Equiaxed alpha-alumina and method for producing the same

Abstract

The application provides alpha-alumina in an isometric shape and a preparation method thereof, and aluminum isopropyl alcohol is hydrolyzed by dropwise adding water in an organic solvent; an organic acid is added in a hydrolysis process to adjust the system to be weakly acidic; a surface-modified double-particle-size alpha-Al2O3 seed mixture is added; aging is performed after the hydrolysis is completed, a core-shell composite precursor is obtained; the core-shell composite precursor is calcined, airflow crushing is performed after cooling, and alpha-alumina in an isometric shape is obtained. The synergistic effect of three mechanisms of "precursor crystal type regulation + double-particle-size seed cooperation + in-situ co-precipitation core-shell structure" systematically solves the technical problems of "high temperature, morphology loss of control, and induced unevenness" in the preparation of alpha-Al2O3 by an alcohol salt hydrolysis method for a long time; and alpha-Al2O3 powder with high alpha conversion rate, isometric crystal grains and 99.999% purity is prepared.

Description

Technical Field

[0001] This application belongs to the field of alumina and its preparation technology, and particularly relates to an equiaxed α-alumina and its preparation method. Background Technology

[0002] α-Al₂O₃ powder possesses advantages such as high melting point, corrosion resistance, good chemical stability, and excellent insulation properties, making it widely used in the manufacture of high-performance ceramic products such as structural ceramics and electronic ceramics. High-purity α-Al₂O₃ powder with a purity of over 99% can meet the purity requirements of various high-end application fields, such as semiconductors and optics, and has broad application prospects and significant market potential. Currently, methods such as sol-gel, precipitation, hydrothermal, and vapor deposition can prepare α-Al₂O₃ powder with high purity and relatively uniform particle size; however, these methods are costly and complex, making them unsuitable for large-scale industrial production. Industrially, calcination is commonly used to prepare α-Al₂O₃ powder. Appropriate calcination temperature and time ensure the precursor is fully converted to α-Al₂O₃. However, calcined α-Al₂O₃ powder is prone to particle growth or agglomeration, reducing powder uniformity. The conversion of other alumina crystal forms to α-Al₂O₃ is generally accompanied by the merging and growth of nanocrystals into worm-like α-Al₂O₃ grains. Furthermore, the Bayer process for industrial aluminum hydroxide typically introduces sodium-containing additives during production, resulting in a finished product containing 0.2%-0.5% Na₂O impurities. Sodium reacts with α-Al₂O₃ at high temperatures to form β-Al₂O₃ (Na₂O•11Al₂O₃) or sodium aluminate, severely degrading the high-temperature insulation and dielectric properties of the product in the electronic ceramics field. Traditional sodium removal methods include acid washing pretreatment or high-temperature mineralizing agent reaction for sodium removal. However, the former only removes surface-adsorbed sodium, while the latter's sodium removal efficiency is unstable due to the type of mineralizing agent and the feeding method. How to efficiently remove sodium impurities while ensuring complete α-phase transformation and fine-grained structure has long been a difficult problem that has troubled those skilled in the art.

[0003] Alkyl salt hydrolysis is one of the important technical routes for preparing high-purity ultrafine α-Al₂O₃. This method involves reacting high-purity metallic aluminum with an alcohol to generate a metal alkoxide, which is then hydrolyzed to obtain a hydrated alumina precursor, followed by calcination to obtain alumina powder. The alumina purity obtained by organic alkyl salt hydrolysis can reach 99.999%, far exceeding the purity of alumina prepared by other methods. Compared with the traditional Bayer process, alkyl salt hydrolysis eliminates the need for sodium-containing additives, thus avoiding the introduction of sodium ions at the source; compared with inorganic aluminum salt precipitation, it avoids the problem of residual harmful anions such as sulfate and chloride ions. As can be seen from the reaction equation, the alkyl salt hydrolysis reaction produces almost no harmful gases, the raw materials are relatively environmentally friendly, and the alcohol used in the reaction can be recycled. Therefore, alkyl salt hydrolysis has natural advantages in the preparation of high-purity alumina.

[0004] However, existing methods for preparing α-Al₂O₃ via alkoxide hydrolysis still face the following technical challenges: the transformation from the transition phase θ-Al₂O₃ to the thermodynamically stable α-Al₂O₃ typically requires temperatures above 1100℃, resulting in high energy consumption; and current technologies struggle to maintain a stable phase transition temperature at lower levels. Regarding seed crystal addition, Chinese patent CN100443409C discloses a process for preparing α-Al₂O₃ using aluminum alkoxide hydrolysis: carboxylic acid or surfactant is added during hydrolysis; α-Al₂O₃ seed crystals and an ammonium salt additive are added during ball milling after hydrolysis; and the product is then obtained through drying and calcination. This patent adds the seed crystals during the ball milling stage after hydrolysis, which is a physical mixing method. The seed crystals and precursor only have physical contact rather than chemical bonding, resulting in poor seed crystal distribution uniformity, low interfacial bonding strength, and an induction effect that cannot be maximized. Chinese patent CN113620328B discloses a method of adding a supernatant containing nano-alumina seeds to aluminum alkoxide to obtain a hydrolysis reaction solution, followed by high-pressure hydrothermal treatment (160-220℃, 12-24 hours), drying, and calcination to produce high-purity alumina nanocrystals. This method adds the seeds before the start of the hydrolysis reaction, which is a pre-mixing method. However, this method requires high-pressure hydrothermal treatment, which demands sophisticated equipment, involves complex processes, and has a long cycle, making it unsuitable for industrial production. In the aforementioned prior art, the bonding between the seeds and the matrix is ​​physical contact rather than chemical bonding. The seed distribution in the raw material system is uneven, resulting in areas without seeds requiring high-temperature, long-term transformation. Some areas are seed-rich, while others lack seeds, leading to poor uniformity in the final product's grain size. Furthermore, existing alkoxide hydrolysis techniques lack systematic research on the selective generation of precursor crystal forms during hydrolysis. Different crystal forms of hydrated alumina precursors (boehmite, gibbsite, gibbsite, etc.) exhibit different kinetic pathways and phase transition temperatures in their transformation to α-Al₂O₃, but current techniques have not yet achieved selective generation of precursor crystal forms through precise control of hydrolysis conditions. Simultaneously, α-Al₂O₃ seeds are typically used directly in existing techniques without surface modification. These seeds have high surface energy and are prone to agglomeration, making it difficult to achieve a monodisperse state during precursor addition. A large number of seeds fail to exert an effective inductive effect due to agglomeration. Additionally, α-Al₂O₃ prepared by existing alkoxide hydrolysis methods easily forms an irregular "worm-like" microstructure during high-temperature calcination, exhibiting severe anisotropic grain growth and a wide grain size distribution. Although the addition of single-size seeds can reduce the phase transition temperature to some extent, the control over the final grain morphology is limited.

