Spheroidal alumina and method for its production and use thereof

Abstract

This invention relates to a quasi-spherical alumina, wherein D50 is 4-22 μm, D90 is 10-40 μm, and (D90-D50) / D50 is 0.5-1.2; it comprises aggregated particles and dispersed particles in a ratio of (5-7):(5-3); and also relates to a method for preparing the quasi-spherical alumina. The quasi-spherical alumina of this invention exhibits good particle size uniformity, high filling efficiency, and excellent thermal conductivity, making it suitable as a thermally conductive filler.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic materials technology, specifically to a spherical alumina with uniform particle size and excellent thermal conductivity, and also to a method for preparing the spherical alumina and its applications. Background Technology

[0002] With the rapid development of the electronics industry, the demand for heat dissipation performance of thermally conductive materials is increasing. Chip heat dissipation mainly relies on the thermally conductive interface material between the heat-generating components and the casing for heat conduction. Thermally conductive interface materials are made by adding thermally conductive fillers to organic matrices such as silicone grease and epoxy resin to create thermally conductive grease, thermally conductive pads, thermally conductive gels, thermally conductive tapes, thermally conductive potting compounds, and thermally conductive phase change materials.

[0003] Because silicone grease and epoxy resin have very low thermal conductivity (0.1-0.3 W / mK), fillers with high thermal conductivity, good insulation, and corrosion resistance are usually added. Common thermally conductive fillers include oxides (alumina, magnesium oxide, zinc oxide, silicon dioxide), nitrides (aluminum nitride, boron nitride), and carbon materials (diamond, graphene, carbon fiber, etc.). Among these, alumina is currently the most mature thermally conductive filler due to its moderate thermal conductivity, chemical stability (resistance to acid and alkali corrosion), good insulation properties, and reasonable cost, and is widely used in various fields such as electronics, aerospace, and automotive.

[0004] When alumina is used as a thermally conductive filler, its thermal conductivity is affected by various factors, such as the material's purity, filling performance, particle size, crystal type and content (α phase %), and shape (morphology). Angular alumina has an irregular morphology, making it difficult to fill in large quantities, and its thickening effect on the matrix is ​​too significant. It is typically used in thermally conductive products with a power rating of 3W or lower, and cannot meet the requirements for high thermal conductivity applications. Spherical alumina has a regular morphology and good filling rate, but it usually has a low α phase content, more impurities and defects, resulting in unsatisfactory thermal conductivity. Furthermore, it requires strict temperature control during production, leading to high production costs and a high unit price; most manufacturers typically add it only in small quantities. In contrast, near-spherical alumina has a higher filling rate and α phase content, resulting in better thermal conductivity and making it a cost-effective thermally conductive filler. Summary of the Invention

[0005] This invention provides a quasi-spherical alumina with a concentrated particle size distribution, which results in high filling efficiency and excellent thermal conductivity when used in thermally conductive fillers.

[0006] In a first aspect, the present invention provides a spherical alumina.

[0007] The spherical alumina has a D50 of 4-22 μm, a D90 of 10-40 μm, and a (D90-D50) / D50 ratio of 0.5-1.2. The spherical alumina includes aggregated particles and dispersed particles. Dispersed particles refer to a single primary crystal, while aggregated particles refer to polycrystalline spherical particles formed by the bonding of two or more primary crystals. The ratio of dispersed particles to aggregated particles is (5-7):(5-3). The spherical alumina is imaged using SEM. When D50 is less than or equal to 10 μm, the magnification is 5,000x; when D50 is greater than 10 μm, the magnification is 1,000x. The number of dispersed particles and aggregated particles in the imaged SEM are calculated, and the sum of their proportions is 100%.

[0008] A second aspect of the present invention provides a method for preparing spherical alumina, comprising the following steps: S1 Wet mixing: The slurry of industrial aluminum-containing raw materials is mixed with calcination aids to obtain a mixed slurry; S2 Slurry Drying: The mixed slurry is dried to obtain a premix; S3 Calcination and Phase Transformation: The premixed material is heated to 700-1100℃ at a heating rate of 5-20℃ / min and held for 20-60 min, then heated to 800-1250℃ at a heating rate of 5-15℃ / min and held for 20-60 min, and then heated to 1300-1700℃ at a heating rate of 3-10℃ / min and held for 2-10 h to obtain calcined alumina; S4 Grinding and Crushing: The calcined alumina is crushed to obtain a crude alumina product with spherical shape; S5 Water washing to remove impurities: The crude spherical alumina is washed with water to obtain spherical alumina.

