A method and device for preparing spherical amorphous powder and spherical Fe-based amorphous powder

Abstract

The present invention discloses a method and device for preparing spherical amorphous powder and spherical Fe-based amorphous powder. The present invention introduces molten liquid into a crucible, and the liquid column flowing out from the nozzle at the bottom of the crucible is atomized into small droplets with uniform particle size under the ultrasonic action of an ultrasonic atomizer; the small droplets move freely for a distance in the inert gas protection zone at the top of the powder collecting tank, and shrink into a sphere under the action of surface tension; the spherical small droplets are then taken away from the inert gas protection zone and into the inert gas atomization zone by the ejected high-pressure nitrogen gas flow, and the air film on the surface of the spherical small droplets is punctured and then rapidly cooled to form spherical amorphous powder. The spherical Fe-based amorphous powder prepared by the above method has the characteristics of high amorphousness, high sphericity, adjustable powder particle size, concentrated powder particle size distribution and low powder oxygen content. It meets the high requirements for amorphous powder used to prepare magnetic powder cores and inductors.

Description

Technical Field

[0001] The invention relates to the technical field of amorphous materials, and in particular to a method and a device for preparing spherical amorphous powder and spherical Fe-based amorphous powder. Background Art

[0002] As electronic equipment and components continue to develop in the direction of miniaturization, high frequency and high current, magnetic powder cores and inductors are required to have excellent frequency stability, extremely low loss, high saturation current characteristics, etc., and higher requirements are also put forward for amorphous powders used to prepare magnetic powder cores and inductors, such as high amorphousness (>95%), high sphericity (>95%), ultrafine powder (D50<15μm), low oxygen content (<300ppm), low impurity content, etc.

[0003] The amorphous powder currently used is mainly prepared by amorphous strip crushing method, gas atomization method and water-gas combined atomization method.

[0004] Strip crushing method: prepare amorphous strip by plane flow casting - anneal the strip - crush the annealed strip into amorphous powder. The amorphous powder prepared by this method: ① The powder particle size is coarse, mainly 100-400 mesh (D50 = 75μm-100μm) powder; ② The powder is flaky and the powder edge is sharp, which can easily puncture the insulation layer during the coating and pressing process, resulting in high inductor power consumption.

[0005] Gas atomization method: The medium frequency melting furnace melts the alloy into molten steel - high-pressure gas atomizes the molten steel into droplets - the droplets solidify to form amorphous powder, and the high-pressure gas plays a dual role of crushing and cooling. The amorphous powder prepared by this method: ① The amorphous degree is low, only a limited number of ingredients can be used to prepare amorphous, and the scope of use is small; ② The powder particle size is coarse, the powder yield of D50<15μm is less than 15%, and the manufacturing cost is high;

[0006] Water atomization method: the medium frequency melting furnace melts the alloy into molten steel - high-pressure water atomizes the molten steel into droplets - the droplets solidify to form amorphous powder, and the high-pressure water plays a dual role of crushing and cooling. The amorphous powder prepared by this method has the following characteristics: ① low sphericity and is a special-shaped powder (the high-pressure water flow has high energy and strong cooling capacity. During the process of high-pressure water flow crushing the molten steel into droplets, the droplets solidify before they have time to shrink); ② high oxygen content in the powder, reaching about 2000ppm (dissolved oxygen in water + water decomposition produces oxygen, which leads to oxidation of the powder).

[0007] The amorphous powder prepared by the above method cannot meet the high requirements for amorphous powder currently required for preparing magnetic powder cores and inductors. Summary of the invention

[0008] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a method and device for preparing spherical amorphous powder and spherical Fe-based amorphous powder.

[0009] The present invention solves the technical problem by adopting the following technical solutions.

[0010] The invention provides a method for preparing spherical amorphous powder, which comprises: atomizing a molten liquid into small droplets under ultrasonic action, allowing the small droplets to fall freely and shrink into a sphere under the protection of an inert gas, and then using a high-pressure airflow to rapidly cool the small droplets into spherical amorphous powder.

[0011] In some embodiments of the present invention, the molten liquid is molten steel, and the temperature of the molten steel is 1300°C-1500°C.

