Preparation method of submicron narrow-distribution spheroidic alpha alumina for heat conduction

Abstract

The invention provides a preparation method of submicron narrow-distribution spheroidic alpha alumina for heat conduction, and belongs to the field of heat conduction filler materials. The method comprises the following steps: mixing boehmite and a crystal appearance control agent to obtain composite powder; calcining the composite powder at low temperature to obtain primary calcined powder; performing wet grinding on the primary calcined powder to obtain ground slurry; carrying out spray drying on the grinding slurry to obtain precursor powder; calcining the precursor powder at high temperature to obtain secondary calcined powder; carrying out wet depolymerization on the secondary calcined powder to obtain depolymerized slurry; carrying out spray drying on the depolymerized slurry to obtain submicron sphere-like alpha alumina spray-dried powder; and carrying out airflow scattering on the submicron sphere-like alpha alumina spray-dried powder to obtain the submicron sphere-like alpha alumina micro powder. The preparation method not only can improve the morphology of crystal grains to be spherical-like, but also has low impurity content and low specific surface area, and has good formability and good thermal conductivity after being applied to heat-conducting products.

Description

Technical Field

[0001] This application relates to the field of thermally conductive filler materials technology, and in particular to a method for preparing submicron narrow-distribution quasi-spherical α-alumina for thermal conductivity. Background Technology

[0002] Submicron-sized spherical alpha-alumina, with its regular and smooth morphology, high infill density, excellent thermal conductivity, and low wear resistance, can fill the voids between micron-sized spherical alumina fillers to construct a denser thermally conductive network, thereby significantly improving the thermal conductivity of composite materials. Therefore, submicron-sized spherical alpha-alumina is widely used in the field of thermal interface materials. With the rapid development of new energy vehicles, the 5G electronics industry, and thermal management, the market's technical requirements for thermally conductive materials continue to rise, and the demand prospects for submicron-sized spherical alpha-alumina are increasingly broad.

[0003] Currently, submicron-sized spherical alumina is mainly prepared by aluminum ammonium carbonate pyrolysis, aluminum alkoxide hydrolysis, sol-gel method, and modified Bayer process. Existing preparation processes generally suffer from complex procedures. Summary of the Invention

[0004] This application provides a method for preparing submicron narrow-distribution quasi-spherical α-alumina for thermal conductivity, which offers a simple process for preparing such material.

[0005] This application provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, the method comprising: Boehmite and a crystal morphology control agent are mixed to obtain a composite powder; the crystal morphology control agent includes at least one of boric acid and aluminum fluoride; The composite powder is calcined at low temperature to obtain a single-calcined powder. The calcined powder is then wet-ground to obtain a grinding slurry; The grinding slurry was spray-dried to obtain spherical α-alumina precursor powder. The spherical α-alumina precursor powder is subjected to high-temperature calcination to obtain secondary calcined powder; the high-temperature calcination includes the following parameters: heating rate of 3℃ / min~5℃ / min, heating endpoint temperature of 1400℃~1450℃, and holding time of 1h~2h. The secondary calcined powder is subjected to wet depolymerization to obtain a depolymerization slurry; The depolymerized slurry was spray-dried to obtain submicron-sized spherical α-alumina spray-dried powder. The submicron-sized spherical α-alumina spray-dried powder is dispersed by airflow to obtain submicron-sized spherical α-alumina micro powder.

[0006] Optionally, the boehmite has a particle size of 20μm to 50μm, and the Na2O mass fraction of the boehmite is ≤150ppm.

[0007] Optionally, the mass of the crystal morphology control agent is 0.04% to 0.08% of the total mass of the composite powder.

[0008] Optionally, in the crystal morphology control agent, the mass of boric acid is 0.03% to 0.05% of the total mass of the composite powder, and the mass of aluminum fluoride is 0.01% to 0.03% of the total mass of the composite powder.

[0009] Optionally, the low-temperature calcination includes the following parameters: heating rate of 3℃ / min to 5℃ / min, heating endpoint temperature of 1000℃ to 1200℃, and holding time of 1h to 2h.

[0010] Optionally, the wet grinding process includes: first, preparing a slurry using a stirring mill, and then grinding it using a sand mill.

[0011] Optionally, the particle size D50 of the solid particles in the grinding slurry is 0.2μm to 0.5μm.

[0012] Optionally, the wet depolymerization process uses a sand mill, wherein the speed of the sand mill is 400 rpm to 600 rpm, the grinding time is 60 min to 100 min, and the particles are ground until the particle size reaches the original crystal size of the product.

[0013] Optionally, the air source for the airflow to disperse the gas is compressed air, and the gas pressure of the compressed air is 0.8MPa to 1.2MPa.

