A method for preparing nano-cathode composite powder by spray drying

Abstract

This invention belongs to the field of materials preparation and new energy technology, specifically a method for preparing nano-cathode composite powder by spray drying, comprising the following steps: step (1) pretreatment of nano-tungsten powder; step (2) preparation of mixed slurry; step (3) ultrasonic atomization; step (4) spray drying of atomized droplets; and step (5) simultaneous reduction calcination to obtain nano-cathode composite powder prepared by spray drying. The nano-cathode composite powder prepared by this invention has a uniformly dispersed multi-component structure and optimized phase composition, which can significantly improve the emission current density of barium tungsten cathodes in practical applications.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation and new energy technology, specifically a method for preparing nano-cathode composite powder by spray drying. Background Technology

[0002] Barium-tungsten cathodes consist of a tungsten-based sponge and cathode salts filling the pores. At operating temperatures, the cathode salts undergo a thermochemical reaction with the tungsten matrix, reducing the work function of the tungsten surface and thus promoting electron escape from the tungsten surface, achieving high-density electron emission. The cathode salts play a crucial role in the performance of barium-tungsten cathodes; their compositional uniformity, phase composition, and content directly affect the emission current density, emission uniformity, and lifetime. Therefore, by precisely controlling and optimizing the production process and synthesis equipment of barium aluminate cathode salts, the overall performance of barium-tungsten cathodes can be significantly improved, enabling them to achieve higher efficiency and longer lifespan in electron emission devices.

[0003] Numerous studies have been conducted both domestically and internationally on the synthesis methods of barium aluminate cathode salts. Their elemental composition mainly includes Ba, Ca, Al, Sc, Y, and O, and their synthesis methods primarily include arc melting, mechanical mixing, sol-gel, and liquid-phase co-precipitation. With technological advancements and increasing application demands, the performance requirements for cathode salts are becoming increasingly stringent. Future development trends will focus on the following core areas: (1) Optimization of cathode salt formulations and synthesis processes: exploring new cathode salt formulations and optimizing synthesis processes to further enhance electron emission performance; (2) Innovation in synthesis processes: improving component uniformity and optimizing phase composition through innovative synthesis processes, thereby improving the performance and stability of cathode salts; (3) Intelligent and integrated development of synthesis equipment: developing new synthesis equipment, such as utilizing microwave technology and plasma technology, to improve the performance, stability, and synthesis efficiency of cathode salts through automation and integration.

[0004] With the rapid development of electronic materials technology in my country, the market demand for high-performance barium tungsten cathode materials is rising sharply. Therefore, research on key equipment and processes for preparing high-performance cathode salt composite powders is of great significance. The uniformity of cathode salt composition is a key factor determining the final product's phase composition, emission current density, and service life. Currently, the non-uniformity of compositional distribution in domestically produced cathode salts has become a major obstacle preventing their performance from matching that of imported products.

[0005] Traditional methods for synthesizing cathode salts have certain limitations. For example, arc melting easily introduces impurities at high temperatures and may be accompanied by component segregation and volatilization; mechanical mixing may introduce impurities due to friction and is difficult to achieve uniform dispersion at the molecular level; while sol-gel and liquid-phase coprecipitation methods have certain advantages, they are insufficient in terms of formulation flexibility and optimization of element addition. CN109926591B discloses a simplified method for preparing barium-tungsten cathodes, which uses microwave sintering technology and liquid-phase coprecipitation to prepare barium-tungsten cathodes, obtaining submicron-sized barium-tungsten cathodes with good porosity structure and generating nanoscale active materials on the surface, thereby improving emission performance. However, the liquid-phase coprecipitation method used in this technical solution has limitations in formulation flexibility and element addition, making it difficult to achieve uniform dispersion of components at the molecular level. Furthermore, this method has limited ability to precisely control the phase composition, resulting in large fluctuations in the effective emitter content, which affects the emission stability and lifespan of the cathode. CN105734332B discloses a method for preparing a porous tungsten bulk material with uniform and controllable pores. This method uses plasma spheroidization treatment and spark plasma sintering technology to prepare a porous tungsten bulk material with uniform pores, significantly improving the impregnation performance and emission stability of the barium tungsten cathode. However, this technical solution mainly focuses on the porosity control of the porous tungsten matrix, while paying less attention to the compositional uniformity and phase composition optimization of the cathode salt. It fails to fundamentally solve the problem of impurities and compositional segregation easily introduced into the cathode salt at high temperatures, limiting its application in high-performance barium tungsten cathodes.

[0006] Furthermore, the phase composition of the cathode salt has a significant impact on its performance. Taking barium aluminate cathode salt as an example, its synthesis products typically consist of multiple phases, among which Ba5CaAl4O 12 The effective emission material is considered the most critical component, and its content is often used to judge the performance of cathode salts. When preparing cathode salts using mechanical mixing, the content of the effective phase typically ranges from 20% to 55 wt%, and there are significant fluctuations in the effective phase content among different batches of synthesized products. Therefore, how to precisely control the phase composition during synthesis to obtain optimal electron emission performance has become a key research focus. Summary of the Invention

[0007] This invention provides a method for preparing nano-cathode composite powder using spray drying. By precisely controlling the pretreatment of raw materials, slurry ratio, and process parameters, it solves the problems of poor uniformity of cathode salt composition, large fluctuations in phase composition, and unstable electron emission performance in existing technologies. The nano-cathode composite powder prepared by this invention has a uniformly dispersed multi-component structure and optimized phase composition, which can significantly improve the emission current density of barium tungsten cathodes in practical applications.

[0008] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0009] Step (1) Pretreatment of nano-tungsten powder:

[0010] Step (1-a) Acid washing: Sonicate the nano-tungsten powder in a 5-10% oxalic acid solution at 60-80℃ for 30-60 min;

[0011] Step (1-b) Surface modification: The tungsten nanoparticles after acid washing in step (1-a) are added to an ethanol aqueous solution containing 1-3 wt% silane coupling agent and reacted at 80°C for 2 h; in the ethanol aqueous solution, water accounts for 5-10% of the total volume;

[0012] Step (1-c) Vacuum drying: Dry at 100-120℃ for 2-4 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0013] Step (2) Prepare the mixed slurry:

[0014] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate (APT) in deionized water at 60-80℃ to a concentration of 20-30 wt%, and add 0.1-0.3 wt% citric acid to obtain an ammonium tungstate solution;

[0015] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution, with a molar ratio of Ba:Ca:Al:Sc:Y = 4.8-5.2:0.9-1.1:3.8-4.2:0.005-0.015:0.003-0.010;

[0016] In step (2-c), pretreated nano-tungsten powder is added, accounting for 30-50 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 1.5-5.0 : 1. The slurry is then prepared by adding 0.1-0.5 wt% ammonium polyacrylate (NH4PAA) dispersant and 5-15 wt% polymethyl methacrylate (PMMA) microspheres (particle size 10-100 μm) pore-forming agent.

[0017] Step (2-d) Adjusting viscosity: Add ethanol to a water-ethanol volume ratio of 6-8:2-4, and homogenize with ultrasound at 200-400 W for 10-20 min until the slurry viscosity is 30-50 cP.

[0018] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 1.5-2.5 MHz and a power density of 50-100 W / cm³. 2 The ultrasonic generator generates atomized droplets (D 50=0.8-1.5 μm);

[0019] Step (4) Spray drying of atomized droplets: Under a nitrogen atmosphere, the inlet temperature is 180-220℃, the outlet temperature is 80-100℃, and the airflow velocity is 0.2-0.5 m / s.

[0020] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, heat to 300-400℃ and hold for 0.5-1h, then heat up and reduce in stages under controlled temperature in a moist hydrogen atmosphere:

[0021] First stage: Hold at 700-750℃ for 0.5-1h, with H2O concentration of 0.3-0.5 vol%, to decompose APT into tungsten trioxide;

[0022] Second stage: Maintain a constant temperature of 800-900℃ for 1.5-3 hours, with an H2O concentration of 0.1-0.2 vol%, to completely reduce tungsten trioxide to metallic tungsten and synthesize Ba5CaAl4O3. 12 The main phase is pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0023] This design is based on the following three points:

[0024] 1. Raw Materials: The pretreatment of nano-tungsten powder involves acid washing to remove the surface oxide layer, followed by coating with a silane coupling agent to form an organic-inorganic interface layer. This interface layer has a dual function: firstly, it reduces the surface energy of the tungsten powder, inhibiting high-temperature agglomeration; secondly, the silanol groups generated by its hydrolysis form a hydrogen bond network with the aluminate precursors in the subsequent slurry, providing chemical bonding anchors for the multiphase heterogeneous structure. Trace amounts of scandium and yttrium ions preferentially occupy aluminum-oxygen octahedral sites due to their high ionic field strength, inhibiting the distortion of the main phase through the lattice pinning effect. Synergistic selection of pore-forming agents and dispersants: The decomposition process of PMMA overlaps with that of ammonium tungstate (APT), and the generated carbon dioxide and water vapor can promote pore formation on the surface of WO3, the decomposition product of ammonium tungstate; while the carboxyl groups of ammonium polyacrylate (NH4PAA) adsorb onto the surface of metal ions through chelation, forming an electrostatic-spatial double layer, enabling the nano-tungsten powder to be uniformly dispersed in the solution.