[0005] In summary, there is an urgent need to develop a method for preparing α-Al₂O₃ powder that enables selective generation of precursor crystal forms, in-situ uniform introduction of seed crystals during the reaction process to form chemical bonding interfaces, and synergistic control of dual-size seed crystals. This method would address the technical challenges of high conversion temperatures, uncontrollable grain morphology, low seed induction efficiency, and poor product uniformity. Summary of the Invention This application provides an equiaxed α-alumina and its preparation method to solve the problems existing in related technologies. The technical solution is as follows: In a first aspect, embodiments of this application provide a method for preparing equiaxed α-alumina, comprising the following steps: Hydrolysis is carried out by adding water dropwise to aluminum isopropoxide solution; during the hydrolysis process, an organic acid is added to adjust the system to be weakly acidic; the solvent in the aluminum isopropoxide solution is an organic solvent; During the hydrolysis reaction, a mixture of surface-modified α-Al2O3 seed crystals with two particle sizes is added; hydrolysis continues. After hydrolysis, aging was performed to obtain a core-shell composite precursor. The core-shell composite precursor was calcined, cooled, and then pulverized by airflow to obtain equiaxed α-alumina.

[0006] In one embodiment, the surface-modified dual-size α-Al2O3 seed mixture is prepared by modifying a first-size α-Al2O3 seed and a second-size α-Al2O3 seed with a mass ratio of (1-3):1 using a silane coupling agent; the first size is 10-30 nm; and the second size is 30-50 nm.

[0007] In one implementation, the modification method is as follows: The first and second α-Al2O3 seed crystals were dispersed in a mixed solvent of ethanol and water, and an organosilane coupling agent was added and stirred to obtain a surface-modified mixture of two seed crystals. The mass of the silane coupling agent is 1%-10% of the total mass of the first-size α-Al₂O₃ seed crystals and the second-size α-Al₂O₃ seed crystals; The volume ratio of ethanol to water is 1:(0.1-10); the reaction temperature is 40-80℃, and the reaction time is 2-6h.

[0008] In one embodiment, aluminum isopropoxide is calcined at 900-1000℃ for 1-3 hours to prepare α-Al2O3 seed crystals with the first particle size. Aluminum isopropoxide was calcined at 1100-1150℃ for 2-3 hours to prepare α-Al2O3 seed crystals with the second particle size.

[0009] In one embodiment, the solvent in the aluminum isopropoxide solution is a mixed organic solvent of isopropanol and n-hexane in a volume ratio of (1-3):1; the mass concentration of the aluminum isopropoxide solution is 0.05-0.5 mol / L; the hydrolysis reaction temperature is 20-40℃; and the molar amount of water added is 2-6 times the molar amount of aluminum isopropoxide.

[0010] In one embodiment, the organic acid is citric acid and / or tartaric acid; the amount of organic acid added is 0.1-2% of the mass of aluminum isopropoxide; and the pH of the system is adjusted to 5.2-5.8.

[0011] In one embodiment, when the hydrolysis reaction reaches a conversion rate of 30%-70%, a surface-modified mixture of α-Al2O3 seed crystals with two particle sizes is added. The mass of the surface-modified, dual-particle-size α-Al₂O₃ seed mixture added is 1-8% of the mass of aluminum isopropoxide; The surface-modified dual-size α-Al2O3 seed mixture was added by spraying: the surface-modified dual-size α-Al2O3 seed mixture was dispersed in ethanol, ultrasonically prepared into a seed suspension, and added to the hydrolysis reaction system by spraying.

[0012] In one embodiment, the aging temperature is 40-60°C, and the aging time is 2-12 hours; Calcination is carried out in an air or inert gas atmosphere; the calcination temperature is 850-950℃, and the calcination time is 1-5h; preferably, the temperature is increased to the calcination temperature at a heating rate of 2-10℃ / min.

[0013] Secondly, embodiments of this application provide an equiaxed α-alumina, which is prepared by the above-described method for preparing equiaxed α-alumina.

[0014] In one embodiment, the grain morphology is equiaxed; D50 is 0.18-0.28 μm; and the α phase conversion rate is 98.5-99.2%.

[0015] The advantages or beneficial effects of the above technical solutions include at least the following: The isometric α-alumina and its preparation method of this application employ a synergistic hydrolysis system of mixed organic solvent and organic acid during the preparation of the precursor by alkoxide hydrolysis. This system directionally generates a highly active precursor with abundant lattice defects and a high specific surface area. Combined with the synergistic induction function of dual-size seed crystals, the constructed precursor has a core-shell composite structure, which greatly enhances the seed dispersibility, morphology control, and induction ability. The final morphology of the grains is precisely locked, taking into account both low-temperature nucleation and morphology control, to form α-alumina with high purity and controllable morphology.

[0016] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of this application will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0017] In the accompanying drawings, unless otherwise specified, the same reference numerals throughout the various drawings denote the same or similar parts or elements. These drawings are not necessarily drawn to scale. It should be understood that these drawings depict only some embodiments disclosed in this application and should not be construed as limiting the scope of this application.

[0018] Figure 1 Electron micrograph of equiaxed α-alumina prepared in Example 5; Figure 2 Electron micrograph of the alumina prepared in Comparative Example 1; Figure 3 The image shows an electron microscope image of the alumina prepared in Comparative Example 2. Figure 4 The image shows an electron microscope image of the alumina prepared in Comparative Example 3. Figure 5 Electron micrograph of the alumina prepared in Comparative Example 4; Figure 6 Electron micrograph of the alumina prepared in Comparative Example 6; Figure 7 The image shows an electron microscope image of the alumina prepared in Comparative Example 7. Detailed Implementation

[0019] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of this application. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0020] This application provides a method for preparing equiaxed α-alumina, comprising the following steps: Hydrolysis is carried out by adding water dropwise to aluminum isopropoxide solution; during the hydrolysis process, an organic acid is added to adjust the system to be weakly acidic; the solvent in the aluminum isopropoxide solution is an organic solvent; During the hydrolysis reaction, a mixture of surface-modified α-Al2O3 seed crystals with two particle sizes is added; hydrolysis continues. After hydrolysis, aging was performed to obtain a core-shell composite precursor. The core-shell composite precursor was calcined, cooled, and then pulverized by airflow to obtain equiaxed α-alumina.

