Preparation method and application of monodisperse high-alpha-phase nano aluminum oxide

Abstract

The invention relates to a preparation method and application of monodisperse high-alpha-phase nano alumina, which comprises the following steps: carrying out wet grinding on industrial alumina by using a sand mill to obtain a sanding slurry, centrifuging the sanding slurry, taking the upper emulsion, adjusting the pH value to 8-10, standing for one night, centrifuging, taking the lower precipitate, and freeze-drying to obtain the monodisperse high-alpha-phase nano alumina. And a nanoscale precursor is obtained. And finally, carrying out heat preservation on the nanoscale precursor in a high-temperature muffle furnace for a period of time to obtain the high-alpha-phase nano aluminum oxide. According to the high-alpha-phase nanometer aluminum oxide powder, the alpha conversion rate is 95% or above, the particle size ranges from 100 nm to 300 nm, the morphology is in a sphere-like shape, good filling performance and heat conductivity coefficient are shown in heat-conducting glue, meanwhile, the characteristic of good weather resistance is achieved, and the requirement for diversification of heat-conducting raw materials in the heat dissipation market of electronic equipment is met.

Description

Technical Field

[0001] This invention belongs to the field of alumina and nanomaterials, specifically relating to a method for preparing monodisperse high α-phase nano-alumina and its application. Background Technology

[0002] Alumina (Al₂O₃) is an inorganic non-metallic material with excellent physicochemical properties. It naturally exists in minerals such as corundum, sapphire, and ruby, and its artificial preparation technology is mature, with wide applications in refractories, ceramics, catalysis, polishing, electronics, and new energy fields. Furthermore, alumina has multiple crystal forms, such as γ, δ, θ, η, and α, and different crystal forms can be transformed by temperature. Among them, α-phase alumina is the most thermodynamically stable crystal form, belonging to the trigonal crystal system, with a corundum structure and a dense atomic arrangement. It possesses characteristics such as high hardness, strong wear resistance, high thermal conductivity, and excellent weather resistance, making it one of the excellent thermal interface materials in the electronics and new energy heat dissipation fields.

[0003] Currently, there are various types of alpha-phase alumina used in the fields of electronic and new energy heat dissipation. Classified by crystal form, they include monocrystalline and polycrystalline alumina; by crystal morphology, they include angular alumina, spherical alumina, spherical alumina, and fibrous alumina; and by particle size, they include nano-alumina, submicron alumina, and micron alumina. Among them, alpha-phase nano-spherical alumina, as a "lubricant" for powder particle size distribution, is widely used in thermal interface materials such as thermally conductive potting compounds, thermally conductive silicone greases, thermally conductive adhesives, and thermally conductive pads. However, alpha-phase nano-alumina currently on the market often suffers from defects such as low alpha conversion rate, irregular morphology, large specific surface area, and severe particle agglomeration. This leads to adverse effects in thermal interface material applications, such as low thermal conductivity, thickened colloidal viscosity, weakened mechanical properties, and decreased aging performance. A nano-alumina with high α conversion rate, regular morphology and uniform particle dispersion is beneficial to improving the performance of thermal interface materials, breaking through current bottlenecks and barriers, and thus meeting the increasingly high requirements of the current electronics and new energy markets for thermal interface materials.

[0004] In the prior art, patent document CN201910885260.4, entitled "A Method for Preparing Low-Sodium Spherical Nano-Alumina Powder," also proposes a method for preparing nano-α-alumina powder. However, this patent does not mention the specific α-conversion rate of α-alumina, and electron scanning images show that the prepared nano-alumina particles have a high degree of agglomeration, failing to address the adverse effects on thermal interface materials such as low thermal conductivity, increased colloidal viscosity, weakened mechanical properties, and decreased aging performance. Therefore, developing a novel nano-alumina with high α-phase conversion rate, regular morphology, and uniform particle dispersion is of crucial significance for improving the performance and optimizing the application of thermal interface materials in the heat dissipation field. Summary of the Invention

[0005] Based on the background technology, current nano-alumina exhibits drawbacks in thermal interface materials, such as low thermal conductivity, increased colloidal viscosity, and decreased aging performance. This invention aims to develop a method for preparing monodisperse high-α-phase nano-alumina, thereby improving its application shortcomings and comprehensively enhancing the performance of thermal interface materials. This method is achieved through the following steps: industrial alumina is milled into submicron-sized raw materials using a sand mill; then, a uniformly dispersed nanoscale precursor is obtained using centrifugal separation and freeze-drying methods; finally, the precursor is calcined at high temperature to obtain monodisperse high-α-phase nano-alumina.

