Bulk anisotropic exchange-spring magnets and method of producing the same

Abstract

A method of preparing a permanent magnet nanocomposite. The method includes melting a precursor alloy having a hard magnetic phase and a magnetically soft phase. The hard magnetic phase has less than a stoichiometric amount of rare earth metal or noble metal. The melted precursor is cast into flakes and milled into a powder. The powder may then be pressure crystalized.

Claims

1. A method of preparing an anisotropic permanent magnet nanocomposite, the method comprising: melting a precursor alloy having a hard magnetic phase and a magnetically soft phase, the hard magnetic phase comprising less than a stoichiometric amount of a rare earth metal or a noble metal; casting the melted precursor alloy into flakes; milling the casted flakes into a powder; and pressure crystalizing the powder by: pressurizing and heating the powder at a crystallization pressure ranging from about 0.5 GPa to about 3 GPa and at a crystallization temperature over a pressurizing time, wherein the powder is pressurized at a rate of about 200 MPa/min; holding the powder at the crystallization temperature and the crystallization pressure over a hold time to promote crystal growth; and rapidly quenching the crystal growth to a temperature less than about 200° C. in less than about a minute.

2. The method of claim 1, wherein the hard magnetic phase comprises: Nd—Fe—B, Sm—Co, Sm—Fe—N, Fe—Pt, or Co—Pt.

3. The method of claim 2, wherein the permanent magnet nanocomposite is SmCo.sub.5, the rare earth metal is Sm, and the stoichiometric amount is about 16.6 at. %.

4. The method of claim 2, wherein the permanent magnet nanocomposite is Sm.sub.2Co.sub.17, the rare earth metal is Sm, and the stoichiometric amount is about 10.5 at. %.

5. The method of claim 2, wherein the permanent magnet nanocomposite is Sm.sub.2Fe.sub.17N.sub.3, the rare earth metal is Sm, and the stoichiometric amount is about 9.1 at. %.

6. The method of claim 2, wherein the permanent magnet nanocomposite is FePt or CoPt, the noble metal is Pt or Co, and the stoichiometric amount is about 50 at. %.

7. The method of claim 2, wherein the permanent magnet nanocomposite is Pr.sub.2Fe.sub.14B, the rare earth metal is Pr, and the stoichiometric amount is about 11.76 at. %.

8. The method of claim 2, wherein the permanent magnet nanocomposite is Pr.sub.2Co.sub.5, the rare earth metal is Pr, and the stoichiometric amount is about 16.6 at. %.

9. The method of claim 2, wherein the permanent magnet nanocomposite is Nd.sub.2Fe.sub.14B, the rare earth metal is Nd, and the stoichiometric amount is about 11.76 at. %.

10. The method of claim 1, wherein the magnetically soft phase comprises: α-Fe, Fe—Co, Fe—N, Co, Ni, or combinations thereof.

11. The method of claim 1, wherein melting the precursor alloy further comprises: arc melting, induction melting, levitation melting, or powder metallurgy processing.

12. The method of claim 1, wherein casting the melted precursor alloy further comprises: melt spinning, splat quenching, or planar flow casting.

13. The method of claim 1, wherein the flakes yielded from casting the melted precursor alloy are amorphous or crystalline.

14. The method of claim 13, wherein milling the casted flakes further comprises cryomilling.

15. The method of claim 1, wherein heating the powder occurs at a rate of about 100 K/min.

16. The method of claim 1, wherein the pressurizing time is less than 5 min.

17. The method of claim 1, wherein the pressurizing time is less than 3 min.

18. The method of claim 1, wherein the hold time is less than 20 min.

19. The method of claim 1, wherein rapidly quenching includes using a gas quench.

20. The method of claim 1, wherein pressurizing and crystalizing the powder comprises inductively heating or resistively heating.

21. The method of claim 1, wherein pressurizing and crystalizing are configured to initiate nucleation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.

(2) FIGS. 1A and 1B are schematic representations of atomic and intergranular exchange in permanent magnet nanocomposites having both rare earth and transition metals.

