Preparation method of nano aluminum oxide aerogel powder with high specific surface area, high dispersion and low agglomeration

Abstract

The invention relates to the field of preparation of inorganic aerogel materials, and particularly discloses a preparation method of nano aluminum oxide aerogel powder with high specific surface area, high dispersion and low agglomeration. The method comprises the following steps: by taking water-phase fiber / feather-shaped alumina sol as a matrix, introducing styrene-based colloidal particles, obtaining composite powder by combining the steps of spray instantaneous freezing and vacuum freeze-drying, and performing segmented degreasing in an oxidizing atmosphere to remove the colloidal particles, thereby obtaining the composite powder. The prepared nano aluminum oxide aerogel powder is fluffy, has good pore connectivity, can be redispersed and has a high specific surface area. The method does not introduce an organic replacement solvent which is mutually soluble with water, and is simple in process flow and suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of inorganic aerogel material preparation technology, specifically to a method for preparing high specific surface area, highly dispersed, and low-agglomeration nano-alumina aerogel powder. The powder is obtained using fibrous and / or feather-like alumina precursors as basic raw materials, through a process route of aqueous phase dispersion-spray instantaneous freezing-vacuum freeze-drying-segmented degreasing under an oxidizing atmosphere. Background Technology

[0002] Aerogel powders, due to their low density and porous structure, are widely used in applications such as thermal insulation filling, composite material reinforcement, and functional carriers. In current market and literature reports, aerogel powders with a "pearl neck (bead-neck-bead) structure" based on nanoparticles are relatively common. These powders have dense network nodes and small connecting necks, offering certain structural advantages in suppressing gas thermal conductivity. However, they also exhibit the following shortcomings in engineering applications:

[0003] Agglomeration and uneven clumping: The granular network is prone to secondary adhesion during drying and packaging, forming agglomerates that are difficult to disperse, affecting metering, mixing and uniform spreading;

[0004] Insufficient bulkiness and poor flowability: interparticle static electricity / van der Waals effect and high surface energy lead to high loose density and easy bridging and locking during feeding.

[0005] Specific surface area decay and unstable pore structure: During normal pressure drying or heat treatment, the particle-particle junction neck is prone to coarsening, which leads to shrinkage or disappearance of the pore structure, making it difficult to achieve both high specific surface area and stable structure.

[0006] In contrast, aerogel powders with fibrous or feather-like skeletons as the main structural components have morphological characteristics such as long bridging, fewer nodes, and easy load-bearing. They can form a more porous, compressible, and anti-bridging skeleton at the same solid content, which is beneficial for improving the dispersibility and spreadability of the powder and reducing the risk of secondary agglomeration during use. However, existing fibrous / feather-like aerogel powder preparation processes still generally have the following problems:

[0007] The shortcomings of atmospheric pressure drying (APD) process: the capillary stress generated at the liquid-gas interface acts on the neck and thin bridge, which easily leads to network shrinkage, caking and "skin effect", causing problems such as severe agglomeration, insufficient fluffiness and decrease in specific surface area;

[0008] The shortcomings of supercritical drying (SCD) process: Although it can avoid capillary stress, it requires multiple solvent replacement steps and high-pressure operation, which has high cost and safety requirements; for aqueous fiber / feather systems, component migration and local degumming are easy to occur during the replacement process, which is not conducive to stable large-scale powder production; the supercritical fluid state can easily carry powder into the circulation system, leading to structural failures, etc.

[0009] The drawbacks of simple freeze-drying process: If only conventional freeze-drying process is used without coordinated control of freezing morphology and post-processing, the convergence and extrusion of ice crystal fronts will cause close-distance bridging and sheet stacking of fibers / sheet bundles, forming agglomerates or shredded felt after drying, and it is still difficult to obtain a powder morphology with high dispersion, low agglomeration, looseness and redispersibility.