[0012] In some embodiments of the present invention, the molten steel is ultrasonically atomized into small droplets at 200 KHz-500 KHz.

[0013] In some embodiments of the present invention, the free-falling distance of the small droplets is 2 cm to 12 cm.

[0014] In some embodiments of the present invention, the inert gas is selected from any one of nitrogen, argon or helium, preferably nitrogen.

[0015] In some embodiments of the present invention, the nitrogen pressure is 3.2 MPa-4.5 MPa.

[0016] The present invention also provides a device for implementing the above method, which mainly includes: a crucible, an ultrasonic atomizer and a powder collecting tank. The crucible is embedded in the ultrasonic atomizer and is tightly connected to the ultrasonic atomizer to form a unified whole. A nozzle is provided at the bottom of the crucible, and a powder collecting tank is connected below the nozzle. The powder collecting tank includes an inert gas protection area at the top and a high-pressure gas atomization area at the bottom, so that the small droplets atomized by the ultrasonic atomizer form spherical small droplets in the powder collecting tank and then are rapidly cooled to form spherical amorphous powder.

[0017] In some embodiments of the present invention, the nozzles at the bottom of the crucible are regularly arranged in the circumferential direction, and the aperture of the nozzles is 0.3 mm-0.8 mm.

[0018] In some embodiments of the present invention, an integrally formed annular air inlet is provided around the outer wall of the high-pressure gas atomization zone at the lower part of the powder collecting tank, and the other end of the air inlet is connected to the high-pressure gas atomizer.

[0019] The present invention also provides a spherical Fe-based amorphous powder prepared by the above method, the spherical Fe-based amorphous powder has an amorphous degree of more than 98%, a sphericity of more than 98%, D50 < 15 μm, a powder yield > 65%, and an oxygen content of less than 200 ppm.

[0020] The present invention has the following beneficial effects:

[0021] The present invention provides a method and device for preparing spherical amorphous powder and spherical Fe-based amorphous powder. The method for preparing spherical amorphous powder provided by the present invention comprises: atomizing a molten liquid into small droplets under ultrasonic action, allowing the small droplets to fall freely and shrink into a sphere under the protection of an inert gas, and then using a high-pressure airflow to rapidly cool the small droplets into spherical amorphous powder. The spherical amorphous powder prepared by the above method has the characteristics of high amorphousness, high sphericity, low powder oxygen content, adjustable powder particle size, and concentrated powder particle size distribution, which can meet the high requirements for the use of amorphous powder. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 It is a schematic diagram of an atomization powder preparation device, with the following figure numbers: 1-powder collecting tank, 2-nozzle, 3-high-pressure gas atomizer, 4-high-pressure gas atomization area, 5-flow guide tube, 6-crucible, 7-ultrasonic atomizer, 8-high-pressure gas pipeline, 9-inert gas protection area;

[0024] Figure 2 A schematic diagram of the nozzle distribution at the bottom of the ultrasonic atomizer device;

[0025] Figure 3 The XRD pattern of the spherical Fe-based amorphous powder prepared in Example 1;

[0026] Figure 4 This is a SEM image of the spherical Fe-based amorphous powder prepared in Example 1;

[0027] Figure 5 This is the particle size distribution diagram of the spherical Fe-based amorphous powder prepared in Example 1. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0029] The following is a detailed description of a method and device for preparing spherical amorphous powder and spherical Fe-based amorphous powder provided in an embodiment of the present invention.

[0030] In a first aspect, an embodiment of the present invention provides a method for preparing spherical amorphous powder, which comprises: atomizing molten steel into small droplets under ultrasonic action, allowing the small droplets to fall freely and shrink into a sphere under the protection of an inert gas, and then using a high-pressure airflow to rapidly cool the spherical droplets into spherical amorphous powder.

