Tungsten-scandium mixed matrix for high-power hot cathode and preparation method and application of tungsten-scandium mixed matrix

Abstract

The invention belongs to the field of microwave vacuum electronic technology and refractory metal powder metallurgy, and particularly relates to a tungsten-scandium mixed matrix for a high-power hot cathode and a preparation method and application of the tungsten-scandium mixed matrix. The porosity of through holes of the tungsten-scandium mixed matrix is 18-30%, the average aperture is 0.3-2.0 microns, the scandium oxide content is 1-10 wt%, the percentage of closed area is smaller than or equal to 1%, and the specific surface area of the tungsten-scandium mixed matrix is smaller than or equal to 1%. And the grain size of the scandium oxide in the mixed matrix is not more than 1 mu m after sintering. The preparation method comprises the steps of tungsten powder grading, grinding and coating, vacuum drying, reduction purification, sieving, blending, cold isostatic pressing forming, hydrogen pre-sintering, precision machining, hydrogen high-temperature sintering and the like. In addition, the tungsten-scandium mixed substrate prepared by the method can be applied to scenes such as high-power terahertz devices and the like, and can replace a conventional M-type impregnated barium-tungsten cathode.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of high-power microwave vacuum electron technology and refractory metal powder metallurgy, and particularly relates to a tungsten-scandium mixed substrate for a high-power hot cathode as well as a preparation method and application thereof. BACKGROUND

[0002] Vacuum microwave emission devices have the function of amplifying microwave power, and are key components of satellite communication, data transmission and microwave imaging devices. As a practical space traveling wave tube, a vacuum microwave emission device must ensure long service life, high efficiency and high reliability, which mainly depends on the performance of the hot cathode. The hot cathode is a cathode that obtains an electron current through the emission of hot electrons from the cathode material. A new generation of cathodes, Ba diffusion cathodes, appeared in the 1950s, which rely on the continuous diffusion of active barium atoms from the inside of the cathode to the surface of the cathode to obtain a low work function, thereby obtaining a larger electron current and increasing the emission current density. To date, the substrate material of the hot cathode has gradually developed from pure metal to mixed substrate. So far, diffusion cathodes have developed into various forms, mainly including reserve cathodes (L-type cathodes), barium tungsten cathodes (B-type cathodes), coated barium tungsten cathodes (M-type cathodes), mixed substrate cathodes (MM-type cathodes), coated mixed substrate cathodes (CMM-type cathodes) and scandium cathodes (scandium acid salt cathodes, scandium tungsten mixed substrate cathodes and top layer scandium-containing cathodes, etc.). An electron source with high emission current density and long service life is one of the challenges faced by the development of high-power vacuum electron devices: electron devices operating in the millimeter wave and terahertz range require a spatial charge limited current density of up to 100 A / cm 2 , the emission requires uniformity, and the service life exceeds 10 kh. Among the currently available hot cathodes, including M-type cathodes, mixed substrate cathodes and coated mixed substrate cathodes, the direct current density is 5-30 A / cm 2 ; however, when the emission current density exceeds 20 A / cm 2 , the service life is short due to the excessive evaporation of the cathode active material at high temperatures, and it still cannot meet the long service life requirement of vacuum electron devices.

[0003] The hot cathode has developed for more than a hundred years, and the real breakthrough in emission capacity is the scandium cathode, which has shown the highest emission capacity among all hot cathodes. Since 1967, Avraam et al. (Non-patent document 1) discovered the barium scandate cathode, several types of scandium cathodes have been developed, including impregnated scandate cathodes, scandium tungsten mixed substrate cathodes, pressed top layer scandium tungsten cathodes and thin film top layer scandium tungsten cathodes. Stratum et al. (Non-patent document 2) of Philips Company added Sc2O3 to the impregnant to prepare a scandate cathode, which achieved an emission current density of 10 A / cm 2Hasker (Non-patent document 3) used W+ScH2 powder to prepare a mixed base scandate cathode of W+Sc2O3, and the pulse emission current density was 100 A / cm 2 at 947℃. Yamamoto (Non-patent document 4) et al. respectively used W+Sc2O3, W+Sc2W3O 12 to improve the top layer scandate cathode by sputtering coating W-based top, and the pulse emission current density was 35 A / cm 2 and 80 A / cm 2 at 850℃. Gartner et al. (Non-patent document 5) prepared a nanoparticle top layer film composed of Re, Sc and Sc2O3411 impregnated salt by laser ablation deposition, and achieved a top layer scandate cathode with excellent performance, reaching an unprecedented pulse emission current density of about 400 A / cm 2 at 965℃.