[0010] In summary, existing technologies urgently need a preparation scheme that can suppress bridging and agglomeration during the freezing stage, reduce the tendency of secondary coagulation after drying, and is suitable for industrial-scale production, while maintaining the advantages of freeze-drying processes such as no displacement and low capillary stress. Summary of the Invention

[0011] To address the technical problems encountered in existing processes, this invention aims to provide a method for preparing nano-alumina aerogel powder with high specific surface area, high dispersion, and low agglomeration, primarily targeting fibrous and / or feather-like alumina framework systems. While retaining the advantages of freeze-drying, such as no displacement required and avoidance of capillary stress at the liquid-gas interface, this method addresses the issues of severe agglomeration, insufficient bulkiness, and easy reduction in specific surface area of ​​existing "pearl-neck" granular powders, as well as the tendency for shrinkage and caking during atmospheric pressure drying, the complexity and high cost of supercritical drying displacement, and the tendency for simple freeze-drying to result in bridging due to ice crystal compression during the freezing stage. This invention proposes a powder-forming path that reduces close-range fiber contact during the freezing stage and gently loosens agglomeration points during post-processing, thereby obtaining a powder morphology suitable for large-scale production and high dispersion quality.

[0012] The mechanism of this invention is as follows: Styrene-based colloidal nanospheres are introduced into the aqueous phase, dispersing them between fibrous and / or feather-like alumina precursors to form uniform, removable physical spacers, which work synergistically with the subsequent spray-freezing-vacuum freeze-oxidative segmented degreasing sequence. Without the addition of colloids, during freezing, the advancing ice crystal fronts drive the liquid phase and push the fibers / sheet bundles towards the frozen surface, easily causing trapping and bridging, resulting in oriented stacking or localized caking. After sublimation, blocky agglomeration and necking loss occur, leading to a decrease in specific surface area and poor dispersibility. With the addition of colloids, the nanospheres provide stable spacing and disperse stress between fibers / sheet bundles. The independent droplets formed by spraying rapidly solidify in the low-temperature medium and stabilize their initial spatial distribution. Ice crystal growth is restricted to traveling between the spacers and the fiber skeleton rather than large-scale stacking, thus significantly reducing close-range contact points. The subsequent vacuum freeze-freezing... Freeze-drying removes the ice phase through sublimation, avoiding capillary stress caused by liquid-gas interface retreat. After freeze-drying, the colloidal particles elastically rebound to an approximately spherical shape under unloaded conditions, maintaining effective spacing and open channels between fibers / sheets in the dry state, preserving fine connections and promoting the formation of a high specific surface area. Finally, the organic colloids are removed by staged heating and holding in an oxidizing atmosphere. The decomposition gases generated during the removal process are discharged along the existing channels, creating a short-term loosening effect in the fiber contact area, further loosening residual micro-contact points without damaging the overall skeleton, thereby reducing secondary adhesion and agglomeration, and improving the high dispersibility and bulkiness of the powder. The entire process does not rely on water-miscible organic displacement solvents, is continuous and scale-up friendly, and the resulting nano-alumina aerogel powder simultaneously meets the key characteristics of high specific surface area, high dispersibility, and low agglomeration.

[0013] Specifically, the preparation method of the high specific surface area, highly dispersed, and low agglomeration nano-alumina aerogel powder includes the following steps:

[0014] a) Preparation of a dispersion containing an alumina precursor in an aqueous phase;

[0015] The alumina precursor is selected from fibrous alumina sol and / or feathery alumina sol, and a homogeneous system is obtained through mechanical dispersion and colloidal stabilization; the dispersion is an aqueous system that does not contain organic displacement solvents miscible with water, so as to avoid capillary shrinkage and displacement burden during subsequent drying.

[0016] b) Add styrene-based colloidal particle dispersion to the dispersion to obtain a composite dispersion;

[0017] The colloidal particles provide uniform physical spacing and maintain dispersion stability in the composite dispersion, thereby reducing close contact and bridging of fibers / sheet bundles during the freezing stage. The volume fraction of the colloidal particles in the composite dispersion can be 0.5% to 3.0%, and the median particle size can be 30 to 300 nm, so as to match the subsequent instantaneous freezing, freeze-drying and removal steps.

