Preparation method of low-sodium high-whiteness spheroidal ultrafine alpha-alumina powder and application thereof

Abstract

This invention discloses a method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder and its applications. The method includes: raw material pretreatment, compounding of a composite mineralizer, ball milling, microwave calcination, ultrafine grinding, surface modification, and sieving and drying steps. The composite mineralizer is composed of boron nitride, rare earth citrate, nano-zirconia, polyethylene glycol, and organophosphonic acid in a specific ratio. This invention, through the synergistic effect of the composite mineralizer combined with a microwave low-temperature calcination process, achieves low-temperature crystal transformation, deep desodiuming, near-spherical morphology control, and ultrafine particle size control of alumina. The resulting powder possesses excellent comprehensive properties such as low sodium content, high whiteness, near-spherical shape, ultrafineness, and low oil absorption value, and can be widely used as a thermally conductive filler in thermal interface materials, electronic packaging materials, and high thermal conductivity composite materials.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic materials technology, and in particular to a method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder and its application. Background Technology

[0002] Alpha-alumina, as a core product in the field of inorganic non-metallic materials, has become a key filler in high-end fields such as thermally conductive interface materials, electronic packaging materials, and high thermal conductivity composite materials due to its inherent advantages such as high melting point, high hardness, excellent chemical stability, and outstanding electrical insulation properties. With the rapid development of electronic devices towards miniaturization, high power, and integration, more stringent and diverse requirements have been placed on the performance of alpha-alumina powder: it not only needs to have low sodium properties to avoid the risk of electrical breakdown caused by ion migration under an electric field, but also needs to meet indicators such as spherical morphology, ultrafine particle size, and high whiteness. At the same time, it must also take into account low oil absorption value and suitable loose and tap densities to improve the filling rate and dispersibility in the polymer matrix, ultimately ensuring the thermal conductivity and processing stability of the composite material.

[0003] Current mainstream processes for preparing α-alumina powder include high-temperature calcination, sol-gel method, and melt spraying method, but all have insurmountable technical bottlenecks. High-temperature calcination, as the main method for large-scale industrial production, typically requires calcination at temperatures above 1300℃ for more than 10 hours. This not only consumes enormous amounts of energy and results in high production costs, but also easily leads to powder particle agglomeration, making it difficult to obtain products with ultra-fine particle size and high sphericity. Furthermore, the halides and borides used in traditional processes are prone to volatilization at high temperatures, producing harmful gases that corrode production equipment and increase environmental treatment costs. Simultaneously, they cannot achieve deep removal of sodium ions, resulting in sodium content in products often exceeding 0.05 wt%, which is insufficient to meet the insulation requirements of high-end electronics applications.

[0004] While the sol-gel method can prepare ultrafine powders, it relies on large amounts of organic solvents and surfactants, increasing raw material costs and the difficulty of subsequent separation and purification. It also easily leads to residual organic impurities, affecting powder whiteness and purity. Furthermore, this process has low production efficiency, is difficult to scale up, and the product is prone to agglomeration, making it difficult to guarantee dispersibility. The melt-jet method uses a high-temperature heat source to melt alumina raw materials and then rapidly cools them to prepare spherical particles. While this can improve sphericity, the melting temperature reaches over 2000℃, resulting in extremely high energy consumption. Rapid cooling of the droplets easily forms internal pores and microcracks, reducing powder density and thermal conductivity. Simultaneously, precise control of ultrafine particle size is difficult, resulting in a wide particle size distribution and high costs for subsequent classification processing.

[0005] In recent years, microwave calcination technology has been applied to the preparation of α-alumina due to its advantages such as uniform volume heating and rapid heating rate, attempting to reduce the calcination temperature. However, the design of composite mineralizer systems in existing technologies is unreasonable, often focusing on a single function and lacking synergistic design for desodiuming, crystal transformation, morphology control, and dispersibility improvement. Some processes have attempted to add additives such as boron nitride and rare earth compounds, but a scientific compounding ratio and mechanism of action have not been established, leading to unbalanced product performance, such as sphericity less than 80%, α-phase conversion rate less than 90%, or although ultrafine particle size can be achieved, the oil absorption value is too high, and the filling rate in the polymer matrix is ​​less than 60%, which cannot meet the preparation requirements of high-end composite materials.

[0006] Furthermore, existing processes lack a systematic design for controlling powder whiteness, resulting in widespread problems such as residual metal impurities and yellowing caused by high-temperature calcination, making it difficult to maintain a whiteness above 94%. Simultaneously, there is a lack of effective means to control indicators such as loose density and tap density, leading to poor powder flowability during storage, transportation, and processing, further affecting the performance consistency of composite materials. To address these issues, the industry urgently needs to develop a rational, energy-efficient, and environmentally friendly preparation technology. This technology should achieve efficient preparation of low-sodium, high-whiteness, near-spherical, ultrafine-particle-size α-alumina powders through innovative composite mineralizer systems and optimized process parameters, while also ensuring low oil absorption and excellent dispersibility. This would meet the application needs of high-end fields such as thermal interface materials and electronic packaging materials, and drive technological upgrades in related industries.

[0007] The above background information is provided only to aid in understanding the inventive concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was disclosed on the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0008] The main objective of this invention is to propose a method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder and its application, in order to solve the technical problems of the prior art, such as high calcination temperature, single function of mineralizer, difficulty in simultaneously achieving low-sodium characteristics of powder with near-spherical morphology / ultrafine particle size / high α-phase conversion rate, high oil absorption value and poor performance stability.

[0009] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0010] A method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder includes the following steps:

[0011] (1) Raw material pretreatment: Dry the industrial alumina powder at 100-120℃ and -0.08--0.1MPa vacuum for 2-4 hours, cool it and pass it through a 200-mesh sieve, collect the sieve material and set it aside;

[0012] (2) Compound mineralizer preparation: Weigh boron nitride, rare earth citrate, nano zirconium oxide, polyethylene glycol and organophosphonic acid according to the mass ratio, mix them evenly to prepare a compound mineralizer; wherein the mass ratio of each component is boron nitride: rare earth citrate: nano zirconium oxide: polyethylene glycol: organophosphonic acid = (1-1.5): (3.5-5): (1-1.5): (1-1.2): (0.5-0.8);

[0013] (3) Mixed ball milling: The industrial alumina powder pretreated in step (1) and the composite mineralizer in step (2) are added to the ball mill at a mass ratio of 100:(4-8). Grinding media are added, and the ball-to-material ratio is controlled at (4-6):1. The ball milling speed is 350-420 r / min, and the ball milling is carried out at 65-73℃ for 4-5 hours with intermittent operation during the process to obtain ball milled powder.

[0014] (4) Microwave calcination: The ball milled powder from step (3) is placed in a microwave calcination furnace and heated to 880-920°C at a heating rate of 8-12°C / min in an air atmosphere. The temperature is then maintained for calcination for 4-6 hours, and the powder is cooled to obtain the calcined product.

[0015] (5) Ultrafine grinding: Place the calcined material from step (4) into an ultrafine ball mill, add grinding media with a diameter of 0.5-3 mm, control the ball-to-material ratio (3-5):1, and the ball milling speed of 470-550 r / min, grind until D50 < 2 μm, and obtain the initial powder.

[0016] (6) Surface modification: Add 0.2-0.4% of the mass of silane coupling agent to the initial powder of step (5), mix for 20-25 min, and perform surface modification treatment;

[0017] (7) Sieving and drying: The modified powder from step (6) is sieved to remove coarse particles. The sieved material is dried at 80-100℃ and -0.08--0.1MPa vacuum for 3-5 hours to obtain low sodium, high whiteness, spherical ultrafine α-alumina powder.

