CN114247887B - Preparation method of field emission micro-nano tungsten emitter - Google Patents
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B22F3/10—Sintering only
- B22F3/1017—Multiple heating or additional steps
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- B22F3/225—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip by injection molding
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- B22F9/00—Making metallic powder or suspensions thereof
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- B22F9/00—Making metallic powder or suspensions thereof
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- B22F9/04—Making metallic powder or suspensions thereof using physical processes starting from solid material, e.g. by crushing, grinding or milling
- B22F2009/044—Making metallic powder or suspensions thereof using physical processes starting from solid material, e.g. by crushing, grinding or milling by jet milling
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Abstract
A forming method of a field emission micro-nano tungsten emitter belongs to the technical field of powder metallurgy. Firstly, the powder state of the tungsten powder is improved by adopting one or more times of air flow milling treatment, and the nearly spherical tungsten powder particles with fine granularity, high dispersion and narrow distribution are obtained, thereby being beneficial to forming a more uniform open pore structure in the forming stage. And secondly, carrying out one or more times of calcination treatment on the treated powder to eliminate internal stress generated in the jet milling process. And uniformly mixing the powder and a binder to prepare a feed material, forming a tungsten blank with a required shape and size on micro-injection forming equipment, and finally preparing the field emission micro-nano tungsten emitter with uniform pores through degreasing and sintering. The invention obviously optimizes the raw material powder and micro powder injection molding process, and the prepared field emission micro-nano tungsten emitter has low impurity content, uniform pores, grain size of less than or equal to 1 mu m, pore diameter of 200-800 nm, porosity of 15-35 percent and open pore porosity of more than 95 percent of total porosity.
Description
Technical Field
The invention belongs to the technical field of powder metallurgy, and particularly relates to a method for forming a field emission micro-nano tungsten emitter.
Background
With the continuous development of modern small satellites, not only attitude and orbit control but also even orbital mobility is required. For this reason, it is necessary to develop a micro propulsion system corresponding thereto. The field emission electric propeller is a novel micro-Newton electric propulsion system, and has the characteristics of small thrust, large-range accuracy and adjustability, high specific impulse, high efficiency, low power consumption cost, compact structure, light weight and the like. The micro-nano tungsten emitter is an important component of the field emission electric propeller, can play a key role in storing and conveying the propellant, and influences the emission performance of the propeller. In order to realize the micro-Newton thrust and uniform emission performance of the propeller, the emitter needs to have fine grains, small pore diameter and good pore connectivity, and the performances are closely related to the appearance, granularity distribution and the like of raw material powder, but the agglomeration of fine-grained powder is serious, the granularity distribution is wide, the shape is irregular, so that the conditions of a large amount of closed pores, uneven pore size and distribution, poor pore shape and the like exist in the final emitter, and the problem becomes a bottleneck problem for restricting the emission performance of the propeller. In addition, the field emission micro-nano tungsten emitter has a complex shape structure, but metal tungsten has high hardness and large brittleness, the performance is sensitive to a processing state, and the micro-nano tungsten electrode with a required shape structure and size is difficult to prepare by adopting a traditional processing technology.