[0006] The monodisperse high-α phase nano-alumina prepared by this invention solves the problems of nanoparticle agglomeration and crystal phase control during the preparation process through a synergistic process chain. The specific technical points and principles are as follows: (1) First, the preparation of uniformly sized nano-precursors is the foundation of the entire process. Industrial alumina is ground into submicron-sized powder using a sand mill, and then nano-sized powder is extracted using centrifugal static separation technology. This step effectively achieves particle size classification. Subsequently, the nano-sized powder is freeze-dried to obtain the nano-precursors. The removal of moisture by low-temperature vacuum sublimation avoids severe agglomeration of the nano-powder due to the liquid bridging effect of water molecules during high-temperature drying, laying the foundation for the next step of high-temperature calcination to obtain powder with uniform particle size and good dispersibility. (2) Two complementary dispersants were used in the sand mill dispersion system to synergistically optimize the sand milling effect, which is beneficial to improving grinding efficiency and reducing the risk of powder agglomeration. Dispersant 1# is a cationic / anionic dispersant that mainly adsorbs on the particle surface. Through electrostatic repulsion, it reduces the viscosity of the slurry and improves the grinding effect and crushing efficiency. Dispersant 2# is a water-miscible short-chain silane coupling agent. Its hydrophilic end binds to the surface of the nanoparticles and coats the surface of the ground nanoparticles, while its hydrophobic end extends outward to achieve the effect of isolation and repulsion, thereby reducing the risk of powder agglomeration. (3) Finally, the key to forming high-α-phase, well-dispersed nano-alumina lies in the two-stage high-temperature sintering method. During the first stage of sintering in the temperature range of 1000-1100℃, the alumina is at the point where the γ-phase begins to rapidly transform into the α-phase, which is also the temperature node where the austenitic ripening phenomenon begins to be significant. By controlling the slow calcination at this temperature, the degree of crystal phase transformation and austenitic ripening can be controlled, effectively suppressing crystal growth and crystal agglomeration. During the second stage of sintering in the temperature range of 1300-1600℃, most of the alumina has been transformed into α-phase nano-alumina with dense atomic arrangement and complete crystal structure. At this time, the influence of austenitic ripening on its crystal structure is significantly reduced, and the phenomena of excessive crystal growth and particle agglomeration are effectively suppressed. This allows the crystals to maintain the nanoscale without significant coarsening under continuous high-temperature conditions, gradually transforming into the complete α-phase, and obtaining nano-alumina products with both high α-phase content and excellent dispersibility.

[0007] The present invention provides a method for preparing monodisperse high α-phase nano-alumina, the specific steps of which are as follows: Step 1: Using industrial alumina as raw material, disperse it in water to form a mixed slurry with a solid content of 30-50%. Pour it into a sand mill, add dispersant 1#, and sand mill at a speed of 1500-2500 r / min for 0.5-2 hours. Then add dispersant 2# and sand mill for 0.5-1 hours to obtain the sand milled slurry. Step 2: Transfer the sand-milled slurry to a centrifuge and centrifuge at a speed of 1500-2000 r / min for 5-20 min. Take the upper emulsion and return the lower sediment to Step 1 for recycling. Step 3: Add pH adjuster to the upper emulsion to adjust the pH to 8-10, let it stand for 12-24 hours, pour off the upper clear liquid, and take the lower precipitate; Step 4: Transfer the lower precipitate to a freeze dryer, load it onto a freeze drying tray for freeze drying, and obtain the dried nanoscale precursor; Step 5: Place the dried nanoscale precursor in a high-temperature sintering furnace and sinter it at two different temperatures. After cooling, nano-alumina powder is obtained.

