Metal treatment – Stock – Magnetic
Reexamination Certificate
2000-06-14
2002-07-16
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C148S301000
Reexamination Certificate
active
06419759
ABSTRACT:
The present invention relates to a multielement rare earth-iron interstitial hard magnetic material having a ThMn
12
type crystal structure. The present invention further relates to processes for producing isotropic and anisotropic magnetic powder, and for producing isotropic and anisotropic magnet.
Currently, the rare earth-iron based material used for producing hard magnet is Nd
2
Fe
14
B, in which the process used for producing Nd
2
Fe
14
B type bonded magnet is melt spinning or HDDR technique. However, the magnetic powder obtained by using these processes is generally isotropic, with the maximum energy product of 60-110 KJ/m
3
(8-13 MGOe). It is an anisotropic magnetic powder having a high magnetic energy product that is sought to be developed. The Nd
2
Fe
14
B type magnet has a low Curie temperature, and is insufficient in anti-oxidation capacity. Furthermore, since spin reorientation occurs at the temperature around 130K, and easy magnetization direction deviates from C axis, the permanent magnetic properties vanish at low temperatures. Iriyama Kyohiko et al. and J. M. D. Coey et al. teach a R
2
Fe
17
N
x
based rare earth-iron-nitrogen permanent magnetic material (Iriyama Kyohiko et al., JP (31) 285741, 88; Iriyama Kyohiko et al., CN 89101552). For R
2
Fe
17
N
x
, easy magnetization axis appears only when R is Sm. Consequently, in the preparation of high performance magnet, the rare earth utilized is mainly Sm that is more costly than Nd or Pr.
In 1990, Yingchang Yang et al. discovered the interstitial atomic effect of nitrogen in R (Fe
1−&agr;
M
&agr;
)
12
type intermetallic compound, wherein R is a rare earth element, M is Ti, V, Mo, Nb, Ga, W, Si, Al, or Mn, &agr; a is from 0.08 to 0.27. The method comprises smelting a master alloy with the above composition, heat treating under nitrogen atmosphere at 350° C. to 600° C., to form a R(Fe
1−&agr;
M
&agr;
)
12
N
x
interstitial type nitride, such as NdTiFe
11
N
x
. The result of the neutron diffraction study shows that nitrogen atoms have entered the 2b interstitial sites of ThMn
12
type crystal structure. Interstitial atoms enhance Fe-Fe exchange, thereby raise the Curie temperature by 200° C., modify 3d electron band structure of Fe, thus the magnetic moment of Fe is increased by 10-20%. Most importantly, interstitial atoms adjust the crystal field interactions of rare earth sites in crystals. Upon accomplishment of the nitrogenation, easy magnetization axis appear in the 1:12 type nitride of Pr, Nd, Tb, Dy, and Ho, which have very strong magneto-crystalline anisotropy fields. Therefore, R(Fe
1−&agr;
M
&agr;
)
12
N
x
, particularly Nd(Fe
1−&agr;
M
&agr;
)N
x
, has intrinsic magnetic properties comparable to that of Nd
2
Fe
14
B, which can be used, besides Nd
2
Fe
14
B, as a rare earth permanent-magnetic material based on Nd instead of Sm. (see, for example, CN ZL90109166.9; Yingchang Yang et al., New Potential Hard Materials —Nd(Fe,Ti)
12
N
x
, Solid State Communications
, 78(1991)317; Neutron Diffraction Study of the Nitrides YTiFe
11
N
x
, Solid State Communications
, 78(1991)313; and Yingchang Yang et al., Magnetocrystalline Anisotropy of YTiFe
11
N
x
, Applied Physics Letters
, 58(1991)2042. Since the publication of these results obtained by Yingchang Yang, there has been disclosed some other patent applications in this field, for example, U.S. Pat. No. 5,403,407 of G. C. Hadjipanayis et al. in 1992. In Hadjipanayis's patent, an alloy with a composition of R
x
Fe
y−w
CO
w
M
z
L
&agr;
is employed, wherein R is a rare earth, M is Cr, Mo, Ti, or V, L is C or N, x is an atomic percent from 5 to 20, y is an atomic percent from 65 to 85, w is an atomic percent of about 20, z is an atomic percent from 6 to 20, and &agr; is an atomic percent from 4 to 15. In this alloy, it is necessary to add 10-20 atomic percent of cobalt. After smelting of alloy, amorphous non-crystal magnetic material is formed by utilizing a high energy ball-mill mechanical alloying method, and a magnetic powder having a coercivity of 160-640 KA/m (2-8 kOe) is obtained by controlling the crystallization temperature. However, the magnetic powder thus obtained is isotropic, with very low remanence (Br), which is 0.3-0.4 T (3-4 KG), and very low maximum magnetic energy product ((BH)
