Rare-earth bonded magnet, material therefor and method for manufacturing a bonded magnet

Abstract: A rare-earth bonded magnet is improved in its heat resistance by coating rare-earth magnetic powder with heat resisting addition polymerized thermosetting resin consisting mainly of triazine resin because oxidation of the rare-earth magnetic powder is prevented or retarded by triazine rings formed in the coating film of the thermosetting resin consisting mainly of the triazine resin. The heat resistance of the bonded magnet is further improved by curing the thermosetting resin in a vacuum and by adding organometallic salt in the thermosetting resin as a metallic catalyser since the coating film is formed more firmly.

Description: This invention relates to a rare-earth bonded magnet used widely for industrial products such as automobiles, business machines, domestic electrification machines and sounder machines, and to a material and a method suitable for manufacturing the rare-earth bonded magnet. Heretofore, there has been widely used Alnico magnets and Ferrite magnets as permanent magnets. However, rare-earth magnets have been developed, which have excellent magnetic properties as compared with the aforementioned magnets, and the application and demand of the rare-earth magnets has increased remarkably in recent years. The rare-earth magnets contain active metals and are easily oxidized. Therefore, the rare-earth magnets of this kind are inferior in their corrosion resistance and heat-resisting properties, especially in an atmosphere at a temperature higher than room temperature. Among the rare-earth magnets, R-Fe-B magnets and R-Fe-N magnets have Fe(iron) for their main element in addition to R(rare-earth metals), and are oxidized more remarkably as compared with Sm-Co magnets. Accordingly, the R-Fe rare-earth magnets have excellent magnetic properties. However, they have serious problems from the view point of their oxidation resistance, their corrosion resistance and their temperature characteristics and heat resistance at a temperature higher than room temperature. Among the rare-earth magnets, sintered magnets are densified by sintering. Therefore, it is possible to improve the heat resistance of sintered magnets considerably by coating the surface of the magnet with, for example, Ni, or resin at the final stage of the magnet manufacturing process. Among the bonded magnets, especially in a magnet manufactured by injection molding using thermoplastic resin such as polyamide resin, it is possible to improve its heat resistance by coating the surface of the magnet in a manner similar to that described above in relation to sintered magnets because the surface of the magnetic powder is covered completely with resin. For bonded magnets, in a magnet manufactured by compression molding using a binder such as thermosetting resin (for example, epoxy resin), metals or the like, a large number of vacancies exist between the powdered magnet material and the binder. Therefore, it is not possible to prevent the magnet from oxidizing owing to the internal vacancies even if the surface of the magnet is coated completely, and oxidation of the magnetic material through the coating layer and the internal vacancies is unavoidable. Consequently, there is a problem in that secular change of the magnetic properties at room temperature and above becomes large and the heat resistance of the magnet is degraded with the passage of time. JP-A-1114006 discloses the use of a triazine derivative having a dithiol group, and a heavy metal of a magnetic substance to form an organic film on the surface of a magnetic powder. JP-A-2117105 discloses a method of manufacturing a bonded magnet which comprises adding a condensed aminotriazine phosphate compound to a mixture of polyamide and magnetic powder material. JP-A-2006573 discloses a corrosion resistant coating material containing a mercaptotriazine derivative and a tanic acid or a derivative thereof. This invention is made in view of the aforementioned problems of the prior art. Accordingly, it is an aim of this invention to provide a rare-earth bonded magnet with improved heat-resistance characteristics by preventing the oxidation of the rare-earth magnetic material as much as possible and decreasing the secular change of the magnetic properties at room temperature and higher temperatures. The present invention provides a material for a rare-earth bonded magnet comprising a rare-earth magnetic powder coated with a heat resisting addition polymerizable thermosetting resin, in an amount of not more than 2 wt% based on the weight of the magnetic powder, which includes cyanato (-R-O-C≡N) groups. The present invention also provides a rare-earth bonded magnet prepared by agglomerating a rare-earth magnetic powder coated with a heat resisting addition