[0018] Preferably, the boron nitride in step (2) is hexagonal boron nitride with a purity ≥99% and D50 <5μm; the rare earth citrate is lanthanum citrate or cerium citrate; the nano-zirconia has an average particle size of 20-100nm; the polyethylene glycol has a number-average molecular weight of 200-2000; and the organophosphonic acid is at least one of aminotrimethylenephosphonic acid, hydroxyethylidene diphosphonic acid, or ethylenediaminetetramethylenephosphonic acid.

[0019] Preferably, the grinding media in step (3) is 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm, compounded in a mass ratio of 2:3:1.

[0020] Preferably, the microwave frequency of the microwave calcining furnace in step (4) is 2450MHz or 915MHz; the temperature fluctuation during calcination is ≤±5℃.

[0021] Preferably, the ultrafine grinding media in step (5) is zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm, compounded in a mass ratio of (2-3):(3-4):(1-2).

[0022] Preferably, the silane coupling agent in step (6) is one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane or vinyltrimethoxysilane, which is diluted with anhydrous ethanol at a ratio of 1:(5-10) and sprayed in before use.

[0023] Preferably, the sodium content of the industrial alumina powder in step (1) is 0.35-0.5wt%, and the average particle size D50 is 50-80μm.

[0024] Preferably, the mass ratio of rare earth citrate to boron nitride in step (2) is (2-4):1.

[0025] Preferably, the powder has a Na₂O content of <0.02wt%, a sphericity >85%, an α-phase conversion rate ≥96%, a fineness D50 <2μm, a whiteness >94%, and a bulk density of 0.5-0.6g / cm³. 3 Tap density: 0.8-0.9 g / cm³ 3 Oil absorption value ≤25mL / 100g.

[0026] This invention also provides the application of low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder as a thermally conductive filler in the preparation of thermally conductive interface materials, electronic packaging materials, or high thermal conductivity composite materials.

[0027] The beneficial effects of this invention compared to the prior art include:

[0028] 1. The composite mineralizer system works synergistically and efficiently, achieving simultaneous optimization of multiple properties.

[0029] The composite mineralizer of this invention is composed of boron nitride, rare earth citrate, nano-zirconia, polyethylene glycol, and organophosphonic acid in a specific ratio. The components work synergistically, overcoming the limitations of traditional mineralizers with their single function. Boron nitride induces heterogeneous nucleation, guiding alumina crystals towards a near-spherical shape while simultaneously improving thermal conductivity. Rare earth citrate acts as a crystal transformation promoter, lowering the activation energy of the α-phase transformation and ensuring its sufficiency. Nano-zirconia refines grains and optimizes crystal morphology, enhancing the structural stability of the powder. Polyethylene glycol disperses and prevents agglomeration, avoiding the formation of hard agglomerates during processing. Organophosphonic acid achieves deep removal of sodium ions through complexation. The components work together seamlessly, eliminating the need for additional functional additives to simultaneously achieve multiple performance goals such as low sodium content, near-spherical shape, and high α-phase conversion rate. This solves the cumbersome problem of traditional processes requiring multiple steps to control individual properties, simplifying the production process and improving its stability.

[0030] 2. Microwave low-temperature calcination process is energy-efficient and highly effective, balancing environmental protection and production efficiency.

[0031] This invention employs microwave calcination technology, combined with the synergistic effect of composite mineralizers, to achieve low-temperature calcination at 880-920℃, significantly reducing energy consumption compared to traditional high-temperature calcination processes. Microwave heating, with its volumetric heating characteristics, allows for simultaneous heating of the material's interior and surface, resulting in uniform temperature distribution. This avoids the uneven calcination problem caused by the external heating and internal cooling of traditional resistance furnaces, ensuring consistent powder properties. Simultaneously, microwave heating offers rapid heating, requiring only 4-6 hours of holding time, shortening the production cycle and improving efficiency. Low-temperature calcination not only reduces energy consumption but also prevents excessive sintering and agglomeration of powder particles at high temperatures, helping to maintain ultrafine particle size and near-spherical morphology. It also reduces the generation of harmful gases during production, minimizing equipment corrosion and environmental pollution, aligning with the concept of green production and addressing the pain points of high energy consumption and significant environmental impact associated with traditional high-temperature calcination.

[0032] 3. The sodium removal mechanism is scientifically sound and the low-sodium performance is stable and reliable.

[0033] This invention constructs a highly efficient sodium removal system through the synergistic effect of organophosphonic acid and other mineralizing agent components, achieving deep removal of sodium ions. Organophosphonic acid possesses strong complexing ability, forming stable complexes with sodium ions in industrial alumina raw materials, which are then effectively separated in subsequent processes. Simultaneously, other components in the composite mineralizing agent optimize the reaction environment, enhancing the complexing efficiency of organophosphonic acid and ensuring the stability of the sodium removal effect. Compared to traditional processes relying on a single mineralizing agent or acid washing for sodium removal, this invention eliminates the need for highly corrosive acids, avoiding equipment corrosion and overcoming the shortcomings of traditional mineralizing agents, such as incomplete sodium removal and large performance fluctuations. The entire sodium removal process is organically integrated with ball milling, calcination, and other processes, eliminating the need for additional complex sodium removal steps. This simplifies the process flow and improves production efficiency while ensuring low sodium performance.

[0034] 4. Precise control of powder morphology and particle size to meet high-end application requirements.

[0035] This invention achieves precise control over the spherical morphology and ultrafine particle size of powders through precise optimization of process parameters and synergistic regulation of composite mineralizers. In the ball milling stage, a gradient ratio of grinding media, combined with specific ball-to-material ratios, rotation speed, and temperature parameters, ensures uniform and refined material. During calcination, the heterogeneous nucleation of boron nitride guides directional crystal growth, while rare earth citrate and nano-zirconia synergistically optimize crystal morphology, causing particles to gradually develop towards a spherical shape. The ultrafine grinding stage further refines the particle size, ensuring the product meets ultrafine requirements. The entire preparation process forms a complete morphology and particle size control system, solving the problems of insufficient powder sphericity and wide particle size distribution in traditional processes. The spherical morphology reduces the frictional resistance of the powder in the polymer matrix, improving dispersibility and filling effect; the ultrafine particle size facilitates the construction of continuous thermal conductive pathways while ensuring the processing performance of the composite material, enabling it to meet the application requirements of high-end fields such as thermal interface materials and electronic packaging materials.

[0036] 5. Excellent whiteness control and consistent performance.

[0037] This invention establishes a comprehensive whiteness assurance system across multiple stages, from raw material selection and process design to equipment selection. The raw material pretreatment process removes coarse particulate impurities through sieving, reducing the impact of impurities in subsequent processes. All components of the composite mineralizer are made from high-purity raw materials to avoid introducing additional impurities. Throughout the production process, the inner walls of equipment in contact with materials are made of inert materials to prevent contamination by metallic impurities. Simultaneously, the low-temperature calcination process avoids oxidation and yellowing of materials at high temperatures, ensuring the whiteness of the powder. Compared to traditional processes where insufficient whiteness is caused by impurity residues and high-temperature yellowing, the powder prepared by this invention exhibits higher whiteness stability, meeting the stringent requirements of high-end applications demanding consistency in appearance and performance.