[0021] In the preparation of alumina by hydrolysis of aluminum isopropoxide, this application precisely controls the hydrolysis system to be weakly acidic and introduces a synergistic system of organic solvent and organic acid; it directionally generates highly active boehmite with abundant lattice defects and high specific surface area; it uses surface-modified dual-size seed crystals to form a chemically bonded core-shell structure through in-situ deposition, which effectively improves the dispersibility and induction efficiency of the seed crystals and precisely locks the final morphology of the grains.

[0022] In one embodiment, aluminum isopropoxide is prepared by distillation after reacting aluminum in anhydrous isopropanol. Preferably, the purity of aluminum isopropoxide is 99.9995%.

[0023] In this embodiment, aluminum powder and anhydrous isopropanol are selected as raw materials, reacted under an inert atmosphere, and then purified by vacuum distillation under pressure. The middle fraction is collected to obtain high-purity aluminum isopropoxide with a purity ≥99.999%. Preferably, the purity of the aluminum powder is ≥99.99%; the reaction temperature is 60-80℃, the reaction time is 5-8h; and the vacuum distillation conditions are 230-250℃ and 1330-1340Pa, with the middle fraction collected.

[0024] In one embodiment, the solvent in the aluminum isopropoxide solution is a mixed organic solvent of isopropanol and n-hexane in a volume ratio of (1-3):1; the mass concentration of the aluminum isopropoxide solution is 0.05-0.5 mol / L. The hydrolysis reaction temperature is 20-40℃; the molar amount of water added is 2-6 times the molar amount of aluminum isopropoxide.

[0025] In one implementation method, the organic acid is citric acid and / or tartaric acid; the amount of organic acid added is 0.1-2% of the mass of aluminum isopropoxide; and the pH of the system is adjusted to 5.2-5.8.

[0026] Existing methods for preparing boehmite by alkoxide hydrolysis are mostly carried out under alkaline conditions (pH>8.5), yielding boehmite with a high α-transformation temperature (α-phase transformation temperature as high as 1350℃ or higher). This application precisely controls the hydrolysis system in a weakly acidic environment of pH=5.2-5.8 and introduces a synergistic system of mixed organic solvents (isopropanol and n-hexane) and organic acids (citric acid and / or tartaric acid). The carboxyl groups in the organic acids selectively adsorb onto specific crystal faces of boehmite through complexation, which not only regulates the precursor crystal morphology but also lowers the lattice energy barrier, ultimately resulting in highly active boehmite with abundant lattice defects and a high specific surface area, laying the foundation for subsequent low-temperature α-phase transformation at 850-950℃.

[0027] This application also uses a surface-modified dual-size α-Al2O3 seed mixture as seed crystals. In one embodiment, the surface-modified dual-size α-Al2O3 seed mixture is prepared by modifying a first-size α-Al2O3 seed crystal and a second-size α-Al2O3 seed crystal with a mass ratio of (1-3):1 using a silane coupling agent; the first particle size is 10-30 nm; and the second particle size is 30-50 nm.

[0028] Single-size seed crystals result in irregular, worm-like grain morphologies. Based on the theoretical discovery that the θ→α phase transition involves a critical crystal size (≈22nm) and a basic crystal size (≈50nm), this application innovatively introduces both 10-30nm small-size seed crystals and 30-50nm large-size seed crystals into the system simultaneously. The small-size seed crystals precisely match the critical nucleation size for the θ→α phase transition, preferentially inducing heterogeneous nucleation of the α phase at low temperatures, significantly reducing the phase transition temperature. The large-size seed crystals match the basic crystal size at the completion of the α phase transition, serving as templates for solid-phase epitaxial growth and precisely locking the final grain morphology. This overcomes the technical bottleneck of traditional single-size seed crystals, which struggle to simultaneously achieve low-temperature nucleation and morphology control.

[0029] In this embodiment, the modification method is as follows: The first and second α-Al2O3 seed crystals were dispersed in a mixed solvent of ethanol and water, and an organosilane coupling agent was added and stirred to obtain a surface-modified mixture of two seed crystals. The mass of the silane coupling agent is 1%-10% of the total mass of the first-size α-Al₂O₃ seed crystals and the second-size α-Al₂O₃ seed crystals; The volume ratio of ethanol to water is 1:(0.1-10); the reaction temperature is 40-80℃, and the reaction time is 2-6h.

[0030] In one embodiment, after the reaction, solid-liquid separation is performed, followed by washing and drying to obtain a surface-modified dual-size seed crystal mixture. In this embodiment, centrifugation is used, with centrifugation at 8000-10000 rpm for 8-15 min; the precipitate is washed 2-4 times with anhydrous ethanol and then vacuum dried at 50-70℃ for 4-10 h.

[0031] Furthermore, traditional α-Al₂O₃ seeds are prone to agglomeration due to their high surface energy, leading to a decrease in induction efficiency. This application introduces a silane coupling agent to modify the surface function of a mixture of seeds with different particle sizes. The silane coupling agent contains both hydrolyzable alkoxy groups and organic functional groups (such as amino groups). On one hand, the silanol groups generated by hydrolysis condense with Al-OH groups on the seed surface to form a covalently bonded graft layer, creating steric hindrance to prevent seed agglomeration. On the other hand, the terminal active amino groups can react with Al in the hydrolysis system... 3+Alternatively, a hydroxyaluminum complex can form a coordination bond, inducing hydrated alumina to preferentially nucleate and grow on the seed crystal surface. This chemically enhances the interfacial bonding between the seed crystal and the precursor, effectively improving the seed crystal's dispersibility and induction efficiency. Preferably, the silane coupling agent is γ-aminopropyltriethoxysilane (KH-550).

[0032] In this embodiment, aluminum isopropoxide is calcined at 900-1000℃ for 1-3 hours to prepare α-Al2O3 seed crystals with the first particle size; Aluminum isopropoxide was calcined at 1100-1150℃ for 2-3 hours to prepare α-Al2O3 seed crystals with the second particle size.

[0033] As one implementation method, when the hydrolysis reaction reaches a conversion rate of 30%-70%, a mixture of surface-modified dual-size α-Al2O3 seed crystals is added.

