Electrodepositing apparatus and preparation of rare earth permanent magnet

Abstract

An electrodepositing apparatus is provided comprising an inner tank (1) filled with an electrodepositing solution, an outer tank (3), a feedback means (4), a rectifying member (5) disposed in the inner tank (1), a means (8) for holding an article (p), a counter electrode (6), and a power supply (9). The electrodepositing solution is circulated in such a way that it overflows the inner tank and is fed back from the outer tank to the inner tank by the feedback means, the flow of the solution is rectified by the rectifying member to keep flat the solution surface in the inner tank, a selected portion of the article is immersed in the solution, and the coating agent is electrodeposited on the selected portion of the article.

Claims

1. A method for preparing a rare earth permanent magnet, comprising the steps of: coating a sintered magnet body having a R.sup.1FeB base composition, wherein R.sup.1 is at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, with a powder comprising at least one member selected from the group consisting of an oxide, fluoride, oxyfluoride, hydride, and rare earth alloy of R.sup.2, wherein R.sup.2 is at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and heat treating the coated magnet body for causing R.sup.2 to be absorbed in the magnet body, wherein the coating step includes the steps of: providing an electrodepositing apparatus comprising an inner tank filled with an electrodepositing solution and adapted to effect electrodeposition on an article immersed in the solution, wherein a peripheral wall of the inner tank is provided, at an upper edge of the peripheral wall, with a plurality of equally spaced apart V-shaped notches across which the electrodepositing solution overflows, an outer tank enclosing the inner tank so that the outer tank receives an overflow of the electrodepositing solution from the inner tank, a feedback system comprising a flow-out pipe extending from the outer tank and a return pipe extending back to the inner tank near the bottom of the inner tank, a rectifying plate disposed at a vertical intermediate position in the inner tank and horizontally extended so as to divide the inner tank into upper and lower compartments, the rectifying plate having a plurality of first apertures distributed so that the rectifying plate rectifies flow of the electrodepositing solution and suppresses waves in the surface of the electrodepositing solution overflowing from the upper rim of the inner tank, an article holder configured to hold the article so that the article is partially immersed in the electrodepositing solution in the inner tank, a counter electrode, with a plurality of second apertures, disposed in the inner tank and opposed to the article which is to be held by the article holder and immersed in the electrodepositing solution, and a power supply for applying a predetermined voltage between the article and the counter electrode, circulating the electrodepositing solution in such a way that it overflows the inner tank into the outer tank and is fed back from the outer tank to the inner tank near its bottom by the feedback system, immersing a selected portion of the magnet body in the electrodepositing solution in the inner tank, the electrodepositing solution comprising a solvent and the powder dispersed in the solvent, and electrodepositing the powder on the surface of the magnet body to form a powder coating on the selected portion of the magnet body by actuating the power supply to apply the predetermined voltage between the article and the counter electrode.

2. The method of claim 1, wherein the inner tank includes a bottom wall, the return pipe having a plurality of orifices in its tubular wall, the return pipe being connected to the feedback system and extended through the inner tank along the bottom wall, and the feedback system feeds the electrodepositing solution into the return pipe to inject the electrodepositing solution into the inner tank through the orifices.

3. The method of claim 2, wherein the orifices are arranged in the return pipe such that their diameter gradually or stepwise decreases from the proximal end to the distal end of the return pipe.

4. The method of claim 1, wherein the counter electrode is a metal plate and disposed on the rectifying plate.

5. The method of claim 4 wherein the counter electrode is a metal disk, the disk being generally frusto-conical shaped at a central portion or over its entirety.

6. The method of claim 1, further comprising at least one of a level meter, thermometer, concentration meter, and flow meter.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically illustrates an electrodepositing apparatus in one embodiment of the invention.

(2) FIG. 2 is a perspective view of the inner tank in the electrodepositing apparatus.

(3) FIG. 3 is a perspective view of one exemplary counter electrode used in the electrodepositing apparatus.

(4) FIG. 4 schematically illustrates an electrodepositing apparatus used in Reference Experiments 1 to 3.

(5) FIGS. 5 A, B and C illustrate the shape and dimensions of counter electrodes used in Experiments 4 to 6, respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS

(6) As used herein, the terms upper, lower, vertical, horizontal and the like are used in conjunction with the view of FIG. 1.