[0008] Based on the above technical solution, the dispersant 1# mentioned in step one is one or more of ammonium polycarboxylate, ammonium polyacrylate, and ammonium dodecylbenzenesulfonate; its mass ratio with industrial alumina raw material is 100:0.5-2. Dispersant 1# is a cationic / anionic dispersant, which can effectively reduce the viscosity of the slurry that has a thickening effect due to grinding and crushing after addition, thereby improving the grinding effect and grinding the powder to the nanoscale.

[0009] Based on the above technical solution, the dispersant 2# mentioned in step one is one or more of methyltrimethoxysilane, methyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, hexyltrimethoxysilane, and hexyltriethoxysilane; its mass ratio with industrial alumina raw material is 100:1-5. Dispersant 2# is a short-chain silane coupling agent that can coat the surface of the ground and broken nanoparticles. Utilizing the steric hindrance and hydrophobic effect of the alkyl groups on the surface, it achieves a mutual repulsion effect between powder particles, thereby reducing the risk of nanoparticle agglomeration.

[0010] Based on the above technical solution, the lower sediment in step two can be recycled multiple times by repeating step one until it is transformed into nanoparticles that float in the aqueous phase and become the "upper emulsion", thereby reducing costs and making it more suitable for industrial production.

[0011] Based on the above technical solution, the pH adjuster mentioned in step three is either ammonia or triethylamine.

[0012] Based on the above technical solution, the freeze-drying conditions in step four are set as follows: slurry thickness is 1-2 cm, pre-freezing temperature is -30 to -40℃, pre-freezing time is 2-4 hours, vacuum degree is 15-30 Pa, cold trap temperature is -60 to -80℃, freezing time is 3-6 h, heating temperature is 20-30℃, and heating time is 5-10 h.

[0013] Based on the above technical solution, the temperature conditions of the two stages of the high-temperature sintering furnace are set as follows: the heating rate is 0.5-2℃ / min, first-stage sintering is carried out at 1000-1100℃ for 1-3 hours, and then the temperature is raised to 1300-1600℃ and held for 2-6 hours for second-stage sintering. The purpose of holding at 1000-1100℃ in the first-stage sintering is to control the degree of crystal phase transformation and austenitic ripening, which can effectively suppress crystal growth and crystal agglomeration. The purpose of holding at 1300-1600℃ in the second-stage sintering is to gradually transform the nano-alumina, which has been mostly transformed into the α phase, into the complete α phase under continuous high temperature.

[0014] Compared with existing patented nano-alumina, the monodisperse high α-phase nano-alumina of the present invention has the following advantages: (1) The monodisperse high α-phase nano-alumina of the present invention solves the problems of particle agglomeration and crystal phase control in the preparation process of nano-alumina by using dispersant synergistic grinding, freeze drying and two-stage high-temperature sintering, which significantly improves the dispersibility of nano-alumina and avoids the defects of traditional powders that are prone to agglomeration and clumping; at the same time, it promotes its α conversion rate to up to 95% and its morphology to be regular, which is conducive to improving the filling performance and thermal conductivity of nano-powders in thermal interface materials. This solution can effectively improve the adverse effects of colloidal viscosity thickening and aging performance degradation of nano-powders in thermal interface materials. (2) The raw material for the monodisperse high α-phase nano alumina of the present invention is industrial alumina, which is inexpensive and abundant. It does not generate a large amount of water vapor during calcination, thus affecting the temperature fluctuation and service life of the high-temperature sintering furnace. Moreover, during the grinding and preparation of the precursor, the precipitates that do not meet the requirements are re-ground, achieving material recycling and further reducing the preparation cost. Attached Figure Description

[0015] Figure 1 This is a scanning electron microscope image of the monodisperse high α-phase nano-alumina prepared in Example 1. Specific implementation methods

[0016] The following embodiments provide a detailed description of the above-mentioned content of the present invention. In particular, it should be noted that several adjustments and improvements can be made based on the principles of the present invention, and these adjustments and improvements are also considered to be within the protection scope of the embodiments of the present invention.

