Rare earth bonded magnet and composition therefor

Abstract: A rare earth bonded magnet formed by bonding a rare earth magnet powder with a binding resin contains the rare earth magnet powder, and a thermoplastic resin serving as the binding resin in an amount within a range of from 1 to 5 wt.%, and preferably further contains an oxidation inhibitor. As the rare earth magnet powder, for example, at least one of an Sm-Co alloy, an R-Fe-B alloy (where, R represents at least a kind of rare earth elements including Y) and an Sm-Fe-N alloy can be appropriately used. As the thermoplastic resin, for example, polyamide, a liquid crystal polymer, or a PPS is appropriately employed. As the oxidation inhibitor, a chelating agent is appropriately applicable. In such a rare earth bonded magnet, the thermoplastic resin covers the outer surface of the rare earth magnet powder 2, and is present so as to prevent particles of the magnet powder from coming into contact with each other. Such a rare earth bonded magnet should preferably have a void ratio of up to 2 vol. %. A rare earth bonded magnet can by manufactured by using a composition for a rare earth bonded magnet comprising a mixture of the rare earth magnet powder, the thermoplastic resin, and the oxidation inhibitor, or a kneaded mass formed by kneading such a mixture, and extruding this composition for a rare earth bonded magnet. The amounts of addition of the thermoplastic resin and the oxidation inhibitor in the composition for a rare earth bonded magnet should be such that, when manufacturing a bonded magnet by extruding the composition for a rare earth bonded magnet, a fluidity sufficient to permit extrusion in forming is ensured. In accordance with the present invention having the construction as described above, there is available a rare earth bonded magnet which is excellent in formability, magnetic properties and has a high mechanical strength, with a slight amount of binding resin.

Description: The present invention relates to a rare earth bonded magnet molded by bonding a rare earth magnet powder with a bonding resin (binder) and a composition of a rare earth bonded magnet to manufacture the same. A rare earth bonded magnet is manufactured by using a mixture (compound) of a rare earth magnet powder and a binding resin (binder), and molding the mixture under pressure into a desired shape. Commonly employed molding methods include the compaction molding method, the injection molding method and the extrusion method. The compaction molding method comprises the steps of packing the compound into a pressing die, compacting the packing with pressure to obtain a molded body, and then heating the molded body for setting the thermosetting resin used as the binding resin, thereby manufacturing a magnet. As compared with the other methods, this method permits molding with a smaller amount of binding resin. The resultant magnet contains a smaller amount of resin, and this is advantageous in enhancing magnetic properties. This method suffers, however, from a low degree of versatility with respect to the magnet shape and a low productivity. The injection molding method comprises the steps of heating the compound to melt the thermoplastic resin of the compound, injecting the resultant melt into a mold while the melt has a sufficient fluidity, and molding the melt into a prescribed shape of magnet. This method is advantageous in that a high degree of versatility with respect to the shape of magnet is available, permitting easy manufacture even of irregular shaped magnets. However, because a high level of fluidity of the melt is required during molding, it is necessary to increase the amount of binding resin, leading to a drawback of poor magnetic properties of the resultant magnet. The extrusion molding method comprises the steps of heating the compound fed into an extruder to melt the thermoplastic resin of the compound, extruding the compound from a mold of the extruder and simultaneously cooling it for solidification, and cutting the resultant long molded body into prescribed lengths, thereby manufacturing magnets. This molding method has advantages of both the compaction molding method and the injection molding method. More specifically, the extrusion molding method permits freely setting a shape of a magnet through selection of a mold, easy manufacture of a thin-walled or long magnet, and because a high level of melt fluidity is not required, allows molding with a smaller amount of added binding resin than that in the injection molding method, thus contributing to the enhancement of magnetic properties. As disclosed in JP-B-56-31841 and JP-B-56-44561, a thermosetting resin such as an epoxy resin has been used as a binding resin contained in the foregoing compound, and because of the properties of thermosetting resins, it has been possible to use such a small amount of addition as from 0.5 to 4 wt.%. When employing a thermoplastic resin as the binding resin, however, the effects of the amount of addition and the state of the resin in the bonded magnet on moldability, magnetic properties and mechanical properties have not as yet been clarified. The document JP-A-59-136909 discloses a composition for a rare earth bonded magnet in the form of a kneeded material composed of 50 to 96 wt.% magnet powder, Sm 2 TM 17 (TM is a transition metal mainly consisting of cobalt) and thermal plastic resin. For producing a magnet from this material, the material is magnetized in a first magnetic field and then injection molded in a second magnetic field, the second magnetic field being lower than the first magnetic field. The document JP-A-53-04013 discloses a composition for a rare earth bonded magnet composed of Nd-Fe-B magnetic powder of 90 to 95 wt.%, an oxidation-preventing agent of 0.005 to 1.0 wt.%, a coupling agent and nylon 13. The magnetic powder is coated with the coupling agent and the oxidation-preventing agent consists of the mixture of hindered phenol, phosphorous and tioether. The document US-A-4,462,919 discloses a composition for a rare earth bonded magnet obtained by coating the surface of rare earth-cobalt powder with a resin in order to prevent its oxidative deterioration, and filling a thermal plastic resin with the rare earth-cobalt powder in an amount of 70 to 97 wt.