[0004] Although scandate cathodes have excellent low-temperature large-current density characteristics, the problems of uneven emission and insufficient ion bombardment resistance have always restricted their practical application. At present, from the development status at home and abroad, the tungsten-scandium mixed matrix material system with large current and long service life has become the development mainstream. Patent document 1 (CN109834266A) proposes a method for preparing an impregnated scandium-containing diffusion cathode by microwave sintering, which is as follows: ammonium metatungstate and scandium nitrate are dissolved in deionized water to prepare a mixed solution, and after liquid-liquid doping, a submicron doped precursor powder is obtained by using sol-gel combined with two-stage hydrogen reduction method; the above-mentioned precursor powder is pressed into a scandium-containing diffusion cathode compact, and after microwave sintering, a uniform and round submicron porous tungsten skeleton structure cathode matrix is obtained; after impregnation, water washing and annealing, an impregnated scandium-containing diffusion cathode is obtained. Test the cathode in a flat panel diode, the pulse emission current density reaches 140 A / cm b at 950℃ 2The emission slope reaches 1.45 or more. However, in the method of the invention, since the tungsten-scandium mixed powder is prepared by liquid-liquid doping, the particle size of the tungsten oxide particles and the nucleation and growth process when the tungsten oxide particles are reduced to tungsten powder are sensitive to solution temperature, pH value, reduction temperature, etc. Therefore, the powder is prone to agglomeration during the liquid phase drying process, and the particle size distribution and the degree of agglomeration are difficult to control, which makes it difficult to obtain a suitable porosity and a low closed porosity during sintering, reduces the proportion of effective interconnected pores in the matrix, and further affects the salt immersion process. In addition, the organic matter added during the preparation of the gel needs to be calcined at high temperature to remove it, which will cause the agglomeration of the nano-oxide powder, especially the ultra-fine nano-scandium oxide powder, which is more prone to agglomeration. Patent document 2 (CN110303165A) proposes a kind of impregnated scandium oxide doped rhenium-tungsten multi-phase mixed base diffusion cathode and preparation method, specifically: ammonium salt containing tungsten element, ammonium salt containing rhenium element, scandium nitrate are mixed, deionized water, ethanol, ethylene glycol, etc. Liquid is used as a blending agent, the solid raw materials are made into a slurry and are stirred thoroughly, and the amount of blending agent is continuously adjusted to improve the uniformity of the mixture. However, since the mixture needs to go through processes such as crystallization precipitation, drying decomposition, hydrogen reduction, etc., although atomic-level or micro-component uniformity can be achieved, the particle size and morphology of the mixed powder are difficult to control, the slurry is prone to agglomeration after drying decomposition and reduction, the decomposition products cause obvious corrosion of the equipment, and the residual acid ions seriously affect the cathode emission performance. This will lead to poor stability of the cathode surface electron emission. In addition, the traditional mechanical mixing method (such as planetary ball milling) cannot completely open the nano-scandium oxide agglomerates, the tungsten particle surface coating effect is poor, and the ideal micron tungsten powder particle surface coated with nano-scandium oxide particles "satellite ball-shaped" W-Sc2O3 composite powder cannot be obtained, which will cause defects such as holes and loose structure in the matrix, affecting the quality of the salt immersion process and the emission stability.

[0005] Patent document 3 (prior invention patent of the present applicant) discloses a tungsten matrix for a hot cathode and a preparation method thereof, specifically: using wet fractionated narrow particle size, medium-fine particle tungsten or tungsten alloy powder as raw material, vacuum degassing, sieving, cold isostatic pressing, composite sintering, then hot isostatic pressing and re-sintering modification, full densification and copper infiltration, finally precision machining and high temperature vacuum decoppering. However, the patent does not involve the grinding and coating preparation process of tungsten-doped nano-powder.

[0006] In summary, in the prior art, the liquid-liquid doping method and the sol-gel method have the problems of poor control of tungsten particle morphology, particle size distribution and agglomeration degree, and the problems of poor nanoscale scandium oxide depolymerization and tungsten particle surface coating effect in the traditional mechanical mixing method are difficult to solve; in addition, the above problems will directly affect the cathode substrate surface morphology and the distribution state of the doping components in the substrate, therefore, realizing the uniform distribution of Sc on the surface of tungsten particles and the surface of the porous tungsten skeleton is the key support for synthesizing tungsten scandium mixed matrix material as the core component of future high-power thermionic cathode, the present application will solve the problem of nanoscale scandium oxide agglomeration, at the same time, prepare "satellite spherical" W-Sc2O3 composite powder with uniform nanoscale scandium oxide particles coated on the surface of micrometer tungsten powder particles, and prepare tungsten scandium mixed matrix.

[0007] Prior art Non-patent document 1: Avraam I F, Anna I S, Irina V J. U.S. patent, US3358178A, 1967. Non-patent document 2: Stratum A J A V, Os J G V, Blatter J R, et al. U.S. patent, US04007393A, 1977. Non-patent document 3: Hasker J, Esdonk J V, Crombeen J E. Applied Surface Science , 1986, 26(2), 173. Non-patent document 4: Yamamoto S, Sasaki S, Taguchi S, et al. Applied Surface Science , 1988, 33(4), 1200. Non-patent document 5: Gartner G, Geittner P, Lydtin H, et al. Applied Surface Science , 1997, 111(2), 11. Patent document Patent document 1: CN109834266A Patent document 2: CN 110303165A Patent document 3: CN108251734B Summary of the application

[0008] In view of the problems existing in the prior art, the main purpose of the present application is to provide a preparation method of a tungsten-scandium mixed base body for a high-power hot cathode, which can solve the problem of nanometer scandium oxide agglomeration in the manufacturing process of a cathode base body, control the morphology of tungsten particles and the uniformity of particle size distribution, obtain "satellite spherical" W-Sc2O3 composite powder in which micron tungsten powder particles are uniformly coated with nanometer scandium oxide particles, and prepare the tungsten-scandium mixed base body for the high-power hot cathode.

[0009] One technical solution of the present application is a tungsten-scandium mixed base body for a high-power hot cathode, characterized in that the tungsten-scandium mixed base body has a through-hole porosity of 18-30%, an average pore size of 0.3-2.0 μm, a scandium oxide content of 1-10 wt%, a closed porosity of ≤1%, and a sintered grain size of the scandium oxide of no more than 1 μm.

[0010] Further, the through-hole porosity is preferably 18% to 26%; the average pore size is preferably 0.5 μm to 2.0 μm; the scandium oxide content is preferably 2 wt% to 8 wt%; and the closed porosity is preferably 0.2% to 1.0%.

