Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2001-07-31
2003-04-29
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C164S423000, C164S463000, C075S331000, C075S334000
Reexamination Certificate
active
06554913
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing magnetic powder, magnetic powder and bonded magnets. More specifically, the present invention relates to a method of manufacturing magnetic powder, magnetic powder manufactured by the method, and a bonded magnet manufactured using the magnetic powder.
2. Description of the Prior Art
Rare-earth magnetic materials formed from alloys containing rare-earth elements have high magnetic properties. Therefore, when they are used for magnetic materials for motors, for example, the motors can exhibit high performance.
Such magnetic materials are normally manufactured by the quenching method using a melt spinning apparatus, for example. Hereinbelow, a description will be made with regard to the manufacturing method using the melt spinning apparatus.
FIG. 21
is a sectional side view which shows the situation caused at or around a colliding section of a molten alloy with a cooling roll in the conventional melt spinning apparatus which manufactures a magnetic material by means of a single roll method.
As shown in this figure, in the conventional method, a magnetic material of a predetermined alloy composition (hereinafter, referred to as “alloy”) is melt and such a molten alloy
60
is injected from a nozzle (not shown in the drawing) so as to be collided with a circumferential surface
530
of a cooling roll
500
which is rotating relative to the nozzle in the direction indicated by the arrow A in FIG.
21
. The alloy which is collided with the circumferential surface
530
is rapidly cooled down (quenched) to be solidified, thereby producing a ribbon-shaped magnetic material (that is, a melt spun ribbon
80
) in a continuous manner. In this regard, it is to be noted that the dotted line in
FIG. 21
indicates a solidification interface
710
of the molten alloy
60
.
In the method described above, since the rare-earth elements are liable to oxidize and when they are oxidized the magnetic properties thereof tend to be lowered, the manufacturing of the melt spun ribbon
80
is normally carried out under an inert gas atmosphere.
However, this causes the case that gas enters between the circumferential surface
530
and the puddle
70
of the molten alloy
60
, which results in formation of dimples (depressions)
9
in the roll contact surface
810
of the melt spun ribbon
80
(that is, the surface of the melt spun ribbon which is in contact with the circumferential surface
530
of the cooling roll
500
). This tendency becomes prominent as the peripheral velocity of the cooling roll
500
becomes large, and in such a case the area occupied by thus formed dimples also becomes larger.
In the case where such dimples
9
(especially, huge dimples) are formed, the molten alloy
60
can not sufficiently contact with the circumferential surface
530
of the cooling roll
500
at the locations of the dimples due to the existence of the entered gas, so that the cooling rate is lowered to prevent rapid solidification. As a result, at portions of the melt spun ribbon where such dimples are formed, the crystal grain size of the alloy becomes coarse, which results in lowered magnetic properties.
Magnetic powder obtained by milling such a melt spun ribbon having the portions of the lowered magnetic properties has larger dispersion or variation in its magnetic properties. Therefore, bonded magnets formed from such magnetic powder can have only poor magnetic properties, and corrosion resistance thereof is also lowered.
SUMMARY OF THE INVENTION
In view of the above problem involved in the prior art, it is an object of the present invention to provide a method of manufacturing magnetic powder which can provide bonded magnets having excellent magnetic properties and reliability. Further, it is also an object of the present invention to provide magnetic powder and bonded magnets having excellent magnetic properties and reliability.
In order to achieve the above object, the present invention is directed to a method of manufacturing magnetic powder in which the magnetic powder is manufactured by milling a ribbon-shaped magnetic material which has been obtained by colliding a molten alloy of a magnetic material to a circumferential surface of a rotating cooling roll so as to cool and then solidify it. This method is characterized in that the cooling roll is formed with gas flow passages as gas expelling means for expelling gas entered between the circumferential surface and a puddle of the molten alloy in the circumferential surface thereof, and, when the average pitch of these gas flow passages is defined as P&mgr;m and the average particle size of the magnetic powder is defined as D&mgr;m, the relationship represented by the formula P<D is satisfied.
According to the above described manufacturing method, it is possible to provide magnetic powder from which bonded magnets having excellent magnetic properties and reliability can be manufactured.
In this method, it is preferred that the average particle size of the magnetic powder lies in the range of 5 to 300 &mgr;m. This makes it possible to provide bonded magnets having especially excellent magnetic properties.
Further, it is also preferred that the average pitch P of the gas flow passages lies in the range of 0.5 &mgr;m or more and less than 100 &mgr;m. When such a cooling roll is used, dispersion in the cooling rates of the molten alloy can be made small irrespective of the contacting portions of molten alloy with the cooling roll, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Furthermore, it is also preferred that the average width of the gas flow passages lies in the range of 0.5 to 90 &mgr;m. When such a cooling roll is used, gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll can be effectively expelled through the passages, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Moreover, it is also preferred that the average depth of the gas flow passages lies in the range of 0.5 to 20 &mgr;m. When such a cooling roll is used, it is also possible to expel gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll effectively through the passages, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Further, in a preferred form of this method, when the average width of the gas flow passages is defined as L
1
and the average depth of the gas flow passages is defined as L
2
, the relationship represented by the formula of 0.5≦L
1
/L
2
≦15 is satisfied. Use of such a cooling roll also makes it possible to expel gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll effectively through the passages, so that it is possible to provide bonded magnets having especially excellent magnetic properties.
In this method, it is preferred that the cooling roll includes a roll base and an outer surface layer provided on an outer peripheral portion of the roll base, and the gas flow passages are provided in the outer surface layer. Use of such a cooling roll also makes it possible to provide bonded magnets having excellent magnetic properties and reliability.
In this case, it is preferred that the outer surface layer of the cooling roll is formed of a material having heat conductivity lower than the heat conductivity of the structural material of the roll base at or around a room temperature. This makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, thereby enabling to provide bonded magnets having especially excellent magnetic properties.
Further, it is also preferred that the heat conductivity of the outer surface layer of the cooling roll at or around a room temperature is equal to or less than 80 W m
−1
K
−1
. This also makes it possibl
Arai Akira
Kato Hiroshi
Harness & Dickey & Pierce P.L.C.
Seiko Epson Corporation
Sheehan John
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