Method of producing a solid resin-coated magnet powder for producing anisotropic bonded magnet

Abstract: There is provided a solid resin-coated composite magnet powder for producing an anisotropic bonded magnet, containing anisotropic magnet powder particles each having a surface coated with a solid resin layer formed thereon. There is also provided a method of producing a magnet of solid resin-coated magnet powder having the steps of mixing an anisotropic magnet powder with a solution or suspension of a resin in a solvent whilst extracting the solvent under reduced pressure so as to produce a granular product and thereafter breaking the granules into individual solid resin-coated magnetic powder particles.

Description: The present invention relates to a method of producing a solid resin-coated magnet powder for producing an anisotropic bonded magnet, and a method of producing the magnet. An anisotropic bonded magnet among bonded magnets is generally formed by mixing an anisotropic magnet powder with a solid resin and granulating the resultant mixture to form particles of a solid resin-coated magnet powder having surfaces coated with a solid resin such as a solid epoxy resin, a polyester resin, a phenolic resin or the like, press-molding the solid resin-coated magnet powder to form a molded product, and curing the solid resin by heating the molded product (refer to Japanese Patent Laid-Open No. 1-281707). The solid resin-coated magnet powder is considered to have better fluidity than that of a liquid resin-coated magnet powder, and can thus be easily cast into a mold having a small thickness However, in an anisotropic bonded magnet, the solid resin-coated magnet powder produced by the above conventional method mainly consists of a solid resin-coated composite magnet powder 5 containing a plurality of anisotropic magnet powder particles 1 coated with a solid resin 2, as shown in Fig. 3. Even though each of the anisotropic magnet powder particles 1 has excellent anisotropy, since a plurality of the anisotropic magnet powder particles 1 are contained with random orientation directions 4, the solid resin-coated composite magnet powder 5 has insufficient anisotropy as a whole, and cannot be oriented so as to exhibit sufficient anisotropy even by molding in a magnetic field. There is thus a problem that an anisotropic bonded magnet having excellent magnetic characteristics cannot be obtained. There is also the problem that a molded product of the conventional solid resin-coated composite magnet powder 5 has a lower density than that of a molded product of liquid resin-coated magnet powder obtained at the same molding pressure, and the bonded magnet obtained exhibits poor magnetic characteristics. JP-A-1-281 707, as referred to above, describes the production of a magnetic powder comprising a composition of rare earth metal, iron and boron which is coated with an organic resin which is solid at room temperature for use in the production of magnets. US-A-4 200 547 describes the production of matrix-bonded permanent magnets by injection moulding a mixture of magnetically-hard particles and a hotmelt polyamide which may preliminarily be melted, then cooled and granulated before injection-moulding. JP-A-1-114 006 is concerned with improving the effects of a corrosion preventative treatment by using a triazine dielectric having a dithiol group and a magnetic heavy metal substance which are compounded to form a film on the surface of a magnetic powder, the magnetic powder thereafter being mixed with a binder resin and moulded under pressure to produce a magnet component. JP-A-1-321 603 proposes to improve the oxidation resistance of a magnetic alloy powder by mixing it with an organic substance dissolved in an organic solvent and thereafter pulverising, and then volatising the solvent. An object of the present invention is to provide a method of producing a solid resin-coated magnet powder for producing an improved anisotropic bonded magnet and a method of producing the anisotropic magnet. In order to achieve the object, the present invention provides a method of producing a solid resin-coated composite magnet powder for producing an anisotropic bonded magnet, comprising an anisotropic magnet powder and a solid resin formed on the surface of the magnet powder. The present invention in particular provides a method of producing a solid resin-coated magnet powder is claimed