A method for preparing a high-density large-particle spheroidal alumina powder

Abstract

This invention discloses a method for preparing high-density, large-particle-size spherical alumina powder, belonging to the field of inorganic non-metallic material preparation technology. The method uses industrial aluminum hydroxide or industrial alumina containing a transition phase as raw material. It involves introducing 1-10% α-alumina seed crystals and 1.0-3.0% composite mineralizer (containing boron and fluorine sources) by mass of the raw materials. After mechanical mixing, the mixture is calcined in air at 1350-1550℃ for 2-8 hours. Finally, the mixture is cooled, dispersed, and sieved to obtain the final product. This invention, through the synergistic effect of seed crystals and composite mineralizer, simultaneously achieves α-phase transformation, densification, grain growth, and morphology control of alumina powder under mild calcination conditions. It successfully prepares alumina powder with an α-phase content ≥96%, true density >3.96 g / cm³, median particle size D50 ≥50 μm, and a regular spherical morphology. This method is simple, low-cost, and the product is suitable for high-filling thermally conductive fillers, showing great promise for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic material preparation technology, specifically relating to a method for preparing high-density, large-particle-size, spherical alumina powder. Background Technology

[0002] Alpha-alumina (Al₂O₃) is an extremely important industrial material. Due to its excellent chemical stability, good insulation, and thermal conductivity, it is widely used in the field of thermally conductive fillers, such as in the preparation of thermally conductive silicone and epoxy resin potting compounds. By controlling the morphology of alumina powder to make it spherical or near-spherical, the powder's filling density and flowability can be effectively improved, thereby significantly enhancing the thermal conductivity of the composite material.

[0003] To better improve the performance of thermally conductive fillers, the industry expects to obtain alumina powder that simultaneously possesses high α-phase content (to ensure high thermal conductivity and stability), large particle size (D50>50μm, to reduce interfacial thermal resistance and increase filling rate), and good spherical morphology (to optimize flowability and bulk density).

[0004] However, existing technologies, especially the mainstream solid-state calcination method in industry, face a series of irreconcilable core contradictions and technical bottlenecks in achieving the above goals: 1. Difficulty in coordinating morphology control and phase transformation-densification processes: While high temperatures help increase density (promoting α-phase transformation) and grain growth, they also cause severe sintering and particle adhesion, leading to morphological deterioration and the inability to obtain near-spherical particles; while low temperatures can maintain morphology to some extent, they cannot complete sufficient phase transformation and densification, resulting in low α-phase content and true density. This problem corresponds to the beneficial effect of "simultaneously completing phase transformation, densification, and morphology control under mild conditions" to be achieved in this invention. 2. Difficulty in effectively obtaining large-diameter single particles: Under the traditional solid-state reaction mass transfer mechanism, grain growth is limited, and strong hard agglomerates easily form between particles, resulting in products that are actually agglomerates of multiple small particles, rather than truly large-diameter single particles. This not only affects the particle size but also seriously impairs the flowability and filling performance of the powder. This problem corresponds to the beneficial effect of this invention in achieving "large-diameter (D50 ≥ 50μm) single spherical particles" by suppressing hard agglomeration. 3. Single function of additives and lack of synergistic effect: Although there have been attempts to use single mineralizers or seed crystals in the prior art, the effects are limited. For example, while seed crystals alone can promote α-phase transformation, they cannot effectively improve morphology and may even aggravate sintering; while certain mineralizers (such as boric acid) alone may have a certain rounding effect on morphology, their effect on promoting α-phase transformation and grain growth is insufficient, and high density and large particle size cannot be obtained. There is a lack of technical means to synergistically reduce the phase transformation energy barrier, promote grain growth, and simultaneously "polish" the morphology. This problem directly corresponds to the core beneficial effect brought about by the "synergistic effect of α-alumina seed crystals and composite mineralizers" in this invention. 4. Difficulty in balancing process economy and product performance: In order to achieve high performance, some methods have to adopt complex processes, harsh conditions, or expensive raw materials, resulting in high production costs and making it difficult to meet the needs of large-scale industrial production. This problem corresponds to the beneficial effects pursued by this invention: "simple process, low cost, and suitable for industrialization".

