Highly heat-conductive spherical alumina powder and method for preparing the same

Abstract

The application relates to the technical field of alumina powder, and particularly discloses high-thermal-conductivity spherical alumina powder and a preparation method thereof. The method comprises the following steps: mixing gamma-Al2O3 or alpha-Al2O3 powder, a composite mineralizer composed of titanium dioxide and bismuth oxide, a bonding agent, a dispersing agent and deionized water, and performing ball milling to prepare slurry; obtaining precursor particles through spray drying; spraying the precursor particles into a flame field of a spheroidizing device by using a carrier gas to perform melting spheroidization; rapidly cooling the product to below 300 DEG C within 2s to obtain an alumina intermediate; and finally performing static calcination in an air atmosphere to obtain high-thermal-conductivity spherical alumina powder. Through the combination of the composite mineralizer and the process of "melting rapid cooling + static calcination", the problems of insufficient alpha phase conversion, small crystal grains and serious phonon scattering in the prior art are solved, the synergy of high sphericity, high alpha phase content and coarse crystal grains is realized, and the intrinsic thermal conductivity of the powder and the filling performance of the composite material are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of alumina powder technology, and more particularly to a high thermal conductivity spherical alumina powder and its preparation method. Background Technology

[0002] In the field of thermal management for electronic devices, thermal interface materials (TIMs) are crucial for bridging the micro-gaps between chips and heat sinks and reducing contact thermal resistance. Their performance largely depends on the thermally conductive filler used. Spherical alumina has become the mainstream filler due to its excellent overall performance, but existing products face bottlenecks:

[0003] 1. Limited Improvement in Thermal Conductivity: Commercially available spherical alumina is mostly prepared via melt spraying. For example, α-phase alumina powder is used as raw material, sphericalized through high-temperature melt spraying, and then the resulting spherical alumina powder is rapidly passed through a high-temperature heating section using the residual heat from sphericalization and heated air as a carrier to obtain spherical α-phase alumina. However, in actual large-scale production, spherical alumina prepared by melt spraying often suffers from incomplete α-phase transformation or fine grains. Numerous grain boundaries become the main source of phonon scattering, severely limiting further improvement in the thermal conductivity of the composite material.

[0004] 2. The contradiction between filler and rheology: Increasing the filler content is a direct way to improve thermal conductivity, but it often leads to a sharp increase in the viscosity of composite materials, which deteriorates the coating, potting and other construction processes.

[0005] Therefore, developing an alumina filler preparation technology that can simultaneously achieve high α-phase content, grain coarsening, and high sphericity without excessively increasing the process temperature has become an urgent need in the industry. Summary of the Invention

[0006] The invention aims to provide a high thermal conductivity spherical alumina powder and its preparation method. By combining a composite mineralizer with a process of "melt rapid cooling + static calcination", the invention solves the problems of insufficient α phase transformation, small grain size and severe phonon scattering in the prior art. It achieves a synergistic effect of high sphericity, high α phase content and coarse grain size, which significantly improves the intrinsic thermal conductivity of the powder and the filling performance of composite materials.

[0007] The technical solution adopted in the invention is:

[0008] A method for preparing highly thermally conductive spherical alumina powder includes the following steps:

[0009] Step S1: Mix γ-Al2O3 powder or α-Al2O3 powder with composite mineralizer, binder, dispersant and deionized water according to the specified ratio, and then ball mill to obtain a uniform slurry; wherein, the composite mineralizer includes titanium dioxide and bismuth oxide, and the amount of each is 0.25~1% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder, respectively;

[0010] Step S2: Spray dry the slurry to obtain alumina precursor particles with a D50 particle size of 50~200μm;

[0011] Step S3: The alumina precursor particles are sprayed into the flame field of the spheroidizing equipment using a carrier gas to perform melt spheroidization; the spheroidized product is discharged from the outlet of the spheroidizing equipment immediately after leaving the flame field and cooled from the flame field temperature to below 300°C within 2 seconds to obtain the alumina intermediate.