[0014] Optionally, the submicron-sized spherical α-alumina powder meets the following performance indicators: α content > 95%, Na₂O ≤ 200 ppm, particle size D50 of 0.8 μm to 1 μm, D90 / D10 ≤ 4, and specific surface area < 3 m². 2 / g, with a spherical grain ratio of >70%, and a viscosity of <3500Pas after filling twice with vinyl silicone oil powder of viscosity 350.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, which can effectively improve the morphology of alumina grains, making the alumina grains present a near-spherical shape. The specific process is as follows: First, the composite powder is treated by a low-temperature calcination process to prepare irregular angular metastable α-alumina with high surface energy. During the phase transformation process of the low-temperature calcination process, a large number of crystal defects are left inside the metastable α-alumina crystal, and the overall internal energy of the metastable α-alumina crystal is at a high level. Subsequently, a high-temperature calcination process provides sufficient thermodynamic driving force to promote surface diffusion of atoms in the near-spherical α-alumina precursor powder crystal. At the same time, the high-temperature calcination process and the crystal morphology control agent work synergistically to not only transform the originally angular alumina particles into near-spherical alumina particles with smoother surfaces and more regular shapes, but also eliminate defects inside the alumina crystal.

[0016] Meanwhile, the near-spherical morphology of alumina particles directly improves the flowability and filling density of submicron near-spherical α-alumina micropowder. Combined with the low impurity content and low specific surface area of ​​the submicron near-spherical α-alumina micropowder itself, when applied to thermal conductive products, it can ensure that the thermal conductive products have excellent molding performance and significantly improve the thermal conductivity of the products, thus meeting the core material usage requirements of the thermal conductive field. Attached Figure Description

[0017] 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.

[0018] 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.

[0019] Figure 1 This is a first microscopic morphology diagram of submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Embodiment 1 of this application; Figure 2 This is a second microstructure image of submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Embodiment 1 of this application; Figure 3 The first microstructure of the submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Comparative Example 1 of this application; Figure 4 The second microstructure of the submicron narrow-distribution quasi-spherical α-alumina for thermal conductivity provided in Comparative Example 1 of this application is shown. Detailed Implementation

[0020] 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.

[0021] 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 to 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.

[0022] This application provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, the method comprising: S1. Boehmite and a crystal morphology control agent are mixed to obtain a composite powder; the crystal morphology control agent includes at least one of boric acid and aluminum fluoride; S2. The composite powder is calcined at low temperature to obtain a single-calcined powder; S3. The calcined powder is wet-ground to obtain a grinding slurry; S4. Spray dry the grinding slurry to obtain spherical α-alumina precursor powder; S5. The spherical α-alumina precursor powder is calcined at high temperature to obtain secondary calcined powder. S6. The calcined powder is subjected to wet depolymerization to obtain a depolymerized slurry; S7. Spray dry the depolymerized slurry to obtain submicron-sized spherical α-alumina spray-dried powder; S8. The submicron-sized spherical α-alumina spray powder is dispersed by airflow to obtain submicron-sized spherical α-alumina micro powder.

[0023] It should be noted that the functions of each step in the preparation method of submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in this application are as follows: S1 Mixing Process: Boehmite is mixed with a crystal morphology control agent to obtain a composite powder. This process is fundamental to the preparation process. The mixing process determines the raw material system and ensures that the crystal morphology control agent is uniformly dispersed in the composite powder, laying the foundation for subsequent precise control of crystal morphology. At the same time, using boehmite with a particle size of 20μm to 50μm and a Na2O mass fraction of ≤150ppm as raw material ensures that the initial state of the raw material is uniform and controls the impurity content from the source, avoiding excessive impurities in the final product.

[0024] S2 Low-Temperature Calcination Process: The composite powder is calcined at low temperatures to obtain low-temperature calcined powder. This process focuses on the initial control of grain size. Its core function is to control the grain size and crystallinity under specific conditions, so that the crystals can complete the initial phase transformation in the composite powder in a relatively mild environment, forming fine α grains, which provide high-quality "initial nuclei" for subsequent high-temperature calcination to optimize the crystal morphology. At the same time, the crystal morphology control agent plays an initial role in the low-temperature calcination stage, laying the foundation for subsequent crystal morphology regularity.

[0025] S3 Wet Grinding Process: The low-temperature calcined powder is wet-ground to obtain a wet-ground slurry. This process is crucial for particle size refinement. The wet grinding process first uses a stirred mill to prepare the low-temperature calcined powder into a slurry, and then uses a sand mill to grind the particles in the slurry. The main function is to refine the low-temperature calcined powder to a suitable particle size, so that the particle size D50 of the ground solid particles reaches 0.2μm to 0.5μm. By refining the particles, the wet-ground slurry is more likely to form a regular spherical structure during subsequent spray drying. At the same time, the wet grinding process can also further promote the uniform contact between the particles and the residual crystal morphology control agent, improving the effect of subsequent processes.

[0026] S4 Spray Drying Process (First Step): The wet grinding slurry is spray-dried to prepare spherical α-alumina precursor powder. This process is an important step in morphology shaping. By utilizing the characteristics of rapid drying and shrinkage of droplets during spray drying, the fine particles in the wet grinding slurry are aggregated into spherical solid particles, which initially fixes the spherical morphology of the particles and preserves the basic structure for subsequent morphology optimization after high-temperature calcination.