[0025] 2. In terms of process, the segmented oxygen control in a wet hydrogen atmosphere forms the thermodynamic core: in the low-temperature stage (700-750℃), a relatively high water vapor concentration (0.3-0.5 vol%) maintains a moderate oxygen partial pressure, stabilizing the APT decomposition product tungsten trioxide in the +4 to +6 valence transition region, avoiding lattice distortion caused by direct deep reduction; in the high-temperature stage (800-900℃), the water vapor concentration is rapidly reduced (0.1-0.2 vol%), lowering the oxygen partial pressure below the critical threshold. At this point, the premixed nano-tungsten powder serves as a nucleation substrate, catalyzing the topological reduction reaction of tungsten trioxide → W through interfacial electron transfer; the spatiotemporal coupling of spray granulation and heat treatment achieves the directional assembly of the microstructure: submicron droplets generated by ultrasonic atomization form a core-shell structure under the action of surface tension—the nano-tungsten powder is encapsulated by the aluminate precursor solution. When entering the wet hydrogen reduction stage, the channels accelerate water molecule transport, allowing the reduction reaction to proceed from the particle surface to the interior, while the aluminate melt simultaneously fills the pores, ultimately forming an interpenetrating network structure of metallic tungsten grains and the oxide matrix.

[0026] 3. The oxalic acid pretreatment of nano-tungsten powder essentially modifies surface properties. Oxalic acid molecules etch high-energy crystal planes, causing them to form WO-Si covalent bridges with silane coupling agents. During vacuum drying, dehydration and condensation occur, forming a three-dimensional silicon-oxygen network. This makes the tungsten powder a heterogeneous nucleation substrate for aluminate precursors. This directly dictates that the slurry preparation must use a water-ethanol mixed solvent (volume ratio 7:3), as pure water hydrolyzes silicon-oxygen bonds, while pure ethanol cannot dissolve the nitrate precursor. The molar ratio of nitrates originates from Ba5CaAl4O3. 12 Oxygen stacking defect regulation in the unit cell: Barium ions fill the twelve-coordinate interstices to stabilize the framework, calcium ions occupy the octahedral sites to inhibit lattice overturning, and scandium / yttrium generate cation vacancy channels through valence bond compensation. Dissolving ammonium tungstate in hot water at 60-80℃ improves ion migration efficiency. NH4PAA dispersant improves the dispersibility of metallic tungsten, forming a thin film during atomization to reduce surface metal ion segregation during the drying process of atomized particles. In the subsequent calcination process, PMMA decomposition and removal occur in two stages: the first stage occurs at 300-400℃, mainly through gas release; the second stage occurs around 600℃, mainly through carbon doping removal. This process overlaps with APT decomposition, and the gas generated during this process creates pores, which helps improve the surface roughness of the tungsten trioxide generated by APT decomposition. In the wet hydrogen reduction system, water vapor achieves multi-level chemical regulation through competitive adsorption and gas-phase equilibrium reaction: at 300-400℃, its molecules form a hydroxylated monolayer (≡W-OH) on the WO3 surface, reducing the solid surface energy from 2.5 J / m³. 2 Reduced to 0.8 J / m 2 Effectively inhibits high-temperature sintering of porous structures; in the 700-750℃ range, a concentration of 0.3-0.5 vol% is maintained at an oxygen partial pressure of 10 vol% through the water-gas reaction (H2O + H2 ⇌ 2H2 + O). -20atm, which directionally reduces WO3 to substoichiometric WO3. 2.9 (Oxygen vacancy concentration ≈ 10) 21 cm -3 Simultaneously, adsorbed water molecules occupy the three-coordinate active sites on the (001) plane of the tungsten lattice, blocking the dissociation and adsorption pathway of hydrogen atoms; after rising to 800-900℃, 0.1-0.2 vol% water vapor reacts with residual carbon in a gasification reaction (C + H2O → CO + H2), eliminating carbon pollution and consuming excess oxygen carrier, causing the oxygen partial pressure to naturally drop to 10. -22 Atm drives the topological reduction of WO2 to metallic tungsten, ultimately achieving a complete phase transition of the porous framework from the oxidized state to the metallic state.

[0027] Preferably, the heating rate in step (5) is 3-5℃ / min, the hydrogen flow rate is 3.5-4.5 L / min, and a heat preservation section of 0.5h is added at 600℃ to decompose the PMMA pore-forming agent.

[0028] As a preferred embodiment, the viscosity of the slurry in step (2) d) is controlled by: solvent composition: water and ethanol volume ratio 7:3;

[0029] Dispersant: NH4PAA addition amount 0.3±0.05 wt%;

[0030] Ultrasonic homogenization: power 300±50 W, time 15±2 min.

[0031] Preferably, in step (3), the temperature of the atomization chamber is 25±2℃, the flow rate of the auxiliary nitrogen gas is 0.2±0.05 m / s, and the droplet size distribution span is (D 90 -D 10 ) / D 50 ≤1.0.

[0032] Preferably, the swirl guide plate of the spray drying tower in step (4) has an inclination angle of 30±5°.

[0033] Step (1) involves pretreating the nano-tungsten powder, which serves as the framework of the composite powder structure, to purify its surface and construct a functionalized surface layer that facilitates subsequent interfacial chemical bonding. This pretreatment step specifically includes a sequentially performed acid washing sub-step, surface modification sub-step, and vacuum drying sub-step.

[0034] The specific preparation method is as follows: Step (1) mainly aims to obtain nano-tungsten powder that can be uniformly dispersed in a salt solution: In the acid washing sub-step, commercially available nano-tungsten powder with an average particle size of 50-100 nm is placed in an oxalic acid aqueous solution with a mass fraction of 5-10%. Oxalic acid, as a dibasic weak acid and an effective metal ion chelating agent, selectively removes the tungsten oxide (mainly tungsten trioxide) layer formed on the surface of the nano-tungsten powder particles due to long-term storage or during the preparation process, as well as any possible iron, molybdenum, and other metal impurity ions, while avoiding excessive corrosion of the tungsten matrix itself. The oxalic acid solution containing nano-tungsten powder is placed in an ultrasonic cleaning device and subjected to ultrasonic treatment with a power of 100-300 watts for 30-60 minutes at a temperature of 60-80°C. The temperature range ensures that the acid washing reaction has a moderate kinetic rate, while the cavitation effect generated by the ultrasound can significantly enhance liquid refluxing and mass transfer, and efficiently remove the passivation layer and contaminants on the particle surface and inside the agglomerates by mechanical stripping. After acid washing, the tungsten powder was repeatedly centrifuged and washed with deionized water until the pH of the supernatant reached 6.8-7.2, followed by filtration. The acid-washed and purified nano-tungsten powder was then immersed in a 1-3 wt% ethanol solution of silane coupling agent (KH-550, i.e., γ-aminopropyltriethoxysilane). Here, ethanol, as the reaction solvent, along with trace amounts of water present in the system, promotes the hydrolysis of the silane coupling agent. The suspension was heated to 80°C under stirring and reacted at this temperature for 2 hours. Under these conditions, the triethoxysilane group in the γ-aminopropyltriethoxysilane molecule hydrolyzes, generating highly reactive silanol groups. These silanol groups then undergo a dehydration condensation reaction with the hydroxylation sites (W-OH) formed on the surface of the nano-tungsten powder particles after acid washing, forming stable WO-Si covalent bonds. This chemical bonding firmly anchors the silane coupling agent molecules to the surface of the tungsten powder. Meanwhile, the other end of the coupling agent molecule is a chemically active aminopropyl group. The terminal amino group acts as a Lewis base, providing an active site for subsequent coordination or electrostatic adsorption with metal cations in the slurry, thus constructing a chemical bridge between the tungsten matrix and the subsequently generated in-situ cathode salt. The surface-modified nano-tungsten powder is filtered and placed in a vacuum drying oven at a temperature of 100-120℃ and a temperature not exceeding 10℃. -2 Drying at a vacuum level of Pa for 2-4 hours thoroughly removes residual ethanol, moisture, and unreacted silane coupling agent from the surface and interparticle spaces of the tungsten powder. At the same time, the oxygen-free environment prevents unintended oxidation of the functionalized tungsten powder surface, thereby obtaining pretreated nano-cathode tungsten powder with a clean, dry surface and specific chemical reactivity.