[0038] 6. The surface modification process is reasonable, improving the adaptability of powder applications.

[0039] This invention adds a surface modification step after ultrafine grinding. A suitable silane coupling agent is selected to treat the powder surface, improving the powder's surface properties through the interaction between the coupling agent molecules and the powder surface groups. The modified powder surface exhibits enhanced oleophilicity and improved compatibility with the polymer matrix, effectively reducing powder agglomeration within the matrix and improving dispersion uniformity. Simultaneously, surface modification also reduces the powder's oil absorption value, decreasing the amount of resin used in composite material preparation and lowering production costs. Compared to traditional processes lacking surface modification or suffering from poor powder-matrix compatibility and insufficient filling rate due to inappropriate modifier selection, this invention's surface modification process is more targeted. The modification parameters are optimized and adjusted to flexibly adapt to various polymer matrices according to different application scenarios, expanding the powder's application range. The entire modification process is simple to operate and seamlessly integrates with subsequent sieving and drying processes, without adding complex production steps. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to specific embodiments. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope and application of the present invention.

[0041] A method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder includes the following steps:

[0042] 1. Raw material pretreatment: Select industrial alumina powder with sodium content of 0.35-0.5wt% and average particle size D50 of 50-80μm, place it in a vacuum drying oven, dry it at 100-120℃ and -0.08--0.1MPa for 2-4 hours, cool it naturally to room temperature, and then pass it through a 200-mesh sieve. Collect the sieve material for later use.

[0043] 2. Compound mineralizer formulation: Weigh each component according to the mass ratio of boron nitride, rare earth citrate, nano zirconium oxide, polyethylene glycol, and organophosphonic acid of 1-1.5:3.5-5:1-1.5:1-1.2:0.5-0.8, place all components in a container and mix evenly to obtain the compound mineralizer.

[0044] 3. Mixed ball milling: According to the mass ratio of industrial alumina powder to composite mineralizer of 100:4-8, the pretreated industrial alumina powder and the compound mineralizer are added to the ball mill. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 4-6:1 and the ball milling speed is 350-420r / min. The ball milling is carried out at a constant temperature of 65-73℃ for 4-5 hours. The ball milling process is intermittent to obtain ball-milled powder.

[0045] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 880-920℃ at a heating rate of 8-12℃ / min in an air atmosphere. After holding at this temperature for 4-6 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0046] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2-3:3-4:1-2. Control the ball-to-material ratio to be 3-5:1 and the ball milling speed to be 470-550 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0047] 6. Surface modification: Weigh out silane coupling agent at a ratio of 0.2-0.4% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:5-10 and spray it into the initial powder. Mix the mixture at a suitable speed for 20-25 minutes to complete the surface modification treatment.

[0048] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven. It is dried at 80-100℃ and -0.08--0.1MPa for 3-5 hours. After natural cooling, low-sodium, high-whiteness, spherical ultrafine α-alumina powder is obtained.

[0049] Technical principle of the invention:

[0050] I. The Individual Role of Core Raw Materials

[0051] The core raw materials of this invention include industrial alumina powder and composite mineralizer. Each raw material plays a specific role in the preparation process, laying the foundation for the performance of the final product.

[0052] Industrial alumina powder serves as the core substrate, providing the aluminum source required for the growth of α-alumina crystals. The uniformity of particle size and control of impurities after pretreatment directly affect the consistency of subsequent crystal growth and are prerequisites for ensuring product purity and particle size distribution.

[0053] The components of the composite mineralizer have distinct and complementary functions: Boron nitride, with its near-spherical morphology and chemical stability, acts as a heterogeneous nucleation center during calcination, guiding the directional growth of alumina crystals along its surface, while its layered structure optimizes the crystal growth environment; rare earth citrate, as a crystal transformation promoter, uses its unique electronic structure to reduce the activation energy of the alumina transition from the transition phase to the α phase, accelerating the crystal transformation process; the high hardness and chemical inertness of nano-zirconia can refine the grains during ball milling and calcination, inhibiting excessive crystal growth and improving the stability of the powder structure; polyethylene glycol, as a dispersant, has its polymer chains adsorbed on the surface of powder particles, forming a steric hindrance effect that hinders particle aggregation; and organophosphonic acid, relying on its strong complexing ability, forms a stable complex with sodium ions in industrial alumina, ensuring deep desodiumification.

[0054] II. Synergistic Mechanisms Among Raw Materials

[0055] The key technological breakthrough of this invention lies in the synergistic effect between the components and raw materials of the composite mineralizer, rather than the independent function of a single component. The effect of 1+1>2 is achieved through the combination of multiple components.

[0056] Synergistic effect of crystal transformation and morphology control: Boron nitride provides a template for crystal growth through heterogeneous nucleation, while rare earth citrate lowers the activation energy of crystal transformation, enabling the crystal to rapidly complete the α-phase transformation at a lower temperature. Simultaneously, nano-zirconia refines the grains and synergistically regulates the anisotropic growth rate of the crystal with boron nitride, resulting in more uniform crystal growth along all crystal directions and ultimately forming a near-spherical morphology. This combination solves the dual problems of insufficient crystal transformation and irregular morphology in traditional processes, achieving both high α-phase conversion and near-spherical morphology simultaneously without the need for additional morphology modifiers or high-temperature treatment.

[0057] Synergistic effect of desodiuming and dispersion to prevent agglomeration: The complexation reaction between organophosphonic acid and sodium ions requires a uniformly dispersed system to proceed fully. The steric hindrance effect of polyethylene glycol ensures good dispersion of industrial alumina powder and mineralizer during ball milling, ensuring sufficient contact between organophosphonic acid and sodium ions and improving the efficiency of complexation desodiuming. At the same time, polyethylene glycol avoids agglomeration of powder due to changes in surface energy during desodiuming, ensuring the independence of particles during subsequent calcination and grinding. This synergistic effect solves the problem of incomplete desodiuming caused by uneven dispersion in traditional desodiuming processes, achieving simultaneous deep desodiuming and improved dispersibility.

[0058] Synergistic effect of stability and processing performance: The grain refinement effect of nano-zirconia combined with the dispersing effect of polyethylene glycol results in uniform particle size and stable structure, reducing crystal defects in subsequent processing; the chemical inertness of boron nitride and the complexing and impurity removal effect of organophosphonic acid work together to reduce the impurity content in the powder and improve electrical insulation performance; rare earth citrate enhances the thermal stability of the powder by optimizing the crystal structure. This multi-component synergy ensures a balanced improvement in the product's performance across multiple dimensions, including low sodium content and high stability.

[0059] III. Necessity and Importance of Process Parameter Selection

[0060] The process parameters of this invention have been systematically optimized. The selection of each parameter is directly related to the role of raw materials, synergistic mechanism and product performance. The reasonable range of parameters is the key to achieving the expected results.

[0061] Raw material pretreatment parameters: The selection of vacuum drying temperature and time must balance the removal of adsorbed water on the surface of the raw materials and the avoidance of increased brittleness caused by excessive drying of the powder. Too high a temperature or too long a time will cause fine powder agglomeration in the subsequent ball milling, while too low a temperature or insufficient time will cause residual moisture to affect the efficiency of the mineralizer. The 200-mesh sieve is used to remove coarse particle impurities in the raw materials to ensure the uniformity of the raw material particle size, which provides a basis for subsequent uniform ball milling and crystal growth.