[0027] S5 High-Temperature Calcination Process: The near-spherical α-alumina precursor powder is calcined at high temperature to obtain secondary calcined powder. This process is the core of improving crystal morphology and crystal quality. The high-temperature calcination process can provide sufficient energy to the atoms or ions in the near-spherical α-alumina precursor powder, enabling them to overcome energy barriers and migrate. Through surface diffusion and grain boundary diffusion, the particle surface is smoothed and the morphology is regularized. In conjunction with the crystal morphology control agent, the irregular particles in the near-spherical α-alumina precursor powder are transformed into near-spherical particles. At the same time, internal crystal defects are eliminated, and the stability and integrity of the crystal structure are improved.

[0028] S6 Wet Depolymerization Process: The secondary calcined powder is wet depolymerized to obtain a depolymerized slurry. This process addresses the agglomeration problem of the secondary calcined powder by using a sand mill to wet depolymerize the powder. The core function is to break up the "hard agglomerates" that may form during the secondary calcination process. Through mechanical shearing force, the agglomerates in the secondary calcined powder are dispersed into individual particles close to the initial primary particle size, restoring and preserving the spherical morphology of the particles, and avoiding agglomeration from affecting the particle size distribution and performance of the final product.

[0029] S7 Spray Drying Process (Second): The depolymerized slurry is spray-dried to prepare submicron-sized spherical α-alumina spray-dried powder. This process is a secondary fixation of morphology. While maintaining the spherical structure of the particles, the depolymerized slurry is transformed into a solid powder that is easy to process in subsequent steps. This ensures that the particles maintain a regular shape before entering the final process and avoids morphological damage due to unstable state.

[0030] S8 Airflow Dispersion Process: The submicron-sized spherical α-alumina spray-dried powder is dispersed by airflow to obtain submicron-sized spherical α-alumina micro powder. This process is the final step in product formation. Compressed air is used to disperse the submicron-sized spherical α-alumina spray-dried powder. Its main function is to further disperse any slight agglomeration that may exist in the submicron-sized spherical α-alumina spray-dried powder, ensuring that the final product particles are evenly dispersed and fully meet the performance indicators such as α content > 95% and particle size D50 of 0.8μm to 1μm.

[0031] In some embodiments, the boehmite has a particle size of 20 μm to 50 μm and a Na2O mass fraction of ≤150 ppm.

[0032] The particle size of the boehmite raw material is selected within the range of 20μm to 50μm. This ensures that the particle size is moderate, facilitating uniform mixing of boehmite and crystal morphology control agent, and avoiding clumping due to excessively fine particles or uneven mixing due to excessively coarse particles. The requirement of a Na₂O mass fraction of ≤150ppm for the boehmite raw material allows for control of impurity content in the final product from the source, ensuring that impurities meet usage requirements and do not affect thermal conductivity. For example, the particle size of boehmite can be 20μm, 23μm, 26μm, 29μm, 32μm, 35μm, 38μm, 41μm, etc.; the Na₂O mass fraction of boehmite can be 80ppm, 90ppm, 100ppm, 110ppm, 130ppm, 140ppm, 150ppm, etc.

[0033] In some embodiments, the mass of the crystal morphology control agent is 0.04% to 0.08% of the total mass of the composite powder.

[0034] The role of the crystal morphology control agent in this application is as follows: By selectively adsorbing onto specific crystal surfaces of alumina crystals, the crystal morphology control agent regulates the anisotropic growth rate of alumina crystals, thereby guiding alumina particles toward the desired "quasi-spherical" morphology.

[0035] In this process, the incorporation of boric acid as a crystal morphology control agent not only inhibits the excessive sintering and growth of alumina particles during calcination but also increases the phase transformation temperature from γ-Al₂O₃ to α-Al₂O₃. Through the aforementioned adsorption and inhibition effects, boric acid effectively passivates the sharp edges and corners on alumina particles that are prone to rapid growth, forcing the alumina particles to develop isotropically as a whole, thus resulting in a smoother morphology. When the amount of boric acid added is less than 0.03% of the total mass of the composite powder, there are insufficient adsorption sites, making it impossible to effectively control the growth of alumina crystals, and the spherical effect is not significant. When the amount of boric acid added is more than 0.05% of the total mass of the composite powder, boric acid may form impurity phases such as aluminoborates with aluminum, reducing the specific surface area and purity of alumina.