%. A magnet is produced from this composition by subjecting the thermal plastic resin to injection molding in a magnetic field. The document JP-A-62-208608 discloses a composition for a bonded magnet consisting of 3 to 20 wt.% nylon, 79.8 to 95 wt.% magnetic powder, 1 to 5 wt.% lubricant and 0.5 to 3 wt.% of a surface treatment agent of an organic metal compound. It is an object of the present invention to provide a rare earth bonded magnet, bonded with a small amount of thermoplastic resin serving as a binding resin, which is excellent in moldability and magnetic properties and has a high mechanical strength, and a composition for a rare earth bonded magnet for the manufacture thereof. These objects are achieved with a rare earth bonded magnet as claimed in claims 1 and a composition for a rare earth bonded magnet as claimed in claim 10. Preferred embodiments of the invention are subject-matter of the dependent claims. Other objects, constructions and effects of the present invention will be better understood from the following detailed description of preferred embodiments and from specific examples presented later. Fig. 1 is an enlarged sectional view illustrating a modelled section of the rare earth bonded magnet of the present invention. First, the rare earth bonded magnet of the present invention will be described. The rare earth bonded magnet of the present invention contains the following rare earth magnet powder and thermoplastic resin, and as required, further contains an oxidation inhibitor. 1. Rare earth magnet powder A rare earth magnet powder should preferably comprise an alloy containing at least one rare earth element and at least one transition metal, and particularly preferable are the following alloys [1] to [5]: [1] An alloy comprising, as main ingredients, Sm (one or more other rare earth elements may additionally be included in which case Sm has the highest proportion among the rare earth elements) and one or more transition metals including Co (hereinafter referred to as "Sm-Co alloy"). [2] An alloy comprising, as main ingredients, R (R represents at least one of rare earth elements including Y), Fe as transition metal (one or more other transition metals may additionally be included in which case Fe has the highest proportion among the transition metals), and B (hereinafter referred to as "R-Fe-B alloy"). [3] An alloy comprising, as main ingredients, Sm lone or more other rare earth elements may additionally be included in which case Sm has the highest proportion among the rare earth elements), Fe (one or more other transition metals may additionally be included in which case Fe has the highest proportion among the transition metals), and N (one or more other interstitial elements may additionally be included in which case N has the highest proportion among the interstitial elements) (hereinafter referred to as "Sm-Fe-N alloy"). [4] An alloy comprising, as main ingredients, R (R represent at least one of rare earth elements including Y) and one or more transition metals such as Fe, and having a magnetic phase on nanometer level (nanocrystalline magnet). [5] An alloy comprising a mixture of at least two of the foregoing compositions [1] to [4]. With this composition, the resultant magnet can have advantages of all the kinds of magnet powder in the mixture, particularly in magnetic properties, thus making it possible to easily obtain more excellent magnetic properties. Especially when mixing two or more kinds of anisotropic magnetic powder, the resultant magnet has an improved degree of alignment. Typical Sm-Co alloys include SmCo 5 and Sm 2 TM 17 (where, TM is a transition metal). Typical R-Fe-B alloys include Nd-Fe-B, Pr-Fe-B, Nd-Pr-Fe-B, Ce-Nd-Fe-B, Ce-Pr-Nd-Fe-B and alloys resulting from partial substitution of Fe of the foregoing alloys by Ni, Co or any other transition metal. Typical Sm-Fe-N alloys include Sm 2 Fe 17 N 3 formed by nitriding an Sm 2 Fe 17 alloy. Rare earth elements used in the magnetic powder include Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Mischmetal, and one or more thereof can be contained. Applicable transition metals include Fe, Co and Ni, and one or more can be contained. With a view to improving magnetic properties, the magnet powder may contain, as required, B, Al, Mo, Cu, Ga, Si, Ti, Ta, Zr, Hf, Ag or Zn. While there is no particular restriction on the average particle diameter of the magnet powder, the average particle diameter should preferably be within a range of from about 0.5 to 50 µm, or more preferably, from 1 to 30 µm. The average particle diameter of the magnet powder can be measured, for example, by the F.S.S.S. (Fischer sub-sieve sizer) method. For the purpose of obtaining a satisfactory moldability in injection molding or extrusion molding with a small amount of binding resin as described later, the particle diameter distribution of the magnet powder should preferably be relatively wide, i.e., dispersed to some extent. This permits reduction of the void ratio of the resultant bonded magnet. In the case of composition [5] as described above, the average particle diameter may differ between individual compositions of magnet powder to be mixed. When using a mixture of two or more kinds of magnet powder of different particle diameters, sufficient mixing and kneading ensures a higher probability of achieving a state in which magnet powder particles of smaller particle diameters come between those of larger particle diameters, thus allowing an increased packing density of magnet powder particles within the compound, hence contributing to the improvement of magnetic properties of the resultant bonded magnet. There is no particular restriction on the method of manufacturing a magnet powder: for example, a product available by making an alloy ingot by melting and casting the alloy and then milling (and screening) this alloy ingot to an appropriate particle size, or a product available by manufacturing a melt spun ribbon (a collection of fine polycrystals) in a melt spinning apparatus for manufacturing amorphous alloy, and milling (and screening) this ribbon into an appropriate particle size may be used. 2. Binding resin (binder) A thermoplastic