High temperature permanent magnets

Details High temperature permanent magnets High temperature permanent magnets

2000-01-03

2002-09-17

King, Roy (Department: 1742)

Metal treatment

Stock

Magnetic

C148S301000

Reexamination Certificate

active

06451132

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to high temperature permanent magnet materials, and more particularly to permanent magnets which have improved magnetic properties at a desired operating temperature.
Permanent magnets containing one or more rare earth elements and transition elements are well known for use in a variety of applications. For example, magnets have been used in motors and generators for aircraft and spacecraft systems. Magnets have also been widely used in actuators, inductors, inverters, magnetic bearings, and regulators for flight control surfaces and other aircraft components. These applications require the magnets to operate at temperatures up to about 300° C.
In recent years, the need has increased for magnetic and electromagnetic materials capable of reliable operation at higher temperatures of from 300° C. to 600° C. For example, the MEA (More Electric Aircraft) initiative has stimulated the development of an Integrated Power Unit (IPU) which utilizes a high-speed, direct-coupled starter/generator and magnetic bearings integrated onto the rotor of a single-shaft gas turbine aircraft engine which permits direct coupling to the turbine shaft, thereby eliminating all gearing and lubrication found in current military and commercial aircraft power units. However, the operating temperature of magnetic materials for such an application is higher than 400° C. Other high temperature applications include replacement of hydraulic-mechanical components in aircraft with permanent magnets. Accordingly, magnetic materials capable of operating at temperatures as high as 400°C. and above are needed for such applications.
Currently, Sm(Co
w
Fe
v
Cu
x
Zr
y
)
z
or RE
2
TM,
17
(where RE represents rare earth metals and TM represents transition metals) permanent magnet materials have been demonstrated as the best magnets for elevated temperature applications. These magnets have satisfied many applications at temperatures up to 300° C. However, such magnets have typically been unable to retain their magnetic properties at elevated temperatures greater than 300° C. For example, the intrinsic coercivity (
i
H
c
) of such magnets has been found to drop substantially at temperatures of 300° C. or greater. More importantly, the extrinsic demagnetization curve for such magnets is not straight, and linear extrinsic demagnetization curves are imperative for dynamic applications. In order to maintain stability at high temperatures, magnets must maintain a high
i
H
c
and a low temperature coefficient of
i
H
c
as well as a linear extrinsic demagnetization curve.
Accordingly, there is still a need in the art for a permanent magnet material which is capable of operating at temperatures higher than 300° C., which exhibits a high
i
H
c
and a low temperature coefficient of
i
H
c
, and which exhibits a linear extrinsic demagnetization curve at high temperatures.
As additional background information, the early development of rare earth magnet alloy systems is discussed in the following papers:
K. Strnat and W. Ostertag, “Program for an in-house investigation of the yttrium-cobalt alloy system”, Technical Memorandum, May 64-4, Projects 7367 and 7360, AFML, Wright-Patterson AFB, Ohio, March, (1964)
K. Strnat and G. Hoffer, “YCo
5
- A promising New Permanent Magnet Material”, USAF Tech. Doc. Rept., Materials Laboratory, WPAFB AFML-TR-65-446, May (1966).
G. Hoffer and K. Strnat, “Magnetiocrystalline Anisotropy of YCO
5
and Y2Co
17
”, IEEE Trans. Magn., Mag-2, 487, Sept., (1966).
K. Strnat, G. Hoff, J. Olson, W. Ostertag, and J. Becker, “A family of new cobalt-base permanent magnet material”, J. Appl. Phys. 38 1001, (1967)
D. Das, “Twenty million energy product samarium-cobalt magnet”, IEEE Trans., Magn. Mag-5, 214, (1969)
M. Benz and D. Martin, “Cobalt-samarium permanent magnets prepared by liquid phase sintering”, Appl. Phys. Lett., 17 176 (1970)
RE
2
TM