(3) FIG. 2 is a graphical representation of a magnetic hysteresis loops for soft magnetic materials, hard magnetic materials, and composite magnetic materials.

(4) FIG. 3 is a flowchart illustrating a method of preparing an anisotropic, exchange-spring magnet according to an embodiment of the present invention.

(5) FIG. 3A is a flowchart illustrating the pressure crystallization subprocess of the method illustrated in FIG. 3 and in accordance with an embodiment of the present invention.

(6) FIG. 4 is a side elevational, exploded view of an exemplary compaction die suitable for embodiments of the present invention.

(7) FIG. 5 is a side elevational view of an exemplary hot press suitable for use with the method of FIGS. 3 and 3A.

(8) FIG. 6 is a graphical representation of X-Ray Diffraction plots of melt spun materials of overquenched flakes prepared by methods in accordance with embodiments of the present invention.

(9) FIG. 7 is a graphical representation of thermomagnetic measurements confirming the presence of Nd.sub.2Fe.sub.14B and α-Fe in the prepared melt spun materials.

(10) FIG. 8 is a graphical representation of Nd.sub.5.9Fe.sub.91B.sub.3.1 alloy, milled with ball-to-powder weight ratio of 4 for 1 hr, 2 hr, 4, hr, and 6 hr.

(11) FIG. 9 graphical representation illustrating the evolution of coercivity as a function of crystallization time for Nd.sub.5.9Fe.sub.91B.sub.3.1 at 560° C. and 750 MPa.

(12) FIG. 10 graphically illustrates thermomagnetic curves for 5 min and 20 min crystallized bulk samples at an external field of 1.8 kOe.

(13) FIGS. 11A-11B are transmission electron microscope bright field images of Nd.sub.5.9Fe.sub.91B.sub.3.1 material crystalized at 560° C. for 5 min.

(14) FIG. 11C is a selected area electron diffraction pattern associated with the TEM images of FIG. 11A.

(15) FIGS. 12A-12C are elemental mapping images of Nd.sub.5.9Fe.sub.91B.sub.3.1 material crystallized at 560° C. for 18 min.

(16) FIG. 13 is a schematic illustration of texture formation.

(17) FIG. 14 is a graphical representation of anisotropic behavior in MH curves taken parallel and perpendicular to the load direction.

(18) FIG. 15 is a graphical representation of heating and cooling effects on magnetization

(19) FIGS. 16A and 16B graphically illustrate a comparison of α-Fe I(110)/I(200) intensity ratios did not indicate a presence of texture for iron.

(20) It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

(21) Referring now to the FIGS., and in particular to FIG. 3, a method 20 of preparing an anisotropic, exchange-spring magnet according to an embodiment of the present invention is shown. At start, a precursor alloy rich in magnetically soft phase content (for example, α-Fe, Fe—Co, Fe—N, Co, Ni, or combinations thereof) and a hard magnetic phase (for example, Nd—Fe—B, Sm—Co, Sm—Fe—N, Fe—Pt, or Co—Pt) having nominal rare earth content (generally less than 11.76 at. %, or specifically in some embodiments ranging between 4.7 at. % and 8.2 at. %, or between 5.9 at. % and 8.2 at. %, or between 7.1 at. % and 8.2 at. %) may be melted (Block 22) (such as by arc melt) and flakes cast (Block 24) to at least a partially amorphous state in the form of flakes. Generally, any range of nominal rare earth content may be used so long as, stoichiometrically, a two-phase material results. For instance, suitable nominal rare earth content less than about 11.76 at. % for Nd.sub.2Fe.sub.14B, less than about 16.6 at. % for SmCo.sub.5, less than about 10.5 at. % for Sm.sub.2Co.sub.17, less than about 9.1 at. % for Sm.sub.2Fe.sub.17N.sub.3, less than about 11.76 at. % for Pr.sub.2Fe.sub.14B, and less than about 16.6 at. % for PrCo.sub.5 may be used (similarly, for noble metal Pt: less than about 50 at. % for each, individually, of FePt and CoPt). Melting (Block 22), according to various embodiment so the present invention may comprise arc melting, induction melting, levitation melting, and powder metallurgy (“PM”) and may be performed multiple times, if desired, until a homogenous alloy results. Flakes may be cast (Block 26) by melt spinning. For example, wheel speeds ranging from 5 m/s to 65 m/s with cooling rates ranging from 10.sup.4 K/s to 10.sup.7 K/s. Alternatively, casting procedures may include splat quenching, planar flow casting, or gas/water atomization with similar cooling rates.