Powder micro-injection molding is a near-net-shape molding technology which introduces the modern plastic injection molding technology into the field of powder metallurgy, and is mainly characterized in that metal powder and a binder are mixed to form a feed material, then the feed material is subjected to injection molding by micro-injection molding equipment to form a green body, and finally the green body is degreased and sintered to obtain a required product. The method has the advantages of capability of directly preparing products with complex size, high precision and complex shape, excellent performance, high finished product rate, good product consistency and the like. The deagglomeration, crushing and refining and surface shaping of the powder can be realized by the jet milling treatment, and the loose packing, tap density and the like of the powder are improved. Chinese patent (CN 105499574A) discloses a method for preparing a special-shaped porous tungsten product with uniform pores, and the prepared porous tungsten product has a complex shape, a uniform pore structure and good pore connectivity. However, the aperture and the skeleton size of the prepared product are larger and respectively reach 1-3,3-6 μm, the grain size is overlarge, and the emitter tip can realize high-efficiency operation only when reaching 1 μm. The sharpness of the field emission micro-nano tungsten emitter tip has great influence on the quality efficiency and the starting voltage of the emitter, and a blunt emitter can cause low quality efficiency, low specific pulse and high starting voltage and cannot be used for a high-efficiency field emission electric propeller. Chinese patent (CN 101623760B) discloses an application of micro-injection molding technology in preparation of tungsten-based alloy products. The emphasis of this patent is on the addition of alloying elements to solve the problem of sinter densification and the use of microinjection molding techniques to solve the problem of micro-part formation, independent of the emitter preparation. Chinese patent (CN 102259189A) discloses a method for preparing a porous cathode substrate, but because powder raw materials are seriously agglomerated without treatment and have larger powder particle sizes, the prepared product has larger pores and uneven distribution, has more closed pores and is not suitable for the application of an emitter.
Therefore, the invention combines the micro-injection molding technology and the jet milling technology to develop a novel field emission micro-nano tungsten emitter preparation method, and can prepare the field emission micro-nano tungsten emitter with excellent pore characteristics, uniform tissue, complex shape, high size precision and good product consistency, wherein the grain size of the prepared emitter is less than or equal to 1 mu m, the aperture is 200-800 nm, and the requirement on the emitter can be completely met.
Disclosure of Invention
The invention aims to provide a method for forming a field emission micro-nano tungsten emitter.
A method for forming a field emission micro-nano tungsten emitter is characterized by comprising the following steps: taking tungsten powder as a raw material, and carrying out one or more times of dispersion grading treatment on the tungsten powder to obtain fine-grained tungsten powder with narrow particle size distribution and nearly spherical shape; and then preparing a field emission micro-nano tungsten emitter green body with a complex shape by powder micro-injection molding, and finally preparing a field emission micro-nano tungsten emitter part with a complex shape by degreasing and sintering, wherein the method comprises the following specific steps:
1. the raw material powder is tungsten powder, the purity is more than 99.9 percent, and the granularity is less than 1 mu m;
2. and (3) carrying out one or more times of air flow grinding treatment on the raw material tungsten powder by adopting a counter-jet type air flow grinding device. The purpose is to realize the crushing of powder agglomeration and the improvement of the sphericity of the powder surface so as to improve the loose packing, tap density and fluidity of the powder. The air flow milling process adopts high-purity nitrogen with the purity higher than 99.9 percent as a milling medium, the pressure in a milling cavity is 0.5 to 0.7MPa, and the frequency of a sorting wheel is 40 to 60Hz. Finally obtaining treated powder;
3. under the protection atmosphere of high-purity hydrogen with the purity of more than 99.9 percent, the treated powder is calcined for one time or more times within the temperature range of 300-550 ℃;
4. uniformly mixing the calcined powder and an organic binder, and preparing a granular feed;
5. according to different application scenes of the field emission electric thruster and different requirements on the shape and the size of an emitter, performing micro-injection molding on a tungsten blank with a required shape and size;
6. soaking the micro-injection molded blank in an organic solvent, and drying to remove part of the binder;
7. and (3) under the protective atmosphere of high-purity hydrogen with the purity of more than 99.9%, carrying out thermal de-sintering treatment on the sample dried in the step (6) in a tungsten filament sintering furnace, and preparing the field emission micro-nano tungsten emitter.
Further, the calcination treatment process in step 3 is as follows: the heating rate is 3-5 ℃/min, and the heat preservation time is 60-300 min.
Further, the proportion of the binder in the step 4 is as follows: 55-60% of microcrystalline wax, 3-10% of polyethylene wax, 10-15% of linear low-density polyethylene, 20-25% of polypropylene and 5-10% of stearic acid.
Further, the organic solvent adopted in the step 6 is trichloroethylene solution, and is soaked for 6-12 hours at the temperature of 40-60 ℃, so that a porous network system is ensured to be formed, and the decomposition and volatilization of the binder in the subsequent thermal desorption sintering process are facilitated.
