Ultrahigh-pressure atomization method for improving tap density and fluidity of nano aluminum powder

Abstract

The invention relates to the technical field of manufacturing of metal nano materials and metal powder, and discloses an ultrahigh-pressure atomization method for improving the tap density and fluidity of nano aluminum powder, and the ultrahigh-pressure atomization method comprises the following steps: preparing a thermodynamic non-equilibrium homogeneous precursor fluid containing 12-hydroxystearic acid and liquid carbon dioxide; supercritical delivery of the fluid to a nozzle and ejection of an impinging molten aluminum stream; the method comprises the following steps: crushing molten aluminum by utilizing expansion shearing force generated by gasification of liquid carbon dioxide, and driving 12-hydroxystearic acid to be subjected to flash rotary joint decomposition on the surface of molten aluminum drops and self-assembled to form a three-dimensional supramolecular fiber network by utilizing a cryogenic effect generated by gasification heat absorption and solubility difference caused by sudden pressure drop; according to the method, a physical steric hindrance cage is constructed before secondary collision of the liquid drops by utilizing a three-dimensional network which is rapidly formed at a microsecond level, so that the fusion of the liquid drops is blocked, and the problems of agglomeration and irregular morphology of the nano aluminum powder are solved.

Description

Technical Field

[0001] This invention relates to an ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder, belonging to the field of manufacturing technology of metal nanomaterials and metal powders. Background Technology

[0002] Currently, ultra-high pressure atomization utilizes the kinetic energy of a high-pressure fluid medium to impact molten metal, shearing and breaking the molten metal stream into micron or nano-sized droplets. This is the mainstream process for preparing highly spherical metal powders, increasing the medium pressure to reduce powder particle size and meeting the basic requirements of additive manufacturing and high-energy propellants for ultrafine metal fuels. Industrial applications have increased requirements for powder packing density and rheological properties. The high shear energy applied during atomization causes the formation of a high-energy turbulent field in the atomization zone. Under high-frequency turbulent conditions, the nascent nano-aluminum droplets have high specific surface energy and Brownian motion speed. The timescale of secondary collisions and mutual adhesion is on the microsecond scale. The droplets are in an active molten or semi-solid state. The high-speed collisions lead to irregular fusion or satellite spherical adsorption between particles, forming dumbbell-shaped or potato-shaped irregular structures. Existing technologies improve powder dispersibility by adding surfactants to the atomization medium or subsequent surface coating. The chemical passivation film formation mechanism depends on the diffusion and migration of solute molecules in the liquid phase medium and the surface adsorption kinetics. The timescale required to complete the coating is lagging behind the physical collision frequency of droplets in the high-energy flow field.

[0003] Besides the morphological defects caused by physical collisions in the flow field, existing technologies for single physical preparation or conventional condensation processes lack in-situ surface structuring intervention, making it difficult to simultaneously achieve microscopic monodispersity and macroscopic packing density of powders. For example, the Chinese invention patent CN102950293B discloses a method for producing nano-aluminum powder, which uses a plasma transfer arc as a heating source to melt and evaporate aluminum, and uses cooling gas to control the condensation of aluminum vapor to obtain nano-powder. Although the gas-phase condensation path can achieve particle size refinement through a rapid cooling mechanism, the particle growth process is still in a disordered heat exchange environment of free particles, lacking methods for directional passivation or steric hindrance construction of high surface energy on the particle surface. The gas flow transport and gas-solid separation process of the newly formed highly active nanoparticles are susceptible to van der Waals forces. Force-dominated processes lead to severe soft agglomeration, forming a loose, porous, grape-like structure. The lack of an in-situ isolation mechanism results in structural defects, directly causing high porosity and tap density in the finished powder, far below the theoretical limit, and extremely poor flowability. This fails to meet the requirements of modern high-energy solid propellants or precision printing processes for high-solid-content powder packing and high-density stacking. The spatiotemporal mismatch between the chemical response velocity of the medium and the physical collision velocity of the flow field prevents passivation methods from constructing a physical isolation layer within the window before secondary agglomeration of droplets. Existing processes for preparing nano-aluminum powder exhibit satellite sphere defects and mechanical interlocking phenomena, resulting in an excessively large angle of repose and a tap density far below the theoretical limit, failing to meet the requirements of high-end manufacturing for high-density powder stacking and self-rheological properties.

[0004] Therefore, how to avoid the problem that the chemical adsorption rate lags behind the physical collision rate dynamics of the high-energy flow field, and how to achieve in-situ surface passivation synchronously with the breakup process within the microsecond timescale of metal droplet breakup, thereby avoiding satellite sphere defects and improving the powder tap density, has become the technical problem to be solved by this invention. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: an ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder, the method comprising the following steps:

[0006] To prepare a thermodynamically non-equilibrium homogeneous precursor fluid, 1.0% to 2.0% by mass of 12-hydroxystearic acid was added to hydrogenated mineral oil with a kinematic viscosity of 2.0 to 5.0 mm² / s at 40°C. The mixture was stirred at 80 to 95°C until completely dissolved to form a base oil phase. Liquid carbon dioxide with a mass fraction of 15% to 25% was injected into the base oil phase. The system was pressurized to 80 to 100 MPa and maintained at 60 to 80°C until 12-hydroxystearic acid and liquid carbon dioxide were completely miscible to form a single homogeneous transparent fluid.