(7) Briefly stated, the electrodepositing apparatus of the invention is such that an article is coated by immersing the article in an electrodepositing solution having a coating agent dispersed or dissolved in a solvent, and applying a voltage between the article and a counter electrode for letting the coating agent deposit on the surface of the article. As mentioned above, a selected portion of the article is immersed in the electrodepositing solution, and electrodeposition is carried out locally on the selected portion of the article. The local electrodeposition ensures to form a uniform coating accurately on the selected portion.

(8) Referring to FIG. 1, the electrodepositing apparatus in one embodiment of the invention is described in detail. The apparatus includes an inner tank 1 and an outer tank 3. The inner tank 1 is a rectangular box consisting of a peripheral wall and bottom wall, which is open at the upper end. The inner tank 1 is filled with an electrodepositing solution 2. The outer tank 3 is a rectangular box consisting of a peripheral wall and bottom wall, which is open at the upper end. The outer tank 3 is larger than the inner tank 1 so that the inner tank 1 is enclosed in the outer tank 3. Thus the outer tank 3 receives the solution 2 overflowing the inner tank 1. The apparatus includes a feedback line 4 connecting a discharge port in the bottom wall of outer tank 3 to return pipes 7 which are extended through the inner tank 1 near the bottom. A pump 41 is disposed in the feedback line 4 for pumping the solution from the outer tank 3 to the bottom of the inner tank 1 through the feedback line 4. The solution 2 is circulated in this way. The feedback line 4, pump 41 and return pipes 7 constitute the feedback means. A flow meter (not shown) may be disposed in the feedback line 4 for monitoring the flow velocity of the solution 2 through the line, whereby the circulating amount or velocity of the solution may be adjusted.

(9) As best shown in FIG. 2, the peripheral wall of the inner tank 1 has the upper rim which is tapered upward from the outside. That is, the peripheral wall upper rim is configured like a cutting edge. The peripheral wall upper rim is provided with a plurality of equally spaced apart V-shaped notches 11. When the solution 2 overflows the upper rim of the inner tank 1, the solution passes through the notches and discharges out equally on the four sides. The notches are effective for inhibiting the surface of the solution 2 in the inner tank from waving under the influence of surface tension so that the surface of the overflowing solution 2 may be kept flat. The depth, V angle, number, and spacing of the notches 11 may be determined as appropriate, depending on the size and shape of the upper rim, the type and flow (or circulating) velocity of the solution and the like. Preferably these parameters are empirically determined by circulating the electrodepositing solution.

(10) In the inner tank 1, a rectifier member 5 in the form of a rectangular plate is disposed at a vertical intermediate (relatively upper) position of the tank and horizontally extended so as to divide the inner tank 1 into upper and lower compartments. As shown in FIG. 2, the rectifier plate 5 is provided with apertures of three sizes, that is, large, middle and small diameter apertures 51, 52, and 53. Specifically, small apertures 53 are uniformly distributed over the entire surface of the rectifier plate 5. Large apertures 51 and middle apertures 52 are uniformly distributed among small apertures 53. Large apertures 51 are distributed in a predetermined zone about the center of the rectifier plate 5 while middle apertures 52 are distributed in a predetermined zone near the periphery of the rectifier plate 5. The distribution of large apertures 51 in a central portion and smaller apertures 52 in a peripheral portion of the rectifier plate 5 is set for the following reason.

(11) After the electrodepositing solution 2 is fed back to the inner tank 1 near the bottom, it flows upward and overflows the upper rim of the inner tank 1. The solution flow in the tank has a tendency that the flow velocity near the peripheral wall is higher than the flow velocity near the center. The differential flow velocity may be offset by arranging apertures in the rectifier plate 5 such that the diameter of apertures (51) near the center is larger than the diameter of apertures (52) near the periphery. This arrangement is effective for preventing the surface of the solution 2 from waving due to the differential flow velocity.

(12) The material of which the rectifier plate 5 is made is not particularly limited and may be selected from a wide variety of materials including metals and synthetic resins. When a counter electrode is secured to the rectifier plate 5 as will be described later, the plate must be made of an insulating synthetic resin such as polyvinyl chloride. It is noted that the rectifier member is not limited to the rectifier plate 5 illustrated herein. For example, a mesh plate or expanded plate may be used, and a plurality of rectifier plates may be combined to form the rectifier member.

