A method for preparing aluminum oxide whiskers

Abstract

The application relates to a preparation method of aluminum oxide whiskers and belongs to the technical field of inorganic ceramic fibers. The method comprises the following steps: mixing and reacting metal aluminum and aluminum chloride hexahydrate to obtain an aluminum oxide sol; sequentially concentrating and aging the aluminum oxide sol to obtain an aged aluminum oxide sol; mixing the aged aluminum oxide sol with a spinning aid to obtain a spinning solution; electrostatically assisting high-pressure air spinning of the spinning solution to obtain aluminum oxide superfine fiber filaments; performing first-stage calcination treatment and second-stage calcination treatment on the aluminum oxide superfine fiber filaments to obtain aluminum oxide superfine fibers; and performing high-speed stirring treatment on the aluminum oxide superfine fibers under the condition that the rotating speed is 3000 rpm-5000 rpm to obtain aluminum oxide whiskers.

Description

Technical Field

[0001] This application relates to the field of inorganic ceramic fiber technology, and in particular to a method for preparing alumina whiskers. Background Technology

[0002] Alumina whiskers are single-crystal, short-fiber alumina materials with a near-perfect crystal structure and high chemical stability. Due to their few crystal defects and high aspect ratio, alumina whiskers exhibit excellent mechanical properties, including high strength, high elastic modulus, high temperature resistance (above 1400℃), and good corrosion resistance. In composite materials, alumina whiskers effectively inhibit crack propagation, strengthen interfacial bonding, and absorb fracture energy, thereby significantly improving the toughness, flexural strength, and thermal shock resistance of the matrix. Therefore, they are widely used in ceramic-based, metal-based, and polymer-based composite materials as an ideal toughening and reinforcing phase, possessing irreplaceable application value in high-end fields such as aerospace, precision machinery, high-temperature filtration, and lithium-ion battery membrane coating.

[0003] Currently, the main methods for preparing alumina whiskers include the vapor-phase method, flux method, sol-gel method, and hydrothermal method. The vapor-phase method uses a metal catalyst to induce the nucleation and growth of a vapor-phase aluminum source on the catalyst surface, resulting in high product purity and excellent aspect ratio. However, it requires high temperatures of 1000–1600℃, demanding sophisticated equipment and relying on precious metal catalysts, leading to high costs, low yields, and difficulty in large-scale industrialization. The flux method mixes the aluminum source with a fluoride flux, using the flux to lower the melting point and promote whisker growth at 900–1200℃. This process is simple and low-cost, but the product is prone to agglomeration, and flux residue is difficult to completely remove, affecting whisker purity and composite material properties. Subsequent impurity removal also increases costs. Sol-gel methods... Gel method via aluminum salt sol Gel preparation of precursors followed by calcination at 600–900℃ yields whiskers, resulting in high product purity and easily controllable morphology. However, the process is cumbersome, time-consuming, and the precursor filtration and drying are difficult, leading to low efficiency on a large scale and hindering industrialization. A hydrothermal method, using a high-pressure environment at 150–300℃, dissolves the precursor... Recrystallization forms whiskers, with mild reaction conditions and uniform product morphology, but high investment in high-pressure equipment and limited single-batch output make it difficult to meet large-scale demand. Summary of the Invention

[0004] This application provides a method for preparing alumina whiskers to solve the following technical problem: how to stably prepare high-purity single-crystal alumina whiskers with controllable diameter and length in a low-cost and industrially suitable manner. This application provides a method for preparing alumina whiskers, the method comprising: Alumina sol is obtained by mixing and reacting metallic aluminum and aluminum chloride hexahydrate. The alumina sol was sequentially concentrated and aged to obtain aged alumina sol. The aged alumina sol is mixed with a spinning aid to obtain a spinning solution; The spinning solution was subjected to electrostatic assisted high-pressure air spinning to obtain alumina ultrafine fiber precursor. The alumina microfiber precursor is subjected to a first-stage calcination treatment and a second-stage calcination treatment to obtain alumina microfiber. The alumina ultrafine fibers were subjected to high-speed stirring at a rotation speed of 3000 rpm to 5000 rpm to obtain alumina whiskers.

[0005] Optionally, the step of sequentially concentrating and aging the alumina sol to obtain aged alumina sol includes: The alumina sol was concentrated to a solid content of 20wt%~25wt% at 40℃~70℃. The concentrated alumina sol was aged at 15°C to 30°C for 24 to 48 hours to obtain aged alumina sol.

[0006] Optionally, the heating rate of the first calcination treatment is 0.5℃ / min to 2℃ / min, the temperature of the first calcination treatment is 500℃ to 800℃, and the time of the first calcination treatment is 1 hour to 3 hours.

[0007] Optionally, the temperature of the second calcination treatment is 1400~1600℃, and the time of the second calcination treatment is 10 seconds~30 seconds.

[0008] Optionally, the temperature of the mixing reaction is 70℃~90℃, and the mixing reaction time is 5 hours~10 hours.

[0009] Optionally, the molar ratio of the metallic aluminum to the aluminum chloride hexahydrate is 3 to 5.

