Patent Application
20200365302
The present invention generally relates to a high-pressure gas atomization (HPGA) process for producing high-quality metal powder and alloy materials including soft magnetic materials.
Melt-spinning has been used to make amorphous ribbons including alloys such as FeSiNbCuB and FeZrNbCu which are commercially available under the names FINEMET and NANOPERM, respectively and CoFeSiNbCuB and FeCoZrBCu which are prototypes as HITPERM. When manufacturing soft magnetic materials, an ingot containing the targeted alloy having the targeted composition is prepared by arc melting in an argon atmosphere. Amorphous ribbons are produced from the ingot using a single wheel melt-spinning technique. The process begins with re-melting of the arc-melted ingot in a boron nitride crucible in an argon atmosphere. A small amount of argon gas is used to quench the molten targeted alloy onto a Cu—Be wheel, which can reach quench speeds of one million ° C./second, in order to obtain an amorphous structure. The melt-spun ribbons are annealed isothermally during a crystallization process to form a two-phase structure that includes nanocrystal that is embedded in an amorphous matrix. The nanocrystals are α-CoFe for HITPERM and a-Fe for FINEMET and NANOPERM.
Because of the eutectic requirements in forming amorphous precursors, melt-spinning is not suitable for some alloy compositions. In some applications, Zr and B are added to improve glass formation and Nb and Cu are needed to reduce the grain size during melt-spinning. Melt-spinning, which an expensive process that requires costly equipment, is not suitable for fabricating FINEMET, NANOPERM and HITPERM alloys at commercial scales. In addition, with the melt-spun process it is usually difficult to define and control both the nature and extent of the grain boundary phase associated with crystal chemistry which is needed to tailor phase constituents and microstructure in order to meet targeted structure and magnetic properties. Furthermore, surface crystallization, which is commonly observed in melt-spun ribbons, is not desirable for soft magnetic materials because it causes texture layers that diminish magnetic performance.
The present invention is based, in part, on the development of a low-cost and scalable high-pressure gas atomization (HPGA) process for producing high-quality metal powder and alloy materials including soft magnetic materials.
In one aspect, the invention is directed to a method of producing soft magnetic materials that 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, which is preferably applied at a pressure of 800 to 1000 psi, quenches or cools the liquid metal at speeds of up to 5×105° C. per second. The soft magnetic alloy powder is spherical-shaped with particle sizes of between 1 μm and 5 μm 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.
The microstructure of the atomized powders from HPGA can be tailored by using low temperature annealing to produce a high induction nanocrystal phase in an amorphous matrix. For example, an α-CoFe nanocrystal phase can be produced in HITPERM and a high induction a-Fe nanocrystal phase can be produced in FINEMET, where the nanocrystal phase has sizes smaller than 100 nm within an amorphous matrix. Magnetic powders containing α-CoFe or a-Fe with sizes smaller than 40 nm oftentimes result in an increase of the Curie temperature of the magnetic materials, thereby increasing operating temperature, and providing low magnetostrictive coefficient without negatively affecting their mechanical properties. Theoretical calculations for metallic ferromagnetic nanoparticle composites suggest that small inter-particles can offer higher resistivity and therefore reduce power loss due to eddy currents.
HPGA is particularly suited for fabricating soft magnetic nanocomposite materials including soft magnet alloys containing α-CoFe (e.g. HITPERM) and soft magnet alloys containing a-Fe (e.g. FINEMET), as well Fe-M-B—Cu (M=Zr or Nb) soft magnet alloys (e.g. NANOPERM). HPGA can produce higher-quality metal and alloys powders at a lower cost than conventional melt-spun. In addition, crystallization of melt-spun amorphous alloys often leads to minority phases and devitrification of the amorphous grain boundary phase because of the high annealing temperatures used in their crystallization process. Furthermore, the grain boundary phases need to be well controlled in order to achieve desired phase constituents and microstructure for targeted magnetic properties, but it is usually difficult to define both the nature and extent of the grain boundary phase associated with crystal chemistry and to tailor phase constituents and microstructure to achieve the targeted structural and magnetic properties.