[0011] Another technical solution of the present application is the preparation method of the above-mentioned tungsten-scandium mixed base body for a high-power hot cathode, which comprises the following steps (refer to Figure 1 the drawing): (1) Tungsten powder grading: micron-grade tungsten powder is subjected to grading treatment in a particle size control device to remove coarse powder and fine powder, so as to obtain narrow particle size tungsten powder with a laser particle size distribution span of no more than 1.2; (2) Grinding and coating: the narrow particle size tungsten powder after grading in step (1), nanometer scandium oxide powder and a certain amount of surfactant are ground and coated with anhydrous ethanol as a dispersant, so as to obtain tungsten-scandium grinding slurry; (3) Vacuum drying: the tungsten-scandium grinding slurry obtained in step (2) is subjected to vacuum drying, so as to obtain "satellite spherical" tungsten-scandium composite powder in which micron tungsten powder particles are coated with nanometer scandium oxide particles; (4) Reduction and purification: the tungsten-scandium composite powder obtained in step (3) is heated in a high-purity hydrogen atmosphere for reduction and purification; (5) Sieving: the tungsten-scandium composite powder after hydrogen reduction and purification in step (4) is subjected to sieving treatment; (6) Batch mixing: the tungsten-scandium composite powder after sieving in step (5) is subjected to batch mixing treatment; (7) Cold isostatic pressing: the tungsten-scandium composite powder after batch mixing in step (6) is loaded into a rubber sleeve mold for cold isostatic pressing, so as to form a tungsten-scandium compact; (8) Hydrogen pre-sintering: the tungsten-scandium compact after cold isostatic pressing in step (7) is subjected to hydrogen pre-sintering, so as to obtain a tungsten-scandium pre-sintered compact; (9) Precision machining: the tungsten-scandium pre-sintered compact obtained in step (8) is subjected to precision machining, so as to obtain a tungsten-scandium pre-sintered body with appropriate dimensions. (10) Hydrogen high-temperature sintering: the tungsten scandium pre-sintered body after precision machining in step (9) is subjected to hydrogen high-temperature sintering, thereby obtaining a tungsten scandium mixed substrate for a high-power thermionic cathode.

[0012] Further, the narrow particle size tungsten powder in step (1) is obtained by dry classification of reduced tungsten powder, and the dry classification is jet classification. In the preparation method of the present application, commercially available medium-fine particle size tungsten powder is used as raw material, wherein the Fisher particle size of the tungsten powder is 1-8 μm. The raw material tungsten powder is subjected to dry classification, and the cathode special tungsten powder after classification is a narrow particle size distribution tungsten powder with a laser particle size distribution span (SPAN=(D 90 -D 10 ) / D 50 ) of not more than 1.2.

[0013] Further, the average particle size of the nano scandium oxide powder in step (2) is 10-100 nm, and the purity is not less than 99.9%. Preferably, the average particle size of the nano scandium oxide powder is 20 nm~80 nm.

[0014] Further, the mass fraction of the nano scandium oxide powder in step (2) relative to the total mass of tungsten powder and scandium oxide is 1-10 wt%; preferably, the mass fraction of scandium oxide is 2%~8wt%.

[0015] Further, the surface active agent in step (2) is dodecanoic acid, octadecanoic acid or oleic acid, and the mass fraction relative to the total mass of tungsten powder and scandium oxide is 0.1-5 wt%; preferably, the mass fraction of the surface active agent can be 0.5%~4wt%.

[0016] Further, the grinding and coating method in step (2) is: taking anhydrous ethanol as a dispersing agent, mixing the classified narrow particle size tungsten powder, nano scandium oxide powder and surface active agent in a grinding and coating machine for 0.5-8 h at a rotation speed of 3000-6000 r / min, to obtain a tungsten scandium grinding slurry. Preferably, the mixing time can be 2 h~7h; the rotation speed is 3000 r / min~5000 r / min.

[0017] Further, the vacuum drying method of step (3) is: the tungsten scandium grinding slurry is dried in a vacuum drying machine, the vacuum degree is 1-10 Pa, the drying time is 5-60 min, and the "satellite spherical" tungsten scandium composite powder with micron tungsten powder particles coated with nano scandium oxide particles is obtained. Under the field emission scanning electron microscope (FESEM) 200 times field of view, the number of nano scandium oxide agglomerates with a size greater than 1 μm is ≤3; under the 1000 times FESEM field of view, the number of nano scandium oxide agglomerates with a size greater than 1 μm is ≤1. Specifically, the vacuum degree is preferably 1 Pa~7 Pa; the time is preferably 10 min~50 min.

[0018] Further, in the reduction purification of step (4), the high-purity hydrogen gas has a dew point of not higher than -60℃, the heating temperature is 400-800℃, and the time is 4-8 h. The preferred heating temperature is 400℃~700℃; the preferred holding time is 5h~8h.

[0019] Further, the screen specification used in the sieving process of step (5) is 100-1000 mesh, and the preferred screen specification used is 300~900 mesh.

[0020] Further, in the batch mixing process of step (6), a three-dimensional mixer is used to fully mix the sieved tungsten scandium composite powder, high-purity argon gas is used as the protective gas throughout the process, the speed is adjusted to 30-100 r / min, and the time is 30-120 min, to finally obtain the high-uniformity purified cathode special tungsten scandium composite powder. The preferred speed is 50 r / min~100r / min; the preferred time is 30min~100min.

[0021] Further, the pressure of the cold isostatic pressing of step (7) is 150-250 MPa, and the holding time is 5-30 min. The preferred pressure is 180 MPa~240 MPa; the preferred time is 10 min~25 min.

[0022] Further, in the hydrogen pre-sintering of step (8), the high-purity hydrogen gas has a dew point of not higher than -60℃, the sintering temperature is 1200-1500℃, and the sintering time is 30-120 min. The preferred sintering temperature is 1300℃~1500℃; the preferred sintering time is 30 min~100 min.

[0023] Further, in the high-temperature hydrogen sintering of step (10), the high-purity hydrogen gas has a dew point of not higher than -60℃, the sintering temperature is 1600-2000℃, and the sintering time is 30-120 min. The preferred sintering temperature is 1700℃~2000℃; the preferred sintering time can be 50 min~120 min.

[0024] The structure is beneficial to uniform distribution of the scandium oxide on the surface of the tungsten skeleton, and solves the agglomeration problem of the nano scandium oxide powder in the manufacturing process of the mixed matrix.

