High Pressure Gas Atomization Process for Preparing Soft Nanocomposite Magnetic Materials

Abstract

High-pressure gas atomization (HPGA) process produces high-quality metal powder and alloy materials including soft magnetic materials. HPGA includes: (a) melting a metal to form a liquid metal; (b) forming a continuous stream of the metal liquid; and (c) directing high-pressure inert gas into the continuous stream of liquid metal to generate droplets of the liquid metal, whereby the droplets solidify to form particles that exhibit soft magnetic properties. The high-pressure inert gas quenches or cools the liquid metal at speeds of up to 510.sup.5 C. per second. The soft magnetic alloy powder is spherical-shaped with particle sizes of between 1 m and 5 m and comprises a mixture of amorphous and microcrystalline phases with a narrow size distribution. These features facilitate consolidation into various products including near-net shape magnets. Annealing yields nanocrystal phases including a-CoFe or a-Fe phase that is embedded in amorphous matrix.

Claims

1. A method of producing soft magnetic materials comprising: (a) melting a metal to form a liquid metal; (b) forming a continuous stream of the metal liquid; and (c) directing high pressure inert gas into the continuous stream of liquid metal to generate droplets of the liquid metal, whereby the droplets solidify to form particles that exhibit soft magnetic properties.

2. The method of claim 1 further comprising (d) annealing the particles at a low annealing temperature.

3. The method of claim 3 wherein step (d) causes crystallization within the particles to form nanocrystal phases of -CoFe or -Fe.

4. The method of claim 3 wherein the nanocrystal phases have diameters that ranges from 5 to 10 nm.

5. The method of claim 3 wherein the annealing temperature ranges from 500 to 600 C.

6. The method of claim 1 wherein in step (b) the liquid metal passes through an elongated channel and exits through an aperture as a melt stream and step (c) comprises impinging inert gas into the melt stream.

7. The method of claim 5 wherein in step (b) the aperture is positioned within a spray chamber and in step (c) the impinging inert gas has a pressure of 800-1000 psi.

8. The method of claim 1 wherein step (c) comprises directing high pressure inert gas from a plurality of directions into the melt stream.

9. The method of claim 1 wherein step (a) comprises melting an alloy.

10. The method of claim 9 wherein the alloy is FeSiNbCuB, FeZrNbCu, CoFeZrCuB, or CoFeSiNbCuB.

11. The method of claim 1 wherein step (a) comprises melting the metal in a vacuum chamber or in an inert environment.

12. The method of claim 1 wherein the droplets solidify into particles at a cooling rate of 110.sup.5 to 510.sup.5 degrees C./s.

13. The method of claim 1 wherein the particles comprise nanocomposites.

14. The method of claim 13 wherein the nanocomposites have diameters in the range of 5 to 10 nm.

15. A method of fabricating soft nanocomposite magnetic materials comprising: (a) melting a metal to form a liquid metal; (b) forming a continuous stream of the metal liquid; (c) directing high pressure inert gas into the continuous stream of liquid metal to generate droplets of the liquid metal, whereby the droplets solidify to form particles that exhibit soft magnetic properties; and (d) annealing the microscale particles at a low annealing temperature to yield soft nanocomposite magnetic materials.

16. The method of claim 15 wherein step (d) causes crystallization within the particles to form nanocrystal phases of -CoFe or -Fe.

17. The method of claim 15 further comprising (e) consolidating the soft nanocomposite magnetic materials.

18. The method of claim 17 wherein step (e) forms magnets.

19. The method of claim 18 wherein the magnets comprises an alloy that is FeSiNbCuB, FeZrNbCu, CoFeZrCuB, or CoFeSiNbCuB.

20. The method of claim 18 wherein the magnets are incorporated in an inverter or converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1A is a HPGA system and FIG. 1B is the nozzles arrangement.

[0015] FIGS. 2A, 2B, 2C and 2D depicts the crystallization process of amorphous (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 and (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 during annealing of (HITPERM) alloy powders.

[0016] FIGS. 3A, 3B, 3C, 3D and 3E are SEM images showing microstructures: Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1, (Cu.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1, and (Cu.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1, respectively.

[0017] FIGS. 4A and 4B are XRD patterns of HITPERM-type (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 and FINEMT-type Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 alloy powders, respectively.

[0018] FIGS. 5A, 5B, 5C and 5D are HRTEM images of (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 and FINEMET-type Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1 alloy powders prepared by HPGA.

[0019] FIGS. 6A and 6B are HRTEM images of (Co.sub.0.35Fe.sub.0.65).sub.73.5Zr.sub.7B.sub.4Cu.sub.1 alloy powders prepared by HPGA.

[0020] FIG. 7 is a hysteresis loop for about 5 m (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 prepared by the HPGA.