[0010] Optionally, the spinning aid is at least one of polyethylene oxide, polyvinyl alcohol, and polyvinylpyrrolidone.

[0011] Optionally, the mass of the spinning aid is 5% to 20% of the mass of the aged alumina sol; the viscosity of the spinning solution is 300 mPa·s to 2000 mPa·s.

[0012] Optionally, the electrostatic assisted high-pressure air spinning meets the following requirements: spinning solution spraying rate of 10mL / h~20mL / h, air flow rate of 10L / min~30L / min, spinning voltage of 10kV~20kV, and spinning distance of 15cm~30cm.

[0013] Optionally, the alumina whiskers have the following characteristics: diameter of 0.2μm to 1μm, length of 5μm to 50μm, and operating temperature of 1200℃ to 1400℃.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing alumina whiskers. The method includes: mixing and reacting metallic aluminum and aluminum chloride hexahydrate to obtain alumina sol; concentrating and aging the alumina sol sequentially to obtain aged alumina sol; mixing the aged alumina sol with a spinning aid to obtain a spinning solution; electrostatically assisted high-pressure air spinning of the spinning solution to obtain alumina microfiber precursors; subjecting the alumina microfiber precursors to a first-stage calcination treatment and a second-stage calcination treatment to obtain alumina microfibers; and subjecting the alumina microfibers to high-speed stirring treatment at a rotation speed of 3000 rpm to 5000 rpm to obtain alumina whiskers. First, metallic aluminum and aluminum chloride hexahydrate are mixed and reacted. The weakly acidic environment generated by the hydrolysis of aluminum chloride hexahydrate causes a redox reaction in metallic aluminum, generating aluminum ions in situ, which together with the aluminum ions provided by aluminum chloride hexahydrate form an alumina sol with controllable polymerization degree. This alumina sol uses inexpensive industrial products as raw materials, eliminating the need for expensive organoaluminum sources or complex sol-gel steps, thus reducing costs from the source. Secondly, the alumina sol is concentrated and aged sequentially to obtain aged alumina sol. The aged alumina sol is then mixed with spinning aids to obtain a spinning solution. Through the combined effects of concentration, aging, and spinning aids, the viscosity of the spinning solution is adjusted to a suitable rheological window for stretching. Next, the spinning solution is subjected to electrostatic-assisted high-pressure air spinning. Under the dual stretching of electrostatic force and high-pressure airflow, the spinning solution forms continuous, uniform, and smooth alumina ultrafine fiber precursors. Next, the alumina microfiber precursor undergoes a first-stage calcination treatment and a second-stage calcination treatment: the first-stage calcination treatment allows spinning auxiliaries and volatile components to escape smoothly, transforming the alumina microfiber precursor into pure and crack-free transition phase alumina microfiber; the second-stage calcination treatment involves flash calcining the pretreated fibers, which have undergone the first-stage calcination treatment and cooling, at a higher temperature for an extremely short time (on the order of seconds). The instantaneous high-temperature pulse provides a huge phase transformation driving force, enabling the transition phase alumina microfiber to complete the transformation to α-alumina in a very short time. At the same time, due to the extremely short treatment time, the number of α-alumina crystal nuclei is effectively limited and the grains do not have time to grow laterally, thus preferentially growing along the fiber axis into continuous single-crystal alumina microfibers, rather than polycrystalline aggregates. This kinetically resolves the contradiction between complete phase transformation and grain coarsening inhibition, ensuring the high purity and single-crystal characteristics of the product. Finally, the alumina ultrafine fibers obtained after two calcination processes are subjected to high-speed stirring. Shear stress is used to cause the continuous single-crystal alumina ultrafine fibers to fracture laterally at stress concentration points, while avoiding excessive crushing. This allows the length of the alumina whiskers formed after fracture to be adjusted to the required range, thus achieving on-demand control of the alumina whisker length.All the raw materials used in the above steps are inexpensive bulk industrial products, and the equipment used is suitable for continuous and large-scale production. The entire process is simple and controllable and has no toxic or harmful emissions. Therefore, high-purity, uniform diameter and controllable length single crystal alumina whiskers were stably prepared in a low-cost and industrially suitable manner. Attached Figure Description The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 A schematic flowchart illustrating a method for preparing alumina whiskers provided in an embodiment of this application; Figure 2 This is a microstructure diagram of alumina whiskers provided in Example 1 of this application. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0019] Figure 1 This is a schematic flowchart illustrating a method for preparing alumina whiskers provided in an embodiment of this application.

[0020] Please see Figure 1 This application provides a method for preparing alumina whiskers, the method comprising: S1. Metallic aluminum and aluminum chloride hexahydrate are mixed and reacted to obtain alumina sol; S2. The alumina sol is concentrated and aged sequentially to obtain aged alumina sol. S3. The aged alumina sol is mixed with a spinning aid to obtain a spinning solution; S4. The spinning solution is subjected to electrostatic assisted high-pressure air spinning to obtain alumina ultrafine fiber precursor. S5. The alumina microfiber precursor is subjected to a first-stage calcination treatment and a second-stage calcination treatment to obtain alumina microfiber. S6. The alumina ultrafine fibers are subjected to high-speed stirring at a rotation speed of 3000 rpm to 5000 rpm to obtain alumina whiskers.