[0012] Step S4: The alumina is statically calcined in an air atmosphere to obtain highly thermally conductive spherical alumina powder.

[0013] Furthermore, the total amount of titanium dioxide and bismuth oxide used does not exceed 1.5% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder. In this invention, by controlling the amount of composite mineralizer, the residual low-melting-point phase or powder sintering caused by excessive composite mineralizer is prevented. Under the premise of ensuring sufficient promotion of α-phase transformation and grain growth, the chemical stability and insulation of the product are ensured.

[0014] Furthermore, the amount of adhesive used is 0.2% to 1.2% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder;

[0015] And / or, the amount of dispersant used is 0.2% to 1.2% of the weight of the γ-Al₂O₃ powder or the α-Al₂O₃ powder. In this invention, the proportion of binder and / or dispersant is precisely controlled to avoid uneven slurry dispersion and granulation difficulties due to insufficient amount, and excessive amount leading to decomposition at high temperatures, generating a large amount of gas, resulting in hollow or broken particles. This range ensures that the alumina precursor particles have high strength and low porosity.

[0016] Furthermore, the adhesive is at least one of polyvinyl alcohol and polyvinylpyrrolidone;

[0017] And / or, the dispersant is at least one of ammonium polyacrylate and ammonium citrate. In this invention, a specific combination of additives that is water-soluble, easily decomposes at high temperatures, and leaves no metal residue is selected to avoid introducing Na. + Ca 2+ The removal of alkali metal ion impurities ensures the high purity and high resistivity of the final high thermal conductivity spherical alumina powder, meeting the requirements of electronic packaging.

[0018] Further, the process parameters for ball milling in step S1 are as follows: alumina ceramic balls are used, the ball-to-material mass ratio is 2~3:1, the rotation speed is 100~200 rpm, the time is 4~8 h, the solid content of the slurry is controlled at 50~70%, and the ball milling temperature is ≤60℃. This invention employs pollution-free ball milling media (alumina ceramic balls) and a low-temperature ball milling process. On the one hand, this ensures uniform mixing of the composite mineralizer with γ-Al2O3 powder or α-Al2O3 powder; on the other hand, it prevents premature phase transformation of γ-Al2O3 powder or slurry flocculation at high temperatures, laying a uniform foundation for subsequent spray drying.

[0019] Furthermore, the process parameters for spray drying in step S2 are: inlet temperature 200~320℃, outlet temperature 100~200℃, negative pressure -100~-200Pa, atomizer speed 8000~20000rpm, and feed pump flow rate 165~335mL / min. In this invention, by controlling the evaporation rate and atomization intensity, near-spherical alumina precursor particles with high solidity, narrow particle size distribution, and good flowability are prepared, avoiding breakage or hollow spheres caused by water boiling in the flame field in step S3.

[0020] Further, the process parameters for molten spheroidization in step S3 are as follows: the carrier gas is oxygen, with a flow rate of 130~140 m³ / h and a powder feeding rate of 220~250 kg / h; the natural gas flow rate is 230~240 m³ / h; the powder residence time is 0.1~1 s; the cooling method is to use a partitioned water-cooled system to rapidly solidify spherical particles, followed by gas-solid separation using a cyclone separator and a bag filter to collect the product. In this invention, by setting appropriate molten spheroidization process parameters, a dense and smooth spherical amorphous / metastable alumina intermediate can be obtained.

[0021] Furthermore, the process parameters for static calcination in step S4 are: temperature 1200~1600℃, time 2~10h. This invention defines an optimal thermodynamic window for grain coarsening; if the temperature is too low, the phase transformation will be incomplete, and if it is too high, hard agglomeration will occur between particles. Under these process parameters, the liquid-phase sintering mechanism of the composite mineralizer is utilized to maximize grain size while maintaining the spherical morphology of the particles.

[0022] A high thermal conductivity spherical alumina powder, prepared by the aforementioned method for preparing high thermal conductivity spherical alumina powder, is an aggregate of primary alumina crystals; wherein the average grain size of the primary alumina crystals is 5~20μm; and the specific surface area is ≤0.1m². 2 / g.