[0036] Aluminum fluoride, used as a crystal morphology control agent, decomposes into HF gas at temperatures above 500°C. This HF gas can flow freely within the pores of alumina particles, selectively dissolving small crystallites or irregular protrusions with high surface energy. This dissolution process effectively smooths the surface of alumina particles, eliminates fine microcrystals and sharp edges, resulting in more regular, near-spherical alumina particle morphology and a more uniform pore size distribution. Additionally, the F produced during the decomposition of aluminum fluoride... - It can significantly reduce the activation energy of the alumina phase transition, and an appropriate amount of F -It can also promote the rearrangement of the internal structure of alumina, forming a more stable alumina crystal structure with fewer defects. When the amount of aluminum fluoride added is less than 0.01% of the total mass of the composite powder, it cannot effectively control the morphology and structure of alumina particles; when the amount of aluminum fluoride added is more than 0.03% of the total mass of the composite powder, it will lead to excessive sintering of alumina and coarsening of grains. In addition, the excess fluorine cannot completely escape during the calcination process and will remain in the alumina in the form of chemical adsorption or surface compounds, affecting product performance.

[0037] In some embodiments, the boric acid in the crystal morphology control agent is 0.03% to 0.05% of the total mass of the composite powder, and the aluminum fluoride is 0.01% to 0.03% of the total mass of the composite powder.

[0038] The total mass of the crystal morphology control agent accounts for 0.04% to 0.08% of the total mass of the composite powder. This proportion allows the crystal morphology control agent to play its full role, ensuring that insufficient dosage does not lead to poor crystal morphology control effect, and also accurately matches the subsequent calcination process to synergistically achieve spherical particle shape. Among them, boric acid accounts for 0.03% to 0.05% of the total mass of the composite powder, which can effectively inhibit the excessive sintering and growth of alumina particles during calcination, while also helping to control the crystal growth direction and promote the isotropic development of particles. Aluminum fluoride accounts for 0.01% to 0.03% of the total mass of the composite powder, which can decompose at high temperature and act on the particle surface, smoothing particle protrusions, eliminating sharp edges, and also promoting the rearrangement of the internal structure of alumina to form a more stable crystal structure. For example, the mass ratio of the crystal morphology control agent to the total mass of the composite powder can be 0.04%, 0.045%, 0.05%, 0.055%, 0.06%, 0.065%, 0.07%, 0.075%, etc.; the mass ratio of boric acid in the crystal morphology control agent to the total mass of the composite powder can be 0.03%, 0.0325%, 0.035%, 0.0375%, 0.04%, 0.0425%, 0.045%, 0.0475%; the mass ratio of aluminum fluoride in the crystal morphology control agent to the total mass of the composite powder can be 0.01%, 0.0125%, 0.015%, 0.0175%, 0.02%, 0.0225%, 0.025%, 0.0275%, etc.

[0039] In some embodiments, low-temperature calcination includes the following parameters: heating rate of 3℃ / min to 5℃ / min, heating endpoint temperature of 1000℃ to 1200℃, and holding time of 1h to 2h.

[0040] The low-temperature calcination process employs a heating rate of 3℃ / min to 5℃ / min, which can slowly control the dehydration process of boehmite, preventing the composite powder particles from cracking due to internal stress caused by excessively rapid dehydration. At the same time, it allows the crystal morphology control agent to penetrate and act on the crystals uniformly, ensuring a stable phase transformation and promoting the uniform formation of fine α-grains. The heating endpoint temperature of 1000℃ to 1200℃ in the low-temperature calcination process provides a suitable thermodynamic environment for the initial phase transformation of the crystals, enabling the initial transformation of γ-Al2O3 to α-Al2O3 without causing premature excessive growth of the grains. The holding time of 1h to 2h in the low-temperature calcination process provides sufficient time for the phase transformation, ensuring sufficient phase transformation and laying a good grain foundation for subsequent high-temperature treatment. For example, the heating rate of low-temperature calcination can be 3℃ / min, 3.25℃ / min, 3.5℃ / min, 3.75℃ / min, 4℃ / min, 4.25℃ / min, 4.5℃ / min, 4.75℃ / min, etc.; the heating endpoint temperature of low-temperature calcination can be 1000℃, 1025℃, 1050℃, 1075℃, 1100℃, 1125℃, 1150℃, 1175℃, etc.; and the holding time of low-temperature calcination can be 1h, 1.1h, 1.2h, 1.3h, 1.5h, 1.6h, 1.7h, 1.8h, etc.

[0041] In some embodiments, high-temperature calcination includes the following parameters: heating rate of 3℃ / min to 5℃ / min, heating endpoint temperature of 1400℃ to 1450℃, and holding time of 1h to 2h.

[0042] The high-temperature calcination process employs a heating rate of 3℃ / min to 5℃ / min, ensuring uniform heating of the spherical α-alumina precursor powder inside and outside the body. This prevents particle cracking caused by thermal stress due to temperature differences and maintains the integrity of the particle morphology. The heating endpoint temperature of 1400℃ to 1450℃ in the high-temperature calcination process provides sufficient energy for atomic migration, promoting the diffusion of high-energy atoms to low-energy regions. This, in conjunction with the crystal morphology control agent, achieves regularization of particle morphology, transforming irregular particles into spherical shapes. The 1h to 2h holding time in the high-temperature calcination process provides sufficient kinetic time for particle morphology optimization and crystal defect elimination, ensuring sufficient atomic migration and rearrangement, resulting in a more stable crystal structure and more regular particle morphology. For example, the heating rate of high-temperature calcination can be 3℃ / min, 3.25℃ / min, 3.5℃ / min, 3.75℃ / min, 4℃ / min, 4.25℃ / min, 4.5℃ / min, 4.75℃ / min, etc.; the heating endpoint temperature of high-temperature calcination can be 1400℃, 1406.25℃, 1412.5℃, 1418.75℃, 1425℃, 1431.25℃, 1437.5℃, 1443.75℃, etc.; and the holding time of high-temperature calcination can be 1h, 1.1h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, etc.

