Method for preparing porous tungsten at low temperature based on reactive sintering

Abstract

The invention discloses a method for preparing porous tungsten at low temperature based on reactive sintering, and relates to the technical field of preparation of barium-tungsten cathode materials. By introducing aluminum as a sintering aid and an adhesive, a self-propagating high-temperature sintering process can be realized, the reaction is self-sustaining, the energy consumption is low, reactants are fully mixed, and the speed is high. And 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, and the mixed oxide generates the required CaO-Al2O3 composite salt while the reaction heat release of aluminum and tungsten oxide promotes sintering. In the sintering process, Al forms a local liquid phase to promote interaction among particles, so that a high-density tungsten material is obtained at a low temperature, generated Al2O3 is compounded with CaO at a high temperature, sintering and salt leaching processes are carried out at the same time, and the preparation cost of the tungsten cathode is greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of barium tungsten cathode materials, and in particular to a method for preparing porous tungsten at low temperature based on reaction sintering. Background Art

[0002] Tungsten is a hard metal with a high melting point, high hardness, high strength and high wear resistance, which makes it very stable in high temperature environments and is therefore widely used in high temperature resistant materials. Tungsten also has relatively stable chemical properties and has good corrosion resistance to most acid, alkali and oxidizing environments. As for its porous body materials, it has the characteristics of high specific surface area, good thermal conductivity, light weight and high strength, high temperature stability, strong permeability, and good energy absorption. It is widely used in catalysts and catalyst carriers, high temperature structural materials, nuclear industry, electrode materials, biomedical engineering, and electronic industry. For example, barium tungsten cathodes are used in electronic and electrical equipment. As a hot cathode, it has the advantages of high emission current density, strong resistance to ion bombardment, and not easy to ignite under high pressure.

[0003] The barium tungsten cathode is mainly composed of a porous tungsten matrix and barium oxide. The barium tungsten cathode can be obtained by immersing the porous tungsten matrix in a barium salt solution, allowing its surface to adsorb or react to form a thin layer of barium oxide. However, the barium tungsten cathode needs to have sufficient stability at high temperatures, and an activator needs to be added to further improve the electron emission efficiency, while reducing the operating temperature of the cathode, increasing the emission current density and extending the cathode life. This can be obtained by adding CaO and Al2O3 to the porous tungsten matrix to form a composite salt at high temperature, in which Al2O3 and CaO serve as stabilizers and activators, respectively.

[0004] Usually, the main methods for impregnating porous tungsten with salt include molten salt impregnation, vacuum impregnation, pressure impregnation, etc. The molten salt impregnation method immerses the porous tungsten material into molten salt, and uses capillary action and pressure difference to make the molten salt penetrate into the porous structure; the vacuum impregnation method is to place the porous tungsten in molten salt under a vacuum environment, use vacuum to remove the air inside the material, and make the molten salt penetrate into the pores under pressure. This method can improve the penetration depth and uniformity of the molten salt; the pressure impregnation method immerses the porous tungsten material into the molten salt by applying external pressure, and can achieve impregnation in a shorter time; these methods require a higher temperature environment, and the vacuum degree and pressure and other conditions need to be precisely controlled during the operation, and high-temperature treatment may cause thermal damage to the material.

[0005] Reaction exothermic sintering is a method that can achieve low-temperature preparation of porous tungsten. Tungsten oxide reacts with active substances, such as aluminum. The reaction releases a large amount of heat to promote sintering, consume specific components and form other components, thereby obtaining a predetermined porous sintered body. Summary of the invention

[0006] In order to solve the defects such as the need for high temperature in the preparation process of barium tungsten cathode, the present invention proposes a method for preparing porous tungsten based on reaction sintering at low temperature. By introducing aluminum as a sintering aid and binder, a self-propagating high-temperature sintering process can be achieved, and the reaction is self-sustaining, low energy consumption, sufficient mixing of reactants, and fast speed. In addition, the generated Al2O3 is a component of aluminate, so CaO can be added to it on the basis of the heat release of the reaction. While the heat release of the reaction between aluminum and tungsten oxide promotes sintering, the mixed oxide generates the required CaO-Al2O3 composite salt.

[0007] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is:

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

[0009] (1) Preparation of mixed powder of tungsten and tungsten oxide: tungsten powder and tungsten oxide powder are uniformly milled to obtain a mixed powder, in which tungsten oxide is uniformly distributed on the surface of tungsten particles;

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

[0011] (3) Powder densification sintering: The precursor powder is densified and sintered using plasma discharge sintering to obtain porous tungsten.