(22) Depending on cooling rates, casting may yield amorphous, crystalline, or overquenched flakes, wherein the latter comprises a crystalline lacking fully developed microstructure and no significant coercivity values. Fully amorphous flakes are not preferred as milling (Block 26) may be difficult, the flakes are ductile, and most of the flakes bonded to the milling media and milling jar during milling. Overquenched flakes did not present such problems.

(23) The formed flakes may then be milled to a fine powder (Block 26). Milling may include, for example, ball milling, planetary, or other milling apparatus having enough impact energy to reduce the size of the flakes. Alternatively still, according to some embodiments of the present invention in which fully crystalline or fully amorphous flakes are used, cryomilling, or other like milling process may be used. Milling provides the benefit of remove background memory with respect to nuclei and crystal growth preference. According to one exemplary embodiment, a SPEX high energy ball mill (“HEBM”) may be used. Ball milling the flakes results in an amorphization of the rare earth and the magnetically soft phase, leaving only a portion of the magnetically soft phase in a crystalline state. A ball-to-powder weight ratio (“BPR”) may range from 1 to 10, although a BPR of 5 may be preferred in some embodiments.

(24) Once the fine powder is obtained, a pressure crystallization process (Block 28) may proceed, which is described in greater detail with reference to FIG. 3A. The pressure crystallization process may be configured to yield a finely distributed uniform microstructure with preferential growth by utilizing uniaxial stress so as to control the growth of an interface between amorphous phase and nanocrystals.

(25) With reference now to FIGS. 4 and 5 as well as FIG. 3B, and at start, a compaction die (such as the tungsten carbide die 40, first and second punch 42, 44) may be positioned within a heating system 46. FIG. 5 illustrates a portion of an exemplary inductively heated hot press that may be used according to some embodiments; however, because of the slower diffusion kinetics observed under pressure, other heating systems, such as a resistively heated consolidation system, may alternatively be used. Generally, with reference to the illustrated embodiment of FIG. 5, the hot press may be configured to apply pressures of up to 1 GPa; however, other hot presses may be used so long as pressures of 0.5 GPa or more may be obtained (or, more particularly, ranging from 0.5 GPa to about 3 GPa). The fine powder within the compaction die are prepared (Block 30) and heated to a crystallization temperature for the particular alloy with a simultaneous ramping (for example, about 100 K/min; however, the ramping may be heating system and sample size dependent) of temperature to a crystallization pressure (for example, 1 GPa at, for example, about 200 MPa/min) over a pressurizing time (typically ranging from 3 min to 5 min) (Block 32). Crystallization temperatures may be determined in a manner that would be understood by those of ordinary skill in the art, for example, by a Differential Scanning Calorimeter (“DSC”). The fine powder and compaction die are held at the crystallization temperature and crystallization pressure for a hold time, which may range vary and depends, in part, on the crystallization pressure (Block 34). After the holding time, the compaction die and powder are rapidly gas quench to a temperature below 200° C. in less than 1 min.

(26) Anisotropic alloys, produced according embodiments of the present invention as described herein, provide several benefits over conventional methods. Alloys resulting from embodiments of the present invention is the annealing/crystallization times necessary for optimum properties. Conventional, overquenched flakes need approximately 3 min annealing to arrive at optimum grain sizes; the alloys produced according to methods and embodiments described herein are obtained after 20 min. While not wishing to be bound by theory, it is believed that the former, conventional alloys comprise nuclei such that annealing drives grain growth alone. By utilizing quasiamorphous precursors, as described herein, nucleation must occur before grain growth may begin. Accordingly, nucleation with limited grain growth takes place within the first few minutes (for example, 5 min) of the pressure crystallization, annealing process. Grain growth thus occurs over the remaining processing time (for example 15 min). Such slower diffusion kinetics, under pressures, make it possible to use resistively heated consolidation systems for hot pressing.