resin is used as the binding resin (binder). When employing a thermosetting resin such as an epoxy resin conventionally used as a binding resin, the poor fluidity during molding leads to a low moldability, an increased void ratio, and low mechanical strength and corrosion resistance. When employing a thermoplastic resin, in contrast, these problems are solved. This provides a wider selection including one giving a high moldability, or one giving higher heat resistance and mechanical strength, varying with the kind and extent of copolymerization. Applicable thermoplastic resins include, for example, polyamide (eg: nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, nylon 6-12, nylon 6-66), thermoplastic polyimide, liquid crystal polymers such as aromatic polymer, polyphenylene oxide, polyphenylene sulfide, polyethylene, polyolefin such as polypropylene, denatured polyolefin, polycarbonate, polymetacrylate, polyether, polyetherketone, polyetherimide, polyacetal, and copolymers, blends and polymer alloys mainly comprising any of the above. One or more of these resins may be used in mixture. Among the thermoplastic resins enumerated above, those mainly comprising one or more of polyamide, liquid crystal polymer and polyphenylene sulfide are preferable because of a satisfactory wettability relative to the surface of the magnet powder, resulting coverage of the outer surface of magnet powder (coated resin state), and a high mechanical strength. Polyamide is preferable also for remarkable improvement of moldability, and liquid crystal polymer and polyphenylene sulfide are preferable also for improvement of heat resistance, respectively. The thermoplastic resins provide a wider selection enabling to select those placing point on moldability, or on heat resistance or mechanical strength. The thermoplastic resin used in the present invention should preferably have a melting point of no more than 400°C, or more preferably, no more than 300°C. A melting point of over 400°C leads to an increase in molding temperature and easier occurrence of oxidation of the magnetic powder or the binding resin. With a view to further improving wettability to the magnet powder surface, fluidity and moldability, the average molecular weight (degree of polymerization) of the thermoplastic resin used in the present invention should preferably be within a range of from about 10,000 to 60,000, or more preferably, from about 12,000 to 30,000. The content of the thermoplastic resin in a bonded magnet should be within a range of from about 1 to 5 wt.%, or preferably, from about 1 to 4,3 wt.%. When adding an oxidation inhibitor described later, the content of the thermoplastic resin should preferably be within a range of from about 1 to 3.8 wt.%, or more preferably, from about 1.0 to 3.6 wt.%. A lower content of the thermoplastic resin makes it difficult to conduct sufficient kneading with the magnet powder during manufacturing, and leads to a lower moldability, to easier occurrence of contact between adjacent particles of magnet powder, thus preventing that a magnet having a low void ratio and a high mechanical strength is obtained. A higher content of the thermoplastic resin results in poorer magnetic properties although moldability is satisfactory. Fig. 1 is an enlarged sectional view illustrating a modelled section of the rare earth bonded magnet of the present invention. In the rare earth bonded magnet 1 of the present invention, as shown in Fig. 1, a thermoplastic resin 3 serving as a binding resin covers the outer surface of the particles of magnet powder 2 in a state that adjacent particles of magnet powder 2 are prevented from coming into contact with each other (hereinafter referred to as the "resin matrix state"). Accordingly, a magnet having a low void ratio, a high mechanical strength, and an excellent corrosion resistance is available even with a relatively small content of the thermoplastic resin as described above. This state of the thermoplastic resin is achievable by setting, during the manufacturing process of the rare earth bonded magnet, appropriate kneading conditions of a composition for the rare earth bonded magnet (mixture of the magnet powder, the binding resin and the like) and appropriate molding conditions of the kneaded mass (compound). 3. Oxidation inhibitor An oxidation inhibitor is an additive added to a composition for a rare earth bonded magnet described later for preventing, upon kneading such a composition, the rare earth magnet powder from being oxidized (deteriorated or denatured) or the binding resin from being oxidized (assumed to occur under the effect of metal ingredients in the rare earth magnet power acting as a catalyst). When adding this oxidation inhibitor, the amount of addition thereof should be within a range of from about 0.05 to 2.5 wt.%. Addition of the oxidation inhibitor brings about the following effects: First, it prevents oxidation of the rare earth magnet powder and the binding resin and maintains a satisfactory wettability of the binding resin relative to the surface of the rare earth magnet powder, so that it is easy to achieve the foregoing resin matrix state with a slight amount of binding resin. Secondly, since it prevents oxidation of the rare earth magnet powder, it contributes to an improvement of magnetic properties of the resultant magnet, and at the same time, serves to improve thermal stability upon kneading and forming the composition for a rare earth bonded magnet, thus ensuring satisfactory moldability with a small amount of binding resin. Since the oxidation inhibitor evaporates or decomposes during an intermediate process such as kneading or molding of the composition for a rare earth bonded magnet, part of it is present in a state of residue in the resultant rare earth magnet. The content of the oxidation inhibitor in the rare earth bonded magnet is therefore within a range of from about 10 to 90%, or more particularly, from about 20 to 80% relative to the amount of the oxidation inhibitor added to the composition for a rare earth bonded magnet. As described above, the oxidation inhibitor not only prevents the rare earth magnet powder and the binding resin from being oxidized during manufacture of a magnet, but also