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Method of fabricating rare-earth sintered magnet and method of fabricating rare-earth bonded magnet

Method of fabricating rare-earth sintered magnet and method of fabricating rare-earth bonded magnet description/claims

1. Field of the Invention

The present invention relates to a method of fabricating a rare-earth sintered magnet and to a method of fabricating a rare-earth bonded magnet.

2. Description of the Related Art

Fabrication of a magnet consisting chiefly of a rare-earth element relies on a general method of sintering. This sintering method includes various processing steps including dissolution of an alloy, thermal treatment, pulverization, press molding, nitriding (if necessary), sintering, thermal treatment, machining, and magnetization.

A bonded magnet that can be molded with great latitude at low cost is fabricated by subjecting a magnet to process steps up to thermal processing using sintering, pulverizing the thermally processed magnet, mixing the pulverized magnet with a resin (such as epoxy resin or nylon), and compression molding or injection molding the mixture. The compression molding or injection molding step yields great latitude in molding a compound at low cost.

Compounds such as SmCo5, Sm2Co17, and Nd2Fe14B from which magnets are fabricated are sintered using a large furnace. Among various steps, a sintering step is especially important. In this sintering step, the large furnace heats the compound by means of a heater rod within the furnace in a vacuum or an inert ambient of Ar. The temperature is elevated or a high temperature is maintained by thermal conduction or radiant heat. Solid-phase and liquid-phase reactions of the molded object are made to progress. Thus, the sintered object is obtained.

However, with the large furnace, a temperature difference is produced between a location close to the heat-generating portion of the heater and a location at a large distance from the heat-generating portion. This affects the crystal grain diameters closely related to the magnetic characteristics of rare-earth magnets. Consequently, the magnetic characteristics suffer from variations.

In view of this problem, a method has been proposed which makes uniform the temperature distribution by arranging coils around a sintering furnace not using a large furnace and performing RF induction heating.

On the other hand, a compound such as rare earth-transition metal-nitride based material used as a magnetic material decomposes into nitrides of the rare-earth material and α-Fe at high temperatures exceeding 650° C. and thus sufficient magnetic characteristics are not obtained. Therefore, it has been impossible to use the compound in bulk form. Accordingly, this material for a magnet is limited to magnetic powder for a bonded magnet.

In an attempt to solve this problem, a method using plasma sintering has been proposed. In particular, the sintering time is shortened. Thermal decomposition produced during sintering is reduced to a minimum.

However, in the method of arranging the coils around the sintering furnace and performing thermal treatment by RF induction heating, large-scale equipment is required to arrange the coils around the large furnace. Furthermore, it is costly to cool the coils. Hence, this method is unsuited for mass production.

In the method using the plasma sintering, the sintering is done instantly but glow discharge is produced between magnetic particles, resulting in oversintering. This leads to increases in the diameters of the crystal grains. As a result, it has been impossible to obtain desired magnetic characteristics.

In this way, the aforementioned methods suffer from various problems. Furthermore, rare earth element-transition metal based magnets and rare earth-transition metal-nitride based magnets sintered by the above-described methods have magnetic characteristics that remain to be considerably lower than their theoretical values. Especially, the present situation is that rare earth-transition metal-nitride based sintered magnets have not been mass-produced.

The present invention has been made to solve the foregoing problems. It is an object of the invention to provide a method of fabricating a rare earth based sintered magnet having excellent magnetic characteristics (i.e., not thermally decomposed even if sintered) using a nitriding step capable of being performed in a reduced time. It is another object of the invention to provide a method of fabricating a rare earth based bonded magnet having excellent magnetic characteristics (i.e., not thermally decomposed even if sintered) using a nitriding step capable of being performed in a reduced time.

One embodiment of the present invention provides a method of fabricating a rare-earth sintered magnet, comprising the steps of: preparing powder of an alloy containing rare-earth element and transition metal; molding the powder of the alloy into a molded article having a desired form; irradiating the molded article with microwave in a vacuum or in an inert gas and sintering the molded article; and heating the molded article in an inert gas.

In one feature of the invention, in the irradiating step, the molded article is irradiated with microwave in nitrogen gas for nitriding and sintering.

In another feature of the invention, the microwave has a frequency higher than 1 GHz and lower than 30 GHz.

In a further feature of the invention, the powder of the alloy is made of particles having an average particle diameter of 2 to 90 μm.

In a yet other feature of the invention, in the irradiating step, a pressure of the nitrogen gas is from 0.1 to 5 MPa.

An other embodiment of the present invention provides a method of fabricating a rare-earth bonded magnet, comprising the steps of: pulverizing a rare-earth sintered magnet fabricated by the method of the present invention into powder of a rare-earth sintered magnet having an average particle diameter of 5 to 90 μm; and mixing the powder of the rare-earth sintered magnet with a resinous binder or a metal binder and compression molding or injection molding the mixture.

According to the present invention, the powder of the rare-earth magnet is allowed to self-heat quickly and selectively. The whole sample can be elevated in temperature uniformly. Therefore, the powder of the rare-earth magnet can be sintered instantly. The heating time can be shortened. This in turn can prevent evaporation of the rare-earth element. Hence, a magnet having a desired composition can be obtained.

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