[0018] c) Spray the composite dispersion and freeze it instantaneously in a low-temperature medium to obtain frozen particles;

[0019] The process involves atomizing individual droplets and rapidly freezing them in liquid nitrogen or a low-temperature nitrogen gas or a mixture of nitrogen at a temperature not exceeding -50°C. This process is used to fix the dispersed structure and limit the pushing of ice crystals.

[0020] d) The frozen particles are subjected to vacuum freeze-drying to obtain a dry powder containing styrene-based colloidal particles;

[0021] Vacuum freeze drying removes the ice phase directly through sublimation, thus avoiding the capillary pull generated by the liquid-gas interface retreat, maintaining open channels and small connecting parts, and preventing the softening and migration of colloids at the end of the drying process.

[0022] e) Perform segmented degreasing by increasing the temperature according to a programmed temperature under an oxidizing atmosphere to remove the styrene-based colloidal particles and obtain inorganic fiber aerogel powder;

[0023] The process involves segmented heating and phased heat preservation to controllably remove colloidal particles from the surface inwards while maintaining the morphology of the skeleton. The final temperature of the heating stage is 250~480℃. The gas released during the removal process is discharged along the existing channels, which has a short-term loosening effect on the fiber contact area to further reduce agglomeration and improve bulkiness and redispersibility.

[0024] The present invention also provides a high specific surface area, highly dispersed, and low-agglomeration nano-alumina aerogel powder, which is prepared according to the above method.

[0025] Beneficial effects

[0026] This invention employs a continuous process of aqueous dispersion-spray instantaneous freezing-vacuum freeze-drying-oxidative segmented degreasing, introducing styrene-based colloidal nanospheres into the aqueous phase as a removable spacer component. Targeting fibrous and / or feather-like alumina framework systems, it synergistically solves the problems of ice crystal pushing, close-range bridging, and post-drying agglomeration that easily occur in freeze-drying powder production, while also considering process scale-up and equipment compatibility. Specifically, it is reflected in:

[0027] 1. High dispersion and low agglomeration: During the dispersion stage, colloidal nanospheres provide uniform physical spacing between fibers / sheet bundles. After the droplets are instantaneously frozen in the low-temperature medium, this spatial distribution is reliably maintained, and ice crystal growth travels more along the spacers and framework rather than pushing fibers to stack, significantly reducing contact points and bridging formed during the freezing period. In the subsequent staged degreasing in an oxidizing atmosphere, the gas released during colloid removal is discharged along the existing channels, producing a short-term loosening effect at local contact points, further weakening the tendency of agglomeration after drying, thereby obtaining a highly dispersed and low-agglomeration powder morphology.

[0028] 2. Reduced microstructure damage: Unlike atmospheric pressure drying, vacuum freeze drying removes the ice phase directly through sublimation, avoiding capillary pull and "skin densification" caused by the liquid-gas interface retreat, and preventing the collapse of small connecting parts and open channels; colloidal particles have a certain degree of resilience in the dry state, which can maintain the effective spacing between fibers / sheet bundles before degreasing, reduce damage to the microstructure during the drying stage, and is conducive to the formation and maintenance of a higher specific surface area.

[0029] 3. Improved powder fluffiness and flowability: Independent droplets formed by spraying are instantaneously frozen and freeze-dried to obtain dry particles with controllable particle size; the presence of spacer components inhibits the aggregation of flaky / lumpy particles, and the resulting powder is fluffy and granular, making it easier to achieve stable feeding, uniform mixing and paving, and improving the usability of the project.

[0030] 4. Scalability: The continuous phase throughout the process is water, which does not introduce organic displacement solvents that are miscible with water, reducing displacement and recovery steps and making it friendly to vacuum systems; the equipment combination of spray + low temperature nitrogen / liquid nitrogen freezing + conventional freeze drying equipment + air atmosphere staged degreasing has high versatility, clear process sequence and controllable window, which facilitates scale-up implementation from laboratory to pilot / mass production.

[0031] 5. Stable morphology and low residue: The degreasing method of segmented heating and staged heat preservation is conducive to removing organic colloids from the surface to the inside, reducing the risk of carbonization residue, while avoiding excessive coarsening of the fine structure of alumina by one high temperature, thus balancing the stability of the skeleton morphology and clean removal.