contributes to improvement of corrosion resistance of the resultant magnet. A chelating agent which is capable of preventing or inhibiting oxidation of the rare earth magnet powder and the binding resin is used as the oxidation inhibitor: for example, a chelating agent such as an amine compound, preferably one which makes the surface of magnet powder inactive be appropriately used. The chelating agent provides a particularly high oxidation prevention effect. As required, a plasticizer which plasticizes the binding resin (for example, stearic acid salt, fatty acid), a lubricant (for example, silicone oil, any of various waxes, fatty acid, alumina, silica, titania or any other inorganic lubricant), and/or other additives such as a molding additive may be contained in the rare earth bonded magnet of the present invention. Addition of at least any of a plasticizer and a lubricant improves fluidity of the material during kneading of the composition for rare earth bonded magnet or during molding of the bonded magnet. For this rare earth bonded magnet of the present invention, the void ratio (ratio of voids 4 to total volume) should preferably be no more than 2 vol.%, or more preferably, no more than 1.5 vol.%. A higher void ratio may result in a decrease in mechanical strength and corrosion resistance, depending upon the chemical composition of the thermoplastic resin, the content thereof, and the chemical composition and particle diameter of the magnet powder. Because of the chemical composition of the magnet powder and the high content of the magnet powder, the rare earth bonded magnet of the present invention is excellent in magnetic properties, irrespective of whether it is an anisotropic magnet or an isotropic magnet. More specifically, when the rare earth bonded magnet of the present invention is molded in the absence of a magnetic field, a maximum magnetic energy product (BH)max of at least 8 MGOe (64 kJ/m 3 ), particularly of at least 9.5 MGOe (76 kJ/m 3 ) can be achieved. When it is formed in a magnetic field, it is possible to achieve a maximum magnetic energy product (BH)max of at least 12 MGOe (96 kJ/m 3 ), particularly of at least 14 MGOe (112 kJ/m 3 ). There is no particular restriction of the shape and size of the rare earth bonded magnet of the present invention: for the shape, for example, applicable shapes include a rod shape, a prism shape, a cylindrical shape, an arch shape, a plate shape and all other shapes, and the size thereof may be large or very small. Now, the composition for a rare earth bonded magnet of the present invention will be described below. The composition for a rare earth bonded magnet of the present invention contains, as main ingredients, the foregoing rare earth magnet powder, the foregoing thermoplastic resin, and preferably though not absolutely necessary the foregoing oxidation inhibitor. In this case, the amount of addition of the thermoplastic resin and the oxidation inhibitor should be such that, when manufacturing the bonded magnet by extruding the composition for a rare earth bonded magnet, a necessary and sufficient fluidity of a melt of that composition is ensured during molding, and particularly, such that a void ratio of no more than 2 vol.% is achieved for the resultant rare earth bonded magnet. These amounts vary with various conditions such as the kind of thermoplastic resin and oxidation inhibitor, if any, the chemical composition of the magnet powder, molding method, temperature, pressure and other molding conditions, and the shape and size of the molded product. In a typical example, the amount of addition of the thermoplastic resin in the composition for a rare earth bonded magnet should preferably be within a range of from about 1 to 3.8 wt.%, or more preferably, from about 1.1 to 3.6 wt.%. The amount of addition of the oxidation inhibitor in the composition for rare earth bonded magnet should preferably be within a range of from about 0.1 to 2.0 wt.%, or more preferably, from about 0.5 to 1.8 wt.%. As the oxidation inhibitor, it is desirable to use a chelating agent as described above, as giving a particularly high oxidation preventive effect. Because the oxidation inhibitor is added to the composition for a rare earth bonded magnet, it is possible to achieve a satisfactory extrusion even with a small amount of added binding resin as described above. With an amount of added thermoplastic resin of less than 1 wt.% in the composition for a rare earth bonded magnet, however, the viscosity of the kneaded mass becomes higher causing an increased torque during kneading, and heat generation tends to accelerate oxidation of the magnet powder and the binding resin. When the amount of added oxidation inhibitor is small, therefore, it becomes impossible to sufficiently inhibit oxidation of the magnet powder and the binding resin, and the increase in viscosity of the kneaded mass (molten resin) causes deterioration of moldability, thus making it impossible to obtain a magnet having a low void ratio and a high mechanical strength. An amount of added thermoplastic resin of more than 3.8 wt.%, while improving moldability, may be disadvantageous for achieving outstanding magnetic properties, depending upon the chemical composition of the magnet powder, particle diameter and other conditions. When the amount of added oxidation inhibitor in the composition for a rare earth bonded magnet is under 0.1 wt.%, on the other hand, there is available only a limited oxidation prevention effect, and if the amount of added thermoplastic resin is small, oxidation of the magnet powder cannot sufficiently be inhibited. With an amount of added oxidation inhibitor of over 2.0 wt.%, the amount of resin relatively decreases, leading to a decreased mechanical strength of the resultant molded product. When the amount of the thermoplastic resin is relatively large, as described above, it is possible to reduce the amount of added oxidation inhibitor, and when the amount of added thermoplastic resin is small, in contrast, it is necessary to increase the amount of added oxidation inhibitor. The total amount of the thermoplastic resin and the oxidation inhibitor should therefore preferably be within a range of from about 1.1 to 4.7 wt.