[0023] The beneficial effects of the invention are:

[0024] 1. Existing technologies rely on residual heat from melting for phase transformation, often resulting in insufficient α-phase transformation. This invention achieves process decoupling through a two-step heat treatment design of "flame field melting and spheroidization" and "independent static calcination." During flame melting and spheroidization, the low-melting-point eutectic properties of the composite mineralizer (bismuth oxide forms a liquid phase at lower temperatures, greatly promoting particle diffusion and migration; titanium dioxide can effectively dissolve in the alumina lattice or accumulate at grain boundaries, reducing the α-phase formation energy barrier and inhibiting normal grain growth) significantly reduce the melting activation energy of alumina, completing particle surface melting and spheroidization within a very short residence time (0.1~1s). Simultaneously, rapid cooling within 2s "freezes" the high-temperature metastable structure, preventing excessive grain growth or microcracks at high temperatures. During static calcination of the alumina intermediate in air, the grain boundary liquid phase formed by the composite mineralizer drives the complete transformation of the residual γ-phase or transition phase into the α-phase at a relatively mild temperature, while simultaneously promoting grain growth.

[0025] 2. Spherical alumina prepared by traditional melt spraying methods results in small grains (typically <10 μm) due to rapid cooling, with numerous grain boundaries becoming phonon scattering centers. This invention utilizes a synergistic mechanism of "composite mineralizer induction + rapid cooling shaping + slow calcination coarsening." Titanium dioxide / bismuth oxide segregates at grain boundaries, lowering the grain boundary energy and allowing grains to merge and grow across multiple original particle boundaries during static calcination. The final product has a large grain size (average grain size 5~20 μm), significantly reducing the number of grain boundaries, thereby significantly reducing the phonon scattering probability and improving the intrinsic thermal conductivity of the powder itself.

[0026] 3. This invention ensures high sphericity and suitable particle size (D50 particle size: 50~200μm) of the precursor through ball milling and spray drying. Flame field melting and spheroidization utilizes surface melting rather than overall vaporization, and is combined with rapid cooling to prevent collapse, resulting in particles with high true sphericity and low void ratio. This alumina powder with high sphericity, high thermal conductivity, and low specific surface area, when used in TIMs, can achieve high bulk density and high packing fraction, significantly increasing the filling amount without significantly increasing viscosity, thus solving the industry pain point of "high thermal conductivity requires high filling, and high filling leads to difficult processing." Attached Figure Description

[0027] 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, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 The image shows an electron microscope (EM) image of alumina powder obtained by static calcination at 1200°C in Example 1.

[0029] Figure 2 The image shows an electron microscope (EM) image of alumina powder obtained by static calcination at 1600°C in Example 1.

[0030] Figure 3 This is an electron microscope image of the alumina powder obtained after static calcination at 1200℃ in Example 2.

[0031] Figure 4 The image shows an electron microscope (EM) image of alumina powder obtained by static calcination at 1600°C in Example 2.

[0032] Figure 5 This is an electron microscope image of the alumina powder obtained after static calcination at 1200℃ in Example 3.

[0033] Figure 6 This is an electron microscope image of the alumina powder obtained after static calcination at 1600°C in Example 3.

[0034] Figure 7 The image shows an electron microscope image of the alumina powder obtained after static calcination at 1200℃ in Comparative Example 1.

[0035] Figure 8 The image shows an electron microscope image of the alumina powder obtained by static calcination at 1600℃ in Comparative Example 1.

[0036] Figure 9 The image shows an electron microscope image of the alumina powder obtained after static calcination at 1200℃ in Comparative Example 2.

[0037] Figure 10 The image shows an electron microscope image of the alumina powder obtained by static calcination at 1600℃ in Comparative Example 2.

[0038] Figure 11 The image shows an electron microscope image of the alumina powder obtained after static calcination at 1200℃ in Comparative Example 3.

[0039] Figure 12 The image shows an electron microscope (EM) image of the alumina powder obtained by static calcination at 1600 °C in Comparative Example 3.

