A method and device for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis

Abstract

The application discloses a method and device for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis, and relates to the technical field of powder material preparation. The application establishes an inside-out heat flow field through the penetrating heating of the microwave on the polar solvent molecules in the liquid drops. Under the traditional heat conduction, the evaporation rate of the liquid drop surface is much greater than the internal solute diffusion rate, so that the Peclet number is greater than 1, thereby causing the surface supersaturation crust. The diffusion rate of the solute to the center is significantly improved by using the microwave coupling effect, so that the Pe number is forced to be below 0.5. The 'bulk phase shrinkage' mode forces the solute to continuously converge to the geometric center of the liquid drop during the solvent evaporation process, which helps to eliminate the 'hollow ball' morphology, and the solid precursor particles with extremely high density are obtained.

Description

Technical Field

[0001] This invention relates to the field of powder material preparation technology, specifically to a process and specialized equipment for preparing highly dense, grain boundary-free, and highly monocrystalline elemental metal powders by using microwave volumetric heating and aerosol pyrolysis coupling technology, and by controlling the solute precipitation kinetics inside droplets and the electromagnetically induced high-temperature recrystallization process. Background Technology

[0002] Spray pyrolysis (SP) technology, as a powder synthesis process involving atomic-level mixing of liquid-phase precursors and rapid, continuous gas-phase reactions, has demonstrated significant technological advantages in the preparation of high-purity metal powders (such as copper, silver, nickel, and tungsten). Its core lies in the process where the precursor liquid, after being atomized into micron-sized droplets, sequentially undergoes solvent evaporation, solute precipitation, thermal decomposition, and reduction crystallization under carrier gas transport. However, in the industrialization process pursuing "high-end electronic-grade single-crystal powders," traditional spray pyrolysis processes using thermal radiation or heat conduction modes are still constrained by the following deep-seated technological bottlenecks: 1. Particle "hollowing" defect caused by solute distribution imbalance In traditional heat conduction heating, droplets are heated from the surface inwards. According to aerosol kinetics, particle morphology is controlled by the ratio of the evaporation rate at the droplet surface to the diffusion rate of the solute towards the center, i.e., the Pelet number (Pe). In traditional processes, the Pelet number is usually much greater than 1, meaning that solvent evaporation at the surface is much faster than solute diffusion inwards, causing the solute to quickly reach supersaturation at the droplet surface and precipitate to form a solid shell. As the residual solvent inside vaporizes upon heating, the internal pressure of the particle increases dramatically, causing the particle to expand, burst, or form a thin-walled, hollow "hollow sphere." This morphological defect directly results in the powder's tap density being only 30%-50% of the theoretical density. In high-end electronic paste applications, this leads to extremely uneven sintering shrinkage, causing cracking or peeling of the circuit coating.

[0003] 2. Polycrystalline structure and grain boundary effect induced by multicentric nucleation The electrical and thermal conductivity of metal powders is closely related to the degree of crystallization within them. In the constant temperature field of a traditional pyrolysis furnace, precursor salts often exhibit a "multi-center concurrent nucleation" characteristic during reduction. Due to the lack of effective control over the nucleation site and growth rate, individual micron-sized particles are often composed of thousands of nano-sized primary grains, forming a typical polycrystalline aggregate structure. This structure is filled with high-density, high-angle grain boundaries. Grain boundaries not only act as centers for electron scattering, significantly increasing the resistivity of the powder, but also provide a fast diffusion path for oxygen atoms to penetrate, making the powder highly susceptible to grain boundary oxidation during storage or processing, severely impairing the material's electrical conductivity and service life.

[0004] 3. The single crystal growth kinetics are hindered by the grain boundary migration barrier. The transformation from "nanopolycrystalline" to "complete single crystal" is essentially a process of grain merging and grain boundary disappearance. Long-range migration of metal atoms and grain boundary engulfment require overcoming a significant thermal activation energy barrier. In traditional spray pyrolysis, the residence time of particles in the high-temperature zone is typically only on the order of seconds, insufficient to support a complete recrystallization process due to insufficient atomic diffusion kinetic energy. Simply extending the residence time or increasing the ambient temperature to forcibly promote grain growth leads to disordered fusion agglomeration of particles through "sintering necks" due to the lack of control over particle surface energy. This transforms the product from monodisperse spherical particles into coarse, blocky agglomerates, negating the original morphological controllability of the spray pyrolysis process. Summary of the Invention

[0005] This invention aims to overcome the technical bottlenecks in the prior art, such as the tendency of spray pyrolysis metal powder to become hollow, polycrystalline, and have many crystallization defects. It provides a microwave-assisted spray pyrolysis method and apparatus that can suppress hollowing through physical heat transfer and use electromagnetic energy fields to induce in-situ repair of metal crystals to promote highly monocrystalline growth.

