JP2006049801A - Corrosion-resistant magnet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a corrosion-resistant magnet capable of preventing corrosion while restoring a magnet surface after machining in a rare earth magnet, and to provide its manufacturing method. <P>SOLUTION: An inactive phase including at least one of a plurality of kinds of elements constituting the rare earth magnet is provided on the surface of the rare earth magnet. The inactive phase is inactivated by exciting the surface and chemically changing an active phase on the surface. The inactive phase contains at least one kind of rare earth metal elements and transition metal elements constituting the rare metal magnet. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a corrosion-resistant magnet in which an active phase on the surface of a rare earth magnet is inactivated and a method for producing the same.

  Rare earth magnets are generally represented by the Re-B-Fe system or Re-Tm-B system (re is one selected from rare earth metals, and Tm is one selected from transition metals). In addition, it is known that the magnetic properties are superior to those of conventional alloy magnets and ferrite magnets.

However, rare earth magnets are easily rusted because they are made of an active metal material, and in particular, in order to improve the magnetic properties, many rare earth metals and transition metals are required not only in the main phase but also in the grain boundary phase. For this reason, corrosion has been an inevitable problem in rare earth magnets.
For example, in a so-called neodymium magnet whose main phase is Nd ₂ Fe ₁₄ B, which is a kind of Re—B—Fe rare earth magnet, an Nd—Fe phase in which a large amount of neodymium is present is formed as a grain boundary phase. This phase is oxidized or reacted with water vapor to form oxides (Nd ₂ O ₃ ) and hydroxides (Nd (OH) ₃ ), and the volume of the grain boundary phase expands. The grain boundary destruction of the body occurs.
In the one hand the surface of the main phase, hydrates of Fe ₂ O ₃ · H ₂ O is formed by the presence of oxygen及水steam, the magnetic properties of the magnet is lowered.
Such a problem is a problem common to rare earth magnets, and it is known that a corrosion phenomenon similar to that of a neodymium magnet also occurs in Pr ₂ Fe ₁₄ B, which is a hot-worked rare earth magnet of the Re—Tm—B system.

For the above problem, a method of forming an oxidation-resistant chemical conversion film by subjecting the surface of the magnet to chemical conversion treatment such as phosphate treatment or chromate treatment (for example, see Patent Document 1), Zn, Al A method of vapor deposition or electroless Ni plating (for example, see Patent Document 2), and a method of binding a magnet and an additive for preventing rust with a binder resin (for example, see Patent Document 3) are known. ing.
Further, as a method of forming a silicon dioxide protective film or a silicate protective film on the surface of the magnet, a technique for forming a resin film on the protective film (see, for example, Patent Document 4), a magnet and a protective film are used as a resin. A technique for binding with a binder (for example, see Patent Document 5) has been studied.

Japanese Patent Laid-Open No. 1-14902 JP-A-64-15301 JP-A-1-147806 JP-A-62-152107 JP-A-8-111306

  In the conventional methods for surface treatment of magnets as described in Patent Documents 1 to 3, the thickness of the coating is increased or the coating is formed a plurality of times in order to eliminate defects in the coating. However, since it is difficult to completely eliminate defects in the coating and it is difficult to completely shield the surface of the magnet from external oxygen and water vapor, the rare earth magnet has been corroded.

In addition, in the method of forming a silicon dioxide protective film or a silicate protective film on the surface of the magnet, the magnet has various sizes and shapes, and the surface of the magnet is often not flat. It was difficult to form a strong protective film.
In particular, in the technique described in Patent Document 4, reactive silyl isocyanate is used, but with this technique, it is difficult to achieve uniform film growth, and it is easy to form a film having irregularities. Moreover, a strong film could not be formed only by physically adsorbing silicate onto the irregularities of the magnet surface. In the technique described in Patent Document 5, a technique for forming a protective film by sol-gel reaction using ethyl silicate or plasma particle chemical vapor deposition is disclosed, but a uniform, dense and strong protective film is formed. I couldn't.
Therefore, since the film itself is easy to peel off, the rare earth magnet has been corroded as a result.

On the other hand, a sintered magnet is generally required to be machined because the dimensional accuracy of the sintered magnet is poor. Even in the rare-earth magnet, unevenness is formed on the processed surface by machining, so that it is necessary to cover the unevenness with a corrosion-resistant coating. However, crystal grain defects are generated on the processed surface by machining, and the crystal grains are likely to fall off. For this reason, the crystal grains that may possibly fall out have been dropped through barrel polishing or the like before forming a corrosion-resistant film on the processed surface, and have undergone complicated steps of polishing, washing, and film formation.
In addition, since the surface may be corroded even during and after polishing, it is necessary to perform the subsequent process in an environment that does not corrode. Was necessary.
Furthermore, since stress during machining may remain, and forming a film in such a state makes it easy to peel off, once the residual stress is removed by annealing or the like in an environment where the magnet does not corrode, It was necessary to form a film after this.
That is, when a corrosion-resistant film is formed on a rare earth magnet, it is necessary to repair defects on the surface caused by machining, which has a problem of increasing the processing cost.