%, or more preferably, from about 1.1 to 4.5 wt.%. Any of the various additives as described above can be added as required to the composition for a rare earth bonded magnet. As the addition of a plasticizer improves fluidity during molding, a similar fluidity level is available with a smaller amount of binding resin. This is also the case with the addition of the lubricant. The amount of addition of each, the plasticizer and the lubricant, should preferably be within a range of from about 0.01 to 0.3 wt.%, or more preferably, from about 0.05 to 0.2 wt.%. With these amounts of addition, the plasticizer and the lubricant can effectively show their respective favorable functions. Applicable forms of the composition for a rare earth bonded magnet include a mixture of a rare earth magnet powder, a thermoplastic resin and an oxidation inhibitor, a kneaded mass formed by kneading the foregoing mixture, and pellets of this kneaded mass (for example, of a particle diameter of from 1 to 12 mm). Use of such a kneaded mass or pellets further improves moldability in extrusion. Kneading of the foregoing mixture is accomplished by means, for example, of a roll mill, a kneader or a twin screw extruder. During the kneading process, the kneading temperature, which is appropriately determined depending on the chemical composition of the thermoplastic resin and properties thereof, should preferably be at least the thermal deformation temperature or the softening temperature (softening point or glass transition point) of the thermoplastic resin. When the thermoplastic resin has a relatively low melting point, the kneading temperature should preferably be near, or higher than, the melting point of the thermoplastic resin. Kneading at such a temperature enhances the kneading efficiency and permits uniform kneading in a shorter period of time. As kneading is conducted with a decreased viscosity of the thermoplastic resin, this easily results in the state in which the thermoplastic resin covers the rare earth magnetic powder particles, and contributes to reduction of the void ratio of the resultant rare earth bonded magnet. The rare earth bonded magnet of the present invention is manufactured, for example, as follows. The composition (mixture) for a rare earth bonded magnet containing the foregoing proportions of rare earth magnet powder, thermoplastic resin, and preferably, oxidation inhibitor is sufficiently kneaded at the above-mentioned kneading temperature by means of a kneader or the like, thereby obtaining a kneaded mass of the composition for a rare earth bonded magnet. Then, the resultant kneaded mass (compound) of the rare earth bonded magnet is extruded by an extruder while heating it at a temperature of at least the melting point of the thermoplastic resin (for a polyamide resin, for example, a temperature of from 120 to 230°C), and after cooling, cut into desired lengths, thereby obtaining rare earth bonded magnets. The kneaded mass subjected to extrusion may be in the form of pellets. Another method comprises the steps of packing a mixture or a kneaded mass (compound) of the composition for a rare earth bonded magnet containing the foregoing rare earth magnet powder, thermoplastic resin, and preferably, oxidation inhibitor into a press mold, and applying a pressure within a range, for example, of from about 0.5 to 3.0 tons/cm 2 (49 to 294 MPa) onto it while heating it to a temperature of at least the melting temperature of the thermoplastic resin (for a polyamide resin, for example, a temperature of from 180 to 200°C) for compression molding, thereby obtaining a rare earth bonded magnet of a desired shape. EXAMPLES Now, the present invention will be described below further in detail by means of examples. Example 1 First, 96 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter: 20 µm), 3.4 wt.% polyamide (melting point: 175°C) and 0.6 wt.% hydrazine-based oxidation inhibitor (chelating agent) were kneaded at 230°C by means of an extruder for kneading, and extruded into a round bar having a diameter of 10 mm at 250°C. At this point, the total kneading disk part length in the barrel of the extruder (standard of kneading intensity) was 15 cm. The resultant round bar was cut into lengths of 7 mm, thereby completing rare earth bonded magnets of the present invention. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 3.6 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 9.5 MGOe (76 kJ/m 3 ), a density p = 6.06 g/cm 3 , and a void ratio of 1.3 vol.%. Example 2 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 96 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 19 µm), 3.0 wt.% polyamide (melting point = 175°C), and 1.0 wt.% hydrazine-based oxidation inhibitor (chelating agent). The kneading torque in the kneader was about 80% of that in Example 1. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 3.3 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 10.3 MGOe (82 kJ/m 3 ), a density p = 6.13 g/cm 3 , and a void ratio of 1.1 vol.%. Example 3 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 96.3 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 18 µm), 2.5 wt.% polyamide (melting point = 175°C), and 1.2 wt.% hydrazine-based oxidation inhibitor (chelating agent). As the total length of the kneading disk part in the kneading extruder was extended to 20 cm, the kneading torque was about 120% of that in Example 1. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 2.9 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 11.6 MGOe (92 kJ/m 3 ), a density ρ = 6.21 g/cm 3 , and a void ratio of 1.2 vol.%. Example 4 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 97.3 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 21 µm), 1.0 wt.% polyamide (melting point = 175°C), 1.5 wt.% hydrazine-based oxidation inhibitor (chelating agent), and 0.2 wt.% zinc isostearate (plasticizer). As the total length of the kneading disk part in the kneading extruder was extended to 30 cm, the kneading torque was about 150% of that in Example 1. r Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 1.3 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 14.3 MGOe (114 kJ/m 3 ), a density ρ = 6.54 g/cm 3 , and a void ratio of 3 vol.