[0035] The main purpose of step (2) is to prepare a multi-component, highly stable mixed slurry containing a liquid precursor and a solid framework. This slurry is the material basis for the subsequent spray drying to form uniform composite particles. This step first prepares a tungsten precursor solution. Weigh ammonium tungstate and dissolve it in deionized water at 60-80℃ to form an ammonium tungstate aqueous solution with a mass concentration of 20-30 wt%. As a high-purity, highly water-soluble tungsten source, APT will be converted into metallic tungsten in situ during subsequent heat treatment. In this process, 0.1-0.3% of citric acid by mass of APT is added to the solution. As a polycarboxyl chelating agent, citric acid can form a stable soluble complex with tungstate ions, effectively inhibiting the local hydrolysis or precipitation of APT in the solution due to pH or temperature fluctuations, thereby ensuring that the tungsten precursor is uniformly dispersed at the molecular level throughout the slurry system; and introducing a cathode active material precursor. In the continuously stirred ammonium tungstate solution, high-purity barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate were added sequentially according to a predetermined stoichiometric ratio. These nitrates were chosen as ideal precursors for barium, calcium, aluminum, scandium, and yttrium due to their excellent water solubility and the fact that their thermal decomposition products are all gases. The molar ratio of each metal element was strictly controlled within the range of Ba:Ca:Al:Sc:Y = 4.8-5.2 : 0.9-1.1 : 3.8-4.2 : 0.005-0.015 : 0.003-0.010. This formulation aims to ensure that the main phase of the final synthesized cathode salt is a pentabaric calcium tetraaluminate with excellent electron emission performance. Trace amounts of scandium and yttrium are introduced as dopants to further reduce work function, suppress barium evaporation, and improve the cathode's resistance to poisoning. Pretreated nano-tungsten powder prepared in step (1) is added to the mixed salt solution, with the addition amount accounting for 30-50 wt% of the total tungsten mass in the final composite powder. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following molar ratio: tungsten to the total metal cations in the mixed salt solution, W : (Ba+Ca+Al+Sc+Y) = 1.5-5.0 : 1. This portion of tungsten powder constitutes the primary rigid framework of the composite particles. Simultaneously, 0.1-0.5% by mass of ammonium polyacrylate is added to the slurry as a highly efficient dispersant. The carboxylate ions on the ammonium polyacrylate molecular chain can adsorb onto the surface of pretreated tungsten powder and any subsequent trace precipitates. Through electrostatic repulsion and steric hindrance, they impart a sufficiently high zeta potential to the particle surface, thereby overcoming van der Waals forces and preventing particle agglomeration in the slurry. Furthermore, 5-15% by weight of polymethyl methacrylate microspheres are added to the slurry solids. These microspheres have an average particle size of 10-100 μm and will decompose and volatilize during subsequent heat treatment, forming a pore structure with a specific size range within and between the final composite powder particles. These pores are crucial for improving the efficiency of the internal active material supply channels during cathode operation, allowing for precise control of the slurry's rheological properties.Anhydrous ethanol is added to the slurry to adjust the volume ratio of water to ethanol in the solvent system to 6-8:2-4. The addition of ethanol aims to reduce the overall surface tension of the slurry to below 40 mN / m, which is beneficial for forming finer and more regular spherical droplets during atomization. Subsequently, the entire slurry system is placed in an ultrasonic homogenizer with a cooling circulation jacket and subjected to high-intensity homogenization treatment at 200-400 watts of ultrasonic power for 10-20 minutes, specifically, 300±50 watts for 15±2 minutes. This high-energy ultrasonic treatment ensures that all solid particles are fully dispersed and uniformly suspended in the liquid phase, ultimately obtaining a stable, macroscopically and microscopically homogeneous slurry with a viscosity controlled within the range of 30-50 cP.

[0036] The purpose of step (3) is to ultrasonically atomize the prepared slurry to generate uniform micron-sized droplets. The slurry is delivered to the atomizing head of the ultrasonic generator at a constant flow rate via a peristaltic pump. The atomizer uses a piezoelectric ceramic transducer with an operating frequency set between 1.5 and 2.5 MHz and a power density of 50-100 W / cm². 2 Under this high-frequency vibration, Rayleigh surface waves are generated on the surface of the slurry film. When the wave amplitude reaches a critical value, the wave crest breaks and is ejected in the form of droplets. The physical principle of this atomization method determines that the generated droplet size distribution is extremely narrow, and D... 50 Atomized droplets with a median particle size between 0.8 and 1.5 μm were used. To ensure the stability of the atomization process, the temperature of the atomization chamber was controlled at 25 ± 2 °C, and an auxiliary nitrogen gas flow rate of 0.2 ± 0.05 m / s was introduced. This gas flow not only carried the droplets into the subsequent drying tower but also prevented unnecessary collisions and coalescence of the droplets in the atomization area. Through this step, the atomic / molecular-level liquid phase homogeneity and nanoscale solid phase dispersion of the original slurry were completely replicated and encapsulated in each independent micron-sized droplet "microreactor," and the droplet size distribution span was defined as (D... 90 -D 10 ) / D 50 It is controlled to be no greater than 1.0.

[0037] Step (4) involves rapid spray drying of the droplets generated by ultrasonic atomization to solidify their uniform internal structure. A nitrogen gas stream carrying the atomized droplets is introduced into the top of a vertical spray drying tower. The interior of the drying tower is under a controlled nitrogen atmosphere to prevent any oxidation reaction during the drying process. Hot nitrogen enters the drying tower via a swirling guide vane at the top. The vane's blade angle is set to 30±5°, a design that creates a stable spiral downward airflow field. This specific flow field structure ensures sufficient residence time and optimized trajectory for the droplets within the tower, facilitating efficient heat and mass exchange with the hot gas stream while preventing particle adhesion to the tower wall. The inlet temperature of the drying tower is controlled at 180-220℃, and the outlet temperature is controlled at 80-100℃. Under these temperature conditions, the water and ethanol solvent in the droplet evaporate rapidly within seconds, while the internal solutes (nitrates, APT) and suspended matter (nano-tungsten powder, PMMA microspheres) quickly precipitate and solidify, forming spherical composite particles with an internal structure consistent with the original droplet. The dried powder is then carried by airflow into a cyclone separator for collection, yielding the intermediate product, the uncalcined composite precursor powder.

[0038] Step (5) involves simultaneous reduction and calcination of the collected composite precursor powder. This step is the core of the invention. Within a single heat treatment cycle, through programmed and precise control of temperature, atmosphere, and heating rate, it synergistically achieves the decomposition of the pore-forming agent, the multi-step reduction of the tungsten precursor, the solid-phase synthesis of the cathode active phase, and the interfacial strengthening between the components. The heating rate of the entire heat treatment process is strictly controlled at 3-5℃ / min, and the total hydrogen flow rate is set at 3.5-4.5L / min. First, the temperature is raised to 300-400℃ and held within this temperature range for 0.5-1h. The main purpose of this stage is to ensure the complete thermal decomposition of the PMMA pore-forming agent, ensuring that it is completely removed before the main reduction reaction occurs, forming an open pore structure, which is beneficial for the entry and exit of subsequent reaction gases. After this, the wet hydrogen reduction and synthesis stage with segmented temperature control is entered. The humid hydrogen atmosphere is achieved by passing high-purity hydrogen (99.999%) through a precisely temperature-controlled bubbling humidification device. The partial pressure of water vapor carried by the hydrogen, i.e., the dew point of the hydrogen, is precisely adjusted by controlling the temperature of the humidifying water. The first stage involves raising the furnace temperature to 700-750℃ and maintaining it at this temperature for 0.5-1 hour. During this stage, the volume concentration of water vapor in the introduced humid hydrogen atmosphere is controlled at 0.3-0.5 vol%, corresponding to a hydrogen dew point of -35 to -30℃. At this higher temperature and higher water vapor partial pressure, ammonium tungstate in the precursor powder completely decomposes into tungsten trioxide. Simultaneously, the oxides generated from the decomposition, such as BaO, CaO, and Al2O3, begin to undergo preliminary solid-phase diffusion and reaction. The higher water vapor concentration here inhibits the premature and rapid reduction of tungsten trioxide to metallic tungsten, instead promoting the formation of volatile mesophases such as WO2(OH)2. This facilitates the migration and rearrangement of tungsten species, laying the foundation for the formation of a uniform tungsten framework network. In the second stage, the furnace temperature is further increased to 800-900℃ and held constant for 1.5-3 hours. During this stage, the volume concentration of water vapor in the introduced wet hydrogen atmosphere is significantly reduced to 0.1-0.2 vol%, corresponding to a hydrogen dew point of -40℃ to -36℃. This reduction in water vapor concentration disrupts the chemical equilibrium established in the previous stage, greatly increasing the reduction potential of hydrogen. This allows tungsten trioxide to be completely and efficiently reduced to metallic tungsten by hydrogen. Since this reduction reaction occurs in situ within the already formed nanoporous oxide matrix, the newly generated ultrafine metallic tungsten particles will achieve atomic-scale close contact and metallurgical bonding with the pretreated nano-tungsten powder serving as the framework and the surrounding cathode salt particles. Simultaneously, at higher temperatures of 800-900℃, the solid-state reaction kinetics between BaO, CaO, and Al2O3 are significantly accelerated, reacting rapidly over extremely short diffusion distances to produce well-crystallized, single-phase Ba5CaAl4O. 12 Active host phase.