  Therefore, rare earth magnets have a material problem that they are easily corroded. In addition to this material problem, it is necessary to repair physical defects in the structure of the machined surface before forming a coating. There was a problem.

  The present invention has been devised in view of the above problems, and it should solve the problem of providing a corrosion-resistant magnet capable of preventing corrosion while repairing a magnet surface after machining in a rare-earth magnet and a method for manufacturing the same. It is to be an issue.

  In order to achieve the above object, a first characteristic configuration of a corrosion-resistant magnet according to the present invention includes an inert phase containing at least one element among a plurality of types of elements constituting the rare earth magnet on the surface of the rare earth magnet. There is in point.

In other words, according to this configuration, the surface of the rare earth magnet is inactive, and the inert phase can block the active phase of the magnet from outside oxygen, water vapor, etc., thereby preventing the rare earth magnet from being corroded. can do.
And since an inert phase contains at least 1 type from the multiple types of elements which comprise a rare earth magnet, it is easy to become familiar with the magnet surface, can raise bond strength, and can make it hard to peel. Therefore, the rare earth magnet can be easily handled even after the inactivated phase is formed.
In the present invention, the rare earth magnet includes magnetic powder and the like regardless of size and shape.

  A second characteristic configuration of the corrosion-resistant magnet according to the present invention is that the inert phase is inactivated by exciting the surface and chemically changing the active phase on the surface.

In other words, according to this configuration, the surface of the rare earth magnet can be excited, and the active phase on the surface of the rare earth magnet can be chemically changed to form a completely inactivated inert phase on the surface of the magnet. The factor can be eliminated, and the surface of the rare earth magnet can be prevented from being corroded.
In addition, a chemical change is a change of a composition and includes not only a compound but also thermal decomposition.

  A third characteristic configuration of the corrosion-resistant magnet according to the present invention is that the inert phase includes at least one element of a rare earth metal element and a transition metal element constituting the rare earth magnet.

That is, according to this configuration, the active phase of the magnet can be blocked from the outside oxygen, water vapor, and the like by the inert phase, and the rare earth magnet can be prevented from being corroded. And since the inert phase contains at least one kind of rare earth metal elements and transition metal elements constituting the rare earth magnet, it can easily become familiar with the magnet surface, increase the bonding strength, and make it difficult to peel off.
The inert phase is preferably formed by setting the surface temperature to be equal to or higher than the boiling points of the rare earth metal element and the transition metal element constituting the surface. That is, according to this means, the temperature of the surface of the rare earth magnet is set to a temperature equal to or higher than a plurality of types of maximum boiling points constituting the surface, thereby sublimating the material constituting the surface of the rare earth magnet from which the active phase is exposed. And chemical changes, that is, thermal decomposition. Then, the sublimated gas is cooled, and some or all of the atoms and molecules obtained by pyrolyzing the material constituting the surface of the rare earth magnet are recombined as they are or further with the same or different atoms and molecules, and then the magnet Since it solidifies on the surface in the excited state, an inactive phase can be formed and the surface can be inactivated.
Then, since part or all of the material constituting the surface of the rare earth magnet is solidified on the excited magnet surface, an alloy phase can be formed at the interface with the inert phase, and the inert phase is separated from the magnet. It becomes difficult to peel off.

  In addition, even when oxides or hydroxides are already formed on the surface of the rare earth magnet, the inert phase is formed while removing impurities by sublimating the material constituting the surface of the rare earth magnet. be able to. Furthermore, even if there is a physical defect on the structure of the machined surface in the sintered magnet, the surface layer including the defective portion can be sublimated to form an inert phase, so that the defect can be repaired. Can prevent corrosion

  A fourth characteristic configuration of the corrosion-resistant magnet according to the present invention is that the inert phase is an amorphous phase composed of one or more elements among a plurality of elements constituting the rare earth magnet.

That is, according to this configuration, the amorphous phase has the same kind of element as the active phase, but has a different composition, and is not affected by oxygen, water vapor, or the like. If the amorphous phase is composed of one or more elements among the plurality of elements constituting the rare earth magnet, a new material is prepared in order to form a new composition using the composition of the surface of the rare earth magnet. There is no need to do.
Further, since the amorphous phase and the magnet have good compatibility, they can be integrated, and the amorphous phase is difficult to peel off from the magnet.