[0034] In the preparation of α-Al₂O₃ by alkoxide hydrolysis, the post-mixing method of adding seed crystals after hydrolysis results in uneven seed dispersion, low interfacial bonding strength, and decreased induction efficiency. Therefore, this application adds a surface-modified seed crystal suspension to the reaction system during the hydrolysis reaction. Utilizing the abundant aluminum hydroxyl oligomers in the middle of hydrolysis, hydrated alumina is deposited in situ on the seed crystal surface to form a chemically bonded core-shell structure. This solves the process problems of seed crystal agglomeration and uneven distribution, and significantly enhances the seed induction and morphology control capabilities.

[0035] Preferably, the seed crystals are added when the hydrolysis reaction reaches a conversion rate of 30%-70%, allowing for dynamic in-situ introduction to construct the core-shell structure. More preferably, they are dynamically added to the reaction system via spraying, resulting in better and more uniform dispersion to form the core-shell structure. The surface-modified dual-particle-size α-Al₂O₃ seed mixture is added via spraying: the surface-modified dual-particle-size α-Al₂O₃ seed mixture is dispersed in ethanol, ultrasonically prepared into a seed suspension, and then added to the hydrolysis reaction system via spraying. The mass concentration of the seed suspension is 0.05-0.5 g / mL.

[0036] In one embodiment, the mass of the surface-modified dual-size α-Al2O3 seed mixture added is 1-8% of the mass of aluminum isopropoxide; As one implementation method, the aging temperature is 40-60℃ and the aging time is 2-12h.

[0037] As one implementation method, after aging, solid-liquid separation is performed, followed by washing with deionized water 2-4 times, then washing with anhydrous ethanol 1-3 times, and vacuum drying at 40-70℃ for 6-24 hours.

[0038] In one embodiment, calcination is carried out in an air or inert gas atmosphere; the calcination temperature is 850-950℃, and the calcination time is 1-5 hours; preferably, the temperature is increased to the calcination temperature at a heating rate of 2-10℃ / min. The calcination temperature of this application can be reduced to 850-950℃.

[0039] This application also provides an equiaxed α-alumina, which is prepared by the above-described method for preparing equiaxed α-alumina.

[0040] In one embodiment, the grain morphology is equiaxed; D50 is 0.18-0.28 μm; and the α phase conversion rate is 98.5-99.2%.

[0041] The following is a further explanation using specific embodiments.

[0042] Example 1 Take 200g of 99.99% pure aluminum powder and 1500mL of anhydrous isopropanol and add them to a three-necked flask. Refrigerate at 70℃ for 6h under nitrogen protection to produce aluminum isopropoxide. After the reaction was completed, the reaction solution was transferred to a vacuum distillation apparatus and purified by vacuum distillation at 240℃ and 1330Pa. The middle fraction was collected to obtain aluminum isopropoxide with a purity of 99.9995%.

[0043] Example 2 High-purity aluminum isopropoxide was calcined at 950℃ and 1100℃ for 2 h to obtain α-Al2O3 seed crystals with particle sizes of 20 nm and 40 nm, respectively. Two types of seed crystals with particle sizes of 20 nm and 40 nm were mixed at a mass ratio of 2:1. 5 g of the mixed seed crystals were dispersed in 100 mL of ethanol / water (volume ratio 1:1) mixed solvent, and 0.5 g of KH-550 silane coupling agent was added. The mixture was stirred in a water bath at 60 °C for 4 h. After the reaction, the mixture was centrifuged at 9000 rpm for 10 min, the precipitate was washed three times with anhydrous ethanol, and then dried under vacuum at 60 °C for 6 h to obtain a surface-modified mixture of two-size seed crystals. Disperse 5g of the modified seed mixture in 50mL of anhydrous ethanol and sonicate for 15min to prepare a seed suspension.

[0044] Example 3 High-purity aluminum isopropoxide was calcined at 950℃ for 1 h and at 1100℃ for 1 h to obtain α-Al2O3 seed crystals with particle sizes of 10 nm and 30 nm, respectively. Two types of seed crystals with particle sizes of 10 nm and 30 nm were mixed at a mass ratio of 1:1. 5 g of the mixed seed crystals were dispersed in 100 mL of ethanol / water (volume ratio 1:1) mixed solvent, and 0.5 g of KH-550 silane coupling agent (seed crystal to silane coupling agent mass ratio 50:1) was added. The mixture was stirred and reacted in a 60 °C water bath for 4 h. After the reaction, the mixture was centrifuged at 9000 rpm for 10 min, the precipitate was washed three times with anhydrous ethanol, and then dried under vacuum at 60 °C for 6 h to obtain a surface-modified mixture of two-size seed crystals. Disperse 5g of the modified seed mixture in 50mL of anhydrous ethanol and sonicate for 15min to prepare a seed suspension.

[0045] Example 4 High-purity aluminum isopropoxide was calcined at 1000℃ for 3 h and at 1150℃ for 3 h to obtain α-Al2O3 seed crystals with particle sizes of 30 nm and 50 nm, respectively. Two types of seed crystals with particle sizes of 30 nm and 50 nm were mixed at a mass ratio of 2:1. 5 g of the mixed seed crystals were dispersed in 100 mL of ethanol / water (volume ratio 1:1) mixed solvent, and 0.5 g of KH-550 silane coupling agent (seed crystal to silane coupling agent mass ratio 10:1) was added. The mixture was stirred and reacted in a 60 °C water bath for 4 h. After the reaction, the mixture was centrifuged at 9000 rpm for 10 min, the precipitate was washed three times with anhydrous ethanol, and then dried under vacuum at 60 °C for 6 h to obtain a surface-modified mixture of two-size seed crystals. Disperse 5g of the modified seed mixture in 50mL of anhydrous ethanol and sonicate for 15min to prepare a seed suspension.

[0046] Example 5 Dissolve 100g of aluminum isopropoxide in a mixed solvent of isopropanol and n-hexane in a volume ratio of 2:1 to prepare a solution with a concentration of 0.3mol / L, and cool it to 30℃. Hydrolysis was carried out by slowly adding deionized water dropwise under stirring conditions, controlling the molar ratio of water to alkoxide to be 4:1. During the dropwise addition, citric acid (1% of the mass of aluminum isopropoxide) was added to adjust the pH of the system to 5.5. When the hydrolysis reaction has been going on for 20 minutes (approximately 50% conversion), the seed crystal suspension from Example 2 is uniformly sprayed into the reaction system through a sprayer while continuously stirring. During the spraying process, the pH of the system is maintained at 5.5 and the temperature at 30°C. After spraying, continue stirring for 30 minutes, then heat to 50℃ and age for 8 hours; filter, wash 3 times with deionized water, then wash 2 times with anhydrous ethanol, and vacuum dry at 60℃ for 12 hours to obtain the core-shell composite precursor. The core-shell composite precursor was placed in an alumina crucible, placed in a muffle furnace, heated to 900°C at a heating rate of 5°C / min, held for 3 hours, and then cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain equiaxed α-Al₂O₃ powder. Electron microscopy and particle size distribution are shown below. Figure 1 As shown.