[0021] In some embodiments, the molar ratio of the metallic aluminum to the aluminum chloride hexahydrate is 3 to 5.

[0022] A 3:5 molar ratio of metallic aluminum to aluminum chloride hexahydrate means that the amount of metallic aluminum is 3:5. In the preparation of alumina sol, metallic aluminum and aluminum chloride hexahydrate react together. In an acidic system (where hydrochloric acid is produced by the hydrolysis of aluminum chloride hexahydrate), metallic aluminum undergoes oxidation to generate aluminum ions. Simultaneously, aluminum chloride hexahydrate provides additional aluminum ions, together forming a hydroxyaluminate complex with a specific degree of polymerization, ultimately transforming into alumina sol. If the molar ratio of metallic aluminum to aluminum chloride hexahydrate is less than 3 (i.e., the proportion of metallic aluminum is too low), there is a relative excess of aluminum chloride hexahydrate in the system. The acidity generated by hydrolysis is too high, leading to an excessively rapid polymerization reaction of aluminum ions. This causes the sol to easily form large precipitate particles, making it impossible to obtain a stable, transparent sol, and reducing the spinnability of the subsequent spinning solution. If the molar ratio of metallic aluminum to aluminum chloride hexahydrate is higher than 5 (i.e., the proportion of metallic aluminum is too high), there will be a large excess of metallic aluminum. During the reaction, a large amount of hydrogen gas will be generated and heat will be released, causing the system temperature to rise sharply. This may lead to localized boiling or uncontrolled sol-gelation, resulting in excessively high and uneven sol viscosity, making subsequent concentration, aging, and spinning impossible.

[0023] In some embodiments, the temperature of the mixing reaction is 70°C to 90°C, and the mixing reaction time is 5 hours to 10 hours.

[0024] The mixed reaction refers to the process of reacting metallic aluminum and aluminum chloride hexahydrate as raw materials to generate alumina sol. In this process, the reaction temperature is controlled within the range of 70℃ to 90℃, and the reaction duration is controlled within the range of 5 hours to 10 hours.

[0025] When the reaction temperature is below 70℃, the reaction rate is too slow, and the passivation film on the surface of the aluminum metal is difficult to be effectively destroyed, resulting in a prolonged reaction induction period. The reaction cannot be completed within 5 to 10 hours, leaving residual aluminum metal. The solid content and aluminum ion concentration of the alumina sol are low, and subsequent concentration to 20wt% to 25wt% requires a longer time and the degree of polymerization of the sol is insufficient. When the reaction temperature is above 90℃, the reaction is too violent, and local overheating leads to uneven hydrolysis and polymerization of aluminum ions, which easily generates large particle precipitates or gel clumps, making the alumina sol turbid or layered, and unable to form a clear and stable sol.

[0026] The reaction between metallic aluminum and aluminum chloride hexahydrate is not instantaneous; sufficient time is required for the aluminum surface to be continuously dissolved by acid and aluminum ions to be generated, while the aluminum ions gradually hydrolyze and polymerize. When the reaction time is less than 5 hours, the metallic aluminum does not react completely, and residual aluminum powder or particles will mix into the alumina sol, resulting in an impure sol. These solid impurities can clog the spinning needles or cause fiber breakage during subsequent spinning. When the reaction time is longer than 10 hours, although the reaction is more complete, the formed alumina sol will undergo over-polymerization, and the viscosity will exceed the range that can be spun subsequently. In addition, prolonged heating increases energy consumption and equipment occupancy time, reducing economic efficiency.

[0027] In some embodiments, the step of sequentially concentrating and aging the alumina sol to obtain aged alumina sol includes: The alumina sol was concentrated to a solid content of 20wt%~25wt% at 40℃~70℃. The concentrated alumina sol was aged at 15°C to 30°C for 24 to 48 hours to obtain aged alumina sol.

[0028] Concentration refers to the process of removing some water from alumina sol to increase the mass percentage of solids (i.e., alumina and its precursors) in the sol. The concentration temperature of 40℃~70℃ refers to the temperature range maintained during the concentration process, including the endpoints of 40℃ and 70℃. When the concentration temperature is below 40℃, the water evaporation rate is too slow, requiring a long time to concentrate to a solid content of 20wt%~25wt%. During prolonged heating, the sol is prone to microbial contamination or localized uneven polymerization. When the concentration temperature is above 70℃, although the evaporation rate is fast, the hydrolysis and polymerization reaction of aluminum ions in the alumina sol is accelerated, easily leading to irreversible over-polymerization or localized gelation, resulting in decreased sol fluidity or even complete solidification, rendering it unusable. A solid content of 20wt%~25wt% means that after concentration, the mass percentage of solids in the alumina sol is 20% to 25% of the total sol mass, including the endpoints of 20wt% and 25wt%.