[0012] As a preferred technical solution of the present invention, in the preparation method:

[0013] The tungsten powder used in step (1) has a purity of more than 99.99% and a particle size of 4 to 9 μm; the tungsten oxide powder used has a purity of more than 99.99% and an average particle size of 500 nm; and the mass ratio of tungsten powder to tungsten oxide powder is 15 to 40:1.

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

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

[0016] 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 mill containing a mixed powder of tungsten and tungsten oxide, the diameter of the grinding balls is 4 mm, the ball-to-powder mass ratio is 5:1, the ball mill is sealed in an argon atmosphere vacuum glove box, and then loaded into a high-performance ball mill for ball milling for 20 minutes at a ball milling speed of 100 rpm.

[0017] The step (3) comprises the following steps: loading the ball-milled powder into a sintering mold, isolating the mold wall and the powder with carbon paper, placing the mold in a discharge plasma sintering furnace, and evacuating the sintering furnace; setting the initial pressure to 10 MPa, the sintering temperature to 1100°C, the heating rate to 100°C / min, the insulation time to 5 to 15 min, and the sintering pressure to 30 to 50 MPa; manually pressurizing the mold to the sintering pressure during the heating process; after insulation, filling the mold with argon to normal pressure, cooling the mold with the furnace in an argon atmosphere, taking out the product, and grinding the carbon paper on the surface with a grinder to finally obtain a porous tungsten cathode material.

[0018] Compared with the conventional salt leaching method of porous tungsten, the method for preparing porous tungsten of the present invention has the following obvious advantages:

[0019] 1. The temperature required for CaO and Al2O3 to generate CaO-Al2O3 composite salt under high temperature environment is above 1200℃. The traditional salt immersion method requires high temperature to melt the composite salt. The thermal stress generated by high temperature may cause the tungsten matrix structure to be destroyed, and the uneven coverage of the molten salt will cause the expansion or contraction to be asynchronous. In contrast, in the present invention, the reaction product Al2O3 and CaO generate composite salt under the sintering temperature and reaction exothermic promotion, which greatly reduces the temperature required for salt immersion.

[0020] 2. The addition of aluminum can significantly reduce the temperature required for sintering porous tungsten. The reaction between tungsten oxide and aluminum is a self-propagating reaction. It reacts rapidly and releases a large amount of heat, so that the temperature reaches the optimal level at the microscopic level, while the macroscopic sintering temperature is significantly reduced, which promotes the sintering between tungsten powders. Compared with traditional high-temperature densification sintering, it reduces the phenomenon of coarse grains.

[0021] 3. In the present invention, the salt directly infiltrates into the tungsten powder, which reduces the waste of materials and improves the utilization rate of materials; additives such as calcium and aluminum are evenly distributed in the tungsten matrix, avoiding the problem of surface enrichment or insufficient internal distribution, and avoiding the component segregation phenomenon that may occur during impregnation.

[0022] 4. During the sintering process, Al forms a local liquid phase, which promotes the interaction between particles, thereby obtaining a higher density tungsten material at a lower temperature. The generated Al2O3 is compounded with CaO at a high temperature, and the sintering and salt immersion processes are carried out simultaneously, which greatly reduces the cost of tungsten cathode preparation. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0027] The present invention proposes a method for preparing porous tungsten at low temperature based on reaction sintering. The reaction sintering of tungsten oxide and aluminum generates a large amount of heat to promote sintering. Tungsten powder, tungsten oxide powder, aluminum powder and calcium oxide powder are ball-milled and mixed. Through spark plasma sintering, the reaction heat promotes the bonding between tungsten particles and promotes the formation of composite salts of aluminum oxide and calcium oxide generated by the reaction.

[0028] The present invention is further described in detail below in conjunction with embodiments and 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 cemented carbide ball mill, and cemented carbide grinding balls were added. The grinding balls had a diameter of 4 mm and a ball-to-powder mass ratio of 5:1. The ball mill was sealed in an argon atmosphere vacuum glove box and then placed in a high-energy ball mill at a rotation speed of 400 rpm for 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 mixed powder of tungsten and tungsten oxide, placed in a cemented carbide ball mill, and cemented carbide grinding balls were added. The grinding balls had a diameter of 4 mm and a ball-to-powder mass ratio of 5:1. The ball mill was sealed in an argon atmosphere vacuum glove box and then placed in a high-energy ball mill at a rotation speed of 100 rpm for 20 min.