polymerizable thermosetting resin, in an amount of not more than 2 wt% based on the weight of the magnetic powder, which includes cyanato (-R-O-C≡N) groups together with a binder. Preferably, the rare-earth magnetic powder is coated with the heat resisting addition polymerizable resin in an amount of from 0.01 to 1.00 wt% based on the weight of the magnetic powder. More preferably, the rare-earth magnetic powder coated with the heat resisting addition polymerizable thermosetting resin is agglomerated together with a binder and a metallic catalyst comprising an organometallic salt. The present invention further provides a method for manufacturing a rare-earth bonded magnet by compacting a rare-earth magnetic powder together with a binder which comprises coating a surface of said rare-earth magnetic powder with a heat resisting addition polymerizable thermosetting resin which includes cyanato (-R-O-C≡N) groups by adding said thermosetting resin into the rare-earth magnetic powder almost at the same time as addition of the binder, subsequently molding a compact by pressing the rare-earth magnetic powder coated with the thermosetting resin and curing the thermosetting resin in said compact. An organometallic salt as a metallic catalyst may be added together with the binder and the heat resisting addition polymerizable thermosetting resin, and the curing of the thermosetting resin may be carried out at a temperature of not lower than 150°C in a vacuum or in an atmosphere or argon. The present invention will now be described in greater detail below. In the present invention, magnetic powder containing rare-earth metals such as R-Fe, R-Fe-B, R-Fe-N and the like are used as rare-earth magnetic powder. As the resin to be coated on the surface of the rare-earth magnetic powder, a heat resisting addition polymerizable thermosetting resin is used which includes cyanato groups -R-O-C≡N. The material for the rare-earth bonded magnet according to this, invention comprises rare-earth magnetic powder coated with heat resisting addition polymerizable thermosetting resin which includes cyanato (-R-O-C≡N) groups, together with binder if required. Various methods of coating may be employed, such as a method of coating the rare-earth magnetic powder by dipping it into a solution (for example, methyl ethyl ketone may be used as a solvent) containing the heat resisting addition polymerizable thermosetting resin, a method of mixing the rare-earth magnetic powder after adding the heat resisting addition polymerizable thermosetting resin in it, and a method of coating the rare-earth magnetic powder by vaporizing the polymerizable thermosetting resin and depositing it on the surface of the magnetic powder. The rare-earth bonded magnet according to this invention is formed by agglomerating rare-earth magnetic powder using binder, which magnetic powder is coated with heat resisting addition polymerizable thermosetting resin together with binder if required. An epoxy resin may be used as the binder. The magnetic powder is molded (agglomerated) into a compact having the desired shape by forming methods such as compression molding and so on. After molding the compact, it is preferable to cure the thermosetting resin added as the binder and the heat resisting addition polymerizable thermosetting resin at a temperature of not lower than 150°C in a non-oxidative atmosphere or in a vacuum. In the curing treatment, the thermosetting resin used as a binder is hardened, while the heat resisting addition polymerizable thermosetting resin is hardened by heating to form triazine rings therein. The triazine ring is remarkably stable to thermal energy, so that the heat resistance of the resin is improved. In order to more uniformly coat the heat resisting addition polymerizable thermosetting resin onto the surfaces of the respective particles of the rare-earth magnetic powder, it is desirable to perform the curing at a temperature of not lower than 150°C in a vacuum. This is because the triazine resin is thereby vapourized temporarily and hardened after depositing on the surface of the rare-earth magnetic powder in a very uniform manner. For manufacturing the rare-earth bonded magnet in such a manner, it is preferable to add an organometallic salt such as zinc octylate, iron acetylacetonate or the like as a metallic catalyst together with the binder and the heat resisting addition polymerizable thermosetting resin. Namely, it is possible to further reduce the secular change of the magnetic porperties because adhesion between the rare-earth magnetic powder and the heat resisting addition polymerizable thermosetting resin is improved by addition of the organometallic salt as the metallic catalyst, and a firm coating film having heat resistance can be obtained. In the present invention, the rare-earth magnetic powder