in claim 1. In the drawings: Fig. 1 is an sectional view showing a solid resin-coated magnet powder obtained by the method of the present invention; Fig. 2 is a sectional view showing the method of producing a solid resin-coated magnet powder in accordance with the invention by breaking a conventional solid resin-coated composite magnet powder; and Fig. 3 is a sectional view showing a conventional solid resin-coated composite magnet powder. As a result of investigation performed by the inventors for producing a solid resin-coated magnet powder which can be oriented more easily than a conventional magnet powder, and which causes an increase in the density of a molded product, the following findings were obtained: (1) When the solid resin 2 of a conventional solid resin-coated composite powder 5 is cracked to produce cracks 3 therein, as shown in Fig. 2, and is separated along the cracks into individual anisotropic magnet powder particles coated with the solid resin, and when the obtained solid resin-coated magnet powder 6 shown in Fig. 1 is molded in a magnetic field, the solid resin-coated magnet powder 6 is easily oriented in the direction of the magnetic field because the respective solid resin-coated magnet powder particles are separated from each other, and the anisotropic bonded magnet produced by using the solid resin-coated magnet powder 6 exhibits excellent magnetic anisotropy. (2) The solid resin-coated magnet powder 6 having the effect described in Paragraph (1) has a high degree of freedom and is easily closely packed, and thus causes an increase in the density of a molded product obtained at the same molding pressure, thereby producing an anisotropic bonded magnet having excellent magnetic characteristics. (3) Although an epoxy resin, a polyester resin, and a phenolic resin, all of which are solid at room temperature are preferred as the solid resin a bismaleimidotriazine resin (referred to as "BT resin" hereinafter) is particularly preferred. In one embodiment of the present invention there is provided a method of producing a solid resin-coated magnet powder for producing an anisotropic bonded magnet comprising anisotropic magnet powder particles which have surfaces coated with a solid resin, which is a BT resin, and which are separated from each other, and a method of producing the magnet powder. The use of the 3T resin permits the solid resin-coated powder 6 shown in Fig. 1 to be easily produced by cracking, and prevents a damage of the anisotropic magnet powder during cracking and deterioration in the magnetic characteristics, particularly coercive force. An anisotropic magnet powder used for producing the solid resin-coated magnet powder for producing an anisotropic bonded magnet is formed by maintaining, at a temperature of 600 to 1200°C, an alloy consisting of, as main components, rare earth elements including Y (referred to as "R" hereinafter), Fe or a component obtained by partially substituting Fe with Co (referred to as "T" hereinafter) and B, and 0.01 to 5.0 atomic % M (M is at least one of Ga, Hf, Nb, Ta, W, Mo, Al, Ti, Si and V) to homogenize the alloy, performing hydrogen treatment of the homogenized alloy by the method below, cooling the alloy to obtain an alloy having a recrystallized fine aggregate structure with a ferromagnetic phase, and finely grinding the alloy obtained. In the hydrogen treatment, the homogenized alloy consisting of R, T and B as main components is caused to occlude hydrogen by heating the alloy to 500°C from room temperature in an atmosphere of hydrogen and maintaining it at 500°C, is caused to further occlude hydrogen by heating the alloy to a predetermined temperature within the range of 750 to 950°C and maintaining it at this temperature to promote the phase transformation thereof, and is forced to release hydrogen therefrom by maintaining the alloy occluding hydrogen at a temperature within the range of 750 to 950°C in a vacuum atmosphere to promote the phase transformation thereof. Each of the thus-formed magnet powder particles consisting of R, T and B as main components has magnetic anisotropy. The magnet powder is kneaded with a solid resin diluted with an organic solvent such as acetone or the like in an atmosphere under a reduced pressure, preferably under a reduced pressure of 100 Torr or less, and the resultant mixture is then granulated to produce the solid resin-coated composite powder 5 coated with