[0006] This invention establishes an outward thermal flow field by penetratingly heating polar solvent molecules (such as water and alcohols) inside a droplet using microwaves. Under conventional heat conduction, the evaporation rate of the droplet surface is much greater than the diffusion rate of the solute inside, resulting in a Pe number greater than 1, which leads to surface supersaturation and crust formation. By utilizing the microwave coupling effect, the diffusion rate of the solute towards the center is significantly increased, forcibly reducing the Pe number to below 0.5. This "bulk contraction" mode forces the solute to continuously converge towards the geometric center of the droplet during solvent evaporation, which helps to eliminate the "hollow sphere" morphology and obtain high-density solid precursor particles.

[0007] When the particles enter the high-temperature reduction crystallization stage, this invention utilizes the strong coupling between the high-frequency electromagnetic field and the defects (vacancies, interstitial atoms) in the metal lattice and the high-energy grain boundaries. This non-thermal effect significantly reduces the apparent activation energy Q required for atoms to cross the grain boundary barrier by providing electromagnetic driving force. At a temperature far below the melting point of the metal, electromagnetic energy induces rapid recrystallization inside the particles, prompting nanoscale small grains to rapidly merge with the grain boundaries through crystal orientation rotation. This controlled "super growth" path enables the particles to achieve a qualitative change from "disordered polycrystalline" to "complete single crystal" within a second-level residence time.

[0008] The technical solution of this invention is as follows: A method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis includes the following steps: (1) Preparation of precursor solution: Mix metal salt, organic complexing agent and polar solvent to obtain precursor solution; (2) Microwave volume drying suppresses hollowing: The precursor solution is atomized, and the atomized droplets are controlled by adjusting the microwave output power density of the microwave drying cavity. The volume heating effect of microwaves on the droplets controls the temperature gradient inside and outside the droplets, so that the solute shrinks uniformly towards the geometric center to obtain solid dry particles. (3) Electromagnetically induced single crystal growth: Dry particles enter a high-temperature single crystal growth furnace and are heated and decomposed under a reducing atmosphere. The synergistic effect of thermal energy and electromagnetic field non-thermal effect reduces the atomic diffusion activation energy Q, induces non-thermal migration and engulfment of grain boundaries, and causes the polycrystalline structure to recrystallize in situ into a single crystal structure. (4) Continuous negative pressure collection: Under a slightly negative pressure environment, the powder enters the spray collector with the airflow and settles into the liquid seal bottle to achieve continuous production.

[0009] The metal salt in step (1) is at least one of nitrate, acetate, chloride or sulfate, and the metal is at least one of copper, silver, nickel, tungsten, molybdenum, gold, rhenium or cobalt; the organic complexing agent is at least one of ethylenediaminetetraacetic acid, ammonia, citric acid, tartaric acid or oxalic acid, and the polar solvent is water, ethanol or ethylene glycol, etc.

[0010] The molar ratio of the organic complexing agent to the metal ions in step (1) is (0.5-2):1; the concentration of metal ions in the precursor solution is controlled to be 0.1-1.5 mol / L.

[0011] The precursor solution in step (1) also contains an antioxidant with a mass fraction of 0.1%-1.0%; the antioxidant is selected from at least one of benzotriazole, ascorbic acid or sodium sulfite.

[0012] The droplet size distribution index of the atomized liquid in step (2) is less than 2.5.

[0013] In step (2), the ratio of the controlled microwave output power to the atomization rate of the precursor solution is 0.5-2.5 kW / (L·h⁻¹).

[0014] The heating and pyrolysis temperature in step (3) is controlled at the metal melting point T. m 0.4-0.9 times; the effective residence time of particles in the high-temperature section is controlled at 2-15 seconds.

[0015] In step (4), the spray liquid of the spray collector contains an antioxidant with a mass fraction of 0.1%-1.0%; the antioxidant is selected from at least one of benzotriazole, ascorbic acid or sodium sulfite, and the liquid-sealed bottle contains the same solution as the spray liquid.