is coated with the heat resisting addition polymerizable thermosetting resin which includes cyanato (-R-O-C≡N) groups on a surface thereof, and the coated magnetic powder is used in this form. Consequently, it is possible to retard or prevent the oxidation of the magnetic material. The heat resistance of the magnet is improved and the secular change of the rare-earth bonded magnet at room temperature and above is reduced remarkably. The present invention is illustrated below with reference to the following non-limiting Examples. EXAMPLE 1 At first, by spraying rare-earth molten magnetic alloy consisting essentially of 28wt% of Nd - 0.9wt% of B - 5.0wt% of Co - Fe (remainder) on the surface of a copper roll rotating at the peripheral speed of 25m/sec, a ribbon of about 30µm in thickness was obtained, and rare-earth magnetic powder was obtained by comminuting the ribbon into a size smaller than 200µm. Subsequently, the rare-earth magnetic powder was annealed for 10 minutes at 550°C. Next, as a binder epoxy resin, which is a thermosetting resin, was added into the annealed rare-earth magnetic powder in an amount as much as 2wt%, and a heat resisting addition polymerizable thermosetting resin which includes cyanato (-R-O-C≡N) groups was added to the magnetic powder in the respective amounts shown in Table 1. Then, these components were mixed uniformly. Zinc octylate was further added in some of the samples as a metallic catalyst in an amount as much as 0.0006wt% of the heat resisting polymerisable thermosetting resin as indicated in Table 1. Then, each of the mixed powders was compressed into a compact of 10mm in diameter and 7mm in height, and cured for 1 hour at 170°C in an atmosphere of argon. Furthermore, rare-earth bonded magnets were obtained by polarizing the cured compacts in a pulsed magnetic field of 50k0e, and open flux values of the polarized magnet were measured, respectively. The open flux values were measured again at room temperature after holding the magnets for 1000 hours at 180°C, whereby the rates of decrease of the open flux values, that is the irreversible demagnetizing factors, were obtained. The results of the measurement of the irreversible demagnetizing factors are shown in Table 1. Sample Number Triazine resin content (wt%) Irreversible demagnetizing factor (%) Without zinc octylate Addition of zinc octylate Conventional example 1 0 55.0 55.0 2 0.01 38.0 32.5 3 0.05 27.5 20.6 4 0.10 10.8 9.9 Inventive example 5 0.20 9.4 8.4 6 0.30 8.5 7.3 7 0.50 8.2 7.3 8 1.00 8.9 8.2 As shown in Table 1, in the case of the conventional, Example No 1 which was not coated with heat resisting polymerizable resin which includes cyanato (-R-O-C≡N) groups on the surface of the rare-earth magnetic powder, the irreversible demagnetizing factor after holding for 1000 hours at 180°C was considerably large. As compared with the above, in the case of Example Nos 2-8 according to this invention which were coated with a heat resisting resin which includes cyanato (-R-O-C≡N) groups, it was confirmed that the irreversible demagnetizing factor becomes considerably smaller when that resin is coated more than some degree. However, it is desirable to coat the magnetic powder with not more than 2wt% since the magnetic properties are degraded if the resin is coated in excess. In addition, it was confirmed that the irreversible demagnetizing factor becomes smaller when an organometallic salt is added. Example 2 By spraying molten rare-earth magnetic alloy consisting essentially of 28wt% of Nd - 0.9wt% of B - 5.0wt% of Co - Fe (remainder) on the surface of a copper roll rotating at the peripheral speed of 25m/sec, a ribbon of about 30µm in thickness was obtained, and rare-earth magnetic powder was obtained by comminuting the ribbon into a size of smaller than 200µm. The thus obtained rare-earth magnetic powder was annealed for 10 minutes at 550°C. Secondly, 2wt% of epoxy resin which is a thermosetting resin, was added to the annealed rare-earth magnetic resin powder as a binder, and resin which includes cyanato (-R-O-C≡N) groups (the heat resisting addition polymerizable thermosetting resin) was added to the magnetic powder in the amounts shown in Table 2. Then, these components were mixed uniformly in a manner similar to Example 1. Zinc octylate was also added in some of samples as a metallic catalyst in an amount as much as 0.0006wt% of the content of the resin which includes cyanato (-R-O-C≡N) groups. Then, each of the mixed powders was compressed into a compact of 10mm in diameter and 7mm in height, and the compact was cured for 1 hour at 170°C in a vacuum. Then, the cured compacts were polarized in a pulsed magnetic field of 50 kOe, and open flux values of each of the polarized magnets were measured. The open flux values were measured again at room temperature after holding the