A method for preparing porous tungsten at low temperature based on reaction sintering

Abstract

The application relates to a method for preparing porous tungsten at low temperature based on reaction sintering, and relates to the technical field of barium-tungsten cathode material preparation. By introducing aluminum as a sintering additive and a binder, a self-sustaining high-temperature sintering process can be realized, the reaction is self-sustaining, energy consumption is low, reactants are fully mixed, and the speed is fast. Moreover, the generated Al2O3 is a component of the tungsten cathode, and the working stability of the cathode is improved, so that CaO required by the cathode can be added on the basis of reaction heat release, the reaction heat release of aluminum and tungsten oxide promotes sintering, and the required CaO-Al2O3 composite salt is generated. In the sintering process, Al forms a local liquid phase, promotes the interaction between particles, and thus a tungsten material with high density is obtained at a lower temperature, the generated Al2O3 is compounded with CaO at high temperature, and the sintering and salt immersion processes are simultaneously performed, so that the cost of tungsten cathode preparation is greatly reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of barium-tungsten cathode material preparation, and particularly relates to a method for preparing porous tungsten at low temperature based on reaction sintering. BACKGROUND

[0002] Tungsten is a hard metal with high melting point, high hardness, high strength and high wear resistance, which makes tungsten very stable in high-temperature environment, and thus is more applied in high-temperature-resistant materials. Tungsten also has relatively stable chemical properties, and has good corrosion resistance to most acids, bases and oxidation environments. For its porous body material, it has high specific surface area, good thermal conductivity, light weight, high strength, high temperature stability, strong permeability and good energy absorption, and is widely applied in the fields of catalysts and catalyst carriers, high-temperature structural materials, nuclear industry, electrode materials, biomedical engineering, electronic industry and the like. For example, barium-tungsten cathode is applied in electronic and electrical equipment as a kind of hot cathode, and has the advantages of large emission current density, strong anti-ion bombardment ability and not easy to spark under high voltage.

[0003] The barium-tungsten cathode is mainly composed of a porous tungsten substrate and barium oxide. The barium-tungsten cathode can be obtained by immersing the porous tungsten substrate into a barium salt solution to make the surface of the porous tungsten substrate adsorb or react with the barium salt to form a thin layer of barium compound. However, the barium-tungsten cathode needs to have sufficient stability at high temperature, and an activator needs to be added to further improve the electron emission efficiency, reduce the working temperature of the cathode, improve the emission current density and prolong the service life of the cathode. This can be achieved by adding CaO and Al2O3 to the porous tungsten substrate in the form of a composite salt, in which Al2O3 and CaO act as a stabilizer and an activator, respectively.

[0004] Generally, the salt immersion method of porous tungsten mainly includes molten salt immersion method, vacuum immersion method and pressure immersion method. The molten salt immersion method immerses the porous tungsten material into molten salt, and uses capillary action and pressure difference to make the molten salt penetrate into the internal porous structure. The vacuum immersion method immerses the porous tungsten into molten salt in a vacuum environment, removes the air inside the material by vacuum, and makes the molten salt penetrate into the pores under pressure. This method can improve the penetration depth and uniformity of the molten salt. The pressure immersion method immerses the porous tungsten material into molten salt by applying external pressure, and can achieve immersion in a short time. These methods all need a high-temperature environment, and the vacuum degree and pressure conditions need to be accurately controlled during the operation process. High-temperature treatment may cause thermal damage to the material.

[0005] Reaction heat release promotes sintering, which is a method for preparing porous tungsten at low temperature. By reacting tungsten oxide with active substances such as aluminum, a large amount of heat is released to promote sintering, specific components are consumed and other components are formed, so that a predetermined porous sintered body is obtained. SUMMARY

[0006] In order to solve the defects such as high temperature required in the preparation process of barium tungsten cathode, the application provides a method for preparing porous tungsten at low temperature based on reaction sintering, which can realize a self-propagating high-temperature sintering process by introducing aluminum as a sintering aid and an adhesive, and has the advantages of self-sustaining reaction, low energy consumption, sufficient mixing of reactants, and fast speed. And the generated Al2O3 is a component of aluminate, so CaO can be added on the basis of reaction heat release, and the reaction heat release of aluminum and tungsten oxide promotes sintering, while the mixed oxide generates the required CaO-Al2O3 composite salt.

