CN114247887B - A preparation method of field emission micro-nano tungsten emitter - Google Patents

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

A preparation method of field emission micro-nano tungsten emitter

technical field

The invention belongs to the technical field of powder metallurgy, and in particular relates to a forming method of a field emission micro-nano tungsten emitter.

Background technique

With the continuous development of modern small satellites, not only attitude and orbit control are required, but even orbital maneuvering capabilities are required. For this reason, it is necessary to develop and research its corresponding micro propulsion system. The field emission electric thruster is a new type of microbull-level electric propulsion system, which has the characteristics of small thrust and wide range of precise adjustment, high specific impulse, high efficiency, low power consumption and cost, compact structure, and light weight. The micro-nano tungsten emitter is an essential part of the field emission electric thruster, which can play a key role in storing and transporting the propellant, and affects the launch performance of the thruster. To achieve micro-new level thrust and uniform emission performance of the thruster, the emitter needs to have fine grains, small pore size and good pore connectivity, and these properties are closely related to the morphology, particle size and particle size distribution of the raw material powder. However, the serious agglomeration of fine-grained powder, wide particle size distribution, and irregular shape will lead to a large number of closed cells, uneven pore size and distribution, and poor pore shape in the final emitter. This has become a bottleneck problem restricting the launch performance of the thruster. . In addition, the shape and structure of field emission micro-nano tungsten emitters are complex, but metal tungsten has high hardness, high brittleness, and performance is sensitive to processing conditions.

Powder micro-injection molding is a near-net-shaping technology that introduces modern plastic injection molding technology into the field of powder metallurgy. It mainly mixes metal powder and binder to form feed, and then injects it into a green body through micro-injection molding equipment. , and finally degreasing and sintering to obtain the desired product. It has the advantages of being able to directly prepare complex, high-dimensional precision, and complex-shaped products, and has the advantages of excellent performance, high yield, and good product consistency. The jet mill treatment can realize the deagglomeration, crushing and refinement of the powder, surface shaping, and improve the loose packing and tap density of the powder. Chinese patent (CN105499574A) discloses a method for preparing heterogeneous porous tungsten products with uniform pores. The prepared porous tungsten products have complex shapes, uniform pore structure and good pore connectivity. However, the pore size and skeleton size of the products prepared by it are relatively large, reaching 1-3, 3-6 μm respectively, and the grain size is too large, and the tip of the emitter needs to reach 1 μm to achieve efficient operation. The sharpness of the field emission micro-nano tungsten emitter tip has a great influence on the mass efficiency and starting voltage of the emitter. A blunt emitter will cause low mass efficiency and low specific pulse and high starting voltage, which cannot be used for high-efficiency field emission electrodes. propeller. Chinese patent (CN101623760B) discloses an application of micro-injection molding technology in the preparation of tungsten-based alloy products. The focus of this patent is to add alloying elements to solve the problem of sintering densification and use micro-injection molding technology to solve the forming problem of micro-parts, which has nothing to do with the preparation of the emitter. Chinese patent (CN102259189A) discloses a method for preparing a porous cathode substrate. However, due to the severe agglomeration of untreated powder raw materials and the large particle size of the powder, the resulting product has large pores and uneven distribution, and there are many closed pores. Suitable for emitter applications.

Therefore, the present invention combines the two technologies of micro-injection molding and jet milling to develop a new field emission micro-nano tungsten emitter preparation method, which can produce products with excellent pore characteristics, uniform structure, complex shape, high dimensional accuracy and The field emission micro-nano tungsten emitter with good consistency, the prepared emitter grain size is ≤1μm, and the aperture is 200-800nm, which can fully meet the requirements for the emitter.

Contents of the invention

The purpose of the present invention is to provide a method for forming a field emission micro-nano tungsten emitter.

A method for forming a field emission micro-nano tungsten emitter, which is characterized in that: using tungsten powder as a raw material, it is subjected to one or more dispersion and classification treatments to obtain fine-grained raw material tungsten powder with narrow particle size distribution and nearly spherical shape; The field emission micro-nano tungsten emitter green body of complex shape is prepared by micro-injection molding, and finally the complex-shaped field emission micro-nano tungsten emitter parts are prepared by degreasing and sintering. The specific steps are as follows:

1. The raw material powder is tungsten powder, the purity is greater than 99.9%, and the particle size is less than 1 μm;

2. Use the counter-jet jet milling device to process the raw material tungsten powder once or more times. The purpose is to realize the crushing of powder agglomeration and the improvement of powder surface sphericity, so as to improve the loose packing, tap density and fluidity of powder. The jet mill process uses high-purity nitrogen with a purity higher than 99.9% as the grinding medium, the pressure in the grinding chamber is 0.5-0.7 MPa, and the frequency of the sorting wheel is 40-60 Hz. Finally, the processed powder is obtained;