[0062] The coordinated control of ball milling parameters—ball-to-material ratio, rotation speed, and temperature—directly affects grinding efficiency and powder dispersion. A ball-to-material ratio that is too small leads to insufficient grinding energy and inadequate powder refinement; a ratio that is too large increases collision losses between grinding media and introduces impurities. Rotation speed must match the size of the grinding media and the characteristics of the material; too high a speed can easily cause over-grinding and agglomeration, while too low a speed will fail to achieve the desired particle size. A ball milling temperature of 65-73℃ can improve the dispersion of polyethylene glycol while preventing premature decomposition of organic components due to excessive temperature, thus ensuring the integrity of the mineralizer. Intermittent operation design is used to release the heat generated during ball milling, preventing agglomeration and mineralizer deactivation caused by localized overheating.

[0063] Microwave calcination parameters: The heating rate must be controlled to match the crystal growth pattern. Too rapid a rate will lead to uneven crystal growth, resulting in defects and irregular morphologies; too slow a rate will prolong the production cycle and reduce efficiency. A calcination temperature of 880-920℃ is the optimal range for the synergistic effect of the composite mineralizer. This range meets the temperature requirements for rare earth citrate to promote crystal transformation while avoiding particle sintering and agglomeration caused by high temperatures, and also matches the heterogeneous nucleation temperature range of boron nitride. The holding time must ensure sufficient crystal transformation and complete crystal growth. Too short a time will result in insufficient α-phase conversion, while too long a time will lead to energy waste and excessive crystal growth.

[0064] Ultrafine grinding and surface modification parameters: The media ratio, ball-to-material ratio, and rotation speed of ultrafine grinding must be precisely matched to the hardness and particle size of the calcined material to achieve precise control of the ultrafine particle size, while avoiding over-grinding that could damage the spherical morphology. The amount of silane coupling agent added in surface modification must be strictly controlled; too little will not achieve sufficient modification, while too much will form multiple layers of coating on the powder surface, affecting thermal conductivity and dispersion performance. The mixing time must ensure sufficient bonding between the coupling agent and the powder surface to guarantee the stability of the modification effect.

[0065] IV. Unexpected Technical Effects Achieved

[0066] By leveraging the synergistic effect of raw materials and precisely matching process parameters, this invention achieves technical results that exceed the expectations of traditional processes, solving multiple industry pain points.

[0067] Firstly, it achieves simultaneous optimization of multiple key performance characteristics. In traditional processes, performance characteristics such as low sodium content, spherical shape, high α-phase conversion rate, and ultrafine particle size are often mutually restrictive and need to be controlled separately through multiple process steps. However, this invention achieves the above performance targets simultaneously in a single process flow by means of the synergistic effect of composite mineralizers, without the need to add additional complex procedures. This simplifies the production process and improves the balance of product performance.

[0068] Secondly, this invention achieves a balance between high α-phase conversion rate and near-spherical morphology under low-temperature conditions. Traditional processes require high-temperature driving for α-phase transformation, which can easily lead to particle sintering and agglomeration, thus destroying the spherical morphology. This invention reduces the crystal transformation temperature through the synergistic effect of composite mineralizers, achieving a high α-phase conversion rate at 880-920℃ while avoiding particle sintering, ensuring near-spherical morphology and ultrafine particle size, and overcoming the contradiction between low temperature and high performance.

[0069] Third, the sodium removal process does not rely on highly corrosive acids. Traditional sodium removal processes often use acid washing, which easily corrodes equipment and affects powder purity. This invention achieves deep sodium removal under mild conditions through the synergistic effect of organophosphonic acid and polyethylene glycol, while ensuring equipment safety and resolving the conflict between sodium removal and environmental protection.

[0070] Fourth, the product's processing adaptability is improved. The appropriate loose density and tapped density obtained through process parameter control, combined with the surface modification effect, greatly improve the dispersion and filling rate of the powder in the polymer matrix. This avoids the processing difficulties caused by agglomeration or density imbalance of traditional ultrafine powders, provides convenience for the subsequent preparation of composite materials, and expands the application scenarios of the product.

[0071] To make the present invention more fully disclosed, more specific embodiments are described below.

[0072] Example 1:

[0073] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.35wt% and an average particle size D50 of 72μm. Place it in a vacuum drying oven and dry it at 100℃ and -0.1MPa for 4h. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0074] 2. Compound mineralizer formulation: The components of boron nitride, lanthanum citrate, nano zirconium oxide, polyethylene glycol, and ethylenediaminetetramethylenephosphonic acid are weighed separately according to a mass ratio of 1.4:4.8:1.4:1.2:0.7. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0075] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:6. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 5.5:1 and the ball milling speed is 400r / min. The ball milling is carried out at a constant temperature of 69℃ for 5 hours. The ball milling is run intermittently during the process to obtain the ball-milled powder.

[0076] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 900°C at a heating rate of 10°C / min in an air atmosphere. After holding at this temperature for 5 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0077] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 3:4:2. Control the ball-to-material ratio to be 4:1 and the ball milling speed to be 510 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0078] 6. Surface modification: Weigh vinyltrimethoxysilane at a ratio of 0.3% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:9 and spray it into the initial powder. Mix the mixture at a suitable speed for 24 minutes to complete the surface modification treatment.

[0079] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 95℃ and -0.1MPa for 4.5h. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0080] Example 2:

[0081] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.42wt% and an average particle size D50 of 58μm. Place it in a vacuum drying oven and dry it at 110℃ and -0.08MPa for 2.5h. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0082] 2. Compound mineralizer formulation: The components of boron nitride, cerium citrate, nano zirconium oxide, polyethylene glycol, and hydroxyethylidene diphosphonic acid are weighed separately according to the mass ratio of 1.1:3.8:1.1:1.0:0.5. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0083] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:4. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 4.5:1 and the ball milling speed is 360r / min. The ball milling is carried out at a constant temperature of 68℃ for 4 hours. The ball milling process is intermittent to obtain the ball-milled powder.

[0084] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 910°C at a heating rate of 9°C / min in an air atmosphere. After holding at this temperature for 4.5 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0085] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2:3:1. Control the ball-to-material ratio to be 3.5:1 and the ball milling speed to be 480 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0086] 6. Surface modification: Weigh γ-glycidyl etheroxypropyltrimethoxysilane at a ratio of 0.25% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:6 and spray it into the initial powder. Mix the mixture at a suitable speed for 20 minutes to complete the surface modification treatment.

[0087] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 85℃ and -0.08MPa for 3.5h. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0088] Example 3:

[0089] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.38wt% and an average particle size D50 of 65μm. Place it in a vacuum drying oven and dry it at 105℃ and -0.09MPa for 3 hours. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0090] 2. Compound mineralizer formulation: The components of boron nitride, lanthanum citrate, nano zirconium oxide, polyethylene glycol, and aminotrimethylene phosphonic acid are weighed separately according to a mass ratio of 1.2:4.2:1.3:1.1:0.6. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0091] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:5. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 5:1, the ball milling speed is 380r / min, and the ball milling is carried out at a constant temperature of 70℃ for 4.5h. The ball milling process is intermittent to obtain ball-milled powder.

[0092] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 920°C at a heating rate of 10°C / min in an air atmosphere. After holding at this temperature for 5 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0093] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2.5:3.5:1.5. Control the ball-to-material ratio to be 4:1 and the ball milling speed to be 500 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0094] 6. Surface modification: Weigh γ-aminopropyltriethoxysilane at a ratio of 0.3% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:8 and spray it into the initial powder. Mix the mixture at a suitable speed for 22 minutes to complete the surface modification treatment.