[0005] Therefore, there is an urgent need in the field to develop a new method for preparing alumina powder with high density (high α phase content), large particle size and good spherical morphology at low cost and high efficiency under relatively mild calcination conditions, in order to overcome the above-mentioned defects of the prior art. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing solid-state calcination methods for preparing alumina powder, which make it difficult to simultaneously achieve high α-phase content, large particle size, and good spherical morphology. This invention provides a simple, low-cost method for preparing high-density, large-particle-size, spherical alumina powder suitable for industrial production.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-density, large-particle-size spherical alumina powder, characterized by comprising the following steps: Step 1: Raw material preparation Using industrial aluminum hydroxide or industrial alumina containing transition phase alumina as raw material, α-alumina seed crystals and composite mineralizers are added; The amount of α-alumina seed crystals added is 1-10% of the total mass of the raw materials; The composite mineralizer contains a boron source and a fluorine source, and its total addition amount is 1.0-3.0% of the total mass of the raw materials; Step 2: Mix The raw materials, seed crystals, and composite mineralizers are mechanically mixed until all components are uniformly dispersed. Step 3: Calcination The uniformly mixed materials are calcined at high temperature in air atmosphere, with a calcination temperature of 1350-1550℃ and a calcination time of 2-8 hours. Step 4: Post-processing After calcination, the material is cooled, dispersed, and sieved to obtain the high-density, large-particle-size spherical alumina powder.

[0008] Furthermore, the boron source in the composite mineralizer is selected from boric acid, and the fluorine source is selected from one or more of aluminum fluoride, ammonium fluoride, magnesium fluoride, or calcium fluoride.

[0009] Furthermore, the heating rate during the calcination process is 1-10℃ / min.

[0010] The alumina powder prepared by the above method has an α phase content of not less than 96%, a true density greater than 3.96 g / cm³, a median particle size D50 of not less than 50 μm, and the particles have a spherical morphology.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Enhanced synergistic effect between seed crystals and mineralizers By introducing α-alumina seed crystals and specific composite mineralizers (including boron and fluorine sources), a dual mechanism of gas-phase transport and liquid-phase promotion is formed during calcination, which effectively reduces the temperature and energy barrier of α-phase transformation, promotes the formation of high-density α-alumina phase, significantly improves particle sphericity, and inhibits the occurrence of solid-phase sintering and hard agglomeration.

[0012] 2. The overall performance of the product has been significantly improved. The prepared alumina powder has high α phase content (not less than 96%), high true density (greater than 3.96 g / cm³), large median particle size (D50 ≥ 50μm) and good spherical morphology (length-to-short axis ratio less than 1.3), with excellent comprehensive performance, and is especially suitable for the field of high-filling-rate thermal conductive materials.

[0013] 3. The calcination process has a wide processing window and mild conditions. By optimizing the calcination temperature (1350–1550℃), time (2–8 hours), and heating rate (1–10℃ / min), phase transformation, grain growth, and morphology control were achieved simultaneously under relatively mild conditions, overcoming the contradiction between morphology degradation caused by high temperature and incomplete phase transformation at low temperature in traditional methods.

[0014] 4. The process is simple, the cost is low, and it is suitable for industrialization. The method described has a simple process flow, requires no complex equipment, and uses common industrial raw materials and auxiliary materials, making the cost controllable. The product is loose and easy to disperse after calcination, requiring no strong crushing, and has low energy consumption, making it highly valuable for industrial promotion. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of the preparation method of the present invention.

[0016] Figure 2 This is a scanning electron microscope (SEM) image of the alumina powder prepared in Example 1.

[0017] Figure 3 This is a photograph of the particle size distribution of the alumina powder prepared in Example 1.

[0018] Figure 4 This is a SEM image of the alumina powder prepared in Comparative Example 1 (without seed crystals and mineralizers).

[0019] Figure 5 This is a SEM image of the alumina powder prepared in Comparative Example 2 (with only seed crystals added and no mineralizer added).

[0020] Figure 6 This is a SEM image of the alumina powder prepared in Comparative Example 3 (with only boric acid mineralizer added). Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto. The method of the present invention can prepare alumina powders with different particle sizes and morphologies by adjusting parameters such as raw material ratio, calcination temperature and time. The following embodiments are only used to illustrate the technical effects of the present invention and are not intended to limit the scope of protection of the present invention.

[0022] Example 1 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1: Raw material preparation: Take 1000g of industrial aluminum hydroxide (ATH) powder as raw material, and add 50g of α-Al2O3 seed crystals (median particle size D50 is about 3.8μm, α phase content is 97.1%, accounting for 5% of the total mass of raw material), 10g of boric acid (H3BO3, accounting for 1.0% of the total mass of raw material), 10g of aluminum fluoride (AlF3, accounting for 1.0% of the total mass of raw material) and 5g of ammonium fluoride (NH4F, accounting for 0.5% of the total mass of raw material) as composite mineralizer.

[0023] Step 2, Mix: Place all the above materials in a mixer and dry ball mill for 2 hours to ensure that each component is evenly dispersed.

[0024] Step 3, calcination: The uniformly mixed material is placed in an alumina crucible and then placed in a box-type high-temperature furnace. The temperature is increased from room temperature to 1470°C at a rate of 5°C / min under air atmosphere, and then held at that temperature for 3 hours.