[0039] After heat treatment, the powder is naturally cooled to room temperature in a flowing pure hydrogen atmosphere, resulting in the nano-cathode composite powder described in this invention. Each particle of this powder contains a composite structure composed of in-situ generated ultrafine tungsten and pretreated nano-tungsten powder, tightly bonded to the nanocrystalline cathode salt. This unique microstructure fundamentally solves the technical bottlenecks of traditional processes, providing an ideal raw material for the preparation of high-performance barium tungsten cathodes.

[0040] Compared to existing technologies, the advantages of this solution are:

[0041] The invention significantly improves the compositional uniformity and phase composition stability of nano-cathode composite powder through component design, combined with spray drying and segmented temperature-controlled reduction processes, effectively solving the problems of impurity introduction and component segregation in traditional methods. Attached Figure Description

[0042] Figure 1 The image shows the SEM microstructure of the nano-cathode composite powder prepared by spray drying in Example 1.

[0043] Figure 2 The image shows the SEM microstructure of the nano-cathode composite powder prepared by spray drying in Example 1, which was then used to make a barium tungsten cathode material. Detailed Implementation

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0045] General Implementation Examples

[0046] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0047] Step (1) Pretreatment of nano-tungsten powder:

[0048] Step (1-a) Acid washing: Sonicate the nano-tungsten powder in a 5-10% oxalic acid solution at 60-80℃ for 30-60 min;

[0049] Step (1-b) Surface modification: The nano-tungsten powder after acid washing in step (1-a) is added to an ethanol aqueous solution containing 1-3 wt% silane coupling agent and reacted at 80℃ for 2 h. The water content in the ethanol solution is 5-10% of the total solution volume.

[0050] Step (1-c) Vacuum drying: Dry at 100-120℃ for 2-4 hours, with a vacuum degree ≤10. -2Pa, to obtain pretreated nano-tungsten powder;

[0051] Step (2) Prepare the mixed slurry:

[0052] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 60-80℃ with a concentration of 20-30 wt%, and add 0.1-0.3 wt% citric acid to obtain an ammonium tungstate solution;

[0053] Step (2-b) Mixed salt solution: Add barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 4.8-5.2 : 0.9-1.1 : 3.8-4.2 : 0.005-0.015 : 0.003-0.010;

[0054] In step (2-c), pretreated nano-tungsten powder is added, accounting for 30-50 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 1.5-5.0 : 1. The slurry is then obtained. Based on the weight of the slurry, 0.1-0.5 wt% ammonium polyacrylate dispersant and 5-15 wt% polymethyl methacrylate microspheres with a particle size of 10-100 μm are added.

[0055] Step (2-d) Adjusting viscosity: Add ethanol to make the water to ethanol volume ratio 6-8:2-4, ultrasonic homogenize at 200-400W for 10-20 minutes, and the slurry viscosity is 30-50 cP.

[0056] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 1.5-2.5 MHz and a power density of 50-100 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =0.8-1.5 μm, atomization chamber temperature 25±2℃, auxiliary nitrogen flow rate 0.2±0.05 m / s, droplet size distribution span value ≤1.0; in the ultrasonic generator, the slurry is pumped into the ultrasonic generator through a liquid pump, and nitrogen is introduced from the gas supply point. The liquid is finely atomized by inducing liquid cavitation effect through high-frequency sound wave vibration.

[0057] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 180-220℃, the outlet temperature is 80-100℃, and the airflow velocity is 0.2-0.5 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30±5°.

[0058] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 3-5℃ / min, the temperature is raised to 300-400℃, and held for 0.5-1h. An additional holding period of 0.5h is added at 600℃, followed by heating. The reduction is carried out in stages with controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 3.5-4.5 L / min.

[0059] First stage: Hold at 700-750℃ for 0.5-1h, with H2O concentration of 0.3-0.5 vol%, to decompose APT into tungsten trioxide;

[0060] Second stage: Hold at 800-900℃ for 1.5-3h, H2O concentration 0.1-0.2 vol%, cool and then pulverize and disperse to obtain nano-cathode composite powder prepared by spray drying;

[0061] Unless otherwise specified, the omitted steps in the subsequent embodiments and comparative examples are the same as those in the general embodiments.

[0062] Example 1

[0063] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0064] Step (1) Pretreatment of nano-tungsten powder:

[0065] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0066] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0067] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0068] Step (2) Prepare the mixed slurry:

[0069] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0070] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0071] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0072] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0073] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0074] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0075] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0076] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0077] The second stage involved maintaining a constant temperature of 850℃ for 2.25 hours, with an H2O concentration of 0.15 vol%. After cooling, the mixture was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying. Figure 1 The image shown is a SEM image of this embodiment. As can be seen from the image, the particle size is less than 200 nm, and even smaller particles, namely Ba5CaAl4O, are uniformly attached to the surface. 12 Particles and tungsten particles from APT decomposition.

[0078] Example 2

[0079] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0080] Step (1) Pretreatment of nano-tungsten powder:

[0081] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 5% oxalic acid solution at 60℃ for 60 min;

[0082] Step (1-b) Surface modification: react at 80°C for 2 h in an ethanol solution containing 1 wt% silane coupling agent KH-550, with water accounting for 5% of the total volume of the ethanol solution;

[0083] Step (1-c) Vacuum drying: Dry at 100℃ for 4 hours, with a vacuum degree ≤10. -2 Pa;

[0084] Step (2) Prepare the mixed slurry:

[0085] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 60℃ with a concentration of 20 wt%, and add 0.1 wt% citric acid to obtain an ammonium tungstate solution;

[0086] Step (2-b) Mixed salt solution: molar ratio Ba:Ca:Al:Sc:Y = 4.8:0.9:3.8:0.005:0.003;

[0087] In step (2-c), pretreated nano-tungsten powder is added, accounting for 30 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 4.0 : 1. Based on the weight of the slurry, 0.1 wt% ammonium polyacrylate and 5 wt% polymethyl methacrylate microspheres with a particle size of 10 μm are added.

[0088] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 6:4, ultrasonically homogenize at 200 W for 20 min, and the slurry viscosity is 30 cP;

[0089] Step (3) Ultrasonic atomization: frequency 1.5 MHz, power density 50 W / cm³ 2 D 50 =0.8 μm, atomization chamber temperature 23℃, auxiliary nitrogen flow rate 0.15 m / s;

[0090] Step (4) Spray drying: inlet temperature 180℃, outlet temperature 80℃, airflow velocity 0.2 m / s, swirl guide plate tilt angle 25°;

[0091] Step (5) Synchronous reduction calcination: The temperature is increased at a rate of 3℃ / min to 300℃ and held for 1 h, then held at 600℃ for 0.5 h, followed by temperature increase and segmented temperature control reduction in a humid hydrogen atmosphere with a hydrogen flow rate of 3.5 L / min.

[0092] Phase 1: Maintain temperature at 700℃ for 1 hour, H2O concentration 0.5 vol%.

[0093] Second stage: The temperature was kept constant at 800℃ for 3 hours, the H2O concentration was 0.2 vol%, and after cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0094] Example 3

[0095] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0096] Step (1) Pretreatment of nano-tungsten powder:

[0097] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 10% oxalic acid solution at 80℃ for 30 min;

[0098] Step (1-b) Surface modification: react at 80°C for 2 h in a 3 wt% silane coupling agent KH-550 ethanol solution, with water accounting for 10% of the total volume of the ethanol solution;

[0099] Step (1-c) Vacuum drying: Dry at 120℃ for 2 hours, with a vacuum degree ≤10. -2 Pa;

[0100] Step (2) Prepare the mixed slurry:

[0101] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 80℃ with a concentration of 30 wt%, and add 0.3 wt% citric acid to obtain an ammonium tungstate solution;

[0102] Step (2-b) Mixed salt solution: molar ratio Ba:Ca:Al:Sc:Y = 5.2:1.1:4.2:0.015:0.010;

[0103] In step (2-c), pretreated nano-tungsten powder is added, accounting for 50 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 2.0 : 1. Based on the weight of the slurry, 0.5 wt% ammonium polyacrylate and 15 wt% polymethyl methacrylate microspheres with a particle size of 100 μm are added.