[0095] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 90℃ and -0.09MPa for 4 hours. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0096] Example 4:

[0097] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.48wt% and an average particle size D50 of 55μm. Place it in a vacuum drying oven and dry it at 115℃ and -0.09MPa for 2 hours. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0098] 2. Compound mineralizer formulation: The components of boron nitride, cerium citrate, nano zirconium oxide, polyethylene glycol, and aminotrimethylene phosphonic acid are weighed separately according to a mass ratio of 1.0:3.5:1.0:1.0:0.5. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0099] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:4. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 4:1 and the ball milling speed is 350r / min. The ball milling is carried out at a constant temperature of 65℃ for 4 hours. The ball milling is run intermittently during the process to obtain the ball-milled powder.

[0100] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 910°C at a heating rate of 8°C / min in an air atmosphere. After holding at this temperature for 4 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0101] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2.5:3:1.5. Control the ball-to-material ratio to be 3:1 and the ball milling speed to be 470 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0102] 6. Surface modification: Weigh γ-aminopropyltriethoxysilane at a ratio of 0.2% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:5 and spray it into the initial powder. Mix the mixture at a suitable speed for 21 minutes to complete the surface modification treatment.

[0103] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 80℃ and -0.08MPa for 3 hours. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0104] Example 5:

[0105] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.45wt% and an average particle size D50 of 78μm. Place it in a vacuum drying oven and dry it at 120℃ and -0.1MPa for 3.5h. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0106] 2. Compound mineralizer formulation: The components of boron nitride, lanthanum citrate, nano zirconium oxide, polyethylene glycol, and hydroxyethylidene diphosphonic acid are weighed separately according to a mass ratio of 1.5:5.0:1.5:1.2:0.8. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0107] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:8. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 6:1 and the ball milling speed is 420r / min. The ball milling is carried out at a constant temperature of 71℃ for 5 hours. The ball milling is run intermittently during the process to obtain the ball-milled powder.

[0108] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 880°C at a heating rate of 12°C / min in an air atmosphere. After holding at this temperature for 6 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0109] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 3:3.5:1.5. Control the ball-to-material ratio to be 5:1 and the ball milling speed to be 550 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0110] 6. Surface modification: Weigh γ-glycidyl etheroxypropyltrimethoxysilane at a ratio of 0.4% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:10 and spray it into the initial powder. Mix the mixture at a suitable speed for 25 minutes to complete the surface modification treatment.

[0111] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 100℃ and -0.1MPa for 5 hours. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0112] Example 6:

[0113] 1. Raw material pretreatment: Select industrial alumina powder with sodium content of 0.40wt% and average particle size D50 of 62μm, place it in a vacuum drying oven, dry it at 108℃ and -0.09MPa for 2.8h, cool it naturally to room temperature, and then pass it through a 200-mesh sieve. Collect the sieve material for later use.

[0114] 2. Compound mineralizer formulation: The components of boron nitride, cerium citrate, nano zirconium oxide, polyethylene glycol, and ethylenediaminetetramethylenephosphonic acid are weighed separately according to a mass ratio of 1.3:4.5:1.2:1.1:0.65. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0115] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:6. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 5.2:1 and the ball milling speed is 390r / min. The ball milling is carried out at a constant temperature of 71℃ for 4.8h. The ball milling process is intermittent to obtain ball-milled powder.

[0116] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 910°C at a heating rate of 10.5°C / min in an air atmosphere. After holding at this temperature for 5.2 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0117] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2.8:3.2:1.8. Control the ball-to-material ratio at 4.2:1 and the ball milling speed at 510 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0118] 6. Surface modification: Weigh vinyltrimethoxysilane at a ratio of 0.32% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:7 and spray it into the initial powder. Mix the mixture at a suitable speed for 23 minutes to complete the surface modification treatment.

[0119] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 92℃ and -0.09MPa for 4.2h. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0120] Example 7:

[0121] 1. Raw material pretreatment: Select industrial alumina powder with a sodium content of 0.37wt% and an average particle size D50 of 68μm. Place it in a vacuum drying oven and dry it at 102℃ and -0.085MPa for 3.2h. After naturally cooling to room temperature, pass it through a 200-mesh sieve and collect the sieve material for later use.

[0122] 2. Compound mineralizer formulation: The components of boron nitride, lanthanum citrate, nano zirconium oxide, polyethylene glycol, and aminotrimethylene phosphonic acid are weighed separately according to a mass ratio of 1.25:4.3:1.35:1.05:0.62. All components are placed in a container and mixed evenly to obtain the compound mineralizer.

[0123] 3. Mixed ball milling: The pretreated industrial alumina powder and the compound mineralizer are added to the ball mill at a mass ratio of 100:5.5. At the same time, 99% ceramic alumina balls are added as grinding media. The grinding media is composed of 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm and 8-10mm in a mass ratio of 2:3:1. The ball-to-material ratio is controlled at 4.8:1 and the ball milling speed is 370r / min. The ball milling is carried out at a constant temperature of 67℃ for 4.3h. The ball milling is run intermittently during the process to obtain ball-milled powder.

[0124] 4. Microwave calcination: The ball-milled powder is placed in a microwave calcination furnace and heated to 900°C at a heating rate of 9.5°C / min in an air atmosphere. After holding at this temperature for 4.8 hours, it is naturally cooled to room temperature to obtain the calcined product.

[0125] 5. Ultrafine grinding: Place the calcined material in an ultrafine ball mill and add zirconia balls as grinding media. The grinding media is composed of zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm in a mass ratio of 2.2:3.6:1.4. Control the ball-to-material ratio to be 3.8:1 and the ball milling speed to be 490 r / min. Continue grinding until the powder fineness D50 < 2 μm to obtain the initial powder.

[0126] 6. Surface modification: Weigh γ-aminopropyltriethoxysilane at a ratio of 0.28% of the initial powder mass. Before use, dilute it with anhydrous ethanol at a ratio of 1:7.5 and spray it into the initial powder. Mix the mixture at a suitable speed for 22.5 minutes to complete the surface modification treatment.

[0127] 7. Sieving and drying: The modified powder is sieved to remove coarse particle impurities. The sieved material is collected and placed in a vacuum drying oven and dried at 88℃ and -0.085MPa for 3.8h. After natural cooling, low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder is obtained.

[0128] Comparative Example 1:

[0129] Only the composite mineralizer did not contain boron nitride. The mass ratios of the other components were adjusted proportionally according to the ratio after boron nitride removal (the mass ratio of lanthanum citrate, nano-zirconium oxide, polyethylene glycol, and ethylenediaminetetramethylenephosphonic acid was 4.8:1.4:1.2:0.7). All other operating steps and process parameters were completely consistent with those in Example 1.

[0130] Comparative Example 2:

[0131] The composite mineralizer was not supplemented with lanthanum citrate. The mass ratios of the other components were adjusted proportionally according to the ratio after the removal of lanthanum citrate (the mass ratio of boron nitride, nano-zirconia, polyethylene glycol, and ethylenediaminetetramethylenephosphonic acid was 1.4:1.4:1.2:0.7). All other operating steps and process parameters were completely consistent with those in Example 1.

[0132] Comparative Example 3:

[0133] The composite mineralizer did not contain nano-zirconia, and the mass ratios of the other components were adjusted proportionally according to the ratio after removing nano-zirconia (the mass ratio of boron nitride, lanthanum citrate, polyethylene glycol, and ethylenediaminetetramethylenephosphonic acid was 1.4:4.8:1.2:0.7). All other operating steps and process parameters were completely consistent with those in Example 1.