[0025] Step 4, Post-processing: After calcination, the material was allowed to cool naturally to room temperature with the furnace. Upon removal, the material was observed to be a loose powder that could be dispersed with only slight vibration. Finally, it was passed through a 100-mesh sieve (approximately 150 μm in diameter), and 2.5 kg of alumina ceramic balls with a diameter of 10-20 mm were added to a ball mill jar, along with 400 g of alumina. The mixture was then ground for 3 hours to obtain the final alumina powder product.

[0026] Product performance test results: The product has an α-phase content of 97.7% (quantitative analysis by XRD), a true density of 3.98 g / cm³, a tap density of 1.68 g / cm³, and a median particle size (D50) of 62.9 μm as measured by a laser particle size analyzer. Scanning electron microscopy (SEM) observation revealed that the particles are distinctly spherical with smooth surfaces and good sphericity (the ratio of the major axis to the minor axis is less than 1.3).

[0027] Example 2 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1: Raw material preparation: Take 1000g of industrial alumina powder containing the θ-Al2O3 transition phase as raw material, and add 30g of α-Al2O3 seed crystals (median particle size D50 is about 4.3μm, α phase content is 95.3%, accounting for 3% of the total mass of raw material), 5g of boric acid (H3BO3, accounting for 0.5% of the total mass of raw material) and 10g of magnesium fluoride (MgF2, accounting for 1.0% of the total mass of raw material) as composite mineralizers.

[0028] Step 2, Mix: Place all the above materials in a mixer and dry ball mill for 3 hours to ensure uniform dispersion of each component. Step 3, calcination: The uniformly mixed material is placed in an alumina crucible and then placed in a box-type high-temperature furnace. The temperature is increased from room temperature to 1530°C at a rate of 8°C / min under air atmosphere, and then held at that temperature for 6 hours.

[0029] Step 4, Post-processing: After calcination, the material was allowed to cool naturally to room temperature with the furnace. Upon removal, the material was observed to be a loose powder that could be dispersed with only slight vibration. Finally, it was passed through a 100-mesh sieve (approximately 150 μm in diameter), and 2.5 kg of alumina ceramic balls with a diameter of 10-20 mm were added to a ball mill jar, along with 400 g of alumina. The mixture was then ground for 3 hours to obtain the final alumina powder product.

[0030] Product performance test results: The product has an α-phase content of 96.6%, a true density of 3.97 g / cm³, a tap density of 1.72 g / cm³, and a median particle size (D50) of 57.8 μm measured by a laser particle size analyzer. SEM observation shows that the particles have regular morphology, are nearly spherical, have a relatively smooth surface, and exhibit excellent sphericity.

[0031] Example 3 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1: Raw material preparation: Take 1000g of industrial aluminum hydroxide (ATH) powder as raw material, and add 80g of α-Al2O3 seed crystals (median particle size D50 is about 5.35μm, α phase content is 96.7%, accounting for 8% of the total mass of raw material), 15g of boric acid (H3BO3, accounting for 1.5% of the total mass of raw material) and 5g of calcium fluoride (CaF2, accounting for 0.5% of the total mass of raw material) as composite mineralizer.

[0032] Step 2, Mix: Place all the above materials in a mixer and dry ball mill for 1.5 hours to ensure that each component is evenly dispersed.

[0033] Step 3, calcination: The uniformly mixed material is placed in an alumina crucible and then placed in a box-type high-temperature furnace. The temperature is increased from room temperature to 1550°C at a rate of 3°C / min under air atmosphere, and then held at that temperature for 8 hours.

[0034] Step 4, Post-processing: After calcination, the material was allowed to cool naturally to room temperature with the furnace. Upon removal, the material was observed to be a loose powder that could be dispersed with only slight vibration. Finally, it was passed through a 100-mesh sieve (approximately 150 μm in diameter), and 2.5 kg of alumina ceramic balls with a diameter of 10-20 mm were added to a ball mill jar, along with 400 g of alumina. The mixture was then ground for 3 hours to obtain the final alumina powder product.

[0035] Product performance test results: The product has an α-phase content of 96.9%, a true density of 3.97 g / cm³, a tap density of 1.75 g / cm³, and a median particle size (D50) of 75.3 μm measured by a laser particle size analyzer. SEM observation shows that the particles are spherical with uniform particle size distribution and good sphericity.

[0036] Comparative Example 1 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1, Raw material preparation (no seed crystals, no mineralizers): Take 1000g of industrial aluminum hydroxide from the same batch as in Example 1, without adding any seed crystals or mineralizers.

[0037] Step 2, calcination: The raw material was placed in an alumina crucible and placed in a box-type high-temperature furnace. The temperature was increased from room temperature to 1470°C at a rate of 5°C / min in air atmosphere and held at that temperature for 3 hours.