[0047] Example 6 100g of aluminum isopropoxide was dissolved in a mixed solvent of isopropanol and n-hexane in a volume ratio of 1:1 to prepare a solution with a concentration of 0.3mol / L, and then cooled to 30°C. Hydrolysis was carried out by slowly adding deionized water dropwise under stirring conditions, controlling the molar ratio of water to alkoxide at 4:1. Tartaric acid (0.5% of the mass of aluminum isopropoxide) was added during the dropwise addition to adjust the pH of the system to 5.2. When the above hydrolysis reaction has been going on for 30 minutes (approximately 70% conversion), the seed crystal suspension of Example 3 is uniformly sprayed into the reaction system through a sprayer under continuous stirring. During the spraying process, the pH of the system is maintained at 5.5 and the temperature at 30°C. After spraying, continue stirring for 30 minutes, then heat to 50℃ and age for 3 hours; filter, wash three times with deionized water and anhydrous ethanol respectively, and vacuum dry at 60℃ for 12 hours to obtain the core-shell composite precursor; The core-shell composite precursor was placed in an alumina crucible, placed in a muffle furnace, heated to 850°C at a heating rate of 2°C / min, held for 5 hours, and then cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain equiaxed α-Al2O3 powder.

[0048] Example 7 100g of aluminum isopropoxide was dissolved in a mixed solvent of isopropanol and n-hexane in a volume ratio of 3:1 to prepare a solution with a concentration of 0.3mol / L, and then cooled to 30°C. Hydrolysis was carried out by slowly adding deionized water under stirring conditions, controlling the molar ratio of water to alkoxide to be 4:1. During the addition process, an equal mass mixture of citric acid and tartaric acid (2% of the mass of aluminum isopropoxide) was added to adjust the pH of the system to 5.8. After the addition was complete, the mixture was aged in a 60°C water bath for 2 hours, then filtered, washed three times with deionized water, then washed twice with anhydrous ethanol, and vacuum dried at 60°C for 12 hours to obtain boehmite precursor powder. When the above hydrolysis reaction has been going on for 10 minutes (about 30% conversion), the seed crystal suspension of Example 4 is sprayed evenly into the reaction system through a sprayer under continuous stirring. During the spraying process, the pH of the system is maintained at 5.5 and the temperature at 30°C. After spraying, continue stirring for 30 minutes, then heat to 50℃ and age for 3 hours; After aging, the product was filtered, washed three times with deionized water and anhydrous ethanol, and then dried under vacuum at 60°C for 12 hours to obtain the core-shell composite precursor. The core-shell composite precursor was placed in an alumina crucible and then placed in a tube furnace. The calcination atmosphere was nitrogen. The temperature was increased to 950°C at a rate of 10°C / min and held for 1 hour. The furnace was then cooled to room temperature to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain equiaxed α-Al2O3 powder.

[0049] Comparative Example 1 The difference between Comparative Example 1 and Example 5 is that there is no pH adjustment and no mixed solvent or organic acid is used. Specifically: Dissolve 100g of aluminum isopropoxide in a mixed solvent of isopropanol and n-hexane in a volume ratio of 2:1 to prepare a solution with a concentration of 0.3mol / L, and cool it to 30℃. Hydrolysis was carried out by slowly adding deionized water dropwise under stirring conditions, with the molar ratio of water to alkoxide controlled at 4:1 and the pH of the system at 7.5. When the hydrolysis reaction has been going on for 20 minutes (approximately 50% conversion), the seed crystal suspension from Example 2 is uniformly sprayed into the reaction system through a sprayer under continuous stirring at a temperature of 30°C. After spraying, continue stirring for 30 minutes, then heat to 50℃ and age for 8 hours; filter, wash 3 times with deionized water, then wash 2 times with anhydrous ethanol, and dry under vacuum at 60℃ for 12 hours to obtain the precursor. The precursor was placed in a corundum crucible, placed in a muffle furnace, heated to 900°C at a heating rate of 5°C / min, held for 3 hours, and then cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain α-Al₂O₃ powder. Electron microscopy results are shown below. Figure 2 As shown.

[0050] Comparative Example 2 The difference between Comparative Example 2 and Example 5 is that only small-particle-size α-Al₂O₃ seeds with a particle size of 20 nm were used. Specifically: High-purity aluminum isopropoxide was calcined at 950℃ for 2 hours to obtain α-Al2O3 seed crystals with a particle size of 20 nm; Take 5g of seed crystals and disperse them in 100mL of ethanol / water (volume ratio 1:1) mixed solvent, add 0.5g of KH-550 silane coupling agent, and stir the reaction in a 60℃ water bath for 4h. After the reaction, the mixture was centrifuged at 9000 rpm for 10 min, the precipitate was washed three times with anhydrous ethanol, and then dried under vacuum at 60 °C for 6 h to obtain a surface-modified seed crystal mixture. Disperse 5g of the modified seed mixture in 50mL of anhydrous ethanol and sonicate for 15min to prepare a seed suspension.

[0051] Other preparation methods are the same as in Example 5. Electron microscopy is shown below. Figure 3 As shown.

[0052] Comparative Example 3 The difference between Comparative Example 3 and Example 5 is that only large-particle-size α-Al₂O₃ seeds with a particle size of 40 nm were used. Specifically: High-purity aluminum isopropoxide was calcined at 1100℃ for 2 hours to obtain α-Al2O3 seed crystals with a particle size of 40 nm. Take 5g of seed crystals and disperse them in 100mL of ethanol / water (volume ratio 1:1) mixed solvent, add 0.5g of KH-550 silane coupling agent, and stir the reaction in a 60℃ water bath for 4h. After the reaction, the mixture was centrifuged at 9000 rpm for 10 min, the precipitate was washed three times with anhydrous ethanol, and then dried under vacuum at 60 °C for 6 h to obtain a surface-modified seed crystal mixture. Disperse 5g of the modified seed mixture in 50mL of anhydrous ethanol and sonicate for 15min to prepare a seed suspension.

[0053] Other preparation methods are the same as in Example 5. Electron microscopy is shown below. Figure 4 As shown.

