A method for manufacturing alumina-coated bismuth oxide powder and its application

Abstract

This invention provides a method for manufacturing bismuth oxide powder coated with alumina. The method involves preparing a bismuth oxide@alumina structure via hydrothermal synthesis, removing the pore-forming agent through subsequent sintering to prepare bismuth oxide@porous alumina, and finally loading a flame retardant onto the porous alumina shell using an impregnation method to prepare bismuth oxide composite particles. These particles are then used as fillers in the preparation of highly flame-retardant polymer materials. This invention, by creating a porous core-shell structure, confines the highly active bismuth oxide, preventing agglomeration of highly active bismuth oxide during polymer material preparation and thus avoiding impact on the stability of the finished product. Simultaneously, under high-temperature conditions, the core layer of bismuth oxide reacts rapidly with the flame retardant loaded on the porous alumina shell to generate a dense bismuth phosphate layer, isolating oxygen and heat to improve the flame-retardant properties of the material. Furthermore, the bismuth oxide@porous alumina structure significantly reduces the density of the filler, which is beneficial for promoting the development of high-performance lightweight materials.

Description

Technical Field

[0001] This invention relates to the field of polymer material preparation technology, specifically to a method for manufacturing alumina-coated bismuth oxide powder and its application in polymer material preparation. Background Technology

[0002] Bismuth oxide is an important inorganic functional material. Its high-temperature decomposition generates free radicals that can interrupt combustion chain reactions. Therefore, in the preparation of polymer materials, adding <10% bismuth oxide powder often improves the flame retardancy of the materials. However, due to its photocatalytic activity, bismuth oxide can generate reactive oxygen species under visible or ultraviolet light irradiation. These reactive oxygen species can oxidize sensitive groups in polymer materials, generating chromophores. The accumulation of oxidation products can cause the material to turn yellow or even brown. Furthermore, when highly reactive bismuth oxide is heated and mixed within polymer materials, it catalyzes the breakage of molecular chains, generating low-molecular-weight degradation products that reduce product performance. This negative impact is significantly amplified with increasing bismuth oxide content and decreasing bismuth oxide powder particle size.

[0003] In the existing technology, Ube Industries of Japan has developed a PA66 composite material with alumina-coated bismuth oxide for use in weather-resistant automotive air intake pipes. However, by encapsulating the bismuth oxide with alumina, the contact between the bismuth oxide and the polymer material is completely isolated. Although this avoids the problem of polymer chain catalytic decomposition caused by the high activity of bismuth oxide during manufacturing and later use, it also greatly limits the flame retardant properties of bismuth oxide. Therefore, how to retain the flame retardant properties of bismuth oxide while reducing its negative effects on materials is an issue that cannot be ignored when applying bismuth oxide as a high-performance filler in polymer materials. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] To address the shortcomings of existing technologies, this invention provides a method for manufacturing alumina-coated bismuth oxide powder and its application, thereby solving the problems mentioned in the background section.

[0006] (II) Technical Solution

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for manufacturing alumina-coated bismuth oxide powder, comprising the following steps:

[0008] ① Prepare an aluminum salt solution by adding bismuth oxide powder, dispersant, and pore-forming agent to the aluminum salt solution and stirring until homogeneous to obtain a reaction solution;

[0009] The aluminum ion concentration is 0.1–0.5 mol / L, the aluminum ion:bismuth ion molar ratio is 3–5:1, the bismuth oxide to dispersant mass ratio is 1:0.01–0.05, and the aluminum salt to pore-forming agent mass ratio is 1:0.05–0.1.

[0010] ② Add the precipitant and complexing agent to the reaction solution in sequence, mix well, transfer to the reaction vessel and seal, and perform hydrothermal reaction at 150-170℃ for 3-6 hours to obtain a solid-liquid mixture;

[0011] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0012] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 400-600℃ at 1-2℃ / min, hold at the temperature for 2-4 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0013] ⑤ Dissolve the flame retardant in the solution, place the bismuth oxide@porous alumina prepared in step ④ in the solution, ultrasonically disperse for 20-30 min, filter to remove the solvent, dry at 60-80℃, and sinter at 200℃ for 2-4 hours to obtain bismuth oxide composite particles.