Further, the heat removal sintering treatment process in the step 7 is to heat up to 400 ℃ at a rate of 0.5-1 ℃/min, preserve heat for 60-120 min, then heat up to 700 ℃ at a rate of 2-3 ℃/min, preserve heat for 60-120 min, heat up to 900 ℃ at a rate of 3-5 ℃/min, preserve heat for 60-120 min, heat up to 1000-1250 ℃ at a rate of 1-3 ℃/min, and preserve heat for 60-180 min.
Furthermore, the size of crystal grains of the sintered field emission micro-nano tungsten emitter is less than or equal to 1 mu m, the aperture is 200-800 nm, the porosity is 15-35%, the pores are uniform, and the connectivity is good.
By adopting the technical scheme, the invention has the beneficial effects that: (1) From the aspect of optimizing powder raw materials, the adopted raw material powder has the particle size of less than 1 mu m, fine particle size and serious powder agglomeration, so that the requirement of micro-injection molding on the raw material powder cannot be met, and the micro-nano electric thruster has the defects of more closed pores of micro-nano tungsten and uneven pores, thereby influencing the performance of the field emission electric thruster. The powder is treated to remove the agglomeration, grading and spheroidizing of powder particles, so that the loose tap density and the fluidity of the powder are improved, and the nearly spherical tungsten powder particles with fine granularity, high dispersion and narrow distribution are obtained. (2) The powder after the jet milling is calcined, so that the internal stress generated in the jet milling process can be eliminated, the powder activity is reduced, and the pore uniformity and the aperture ratio can be further improved in the subsequent sintering process. (3) Uniformly mixing the calcined powder and an organic binder, and preparing a granular feed; according to different application scenes of the field emission electric thruster and different requirements on the shape and the size of an emitter, a tungsten blank body with the required shape and size is formed by micro-injection; the micro-nano tungsten emitter is soaked in an organic solvent, dried, and subjected to thermal desorption sintering treatment in a tungsten filament sintering furnace in a segmented sintering and slow heating mode after partial binder is removed, so that the porosity of the micro-nano tungsten emitter can be accurately controlled. If the heating rate is too high, the emitter is deformed and cracked due to too high temperature, and waste products are directly caused, otherwise, the heating rate is too low, the heat preservation time is too short, and carbon residue is generated in the field emission micro-nano tungsten emitter, so that the generation performance of the emitter is influenced. (4) The powder micro-injection molding is suitable for the precise manufacturing of micro parts with micron-sized characteristic sizes, subsequent processing is not needed, the material utilization rate is high, and the field emission micro-nano tungsten emitter with the final shape can be directly prepared. (5) The prepared field emission micro-nano tungsten emitter has low impurity content, uniform pores, grain size of less than or equal to 1 mu m, pore diameter of 200-800 nm, porosity of 15-35 percent and open pore porosity of more than 95 percent of total porosity.
Drawings
FIG. 1 is an XRD of tungsten powder before and after jet milling treatment,
fig. 2 is a fracture SEM of the prepared field emission micro-nano tungsten emitter.