(13) A counter electrode 6 in the form of a rectangular metal plate is disposed on the upper surface of the rectifier plate 5 at its center. The counter electrode 6 is also provided uniformly with a plurality of apertures so that the electrodepositing solution 2 may pass therethrough. The counter electrode 6 may be made of a conductive metal plate such as stainless steel. The shape of the counter electrode 6 may be determined, depending on the shape of the article to be treated, the portion of the article subject to electrodeposition, the state of the article during immersion, the solvent of the solution, the type of coating agent, and various electrodepositing conditions. For example, a perforated metal plate may be worked into a cylinder or rectangular box. The counter electrode may also be a perforated metal disk 61 having a central portion worked into a frusto-conical shape as shown in FIG. 3.

(14) The inventors have confirmed that the counter electrode 61 in the form of a perforated metal disk having a frusto-conical central portion as shown in FIG. 3 is especially effective for improving the uniformity of a coating thickness or weight. Particularly when a powder comprising an oxide of R.sup.2 (wherein R.sup.2 is at least one element selected from rare earth elements inclusive of Y and Sc) or the like is locally electrodeposited on the surface of a sintered magnet body of a R.sup.1FeB base composition (wherein R.sup.1 is at least one element selected from rare earth elements inclusive of Y and Sc), the counter electrode 61 is effective for preventing a particle coating from becoming uneven or a coating weight from varying.

(15) The size of the counter electrode 6 is not particularly limited and may be determined as appropriate. Typically the size of the counter electrode 6 is set ? to 3 times the size of an article p to be treated. When the counter electrode is of very large size, the rectifier plate 5 may be made of a conductive metal such as stainless steel so that the rectifier plate 5 may also serve as the counter electrode. As long as the counter electrode 6 is positioned on the rectifier plate 5, the electrode 6 may be disposed contiguous to or spaced apart from the rectifier plate 5.

(16) As shown in FIG. 2, two return pipes 7 are disposed in a lower portion of the inner tank 1 and extended through the tank along the bottom. The return pipes 7 are connected to the feedback line 4 of the feedback means. The return pipe 7 has a plurality of orifices (not shown) uniformly distributed in its tubular wall. Once the electrodepositing solution 2 is fed back to the return pipes 7, it is injected through the orifices and introduced into the inner tank 1 near the bottom. As shown in FIG. 2, the return pipes 7 are spaced apart a distance and extended parallel in the inner tank 1 along the bottom. The return pipes 7 have proximal ends which are extended outside the inner tank 1 and connected to the feedback line 4 via a manifold, and distal ends which are closed.

(17) Though not shown, the orifices in the return pipe 7 are uniformly distributed in the lower side of the tubular wall so that the solution 2 may be injected toward the bottom of the inner tank 1. There is a tendency that the discharge amount of the solution injected through those orifices on the distal end side is larger than the discharge amount of the solution injected through those orifices on the proximal end side connected to the feedback line 4. For correcting the difference in discharge amount, the orifices are preferably arranged in the return pipe such that their diameter may gradually or stepwise decrease from the proximal end to the distal end of the return pipe. Although two return pipes 7 are shown, the number of return pipes is not critical.

(18) In FIG. 1, the apparatus further includes a holding means in the form of a mechanical clamp 8 for holding the article p so that the article p may be partially immersed in the electrodepositing solution 2 in the inner tank 1. The mechanical clamp 8 is connected to a robot arm, for example, so that it may be moved in any directions including vertical and lateral directions. The clamp 8 tightly holds the article p in the predetermined attitude so that the article may be immersed in the solution from above, kept immersed in a stable manner, and then pulled up. The clamp 8 enables to adjust the immersion depth or extent of the article p which is partially immersed in the electrodepositing solution and the lateral position of the article p relative to the counter electrode 6. The holding means is not limited to the mechanical clamp illustrated above, as long as it holds the article p in the predetermined attitude tightly and translates the article in at least vertical direction so that the article p may be vertically moved into and out of the solution, and enables to adjust the immersion depth or extent of the article p in the solution.

(19) Though not shown, the mechanical clamp 8 has a probe which is brought in pressure contact with the article when the clamp holds the article. Electricity is conducted from a DC power supply 9 (to be described below) to the article p via the probe. The probe or conductive means to the article may be omitted if the holding means itself provides for electric conduction to the article.