[0032] 6. Adaptable to different precursors: This scheme does not rely on a single morphological condition and is suitable for both fibrous alumina sol with a higher aspect ratio and easy entanglement, and feathery alumina sol with easy bridging. High specific surface area, high dispersion, and low agglomeration of nano alumina aerogel powder can be obtained through the same process sequence, which enhances the versatility and stability of the process.

[0033] In summary, without altering the advantages of freeze-drying—namely, "no displacement and low capillary stress"—this invention achieves full-process control of key structural factors (spacing, connection points, and pore connectivity) through the sequential coordination of the dispersion, freezing, drying, and removal stages. This simultaneously achieves the technical goals of high specific surface area, high dispersion, and low agglomeration, and possesses good engineering feasibility and scalability prospects. Detailed Implementation

[0034] The preparation method of the present invention includes steps a) to e) as follows:

[0035] a) Preparation of a dispersion containing an alumina precursor in an aqueous phase;

[0036] b) Add styrene-based colloidal particle dispersion to the dispersion to obtain a composite dispersion;

[0037] c) Spray the composite dispersion and freeze it instantaneously in a low-temperature medium to obtain frozen particles;

[0038] d) The frozen particles are subjected to vacuum freeze-drying to obtain a dry powder containing styrene-based colloidal particles;

[0039] e) Perform segmented degreasing by increasing the temperature in an oxidizing atmosphere to remove the styrene-based colloidal particles and obtain alumina aerogel powder.

[0040] Furthermore, in step a):

[0041] The alumina precursor is preferably a fibrous alumina sol and / or a feather-like alumina sol; this type of morphology has the characteristics of long bridging paths and few nodes, which is beneficial for maintaining a loose skeleton and improving redispersibility during subsequent drying and heat treatment. In step a), the dispersion of the alumina precursor can be made uniform by mechanical stirring or other means. Step a) uses a pure aqueous phase as the continuous phase, and does not contain any water-miscible organic displacement solvents (such as alcohols, ketones, ethers, etc.). The specific reasons are as follows: First, the dielectric constant and ionic environment of the aqueous phase help to form a stable electrostatic repulsion and good wetting between the fibrous / feather-like alumina sol and the styrene-based colloidal particles, avoiding the destruction / demulsification of the emulsion layer, double-layer compression, and sudden changes in the solvation environment caused by the addition of miscible organic solvents. This reduces the aggregation of colloidal particles and the flocculation of alumina sol near the isoelectric point, maintaining the uniform dispersion state required for atomization and freezing. Second, at typical cold trap temperatures of -50~-80℃, the saturated vapor pressure of water vapor is low and can be fully condensed and captured by the cold trap. In contrast, water-miscible organic solvents tend to generate higher vapor pressures and co-evaporation penetration, which can enter the pre-stage pumping channel, causing vacuum pump oil contamination, reduced pump efficiency, and safety / environmental risks. Therefore, strictly using a pure aqueous phase in step a) is beneficial for dispersion stability and controllable atomization, while also ensuring that the cold trap effectively retains the volatile phase, avoids vacuum system contamination, reduces displacement and recovery steps, and improves process safety and scale-up feasibility. The homogeneity and rheological characteristics required for subsequent spray-on instantaneous freezing can be obtained through conventional shear dispersion and colloidal stabilization methods, laying the foundation for consistent implementation of steps c) to e).

[0042] Furthermore, in step b):

[0043] The styrene-based colloidal particle dispersion is added to the dispersion obtained in step a) to form a composite dispersion. The solid content of the styrene-based colloidal particle dispersion is 6-20 wt.%, and the volume percentage of the styrene-based colloidal particle dispersion relative to the composite dispersion is 0.3-5 vol.%. After mixing, the mixture can be homogenized by mechanical stirring or other methods. The colloidal particles are stably dispersed in the aqueous phase, providing uniform physical spacing between fibers / feathers, reducing the probability of close contact points and bridging during freezing, and inhibiting subsequent agglomeration from the source. To ensure compatibility with the alumina aqueous system, the pH of the colloidal dispersion is pre-adjusted before mixing, preferably to 7.0-9.0 before mixing with the dispersion containing the alumina precursor. This places it in a stable region that matches the surface charge of alumina, thereby preventing flocculation and maintaining uniform distribution upon addition. The volume fraction of colloidal particles can be 0.5% to 3.0%, and the median particle size can be 30 to 300 nm, so as to balance the interval effect during freezing and the controllability during subsequent removal; this range is coordinated with the sequence of instantaneous freezing, lyophilization and segmented degreasing in steps c) to e).

