Patent Application
20120001711
The present invention relates to a lower cost permanent magnet having improved magnetic performance at temperatures above 125 degrees C. up to 200 degrees C.
Currently, high performance permanent magnets are based on two types of permanent magnet materials. One type of permanent magnet material is based on Nd2Fe14B, while the other type is based on Sm2Co17 and SmCo5. At room temperature, Nd2Fe14B based magnets enjoy a considerable advantage over the Sm—Co type in terms of cost and performance. However, as a result of both a low Curie temperature and a large temperature dependence of the magnetocrystalline anisotropy, the performance of Nd2Fe14B magnets drops off rapidly with temperature above 100 degrees C.
Past workers have attempted to improve the properties of Nd2Fe14B based magnets by alloy additions. The partial substitution of Co for Fe raises the Curie temperature, while the temperature dependence of the magnetocrystalline anisotropy is improved by the addition of a heavy rare earth element, such as Dy. Unfortunately, the addition of Co lowers magnetocrystalline anisotropy, while the addition of Dy lowers saturation magnetization. Furthermore, the amounts of elemental substitutions in the alloy is limited by phase diagram considerations.
High torque permanent magnet electric drive motors now are limited in operation temperature by the temperature dependence of the permanent magnets. This is true even for motors operating in an ambient temperature environment as a result of heating due to energy losses in the motor. For example, high torque permanent magnet electric motors now are graded based on coercivity Hci and temperature coefficient of Hci by the following nomenclature adopted by Chinese and European manufacturers:
The maximum operating temperature for the grades is given below
M grade—100 degrees C.
H grade—120 degrees C.
SH grade—150 degrees C.
UH grade—180 degrees C.
EH grade—200 degrees C.
AH grade—230 degrees C.
AH′ grade—230 degrees C.
Many commercial Nd2Fe14B based permanent magnets used today contain increasing amounts of Dy indicated above in order to achieve required magnetic properties at the higher operating temperatures demanded of the magnet. In order to obtain a higher energy product, (BH)max, a reasonable coercivity (Hci) must be obtained. Therefore, a sufficient amount of Dy is added to the Nd2Fe14B based magnets to this end, although its excessive cost may double the total cost of most conventional magnet alloys. For example, commercial permanent magnets used in Toyota Prius automobiles include above 6 weight % Dy in the magnet composition to achieve desired high temperature operating performance above 150 degrees C. In particular, the permanent magnets employed in the Prius 2004, Prius 2010, Camry 2007, and Camry 2010 automobiles include 8.2, 7.4, 5.9, and 5.7 weight % Dy, respectively.
However, the average mine output for Dy ore is not keeping pace with this increased commercial usage such that Dy may be in short supply in the future unless the Dy content of commercial high temperature permanent magnets is reduced. The cost of Co, which can be another substitute alloying element to improve magnetic properties, is also significant (currently about 10% of the Dy cost) and it would be desirable to reduce the Co content.
Thus, it would be very desirable to reduce a significant amount of the Dy and any Co in commercial Nd2Fe14B based permanent magnets but without adversely affecting the higher operating temperature grade levels such as SH, UH, or EH.
The present invention provides in one embodiment a permanent magnet that is operable above about 125 C to about 200 C while reducing or eliminating the amount of Dy present in the magnet composition. That is, the present invention provides a permanent magnet of the SH, UH, or EH grade type with reduced or no Dy in the composition. The permanent magnet can be a isotropic bonded magnet that includes permanent magnet particles bonded together by a binder or a anisotropic sintered magnet formed by crushing ingot or strip cast material, aligning and pressing the powder in a field, and sintering.
In an illustrative embodiment of the invention, the permanent magnet comprises a major phase represented by MRE2Fe14B wherein said MRE comprises two or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y (1) wherein one of the rare earth elements is chosen from one or more of La, Ce, Pr, Nd, Eu, and Gd in an amount not exceeding 45 atomic % of the magnet and (2) wherein at least 50% atomic % of MRE comprises Y and at least one of Dy, Ho, and Tb wherein the total content of the at least one of Dy, Ho, and Tb does not exceed 4 weight % (1.6 atomic %) of the total mass of the magnet. In preferred illustrative embodiments of the invention, the temperature coefficient of Hci is about −0.30 to −0.35 for such permanent magnets, this range being well below the temperature coefficients of Hci set forth above for grades M, H, SH, UH, EH, AH, and AH′.
In a particular illustrative embodiment of the invention, at least 50% atomic % of MRE comprises Y and Dy wherein Dy is present in an amount not exceeding 4 weight %, preferably not exceeding 2.75 weight % (Dy is 2.26 weight % for the peak (BH)max) of the total mass of the magnet. A preferred range of Dy comprises 0 to 4 weight % (0 to 1.6 atomic %). The ratio of Y/Dy typically is at least 6:1.
In another particular illustrative embodiment of the invention, the permanent magnet comprises a major phase represented by MRE2Fe14B wherein said MRE comprises two or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y (1) wherein one of the rare earth elements is chosen from one or more of La, Ce, Pr, Nd, Eu, and Gd in an amount not exceeding 45 atomic % of the magnet and (2) wherein MRE consists of Y and is free of Dy, Ho, and/or Tb, except for impurity content thereof that does not exceed 0.2 weight %. (0.07 atomic %) of the total mass of the magnet.