[0038] Step 3, Post-processing: After calcination, allow the material to cool naturally to room temperature with the furnace. Upon removal, the material is found to be severely agglomerated and requires vigorous crushing to disperse.

[0039] Product performance test results: The α phase content was only 86.4%, the true density was 3.92 g / cm³, the D50 was 2.24 μm, and SEM showed that the particle morphology was extremely irregular, mostly angular, and there were a large number of hard agglomerates.

[0040] Comparative Example 2 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1, Raw material preparation (seed crystals only): Take 1000g of industrial aluminum hydroxide from the same batch as in Example 1, add only 50g of α-Al2O3 seed crystals (accounting for 5% of the raw material), and do not add any mineralizer.

[0041] Step 2, Mix: Place all the above materials in a mixer and dry ball mill for 2 hours to ensure that each component is evenly dispersed.

[0042] Step 3, calcination: The uniformly mixed material is placed in an alumina crucible and then placed in a box-type high-temperature furnace. The temperature is increased from room temperature to 1470°C at a rate of 5°C / min under air atmosphere, and then held at that temperature for 3 hours.

[0043] Step 4, Post-processing: After calcination, allow the material to cool naturally to room temperature with the furnace. Remove the material; severe sintering has resulted in hard agglomerates, which need to be crushed and sieved. Add 2.5 kg of alumina ceramic balls (10-20 mm in diameter) and 400 g of alumina to a ball mill jar, and grind for 3 hours.

[0044] Product performance test results: The α phase content was 93.7%, but the true density was 3.95 g / cm³, the D50 was 5.23 μm, and SEM showed that the particles were irregular polygonal and the particles were obviously adhered to each other.

[0045] Comparative Example 3 A method for preparing high-density, large-particle-size spherical alumina powder includes the following steps: Step 1, Raw material preparation (boric acid mineralizer only): Take 1000g of industrial aluminum hydroxide from the same batch as in Example 1, add only 10g of boric acid (accounting for 1.0% of the raw material), and do not add seed crystals or other fluoride mineralizers.

[0046] Step 2, Mix: Place all the above materials in a mixer and dry ball mill for 2 hours to ensure that each component is evenly dispersed.

[0047] Step 3, calcination: The uniformly mixed material is placed in an alumina crucible and then placed in a box-type high-temperature furnace. The temperature is increased from room temperature to 1470°C at a rate of 5°C / min under air atmosphere, and then held at that temperature for 3 hours.

[0048] Step 4, Post-processing: After calcination, allow the material to cool naturally to room temperature with the furnace. Remove the material; it is relatively loose and can be dispersed with gentle vibration.

[0049] Product performance test results: The α phase content was 95.2%, the true density was 3.96 g / cm³, the D50 was 3.62 μm, and SEM showed that the particle morphology was somewhat rounded and irregular. The particle size was far from meeting the requirements for large particle size, and the sphericity was generally poor.

[0050] The method described in this invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A method for preparing high-density, large-particle-size spherical alumina powder, characterized in that, Includes the following steps: Step 1, Raw material preparation: Use industrial aluminum hydroxide or industrial alumina containing transition phase alumina as raw material, and add α-alumina seed crystals and composite mineralizer; The amount of α-alumina seed crystals added is 1-10% of the total mass of the raw materials; The composite mineralizer contains a boron source and a fluorine source, and its total addition amount is 1.0-3.0% of the total mass of the raw materials; Step 2, Mixing: Mechanically mix the raw materials, α-alumina seeds and composite mineralizer from Step 1 until all components are uniformly dispersed; Step 3, calcination: The material that was mixed evenly in step 2 is calcined at a high temperature in air atmosphere. The calcination temperature is 1350-1550℃ and the calcination time is 2-8 hours. Step 4, Post-processing: Cool, disperse, and sieve the calcined material from Step 3 to obtain the high-density, large-particle-size spherical alumina powder.

2. The method for preparing high-density, large-particle-size spherical alumina powder according to claim 1, characterized in that, The boron source in the composite mineralizer is selected from boric acid.

3. The method for preparing high-density, large-particle-size spherical alumina powder according to claim 1 or 2, characterized in that, The fluorine source in the composite mineralizer is selected from one or more of aluminum fluoride, ammonium fluoride, magnesium fluoride, or calcium fluoride.

4. The method for preparing high-density, large-particle-size spherical alumina powder according to claim 1, characterized in that, The calcination process in step 3 has a heating rate of 1-10℃ / min.

5. The method for preparing high-density, large-particle-size spherical alumina powder according to claim 1, characterized in that, The alumina powder prepared by the method has an α-phase content of not less than 96%, a true density greater than 3.96 g / cm³, a median particle size D50 of not less than 50 μm, and the particles exhibit a near-spherical morphology.

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