[0021] FIG. 8 is a hysteresis loop for annealed (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1.

[0022] FIG. 9 is a hysteresis loop for about 5 m FINEMET-type Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1.

[0023] FIG. 10 is a hysteresis loop for annealed FINEMET-type Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] FIGS. 1A and 1B illustrate a high-pressure gas atomization (HPGA) system for preparing soft magnetic materials. Selected metals such as alloy ingots are melted in a ceramic (e.g., alumina) crucible 4 of a furnace 2 to yield a liquid metal. Typically, the metal is superheated to a temperature of 1500-1550 C. (or higher) and held at this temperature for about 3 minutes to insure complete homogenization of the melt temperature and composition. The metal is preferably heated in vacuum or an inert gas environment. Vacuum pump 8 is connected to furnace 2 via valves 12, 14 and to an atomization spray chamber 20 via valves 16, 14. The liquid metal flow through an elongated channel 6 that is connected to nozzle arrangement 18. The length of channel 6 is preferably sufficiently long that the liquid metal flows through the exit of the channel 6 as a continuous stream of liquid metal into the atomization chamber 20.

[0025] As shown in FIG. 1B, the nozzle arrangement has an exit for the channel 6 at the melt feed tube tip 28. Gas is supplied through conduits 22, 24, each of which has an outlet or nozzle that directs a jet of high-pressure gas into the stream of liquid metal. An atomization gas source 32 supplies inert as such as ultra-high pure Ar or N.sub.2, at a pressure of 800-1000 psi, through valve 30 to the gas conduits. Instead of employing a plurality of discrete gas nozzles that direct jets of high-pressure gas into the liquid metal, an annular ring can be employed to direct a ring-shaped stream of gas into the liquid metal. As the high-pressure gas streams impinge on the liquid metal stream, the liquid metal atomizes and breaks into liquid metal droplets. As the droplets continue to descend down the atomization chamber 20, the droplets solidify into spherical metal powder particles. The spray chamber 20 is preferably backfilled with Ar or Nz.

[0026] In a preferred embodiment as depicted in FIG. 1B, the liquid metal exits the aperture of channel 6 and flows a certain distance under gravity or pressure before being impacted by the high-pressure gas within the atomization region 26 that is within the upper portion atomization chamber 20. Channel 6 preferably has an elongated bore with a length of 10.4 mm and inner diameter of 1.32 mm (0.408 inch). Channels with longer lengths and wider diameters are used in preparing larger quantities of soft magnetic materials. The elongated bore assures that the liquid metal exits into the atomization chamber 20 as a continuous stream. The droplets cool and solidify rapidly as they fall downward inside atomization chamber 20. As a result of the impingement of the high-pressure gas onto the melt stream, the cooling rate of the droplets range from 110.sup.5 to 510.sup.5 C./s.

[0027] Powder removed from a primary cyclone collector 34, such as an electrostatic precipitator powder collector (ESP), and secondary cyclone 36 can be further sieved in ambient environment using standard methods into different size particles, including powders with diameters that range between 1 m to 5 m. A wet scrubber 38 removes remaining materials from the exhaust gas.

[0028] The powders can comprise a mixture of amorphous and microcrystal phases that can be further processed. In the case of FINEMET, NANOPERM, and HITPERM powders, it has been demonstrated that a low temperature annealing will cause nanocrystal phases such as -CoFe and a-Fe to become embedded in an amorphous matrix that is formed during the crystallization process. Typically, the annealing temperature is from 300 to 600 C. and preferably from 500 to 600 C. Annealing at a low annealing temperature causes crystallization within the particles to form nanocrystal phases of -CoFe or a-Fe with diameters that range from 5 to 10 nm. With respect to the preparation of nanocomposites, HPGA can generate a mixture of amorphous and microcrystalline powders, so that the microstructure of the atomized powders can be easily tailored by lowering the annealing temperature than the temperature used for melt-spinning of ribbons to produce high induction -CoFe phase on HITPERM and a-Fe phase in FINEMET and NANOPERM with sizes smaller than 10 nm. This results in an increase of the Curie temperature for the soft magnetic materials, thereby increasing operating temperatures, high induction, low magnetostrictive coefficients, and low hysteretic and eddy current losses, without compromising the mechanical properties.