(20) Also shown in FIG. 1 is a DC power supply 9 which is electrically connected to the counter electrode 6 and the probe of the mechanical clamp 8 for applying a predetermined voltage between the article p held by the clamp 8 and the counter electrode 6. Although FIG. 1 is illustrated with the article p made a cathode and the counter electrode 6 made an anode, the polarity of applied voltage may be set depending on the polarity of the coating agent in the electrodepositing solution.

(21) Also shown in FIG. 1 is a level meter 10 for detecting the surface of the electrodepositing solution in the outer tank 3. The volume of the electrodepositing solution is managed by means of the level meter 10. Though not shown, a thermometer, concentration meter or another meter may be installed for monitoring the electrodepositing solution. Also if desired, there may be installed a chiller for controlling the temperature of the solution, a filter for removing foreign matter from the solution, or the like.

(22) Now it is described how to use and operate the electrodepositing apparatus illustrated above, with reference to an example wherein a selected portion of a sintered magnet body having a R.sup.1FeB base composition (wherein R.sup.1 is at least one element selected from rare earth elements inclusive of Y and Sc) is immersed in an electrodepositing solution of a particle powder dispersed in a solvent, the powder containing an oxide, fluoride, oxyfluoride, hydride or rare earth alloy of R.sup.2 (wherein R.sup.2 is at least one element selected from rare earth elements inclusive of Y and Sc), and electrodeposition is effected to deposit particles on the magnet body surface to form a powder coating on the selected portion of the magnet body.

(23) An electrodepositing solution of the powder dispersed in a solvent is supplied to the inner and outer tanks 1 and 3. The pump 41 is actuated so that the electrodepositing solution 2 may circulate through the apparatus. The solution is pumped from the outer tank 3 to the return pipes 7 through the feedback line 4 and injected into the inner tank 1 through the orifices (not shown) in the return pipes 7. The solution flows upward in the inner tank 1, overflows the upper rim of the inner tank 1, and falls down into the outer tank 3.

(24) The solution 2 flowing in the inner tank 1 is rectified or straightened by the rectifier plate 5, after which the solution overflows the upper rim of the inner tank 1 across the V-shaped notches 11 in the rim. The notches 11 function to minimize the influence of surface tension so that the solution 2 overflowing the inner tank 1 may keep its surface flat. Thus the solution 2 defines a substantially flat surface along the upper rim of the inner tank 1.

(25) The substantially flat surface of the solution 2 refers to a liquid surface consisting of waves having a crest-valley height of preferably up to 3 mm, more preferably up to 1 mm, which is a mirror-like surface. Then the immersion depth or extent of the sintered magnet body (article) p can be adjusted in the millimeter order.

(26) The circulating amount of the electrodepositing solution 2 may be determined as appropriate depending on the dimensions of the inner tank 1. For the inner tank 1 having a volume of 20 to 50 L, for example, the solution may be circulated at a flow rate of 10 to 250 L/min, preferably 20 to 100 L/min, and more preferably 30 to 60 L/min. If the circulating amount is too small, powder particles may settle down at weak flow zones in the tanks. If the circulating amount is too large, the flow volume across the upper rim of the inner tank 1 becomes large so that the solution surface may become wavy to interfere with uniform electrodeposition on the selected portion.

(27) When the electrodepositing solution 2 is circulated by means of the pump 41, the pump 41 may be controlled by an inverter. The inverter control ensures that the pump 41 is operated for slow circulation at a flow rate of up to 30 L/min, for example, in the quiescent period, and the pump 41 is operated for proper circulation at a flow rate of 30 to 60 L/min in the electrodepositing period. Then electrodeposition can be continued while the particles are kept fully dispersed in the solution and the electric power consumed is saved.

(28) While the electrodepositing solution 2 is circulated in this way, the mechanical clamp 8 is manipulated so as to hold the sintered magnet body (article) p and to move down the magnet body to immerse it in the solution in the inner tank 1 to a predetermined depth, thereby bringing the necessary portion of the magnet body p in contact with the solution 2. That is, the selected portion of the magnet body p is immersed in the solution to a certain depth below the surface. In the immersed state, the DC power supply 9 is actuated to apply a predetermined voltage between the magnet body p and the counter electrode 6 for a predetermined time for causing the powder (dispersed in the solution) to deposit on the immersed portion of the magnet body p to form a powder coating.