[0017] The present invention specifically discloses the following specific embodiments:

[0018] Example 1 Step 1: Disperse 1 kg of industrial alumina in water to form a mixed slurry with a solid content of 30%, pour it into a sand mill, add 10 g of ammonium dodecylbenzenesulfonate, and sand mill at 2500 r / min for 1 hour. Then add 30 g of propyltriethoxysilane and sand mill for 1 hour to obtain the sand milled slurry. Step 2: Transfer the sand-milled slurry to a centrifuge and centrifuge for 15 minutes at a speed of 2000 r / min. Take the upper emulsion and repeat Step 1 to recycle the lower sediment. Step 3: Add ammonia to the upper emulsion to adjust the pH to 9, let it stand for 16 hours, pour off the upper clear liquid, and take the lower precipitate. Step 4: Transfer the lower precipitate to a freeze dryer, load it with a freeze drying tray, control the slurry thickness to 1 cm, set the pre-freezing temperature to -30℃, the pre-freezing time to 3 hours, the vacuum degree to 15 Pa, the cold trap temperature to -60℃, the freezing time to 5 hours, the heating temperature to 30℃, and the heating time to 6 hours. After heating, a dry nanoscale precursor is obtained. Step 5: Place the dried nanoscale precursor in a high-temperature sintering furnace, heat at a rate of 1℃ / min, hold at 1100℃ for 3 hours, then heat to 1400℃ and hold for 2 hours. After cooling, nano-alumina powder is obtained.

[0019] Example 2 Step 1: Disperse 10 kg of industrial alumina in water to form a mixed slurry with a solid content of 40%, and pour it into a sand mill. Add 100 g of ammonium polycarboxylate and sand mill at 2200 r / min for 1 hour. Then add 300 g of a mixed dispersant of methyltriethoxysilane and propyltriethoxysilane in a mass ratio of 1:1 and sand mill for 1 hour to obtain the sand milled slurry. Step 2: Transfer the sand-milled slurry to a centrifuge and centrifuge for 15 minutes at a speed of 1500 r / min. Take the upper emulsion and repeat Step 1 to recycle the lower sediment. Step 3: Add ammonia to the upper emulsion to adjust the pH to 10, let it stand for 16 hours, then pour off the upper clear liquid and take the lower precipitate. Step 4: Transfer the lower precipitate to a freeze dryer, load it onto a freeze drying tray, control the slurry thickness to 2 cm, set the pre-freezing temperature to -40℃, the pre-freezing time to 4 hours, the vacuum degree to 20 Pa, the cold trap temperature to -60℃, the freezing time to 6 hours, the heating temperature to 30℃, and the heating time to 8 hours. After heating, a dry nanoscale precursor is obtained. Step 5: Place the dried nanoscale precursor in a high-temperature sintering furnace, heat at a rate of 2℃ / min, hold at 1000℃ for 3 hours, then heat to 1500℃ and hold for 3 hours. After cooling, nano-alumina powder is obtained.

[0020] Example 3 Step 1: Disperse 50kg of industrial alumina in water to form a mixed slurry with a solid content of 30%, pour it into a sand mill, add 500g of ammonium polyacrylate, and sand mill at 2000r / min for 1.5h. Then add 2500g of methyltriethoxysilane and sand mill for 1h to obtain the sand milled slurry. Step 2: Transfer the sand-milled slurry to a centrifuge and centrifuge for 20 minutes at a speed of 2000 r / min. Take the upper emulsion and repeat Step 1 to recycle the lower sediment. Step 3: Add ammonia to the upper emulsion to adjust the pH to 9, let it stand for 16 hours, pour off the upper clear liquid, and take the lower precipitate. Step 4: Transfer the lower precipitate to a freeze dryer, load it onto a freeze drying tray, control the slurry thickness to 1.5 cm, set the pre-freezing temperature to -35℃, the pre-freezing time to 3 hours, the vacuum degree to 20 Pa, the cold trap temperature to -70℃, the freezing time to 4 hours, the heating temperature to 30℃, and the heating time to 6 hours. After heating, a dry nanoscale precursor is obtained. Step 5: Place the dried nanoscale precursor in a high-temperature sintering furnace, raise the temperature at a rate of 0.5℃ / min, hold at 1000℃ for 3 hours, then raise the temperature to 1300℃ and hold for 6 hours. After cooling, nano-alumina powder is obtained.