[0040] Figure 13 The image shows an electron microscope image of the alumina powder obtained in Comparative Example 4.

[0041] Figure 14 The image shows the XRD pattern of the high thermal conductivity spherical alumina powder obtained by static calcination at 1450℃ in Example 1. Detailed Implementation

[0042] The embodiments of the invention will now be described in detail with reference to the accompanying drawings.

[0043] Example 1

[0044] A method for preparing highly thermally conductive spherical alumina powder includes the following steps:

[0045] Step S1, Slurry preparation: Take 1 kg of commercial α-Al2O3 powder (D50 particle size = 5~7 μm), add 7.5 g of TiO2 (0.75% of the α-Al2O3 powder mass) and 7.5 g of Bi2O3 (0.75% of the α-Al2O3 powder mass), add 5 g of PVA (0.5% of the α-Al2O3 powder mass), 5 g of ammonium polyacrylate (0.5% of the α-Al2O3 powder mass), and deionized water, controlling the solid content of the slurry to 65%. Use Al2O3 ceramic balls with a ball-to-powder ratio of 2.5:1, ball mill at 150 rpm for 6 hours at a ball milling temperature of 55℃.

[0046] Step S2, granulation: spray drying, inlet temperature 260℃, outlet temperature 130℃, negative pressure -150Pa, atomization speed 10000rpm, flow rate 200mL / min, to obtain alumina precursor particles with D50 particle size = 85~87μm.

[0047] Step S3, Spheroidization: O2 carrier gas 130m 3 / h, powder delivery rate 220kg / min, natural gas flow rate 230m³ / h 3 / h, powder residence time is about 1s, and the outlet is rapidly cooled to <300℃ within 2s by indirect water cooling to obtain alumina intermediate.

[0048] Step S4, Static Calcination: Static calcination at 1200℃~1600℃ for 4 hours in air atmosphere to obtain highly thermally conductive spherical alumina powder (an aggregate of alumina precursor crystals). The test results of highly thermally conductive spherical alumina powder prepared at different static calcination temperatures are shown in Table 1 below. For the thermal conductivity and extrusion rate tests, the highly thermally conductive spherical alumina powder was filled with 94.5 vol% of silicone grease (vinyl silicone oil RH-Vi1323, viscosity 302.29 cs, this silicone grease was used in subsequent examples and control examples).

[0049] Table 1 Test Results

[0050] Example 2

[0051] A method for preparing highly thermally conductive spherical alumina powder includes the following steps:

[0052] Step S1, pulping: Commercial α-Al2O3 powder was used, and TiO2, 1% and Bi2O3, 1% were added. The remaining parameters were the same as in Example 1, and the solid content of the pulp was controlled to be 65%.

[0053] Steps S2 and S3 are the same as in Example 1.

[0054] Step S4, Static Calcination: Alumina precursor particles with a D50 particle size of 85~87μm were statically calcined in air at 1200℃~1600℃ for 4h to obtain highly thermally conductive spherical alumina powder. The test results of highly thermally conductive spherical alumina powder prepared at different static calcination temperatures are shown in Table 2 below. For the thermal conductivity and extrusion rate tests, the highly thermally conductive spherical alumina powder was filled with 94.5 vol% silicone grease.

[0055] Table 2 Test Results

[0056] Example 3

[0057] A method for preparing highly thermally conductive spherical alumina powder includes the following steps:

[0058] Step S1, pulping: commercial α-Al2O3 powder was used, and TiO2, 0.25% and Bi2O3, 0.25% were added. The remaining parameters were the same as in Example 1, and the solid content of the pulp was controlled to be 65%.

[0059] Steps S2 and S3 are the same as in Example 1.

[0060] Step S4, Static Calcination: Alumina precursor particles with a D50 particle size of 85~87μm were statically calcined in air at 1200℃~1600℃ for 4h to obtain highly thermally conductive spherical alumina powder. The test results of highly thermally conductive spherical alumina powder prepared at different static calcination temperatures are shown in Table 3 below. For the thermal conductivity and extrusion rate tests, the highly thermally conductive spherical alumina powder was filled with 94.5 vol% silicone grease.