[0009] A third aspect of the invention provides the use of near-spherical alumina according to the invention, or near-spherical alumina prepared by the method according to the invention, as a thermally conductive filler in thermally conductive grease, thermally conductive pads, thermally conductive gels, thermally conductive tapes, thermally conductive potting compounds, and thermally conductive phase change materials.

[0010] The advantages of this invention are: 1. This invention achieves the goal of improving the viscosity and thermal conductivity of spherical alumina in practical applications by adjusting the aggregation degree of aggregated particles to ensure good consistency between aggregated and dispersed particle sizes, and by adjusting the particle sizes of D90 and D50 to satisfy the relationship 0.5≤(D90-D50) / D50≤1.2, while maintaining a moderate specific surface area.

[0011] 2. Furthermore, the preparation method of the product of the present invention particularly includes operations such as wet grinding and mixing, staged heating and calcination, and water washing to remove fine powder, so that the raw materials are mixed more uniformly, the grain growth is more uniform, and the amount of fine powder particles generated is minimized, thereby obtaining spherical alumina with particle size, particle size distribution, specific surface area and viscosity within the range described in the present invention.

[0012] 3. This preparation method also has the advantages of being simple to operate, energy-saving, and easy to industrialize. Attached Figure Description

[0013] Figure 1 The image shows an example SEM image of the spherical alumina prepared in Example 1 of this invention, magnified 5,000 times. The dashed lines represent dispersed particles, and the solid lines represent aggregated particles.

[0014] Figure 2 This is an example SEM image of the near-spherical alumina prepared in Example 2 of the present invention, magnified 1,000 times.

[0015] Figure 3 This is an example SEM image of the near-spherical alumina prepared in Comparative Example 2 of the present invention, magnified 5,000 times.

[0016] Figure 4 This is a schematic flowchart of the method for preparing spherical alumina according to the present invention. Detailed Implementation

[0017] In this invention, the term "quasi-spherical" refers to a diameter ratio in the range of 0.8-1.5, determined by the following method: that is, the ratio of the diameters in any two mutually perpendicular directions in a scanned image of a single particle; the closer the value is to 1, the closer it is to a sphere.

[0018] In this invention, the term "dispersed particle" refers to a single primary crystal. Specifically, a "primary crystal" refers to a single, well-defined, spherical particle in a scanning electron microscope (SEM) image, as determined by the methods described below.

[0019] In this invention, the term "aggregated particle" refers to a polycrystalline particle formed by the mutual bonding of two or more primary crystals, with a particle size of 4-22 μm, as determined by the method described in the following examples.

[0020] In this invention, D10 refers to the particle size value when the volume cumulative distribution reaches 10%, that is, 10% of the particles in the sample have a particle size smaller than this value, reflecting the proportion of fine-end particles in the powder; D90 refers to the particle size value when the volume cumulative distribution reaches 90%, that is, 90% of the particles have a particle size smaller than this value, used to characterize the upper limit of the distribution of coarse-end particles; D50 refers to the particle size value when the volume cumulative distribution reaches 50%, that is, 50% of the particles have a particle size smaller than this value, used to characterize the average particle size.

[0021] Spherical alumina Currently, the thermal conductivity of near-spherical alumina with particle sizes in the micrometer range is not ideal, especially when it is compounded with fillers of other particle sizes. The thermal conductivity of the final product is unstable. This is mainly because near-spherical alumina with micrometer particle sizes has poor particle dispersibility, a wide particle size distribution, and difficult morphology control. Furthermore, it is difficult to adjust the specific surface area and viscosity to the desired level, which affects the filling efficiency and causes the thermal conductivity of the final product after compounding to fail to meet the expected performance.

[0022] Through numerous experiments, the inventors were surprised to discover that when the particle size of the dispersed particles (i.e., the original crystals) is 2-20 μm and the aggregated particles are formed by the adhesion of two or more original crystals, the particle size of the aggregated particles is close to that of the dispersed particles. Thus, spherical alumina with good particle size uniformity can be obtained.

[0023] Furthermore, the inventors have discovered that by adjusting the particle size of D90 and D50, while ensuring that they satisfy the relationship 0.5≤(D90-D50) / D50≤1.2, and by adjusting the specific surface area, the particle size distribution, particle dispersibility, and flowability of spherical alumina can be optimized; thereby increasing the filling rate, improving thermal conductivity, and ensuring the stability of the thermal conductivity of the final compounded product.

