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

Abstract

A method of fabricating a rare-earth based sintered magnet having improved magnetic and mechanical characteristics is offered. Also, a method of fabricating a rare-earth bonded magnet having improved magnetic and mechanical characteristics is offered. The method of fabricating the rare-earth based sintered magnet is started with preparing powder of an alloy including a rare-earth element and a transition metal. The powder of the alloy is mixed with an additive. The mixture is compression molded and irradiated with microwaves to cause the powder to self-heat. As a result, the mixture is sintered.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]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.[0003]2. Description of the Related Art[0004]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.[0005]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 lat...

AI Technical Summary

Benefits of technology

[0014]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.

[0021]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.

[0022]Furthermore, only the sample is heated. Surroundings of the sample are not elevated in temperature. Consequently, after the sintering, the rate at which the sample is cooled can be increased. This in turn achieves a sintered rare-earth based magnet having excellent magnetic characteristics having no depositions. In addition, the step of homogenizing the quality of the sintered object produces a uniform phase that is important for the coercive force of the sintered rare-earth based magnet.

[0023]According to the present invention, nitriding and sintering can be carried out at the same time by irradiating the molded object within an ambient of nitrogen with microwaves. In consequence, the time taken to perform the nitriding step can be shortened. Moreover, the molded object can be nitrided uniformly up to its inner depths, thus producing a rare-earth magnet having high magnetic characteristics. Nitrogen atoms can be moved into stable sites between crystal lattice points by performing the homogenizing step. Consequently, a stable rare-earth sintered magnet having high magnetic characteristics can be obtained.

[0025]According to the invention, grains of the powder of the alloy have an average grain diameter of 2 to 90 μm. Consequently, the powder of the rare-earth magnet is suppressed from becoming smaller particles each having a single magnetic domain. In addition, oxidation and excessive nitriding of the powder can be suppressed. The powder of the alloy can be uniformly nitrided.

[0027]According to the invention, a rare-earth bonded magnet is fabricated, using powder of a rare-earth magnet and a resinous binder or a metal binder. As a consequence, a bonded magnet having excellent magnetic characteristics can be obtained.

Problems solved by technology

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.

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.

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.

Especially, the present situation is that rare earth-transition metal-nitride based sintered magnets have not been mass-produced.

Images