[0031] The embodiment of the present invention provides a method for preparing spherical amorphous powder, which includes: atomizing a molten liquid into small droplets through ultrasonic treatment, using ultrasonic waves to refine the grains of amorphous materials, improve their mechanical properties and thermal stability, and promote the recrystallization process of amorphous materials, thereby improving their processing performance and electrical properties. Then, the small droplets are allowed to fall freely in an inert gas protection area, and the small droplets shrink into a sphere under the action of gravity and surface tension. This process is similar to the formation process of "water droplets"; and then the air film on the surface of the small droplets is cold-punctured by high-pressure airflow and the spherical small droplets are rapidly cooled to form spherical amorphous powder.

[0032] In some optional embodiments, the molten liquid is molten steel, and the temperature of the molten steel is 1300-1500° C. Alternatively, the temperature of the molten steel may be 1300° C., 1350° C., 1400° C., 1500° C., and any other value between 1300° C. and 1500° C.

[0033] In some optional embodiments, the molten steel is ultrasonically atomized into small droplets at 200KHz-500KHz. The particle size of the small droplets can be controlled by controlling the ultrasonic frequency, and optionally, the ultrasonic frequency can be 200KHz, 250KHz, 300KHz, 350KHz, 400KHz, 450KHz, 500KHz and any other value between 200KHz-500KHz.

[0034] In some optional embodiments, the free-falling distance of the small droplets is 2 cm-12 cm. Optionally, the free-falling distance of the small droplets can be 2 cm, 4 cm, 6 cm, 8 cm, 10 cm, 12 cm and any other value between 2 cm-12 cm.

[0035] In some optional embodiments, the inert gas is selected from any one of nitrogen, argon or helium, preferably nitrogen, and the nitrogen pressure is 3.2MPa-4.5MPa. Optionally, the nitrogen pressure can be 3.2MPa, 3.4MPa, 3.6MPa, 3.8MPa, 4.0MPa, 4.2MPa, 4.5MPa and any other value between 3.2MPa-4.5MPa.

[0036] In a second aspect, an embodiment of the present invention further provides a device for implementing the above method, such as Figure 1 As shown, it mainly includes: a crucible 6, an ultrasonic atomizer 7 and a powder collecting tank 1. The crucible 6 is embedded in the ultrasonic atomizer 7 and is closely connected with the ultrasonic atomizer 7 to form a unified whole. A nozzle 2 is arranged at the bottom of the crucible 6, and a powder collecting tank 1 is connected below the nozzle 2. The powder collecting tank 1 includes an inert gas protection area 9 at the top and a high-pressure gas atomization area 4 at the bottom, so that the small droplets atomized by the ultrasonic atomizer 7 shrink in the powder collecting tank 1 to form spherical small droplets and then cool to form spherical amorphous powder.

[0037] In the process of preparing spherical amorphous powder by using the above-mentioned preparation device, the smelted molten steel can be introduced into the crucible 6 through the guide tube 5, the crucible 6 is embedded in the ultrasonic atomizer 7 and is closely connected to the ultrasonic atomizer 7 to form a unified whole, the liquid level of the molten steel is 1 / 3-2 / 3 of the depth of the crucible 6, the molten steel flows out through the nozzle 2, the ultrasonic atomizer 7 atomizes the liquid column of the molten steel into small droplets with uniform particle size, the small droplets move a distance in the inert gas protection zone 9, and shrink into balls under the action of surface tension, and then the high-pressure airflow is ejected from the high-pressure gas atomizer 3 at a supersonic speed through the high-pressure gas pipeline 8 into the powder collecting tank 1, the ejected high-pressure and high-speed airflow takes the spherical droplets away from the inert gas protection zone 9 and enters the high-pressure gas atomization zone 4, the high-pressure airflow in the high-pressure gas atomization zone 4 punctures the air film on the surface of the spherical droplets and rapidly cools them to form spherical amorphous powder, and the spherical amorphous powder is collected, graded, and batched to obtain the target product.

[0038] In some alternative embodiments, see Figure 2 The nozzles 2 at the bottom of the crucible 6 are regularly arranged along the circumferential direction, and the aperture of the nozzle 2 is 0.3 mm-0.8 mm. The diameter of the molten steel column is controlled by controlling the aperture of the nozzle 2. Optionally, the aperture of the nozzle 2 is 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, and any other value between 0.3 mm and 0.8 mm.

