A method for preparing sheet-like α-Al2O3

Abstract

The application discloses a preparation method of flaky alpha-Al2O3 powder, and aims to provide a preparation method of flaky alpha-Al2O3 powder which is simple in synthesis method, time-saving, low in calcination temperature, smooth in surface, good in dispersity, large in diameter-thickness ratio and controllable in particle size, and belongs to the technical field of inorganic materials. The method comprises the following steps: reacting a solid-phase aluminum salt with an organic urea in an acidified solution at room temperature to obtain a metal aluminum complex aqueous solution; adding soluble fused salt and an inorganic salt additive into the metal aluminum complex obtained in step 1), then adding hexafluoroaluminate, stirring and heating at 60-120 DEG C to obtain a semi-transparent solution A; after the semi-transparent solution A is dried in an oven, the powder B is obtained by grinding with a mortar; the uniformly mixed powder B is placed in an alumina ceramic crucible and sent into a muffle furnace to perform high-temperature sintering to obtain the flaky alpha-Al2O3 powder.

Description

Technical Field

[0001] This invention relates to a method for preparing Al2O3, specifically a method for preparing sheet-like α-Al2O3, belonging to the field of inorganic materials technology. Background Technology

[0002] α-Al₂O₃, due to its excellent properties such as high strength, high hardness, corrosion resistance, high temperature resistance, oxidation resistance, and good insulation, is widely used in various fields including electronics and information technology, environmental protection, aerospace, sensing, energy storage and conversion, lighting and display, biomedicine and detection, imaging, and catalysis. Currently reported monodisperse α-Al₂O₃ shapes include rods, spheres, and flakes. Flake α-Al₂O₃ has a unique two-dimensional structure, with radial dimensions ranging from a few micrometers to tens of micrometers and thicknesses from a few nanometers to hundreds of nanometers. Due to its smooth surface, it is used as an abrasive ion in high-end wafer polishing slurries for wafer polishing; its high refractive index is used as a substrate for pearlescent pigments; and its high aspect ratio provides a large contact area, allowing it to be combined with other materials (such as in the preparation of refractory composites and shell-like materials).

[0003] Currently, the mainstream methods with industrial potential for synthesizing sheet-like α-Al2O3 are mainly the sol-gel method, precipitation method, hydrothermal method, molten salt method, and solid-phase method.

[0004] The molten salt method refers to a method of growing materials using reactants in a molten salt environment at high temperatures. The high-temperature molten salt provides an excellent growth environment for crystal nucleation and growth. The melting point of the molten salt in the molten salt method is lower than that of the calcined material, effectively reducing the sintering temperature. Furthermore, the good fluidity of the molten salt in its molten state provides favorable conditions for the diffusion of various components. Compared to conventional methods, the molten salt method offers advantages such as inexpensive raw materials, high flexibility, low synthesis temperature, short reaction cycle, controllable crystal morphology, uniform powder composition without segregation, high phase purity, and simple process with minimal pollution.

[0005] CN101891226A describes a process where aluminum-containing double salts, soluble alkali metal sulfates, titanium salts, phosphoric acid, or soluble phosphates are thoroughly dry-mixed, pre-calcined, and then treated with molten salt at a high temperature of 1000-1200℃ to obtain flake-like α-Al₂O₃ with an average particle size of 5-80 μm, a thickness of 0.1-1 μm, and a particle size-to-thickness ratio of 30-500. CN112479241B describes a process where aluminum hydroxide is mixed with a strong alkaline solution, acid is added dropwise, and then aged to obtain flake-like aluminum hydroxide. The flake-like aluminum hydroxide is then mixed with molten salt and calcined at 1000-1100℃ to finally obtain flake-like alumina with an average particle size of approximately 10 μm and a thickness of approximately 0.5 μm, but its surface is not very smooth and contains some particles. WO2006101306A1 selected water-soluble aluminum salts or composite aluminum salts as aluminum sources, sodium sulfate and potassium sulfate as molten salts, and controlled the shape and size of alumina by additives such as titanium salts, phosphates, zinc salts and tin salts, and obtained flake alumina at 1000-1200℃. CN104986786A Preparation of mixed precursor solution: A precursor solution is obtained by uniformly mixing water-soluble aluminum salt, soluble salt, nano-alumina seed crystals and / or plate-like α-Al2O3 seed crystals; an alkaline aqueous solution is then added to the solution, and the final pH is controlled to be 5.0-9.0 at the end of the hydrolysis reaction to obtain a mixed gel containing hydrolysis products; the dried mixed gel is calcined at 850-1400 °C to obtain plate-like α-Al2O3 powder with a major diameter greater than or equal to 4 μm, a thickness less than or equal to 0.1 μm, and an aspect ratio greater than or equal to 40; CN105858694A Adds molten salt and Na3FSO4 crystals to an aluminum-containing raw material to obtain a mixture, and then calcines the mixture at 650-1000 °C to obtain plate-like α-Al2O3, but its surface is very rough.