%. Example 5 The compound of Example 1 was continuously extruded under the same conditions as in Example 1 into a cylinder having an outside diameter of 18 mm and a wall thickness of 0.8 mm, and the resultant cylinder was cut into lengths of 7 mm, thereby manufacturing cylindrical rare earth bonded magnets. The resultant rare earth bonded magnets had almost the same chemical composition and properties as in Example 1. Example 6 The compound of Example 3 was continuously extruded under the same conditions as in Example 3 into a cylinder having an outside diameter of 18 mm and a wall thickness of 0.8 mm, and the resultant cylinder was cut into lengths of 7 mm, thereby manufacturing cylindrical rare earth bonded magnets. The resultant rare earth bonded magnets had almost the same chemical composition and properties as in Example 3. Example 7 The compound of Example 4 was compression-molded by a press molding machine at a temperature of 225°C under a pressure of 1 ton/cm 2 into a cylindrical rare earth bonded magnet having an outside diameter of 18 mm, a wall thickness of 0.8 mm and a length of 7 mm. The resultant rare earth bonded magnet had almost the same chemical composition and properties as in Example 4. Example 8 A magnet powder comprising Sm (Co 0.604 Cu 0.06 Fe 0.32 Zr 0.016 ) 8.3 in an amount of 96 wt.% (average particle diameter = 24 µm) 3.4 wt.% polyamide (melting point = 175°C) and 0.6 wt.% hydrazine-based oxidation inhibitor (chelating agent) were kneaded in a kneader at 230°C. The total length of the kneading disk part in the kneading extruder was extended to 30 cm, and the kneading torque was about 120% of that in Example 1. The resultant compound was compression-molded by means of a press molding machine in an alignment magnetic field of 15 kOe (11.937 kA/cm) at a temperature of 230°C under a pressure of 1 ton/cm 2 (98 MPa), thereby manufacturing a cylindrical rare earth bonded magnet having an outside diameter of 18 mm, a wall thickness of 0.8 mm and a length of 7 mm. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 3.6 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 20.7 MGOe (165 kJ/m 3 ), a density p = 7.35 g/cm 3 , and a void ratio of 1.5 vol.%. Example 9 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1, except that the chemical composition of the compound comprised 96 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particles diameter = 17 µm), 3.0 wt.% liquid crystal polymer (melting point = 180°C), and 1.0 wt.% hydrazine-based oxidation inhibitor (chelating agent) at a kneading temperature of 250°C. The kneading torque was about 125% of that in Example 1. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a liquid crystal polymer content of 3.4 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 9.8 MGOe (78 kJ/m 3 ), a density ρ = 6.14 g/cm 3 , and a void ratio of 1.3 vol.%. Example 10 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 96.2 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 19 µm), 2.5 wt.% liquid crystal polymer (melting point = 180°C), and 1.3 wt.% hydrazine-based oxidation inhibitor (chelating agent), at a kneading temperature of 250°C. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a liquid crystal polymer content of 3.0 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy average product (BH)max = 10.4 MGOe (83 kJ/m 3 ), a density ρ = 6.17 g/cm 3 , and a void ratio of 1.2 vol.%. Example 11 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 97 wt.% magnet powder comprising Sm 2 Fe 17 N 3 (average particle diameter = 1.5 µm), 1.4 wt.% polyamide (melting point = 175°C), 1.4 wt.% hydrazine-based oxidation inhibitor (chelating agent), and 0.2 wt.% zinc isostearate (plasticizer). The total length of the kneading disk part in the kneading extruder was set at 30 cm. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 1.8 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 21.3 MGOe (170 kJ/m 3 ), a density ρ = 6.6 g/cm 3 , and a void ratio of 1.2 vol.%. Example 12 A rare earth bonded magnet of the present invention was manufactured in the same manner as in Example 1 except that the chemical composition of the compound comprised 96.2 wt.% magnet powder comprising Sm 2 Fe 17 N 3 (average particle diameter = 1.0 µm), 2.0 wt.% liquid crystal polymer (melting point = 180°C), 1.5 wt.% hydrazine-based oxidation inhibitor (chelating agent), 0.2 wt.% zinc isostearate (plasticizer), and 0.1 wt.% silicone oil (lubricant) at a kneading temperature of 250°C. The total length of the kneading disk part in the kneading extruder was set at 30 cm. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a liquid crystal polymer content of 2.5 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 20.6 MGOe (164 kJ/m 3 ), a density ρ = 6.54 g/cm 3 , and a void ratio of 1.1 vol.%. Example 13 A magnet powder comprising Sm (Co 0.604 Cu 0.06 Fe 0.32 Zr 0.016 ) 8.3 in an amount of 72 wt.% (average particle diameter = 22 µm), 24 wt.% magnet powder comprising Sm 2 Fe 17 N 3 (average particle diameter = 1.2 µm), 3.4 wt.% polyamide (melting point = 175°C), and 0.6 wt.% hydrazine-based oxidation inhibitor (chelating agent) were kneaded in a kneader at 230°C. The total length of the kneading disk part in the kneading extruder was 30 cm, and the kneading torque was about 140% of that in Example 1. The resultant compound was compression-molded by means of a press molding machine in an alignment magnetic field of 15 kOe (11.937 kA/cm) at a temperature of 230°C under a pressure of 1 ton/cm 2 (98 MPa), thereby manufacturing a cylindrical rare earth bonded magnet having an outside diameter of 18 mm, a wall thickness of 0.8 mm, and a length of 7 mm. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyamide content of 3.7 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 22.5 MGOe (179 kJ/m 3 ), a density ρ = 7.27 g/cm 3 , and a void ratio of 1.1 vol.%. Example 14 A magnet powder comprising Sm (Co 0.604 Cu 0.06 Fe 0.32 Zr 0.016 ) 8.3 in an amount of 50 wt.% (average particle diameter 22 µm), 27.3 wt.% magnet powder comprising Sm 2 Fe 17 N 3 (average particle diameter = 1.2 µm), 20 wt.% anisotropic Nd-Fe-B-based magnet powder (average particle diameter = 17 µm), 1.0 wt.% polyphenylene sulfide, 1.5 wt.% hydrazine-based oxidation inhibitor (chelating agent), and 0.2 wt.