[0043] In some embodiments, wet grinding includes: first, preparing a slurry by stirring, and then grinding with a sand mill.

[0044] In some embodiments, the particle size D50 of the solid particles in the grinding slurry is 0.2 μm to 0.5 μm.

[0045] The wet grinding process employs a combination of "stirred mill slurry preparation + sand mill grinding." The stirred mill first mixes the low-temperature calcined powder and liquid at a liquid-to-solid ratio of 3:1 to form a uniform slurry, creating favorable conditions for subsequent sand mill grinding. The sand mill grinding precisely refines the particles to a D50 of 0.2μm to 0.5μm, ensuring that the particle size meets the requirements for subsequent spray drying and resulting in a more uniform particle surface, which is beneficial for forming regular spherical shapes. For example, the solid particle size D50 in the grinding slurry can be 0.2μm, 0.2375μm, 0.275μm, 0.3125μm, 0.35μm, 0.3875μm, 0.425μm, 0.4625μm, etc.

[0046] In some embodiments, wet depolymerization is performed using a sand mill, wherein the speed of the sand mill is 400 rpm to 600 rpm, the grinding time is 60 min to 100 min, and the particles are ground until the particle size reaches the original crystalline size of the product.

[0047] The wet deagglomeration process uses a sand mill, with the mill speed controlled at 400 rpm to 600 rpm and the grinding time at 60 min to 100 min. This combination of parameters effectively disperses hard agglomerates of the secondary calcined powder through appropriate mechanical shear force, while precisely controlling the degree of grinding to ensure that the particle size returns to the original crystalline size of the product. This preserves the spherical structure of the particles while ensuring effective dispersion. For example, the mill speed during wet deagglomeration can be 400 rpm, 425 rpm, 450 rpm, 475 rpm, 500 rpm, 525 rpm, 550 rpm, 575 rpm, etc.; and the grinding time can be 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, etc.

[0048] In some implementations, the gas source for the airflow dispersion is compressed air, and the gas pressure of the compressed air is 0.8MPa to 1.2MPa.

[0049] The airflow agitation process uses compressed air as the gas source, with the gas pressure controlled between 0.8 MPa and 1.2 MPa. This pressure range provides sufficient dispersing force to thoroughly break up the slight agglomerations in the submicron-sized spherical α-alumina spray-dried powder, while avoiding particle breakage due to excessive pressure or insufficient dispersion due to excessively small pressure. The final product is uniformly dispersed and morphologically intact submicron-sized spherical α-alumina powder. For example, the compressed air pressure during airflow agitation can be 0.8 MPa, 0.85 MPa, 0.9 MPa, 0.95 MPa, 1.0 MPa, 1.05 MPa, 1.1 MPa, or 1.15 MPa.

[0050] In some embodiments, the submicron-sized spherical α-alumina powder meets the following performance indicators: α content > 95%, Na₂O ≤ 200 ppm, particle size D50 of 0.8 μm to 1 μm, D90 / D10 ≤ 4, and specific surface area < 3 m². 2 / g, with a spherical grain ratio of >70%, and a viscosity of <3500Pas after filling twice with vinyl silicone oil powder of viscosity 350.

[0051] The preparation method provided in this application can effectively improve the morphology of alumina grains, making them spherical. The specific process is as follows: First, the composite powder is treated by a low-temperature calcination process to prepare irregular angular metastable α-alumina with high surface energy. During the phase transformation process of the low-temperature calcination process, a large number of crystal defects remain inside the metastable α-alumina crystal, and the overall internal energy of the metastable α-alumina crystal is at a high level. Subsequently, a high-temperature calcination process provides sufficient thermodynamic driving force to promote surface diffusion of atoms in the spherical α-alumina precursor powder crystal. At the same time, the high-temperature calcination process, in conjunction with the crystal morphology control agent, not only transforms the originally angular alumina particles into smoother, more regularly shaped spherical alumina particles, but also eliminates defects inside the alumina crystal.

[0052] The near-spherical morphology of alumina particles directly improves the flowability and filling density of submicron near-spherical α-alumina micropowder. Combined with the low impurity content and low specific surface area of ​​submicron near-spherical α-alumina micropowder, when applied to thermal conductive products, it can ensure that the thermal conductive products have excellent molding properties and significantly improve their thermal conductivity, thus meeting the core material requirements of the thermal conductive field.