A permanent magnet material having the compositional features described above pursuant to the invention can be produced by rapid solidification processes such as melt spinning or planar flow casting to make ribbon, flake, and fragments thereof or by melt atomization such as gas or centrifugal atomization to produce spherical powders which are used for bonded magnets wherein the magnet material is mixed with binder and formed to a magnet shape to provide a bonded permanent magnet of reduced cost. Alternately, the material can be chill cast and crushed for the fabrication of a sintered permanent magnet. A permanent magnet pursuant to the invention exhibits a reduced temperature dependence of magnetocrystalline anisotropy and saturation magnetization as compared that of a permanent magnet based on Nd2Fe14B at temperatures above about 125 degrees C. to about 200 degrees C.
The above and other advantages of the present invention will become apparent from the following detailed description taken with the following drawings.
The present invention provides a permanent magnet material that exhibits a reduced temperature dependence of magnetocrystalline anisotropy and saturation magnetization as compared to that of a permanent magnet based on Nd2Fe14B at temperatures above about 125 to about 200 degrees C. while having a magnet composition in which the amount of Dy or like alloying element, such as Ho and Tb, is reduced or eliminated.
The present invention provides in one embodiment a permanent magnet that comprises a major phase (greater than 50 volume % of the magnet) represented by MRE2(Fe, Co)14B wherein said MRE comprises two or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y (1) wherein one of the rare earth elements is chosen from one or more of La, Ce, Pr, Nd, Eu, and Gd in an amount not exceeding 45 atomic % of the magnet and (2) wherein at least 50% atomic % of MRE comprises Y and at least one of Dy, Ho, and Tb wherein the total content of the at least one of Dy, Ho, and Tb does not exceed 4 weight % (1.6 atomic %) of the total mass of the magnet.
In a particular illustrative embodiment of the invention, at least 50% atomic % of MRE comprises Y and Dy wherein Dy is present in an amount not exceeding 4 weight %, preferably not exceeding 2.75 weight % (1.04 atomic %), of the total mass of the magnet. The ratio of Y/D typically is at least 6:1. An exemplary permanent magnet is represented by (Nd0.45Y0.55-xDyx)2(Fe, Co)14B where x is 0 to 0.14 with the proviso that some the MRE (Nd, Y, Dy) can be substituted with Zr or other carbide-forming element to provide grain-boundary pinning carbide precipitates as described in U.S. Pat. No. 5,486,240, which is incorporated herein by reference.
In another particular illustrative embodiment of the invention, the permanent magnet comprises a major phase represented by MRE2(Fe, Co)14B wherein said MRE comprises two or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y (1) wherein one of the rare earth elements is chosen from one or more of La, Ce, Pr, Nd, Eu, and Gd in an amount not exceeding 45 atomic % of the magnet and (2) wherein MRE consists only of Y and is devoid or free of Dy, Ho, and/or Tb, except for impurity content thereof that does not exceed 0.2 weight %. (0.07 atomic %) of the total mass of the magnet. An exemplary Dy/Ho/Tb-free permanent magnet is represented by (Nd0.45Y0.55)2(Fe, Co)14B.
The permanent magnet alloy or composition can be altered by the inclusion of additional elements which either substitute in the MRE2Fe14B phase in order to modify its intrinsic properties or which form secondary phases which control grain structure and/or modify the magnetic and/or corrosion properties of the final permanent magnet. For example, Al may be substituted for part of the transition element Fe, Co. The optional inclusion of Si, Cu, Zn, Ga, Ti, Zr, Hf, V, Nb, Ta, Mo, and W, singly or in combination, in the alloy are examples of elements which control the grain boundaries and rapid solidification properties of the alloy. The addition of one or more of Zr, Hf, V, Nb, Ta, Mo, and W can be employed to form respective borides, carbides, and nitrides to control the grain boundaries and rapid solidification properties of the alloy as described for example in U.S. Pat. No. 5,486,240. When the permanent magnet material is to be used to make sintered magnets, the inclusion in the material of one or more sintering aids, such as a powder of the Dy—Fe eutectic, which is blended with the magnet alloy powder, is envisioned.
The permanent magnet material may optionally include excess transition metal (Fe, Co) and/or B to form relatively large fractions of soft magnetic phase or soft magnetic (Fe, Co)3B phase together with the hard magnetic MRE2Fe14B phase in the permanent magnet material so long as useable coercivity is maintained. Such a modified permanent magnet material can find use in fabrication of exchange spring type of permanent magnets.
A permanent magnet material pursuant to the invention can be made in the form of a rapidly solidified ribbon by conventional melt spinning, in the form of rapidly solidified pulverized particulates by conventional melt spinning of a ribbon followed by pulverization of the ribbon, in the form of atomized generally spherical powder particulates by melt atomization such as gas or centrifugal atomization, and in other rapidly solidified forms by processes used to produce rapidly solidified permanent magnet materials. Melt spinning is described in U.S. Pat. No. 4,496,395 and others, the teachings of which are incorporated herein by reference. Gas atomization is described in U.S. Pat. No. 5,242,508; U.S. Pat. No. 5,372,629; U.S. Pat. No. 5,811,187; and others, the teachings of which are incorporated herein by reference.