[0014] As a further preferred option, the aluminum salt is one or more of aluminum nitrate, aluminum chloride, and aluminum sulfate.

[0015] As a further preferred option, the dispersant is one or more of sodium dodecyl sulfate, polyacrylic acid, polyethylene glycol, and citric acid, more preferably citric acid.

[0016] As a further preferred option, the pore-forming agent is polyethylene glycol or polyvinylpyrrolidone.

[0017] As a further preferred option, the precipitant is one or more of ammonia, urea, and ammonium carbonate, with urea being more preferred.

[0018] As a further preferred option, the complexing agent is one or more of citric acid, chitosan, EDTA, and PAA.

[0019] As a further preferred option, the flame retardant is one of halogen-based flame retardants, phosphorus-based flame retardants, or nitrogen-based flame retardants, and more preferably a phosphorus-based flame retardant.

[0020] An application of alumina-coated bismuth oxide powder: the alumina-coated bismuth oxide powder prepared by the above method is used as a high-functional filler in the preparation of highly flame-retardant polymer materials.

[0021] (III) Beneficial Effects

[0022] This invention provides a method for manufacturing alumina-coated bismuth oxide powder, which has the following advantages:

[0023] This invention utilizes a hydrothermal reaction to encapsulate bismuth oxide on a porous core-shell structure with alumina, which has high thermal conductivity and better thermal stability. By removing the template, a highly active bismuth oxide is confined, preventing the agglomeration of highly active bismuth oxide powder during polymer preparation, which would affect the stability of the finished product. Simultaneously, a flame retardant is loaded onto the porous shell. During the use of the material, the porous silica reacts and adsorbs bismuth ions free from the core, preventing the bismuth ions from reacting with the polymer to form chromophores and causing the material to yellow. When the material is in a high-temperature environment or when combustion occurs, the bismuth oxide in the core reacts rapidly with the flame retardant loaded on the porous alumina shell to form a dense bismuth phosphate layer, which isolates oxygen and heat, thereby improving the flame retardancy of the material.

[0024] Meanwhile, the bismuth oxide@porous alumina structure greatly reduces the density of the filler, which is conducive to promoting the development of high-performance lightweight materials. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the preparation process of alumina-coated bismuth oxide powder according to the present invention. Detailed Implementation

[0026] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0027] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, examples of various specific processes and materials are provided, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0028] This invention provides a method for manufacturing alumina-coated bismuth oxide powder, comprising the following steps:

[0029] ① Prepare an aluminum salt solution by adding bismuth oxide powder, dispersant, and pore-forming agent to the aluminum salt solution and stirring until homogeneous to obtain a reaction solution;

[0030] The aluminum ion concentration is 0.1–0.5 mol / L, the aluminum ion:bismuth ion molar ratio is 3–5:1, the bismuth oxide to dispersant mass ratio is 1:0.01–0.05, and the aluminum salt to pore-forming agent mass ratio is 1:0.05–0.1.

[0031] In this invention, the bismuth oxide is industrial-grade bismuth oxide produced by Sinopharm Group, model 10023418, with a purity of 99.5% and a particle size distribution range of 3-10 μm.

[0032] The aluminum salt is one or more of aluminum nitrate, aluminum chloride, and aluminum sulfate; the dispersant is one or more of sodium dodecyl sulfate, polyacrylic acid, polyethylene glycol, and citric acid, more preferably citric acid; the pore-forming agent is polyethylene glycol or polyvinylpyrrolidone.

[0033] It should be noted that all raw materials not explicitly described in this invention are common commercially available products.