[0134] Comparative Example 4:

[0135] Only the composite mineralizer did not contain polyethylene glycol. The mass ratios of the other components were adjusted proportionally according to the ratio after removing polyethylene glycol (the mass ratio of boron nitride, lanthanum citrate, nano-zirconium oxide, and ethylenediaminetetramethylenephosphonic acid was 1.4:4.8:1.4:0.7). All other operating steps and process parameters were completely consistent with those in Example 1.

[0136] Comparative Example 5:

[0137] Only the composite mineralizer did not contain organophosphonic acid. The mass ratios of the other components were adjusted proportionally according to the ratios after the removal of organophosphonic acid (the mass ratios of boron nitride, lanthanum citrate, nano-zirconia, and polyethylene glycol were 1.4:4.8:1.4:1.2). All other operating steps and process parameters were completely consistent with those in Example 1.

[0138] Comparative Example 6:

[0139] The only difference is that a constant temperature condition was not used during the ball milling process, and the temperature naturally rose during the milling process. All other operating steps and process parameters were completely consistent with those in Example 1.

[0140] Comparative Example 7:

[0141] The only difference was that the heating rate of microwave calcination was adjusted to 6°C / min, while all other operating steps and process parameters were completely consistent with those in Example 1.

[0142] Comparative Example 8:

[0143] The only difference was that the microwave calcination temperature was adjusted to 850°C, while all other operating steps and process parameters were exactly the same as in Example 1.

[0144] Comparative Example 9:

[0145] The only difference is that the zirconia balls used in the ultrafine grinding were not compounded according to the specified ratio, and only a single specification of zirconia balls with a diameter of 1-2 mm were used. All other operating steps and process parameters were completely consistent with those in Example 1.

[0146] Comparative Example 10:

[0147] The only difference is that the surface modification treatment was not performed. The initial powder after ultrafine grinding was directly sieved and dried. All other operation steps and process parameters were completely consistent with those in Example 1.

[0148] Comparative Example 11:

[0149] The only difference was that the mass ratio of each component in the composite mineralizer was adjusted to boron nitride: lanthanum citrate: nano zirconium oxide: polyethylene glycol: ethylenediaminetetramethylenephosphonic acid = 0.8:6.0:0.8:0.8:0.4. All other operating steps and process parameters were completely consistent with those in Example 1.

[0150] Comparative Example 12:

[0151] The only difference was that the rotation speed of the ball mill was adjusted to 300 r / min, while all other operating steps and process parameters were exactly the same as in Example 1.

[0152] Comparative Example 13:

[0153] The only difference was that the mass ratio of industrial alumina powder to composite mineralizer was adjusted to 100:2, while all other operating steps and process parameters were completely consistent with those in Example 1.

[0154] Comparative Example 14:

[0155] The only difference was that the microwave calcination holding time was adjusted to 3 hours; all other operating steps and process parameters were completely consistent with those in Example 1.

[0156] Comparative Example 15:

[0157] The amount of silane coupling agent added was adjusted to 0.1% only during surface modification; all other operating steps and process parameters were completely consistent with those in Example 1.

[0158] Single-factor experiment:

[0159] Eight single-factor experiments were designed. In each experiment, only the target parameter was changed, while the other parameters remained the same as in Example 1.

[0160] The experiment used Na2O content, sphericity, whiteness, and oil absorption as the core evaluation indicators, and the testing methods all adopted the latest national standards, as detailed below:

[0161] Na2O content: GB / T6609.4-2004 "Chemical analysis methods and physical property determination methods for alumina - Part 4: Determination of sodium oxide content by flame atomic absorption spectrometry";

[0162] Sphericity: GB / T21649.1-2024 "Particle size analysis, image analysis method - Part 1: static image analysis method";

[0163] Whiteness: GB / T5950-2008 "Methods for measuring the whiteness of building materials and non-metallic mineral products";

[0164] Oil absorption value: GB / T5211.15-2014 "General Test Methods for Pigments and Extenders - Part 15: Determination of Oil Absorption".

[0165] Experiment 1: Single-factor experiment on the addition ratio of composite mineralizer (industrial alumina: composite mineralizer)

[0166] Parameter range: 100:4, 100:5, 100:6, 100:7, 100:8. Experimental results are shown in Table 1.

[0167]

[0168] Conclusion Analysis: When the addition ratio is <100:5, the amount of mineralizer is insufficient, the organophosphonic acid cannot fully complex sodium ions, and the Na2O content is too high; the heterogeneous nucleation effect of boron nitride and the crystal form promotion effect of lanthanum citrate are insufficient, resulting in low sphericity and α-phase conversion rate; the dispersion effect of polyethylene glycol is limited, and powder agglomeration leads to a larger D50 and increased oil absorption value. When the addition ratio is >100:7, excess mineralizer will form a coating layer on the powder surface, hindering the ultrafine grinding effect, and the D50 will increase slightly; excess organic components increase the powder gaps, the oil absorption value increases slightly, and the excess mineralizer does not bring about substantial performance improvement, only causing raw material waste. Optimal parameter: 100:6.

[0169] Experiment 2: Single-factor experiment on mixing ball milling temperature

[0170] Parameter range: 65℃, 67℃, 69℃, 71℃, 73℃. The experimental results are shown in Table 2.

[0171]

[0172] Conclusion and Analysis: When the temperature is below 67℃, the dispersing effect of polyethylene glycol (PEG) increases with increasing temperature. At lower temperatures, the dispersing effect is insufficient, leading to powder agglomeration and a higher D50. Insufficient contact between the mineralizer and alumina powder limits the sodium removal efficiency and crystal transformation effect, resulting in higher Na2O content and lower α-phase conversion rate. When the temperature is above 71℃, excessively high temperatures cause slight decomposition of PEG, resulting in the loss of some dispersing and anti-agglomeration functions, slight powder agglomeration, increased D50, and higher oil absorption value. Simultaneously, high temperatures may reduce the activity of the organic mineralizer, slightly decreasing sphericity and α-phase conversion rate. Optimal parameter: 69℃.

[0173] Experiment 3: Single-factor experiment on microwave calcination temperature

[0174] Parameter range: 880℃, 890℃, 900℃, 910℃, 920℃. Experimental results are shown in Table 3.

[0175]

[0176] Conclusion and Analysis: When the temperature is below 890℃, the low temperature leads to insufficient activation energy for the α-phase transformation, resulting in a low α-phase conversion rate; the complexation reaction between organophosphonic acid and sodium ions is incomplete, leading to a high Na₂O content; and the heterogeneous nucleation effect of boron nitride is insufficient, resulting in low sphericity. When the temperature is above 910℃, the high temperature accelerates crystal growth, but the anisotropic growth rate of the crystals becomes unbalanced, and the sphericity decreases; at the same time, the high temperature easily leads to slight sintering and agglomeration of powder particles, increasing D50 and oil absorption value; excessively high temperatures can also cause slight yellowing of the powder and a gradual decrease in whiteness. Optimal parameter: 900℃.

[0177] Experiment 4: Single-factor experiment on microwave calcination holding time

[0178] Parameter range: 4h, 4.5h, 5h, 5.5h, 6h. Experimental results are shown in Table 4.