[0025] An application, the high-power tungsten scandium mixed matrix for a hot cathode is used in a high-power terahertz device.

[0026] According to the high-power tungsten scandium mixed matrix for a hot cathode, the electron emission capacity and emission uniformity of the scandium cathode depend on the distribution and uniformity of the scandium on the cathode surface to a great extent. The research shows that the diffusion rate of Sc is much lower than that of Ba, and the factors beneficial to the improvement of the Sc surface uniformity are as follows: one is the dispersion distribution of the small size (nano level) scandium oxide in the matrix, which makes the reaction of Sc2O3, W and other substances evenly spread throughout the matrix, so that Sc can be fully and uniformly released in the manufacturing and activation process; two is that the W particle size of the composed matrix is concentrated, and the average particle size is small, which can make the diffusion path in the porous W matrix be more and the distance be shorter, and is beneficial to the diffusion of the active Sc to the surface along the connected pores and the formation of uniform coverage on the tungsten particle surface, and further enhances the ion bombardment resistance.

[0027] Furthermore, the preparation method according to the present application is different from the conventional process in that the nano-scanium oxide powder, the narrow-size tungsten powder and a certain amount of surfactant are ground and coated with anhydrous ethanol as a dispersant, so that the micron tungsten powder particle surface is uniformly coated with nano-scanium oxide particles to form a "satellite spherical" structure. The particle size and distribution state of the tungsten powder and the scandium oxide powder are strictly controlled through jet classification, grinding and coating, reduction and purification and other processes in the preparation process, thereby solving the agglomeration problem of the nano-scanium oxide powder in the mixed matrix manufacturing process, and uniformly coating the scandium oxide on the surface of the tungsten powder particles and in the mixed matrix framework. Specifically, the tungsten powder with uniform size and narrow particle size distribution is obtained by jet classification; the nano-scanium oxide powder is uniformly coated on the surface of the tungsten powder under the action of mechanical forces such as centrifugal force, friction force and shear force by the grinding and coating method; the powder agglomeration is inhibited by the surfactant and vacuum drying, and the "satellite spherical" tungsten scandium composite powder with uniform size, narrow particle size distribution and tungsten powder particles uniformly coated with nano-scanium oxide particles is obtained. Due to the uniform distribution of nano-scanium oxide on the surface of micron tungsten powder particles and in the mixed matrix, the deformation is uniform, stable and controllable when the matrix shrinks in volume in all directions during cold isostatic pressing, hydrogen pre-sintering and hydrogen high-temperature sintering, the consistency of pressing and sintering is high, the repeatability is good, the sintered scandium oxide particles are small and uniform, and thus the tungsten scandium mixed matrix with moderate porosity, small and narrow pore size distribution, low closed porosity, low impurity content and good machining performance can be obtained, which ensures the structural stability of subsequent impregnation and precision machining, and the high cathode yield, and is suitable for batch preparation and production.

[0028] The through-hole porosity of the tungsten scandium mixed matrix prepared by the present application is 18-30%, the average pore size is 0.3-2.0 μm, the scandium oxide content is 1-10 wt%, the closed porosity is ≤1%, and the sintered scandium oxide grain size in the mixed matrix is not more than 1 μm. As can be seen, the tungsten scandium mixed matrix of the present application has the advantages of moderate porosity, small and narrow pore size distribution, uniform framework composition, low closed porosity and low impurity content, and thus can be applied to high-power terahertz devices and other scenarios, and is expected to replace the conventional M-type impregnated barium tungsten cathode. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The process flow chart of the preparation method of the present application; Figure 2 The field emission scanning electron microscope (FESEM) photo of the nano-scanium oxide powder particles (200 times) used in Example 1 of the present application; Figure 3 The field emission scanning electron microscope (FESEM) photo of the ground and coated tungsten scandium composite powder (200 times) of Example 1 of the present application; Figure 4This is a field emission scanning electron microscope (FESEM) image of the ground and coated tungsten-scandium composite powder (1000x magnification) of Example 1 of the present invention; Figure 5 This is a field emission scanning electron microscope (FESEM) image of the ground and coated tungsten-scandium composite powder (20,000x magnification) of Example 1 of the present invention; Figure 6 These are photos of the energy dispersive spectroscopy (EDS) results of the tungsten-scandium composite powder ground and coated in Example 1 of the present invention. Figures (ae) show the EDS overlay, W distribution, O distribution, Sc distribution, and XRD pattern, respectively. Figure 7 This is a 500x field emission scanning electron microscope (FESEM) image of the tungsten-scandium mixed matrix microstructure of Example 1 of the present invention; Figure 8 This is a 1000x field emission scanning electron microscope (FESEM) image of the tungsten-scandium mixed matrix microstructure of Example 1 of the present invention; Figure 9 This is a 5000x field emission scanning electron microscope (FESEM) image of the tungsten-scandium mixed matrix microstructure of Example 1 of the present invention. Figure 10 This is a pore size distribution curve of the porous tungsten-scandium hybrid substrate used for the hot cathode in Embodiment 1 of the present invention, obtained by mercury intrusion porosimetry.

[0030] Figure 11 This is a field emission scanning electron microscope (FESEM) image of the tungsten-scandium mixed matrix microstructure of Comparative Example 1 of this invention, magnified 1000 times.

[0031] Figure 12 This is a field emission scanning electron microscope (FESEM) image of the microstructure of commercially available tungsten powder at 10,000x magnification, which is Comparative Example 2 of this invention. Detailed Implementation

[0032] The following is in conjunction with the appendix Figures 1-12 The present invention will be described in detail below with specific embodiments. The embodiments shown below are merely illustrative and are not intended to limit the scope of protection of the present invention.

[0033] All mass percentages mentioned in this invention refer to the total mass of tungsten powder and scandium oxide powder.