  The feature means of the method for producing a corrosion-resistant magnet according to the present invention is that the surface of the rare earth magnet is irradiated with an electron beam so that the temperature of the surface is equal to or higher than the highest boiling point of a plurality of types of elements constituting the surface. By sublimating the material to be inactivated, an inert phase containing at least one element among the plurality of elements constituting the rare earth magnet is provided on the surface to inactivate the surface, and then irradiated with an electron beam. Thus, the inert phase is gradually cooled.

That is, according to this means, an inert phase can be formed on the surface of the rare earth magnet and the active phase can be deactivated, so that the rare earth magnet can be prevented from being corroded.
Then, after providing an inert phase on the surface of the rare earth magnet to inactivate the surface, the electron beam is further irradiated once or a plurality of times to gradually cool the inert phase to relieve thermal stress, thereby reducing the electron beam It is possible to relieve the thermal stress on the surface of the magnet generated by the collision. Thereby, it can prevent that mechanical defects, such as a crack, arise in an inert phase.

The corrosion-resistant magnet according to the present invention has an inert phase containing at least one element among elements constituting the rare earth magnet on the surface of the rare earth magnet. Thereby, the surface of the rare earth magnet can be inactivated and the rare earth magnet can be prevented from being corroded. And since an inert phase contains at least 1 sort (s) among the elements which comprise a rare earth magnet, it is easy to become familiar with the magnet surface, can raise bond strength, and can make it difficult to peel.
Rare earth magnets are generally represented by Re-B-Fe or Re-Tm-B systems (where Re represents one selected from rare earth metals and Tm represents one selected from transition metals). As described above, it is constituted by a plurality of types of elements such as Re, B, Fe or the like or Re, Tm, B or the like. For this reason, the said inert phase should just contain at least 1 type of element among the multiple types of elements which comprise a rare earth magnet.

  In addition, the inert phase is preferably inactivated by exciting the surface of the rare earth magnet and chemically changing the active phase on the surface of the rare earth magnet. That is, the active phase can be inactivated by exciting the surface of the magnet, and a corrosion-resistant magnet can be obtained.

  In particular, the inert phase is preferably formed by setting the temperature of the surface of the rare earth magnet to be equal to or higher than the maximum boiling point of a plurality of elements constituting the surface. As excitation of the surface of the rare earth magnet, the surface of the rare earth magnet is brought to a temperature equal to or higher than the highest boiling point of a plurality of elements constituting the surface, whereby the active phase is exposed as shown in FIG. A substance constituting the surface of the magnet can be sublimated and chemically changed, that is, thermally decomposed. Then, the sublimated gas is cooled, and some or all of the atoms and molecules obtained by pyrolyzing the material constituting the surface of the rare earth magnet are solidified on the surface in the excited state of the magnet. As shown, an inert phase can be formed to inactivate the surface. Of course, the pyrolyzed atoms and molecules may further form an inert phase after recombination with the same or different atoms and molecules. Then, since part or all of the material constituting the surface of the rare earth magnet is solidified on the excited magnet surface, an alloy phase can be formed at the interface with the inert phase, and the inert phase is separated from the magnet. It becomes difficult to peel off.

  In addition, since rare earth magnets have an active phase, oxides and hydroxides may already be formed in the atmosphere. However, even in such a case, it is possible to form an inert phase while removing impurities by sublimating a substance constituting the surface. Furthermore, even if there is a physical defect on the structure of the machined surface in the sintered magnet, the surface layer including the defective portion can be sublimated to form an inert phase, so that the defect can be repaired. It is possible to prevent corrosion.

  The means for exciting the surface of the rare earth magnet, particularly the means for forming the surface temperature of the rare earth magnet above the maximum boiling point of the plural elements constituting the surface is not particularly limited, but an electron beam is applied to the surface of the rare earth magnet. Means for irradiation can be mentioned. Thereby, the accelerated electron beam can collide with the magnet surface, and the substance which comprises a surface can be sublimated. According to this means, the thickness of the surface layer to be sublimated can be arbitrarily determined by controlling the energy density of the electron beam, the acceleration of the electron beam, the irradiation time, and the like. For example, when the thickness of the surface layer that needs to be modified is large, such as when there is a large structural physical defect, the energy density of the electron beam is increased, the acceleration of the electron beam is increased, and the irradiation time is increased. What is necessary is just to enlarge the total energy amount given to the magnet surface by making it long. Since rare earth magnets have an active phase as described above and easily form oxides, hydroxides, and the like, it is preferable to perform electron beam irradiation in an environment where oxygen and water vapor are blocked. It is preferable to carry out in inert gas atmosphere. Further, when performed in an atmosphere of nitrogen, hydrocarbon, or the like, an inert phase in which thermally decomposed atoms or molecules are nitrided or carbonized can be formed.