[0024] In a first aspect, the present invention provides a near-spherical alumina with a D50 of 4-22 μm, preferably 6-13 μm; a D90 of 10-40 μm, preferably 13-24 μm; a (D90-D50) / D50 ratio of 0.5-1.2, preferably 0.6-1.0; and a specific surface area of ​​0.1-0.55 m². 2 / g, preferably 0.1-0.35 m 2 / g.

[0025] The spherical alumina disclosed in this invention has a thermal conductivity of 1.4-3.2 W / (m·K), preferably 1.6-3.2 W / (m·K).

[0026] The spherical alumina disclosed in this invention has an α-phase content of over 95% and a diameter ratio of 0.8-1.5. If the α-phase content is too low, the thermal conductivity of the product will be low; if the diameter ratio is not within the above range, the morphology of the product will be irregular, and both the filling efficiency and thermal conductivity will be low.

[0027] The spherical alumina disclosed in this invention contains more than 98% Al2O3 by mass, and has high purity.

[0028] The spherical alumina disclosed in this invention contains one or more elements selected from boron, fluorine, calcium, lanthanum, and yttrium, and the total amount of these elements accounts for less than or equal to 1% of the mass percentage of the spherical alumina, for example, 0.1%-1%. Excessive content of these elements will result in excessive impurities remaining after calcination.

[0029] Preparation method Existing preparation methods for spherical alumina include high-temperature secondary calcination, homogeneous precipitation, sol-gel method, and template method.

[0030] Among them, homogeneous precipitation and sol-gel methods can prepare near-spherical α-alumina with a primary crystal size in the micrometer range. However, these two methods use organic solvents and surfactants, which are not conducive to the separation and drying of alumina products. Furthermore, they result in severe agglomeration after calcination, leading to high costs and making it difficult to achieve large-scale production.

[0031] High-temperature secondary calcination is a commonly used method for preparing spherical alumina. However, due to equipment limitations, the mixing of raw materials and calcination aids is uneven. The spherical alumina obtained by calcination has a wide particle size distribution after crushing, with many incomplete and fine particles, resulting in a high specific surface area and high viscosity when used as a filler. Moreover, the secondary calcination process is complex, time-consuming, and costly.

[0032] Through extensive experimentation, the inventors discovered that by employing wet grinding and mixing, staged heating and calcination, and water washing to remove fine powder, the mixing of industrial aluminum-containing raw materials and calcination aids can be made more uniform, resulting in more uniform grain growth and minimizing the amount of fine powder particles produced. This is beneficial for obtaining spherical alumina with particle size, particle size distribution, specific surface area, and viscosity all within the range described in this invention.

[0033] A second aspect of the present invention provides a method for preparing spherical alumina according to the first aspect of the present invention, comprising: S1 Wet mixing: The slurry of industrial aluminum-containing raw materials is mixed with calcination aids to obtain a mixed slurry; S2 Slurry Drying: The mixed slurry is dried to obtain a premix; S3 Calcination and Phase Transformation: The premixed material is heated to 700-1100℃ at a heating rate of 5-20℃ / min and held for 20-60 min, then heated to 800-1250℃ at a heating rate of 5-15℃ / min and held for 20-60 min, and then heated to 1300-1700℃ at a heating rate of 3-10℃ / min and held for 2-10 h to obtain calcined alumina; S4 Grinding and Crushing: The calcined alumina is crushed to obtain a crude alumina product with spherical shape; S5 Water washing to remove impurities: The crude spherical alumina is washed with water to obtain spherical alumina.

[0034] [S1 Wet Mixing] Optionally, in S1, the slurry of industrial aluminum-containing raw material is made by mixing industrial aluminum-containing raw material with pure water (conductivity <50 μS / cm), and the solid content of the slurry is 30-60%, preferably 40-50%.

[0035] This allows for wet mixing with calcination aids, resulting in a more uniformly mixed material. This is beneficial in subsequent operations and steps, especially in grinding and staged heating calcination, by reducing the generation of broken fine powder and uneven grain growth. This is conducive to obtaining spherical particles with a smaller degree of aggregation of dispersed particles, thus achieving a similar particle size between dispersed and aggregated particles.