[0034] like Figure 1 As shown, the method for preparing the tungsten-scandium hybrid substrate for the high-power hot cathode of the present invention specifically includes the following steps: (1) Tungsten powder classification: Commercially available tungsten powder with medium and fine particle size of 1-8 μm is classified by dry method in particle size control equipment to remove coarse and fine powder and obtain narrow particle size tungsten powder with a laser particle size distribution span of no more than 1.2.

[0035] (2) Milling coating: the step (1) classified narrow particle size tungsten powder, 1-10wt% of the mass fraction of the particle size of 10-100 nm of scandium oxide powder and 0.1-5wt% of the mass fraction of the surface active agent are mixed in the milling coating machine for 0.5-8 h, the rotation speed is 3000-6000 r / min, and the tungsten scandium milling slurry is obtained.

[0036] (3) Vacuum drying: the tungsten scandium milling slurry obtained in step (2) is vacuum dried in a vacuum drying machine, the vacuum degree is 1-10 Pa, the drying time is 5-60 min, and the "satellite ball-shaped" tungsten scandium composite powder with micron tungsten powder particle surface coated with nano scandium oxide particles is obtained.

[0037] (4) Reduction purification: the tungsten scandium composite powder obtained in step (3) is heated in high-purity hydrogen atmosphere for reduction purification. The dew point of high-purity hydrogen is not higher than-60℃, the heating temperature is 400-800℃, and the time is 4-8 h.

[0038] (5) Sieving: the tungsten scandium composite powder after reduction purification in step (4) is sieved, and the screen specification used in the sieving treatment is 100-1000 mesh.

[0039] (6) Batch mixing: the tungsten scandium composite powder after sieving in step (5) is batch mixed, the three-dimensional mixer is used to mix the tungsten scandium composite powder after sieving, the high-purity argon gas is used as the protective gas in the whole process, the speed is adjusted to 30-100 r / min, the time is 30-120 min, and finally the high-uniformity and purified tungsten scandium composite powder is obtained.

[0040] (7) Cold isostatic pressing: the tungsten scandium composite powder after batch mixing in step (6) is loaded into a rubber sleeve mold for cold isostatic pressing. The pressure of cold isostatic pressing is 150-250 MPa, and the pressure holding time is 5-30 min.

[0041] (8) Hydrogen pre-sintering: the tungsten scandium compact after cold isostatic pressing in step (7) is subjected to hydrogen pre-sintering to obtain a tungsten scandium pre-sintered compact. The dew point of high-purity hydrogen used in hydrogen pre-sintering is not higher than-60℃, the sintering temperature is 1200-1500℃, and the sintering time is 30-120 min.

[0042] (9) Precision machining: the tungsten scandium pre-sintered compact obtained in step (8) is subjected to precision machining to obtain a tungsten scandium pre-sintered body with appropriate size. (10) Hydrogen high-temperature sintering: the tungsten scandium pre-sintered body after precision machining in step (9) is subjected to hydrogen high-temperature sintering, the dew point of high-purity hydrogen used is not higher than-60℃, the sintering temperature is 1600-2000℃, the sintering time is 30-120 min, and the tungsten scandium mixed substrate for high-power hot cathode is obtained.

[0043] It should be noted that the surfactants and dispersants added during the preparation process are dispersed and disappear during the subsequent sintering process, and therefore only the mass of the tungsten powder and the scandium oxide powder is calculated in the present application. The tungsten scandium mixed substrate for a high-power thermionic cathode obtained by the above preparation method has the following specific composition and mass percentage: 1-10 wt% scandium oxide and the balance tungsten. The tungsten scandium mixed substrate has a through-hole porosity of 18-30%, an average pore size of 0.3-2.0 μm, a scandium oxide content of 1-10 wt%, and a closed porosity of ≤1%. The sintered grain size of the scandium oxide in the mixed substrate is not more than 1 μm.

[0044] The present application is described in detail below by way of examples and drawings.

[0045] In Example 1, commercially available tungsten powder having an average particle size of 3.5 μm was subjected to dry jet classification treatment, and the laser particle size distribution span of the classified tungsten powder was 1.15. Nanometer scandium oxide particles having a particle size of 40 nm were prepared, and the scandium oxide powder was added in an amount of 5 wt% of the total mass. Then, the classified tungsten powder, the scandium oxide powder having a particle size of 50 nm, and dodecanoic acid having a mass percentage of 0.5 wt% were mixed in a grinding and coating machine for 3 h at a rotation speed of 3500 r / min, using anhydrous ethanol as a dispersant, to obtain tungsten scandium grinding slurry. The tungsten scandium grinding slurry was vacuum dried in a vacuum dryer at a vacuum degree of 5 Pa for 30 min, to obtain “satellite ball-shaped” W-Sc2O3 tungsten scandium composite powder in which micron tungsten powder particles were coated with nanometer scandium oxide particles. The tungsten scandium composite powder was reduced and purified at 800°C for 5 h in a high-purity hydrogen atmosphere having a dew point of not higher than -60°C. The tungsten scandium composite powder was sieved through a 400-mesh sieve and mixed in a three-dimensional mixing device under the protection of a high-purity argon atmosphere. The entire process was carried out under the protection of high-purity argon gas, the mixing rate was 35 r / min, and the mixing time was 30 min, to finally obtain high-uniformity cathode special tungsten scandium composite powder. The tungsten scandium composite powder was then uniformly loaded into a φ10×150 mm polyurethane mold, cold isostatic pressed at a pressure of 250 MPa for 5 min, to obtain a tungsten scandium rod blank with a regular shape. The tungsten scandium rod blank was placed in a high-temperature tungsten mesh furnace and sintered at 1300°C for 30 min in a high-purity hydrogen atmosphere, to obtain a pre-sintered substrate with suitable porosity and pore size distribution. After the pre-sintered substrate was precisely machined to a suitable size, it was sintered at 1800°C for 50 min in a high-purity hydrogen atmosphere. Finally, the following tungsten scandium mixed substrate 1 was obtained: the scandium oxide content was 5 wt%, the through-hole porosity was 24%, the average pore size (by mercury porosimetry) was 0.5 μm, the closed porosity was 0.2%, and the sintered grain size of the scandium oxide in the mixed substrate was not more than 1 μm.