  The modification of the surface of the magnet that forms an inert phase by irradiation with an electron beam gives energy to sublimate the surface layer of the magnet to the surface of the magnet. The surface of the magnet generates thermal stress due to electron beam collision. When this thermal stress is large, mechanical defects such as cracks are generated in the inert phase of the modified magnet surface. There is a fear. For this reason, as an electron beam irradiation means, an inert phase is provided on the surface of the rare earth magnet to inactivate the surface, and then the electron beam is irradiated once or a plurality of times to gradually cool the inert phase and heat. It is preferable to relieve stress. Thereby, it can prevent that a crack etc. enter into an inactive phase. Furthermore, preheating may be performed by irradiating the electron beam once or a plurality of times before providing the inert phase, and thereby the thermal stress can be relieved.

  Further, a film may be provided on the surface of the inert phase. For example, if there are cracks on the surface of the inert phase, a film is provided on the surface of the inert phase to cover the cracks, so that oxygen, water vapor, etc. enter from the cracks and corrode the magnet. This is convenient. For example, when a film is formed by magnetron sputtering, for cracks having a width of several molecular levels or more, fine particles that have been sputtered into the cracks are deposited and deposited while filling the cracks with fine particles. A thin film is formed on the surface. When the crack width is smaller than the sputtering particles, the sputtering particles do not enter the crack, and the sputtering particles are deposited on the surface to form a thin film. The coating is preferably a substance having a sublimation point close to that of the inert phase and having corrosion resistance. From such a viewpoint, a metal material such as titanium or cobalt, or a cermet that is a composite composed of a metal component and a ceramic component, such as a titanium carbonitride cermet, can be used.

As described above, it has been found that corrosion can be prevented by providing an inert phase on the surface of the rare earth magnet.
Further, as a rare earth magnet, a neodymium / iron / boron sintered magnet (hereinafter referred to as a neodymium magnet) having the highest magnet energy will be specifically described as an example. As shown in the electron micrograph of the internal structure in Fig. 2, the neodymium magnet is composed of the main phase of the ferromagnetic phase represented by the composition formula of Nd ₂ Fe ₁₄ B and the Nd-Fe alloy phase rich in the neodymium which is the grain boundary phase. And have. These phases are most active and corrode from these phases on the surface of the magnet in the atmosphere. Therefore, in order to improve the corrosion resistance of neodymium magnets, a composition different from the composition of the surface before treatment is obtained by sublimating and thermally decomposing the surface phase by electron beam irradiation, etc., and then solidifying on the surface of the magnet. It is preferable to form an inert phase of

  In this case, as described above, by irradiating an electron beam in an inert gas atmosphere to sublimate a substance constituting the surface, it is decomposed to the atomic or molecular level, and boron, carbon, etc. are contained in the inert gas. And heavy neodymium and iron will settle on the magnet surface. Then, an inactive amorphous phase composed of neodymium and iron is continuously formed while forming an alloy phase at the interface with the surface of the activated neodymium magnet. Of course, what can form such an amorphous phase is not limited to a neodymium magnet. In the case of a rare earth magnet, it is also possible to form an amorphous phase composed of one kind or a plurality of elements among a plurality of kinds of elements constituting the rare earth magnet. Such an amorphous phase has a composition different from the composition of the magnet surface before processing, and can be made unaffected by oxygen, water vapor, etc., and thus is suitable as an inert phase. . If the amorphous phase is composed of one or more kinds of elements constituting the rare earth magnet, it can be formed by reconstructing the composition of the surface of the rare earth magnet. There is no mixing, and there is no need to prepare raw materials separately. Since the amorphous phase and the magnet have good compatibility, they can be integrated, and the amorphous phase is difficult to peel off from the magnet.

  Since the boiling point of neodymium is 3027 ° C. and the boiling point of iron is 2750 ° C., the surface temperature needs to be 3000 ° C. or higher in order to sublimate the material on the surface of the neodymium magnet. In this case, since the boiling point of boron is 2550 ° C., it diffuses after being thermally decomposed before being cooled. Thus, since a neodymium magnet raises the surface of the magnet to a high temperature, thermal stress is likely to occur. For this reason, even after forming an inert phase such as an amorphous phase on the surface of the magnet, thermal stress can be alleviated by irradiating with one or more electron beams continuously or intermittently and gradually cooling. Is preferred.