[0036] Optionally, the industrial aluminum-containing raw material is industrial γ-phase alumina and / or industrial aluminum hydroxide, with a particle size D50 of 10-40 μm, preferably 20-30 μm or 25-35 μm.

[0037] Selecting industrial aluminum-containing raw materials within the specified particle size range is beneficial for obtaining spherical alumina products with the desired particle size.

[0038] Optionally, the calcination aid includes a calcination phase transformation aid, a crystal growth aid, and a morphology modifier for calcination; preferably, the calcination phase transformation aid is selected from boric acid and / or boron oxide; the crystal growth aid is selected from at least one of ammonium chloride, aluminum fluoride, calcium fluoride or other calcium halide salts, and calcium oxide; and the morphology modifier is selected from at least one of yttrium and / or lanthanum oxide.

[0039] In one specific implementation, the calcination phase transformation aid is boric acid, the crystal growth aid is calcium fluoride, and the morphology modifier is lanthanum fluoride.

[0040] By adding calcination aids, especially morphology modifiers, it is beneficial to control the morphology of alumina to be spherical.

[0041] Optionally, the mass ratio of calcination phase transformation aid: crystal growth aid: morphology modifier is (5-1):(5-1):1, preferably (2-1):(2-1):1.

[0042] Optionally, the total amount of the calcination aid is 1-5% of the mass percentage of the industrial aluminum-containing raw material, preferably 1.5-4.5%. If the content of the calcination aid is too high, too much residue will remain in the finished product. If the content of the calcination aid is too low, it is not conducive to obtaining alumina products with a near-spherical morphology, nor is it conducive to controlling the aggregation degree of dispersed particles, thereby obtaining aggregated and dispersed particles with similar particle sizes.

[0043] Optionally, in step S1, the mixed slurry is ground, which can be achieved by methods commonly used in the art. The grinding is performed using a vertical stirred mill or a high-speed ball mill, and the grinding media is selected from at least one of zirconium oxide or alumina. The diameter of the grinding media is 4-8 mm, and the mass ratio of the grinding media to the industrial aluminum-containing raw material is (3-8):1, preferably 5:1.

[0044] [S2 Slurry Drying] Optionally, in S2, the method for drying the mixed slurry is not particularly limited, and drying methods known in the art can be used. Spray drying is preferred, and the equipment used is a spray drying tower. Regarding the rotation speed, drying time, and nozzle orifice diameter, those skilled in the art can determine these parameters according to conventional methods; the inlet temperature is 170-210°C, and the outlet temperature is 150-200°C.

[0045] [S3 Calcination Phase Inversion] In S3, a staged heating and calcination method is adopted, which is a single calcination with a three-stage heating method. Each stage is carried out by heating to a specific temperature at a specific heating rate and then holding at that temperature for a period of time.

[0046] This staged heating results in a small temperature difference within the furnace, avoiding the formation of irregularly shaped particles due to localized temperature variations, which is conducive to generating regular spherical morphologies. Moreover, it is beneficial to produce the desired aggregated particle structure—composed of two or more primary crystals bonded together—thereby reducing the difference in particle size between aggregated and dispersed particles, resulting in good particle size consistency and good particle dispersibility.

[0047] In contrast, existing calcination methods, including one-step or two-step calcination, lack control over the heating rate, which easily leads to uneven heating, large local temperature differences, uneven grain growth, and difficulty in controlling morphology. Furthermore, for two-step calcination, the calcined product needs to be cooled, removed, and then mixed and ball-milled between the two calcinations. Especially after the ball-milling operation and before the second calcination, a heat-holding operation is also included. Obviously, this method is complex, time-consuming, and energy-intensive. Moreover, if the first calcination temperature is too low or too high, a relatively regular spherical alumina product cannot be obtained. In contrast, the preparation method of the present invention can obtain alumina with a spherical morphology, resulting in a smaller degree of aggregation of dispersed particles, thereby achieving a similar particle size between dispersed and aggregated particles.

[0048] Optionally, in step S3, the temperature is increased to 700-1100℃ at a heating rate of 5-15℃ / min, preferably 5-10℃ / min, and held for 30-60 min. Then, the temperature is increased to 800-1250℃ at a heating rate of 5-15℃ / min, preferably 5-10℃ / min, and held for 30-60 min. Subsequently, the temperature is increased to 1300-1700℃ at a heating rate of 3-10℃ / min, preferably 3-8℃ / min, and held for 4-6 h. This helps to further improve the uniformity of the calcined alumina particle size, resulting in a narrower particle size distribution and fewer finely pulverized particles.

