Preparation method of porous tungsten sponge body of barium tungsten cathode

Abstract

The invention relates to the field of microwave electronic vacuum, and discloses a powder injection molding preparation method of a barium-tungsten cathode porous tungsten sponge body, which comprises the following steps: S1, mixing tungsten powder and a binder according to a required proportion, and granulating to prepare a feed for injection molding; s2, the feed is added into an injection machine for injection molding, and an injection blank is prepared; s3, the blank is subjected to solvent degreasing or catalytic degreasing; s4, the degreased green body is subjected to thermal degreasing and presintering treatment; and S5, the pre-sintered porous tungsten is placed in a vacuum sintering furnace or an atmosphere protection sintering furnace for high-temperature sintering. A metal powder injection molding technology is adopted, and a near-net forming process is introduced into preparation of the barium-tungsten cathode porous tungsten sponge body, so that the utilization rate of raw materials is increased; and the porous tungsten sponge body prepared by the method has few closed pores and uniform pores, so that the emission performance and emission stability of the barium-tungsten cathode can be remarkably improved, and the service life of the barium-tungsten cathode can be prolonged.

Description

Technical Field

[0001] The present invention relates to the field of microwave electronic vacuum, and specifically to a method for preparing a porous tungsten sponge of a barium tungsten cathode. Background Art

[0002] Vacuum electronic devices are widely used in broadcasting, radar, communications and other fields. As the core component of vacuum electronic devices, the performance of barium tungsten cathode (electron emission uniformity, emission current density, working life, etc.) has a direct impact on the performance of vacuum electronic devices. With the development of vacuum electronic devices towards high power, high frequency, and long life, higher requirements are also placed on the performance of barium tungsten cathodes. The characteristics of tungsten powder and the preparation technology of porous tungsten matrix are the main factors affecting the microstructure of porous tungsten matrix. Improving the electron emission density of barium tungsten cathodes and extending their life have always been the research focus of cathode materials.

[0003] The porous tungsten matrix serves as a "reserve room" for active substances. Active substances can migrate and diffuse to the cathode surface through its internal interconnected pores, which plays a role in storing and emitting active substances. It is the key "pillar" of high-performance barium tungsten cathodes. The internal organizational structure of the porous tungsten matrix, such as its internal porosity, pore size, pore uniformity and pore morphology, will have a direct impact on the electron emission performance and life of the barium tungsten cathode. Therefore, the study of porous tungsten matrix with uniform and controllable pores for barium tungsten cathodes has important scientific significance and application value.

[0004] The traditional method of preparing porous tungsten sponge for barium tungsten cathode mostly adopts the method of compression sintering. The porous tungsten sponge prepared by this method has a large number of closed pores and the sample shape is simple. Since porous tungsten sponge is a brittle material and is not easy to process, in order to prepare porous tungsten sponge of different shapes, the tungsten sponge is generally subjected to high-temperature copper infiltration treatment, and then the copper-infiltrated blank is machined to obtain the desired shape. Finally, the copper in the pores is removed by chemical corrosion and high-temperature volatilization to obtain a porous tungsten sponge of the desired shape. The porous tungsten sponge prepared by this method is often difficult to completely remove the copper in the pores. The remaining copper will contaminate the subsequent impregnation of active salts, thereby affecting the emission performance of the hot cathode and affecting the service life of the hot cathode. Summary of the invention

[0005] In order to solve the technical problems raised in the background technology, the present invention provides a method for preparing a porous tungsten sponge body of a barium tungsten cathode by using a powder injection molding process.

[0006] The present invention is implemented by the following technical scheme: The present invention specifically provides a method for preparing a porous tungsten sponge of a barium tungsten cathode by a powder injection molding process. The method mainly adopts an injection molding process, firstly, tungsten powder and a binder are mixed in a certain proportion to prepare a feed, degrease, and pre-sinter, so as to obtain a porous tungsten skeleton of a desired shape and a certain strength, and then the pre-sintered porous tungsten skeleton is placed in a high-temperature vacuum sintering furnace for sintering, and finally a tungsten sponge with a porosity between 20-30% and a closed porosity of less than 1% is obtained.