[0054] Comparative Example 4 The difference between Comparative Example 4 and Example 5 is that the seed crystal suspension from Example 2 was added before hydrolysis; specifically: Dissolve 100g of aluminum isopropoxide in a mixed solvent of isopropanol and n-hexane in a volume ratio of 2:1 to prepare a solution with a concentration of 0.3mol / L, and cool it to 30℃. The seed crystal suspension of Example 2 was uniformly sprayed into the reaction system through a sprayer under continuous stirring, while maintaining the pH of the system at 5.5 and the temperature at 30°C during the spraying process; Hydrolysis was carried out by slowly adding deionized water dropwise under stirring, controlling the molar ratio of water to alkoxide at 4:1. During the addition, citric acid (1% of the mass of aluminum isopropoxide) was added to adjust the pH of the system to 5.5. The reaction was stirred for another 30 min, and then the temperature was raised to 50 °C for aging for 8 h. The mixture was filtered, washed three times with deionized water, and then washed twice with anhydrous ethanol. The mixture was then vacuum dried at 60 °C for 12 h to obtain the precursor. The precursor was placed in a corundum crucible, placed in a muffle furnace, heated to 900°C at a heating rate of 5°C / min, held for 3 hours, and then cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain α-Al₂O₃ powder. Electron microscopy results are shown below. Figure 5 As shown.

[0055] Comparative Example 5 The difference between Comparative Example 5 and Example 5 is that no seed crystals are added; specifically: Dissolve 100g of aluminum isopropoxide in a mixed solvent of isopropanol and n-hexane in a volume ratio of 2:1 to prepare a solution with a concentration of 0.3mol / L, and cool it to 30℃. Hydrolysis was carried out by slowly adding deionized water dropwise under stirring conditions, controlling the molar ratio of water to alkoxide to be 4:1. During the dropwise addition, citric acid (1% of the mass of aluminum isopropoxide) was added to adjust the pH of the system to 5.5. After the addition was completed, the mixture was aged at 50℃ for 8 hours; filtered, washed 3 times with deionized water, then washed 2 times with anhydrous ethanol, and dried under vacuum at 60℃ for 12 hours to obtain the precursor. The precursor was placed in a corundum crucible, placed in a muffle furnace, heated to 900°C at a heating rate of 5°C / min, held for 3 hours, and then cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain α-Al2O3 powder.

[0056] Comparative Example 6 The difference between Comparative Example 6 and Example 5 is that no seed crystals were added and conventional high-temperature calcination was performed, specifically: Dissolve 100g of aluminum isopropoxide in a mixed solvent of isopropanol and n-hexane in a volume ratio of 2:1 to prepare a solution with a concentration of 0.3mol / L, and cool it to 30℃. Hydrolysis was carried out by slowly adding deionized water dropwise under stirring conditions, controlling the molar ratio of water to alkoxide to be 4:1. During the dropwise addition, citric acid (1% of the mass of aluminum isopropoxide) was added to adjust the pH of the system to 5.5. After the addition was completed, the mixture was aged at 50℃ for 8 hours; filtered, washed 3 times with deionized water, then washed 2 times with anhydrous ethanol, and dried under vacuum at 60℃ for 12 hours to obtain the precursor. The precursor was placed in a corundum crucible, placed in a muffle furnace, heated to 1200℃ at a heating rate of 5℃ / min, held for 3h, and cooled to room temperature with the furnace to obtain α-Al2O3 powder. The calcined product was subjected to air jet milling to obtain α-Al₂O₃ powder. Electron microscopy results are shown below. Figure 6 As shown.

[0057] Comparative Example 7 The difference between Comparative Example 7 and Example 5 is that the seed crystals are not surface modified, specifically: High-purity aluminum isopropoxide was calcined at 950℃ and 1100℃ for 2 h to obtain α-Al2O3 seed crystals with particle sizes of 20 nm and 40 nm, respectively. Two types of seed crystals with particle sizes of 20 nm and 40 nm were mixed at a mass ratio of 2:1. 5 g of the mixed seed crystals were dispersed in 50 mL of anhydrous ethanol and ultrasonically treated for 15 min to prepare a seed crystal suspension. Other preparation methods are the same as in Example 5. Electron microscopy is shown below. Figure 7 As shown.

[0058] The alumina of Examples 5-7 and Comparative Examples 1-7 were subjected to relevant tests, and the results are shown in Table 1.

[0059] Table 1

[0060] Examples 5-7 of this application obtained α-Al2O3 powder with excellent comprehensive performance through precursor crystal form selective control, dual-size seed synergistic induction, and dynamic in-situ co-precipitation of core-shell structure.

[0061] Example 5 showed an α-phase conversion rate of 99.2%, far exceeding the 92-96% of the traditional process, and achieved a near-complete θ→α phase transformation through seed induction at a low temperature of 900℃. The original crystal size was only 0.21 μm, with fine and uniform grains, significantly smaller than the 1.0-2.0 μm of the traditional high-temperature process, proving that the dual-size seed synergistic system effectively suppressed excessive grain growth; indicating fine grains, no severe sintering agglomeration, and good particle dispersion; the grains were equiaxed (e.g., ...). Figure 1 As shown in the figure, the epitaxial template effect of the large-size seed crystal (40nm) effectively suppressed the worm-like structure. This embodiment achieved the best balance between conversion rate, particle size and morphology under medium temperature and time conditions of 900℃ / 3h, which is suitable for application scenarios with the highest requirements for comprehensive performance.

[0062] Example 6 verifies the feasibility of the process at lower temperatures. The α-phase conversion rate in this example is 98.5%, slightly lower than in Example 5, but still above 98%, indicating that even at 850℃, a high degree of phase transformation completion can still be achieved by extending the holding time to 5 hours. The original crystal size is only 0.18 μm, the smallest among the three examples. This is due to the combined effect of the slow atomic diffusion rate and weak grain growth driving force at the lower temperature of 850℃, and the small-sized seed combination (10+30 nm) limiting the upper limit of the grain size. The specific surface area reaches 10.2 m². 2 / g, the highest among the three examples, indicates that the low-temperature calcination at 850℃ effectively suppressed the formation of sintering necks and particle fusion between grains, and more micropores were retained when the precursor removed structural water; the grains were equiaxed, proving that even at a low temperature of 850℃, the epitaxial template effect of the large seed (30nm) was still effective.