[0006] The above-mentioned works all used the molten salt method to prepare sheet-like alumina, but they were basically prepared at high temperatures above 1000℃, and the surface of the prepared sheet-like α-Al2O3 was not smooth.

[0007] CN 115974111 A discloses a controllable preparation method for plate-like alumina. The method involves mixing and grinding aluminum salt and urea until homogeneous, then drying to obtain a urea-aluminum complex. The urea-aluminum complex and a soluble molten salt are then mixed homogeneously and calcined to obtain plate-like α-Al₂O₃ seed crystals. Hydrodynamic classification is used to screen for different particle sizes, obtaining plate-like α-Al₂O₃ seed crystals with varying particle sizes. The plate-like α-Al₂O₃ seed crystals, aluminum salt, soluble molten salt, and a crystal growth regulator solution are then mixed to obtain suspension A. Phosphate was added to an alkaline aqueous solution and mixed to obtain solution B. Solution B was then added to suspension A under continuous stirring, controlling the pH at the end of the hydrolysis reaction to be 6.5-7.5, resulting in a gel mixture containing hydrolysis products. This mixture was dried to obtain a solid gel mixture. The solid gel mixture was then calcined at 900-1300℃ for 2-6 hours at a heating rate of 1-10℃ / min. The calcined solid was washed with water to remove soluble molten salt and then dried to obtain flake-like α-Al₂O₃ powder. This method, based on the molten salt method, utilizes hydraulic classification to achieve controllable preparation of seed crystal particle size, thus improving particle size control and enabling adjustable seed crystal diameter. This is beneficial for preparing uniformly sized flake-like alumina, but the surface smoothness is still not ideal.

[0008] Therefore, the preparation of smooth, uniformly sized, and well-dispersed flaky α-Al2O3 at low temperatures has become a technical challenge in the industry. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a metallic aluminum complex. Based on the molten salt method, this metallic aluminum complex is used to add hexafluoroaluminate to prepare smooth and well-dispersed flake-like α-Al2O3 at low temperature.

[0010] Therefore, the technical solution provided by this invention is as follows:

[0011] A method for preparing sheet-like α-Al₂O₃ comprises the following steps:

[0012] Step 1) Preparation of aluminum complex

[0013] (1) React solid aluminum salt with organic urea in an acidified solution at room temperature for 10-30 min to obtain an aqueous solution of metallic aluminum complex;

[0014] The molar ratio of the solid aluminum salt to the organic urea is 1:1 to 1:6;

[0015] Step 2) Preparation of sheet-like α-Al2O3

[0016] (1) Add soluble molten salt and inorganic salt additives to the aluminum complex obtained in step one), then add hexafluoroaluminate, stir and heat to 60-120℃, react for 15-45 min to obtain a semi-transparent solution A.

[0017] The molar ratio of the solid aluminum salt, soluble molten salt, inorganic salt additive, and hexafluoroaluminate is 100:100:1:0.5-100:100:600:5:10.

[0018] (2) After drying the translucent solution A in an oven, grind it evenly with a mortar and pestle to obtain powder B;

[0019] (3) Place the uniformly mixed powder B into an alumina ceramic crucible and send it into a muffle furnace for high-temperature sintering. The sintering temperature is 800-1200℃, the heating rate is 1-10℃ / min, and the holding time is 2-10h.

[0020] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, the solid aluminum salt described in steps one and two is an oxoacid salt of aluminum and / or a halide salt of aluminum.

[0021] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, the organic urea in step one) is urea or thiourea.

[0022] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, step two) involves stirring using magnetic stirring or mechanical stirring.

[0023] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, the soluble molten salt in step two is one or any combination of sodium chloride, sodium sulfate, sodium fluoride, potassium chloride, potassium sulfate, potassium fluoride, lithium chloride, lithium sulfate, and lithium fluoride.

[0024] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, the inorganic salt additive in step two) is one or any combination of sodium phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, and sodium trimetaphosphate.

[0025] Preferably, in the above-mentioned method for preparing sheet-like α-Al2O3, the hexafluoroaluminate mentioned in step two) includes cryolite, potassium cryolite, or a combination thereof.