(27) The following examples illustrate particular properties and advantages of some of the embodiments of the present invention. Furthermore, these are examples of reduction to practice of the present invention and confirmation that the principles described in the present invention are therefore valid but should not be construed as in any way limiting the scope of the invention.

Example 1—Preparation and Crystallization

(28) Iron rich Nd—Fe—B alloys with nominal Nd contents (between 8.2 at. % and 5.9 at. %) were melt-spun to a partially amorphous state in the form of flakes. The flakes were ball milled to a fine powder form using a SPEX high energy ball mill (“HEBM”), resulting in an amorphization of Nd and B, leaving only a portion of the α-Fe in a crystalline state. A ball-to-powder weight ratio (“BPR”) of 5 was employed for the milling studies. Crystallization temperatures were determined by a Differential Scanning Calorimeter (“DSC”) (Perkin Elmer, Inc., Waltham, Mass.). High pressure crystallization studies were carried out using an inductively heated hot press under pressures as high as 1 GPa. Thermomagnetic, M(T), measurements were carried out using a Vibrating Sample Magnetometer (“VSM”) (Lake Shore Cryotronics, Inc., Westerville, Ohio) equipped with a high temperature furnace. A diffractometer (Bruker Corp., Billerica, Mass.) was used for structural characterizations. The compacted samples were examined in a CM200 Transmission Electron Microscope (“TEM”) (Koninklijke Philips N.V., Amsterdam).

Example 2—Cast Flakes

(29) Melt spinning yielded overquenched flakes with no significant coercivity values. FIG. 6 illustrates X-Ray Diffraction (“XRD”) plots of these melt spun materials. All three compositions were of Nd.sub.2Fe.sub.14B and α-Fe in varying ratios, i.e., the higher the Nd content the higher the Nd.sub.2Fe.sub.14B fraction. No intermediate phases were detected other than the two main phases.

(30) The presence of the Nd.sub.2Fe.sub.14B and α-Fe was confirmed by thermomagnetic measurements, which are graphically illustrated in FIG. 7.

(31) VSM is more sensitivity to the detection of minor ferromagnetic phases than thermomagnetic measurements. The results of VSM measurements indicated fully crystallized cast flakes having only two phases.

(32) Volume fraction ratios were estimated from thermomagnetic measurements and revealed iron vol. % of approximately 30.8, 40.6, and 49.9 for alloys with Nd vol. % contents of 8.2, 7.1, and 5.9, respectively.

(33) FIG. 8 is a graphical representation of the Nd.sub.5.9Fe.sub.91B.sub.3.1 alloy, milled with ball-to-powder weight ratio of 4 for 1 hr, 2 hr, 4, hr, and 6 hr. The illustrated data demonstrates a minimum of about two hours for milling to transform the Nd.sub.2Fe.sub.14B into the amorphous state was identified by periodically monitoring crystallinity during high energy ball milling experiments. The crystallization temperature of Nd.sub.2Fe.sub.14B in ball milled samples was determined to be 560° C. by DSC measurement.

Example 3—Pressure Crystallization

(34) Pressure crystallization was carried out using tungsten carbide compaction dies. Typical runs consisted of (1) about 5 min of heating to 560° C. with simultaneous ramping of pressure, (2) a predetermined holding time at 560° C. and the pressure 1 GPa, and (3) a gas quench to a temperature below 200° C. in less than 1 min.

(35) FIG. 9 is a graphical representation illustrating the evolution of coercivity as a function of crystallization time for Nd.sub.5.9Fe.sub.91B.sub.3.1 at 560° C. and 750 MPa. Particular results for coercivity are provided in Table 1, below.

