted three times. In Examples 1 to 4, the slurry coating processing and drying processing were repeated three times, but the preheating processing was limited to only once in the first round of the slurry coating processing and drying processing.

(45) From the entire coated surface of each sintered magnet body so obtained, powder was removed with a scraper, and its weight was measured. Assuming that the coat weight at which the increasing effect for coercivity reaches the peak is 1.00, the proportions of coat weights per unit area are presented in Table 1.

(46) TABLE-US-00001 TABLE 1 Proportion of Heating before coat weight on coating coated surface Comparative Example No heating 0.72 Example 1 30° C. 0.89 Example 2 40° C. 0.97 Example 3 60° C. 1.01 Example 4 80° C. 1.03

(47) As appreciated from Table 1, the prewarming or preheating of sintered magnet bodies has been confirmed to lead to an increase in the coat weight of the powder owing to the formation of coatings through prompt drying of the solvent without dripping of the slurry to surfaces other than the surfaces to be coated. In contrast, the coat weight in the Comparative Example was small although roll coating was conducted similarly. This can be attributed to dripping of as much the slurry as the decrease in the coat weight to side surfaces of the sintered magnet bodies.

Example 5

(48) A magnet body with a coating of a powder of dysprosium fluoride formed thereon in a similar manner as in Example 3 was subjected to heat treatment at 900° C. for five hours in an Ar atmosphere, and was then subjected to aging treatment at 500° C. for one hour, followed by quenching to obtain a rare earth magnet. Magnet bodies of 2 mm×2 mm×2 mm were cut out from the nine points indicated in FIG. 4, and were then measured for coercivity. The results are presented in Table 2.

(49) 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 5

(50) As appreciated from Table 2, the preheating of magnet bodies before coating can uniformly deposit and form coatings of the powder without dripping of the slurry to surfaces other than the surfaces to be coated, and moreover the roll coating can promote homogenization within coatings, and can effectively use the expensive rare earth compound or compounds without waste. In addition, the increasing effect for coercivity at the coated surfaces is free of irregularity and is very stable.

REFERENCE SIGNS LIST

(51) 1 transport conveyor 2 slurry coating means 21 coating roll 211 horizontal shaft 212 vertical shaft 22 slurry feed unit 221 slurry overflow tank 222 slurry receiving tank 223 slurry feed tray 224 pump 225 return pipe 23 ultrasonic cleaner 3 preheater means 31 infrared heater 32 reflection sheet 4 cleaning means 41 air knife 42 dust collecting duct 5 dryer means 51 infrared heater 52 exhaust duct m sintered magnet body s slurry