[0049] Optionally, in S3, the calcination equipment is a high-temperature furnace or kiln (e.g., pusher kiln, tunnel kiln, rotary kiln, shuttle kiln).

[0050] [S4 Grinding and Crushing] In S4, the crushing equipment is a ball mill with a rotation speed of 150-600 rpm, preferably 150-300 rpm, and a duration of 20-60 min.

[0051] At least one of zirconium oxide and alumina is used as the grinding medium, the diameter of the grinding medium is 2-10 mm, and the mass ratio of its content to calcined alumina is 4-10:1, preferably 4:1-6:1.

[0052] Given the previously implemented steps, particularly S1 and S3, the material is uniformly mixed and the grains grow uniformly. Therefore, the calcined alumina obtained for crushing has good uniformity and dispersibility, with fewer finely crushed particles. This is beneficial because in step S4, the crushing equipment can operate at a lower speed and with a shorter crushing time. This saves energy while allowing the particle size of D50 and D90 to be adjusted and satisfy the relationship 0.5≤(D90-D50) / D50≤1.2 and the requirements of the product of this invention, thereby achieving the goal of improving the filling efficiency and thermal conductivity of the product.

[0053] [S5 water wash removes fine particles] In S5, the water washing process includes stirring water with the crude spheroidal alumina, letting it stand for 1-5 minutes, and then pouring out the supernatant.

[0054] Specifically, water and the crude spherical alumina are mixed in a mass ratio of (5-10):1 and stirred at a speed of 10-100 rpm for 5-30 minutes. Then, the mixture is allowed to stand for 1-8 minutes, and the turbid liquid at the top is poured off. This washing step is repeated several times until the liquid at the top changes from turbid to clear. This washing step is usually repeated 2-5 times.

[0055] Therefore, by utilizing the difference in settling velocity between finely ground particles and other particles (such as dispersed particles and aggregated particles), the finely ground particles generated due to over-grinding during the crushing step can be reduced or removed, thereby achieving the requirement of fewer fine particles in the product.

[0056] In addition, water washing helps to reduce the content of other elements in the product due to substances introduced during the process, such as calcination aids, thereby improving product purity and reducing product conductivity.

[0057] S5 also includes the dried spherical alumina, which can be dried using methods known in the art, such as oven drying at a temperature of 80-140°C for 2-6 hours.

[0058] Using the method of the present invention, the materials are mixed evenly and the grains grow evenly during the preparation process, so that the particle size of the obtained alumina is basically consistent. This is beneficial to reduce the intensity and time of crushing in the subsequent crushing step, so that the fine crushed particles can be generated as little as possible. At the same time, the consistency of the particle size of the crushed alumina is maintained, and most of the particles are concentrated in a certain specific particle size range.

[0059] The advantages of this are twofold: firstly, a simple water washing step can be used, eliminating the need for cumbersome grading steps, to significantly reduce or remove finely pulverized particles and retain particles with consistent size to the greatest extent possible. Secondly, the particle size distribution difference between the crushed alumina particles and the product obtained after subsequent water washing is very small. Therefore, by monitoring the particle size of the crushed alumina particles, the target particle size of the final product can be controlled, ensuring that the D90 and D50 values ​​not only fall within a specific range but also satisfy a specific relationship, while the specific surface area is within the range described in this invention.

[0060] In this invention, the size of the primary crystals can be determined by observing the SEM test image of the calcined alumina obtained through the calcination step. This size can be set as the target particle size in the subsequent crushing process to monitor the particle size of the crushed particles in real time. This can be achieved by taking samples at regular intervals during the crushing process for particle size testing. The parameters tested are, for example, D10 / D50 / D90, etc. The equipment used is a Malvern 3000 series laser particle size analyzer.

[0061] use The spherical alumina of this invention has good particle uniformity, low viscosity in practical applications, and excellent thermal conductivity. When filled into organic matrices such as silicone grease and epoxy resin, it can achieve good filling efficiency and obtain a thermally conductive interface material with excellent and stable thermal conductivity, which can be applied in the fields of electronics, aerospace, and automobiles.

[0062] Example Unless otherwise specified, all raw materials used are commercially available and can be used directly without further processing; unless otherwise specified, the experimental conditions used are at room temperature and pressure.