[0021] Blank example The blank example is commercially available nano-alumina with a particle size of 100-300nm and an α conversion rate of 60%.

[0022] Comparative Example 1 Comparative Example 1 is based on Example 1, except that dispersant 1# is not introduced in step one, and all other parameters and operating procedures are the same as in Example 1.

[0023] Comparative Example 2 Comparative Example 2 is based on Example 1, except that dispersant 2# is not introduced in step one, and all other parameters and operating procedures are the same as in Example 1.

[0024] Comparative Example 3 Comparative Example 3 is based on Example 1, except that dispersant 1# and dispersant 2# are not introduced in step one, and all other parameters and operating procedures are the same as in Example 1.

[0025] Comparative Example 4 Comparative Example 4 is based on Example 1, except that in step four, instead of freeze drying, a forced-air drying oven is used for drying at 120°C for 5 hours. All other parameters and operating procedures are the same as in Example 1.

[0026] Comparative Example 5 Comparative Example 5 is based on Example 1, except that in step five, the temperature is not kept at 1100°C, but is directly increased from room temperature to 1400°C at a rate of 1°C / min for 2 hours. All other parameters and operating procedures are the same as in Example 1.

[0027] To illustrate the specific application effects of the above examples, the products corresponding to the embodiments, blank examples, and comparative examples were tested for particle size (D50 and D100), oil absorption value, thermal conductivity, viscosity, weather resistance at 150℃, and α conversion rate, respectively. The colloidal state after passing the weather resistance test was observed for comparison and analysis. Table 1 below shows the test performance of the nano-alumina of each embodiment, blank example, and comparative example, demonstrating the beneficial effects of the monodisperse high α-phase nano-alumina of the present invention in organosilicon thermal conductive gels for thermal interfaces.

[0028] The performance testing methods are as follows: 1) Particle size test method: Take 1g of powder and place it in a 50ml measuring cup, add 30ml of water, sonicate in an ultrasonic oscillator for 5min, and then slowly add it to the LS-609 laser particle size analyzer for testing, and take the D50 and D100 particle size parameters. 2) Oil absorption value test method: Dibutyl phthalate is used as the oil phase to impregnate magnesium oxide. The centrifuge tube containing the oil powder sample is ultrasonically dispersed for a period of time. Then, the sample is centrifuged to remove the remaining dibutyl phthalate. After inverting the centrifuge tube for half an hour, the inorganic powder impregnated with oil is obtained. During the process, the mass of the powder and the mass of the remaining dibutyl phthalate are recorded, and the oil absorption value of the powder can be calculated. For easy conversion and comparison, the unit of oil absorption value is set to g / 100g, that is, 100g of alumina can adsorb xg of dibutyl phthalate. 3) Thermal conductivity test method: 350 parts of the sample were filled into 350cp vinyl silicone oil and stirred at 900r / min for 2min in a vacuum degassing machine to obtain thermally conductive gel. The thermal conductivity of the above silicone was tested using a DRL-Ⅲ thermal conductivity meter. The sample diameter was 20mm, the thickness was 2mm, and the test pressure was 30N. 4) Viscosity test method: 300 parts of the sample were filled into 350cp vinyl silicone oil and stirred at 900r / min for 2min in a vacuum degassing machine to obtain thermally conductive gel. The viscosity value of the thermally conductive gel was tested using a DV-2T digital display rotational viscometer, and the viscosity value at 12r / min was taken using a No. 97 rotor. 5) 150℃ Weathering Test: Using a DHG-9070A forced-air drying oven, the thermal conductive gel was applied to a 10cm*10cm glass surface with a thickness of 0.2mm. The surface was then placed horizontally in a 150℃ environment to test its aging performance. The observation period was 200-500 hours. After the weathering test, the surface was removed and its condition was observed to see if it remained unchanged, cracked, or powdered. 6) α-conversion rate test: The method refers to YS / T 976-2014 "Determination of α-Al2O3 content in calcined α-type alumina by X-ray diffraction". Under the same X-ray diffraction conditions, the net intensity of the diffraction integral of the α-Al2O3 (012) and (116) crystal planes of the reference sample and the standard sample was measured respectively, and the intensity percentage was calculated by the external standard direct analysis method.