  Moreover, the structural physical defect which a neodymium magnet has may have reached the depth of 10 micrometers from the surface. In this case, if the irradiation is an electron beam, the surface layer having a thickness of 10 μm is repaired and an inert phase having corrosion resistance is formed by controlling the energy density, acceleration, irradiation time, etc. of the electron beam. Can do.

Here, an example of a method of irradiating the surface of the rare earth magnet with an electron beam will be specifically described. However, it is not limited to this.
For the electron beam irradiation, a conventionally known method can be applied, and for example, it can be performed by an apparatus shown in FIG. The electron beam irradiation apparatus includes a vacuum vessel 1, a scroll pump 2, a turbo molecular pump 3, and an inert gas cylinder 4. A solenoid 5 is provided outside the vacuum vessel 1, a cathode 8 is provided above the inside of the vacuum vessel 1, and a ring-shaped anode 6 is provided below the cathode 8.

A method for irradiating the surface of the magnet with an electron beam using such an apparatus is shown in FIG. First, the sample 12 which is a magnet is arranged on the sample stage 10 of the vacuum vessel 1 (S1). And the scroll pump 2 and the turbo-molecular pump 3 are operated, and the inside of the vacuum vessel 1 is pressure-reduced so that it may become 10 ^<-2 > Pa or less (S2). After depressurization, as an inert gas, for example, argon gas is introduced into the vacuum vessel 1 and maintained at a pressure of about 10 ⁻¹ Pa (S3). Thereafter, a current is passed through the solenoid 5 to generate a strong pulse magnetic field at the cathode 8 (S4). When the magnetic field is generated, a positive pulse voltage is applied to the anode 6 to accelerate free electrons in the vacuum chamber 1 (S5). ). The electrons repeatedly collide with the argon gas in the vacuum chamber 1, ionize the argon gas molecules, and generate an anode plasma 7 in the vicinity of the anode 6. When the amount of the anode plasma 7 reaches the maximum state, a pulse acceleration voltage having a negative pulse width of about 2 μsec is applied to the cathode 8 (S6).

As a result, the anode plasma 7 becomes a virtual anode, and the apparent distance between the anode and the cathode 8 is shorter than when there is no plasma. Then, a strong electric field concentrates on the tip of the cathode 8 and a high electric field is formed on the tip of the cathode 8, whereby electrons are explosively emitted from the surface of the cathode 8 to generate a high-density cathode plasma 9. Since the cathode plasma 9 is formed near the cathode 8 and the anode plasma 8 is formed around the anode 6, each of them functions as a negative electrode and a positive electrode having a volume. As a result, a high electric field is concentrated between the two, and electrons emitted from the cathode plasma 9 are accelerated by this electric field, and the surface of the sample 12 is irradiated with a high-density electron beam 11.
The energy density of the electron beam irradiated on the surface of the magnet can be determined by controlling the current flowing through the solenoid 5 and the voltage applied to the anode 5. The acceleration of the electron beam can be controlled by the voltage applied to the cathode 8.

  On the other hand, as another means for exciting the surface of the rare earth magnet, for example, plasma particles may be irradiated to the magnet surface by air discharge. In this case, it is preferable that the plasma particles contain at least one of nitrogen ions and carbon ions. Such plasma particles are brought into contact with the surface of the rare earth magnet to excite the surface, and by reacting the plasma particles with the active phase on the surface of the rare earth magnet, the active phase is chemically changed, that is, nitrided or carbonized. Can be inactivated. Plasma particles can be generated by discharging in the presence of a gas. As the discharge, glow discharge, corona discharge, arc discharge, and the like are known and can be arbitrarily selected.

  The gas for generating plasma particles can be arbitrarily selected depending on the type of plasma particles. For example, nitrogen as a gas for generating nitrogen ions, and hydrocarbons such as methane and propane can be preferably applied as a gas for generating carbon ions. These gases preferably do not contain oxygen and water vapor for the same reason as in the case of electron beam irradiation. For example, a mixture of dry nitrogen, methane, propane, etc. in an inert gas such as argon should be used. Is preferred.

  In the case where oxides or hydroxides are already formed on the magnet surface, the means for irradiating plasma particles may remove impurities such as oxides in advance and activate the active phase. preferable. Then, after removing impurities and cleaning, it is preferable to continuously inactivate the active phase with plasma particles as it is in an environment cut off from the outside.