Detailed Description
Example 1
1) The raw material powder is tungsten powder with Fisher particle size of 0.3 mu m and purity of more than 99.9 percent;
2) Firstly, cleaning an air flow mill device, opening a main control preheating device for 10-30 min, charging high-purity nitrogen with the purity of more than 99.9%, starting a grinding valve, adjusting the grinding pressure to 0.5Mpa, adding raw material tungsten powder, feeding the tungsten powder into a grinding chamber for deagglomeration, adjusting the frequency of a sorting wheel to 40Hz, collecting all powder, and repeating the operation to obtain twice air flow mill processing powder;
3) Heating the powder subjected to the jet milling treatment to 300 ℃ at a speed of 5 ℃/min under a high-purity hydrogen atmosphere with the purity of more than 99.9 percent, preserving the temperature for 180min, and performing primary calcination treatment to eliminate the internal stress generated in the jet milling process;
4) Uniformly mixing the calcined powder with an organic binder (55% of microcrystalline wax, 3% of polyethylene wax, 10% of linear low-density polyethylene, 25% of polypropylene and 7% of stearic acid), and preparing into granular feed, wherein the volume fraction of the powder in the feed is 40%;
5) Feeding the granular material on a micro-injection molding machine, and performing micro-injection molding to obtain a tungsten blank with a required shape and size, wherein the injection parameters are that the injection temperature is 170 ℃, the mold temperature is 60 ℃, the injection pressure is 100MPa, and the pressure maintaining pressure is 50MPa;
6) The organic binder in the tungsten green body is completely removed by adopting a proper process, and the degreasing process is to soak the tungsten green body in a trichloroethylene solution at the temperature of 45 ℃ for 7 hours, so that a porous network system is ensured to be formed, and the decomposition and volatilization of the binder in the thermal degreasing process are facilitated; and then carrying out thermal desorption sintering in a tubular furnace under the protective atmosphere of high-purity hydrogen with the purity of more than 99.9 percent, wherein the temperature is increased to 400 ℃ at the speed of 0.5 ℃/min to avoid the deformation or cracking of a sample caused by the overhigh degreasing speed, the temperature is kept for 60min, then the temperature is increased to 700 ℃ at the speed of 2 ℃/min, the temperature is kept for 60min, then the temperature is increased to 900 ℃ at the speed of 3 ℃/min, the temperature is increased to 1200 ℃ at the speed of 2 ℃/min after the temperature is kept for 60min, and finally the prepared pure tungsten porous field emission emitter has low impurity content, uniform pores, the grain size of 0.5 mu m, the average pore diameter of 320nm, the open porosity of 18 percent and the open porosity of more than 95 percent of the total porosity.
Example 2
1) The raw material powder is tungsten powder, the Fisher size is 0.5 mu m, and the purity is more than 99.9 percent;
2) Firstly cleaning the jet mill equipment, opening the master control for preheating for 10-30 min, charging high-purity nitrogen with the purity of more than 99.9%, starting a grinding valve, adjusting the grinding pressure to 0.6Mpa, adding raw material tungsten powder, feeding the tungsten powder into a grinding chamber for deagglomeration, adjusting the frequency of a sorting wheel to 50Hz, collecting all powder, and then repeating the operation to obtain primary jet mill treated powder.
3) Heating the powder subjected to the jet milling treatment to 400 ℃ at a speed of 5 ℃/min under a high-purity hydrogen atmosphere with the purity of more than 99.9 percent, preserving the heat for 60min, and performing twice calcining treatment to eliminate the internal stress generated in the jet milling process;
4) Uniformly mixing the calcined powder with an organic binder (55% of microcrystalline wax, 6% of polyethylene wax, 10% of linear low density polyethylene, 24% of polypropylene and 5% of stearic acid), and preparing into granular feed, wherein the volume fraction of the powder in the feed is 46%;
5) Feeding the granular material on a micro-injection molding machine, and performing micro-injection molding to obtain a tungsten blank with a required shape and size, wherein the injection parameters are that the injection temperature is 175 ℃, the mold temperature is 70 ℃, the injection pressure is 110MPa, and the pressure maintaining pressure is 55MPa;
6) The organic binder in the tungsten green body is completely removed by adopting a proper process, and the degreasing process is to soak the tungsten green body in a trichloroethylene solution at the temperature of 60 ℃ for 6 hours, so that a porous network system is ensured to be formed, and the decomposition and volatilization of the binder in the thermal degreasing process are facilitated; and then carrying out thermal desorption sintering in a tube furnace under the protective atmosphere of high-purity hydrogen with the purity of more than 99.9 percent, wherein the temperature is raised to 400 ℃ at the speed of 1 ℃/min to avoid deformation or cracking of a sample caused by overhigh degreasing speed, the temperature is kept for 60min, then the temperature is raised to 700 ℃ at the speed of 3 ℃/min, the temperature is kept for 70min, then the temperature is raised to 900 ℃ at the speed of 5 ℃/min, the temperature is kept for 60min, the temperature is raised to 1250 ℃ at the speed of 3 ℃/min, the temperature is kept for 60min, and finally the prepared pure tungsten porous field emission emitter has low impurity content, uniform pores, the grain size of 0.7 mu m, the average pore diameter of 430nm, the porosity of pores of 21 percent and the porosity of pores of more than 96 percent of the total porosity.