[0029] Performance tests were performed on each embodiment, blank example, and comparative example, and the test results are shown in Table 1.

[0030] Table 1

[0031] As can be seen from the data in Table 1: (1) A comparison of the oil absorption value, viscosity value, and aging resistance of the blank example and Examples 1 to 3 shows that the blank example has severe powder particle agglomeration. The particles "bridge" each other due to electrostatic and van der Waals forces, resulting in an increased specific surface area. This leads to poor compatibility of the blank powder in the polymer matrix, resulting in higher oil absorption and viscosity values ​​of the prepared thermal conductive gel. Due to the high viscosity, the thermal conductive gel is prone to cracking or even complete pulverization during high-temperature aging. This is because the uneven distribution of powder particles in the polymer matrix causes uneven stress due to thermal expansion at high temperatures, which in turn leads to colloid cracking. In contrast, the nano-alumina in Examples 1 to 3 of this invention has undergone multi-stage processing, which improves the defects of powder agglomeration and agglomeration, enhances its compatibility in the polymer matrix, and reduces its uneven distribution in the matrix. Therefore, it has certain improvements in technical parameters such as oil absorption value, viscosity value, aging resistance time, and aging resistance. (2) A comparison of the α-conversion rate and thermal conductivity of the blank example and Examples 1 to 3 shows that the blank example has a lower α-conversion rate and contains other different phase crystal forms, resulting in a lower thermal conductivity of the thermally conductive gel. In contrast, the nano-alumina of Examples 1 to 3 of this invention has an α-conversion rate as high as 95%, complete crystal form, and low crystal defects, which reduces the difficulty of phonon propagation in the crystal lattice. Specifically, it increases the phonon group velocity and reduces the phonon mean free path, ultimately leading to a significant increase in thermal conductivity. In summary, the examples demonstrate that the monodisperse high-α-phase nano-alumina prepared in this invention exhibits excellent overall performance in thermally conductive gels for thermal interface materials. (3) From the comparison of particle size D50 and D100, oil absorption value, thermal conductivity, viscosity value and aging resistance of Comparative Examples 1 to 3 and Example 1, it can be seen that in Comparative Example 1, due to the lack of dispersant 1#, the particles could not be fully dispersed during the dispersion and grinding process in step 1, resulting in a sand-milled slurry that could not reach the nanoscale. Consequently, it could not be converted into nanoscale alumina in the subsequent calcination step. Although other performance parameters were better, the particle size of the final product exceeded the nanoscale range. In Comparative Example 2, the role of dispersant 2# is to disperse and repel. However, without dispersant 2#, the particles could not be effectively blocked and separated after grinding in step 1, causing the powder to return to an agglomerated state after grinding and breaking. Similarly, this resulted in the particle size of the powder growing during the subsequent calcination step, preventing it from reaching the nanoscale. In Comparative Example 3, both dispersant 1 and dispersant 2# were missing. During the dispersion and grinding process in step one, the particles were not sufficiently dispersed and did not play a role in combing and repelling. This resulted in severe particle agglomeration, which led to rapid crystal growth in the final calcination step. The final product had a D50 particle size of 4μm, which does not belong to the category of nanoparticles. (4) From the comparison of particle size D50 and D100, oil absorption value, thermal conductivity, viscosity value and aging resistance of Comparative Example 4 and Example 1, it can be seen that if the freeze drying in step 4 is replaced by high-temperature drying in a forced-air drying oven, the powder will re-agglomerate due to the "liquid bridge" force during the drying process, resulting in severe agglomeration of the precursor. This will destroy the coating and isolation layer formed by the dispersant on the particle surface, ultimately causing the particles to still grow excessively during the calcination process in step 5 and fail to reach the nanoscale. Moreover, it will cause a sharp increase in viscosity value in the organosilicon matrix and a decrease in aging resistance. (5) From the comparison of particle size D50 and D100, oil absorption value, thermal conductivity, viscosity value and aging resistance of Example 5 and Example 1, it can be seen that the secondary calcination in step 5 effectively controls the degree of particle growth during the crystal transformation process. Therefore, although Example 5 effectively controls the agglomeration of the precursor through the optimization of steps 1 to 4, if the control of the first low-temperature calcination is lacking, the particles still cannot reach the nanoscale of 300-400nm. Moreover, due to the direct heating to 1400℃, the particles agglomerate to a certain extent, which increases the oil absorption value, thereby leading to an increase in viscosity value in the organosilicon matrix and a decrease in aging resistance.