magnets for 1000 hours at 180°C, whereby the rates of decrease of the open flux values, that is the irreversible demagnetizing factors were obtained. The results of the measurement of the irreversible demagnetizing factors are shown in Table 2. Sample Number Triazine resin content (wt%) Irreversible demagnetizing factor (%) Without zinc octylate Addition of zinc octylate Conventional example 9 0 53.5 53.5 10 0.01 13.6 11.5 11 0.05 10.8 9.1 12 0.10 9.3 8.8 Inventive example 13 0.20 8.5 8.4 14 0.30 8.2 7.7 15 0.50 7.9 7.5 16 1.00 8.0 7.8 As shown in Table 2, in the case of performing the curing in a vacuum, it was confirmed that the irreversible demagnatizing factor after holding for 1000 hours at 180°C becomes smaller and the heat resistance of the magnet is further improved since the resin which includes cyanato (-R-O-C≡N) groups is coated more uniformly on the respective surfaces of the magnetic powder. Example 3 After obtaining rare-earth magnetic powder consisting essentially of 31.1wt% of Nd - 1.0wt% of B - Fe (remainder) in the same manner as Example 1, 2.0wt% of epoxy resin as a binder and 0.3wt% of resin which includes cyanato (-R-O-C≡N) groups (the heat resisting addition polymerizable thermosetting resin) were added to the annealed rare-earth magnetic powder, and those components were mixed uniformly. In some of the samples, zinc octylate was further added as a metallic catalyst in an amount as much as 0.0006wt% of the content of the resin which includes cyanato (-R-O-C≡N) groups. Next, each of the mixed powders was compressed into a compact of 10mm in diameter and 7mm in height. The compacts were cured for 1 hour at 170°C, either in air, in an atmosphere of argon, or in a vacuum as indicated in Table 3 below. Then, the cured compacts were polarized in a pulsed magnetic field of 50kOe, and open flux values of the polarized magnets were measured, respectively. The rates of decrease of the open flux values, that is, the irreversible demagnetizing factors were obtained by measuring the open flux values at room temperature after holding them for 1000 hours at 180 C. The measured results are shown in Table 3. Sample Number Triazine resin content (wt%) Zinc octylate Curing Irreversible demagnetizing factor (%) Conventional example 17 0 None in Air 32.5 18 0 None in Argon 41.5 19 0 None in Vacuum 49.0 Inventive example 20 0.3 None in Air 4.9 21 0.3 None in Argon 5.1 22 0.3 None in Vacuum 3.5 23 0.3 Addition in Air 4.4 24 0.3 Addition in Argon 4.6 25 0.3 Addition in Vacuum 3.0 As is evident from Table 3, the conventional Examples (Nos. 17-19) which contain no resin which includes cyanato (-R-O-C≡N) groups or organometallic salt showed large irreversible demagnetizing factors after holding for 1000 hours at 180°C. As compared with that Example, in Example Nos 20-22 which contain resin which includes cyanato (-R-O-C≡N) groups but no organometallic salt and Example Nos 23-25 which contain resin which includes cyanato (-R-O-C≡N) groups and an organometallic salt, considerably smaller irreversible demagnetizing factors are observed after holding them for 1000 hours at 180°C. It has been confirmed that the irreversible demagnetizing factor of the magnet cured in a vacuum is smaller than that of magnet cured in the air or an atmosphere of argon, and curing in a vacuum is effective for further improving the heat resistance of the magnet. Furthermore, it has become clear that it is possible to further decrease the irreversible demagnetizing factor by adding an organometallic salt as a metallic catalyst. Example 4 An ingot having a composition represented by Sm2Fe17 was subjected to a homogenizing treatment by.heating it for 24 hours at a temperature of 1100°C, and grinding mechanically into powder of the size passing 120 mesh. Then, the powder was subjected to nitriding by heating for 5 hours at a temperature of 550°C in an atmosphere of nitrogen. Secondly, fine rare-earth magnetic powder was obtained by comminuting the nitrided powder into particles of 3µm in mean diameter. Subsequently, to the rare-earth magnetic powder was added 2wt% of epoxy resin as a binder, and 0.3wt% of resin which includes cyanato (-R-O-C≡N) groups (the heat resisting addition polymerizable thermosetting resin). In some samples, iron acetylacetone was further added as a metallis catalyst in an amount as much as 0.0015wt% of the resin which includes cyanato (-R-O-C≡N) groups. Then, each of the mixed powders was compressed into a compact of 10mm in a diameter and 7mm in height in a vertical magnetic field of 15kOe, and the compacts were cured for 1 hour at 170°C, either in an atmosphere of argon or in a vacuum as indicated in Table 4. The results shown in Table 4 were obtained by measuring the irreversible demagnetizing factors after holding for 1000 hours at 180°C in the same manner as Example 1. The