3. Under the protective atmosphere of high-purity hydrogen with a purity greater than 99.9%, the processed powder is calcined once or several times in the temperature range of 300°C to 550°C;

4. Mix the calcined powder and organic binder evenly, and make granular feed;

5. According to the different application scenarios of field emission electric thrusters, the requirements for the shape and size of the emitter are different, and the tungsten body of the shape and size required for micro-injection molding;

6. Soak the micro-injection molding body in an organic solvent and then dry it to remove part of the binder;

7. Under the protective atmosphere of high-purity hydrogen with a purity greater than 99.9%, the sample dried in step 6 is subjected to thermal desintering treatment in a tungsten wire sintering furnace to prepare a field emission micro-nano tungsten emitter.

Further, the calcination process in step 3 is as follows: the heating rate is 3-5° C./min, and the holding time is 60-300 min.

Further, the ratio of the binder in step 4 is: microcrystalline wax is 55% to 60%, polyethylene wax is 3% to 10%, linear low density polyethylene is 10% to 15%, polypropylene is 20% to 25% and 5% to 10% stearic acid.

Further, the organic solvent used in step 6 is a trichlorethylene solution, soaked at a temperature of 40-60°C for 6-12 hours to ensure the formation of a porous network system, which is conducive to the decomposition of the binder in the subsequent thermal desintering process and volatile.

Further, the thermal desintering process in step 7 is to raise the temperature to 400°C at 0.5-1°C/min, keep it warm for 60-120min, then raise the temperature to 700°C at 2-3°C/min, hold it for 60-120min, and then heat it at 3°C/min to 700°C. Raise the temperature to 900°C at ~5°C/min, keep it warm for 60-120 minutes, then raise the temperature to 1000-1250°C at 1-3°C/min, and keep it warm for 60-180 minutes.

Further, the sintered field emission micro-nano tungsten emitter has a grain size of ≤1 μm, a pore diameter of 200-800 nm, a porosity of 15-35%, uniform pores and good connectivity.

By adopting the aforementioned technical scheme, the beneficial effects of the present invention are as follows: (1) From the perspective of optimizing powder raw materials, the present invention cannot meet the requirements of microinjection molding because the particle size of the raw material powder used is less than 1 μm, the particle size is small and the powder agglomeration is serious. The demand for raw material powder will cause many closed cells and uneven pores in the micro-nano tungsten electrode, which will affect the performance of the field emission electric thruster. By treating the powder, the agglomeration, classification and spheroidization of the powder particles can be released, the loose packing density and fluidity of the powder can be improved, and fine-grained, highly dispersed, narrowly distributed nearly spherical tungsten powder particles can be obtained. (2) Calcining the jet-milled powder can eliminate the internal stress generated during the jet-milling process, reduce the activity of the powder, and further improve the pore uniformity and porosity in the subsequent sintering process. (3) Mix the calcined powder with an organic binder evenly, and make granular feed; according to different application scenarios of field emission electric propulsion, the requirements for the shape and size of the emitter are different, and the shape required for micro-injection molding The tungsten body of the same size; soaked in an organic solvent and then dried, after removing part of the binder, the thermal desintering process is carried out in a tungsten wire sintering furnace by segmental sintering and slow temperature rise, which can realize the Precise control of the porosity of micro-nano tungsten emitters. And if the heating rate is too fast, the emitter will be deformed and cracked if the temperature is too high, which will directly cause waste products. On the contrary, if the heating rate is too slow and the holding time is too short, carbon residue will be generated in the prepared field emission micro-nano tungsten emitter, which will affect the emission. Extreme performance. (4) Powder micro-injection molding is suitable for precision manufacturing of micro-parts with characteristic dimensions of micron scale, without subsequent processing, high material utilization rate, and can directly prepare field emission micro-nano tungsten emitters in the final shape. (5) The prepared field emission micro-nano tungsten emitter has low impurity content, uniform pores, grain size ≤1 μm, pore diameter of 200-800nm, porosity of 15-35%, and open porosity accounting for more than 95% of the total porosity .

Description of drawings

Figure 1 is the XRD of tungsten powder before and after jet milling,

Figure 2 is a fractured SEM of the prepared field emission micro-nano tungsten emitter.