[0044] Furthermore, in step c):

[0045] The composite dispersion is atomized into independent droplets and immediately brought into contact with a cryogenic medium, allowing it to solidify rapidly. The cryogenic medium is liquid nitrogen or a cryogenic gas containing nitrogen or a mixture of nitrogen and liquid nitrogen at a temperature not exceeding -50°C. The independent droplet-level units facilitate the immediate fixation of the aforementioned dispersion structure during freezing, limiting the macroscopic advancement of ice crystals and fiber compression caused by liquid confluence. The cryogenic gas / liquid medium provides sufficient cold-end capacity, allowing ice crystal growth to primarily occur between colloidal particles and the framework, reducing orientation stacking and localized caking. This step yields a granular frozen precursor, facilitating heat and mass transfer in step d) and laying the foundation for obtaining a fluffy powder morphology.

[0046] Furthermore, in step d):

[0047] Vacuum freeze-drying is performed on the frozen precursor to directly remove the ice phase via sublimation. Unlike atmospheric pressure drying, the sublimation path bypasses the liquid-gas interface, significantly reducing the impact of capillary stress on stress-sensitive areas (necks, thin bridges) and preserving open channels and fine connections. The vacuum freeze-drying process includes a main sublimation stage and a desorption drying stage. The vacuum level required for the main sublimation stage is less than or equal to 50 Pa, and the vacuum level required for the desorption drying stage is less than or equal to 100 Pa. During the drying process, the styrene-based colloidal particles remain as dry particles with a certain degree of elasticity, maintaining effective spacing between fibers / feathers before removal. This avoids re-adhesion and channel blockage caused by surface densification or local softening at the end of drying, thus creating open channels for subsequent clean removal.

[0048] Furthermore, in step e):

[0049] The process involves segmented heating and phased holding under an oxidizing atmosphere, preferably involving multiple heating and holding stages. For example, heating from room temperature to temperature B1 at a rate of A1 and holding for time C1, then heating from room temperature to temperature B2 at a rate of A2 and holding for time C2, and so on, until finally heating from room temperature to temperature Bn at a rate of An and holding for time Cn, where n ≥ 3. The final temperature of the last stage (i.e., temperature Bn) is 250~480℃, removing styrene-based colloidal particles from the surface inwards. The segmented approach allows for the removal of the outer layer first, followed by the inner layer, and, combined with the interconnected channels formed in step d), allows the gas generated during the removal process to escape smoothly along the channels. This gas release creates a short-term loosening effect in the fiber contact area, reducing residual micro-adhesion points without damaging the overall skeleton, further reducing the tendency for post-drying agglomeration and improving the bulkiness and redispersibility of the powder. An oxidizing environment helps reduce the risk of organic residues and carbonization, while suppressing excessive coarsening of the alumina fine structure under segmented temperature control, achieving a balance between cleaning and skeleton stability, and meeting the requirements of consistency and repeatability for engineering scale-up.

[0050] This invention also provides a high specific surface area, highly dispersed, and low-agglomeration nano-alumina aerogel powder, which is prepared according to the above-described method. Its loose bulk density is 0.05-0.08 g / cm³. 3 Tap density: 0.10-0.13 g / cm³ 3 Particle size D10 / D50 / D90≈420 / 770 / 1180 μm; BET specific surface area 250-400 m² 2 / g; average mesopore diameter (BJH) 16-30 nm; macropore peak 2-16 μm; residual carbon ≤0.05 wt.%.