Example 3
1) The raw material powder is tungsten powder, the Fisher size is 0.8 mu m, and the purity is more than 99.9%;
2) Firstly, cleaning the jet mill equipment, opening a master control for preheating for 10-30 min, charging high-purity nitrogen with the purity of more than 99.9%, starting a grinding valve, adjusting the grinding pressure to be 0.7Mpa, adding raw material tungsten powder, feeding the tungsten powder into a grinding chamber for deagglomeration, adjusting the frequency of a sorting wheel to be 60Hz, collecting all powder, and repeating the operation to obtain powder treated by the jet mill for three times;
3) Heating the powder subjected to the jet milling treatment to 500 ℃ at a speed of 5 ℃/min under a high-purity hydrogen atmosphere with the purity of more than 99.9 percent, preserving the heat for 100min, and carrying out three times of calcination treatment to eliminate the internal stress generated in the jet milling process;
4) Uniformly mixing the calcined powder with an organic binder (60% of microcrystalline wax, 3% of polyethylene wax, 10% of linear low-density polyethylene, 22% of polypropylene and 5% of stearic acid), and preparing into granular feed, wherein the volume fraction of the powder in the feed is 50%;
5) Feeding the granular material on a micro-injection molding machine, and performing micro-injection molding to obtain a tungsten blank with a required shape and size, wherein the injection parameters are that the injection temperature is 180 ℃, the mold temperature is 80 ℃, the injection pressure is 120MPa, and the pressure maintaining pressure is 60MPa;
6) The organic binder in the tungsten green body is completely removed by adopting a proper process, and the degreasing process is to soak the tungsten green body in a trichloroethylene solution at 50 ℃ for 10 hours, so that a porous network system is ensured to be formed, and the decomposition and volatilization of the binder in the thermal degreasing process are facilitated; and then carrying out thermal desorption sintering in a tubular furnace under the protective atmosphere of high-purity hydrogen with the purity of more than 99.9 percent, wherein the temperature is raised to 400 ℃ at the speed of 1 ℃/min to avoid deformation or cracking of a sample caused by overhigh degreasing speed, the temperature is kept for 60min, then the temperature is raised to 700 ℃ at the speed of 3 ℃/min, the temperature is kept for 60min, then the temperature is raised to 900 ℃ at the speed of 5 ℃/min, the temperature is kept for 60min, the temperature is raised to 1250 ℃ at the speed of 3 ℃/min, the temperature is kept for 1800min, and finally the prepared pure tungsten porous field emission emitter has low impurity content, uniform pores, the grain size of 0.9 mu m, the average pore diameter of 450nm, the open porosity of 23 percent and the open porosity of more than 98 percent of the total porosity.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (4)
1. A forming method of a field emission micro-nano tungsten emitter is characterized in that the field emission micro-nano tungsten emitter is prepared by a method combining jet milling treatment and micro injection forming, and the method comprises the following specific steps:
1) The raw material powder is tungsten powder, the purity is more than 99.9 percent, and the granularity is less than 1 mu m;
2) Carrying out one or more times of air flow grinding treatment on the raw material tungsten powder by adopting a counter-jet type air flow grinding device; the purpose is to realize the crushing of powder agglomeration and the improvement of the sphericity of the powder surface so as to improve the loose filling, tap density and fluidity of the powder; in the process of the jet mill, high-purity nitrogen with the purity higher than 99.9 percent is used as a grinding medium, the air pressure in a grinding cavity is 0.5 to 0.7MPa, and the frequency of a sorting wheel is 40 to 60Hz; finally obtaining treated powder;