Detailed ways

Example 1

1) The raw material powder is tungsten powder with a Fischer particle size of 0.3 μm and a purity greater than 99.9%;

2) First clean the jet mill equipment, turn on the main control to preheat for 10-30 minutes, fill in high-purity nitrogen with a purity greater than 99.9%, start the grinding valve, adjust the grinding pressure to 0.5Mpa, add raw tungsten powder into the grinding chamber to deagglomerate, and Adjust the frequency of the sorting wheel to 40Hz. After collecting all the powder, repeat the above operation to obtain the jet milled powder twice;

3) The jet-milled powder is heated up to 300°C at 5°C/min in an atmosphere of high-purity hydrogen with a purity greater than 99.9%, kept for 180min, and then calcined once to eliminate the internal stress generated during the jet-milling process;

4) The calcined powder is mixed with an organic binder (microcrystalline wax is 55%, polyethylene wax is 3%, linear low density polyethylene is 10%, polypropylene is 25% and stearic acid is 7%) Evenly refined, and made into granular feed, the volume fraction of powder in the feed is 40%;

5) Feed the granular material on the micro-injection molding machine, and micro-inject the tungsten body of the desired shape and size. The injection parameters are as follows: injection temperature is 170°C, mold temperature is 60°C, injection pressure is 100MPa, holding pressure 50MPa;

6) Remove the organic binder in the tungsten body by proper process. The degreasing process is soaking in trichlorethylene solution at 45°C for 7 hours to ensure the formation of a porous network system, which is conducive to the thermal degreasing process. Decomposition and volatilization of the caking agent; then thermal desintering is carried out in a tube furnace under a protective atmosphere of high-purity hydrogen with a purity greater than 99.9%. The process is to raise the temperature to 400°C at 0.5°C/min to avoid excessive degreasing speed and cause the sample Deformation or cracking, keep warm for 60 minutes, then raise the temperature at 2°C/min to 700°C, keep warm for 60 minutes, then raise the temperature to 900°C at 3°C/min, keep warm for 60 minutes, raise the temperature to 1200°C at 2°C/min, keep warm for 60 minutes, and finally The prepared pure tungsten porous field emission emitter has low impurity content, uniform pores, a grain size of 0.5 μm, an average pore diameter of 320 nm, an open porosity of 18%, and the open porosity accounts for more than 95% of the total porosity.

Example 2

1) The raw material powder is tungsten powder, the Fischer particle size is 0.5 μm, and the purity is greater than 99.9%;

2) First clean the jet mill equipment, turn on the main control to preheat for 10-30 minutes, fill in high-purity nitrogen with a purity greater than 99.9%, start the grinding valve, adjust the grinding pressure to 0.6Mpa, add raw tungsten powder into the grinding chamber to deagglomerate, and Adjust the frequency of the sorting wheel to 50Hz. After collecting all the powder, repeat the above operation to obtain a jet mill treatment powder.

3) The jet milled powder is heated up to 400°C at a rate of 5°C/min in a high-purity hydrogen atmosphere with a purity greater than 99.9%, and kept for 60 minutes, and then calcined twice to eliminate the internal stress generated during the jet milling process ;

4) The calcined powder is mixed with an organic binder (microcrystalline wax is 55%, polyethylene wax is 6%, linear low density polyethylene is 10%, polypropylene is 24% and stearic acid is 5%) Evenly refined, and made into granular feed, the volume fraction of powder in the feed is 46%;

5) Feed the granular material on the micro-injection molding machine, and micro-inject the tungsten body of the required shape and size. The injection parameters are as follows: injection temperature is 175°C, mold temperature is 70°C, injection pressure is 110MPa, holding pressure is 55MPa;

6) Remove the organic binder in the tungsten body by proper process. The degreasing process is soaking in trichlorethylene solution at 60°C for 6 hours to ensure the formation of a porous network system, which is helpful for sticking during thermal degreasing. Decomposition and volatilization of the caking agent; then thermal desintering is carried out in a tube furnace under a protective atmosphere of high-purity hydrogen with a purity greater than 99.9%. The process is to raise the temperature to 400°C at 1°C/min to avoid excessive degreasing speed and cause the sample Deformation or cracking, keep warm for 60 minutes, then raise the temperature to 700°C at 3°C/min, keep hold for 70min, then raise the temperature to 900°C at 5°C/min, keep hold for 60min, raise the temperature to 1250°C at 3°C/min, keep hold for 60min, and finally prepare The pure tungsten porous field emission emitter has low impurity content, uniform pores, grain size of 0.7μm, average pore diameter of 430nm, open porosity of 21%, and open porosity accounting for more than 96% of the total porosity.