[0046] The structure of the specific tungsten scandium mixed substrate can be seen from the attached Figures 2-10It can be seen from the analysis photos that: in particular, Figure 2 The field emission scanning electron microscope (FESEM) photo of the nano scandium oxide powder particles (200 times) used in Example 1 of the present application, and Figures 3-5 Compared with the field emission scanning electron microscope (FESEM) photos of the 200 times, 1000 times and 20000 times of the grinding coated tungsten scandium mixed powder of Example 1 of the present application, Figure 2 The nano scandium oxide powder particles in the present application show uneven agglomerated blocks, while at the same magnification, Figure 3 The tungsten scandium composite powder after the strict classification grinding coating of the present application shows uniform particle size and uniform distribution, and no agglomerates are observed; from Figure 4 It can be seen from the high magnification grinding coated tungsten scandium composite powder photo shown in the present application that the nano scandium oxide is uniformly distributed on the surface of the tungsten powder, forming a uniformly distributed tungsten scandium composite powder, and the agglomeration problem is basically solved. In addition, Figure 5 It can be observed from the 20000 times field emission scanning electron microscope (FESEM) photo of the grinding coated tungsten scandium composite powder of the present application that the nano scandium oxide powder is uniformly coated on the surface of the tungsten particles to form a "satellite spherical" structure, and the uniform distribution of the nano scandium oxide on the surface of the tungsten particles is determined. In addition, Figure 6 The EDS spectrum result photo of the grinding coated tungsten scandium composite powder of Example 1 of the present application, figures (a-e) are EDS superimposed graph, W distribution graph, O distribution graph, Sc distribution graph and XRD graph, respectively, from which it can be observed that Sc and O in the tungsten scandium composite powder are highly coincident, and no impurity peak is found in the XRD spectrum, and from (a)~(d) it can be seen that the nano scandium oxide is dispersedly distributed, uniform as a whole, and there is no scandium oxide agglomerate.

[0047] Figures 7-9 The field emission scanning electron microscope (FESEM) photos of the tungsten scandium mixed matrix of Example 1 of the present application at 500 times, 1000 times and 5000 times can be seen that the tungsten scandium mixed matrix after high temperature hydrogen sintering continues the overall uniformity of the tungsten scandium composite powder, the deformation of the matrix in each direction is uniform, stable and controllable when the volume shrinks, the sintering consistency is high, and the scandium oxide particles after sintering are small and uniform. Especially from Figure 9 It can be seen from the high magnification photo of the present application that the sintering neck of the tungsten scandium mixed matrix is complete and smooth, the hole size is close, and the through holes are many. The scandium oxide particles adhere to the vicinity of the sintering neck, thereby inhibiting the migration and diffusion of tungsten and improving the structural stability of the cathode at high temperature.

[0048] Figure 10The photos of the grinding coated tungsten scandium mixed matrix mercury injection method pore size distribution curve results of Example 1 of the present application are shown in the figure. From the data in the figure, it can be seen that the tungsten scandium mixed matrix has moderate porosity (24.16%), small pore size (0.54 μm) and narrow distribution, low closed pore rate, average pore size (mercury injection method) 0.53 μm, low impurity content, and good machining performance, which is beneficial to the subsequent impregnation of the emitting salt and the structural stability of precision machining.

[0049] Example 2, commercially available tungsten powder with an average particle size of 4.2 μm is subjected to dry jet classification, and the classification of the tungsten powder has a laser particle size distribution span (SPAN) of 1.05; the nano scandium oxide particle size is 30 nm, and the scandium oxide powder is added at a proportion of 4 wt% of the total mass, with anhydrous ethanol as a dispersant, the classified tungsten powder, the scandium oxide powder with a particle size of 30 nm, and 1 wt% of dodecanoic acid are mixed in a grinding coating machine for 4 h at a speed of 4800 r / min, to obtain a tungsten scandium grinding slurry. The tungsten scandium grinding slurry is vacuum dried in a vacuum dryer at a vacuum degree of 6 Pa for 30 min, to obtain a "satellite ball-shaped" W-Sc2O3 composite powder with micron tungsten powder particles coated with nano scandium oxide particles on the surface. The tungsten scandium composite powder is purified at 700℃ under high-purity hydrogen atmosphere for 4 h; it is sieved through a 300 mesh sieve and mixed in a three-dimensional mixing device under the protection of high-purity argon gas, the different batches of tungsten scandium composite powder are mixed thoroughly, the whole process is protected by high-purity argon gas, the mixing rate is 40 r / min, and the time is 40 min, to finally obtain a high-uniformity tungsten scandium composite powder for cathodes. The tungsten scandium composite powder is uniformly loaded into a φ10x150 mm polyurethane mold, and cold isostatic pressing is performed at a pressure of 200 MPa for 10 min to obtain a tungsten scandium rod blank with regular shape; the tungsten scandium rod blank is placed in a high-temperature tungsten mesh furnace and sintered at 1400℃ under high-purity hydrogen atmosphere for 30 min, to obtain a pre-sintered matrix with suitable porosity and pore size distribution; after the pre-sintered matrix is precisely machined to a suitable size, it is sintered at 1900℃ under high-purity hydrogen atmosphere for 40 min; finally, the tungsten scandium mixed matrix 2 of the present application is obtained, which has a scandium oxide content of 4 wt%, a through-hole porosity of 22%, an average pore size (mercury injection method) of 0.6 μm, and a closed pore rate of 0.7%, and the scandium oxide sintered in the mixed matrix has a grain size of not more than 1 μm.