[0029] The HPGA process was used to make soft magnetic nanocomposite materials including: (i) soft magnet alloys containing -CoFe (e.g. HITPERM), (2) soft magnet alloys containing a-Fe (e.g. FINEMET), and (3) Fe-M-BCu (M=Zr or Nb) soft magnet alloys (e.g. NANOPERM). In particular, (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1 and (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders that have -CoFe and a-Fe nanocrystal phases embedded in an amorphous matrix were prepared. FINEMET, NANOPERM or HITPERM compositions that contain an a-Co.sub.35Fe.sub.65 phase facilitates the formation of an amorphous alloy with HPGA by satisfying the eutectic requirements for the formation of amorphous precursors. In addition, the presence of the a-Co.sub.35Fe.sub.65 phase enhances their magnetic properties such as maximum saturation magnetization. The resultant nanocomposite powders would contain -CoFe and a-Fe nanocrystal phase embedded in amorphous matrix for both (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 and Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1 powders respectively. A subsequent low temperature annealing facilitates the formation of the -CoFe in both (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu and (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders and of the a-Fe nanocrystal in Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1 from amorphous phase, which can therefore lead to a decrease in H.sub.c and power loss of nanocomposites for use in power electronics and hybrid electric vehicles.

[0030] In preparing (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, alloy ingots containing Co, Fe, Si, Nb, B and Cu were prepared by plasma arch melting a mixture of these constituent elemental metals. The relative molar amounts of each metal in the mixture were in proportional to that of the soft magnet alloy. The ingots with the pre-alloyed compositions were induction-heated in an alumina crucible to a superheat of 1500-1550 C. The powders recovered from the HPGA process were annealed at a temperature of 2 for 5 hours to crystallize the -CoFe and a-Fe phases from the amorphous matrix. The FINEMET powders from the HPGA process had a saturation magnetization (M.sub.s) of 85 emu/g and Hc of 1.4 Oe. In comparison, FINEMET made by melt-spun process typically has a M.sub.s of 100 emu/g and Hc of 0.011 Oe.

[0031] In preparing Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1, alloy ingots containing Fe, Si, Nb, B and Cu were prepared by plasma arch melting a mixture of these constituent elemental metals. The relative molar amounts of each metal in the mixture were in proportional to that of the soft magnet alloy. The ingots with the pre-alloyed compositions were induction-heated in an alumina crucible to a superheat of 1500-1550 C. The powders recovered from the HPGA process were annealed at a temperature of 600 C. for 2-5 hours to crystallize the -CoFe and a-Fe phases from the amorphous matrix. The Co-added FINEMET powders recovered from the HPGA had a M.sub.s of 95 emu/g and Hc of 4 Oe. In comparison, Co-added FINEMET made by melt-spun process typically has a M.sub.s of 150 emu/g and Hc of 1-2.5 Oe range.

[0032] In preparing (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1, alloy ingots containing Co, Fe, Zr, B and Cu were prepared by plasma arch melting a mixture of these constituent elemental metals. The relative molar amounts of each metal in the mixture were in proportional to that of the soft magnet alloy. The ingots with the pre-alloyed compositions were induction-heated in an alumina crucible to a superheat of 1500-1550 C. The powders recovered from the HPGA process were annealed at a temperature of 600 C. for 2-5 hours to crystallize the -CoFe and a-Fe phases from the amorphous matrix.

[0033] FIGS. 2A, 2B, 2C and 2D illustrate the crystallization or nucleation process of amorphous (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 and (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 powders that occurs in the annealing stage. FIG. 2A depicts the amorphous matrix of the alloy powders derived from the HPGA process. As annealing proceeds, Cu clustering forms within the matrix as shown in FIG. 2B. Next, heterogeneous nucleation of the -CoFe nanocrystalline phase appears as shown in FIG. 2C. Finally, a nanocomposite alloy containing the -CoFe phase within the amorphous matrix develops as shown in FIG. 2D. Similarly, in the annealing of FILAMENT alloy powders, crystallization or nucleation of the a-Fe phase within the amorphous phase also occurs. The microstructure of atomized powders can be modified and tailored by annealing at lower temperatures. In particular, lower annealing temperatures can prevent formation of minority phases and devitrification of the amorphous grain boundary phase caused by high annealing temperatures, which often takes place in melt-spun process.

[0034] FIGS. 3A-3E are scanning electron microscope images of Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1, (Cu.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1, (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1, and (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders. The particles exhibit uniform spherical shapes and have sizes from about 1 m to 5 m.

[0035] FIGS. 4A and 4B are X-ray diffraction patterns of (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 and Fe.sub.73.5Si.sub.13.5Nb.sub.3B.sub.9Cu.sub.1 powders, respectively, that were prepared by HPGA. In FIG. 4A the XRD peak indicates the presence to the -CoFe phase within an amorphous phase. In FIG. 4B the XRD peak indicates the presence to the a-Fe phase within an amorphous phase.

[0036] FIGS. 5A and 5B are high-resolution transmission electron microscopy images of (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 alloy powders prepared by HPGA. FIG. 5A shows -CoFe nanocrystals embedded in an amorphous matrix. FIG. 5B shows (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 nanoparticles with a particles size of about 10 nm. FIGS. 5C, and 5D are HRTEM images of FINEMT-type Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 alloy powders prepared by HPGA. FIG. 5C shows a-Fe nanocrystals embedded in an amorphous matrix and FIG. 5D shows Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 nanoparticles with a particle size of about 5-10 nm.