[0007] The specific implementation method of the present invention is as follows:

[0008] A method for preparing a barium tungsten cathode porous tungsten sponge by powder injection molding, the preparation method comprising the following steps:

[0009] Step S1: tungsten powder and wax-based binder or plastic-based binder are mixed and granulated in a required proportion to prepare wax-based feed or plastic-based feed for injection molding;

[0010] Step S2: feeding the feed into the injection molding machine for injection molding to prepare the injection blank;

[0011] Step S3: removing the binder in the injection blank by solvent degreasing or catalytic degreasing;

[0012] Step S4: thermally debinding and pre-sintering the debinded green body;

[0013] Step S5: placing the pre-sintered porous tungsten in a vacuum sintering furnace or an atmosphere protection sintering furnace for high-temperature sintering.

[0014] Preferably, the tungsten powder has a particle size of 5-10 μm, and the tungsten powder is ball milled to obtain high-energy tungsten powder.

[0015] Preferably, the mass proportions of the components in the wax-based adhesive are: 50-70% paraffin, 10-30% high-density polyethylene, 10-20% polypropylene, 1-10% antioxidant, and 1-4% cosolvent.

[0016] Preferably, the mass proportions of the components in the plastic-based adhesive are: 60-90% copolymer formaldehyde, 5-10% heat-resistant polystyrene, 1-5% compatibilizer, and 1-5% antioxidant.

[0017] Preferably, the degreasing solvent used in the solvent degreasing in step S3 is n-propane bromide, the degreasing time is 12-24 hours, and the degreasing temperature is 40-60°C.

[0018] Preferably, the catalytic degreasing in step S3 is performed using a catalytic degreasing furnace, the catalyst is oxalic acid, the degreasing temperature is 100-150° C., and the time is 8-12 hours.

[0019] Preferably, the degreasing temperature of the thermal degreasing in step S4 is 200-300°C, 400-500°C, 500-600°C, respectively, and the temperature is kept for 1-2 hours, and the residual binder is removed by graded degreasing;

[0020] The pre-sintering temperature is 1200-1400°C and the sintering time is 2-4 hours, so that the blank has a certain strength;

[0021] Both thermal debinding and pre-sintering use a molybdenum wire sintering furnace and sintering in a hydrogen environment.

[0022] Preferably, the sintering temperature is 1700-1900° C., and the sintering time is 1-2 hours.

[0023] Through the above scheme, the barium tungsten cathode porous tungsten sponge prepared by the present invention is finally tested by a mercury intrusion instrument, and the pores are uniform, the closed porosity is low, and the porosity is 20-30%.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention adopts metal powder injection molding technology and introduces near-net forming technology into the preparation of porous tungsten sponge of barium tungsten cathode, thereby improving the utilization rate of raw materials; and the porous tungsten sponge prepared by this method has fewer closed pores and uniform pores, which can significantly improve the emission performance and emission stability of the barium tungsten cathode and extend the service life of the barium tungsten cathode; in addition, the injection molding process can customize different molds according to the size and shape of the parts, and the required parts can be prepared by the corresponding molds, which can avoid machining or only require a small amount of machining, greatly reducing the production cost and improving production efficiency and material utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The process flow chart of the preparation method proposed by the present invention;

[0027] Figure 2 The fracture microstructure of the tungsten sponge prepared in Example 1;

[0028] Figure 3 This is the fracture microstructure of the tungsten sponge prepared in Example 2. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form a new embodiment.

[0030] Embodiment 1:

[0031] The present embodiment provides a method for preparing a barium tungsten cathode porous tungsten sponge by powder injection molding, comprising the following steps:

[0032] Step S1: Tungsten powder processing method: ball milling commercial tungsten powder with a Fisher particle size of about 10 μm to obtain high-energy tungsten powder.