[0061] Table 3 Test Results

[0062] Compare with Example 1

[0063] A method for preparing alumina powder, comprising the following steps:

[0064] Step S1, pulping: commercial α-Al2O3 powder was used, with TiO2, 0.2% and Bi2O3, 0.2% added. The remaining parameters were the same as in Example 1, and the solid content of the pulp was controlled to be 65%.

[0065] Steps S2 and S3 are the same as in Example 1.

[0066] Step S4, Static Calcination: Alumina precursor particles with a D50 particle size of 85~87μm were statically calcined in air at 1200℃~1600℃ for 4h to obtain alumina powder. The test results of alumina powder prepared at different static calcination temperatures are shown in Table 4 below. For the thermal conductivity and extrusion rate tests, the alumina powder was filled with silicone grease at a concentration of 94.5 vol%.

[0067] Table 4 Test Results

[0068] Compare with Example 2

[0069] A method for preparing alumina powder, comprising the following steps:

[0070] Step S1, pulping: Except for not adding any mineralizer (TiO2 / Bi2O3), the other parameters are the same as in Example 1, and the solid content of the pulp is controlled at 65%.

[0071] Steps S2 and S3 are the same as in Example 1.

[0072] Step S4, Static Calcination: Alumina precursor particles with a D50 particle size of 85~87μm were statically calcined in air at 1200℃~1600℃ for 4h to obtain alumina powder. The test results of alumina powder prepared at different static calcination temperatures are shown in Table 5 below. For the thermal conductivity and extrusion rate tests, the alumina powder was filled with silicone grease at a concentration of 94.5 vol%.

[0073] Table 5 Test Results

[0074] Compare with Example 3

[0075] A method for preparing alumina powder, comprising the following steps:

[0076] Step S1, pulping: The preparation process is the same as in Example 1, but TiO2, 1.5% and Bi2O3, 1.5% are added, and the solid content of the pulp is controlled at 65%.

[0077] Steps S2 and S3 are the same as in Example 1.

[0078] Step S4, Static Calcination: Alumina precursor particles with a D50 particle size of 85~87μm were statically calcined in air at 1200℃~1600℃ for 4h to obtain alumina powder. The test results of alumina powder prepared at different static calcination temperatures are shown in Table 6 below. For the thermal conductivity and extrusion rate tests, the alumina powder was filled with silicone grease at a concentration of 94.5 vol%.

[0079] Table 6 Test Results

[0080] Compare with Example 4

[0081] A method for preparing alumina powder, comprising the following steps:

[0082] Step S1, pulping: Except for not adding any mineralizer (TiO2 / Bi2O3), the pulp solid content is 65%, and the rest is the same as step S1 in Example 1.

[0083] Step S2, granulation: spray drying, inlet temperature 260℃, outlet temperature 130℃, negative pressure -150Pa, atomization speed 10000rpm, flow rate 200mL / min, to obtain alumina precursor particles with D50 particle size = 85~87μm.

[0084] Step S3, Spheroidization: O2 carrier gas 130m 3 / h, powder delivery rate 220kg / h, natural gas flow rate 230m³ / h 3 / h, alumina powder was obtained. The test results of the alumina powder are shown in Table 7 below. Among them, the thermal conductivity and extrusion rate were tested with alumina powder filled in silicone grease at 94.5 vol%.

[0085] Table 7 Test Results

[0086] By comparing the data of the examples (with added compound mineralizer) and the control examples (without compound mineralizer or with excessive compound mineralizer), the following conclusions can be drawn:

[0087] 1. The key role of compound mineralizer dosage: balancing the promoting effect and side effects.