(29) Electric conduction conditions may be determined as appropriate and are not particularly limited. Typically, a voltage of 1 to 300 volts, especially 5 to 50 volts is applied for 1 to 300 seconds, especially 5 to 60 seconds. Also the temperature of the electrodepositing solution is not particularly limited. Typically the solution is set at 10 to 40? C. Manipulation should preferably be such that the mechanical clamp 8 may not contact with the electrodepositing solution, especially during electrodepositing operation.

(30) Although the magnet body p is made a cathode and the counter electrode 6 made an anode in the arrangement of FIG. 1, the polarity may be changed depending on the composition of the electrodepositing solution 2. In this embodiment, the electrodepositing solution is prepared by dispersing a powder containing an oxide, fluoride, oxyfluoride, hydride, or rare earth alloy of R.sup.2 (wherein R.sup.2 is at least one element selected from rare earth elements inclusive of Y and Sc) in water or a suitable organic solvent, and adding a surfactant and other additives, if desired. Since the polarity of the powder in the electrolytic solution changes with the presence/absence and type of the surfactant, the polarity of the magnet body p and counter electrode 6 may be set depending on these conditions.

(31) Once electrodeposition is completed by electric conduction for the predetermined period, the magnet body p is pulled up from the solution in the inner tank 1, spun or air blown to remove extra droplets, and then dried in a suitable manner.

(32) As described above, the electrodepositing apparatus ensures that a selected portion of a sintered magnet body (article) p is immersed in the electrodepositing solution, and electrodeposition is effected to deposit the powder locally on the necessary portion of the magnet body. During the operation, the surface of the electrodepositing solution overflowing the inner tank is kept as a substantially flat surface free of substantial waves or curves, specifically as a mirror-like surface including waves of up to 1 mm as will be demonstrated in Experiments 1 to 3. The immersion depth or extent may be adjusted in the millimeter order. Thus a satisfactory powder coating may be formed only on the necessary portion of the magnet body, and the amount of expensive powder consumed be significantly saved.

(33) After a local powder coating is deposited on the necessary portion of the magnet body as described above, the coated magnet body is heat treated by the standard technique. This heat treatment is referred to as absorption treatment. Through the absorption treatment, R.sup.2 in the powder deposited on the magnet surface is concentrated in the rare earth-rich grain boundary component within the magnet so that R.sup.2 is incorporated in a substituted manner near a surface layer of R.sub.2Fe.sub.14B primary phase grains. The absorption treatment effectively increases the coercive force of the RFeB sintered magnet without substantial sacrifice of remanence (or residual magnetic flux density). Since electrodeposition is carried out using the apparatus of the invention, the absorption treatment can be locally assigned to the selected area of the magnet where coercive force is required. Then, the amount of expensive powder used is effectively saved. The magnetic performance available on the necessary portion of the magnet body is comparable to that obtained from the overall coverage of a magnet body with the powder and subsequent absorption treatment. If desired, the absorption treatment may be followed by aging treatment at a temperature which is below the absorption treatment temperature.

(34) Experiments were carried out to demonstrate the benefits of the electrodepositing apparatus of the invention.

(35) Preparation of Sintered Magnet Body

(36) 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% by weight, Si having a purity of 99.99% by weight, and ferroboron, radio-frequency heating in an argon atmosphere for melting, and casting the alloy melt on a copper single roll. The alloy consisted of 14.5 atom % of Nd, 0.2 atom % of Cu, 6.2 atom % of B, 1.0 atom % of Al, 1.0 atom % 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.

(37) Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5 ?m. The fine powder was compacted in a nitrogen atmosphere under a pressure of about 1 ton/cm.sup.2 while being oriented in a magnetic field of 15 kOe. The green compact was then placed in a sintering furnace with an argon atmosphere where it was sintered at 1,060? C. for 2 hours, obtaining a sintered magnet block. The magnet block was machined on all the surfaces into a block magnet body. It was cleaned in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried. There were obtained block magnet bodies of three types, magnet body A of 90 mm long?40 mm wide?22 mm thick, magnet body B of 90 mm long?35 mm wide?30 mm thick, and magnet body C of 90 mm long?40 mm wide?30 mm thick.

(38) Preparation of Electrodepositing Solution

(39) Terbium oxide powder having an average particle size of 0.2 ?m was thoroughly mixed with deionized water at a weight fraction of 40% to form a slurry having terbium oxide particles dispersed therein. The slurry served as an electrodepositing solution.