[0016] The present invention also provides an apparatus for preparing dense single crystal metal powder by microwave-assisted spray pyrolysis, comprising an atomizer 1, a microwave drying chamber 2, a high-temperature single crystal growth furnace 3, a spray collector 4, a microwave generator 7, a blower 9, an induced draft fan 10, and a liquid seal bottle 11. A drying tube is installed inside the microwave drying cavity 2, and a metal cut-off waveguide 6 for shielding microwave leakage is coaxially installed on the drying tube through the wall panel of the microwave drying cavity 2. The atomizer 1 and blower 9 are connected to the metal cut-off waveguide 6 at one end of the microwave drying chamber 2. The metal cut-off waveguide 6 at the other end of the microwave drying chamber 2 is connected to the high-temperature resistant waveguide 5 inside the high-temperature single crystal growth furnace 3. The high-temperature resistant waveguide 5 is connected to the side of the spray collector 4. The top of the spray collector 4 is equipped with a nozzle. The top of the spray collector 4 is also connected to the induced draft fan 10. The other end of the induced draft fan 10 is connected to the liquid seal bottle 11. The microwave generator 7 is coupled to the microwave drying cavity 2 via the microwave compensation antenna 8, and is also coupled to the high-temperature resistant wave-transmitting tube 5 inside the high-temperature single crystal growth furnace 3 via the microwave coupling antenna 12.

[0017] The high-temperature single crystal growth furnace 3 is equipped with a reducing atmosphere supply port for introducing a reducing mixed gas containing H2. Specifically, the reducing mixed gas is a mixture of hydrogen and nitrogen, wherein the volume fraction of hydrogen is not less than 5%.

[0018] The length L and inner diameter D of the metal cutoff waveguide 6 satisfy the relationship L / D≥10.

[0019] The liquid-sealed bottle 11 is isolated from the atmosphere by a U-shaped liquid-sealing structure.

[0020] The device of this invention adopts a modular design: it consists of a multi-field coupling device, including a high-frequency ultrasonic / pneumatic dual-fluid atomizer (producing narrow-distribution droplets), a microwave shielded volume drying chamber (achieving solidification conversion), an induction-enhanced high-temperature single crystal growth furnace (completing reduction and single crystallization), and a multi-stage micro-negative pressure spray collector connected in sequence. The modules are connected by a quartz reaction tube to ensure the continuity of material flow.

[0021] This invention addresses the intrinsic electromagnetic safety and cutoff waveguide design challenge in continuous-flow microwave systems. It incorporates a stainless steel cutoff waveguide, fully welded coaxially at the entrance and exit points of the drying tube within the microwave drying cavity. By precisely designing the length L and inner diameter D of the cutoff waveguide to achieve an aspect ratio L / D ≥ 10, and utilizing the exponential attenuation of electromagnetic waves within a subwavelength channel, the microwave leakage outside the drying cavity is controlled to within 0.5 mW / cm². 2 Within.

[0022] This invention features precise control of single crystal growth kinetics. The high-temperature single crystal growth furnace incorporates a multi-segment independent temperature control system and is equipped with multiple supply ports for reducing atmosphere (H2 / N2). The control is achieved by real-time adjustment of microwave power density and ambient thermal field temperature (controlled between 0.4-0.9T). m Within a certain range, a precise energy coupling window is established, which is designed to ensure that metal atoms obtain sufficient migration energy to eliminate internal grain boundaries, while controlling the residence time (2-15 seconds) through a micro-negative pressure flow field to prevent interparticle sintering and agglomeration.

[0023] The product prepared by this invention exhibits a high degree of single-crystallization, with each particle being a single crystal. SEM testing shows that the internal crystal orientation deviation of the particles is extremely small, essentially eliminating grain boundaries. This allows the powder to exhibit electrical and thermal conductivity close to the theoretical value in applications, and it also possesses excellent resistance to electromigration.

[0024] This invention significantly suppresses the "apple-shaped" collapse or fragmentation problem commonly found in traditional spray pyrolysis. The powder is dense and pore-free, with a compaction density reaching 80%-98% of the theoretical density. This greatly reduces the shrinkage rate during the sintering process of electronic pastes and improves the precision of components.

[0025] This invention employs a cutoff waveguide design in conjunction with a micro-negative pressure power system, which not only solves the safety risks of microwave radiation but also prevents the leakage of high-temperature reducing gases. The system supports 24 / 7 continuous operation, and the annual production capacity of a single line can reach the ton level, making it extremely valuable for commercial transformation. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the overall structure of the apparatus for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to the present invention; In the diagram: 1-Atomizer; 2-Microwave drying chamber; 3-High-temperature single crystal growth furnace; 4-Spray collector; 5-High-temperature resistant waveguide; 6-Metal cutoff waveguide; 7-Microwave generator; 8-Microwave compensation antenna; 9-Blower; 10-Exhaust fan; 11-Liquid seal bottle; 12-Microwave coupling antenna. Figure 2 A comparison of the evolution of Pelet number and particle morphology during droplet drying; Figure 3 Comparison diagram of the kinetic mechanism of single crystal growth of metal particles; Figure 4 The simulated distribution of electromagnetic field strength attenuation inside a metal cutoff waveguide; Figure 5 The XRD patterns are of the products of Example 2 and Comparative Example 1 of this invention; Figure 6 SEM images of single-crystal metal powder prepared in Example 2. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments.