following were typical magnetic properties of these rare-earth bonded magnets; Br (residual magnetic flux density) : 8.0KG, iHc (coercive force) : 8.5kOe, (BH)max (maximum energy product) : 11.8MGOe. Sample Number Triazine resin content (wt%) Iron acetyl-acetonate Curing Irreversible demagnetizing factor (%) Conventional example 26 0 None in Argon 13.5 Inventive example 27 0.3 None in Argon 4.3 28 0.3 None in Vacuum 2.5 29 0.3 Addition in Argon 3.1 30 0.3 Addition in Vacuum 2.0 As shown in Table 4, conventional Example No 26 which contains no resin which includes cyanato (-R-O-C≡N) groups or organometallic salt exhibited a large irreversible demagnetizing factor after holding for 1000 hours at 180°C. As compared with the above conventional Example, the inventive Example Nos 27 and 28 which contain resin which includes cyanato (-R-O-C≡N) groups but no organometallic salt, and the inventive Example Nos 29 and 30 which contain both resin which includes cyanato (-R-O-C≡N) groups and an organometallic salt, exhibited considerably smaller irreversible demagnetizing factors after holding for 1000 hours at 180°C. It has been confirmed that the irreversible demagnetizing factor becomes smaller in the case of performing the curing in a vacuum, and that it is effective to carry out the curing in a vacuum for further improving the heat resistance of the magnet. Additionally, it was also confirmed that it is possible to further decrease the irreversible demagnetizing factor by adding an organometallic salt as a metallic catalyst. As mentioned above, in accordance with the present invention, it is possible to prevent the oxidation of the rare-earth magnetic powder which is easily oxidized in itself. Furthermore, the long term (i.e., secular) change of the magnetic properties of the rare-earth bonded magnet at room temperature and at temperatures higher than room temperature becomes smaller. Accordingly, an excellent effect can be obtained since it is possible to provide a rare-earth bonded magnet having improved heat resistance. In this specification the phrase "almost at the same time" means that the thermosetting resin is added into the rare-earth magnetic powder before, at the same time, or after the addition of the binder.

Claims: A material for a rare-earth bonded magnet comprising a rare-earth magnetic powder coated with a heat resisting addition polymerizable thermosetting resin, in an amount of not more than 2 wt% based on the weight of the magnetic powder, which includes cyanato (-R-O-C≡N) groups. The material of claim 1, wherein the rare-earth magnetic powder is coated with the heat resisting addition polymerizable resin in an amount of from 0.01 to 1.00 wt% based on the weight of the magnetic powder. A rare-earth bonded magnet prepared by agglomerating a rare-earth magnetic powder coated with a heat resisting addition polymerizable thermosetting resin, in an amount of not more than 2 wt% based on the weight of the magnetic powder , which includes cyanate (-R-O-C≡N) groups together with a binder. The rare-earth bonded magnet of claim 3, wherein the rare-earth magnetic powder is coated with the heat resisting addition polymerizable resin in an amount of from 0.01 to 1.00 wt% based on the weight of the magnetic powder. The rare-earth bonded magnet of claim 3, wherein said rare-earth magnetic powder coated with a heat resisting addition polymerizable thermosetting resin is agglomerated together with a binder and a metallic catalyst comprising an organometallic salt. A method for manufacturing a rare-earth bonded magnet by compacting a rare-earth magnetic powder together with a binder which comprises coating a surface of said rare-earth magnetic powder with a heat resisting addition polymerizable thermosetting resin which includes cyanate (-R-O-C≡N) groups by adding said thermosetting resin into the rare-earth magnetic powder almost at the same time as addition of the binder, subsequently molding a compact by pressing the rare-earth magnetic powder coated with the thermosetting resin and curing the thermosetting resin in said compact. The method of claim 6, wherein an organometallic salt as a metallic catalyst is added together with the binder and the heat resisting addition polymerizable thermosetting resin into the rare-earth magnetic powder. The method of claim 6 or claim 7, wherein said curing of the thermosetting resin is carried out at a temperature of not lower than 150°C in an atmosphere of argon. The method of claim 6, or claim 7, wherein said curing of the thermosetting resin is carried out at a temperature of not lower than 150°C in a vacuum.

Inventors: FURUYA TAKASHI, HAYASHI NAOKI, FURUYA, TAKASHI, HAYASHI, NAOKI, HAYASHI, NAOKI, NAKATSUGAWA HEIM D-205,
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1992
Main classification: H01F1
IPC codes: H01F1, H01F41