[0053] In summary, the method for preparing submicron narrow-distribution quasi-spherical α-alumina provided in this application has significant advantages in process design, performance control, product quality, and practical application.

[0054] In terms of process design, this method constructs a complete technology chain of "raw material pretreatment—crystal nucleus construction—morphology shaping—performance optimization—product molding". Boehmite is selected as the raw material and combined with a specific crystal morphology control agent. The pre-uniform dispersion of the control agent lays the foundation for subsequent morphology control. The unique combination of two calcinations and two spray drying processes respectively achieves crystal nucleus construction and morphology optimization, and preliminary morphology fixation and stable maintenance. Combined with particle size control through wet grinding and dispersion treatment through wet deagglomeration and airflow agitation, a complete process flow is formed that requires no special equipment, is easy to operate, and is environmentally friendly and low-cost.

[0055] In terms of performance regulation, this method establishes a dual regulation mechanism through the synergistic effect of crystal morphology control agents and two calcinations. Boric acid inhibits excessive sintering of particles and blunts sharp edges, while aluminum fluoride decomposes at high temperatures to smooth the particle surface and promote crystal rearrangement. Combined with the fine assembly units formed by low-temperature calcination and the atomic migration energy provided by high-temperature calcination, a precise transformation from angular to near-spherical shapes is achieved, while eliminating crystal defects. Wet grinding precisely controls particle size, and wet deagglomeration and airflow dispersion effectively solve the agglomeration problem, ensuring that the product has a narrow distribution and high dispersibility.

[0056] The resulting product possesses multiple synergistic performance advantages: strict raw material control and clean processes ensure low impurity content; high-temperature calcination leads to densification, resulting in a low specific surface area; and the near-spherical morphology imparts excellent flowability and filling density. These characteristics mutually reinforce each other, jointly enhancing the overall performance of the product.

[0057] In practical applications, this product perfectly matches the dual requirements of filler molding performance and thermal conductivity in the thermal conductivity field. Excellent flowability and filler density ensure uniform dispersion in the matrix, improving molding quality; the synergistic effect of its near-spherical morphology, low impurities, and low specific surface area creates highly efficient thermal conductivity channels. Furthermore, its simple manufacturing process, controllable cost, and ease of industrial production provide the thermal conductivity materials industry with a cost-effective and stable new raw material option.

[0058] 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 standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0059] Example 1 This embodiment provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, comprising the following steps: S1. Weigh and mix boehmite, aluminum fluoride (a crystal morphology control agent), and boric acid in a mass ratio of 100:0.01:0.05 to obtain a composite powder; S2. The composite powder is heated from room temperature to 1200℃ at a heating rate of 5℃ / min and held at that temperature for 2 hours for low-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the first-calcined powder. S3. After wet grinding the calcined powder in a stirred mill for 1 hour, it is then put into a sand mill for ultrafine grinding for 40 minutes to obtain a grinding slurry; the final particle size of the grinding slurry is 0.4μm. S4. Spray dry the grinding slurry to obtain spherical α-alumina precursor powder; S5. The spherical α-alumina precursor powder is heated from room temperature to 1450℃ at a heating rate of 5℃ / min and held at that temperature for 2h for high-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the secondary calcined powder. S6. The calcined powder is subjected to wet depolymerization using a sand mill at a speed of 800 rpm for 60 minutes to obtain a depolymerized slurry. The particle size of the depolymerized slurry is the original crystal size of the product. S7. Spray dry the depolymerized slurry to obtain submicron-sized spherical α-alumina spray-dried powder; S8. The submicron-sized spherical α-alumina spray powder is dispersed by airflow. The air source is compressed air with a gas pressure of 0.8 MPa to obtain submicron-sized spherical α-alumina micro powder.

[0060] Example 2 This embodiment provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, comprising the following steps: S1. Weigh and mix borosilicate, boric acid (a crystal morphology control agent), and aluminum fluoride in a mass ratio of 100:0.05:0.03 to obtain a composite powder; S2. The composite powder is heated from room temperature to 1000℃ at a heating rate of 5℃ / min and held at that temperature for 2 hours for low-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the first-calcined powder. S3. After wet grinding the calcined powder in a stirring mill for 1 hour, it is then put into a sand mill for ultrafine grinding for 40 minutes to obtain a grinding slurry; the final particle size of the grinding slurry is 0.48μm. S4. Spray dry the grinding slurry to obtain spherical α-alumina precursor powder; S5. The spherical α-alumina precursor powder is heated from room temperature to 1450℃ at a heating rate of 5℃ / min and held at that temperature for 2h for high-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the secondary calcined powder. S6. The calcined powder is subjected to wet depolymerization using a sand mill at a speed of 500 rpm for 80 minutes to obtain a depolymerized slurry. The particle size of the depolymerized slurry is the same as the original crystal size of the product. S7. Spray dry the depolymerized slurry to obtain submicron-sized spherical α-alumina spray-dried powder; S8. The submicron-sized spherical α-alumina spray powder is dispersed by airflow. The air source is compressed air with a gas pressure of 1.0 MPa to obtain submicron-sized spherical α-alumina micro powder.