Claims

1. A method for preparing monodispersed nanometer alumina of high alpha phase, characterized in that, The method comprises the following steps: Step one: using industrial alumina as raw material, dispersing it in water to form a mixed slurry with a solid content of 30-50%, pouring it into a sand mill, adding dispersant 1# and sand milling at a speed of 1500-2500 r / min for 0.5-2 h, then adding dispersant 2# and sand milling for 0.5-1 h to obtain a sand milling slurry; Step two: transferring the sand milling slurry to a centrifuge and centrifuging at a speed of 1500-2000 r / min for 5-20 min, taking the upper emulsion, and returning the lower sediment to step one for recycling; Step three: adding a pH adjuster to the upper emulsion to adjust the pH to 8-10, standing for 12-24 h, then pouring out the upper clear liquid and taking the lower sediment; Step four: transferring the lower sediment to a freeze-drying machine, loading it on a freeze-drying tray and freeze-drying to obtain dry nanoscale precursors; Step five: placing the dry nanoscale precursors in a high-temperature sintering furnace, sintering at two different temperatures, and cooling to obtain nanoscale alumina powder.

2. The method for preparing monodisperse high-α phase nano-alumina according to claim 1, characterized in that, The dispersant 1# in step one is one or more of polycarboxylic acid ammonium, polyacrylamide, and ammonium dodecylbenzenesulfonate; the mass ratio of the dispersant 1# to the industrial alumina raw material is 100:0.5-2.

3. The method for preparing monodisperse high α-phase nano-alumina according to claim 1, characterized in that, The dispersant 2# in step one is one or more of methyltrimethoxysilane, methyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, hexyltrimethoxysilane, and hexyltriethoxysilane; the mass ratio of the dispersant 2# to the industrial alumina raw material is 100:1-5.

4. The method for preparing monodisperse high-α phase nano-alumina according to claim 1, characterized in that, The pH adjuster in step three is one of ammonia or triethylamine.

5. The method for preparing monodisperse high-α phase nano-alumina according to claim 1, characterized in that, In step four, the freeze-drying conditions are set as follows: the slurry thickness is 1-2 cm, the pre-freezing temperature is set to -30 to -40℃, the pre-freezing time is 2-4 hours, the vacuum degree is 15-30 Pa, the cold trap temperature is -60 to -80℃, the freezing time is 3-6 h, the heating temperature is 20-30℃, and the heating time is 5-10 h.

6. The method for preparing monodisperse high-α phase nano-alumina according to claim 1, characterized in that, In step five, the two-stage temperature conditions of the high-temperature sintering furnace are set as follows: the heating rate is 0.5-2℃ / min, first sintering at 1000-1100℃ for 1-3 h, and then sintering at 1300-1600℃ for 2-6 h.

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