[0063] Example 7 verifies the feasibility of the invention under rapid production conditions. The α-phase conversion rate in this example is 99.0%, slightly higher than Example 6 and essentially the same as Example 5, demonstrating that the greater thermodynamic driving force provided by 950℃ can complete the phase transformation within 1 hour, and the holding time is shortened to 1 / 3 of that in Example 5; the primary crystal size is 0.28 μm, the largest among the three examples. This is a combined result of the accelerated atomic diffusion rate and increased grain growth driving force at the higher temperature of 950℃, and the setting of a higher upper limit for the particle size using a large-size seed combination (30+50nm); the specific surface area is 6.8 m². 2 / g, the lowest among the three embodiments, indicates that at the higher temperature of 950℃, sintering necks are easily formed between grains, increasing particle fusion and reducing micropores; the grains are equiaxed, proving that large seed crystals (50nm) can still effectively lock the grain morphology at 950℃, without over-sintering. This embodiment demonstrates the production flexibility of the present invention under "higher temperature, shorter time" conditions. The three embodiments show a clear progressive trend: "lower temperature → finer grains, higher specific surface area, longer holding time; higher temperature → faster phase transformation, coarser grains, lower specific surface area."

[0064] Comparative Example 1 (no pH adjustment, no mixed solvent or organic acid used, pH≈7.5) used a mixture of boehmite and gibbsite, rather than pure boehmite. The α-phase conversion rate of this precursor after calcination at 900℃ for 3h was only 65%, with a large amount of transition phase (θ-Al₂O₃) remaining, far lower than the 99.2% of Example 5. This is because the un-pH-controlled precursor has low reactivity and few lattice defects, making it difficult to complete the phase transformation at low temperatures even with the addition of seed crystals. The grain morphology is irregular (e.g., ...). Figure 2 As shown in the figure, the product is a mixture of θ and α phases. This comparative example demonstrates that precise control of pH = 5.2-5.8 is not only crucial for the selective generation of highly active boehmite precursors, but also directly affects the purity level of the product; without this step, low-temperature phase transition cannot be achieved.

[0065] Comparative Example 2 (single small-diameter seed crystal 20 nm, without dual-diameter synergy) showed an α-phase conversion rate of 98.8%, close to the 99.2% of Example 5, demonstrating that small seed crystals (10-30 nm) cover the critical crystal diameter for the θ→α phase transition (≈22 nm) and can effectively reduce the phase transition temperature when used alone. However, its grains exhibit a distinct worm-like irregular structure (e.g., Figure 3As shown, the grain size distribution is extremely wide (150-400 nm). This is because of the lack of an epitaxial template effect from a large seed crystal (30-50 nm). During the phase transition, the grains lose their morphological constraints and grow freely along the anisotropic direction. This comparative example demonstrates that although a single small seed crystal can lower the phase transition temperature, it cannot control the grain morphology; the synergy of two grain sizes is a necessary condition for achieving equiaxed grains.

[0066] The α-phase conversion rate of Comparative Example 3 (single large-size seed crystal 40 nm, without dual-size synergy) was only 82%, far lower than the 99.2% of Example 1. This is because the nucleation density of the single large seed crystal is low (few seed crystal particles per unit mass), resulting in insufficient heterogeneous nucleation sites and a large amount of θ-Al₂O₃ failing to undergo induced phase transformation and remaining. Its grain morphology is relatively regular (e.g., ...). Figure 4 As shown in the figure, the epitaxial template effect of large seed crystals is still effective, but insufficient nucleation leads to incomplete phase transformation. This comparative example demonstrates that although a single large seed crystal can provide a morphological template, its nucleation density is low and its induction ability at 900℃ is insufficient; small seed crystals are a prerequisite for ensuring high conversion rates.

[0067] The α-phase conversion rate of Comparative Example 4 (with seed crystals added before hydrolysis) was only 72%, a decrease of 27.2 percentage points compared to Example 5. The seed crystals were added to the reaction system before the hydrolysis reaction began. At this time, the alkoxide molecules in the system had not yet hydrolyzed, and there were no aluminum hydroxyl oligomers. Although the seed crystal surface was modified with a silane coupling agent, in an environment where a large number of alkoxide molecules and organic solvent molecules coexisted, the active sites on the seed crystal surface were preferentially occupied by alkoxide or solvent molecules. If the seed crystals remained in the acidic hydrolysis environment for a long time (throughout the entire hydrolysis process), the surface modification layer might undergo hydrolysis and desorption, further losing its inductive activity. When the hydrolysis reaction progressed to the middle and late stages and a large number of aluminum hydroxyl oligomers were generated, the seed crystal surface was already encapsulated by solvent molecules, making it difficult for newly formed aluminum hydroxyl species to effectively deposit on the seed crystal surface. Hydrated alumina mainly nucleates itself in solution, forming independent particles, and its relationship with the seed crystal is only physically mixed, unable to form a chemically bonded core-shell structure. Since the hydrated alumina and the seed crystals only physically mix and do not form a core-shell structure, the seed crystals cannot effectively induce phase transformation and morphology in the surrounding matrix during calcination. In some areas, seed crystals are enriched, inducing the formation of a small amount of α-phase; however, most areas lack effective seed crystals and rely solely on spontaneous nucleation, resulting in irregular morphology (e.g., ...). Figure 5 As shown in the figure, the grain size is not uniform.

[0068] Comparative Example 5 (without seed crystals) showed an α-phase conversion rate of <5% after calcination at 900℃ for 3 hours, with almost all phases being θ-Al₂O₃. This is because at 900℃ without seed crystals, the critical nucleation free energy for the θ→α phase transition is extremely high, making it impossible to overcome the nucleation barrier, and the phase transition hardly occurs. This comparative example demonstrates that seed crystals are the core element of the low-temperature technology of this invention—within the temperature range of 850-950℃, there is no α-phase transition without seed crystals.

[0069] Comparative Example 6 (conventional high-temperature calcination, 1200℃ / 3h, without seed crystals) showed an α-phase transformation rate of 99%, achieving complete phase transformation through high-temperature thermal driving force, comparable to Example 5. However, its original crystals were >1μm and severely agglomerated. At 1200℃, atomic diffusion was intense, leading to rapid grain growth and the formation of strong sintering necks between primary particles, resulting in agglomeration into dense bulk materials (such as...). Figure 6 As shown in the figure, even air jet milling cannot restore the equiaxed dispersion state. This comparative example demonstrates that although traditional high-temperature processes can achieve complete phase transformation, they are energy-intensive and produce coarse agglomerates. The present invention can achieve the same conversion rate at 900℃, with fine grains (0.22μm) and good dispersion, and the calcination temperature is reduced by 300℃, resulting in significant energy savings.