[0028] Example 1 An apparatus for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis, such as Figure 1 As shown, it includes an atomizer 1, a microwave drying chamber 2, a high-temperature single crystal growth furnace 3, a spray collector 4, a high-temperature resistant waveguide 5, a metal cutoff waveguide 6, a microwave generator 7, a microwave compensation antenna 8, a blower 9, an exhaust fan 10, a liquid-sealed bottle 11, and a microwave coupling antenna 12. A drying tube is installed inside the microwave drying cavity 2. A metal cut-off waveguide 6 for shielding microwave leakage is coaxially installed on the drying tube through the wall panel of the microwave drying cavity 2. The length L of the metal cut-off waveguide 6 and the inner diameter D satisfy the relationship L / D≥10. The atomizer 1 and blower 9 are connected to the metal cut-off waveguide 6 at one end of the microwave drying chamber 2. The metal cut-off waveguide 6 at the other end of the microwave drying chamber 2 is connected to the high-temperature resistant waveguide 5 inside the high-temperature single crystal growth furnace 3. The high-temperature resistant waveguide 5 is connected to the side of the spray collector 4. The top of the spray collector 4 is equipped with a nozzle. The top of the spray collector 4 is also connected to the induced draft fan 10. The other end of the induced draft fan 10 is connected to the liquid seal bottle 11. The liquid seal bottle 11 is isolated from the atmosphere through a U-shaped liquid seal structure. The microwave generator 7 is coupled to the microwave drying cavity 2 via the microwave compensation antenna 8, and is also coupled to the high-temperature resistant wave-transparent tube 5 in the high-temperature single crystal growth furnace 3 via the microwave coupling antenna 12, for generating an auxiliary electromagnetic field with a controlled frequency; the microwave compensation antenna 8 and the microwave coupling antenna 12 are connected to the microwave generator 7 or an independent microwave source. The high-temperature single crystal growth furnace 3 is equipped with a reducing atmosphere supply port for introducing a reducing mixed gas containing H2. Specifically, the reducing mixed gas is a mixture of hydrogen and nitrogen, wherein the volume fraction of hydrogen is not less than 5%. The pressure inside the microwave drying chamber 2 and the high-temperature single crystal growth furnace 3 can be adjusted by the power of the blower 9, the induced draft fan 10, and the reducing atmosphere supply. Temperature and pressure sensors are installed at the connection between the high-temperature resistant waveguide 5 and the spray collector 4 to monitor the temperature and pressure inside the high-temperature single crystal growth furnace 3.

[0029] Example 2 A method for preparing dense submicron-sized single-crystal silver (Ag) powder by microwave-assisted spray pyrolysis, using the apparatus of Example 1, includes the following steps: (1) Preparation of precursor solution and collection solution: Prepare a silver nitrate solution with a concentration of 0.2 mol / L, add an appropriate amount of ammonia water with a mass fraction of 22% to adjust the pH value to 9-10 to form silver ammonia complex ions, wherein the molar ratio of ammonia water to silver ions is 1.5:1, and obtain the precursor solution. Load the precursor solution into atomizer 1. Prepare an aqueous solution of 0.5% ascorbic acid by mass as the collection solution and the spray solution. Put the collection solution into the liquid seal bottle 11 and the spray solution into the sprayer connected to the spray nozzle at the top of the spray collector 4. (2) Microwave volumetric drying to suppress hollowing: The precursor solution is atomized using an atomizer 1, with a droplet size distribution index of less than 2.5. The atomized droplets enter the drying tube inside the microwave drying cavity 2. By adjusting the microwave output power density of the microwave drying cavity 2, mainly by adjusting the microwave compensation antenna 8 inside the microwave drying cavity 2 and outside the drying tube, and the microwave generator 7 connected to the metal cutoff waveguide 6, the ratio of the microwave output power inside the microwave drying cavity 2 to the atomization feed rate of the precursor solution is made to be 1.0 kW / (L·h). -1 By utilizing the volume heating effect of microwaves on droplets, the solute is uniformly contracted towards the geometric center to obtain solid dry particles. (3) Electromagnetically induced single crystal growth: Dry particles are introduced into the high-temperature single crystal growth furnace 3 and heated and decomposed under a reducing atmosphere (the reducing atmosphere is a mixture of hydrogen and nitrogen, with hydrogen accounting for 10% by volume). The temperature of the high-temperature single crystal growth section is 820°C (approximately 0.85T). m The carrier gas flow rate ensures that the particles reside in the high-temperature section for 5 seconds. By utilizing the synergistic effect of thermal energy and electromagnetic field non-thermal effects, the atomic diffusion activation energy Q is reduced, inducing non-thermal migration and engulfment of grain boundaries, thereby enabling the polycrystalline structure to recrystallize in situ into a single-crystal structure. (4) Continuous negative pressure collection: The pressure in the microwave drying chamber 2 and the high temperature single crystal growth furnace 3 is adjusted to a slight negative pressure (the system maintains a negative pressure of -200Pa) by the power of the blower 9, the induced draft fan 10 and the reducing atmosphere supply. Under the slight negative pressure environment, the powder enters the spray collector 4 with the airflow and settles into the liquid seal bottle 11 under the action of the induced draft fan 10 to achieve continuous production of submicron-level single crystal silver (Ag) powder.