[0029] The concentrated alumina sol is in a non-equilibrium state, with unevenly distributed colloidal particles. Static aging at 15℃~30℃ (room temperature range) allows the colloidal particles to slowly collide and rearrange through Brownian motion, forming a more uniform three-dimensional network structure. Below 15℃, the particle movement is too slow, resulting in low aging efficiency and insufficient structural homogenization within 24~48 hours. Above 30℃, the particle movement is too fast, easily leading to excessive collisions, causing localized gelation or precipitation, and disrupting the sol's homogeneity. The aging temperature range of 15℃~30℃ is considered mild, allowing the system to reach a more thermodynamically stable state without significantly altering the degree of polymerization. This temperature range matches the subsequent mixing temperature (room temperature) with spinning auxiliaries, eliminating the need for additional heating or cooling.

[0030] In some embodiments, the spinning aid is at least one of polyethylene oxide, polyvinyl alcohol, and polyvinylpyrrolidone.

[0031] In the preparation process of alumina whiskers in this application embodiment, aged alumina sol is mixed with spinning aids to obtain a spinning solution. The core function of the spinning aids is to regulate the rheological properties of the spinning solution, enabling it to stably form continuous and uniform alumina ultrafine fiber precursors during electrostatically assisted high-pressure air spinning. Specifically, the spinning aids increase the viscosity of the spinning solution and improve its stretchability, suppressing the breakage of the spinning jet and droplet splashing, ensuring that the fiber precursors have a uniform diameter and complete morphology. Polyethylene oxide, polyvinyl alcohol, and polyvinylpyrrolidone are all water-soluble polymers with different molecular structures and physicochemical properties, but all can function as spinning aids in the alumina sol system. The three aids can be used alone or in combination of any two or three.

[0032] In some embodiments, the mass of the spinning aid is 5% to 20% of the mass of the aged alumina sol; the viscosity of the spinning solution is 300 mPa·s to 2000 mPa·s.

[0033] When the mass of spinning aids accounts for less than 5% of the mass of aged alumina sol, the amount of spinning aids added is insufficient and cannot effectively improve the spinnability of the spinning solution. At this point, the viscosity of the spinning solution is too low, typically below 300 mPa·s. This results in difficulty forming continuous alumina microfiber filaments after the spinning solution is ejected during electrostatically assisted high-pressure air spinning; instead, droplets or short fiber segments form, with uneven fiber diameters and a tendency to form beaded structures. When the mass of spinning aids accounts for more than 20% of the mass of aged alumina sol, the amount of spinning aids added is excessive, and the viscosity of the spinning solution is too high, typically exceeding 2000 mPa·s. A spinning solution with excessively high viscosity has poor fluidity, easily causing spinneret clogging in electrostatic spinning equipment, leading to an unstable spinning process and resulting in large and uneven diameter alumina microfiber filaments.

[0034] The viscosity of the spinning solution, ranging from 300 mPa·s to 2000 mPa·s, is crucial for ensuring the stable production of uniform fiber precursors in electrostatically assisted high-pressure air spinning. When the viscosity of the spinning solution is below 300 mPa·s, surface tension dominates, preventing the formation of a stable Taylor cone in electrostatically assisted high-pressure air spinning, resulting in frequent fiber breakage and inability to collect continuously. When the viscosity of the spinning solution is above 2000 mPa·s, the extrusion resistance is too high, making it difficult for the airflow to stretch the spinning solution into fine fibers, leading to a significant increase in fiber precursor diameter and a non-uniform, bamboo-like morphology.

[0035] In some embodiments, the electrostatic assisted high-pressure air spinning meets the following requirements: spinning solution ejection rate of 10 mL / h to 20 mL / h, air flow rate of 10 L / min to 30 L / min, spinning voltage of 10 kV to 20 kV, and spinning distance of 15 cm to 30 cm.

[0036] Electrostatic assisted high-pressure air spinning is a spinning technology that couples the effects of electrostatic force and high-pressure airflow stretching force. The spinning solution ejection rate refers to the volumetric flow rate of the spinning solution extruded from the spinneret of the electrostatic assisted high-pressure air spinning equipment per unit time. When the spinning solution ejection rate is below 10 ml / h, the amount of spinning solution extruded is too small. Under the combined action of spinning voltage and airflow, the fibers are overstretched, resulting in alumina microfiber precursors that are too thin and easily break, with poor fiber continuity and difficulty in stable collection. When the spinning solution ejection rate is above 20 ml / h, the amount of spinning solution extruded is too large. The airflow and spinning voltage cannot fully stretch the excess spinning solution into fine fibers, leading to alumina microfiber precursors with large and uneven diameters, even exhibiting droplet-like or beaded structures, affecting the diameter uniformity and length controllability of the obtained alumina whiskers.

[0037] Airflow rate refers to the volumetric flow rate of the high-pressure airflow in electrostatically assisted high-pressure air spinning equipment. This airflow applies mechanical stretching force to the spinning jet at the spinneret and promotes solvent evaporation. When the airflow rate is below 10 liters per minute, the stretching effect of the airflow on the spinning solution is insufficient, and the jet formed by the spinning solution under the action of the electric field force cannot be sufficiently refined. The resulting alumina microfiber precursors have a larger diameter, and the fibers are prone to adhesion. When the airflow rate is above 30 liters per minute, the airflow velocity is too high, which enhances the disturbance to the spinning solution jet, causing the precursor fibers to swing violently or break during flight. The diameter uniformity of the precursor fibers decreases, and an airflow rate above 30 liters per minute increases the equipment load and operating costs.

