Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-09-27
2004-11-23
Sheehan, John P (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S101000, C148S302000
Reexamination Certificate
active
06821357
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to permanent magnets, R—TM—B based permanent magnets, where R is a rare earth element embracing Y and TM is a transition metal, and, more particularly, to a starting material thereof, an intermediate product thereof and an ultimate product thereof.
Additionally, this invention relates to rare-earth magnetic powders for bonded magnets and a manufacturing method thereof.
BACKGROUND
The mechanism used for generating the coercivity in permanent magnets currently under use may be enumerated by single magnetic domain particle type, nucleation type and pinning type mechanisms. Of these, the nucleation type coercivity generating mechanism has been introduced in order to account for generation of large coercivity in a sintered magnet having a crystal grain size not less than the single magnetic domain particle size, and is based on the theory that facility of nucleation of an demagnetizing field in the vicinity of the crystal grain boundary determines the coercivity of the crystal grain in question. This type of the magnet has peculiar magnetization properties that, while saturation of a lower impressed magnetic field, a magnetic field not less than the saturation magnetization needs to be applied to obtain sufficient coercivity. It may be presumed that the high magnetic field can drive off any demagnetizing field left in the crystal grain completely by a high magnetic field thus producing high coercivity. Examples of the magnet having the nucleation type coercivity generating mechanism include SmCo
5
-based or Nd—Fe—B-based sintered magnets.
The R—TM—B based permanent magnet has superior magnetic properties, and is finding a wide field of usages. There are a variety of manufacturing methods for the R—TM—B based permanent magnet, the most representative one being a sintering method and a rapid solidification method. The sintering method, as disclosed in Japanese Laying-Open Patent Kokai JP-A-59-46008, is a method consisting in pulverizing an ingot of a specified composition to fine powders of single crystals with a mean particle size of several &mgr;m, consolidating the powders to an optional shape under magnetic orientation in a magnetic field, and sintering the green compact to a bulk magnet. The rapid solidification method, disclosed in Japanese Patent Kokai JP-A-60-9852, is a method consisting in rapidly solidifying an alloy of a specified composition by a method such as roll quenching method to an amorphous state followed by heat treatment to precipitate fine crystal grains. The magnet alloy obtained by routinely mixed with a resin and molded to produce bonded magnets.
Rare earth magnetic powders having the coercivity generating mechanism of the pinning type, such as Sm
2
Co
17
, can be processed into magnetic powders suitable for bonded magnets simply by pulverizing a molten ingot of a pre-set composition. On the other hand, in rare earth magnetic powders having the coercivity generating mechanism of the nucleation type, practically useful coercivity is not produced unless the crystal grain size of the powdered particles is set so as not to be larger than the single magnetic domain particle size. Thus, as a manufacturing method in which the Nd
2
Fe
14
B crystal grain size in the powdered particles is less than the single magnetic domain particle size, there are currently used a rapid solidification method and a HDDR (hydrogenation-decomposition-dehydrogenation-recombination) method.
SUMMARY OF THE DISCLOSURE
The present inventors have found that the conventional techniques concerning the above-mentioned nucleation type magnet has the following disadvantages. That is, while it has been predicted that, in the conventional techniques, the coercivity of the nucleation type magnet is governed by nucleation of the demagnetizing field, sufficient information has not been acquired as to specified means for suppressing coercivity. For instance, while it has been known that the presence of the Nd-rich grain boundary phase operates to improve the coercivity in the Nd—Fe—B based sintered magnet, its detailed mechanism has not been clarified.
In the above-described conventional techniques, sample preparation and evaluation are repeatedly carried out to optimize various conditions of the manufacturing process of the magnet to improve the magnetic properties of the magnet by an empirical route. However, with such an empirical method, it is difficult to achieve drastically improved magnetic properties. Moreover, if plural permanent magnets of different compositions are produced, the sample preparation and evaluation of the different magnets need to be repeatedly carried out for the respective magnets.
In the above-described a manufacturing method in which the Nd
2
Fe
14
B crystal grain size in the powdered particles is less than the single magnetic domain particle size, the rapid solidification method and the HDDR method suffer from the defect that the investment costs for production equipment are high and the manufacturing conditions are severe to raise the cost.
It is an object of the present invention to provide a guide or key for the designing of high magnetic performance.
It is another object of the present invention to provide a magnet having high magnetic performance.
It is a further object of the present invention to provide rare-earth magnetic powders for bonded magnets having high magnetic properties, and which can be manufactured inexpensively, and a manufacturing method thereof.
Heretofore, the structure of an interface governing the magnetic properties of a magnet, in particular its coercivity, between the major phase and the grain boundary phase, has not been clarified. In the present specification, the “major phase” means the “phase exhibiting the ferromagnetism”. The major phase desirably accounts for not less than one half of the entire phase. Thus, in the conventional technique, various conditions of the magnet manufacturing process are optimized for empirically improving the magnetic properties of the magnet. This empirical technique is not only time-consuming and costly but also is encountered with limitations in further improving the magnetic properties.
The present inventors have conducted researches into the fundamental problem of what should be the ideal interface structure, without relying upon the empirical technique, and found that, in a variety of magnetic materials exhibiting nucleation type coercivity generating mechanism, the ease with which nucleation occurs depends on the magnitude of the magnetocrystalline anisotropy in the vicinity of the outermost magnitude of the anisotropy constant K
1
in the vicinity of the outermost shell to be at least equal or larger than that in an interior region, the nucleation can be suppressed to improve coercivity of the magnet. This finding has led to completion of the present invention.
THE FIRST GROUP OF THE PRESENT INVENTION
In a first aspect of the first group of the present invention, the ferromagnetic phase is matched with the grain boundary phase. In its second aspect of the first group, the atomic arrangement (orientation) is regular on both sides of an interface between the ferromagnetic phase and the grain boundary phase. In its third aspect of the first group, the grain boundary phase has a crystal type, a plane index and azimuthal index(crystal orientation) matched to the ferromagnetic phase. In its fourth aspect of the first group, the magnetocrystalline anisotropy at a lattice point of said ferromagnetic phase neighboring to the interface with the grain boundary phase is not less than one-half the magnetocrystalline anisotropy at the lattice point interior of said ferromagnetic phase.
In its fifth aspect of the first group, the magnetocrystalline anisotropy in the outermost shell of the ferromagnetic particles is not less than one-half that in the interior thereof. In its sixth aspect of the first group, the ferromagnetic crystal grains is higher than that in the interior thereof. In its seventh aspect of the first group, the magnetocrystalline anisotropy of the outer s
Makita Ken
Yamashita Osamu
Sheehan John P
Sumitomo Special Metals Co. Ltd.
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