[0007] A supercritical transport and transient decompression field is established. A single homogeneous transparent fluid is transported to the atomizing nozzle through a high-pressure pump. The single homogeneous transparent fluid undergoes adiabatic expansion the instant it exits the atomizing nozzle and impacts the molten aluminum flow. The volume expansion shear force generated by the vaporization of liquid carbon dioxide breaks the molten aluminum flow into aluminum droplets. The Joule-Thomson effect generated by the heat absorption of liquid carbon dioxide vaporization reduces the temperature of the jet core region to -20°C to -30°C within 50 microseconds.

[0008] Triggering flash swirl decomposition and in-situ network formation, the supersaturation driving force is constructed by utilizing the solubility difference of 12-hydroxystearic acid under pressure of 80-100 MPa and low temperature environment. This drives the 12-hydroxystearic acid to undergo swirl decomposition on the surface of aluminum droplets and self-assemble into a three-dimensional supramolecular fiber network. Since the formation rate of the three-dimensional supramolecular fiber network is faster than the mean free path flight time of aluminum droplets in the turbulent field, a physical hazard cage is constructed before the aluminum droplets undergo secondary collision to prevent the fusion between droplets.

[0009] Preferably, the 12-hydroxystearic acid molecule contains a hydroxyl functional group, which induces anisotropic growth of 12-hydroxystearic acid during precipitation through intermolecular hydrogen bonding in the nonpolar environment of hydrogenated mineral oil; the three-dimensional supramolecular fiber network is composed of multiple 12-hydroxystearic acid fibers with diameters of 50 nm to 200 nm through physical entanglement and hydrogen bonding, and the coverage of the three-dimensional supramolecular fiber network on the surface of the aluminum droplet is not less than 85%.

[0010] Preferably, the hydrogenated mineral oil is a mixture of alkanes with a carbon number distribution between 10 and 15, and the flash point of the hydrogenated mineral oil is not lower than 140 degrees Celsius; the single homogeneous transparent fluid is maintained in a turbulent state with a Reynolds number greater than 50,000 before entering the atomizing nozzle, to ensure the uniformity of the microscopic distribution of 12-hydroxystearic acid in the single homogeneous transparent fluid.

[0011] Preferably, the local cooling rate generated by the Joule-Thomson effect is controlled by the mass fraction of liquid carbon dioxide in a single homogeneous transparent fluid and the pressure difference before and after the atomizing nozzle. The local cooling rate satisfies the thermodynamic condition that the nucleation rate of 12-hydroxystearic acid is less than its crystal growth rate, thereby inducing the formation of fibrous crystals with an aspect ratio greater than 20:1.

[0012] Preferably, the supersaturation driving force is characterized by the dimensionless supersaturation ratio S, which is calculated according to the following formula: S = C total / C eq ≥10 3 Among them, C total C represents the initial solubility concentration of 12-hydroxystearic acid in a single homogeneous transparent fluid. eq The equilibrium solubility of 12-hydroxystearic acid at -20°C and normal pressure is given; the dimensionless supersaturation ratio S is set to ensure that the spindle decomposition process is in the diffusion-controlled kinetic range, so that the formation time of the three-dimensional supramolecular fiber network is less than the mean free path flight time of aluminum droplets in the turbulent field.

[0013] Preferably, the adiabatic expansion process is accompanied by a solvent micro-explosion effect, and the volume expansion ratio of liquid carbon dioxide exceeds 400 times during the transformation from the supercritical state to the gaseous state. The solvent micro-explosion effect generates secondary turbulent pulsations at the boundary layer of the aluminum droplet. The secondary turbulent pulsations destroy the oxide film precursor on the surface of the aluminum droplet and promote the coordination adsorption of the carboxyl terminus of 12-hydroxystearic acid with aluminum atoms on the surface of the aluminum droplet.

[0014] Preferably, the step of preparing the thermodynamically non-equilibrium homogeneous precursor fluid further includes: adding 0.1% to 0.3% by mass of a fluorocarbon surfactant to the base oil phase before adding liquid carbon dioxide. The fluorocarbon surfactant is used to reduce the interfacial tension between 12-hydroxystearic acid and hydrogenated mineral oil, thereby reducing the defect density of the three-dimensional supramolecular fiber network during its formation.

[0015] Preferably, the atomizing nozzle adopts a tightly coupled annular slit structure, with the molten aluminum flow located at the center, and a single homogeneous transparent fluid ejected through the annular slit; the mass flow rate ratio of the single homogeneous transparent fluid to the molten aluminum flow is controlled to be 1:1 to 2:1, ensuring that the cooling capacity of the jet core region is sufficient to offset the latent heat released by the molten aluminum and maintain the subcooling required for the decomposition of the spindle.

[0016] Preferably, the method further includes: collecting the atomized powder in a collection tank protected by an inert gas, wherein the hydrogenated mineral oil on the surface of the powder volatilizes during subsequent heat treatment, and the three-dimensional supramolecular fiber network is carbonized in situ to form a carbonaceous coating layer with a thickness of 2 to 5 nanometers.