[0034] ② Add the precipitant and complexing agent to the reaction solution in sequence, mix well, transfer to the reaction vessel and seal, and perform hydrothermal reaction at 150-170℃ for 3-6 hours to obtain a solid-liquid mixture;

[0035] The molar ratio of precipitant to aluminum ions is 3 to 5:1. The precipitant is one or more of ammonia, urea, and ammonium carbonate, with urea being more preferred. The reaction process is gentle and the particle size is more uniform.

[0036] The molar ratio of the complexing agent to aluminum ions is 0.5 to 2:1, and the complexing agent is one or more of citric acid, chitosan, EDTA, and PAA.

[0037] Chitosan needs to be dissolved in a small amount of dilute acetic acid before it can be added as a complexing agent.

[0038] It should be noted that the initial pH value should be controlled between 3 and 5 to avoid premature precipitation of Al ions. The hydrothermal reactor must be strictly sealed to prevent the decomposition products from escaping and affecting the morphology of the products.

[0039] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0040] During washing, the sample is first washed three times with an ethanol-water solution (1:1) to remove unreacted precipitants and complexing agents, and then briefly soaked in 0.1M HNO3 (10 min) to remove loose Al2O3 particles from the surface.

[0041] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 400-600℃ at 1-2℃ / min, hold at the temperature for 2-4 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0042] ⑤ Dissolve the flame retardant in the solution, place the bismuth oxide@porous alumina prepared in step ④ in the solution, ultrasonically disperse for 20-30 min, filter to remove the solvent, dry at 60-80℃, and sinter at 200℃ for 2-4 hours to obtain bismuth oxide composite particles.

[0043] The flame retardant is one of halogen-based flame retardants, phosphorus-based flame retardants, or nitrogen-based flame retardants, and is more preferably a phosphorus-based flame retardant.

[0044] Phosphorus-based flame retardants include one or more of ammonium polyphosphate, ammonium hypophosphite, phosphate esters, DOPO derivatives, polycyanamide polyphosphate (MPP), and aluminum hypophosphite.

[0045] Understandably, depending on the flame retardant chosen, a suitable solution is selected as the solvent. For example, ammonium polyphosphate and ammonium hypophosphite can be dissolved in water or ethanol, phosphate esters can be dissolved in ethanol, and DOPO derivatives and MPP can be dissolved in organic solvents such as acetone, DMF, toluene, etc.

[0046] Furthermore, considering safety, environmental protection, and operating costs, a better option is an aqueous solution of ammonium polyphosphate or ammonium hypophosphite.

[0047] An application of alumina-coated bismuth oxide powder: bismuth oxide composite particles prepared using the above method are used as high-functionality fillers in the preparation of highly flame-retardant polymer materials.

[0048] Weigh 5% to 15% of the total mass of bismuth oxide composite particles into the polymer material system, mix them with the raw materials, and then melt-blend, extrude, cool, dry, and pelletize to obtain a highly flame-retardant polymer material.

[0049] It should be noted that in step ⑤, the mass of the flame retardant accounts for 5% to 20% of the mass of the raw materials used to prepare the polymer matrix.

[0050] Among them, ammonium polyphosphate, ammonium hypophosphite, aluminum hypophosphite, and phosphate esters account for 15% to 20%; DOPO derivatives and polycyanamide polyphosphate (MPP) account for 5% to 20%.

[0051] To further understand the present invention, the bismuth oxide composite particles provided by the present invention will be described below with reference to the embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0052] Example 1

[0053] ① Weigh 56.25g of aluminum nitrate to prepare an aluminum salt solution. Add 23.3g of bismuth oxide powder, 0.23g of citric acid and 2.8g of polyethylene glycol to the aluminum salt solution and stir until homogeneous to obtain a reaction solution.

[0054] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 9g of urea and 13.4g of PAA to the reaction solution in sequence, mix well, transfer to a reaction vessel and seal, and hydrothermally react at 150℃ for 6 hours to obtain a solid-liquid mixture.