[0039] In some optional embodiments, an integrally formed annular air inlet is formed around the outer wall of the high-pressure gas atomization area 4 at the lower part of the powder collecting tank 1 , and the other end of the air inlet is connected to the high-pressure gas atomizer 3 .

[0040] In some optional embodiments, the method for preparing spherical Fe-based amorphous powder using the above-mentioned device comprises the following steps:

[0041] a. Put the prepared raw materials into the vacuum melting furnace for melting, deoxidation and slag removal;

[0042] b. The molten steel at a temperature of 1300° C. to 1500° C. is introduced into the crucible 6 through the flow guide pipe 5. A large number of regularly arranged nozzles 2 (with a diameter of 0.3 mm to 0.8 mm) are provided at the bottom of the crucible 6;

[0043] c. The molten steel flows out from the nozzle 2 and is broken into small droplets under the ultrasonic action of the ultrasonic atomizer 7 (ultrasonic frequency is 200KHz-500KHz), and then enters the powder collecting tank 1;

[0044] d. The small droplets move freely for a distance (2cm-12cm) in the argon environment in the inert gas protection zone 9 at the top of the powder collecting tank 1. During the movement, the small droplets shrink into a spherical shape under the action of surface tension, and then enter the high-pressure gas atomization zone 4 at the bottom of the powder collecting tank 1 (when the gas is nitrogen, 3.2MPa<nitrogen pressure<4.5MPa). The spherical small droplets move downward rapidly under the high-pressure and high-speed nitrogen gas flow and gravity.

[0045] e. During the movement, the high-pressure and high-speed nitrogen gas flow punctures the air film on the surface of the droplets and rapidly cools the small droplets to form Fe-based amorphous powder;

[0046] f. Collect and classify the Fe-based amorphous powder to obtain the target product; wherein:

[0047] (1) Amorphous degree > 98% or more: In step e, a layer of air film is formed on the surface of the small droplets, which moves downward under the action of gravity. When the small droplets come into contact with the high-pressure and high-speed nitrogen gas flow, the movement speed of the small droplets is about 10 m / s, and the movement speed of the high-pressure and high-speed nitrogen gas flow is about 650 m / s. There is a large relative movement between the two. The high-pressure and high-speed nitrogen gas flow instantly tears the air film on the surface of the droplets and rapidly cools the droplets, so the powder has a high amorphous degree.

[0048] (2) Sphericity>98% or more: In steps c and d, the steel liquid column flowing out of the nozzle 2 at the bottom of the crucible 6 is broken into small droplets under the ultrasonic action of the ultrasonic atomizer 7. The small droplets move freely for a distance in the inert gas protection zone 9 at the top of the powder collecting tank 1. During the movement, the small droplets shrink into balls under the action of surface tension. The movement time is controlled to be greater than the spheroidization time to ensure that the small droplets have enough time to spheroidize, so the powder has high sphericity.

[0049] (3) Powder particle size is adjustable and powder particle size distribution is concentrated, D50 < 15 μm powder, yield > 65%: In steps b and c, the diameter of the steel liquid column is controlled by controlling the aperture of the nozzle 2, and the length of the broken steel liquid column is controlled by controlling the ultrasonic frequency of the ultrasonic atomizer 7. Each broken steel liquid column forms a small droplet. The volume of the small droplet, that is, the diameter of the powder after shrinking into a sphere, is controlled by determining the diameter and length of the steel liquid column. Therefore, the powder particle size is controllable and the distribution is concentrated.

[0050] (4) Powder oxygen content <200ppm: In steps a, d and e, high-purity raw materials are selected for vacuum melting, and the molten steel is deoxidized and deslaged. The oxygen content in the molten steel is controlled below 50ppm. The atomization and cooling processes are carried out under an inert atmosphere. The entire system is in an inert gas atmosphere. The oxygen content in the system is extremely low, so the product has a low oxygen content.

[0051] In the third aspect, an embodiment of the present invention also provides a spherical Fe-based amorphous powder prepared by the above method, the spherical Fe-based amorphous powder has an amorphous degree of more than 98%, a sphericity of more than 98%, D50 <15μm, a powder yield >65%, and an oxygen content of less than 200ppm.