[0017] Preferably, the tap density of the nano-aluminum powder prepared by this method reaches 55% to 65% of its theoretical density, and the angle of repose of the nano-aluminum powder is less than 30 degrees; the nano-aluminum powder particles exhibit a point contact stacking mode due to the presence of a three-dimensional supramolecular fiber network, and this stacking mode exhibits shear-thinning rheological properties when subjected to shear force.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. This invention utilizes the volume expansion of thermosensitive phase change components and the solubility sensitivity of flash crystallization components to construct a synchronous passivation mechanism that is strictly matched with the time scale of metal droplet breakage. During the microsecond window period when molten aluminum is broken by pneumatic shearing, the thermosensitive phase change components rapidly vaporize and absorb heat, causing the ambient temperature in the jet core area to drop instantaneously. This drives the flash crystallization components dissolved in the carrier oil to precipitate explosively due to the cliff-like drop in solubility. The phase change thermodynamics drives the precipitation rate to be orders of magnitude faster than the velocity of secondary collisions caused by Brownian motion or turbulence in the droplets. This ensures that before the surface of the nascent aluminum droplets captures satellite spheres or undergoes irregular fusion, the droplets self-assemble in situ to form a rigid three-dimensional nanofiber network, physically blocking the adhesion path between droplets. This forces the droplets to shrink into regular spheres under the action of surface tension, thus eliminating irregular shapes and satellite sphere defects in the prepared metal powder and achieving random dense packing close to the theoretical limit.

[0020] 2. By constructing a three-dimensional steric structure in situ on the particle surface, the macroscopic rheological behavior of nano-metal powder is reshaped at the microscopic physical interaction level. Unlike the two-dimensional molecular adsorption film formed by traditional surface modifiers, this scheme generates a fibrous network structure in a rapid cooling environment of the flash crystallization component, providing a solid and thick elastic isolation space between particles. The isolation structure shields the strong van der Waals forces on the surface of high specific surface area nano-metal particles, changing the particle packing contact interface from rigid contact of high friction coefficient metal oxides to elastic point contact of low friction coefficient organic matter. The contact mode is fundamentally changed, eliminating the mechanical interlocking and bridging tendency of powder static packing or dynamic transportation process, so that it exhibits fluid-like natural flowability without the addition of external flow aids, meeting the process requirements of powder bed density and uniformity for high-precision additive manufacturing powder laying or high solid content propellant filling.

[0021] 3. This invention establishes a steady-state control mechanism based on the thermodynamic properties of the medium, reducing the sensitivity of the ultra-high pressure atomization process to machining accuracy and equipment operating conditions. The spheroidization and anti-agglomeration effects of metal droplets do not solely depend on the geometric accuracy of the nozzle channel or the mechanical stability of the high-pressure pump set, but are controlled by the thermodynamic matching relationship between the expansion endothermic coefficient of the phase change component and the solubility curve of the crystalline component in the composite fluid formulation. The physicochemical constants of the phase change enthalpy and solubility curve of the substance are constant and do not drift with the equipment operating time, enabling the process to resist external interference caused by nozzle wear, pressure pulsation or slight environmental changes, and ensuring that the particle size distribution, morphological characteristics and surface condition of different batches of metal powder remain highly consistent under long-cycle, continuous industrial production conditions. Attached Figure Description

[0022] Figure 1 This is a flow chart of the ultra-high pressure atomization and in-situ network formation process based on thermodynamic non-equilibrium media of the present invention. Figure 2 This is a curve showing the bidirectional regulation of the amount of 12-hydroxystearic acid added on the density and flowability of nano-aluminum powder in this invention. Figure 3 This is a schematic diagram illustrating the formation mechanism of the tightly coupled annular nozzle structure and the physical potential barrier cage on the surface of the aluminum droplet according to the present invention. Detailed Implementation

[0023] 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. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] This invention provides an ultra-high pressure atomization method to improve the tap density and flowability of nano-aluminum powder, comprising four core process stages: preparation and homogenization of the composite atomization medium, high-pressure transport under supercritical conditions, micro-explosion fragmentation and cryogenic activation induced by transient decompression, and construction of an in-situ self-assembled three-dimensional steric network. The thermodynamic preparation and homogenization of the composite atomization medium aims to obtain a fluid system that is homogeneous under ultra-high pressure but undergoes phase separation under normal pressure. Hydrogenated mineral oil is selected as the base carrier phase, and its kinematic viscosity at 40°C is limited to 2.0 mm². 2 / s to 5.0mm 2 / s, this viscosity range ensures sufficient momentum transfer efficiency to maintain the flatness of the jet core region, while guaranteeing the diffusion and dissolution rate of liquid carbon dioxide. The hydrogenated mineral oil is dehydrated to a water content below 10 ppm before use. 12-hydroxystearic acid is selected as the flash crystallization component. In a mixing tank equipped with temperature control and mechanical stirring, the hydrogenated mineral oil is heated to 85°C to 95°C. 12-HSA solid powder is added at a ratio of 1.0% to 2.0% of the base carrier oil mass. The stirrer is started and stirred at 300 rpm to 500 rpm for 20 to 30 minutes until the solution is completely transparent and homogeneous. After the initial solution is prepared, a high-pressure metering pump is used to deliver the solution to the inlet of the static mixer. At the same time, liquid carbon dioxide is injected as a thermosensitive phase change component through another high-pressure pump. The injection ratio of liquid carbon dioxide is controlled at 15% to 25% of the total mass of the final composite fluid. The mixing system is then pressurized to a pressure range of 80 MPa to 100 MPa and the temperature is maintained at 60°C to 80°C. Under these pressure and temperature conditions, carbon dioxide is in a supercritical fluid state and forms a single homogeneous transparent fluid with mineral oil and 12-hydroxystearic acid. The pressure setpoint must be higher than the critical miscibility pressure of the ternary system of carbon dioxide, mineral oil and 12-hydroxystearic acid to prevent the system from stratifying.