% zinc isostearate (plasticizer) were kneaded by means of a kneader at 300°C. The kneading disk part in the kneading extruder had a total length of 30 cm, and the kneading torque was about 170% of that in Example 1. The resultant compound was compression-molded by means of a press molding machine in an alignment magnetic field of 18 kOe (14.324 kA/cm) at a temperature of 300°C under a pressure of 2 tons/cm 2 (196 MPa), thereby manufacturing a round bar-shaped rare earth bonded magnet having a diameter of 10 mm and a length of 7 mm. Analysis of the chemical composition of the resultant rare earth bonded magnet showed a polyphenylene sulfide content of 1.4 wt.%. Evaluation of properties of this rare earth bonded magnet showed a magnetic energy product (BH)max = 20.1 MGOe (160 kJ/m 3 ), a density ρ = 7.20 g/cm 3 and a void ratio of 1.1 vol.%. All the rare earth bonded magnets of the foregoing Examples 1 to 14 were confirmed to be excellent in moldability, with a low void ratio, a high mechanical strength and excellent magnetic properties. Comparative Example 1 Each of the following three materials: (a) 99.0 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 19 µm) + 1.0 wt.% epoxy resin; (b) 97.0 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 20 µm) + 3.0 wt.% epoxy resin; (c) 95.0 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 21 µm) + 5.0 wt.% epoxy resin, was kneaded by a kneading extruder at room temperature, and the resultant compound was compaction-molded in a press molding machine under a pressure of 5 tons/cm 2 (490 MPa), thereby forming a rod-shaped block having a diameter of 10 mm and a length of 7 mm. Subsequently, the molded block was cured at 150°C for one hour for setting the resin, thereby manufacturing a rare earth bonded magnet. Because bonding between the magnet powder and the binding resin (thermoplastic resin) was insufficient in the rare earth bonded magnet obtained from (a), the magnet powder came off in response to a slight shock, making it impossible to apply this magnet in practical use. For the rare earth bonded magnet manufactured from (b), the viscosity decreased during heat treatment causing discharge of the resin, and after setting of the resin, the discharged resin covered the surface of the magnet, thus making it impossible to evaluate magnetic properties. For the rare earth bonded magnet derived from (c), the shape could not be maintained upon taking out the block from the press die, thus making it impossible to accomplish molding. Comparative Example 2 In this Example, 99.1 wt.% Nd-Fe-B-based magnet powder (rapid-quenched Nd 12 Fe 82 B 6 powder; average particle diameter = 22 µm), and 0.9 wt.% polyamide (melting point = 175°C) were kneaded by a kneader at 230°C. At this point, the kneading torque was very large, making it impossible to permit sufficient kneading, and heat generation led to serious oxidation of the magnet powder. Extrusion could not be accomplished from the resultant compound. Accordingly, the foregoing compound was compression-molded in a press forming machine at 230°C under a pressure of 3.0 tons/cm 2 (294 MPa) into a cylindrical rare earth bonded magnet having a diameter of 10 mm, and a length of 7 mm. For the resultant rare earth bonded magnet, magnetic properties were tried to but could not be measured: exposing the magnet to a magnetic field shock caused peeling and falling apart (separating) of the magnet powder, thus making it impossible to practically use this magnet. Examples 15 to 26 There were provided seven kinds of rare earth magnet powder having any of the following seven chemical compositions (1) to (7), the following three kinds of thermoplastic resin (binding resin) A, B and C, a hydrazine-based oxidation inhibitor (chelating agent), zinc stearate (plasticizer) and a silicone oil (lubricant), and were mixed in prescribed combinations. Each of the resultant mixtures was kneaded under the conditions shown in annexed Tables 1 and 2, and the resultant composition (compound) for a rare earth bonded magnet was molded under the molding conditions shown in these Tables, thereby obtaining respective rare earth bonded magnets of the present invention. The shape, size, chemical composition, state and properties of the resultant magnets are shown in Tables 3 and 4. (1) Rapid-Quenched Nd 12 Fe 82 B 6 powder (average particle diameter = 19 µm); (2) Rapid-Quenched Nd 3 Pr 4 Fe 82 B 6 powder (average particle diameter = 18 µm); (3) Rapid-Quenched Nd 12 Fe 78 Co 4 B 6 powder (average particle diameter = 20 µm); (4) Sm(Co 0.604 Cu 0.06 Fe 0.32 Zr 0.016 ) 8.3 powder (average particle diameter = 22 µm); (5) Sm 2 Fe 17 N 3 powder (average particle diameter = 2 µm); (6) Anisotropic Nd 13 Fe 69 C 11 B 6 powder based on the HDDR method (average particle diameter = 30 µm); (7) Nanocrystalline Nd 5.5 Fe 66 B 18.5 Co 5 Cr 5 powder (average particle diameter = 15 µm). A. Polyamide (nylon 12); melting point: 175°C; B. Liquid crystal polymer; melting point: 180°C; C. Polyphenylene sulfide (PPS); melting point: 280°C. The state of resin as shown in Tables 3 and 4 was evaluated by cutting the resultant magnet and taking a photograph with an electron microscope (magnification of 100) of the sectional surface. The mechanical strength in Tables 3 and 4 was evaluated by separately preparing test pieces having an outside diameter of 15 mm and a height of 3 mm, subjecting each such test piece to press molding in the absence of a magnetic field at any of the molding temperatures shown in Tables 1 and 2 under a pressure of 1.5 tons/cm 2 (147 MPa), and evaluating the mechanical strength by shearing by punching. The values of corrosion resistance shown in Tables 3 and 4 are the results of an acceleration test carried out on the resultant rare earth bonded magnets in an isothermal/isohumidity vessel under conditions including 80°C and 90%RH. The corrosion resistance was evaluated by means of the time before occurrence of rust with four marks: A, B, C and D, A representing the longest, D the shortest time. Comparative Examples 3 and 4 A mixture of the rare earth magnet powder having the chemical composition (1) above and epoxy resin (thermosetting resin) was kneaded under the conditions shown in Table 2. The resultant compound