[0050] The tungsten scandium grinding slurry of Example 2 and the obtained tungsten scandium mixed matrix have similar surface structures to those of Example 1, and are not described again. Figures 2-10

[0051] ​Example 3, commercially available tungsten powder with an average particle size of 6.0 μm was subjected to dry jet milling, and the laser particle size SPAN of the milled tungsten powder was 1.10; the nano-sized scandium oxide had a particle size of 70 nm, and was added in an amount of 4 wt% of the total mass; anhydrous ethanol was used as a dispersant, the milled tungsten powder, the scandium oxide powder with a particle size of 70 nm, and 1.5 wt% of dodecanoic acid were mixed in a grinding and coating machine at a rotation speed of 5500 r / min for 8 h to obtain a tungsten scandium grinding slurry. The tungsten scandium grinding slurry was subjected to vacuum drying in a vacuum drying machine at a vacuum degree of 10 Pa for 60 min to obtain "satellite ball-shaped" W-Sc2O3 composite powder in which micron-sized tungsten powder particles were coated with nano-sized scandium oxide particles. The tungsten scandium composite powder was subjected to a replacement purification process at 600°C under a high-purity hydrogen atmosphere for 8 h; the powder was sieved through a 400-mesh sieve and was mixed in a three-dimensional mixing device under the protection of a high-purity argon atmosphere, the different batches of tungsten scandium composite powder were mixed thoroughly, the whole process was carried out under the protection of high-purity argon gas, the mixing rate was 50 r / min, and the mixing time was 60 min, to obtain high-uniformity tungsten scandium composite powder for cathodes. The powder was uniformly loaded into a φ10x150 mm polyurethane mold, and was cold isostatic pressed at a pressure of 150 MPa for 30 min to obtain tungsten scandium rod blanks with regular shapes; the tungsten scandium rod blanks were placed in a high-temperature tungsten mesh furnace, and were sintered at 1500°C under a high-purity hydrogen atmosphere for 120 min to obtain a pre-sintered matrix with suitable porosity and pore size distribution; after the pre-sintered matrix was precisely machined to a suitable size, it was sintered at 2000°C under a high-purity hydrogen atmosphere for 120 min; finally, the tungsten scandium mixed matrix of the application was obtained, the scandium oxide content was 4 wt%, the open porosity was 26%, the average pore size (by mercury porosimetry) was 0.8 μm, the closed porosity was 0.7%, and the sintered scandium oxide grains in the mixed matrix had a size of no more than 1 μm.

[0052] Comparative Example 1, except that the tungsten powder and the scandium oxide powder were mixed by a planetary ball milling method, the other processes were the same as in Example 1. Finally, a tungsten scandium mixed matrix was obtained, the scandium oxide content was 5 wt%, the open porosity was 22%, the average pore size (by mercury porosimetry) was 1.8 μm, the closed porosity was 2.5%, and the sintered scandium oxide grains in the mixed matrix had a size of more than 40 μm. Figure 11The image shown is a 1000x field emission scanning electron microscope (FESEM) image of the tungsten-scandium mixed matrix prepared in this comparative example. It is evident that in the tungsten-scandium mixed matrix prepared using the conventional mechanical mixing method (planetary ball milling), scandium oxide exhibits severe agglomeration after sintering, with agglomerate sizes exceeding 40 μm. The scandium oxide distribution is uneven, and local shrinkage is uneven. Compared to Comparative Example 1, in Example 1, due to the use of a grinding and coating method with anhydrous ethanol as a dispersant and dodecanoic acid as a surfactant, the resulting "satellite-shaped" W-Sc2O3 composite powder, in which micron-sized tungsten powder particles are coated with nano-scandium oxide particles, shows that under a 200x field of view using FESEM, the number of nano-scandium oxide agglomerates larger than 1 μm is ≤3 (…). Figure 3 As shown); under 1000x FESEM field of view, the number of scandium oxide nanoparticles larger than 1 μm is ≤1 ( Figure 4 Furthermore, in the tungsten-scandium composite powder prepared by the grinding and coating method, the scandium oxide grain size after sintering does not exceed 1 μm and exhibits a uniform granular shape. Figures 7-9 (As shown). Therefore, from Figures 2-9 The process of change shows that, compared with the traditional planetary ball milling method, the grinding and coating method can control the particle size and distribution of tungsten powder and scandium oxide powder during the preparation process. It can effectively solve the problem of agglomeration of nano scandium oxide powder in the process of manufacturing the mixed matrix, so that nano scandium oxide is fully and evenly distributed on the surface of tungsten powder particles and in the mixed matrix. In other words, it improves the problem of uneven emission caused by uneven scandium oxide distribution from the source.

[0053] Comparative Example 2 was identical to Example 2, except that the tungsten powder was not subjected to dry jet classification and was directly used as commercially available tungsten powder with a Fisher particle size of 1-8 μm. The final product was a tungsten-scandium mixed matrix with a scandium oxide content of 4 wt%, a porosity of 19%, an average pore size (mercury intrusion porosimetry) of 1.2 μm, and a closed-pore ratio of 3.5%. It is evident that the porosity of this matrix was significantly lower than that of the tungsten-scandium mixed matrix in Example 2, while the closed-pore ratio was significantly higher.

[0054] In addition, in Comparative Example 2, such as Figure 12 As shown, commercially available tungsten powder has uneven particle size and contains many hard agglomerates, resulting in poor dispersibility and a wide particle size distribution. This makes it prone to defects such as sintering deformation and high closed-porosity during subsequent processes such as cold isostatic pressing, precision machining, and high-temperature hydrogen sintering. This is because tungsten powder is the most important component of the tungsten-scandium mixed matrix framework, and suitable tungsten powder size and particle size distribution are prerequisites for obtaining a high-quality tungsten-scandium mixed matrix. Furthermore, since the grinding and coating method can only change the distribution state of scandium oxide and eliminate nano-scandium oxide agglomerates, but cannot adjust the size and particle size distribution of tungsten powder, dry jet classification of commercially available tungsten powder is also an important step in forming a uniform distribution.