[0063] Example 1 S1: Wet mixing 2 kg of industrial γ-phase alumina (purchased from Maclean) with a D50 of 20 μm was mixed with 3 kg of pure water (self-made, conductivity <50 μS / cm) to prepare a slurry with a solid content of 40%. 20 g of boric acid (purchased from Maclean), 10 g of calcium fluoride (purchased from Shanghai Lingfeng), and 10 g of lanthanum oxide (purchased from Maclean) were added and mixed. The mixture was then added to a vertical stirred mill, using 10 kg of 5 mm zirconium oxide (purchased from Saint-Gobain) as the grinding medium. The mixture was wet-mixed at 150 rpm for about 2 hours to obtain a mixed slurry.

[0064] S2: Slurry drying The mixed slurry obtained in step S1 above is spray-dried at a feed temperature of 180°C and an outlet temperature of 150°C to obtain a premix.

[0065] S3: Calcination phase inversion The premix obtained in step S2 above was placed in a sagger and calcined in a high-temperature furnace in stages. The stage temperatures and holding times were as follows: the temperature was increased to 750°C at a heating rate of 6°C / min and held for 30 min; the temperature was increased to 850°C at a heating rate of 5°C / min and held for 30 min; then the temperature was increased to 1300°C at a heating rate of 4°C / min and held for 4 h to obtain calcined alumina.

[0066] S4: Grinding and crushing The calcined alumina obtained in step S3 above was crushed using 5 mm zirconium oxide as the grinding medium at a material-to-ball ratio of 1:5. The ball mill was used at a speed of 200 r / min for 30 min to obtain a near-spherical alumina crude product with particle sizes D50=5.8 μm and D90=11.2 μm.

[0067] S5: Water washing removes fine particles Water was mixed with the crude spheroidal alumina obtained in step S4 at a mass ratio of 6:1. The mixture was stirred in an electric stirrer at a speed of 30 r / min for 10 min, and then allowed to stand for 5 min. The upper layer of suspended liquid was poured off. This process was repeated 3 times until the upper liquid became clear. The lower solid was dried at 100 °C to obtain spheroidal alumina with particle sizes D50 = 6.27 μm and D90 = 11.26 μm.

[0068] Example 2 S1: Wet mixing D50 consists of 2 kg of 30 μm industrial γ-phase alumina (purchased from Maclean's), 3 kg of pure water (self-made, conductivity <50 μS / cm) to prepare a slurry with 40% solids content. 40 g of boric acid (purchased from Maclean's), 20 g of calcium fluoride (purchased from Shanghai Lingfeng), and 20 g of lanthanum oxide (purchased from Maclean's) are added and mixed. The mixture is then added to a vertical stirred mill, using 10 kg of 5 mm zirconium oxide (purchased from Saint-Gobain) as the grinding medium. The mixture is wet-mixed at 150 rpm for about 2 hours to obtain a mixed slurry.

[0069] S2: Slurry drying The mixed slurry obtained in step S1 above is spray-dried at a feed temperature of 180°C and an outlet temperature of 150°C to obtain a premix.

[0070] S3: Calcination phase inversion The premix obtained in step S2 above was placed in a sagger and calcined in a high-temperature furnace in stages. The stage temperatures and holding times were as follows: the temperature was increased to 1050℃ at a heating rate of 15℃ / min and held for 50 min; the temperature was increased to 1250℃ at a heating rate of 15℃ / min and held for 60 min; and then the temperature was increased to 1700℃ at a heating rate of 10℃ / min and held for 6 h to obtain calcined alumina.

[0071] S4: Grinding and crushing The calcined alumina obtained in step S3 was crushed using a ball mill with 5 mm zirconium oxide as the grinding medium at a material-to-ball ratio of 1:5. The milling time was 30 min at a speed of 200 r / min, yielding particle sizes of D50 = 18.1 μm and D90 = 35.9 μm. of Crude alumina with spherical shape.

[0072] S5: Water washing removes fine particles Water was mixed with the crude spheroidal alumina obtained in step S4 at a mass ratio of 6:1. The mixture was stirred in an electric stirrer at a speed of 30 r / min for 10 min, and then allowed to stand for 5 min. The upper layer of suspended liquid was poured off. This process was repeated three times with water until the upper liquid became clear. The lower solid was dried at 100 °C to obtain spheroidal alumina with particle sizes D50 = 20.3 μm and D90 = 38.1 μm.