[0051] To further illustrate the technical solution and beneficial effects of the present invention, the present invention will be described in detail below with reference to specific embodiments. It should be noted that the embodiments described are only used to illustrate the technical content and practical application effects of the present invention, and are not intended to limit the scope of protection of the present invention. Various modifications, substitutions, optimizations, or equivalent transformations made by those skilled in the art based on the disclosure of the present invention without departing from its essential spirit should all fall within the scope of protection of the present invention.

[0052] Example 1:

[0053] The preparation method of fibrous alumina aerogel powder includes the following steps:

[0054] Step 101: Prepare a dispersion containing alumina precursor.

[0055] Take fibrous alumina sol (solid content of 20.0 wt.% based on Al2O3), add deionized water to prepare 1.50 L of aqueous dispersion, reduce the alumina solid content to 8 wt.%, stir at 600 rpm for 15 min, and then shear disperse at 6000 rpm for 5 min.

[0056] Step 102: Add styrene-based colloidal particle dispersion to obtain composite dispersion.

[0057] A styrene-based colloidal particle dispersion (D50 = 80 nm, solid content 15 wt.%) was slowly added dropwise to the stirred aqueous dispersion at a rate of 0.5 vol.% (based on the volume of the composite dispersion). The pH of the colloidal dispersion was pre-adjusted to 8.0 with ammonia before addition. The pH of the mixed system was controlled to be 7.8-8.2. After stirring at 400 rpm for 10 min, the mixture was allowed to stand for 5 min to remove bubbles, yielding the composite dispersion. The viscosity of the composite dispersion at 25℃ was approximately 120 mPa·s.

[0058] Step 103: Spray and instantly freeze in a cryogenic medium.

[0059] The liquid is atomized through a 0.60 mm atomizing nozzle, with a liquid inlet rate of 0.50 L / min. The droplets enter liquid nitrogen (-196℃) from the nozzle to a depth of 20 cm above the liquid surface and freeze within ≤1 s. After remaining in liquid nitrogen for 10 min, the liquid is transferred to a -80℃ pre-cooling tray to obtain the frozen material.

[0060] Step 104: Vacuum freeze drying.

[0061] Control the frozen material layer thickness to ≤8 mm; during the main sublimation stage, control the chamber pressure to be below 25 Pa and slowly raise the plate temperature from -30℃ to -10℃ over 6 hours; during the desorption drying stage, control the chamber pressure to be below 80 Pa and slowly raise the plate temperature to 45℃ over 8 hours. Sieve to obtain composite powder of 300-1000 μm size.

[0062] Step 105: Oxidative segmented degreasing.

[0063] The composite powder was spread in a degreasing pan with a thickness controlled to ≤8 mm, and the air atmosphere was controlled. The heating program was controlled as follows: RT to 120℃ (heating rate 1.0℃ / min, hold for 60 min), to 220℃ (heating rate 1.0℃ / min, hold for 60 min), to 320℃ (heating rate 0.8℃ / min, hold for 60 min), to 400℃ (heating rate 0.5℃ / min, hold for 90 min), to 460℃ (heating rate 0.5℃ / min, hold for 30 min), and then furnace cooled. Fiberous alumina aerogel powder product was obtained.

[0064] The characterization results are as follows:

[0065] Loose packing density: 0.065 g / cm³ 3 Tap density: 0.118 g / cm³ 3 Particle size D10 / D50 / D90≈320 / 610 / 980 μm; BET specific surface area 360 m² 2 / g; average mesopore diameter (BJH) 18.7 nm; macropore peak 2-6 μm; residual carbon ≤0.05wt.%.

[0066] Example 2:

[0067] The preparation method of feather-like alumina aerogel powder includes the following steps:

[0068] Step 201: Prepare a dispersion containing alumina precursor.

[0069] Take feather-like alumina sol (25 wt.% as Al2O3), add deionized water to prepare 2.20 L of aqueous dispersion, reduce the alumina solid content to 6 wt.%, stir at 500 rpm for 10 min, and then shear at 5000 rpm for 8 min.

[0070] Step 202: Add styrene-based colloidal particle dispersion to obtain composite dispersion.