[0179]

[0180] Conclusion and Analysis: When the holding time is less than 4.5 hours, insufficient holding leads to incomplete α-phase transformation and a low α-phase conversion rate; the complexation reaction between organophosphonic acid and sodium ions is incomplete, resulting in a high Na₂O content; insufficient crystal growth time leads to low sphericity, incomplete powder agglomeration, and a high oil absorption value. When the holding time is greater than 5.5 hours, extending the holding time has a slight effect on improving the α-phase conversion rate and Na₂O content, but it can lead to excessive crystal growth and a slight increase in D50; at the same time, prolonged calcination may cause slight oxidation of the powder surface, a slight decrease in sphericity, and increased energy consumption and production cycle. Optimal parameter: 5 hours.

[0181] Experiment 5: Single-factor experiment on microwave calcination heating rate

[0182] Parameter range: 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min. The experimental results are shown in Table 5.

[0183]

[0184] Conclusion and Analysis: When the heating rate is <9℃ / min, the slow heating leads to a prolonged production cycle, uneven crystal growth rate, and low sphericity; the reaction rate between the mineralizer and the powder is slow, the desodiuming and crystal transformation effects are not optimal, and the Na2O content is high. When the heating rate is >11℃ / min, the rapid heating causes thermal stress inside the powder, resulting in crystal defects and a decrease in sphericity; excessively high local temperatures can easily cause slight sintering and agglomeration of particles, increasing D50 and oil absorption value; at the same time, rapid heating accelerates the volatilization of organophosphonic acid, reduces desodiuming efficiency, and increases Na2O content. Optimal parameter: 10℃ / min.

[0185] Experiment 6: Single-factor experiment on the ratio of ultrafine grinding balls to powder

[0186] Parameter range: 3:1, 3.5:1, 4:1, 4.5:1, 5:1. Experimental results are shown in Table 6.

[0187]

[0188] Conclusion and Analysis: When the ball-to-particle ratio is <4:1, the grinding energy is insufficient to fully break up the agglomerates of the calcined material, resulting in a higher D50. The powder particles have a rough surface and a higher oil absorption value. However, the ball-to-particle ratio has no significant effect on the Na2O content, α-phase conversion rate, and whiteness, as these properties have already been determined in the previous process. When the ball-to-particle ratio is >4.2:1, the grinding energy is excessive. Over-grinding will destroy the spherical morphology of the crystals, reducing the sphericity. The wear of the grinding media is aggravated, which may introduce trace impurities, resulting in a slight decrease in whiteness. At the same time, the reduction in D50 is limited, but it increases equipment wear and energy consumption. Optimal parameter: 4.2:1.

[0189] Experiment 7: Single-factor experiment on the amount of surface-modified silane coupling agent added

[0190] Parameter range: 0.2%, 0.25%, 0.3%, 0.35%, 0.4%. Experimental results are shown in Table 7.

[0191]

[0192] Conclusion and Analysis: When the addition amount is <0.25%, the coupling agent is insufficient and cannot completely cover the powder surface, resulting in poor modification effect; the powder surface lacks sufficient oleophilicity, dispersibility is poor, and the oil absorption value is high; D50 increases slightly due to agglomeration. When the addition amount is >0.35%, excessive coupling agent forms a multi-layer coating on the powder surface, increasing the risk of particle adhesion; D50 increases slightly, and the oil absorption value increases slightly; excess coupling agent does not improve the modification effect, only wastes raw materials, and may affect the whiteness of the powder. The amount of coupling agent added has no significant effect on Na2O content, sphericity, and α-phase conversion rate, as these properties are determined by the previous process. Optimal parameter: 0.3%.

[0193] Experiment 8: Single-factor experiment on ultrafine grinding speed

[0194] Parameter range: 470 r / min, 490 r / min, 510 r / min, 530 r / min, 550 r / min. The experimental results are shown in Table 8.

[0195]

[0196] Conclusion and Analysis: When the rotational speed is <490 r / min, the grinding kinetic energy is insufficient, the powder is not finely refined, and the D50 is too large; particle agglomeration is not completely eliminated, and the oil absorption value is too high. When the rotational speed is >530 r / min, the excessive speed leads to increased collision between the grinding media and the powder, destroying the near-spherical morphology of the crystals and reducing the near-sphericity; the excessively high speed increases the powder temperature, and slight agglomeration leads to a slight increase in the oil absorption value; at the same time, increasing the speed has a limited effect on reducing D50, but increases energy consumption and equipment wear. The rotational speed has no significant effect on Na2O content, α-phase conversion rate, and whiteness, as these properties have been determined in the previous process. Optimal parameter: 510 r / min.

[0197] Testing and data analysis of material performance indicators:

[0198] 1. Test Design

[0199] The powders prepared in 7 examples and 15 comparative examples were tested according to the latest national standards for 8 core indicators, including Na2O content, sphericity, α-phase conversion rate, D50, whiteness, oil absorption value, loose density, and tap density. The test results are shown in Table 9 (examples) and Table 10 (comparative examples).

[0200] 2. Performance test results of the example (Table 9)

[0201]

[0202] 3. Comparative Performance Test Results (Table 10)

[0203]

[0204] 4. Data Comparison and Analysis and Theoretical Analysis:

[0205] (1) Internal performance consistency analysis of the embodiment

[0206] The performance indicators of the seven embodiments showed small fluctuations: Na2O content 0.011-0.018wt%, sphericity 85.4%-88.5%, α-phase conversion rate 96.3%-97.6%, D50 1.42-1.64μm, whiteness 94.4%-95.3%, oil absorption value 22.8-25.3mL / 100g, loose density 0.55-0.59g / cm³, and tapped density 0.83-0.88g / cm³. This indicates that the process parameters of the present invention are stable and reliable, and the performance consistency is good during mass production, meeting the stability requirements of industrial production.

[0207] (2) Performance comparison analysis of the examples and comparative examples

[0208] ①The necessity of composite mineralizer components

[0209] Comparative Examples 1-5, each lacking one component of the composite mineralizer, all exhibited varying degrees of performance degradation.

[0210] The highest Na2O content was found in the absence of organophosphonic acid (Comparative Example 5), which was 3.5 times that of Example 1. Since organophosphonic acid is the core component of complex desodiumization, deep desodiumization cannot be achieved when it is missing.

[0211] The α-phase conversion rate was lowest when lanthanum citrate was missing (Comparative Example 2), which was 4.3% lower than that of Example 1. This is because lanthanum citrate can reduce the activation energy of the α-phase transformation, and the crystal transformation is insufficient after its absence.

[0212] The sphericity of the sample lacking boron nitride (Comparative Example 1) was only 79.8%, which was 8.7% lower than that of Example 1, because the heterogeneous nucleation of boron nitride is the key to the formation of spheric morphology;

[0213] The oil absorption value of the sample lacking polyethylene glycol (Comparative Example 4) reached 27.8 mL / 100g, which is 5.0 mL / 100g higher than that of Example 1. Due to the lack of the dispersing and anti-agglomeration effect of polyethylene glycol, the powder agglomeration was severe.

[0214] The D50 of the missing nano-zirconia (Comparative Example 3) is greater than that of Example 1 because nano-zirconia can refine the grains, resulting in uneven crystal growth after its absence.

[0215] ② Rationality of process parameters (Comparative Example 6-15)

[0216] Comparative Examples 6-15 demonstrate the necessity of parameter ranges by changing a single process parameter:

[0217] Mixed ball milling without temperature control (Comparative Example 6): D50 and oil absorption value were higher than those in Example 1. Due to temperature fluctuations, the dispersion effect of polyethylene glycol was destroyed, resulting in powder agglomeration.