[0026] Furthermore, in the above-mentioned method for preparing sheet-like α-Al2O3, the hexafluoroaluminate described in step two) was ball-milled for different times (0-12h).

[0027] Compared with the prior art, the technical solution provided by the present invention has the following technical advantages:

[0028] 1. The technical solution provided by this invention involves the reaction of solid aluminum salt and organic urea in an acidified solution at room temperature. The technical solution provided by this application involves the reaction under acidic conditions, which effectively inhibits the hydrolysis of aluminum salt and allows it to better form a complex with urea.

[0029] 2. The technical solution provided by the present invention prepares a metallic aluminum complex by a one-pot solution method, and then uses this complex to obtain flake-shaped α-Al2O3 powder by calcination at 800℃~1200℃ based on the molten salt method. It has the advantages of cheap raw materials, simple process, time saving and low energy consumption.

[0030] 3. The technical solution provided by this invention introduces hexafluoroaluminates of different particle sizes, which helps the γ-phase alumina dissolve in the molten salt and accelerates the diffusion and mass transfer rate in the molten salt. By precisely controlling its dosage, the morphology of the plate-like α-Al2O3 can be controlled, reducing agglomeration and obtaining plate-like α-Al2O3 powder with narrow particle size distribution, smooth surface, good monodispersity, and high aspect ratio.

[0031] In summary, the technical solution provided in this application successfully synthesized well-dispersed lamellar α-Al₂O₃ single crystals at a low temperature of 800℃ by introducing cryolite and sodium phosphate into a sodium sulfate-potassium sulfate molten salt, using an aluminum-urea complex as a precursor. In this molten salt system, we overcame the problem of crystal agglomeration and reduced interfacial energy during low-temperature growth by introducing trace amounts of sodium phosphate and cryolite. This synthesis method is simple, easy to implement, and time-saving. Furthermore, the raw materials are inexpensive, and the synthesis temperature is low, which aligns well with the industrial production concept of energy conservation and emission reduction, and has great potential for industrial-scale production. Attached Figure Description

[0032] Figure 1 The image shows a field emission scanning electron microscope (FEM) image of the sheet-like alumina prepared in Comparative Example 2.

[0033] Figure 2 The XRD diffraction pattern of the sheet-like alumina prepared in Comparative Example 2 is shown.

[0034] Figure 3 The image shows a field emission scanning electron microscope image of the sheet-like alumina prepared in Comparative Example 3.

[0035] Figure 4 The image shown is a field emission scanning electron microscope image of the sheet-like alumina prepared in Example 1.

[0036] Figure 5 The XRD diffraction pattern of the sheet-like alumina prepared in Example 2;

[0037] Figure 6 Comparison of XRD diffraction patterns of the sheet-like alumina prepared in Comparative Examples 3 and 4, 5 and 6;

[0038] Figure 7 The XRD diffraction patterns of the sheet-like alumina prepared in Examples 1, 7, 8, and 9 are shown in comparison. Detailed Implementation

[0039] The claims of the present invention will be further described in detail below with reference to specific embodiments, but this does not constitute any limitation on the present invention. Any limited modifications made by any person within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0040] Example 1

[0041] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.05 g of cryolite and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as Al.

[0042] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0043] The morphology and thickness of the powder were characterized and analyzed using a field emission scanning electron microscope (SEM, ZEISS Gemini SEM450), such as... Figure 4 As shown; the phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalytical X'PertPro), and the measured data are as follows. Figure 5 As shown.

[0044] Example 2

[0045] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹-1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were then added, and heating was initiated. The temperature was raised to 80°C, and after the molten salt had completely dissolved and the solution became clear and transparent, 0.05 g of cryolite and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 1000°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A₂.

[0046] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 6 As shown;

[0047] Example 3

[0048] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were then added, and heating was initiated. The temperature was raised to 80°C, and after the molten salt had completely dissolved and the solution became clear and transparent, 0.05 g of cryolite and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 1200°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A₃.

[0049] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 6 As shown;

[0050] Example 4

[0051] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.05 g of cryolite (milled for 2 hours) and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A₄.

[0052] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0053] Example 5

[0054] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.05 g of cryolite (milled for 4 hours) and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A5.

[0055] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. i0 D 50 and D 90 The measured data are shown in Table 1.

[0056] Example 6

[0057] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetic stirring in a dilute nitric acid solution for 10 minutes at room temperature. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.05 g of cryolite (milled for 8 hours) and 0.025 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A6.

[0058] Example 7

[0059] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.01 g of cryolite and 0.05 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A7.

[0060] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown; the D of alumina thin films was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0061] Example 8

[0062] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.01 g of cryolite and 0.1 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A8.