[0071] A styrene-based colloidal particle dispersion (20 wt.% solids) with a D50 of 180 nm was slowly added dropwise to the stirred aqueous dispersion at a rate of 2.0 vol.% over 8 min. The pH of the colloidal dispersion was pre-adjusted to 8.5 before addition. The pH of the composite dispersion was controlled between 8.2 and 8.6. After stirring with a paddle at 400 rpm for 10 min, the mixture was allowed to stand for 5 min to remove bubbles. The viscosity of the composite dispersion at 25℃ was approximately 210 mPa·s.

[0072] Step 203: Spray and instantly freeze in the cryogenic gas.

[0073] Atomization was achieved using a 0.50 mm orifice nozzle with a liquid inlet rate of 0.80 L / min. The cryogenic medium was cryogenic nitrogen (cavity temperature -60℃, nitrogen flow rate 25 L / min, gas velocity 2.5 m / s). The droplets froze within 2 seconds. The frozen material was collected after being kept at -60℃ for 15 minutes.

[0074] Step 204: Vacuum freeze drying.

[0075] The frozen material layer thickness was controlled to ≤10 mm; during the main sublimation stage, the chamber pressure was controlled at 30 Pa, and the plate temperature was slowly increased from -28℃ to -12℃ over 7 hours; during the desorption drying stage, the chamber pressure was controlled at 100 Pa, and the plate temperature was slowly increased from -5℃ to 35℃ over 5 hours. The composite powder of 250–850 μm size was obtained by sieving.

[0076] Step 205: Oxidative segmented degreasing.

[0077] The composite powder was spread in a degreasing pan with a thickness controlled to ≤10 mm, and the air atmosphere was controlled. The heating program was as follows: from room temperature to 120℃ (heating rate 1.0℃ / min, hold for 60 min), then to 220℃ (heating rate 1.0℃ / min, hold for 60 min), then to 320℃ (heating rate 0.8℃ / min, hold for 90 min), then to 420℃ (heating rate 0.5℃ / min, hold for 90 min), and finally to 480℃ (heating rate 0.5℃ / min, hold for 30 min); furnace cooling was then performed. Feather-like alumina aerogel powder product was obtained.

[0078] The characterization results are as follows:

[0079] Loose bulk density: 0.078 g / cm³ 3 Tap density: 0.136 g / cm³ 3Particle size D10 / D50 / D90≈280 / 520 / 860 μm; BET specific surface area 315 m² 2 / g; average mesopore size (BJH) 22.4 nm; macropore peak 3–8 μm; residual carbon ≤0.05wt.%.

[0080] Example 3:

[0081] The preparation method of fiber / feather blended alumina aerogel powder includes the following steps:

[0082] Step 301: Prepare a dispersion containing alumina precursor.

[0083] Fiber-like and feather-like alumina sol were mixed at a mass ratio of 1:1 to prepare 2.50 L of aqueous dispersion (4 wt.% of Al2O3). The dispersion was first stirred with a paddle at 600 rpm for 12 min, and then sheared at 5500 rpm for 6 min.

[0084] Step 302: Add styrene-based colloidal particle dispersion to obtain composite dispersion.

[0085] A styrene-based colloidal particle dispersion (D50 = 300 nm, solid content 15 wt.%) was slowly added dropwise to the stirred aqueous dispersion at a rate of 3.0 vol.% over 10 min. The pH of the colloidal dispersion was pre-adjusted to 7.5 before addition. The pH of the composite dispersion was controlled between 7.4 and 7.8. The viscosity of the composite dispersion at 25°C was approximately 180 mPa·s.

[0086] Step 303: Spray and instantly freeze in the low-temperature gas.

[0087] A 0.70 mm atomizing nozzle was used, with a liquid inlet rate of 0.70 L / min. The cryogenic medium was a mixture of nitrogen and air (cavity temperature -55℃, mixed gas flow rate 20 L / min, gas velocity 2.0 m / s). The droplets froze within 2 s; the droplet diameter (D[3,2]) was measured online to be 160-230 μm. The frozen material was collected after being kept at -55℃ for 10 min.

[0088] Step 304: Vacuum freeze drying.