[0073] Example 3 The difference from Example 1 is that yttrium oxide is used instead of lanthanum oxide in S1.

[0074] Example 4 The difference from Example 1 lies in step S3, calcination and phase inversion, as follows: The premix obtained in step S2 above is placed in a sagger and calcined in a high-temperature furnace in stages. The stage temperatures and holding times are as follows: the temperature is increased to 750°C at a heating rate of 15°C / min and held for 30 min; the temperature is increased to 850°C at a heating rate of 15°C / min and held for 30 min; then the temperature is increased to 1300°C at a heating rate of 10°C / min and held for 4 h to obtain calcined alumina.

[0075] Comparative Example 1 The difference from Example 1 is that step S1 involves wet mixing, in which no calcination aid is used.

[0076] Comparative Example 2 The difference from Example 1 is that step S5, water washing to remove fine particles, is not included.

[0077] The conductivity of the product in Example 1 was measured to be 26.5 μs / cm; the conductivity of the product in Comparative Example 2 was 159.6 μs / cm.

[0078] Performance testing 1. Measurement Method 1.1 Methods for determining the particle size of particles (aggregated particles, dispersed particles) First, attach conductive tape to the sample stage, use a toothpick to pick up a small amount of spherical alumina product, shake it onto the tape to distribute the powder evenly on the tape; then blow away the excess powder with a syringe; place the sample stage with the powder attached into an ion sputtering instrument to deposit 5 nm thick gold to prepare the sample to be tested. Then, using a scanning electron microscope (Zeiss, model 360), randomly selected powder particles from the above-mentioned test samples were subjected to SEM imaging to obtain SEM images; for spherical alumina products with particle D50≤10μm, 5,000x magnification was selected, and for D50>10μm, 1,000x magnification was selected.

[0079] Observe the image to show the complete particle (as in the example). Figure 1 ): Dispersed particles are single, spherical particles with distinct boundaries; aggregated particles are polycrystalline spherical particles formed by the bonding of two or more primary crystals.

[0080] 1.2 Particle size determination methods (D10, D50, D90) The powder was diluted 10,000 times, ultrasonically dispersed for 3-5 minutes, and then added to a Malvern 3000+ laser particle size analyzer. The measuring medium was water, and the refractive index of alumina was 1.764.

[0081] 1.3 Specific surface area The specific surface area was calculated using the BET theory based on the adsorption isotherm determined by the gas adsorption method. Specifically, the nitrogen low-temperature adsorption method was used for the test.

[0082] 1.4 Viscosity The sample was prepared by uniformly mixing 350 cps silicone oil with the sample at an oil-to-powder ratio of 1:5 and then testing it on a rheometer (Anton Paar MCR703).

[0083] 1.5 Thermal conductivity The spherical alumina product prepared above, component A (bisphenol F type epoxy resin), and component B (m-phenylenediamine, aromatic curing agent) were added to a mixing container at a mass ratio of 5:1:0.1. The mixture was then uniformly mixed with a spatula for 10-15 minutes to form a uniform viscous paste. After vacuum degassing, the paste was transferred between two layers of fluoroplastic film and pressed into 2 mm sheets using a roller press. The sheets were then placed in a 120°C oven for curing for 2 hours. After curing, the sheets were removed and tested using a thermal conductivity meter (Hot Disk TPS 2200 S).

[0084] 1.6 Electrical conductivity Disperse 10g of spherical alumina product into 100g of pure water (conductivity <50 μS / cm), stir for 30min, and test using a conductivity meter.

[0085] 1.7 Diameter ratio The ratio of the diameters of any two mutually perpendicular directions of 20 randomly selected individual particles in the scanned image is considered to be closer to 1, indicating a spherical shape.

[0086] 1.8 Elemental content (boron, fluorine, calcium, lanthanum, yttrium) and alumina content Detection was performed using an XRF spectrometer.

[0087] 1.9 α-phase content Tested using XRD.

[0088] 2. Results Data The alumina products obtained in the examples and comparative examples were measured according to the above-described measurement methods.

[0089] Figure 1 The image shown is a SEM image of the near-spherical alumina prepared in Example 1 at 5,000x magnification. Figure 1 In the diagram, dispersed particles and aggregated particles are schematically labeled, by... Figure 1 It can be seen that dispersed particles are the majority, and their morphology is that of single spherical particles; while aggregated particles are formed by the mutual adhesion of dispersed particles; the particle size of dispersed particles and aggregated particles is similar, in the range of 4 to 6 μm, and the degree of particle dispersion is good.