[0030] The submicron-sized single-crystal silver (Ag) powder obtained in this embodiment has an average particle size D. 50 The particle size distribution index is 1.8, the XRD diffraction peak is extremely sharp, and the full width at half maximum (FWHM) of the Cu target (111) plane is only 0.224°. The internal crystal orientation deviation of a single particle is only 0.85° as determined by EBSD, which shows extremely high single-crystal phase continuity. The resistivity test result of the silver paste made from the powder is 1.69 micro ohms / □ (very close to the theoretical conductivity of pure silver).

[0031] Example 3 A method for preparing highly conductive dense single-crystal copper (Cu) powder by microwave-assisted spray pyrolysis, using the apparatus of Example 1, includes the following steps: (1) Preparation of precursor solution and collection solution: Prepare a 1 mol / L copper nitrate solution, add EDTA as a complexing agent, wherein EDTA reacts with Cu 2+ The molar ratio is 1.2:1 to obtain a precursor solution, which is then loaded into atomizer 1. Prepare an aqueous solution of 0.5% benzotriazole by mass as a collection liquid and a spray liquid. Put the collection liquid into a liquid-sealed bottle 11 and the spray liquid into a sprayer connected to the spray nozzle at the top of the spray collector 4. (2) Microwave volume drying to suppress hollowing: The precursor solution is atomized using an atomizer 1 with an ultrasonic atomization frequency of 1.7MHz and a droplet size distribution index of less than 2.5. The atomized droplets enter the drying tube inside the microwave drying cavity 2. By adjusting the microwave output power density of the microwave drying cavity 2, mainly by adjusting the microwave compensation antenna 8 inside the microwave drying cavity 2 and outside the drying tube, and the microwave generator 7 connected to the metal cutoff waveguide 6, the ratio of the microwave output power inside the microwave drying cavity 2 to the atomization feed rate of the precursor solution is 1.5kW / (L·h). -1 By utilizing the volume heating effect of microwaves on droplets, the solute is uniformly contracted towards the geometric center to obtain solid dry particles. (3) Electromagnetically induced single crystal growth: Dry particles are introduced into the high-temperature single crystal growth furnace 3 and heated and decomposed under a reducing atmosphere (the reducing atmosphere is a mixture of hydrogen and nitrogen, with hydrogen accounting for 10% by volume). The temperature of the high-temperature single crystal growth section is 920°C (approximately 0.85T). m The carrier gas flow rate ensures that the particles reside in the high-temperature section for 5 seconds. By utilizing the synergistic effect of thermal energy and electromagnetic field non-thermal effects, the atomic diffusion activation energy Q is reduced, inducing non-thermal migration and engulfment of grain boundaries, thereby enabling the polycrystalline structure to recrystallize in situ into a single-crystal structure. (4) Continuous negative pressure collection: The pressure in the microwave drying chamber 2 and the high temperature single crystal growth furnace 3 is adjusted to a slight negative pressure (the system maintains a negative pressure of -200Pa) by the power of the blower 9, the induced draft fan 10 and the reducing atmosphere supply. Under the slight negative pressure environment, the powder enters the spray collector 4 with the airflow and settles into the liquid seal bottle 11 under the action of the induced draft fan 10 to achieve continuous production of highly conductive dense single crystal copper (Cu) powder.