HPGA is more flexible than melt-spinning in that a greater variety of compositions can be used in forming the amorphous phase, in alloying and in controlling the impurity levels. In addition, high powder production rates of HPGA process make this technology viable and competent, both economically and technically, and make it flexible to control powder size, composition, size distribution, shape and surface morphology during HPGA process. Furthermore, HPGA can produce highly alloyed specialty-alloy-powders because the atomized powders are preferably pre-alloyed and exhibit high composition homogeneity. Finally, surface crystallization, which is commonly observed in melt-spun ribbons, is not desirable for soft magnetic performance because it causes texture layers and has a negative effect on magnetic properties; surface crystallization can be minimized with HPGA.
HPGA systems are less expensive to construct and operate than melt-spinning devices; moreover, HPGA can be readily scaled-up to produce large quantities of alloy powder ranging from 15 kg to 25 kg per batch or more. HPGA yields fine, clean, spherical-shaped, microscale sized powders can be compacted into high density products.
As shown in
In a preferred embodiment as depicted in
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 μm to 5 μm. A wet scrubber 38 removes remaining materials from the exhaust gas.
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 α-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 α-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 α-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.
The HPGA process was used to make soft magnetic nanocomposite materials including: (i) soft magnet alloys containing α-CoFe (e.g. HITPERM), (2) soft magnet alloys containing a-Fe (e.g. FINEMET), and (3) Fe-M-B—Cu (M=Zr or Nb) soft magnet alloys (e.g. NANOPERM). In particular, (Co0.35Fe0.65)73.5Si15.5Nb3B7Cu1, Fe73.5Si13.5Nb3B9Cu1 and (Co0.35Fe0.65)88Zr7B4Cu1 powders that have α-CoFe and a-Fe nanocrystal phases embedded in an amorphous matrix were prepared. FINEMET, NANOPERM or HITPERM compositions that contain an a-Co35Fe65 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-Co35Fe65 phase enhances their magnetic properties such as maximum saturation magnetization. The resultant nanocomposite powders would contain α-CoFe and a-Fe nanocrystal phase embedded in amorphous matrix for both (Co0.35Fe0.65)73.5Si15.5Nb3B7Cu1 and Fe73.5Si13.5Nb3B9Cu1 powders respectively. A subsequent low temperature annealing facilitates the formation of the α-CoFe in both (Co0.35Fe0.65)73.5Si15.5Nb3B7Cu and (Co0.35Fe0.65)88Zr7B4Cu1 powders and of the a-Fe nanocrystal in Fe73.5Si13.5Nb3B9Cu1 from amorphous phase, which can therefore lead to a decrease in Hc and power loss of nanocomposites for use in power electronics and hybrid electric vehicles.
In preparing (Co0.35Fe0.65)73.5Si15.5Nb3B7Cu1, 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 α-CoFe and a-Fe phases from the amorphous matrix. The FINEMET powders from the HPGA process had a saturation magnetization (Ms) of 85 emu/g and Hc of 1.4 Oe. In comparison, FINEMET made by melt-spun process typically has a Ms of 100 emu/g and Hc of 0.01−1 Oe.
In preparing Fe73.5Si13.5Nb3B9Cu1, 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 α-CoFe and a-Fe phases from the amorphous matrix. The Co-added FINEMET powders recovered from the HPGA had a Ms of 95 emu/g and Hc of 4 Oe. In comparison, Co-added FINEMET made by melt-spun process typically has a Ms of 150 emu/g and Hc of 1-2.5 Oe range.
In preparing (Co0.35Fe0.65)88Zr7B4Cu1, 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 α-CoFe and a-Fe phases from the amorphous matrix.
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.
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 (10−6 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.
Microscale HITPERM-type (Co0.35Fe0.65)88Zr7B4Cu1 powders with sizes around 16 μm 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 α-CoFe, which contributes a large magnetic moment to the soft magnetic (Co0.35Fe0.65)88Zr7B4Cu1 powders, forms in the crystallization process in the amorphous and partially crystallized matrix. Hc of the (Co0.35Fe0.65)88Zr7B4Cu1 powders prepared by HPGA was a little high than that of (Co0.35Fe0.65)73.5Si15.5Nb3B7Cu1.
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.
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.
This invention was made with Government support under SBIR Grant No. DE-SC0012055 awarded by the U.S. Department of Energy to Aegis Technology Inc. The Government has certain rights to the invention.