[0104] Step (2-d) Adjusting viscosity: Add ethanol to make the water to ethanol volume ratio 8:2, ultrasonically homogenize at 400 W for 10 min, and the slurry viscosity is 50 cP;

[0105] Step (3) Ultrasonic atomization: frequency 2.5 MHz, power density 100 W / cm³ 2 D 50=1.5 μm, atomization chamber temperature 27℃, auxiliary nitrogen flow rate 0.25 m / s;

[0106] Step (4) Spray drying: inlet temperature 220℃, outlet temperature 100℃, airflow velocity 0.5 m / s, swirl guide plate tilt angle 35°;

[0107] Step (5) Synchronous reduction calcination: The temperature is increased at a rate of 5℃ / min to 400℃ and held for 0.5h, then held at 600℃ for 0.5h, followed by temperature increase and segmented temperature-controlled reduction in a humid hydrogen atmosphere with a hydrogen flow rate of 4.5 L / min.

[0108] Phase 1: Maintain a constant temperature of 750℃ for 0.5 hours, with an H2O concentration of 0.3 vol%.

[0109] Second stage: The temperature was kept constant at 900℃ for 1.5h, the H2O concentration was 0.1 vol%, and after cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0110] Example 4

[0111] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0112] Step (1) Pretreatment of nano-tungsten powder:

[0113] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 6% oxalic acid solution at 65℃ for 50 min;

[0114] Step (1-b) Surface modification: react at 80°C for 2 h in an ethanol solution containing 1.5 wt% silane coupling agent KH-550, with water accounting for 6% of the total volume of the ethanol solution;

[0115] Step (1-c) Vacuum drying: Dry at 105℃ for 3.5h, with a vacuum degree ≤10. -2 Pa;

[0116] Step (2) Prepare the mixed slurry:

[0117] Step (2-a) Dissolving ammonium tungstate: Ammonium tungstate is dissolved in deionized water at 65°C with a concentration of 22 wt%, and 0.15 wt% citric acid is added to obtain an ammonium tungstate solution;

[0118] Step (2-b) Mixed salt solution: molar ratio Ba:Ca:Al:Sc:Y = 4.9:0.95:3.9:0.008:0.004;

[0119] In step (2-c), pretreated nano-tungsten powder is added, accounting for 35 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.2 wt% ammonium polyacrylate and 7 wt% polymethyl methacrylate microspheres with a particle size of 30 μm are added.

[0120] Step (2-d) Viscosity adjustment: Add ethanol to make the water to ethanol volume ratio 6.5:3.5, ultrasonic homogenize at 250 W for 18 min, and the slurry viscosity is 35 cP;

[0121] Step (3) Ultrasonic atomization: frequency 1.8 MHz, power density 60 W / cm³ 2 D 50 =1.0 μm, atomization chamber temperature 24℃, auxiliary nitrogen flow rate 0.18 m / s;

[0122] Step (4) Spray drying: inlet temperature 190℃, outlet temperature 85℃, airflow velocity 0.3 m / s, swirl guide plate tilt angle 28°;

[0123] Step (5) Synchronous reduction calcination: The temperature is increased at a rate of 3.5℃ / min to 320℃ and held for 0.8h, then held at 600℃ for 0.5h, followed by temperature increase and segmented temperature-controlled reduction in a humid hydrogen atmosphere with a hydrogen flow rate of 3.8 L / min.

[0124] Phase 1: Constant temperature at 710℃ for 0.9 hours, H2O concentration 0.45 vol%

[0125] Second stage: The mixture was kept at 820℃ for 2.5 hours with an H2O concentration of 0.18 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0126] Example 5

[0127] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0128] Step (1) Pretreatment of nano-tungsten powder:

[0129] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 9% oxalic acid solution at 75℃ for 40 min;

[0130] Step (1-b) Surface modification: react at 80°C for 2 h in an ethanol solution containing 2.5 wt% silane coupling agent KH-550, with water accounting for 9% of the total volume of the ethanol solution;

[0131] Step (1-c) Vacuum drying: Dry at 115℃ for 2.5h, with a vacuum degree ≤10. -2 Pa;

[0132] Step (2) Prepare the mixed slurry:

[0133] Step (2-a) Dissolving ammonium tungstate: Ammonium tungstate is dissolved in deionized water at 75°C with a concentration of 28 wt%, and 0.25 wt% citric acid is added to obtain an ammonium tungstate solution;

[0134] Step (2-b) Mixed salt solution: molar ratio Ba:Ca:Al:Sc:Y = 5.1:1.05:4.1:0.012:0.008;

[0135] In step (2-c), pretreated nano-tungsten powder is added, accounting for 45 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. The slurry is then prepared by adding 0.4 wt% ammonium polyacrylate and 12 wt% polymethyl methacrylate microspheres with a particle size of 80 μm.

[0136] Step (2-d) Viscosity adjustment: Add ethanol to make the water to ethanol volume ratio 7.5:2.5, ultrasonic homogenize at 350 W for 12 min, and the slurry viscosity is 45 cP;

[0137] Step (3) Ultrasonic atomization: frequency 2.2 MHz, power density 90 W / cm³ 2 D 50 =1.4 μm, atomization chamber temperature 26℃, auxiliary nitrogen flow rate 0.22 m / s;

[0138] Step (4) Spray drying: inlet temperature 210℃, outlet temperature 95℃, airflow velocity 0.4 m / s, swirl guide plate tilt angle 32°;

[0139] Step (5) Synchronous reduction calcination: The temperature is increased at a rate of 4.5℃ / min to 380℃ and held for 0.6h. After reaching 600℃, the temperature is controlled in stages in a humid hydrogen atmosphere with a hydrogen flow rate of 4.2 L / min.

[0140] Phase 1: Constant temperature at 740℃ for 0.6 hours, H2O concentration 0.35 vol%

[0141] Second stage: constant temperature at 880℃ for 2.0h, H2O concentration of 0.12 vol%, followed by cooling and pulverization to obtain nano-cathode composite powder prepared by spray drying.

[0142] Comparative Example 1

[0143] The difference from Example 1 is that hydrochloric acid was used as the pickling medium:

[0144] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0145] Step (1) Pretreatment of nano-tungsten powder:

[0146] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% hydrochloric acid solution at 70℃ for 45 min;

[0147] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0148] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0149] Step (2) Prepare the mixed slurry:

[0150] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0151] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0152] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0153] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0154] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0155] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0156] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0157] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0158] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0159] Comparative Example 2

[0160] The difference from Example 1 is that it did not undergo silane surface modification:

[0161] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0162] Step (1) Pretreatment of nano-tungsten powder:

[0163] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0164] Step (1-b) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0165] Step (2) Prepare the mixed slurry:

[0166] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0167] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0168] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0169] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0170] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0171] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0172] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0173] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0174] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0175] Comparative Example 3

[0176] The difference from Example 1 is that in step (1-c), drying in air:

[0177] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0178] Step (1) Pretreatment of nano-tungsten powder:

[0179] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0180] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0181] Step (1-c) Drying: Dry at 110℃ for 3 hours, then air dry to obtain pretreated nano-tungsten powder;

[0182] Step (2) Prepare the mixed slurry:

[0183] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0184] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0185] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0186] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0187] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0188] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0189] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0190] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0191] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0192] Comparative Example 4

[0193] The difference from Example 1 is that nano-tungsten powder was not used:

[0194] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0195] Step (1) Pretreatment of nano-tungsten powder:

[0196] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0197] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0198] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0199] Step (2) Prepare the mixed slurry:

[0200] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0201] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution, with a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006; the molar ratio of tungsten to the total metal cations in the mixed salt solution, W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1;

[0202] In step (2-c), add 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm.