[0052] The present invention will be further described below in conjunction with the embodiments.

[0053] like Figure 1 As shown, the device for preparing spherical Fe-based amorphous powder mainly includes: a ceramic crucible 6, an ultrasonic atomizer 7 and a powder collecting tank. The ceramic crucible 6 contains molten steel liquid. The ceramic crucible 6 is embedded in the ultrasonic atomizer 7 and is closely connected to the ultrasonic atomizer 7 to form a unified whole. The bottom of the ceramic crucible 6 is provided with a nozzle 2, and the nozzles 2 are regularly arranged along the circumferential direction (see Figure 2 ), a powder collecting tank is connected below the nozzle 2, and the powder collecting tank includes an upper inert gas protection zone 9 and a lower high-pressure gas atomization zone 4, an air inlet pipe is integrally formed on one side of the outer wall of the high-pressure gas atomization zone 4 below the inert gas protection zone 9 of the powder collecting tank, and the other end of the air inlet pipe is connected to the high-pressure gas atomizer 3, so that the small droplets atomized by the ultrasonic atomizer 7 form spherical small droplets in the powder collecting tank and then are cooled to form amorphous powder.

[0054] A method for preparing spherical Fe-based amorphous powder using the above device comprises the following steps:

[0055] The smelted molten steel is introduced into the ceramic crucible 6 through the guide tube 5. The ceramic crucible 6 is embedded in the ultrasonic atomizer 7 and is closely connected to the ultrasonic atomizer 7 to form a unified whole. The liquid level of the molten steel is 1 / 3-2 / 3 of the depth of the ceramic crucible 6. The molten steel flows out through the nozzle 2. The molten steel liquid column of the ultrasonic atomizer 7 is atomized into small droplets with uniform particle size. The small droplets move a distance in the inert gas protection zone 9 and shrink into balls under the action of surface tension. After that, the high-pressure and high-speed inert gas gas flow is ejected from the high-pressure gas atomizer 3 at a supersonic speed and enters the powder collecting tank 1 through the inert gas flow pipe 8. The high-pressure and high-speed inert gas gas flow takes the spherical small droplets away from the inert gas protection zone 9 and enters the high-pressure gas atomization zone 4 at the lower part of the powder collecting tank 1. The high-speed and high-pressure gas flow in the high-pressure gas atomization zone 4 punctures the air film on the surface of the spherical small droplets and rapidly cools them to form spherical Fe-based amorphous particles. The spherical Fe-based amorphous powder is collected, graded, and batched to obtain the target product.

[0056] Table 1 below shows the main process parameters and test results of the embodiments and comparative examples, wherein:

[0057] The method for detecting the sphericity of Fe-based amorphous powder is as follows: using an X-ray diffractometer to analyze the composition of the product, using a laser particle size analyzer to measure the average particle size D50 of the product, using an electron scanning electron microscope to observe the morphology of the product, and randomly selecting 50 particles from the product, measuring the aspect ratio (DL) and short diameter ratio (DS) of each particle, and using the average value of DS / DL to calculate the sphericity of the product.

[0058] The method for detecting the amorphous degree of Fe-based amorphous powder is as follows: reference detection standard: EN 13925 Non-destructive testing - X-ray diffraction detection equipment for polycrystalline and amorphous materials: Bruker D8, detection parameters: scanning angle: 20-80°, scanning speed: 10° / min.

[0059] Table 1

[0060]

[0061] From Table 1 and Figure 3-5 It can be seen that the spherical Fe-based amorphous powders prepared by the schemes provided in Examples 1-5 of the present invention have high sphericity, concentrated powder particle size distribution, and basically no agglomeration phenomenon.

[0062] Compared with Examples 1-5, during the preparation process of the Fe-based amorphous powder in Comparative Examples 1 and 7, as the nozzle diameter increases and the ultrasonic frequency decreases, the ultrasonic capacity weakens and the molten steel liquid cannot be broken into smaller droplets, resulting in the particle size of the small droplets becoming larger, and the amorphous degree of the large-size droplets after extremely rapid cooling will also decrease.