[0088] In Examples 1 (TiO2, 0.75% + Bi2O3, 0.75%, total 1.5%) and 3 (TiO2, 0.25% + Bi2O3, 0.25%, total 0.5%), when statically calcined at 1200~1600℃, the primary crystal grains reached 5~20μm (coarse grains reduce phonon scattering), the thermal conductivity was up to 7.1W / (m·K), the extrusion rate was up to 24.66 g / min (maintaining good processability even with high filling content), and the specific surface area was down to 0.04m² / g (low specific surface area reduces interfacial thermal resistance), fully meeting the design goal of "high thermal conductivity + easy processing".

[0089] Compared with Example 1 (TiO2, 0.2% + Bi2O3, 0.2%, total 0.4%), the composite mineralizer could not fully promote the α phase transformation and grain growth. The original crystal grains were only 3~7μm (with increased fine grain boundaries and intensified phonon scattering). The highest thermal conductivity was 6.8W / (m·K), and the highest extrusion rate was 21.86 g / min. This also shows that "0.25%~1% of a single component" is the optimal range for the composite mineralizer to play its role.

[0090] Compared with Example 3 (TiO2, 1.5% + Bi2O3, 1.5%, total 3%), although the short-term grain size reached 15~30μm, the thermal conductivity dropped to 6.8 W / (m·K) at 1600℃ and the extrusion rate dropped to 16.44g / min. This shows that the total amount of composite mineralizer exceeding 3% will cause side effects, destroy the stability of powder, and the processability is significantly reduced under high filling amount.

[0091] 2. The necessity of process decoupling: melt rapid cooling and shaping + static calcination phase transformation / grain growth

[0092] In contrast to Example 4, which did not involve static calcination but only spheroidization through melting, the original crystal grains were only 2.3 μm (with dense fine grain boundaries), with a thermal conductivity of 6.2 W / (m·K) and an extrusion rate of 13.52 g / min. This demonstrates that "static calcination" is the key step in achieving complete transformation of the α phase and grain coarsening—flame melting alone cannot overcome the bottleneck of traditional technology.

[0093] In contrast to Example 2, which lacked a composite mineralizer, the α-phase transformation was insufficient after static calcination, with the original crystal grains ranging from 2 to 7 μm. The highest thermal conductivity was 6.6 W / (m·K), and the highest extrusion rate was 18.72 g / min. This demonstrates that the composite mineralizer is the core element for "promoting α-phase transformation and grain coarsening," and neither can be omitted.

[0094] 3. Effectiveness of process parameter windows: Ensuring the performance of precursors and final products.

[0095] Example 1 employed ball milling with alumina ceramic balls at a ball-to-material ratio of 2.5:1, at 150 rpm for 6 hours, with a solid content of 65% and a temperature of 55°C, to ensure a uniform and uncontaminated slurry. Spray drying was then performed at an inlet temperature of 260°C, an outlet temperature of 130°C, a negative pressure of -150 Pa, atomization at 10000 rpm, and a flow rate of 200 mL / min to obtain a precursor with a D50 of 85-87 μm (high solidity and good flowability), laying a uniform foundation for subsequent melt spheroidization. Comparative Examples 1-3 maintained these parameters, but still exhibited poor performance due to issues with the mineralizer, demonstrating that precursor quality is a prerequisite for subsequent processes.

[0096] Example 1 employed a process of melting and spheroidizing using "130 m³ / h of O2 carrier gas, 220 kg / h of powder feed rate, 230 m³ / h of natural gas, 1 s residence time, and 2 s rapid cooling to <300°C" to ensure surface melting and spheroidization and "freezing" of the metastable structure. Static calcination was performed at "1200~1600°C for 4 h", reaching its optimal value at 1550°C (thermal conductivity 7.0 W / (m·K), extrusion 24.66 g / min), verifying that "1200~1600°C for 2~10 h" is the best window for grain coarsening without hard agglomeration.

[0097] Figure 1 and Figure 2 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Example 1. Figure 3 and Figure 4 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Example 2. Figure 5 and Figure 6 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Example 3. Figure 7 and Figure 8 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Comparative Example 1. Figure 9 and Figure 10 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Comparative Example 2. Figure 11 and Figure 12 The image shows electron microscope (EM) images of alumina powder obtained by static calcination at 1200°C and 1600°C in Comparative Example 3. Figure 13 The image shows an electron microscope image of the alumina powder obtained in Comparative Example 3.