[0063] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown; the D of alumina thin films was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0064] Example 9

[0065] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.01 g of cryolite and 0.25 g of sodium phosphate dodecahydrate were weighed and added, and heating was continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as A9.

[0066] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown; the D of alumina thin films was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0067] Comparative Example 1

[0068] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of deionized water and stir magnetically for 10min at room temperature to obtain a clear solution. Then weigh 2.45g of sodium sulfate and 3.00g of potassium sulfate and add them to the above solution, then turn on the heater. Heat to 80℃ and maintain for 25min. After heating, transfer the solution to a petri dish and dry in an oven (120℃, 2h). After drying, transfer the dried cake to a mortar and grind it into a fine powder. Transfer the mixed powder to an alumina ceramic crucible and keep it at 800℃ for 5h, then allow it to cool naturally. Wash the added molten salt in 90℃ hot water until the conductivity is less than 20μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B1.

[0069] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0070] Comparative Example 2

[0071] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, heating continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B₂.

[0072] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0073] The morphology and thickness of the powder were characterized and analyzed using a field emission scanning electron microscope (SEM, ZEISS Gemini SEM450), such as... Figure 1 As shown; the phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalytical X'PertPro), and the measured data are as follows. Figure 2 As shown.

[0074] Comparative Example 3

[0075] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.05 g of cryolite was weighed and added, and heating continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·om using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B3.

[0076] The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0077] The morphology and thickness of the powder were characterized and analyzed using a field emission scanning electron microscope (SEM, ZEISS Gemini SEM450), such as... Figure 3 As shown.

[0078] Comparative Example 4

[0079] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, 0.1 g of cryolite was weighed and added, and heating continued for 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B4.

[0080] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown. The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0081] Comparative Example 5

[0082] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1 A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. Then, 2.45 g of sodium sulfate, 3.00 g of potassium sulfate, and 0.25 g of cryolite were weighed and added to the solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt was completely dissolved and the solution became clear and transparent, heating continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). After drying, the dried cake was transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm using a conductivity meter. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B5.

[0083] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown. The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 50 and D 90 The measured data are shown in Table 1.

[0084] Comparative Example 6

[0085] Dissolve 5g of aluminum nitrate (Al(NO3)3·9H2O) and 4.8g of urea (CON2H4) in 60ml of 0.1mol·L⁻¹ -1A clear solution was obtained by magnetically stirring a dilute nitric acid solution at room temperature for 10 minutes. 2.45 g of sodium sulfate and 3.00 g of potassium sulfate were added to this solution, and heating was initiated. The temperature was raised to 80°C, and after the molten salt had completely dissolved and the solution became clear and transparent, 0.5 g of cryolite was added, and heating continued for another 25 minutes. After heating, the solution was transferred to a petri dish and dried in an oven (120°C, 2 hours). The dried cake was then transferred to a mortar and ground into a fine powder. The mixed powder was then transferred to an alumina ceramic crucible and kept at 800°C for 5 hours, followed by natural cooling. The added molten salt was washed in hot water at 90°C until the conductivity was measured to be less than 20 μS·cm. -1 After washing, the powder was transferred to a petri dish and dried in an oven to obtain flake-like α-Al₂O₃ powder, denoted as B6.

[0086] The phase composition of the powder was analyzed using X-ray powder diffraction (XRD, PANalyticalX'PertPro), and the measured data are as follows: Figure 7 As shown. The D-value of alumina sheets was evaluated using a Truth Optics LT2200E laser particle size analyzer. 10 D 5o and D 90 The measured data are shown in Table 1.

[0087] Table 1

[0088]

[0089]

[0090] As can be seen from the table above, compared to B3, the particle sizes D10, D50, and D90 of B4, B5, and B6 increase with the increase of cryolite content. The significant increase in D90 is due to the formation of large crystalline sodium aluminate (Na2Al) crystals. 2x O 3x+1 ).analyze Figure 6 Without the addition of acid, B1 showed a very weak α-Al₂O₃ diffraction peak, along with other unattributed impurity peaks. With the addition of 1% wt cryolite, the XRD diffraction peaks were very pure, with almost no other impurity peaks. However, when the amount added exceeded 1% wt, mullite-like diffraction peaks appeared. Based on the combined results of electron microscopy, XRD, and testing, a cryolite addition of 1% wt yielded the best product.

[0091] Comparing Examples B2 and B3, it can be seen that the addition of cryolite not only improves the dispersibility of the flake-like α-Al2O3 but also improves its surface morphology, making the previously particle-covered surface very smooth.