max
), which is 8-16 KJ/m
3
(1-2 MGOe). This does not meet the requirement of practical application. As is well known, the parameters used to denote the performance of a permanent magnetic material include remanence Br, coercivity iHc and bHc, and maximum magnetic energy product (BH)
max
. In these parameters, the maximum magnetic energy product is an overall indication of permanent magnetism, which represents the overall performance of magnet. In the above said patents, only the intrinsic magnetism of the material, such as saturation magnetization intensity (Ms), Curie temperature (Tc) and anisotropy field of magnetic moment (Ha) are dealt with, while the fundamental performance of permanent magnet are not. In other words, there is not disclosed a method to achieve higher remanence (Br) and higher maximum magnetic energy product ((BH)
max
). Each of remanence (Br), coercivity (iHc and bHc) and maximum magnetic energy product ((BH)
max
), which represents the performance of permanent magnetic materials, is structure sensitive. Theoretically, these parameters depend on the structure of magnetic domain and the process of demagnetization. Technically, these parameters depend on the microstructure of the material and the process of its production. This is a very special and very complicated problem that needs to be solved. It is just for this reason that such a category of materials has not been put into practical use since the discovery of the interstitial atomic effect in 1:12 type alloy by Yingchang Yang et al. a decade ago.
It is an object of this invention to provide a multielement rare earth-iron interstitial type permanent magnetic material having a ThMn12 crystal structure. The permanent magnetic material of the present invention has high remanence, high coercivity and high magnetic energy product. There is also provided a process for producing the permanent magnetic material of the present invention.
To this object, the composition of the 1:12 type nitride master alloy is modified based on the result obtained in the study of the magnetic domain structure and the magnetization reversal mechanism of 1:12 type nitride. It is expanded to a multielement alloy, which is featured in that an easily pulverizable alloy with better single-phase property can be produced. This is fundamental for producing high performance magnets. On the other hand, by using the process of the present invention, the activity of alloy is enhanced, the temperature of gas-solid phase reaction is lowered, and complete nitrogenation is ensured. Thereby the magnetism of the material is greatly enhanced, the content of rare earth metal is lowered, and the need to dope with expensive metals such as cobalt is eliminated. By using the process of the present invention, an anisotropic magnetic powder and a magnet having high remanence, high coercivity, and high magnetic energy product can be produced.
Specifically, there is provided a multielement rare earth-iron interstitial permanent magnetic material represent by the following formula:
(R
1−&agr;
R′
&agr;
)
x
(Mo
1−&bgr;
M
&bgr;
)
y
Fe
100−x−y−z
l
z
wherein, R is a light rare earth element selected from the group consisting of Pr, Nd, Pr—Nd concentrated material and mixtures of Pr and Nd of any composition; R′ is a heavy rare earth element selected from the group consisting of Gd, Tb, Dy, Ho, Er, Y and a mixture of thereof; &agr; is from 0.01 to 0.14; x is an atomic percent from 4 to 15; M is an element of IIIA, IVA, IVB, VB, VIB and VIIB families in the periodic table selected from the group consisting of B, Ti, V, Cr, Mn, W, Si, Al, Ga, Nb, Ta, Sr, Zr, and mixtures of thereof; &bgr; is from 0.01 to 0.98; y is an atomic percent from 3 to 20; l is an element occupying the above
Cheng Benpei
Ge Senlin
Yang Yingchang
Knobbe Martens Olson & Bear LLP
Sheehan John
Yang Yingchang
LandOfFree
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