the solid resin 2 and containing a plurality of anisotropic magnet powder particles 1, as shown in Fig. 3. A R-Fe-B anisotropic magnet powder obtained by grinding a full-dense magnet which is made anisotropic by plastic working, anisotropic magnet powder of SmCo 5 , Sm 2 Fe 17 or Sm-Fe-N other than the magnet powder consisting of R, T and 3 as main components can be used as a magnet powder. When the solid resin-coated composite magnet powder 5 is ground together with ceramic balls of alumina, glass, or the like or plastic balls with a density of 5 g/cm 3 or less in a grinder such as a ball mill or an attritor mill, the cracks 3 are produced in the magnet powder 5, as shown in Fig. 2, and the magnet powder 5 is separated into the individual anisotropic magnet powder particles. The ceramic balls or plastic balls must be used in this operation, and the use of balls of a hard metal or stainless steel with a density of 5 g/cm 3 or more undesirably causes grinding of the magnet powder particles due to the high specific gravity thereof. The thus-obtained solid resin-coated magnet powder 6 comprises the anisotropic magnet powder particles 1 each of which exhibits magnetic anisotropy and coated with the solid resin 2, as shown in Fig. 2. When the solid resin-coated magnet powder 6 is filled in a mold and press-molded in a magnetic field, a bonded magnet exhibiting excellent anisotropy can be produced because all the magnet powder particles 1 of the magnet powder 6 are separated and are thus easily oriented in the direction of the magnetic field applied. In order to produce the excellent anisotropic bonded magnet, it is necessary that a raw material powder contains at least 50 % of the solid resin-coated magnet powder 6. In addition, since the solid resin is used in the solid resin-coated magnet powder, the magnet powder has good fluidity, and the density of the molded product is increased to the same level as that of a product formed using a liquid resin. Certain examples of the method of the invention are described in detail below with reference to examples. Example 1 An ingot formed by melting and casting in an atmosphere of Ar gas using a high-frequency furnace and consisting of 28.0 % by weight Nd, 15.0 % by weight Co, 1.0 % by weight B, 0.1 % by weight Zr, 0.5 % by weight Ga, and the balance comprising Fe and inevitable impurities was homogenized by maintaining the ingot at a temperature of 1150°C. Hydrogen treatment was then performed by the method below. The homogenized ingot was caused to occlude hydrogen by heating the ingot to 500°C from room temperature in an atmosphere of hydrogen and maintaining it at 500°C, and was then caused to further occlude hydrogen by heating the ingot to a temperature of 850°C and maintaining it at this temperature to promote phase transformation thereof. The hydrogen occluded the ingot was then forced to be released from the ingot by maintaining the ingot at a temperature of 850°C in a vacuum atmosphere to promote the phase transformation thereof. After cooling, the ingot was ground under a flow of Ar gas to produce an Nd-Fe-B anisotropic magnet powder having an average particle size of 80 µm. On the other hand, a BT resin solution obtained by adding 10 g of solid BT resin to 100 g of acetone and dissolving the resin in acetone was added to the Nd-Fe-B magnet powder at a ratio of resin component of 3 % by weight, and was then kneaded in an atmosphere under a reduced pressure of 1 Torr or less so that the solid BT resin layer was formed on the surfaces of the Nd-Fe-B magnet powder particles while the acetone was completely volatilized. The magnet powder was then granulated to form a solid BT resin-coated magnet powder. The thus-formed solid BT resin-coated magnet powder comprised a bulk solid BT resin-coated composite magnet powder containing a plurality of Nd-Fe-B magnet powder particles, as shown in Fig. 3. The solid BT resin-coated composite magnet powder was thus placed in a pot together with alumina balls, and was cracked by rotating a ball mill for 20 minutes. As a result of SEM observation of the thus-obtained the solid resin-coated magnet powder, it was found that the content of the solid resin-coated magnet powder containing anisotropic magnet powder particles each of which exhibited magnetic anisotropy and was coated with the solid resin was 90 % or more. The solid resin-coated magnet powder was filled in a mold without