[0098] pass Figures 1-13 By comparison, the following conclusions can be drawn:

[0099] 1. Sphericity maintenance: Synergy between process decoupling and dosage control

[0100] Examples 1-3 ( Figures 1-6 At 1200℃ and 1600℃, the particles were all regularly spherical with a dense surface and no collapse, and the sphericity was >0.95% (meeting the high filling requirements); Control Example 3 ( Figures 11-12 Excessive use of composite mineralizer (3% total) leads to obvious sintering and agglomeration (hard agglomeration) between particles at 1600℃, which destroys sphericity. In other words, "total amount of composite mineralizer ≤2% (more preferably ≤1.5%) + static calcination at 1200~1600℃" is the key to maintaining high sphericity - excessive use or excessively high temperature will lead to hard agglomeration.

[0101] 2. Grain coarsening effect: The core role of composite mineralizers

[0102] Examples 1-3 ( Figures 1-6 At 1200℃, grains have already begun to grow. Figure 1 , 3 5), at 1600℃ the grain size increases significantly to about 20μm. Figure 2 , 4 6), coarse grains significantly reduce the number of grain boundaries and decrease phonon scattering; compared to Example 1 ( Figures 7-8 ): The composite mineralizer was insufficient, and the grain size was still only about 6 μm at 1600℃ (with many fine grain boundaries); Comparative Example 2 ( Figures 9-10 (Without composite mineralizer, the grain size is still around 6μm at 1600℃, which indicates that composite mineralizer is a necessary condition for "grain coarsening to 5~20μm", solving the pain point of "fine grains" in traditional technology.)

[0103] 3. The dangers of excessive mineralizers: a direct manifestation of microstructure.

[0104] Compare with Example 3 ( Figures 11-12 In the process, a large number of sintering necks (hard agglomerates) appear between particles at 1600℃. Figure 13 The presence of obvious impurity phases at the grain boundaries (presumably residual phases of excessive composite mineralizer) corresponds perfectly to the data conclusion that "excessive use leads to residual low-melting-point phases and performance degradation," which also explains the necessity of "total amount of composite mineralizer ≤2% (more preferably ≤1.5%)."

[0105] By using a composite mineralizer (TiO2+Bi2O3, total dosage ≤2%, more preferably ≤1.5%), the energy barrier for α-phase formation is lowered, promoting grain coarsening; by using molten rapid cooling, the metastable structure is frozen, maintaining sphericity; and by using static calcination, the complete transformation of the α-phase is achieved, resulting in grain coarsening (5~20μm). Ultimately, a synergistic effect of "high sphericity, high α-phase content, and coarse grains (5~20μm)" is achieved, significantly improving the intrinsic thermal conductivity of the powder (≥6.6 W / (m·K)) and the processability of the composite material (extrusion rate ≥16 g / min), solving the industry pain point that "high thermal conductivity requires high filler, and high filler leads to difficult processing".

[0106] Figure 14 This is the XRD pattern of the high thermal conductivity spherical alumina powder obtained by static calcination at 1450℃ in Example 1. The data in the figure indicate that it is a pure α phase with no other impurities.