[0033] The tungsten powder processed by this method is nearly spherical in shape, the powder particles are dispersed, and the tap density is high.

[0034] Step S2: Preparation of tungsten blank: In this case, high-energy tungsten powder after ball milling is used, and the tungsten powder is mixed with a wax-based binder to prepare a wax-based feed, which is then injected on an injection molding machine to obtain a tungsten blank.

[0035] The detailed preparation method is as follows:

[0036] Step S2-1: Preparation of wax-based feedstock: first add a certain amount of tungsten powder into the mixer, preheat and stir at 180°C for 10-20 minutes, then add high melting point binder polypropylene and high density polyethylene into the mixer, pressurize and mix at 180°C for 30-50 minutes to fully melt the high melting point binder and mix it with the tungsten powder; then add low melting point binder: paraffin, co-solvent, antioxidant into the mixer and lower the temperature to 160°C, pressurize and mix for 30-40 minutes, after the binder and tungsten powder are fully mixed, take out from the mixer, cool and crush to obtain the required wax-based feedstock.

[0037] Step S2-2: Preparation of tungsten blank: Add the crushed wax-based feed material into the injection molding machine, and perform injection molding on the feed material to obtain the required tungsten blank parts.

[0038] Among them, the injection pressure of the injection machine is 30-70Bar, the holding pressure is 40Bar, the injection temperature is 160-170℃, and the mold temperature controller temperature is 40-60℃.

[0039] Step S3: solvent degreasing and thermal degreasing: the injection molded green body is subjected to solvent degreasing to remove paraffin wax, co-solvent and antioxidant in the green body, thereby preventing the green body from having defects such as collapse, cracking and large holes during thermal degreasing and pre-sintering;

[0040] The solvent degreasing uses n-propane bromide as the degreasing solvent, and the green body is completely placed in the degreasing solvent at a solvent temperature of 50° C. for 10 hours.

[0041] Step S4: thermal debinding and pre-sintering are performed together, heating to 200-300℃, 400-500℃, 500-600℃ in a hydrogen atmosphere and keeping the temperature for 1h respectively. This stage is the thermal debinding stage, and the main purpose is to remove the remaining binder in the green body. Then heating to 1400℃ and keeping the temperature for 2h, this stage is the pre-sintering stage, and the main purpose is to ensure that the binder in the green body is completely removed and prepare a tungsten skeleton with a certain strength.

[0042] Step S5: high temperature sintering: placing the pre-sintered tungsten skeleton in a vertical high temperature furnace, with a sintering temperature of 1800° C. and a sintering time of 2 h.

[0043] Step S6: The injection port is removed by machining, and impurities introduced into the surface pores by machining are removed by ultrasonic vibration cleaning, so as to finally obtain the desired porous tungsten sponge.

[0044] After ultrasonic cleaning, the porosity and open porosity of the part were tested by Archimedes drainage method and mercury intrusion porosimeter respectively. The porosity of the part tested by Archimedes drainage method was 24.7%, and the open porosity of the part tested by mercury intrusion porosimeter was 24.5%. The pore size distribution of the part was uniform, with an average pore size of 1.02μm. The part had almost no closed pores, and the closed porosity was only 0.2%, which met the basic requirements of the barium tungsten cathode for the porosity of the porous tungsten matrix.

[0045] Embodiment 2:

[0046] The difference between this embodiment and embodiment 1 is that the wax-based adhesive is replaced with a plastic-based adhesive, and the specific implementation steps are as follows:

[0047] Step S1: Tungsten powder processing method: grind the commercially available nearly spherical tungsten balls with a particle size of about 10 μm to obtain high-energy tungsten powder.

[0048] Step S2: Preparation of tungsten blank: In this case, high-energy tungsten powder after ball milling is used, and the tungsten powder is evenly mixed with a plastic-based binder to prepare a plastic-based feed, which is then injected on an injection molding machine to obtain a tungsten blank.