[0025] Supercritical transport and injection are performed, with a heating system installed in the transport pipeline to maintain the fluid temperature between 60°C and 70°C and control pressure fluctuations within ±0.5MPa. Before the fluid enters the tightly coupled annular slit atomizing nozzle, the Reynolds number is maintained at a turbulent state greater than 50,000. The composite fluid is ejected through the annular slit and impacts the central molten aluminum flow. At the instant of ejection from the nozzle, the ambient pressure drops to atmospheric pressure within less than 10μs. Carbon dioxide molecules dissolved in the oil phase undergo explosive vaporization, with a volume expansion ratio exceeding 400 times. This micro-explosion effect generates high-frequency pressure pulses. Dynamic and shear forces tear the molten aluminum flow into droplets of micron and nanometer scale. Accompanying the endothermic vaporization process, the Joule-Thomson effect causes the local temperature in the jet core region to drop to -20°C to -30°C within 50 μs. Under these cryogenic and supersaturated conditions, dissolved 12-hydroxystearic acid undergoes flash cyclotron decomposition and in-situ three-dimensional network formation. Because 12-hydroxystearic acid has high solubility in supercritical oil systems at 80 MPa and 80°C, but extremely low solubility at atmospheric pressure and -20°C, a sudden temperature and pressure change exceeding 10°C occurs at the nozzle outlet. 3With a dimensionless supersaturation ratio, the precipitated 12-hydroxystearic acid molecules are driven by hydrogen bonds to grow anisotropically along a one-dimensional direction, self-assembling to form fibrous crystals with diameters ranging from 50 nm to 200 nm. The fibrous crystals are physically entangled with each other, constructing a three-dimensional supramolecular fiber network around the aluminum droplet. The formation time of this network is less than 100 μs, which is faster than the mean free path flight time of the aluminum droplet during secondary collisions in the turbulent field, thus forming a physical steric cage before the droplets come into contact with each other.

[0026] Finally, the powder is collected. The atomized powder is carried by the airflow into a cyclone separator protected by inert gas for collection. The resulting nano-aluminum powder is coated with a composite layer consisting of a 12-hydroxystearic acid fiber network and residual mineral oil, which shields the van der Waals forces between nanoparticles, transforming the rigid contact between particles into elastic point contact of the organic fiber layer. The tap density of the resulting nano-aluminum powder reaches 55% to 65% of its theoretical density, and the angle of repose is less than 30°. ∘ It exhibits fluid-like natural flow characteristics without the addition of external flow aids. Regarding the determination of key process parameters, the preferred addition amount of 12-hydroxystearic acid is 1.2% to 1.5%. Too low an addition amount will result in a sparse fiber network and insufficient coverage; too high an addition amount will increase the risk of nozzle clogging due to gelation of the medium at low temperatures. The preferred proportion of liquid carbon dioxide is 20% to 22%. Too low a proportion will result in insufficient expansion work and insufficient cooling rate; too high a proportion will make it difficult for the system to maintain homogeneity at 80 MPa. The process involves dimensionless supersaturation ratio and engineering definition of the working concentration of 12-hydroxystearic acid. Based on the high-pressure rheology offline phase boundary scanning procedure, a precursor sample sequence of 12-hydroxystearic acid mass fraction was prepared in a pressure environment of 80 MPa to 100 MPa with a gradient increase of 0.1%. The transient cooling rate of the nozzle outlet was simulated by a high-pressure capillary rheometer for temperature and concentration variation shear test, and the complex viscosity of the system was subjected to two orders of magnitude or more exponential jumps in temperature and concentration within 100 microseconds. The degree coordinate point is defined as the physical critical threshold of the spindle decomposition. The actual solute working concentration in production is locked at 90% to 95% of the critical threshold. The procedure directly links the measurable rheological mutation characteristics with the thermodynamic driving force index, eliminating the possible deviations that may arise from relying on theoretical solubility model calculations. The thermodynamic steady state establishment of the atomization flow field follows the step-by-step dynamic optimization logic of pressure and flow rate coupling. The system prioritizes pumping pure hydrogenated mineral oil to build the baseline annular pressure drop. The liquid carbon dioxide injection rate is controlled to increase in steps of 0.5% of the total mass flow rate. The high-frequency dynamic pressure sensor is used to collect the fluid pulsation spectrum at the nozzle inlet in real time until the pressure fluctuation characteristics change from disordered broadband noise to a quasi-periodic oscillation signal with a single characteristic frequency. At the same time, an infrared thermal imager is used to capture the stable temperature field distribution in the jet breakup core area in the target range of -20 degrees Celsius to -30 degrees Celsius. The current combination of carbon dioxide injection ratio and precursor pressure is determined to be the process locking parameters that trigger the solvent micro-explosion effect and the cryogenic network formation mechanism working together.