[0070] Comparative Example 7 (unmodified seed surface): Unmodified α-Al₂O₃ seeds, due to their high surface energy, easily aggregated in the suspension. When sprayed into the reaction system, the aggregated seeds could not achieve uniform dispersion, resulting in seed enrichment in some areas and seedlessness in others. Although the enriched areas could form local induction, the stacking of seeds within the agglomerates reduced the effective induction area. In the seedless areas, spontaneous nucleation was necessary to complete the θ→α phase transition, but at a calcination temperature of 900°C, the homogeneous nucleation barrier of α-Al₂O₃ was extremely high, making phase transition almost impossible. Comparing Example 6 (modified seed, conversion rate 99.2%) and Comparative Example 7 (unmodified seed, conversion rate 91%), it can be seen that the unmodified seeds resulted in a reduction in the effective induction area due to agglomeration, and the α-phase conversion rate decreased by 8.2 percentage points, confirming the decisive influence of surface modification on seed dispersion and induction efficiency.

[0071] In regions lacking effective seed induction, α-Al₂O₃ undergoes anisotropic growth, forming a small number of worm-like grains, resulting in decreased product morphology uniformity. The product changes from a purely equiaxed structure in Example 5 to a coexistence of equiaxed and worm-like grains in Comparative Example 7 (e.g.,...). Figure 7 (As shown). Meanwhile, the θ→α phase transition in these regions requires higher thermal driving force or longer grain growth time, resulting in abnormal growth of some grains, with the median grain size increasing from 220 nm in Example 5 to 240 nm in Comparative Example 7.

[0072] In summary, this application systematically solves the three major technical challenges of "high temperature, morphology loss of control, and inhomogeneous induction" that have long existed in the preparation of α-Al2O3 by alkoxide hydrolysis through the synergistic effect of three mechanisms: "precursor crystal form regulation (weak acid conditions, pH 5.2-5.8) + dual-size seed synergy (10-30nm small seeds are responsible for low-temperature nucleation, and 30-50nm large seeds are responsible for morphology locking) + in-situ co-precipitation of core-shell structure (chemical bonding interface)". This application can obtain high-quality products with α-conversion rate ≥98.5%, equiaxed morphology, original crystal ≤0.28μm, and purity ≥99.999% under different temperature-time process windows (850℃ / 5h, 900℃ / 3h, 950℃ / 1h). The comparative examples demonstrate from the opposite perspective that the three core technical features (pH regulation, dual-size synergy, and in-situ co-precipitation) are irreplaceable, and the absence of any one feature leads to a significant decrease in performance.

[0073] This application achieves α-Al2O3 powder with 99.2% conversion rate, 0.22μm equiaxed grains, and 99.999% purity at a low temperature of 900℃. The overall performance is significantly better than that of traditional high-temperature processes (1200℃, >1μm coarse agglomerates). This provides a complete technical solution for the preparation of high-end α-Al2O3 powder by alkoxide hydrolysis, which is low-temperature, high-purity, and has controllable morphology.

[0074] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of those different embodiments or examples.

[0075] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0076] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in this application, and these should all be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing equiaxed α-alumina, characterized in that, Includes the following steps: Hydrolysis is carried out by adding water dropwise to aluminum isopropoxide solution; during the hydrolysis process, an organic acid is added to adjust the system to be weakly acidic; the solvent in the aluminum isopropoxide solution is an organic solvent; During the hydrolysis reaction, a mixture of surface-modified α-Al2O3 seed crystals with two particle sizes is added; hydrolysis continues. After hydrolysis, aging was performed to obtain a core-shell composite precursor. The core-shell composite precursor was calcined, cooled, and then pulverized by airflow to obtain equiaxed α-alumina.

2. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, The surface-modified dual-size α-Al2O3 seed mixture was prepared by modifying the first-size α-Al2O3 seed and the second-size α-Al2O3 seed with a mass ratio of (1-3):1 using a silane coupling agent; the first size was 10-30 nm; and the second size was 30-50 nm.

3. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, The modification method is as follows: The first and second α-Al2O3 seed crystals were dispersed in a mixed solvent of ethanol and water, and an organosilane coupling agent was added and stirred to obtain a surface-modified mixture of two seed crystals. The mass of the silane coupling agent is 1%-10% of the total mass of the first-size α-Al₂O₃ seed crystals and the second-size α-Al₂O₃ seed crystals; The volume ratio of ethanol to water is 1:(0.1-10); the reaction temperature is 40-80℃, and the reaction time is 2-6h.

4. The method for preparing equiaxed α-alumina according to claim 2 or 3, characterized in that, Aluminum isopropoxide was calcined at 900-1000℃ for 1-3 hours to prepare α-Al2O3 seed crystals with the first particle size. Aluminum isopropoxide was calcined at 1100-1150℃ for 2-3 hours to prepare α-Al2O3 seed crystals with the second particle size.

5. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, The solvent in the aluminum isopropoxide solution is a mixed organic solvent of isopropanol and n-hexane in a volume ratio of (1-3):1; the mass concentration of the aluminum isopropoxide solution is 0.05-0.5 mol / L; the hydrolysis reaction temperature is 20-40℃; and the molar amount of water added is 2-6 times the molar amount of aluminum isopropoxide.

6. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, The organic acid is citric acid and / or tartaric acid; the amount of organic acid added is 0.1-2% of the mass of aluminum isopropoxide; the pH of the system is adjusted to 5.2-5.

8.

7. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, When the hydrolysis reaction reaches a conversion rate of 30%-70%, a mixture of surface-modified α-Al2O3 seed crystals with two particle sizes is added. The mass of the surface-modified, dual-particle-size α-Al₂O₃ seed mixture added is 1-8% of the mass of aluminum isopropoxide; The surface-modified dual-size α-Al2O3 seed mixture was added by spraying: the surface-modified dual-size α-Al2O3 seed mixture was dispersed in ethanol, ultrasonically prepared into a seed suspension, and added to the hydrolysis reaction system by spraying.

8. The method for preparing equiaxed α-alumina according to claim 1, characterized in that, The aging temperature is 40-60℃, and the aging time is 2-12 hours; Calcination is carried out in an air or inert gas atmosphere; the calcination temperature is 850-950℃, and the calcination time is 1-5h; preferably, the temperature is increased to the calcination temperature at a heating rate of 2-10℃ / min.

9. An equiaxed α-alumina, characterized in that, It is prepared by the method for preparing equiaxed α-alumina according to any one of claims 1-8.

10. The equiaxed α-alumina according to claim 9, characterized in that, The grain morphology is equiaxed; D50 is 0.18-0.28 μm; α phase conversion rate is 98.5-99.2%.

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