[0061] Example 3 This embodiment provides a method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, comprising the following steps: S1. Weigh and mix boehmite, aluminum fluoride (a crystal morphology control agent), and boric acid in a mass ratio of 100:0.03:0.03 to obtain a composite powder; S2. The composite powder is heated from room temperature to 1100℃ at a heating rate of 5℃ / min and held at that temperature for 2 hours for low-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the first-calcined powder. S3. After wet grinding the calcined powder in a stirring mill for 1 hour, it is then put into a sand mill for ultrafine grinding for 40 minutes to obtain a grinding slurry; the final particle size of the grinding slurry is 0.5μm. S4. Spray dry the grinding slurry to obtain spherical α-alumina precursor powder; S5. The spherical α-alumina precursor powder is heated from room temperature to 1400℃ at a heating rate of 5℃ / min and held at that temperature for 2h for high-temperature calcination. Then it is cooled to room temperature with the furnace to obtain the secondary calcined powder. S6. The calcined powder is subjected to wet depolymerization using a sand mill at a speed of 500 rpm for 80 minutes to obtain a depolymerized slurry. The particle size of the depolymerized slurry is the same as the original crystal size of the product. S7. Spray dry the depolymerized slurry to obtain submicron-sized spherical α-alumina spray-dried powder; S8. The submicron-sized spherical α-alumina spray powder is dispersed by airflow. The air source is compressed air with a gas pressure of 0.8 MPa to obtain submicron-sized spherical α-alumina micro powder.

[0062] Comparative Example 1 This comparative example provides a method for preparing α-alumina micro powder, including the following steps: S1. Weigh and mix boehmite and aluminum fluoride (a crystal morphology control agent) at a mass ratio of 100:0.03 to obtain composite powder; S2. The composite powder is heated from room temperature to 900℃ at a heating rate of 5℃ / min and held at that temperature for 2 hours for low-temperature calcination. Then it is cooled to room temperature in the furnace to obtain the first-calcined powder. S3. After wet grinding the calcined powder in a stirred mill for 1 hour, it is then put into a sand mill for ultrafine grinding for 40 minutes to obtain a grinding slurry; the final particle size of the grinding slurry is 0.3μm. S4. Spray dry the grinding slurry to obtain spherical α-alumina precursor powder; S5. The spherical α-alumina precursor powder is heated from room temperature to 1300℃ at a heating rate of 5℃ / min and held at that temperature for 2h for high-temperature calcination. Then it is cooled to room temperature with the furnace to obtain the secondary calcined powder. S6. The calcined powder is subjected to wet depolymerization using a sand mill at a speed of 500 rpm for 80 minutes to obtain a depolymerized slurry. The particle size of the depolymerized slurry is the same as the original crystal size of the product. S7. Spray dry the depolymerized slurry to obtain submicron-sized spherical α-alumina spray-dried powder; S8. The submicron-sized spherical α-alumina spray powder is dispersed by airflow. The air source is compressed air with a gas pressure of 0.8 MPa to obtain submicron-sized spherical α-alumina micro powder.

[0063] Comparative Example 2 This comparative example provides a method for preparing α-alumina micro powder, including the following steps: S1. Weigh and mix boehmite, aluminum fluoride (a crystal morphology control agent), and boric acid in a mass ratio of 100:0.03:0.03 to obtain a composite powder; S2. The composite powder is heated from room temperature to 1450℃ at a heating rate of 5℃ / min and held at that temperature for 2 hours for calcination. Then it is cooled to room temperature in the furnace to obtain calcined powder. S3. After wet grinding the calcined powder in a stirring mill for 1 hour, it is then put into a sand mill for wet grinding. The sand mill rotates at 500 rpm and grinds for 60 minutes to obtain a grinding slurry. The particle size of the grinding slurry is the original crystal size of the product. S4. Spray dry the grinding slurry to obtain α-alumina spray-dried powder; S5. The α-alumina spray powder is dispersed by airflow. The air source is compressed air with a pressure of 0.8 MPa to obtain α-alumina micro powder.

[0064] The performance of the submicron-sized spherical alumina powders prepared in Examples 1-3 and Comparative Examples 1-2 was measured, and the results are shown in Table 1.

[0065] Table 1 Properties of submicron-sized spherical alumina powder

[0066] As shown in Table 1, the submicron-sized spherical α-alumina powders from Examples 1-3 meet the following performance indicators: α content > 95%, Na₂O ≤ 200 ppm, particle size D50 of 0.8 μm to 1 μm, D90 / D10 ≤ 4, and specific surface area < 3 m². 2 / g, with a spherical grain ratio of >70%, and a viscosity of <3500Pas after filling twice with vinyl silicone oil powder of viscosity 350.