[0027] Example 1: In high-solids-content loading applications for high-energy solid propellants, there is a technical challenge of agglomeration of nano-metal fuels under high loads, leading to loading voids and unstable combustion. It requires that nano-aluminum powder maintain uniform dispersion and dense packing in high-viscosity slurries with a volume fraction exceeding 50%. However, traditional nano-aluminum powder, due to its high surface energy, easily forms wet, sandy clumps during mixing, hindering the increase in loading density. This invention employs an ultra-high-pressure atomization method based on a thermodynamically non-equilibrium composite medium. By configuring a composite fluid containing 1.5% by mass of 12-hydroxystearic acid and 22% by mass of liquid carbon dioxide, homogenization is achieved at a pressure of 85 MPa. When this composite fluid is ejected through a nozzle and impacts the molten aluminum flow, the instantaneous... The vaporization and expansion of the state tore the aluminum liquid flow into micron and nano-droplets within 10 μs. The resulting Joule-Thomson effect caused the local temperature to plummet to -30°C. In this cryogenic environment, the dissolved 12-hydroxystearic acid underwent flash cyclohexane decomposition. Taking advantage of the precipitous drop in solubility at normal pressure and low temperature, it self-assembled in situ on the surface of the aluminum droplets to form a three-dimensional supramolecular fiber network. This constructed a physical spectral barrier before the aluminum droplets underwent secondary collisions, effectively preventing the fusion between droplets. The test showed that the tap density of the obtained nano-aluminum powder reached 62% of the theoretical density, and the angle of repose was 26°. During the preparation of the propellant slurry, the powder exhibited macroscopic fluidity similar to that of a fluid. Even under high shear mixing conditions, no agglomeration occurred, achieving an increase in the solid content of the slurry and uniformity of the filling density.

[0028] Example 2: This example aims to conduct a rigorous comparative experiment on an ultra-high pressure atomization test platform with a rated pressure of 150 MPa. This platform is equipped with a PID temperature-controlled melting crucible, a high-pressure plunger pump assembly, a precision mass flow meter, and a dynamic light scattering particle size analyzer. To simulate the uncertainties of a real industrial environment, the aluminum melt melting temperature was set to 850 ± 10℃. Background airflow disturbances with a flow rate fluctuation of ± 2 m / s were artificially introduced into the atomization chamber to test the anti-interference capability of the network formation mechanism. The base carrier oil used in the experiment was hydrogenated mineral oil, with a measured kinematic viscosity of 4.2 mmHg at 40℃. 2 / s, the flash crystallization component uses 98% pure 12-hydroxystearic acid, and the thermosensitive phase change component uses industrial-grade liquid carbon dioxide; to construct a legally valid chain of comparative evidence, the experimental design includes the following three dimensions: the prior art control group (hereinafter referred to as the control group), which uses the traditional nitrogen atomization process, does not add 12-hydroxystearic acid, and relies solely on gas kinetic energy for breakage; the core sample group of this invention (hereinafter referred to as the test group) adopts the complete technical solution of this invention, with the amount of 12-hydroxystearic acid added set at 1.5% and the proportion of liquid carbon dioxide at 22%; and finally, the parameter boundary verification group, used to verify the critical effect of the parameter range in the claims, wherein the amount of 12-hydroxystearic acid added in group B1 is 0.5% ( (Below the lower limit), the 12-hydroxystearic acid addition of group B2 was 2.5% (above the upper limit); after the experiment started, the microscopic morphology and macroscopic physical properties of the powders in each group were recorded. In the control group, due to the lack of in-situ steric hindrance mechanism, a large number of random collisions occurred during the flight of the nascent droplets. Scanning electron microscopy (SEM) observation showed that more than 40% of the particles were dumbbell-shaped or attached with tiny satellite spheres, resulting in severe mechanical interlocking between the powders. In the experimental group, SEM images showed that the particles had extremely high sphericity and were coated with a uniform organic fiber network. The presence of this fiber network not only shielded the direct contact of the metal surface, but also reduced the friction coefficient between the particles. The following is a comparison of the measured data of the key performance indicators of each group, see Table 1.