[0033] (3) Powder densification sintering: The ball-milled precursor powder is loaded into a graphite sintering mold, and the mold wall and the powder are isolated with carbon paper. 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 is set to 1100°C, the heating rate is set to 100°C / min, the holding time is set to 10 min, and the sintering pressure is set to 40 MPa. During the heating process, the pressure is manually increased to the sintering pressure. After the holding period is completed, argon is filled to normal pressure, and the mold is cooled in an argon atmosphere. The product is taken out, and the carbon paper on the surface is polished with a grinder to finally obtain a porous tungsten cathode material.

[0034] Comparative Example 1

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

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

[0037] (2) Powder densification sintering: The ball-milled precursor powder is loaded into a graphite sintering mold, and the mold wall and the powder are isolated with carbon paper. 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 is set to 1100°C, the heating rate is set to 100°C / min, the holding time is set to 10 min, and the sintering pressure is set to 40 MPa. During the heating process, the pressure is manually increased to the sintering pressure. After the holding period is completed, argon is filled to normal pressure, and the mold is cooled in the argon atmosphere. The product is taken out, and the carbon paper on the surface is polished with a grinder to finally obtain a porous tungsten cathode material.

[0038] pass Figures 1 to 3 It can be seen that the peak of the 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 generated CaO-Al2O3 composite salt under high temperature environment.

[0039] pass Figure 4 By comparison, it can be found that CaO and Al2O3 are still distributed in the material respectively, and the distribution of Al2O3 and CaO are different, indicating that no composite reaction occurs.

[0040] The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by 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: Here are the steps: (1) Preparation of mixed powder of tungsten and tungsten oxide: tungsten powder and tungsten oxide powder are uniformly milled to obtain a mixed powder, in which tungsten oxide is uniformly distributed on the surface of tungsten particles; (2) Preparation of precursor powder: Mix the mixed powder with Al and CaO powders, and ball mill to obtain precursor powder; (3) Powder densification sintering: The precursor powder is densified and sintered using plasma discharge sintering to obtain porous tungsten.

2. The method according to claim 1, characterized in that The tungsten powder used in step (1) has a purity of more than 99.99% and a particle size of 4 to 9 μm; the tungsten oxide powder used has a purity of more than 99.99% and an average particle size of 500 nm; and the mass ratio of tungsten powder to tungsten oxide powder is 15 to 40:

1.

3. The method according to claim 1, characterized in that The mixing parameters for preparing the mixed powder by ball milling in step (1) are as follows: tungsten powder and tungsten oxide powder are added to a cemented carbide ball mill, the diameter of the grinding balls is 4 mm, the ball-to-powder mass ratio is 5:1, the ball mill is sealed in an argon atmosphere vacuum glove box, and then loaded into a high-performance ball mill for ball milling for 24 hours, and the ball milling speed is 400 rpm.

4. The method according to claim 1, characterized in that The Al powder used in step (2) has a purity of more than 99.99% and a particle size of 3 to 7 μm; the CaO powder used has a purity of more than 99.99% and an average particle size of 3 to 7 μm; the molar ratio of the added Al powder, CaO powder and the tungsten oxide powder added in step (1) is 2:0.5:

1.

5. The method according to 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 mill containing a mixed powder of tungsten and tungsten oxide, the diameter of the grinding balls is 4 mm, the ball-to-powder mass ratio is 5:1, the ball mill is sealed in an argon atmosphere vacuum glove box, and then loaded into a high-performance ball mill for ball milling for 20 minutes at a ball milling speed of 100 rpm.

6. The method according to claim 1, characterized in that The step (3) comprises the following steps: loading the ball-milled powder into a sintering mold, isolating the mold wall and the powder with carbon paper, placing the mold in a discharge plasma sintering furnace, and evacuating the sintering furnace; setting the initial pressure to 10 MPa, the sintering temperature to 1100°C, the heating rate to 100°C / min, the insulation time to 5 to 15 min, and the sintering pressure to 30 to 50 MPa; manually pressurizing the mold to the sintering pressure during the heating process; after insulation, filling the mold with argon to normal pressure, cooling the mold with the furnace in an argon atmosphere, taking out the product, and grinding the carbon paper on the surface with a grinder to finally obtain a porous tungsten cathode material.

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