[0107] It should be noted that although the embodiments do not exhaustively list all process parameters for spray drying, melt spheroidization, and static calcination, the specification clearly defines the key process windows: In the spray drying stage, by controlling the inlet temperature to 200-320℃, the outlet temperature to 100-200℃, the negative pressure to -100 to -200Pa, the atomizer speed to 8000-20000 rpm, and the feed pump flow rate to 165-335 mL / min, near-spherical precursor particles with high solidity, narrow particle size distribution (D50 of 50-200μm), and good flowability are obtained, avoiding hollowness or breakage during subsequent spheroidization; in the melt spheroidization stage, by setting the carrier gas (oxygen) flow rate to 130-140 m³ / h, the powder feeding rate to 220-250 kg / h, and the natural gas flow rate to 230-240... The process involves a flow rate of m³ / h, allowing the powder to undergo surface melting in a high-temperature flame field for 0.1–1 s. This is followed by rapid cooling to below 300°C within 2 s using a partitioned water cooling system. This "freezes" the metastable structure while maintaining particle sphericity (>0.95%) and surface density, laying the structural foundation for grain coarsening and α-phase transformation in the subsequent static calcination stage. The static calcination temperature is 1200–1600°C, and the time is 2–10 h. If the temperature is too low, the phase transformation will be incomplete; if it is too high, hard agglomeration will occur between particles. Under these process parameters, the liquid-phase sintering mechanism of the composite mineralizer maximizes grain size while maintaining the spherical morphology of the particles. Those skilled in the art can anticipate the corresponding technical effects based on the foregoing embodiments and comparative examples. In other words, the high thermal conductivity spherical alumina powder prepared by the method of this invention is an aggregate of alumina primary crystals; wherein the average grain size of the alumina primary crystals is 5–20 μm; and the specific surface area is ≤0.1 m². 2 / g.

Claims

1. A method for preparing highly thermally conductive spherical alumina powder, characterized in that, Includes the following steps: Step S1: Mix γ-Al2O3 powder or α-Al2O3 powder with composite mineralizer, binder, dispersant and deionized water according to the specified ratio, and then ball mill to obtain a uniform slurry; wherein, the composite mineralizer includes titanium dioxide and bismuth oxide, and the amount of each is 0.25~1% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder, respectively; Step S2: Spray dry the slurry to obtain alumina precursor particles with a D50 particle size of 50~200μm; Step S3: The alumina precursor particles are sprayed into the flame field of the spheroidizing equipment using a carrier gas to perform melt spheroidization; the spheroidized product is discharged from the outlet of the spheroidizing equipment immediately after leaving the flame field and cooled from the flame field temperature to below 300°C within 2 seconds to obtain the alumina intermediate. Step S4: The alumina is statically calcined in an air atmosphere to obtain highly thermally conductive spherical alumina powder.

2. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The total amount of titanium dioxide and bismuth oxide used shall not exceed 1.5% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder.

3. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The amount of adhesive used is 0.2~1.2% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder; And / or, the amount of the dispersant is 0.2 to 1.2% of the weight of the γ-Al2O3 powder or the α-Al2O3 powder.

4. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The adhesive is at least one of polyvinyl alcohol and polyvinylpyrrolidone; And / or, the dispersant is at least one of ammonium polyacrylate and ammonium citrate.

5. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The process parameters for ball milling in step S1 are as follows: alumina ceramic balls are used, the ball-to-material mass ratio is 2~3:1, the rotation speed is 100~200 rpm, the time is 4~8 h, the solid content of the slurry is controlled to be 50~70%, and the ball milling temperature is ≤60℃.

6. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The process parameters for spray drying in step S2 are as follows: inlet temperature 200~320℃, outlet temperature 100~200℃, negative pressure -100~-200Pa, atomizer speed 8000~20000rpm, and feed pump flow rate 165~335mL / min.

7. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The process parameters for molten spheroidization in step S3 are as follows: the carrier gas is oxygen, with a flow rate of 130~140 m³ / h and a powder feeding rate of 220~250 kg / h; the natural gas flow rate is 230~240 m³ / h; the powder residence time is 0.1~1 s; and the cooling method is indirect water cooling to below 300°C.

8. The method for preparing high thermal conductivity spherical alumina powder according to claim 1, characterized in that, The process parameters for static calcination in step S4 are: temperature 1200~1600℃, time 2~10h.

9. A high thermal conductivity spherical alumina powder, prepared by the method for preparing high thermal conductivity spherical alumina powder according to any one of claims 1 to 8, characterized in that, It is an aggregate of primary alumina crystals; wherein the average grain size of the primary alumina crystals is 5~20μm; and the specific surface area is ≤0.1m². 2 / g.

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