[0218] Microwave calcination heating rate is too low (Comparative Example 7): α phase conversion rate is low, and the crystal transformation rate is uneven due to slow heating, resulting in incomplete reaction.

[0219] Microwave calcination temperature is too low (Comparative Example 8): Sphericity and α-phase conversion rate decrease because the temperature does not reach the optimal range for the synergistic effect of composite mineralizers, and the heterogeneous nucleation and crystal transformation effects are limited.

[0220] Single-size zirconia balls for ultrafine grinding (Comparative Example 9): D50 and oil absorption value increased significantly. Due to the lack of gradient media ratio, the grinding refinement and morphology protection were unbalanced, resulting in particle agglomeration and rough surface.

[0221] No surface modification (Comparative Example 10): The oil absorption value was the highest among all samples. Due to insufficient oleophilicity of the powder surface, poor compatibility with the organic matrix, and severe agglomeration.

[0222] The composite mineralizer ratio was out of range (Comparative Example 11): Multiple properties declined across the board, with Na2O content, sphericity, and α-phase conversion rate all failing to meet the standards, due to the imbalance of component ratios destroying the synergistic effect.

[0223] The ball mill speed was too low (Comparative Example 12): D50 (1.85μm) was too high, the oil absorption value increased, and due to insufficient grinding energy, the powder was not fined enough and the particles agglomerated.

[0224] The proportion of mineralizer added was too low (Comparative Example 13): The performance was the worst. The sphericity, α-phase conversion rate and oil absorption value all deviated significantly from the level of the example. Due to insufficient mineralizer, it could not perform the functions of desodiuming and morphology control.

[0225] Microwave calcination holding time is too short (Comparative Example 14): α phase conversion rate and sphericity are low. Due to insufficient crystal growth and crystal form transformation time, the performance is not optimal.

[0226] Insufficient addition of silane coupling agent (Comparative Example 15): The oil absorption value was higher than that of Example 1. This was because the coupling agent did not completely cover the powder surface, resulting in insufficient modification effect and poor dispersibility.

[0227] Theoretical Analysis: The example demonstrates how the synergistic effect of the components of the composite mineralizer, combined with microwave low-temperature calcination and precise process parameter control, simultaneously achieves balanced optimization of multiple properties, including low sodium content, near-spherical shape, ultrafine particle size, and suitable density. In contrast, the comparative example, due to the absence of a single component or deviation of process parameters from the optimal range, disrupts the synergistic effect of the system, leading to incomplete sodium removal, irregular morphology, and uncontrolled particle size and density. This proves the necessity and rationality of the composite mineralizer system and process design of this invention, effectively addressing the pain point of mutual performance constraints in existing technologies.

[0228] The above description, in conjunction with preferred embodiments, provides a further detailed explanation of the present invention and should not be construed as limiting the specific implementation of the invention to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the inventive concept, and all such substitutions or modifications should be considered within the scope of protection of the present invention.

[0229] Although the invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made without departing from the spirit and scope of the invention. Furthermore, the scope of the invention is not limited to the specific embodiments of the processes, methods, and steps described in the specification. From the disclosure of this invention, those skilled in the art will readily utilize existing or future processes, methods, steps that substantially perform the same function or achieve the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to cover such processes, methods, steps.

Claims

1. A method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder, characterized in that: Includes the following steps: (1) Raw material pretreatment: Dry the industrial alumina powder at 100-120℃ and -0.08--0.1MPa vacuum for 2-4 hours, cool it and pass it through a 200-mesh sieve, collect the sieve material and set it aside; (2) Compound mineralizer formulation: Boron nitride, rare earth citrate, nano-zirconia, polyethylene glycol, and organophosphonic acid were weighed according to the following mass ratio and mixed evenly to obtain the compound mineralizer; the mass ratio of each component was boron nitride: rare earth citrate: nano-zirconia: Polyethylene glycol:organophosphonic acid = (1-1.5):(3.5-5):(1-1.5):(1-1.2):(0.5-0.8); (3) Mixed ball milling: The industrial alumina powder pretreated in step (1) and the composite mineralizer in step (2) are added to the ball mill at a mass ratio of 100:(4-8). Grinding media are added, and the ball-to-material ratio is controlled at (4-6):

1. The ball milling speed is 350-420 r / min, and the ball milling is carried out at 65-73℃ for 4-5 hours with intermittent operation during the process to obtain ball milled powder. (4) Microwave calcination: The ball milled powder from step (3) is placed in a microwave calcination furnace and heated to 880-920°C at a heating rate of 8-12°C / min in an air atmosphere. The temperature is then maintained for calcination for 4-6 hours, and the powder is cooled to obtain the calcined product. (5) Ultrafine grinding: Place the calcined material from step (4) into an ultrafine ball mill, add grinding media with a diameter of 0.5-3 mm, control the ball-to-material ratio (3-5):1, and the ball milling speed of 470-550 r / min, grind until D50 < 2 μm, and obtain the initial powder. (6) Surface modification: Add 0.2-0.4% of the mass of silane coupling agent to the initial powder of step (5), mix for 20-25 min, and perform surface modification treatment; (7) Sieving and drying: The modified powder from step (6) is sieved to remove coarse particles. The sieved material is dried at 80-100℃ and -0.08--0.1MPa vacuum for 3-5 hours to obtain low sodium, high whiteness, spherical ultrafine α-alumina powder.

2. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The boron nitride mentioned in step (2) is hexagonal boron nitride with a purity ≥99% and D50 <5μm; the rare earth citrate is lanthanum citrate or cerium citrate; the nano-zirconia has an average particle size of 20-100nm; the polyethylene glycol has a number average molecular weight of 200-2000; and the organophosphonic acid is at least one of aminotrimethylenephosphonic acid, hydroxyethylidene diphosphonic acid, or ethylenediaminetetramethylenephosphonic acid.

3. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The grinding media mentioned in step (3) are 99% ceramic alumina balls with diameters of 3-5mm, 5-8mm, and 8-10mm, compounded in a mass ratio of 2:3:

1.

4. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The microwave frequency of the microwave calcining furnace mentioned in step (4) is 2450MHz or 915MHz; the temperature fluctuation during the calcination process is ≤±5℃.

5. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The ultrafine grinding media mentioned in step (5) are zirconia balls with diameters of 0.5-1 mm, 1-2 mm, and 2-3 mm, compounded in a mass ratio of (2-3):(3-4):(1-2).

6. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The silane coupling agent mentioned in step (6) is one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane or vinyltrimethoxysilane. Before use, it is diluted with anhydrous ethanol at a ratio of 1:(5-10) and then sprayed in.

7. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The sodium content of the industrial alumina powder in step (1) is 0.35-0.5wt%, and the average particle size D50 is 50-80μm.

8. The method for preparing low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 1, characterized in that: The mass ratio of rare earth citrate to boron nitride in step (2) is (2-4):

1.

9. The low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder prepared by the method according to any one of claims 1-8, characterized in that: The powder has a Na₂O content of <0.02wt%, a sphericity >85%, an α-phase conversion rate ≥96%, a fineness D50 <2μm, a whiteness >94%, and a bulk density of 0.5-0.6g / cm³. 3 Tap density: 0.8-0.9 g / cm³ 3 Oil absorption value ≤25mL / 100g.

10. The application of the low-sodium, high-whiteness, near-spherical ultrafine α-alumina powder according to claim 9 as a thermally conductive filler in the preparation of thermally conductive interface materials, electronic packaging materials, or high thermal conductivity composite materials.

Images