[0203] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0204] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0205] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0206] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0207] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0208] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0209] Comparative Example 5

[0210] The difference from Example 1 is that APT was not added:

[0211] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0212] Step (1) Pretreatment of nano-tungsten powder:

[0213] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0214] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0215] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0216] Step (2) Prepare the mixed slurry:

[0217] Step (2-a) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006 to obtain the ammonium tungstate solution;

[0218] In step (2-b), pretreated nano-tungsten powder is added, and the slurry after the addition of pretreated nano-tungsten powder must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1 to obtain the slurry. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0219] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0220] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0221] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0222] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0223] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0224] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0225] Comparative Example 6

[0226] The difference from Example 1 is that the total molar amount of tungsten is 5.5 times the total molar amount of the other metals:

[0227] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0228] Step (1) Pretreatment of nano-tungsten powder:

[0229] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0230] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0231] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0232] Step (2) Prepare the mixed slurry:

[0233] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0234] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0235] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 5.5 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0236] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0237] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0238] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0239] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0240] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0241] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0242] Comparative Example 7

[0243] The difference from Example 1 is that the molar amount of tungsten is 1 times the total molar amount of the other metals:

[0244] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0245] Step (1) Pretreatment of nano-tungsten powder:

[0246] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0247] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0248] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0249] Step (2) Prepare the mixed slurry:

[0250] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0251] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0252] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 1.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0253] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0254] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0255] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0256] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0257] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0258] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0259] Comparative Example 8

[0260] The difference from Example 1 is that only air pressure spraying is used:

[0261] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0262] Step (1) Pretreatment of nano-tungsten powder:

[0263] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0264] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0265] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0266] Step (2) Prepare the mixed slurry:

[0267] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0268] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0269] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0270] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0271] Step (3) Pneumatic spraying: The nozzle diameter is 0.4 mm; the outlet flow rate is 220 m / s; and the mass flow rate is 0.8 kg / h·L slurry.

[0272] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0273] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0274] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0275] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0276] Comparative Example 9

[0277] The difference from Example 1 is that the swirl deflector is at a 45° angle:

[0278] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0279] Step (1) Pretreatment of nano-tungsten powder:

[0280] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0281] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0282] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0283] Step (2) Prepare the mixed slurry:

[0284] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0285] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0286] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0287] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0288] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0289] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 45°;

[0290] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0291] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0292] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0293] Comparative Example 10

[0294] The difference from Example 1 is that the tilt angle of the swirl deflector is 20°:

[0295] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0296] Step (1) Pretreatment of nano-tungsten powder:

[0297] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0298] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0299] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0300] Step (2) Prepare the mixed slurry:

[0301] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0302] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0303] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0304] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0305] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0306] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0307] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0308] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0309] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0310] Comparative Example 11

[0311] The difference from Example 1 is that dry hydrogen is used for reduction:

[0312] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0313] Step (1) Pretreatment of nano-tungsten powder:

[0314] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0315] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0316] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0317] Step (2) Prepare the mixed slurry:

[0318] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0319] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0320] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0321] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0322] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0323] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0324] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, and an additional holding period of 0.5h is added at 600℃. Then the temperature is raised, and the reduction is carried out in stages with controlled temperature in a hydrogen atmosphere. The hydrogen flow rate is 4.0 L / min.

[0325] Phase 1: Maintain a constant temperature of 725℃ for 0.75 hours;

[0326] Second stage: The temperature was kept constant at 850℃ for 2.25 hours, and after cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0327] Comparative Example 12

[0328] The difference from Example 1 is that the reduction was carried out in stages with controlled temperature in a humid hydrogen atmosphere. The H2O concentration was 0.15 vol% in the first stage and 0.40 vol% in the second stage.

[0329] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0330] Step (1) Pretreatment of nano-tungsten powder:

[0331] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0332] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0333] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0334] Step (2) Prepare the mixed slurry:

[0335] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0336] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0337] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0338] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0339] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0340] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0341] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0342] Phase 1: Constant temperature at 725℃ for 0.75 h, H2O concentration 0.15 vol%

[0343] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.40 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0344] Comparative Example 13

[0345] The difference from Example 1 is that ammonium polyacrylate and polymethyl methacrylate microspheres were not added:

[0346] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0347] Step (1) Pretreatment of nano-tungsten powder:

[0348] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0349] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0350] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0351] Step (2) Prepare the mixed slurry:

[0352] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0353] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0354] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following condition: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1 to obtain the slurry.

[0355] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 7:3, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 40 cP;

[0356] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0357] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0358] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0359] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0360] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0361] Comparative Example 14

[0362] The difference from Example 1 is that the slurry viscosity is too low, at 25 cP.

[0363] A method for preparing nano-cathode composite powder by spray drying includes the following steps:

[0364] Step (1) Pretreatment of nano-tungsten powder:

[0365] Step (1-a) Acid washing: Nano-tungsten powder is ultrasonically treated in 7.5% oxalic acid solution at 70℃ for 45 min;

[0366] Step (1-b) Surface modification: The reaction was carried out at 80°C for 2 hours in an ethanol solution containing 2 wt% silane coupling agent KH-550, with water accounting for 7.5% of the total volume of the ethanol solution;

[0367] Step (1-c) Vacuum drying: Dry at 110℃ for 3 hours, with a vacuum degree ≤10. -2 Pa, to obtain pretreated nano-tungsten powder;

[0368] Step (2) Prepare the mixed slurry:

[0369] Step (2-a) Dissolving ammonium tungstate: Dissolve ammonium tungstate in deionized water at 70°C with a concentration of 25 wt%, and add 0.2 wt% citric acid to obtain an ammonium tungstate solution;

[0370] Step (2-b) Mixed salt solution: Barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate, and yttrium nitrate are added to the ammonium tungstate solution in a molar ratio of Ba:Ca:Al:Sc:Y = 5.0:1.0:4.0:0.01:0.006;

[0371] In step (2-c), pretreated nano-tungsten powder is added, accounting for 40 wt% of the total tungsten element. After the addition of the pretreated nano-tungsten powder, the slurry must meet the following requirements: the molar ratio of tungsten element to the total metal cations in the mixed salt solution W : (Ba+Ca+Al+Sc+Y) = 3.0 : 1. Based on the weight of the slurry, 0.3 wt% ammonium polyacrylate and 10 wt% polymethyl methacrylate microspheres with a particle size of 50 μm are added.

[0372] Step (2-d) Viscosity adjustment: Add ethanol to a water to ethanol volume ratio of 6:4, ultrasonically homogenize at 300 W for 15 min, and the slurry viscosity is 25 cP;

[0373] Step (3) Ultrasonic atomization: The slurry is atomized at a frequency of 2.0 MHz and a power density of 75 W / cm³. 2 The ultrasonic generator produces atomized droplets, D 50 =1.2 μm, atomization chamber temperature 25℃, auxiliary nitrogen flow rate 0.2 m / s, droplet size distribution span value ≤1.0;

[0374] Step (4) Spray drying of atomized droplets: Under nitrogen atmosphere, the inlet temperature is 200℃, the outlet temperature is 90℃, and the airflow velocity is 0.35 m / s; the swirl guide plate of the spray drying tower is tilted at an angle of 30°;

[0375] Step (5) Synchronous reduction calcination: Under a nitrogen atmosphere, the heating rate is 4℃ / min, the temperature is raised to 350℃, held for 0.75h, then raised to 600℃ and held for 0.5h, and then the temperature is raised again. The reduction is carried out in stages under controlled temperature in a humid hydrogen atmosphere, with a hydrogen flow rate of 4.0 L / min.

[0376] Phase 1: Constant temperature at 725℃ for 0.75 hours, H2O concentration 0.4 vol%

[0377] Second stage: The mixture was kept at 850℃ for 2.25 hours with an H2O concentration of 0.15 vol%. After cooling, it was pulverized and dispersed to obtain nano-cathode composite powder prepared by spray drying.

[0378] Detection method:

[0379] Powder composition uniformity analysis: Three points were randomly selected from different locations for each sample, and the composition was tested by X-ray fluorescence spectroscopy. The average composition and standard deviation of each sample were calculated, as shown in Table 1.

[0380] Table 1. Component detection results for the examples and comparative examples

[0381]

[0382] Sample preparation and performance testing:

[0383] Sample preparation: First, powder pretreatment is required. The nano-cathode composite powder is mixed with 0.8 wt% of polyvinyl alcohol (PVA) dispersant, and an appropriate amount of anhydrous ethanol is added at a solid-liquid ratio of 1:4. The mixture is ultrasonically dispersed at 250 W for 45 min. Then, the dispersed slurry is vacuum dried at 70 °C and a vacuum degree of 10. -2 Ethanol was removed from the powder, and the resulting dry powder was passed through a 250-mesh sieve using a vibrating sieve to remove hard agglomerated particles, yielding fine powder. The sieved powder was then loaded into a hard alloy mold with graphite powder as a release agent coated on the inner wall. The mold was first pre-pressed at 75 MPa and held for 1.5 min, followed by a main press at 250 MPa and held for 7 min, ensuring the density of the formed blank was controlled at 55% of the theoretical density. Sintering was then performed. This crucial step required a wet hydrogen atmosphere with a H2 to H2O volume ratio of 92:8 and a gas flow rate controlled at 3.5 L / min. The heating program consisted of raising the temperature from room temperature to 350 °C at a rate of 7 °C / min and holding for 2.5 h for low-temperature degreasing, raising it to 1300 °C at a rate of 3 °C / min and holding for 4 h for medium-temperature densification, and finally raising it to 1700 °C at a rate of 1.5 °C / min and holding for 3 h for high-temperature sintering. Afterwards, the temperature was increased by 7 °C / min during furnace operation. The material was cooled to room temperature at a certain rate. In subsequent processing, the sintered cathode material surface was first ground with a diamond wheel with a grit size of 1500 mesh, and then polished with polishing paste with a grit size of 0.8 μm to achieve a surface roughness Ra of 0.08 μm. This was done under a vacuum of 10... -4 Activation was performed by heating to 1100℃ and holding for 1.5 hours in a vacuum furnace with a partial pressure of 5 × 10⁻⁶ Pa, during which a small amount of O₂ was introduced and the partial pressure was 5 × 10⁻⁶ Pa. -3 A thin oxide layer is formed using Pa, followed by vacuum brazing. A 0.8 mm diameter tungsten wire is connected to the cathode material using a high-temperature Pt-Rh alloy to ensure a contact resistance of 10 Ω·cm. -3 Ω, to obtain the sample; Figure 2 This is a SEM microstructure image of the barium tungsten cathode material prepared by spray drying of nano-cathode composite powder obtained in Example 1. The arrow in the image points to Ba5CaAl4O. 12 The active main phase is relatively evenly distributed, as can be seen from the figure.