[0032] The highly conductive dense single-crystal copper (Cu) powder prepared in this embodiment was observed by SEM to show that the particles had no hollow shells and the measured compaction density was 8.18 g / cm³, reaching 91% of the theoretical density. The EBSD orientation mapping showed that the internal color of a single particle was uniform and there were no obvious grain boundaries. The SAED spots were regular lattice, confirming that it was a single-crystal structure.

[0033] Example 4 A method for preparing MLCC-grade single-crystal nickel (Ni) powder by microwave-assisted spray pyrolysis, using the apparatus of Example 1, includes the following steps: (1) Preparation of precursor solution and collection solution: Prepare a nickel acetate solution with a concentration of 0.8 mol / L, add a citric acid solution with a mass fraction of 5% as a complexing agent, wherein the molar ratio of citric acid to nickel ions is 1:1, and obtain the precursor solution. Load the precursor solution into the atomizer 1. Prepare an aqueous solution of sodium sulfite with a mass fraction of 0.5% as the collection liquid and the spray liquid. Put the collection liquid into the liquid seal bottle 11 and the spray liquid into the sprayer connected to the spray nozzle at the top of the spray collector 4. (2) Microwave volumetric drying to suppress hollowing: The precursor solution is atomized using an atomizer 1 with an ultrasonic atomization frequency of 1.7MHz and a droplet size distribution index of less than 2.5. The atomized droplets enter the drying tube inside the microwave drying cavity 2. By adjusting the microwave output power density of the microwave drying cavity 2, mainly by adjusting the microwave compensation antenna 8 inside the microwave drying cavity 2 and outside the drying tube, and the microwave generator 7 connected to the metal cutoff waveguide 6, the ratio of the microwave output power inside the microwave drying cavity 2 to the precursor solution atomization feed rate is 1.0kW / (L·h). -1 By utilizing the volume heating effect of microwaves on droplets, the solute is uniformly contracted towards the geometric center to obtain solid dry particles. (3) Electromagnetically induced single crystal growth: Dry particles are introduced into the high-temperature single crystal growth furnace 3 and heated and decomposed under a reducing atmosphere (a mixture of hydrogen and nitrogen, with hydrogen accounting for 15% by volume). The temperature of the high-temperature single crystal growth section is 1150°C (approximately 0.79T). m The carrier gas flow rate ensures that the particles reside in the high-temperature section for 5 seconds. By utilizing the synergistic effect of thermal energy and electromagnetic field non-thermal effects, the atomic diffusion activation energy Q is reduced, inducing non-thermal migration and engulfment of grain boundaries, thereby enabling the polycrystalline structure to recrystallize in situ into a single-crystal structure. (4) Continuous negative pressure collection: The pressure in the microwave drying chamber 2 and the high temperature single crystal growth furnace 3 is adjusted to a slight negative pressure (the system maintains a negative pressure of -200Pa) by the power of the blower 9, the induced draft fan 10 and the reducing atmosphere supply. Under the slight negative pressure environment, the powder enters the spray collector 4 with the airflow and settles into the liquid seal bottle 11 under the action of the induced draft fan 10 to achieve continuous production of MLCC grade single crystal nickel (Ni) powder.

[0034] The MLCC-grade single-crystal nickel (Ni) powder prepared in this embodiment was observed through TEM cross-section. The particles were solid and there was no grain boundary / dislocation accumulation. The resulting powder was heated at 250°C in air for 2 hours. The weight gain was only 0.367%, which is far superior to that of polycrystalline nickel powder, showing the excellent chemical stability of the single-crystal structure.