[0092] Comparing B3 and A1, it can be seen that the dispersibility further improved after introducing 0.5% wt of sodium phosphate on a base of fixed 1% wt. cryolite. Figure 4 It can be seen that the grown patches are monodisperse and lie flat under the lens.

[0093] Compared to A1, A7, A8, and A9 changed the amount of sodium phosphate used to 1% wt., 2% wt., and 5 wt., respectively. Table 1 shows that changing the amount of sodium phosphate had almost no effect on the alumina particle size. Figure 7 The XRD results also show that the impurity content is very low, and the impurity content does not change significantly with the amount of sodium phosphate added. In summary, when the amount of cryolite is fixed at 1% wt., the amount of sodium phosphate added at 0.5% wt. can improve the monodispersity of the prepared flake alumina.

[0094] Compared to A1, cryolite with additives of 1 wt% cryolite and 0.5 wt% sodium phosphate was added, followed by cryolite milled for different times: C1 (0 h), C2 (2 h), C3 (4 h), and C4 (8 h). Analysis of Table 1 shows that as the milling time increases, the size of the cryolite decreases, but also approaches a limit. Comparing A1, A4, A5, and A6, using smaller cryolite resulted in smaller flakes and a narrower particle size distribution. Therefore, different particle sizes of flake alumina can be obtained by controlling the particle size of the introduced cryolite.

[0095] In summary, well-dispersed lamellar α-Al₂O₃ single crystals were successfully synthesized at a low temperature of 800℃ by introducing cryolite and sodium phosphate into a sodium sulfate-potassium sulfate molten salt, using an aluminum-urea complex as a precursor. In this molten salt system, the introduction of trace amounts of sodium phosphate and cryolite overcame the problem of crystal agglomeration and reduced interfacial energy during low-temperature growth. This synthesis method is simple, easy to implement, and time-saving. Furthermore, the raw materials are inexpensive, and the synthesis temperature is low, which aligns well with the industrial production philosophy of energy conservation and emission reduction, making it highly promising for industrial-scale production.

Claims

1. A method for preparing sheet-like α-Al₂O₃, characterized in that, The following steps are performed sequentially: Step 1) Preparation of aluminum complex Solid aluminum salts and organic urea were reacted in an acidic solution at room temperature for 10-30 min to obtain an aqueous solution of metallic aluminum complex. The molar ratio of the solid aluminum salt to the organic urea is 1:1 to 1:6; The acid in the acidic solution is hydrochloric acid, nitric acid, or sulfuric acid; Step 2) Preparation of sheet-like α-Al2O3 (1) Add soluble molten salt and inorganic salt additives to the aluminum complex obtained in step one), then add hexafluoroaluminate, stir and heat to 60-120℃, react for 15-45 min to obtain a semi-transparent solution A; The mass ratio of the solid aluminum salt, soluble molten salt, inorganic salt additive, and hexafluoroaluminate is 5.00:5.45:0.05:0.025, or 5.00:5.45:0.05:0.025, or 5.00:5.45:0.01:0.

25. (2) After drying the translucent solution A in an oven, grind it evenly with a mortar and pestle to obtain powder B; (3) Place the uniformly mixed powder B into an alumina ceramic crucible and send it into a muffle furnace for high-temperature sintering. The sintering temperature is 800-1200℃, the heating rate is 1~10℃ / min, and the holding time is 2-10h. The particle size of the hexafluoroaluminate mentioned in step two) is 0.2-1.5 μm; Step 2) The inorganic salt additive mentioned is one or any combination of sodium phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, and sodium trimetaphosphate.

2. The method for preparing sheet-like α-Al₂O₃ according to claim 1, characterized in that, The solid aluminum salts mentioned in steps one and two are oxoacid salts of aluminum and / or halide salts of aluminum.

3. The method for preparing sheet-like α-Al₂O₃ according to claim 1, characterized in that, The organic urea mentioned in step one is urea or thiourea.

4. The method for preparing sheet-like α-Al₂O₃ according to claim 1, characterized in that, Step 2) describes a stirring method that can be either magnetic stirring or mechanical stirring.

5. The method for preparing sheet-like α-Al₂O₃ according to claim 1, characterized in that, Step 2) The soluble molten salt mentioned is one or any combination of sodium chloride, sodium sulfate, sodium fluoride, potassium chloride, potassium sulfate, potassium fluoride, lithium chloride, lithium sulfate and lithium fluoride.

6. The method for preparing sheet-like α-Al₂O₃ according to claim 1, characterized in that, The hexafluoroaluminate mentioned in step two includes cryolite, potassium cryolite, or a combination thereof.

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