(40) Experiments 1 to 3

(41) The electrodepositing solution was supplied to the electrodepositing apparatus in FIGS. 1 and 2. The solution was circulated at a flow rate of 45 L/min and kept at a temperature of 21? C. while the solution overflowed the inner tank 1 of 15 L volume. The surface of the overflowing solution was controlled as a mirror-like surface including waves with a height of up to 1 mm. The block magnet body A (depicted as article p) was held by the mechanical clamp 8, moved down in thickness direction and immersed in the solution to a depth of 2 mm from the overflow surface. The magnet body p was spaced apart 20 mm from the counter electrode 6 of stainless steel SUS304. With the counter electrode 6 made an anode and the magnet body p made a cathode, a DC voltage of 10 volts was applied for 10 seconds to effect electrodeposition. The magnet body was pulled out of the solution and immediately dried in hot air. The magnet body p was turned up-side-down. The same operations as above were repeated. In this way, a thin coating of terbium oxide was deposited only on the front and back surfaces of the magnet body p.

(42) Electrodeposition was similarly carried out on magnet bodies B and C. For all magnet bodies A, B and C, the area density of terbium oxide deposited was 85 ?g/mm.sup.2 on both the front and back surfaces.

(43) Each of the magnet bodies A, B and C having a thin coating of terbium oxide particles locally deposited thereon was subjected to absorption treatment in an argon atmosphere at 900? C. for 5 hours. It was then subjected to aging treatment at 500? C. for one hour, and quenched, obtaining a magnet body. From six areas on the surface of the magnet body, pieces of 2 mm?6.4 mm?7 mm were cut out and measured for magnetic properties. An increase of coercive force to about 660 kA/m due to the absorption treatment was confirmed, as reported in Table 1.

(44) Comparative Experiments 1 to 3

(45) The rectifier plate 5 was removed from the electrodepositing apparatus shown in FIGS. 1 and 2. The notches 11 in the peripheral wall upper rim of the inner tank 1 were buried to give a flat upper rim. Otherwise as in Experiments 1 to 3, the electrodepositing solution 2 was circulated through the apparatus while it overflowed the inner tank 1. The surface of the overflowing solution included waves with a height of 1 to 5 mm. As in Experiments 1 to 3, each of block magnet bodies A, B and C was partially immersed in the solution. Electrodeposition was carried out on both surfaces of the magnet body. The magnet body was covered with a thin coating of terbium oxide only on front and back surfaces. The area density of terbium oxide deposited was 85 ?g/mm.sup.2 on both the front and back surfaces.

(46) Each magnet body having a thin coating of terbium oxide particles locally deposited on its surface was subjected to absorption treatment and aging treatment as in Experiments 1 to 3. Magnet pieces were similarly cut out and measured for magnetic properties. An increase of coercive force to about 660 kA/m due to the absorption treatment was confirmed, as reported in Table 1.

Reference Experiments 1 to 3

(47) Electrodeposition was carried out under the same conditions as in Experiments 1 to 3 except that as shown in FIG. 4, a magnet body (depicted as article p) was longitudinally and entirely immersed in the electrodepositing solution 2 and interposed between a pair of counter electrodes 6 at a spacing of 20 mm and the solution 2 was stirred. A thin coating of terbium oxide was deposited on the entire surfaces of each of magnet bodies A, B and C. The area density of terbium oxide deposited was 85 ?g/mm.sup.2.

(48) The magnet body having a thin coating of terbium oxide particles deposited on the entire surfaces (i.e., overall coverage) was subjected to absorption treatment and aging treatment as in Experiments 1 to 3. Magnet pieces were cut out of the magnet body and measured for magnetic properties. An increase of coercive force to about 660 kA/m due to the absorption treatment was confirmed.

(49) The conditions and results of Experiments 1 to 3, Comparative Experiments 1 to 3, and Reference Experiments 1 to 3 are tabulated in Table 1. The powder consumption, which is an amount of powder deposited, is computed from a weight gain of a magnet body before and after electrodeposition. The increase of coercive force is an average of 6 magnet pieces.