[0049] The detailed preparation method is as follows:

[0050] Step S2-1: Preparation of plastic-based feed: first add a certain amount of tungsten powder into the mixer, preheat and stir at 180°C for 10-20 minutes, then add copolymer formaldehyde and heat-resistant polystyrene into the mixer, and mix under pressure at 180°C for 20-40 minutes to fully melt the binder and mix it with the tungsten powder; then add the compatibilizer, antioxidant and paraffin into the mixer, and mix under pressure for 30-40 minutes. After the binder and tungsten powder are fully mixed, take them out from the mixer, cool and crush them to obtain the required plastic-based feed.

[0051] Step S2-2: Preparation of tungsten blank: Add the crushed plastic-based feed into the injection molding machine and perform injection molding on the feed to obtain the required tungsten blank parts.

[0052] Among them, the injection pressure of the injection machine is 40-90Bar, the holding pressure is 40Bar, the injection temperature is 160-170℃, and the mold temperature controller temperature is 60-100℃.

[0053] Step S3: catalytic debinding and thermal debinding: catalytic debinding is performed on the injection molded green body to remove the copolymerized formaldehyde in the green body and prevent the green body from having defects such as collapse, cracking, and large holes during the thermal debinding and pre-sintering process;

[0054] Among them, catalytic degreasing uses an oxalic acid degreasing furnace, oxalic acid is used as a catalyst, the degreasing time is 10-12h, and the temperature is 100-150℃.

[0055] Step S4: thermal debinding and pre-sintering are performed together, heating to 200-300℃, 400-500℃, 500-600℃ in a hydrogen atmosphere and keeping the temperature for 1h respectively. This stage is the thermal debinding stage, and the main purpose is to remove the remaining binder in the green body. Then heating to 1400℃ and keeping the temperature for 2h, this stage is the pre-sintering stage, and the main purpose is to ensure that the binder in the green body is completely removed and prepare a tungsten skeleton with a certain strength.

[0056] Step S5: high temperature sintering: placing the pre-sintered tungsten skeleton in a vertical high temperature furnace, with a sintering temperature of 1800° C. and a sintering time of 2 h.

[0057] Step S6: The injection port is removed by machining, and impurities introduced into the surface pores by machining are removed by ultrasonic vibration cleaning, so as to finally obtain the desired porous tungsten sponge.

[0058] After ultrasonic cleaning, the porosity and open porosity of the part were tested by Archimedes drainage method and mercury intrusion porosimeter respectively. The porosity of the part tested by Archimedes drainage method was 24.5%, and the open porosity of the part tested by mercury intrusion porosimeter was 24.3%. The pore size distribution of the part was uniform, with an average pore size of 1.02μm. The part had almost no closed pores, and the closed porosity was only 0.2%, which met the basic requirements of the barium tungsten cathode for the porosity of the porous tungsten matrix.

[0059] Embodiment 3:

[0060] The difference between this embodiment and embodiment 1 is that the tungsten powder is replaced with commercially available ordinary tungsten powder with a Fisher particle size of about 5 μm and ball-milled to obtain high-energy tungsten powder, thereby increasing the tap density of the powder and the loading capacity of the feed.

[0061] Since the powder is a common tungsten powder on the market, the tungsten powder is severely agglomerated and the powder particles are irregular polygons, so the tap density of the powder is low, only 6-7g / cm 3 After ball milling, the tap density of tungsten powder is significantly increased to 9-11 g / cm 3 .

[0062] After ultrasonic cleaning, the porosity and open porosity of the part were tested by Archimedes drainage method and mercury intrusion instrument respectively. The porosity of the part tested by Archimedes drainage method was 26.6%, and the open porosity of the part tested by mercury intrusion instrument was 25.7%. Due to the serious agglomeration of the powder, a certain amount of closed pores were produced after high-temperature sintering, with a closed porosity of 0.9%, and a relatively uniform pore size distribution, with an average pore size of 0.98μm, which met the basic requirements of the barium tungsten cathode for the porosity of the porous tungsten matrix.