[0007] In order to achieve the above purpose, the technical scheme adopted by the application is:

[0008] A method for preparing porous tungsten at low temperature based on reaction sintering, the steps are as follows:

[0009] (1) Preparation of tungsten and tungsten oxide mixed powder: uniformly mix tungsten powder and tungsten oxide powder by ball milling to obtain mixed powder, and uniformly distribute tungsten oxide on the surface of tungsten particles in the mixed powder;

[0010] (2) Preparation of precursor powder: mix the mixed powder with Al and CaO powder, and uniformly ball mill to obtain the precursor powder;

[0011] (3) Powder densification sintering: densification sintering of the precursor powder by plasma discharge sintering to obtain porous tungsten.

[0012] As a preferred technical scheme of the application, in the preparation method:

[0013] The purity of the tungsten powder used in step (1) is above 99.99%, and the particle size is 4-9 μm; the purity of the tungsten oxide powder used is above 99.99%, and the average particle size is 500 nm; the mass ratio between the tungsten powder and the tungsten oxide powder is 15-40:1.

[0014] The ball milling mixing parameters for preparing the mixed powder in step (1) are as follows: tungsten powder and tungsten oxide powder are added to a hard alloy ball mill jar, the diameter of the milling balls is 4 mm, the mass ratio of the balls to the powder is 5:1, the ball mill jar is sealed in an argon atmosphere vacuum glove box, and then loaded into a high-performance ball mill for ball milling for 24 h at a rotation speed of 400 rpm.

[0015] The purity of the Al powder used in step (2) is above 99.99%, and the particle size is 3-7 μm; the purity of the CaO powder used is above 99.99%, and the average particle size is 3-7 μm; the molar ratio of the added Al powder and CaO powder to the tungsten oxide powder added in step (1) is 2:0.5:1.

[0016] The step (2) prepares the precursor powder by ball milling with the parameters as follows: Al and CaO powders are added into a hard alloy ball milling tank containing tungsten and tungsten oxide mixed powders, the diameter of the milling ball is 4mm, the mass ratio of the ball and the powder is 5:1, the ball milling tank is sealed in an argon atmosphere vacuum glove box, and then is loaded into a high-performance ball mill for ball milling for 20 minutes at a rotation speed of 100 rpm.

[0017] The step (3) loads the ball-milled powder into a sintering mold, uses carbon paper to insulate the mold wall from the powder, and places the sintering mold in a spark plasma sintering furnace for vacuum treatment; the initial pressure is set to 10 MPa, the sintering temperature is 1100 DEG C, the heating rate is 100 DEG C / min, the holding time is 5-15 min, and the sintering pressure is 30-50 MPa; the sintering pressure is manually increased during the heating process; after the holding is completed, argon is filled to normal pressure, the product is cooled in the argon atmosphere, the surface carbon paper is polished by a grinding machine, and finally the porous tungsten cathode material is obtained.

[0018] Compared with the traditional salt immersion method of porous tungsten, the method for preparing porous tungsten has the following obvious advantages:

[0019] 1. The temperature required for CaO and Al2O3 to generate CaO-Al2O3 composite salt in a high-temperature environment is above 1200 DEG C. The traditional salt immersion method needs to melt the composite salt at high temperature, and the thermal stress generated at high temperature may cause the structure of the tungsten matrix to be damaged, and uneven coverage of the molten salt may cause the phenomenon that expansion and contraction are not synchronized. In contrast, in the present application, Al2O3 and CaO react to generate a composite salt at the sintering temperature and under the heat release of the reaction, thereby greatly reducing the temperature required for salt immersion.

[0020] 2. The addition of aluminum can significantly reduce the temperature required for sintering of porous tungsten. The reaction between tungsten oxide and aluminum is a self-propagating reaction, which is rapid and releases a large amount of heat, so that the temperature on the micro level is optimal, and the macro sintering temperature is significantly reduced, thereby promoting the sintering between tungsten powders, and reducing the phenomenon of coarse grains compared with the traditional high-temperature densification sintering.