any treatment, and was press-molded under a pressure of 6 ton/cm 2 in a magnetic field of 20 KOe to produce a molded product having a length of 10 mm, a width of 10 mm and a height of 10 mm. The molded product obtained was then hardened by maintaining it at a temperature of 150°C for 2 hours to produce an anisotropic bonded magnet 1. Conventional Example 1 For comparison, the solid BT resin-coated composite magnet powder produced in Example 1 was filled in a mold without being cracked, and was processed under the same conditions as those in Example 1 to produce a conventional anisotropic bonded magnet 1. The density, residual flux density Br, coercive force iHc, and maximum energy product (BH)max of the anisotropic bonded magnet 1 made by the method of the invention and the conventional anisotropic bonded magnet 1 were measured. The results of measurement are shown in Table 1. Kind Density (g/cm 3 ) Magnetic Characteristics Br (KG) iHc (KOe) (BH) max (MGOe) Anisotropic bonded magnet 1 made by the method of this invention 6.21 9.3 13.6 19.4 Conventional anisotropic bonded magnet 1 6.03 8.6 13.7 16.1 Example 2 A full-dense magnet which was made anisotropic by plastic working was ground to prepare an Nd-Fe-B plastically worked magnet powder as an anisotropic magnet powder. The Nd-Fe-B magnec powder was used for producing a solid BT resin-coated composite magnet powder. The composite magnet powder produced was cracked by the same method as that in Example 1 to produce a solid resin-coated magnet powder. As a result of SEM observation of the obtained solid resin-coated magnet powder, it was found that the content of the solid resin-coated magnet powder containing anisotropic magnet powder particles exhibiting magnetic anisotropy and coated with the solid resin was 80 % or more. An anisotropic bonded magnet 2 was produced by using the solid resin-coated magnet powder under the same conditions as those in Example 1. Conventional Example 2 For comparison, the solid BT resin-coated composite magnet powder produced in Example 2 was filled in a mold without cracking, and was then processed by the same method as that in Example 2 to produce a conventional anisotropic bonded magnet 2. The density, residual flux density Br, coercive force iHc, and maximum energy product (BH)max of the anisotropic bonded magnet 2 made by the method of the invention and the conventional anisotropic bonded magnet 2 were measured. The results of measurement are shown in Table 2. Kind Density (g/cm 3 ) Magnetic Characteristics Br (KG) iHc (KOe) (BH) max (MGOe) Anisotropic bonded magnet 2 made by the method of this invention 6.16 8.6 13.5 16.4 Conventional anisotropic bonded magnet 2 6.02 7.4 13.5 12.2 Example 3 An Sm 2 Co 17 magnet powder was prepared as an anisotropic magnet powder, and was used for producing a solid resin-coated composite magnet powder. The composite magnet powder produced was cracked by the same method as that in Example 1 to produce a solid resin-coated magnet powder. As a result of SEM observation of the obtained solid resin-coated magnet powder, it was found that the content of the solid resin-coated magnet powder containing anisotropic magnet powder particles each exhibiting magnetic anisotropy and coated with the solid resin was 90 % or more. An anisotropic bonded magnet 3 was produced by using the solid resin-coated magnet powder under the same conditions as those in Example 1. Conventional Example 3 For comparison, the solid BT resin-coated composite magnet powder produced in Example 3 was filled in a mold without cracking, and was then processed by the same method as that in Example 3 to produce a conventional anisotropic bonded magnet 3. The density, residual flux density Br, coercive force iHc, and maximum energy product (BH)max of the anisotropic bonded magnet 3 made by the method of the invention and the conventional anisotropic bonded magnet 3 were measured. The results of measurement are shown in Table 3. Kind Density (g/cm 3 ) Magnetic Characterisitics Br (KG) iHc (KOe) (BH) max (MGOe) Anisotropic bonded magnet 3 made by the method of this invention 7.11 8.1 11.5 15.0 Conventional anisotropic bonded magnet 3 7.00 7.3 11.7 11.8 Example 4 An Sm-Fe-N magnet powder was prepared as an anisotropic magnet powder, and was used for producing a solid BT resin-coated composite magnet powder. The composite magnet powder produced was cracked by the same method as that in Example 