As the cleaning before the inactivation treatment of the active phase, so-called presputtering in which reducing plasma particles are activated by bringing them into contact with the surface of the magnet can be preferably applied. And by making this reducing plasma particle collide with the surface of a magnet, an oxide, a hydroxide, etc. can be reduced and the surface can be activated. When the reducing particles collide with the surface, the surface energy can be increased, thereby promoting the surface reduction reaction and the surface inactivation reaction.
The reducing plasma particles can be generated by performing a discharge treatment in the presence of hydrogen or ammonia.

  The inert phase formed by irradiating the surface of the magnet with plasma particles can be made extremely thin on the order of microns. Specifically, taking glow discharge as an example, plasma particles such as nitrogen ions or carbon ions generated by glow discharge are accelerated, and the accelerated plasma particles are driven into the surface of the rare earth magnet. The depth at which the active phase reacts is determined by the depth of implantation. For this reason, the speed at which plasma particles are accelerated is determined by the conditions that cause glow discharge, and the depth at which nitrides and carbides are formed depends on the speed at which the plasma particles are injected and the surface temperature of the magnet. Can be changed in the order of micron order. Further, according to this method, the active phase can be inactivated with a substantially constant thickness regardless of irregularities such as the surface shape of the rare earth magnet and the structural physical defects.

  Furthermore, in this method, plasma particles are injected into the active phase on the surface of the rare earth magnet, and a compound based on a chemical reaction is formed between the plasma particles and the active phase, thereby forming an inert phase having a strong binding force. I can do it. Specifically, for example, nitrogen ions or carbon ions activated by a glow discharge phenomenon are implanted into the surface of the rare earth magnet. As a result, nitrides or carbides are formed in the active phase, and the nitrides or carbides have a strong bonding force with the active phase that does not react. Moreover, the formed nitrides and carbides are chemically stable materials.

  An example of a method for irradiating the surface of the magnet with air particles by air discharge to form an inert phase on the surface will be described below.

  Conventionally known methods such as arc discharge, corona discharge, glow discharge, and silent discharge can be applied to the plasma irradiation by the air discharge, and can be performed by, for example, a direct current glow discharge apparatus shown in FIG. This apparatus has a low pressure vessel 21 made of stainless steel. The low pressure vessel 21 is provided with an exhaust port 22 for depressurizing the inside of the vessel and a supply port 23 for supplying gas from the outside. An exhaust pump (not shown) is installed at the port 22. A heater 26 for raising the temperature of the entire container is provided outside the low-pressure container 21. A cathode stage 25 is provided inside, and the low-pressure vessel 21 and the cathode stage 25 are connected to a DC power source 24. The sample 27, which is a rare earth magnet, is placed in the low-pressure vessel 21 while being suspended from the cathode base 25 on the wire. This wire is electrically connected to the cathode base 25.

  Further, when the sample 27 is powder, for example, an apparatus shown in FIG. 6 can be applied. That is, the shape of the cathode base 25 is different from the apparatus shown in FIG. The cathode base 25 has a flat plate provided with a large number of holes, and is configured to be able to vibrate. The sample 27 is disposed on this flat plate.

An example of a method for irradiating the surface of the rare earth magnet with plasma particles using such an apparatus is as follows. First, a sample 27 that is a magnet is placed in the low-pressure vessel 21. Then, the inside of the low-pressure vessel 21 is decompressed to about 10 ⁻⁴ Torr. After reducing the pressure, a reducing atmosphere gas is introduced, and the pressure in the container is maintained at 1 Torr or less. Thereafter, the low-pressure vessel 21 is heated to about 150 ° C., a voltage of 600 V is applied between the electrodes to generate reducing gas plasma particles in the low-pressure vessel 21, and the sample 27 is irradiated. After the irradiation, the gas in the low-pressure vessel 21 is once exhausted, and an atmospheric gas for generating plasma particles is introduced so as to have a pressure of 3 to 5 Torr. A voltage of 600 to 700 V is applied between the electrodes, plasma particles are generated in the low-pressure vessel 21, and the sample 27 is irradiated. In this method, a reducing atmosphere gas and an atmosphere gas for generating plasma particles can be arbitrarily selected, and the kind and nature of the inert phase formed on the surface of the rare earth magnet are determined by the kind of these gases.

  The corrosion-resistant magnet thus obtained can be made excellent in corrosion resistance by inactivating the active phase on the surface to form an inert phase. And since an inert phase is chemically stable and the coupling | bonding with the magnet surface is strong, handling of a rare earth magnet is easy and the environmental condition which can be used spreads.