[0038] Spinning voltage refers to the DC high-voltage potential difference applied between the spinneret and the receiving device in an electrostatically assisted high-voltage air spinning device. The electrostatic field causes charges to accumulate on the surface of the spinning solution, generating electrostatic repulsion. This overcomes surface tension, forming a Taylor cone and emitting a jet, which is one of the core driving forces for fiber refinement. When the spinning voltage is below 10 kV, the electrostatic field is insufficient to overcome the surface tension and viscosity of the spinning solution, preventing the stable formation of the Taylor cone. The spinning solution, after being ejected, cannot form a continuous jet but instead drips, making it impossible to obtain continuous alumina ultrafine fiber filaments. When the spinning voltage is above 20 kV, the electrostatic field is too strong, causing excessive acceleration of the jet in the electric field, resulting in excessive fiber stretching. This leads to excessively thin fiber filaments and generates numerous whipping instabilities, making the fiber filaments prone to breakage. Furthermore, spinning voltages above 20 kV increase equipment insulation requirements and operational safety risks.

[0039] Spinning distance refers to the straight-line distance between the tip of the spinneret and the receiving device in an electrostatically assisted high-pressure air spinning device. When the spinning distance is less than 15 cm, the jet's flight time in the electric field is too short, resulting in insufficient solvent evaporation. The fiber filaments still contain a significant amount of solvent when they reach the receiving device, causing the fibers to adhere or fuse together, preventing the formation of independent alumina microfiber filaments. When the spinning distance is greater than 30 cm, the jet's flight distance is too long, the electric field strength decreases with increasing distance, the stretching effect weakens, and the fiber filament diameter becomes thicker and unevenly distributed. Furthermore, a spinning distance greater than 30 cm increases the equipment's footprint and the difficulty of fiber collection.

[0040] The four parameters—spinning solution ejection rate, air flow rate, spinning voltage, and spinning distance—must be adjusted holistically. Increasing the spinning solution ejection rate requires a corresponding increase in air flow rate to provide sufficient stretching force to refine the jet; conversely, decreasing the ejection rate allows for a decrease in air flow rate. Increasing the ejection rate also necessitates a higher spinning voltage to enhance the electric field force against the increased jet inertia; the reverse is also true. Increased air flow rate leads to faster jet acceleration, allowing for a slightly increased spinning distance to fully utilize the stretching zone; conversely, a decreased air flow rate allows for a smaller spinning distance. The spinning voltage and distance must maintain the electric field strength within a safe and effective range; increasing the spinning distance requires a corresponding increase in spinning voltage.

[0041] In some embodiments, the heating rate of the first calcination stage is 0.5℃ / min to 2℃ / min, the temperature of the first calcination stage is 500℃ to 800℃, and the time of the first calcination stage is 1 hour to 3 hours.

[0042] The calcination process used in this application is a two-stage process: the first stage involves conventional slow calcination with a heating rate of 0.5-2℃ / min (temperature 500-800℃, time 1-3 hours). After cooling, a second stage of calcination is performed (temperature 1400-1600℃, time 10-30 seconds). The core function of the first stage of calcination is to slowly remove moisture, spinning aids, and volatile components (such as nitrogen oxides and hydrogen chloride) from the alumina microfiber filaments. Simultaneously, it transforms amorphous alumina or transition phase alumina into an intermediate phase with certain crystal nuclei, laying the foundation for whisker growth in the second stage of calcination. The heating rate refers to the temperature increase per unit time during the rise from room temperature to the temperature of the first stage of calcination. When the heating rate of the first stage of calcination is less than 0.5℃ / min, the heating is too slow. Although this facilitates the smooth discharge of gaseous products, it significantly prolongs the entire first stage of calcination time, leading to a substantial decrease in production efficiency, increased energy consumption, and making it economically infeasible. Meanwhile, a heating rate below 0.5℃ / min causes the alumina microfiber precursor to remain in the low-temperature zone for too long, resulting in excessive densification of the amorphous alumina within the fiber. This, in turn, inhibits the preferential growth of whiskers during the subsequent second-stage calcination process, leading to a larger diameter and lower aspect ratio of the resulting alumina whiskers. When the heating rate of the first-stage calcination process exceeds 2℃ / min, the rapid heating causes a large release of moisture, spinning aid decomposition products, and gases generated from aluminum salt decomposition within the alumina microfiber precursor in a short period. Since the fiber diameter is only on the micrometer or submicrometer scale, the gases do not have enough time to diffuse and escape from the fiber interior, resulting in high-pressure bubbles inside the fiber, which in turn causes fiber expansion, bubbling, cracking, and even bursting.

[0043] When the heating rate of the first calcination stage exceeds 2℃ / min, the rapid temperature rise causes a large release of moisture, spinning aid decomposition products, and gases generated from the decomposition of aluminum salts within the alumina microfiber precursor fibers in a short period. Since the fiber diameter is only on the micrometer or submicrometer scale, the gases cannot diffuse out quickly enough, leading to high-pressure bubbles inside the fiber. This causes the fiber to expand, blister, crack, and even burst. The calcined alumina microfibers exhibit numerous cracks and pores on their surface, destroying fiber integrity. Consequently, smooth and intact alumina whiskers cannot be formed during subsequent grinding and dispersion processes, resulting in fragmented or powdery products.