[0384] Sample performance testing,

[0385] 1. Visual inspection: Soak in water, wipe the surface dry, and observe whether there are any cracks or water stains on the surface;

[0386] 2. Room Temperature Conductivity: Room temperature conductivity is measured using the four-probe method. The prepared cathode material is processed into a 10mm × 10mm × 2mm sheet sample. Conductive silver paste is used to ensure good contact between the probes of the four-probe instrument and the sample surface. At room temperature (25℃), a constant current is applied through the instrument, and the voltage drop between the probes is measured. The conductivity is calculated using the formula and must be greater than 10. 5 S / m;

[0387] 3. High-temperature (900℃) oxidation resistance and electron emission current density: The cathode material was processed into a cylindrical sample with a diameter of 5mm and a length of 20mm. One end was ground into a hemispherical emitting surface (radius of curvature 2mm), and a 0.5mm diameter tungsten wire was welded to the other end as a lead. The emitting surface was polished to a mirror finish with 1000-grit polishing paste, cleaned and dried, and then subjected to a vacuum of 10... -4 In a vacuum furnace at 1000℃ for 1 hour, the gas was degassed to remove adsorbed gaseous impurities from the surface; an electron emission testing system with a diode structure was used, with a nickel sheet (15mm in diameter) as the anode, placed parallel to the cathode emitting surface, and the electrode spacing fixed at 2mm; the test vacuum chamber was evacuated to 1000℃. -5 Below Pa, the cathode is heated to a working temperature of 900℃ using a high-frequency induction heating device, and the temperature is monitored in real time by an optical pyrometer; a DC voltage of 300V is applied to the anode, and the emission current (I) is measured using a micro-ammeter. The value is recorded after the current stabilizes for 3 minutes; the emission current density is calculated according to the formula: J = I / A, where A is the cathode emitting surface area; at the set temperature, if the electron emission current density is ≥10A / cm², the emission current density is considered to be within acceptable limits. 2 If the current fluctuation is ≤5% within 30 minutes of continuous testing, the transmission performance is deemed qualified.

[0388] 4. Vickers hardness test: required to be greater than 300 HV.

[0389] The performance test results are shown in Table 2.

[0390] Table 2. Performance test data results for the examples and comparative examples.

[0391]

[0392] This invention significantly improves the overall performance of cathode composite powder through deep synergy between component design and process optimization. At the component level, the pretreatment stage uses oxalic acid washing to precisely remove the oxide layer on the surface of the nano-tungsten powder, preventing corrosion of the metal matrix. Simultaneously, the silane coupling agent KH-550 forms WO-Si covalent bonds on the tungsten powder surface, constructing an organic-inorganic interface layer. This reduces surface energy, inhibiting high-temperature agglomeration, and enhances interfacial bonding through hydrogen bonding between silanol groups and aluminate precursors. In slurry preparation, the molar ratio of W to the metal cations Ba+Ca+Al+Sc+Y is strictly controlled at 1.5-5.0:1 to ensure the stability of the main phase Ba5CaAl4O 12 The required proportions of barium, calcium, and aluminum elements are synthesized, and ammonium polyacrylate (MPA) dispersant and polymethyl methacrylate (PMMA) pore-forming agent are added to regulate the rheological properties of the slurry and the pore structure after calcination. Ultrasonic homogenization ensures uniform dispersion of the liquid precursor and solid tungsten powder, forming stable core-shell atomized droplets. At the process level, simultaneous reduction calcination employs a segmented temperature control strategy: 700-750℃ with low water vapor concentration (0.3-0.5 vol%) stabilizes the decomposition of APT into tungsten trioxide, avoiding premature reduction and lattice distortion; 800-900℃ rapidly reduces the water vapor concentration (0.1-0.2 vol%) to below the critical threshold oxygen partial pressure, promoting the topological reduction of tungsten trioxide on the premixed nano-tungsten powder substrate, forming an interpenetrating network of metallic tungsten grains and the oxide matrix; nitrogen protection and a moist hydrogen atmosphere suppress oxidation reactions, ensuring uniform distribution of the active main phase. The synergistic effect of the above components and processes results in a final product with a component value deviation of only 0.1-1.3% at different points, achieving component uniformity and electron emission activity, thus exhibiting excellent room temperature conductivity, emission current density at 900℃, and Vickers hardness.

[0393] The performance of each comparative example deteriorated significantly due to deviations in component design or process parameters from the optimized range of Example 1. Comparative Example 1 used 7.5% hydrochloric acid instead of oxalic acid for pickling. Hydrochloric acid, as a strong acid, not only stripped the oxide layer from the tungsten powder surface but also excessively corroded the tungsten matrix, damaging surface hydroxylation sites. This made it difficult for the silanol groups of the subsequent silane coupling agent KH-550 to form stable WO-Si covalent bonds, resulting in decreased interfacial adhesion. This led to uneven dispersion of the tungsten powder in the slurry, easy agglomeration during high-temperature reduction, and significantly higher component tolerances (e.g., Ba tolerance reached 1.2%) compared to Example 1, with the emission current density dropping to 8.5 A / cm². 2 Furthermore, microcracks appeared in some areas.

[0394] Comparative Example 2, without silane coupling agent modification, did not form an organic-inorganic interface layer on the surface of the nano-tungsten powder. It exhibited high surface energy and lacked hydrogen bond network anchors for binding with the aluminate precursor. During slurry preparation, the tungsten powder easily agglomerated due to van der Waals forces, and ultrasonic homogenization could not completely disperse it. This resulted in tungsten-rich and oxide-rich regions within the particles after spray drying, with compositional standard deviations (e.g., Al standard deviation reaching 1.3%) more than twice that of Example 1. During high-temperature reduction, the non-chemically bonded interfaces were prone to cracking, resulting in an emission current density of only 6.2 A / cm². 2 The Vickers hardness drops to 260 HV.

[0395] Comparative Example 3 used air drying instead of vacuum drying. At 110°C, the oxygen in the air caused the cleaned tungsten powder surface after acid washing to re-oxidize and form a thin WO3 layer. This oxide layer blocked the bonding reaction between the silane coupling agent and the tungsten powder, leading to weakened interfacial bonding. Simultaneously, the oxide layer required additional hydrogen consumption during the reduction stage, resulting in incomplete local reduction, fluctuations in the valence state of tungsten, and a decrease in room temperature conductivity to 0.9 × 10⁻⁶. 5 S / m, emission current density at 900℃ is only 7.8 A / cm. 2 Oxidation discoloration spots also appeared on the surface.

[0396] Comparative Example 4: Without the addition of nano-tungsten powder, relying solely on APT to provide the tungsten source, and lacking pre-treated tungsten powder as a heterogeneous nucleation substrate, the WO3 generated by APT decomposition easily forms coarse tungsten particles during reduction, which cannot react with Ba5CaAl4O3. 12 The main phase forms an interpenetrating network. Structurally, it is porous due to the lack of a nano-tungsten powder framework, resulting in a room-temperature conductivity of only 0.6 × 10⁻⁶. 5 The emission current density drops sharply to 5.5 A / cm². 2 Its Vickers hardness is only 250 HV, far lower than the 350 HV of Example 1.

[0397] Comparative Example 5, without the addition of APT, relied solely on nano-tungsten powder for tungsten, resulting in insufficient total tungsten and a lack of the synergistic pore-forming effect of APT and PMMA decomposition (the overlap of APT decomposition and PMMA gas production promotes WO3 pore formation), leading to a porosity decrease of over 30%. Simultaneously, the imbalance in the ratio of tungsten powder to oxides disrupted the balance between conductivity and emission activity, resulting in an emission current density of only 4.2 A / cm². 2 The material is brittle and easily broken.