Hereinafter, examples of the corrosion-resistant magnet according to the present invention will be described.
The surface corrosion resistance required for rare earth magnets varies depending on the environment in which the magnet is used, and is divided into surface strength, chemical stability on the magnet surface, and gas permeability. The more these corrosion resistances are combined, the more general-purpose corrosion-resistant magnets become.
The strength of the surface layer is an index indicating the ease of handling of the rare earth magnet. For example, when the magnet is assembled to a motor, the surface layer is less likely to be destroyed even if the magnet is dropped or hit against another part. Therefore, it is required to have mechanical strength according to the handling of the magnet.
As for the chemical stability of the surface, in order to prevent the surface layer from being destroyed by the reaction product as well as the gas permeability, the surface of the magnet has salt water, condensation, acid / alkali solution, dissolved metal ions. It is required that it does not react with liquids.
Moreover, especially about gas permeability, it is calculated | required that hydrogen gas and water vapor | steam do not permeate | transmit. That is, when these gases permeate the inert phase, the rare earth magnet and the gas react to form a compound as described above. When the compound is formed, the volume expands, grain boundary destruction occurs, and the surface layer is destroyed. Therefore, the inert phase formed on the surface of the magnet is required to have a dense structure that does not allow hydrogen gas to permeate.

Therefore, first, it was examined by X-ray diffraction whether an active phase remained on the surface of the corrosion-resistant magnet. Thereby, since the composition of each phase constituting the magnet is known in advance, the composition of the surface can be analyzed by X-ray diffraction. And since the thickness which X-ray permeate | transmits can be changed by controlling the intensity | strength of the X-ray to irradiate, the composition of a thickness direction can also be analyzed. The denseness of the inert phase of the corrosion-resistant magnet was observed with a cross-sectional electron microscope.
Then, for those having good results, the performance was evaluated under the conditions shown in Table 1. In other words, the mechanical strength between the magnet and the inert phase is evaluated from the drop impact performance, and the chemical stability is the reactivity of the immersion in physiological saline and the reactivity of the salt fixed on the surface in high temperature and high humidity conditions. It was evaluated from the combination. The denseness of the structure was evaluated by the hydrogen gas permeability.

  Using a neodymium magnet, an inert phase was formed on the surface under the conditions shown in Table 2 to produce a corrosion-resistant magnet.

  In Examples 1 and 2, the electron beam was irradiated so that the distance from the surface of the magnet to 10 μm was about 1400 ° C. In Example 1, the electron beam was irradiated only once. In Example 2, an electron beam was continuously irradiated before and after electron beam irradiation under the same conditions as in Example 1 in order to relieve the thermal stress on the magnet surface.

  In Examples 3 to 5, the electron beam was irradiated so that the distance from the surface of the magnet to 10 μm was about 3000 ° C. In Example 3, the electron beam was irradiated only once. As a result, when the electron beam was irradiated, the temperature rapidly increased to about 3000 ° C. with a slight delay time determined by the heat capacity and thermal conductivity of the magnet, and was rapidly cooled close to the temperature inside the container with a slight delay time.

  In Example 4, in order to relieve the thermal stress on the magnet surface, the electron beam was continuously irradiated once before and after the electron beam irradiation under the same conditions as in Example 3. That is, the energy density of the electron beam is such that the time for heating the surface of the magnet to about 3000 ° C. and the time for cooling from about 3000 ° C. to the temperature in the container are twice the time for maintaining at about 3000 ° C. The electron beam was irradiated while changing acceleration and acceleration continuously. Thereby, compared with Example 3, a temperature increase rate and a cooling rate become small. Since the thermal stress is determined by the product of the temperature difference and the specific heat of the sample, the generated thermal stress can be remarkably reduced and the generation of cracks can be suppressed. This can be realized by controlling the pulse generation circuit for generating a high voltage by a synchronization circuit for controlling the time and time when the pulse is generated, and controlling the magnitude and time of the anode current and the cathode voltage.

  In Example 5, the electron beam was continuously irradiated twice before and after the electron beam irradiation under the same conditions as in Example 3. That is, the process of raising the temperature to about 3000 ° C. was divided into two stages. Up to about 2500 ° C., the temperature was rapidly increased with a slight delay time corresponding to the heat capacity and thermal conductivity of the magnet, and then the electron beam was irradiated for a predetermined time so that the surface temperature was maintained at about 2500 ° C. Thereafter, the temperature was raised to about 3000 ° C. in substantially the same time as that maintained at about 3000 ° C. And the process of cooling to the temperature in a container was performed similarly. In addition, since the electron beam irradiation irradiates the surface with a high-density electron beam and excites the surface to change the surface state, the change in the excited state is determined by the differential value of the total energy of the irradiated electron beam. . For this reason, by making the differential value of the energy change as close to zero as possible, the thermal stress on the irradiated surface due to the change in the load condition can be relaxed. Further, the preheating effect and the slow cooling effect are larger as the heating rate and the cooling rate are smaller, but based on the viewpoint of manufacturing cost and the restriction of time control, the sintering temperature that determines the composition of the magnet is around 1300 degrees. The heating rate and cooling rate described above were reduced as the temperature increased.