[0044] The temperature of the first calcination stage refers to the temperature value maintained during the isothermal phase after the initial heating. Controlling the temperature of the first calcination stage within the range of 500℃ to 800℃ ensures complete decomposition and oxidation removal of spinning auxiliaries, and the complete conversion of aluminum salts into alumina, while preventing premature excessive nucleation of α-Al₂O₃. Within this temperature range, the alumina microfiber precursor transforms into pure alumina fibers composed of transition phases such as γ-Al₂O₃ or θ-Al₂O₃. These fibers retain an amorphous or fine-grained structure, making them suitable for rapid whisker growth induced by flash calcination in the second calcination stage.

[0045] The first calcination treatment time refers to the duration of maintaining a constant temperature after reaching the first calcination treatment temperature. Controlling the first calcination treatment time within the range of 1 to 3 hours ensures that organic matter and volatile components are fully removed, the internal temperature of the fiber is uniform, and excessive grain coarsening of the transition phase alumina and premature nucleation of α-Al2O3 are avoided.

[0046] In some embodiments, the temperature of the second calcination treatment is 1400~1600℃, and the time of the second calcination treatment is 10 seconds~30 seconds.

[0047] The second stage of calcination is performed at a temperature of 1400℃~1600℃ for 10 to 30 seconds. The core function of this second stage is to induce the transformation of the alumina ultrafine fibers, primarily composed of transition phase alumina (such as γ-Al₂O₃ or θ-Al₂O₃), obtained from the first stage of calcination into α-Al₂O₃ single-crystal whiskers within a very short time. This process achieves rapid nucleation and directional growth through high-temperature flash calcination, avoiding excessive grain coarsening and polycrystalline aggregation.

[0048] In some embodiments, the alumina whiskers meet the following requirements: diameter of 0.2 μm to 1 μm, length of 5 μm to 50 μm, and operating temperature of 1200℃ to 1400℃.

[0049] The alumina whiskers prepared using the method described in this application have specific geometric dimensions (diameter and length) and operating temperature performance, specifically: diameter 0.2 μm ~ 1 μm, length 5 μm ~ 50 μm, and operating temperature 1200℃ ~ 1400℃. The diameter refers to the width of the cross-section of the alumina whisker. For whiskers with an approximately circular cross-section, the diameter is the diameter of the circular cross-section; for whiskers with a non-circular cross-section, the diameter is the equivalent diameter. The length refers to the axial extension of the alumina whisker, i.e., the longest distance from one end to the other. The operating temperature refers to the temperature range within which the alumina whisker can operate stably for a long time in air without significant grain growth or performance degradation. These parameter ranges represent both the technical effects achievable by the method described in this application and the performance guarantee for the whiskers' applicability in applications such as lithium-ion battery separator coating and high-toughness ceramic preparation.

[0050] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0051] Example 1 Add 500g of deionized water to a 1L beaker, then add 30.0g of aluminum chloride hexahydrate and 13g of metallic aluminum powder sequentially. React at 90℃ for 5 hours to obtain alumina sol. Concentrate the alumina sol at 60℃ to a solid content of 20wt%, then age it at room temperature (25℃) for 24 hours to obtain aged alumina sol. Add 20% (by weight) of polyvinyl alcohol (spinning aid) to the aged alumina sol, mix thoroughly, and obtain a spinning solution with a viscosity of 2000 mPa·s. Add the prepared spinning solution to an electro-electro-spinning apparatus for electrostatic assisted high-pressure air spinning. The process conditions for electrostatic assisted high-pressure air spinning are: spinning solution ejection rate 10mL / h, air flow rate 20L / min, spinning voltage 15kV, and spinning distance 20cm. Collect the alumina ultrafine fiber precursor.

[0052] The prepared alumina ultrafine fiber precursor was heated from room temperature to 600℃ at a heating rate of 0.5℃ / min and held at that temperature for 2 hours (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1500℃ for 20 seconds (second stage calcination treatment) and cooled to room temperature in air to obtain alumina ultrafine fibers. The prepared alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 3000 rpm to obtain alumina whiskers.

[0053] The obtained alumina whiskers were observed by scanning electron microscopy (see...). Figure 2The alumina whiskers are 0.4 μm in diameter and 20 μm in length, with a smooth surface and no cracks or other defects. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation determined that the service temperature of these alumina whiskers is 1300℃.

[0054] Example 2 500g of deionized water was added to a 1L beaker, followed by 30.0g of aluminum chloride hexahydrate and 16g of aluminum powder. The mixture was reacted at 80℃ for 8 hours to obtain alumina sol. The alumina sol was concentrated at 65℃ to a solid content of 21wt%, and then aged at room temperature (25℃) for 36 hours to obtain aged alumina sol. 5% (by weight) of polyethylene oxide (spinning aid) was added to the aged alumina sol, and after thorough mixing, a spinning solution with a viscosity of 500 mPa·s was obtained. The prepared spinning solution was then fed into an electrostatic spinning apparatus for electrostatic assisted high-pressure air spinning. The electrostatic assisted high-pressure air spinning process conditions were: spinning solution ejection rate 15mL / h, air flow rate 10L / min, spinning voltage 10kV, and spinning distance 30cm. Alumina ultrafine fiber precursors were collected.