[0029] Table 1: Comparison of Powder Properties in Each Experimental Group

[0030] Comparing the data from the control and experimental groups, the tap density of the experimental group increased by approximately 50% (from 42.5% to 63.8%), and the angle of repose decreased by nearly half. This confirms the precise temporal and spatial synergy between the ultrafine droplets provided by solvent microbursts and the physical isolation provided by in-situ networking, successfully blocking the droplet fusion path in the high-energy turbulent field. Regarding the rationality of the parameter boundaries, when the amount of 12-hydroxystearic acid added in group B1 was only 0.5%, the tap density, although improved, still did not reach the ideal level (48.6%), and the satellite sphere defect rate was relatively high (28.5%). This indicates that too low a concentration cannot form a dense fiber cage with a coverage of more than 85%, and cannot effectively shield the collisions between droplets. When the addition amount in group B2 is increased to 2.5%, although the defect rate is extremely low (1.8%), the nozzle pressure is observed to pulsate violently during the test, and the powder yield decreases. This is attributed to the sharp increase in viscosity of the supersaturated solution at low temperature, which causes the fluid to gel prematurely inside the nozzle, hindering the continuous atomization process. This proves that the addition amount range of 1.0% to 2.0% is the optimal working window for balancing the network density and process stability.

[0031] Example 3: This example combines Figures 1 to 3This paper describes an ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder, as follows: Figure 1 As shown, the process involves introducing 12-hydroxystearic acid, hydrogenated mineral oil, and liquid carbon dioxide at the raw material input end. During the formulation stage, a single, homogeneous, transparent, thermodynamically non-equilibrium homogeneous precursor fluid is constructed under pressures of 80 MPa to 100 MPa and temperatures of 60°C to 80°C. This fluid enters a tightly coupled annular nozzle atomization unit, where it undergoes instantaneous adiabatic expansion at the nozzle outlet and jets into molten aluminum at 850°C. During this process, the system triggers multi-physics coupling effects, specifically including a solvent micro-explosion effect with a volume expansion ratio exceeding 400 times and a Joule-Thomson effect that cools the environment to -30°C within 50 μs, with a dimensionless supersaturation ratio S ≥ 10. 3 As a key control point, this process drives the flash spinneret decomposition and in-situ networking mechanism to generate a three-dimensional supramolecular fiber network, namely a physical steric cage, which aims to prevent droplet fusion and eliminate satellite sphere defects, ultimately obtaining high-performance nano-aluminum powder with high tap density of 63.8%, fluid-like flowability and point contact stacking characteristics.

[0032] like Figure 2 As shown, this chart is built on a two-dimensional coordinate system with the 12-HSA addition amount as the x-axis and the numerical value as the y-axis. The x-axis value ranges from 0.5% to 2.5%. The solid line in the chart represents the tap density in percentage of theoretical density, and the value shows a trend of first rising and then falling with the increase of the addition amount, reaching a peak at the addition amount of 1.5%. The dashed line in the chart represents the angle of repose in degrees, and its value shows a trend of first falling and then rising with the increase of the addition amount, reaching a trough at the addition amount of 1.5%. The overlap of the extreme points of the two curves on the x-axis objectively reveals the synergistic optimization window of this process parameter on the powder flowability and density; Figure 3 As shown in the diagram, the top of the diagram displays a nozzle structure containing a composite fluid inlet, an aluminum liquid inlet, and a composite fluid outlet. The composite fluid, consisting of 12-hydroxystearic acid, liquid CO2, and mineral oil, is ejected from the nozzle outlet and converges with the molten aluminum liquid in the center at the impact and breakup zone below. The lower half of the diagram details the process of droplet breakup and fiber network formation, showing how the nascent aluminum droplets evolve into aluminum particle cores through breakup. Within less than 50 μs, they undergo flash cyclotron decomposition, and finally, a three-dimensional fiber network, or physical steric cage, is formed on the surface of the aluminum particle core through in-situ self-assembly, thereby completing the surface structure reshaping of the nano-metal particles.

[0033] Example 4: Before starting large-scale continuous industrial production, to ensure the robustness of the process under fluctuations in raw material batches and to accurately determine the optimal addition ratio of 12-hydroxystearic acid, this example implements a standardized rheological and morphological orthogonal calibration procedure. This aims to resolve the engineering contradiction between the risk of nozzle clogging caused by low-temperature gelation of the medium and the risk of agglomeration caused by insufficient steric hindrance network coverage. This determines the process window that ensures both jet stability and efficient spheroidization. A low-temperature rheological property test sequence based on Joule-Thomson effect simulation is established. A set of base carrier oil samples is prepared, with the mass fraction of 12-hydroxystearic acid set in a gradient sequence from 0.8% to 2.2% with a step size of 0.2%. Using a high-pressure rheometer equipped with a temperature control unit, each set of samples is pressurized to 85 MPa and homogenized to simulate the temperature and pressure abrupt change at the atomizing nozzle outlet. Simultaneously with pressure release, the test temperature is instantly reduced to -30°C and maintained constant. The results are recorded for each set of samples at a shear rate of 1000 s⁻¹. -1 The curves showing the evolution of apparent viscosity over time under certain conditions reveal that when the addition amount is below 1.2%, the system viscosity remains relatively stable under cryogenic conditions, exhibiting Newtonian fluid characteristics, indicating that a long-range ordered structure has not been formed. When the addition amount reaches 1.4% or higher, the apparent viscosity of the system exhibits an exponential jump within a timescale of 10 μs to 100 μs. This rheological mutation objectively confirms that the solute molecules undergo explosive spinolysis and self-assembly to form a three-dimensional supramolecular fiber network with shear resistance under the drive of this supercooling. Therefore, the concentration of 1.4% corresponding to the viscosity mutation is defined as the thermodynamic critical lower limit for the formation of an effective physical steric cage.