[0067] Comparative Example 1 suffered from a series of adverse reactions due to severely insufficient calcination temperatures (900℃ and 1300℃). Its α-phase content (93.6%) was the lowest among all groups, confirming incomplete phase transformation; it reached as high as 4.264 m. 2 The specific surface area of ​​ / g and the oil absorption value of 24.8% both indicate that the particles have irregular morphology and numerous and well-developed surface defects; while the wide particle size distribution of 6.4 and the ultra-high viscosity of 6580cp directly reflect its poor particle morphology and poor dispersibility, resulting in extremely poor flowability in the application system.

[0068] Comparative Example 2, due to its one-step high-temperature calcination (1450℃), lacked the crucial low-temperature calcination nucleation stage, resulting in severe sintering and abnormal grain growth. This led to the worst grain size distribution (D90 / D10=8.2) and the highest specific surface area (5.846m²) despite meeting the α-phase content standard. 2The presence of numerous hard agglomerates and broken, irregular small particles ( / g) indicates the presence of these particles. Ultimately, the relatively high viscosity of 4860 cp and oil absorption value of 22.6% demonstrate that the powder's filling and flowability remain unsatisfactory, failing to meet the application requirements for high thermal conductivity fillers.

[0069] Figure 1 This is a first microscopic morphology diagram of submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Embodiment 1 of this application; Figure 2 This is a second microstructure image of the submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Embodiment 1 of this application. Figure 1 and Figure 2 It can be seen that the α-alumina provided in Example 1 has excellent dispersibility and no obvious agglomeration; the particle size is uniform, with a size in the submicron range; the particle morphology is regular and has a spherical feature.

[0070] Figure 3 The first microstructure of the submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Comparative Example 1 of this application; Figure 4 This is a second microstructure image of the submicron narrow-distribution near-spherical α-alumina for thermal conductivity provided in Comparative Example 1 of this application. Figure 3 and Figure 4 It can be seen that the α-alumina in Comparative Example 1 has an irregular morphology, with a very low proportion of spherical particles; a wide particle size distribution and large size differences; severe agglomeration and poor dispersibility.

[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 herein.

Claims

1. A method for preparing submicron narrow-distribution near-spherical α-alumina for thermal conductivity, characterized in that, The method includes: Boehmite and a crystal morphology control agent are mixed to obtain a composite powder; the crystal morphology control agent includes at least one of boric acid and aluminum fluoride; The composite powder is calcined at low temperature to obtain a single-calcined powder. The calcined powder is then wet-ground to obtain a grinding slurry; The grinding slurry was spray-dried to obtain spherical α-alumina precursor powder. The spherical α-alumina precursor powder is subjected to high-temperature calcination to obtain secondary calcined powder; the high-temperature calcination includes the following parameters: heating rate of 3℃ / min~5℃ / min, heating endpoint temperature of 1400℃~1450℃, and holding time of 1h~2h. The secondary calcined powder is subjected to wet depolymerization to obtain a depolymerization slurry; The depolymerized slurry was spray-dried to obtain submicron-sized spherical α-alumina spray-dried powder. The submicron-sized spherical α-alumina spray-dried powder is dispersed by airflow to obtain submicron-sized spherical α-alumina micro powder.

2. The method according to claim 1, characterized in that, The boehmite has a particle size of 20μm to 50μm, and the Na2O mass fraction of the boehmite is ≤150ppm.

3. The method according to claim 1, characterized in that, The mass of the crystal morphology control agent is 0.04% to 0.08% of the total mass of the composite powder.

4. The method according to claim 3, characterized in that, In the crystal morphology control agent, the mass of boric acid is 0.03% to 0.05% of the total mass of the composite powder, and the mass of aluminum fluoride is 0.01% to 0.03% of the total mass of the composite powder.

5. The method according to claim 1, characterized in that, The low-temperature calcination includes the following parameters: heating rate of 3℃ / min to 5℃ / min, heating endpoint temperature of 1000℃ to 1200℃, and holding time of 1h to 2h.

6. The method according to claim 1, characterized in that, The wet grinding process includes: first, preparing a slurry using a stirring mill, and then grinding it using a sand mill.

7. The method according to claim 1, characterized in that, The particle size D50 of the solid particles in the grinding slurry is 0.2μm to 0.5μm.

8. The method according to claim 1, characterized in that, The wet depolymerization process uses a sand mill, wherein the speed of the sand mill is 400 rpm to 600 rpm, the grinding time is 60 min to 100 min, and the particles are ground until the particle size reaches the original crystal size of the product.

9. The method according to claim 1, characterized in that, The air source that the airflow disperses is compressed air, and the gas pressure of the compressed air is 0.8MPa to 1.2MPa.

10. The method according to claim 1, characterized in that, The submicron-sized spherical α-alumina powder meets the following performance indicators: α content > 95%, Na₂O ≤ 200 ppm, particle size D50 of 0.8 μm to 1 μm, D90 / D10 ≤ 4, and specific surface area < 3 m². 2 / g, with a spherical grain ratio of >70%, and a viscosity of <3500Pas after filling twice with vinyl silicone oil powder of viscosity 350.

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