[0055] The prepared alumina ultrafine fiber precursor was heated from room temperature to 500°C at a heating rate of 1°C / min and held at that temperature for 3 hours (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1400°C for 30 seconds (second stage calcination treatment) and cooled to room temperature in air to obtain alumina ultrafine fibers. The obtained alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 4000 rpm to obtain alumina whiskers.

[0056] The obtained alumina whiskers had a diameter of 0.2 μm and a length of 10 μm, with a smooth surface and no defects such as cracks. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation showed that the alumina whiskers could be used at 1400℃.

[0057] Example 3 Add 500g of deionized water to a 1L beaker, then add 30.0g of aluminum chloride hexahydrate and 11g of aluminum powder. React at 70℃ for 10 hours to obtain alumina sol. Concentrate the alumina sol at 50℃ to a solid content of 22wt%, then age it at room temperature (25℃) for 48 hours to obtain aged alumina sol. Add 15% (by weight) of polyvinylpyrrolidone (spinning aid) to the aged alumina sol, mix thoroughly, and obtain a spinning solution with a viscosity of 1500 mPa·s. Add the prepared spinning solution to an electro-electro-spinning apparatus for electrostatic assisted high-pressure air spinning. The process conditions for electrostatic assisted high-pressure air spinning are: spinning solution ejection rate 20mL / h, air flow rate 30L / min, spinning voltage 20kV, and spinning distance 15cm. Collect the alumina ultrafine fiber precursor.

[0058] The prepared alumina ultrafine fiber precursor was heated from room temperature to 700℃ at a heating rate of 1.5℃ / min and held at that temperature for 1 hour (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1600℃ for 10 seconds (second stage calcination treatment) and cooled to room temperature in air to obtain alumina ultrafine fibers. The obtained alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 5000 rpm to obtain alumina whiskers.

[0059] The obtained alumina whiskers had a diameter of 0.5 μm and a length of 30 μm, with a smooth surface and no defects such as cracks. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation showed that the alumina whiskers could be used at 1200℃.

[0060] Example 4 500g of deionized water was added to a 1L beaker, followed by 30.0g of aluminum chloride hexahydrate and 14g of aluminum powder. The mixture was reacted at 80℃ for 9 hours to obtain alumina sol. The alumina sol was concentrated at 45℃ to a solid content of 25wt%, and then aged at room temperature (25℃) for 36 hours to obtain aged alumina sol. Polyvinyl alcohol (spinning aid) at 20% of the mass of the aged alumina sol was added to the aged alumina sol, and after thorough mixing, a spinning solution with a viscosity of 2000 mPa·s was obtained. The prepared spinning solution was added to an electro-electro-spinning apparatus for electrostatic assisted high-pressure air spinning. The process conditions for electrostatic assisted high-pressure air spinning were: spinning solution ejection rate 15mL / h, air flow rate 15L / min, spinning voltage 15kV, and spinning distance 25cm. Alumina ultrafine fiber precursors were collected.

[0061] The prepared alumina ultrafine fiber precursor was heated from room temperature to 800°C at a heating rate of 2°C / min and held at that temperature for 1 hour (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1550°C for 15 seconds (second stage calcination treatment) and cooled to room temperature in air to obtain alumina ultrafine fibers. The obtained alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 3500 rpm to obtain alumina whiskers.

[0062] The obtained alumina whiskers had a diameter of 0.7 μm and a length of 40 μm, with a smooth surface and no defects such as cracks. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation showed that the alumina whiskers could be used at 1300℃.

[0063] Example 5 Add 500g of deionized water to a 1L beaker, then add 30.0g of aluminum chloride hexahydrate and 12g of aluminum powder. React at 90℃ for 6 hours to obtain alumina sol. Concentrate the alumina sol at 50℃ to a solid content of 23wt%, then age it at room temperature (25℃) for 24 hours to obtain aged alumina sol. Add 10% (by weight) of polyethylene oxide (spinning aid) to the aged alumina sol and mix thoroughly to obtain a spinning solution with a viscosity of 1000 mPa·s. Add the prepared spinning solution to an electro-electro-spinning apparatus for electrostatic assisted high-pressure air spinning. The process conditions for electrostatic assisted high-pressure air spinning are: spinning solution ejection rate 10mL / h, air flow rate 25L / min, spinning voltage 20kV, and spinning distance 15cm. Collect the alumina ultrafine fiber precursor.

[0064] The prepared alumina ultrafine fiber precursor was heated from room temperature to 550°C at a heating rate of 1°C / min and held at that temperature for 1.5 hours (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1500°C for 20 seconds (second stage calcination treatment), and then cooled to room temperature in air to obtain alumina ultrafine fibers. The prepared alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 4000 rpm to obtain alumina whiskers.

[0065] The obtained alumina whiskers had a diameter of 1.0 μm and a length of 50 μm, with a smooth surface and no defects such as cracks. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation showed that the alumina whiskers could be used at 1200℃.