[0063] Compared with Examples 1-5, during the preparation of the Fe-based amorphous powder in Comparative Example 2, the increase in the diameter of the nozzle 2 will lead to an increase in the particle size of the small droplets. At the same time, the free fall distance of the small droplets is too long, resulting in partial crystallization of the metastable amorphous material and reducing the amorphous degree of the amorphous powder.

[0064] Compared with Examples 1-5, during the preparation process of the Fe-based amorphous powder in Comparative Examples 3 and 6, the molten steel temperature is too low or the nozzle diameter is too small, and the molten steel has insufficient fluidity, which may cause nozzle clogging and make it impossible to prepare the Fe-based amorphous powder.

[0065] Compared with Examples 1-5, during the preparation process of the Fe-based amorphous powder in Comparative Example 4, the increase in nozzle diameter will lead to an increase in the particle size of the small droplets. At the same time, the free fall distance of the small droplets is too short, and the molten steel liquid has no time to shrink into spherical particles, which causes the sphericity of the spherical small droplets to decrease sharply and the particle size to increase.

[0066] Compared with Examples 1-5, during the preparation process of the Fe-based amorphous powder in Comparative Example 5, the excessively high molten steel temperature will lead to a decrease in the sphericity of the amorphous powder and an increase in the oxygen content. This is mainly due to: under high temperature conditions, the surface tension of the small droplets decreases, which weakens the spheroidization effect and leads to a decrease in sphericity. In addition, high temperature may also promote the occurrence of oxidation reactions, further increasing the oxygen content in the molten steel.

[0067] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing spherical amorphous powder, characterized in that: It includes: The molten liquid is atomized into small droplets under the action of ultrasound, and the small droplets are allowed to fall freely and shrink into a sphere under the protection of an inert gas. The spherical small droplets are then rapidly cooled into spherical amorphous powder using a high-pressure airflow.

2. The method according to claim 1, characterized in that The molten liquid is molten steel, and the temperature of the molten steel is 1300°C-1500°C.

3. The method according to claim 2, characterized in that The molten steel is ultrasonically atomized into small droplets at 200KHz-500KHz.

4. The method according to claim 1, characterized in that The free-falling distance of the small liquid droplets is 2 cm to 12 cm.

5. The method according to claim 1, characterized in that The inert gas is selected from any one of nitrogen, argon or helium, preferably nitrogen.

6. The method according to claim 5, characterized in that The nitrogen pressure is 3.2MPa-4.5MPa.

7. A device for implementing the method according to any one of claims 1 to 6, characterized in that: The invention mainly comprises: a crucible (6), an ultrasonic atomizer (7) and a powder collecting tank (1); the crucible (6) is embedded in the ultrasonic atomizer (7) and is closely connected with the ultrasonic atomizer (7) to form a unified whole; a nozzle (2) is arranged at the bottom of the crucible (6); and the powder collecting tank (1) is connected to the bottom of the nozzle (2); the powder collecting tank (1) comprises an inert gas protection zone (9) at the top and a high-pressure gas atomization zone (4) at the bottom, so that the small droplets atomized by the ultrasonic atomizer (7) shrink in the powder collecting tank (1) to form spherical small droplets and then rapidly cool to form spherical amorphous powder.

8. The device according to claim 7, characterized in that The nozzles (2) at the bottom of the crucible (6) are regularly arranged in the circumferential direction, and the aperture of the nozzles (2) is 0.3 mm-0.8 mm.

9. The device according to claim 7, characterized in that An integrally formed annular air inlet is formed around the outer wall of the high-pressure gas atomization area (4) at the lower part of the powder collecting tank (1), and the other end of the air inlet is connected to the high-pressure gas atomizer (3).

10. A spherical Fe-based amorphous powder prepared by the method according to any one of claims 1 to 6, characterized in that: The spherical Fe-based amorphous powder has an amorphous degree of more than 98%, a sphericity of more than 98%, D50 less than 15 μm, a powder yield greater than 65%, and an oxygen content less than 200 ppm.

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