(36) FIG. 10 graphically illustrates thermomagnetic curves for 5 min and 20 min crystallized bulk samples at an external field of 1.8 kOe. Experimentally determined background Fe magnetization for each measurement is shown for quantifying the volume ratios of Nd.sub.2Fe.sub.14B and α-Fe from saturation magnetization, M.sub.s, values, which are specifically noted in Table 1, below. An approximately 2% difference in M.sub.s values of 5 min and 20 min crystallized bulk samples indicates that nearly all crystallization occurs within the first 5 min. The increasing coercivity with increased crystallization time (FIG. 9) is mainly due to grain growth.

(37) TABLE-US-00001 TABLE 1 CRYSTALLIZATION H.sub.c M.sub.s @ 18 kOe ALLOY PRESSURE (GPa) (kOe) (emu/g) Nd.sub.8.2Fe.sub.87.4B.sub.4.4 1 2.58 160.27 Nd.sub.7.1Fe.sub.89.2B.sub.3.4 0.625 2.01 169.76 Nd.sub.5.9Fe.sub.91B.sub.3.1 1 1.66 178.63

(38) Samples crystallized up to 20 min were characteristic of a fully exchange coupled system. Beyond the crystallization time of 20 min, loops became constricted, which indicates improper coupling due to grain overgrowth.

Example 4—Comparison with Conventional Alloys

(39) For conventional isotropic alloys, such as those described by A. INOUE et al., “Hard magnetic properties of Nd—Fe—B alloys containing intergranular amorphous phase,” IEEE Trans. Magn., Vol. 31 (1995) 3626-3628 and Y. Q. WU et al., “Microstructural characterization of an α-Fe/Nd.sub.2Fe.sub.14B.sub.1 nanocomposite with a remaining amorphous phase,” J. Appl. Phys., Vol. 87 (2000) 8658-8665, an annealing time in overquenched flakes of approximately 3 min is usually sufficient to arrive at optimum grain sizes. Similar grain size ranges for alloys prepared using methods according to embodiments of the present invention described herein are obtained after 20 min.

(40) These conventional alloys are already populated with nuclei such that annealing provides a driving force only for the grain growth. For alloys prepared using methods according to embodiments of the present invention described herein with quasiamorphous precursors, initiation of nucleation occurs before grain growth. For example, it is discernable from FIG. 10 that nucleation and some grain growth takes place within the first 5 min of pressure crystallization; the next 15 min are required for grain growth. This is a clear indication of slower, diffusion kinetics under pressure.

(41) FIGS. 11A and 11B are transmission electron microscope (“TEM”) bright field images of Nd.sub.5.9Fe.sub.91B.sub.3.1 material crystalized at 560° C. for 5 min. FIG. 11C is the selected area electron diffraction pattern associated with the TEM images of FIG. 11A.

(42) FIGS. 12A-12C are elemental mapping images of Nd.sub.5.9Fe.sub.91B.sub.3.1 material crystallized at 560° C. for 18 min.

Example 5—Crystallographic Alignment

(43) Growth of a crystalline interface in an amorphous matrix occurs along crystallographic directions that minimize strain energy. The idea of texture formation under pressure (schematically illustrated in FIG. 13) relies on modification of the strain energies at the growth interface by application of an external stress and promoting preferential growth along certain crystallographic directions. Distinct microstructures obtained by TEM images in which alternating layers of α-Fe and Nd.sub.2Fe.sub.14B are realized by pressure crystallization. Elemental mapping by STEM (Scanning Transmission Electron Microscope) reveals two distinct regions and length scales in which α-Fe and Nd.sub.2Fe.sub.14B phases are intermixed. On a larger scale, alternating layers of α-Fe and Nd.sub.2Fe.sub.14B are still visible. Within the Nd.sub.2Fe.sub.14B bands, a finer scale distribution of α-Fe also exists. These layered structure translates to an anisotropic behavior in MH curves taken parallel and perpendicular to the load direction (data being graphically shown in FIG. 14). Heating and cooling effects on magnetization are graphically illustrated in FIG. 15. FIGS. 14 and 15 are not corrected for demagnetization fields.