was molded under the molding conditions shown in Table 2. The molding product was subjected to a heat treatment at 150°C for one hour for resin setting, thereby obtaining a rare earth bonded magnet. The shape, size, chemical composition, state and properties of the resultant magnets are shown in Table 4. In Table 4, evaluation of the state of resin, mechanical strength (the test pieces were press-molded at room temperature under a pressure of 7 tons/cm 2 (686 MPa)) and corrosion resistance was carried out in the same manner as in the above Example. Comparative Example 5 The mixture of the rare earth magnet powder having the chemical composition (1) above and the thermosetting resin A above was kneaded under the conditions shown in Table 2. The resultant compound was molded under the forming conditions shown in Table 2. The shape, size, chemical composition, state and properties of the resultant magnets are shown in Table 4. Evaluation of the state of resin and the like in Table 4 was conducted in the same manner as in the above Example. As shown in the annexed Tables, it was confirmed that the rare earth bonded magnets of Examples 15 to 26 were excellent in shape, with a low void ratio and a high mechanical strength, and were excellent in magnetic properties and corrosion resistance. In the rare earth bonded magnet of Comparative Example 3, in contrast, insufficient bonding between the magnet powder and the binding resin caused the magnet powder to come off under the effect of shock acting during measurement of magnetic properties, with a low corrosion resistance such that the magnet could not be used in practice. In Comparative Example 4, there occurred an abnormality of discharge of resin onto the outer surface of the magnet. In Comparative Example 5, it was difficult to knead the mixture and oxidation of the magnet powder was so violent as to cause ignition. According to the present invention, as is clear from the results described above, it is possible to provide a rare earth bonded magnet which is excellent in moldability and corrosion resistance, has a high mechanical strength, and is excellent in magnetic properties, with a slight amount of resin.

Claims: A rare earth bonded magnet which contains a rare earth magnet powder, a thermoplastic resin, and an oxidation inhibitor, wherein the content of said thermoplastic resin is within a range of from 1 to 5 wt.%, and said thermoplastic resin covers the outer surface of the particles of said rare earth magnet powder so as to prevent adjacent particles of rare earth magnet powder from coming into contact with each other, characterized in that said oxidation inhibitor is a chelating agent. The magnet according to claim 1, wherein said thermoplastic resin has wettability to the surface of said particles of rare earth magnet powder. The rare earth bonded magnet according to claim 1 or 2, wherein the content of said thermoplastic resin is within a range of from 1 to 3.8 wt.%. The magnet according to any one of the preceding claims, wherein the void ratio is no more than 2 vol.%. The magnet according to any one of the preceding claims, wherein said thermoplastic resin has a melting point of no more than 400°C. The magnet according to any one of the preceding claims, wherein said thermoplastic resin is any of polyamide, a liquid crystal polymer, and polyphenylene sulfide. The magnet according to any one of the preceding claims, wherein said rare earth magnet powder comprises at least one selected from the group consisting of: a first composition comprising, as main ingredients, one or more rare earth elements mainly including Sm and one or more transition metals mainly including Co; a second composition comprising, as main ingredients R, one or more transition metals mainly including Fe, and B, wherein R represents at least one rare earth element including Y; and a third composition comprising, as main ingredients, one or more rare earth elements mainly including Sm, one or more transition metals mainly including Fe, and one or more interstitial elements mainly including N. The magnet according to any one of the preceding claims, wherein said rare earth magnet powder is a mixture of two or more kinds of magnet powder having different chemical compositions and/or average particle diameters. A composition for a rare earth bonded magnet, which comprises a rare earth magnet powder, a thermoplastic resin and an oxidation inhibitor, wherein the amount of addition of said thermoplastic resin is within a range of from 1 to 5 wt.% such that, when the composition is extruded, a fluidity during molding sufficient to permit extrusion is ensured, characterized in that said oxidation inhibitor is a chelating agent. The composition according to claim 9, wherein the amount of addition of said thermoplastic resin in said composition is no less than 1 wt.% but less than 3.8 wt.%, and the amount of addition of said oxidation inhibitor is within a range of from 0.1 to 2.0 wt.%. The composition according to claim 9 or 10, wherein the total amount of addition of said thermoplastic resin and said oxidation inhibitor is within a range of from 1.1 to 4.7 wt.%. The composition according to any one of claims 9 to 11, wherein said composition further contains a plasticizer and/or a lubricant, The composition according to any one of claims 9 to 12, wherein: the amounts of addition of said thermoplastic resin and said oxidation inhibitor are such that, a rare earth bonded magnet having a void ratio of no more than 2 vol.% is obtainable from the composition. The composition according to claims 10 and 13, wherein: said thermoplastic resin is any one of polyamide, a liquid crystal polymer and polyphenylene sulfide; and said oxidation inhibitor is a chelating agent which makes the surface of magnet powder inactive. The composition according to any one of claims 9 to 14, being a kneaded mass formed by kneading said rare earth magnet powder, said thermoplastic resin, and said oxidation inhibitor, at a temperature at which said thermoplastic resin melts or softens.

Inventors: AKIOKA KOJI, IKUMA KEN, SHIRAI HAYATO, AKIOKA, KOJI, IKUMA, KEN, SHIRAI, HAYATO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1996
Main classification: H01F1
IPC codes: H01F1