[0034] To verify the correlation between atomization integrity and product defect rate, actual ultra-high pressure atomization experiments were conducted using the same gradient media sequence. A high-speed camera was used to monitor the Rayleigh breakup length and mist cone stability of the jet exiting the nozzle. Satellite ball defect rates were statistically analyzed on the collected aluminum powder samples. The results showed that the satellite ball defect rate decreased with increasing 12-hydroxystearic acid concentration, dropping below 3.0% at a concentration of 1.4%, which highly coincides with the network formation critical point in rheological testing. When the concentration further increased to above 2.0%, the jet core... Intermittent pressure pulsations and impaired filament splitting began to appear in the core region, indicating that excessively high solid network density hindered the normal breakup and clamping of droplets. Considering both rheological safety and morphological quality boundaries, the optimal addition amount of 12-hydroxystearic acid was determined to be 1.4% to 1.6% for this specific batch of raw materials and under cryogenic conditions of -30°C. This calibration procedure transforms empirical parameter selection into a deterministic decision based on physicochemical measurement data, ensuring that each batch of product achieves a precise thermodynamic balance between network protection and fluid transport.

[0035] Example 5: When applying the ultra-high pressure atomization method of the present invention to a new production line or when changing raw material batches, in order to ensure the precise matching of core process parameters with specific equipment conditions and raw material characteristics, a standardized on-site deployment pre-calibration and debugging procedure needs to be implemented. This procedure aims to eliminate uncertainties introduced by equipment manufacturing tolerances, raw material property fluctuations, and environmental differences. The solubility curves of the base carrier oil and flash crystallization components are calibrated on-site. Hydrogenated mineral oil and 12-hydroxystearic acid raw materials actually used on-site are used to simulate the preparation process of the atomization precursor in a laboratory autoclave. A series of temperature gradients (e.g., 60℃ to 90℃, step size 5℃) and pressure gradients (e.g., 80MPa to 100MPa, step size 5MPa) are set. The critical miscibility temperature and pressure boundary under different ratios are measured using a cloud point analyzer. Based on the calibration results, the temperature control and pressure control parameters on the production line are corrected to ensure that the precursor is always in the homogeneous stable region during the transportation process, with a safety margin of at least 5℃ and 2MPa from the phase separation boundary.

[0036] The cold-state flow field characteristics of the atomizing nozzle were verified and the hot-state test run was fine-tuned. Cold-state injection was performed using pure carrier oil. The velocity distribution and mist cone angle of the nozzle outlet flow field were measured using a laser Doppler velocimeter (LDV) or high-speed photography technology to verify whether the nozzle machining accuracy and installation coaxiality met the design requirements. Based on this, liquid carbon dioxide was introduced for hot-state test run. Starting from the lower limit of the design ratio (15%), the carbon dioxide injection rate was gradually increased. The pressure drop changes and jet morphology before and after the nozzle were monitored. When a stable supersonic expansion shock wave structure was observed in the jet core area and the particle size distribution D50 of the collected primary powder first entered the submicron range, the carbon dioxide mass flow rate ratio and system pressure value at this time were recorded and established as the benchmark operating parameters of the production line under the current operating conditions.

[0037] Example 6: Regarding the core process parameters involved in the technical solution of this invention, especially the mass fraction setting of the thermosensitive phase change component in the composite medium, in order to ensure that the optimal atomization and network formation effect can be stably obtained under different engineering environments, this example establishes an online parameter adaptive calibration procedure based on the coupled response of fluid dynamics and thermodynamics. This procedure aims to solve the process deviation caused by factors such as fluctuations in production environment temperature and initial state of raw materials. The optimal operating parameters are dynamically locked through a real-time feedback mechanism, and the initial setting of mass fraction and disturbance test logic are established. Based on the mass fraction range of 15% to 25%, combined with the current ambient temperature and the measured viscosity of the base carrier oil, an intermediate value is set as the initial mass fraction. After the atomization system is started and enters steady-state operation, a micro-disturbance test is performed: the mass fraction of the thermosensitive phase change component is adjusted in both upward and downward directions with a step size of 0.5%. After each adjustment, the Sotter mean diameter of the atomized droplets is monitored in real time using an online laser particle size analyzer.

[0038] Secondly, a multi-objective optimization parameter locking criterion is constructed. The real-time collected SMD data and the minimum temperature data of the jet core area are input to the central control unit. The control unit performs real-time calculations based on the preset evaluation function. This function aims to find a balance point that minimizes the SMD value (representing the highest crushing efficiency) while the temperature of the jet core area can be stably maintained in the target range of -20℃ to -30℃ (representing the best cryogenic effect). When the fluctuation of the evaluation function value within three consecutive sampling cycles is less than 2%, it is determined that the system has found the optimal operating point under the current working conditions. Then, the mass fraction of the thermosensitive phase change component at this time is locked as the set value for formal production. If the ambient temperature or raw material characteristics drift, the system will automatically trigger a new round of calibration cycle to ensure that the process is always in the optimal state, transforming empirical parameter settings into deterministic control logic based on real-time data feedback.