[0063] Comparative Example 1:

[0064] The difference between Comparative Example 1 and Example 3 is that the tungsten powder used is commercially available common tungsten powder with a Fisher particle size of about 5 μm, without any treatment.

[0065] Since the powder is a common tungsten powder on the market, the tungsten powder is severely agglomerated and the powder particles are irregular polygons, so the tap density of the powder is low, only 6-7g / cm 3 .

[0066] After ultrasonic cleaning, the porosity and open porosity of the part were tested by Archimedes drainage method and mercury intrusion instrument respectively. The porosity of the part tested by Archimedes drainage method was 28.5%, and the open porosity of the part tested by mercury intrusion instrument was 23.4%. Due to the serious agglomeration of the powder, a certain amount of closed pores were produced after high-temperature sintering, with a closed porosity of 5.1%, and the pore size distribution was relatively uneven, with an average pore size of 1.21μm.

[0067] The above-mentioned embodiments are only some preferred embodiments of the present invention, and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present invention shall fall within the scope of protection required by the present invention.

Claims

1. A method for preparing a porous tungsten sponge of a barium tungsten cathode, characterized in that: The preparation method thereof comprises the following steps: Step S1: tungsten powder and a binder are mixed and granulated in a required ratio to prepare a wax-based feed or a plastic-based feed for injection molding, and the binder is a wax-based binder or a plastic-based binder; Step S2: feeding the feed into the injection molding machine for injection molding to prepare the injection blank; Step S3: removing the binder in the injection blank by solvent degreasing or catalytic degreasing; Step S4: thermally debinding and pre-sintering the debinded green body; Step S5: placing the pre-sintered porous tungsten in a vacuum sintering furnace or an atmosphere protection sintering furnace for high-temperature sintering.

2. The method for preparing the porous tungsten sponge of the barium tungsten cathode according to claim 1, characterized in that: The tungsten powder has a Fisher particle size of 5-10 μm, and the tungsten powder is ball-milled to obtain high-energy tungsten powder.

3. The method for preparing the porous tungsten sponge of the barium tungsten cathode according to claim 1, characterized in that: The specific proportion of the binder used in the feed prepared in step S1 is as follows: The mass proportions of the components in the wax-based adhesive are: 50-70% paraffin, 10-30% high-density polyethylene, 10-20% polypropylene, 1-10% antioxidant, and 1-4% cosolvent; The mass proportions of the components in the plastic-based adhesive are: 60-90% of copolymer formaldehyde, 5-10% of heat-resistant polystyrene, 1-5% of compatibilizer, and 1-5% of antioxidant.

4. The method for preparing the porous tungsten sponge of the barium tungsten cathode according to claim 1, characterized in that: In the step S3, the injection blank prepared by the wax-based feedstock is degreased by solvent degreasing, and the injection blank prepared by the plastic-based feedstock is degreased by catalytic degreasing, wherein: The degreasing solvent used in solvent degreasing is n-propane bromide, the degreasing time is 12-24h, and the degreasing temperature is 40-60℃. Catalytic degreasing uses a catalytic degreasing furnace for degreasing, the catalyst is oxalic acid, the degreasing temperature is 100-150℃, and the time is 8-12h.

5. The method for preparing the porous tungsten sponge of the barium tungsten cathode according to claim 1, characterized in that: In step S4, thermal degreasing is performed, i.e., the temperature is kept at 200-300° C., 400-500° C., and 500-600° C. for 1-2 hours respectively, and the remaining binder is removed by graded degreasing; The pre-sintering temperature is 1200-1400°C and the sintering time is 2-4h, so that the sample has a certain strength; Both thermal debinding and pre-sintering use a molybdenum wire sintering furnace and sintering in a hydrogen atmosphere.

6. The method for preparing the porous tungsten sponge of the barium tungsten cathode according to claim 1, characterized in that: In step S5, the sintering temperature is 1700-1900° C., the holding time is 1-2 hours, and the sintering conditions are vacuum sintering or atmosphere sintering.

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