17
type magnets were initiated from the investigation of R
2
(Co, Fe)
17
alloy by A. E. Ray and K. J. Strnat in 1972. However, numerous attempts to develop high
i
H
c
in these stoichiometric 2:17 alloys were generally unsuccessful and attention was then focused on Sm(Co
.85
Cu
.15
)
6.8
(Nagel et al., 1975) and Sm(Co
.85
Fe
.05
Cu
.10
)
8
(Tawara et al., 1976) with Br=10-11 kG, H
c
=4-6 kOe, and (BH)
max
=26 MGOe. Sm(Co
.68
Fe
.28
Cu
.1
Zr
.01
)
7.4
with 30 MGOe was achieved in 1977 (Ojima et al., 1977). Research and development in the 1970's resulted in RE
2
TM
17
type magnets with high energy product, where RE represents rare earth metals, such as Sm, Pr, Gd, Ho, Er, Ce, Y, Nd, and TM represents transition metals such as: Co, Fe, Cu, Zr, Hf, Ti, Mn, Nb, Mo, W, and other transition metals. Particularly preferred high performance magnets for the applications noted above are RE=Sm, Gd, Dy and TM=Co, Fe,Cu, and Zr, having the crystal structure of Sm
2
Co
17
. Most RE-TM magnets can be used at 250° C., and some of these magnets can perform well up to 330° C.
Some of these magnets are described in U.S. Pat. Nos. 4,210,471; 4,213,803; 4,284,440; 4,289,549; 4,497,672; 4,536,233; 4,565,587, 4,746,378, and 5,781,843. See also U.S. Pat. Nos. 3,748,193, 3,947,295; 3,970,484; 3,977,917; 4,172,717; 4,211,585; 4,221,613; 4,375,996; 4,382,061 and 4,578,125.
Publications relating to RE
2
TM
17
type magnets are listed below:
A. E. Ray and K. J. Strnat, IEEE Trans. Magn., Mag-8, 518, 1972
Nagel, Perry and Menth, IEEE Trans. Magn. Mag-11, 1423, 1975
Tawara and Strnat, “Rare earth Cobalt permanent magnets near the 2:17 composition”, IEEE Trans. Magn. Mag-12, 954, 1976
Ojima, Tomizawa, Yoneyama, and Hori, “Magnetic properties of a new type of rare earth magnets Sm
2
(Co,Cu,Fe,M)
17
,” IEEE Trans. Magn. Mag-13, 1317, 1977
A. E. Ray, “The development of high energy product permanent magnets from 2:17 RE-TM alloys”, IEEE Trans, Ma-20, 1615, (1984)
Marlin S. Walmer, “A comparison of temperature compensation in SmCo
5
and RE
2
TM
17
as measured in a permeameter, a traveling wave tube and an inertial device over the temperature range of −60° to 200° C.”, Proceedings of the 9
th
International workshop on rare earth magnets and their applications, Bad Soden, Germany, 131-140 (1987)
H. F. Mildrum and K. D. Wong, “Stability and temperature cycling behavior of RE-Co magnets”, Proceedings of the 9
th
International workshop on rare earth magnets and their applications, Bad Soden, Germany, 35-54 (1987)
J. Fidler, et al., “Analytical Electron microscope study of high and low coercivity SmCo 2:17 magnets”, Mat. Res. Sol. Sym. Proc. 96, 1987
Popov et al., “Inference of copper concentration on the magnetic properties and structure of alloys”, Phys. Met. Metall., 60 (2), 18-27, (1990)
A. E. Ray and S. Liu, “Recent progress in 2:17 type permanent magnets”, J. Material Engineering and Performance, 1, 183-192, (1992)
Extrinsic demagnetization curves for prior art Sm-TM magnet materials are set forth in
FIG. 1
which shows that linear extrinsic demagnetization curves existed up to about 330° C. The curves become non-linear above 330° C.
FIG. 2
illustrates the recoil process for a magnet with a nonlinear extrinsic demagnetization curve, when the demagnetization force drives the magnet past the “knee” of the curve and back to zero magnetic strength.
Further work has been done on RE-TM magnets for use at temperatures above 300° C. References related to these high temperature RE-TM magnets are listed below:
Marlin S. Walmer and Michael H. Walmer, “Knee formation of high Co content 2:17 magnets for MMC high temperature applications”, EEC internal report, May, 1995
S. Liu and E. P. Hoffman, “Application-oriented characterization of Sm
2
(Co,Fe,Cu,Zr)
17
permanent magnets,” IEEE Trans. Magn., 32, 5091, (1996).
B. M. Ma, Y. L. Liang, J. Patel, D. Scott, and C. O. Bounds, “The effect of Fe content on the temperature dependent magnetic properties of Sm(Co,Fe,Cu,Zr)z and SmCo
5
sintered magnets at 450° C.,” IEEE Trans. Magn., 32,4377 (1996).
S. Liu, G. P. Hoffman, and J. R. Brown, “Long-term aging of Sm
2

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