[0037] FIGS. 6A and 6B are HRTEM images of (Co.sub.0.35Fe.sub.0.65).sub.73.5Zr.sub.7B.sub.4Cu.sub.1 alloy powders made by HPGA. The microstructural features are similar to as ones for (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 alloy powders prepared by the identical HPGA process as shown in FIGS. 5A and 5B.

[0038] FIG. 7 is a hysteresis loop of about 5.5 m (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 prepared by HPGA process. In this diagram, H is the applied magnetic field in Oersteds and M is the magnetization. Hc, which is the coercive force, is about 12 Oe.

[0039] FIG. 8 is a hysteresis loop of (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 powder made by HPGA and annealed a temperature of 600 C. for 2 hrs under Ar/H.sub.2 atmosphere. Hc is about 4.4 Oe, which is comparable to similar alloys prepared by melt-spun.

[0040] FIG. 9 is a hysteresis loop of 5.5 m Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 powder made by HPGA. The M.sub.s is 64 emu/g and the H.sub.e is 9.02 Oe,

[0041] FIG. 10 is a hysteresis loop for Fe.sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1 powder prepared by HPGA and annealed at a temperature of 600 C. for 2 hrs under Ar/H.sub.2 atmosphere. The Hc is about 1.5 Oe, which is comparable to similar alloys prepared by melt-spun process.

[0042] Powdered alloys produced by HPGA can also be milled before undergoing annealing. For example, high-energy mechanical ball milling under liquid nitrogen (cryomilling) can be employed to break-down the HPGA-prepared microscale materials into nano-sized powder. High-energy ball milling, which has a much higher ratio of milling balls/powders, as compared to a conventional ball milling process, is an efficient means to generate a variety of nanostructured powder materials with several advantages as applicability to essentially all classes of materials and may be used for easy scaling from small to large quantities of materials. With milling, heavy cyclic deformation is induced in powders, which promotes (1) the formation of nanostructures by the structural decomposition of coarser-grained structures as a result of severe plastic deformation, and (2) penetration of nano-size particulates (nanoparticles) into the powders of other constituents. This then forms a nanocomposite at the single particle level. The introduction of liquid nitrogen into high-energy ball milling, in the cryomilling process, represents a new development for the cost-effective synthesis of nanostructured powders. Cryomilling can further increase the synthesis efficiency and simultaneously minimize the oxidation/contamination of the milled materials.

[0043] To avoid possible impurity-contamination in this milling approach, ceramic balls may be used instead of stainless-steel ones to minimize any Fe contamination. In addition, milling under an Ar atmosphere may be used to avoid possible oxidation. Before consolidation, the cryomilled powders or powder mixtures are degassed in vacuum (10.sup.6 torr) to evacuate the potential trapped gas during the cryomilling process. The pressing of samples is carried out under processing conditions of 200 MPa, 340-500 C. and sintering times ranging from 30-60 minutes in an Argon atmosphere. The resultant powders are then sintered into a bulk sample with nearly theoretical density, by controlling the sintering atmosphere, sintering temperature and sintering time.

[0044] Microscale HITPERM-type (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders with sizes around 16 m were produced by HPGA and the sphere-shaped particles underwent high-energy mechanical ball milling into nanoscale powders in a protective atmosphere such as stearic acid in order to avoid oxidation. The nanopowders were annealed at a low temperature of 600 C., so that -CoFe, which contributes a large magnetic moment to the soft magnetic (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders, forms in the crystallization process in the amorphous and partially crystallized matrix. Hc of the (Co.sub.0.35Fe.sub.0.65).sub.88Zr.sub.7B.sub.4Cu.sub.1 powders prepared by HPGA was a little high than that of (Co.sub.0.35Fe.sub.0.65).sub.73.5Si.sub.15.5Nb.sub.3B.sub.7Cu.sub.1.

[0045] The soft magnetic materials produced by HPGA can be consolidated into various lightweight bulk magnets that can be employed, for example, in small lightweight converters and inverters. For example, the final magnet of the bulk nanocomposites with optimized magnetic properties and targeted shape and size can be made with Rapid Hot Pressing (RHP) and Hot Isostatic Pressing (HIP). RHP is particularly suited for consolidating nanostructured powders into near-net shape samples with dense microstructures. Compared to conventional hot pressing, RHP equipped with rapid induction heating is better in retaining fine-grain microstructure. Because of significantly reduced sintering temperature and time, grain growth is significantly suppressed and the final products with fine grain size and high density can be achieved.

[0046] The foregoing has described the principles, preferred embodiment and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of present invention as defined by the following claims.