[0090] Figure 2 The image shown is a SEM image of the near-spherical alumina prepared in Example 2 at 1000x magnification. Figure 2 The study showed dispersed and aggregated particles with similar particle sizes, ranging from 14 to 22 μm, and with good dispersion.

[0091] Figure 3 The image shows a SEM image of the near-spherical alumina prepared in Comparative Example 2 at 5,000x magnification. From... Figure 3 It is clearly visible that there are many adsorbed powder particles on the surface of the intact crystal grains.

[0092] The measured data for the alumina products are shown in the table below. As can be seen from the data in the table, compared with Comparative Examples 1-2, the alumina products prepared in Examples 1-4 of this invention are all spherical, with higher uniformity in particle size and lower specific surface area, thereby significantly improving the viscosity and thermal conductivity of alumina when used as a filler. At the same time, the alumina content, other element (boron, fluorine, calcium, lanthanum, yttrium) content and α phase content of the products are maintained at a comparable or better level.

Claims

1. Spheroidal-like alumina, characterized in that, D50 is 4-22 μm, D90 is 10-40 μm, and (D90-D50) / D50 is 0.5-1.2; the spherical alumina comprises dispersed particles and aggregated particles. The dispersed particles refer to a single primary crystal, and the aggregated particles refer to polycrystalline spherical particles formed by the bonding of two or more primary crystals together; the ratio of the number of dispersed particles to aggregated particles is (5-7):(5-3). Spherical alumina was imaged using SEM. When D50 was less than or equal to 10 μm, the magnification was 5,000 times; when D50 was greater than 10 μm, the magnification was 1,000 times. The number of dispersed particles and the number of aggregated particles were calculated in the imaged SEM, and the sum of the two was 100%.

2. Spheroidal shaped alumina according to claim 1 having a specific surface area of 0.1 - 0.55 m 2 / g.

3. The near-spherical alumina according to claim 1 is prepared by uniformly mixing 350cps silicone oil with near-spherical alumina at an oil-to-powder ratio of 1:5, with a viscosity of 10-60 Pa·s.

4. The near-spherical alumina according to claim 1, wherein the near-spherical alumina has a thermal conductivity of 1.4-3.2 W / (m·K).

5. The near-spherical alumina according to claim 1, wherein the α-phase content of the near-spherical alumina is more than 95%, and the diameter ratio is 0.8-1.

5.

6. The near-spherical alumina according to claim 1, wherein the mass percentage of alumina Al2O3 is 98% or more.

7. The near-spherical alumina according to claim 1, comprising one or more elements selected from boron, fluorine, calcium, lanthanum, and yttrium, wherein the total amount of said elements accounts for 0.1%-1% of the mass percentage of said near-spherical alumina.

8. A method for preparing near-spherical alumina, comprising: S1 Mixing: The slurry of industrial aluminum-containing raw materials is mixed with calcination aids to obtain a mixed slurry; S2 Drying: The mixed slurry is dried to obtain a premix; S3 Calcination: The premixed material is heated to 700-1100℃ at a heating rate of 5-20℃ / min and held for 20-60 min, then heated to 800-1250℃ at a heating rate of 5-15℃ / min and held for 20-60 min, and then heated to 1300-1700℃ at a heating rate of 3-10℃ / min and held for 2-10 h to obtain calcined alumina; S4 Crushing: The calcined alumina is crushed to obtain a semi-finished spherical alumina product; S5 Water Washing: The crude spherical alumina is washed with water to obtain spherical alumina.

9. The method according to claim 8, wherein the calcination aid comprises a calcination phase transformation aid, a crystal growth aid, and a morphology modifier; preferably, the calcination phase transformation aid is selected from boric acid and / or boron oxide; the crystal growth aid is selected from at least one of ammonium chloride, aluminum fluoride, calcium fluoride or other calcium halide salts, and calcium oxide; and the morphology modifier is selected from at least one of yttrium and / or lanthanum oxide.

10. The near-spherical alumina according to any one of claims 1-7, and / or the near-spherical alumina prepared by the method according to any one of claims 8-9, for use as thermally conductive fillers in thermally conductive grease, thermally conductive pads, thermally conductive gels, thermally conductive tapes, thermally conductive potting compounds, and thermally conductive phase change materials.

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