Metal treatment – Stock – Magnetic
Reexamination Certificate
1999-10-14
2002-10-22
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C075S244000
Reexamination Certificate
active
06468365
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an R—T—B sintered permanent magnet having high coercivity, residual magnetic flux density and maximum energy product.
DESCRIPTION OF PRIOR ART
With respect to R—T—B sintered permanent magnets, wherein R is at least one rare earth element including Y, and T is Fe or Fe and Co, those having maximum energy products of about 40 MGOe are mass-produced. Means for adjusting the alloy compositions of the R—T—B sintered permanent magnets include a single method and a blend method.
The single method is a method for producing an R—T—B sintered permanent magnet using an ingot adjusted to have a main component composition of an R—T—B sintered permanent magnet at a melting and/or casting stage, through the steps of pulverization, molding in a magnetic field, sintering and heat treatment. The resultant R—T—B sintered permanent magnet is subjected to predetermined machining and surface treatment for use in practical applications.
The blend method is a method for producing an R—T—B sintered permanent magnet through the steps of mixing of two or more types of R—T—B sintered permanent magnet powder having different compositions at such a formulation as to provide the final R—T—B sintered permanent magnet with a desired main component composition, pulverization, if necessary, and further molding in a magnetic field, sintering, heat treatment and surface treatment.
The above single method can relatively easily provide sintered permanent magnets with a high coercivity iHc, their residual magnetic flux density Br and maximum energy product (BH)
max
are low, unsuitable for applications requiring high Br and (BH)
max
.
Though conventionally proposed as applications of the blend method are an R—T—B sintered permanent magnet produced from an R—T alloy having a high R content and an R—T—B alloy having a low R content (Japanese Patent Laid)pen No. 7-122413), and an R—T—B sintered permanent magnet in which Ga, C and O are segregated in an R-rich phase and its vicinity (Japanese Patent Laid-Open No. 9-232121). However, there is still room for improvement to make them suitable for high-Br, high (BH)
max
applications. Particularly with respect to heavy rare earth elements having large influence on magnetic properties, their optimum concentration distributions in main phase particles and their control have not yet been made clear.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a high-performance R—T—B sintered permanent magnet suitable for applications requiring high Br and (BH)
max
.
Thus, the R—T—B sintered permanent magnet according to the present invention has a composition comprising 28-33 weight % of R, and 0.5-2 weight % of B, the balance being substantially T and inevitable impurities, wherein R is at least one rare earth element including Y, at least one heavy rare earth element selected from the group consisting of Dy, Th and Ho being indispensable, and T is Fe or Fe and Co, the permanent magnet having a crystal structure comprising first R
2
T
14
B-type, main-phase crystal grain particles having a higher heavy rare earth element concentration than that of a crystal grain boundary phase, and second R
2
T
14
B-type, main-phase crystal grain particles having a lower heavy rare earth element concentration than that of the crystal grain boundary phase.
In a preferred embodiment of the present invention, the R—T—B sintered permanent magnet has a composition comprising 28-33 weight % of R, 0.5-2 weight % of B, and 0.01-0.6 weight % of M
1
, wherein M
1
is at least one element selected from the group consisting of Nb, Mo, W, V, Ta, Cr, Ti, Zr and Hf, the balance being substantially T and inevitable impurities.
In another preferred embodiment of the present invention, the R—T—B sintered permanent magnet has a composition comprising 28-33 weight % of R, 0.5-2 weight % of B, 0.01-0.6 weight % of M
1
, and 0.01-0.3 weight % of M
2
, the balance being substantially T and inevitable impurities, wherein M, is at least one element selected from the group consisting of Nb, Mo, W, V, Ta, Cr, Ti, Zr and Hf, and M
2
is at least one element selected from the group consisting of Al, Ga and Cu.
In a further preferred embodiment of the present invention, the R—T—B sintered permanent magnet comprises more than 31% and 33% or less by weight of R, with 0.6 weight % or less of oxygen, 0.15 weight % or less of carbon, 0.03 weight % or less of nitrogen and 0.3 weight % or less of Ca as inevitable impurities.
In a still further preferred embodiment of the present invention, the R—T—B sintered permanent magnet comprises 28-31 weight % of R with 0.25 weight % or less of oxygen, 0.15 weight % or less of carbon, 0.15 weight % or less of nitrogen and 0.3 weight % or less of Ca as inevitable impurities.
The R—T—B sintered permanent magnet of the present invention is produced, for instance, by the steps of mixing of two types or more of alloy powder having substantially the same composition except for the difference in a ratio of heavy rare earth elements (Dy, etc.)/light rare earth elements (Nd, Pr, etc.) with the same total amount of the rare earth elements, molding in a magnetic field, sintering, heat treatment, and if necessary, machining, finish working such as barreling, etc., and surface treatment such as Ni plating, etc. Depending on the compositions of the above two types or more of alloy powder and the final composition of the R—T—B sintered permanent magnet, the optimum sintering conditions are selected to strictly control the diffusion of heavy rare earth elements such as Dy in the crystal structure of the sintered magnet. As a result, the crystal structure has a characteristic concentration distribution of heavy rare earth elements such as Dy in the R
2
T
14
B-type, main-phase crystal grain particles (substantially in center portions) and the crystal grain boundary phase, containing R
2
T
14
B-type, main-phase crystal grain particles having a higher concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase, and R
2
T
14
B-type, main-phase crystal grain particles having a lower concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase.
The R—T—B sintered permanent magnet having such a sintered crystal structure has extremely larger Br and (BH)
max
than those of the R—T—B sintered permanent magnet produced by the single method, though its coercivity iHc is slightly smaller than that of the latter. Though the correlation between such high magnetic properties and the concentration distribution of heavy rare earth elements such as Dy has not been fully clarified yet, it is presumed that the R
2
T
14
B-type, main-phase crystal grain particles having a higher concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase contributes to achieving high Br, while the R
2
T
14
B-type, main-phase crystal grain particles having a lower concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase contributes to achieving high iHc close to that obtained by the single method.
REFERENCES:
patent: 5034146 (1991-07-01), Ohashi et al.
patent: 5055129 (1991-10-01), Ghandehari
patent: 5405455 (1995-04-01), Kusunoki et al.
patent: 5595608 (1997-01-01), Takebuchi et al.
patent: 07122413 (1993-10-01), None
patent: 09232121 (1996-02-01), None
Kawata Tsunehiro
Uchida Kimio
Hitachi Metals Ltd.
Oltmans Andrew L.
Sheehan John
Sughrue & Mion, PLLC
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