Example 3

1) The raw material powder is tungsten powder, the Fischer particle size is 0.8μm, and the purity is greater than 99.9%;

2) First clean the jet mill equipment, turn on the main control to preheat for 10 to 30 minutes, fill in high-purity nitrogen with a purity greater than 99.9%, start the grinding valve, adjust the grinding pressure to 0.7Mpa, add raw tungsten powder into the grinding chamber to deagglomerate, and Adjust the frequency of the sorting wheel to 60Hz. After collecting all the powder, repeat the above operation to obtain three jet milling powders;

3) The jet-milled powder is heated up to 500°C at 5°C/min in a high-purity hydrogen atmosphere with a purity greater than 99.9%, kept at a temperature of 100min, and calcined three times to eliminate the internal stress generated during the jet-milling process;

4) The calcined powder is mixed with an organic binder (microcrystalline wax is 60%, polyethylene wax is 3%, linear low density polyethylene is 10%, polypropylene is 22% and stearic acid is 5%) Evenly refined, and made into granular feed, the volume fraction of powder in the feed is 50%;

5) Feed the granular material on the micro-injection molding machine, and micro-inject the tungsten body of the desired shape and size. The injection parameters are as follows: the injection temperature is 180°C, the mold temperature is 80°C, the injection pressure is 120MPa, and the holding pressure is 60MPa;

6) The organic binder in the tungsten body is removed by an appropriate process. The degreasing process is soaking in a trichlorethylene solution at 50°C for 10 hours to ensure the formation of a porous network system, which is conducive to the thermal degreasing process. Decomposition and volatilization of the caking agent; then thermal desintering is carried out in a tube furnace under a protective atmosphere of high-purity hydrogen with a purity greater than 99.9%. The process is to raise the temperature to 400°C at 1°C/min to avoid excessive degreasing speed and cause the sample Deformation or cracking, keep warm for 60 minutes, then raise the temperature to 700°C at 3°C/min, keep hold for 60min, then raise the temperature to 900°C at 5°C/min, keep hold for 60min, raise the temperature to 1250°C at 3°C/min, keep hold for 1800min, and finally prepare The pure tungsten porous field emission emitter has low impurity content, uniform pores, grain size of 0.9 μm, average pore diameter of 450nm, open porosity of 23%, and open porosity accounting for more than 98% of the total porosity.

The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this field Those skilled in the art, without departing from the scope of the technical solution of the present invention, may use the method and technical content disclosed above to make some changes or modifications to equivalent embodiments with equivalent changes, but if they do not depart from the technical solution of the present invention, Any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solutions of the present invention.

Claims (4)

1. A forming method for a field emission micro-nano tungsten emitter, characterized in that the field emission micro-nano tungsten emitter is prepared by combining jet mill processing and micro-injection molding, and the specific steps are: 1) The raw material powder is tungsten powder, the purity is greater than 99.9%, and the particle size is less than 1 μm; 2) Using the counter-jet jet milling device, the raw material tungsten powder is subjected to one or more jet milling treatments; the purpose is to achieve 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; the jet mill process uses high-purity nitrogen with a purity higher than 99.9% as the grinding medium, the air pressure in the grinding chamber is 0.5-0.7MPa, and the frequency of the sorting wheel is 40-60Hz; finally the processed powder is obtained; 3) Under the protective atmosphere of high-purity hydrogen with a purity greater than 99.9%, the processed powder is calcined once or more times in the temperature range of 300°C to 550°C; 4) Mix the calcined powder and organic binder evenly, and make granular feed; The ratio of the binder in step 4) is: 55%-60% for microcrystalline wax, 3%-10% for polyethylene wax, 10%-15% for linear low-density polyethylene, and 20% for polypropylene ～25% and stearic acid 5%～10%; 5) According to different application scenarios of field emission electric propulsion, the requirements for the shape and size of the emitter are different, and the shape and size of the tungsten body required for micro-injection molding; 6) Soak the microinjection molded body in an organic solvent and then dry it to remove part of the binder; 7) Under the protective atmosphere of high-purity hydrogen with a purity greater than 99.9%, the sample dried in step 6) is subjected to thermal desintering treatment in a tungsten wire sintering furnace to prepare a field emission micro-nano tungsten emitter; Step 7) The medium-heat desintering treatment process is to raise the temperature to 400°C at 0.5-1°C/min, keep it warm for 60-120min, then raise the temperature to 700°C at 2-3°C/min, keep it warm for 60-120min, and then heat it at 3-5°C/min. ℃/min to 900°C, keep warm for 60-120min, then raise the temperature to 1000-1250°C at 1-3°C/min, keep warm for 60-180min. 2. The forming method of field emission micro-nano tungsten emitter according to claim 1, characterized in that the calcination process in step 3) is as follows: the heating rate is 3-5°C/min, and the holding time is 60-300min. 3. The forming method of field emission micro-nano tungsten emitter as claimed in claim 1, characterized in that the organic solvent adopted in step 6) is a trichlorethylene solution, soaked at 40-60°C for 6-12h to ensure The formation of a porous network system is conducive to the decomposition and volatilization of the binder during the subsequent thermal desintering process. 4. The forming method of field emission micro-nano tungsten emitter as claimed in claim 1, characterized in that the sintered field emission micro-nano tungsten emitter has a grain size of ≤1 μm, an aperture of 200-800 nm, and a porosity of 15-35%. , uniform pores and good connectivity.

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