[0021] 3. In the present application, the salt directly penetrates into the tungsten powder, reducing the waste of materials and improving the utilization rate of materials; the additives such as calcium and aluminum are uniformly distributed in the tungsten matrix, avoiding the problems of surface enrichment or insufficient internal distribution, and avoiding the composition segregation phenomenon that may occur in impregnation.

[0022] 4. In the sintering process, Al forms a local liquid phase, promoting the interaction between particles, so that a tungsten material with high density is obtained at a lower temperature, the generated Al2O3 is compounded with CaO at high temperature, and the sintering and salt immersion processes are carried out at the same time, thereby greatly reducing the cost of tungsten cathode preparation. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 XRD pattern of the porous tungsten cathode material prepared in Example 1.

[0024] Figure 2 SEM pattern of the porous tungsten cathode material prepared in Example 1.

[0025] Figure 3 EDS pattern of the porous tungsten cathode material prepared in Example 1.

[0026] Figure 4 XRD pattern of the porous tungsten cathode material prepared in Comparative Example 1. DETAILED DESCRIPTION

[0027] The present application proposes a method for preparing porous tungsten at low temperature based on reaction sintering, which generates a large amount of heat through the reaction sintering of tungsten oxide and aluminum to promote sintering, and uses tungsten powder, tungsten oxide powder, aluminum powder and calcium oxide powder for ball milling. Through spark plasma sintering, the reaction exothermic promotes the bonding between tungsten particles and the generation of composite salt of aluminum oxide and calcium oxide.

[0028] The present application will be further described in detail below in combination with the embodiments and the accompanying drawings.

[0029] Example 1

[0030] The method for preparing the porous tungsten cathode material in this embodiment is as follows:

[0031] (1) Preparation of tungsten and tungsten oxide mixed powder: 53.18 g of W powder and 2.319 g of WO3 powder were placed in a hard alloy ball mill jar, hard alloy grinding balls were added, the ball diameter was 4 mm, the ball to powder mass ratio was 5:1, the ball mill jar was sealed in an argon atmosphere vacuum glove box, and then placed in a high-energy ball mill, the rotation speed was 400 rpm, and the ball milling time was 24 h.

[0032] (2) Preparation of precursor powder: 0.541 g of Al powder and 0.28 g of CaO powder were added to the tungsten and tungsten oxide mixed powder, placed in a hard alloy ball mill jar, hard alloy grinding balls were added, the ball diameter was 4 mm, the ball to powder mass ratio was 5:1, the ball mill jar was sealed in an argon atmosphere vacuum glove box, and then placed in a high-energy ball mill, the rotation speed was 100 rpm, and the ball milling time was 20 min.

[0033] (3) Powder densification sintering: The ball-milled precursor powder is loaded into a graphite sintering mold, and carbon paper is used to isolate the mold wall from the powder. The mold is placed in a discharge plasma sintering furnace and the furnace is evacuated. The initial pressure is set to 10 MPa, the sintering temperature to 1100℃, the heating rate to 100℃ / min, the holding time to 10 min, and the sintering pressure to 40 MPa. The pressure is manually increased to the sintering pressure during the heating process. After the holding time is completed, argon gas is introduced to atmospheric pressure and the furnace is cooled under an argon atmosphere. The product is taken out and the carbon paper on the surface is polished with a grinding wheel to finally obtain a porous tungsten cathode material.

[0034] Comparative Example 1

[0035] The method for preparing porous tungsten cathode material in this comparative example is as follows:

[0036] (1) Preparation of precursor powder: 52.697g of W powder, 1.022g of Al2O3 powder and 0.281g of CaO powder were placed in a cemented carbide ball milling jar, and cemented carbide grinding balls with a diameter of 4mm and a ball-to-powder mass ratio of 5:1 were added. The ball milling jar was sealed in an argon atmosphere vacuum glove box and then placed in a high-energy ball mill at a speed of 100rpm for 20min.