[0398] In Comparative Example 6, the molar ratio of W to the total metal cations was 5.5:1, exceeding the range of 1.5-5.0:1. Excessive tungsten led to the formation of Ba5CaAl4O3. 12The main phase is sparsely dispersed, resulting in insufficient emission points; the emission current density drops to 9.2 A / cm² at 900℃. 2 Comparative Example 7: The ratio was 1.0:1. Insufficient tungsten content resulted in decreased conductivity, with a room temperature conductivity of only 0.7 × 10⁻⁶. 5 S / m, and the oxide phase is prone to agglomeration, which does not meet the stability requirements.

[0399] Comparative Example 8 uses pneumatic spraying instead of ultrasonic atomization. Pneumatic spraying relies on high-pressure airflow to break up the liquid film, resulting in droplet particle size D. 50 The droplets reached 5-8 μm in size, with a distribution span > 2.0, significantly larger than the ≤ 1.0 of Example 1. The excessively large droplets caused uneven solute migration during drying, resulting in core-shell segregation, increased compositional tolerances (e.g., a tolerance of 1.1% for Ca), and a decrease in emission current density to 8.0 A / cm². 2 The particles showed obvious aggregation.

[0400] Comparative Example 9: The guide vane tilt angle was 45°, exceeding the optimization range of 30±5°. This resulted in excessively high hot nitrogen spiral velocity, causing droplets to remain in the drying tower for less than 0.5 seconds. The solvent did not evaporate completely, forming "soft sphere" particles that were prone to deformation during subsequent reduction. Comparative Example 10: The tilt angle was 20°, resulting in excessively slow airflow velocity. This increased droplet collision and coalescence rate by 30% and caused uneven particle size. Both of these factors widened the compositional standard deviation, and the emission current density decreased to 9.0 A / cm². 2 and 8.8A / cm 2 .

[0401] Comparative Example 11 used dry hydrogen reduction, without water vapor to regulate oxygen partial pressure. During the low-temperature stage (700-750℃), the oxygen partial pressure was too low, causing WO3 from APT decomposition to be directly and deeply reduced to WO2, skipping the WO3 phase. 2.9 In the substoichiometric transition state, the lattice distortion is severe; in the high-temperature stage, due to the excessively strong reduction potential, tungsten particles grow abnormally, the phase composition fluctuates greatly, and the emission current density is only 5.8 A / cm³. 2 Furthermore, the fluctuation reached 12% within 30 minutes.

[0402] Comparative Example 12: Reversing the wet hydrogen concentration (0.15 vol% in the first stage and 0.40 vol% in the second stage), the low water vapor concentration at low temperature could not maintain the stability of WO3, leading to premature reduction and particle sintering; while the high water vapor concentration at high temperature inhibited the reduction of WO2→W, leaving residual tungsten oxide phase and reducing the conductivity to 0.6 × 10⁻⁶. 5 S / m, with an emission current density of only 5.0 A / cm. 2 The characteristic of insufficient reduction is obvious.

[0403] Comparative Example 13, without the addition of ammonium polyacrylate and PMMA, lacked the carboxyl chelating effect of NH4PAA. The zeta potential of the nano-tungsten powder in the slurry dropped to -20 mV, leading to agglomeration. Without PMMA to create pores, the porosity was less than 20%, lower than the 35% of Example 1. Hydrogen and water vapor transport was hindered during reduction, resulting in deteriorated compositional uniformity and a decrease in emission current density to 7.5 A / cm². 2 Its Vickers hardness is only 275 HV.

[0404] Comparative Example 14: The slurry viscosity was 25 cP, lower than the 30-50 cP range. This low viscosity resulted in poor liquid film stability during ultrasonic atomization, and droplets easily coalesced to form D... 50 Large droplets of 3-4 μm in size develop a density gradient inside after drying. During high-temperature reduction, stress concentration causes microcracks, reducing the emission current density to 8.2 A / cm². 2 The conductivity fluctuates by 15%.

[0405] In summary, the comparative example suffered from an imbalance in component ratios, missing interfacial bonds, deviations in process parameters, or omissions of key steps, resulting in a simultaneous decrease in component uniformity, microstructure density, and main phase activity, ultimately leading to performance inferior to Example 1.

Claims

1. A method for producing a nanocathode composite powder by spray drying, characterized by, The method comprises the following steps: Step (1) pretreatment of nano-tungsten powder comprises the following steps: Step (1-a) acid pickling: the nano-tungsten powder is ultrasonically treated in 5-10% oxalic acid solution at 60-80℃ for 30-60min; Step (1-b) surface modification: the nano-tungsten powder after acid pickling in step (1-a) is put into an ethanol aqueous solution containing 1-3 wt% silane coupling agent, and is reacted at 80℃ for 2h; in the ethanol aqueous solution, water accounts for 5-10% of the total volume; Step (1-c) vacuum drying: drying at 100-120 °C for 2-4 h under a vacuum of ≤10 -2 Pa, obtaining pretreated nano-tungsten powder; Step (2) preparation of mixed slurry: Step (2-a) dissolution of ammonium tungstate: ammonium tungstate is dissolved in deionized water at 60-80℃, with a concentration of 20-30 wt%, and 0.1-0.3 wt% citric acid is added to obtain an ammonium tungstate solution; Step (2-b) mixing of salt solution: the ammonium tungstate solution is added with barium nitrate, calcium nitrate, aluminum nitrate, scandium nitrate and yttrium nitrate in a molar ratio of Ba:Ca:Al:Sc:Y = 4.8-5.2:0.9-1.1:3.8-4.2:0.005-0.015:0.003-0.010; Step (2-c) addition of pretreated nano-tungsten powder to obtain a slurry, the total molar ratio of tungsten element to metal cation in the mixed salt solution is W:(Ba+Ca+Al+Sc+Y) = 1.5-5.0:1, 0.1-0.5 wt% dispersant and 5-15 wt% pore-forming agent are added according to the weight of the slurry; the pretreated nano-tungsten powder accounts for 30-50 wt% of the total amount of tungsten element; Step (2-d) adjustment of slurry viscosity to 30-50 cP; Step (3) ultrasonic atomization: the slurry is subjected to ultrasonic atomization at a frequency of 1.5-2.5 MHz and a power density of 50-100 W / cm 2 The ultrasonic generator generates atomized liquid droplets; Step (4) atomization droplet spray drying: under a nitrogen atmosphere, the inlet temperature is 180-220℃, the inclination angle of the cyclone guide plate of the spray drying tower is 30±5°, the outlet temperature is 80-100℃, and the gas flow rate is 0.2-0.5 m / s for drying; Step (5) synchronous reduction and calcination: under a nitrogen atmosphere, the temperature is raised to 300-400℃, and the temperature is kept for 0.5-1h, then the temperature is raised, and the wet hydrogen atmosphere is reduced to obtain a nano-cathode composite powder prepared by spray drying; The reduction in the wet hydrogen atmosphere comprises: First stage: constant temperature at 700-750℃ for 0.5-1h, H2O concentration 0.3-0.5 vol%; Second stage: constant temperature at 800-900℃ for 1.5-3h, H2O concentration 0.1-0.2 vol%.

2. The method for preparing a nanocathode composite powder by spray drying according to claim 1, wherein the spray drying is performed under the conditions of a spray pressure of 10 to 30 kg / cm2, a spray temperature of 150 to 250°C, and a drying temperature of 80 to 150°C. In step (2-c), the dispersant is ammonium polyacrylate; the pore-forming agent is polymethyl methacrylate microspheres with a particle size of 10-100 μm.

3. The method for preparing a nanocathode composite powder by spray drying according to claim 1, wherein the spray drying is performed under the conditions of a spray pressure of 10 to 30 kg / cm2, a spray temperature of 150 to 250°C, and a drying temperature of 80 to 150°C. In step (2-d), the method for adjusting the slurry viscosity to 30-50 cP is: adding ethanol to a water-ethanol volume ratio of 6-8:2-4, and ultrasonically homogenizing for 10-20min at 200-400W.

4. The method for preparing a nanocathode composite powder by spray drying according to claim 1, wherein the spray drying is performed under the conditions of a spray pressure of 10 to 30 kg / cm2, a spray temperature of 150 to 250°C, and a drying temperature of 80 to 150°C. Step (3) After ultrasonic atomization, atomized droplets D 50 = 0.8-1.5 μm.

5. The method of claim 1, wherein the nanocathode composite powder is prepared by spray drying, and In step (3), the temperature of the atomization chamber is 25±2℃, and the auxiliary nitrogen flow rate is 0.2±0.05 m / s.

6. The method of claim 1, wherein the nanocathode composite powder is prepared by spray drying, and In step (5), the temperature rising rate is 3-5℃ / min during the synchronous reduction and calcination process.

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