[0039] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for improving the tap density and flowability of nano-aluminum powder using ultra-high pressure atomization, characterized in that, The method includes the following steps: To prepare a thermodynamically non-equilibrium homogeneous precursor fluid, 1.0% to 2.0% by mass of 12-hydroxystearic acid was added to hydrogenated mineral oil with a kinematic viscosity of 2.0 to 5.0 mm² / s at 40°C. The mixture was stirred at 80 to 95°C until completely dissolved to form a base oil phase. Liquid carbon dioxide with a mass fraction of 15% to 25% was injected into the base oil phase. The system was pressurized to 80 to 100 MPa and maintained at 60 to 80°C until 12-hydroxystearic acid and liquid carbon dioxide were completely miscible to form a single homogeneous transparent fluid. A supercritical transport and transient decompression field is established. A single homogeneous transparent fluid is transported to the atomizing nozzle through a high-pressure pump. The single homogeneous transparent fluid undergoes adiabatic expansion the instant it exits the atomizing nozzle and impacts the molten aluminum flow. The volume expansion shear force generated by the vaporization of liquid carbon dioxide breaks the molten aluminum flow into aluminum droplets, causing the temperature in the jet core area to drop to -20°C to -30°C within 50 microseconds. Triggering flash spinolysis and in-situ networking, 12-hydroxystearic acid undergoes spinolysis on the surface of aluminum droplets and self-assembles to form a three-dimensional supramolecular fiber network. The formation rate of the three-dimensional supramolecular fiber network is faster than the mean free path flight time of the aluminum droplets in the turbulent field, thus constructing a physical hazard cage to prevent the fusion between droplets before the aluminum droplets undergo secondary collisions.

2. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, 12-hydroxystearic acid molecules contain hydroxyl functional groups. In the nonpolar environment of hydrogenated mineral oil, the hydroxyl functional groups induce anisotropic growth of 12-hydroxystearic acid during precipitation through intermolecular hydrogen bonding. The three-dimensional supramolecular fiber network is composed of multiple 12-hydroxystearic acid fibers with diameters of 50 nm to 200 nm that are physically entangled and cross-linked by hydrogen bonds. The coverage of the three-dimensional supramolecular fiber network on the surface of aluminum droplets is not less than 85%.

3. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, Hydrogenated mineral oil is a mixture of alkanes with a carbon number distribution between 10 and 15, and the flash point of the hydrogenated mineral oil is not lower than 140 degrees Celsius; the single homogeneous transparent fluid is kept in a turbulent state with a Reynolds number greater than 50,000 before entering the atomizing nozzle to ensure the uniformity of the microscopic distribution of 12-hydroxystearic acid in the single homogeneous transparent fluid.

4. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The local cooling rate generated by the Joule-Thomson effect is controlled by the mass fraction of liquid carbon dioxide in a single homogeneous transparent fluid and the pressure difference before and after the atomizing nozzle. The local cooling rate satisfies the thermodynamic condition that the nucleation rate of 12-hydroxystearic acid is less than its crystal growth rate, forming fibrous crystals with an aspect ratio greater than 20:

1.

5. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The driving force of supersaturation is characterized by the dimensionless supersaturation ratio S, which is calculated according to the following formula: S = C total / C eq ≥10 3 in, C total C represents the initial solubility concentration of 12-hydroxystearic acid in a single homogeneous transparent fluid. eq The equilibrium solubility of 12-hydroxystearic acid at -20°C and normal pressure.

6. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The adiabatic expansion process is accompanied by a solvent micro-explosion effect, and the volume expansion ratio of liquid carbon dioxide exceeds 400 times during the process of changing from the supercritical state to the gaseous state. The solvent micro-explosion effect generates secondary turbulent pulsations in the boundary layer of aluminum droplets, and the secondary turbulent pulsations destroy the oxide film precursor on the surface of aluminum droplets.

7. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The steps for preparing the thermodynamically non-equilibrium homogeneous precursor fluid also include: adding 0.1% to 0.3% by mass of a fluorocarbon surfactant to the base oil phase before adding liquid carbon dioxide. The fluorocarbon surfactant is used to reduce the interfacial tension between 12-hydroxystearic acid and hydrogenated mineral oil, thereby reducing the defect density of the three-dimensional supramolecular fiber network during its formation.

8. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The atomizing nozzle adopts a tightly coupled annular slit structure, with the molten aluminum flow located in the center and a single homogeneous transparent fluid ejected through the annular slit; the mass flow rate ratio of the single homogeneous transparent fluid to the molten aluminum flow is controlled to be 1:1 to 2:

1.

9. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The method also includes: collecting the atomized powder in a collection tank protected by an inert gas, the hydrogenated mineral oil on the surface of the powder volatilizing during subsequent heat treatment, and the three-dimensional supramolecular fiber network being carbonized in situ to form a carbonaceous coating layer with a thickness of 2 to 5 nanometers.

10. The ultra-high pressure atomization method for improving the tap density and flowability of nano-aluminum powder according to claim 1, characterized in that, The tap density of the nano-aluminum powder prepared by this method reaches 55% to 65% of its theoretical density, and the angle of repose of the nano-aluminum powder is less than 30 degrees.

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