[0089] The frozen material layer thickness was controlled to be ≤5 mm; during the main sublimation stage, the chamber pressure was controlled at 10 Pa, and the plate temperature was slowly increased from -36℃ to -12℃ over 4 hours; during the desorption drying stage, the chamber pressure was controlled at 40 Pa, and the plate temperature was slowly increased to 50℃ over 6 hours. The composite powder of 400–1200 μm size was obtained by sieving.

[0090] Step 305: Oxidative segmented degreasing.

[0091] The composite powder was spread in a degreasing pan with a thickness controlled to ≤12 mm, and the process was carried out in air. The heating program was as follows: from room temperature to 120℃ (heating rate 1.0℃ / min, hold for 60 min), then to 220℃ (heating rate 1.0℃ / min, hold for 60 min), then to 320℃ (heating rate 1.2℃ / min, hold for 45 min), then to 400℃ (heating rate 0.8℃ / min, hold for 90 min), and finally to 400℃ (heating rate 0.5℃ / min, hold for 45 min). This yielded a fiber / feather blended alumina aerogel powder product.

[0092] The characterization results are as follows:

[0093] Loose packing density: 0.058 g / cm³ 3 Tap density: 0.112 g / cm³ 3 Particle size D10 / D50 / D90≈420 / 770 / 1180 μm; BET specific surface area 270 m² 2 / g; average mesopore diameter (BJH) 26.1 nm; macropore peak 5-12 μm; residual carbon ≤0.05wt.%.

[0094] The above embodiments demonstrate that the present invention, through the synergistic regulation of key parameters (precursor morphology and solid content, colloidal particle size and dosage, freezing and lyophilization conditions, segmented degreasing procedures, etc.), can achieve the regulation of powder structure and properties, enabling the obtained nano-alumina aerogel powder to achieve a stable balance between high dispersion, low agglomeration and high specific surface area, fully verifying the effectiveness and feasibility of the technical solution of the present invention.

[0095] It should be noted that, within the scope of protection of this invention, even if some simple process changes are made, they do not depart from the essence of this invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of protection of the claims of this application.

Claims

1. A method for preparing high specific surface area, highly dispersed, and low agglomerated nano-alumina aerogel powder, characterized in that, The method comprises the following steps: a) preparing a dispersion of alumina precursor in an aqueous phase, the alumina precursor being selected from a fibrous alumina sol and / or a feather-like alumina sol; b) adding a dispersion of styrene-based colloidal particles to the dispersion to obtain a composite dispersion, wherein the volume fraction of the styrene-based colloidal particles in the composite dispersion is 0.5% to 3.0%, and the median particle size is 30 to 300 nm; c) spraying the composite dispersion and instantaneously freezing in a low-temperature medium to obtain frozen particles; d) performing vacuum freeze-drying on the frozen particles to obtain a dry powder containing styrene-based colloidal particles; e) performing staged debinding under an oxidizing atmosphere by programmed temperature rising to remove the styrene-based colloidal particles to obtain a high-surface-area, highly dispersed and low-agglomerated nano-alumina aerogel powder.

2. The method according to claim 1, wherein the aqueous phase in step a) does not contain water-miscible organic displacement solvents.

3. The method according to claim 1 or 2, wherein the styrene-based colloidal particles are prepared by soap-free emulsion polymerization and / or seed emulsion polymerization.

4. The method according to any one of the preceding claims, wherein the pH of the dispersion of styrene-based colloidal particles is adjusted to 7.0 to 9.0 before mixing with the dispersion of alumina precursor.

5. The method according to any one of the preceding claims, wherein the low-temperature medium in step c) is liquid nitrogen or low-temperature nitrogen gas or mixed nitrogen gas with a temperature not higher than -50°C.

6. The method according to any one of the preceding claims, wherein step e) is performed in an air atmosphere by using a programmed temperature rising method with staged temperature rising and holding at each stage, and the final temperature of the last stage is 250 to 480°C.

7. A high surface area, highly dispersed, low agglomerated nano alumina aerogel powder, characterized in that, The method is prepared according to any one of claims 1 to 6.