[0035] Example 5 A method for preparing dense, refractory single-crystal tungsten (W) powder by microwave-assisted spray pyrolysis, using the apparatus of Example 1, includes the following steps: (1) Preparation of precursor solution and collection solution: Prepare a 0.5 mol / L ammonium metatungstate (AMT) solution, add 5% oxalic acid solution as a complexing agent, wherein the molar ratio of oxalic acid to tungsten ions is 0.5:1, and obtain the precursor solution. Load the precursor solution into atomizer 1. Prepare an aqueous solution of sodium sulfite with a mass fraction of 0.5% as the collection liquid and the spray liquid. Put the collection liquid into the liquid seal bottle 11 and the spray liquid into the sprayer connected to the spray nozzle at the top of the spray collector 4. (2) Microwave volume drying to suppress hollowing: The precursor solution is atomized using an atomizer 1 with an ultrasonic atomization frequency of 1.7MHz and a droplet size distribution index of less than 2.5. The atomized droplets enter the drying tube inside the microwave drying cavity 2. By adjusting the microwave output power density of the microwave drying cavity 2, mainly by adjusting the microwave compensation antenna 8 inside the microwave drying cavity 2 and outside the drying tube, and the microwave generator 7 connected to the metal cutoff waveguide 6, the ratio of the microwave output power inside the microwave drying cavity 2 to the atomization feed rate of the precursor solution is 2.5kW / (L·h). -1 By utilizing the volume heating effect of microwaves on droplets, the solute is uniformly contracted towards the geometric center to obtain solid dry particles. (3) Electromagnetically induced single crystal growth: Dry particles are introduced into the high-temperature single crystal growth furnace 3 and heated and decomposed under a reducing atmosphere (a mixture of hydrogen and nitrogen, with hydrogen accounting for 15% by volume). The temperature of the high-temperature single crystal growth section is 1450°C (approximately 0.79T). m The carrier gas flow rate ensures that the particles stay in the high-temperature section for 5 seconds. By utilizing the synergistic effect of thermal energy and the non-thermal effect of electromagnetic field, the atomic diffusion activation energy Q is reduced. A high-strength auxiliary electromagnetic potential field is established in the single crystal growth section through microwave compensation antenna, which induces rapid migration of grain boundaries, so that the polycrystalline structure is recrystallized in situ into a single crystal structure. (4) Continuous negative pressure collection: The pressure in the microwave drying chamber 2 and the high temperature single crystal growth furnace 3 is adjusted to a slight negative pressure (the system maintains a negative pressure of -200Pa) by the power of the blower 9, the induced draft fan 10 and the reducing atmosphere supply. Under the slight negative pressure environment, the powder enters the spray collector 4 with the airflow and settles into the liquid seal bottle 11 under the action of the induced draft fan 10 to achieve continuous production of refractory single crystal tungsten (W) powder.

[0036] This embodiment successfully prepared solid spherical tungsten powder, eliminating the "garnet-like" nanocrystal aggregation morphology commonly found in traditional SP process for tungsten powder preparation. The crystal orientation deviation within a single particle was measured to be less than 0.45°, exhibiting obvious coarse grain single crystal characteristics. The compaction density reached 15.6 g / cm³, achieving 80.8% of the theoretical density.

[0037] Comparative Example 1 Traditional heat conduction spray pyrolysis method (compared with Example 2) Using the exact same precursor solution formulation and high-temperature section temperature as in Example 2, but with microwave assistance turned off during the drying stage and a traditional resistance wire external infrared radiation heating mode adopted, the final product produced a large number of hollow spheres and burst fragments. Due to the initial crust formation on the surface, the pressure generated by the vaporization of the internal solvent broke the particles. The proportion of solid particles was less than 25%. It was found that each particle was composed of a large number of nanocrystals with a diameter of about 20-40 nm that were randomly stacked, with extremely high density of grain boundaries. The resistivity test result of the silver paste made from the powder was 3.04 micro ohms / □.

[0038] Figure 2 The diagram shows a comparison of the Pelet number and particle morphology evolution during the droplet drying process. In (a) the traditional heat conduction mode (Pe>1), the solute precipitates on the surface and forms a "hard shell" due to the rapid surface evaporation, and the internal solvent vaporization leads to the hollow and bursting morphology evolution of the particles. In (b) the microwave volume heating mode of the present invention (Pe<0.5), the solute always converges towards the geometric center due to the synchronous heating inside and outside, and finally obtains the evolution mechanism of solid and dense particles.

[0039] Figure 3 The diagram shows a comparison of the growth kinetics of single crystallization of metal particles. (a) shows the polycrystalline structure under traditional high-temperature pyrolysis, in which the particles are composed of a large number of nanocrystal clusters with interlaced grain boundaries and a high density of dislocations. (b) shows the single crystal structure induced by the electromagnetic field of this invention, in which the grain boundaries undergo a leapfrog migration and disappear under the non-thermal effect of microwaves, and the small grains merge into a complete single crystal.

[0040] Figure 4 The diagram shows the simulated distribution of electromagnetic field strength attenuation inside a cutoff waveguide. The curve illustrates the process by which the field strength attenuates exponentially to the background level after microwave energy enters a cutoff waveguide with an L / D>10.