1 to produce a solid resin-coated magnet powder. As a result of SEM observation of the obtained solid resin-coated magnet powder, it was found that the content of the solid resin-coated magnet powder containing anisotropic magnet powder particles each exhibiting magnetic anisotropy and coated with the solid resin was 50 %. An anisotropic bonded magnet 4 was produced by using the solid resin-coated magnet powder under the same conditions as those in Example 1. Conventional Example 4 For comparison, the solid BT resin-coated composite magnet powder produced in Example 4 was filled in a mold without cracking, and was then processed by the same method as that in Example 4 to produce a conventional anisotropic bonded magnet 4. The density, residual flux density Br, coercive force iHc, and maximum energy product (BH)max of the anisotropic bonded magnet 4 made by the method of the invention and the conventional anisotropic bonded magnet 4 were measured. The results of measurement are shown in Table 4. Kind Density (g/cm 3 ) Magnetic Characteristics Br (KG) iHc (KOe) (BH) max (MGOe) Anisotropic bonded magnet 4 made by the method of this invention 5.72 8.0 7.5 12.1 Conventional anisotropic bonded magnet 4 5.57 7.2 7.7 9.8 The results shown in Tables 1 to 4 reveal that the anisotropic bonded magnet produced by using a solid resin-coated magnet powder in accordance with certain embodiments of the method of the invention exhibits maximum energy product (BH)max and magnetic characteristics which are better than those of the conventional anisotropic bonded magnet produced by using the conventional solid resin-coated composite magnet powder. Examples 5 to 7 A solid resin-coated magnet powder was produced by using the Nd-Fe-B magnet powder produced in Example 1 and each of solid epoxy and solid polyester resins as a resin. Anisotropic bonded magnets 5 and 6 were respectively produced by using the produced solid resin-coated magnet powders by the same method as that in Example 1, and were compared with the anisotropic bonded magnet 1 produced by using the solid BT resin in Example 1. The results of comparison are shown in Table 5. Kind Coating resin is parenthesized. Density (g/cm 3 ) Magnetic Characteristics Br (KG) iHc (KOe) (BH) max (MGOe) Anisotropic bonded magnet 1 made by the method of this invention (BT resin) 6.21 9.3 13.6 19.4 Anisotropic bonded magnet 5 made by the method of this invention (epoxy resin) 6.18 9.2 11.5 18.0 Anisotropic bonded magnet 6 made by the method of this invention (polyester resin) 6.19 9.2 11.7 17.5 The results shown in Table 5 reveal that magnetic characteristics of the anisotropic bonded magnet produced by using the solid BT resin are better than those of the anisotropic bonded magnet produced by using the solid epoxy resin or solid polyester resin. It is thus found that the solid BT resin is more preferred as a solid resin than the solid epoxy resin and the solid polyester resin. As described above, a solid resin-coated magnet powder in accordance with certain embodiments of the method of the present invention can provide a bonded magnet exhibiting excellent magnetic anisotropy, as compared with a conventional bonded magnet, and has excellent industrial effects.

Claims: A method of producing a solid resin-coated magnet powder comprising individual anisotropic magnet powder particles each having a surface coated with solid resin, which method comprises mixing an anisotropic magnet powder with a solution or suspension of a resin in a solvent whilst extracting the solvent under reduced pressure so as to produce a granular product, characterized in that the granules of the granular product (5) are thereafter broken into individual solid resin-coated magnetic powder particles (6) by grinding with ceramic or plastic balls having a density of 5g/cm 3 or less. A method as claimed in claim 1 wherein the said solid resin is a bismaleimidotriazine resin. A method of producing an anisotropic bonded magnet comprising producing a solid resin-coated magnet powder by the method according to claim 1 or claim 2 and preparing the magnet therefrom.

Inventors: NAKAYAMA RYOJI, TAKESHITA TAKUO, WATANABE MUNEAKI, NAKAYAMA, RYOJI, TAKESHITA, TAKUO, WATANABE, MUNEAKI, NAKAYAMA, RYOJI CHUO-KENKYUSHO, TAKESHITA, TAKUO CHUO-KENKYUSHO, WATANABE, MUNEAKI CHUO-KENKYUSHO
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1993
Main classification: H01F1
IPC codes: H01F1