(44) Table 2, below, lists grain sizes of 5.9 at. % and 8.2 at. % Nd that were pressure crystallized under 1 GPa pressure for 20 minutes. A Scherrer analysis of the XRD patterns taken on surfaces parallel and perpendicular to a load direction showed different grain sizes in different directions. For samples containing 8.2 at. % Nd, average grain size observed in the parallel direction were 0.5 times as much as average grain size observed in the perpendicular direction. For the 8.2 at. % Nd sample, the difference in grain size was about 0.75%. Despite its layered morphology, observed differences in grain sizes were not as pronounced in α-Fe. From TEM images, such diminished difference in observed grain size of the α-Fe was likely due the α-Fe layers comprising mostly equiaxed subgrains.

(45) Comparison of the α-Fe I(110)/I(200) intensity ratios did not indicate a presence of texture for iron. However, a stronger (113) and a diminished (220) reflection of Nd.sub.2Fe.sub.14B (FIGS. 16A and 16B) for loading in the parallel direction strongly suggests that a presence of external stress does promote preferential growth.

(46) A similar crystalline alignment occurs in the Nd.sub.2Fe.sub.14B system during die-upsetting process. In this process, overquenched Nd—Fe—B is first hot compacted to a near full density during which full crystallization takes place. During the die-upsetting step, the fully dense compact is hot deformed uniaxially to the half of its original height. During the hot deformation, grains grow by an order of magnitude into platelet shaped grains while the “c” axis is aligned parallel to the stress direction. This alignment is explained by preferential growth of grains whose “c” axis coincides with the load direction at the expense of grains whose “c” axis do not. It is highly likely that a similar preferential growth mechanism is responsible for the pressure crystallized samples.

(47) Once the first Nd.sub.2Fe.sub.14B nuclei appear, in this case most likely by heterogeneous nucleation, the planes whose surface energy is lowered by the external load grow faster than the others. Die-upsetting requires presence of a Nd-rich intergranular phase and final grain sizes are on the order of several hundred nanometers. Grain sizes of the pressure crystallized samples on the other hand can be tailored to the optimal values to obtain the highest coercivities.

(48) The obtained coercivities of 1.98 kOe and 2.5 kOe for the samples with 5.9 at. % and 8.2 at. % Nd, respectively, were comparable to the reported coercivities of isotropic alloys with similar Nd content. A. INOUE et al. (supra) reported a coercivity of 3.01 kOe for a Nd.sub.8Fe.sub.88B.sub.4 alloy while Y. LIU et al., “Development of crystal texture in Hd-lean amorphous Nd.sub.9Fe.sub.85B.sub.6 under hot deformation,” Appl. Phys. Lett., Vol. 94 (2009) 172502, reported coercivities of about 3.2 kOe for a Nd.sub.8Fe.sub.85B.sub.6 alloy. Y. Q. WU et al. (supra) reported a coercivity value of 3.6 kOe for a Co containing Nd.sub.8Fe.sub.78Co.sub.8B.sub.6 alloy with higher boron content than the alloys studied in this work.

(49) TABLE-US-00002 TABLE 2 CRYSTALLIZATION D.sub.avNdFeB D.sub.av-NdFeB D.sub.av α-Fe D.sub.av α-Fe ALLOY PRESSURE (GPa) (nm) Parallel (nm) Perpendicular (nm) Parallel (nm) Perpendicular Nd.sub.8.2Fe.sub.87.4B.sub.4.4 1 10.9 21.3 13.7 18.2 Nd.sub.8.2Fe.sub.87.4B.sub.4.4 0.5 11.7 14.1 14.1 13.5 Nd.sub.8.2Fe.sub.87.4B.sub.4.4 0 28.9 28.9 68.9 68.9 Nd.sub.5.9Fe.sub.91B.sub.3.1 1 11.7 15.2 19.5 18

(50) As presented herein, methods of preparing bulk permanent magnetic nanocompositions having decreased rare earth metal composition are described. For example, conventionally the amount of Nd in a stoichiometric Nd.sub.2Fe.sub.14B magnet is 11.76 at. %. As provided herein, a nanocomposite having 5.9 at. % content Nd exhibited permanent magnetic properties.

(51) While the present invention has been illustrated by a description of one or more embodiments thereof and while these embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.