3) Under the protection atmosphere of high-purity hydrogen with the purity of more than 99.9 percent, the treated powder is calcined for one time or more times within the temperature range of 300-550 ℃;
4) Uniformly mixing the calcined powder and an organic binder, and preparing a granular feed;
the proportion of the binder in the step 4) is as follows: 55-60% of microcrystalline wax, 3-10% of polyethylene wax, 10-15% of linear low density polyethylene, 20-25% of polypropylene and 5-10% of stearic acid;
5) According to different application scenes of the field emission electric thruster and different requirements on the shape and the size of an emitter, performing micro-injection molding on a tungsten blank with a required shape and size;
6) Soaking the micro-injection molding blank in an organic solvent, and drying to remove part of the binder;
7) Carrying out thermal de-sintering treatment on the dried sample in the step 6) in a tungsten filament sintering furnace under the protective atmosphere of high-purity hydrogen with the purity of more than 99.9% to prepare a field emission micro-nano tungsten emitter;
the heat removal sintering treatment process in the step 7) is to heat up to 400 ℃ at a speed of 0.5-1 ℃/min, preserve heat for 60-120 min, then heat up to 700 ℃ at a speed of 2-3 ℃/min, preserve heat for 60-120 min, heat up to 900 ℃ at a speed of 3-5 ℃/min, preserve heat for 60-120 min, heat up to 1000-1250 ℃ at a speed of 1-3 ℃/min, and preserve heat for 60-180 min.
2. The method for forming a field emission micro-nano tungsten emitter according to claim 1, wherein the calcining treatment process in the step 3) is as follows: the heating rate is 3-5 ℃/min, and the heat preservation time is 60-300 min.
3. The method for forming a field emission micro-nano tungsten emitter according to claim 1, wherein the organic solvent used in the step 6) is a trichloroethylene solution, and the solution is soaked for 6 to 12 hours at a temperature of 40 to 60 ℃ to ensure that a porous network system is formed, thereby facilitating the decomposition and volatilization of the binder in the subsequent thermal desorption sintering process.
4. The method for forming a field emission micro-nano tungsten emitter according to claim 1, wherein the size of crystal grains of the sintered field emission micro-nano tungsten emitter is less than or equal to 1 μm, the aperture is 200-800 nm, the porosity is 15-35%, the pores are uniform, and the connectivity is good.
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| CN102485380A (en) * | 2010-12-06 | 2012-06-06 | 北京有色金属研究总院 | Preparation method of submicron nearly spherical tungsten powder |
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| CN105382255A (en) * | 2015-12-14 | 2016-03-09 | 北京科技大学 | Nano tungsten powder injection molding method |
| CN105478776A (en) * | 2015-12-14 | 2016-04-13 | 北京科技大学 | Method for preparing high-density pure tungsten product through low-temperature sintering |
| CN105499574A (en) * | 2015-12-16 | 2016-04-20 | 北京科技大学 | Method for preparing specially-shaped porous tungsten product with uniform pores |
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Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102485380A (en) * | 2010-12-06 | 2012-06-06 | 北京有色金属研究总院 | Preparation method of submicron nearly spherical tungsten powder |
| JP2015078423A (en) * | 2013-10-16 | 2015-04-23 | サムソン エレクトロ−メカニックス カンパニーリミテッド． | Nickel nano powder and production method thereof |
| CN105382255A (en) * | 2015-12-14 | 2016-03-09 | 北京科技大学 | Nano tungsten powder injection molding method |
| CN105478776A (en) * | 2015-12-14 | 2016-04-13 | 北京科技大学 | Method for preparing high-density pure tungsten product through low-temperature sintering |
| CN105499574A (en) * | 2015-12-16 | 2016-04-20 | 北京科技大学 | Method for preparing specially-shaped porous tungsten product with uniform pores |
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