[0066] Comparative Example 1 500g of deionized water was added to a 1L beaker, followed by 30.0g of aluminum chloride hexahydrate and 13g of aluminum powder. The mixture was reacted at 80℃ for 7 hours to obtain alumina sol. The alumina sol was concentrated at 60℃ to a solid content of 22wt%, and then aged at room temperature (25℃) for 48 hours to obtain aged alumina sol. Polyvinylpyrrolidone (spinning aid) at 5% of the mass of the aged alumina sol was added to the aged alumina sol, and after thorough mixing, a spinning solution with a viscosity of 500 mPa·s was obtained. The prepared spinning solution was added to an electro-electro-spinning apparatus for electrostatic assisted high-pressure air spinning. The electrostatic assisted high-pressure air spinning process conditions were: spinning solution ejection rate 15mL / h, air flow rate 10L / min, spinning voltage 0kV, and spinning distance 30cm. Alumina ultrafine fiber precursors were collected.

[0067] The prepared alumina ultrafine fiber precursor was heated from room temperature to 500°C at a heating rate of 1°C / min and held at that temperature for 3 hours (first stage calcination treatment), then cooled to room temperature; subsequently, it was flash-calcined at 1400°C for 30 seconds (second stage calcination treatment) and cooled to room temperature in air to obtain alumina ultrafine fibers. The obtained alumina ultrafine fibers were then subjected to high-speed stirring at a speed of 4000 rpm to obtain alumina whiskers.

[0068] The obtained alumina whiskers had a diameter of 5 μm and a length of 120 μm, with a smooth surface and no defects such as cracks. Thermogravimetric-differential thermal analysis and post-high-temperature SEM observation showed that the alumina whiskers could be used at 1400℃.

[0069] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: The diameter and length of alumina whiskers can be precisely controlled. The embodiments of this application enable flexible control of alumina whisker dimensions within a diameter range of 0.2 micrometers to 1 micrometer and a length range of 5 micrometers to 50 micrometers, meeting the requirements of different application scenarios regarding whisker aspect ratio.

[0070] The raw materials are inexpensive and environmentally friendly. This invention uses metallic aluminum and aluminum chloride hexahydrate as raw materials, eliminating the need for expensive organoaluminum sources or precious metal catalysts. The reaction of metallic aluminum with aluminum chloride hexahydrate produces alumina sol, with few byproducts. The entire preparation process does not introduce toxic or harmful substances, meeting green and environmentally friendly requirements.

[0071] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for preparing alumina whiskers, characterized in that, The method includes: Alumina sol is obtained by mixing and reacting metallic aluminum and aluminum chloride hexahydrate. The alumina sol was sequentially concentrated and aged to obtain aged alumina sol. The aged alumina sol is mixed with a spinning aid to obtain a spinning solution; The spinning solution was subjected to electrostatic assisted high-pressure air spinning to obtain alumina ultrafine fiber precursor. The alumina microfiber precursor is subjected to a first-stage calcination treatment and a second-stage calcination treatment to obtain alumina microfiber. The alumina ultrafine fibers were subjected to high-speed stirring at a rotation speed of 3000 rpm to 5000 rpm to obtain alumina whiskers.

2. The method according to claim 1, characterized in that, The step of sequentially concentrating and aging the alumina sol to obtain aged alumina sol includes: The alumina sol was concentrated to a solid content of 20wt%~25wt% at 40℃~70℃. The concentrated alumina sol was aged at 15°C to 30°C for 24 to 48 hours to obtain aged alumina sol.

3. The method according to claim 1, characterized in that, The heating rate of the first calcination stage is 0.5℃ / min to 2℃ / min, the temperature of the first calcination stage is 500℃ to 800℃, and the calcination time of the first stage is 1 hour to 3 hours.

4. The method according to claim 1, characterized in that, The temperature of the second calcination treatment is 1400~1600℃, and the time of the second calcination treatment is 10 seconds~30 seconds.

5. The method according to claim 1, characterized in that, The temperature of the mixing reaction is 70℃~90℃, and the time of the mixing reaction is 5 hours~10 hours.

6. The method according to claim 1, characterized in that, The molar ratio of metallic aluminum to aluminum chloride hexahydrate is 3 to 5.

7. The method according to claim 1, characterized in that, The spinning aid is at least one of polyethylene oxide, polyvinyl alcohol, and polyvinylpyrrolidone.

8. The method according to claim 1, characterized in that, The mass of the spinning aid is 5% to 20% of the mass of the aged alumina sol; the viscosity of the spinning solution is 300 mPa·s to 2000 mPa·s.

9. The method according to claim 1, characterized in that, The electrostatic assisted high-pressure air spinning meets the following requirements: spinning solution ejection rate of 10mL / h~20mL / h, air flow rate of 10L / min~30L / min, spinning voltage of 10kV~20kV, and spinning distance of 15cm~30cm.

10. The method according to claim 1, characterized in that, The alumina whiskers meet the following requirements: diameter of 0.2μm~1μm, length of 5μm~50μm, and operating temperature of 1200℃~1400℃.

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