The rapidly solidified permanent magnet material or a magnet made from such particulates may optionally be heat treated at an elevated temperature for a time to form crystallites of the MRE2Fe14B phase in the material of desired crystallite size (grain size) to improve intrinsic coercivity, energy product, and other magnetic properties in the event the material does not already possess desired magnetic properties. For example, rapidly solidified permanent magnet ribbon discharged from a melt spinning wheel may or may not have desired magnetic properties depending on the parameters used during melting spinning. In the event the magnetic properties are less than desired in the melt spun condition, then the material can be annealed at an elevated temperature for a time to improve properties.
Particulates of the permanent magnet material can be conventionally pressed to a magnet shape and sintered to form an anisotropic permanent magnet. Alternately, particulates of the permanent magnet material can be mixed with a high temperature polymer binder, such polyphenyl-sulfide, and molded to a magnet shape to provide a bonded isotropic or anisotropic permanent. magnet of reduced cost. Further, particulates of the permanent magnet material can be mixed with a metallic binder, such as aluminum or other metal or alloy that melts below about 1000 degrees C., to form a bonded isotropic or anisotropic permanent magnet of reduced cost. In the permanent magnet, the metallurgical grain size of the MRE2Fe14B phase is between 10 nm and 30 microns.
The following Examples are offered to illustrate but not limit the invention:
This example demonstrates enhancement of magnetic properties while reducing the Dy content of the following permanent magnet base alloy composition represented by:
{[Nd0.45(YrDy1)1/r+1*0.55]2Zr0.3Co1Fe13B1}0.98+Zr0.01C0.01
Zr is included as a partial substitution for the MRE (Nd, Y, Dy) in order to form grain boundary-pinning precipitates in the magnet as described in U.S. Pat. No. 5,486,240, which is incorporated herein by reference.
In this Example the Y:Dy ratio of the base alloy composition is systematically changed to include r=1, 2, 3, 4, 5, 6, 8, and 12 wherein its equivalent weight % of Dy is 8, 5.4, 4.1, 3.3, 2.75, 2.36, 1.84, and 1.28, respectively. The Co content partially substitutes for the Fe14 part of the 2-14-1 composition formula, i.e., from (Fe12.5+Co1.5) to (Fe13+Co1.0).
The sample compositions in weight are set forth below:
Weight % for sample compositions
Initial ingots of this series of magnet alloy compositions were prepared by cold hearth arc-melting in a highly purified (low oxygen) Ar atmosphere. To convert each alloy into rapidly quenched ribbon, a 9 g sample of each ingot was melted in a quartz crucible in ⅓ atm. of high purity He gas and ejected onto a single Cu wheel at a wheel speed of 25 m/s. As-spun ribbons were annealed in a highly purified Ar atm. at 700-750° C. for 15 min. Hysteresis loop measurements were performed using a Quantum Design MPMS SQUID magnetometer with a maximum applied field of 5.0 T.
As mentioned above, the ratio of Y:Dy was changed incrementally from r=1 to 12. Its equivalent atomic percentage of Y is 3.1, 4.2, 4.6, 5, 5.2, 5.3, 5.5 and 5.7, respectively. In addition, when Dy is not added, Y is 6.23 at. %. The permanent magnet composition is represented by [(Nd0.45Y0.55)2Zr0.3Co1.0Fe13B1]0.98+Zr0.01C0.01 when Dy is omitted; i.e. the magnet is devoid or free of Dy.
The typical hysteresis loops of samples with r=3, 6 and 12 at 300K and 400K are shown in
Permanent magnets pursuant to the invention are advantageous in having a lower temperature dependence of magnetic properties than the grades M, H, SH, UH, EH, AH, and AH′ set forth above. For example, from
[Hci(400K)−Hci(300K)]/[Hci(300K)×(400K−300K)].
The temperature coefficients set forth above for grades M, H, SH, UH, EH, AH, and AH′ were calculated using the standard equation: [Hci(100 C)−Hci(20 C)]/[Hci(20 C)×(100−20)].
Applicants equation incorporating higher temperature data sampling will cause the calculated temperature coefficients to be somewhat higher than the standard calculation, but even when calculated in this way, are much lower than those for grades M, H, SH, UH, EH, AH, and AH′.
Although the invention has been described above with respect to certain illustrative embodiments, those skilled in the art will appreciate that changes, additions, and modifications can be made therein within the scope of invention as set forth in the appended claims.
This application is a continuation-in-part of copending Ser. No. 11/126,484 filed May 11, 2005, the disclosure of which is incorporated herein by reference.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-82 between the U.S. Department of Energy and Iowa State University, Ames, Iowa, which contract grants to Iowa State University Research Foundation, Inc. the right to apply for this patent.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11126484 | May 2005 | US |
| Child | 13068572 | US |