  In Example 6, a cobalt film having a thickness of 1 μm was formed on the surface of the corrosion-resistant magnet produced in Example 5 by magnetron sputtering. That is, an inert phase was formed while repairing the surface of the magnet by electron beam irradiation, and then a 1 μm thin film was formed thereon by magnetron sputtering.

About the corrosion-resistant magnet produced by the above method, the surface modification state was examined from the results of the surface state, X-ray diffraction, and the enlarged photograph of the cross section by an electron microscope.
As a result, in the magnets produced according to Examples 1 and 2, crystals remained on the surface even after electron beam irradiation. From this result, it was found that even if the surface of a neodymium magnet was dissolved and recrystallized, the composition of the magnet was not changed, and the active phase remained and could not be inactivated.

  In the magnets produced in Examples 3 to 6, a dense amorphous phase was formed on the magnet surface, and no crystal phase of the magnet was observed. However, in the magnet produced in Example 3, visual cracks were confirmed on the surface of the amorphous phase. About the magnet produced in Examples 4-6, the crack was not able to be confirmed visually.

The magnets produced in Examples 4 to 6 were further subjected to the four types of tests shown in Table 1 to evaluate the formed amorphous phase.
As a result, it was found that in the case of a drop impact from a height of 1 m, the surface layer of any magnet did not break and had a strong bonding force with the magnet.
In addition, in the combined load test of the reactivity between immersion in salt water and standing in a high temperature and high humidity state, no change in the surface was observed in all the magnets. Therefore, it was found that the magnets of Examples 4 to 6 did not have surface defects that allow liquid or water vapor to enter.

In the test of hydrogen gas permeability 1, in the magnet of Example 4, a site where the surface was discolored in a state where the surface was locally dispersed was confirmed. For Examples 5 and 6, no permeation of hydrogen gas was observed.
Further, when the hydrogen gas permeability 2 test for forcibly transmitting hydrogen gas was performed on the magnets of Examples 5 and 6, no permeation was confirmed in the magnet of Example 6, but the magnet of Example 5 In, discoloration dispersed on the surface was observed.
For example, in application to a motor used in a fuel cell, it is not necessary to permeate 1 MPa of hydrogen gas. Therefore, it was found that the surface condition of the magnet of Example 5 can be sufficiently used. In the case of more severe use conditions, the magnet of Example 6 can be used. Of course, the magnets of Examples 3 and 4 can be sufficiently applied depending on the application.

  Since the corrosion-resistant magnet according to the present invention has excellent corrosion resistance, it can be applied not only to applications where conventional rare-earth magnets are used, but also to various applications such as applications that could not be applied because of easy corrosion.

Schematic diagram of cross section of rare earth magnet Electron micrograph of neodymium magnet The figure explaining an electron beam irradiation device Diagram explaining electron beam irradiation method Diagram explaining glow discharge device Diagram explaining glow discharge device

Explanation of symbols

1 Vacuum container 6 Anode 8 Cathode 11 Electron beam 12 Sample

Claims (5)

  A corrosion-resistant magnet comprising an inert phase containing at least one element among a plurality of elements constituting the rare earth magnet on the surface of the rare earth magnet.   The corrosion-resistant magnet according to claim 1, wherein the inert phase is inactivated by exciting the surface and chemically changing the active phase on the surface.   The corrosion-resistant magnet according to claim 1, wherein the inert phase includes at least one element selected from a rare earth metal element and a transition metal element constituting the rare earth magnet.   The corrosion-resistant magnet according to any one of claims 1 to 3, wherein the inert phase is an amorphous phase composed of one or more elements among a plurality of elements constituting the rare earth magnet.   The rare earth magnet is constituted by irradiating the surface of the rare earth magnet with an electron beam to raise the temperature of the surface above the maximum boiling point of a plurality of kinds of elements constituting the surface and sublimating the substance constituting the surface. A method for producing a corrosion-resistant magnet, wherein an inert phase containing at least one element among a plurality of elements is provided on the surface to inactivate the surface, and then the inert phase is gradually cooled by irradiation with an electron beam. .