Metal treatment – Stock – Magnetic
Reexamination Certificate
2000-03-20
2001-11-20
Sheehan, John P (Department: 1742)
Metal treatment
Stock
Magnetic
C148S101000, C419S012000, C075S244000
Reexamination Certificate
active
06319336
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a permanent magnet alloy based on R (wherein R represents yttrium (Y) or a rare-earth element), boron (B), carbon (C), cobalt (Co) and iron (Fe), which exhibits particularly improved heat resistance as such that little degradation occurs on the magnetic force even in case it is used under ambient at a temperature as high as 200° C.
BACKGROUND OF THE INVENTION
A Sm-Co based magnet is known as a rare-earth magnet having improved heat resistance, but is expensive. The term “heat resistance” as referred herein particularly signifies that the magnetic force of the magnet does not degrade by heat. As disclosed in Japanese Patent Public Disclosure No. 4-116144 (Patent No. 2740981), one of inventors of the present invention and others have proposed a R—B—C—Co—Fe based permanent magnet alloy as a rare-earth magnet reduced in cost and yet improved in heat resistance. This magnet alloy contains carbon (C) as an essential alloy element, and utilizes a combination of a light rare-earth element and a heavy rare-earth element for the rare-earth element (R). The disclosure teaches that the irreversible demagnetization of the magnet alloy is improved remarkably (i.e., the negative irreverible demagnetization values approach 0%) by the incorporation of C, and that the irreversible demagnetization is further improved by partly incorporating a heavy rare-earth element for R.
OBJECT OF THE INVENTION
In case a permanent magnet is assembled in appliances that are installed in the vicinity of a heat-emitting source, it is essential that the magnetic force of the permanent magnet does not drop when brought to higher temperatures, i.e., that the residual magnetic flux density (Br) does not undergo degradation when heated. However, there are cases in which the magnet is used under conditions as such that the operation temperature approaches ca. 200° C. (e.g., automobile engine appliances are operated at ca. 200° C., and as a matter of course, the same holds true for motors of electric automotive vehicles). Then, Sm—Co based magnets are the only type of known magnets applicable to this field. However, as stated above, Sm—Co based magnets are expensive, and the ordinary Nd—Fe(Co)—B based rare-earth magnets are unfeasible for such high temperature (e.g., 200° C.) applications.
Although the aforementioned Japanese Patent Public Disclosure No. 4-116144 teaches that the incorporation of C (carbon) as the alloying element in a permanent magnet improves the irreversible demagnetization and that the partial replacement of R by a heavy rare-earth element further improves the irreversible demagnetization, there is not shown any magnet that does not undergo demagnetization when heated to 200° C.
In the light of such circumstances, an object of the present invention is to provide a permanent magnet having improved heat resistance feasible for use at such a high temperature of 200° C., yet at a low production cost.
SUMMARY OF THE INVENTION
In order to achieve the object above based on the fundamental concept of incorporating C to improve the heat resistance of the permanent magnet alloy, as proposed in Japanese Patent Public Disclosure No. 4-116144, the present inventors investigated and studied the influence of each of the heavy rare-earth elements on the heat resistance. As a result, in addition to the incorporation of the basic rare-earth elements such as Nd and Pr, it has been newly found that the addition of Dy and Tb in combination and in proper quantities, particularly when they are added in relation to each other, greatly improves the heat resistance of the permanent magnet.
Thus, in accordance with the present invention, there is provided a permanent magnet alloy having an improved heat resistance comprising, in terms of percent by atom ( at. %),
0.1 to 15 at. % C,
0.5 to 15 at. % B,
provided that C and B in total account for 2 to 30 at. %:
40%at.or less Co (exclusive of zero percent),
0.5 to 5 at. % in total of Dy and Tb, preferably, the ratio Tb( at. %)/Dy( at. %) is in the range of from 0.1 to 0.8;
8 to 20 at. % R, where R represents at least one element selected from the group consisting of Nd, Pr, Ce, La, Y, Gd, Ho, Er, and Tm;
with the balance being Fe and unavoidable impurities.
The heat resistance of the permanent magnet alloy is characterized by that the irreversible demagnetization (200° C.) according to the following equation (1) is in the range of 0% to−20%, preferably 0 to−15%, where iHc is 13 KOe or higher:
Irreversible Demagnetization (at 200° C.)=100×(A
200
−A
25
)/A
25
(1)
where, A
25
represents a flux value of a magnet measured at room temperature (25° C.), on a specimen prepared into a shape as such that its permeance coefficient Pc be 1, and magnetized at 50 KOe; and
A
200
represents a flux value of a magnet measured on the same specimen subjected to the measurement of A
25
, which was maintained at 200° C. for 120 minutes and then cooled to room temperature (25° C.), for the measurement.
In particular, a permanent magnet alloy having an irreversible demagnetization in the range of 0 to−20% can be obtained by properly selecting the combination of Dy and Tb, e.g., a case in which Dy and Tb in total account for 0.5 to 5 at. % and in which Dy is in the range of 0.3 to 4.9 at. % and Tb is in the range of 0.1 to 4.7 at. % (i.e., the compositional area defined by points A, B, C and D plotted in FIG.
1
). Furthermore, a permanent magnet alloy having an irreversible demagnetization in the range of 0 to−15% can be obtained by controlling the content of Dy and Tb to fall in the range defined by points B, C, H, E, F and G plotted in FIG.
1
.
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Kamada Masami
Obata Michio
Sato Yuichi
Dowa Mining Co. Ltd.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Sheehan John P
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