(50) TABLE-US-00001 TABLE 1 Magnet State of Increase of Powder Relative dimensions Depositing overflow coercive force consumption powder (mm) range surface (kA/m) (g/body) consumption* Experiment 1 90 ? 40 ? 22 local mirror-like 660 0.700 63.75 surface with waves ?1 mm 2 90 ? 35 ? 30 local mirror-like 661 0.621 52.94 surface with waves ?1 mm 3 90 ? 40 ? 30 local mirror-like 659 0.700 54.90 surface with waves ?1 mm Comparative 1 90 ? 40 ? 22 local waves of 660 0.778 70.85 Experiment 1-5 mm 2 90 ? 35 ? 30 local waves of 662 0.698 59.51 1-5 mm 3 90 ? 40 ? 30 local waves of 658 0.786 61.65 1-5 mm Reference 1 90 ? 40 ? 22 overall 662 1.098 100 Experiment 2 90 ? 35 ? 30 overall 664 1.173 100 3 90 ? 40 ? 30 overall 633 1.275 100 *Relative powder consumption is a powder consumption in Experiment relative to the powder consumption in Reference Experiment which is 100.

(51) As seen from Table 1, the electrodepositing apparatus of the invention ensures that local (or partial) electrodeposition is carried out accurately while controlling the surface of the electrodepositing solution flat and maintaining the accurate depth of immersion. The amount of terbium oxide powder consumed is saved. The increase of coercive force is comparable to that resulting from the overall coverage.

Experiment 4

(52) As in Preparation of sintered magnet body section, a block magnet body D of 85 mm long?45 mm wide?20 mm thick was obtained. Electrodeposition was carried out on magnet body D as in Experiment 1 except that a counter electrode 61 consisting of a frusto-conical center and an annular flange as shown in FIG. 3 was used instead of the counter electrode 6 in FIGS. 1 and 2. Electrodeposition was carried out using counter electrodes 61 of four types having a different set of dimensions r1, r2 and h shown in FIG. 5 (A). For all the counter electrodes 61, the flange had an outer diameter of 100 mm.

(53) Using a fluorescent X-ray coating thickness gauge, the coating weight of particles on the coated surface (i.e., major surface of 85 mm?45 mm) of each magnet body was measured at 630 equally spaced apart points in a matrix of 18?35 points. A proportion (%) of those points having a coating weight of 90 to 120 ?g/mm.sup.2, within a coating weight range of 30 ?g/mm.sup.2, was computed. A variation of coating weight is represented by the standard deviation. The results are shown in Table 2.

Experiments 5 and 6

(54) Electrodeposition was carried out as in Experiment 4, aside from using a counter electrode consisting of a central cylindrical protrusion and an annular flange as shown in FIG. 5 (B) or a counter electrode in the form of a rectangular plate as shown in FIG. 5 (C). For each case, electrodeposition was carried out using counter electrodes of three types having a different set of dimensions d and h in FIG. 5 (B) or dimensions a, b and c in FIG. 5 (C). As in Experiment 4, a proportion (%) of those points having a coating weight of 90 to 120 ?g/mm.sup.2, within a coating weight range of 30 ?g/mm.sup.2, was computed. A variation of coating weight is represented by the standard deviation. The results are shown in Table 2.

(55) It is noted that each of the counter electrodes used in Experiments 4, 5 and 6 was made of stainless steel SUS304 and perforated with equally spaced apart apertures.

(56) TABLE-US-00002 TABLE 2 Proportion within Counter electrode a coating weight Dimensions range of 30 ?g/ Standard Shape (mm) mm.sup.2 (%) deviation Experi- Frusto-conical r1 = 20, r2 = 83.5 9.6 ment 4 protrusion 10, h = 5 r1 = 30, r2 = 98.3 5.2 15, h = 5 r1 = 4, r2 = 97.6 7.1 20, h = 5 r1 = 40, r2 = 95.1 8.2 20, h = 10 Experi- Cylindrical d = 30, h = 2 26.1 28.8 ment 5 protrusion d = 45, h = 2 52.1 22.2 d = 60, h = 2 65.7 18.3 Experi- Rectangular a = 30, b = 34.8 26.7 ment 6 plate 30, c = 2 a = 40, b = 56.4 21.4 40, c = 2 a = 50, b = 70.7 17.6 50, c = 2

(57) As seen from Table 2, the counter electrode 61 of frusto-conical shape is effective for reducing the unevenness of powder coating (or variation of coating weight).

(58) Japanese Patent Application No. 2014-029677 is incorporated herein by reference.

(59) 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.