[0055] In summary, the tungsten-scandium mixed matrix prepared by the preparation method has the advantages of moderate porosity, small pore size, narrow pore size distribution, low closed pore rate, low impurity content and good machining performance, which is conducive to obtaining ideal cathode size and higher cathode yield, and is suitable for batch preparation and production. Based on this, the mixed matrix of the application can be applied to high-power terahertz devices and other scenes, and is expected to replace conventional M-type immersed barium tungsten cathodes.

[0056] The above description of disclosed embodiments enables those skilled in the art to carry out or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A tungsten-scandium hybrid substrate for a high-power hot cathode, characterized in that, The tungsten-scandium mixed matrix has a through-pore porosity of 18-30%, an average pore diameter of 0.3-2.0 μm, a scandium oxide content of 1-10 wt%, a closed-pore rate of ≤1%, and a scandium oxide grain size of no more than 1 μm after sintering.

2. A method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode, characterized in that, Includes the following steps: (1) Tungsten powder classification: Micron-sized tungsten powder is classified in a particle size control device to remove coarse and fine powder, and narrow-particle tungsten powder with a laser particle size distribution span of no more than 1.2 is obtained; (2) Grinding and coating: Using anhydrous ethanol as a dispersant, the narrow-particle tungsten powder, nano-scandium oxide powder and surfactant after classification in step (1) are ground and coated to obtain tungsten-scandium grinding slurry; (3) Vacuum drying: The tungsten-scandium grinding slurry obtained in step (2) is vacuum dried to obtain "satellite spherical" tungsten-scandium composite powder with nano-scandium oxide particles coated on the surface of micron-sized tungsten powder particles; (4) Reduction and purification: The tungsten-scandium composite powder obtained in step (3) is heated in a high-purity hydrogen atmosphere for reduction and purification; (5) Sieving: The tungsten-scandium composite powder after hydrogen purification and reduction in step (4) is sieved; (6) Batch Combination: The tungsten-scandium composite powder after sieving in step (5) is batch-combined; (7) Cold isostatic pressing: The tungsten-scandium composite powder after batching in step (6) is loaded into the rubber sleeve mold and cold isostatic pressing is performed to form a tungsten-scandium blank; (8) Hydrogen pre-sintering: The tungsten scandium blank formed by cold isostatic pressing in step (7) is subjected to hydrogen pre-sintering to obtain a tungsten scandium pre-sintered blank; (9) Precision machining: The tungsten scandium presintered billet obtained in step (8) is precision machined to obtain a tungsten scandium presintered body with suitable dimensions; (10) High-temperature hydrogen sintering: The tungsten-scandium pre-sintered body after precision machining in step (9) is subjected to high-temperature hydrogen sintering to obtain a tungsten-scandium mixed matrix for high-power hot cathode.

3. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The narrow-particle-size tungsten powder in step (1) is obtained by dry grading of commercially available tungsten powder with a Fisher particle size of 1-8 μm and medium to fine particle size.

4. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The average particle size of the nano-scandium oxide powder in step (2) is 10-100 nm, the purity is not less than 99.9%, and its mass ratio relative to the total mass of tungsten powder and scandium oxide powder is 1-10 wt%.

5. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The surfactant in step (2) is dodecanoic acid, octadecanoic acid or oleic acid, and its mass percentage relative to the total mass of tungsten powder and scandium oxide powder is 0.1-5 wt%.

6. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The grinding and coating method in step (2) is as follows: using anhydrous ethanol as a dispersant, the graded narrow-particle tungsten powder, nano-scandium oxide powder and surfactant are ground and mixed in a grinding and coating machine for 0.5-8 h at a speed of 3000-6000 r / min to obtain tungsten-scandium grinding slurry.

7. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The vacuum drying method in step (3) is as follows: the tungsten scandium grinding slurry is dried using a vacuum dryer with a vacuum degree of 1-10 Pa and a drying time of 5-60 min.

8. The method for preparing a tungsten-scandium hybrid matrix for a high-power hot cathode according to claim 2, characterized in that, In the reduction and purification process of step (4), the high-purity hydrogen gas used has a dew point of no higher than -60℃, the heating temperature is 400-800℃, and the time is 4-8 h.

9. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, In step (6), during the batch processing, a three-dimensional mixer is used to fully mix the sieved tungsten-scandium composite powder. High-purity argon is used as the protective gas throughout the process. The speed is adjusted to 30-100 r / min and the time is 30-120 min, finally obtaining a highly uniform tungsten-scandium composite powder for the purified cathode.

10. The method for preparing a tungsten-scandium hybrid substrate for a high-power hot cathode according to claim 2, characterized in that, The pressure of the cold isostatic pressing in step (7) is 150-250 MPa, and the holding time is 5-30 min.

11. The method for preparing a tungsten-scandium hybrid matrix for a high-power hot cathode according to claim 2, characterized in that, In step (8), the high-purity hydrogen used in the hydrogen pre-sintering has a dew point of no higher than -60°C, the pre-sintering temperature is 1200-1500°C, and the pre-sintering time is 30-120 min.

12. The method for preparing a tungsten-scandium hybrid matrix for a high-power hot cathode according to claim 2, characterized in that, In step (10), the high-purity hydrogen used in the high-temperature hydrogen sintering has a dew point of no higher than -60°C, a sintering temperature of 1600-2000°C, and a sintering time of 30-120 min.

13. An application in which the tungsten-scandium hybrid substrate for high-power hot cathode as described in claim 1 or the tungsten-scandium hybrid substrate for high-power hot cathode prepared by any one of claims 2 to 12 is used in a high-power terahertz device.

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