[0037] (2) Powder densification sintering: The ball-milled precursor powder is loaded into a graphite sintering mold, and carbon paper is used to isolate the mold wall from the powder. The mold is placed in a discharge plasma sintering furnace and the furnace is evacuated. The initial pressure is set to 10 MPa, the sintering temperature to 1100℃, the heating rate to 100℃ / min, the holding time to 10 min, and the sintering pressure to 40 MPa. The pressure is manually increased to the sintering pressure during the heating process. After the holding time is completed, argon gas is introduced to atmospheric pressure and the furnace is cooled under an argon atmosphere. The product is taken out and the carbon paper on the surface is polished with a grinding wheel to finally obtain a porous tungsten cathode material.

[0038] pass Figures 1 to 3 It can be seen that the peak of composite salt was detected in the prepared porous tungsten cathode material, and the distribution of Al and Ca was observed to be consistent, proving that CaO and Al2O3 formed CaO-Al2O3 composite salt under high temperature environment.

[0039] pass Figure 4 The comparison reveals that CaO and Al2O3 remain separately distributed within the material, with Al2O3 and CaO exhibiting different distributions, indicating that no recombination reaction has occurred.

[0040] The above description is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing porous tungsten at low temperature based on reaction sintering, characterized in that, The steps are as follows: (1) Preparation of tungsten and tungsten oxide mixed powder: Tungsten powder and tungsten oxide powder are ball-milled to obtain a mixed powder, in which tungsten oxide is evenly distributed on the surface of tungsten particles; The tungsten powder used has a purity of 99.99% or higher and a particle size of 4~9 µm; the tungsten oxide powder used has a purity of 99.99% or higher and an average particle size of 500 nm; the mass ratio between tungsten powder and tungsten oxide powder is 15~40:

1. (2) Preparation of precursor powder: The mixed powder is mixed with Al and CaO powder and ball-milled to obtain precursor powder; The Al powder used has a purity of 99.99% or higher and a particle size of 3~7 µm; the CaO powder used has a purity of 99.99% or higher and an average particle size of 3~7 µm; the molar ratio between the added Al powder, CaO powder and the tungsten oxide powder added in step (1) is 2:0.5:

1. (3) Powder densification sintering: The precursor powder is densified by plasma discharge sintering at a temperature of 1100 ℃ to obtain porous tungsten.

2. The method as described in claim 1, characterized in that, The mixing parameters for preparing the mixed powder in step (1) are as follows: tungsten powder and tungsten oxide powder are added to the cemented carbide ball milling jar. The diameter of the grinding ball is 4 mm and the mass ratio of the ball to the powder is 5:

1. The ball milling jar is sealed in an argon atmosphere vacuum glove box and then loaded into a high-performance ball mill for ball milling for 24 h at a ball milling speed of 400 rpm.

3. The method as described in claim 1, characterized in that, The ball milling parameters for preparing the precursor powder in step (2) are as follows: Al and CaO powders are added to a cemented carbide ball milling jar containing a mixture of tungsten and tungsten oxide powders. The diameter of the grinding balls is 4 mm and the mass ratio of the balls to the powder is 5:

1. The ball milling jar is sealed in an argon atmosphere vacuum glove box and then placed in a high-performance ball mill for ball milling for 20 minutes at a speed of 100 rpm.

4. The method as described in claim 1, characterized in that, In step (3), the ball-milled powder is loaded into a sintering mold, and carbon paper is used to isolate the mold wall from the powder. The mold is placed in a discharge plasma sintering furnace, and the sintering furnace is evacuated. The initial pressure is set to 10 MPa, the sintering temperature to 1100 ℃, the heating rate to 100 ℃ / min, the holding time to 5~15 min, and the sintering pressure to 30~50 MPa. During the heating process, the pressure is manually increased to the sintering pressure. After the holding time is completed, argon gas is introduced to atmospheric pressure, and the furnace is cooled under an argon atmosphere. The product is taken out, and the carbon paper on the surface is polished with a grinding wheel to finally obtain a porous tungsten cathode material.

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