Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2003-07-30
2004-11-09
Mai, Ngoclan T. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S246000, C148S101000, C148S302000, C419S065000, C419S066000
Reexamination Certificate
active
06814776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an iron-based rare-earth alloy powder, which can be used effectively as a material for a bonded magnet, and a method of making the alloy powder. The present invention also relates to a bonded magnet made from the rare-earth alloy powder and further relates to various types of electric equipment including the bonded magnet.
2. Description of the Related Art
A bonded magnet is currently used in various types of electric equipment including motors, actuators, loudspeakers, meters and focus convergence rings. A bonded magnet is a magnet obtained by mixing together a magnet powder and a binder (such as a rubber or a resin) and then compacting and setting the mixture.
An iron-based rare-earth alloy (e.g., Fe-R-B based, in particular) nanocomposite magnet has recently been used more and more often as a magnet powder for a bonded magnet because such a magnet powder is relatively cost effective. The Fe-R-B based nanocomposite magnet is an iron-based alloy permanent magnet in which nanometer-scale crystals of iron-based borides (e.g., Fe
3
B, Fe
23
B
6
and other soft magnetic phases) and those of an R
2
Fe
14
B phase as a hard magnetic phase are distributed uniformly within the same metal structure and are magnetically coupled together via exchange interactions.
The nanocomposite magnet includes soft magnetic phases and yet exhibits excellent magnet performance due to the magnetic coupling between the soft and hard magnetic phases. Also, since there are those soft magnetic phases including no rare-earth elements R such as Nd, the total percentage of the rare-earth elements R can be relatively low. This is advantageous for the purposes of reducing the manufacturing cost of magnets and supplying the magnets constantly. Furthermore, since the magnet includes no R-rich phases in the grain boundary, the magnet also excels in anticorrosiveness.
Such a nanocomposite magnet is obtained by solidifying a molten material alloy (i.e., “molten alloy”) by a rapid cooling process and then subjecting the rapidly solidified alloy to an appropriate heat treatment process. A single roller method is often used to rapidly cool the molten alloy. The single roller method is a method of cooling and solidifying a molten alloy by bringing the alloy into contact with a rotating chill roller. In this method, the resultant rapidly solidified alloy has the shape of a thin strip (or ribbon), which is elongated in the peripheral velocity direction of the chill roller. This method of rapidly cooling a molten alloy by bringing the alloy into contact with the surface of a solid is called a “melt-quenching process”.
On the other hand, in preparing a conventional extensively used powder for a bonded magnet, a rapidly solidified alloy thin strip with a thickness of 50 &mgr;m or less (typically about 20 &mgr;m to about 40 &mgr;m) is obtained at a roller surface peripheral velocity of 15 m/s or more The rapidly solidified alloy thin strip obtained in this manner is thermally treated and then pulverized to a mean particle size of 300 &mgr;m or less (typically about 150 &mgr;m) to be a rare-earth alloy powder for a permanent magnet. The particles of the rare-earth alloy powder obtained in this manner have a flat shape and have aspect ratios that are less than 0.3. As used herein, the “aspect ratio” means the ratio of the minor-axis size of a powder particle to the major-axis size thereof. The rare-earth alloy powder or magnet powder obtained by the melt-quenching process described above will be simply referred to herein as a “conventional rapidly solidified rare-earth alloy powder” or a “conventional rapidly solidified magnet powder”. An Fe-R-B based MQ powder available from Magnequench International Inc. (which will be referred to herein as “MQI Inc.”) is widely known as a typical conventional rapidly solidified magnet powder.
By mixing the conventional rapidly solidified rare-earth alloy powder with a resin (or rubber), a compound to make a magnet (which will be simply referred to herein as a “compound”) is prepared. An additive such as a lubricant is sometimes mixed with this compound. Thereafter, by compacting the resultant compound into a desired shape by a compression, extrusion or injection molding process, for example, and then by magnetizing the compact, a bonded magnet is obtained as a compact for a permanent magnet (which will be sometimes referred to herein as a “permanent magnet body”). It should be noted that a rare-earth alloy powder to exhibit desired permanent magnet performance when magnetized or a magnetized rare-earth alloy powder will be sometimes referred to herein as a “permanent magnet powder” or simply “magnet powder (or magnetic powder”.
The conventional rapidly solidified magnet powder has a flat particle shape as described above. Accordingly, a compound obtained by mixing the conventional rapidly solidified magnet powder with a resin (or rubber) powder exhibits poor flowability or packability during the compaction process thereof. To achieve flowability that is high enough to perform the compaction process smoothly, the percentage of the resin or rubber may be increased. In that case, however, the magnet powder percentage is limited. Or only limited compaction methods and/or compact shapes are available to compact such a material with poor flowability.
Recently, as various types of electric equipment have further reduced their sizes and further improved their performance, it has become more and more necessary to make magnets having an even smaller size and even higher performance. For that purpose, there is a growing demand for a compound that exhibits so high flowability as to fill even a small gap (e.g., with a width of about 2 mm) just as intended. For example, as in an IPM (interior permanent magnet) type motor including a magnet embedded rotor as disclosed in Japanese Laid-Open Publication No. 11-206075, a demand for a compound with high flowability goes on increasing.
Also, when the conventional rapidly solidified magnet powder is used, the magnet powder percentage (i.e., the ratio of the volume of magnet powder to that of overall bonded magnet) is at most about 80% when the powder is compacted by compression and at most about 65% when the powder is compacted by injection molding. The magnet powder percentage will determine the performance of permanent magnets as final products. Thus, to improve the performance of permanent magnets, the magnet powder percentage is preferably increased.
To increase the flowability of the conventional rapidly solidified magnet powder, Japanese Laid-Open Publication No. 5-315174 proposes a method in which a magnet powder obtained by a gas atomization process is used. According to this publication, the magnet powder prepared by the gas atomization process has almost granular particles. Thus, by adding this magnet powder to the conventional rapidly solidified magnet powder, the flowability can be increased. However, it is difficult to make a magnet powder exhibiting sufficient magnetic properties by a gas atomization process. Thus, this method is far from being an industrially applicable method. The reason is as follows. Specifically, the gas atomization process results in a lower cooling rate than the melt-quenching process described above. Accordingly, only very fine particles can satisfy the rapid cooling conditions that should be met to obtain particles with sufficient magnetic properties. Also, a melt of the rare-earth alloy having the composition disclosed in the publication identified above has a relatively high viscosity. Thus, it is hard to obtain fine particles. Consequently, according to the method disclosed in the publication identified above, the yield of those fine particles having sufficient magnetic properties is very low and the productivity is also very bad because a classification process step must be carried out to obtain particles with a desired particle size.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, a primary object of the
Hirosawa Satoshi
Kanekiyo Hirokazu
Kitayama Hirokazu
Miyoshi Toshio
Keating & Bennett, LLP.
Mai Ngoclan T.
Neomax Co. Ltd.
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