[0041] Figure 5 This is a schematic diagram comparing the XRD microstructure of the products of Example 2 and Comparative Example 1 of the present invention. (A) The XRD pattern of the single-crystal silver powder prepared in Example 2 shows sharp diffraction peaks, no impurity peaks, and a significantly narrowed full width at half maximum (FWHM). (B) The XRD pattern of the polycrystalline silver powder prepared in Comparative Example 1 shows broad diffraction peaks with a large amount of noise, indicating the presence of a large number of randomly oriented grains. (C) Standard diffraction peaks of single-crystal silver particles.

[0042] Figure 6 The image shows a SEM image of the single-crystal metal powder prepared in Example 2. As can be seen from the image, the powder material has distinct edges and corners and no obvious grain boundaries.

[0043] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis, characterized in that, Includes the following steps: (1) Mix the metal salt, organic complexing agent and polar solvent to obtain a precursor solution; (2) The precursor solution is atomized and the atomized droplets enter the microwave drying chamber. The volume heating effect of microwaves on the atomized droplets is used to control the temperature gradient inside and outside the droplets, so that the solute shrinks uniformly towards the geometric center to obtain solid dry particles. (3) Dry particles enter a high-temperature single crystal growth furnace and are heated and decomposed under a reducing atmosphere. The synergistic effect of thermal energy and electromagnetic field non-thermal effect reduces the atomic diffusion activation energy Q, induces non-thermal migration and engulfment of grain boundaries, and causes the polycrystalline structure to recrystallize in situ into a single crystal structure. (4) Under a slightly negative pressure environment, the powder enters the spray collector with the airflow and settles into the liquid seal bottle to achieve continuous production.

2. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, The metal salt in step (1) is at least one of nitrate, acetate, chloride or sulfate, and the metal is at least one of copper, silver, nickel, tungsten, molybdenum, gold, rhenium or cobalt; the organic complexing agent is at least one of ethylenediaminetetraacetic acid, ammonia, citric acid, tartaric acid or oxalic acid, and the polar solvent is water, ethanol or ethylene glycol.

3. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, The molar ratio of the organic complexing agent to the metal ions in step (1) is (0.5-2):1; the concentration of metal ions in the precursor solution is 0.1-1.5 mol / L.

4. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, The precursor solution in step (1) also contains an antioxidant with a mass fraction of 0.1%-1.0%; the antioxidant is at least one of benzotriazole, ascorbic acid or sodium sulfite.

5. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, The ratio of microwave output power to precursor solution atomization rate in step (2) is 0.5-2.5 kW / (L·h⁻¹).

6. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, The heating and pyrolysis temperature in step (3) is at the metal melting point T. m 0.4-0.9 times; the residence time of particles in the high-temperature section is 2-15 seconds.

7. The method for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 1, characterized in that, In step (4), the spray liquid of the spray collector contains an antioxidant with a mass fraction of 0.1%-1.0%; the antioxidant is at least one of benzotriazole, ascorbic acid or sodium sulfite.

8. An apparatus for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis, characterized in that, Includes an atomizer (1), a microwave drying chamber (2), a high-temperature single crystal growth furnace (3), a spray collector (4), a microwave generator (7), a blower (9), an induced draft fan (10), and a liquid seal bottle (11). A drying tube is installed inside the microwave drying cavity (2), and a metal cut-off waveguide (6) for shielding microwave leakage is coaxially installed through the wall panel of the microwave drying cavity (2). The atomizer (1), blower (9) is connected to the metal cut-off waveguide (6) at one end of the microwave drying chamber (2), and the metal cut-off waveguide (6) at the other end of the microwave drying chamber (2) is connected to the high-temperature resistant waveguide (5) in the high-temperature single crystal growth furnace (3). The high-temperature resistant waveguide (5) is connected to the side of the spray collector (4). The top of the spray collector (4) is equipped with a nozzle. The top of the spray collector (4) is also connected to the induced draft fan (10). The other end of the induced draft fan (10) is connected to the liquid seal bottle (11). The microwave generator (7) is coupled to the microwave drying cavity (2) through the microwave compensation antenna (8), and is also coupled to the high-temperature resistant wave tube (5) in the high-temperature single crystal growth furnace (3) through the microwave coupling antenna (12).

9. The apparatus for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 8, characterized in that, The high-temperature single crystal growth furnace (3) is equipped with a reducing atmosphere supply port for introducing a reducing mixed gas containing H2.

10. The apparatus for preparing dense single-crystal metal powder by microwave-assisted spray pyrolysis according to claim 8, characterized in that, The length L and inner diameter D of the metal cutoff waveguide (6) satisfy the relationship L / D≥10.

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