[0055] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0056] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 400℃ at 1℃ / min, hold at the temperature for 4 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0057] ⑤ Dissolve 14.9g of ammonium polyphosphate in water to make a solution. Place the bismuth oxide@porous alumina prepared in step ④ into the solution, ultrasonically disperse for 25min, filter to remove the solvent, dry at 80℃, and sinter at 200℃ for 2 hours to obtain bismuth oxide composite particles.

[0058] ⑥ The bismuth oxide composite particles prepared in step ⑤ are mixed with the raw materials at 5% of the total mass of the polymer material system, and then melt-blended, extruded, cooled, dried, and pelletized to obtain a highly flame-retardant polymer material.

[0059] Example 2

[0060] ① Weigh 20g of aluminum chloride to prepare an aluminum salt solution. Add 23.3g of bismuth oxide powder, 0.32g of polyacrylic acid and 1g of polyethylene glycol to the aluminum salt solution and stir until homogeneous to obtain a reaction solution.

[0061] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 26.25g of ammonia and 14.4g of citric acid to the reaction solution in sequence, mix well, transfer to a reaction vessel and seal, and hydrothermally react at 170℃ for 3h to obtain a solid-liquid mixture;

[0062] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0063] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 600℃ at 1.5℃ / min, hold at the temperature for 2 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0064] ⑤ Dissolve 28.3g MPP in acetone to make a solution, place the bismuth oxide@porous alumina prepared in step ④ into the solution, ultrasonically disperse for 30min, filter to remove the solvent, dry at 60℃, and sinter at 200℃ for 4 hours to obtain bismuth oxide composite particles.

[0065] ⑥ Mix the bismuth oxide composite particles prepared in step ⑤ with the raw materials at 10% of the total mass of the polymer material system, then melt-blend and extrude, cool, dry, and pelletize to obtain a highly flame-retardant polymer material.

[0066] Example 3

[0067] ① Weigh 54.3g of aluminum sulfate to prepare an aluminum salt solution. Add 23.3g of bismuth oxide powder, 1.165g of polyethylene glycol and 2.7g of polyvinylpyrrolidone to the aluminum salt solution and stir until homogeneous to obtain a reaction solution.

[0068] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 45g of urea and 30.7g of chitosan-dilute acetic acid solution to the reaction solution in sequence, mix well, transfer to the reaction vessel and seal, and hydrothermally react at 160℃ for 5h to obtain a solid-liquid mixture.

[0069] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0070] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 500℃ at 2℃ / min, hold at the temperature for 3 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0071] ⑤ Dissolve 18.3g of ammonium hypophosphate in ethanol to make a solution. Place the bismuth oxide@porous alumina prepared in step ④ into the solution, ultrasonically disperse for 20min, filter to remove the solvent, dry at 70℃, and sinter at 200℃ for 3 hours to obtain bismuth oxide composite particles.

[0072] ⑥ Mix the bismuth oxide composite particles prepared in step ⑤ with the raw materials at 15% of the total mass of the polymer material system, then melt-blend and extrude, cool, dry, and pelletize to obtain a highly flame-retardant polymer material.

[0073] Comparative Example 1

[0074] ① Weigh 20g of aluminum chloride to prepare an aluminum salt solution, add 23.3g of bismuth oxide powder and 0.32g of polyacrylic acid to the aluminum salt solution, and stir until homogeneous to obtain a reaction solution;

[0075] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 26.25g of ammonia and 14.4g of citric acid to the reaction solution in sequence, mix well, transfer to a reaction vessel and seal, and hydrothermally react at 170℃ for 3h to obtain a solid-liquid mixture;

[0076] ③ The solid-liquid mixture from step ② is filtered, washed, dried, and then placed in a muffle furnace. The temperature is increased to 600℃ at 1.5℃ / min and held at that temperature for 2 hours. After that, it is cooled to room temperature with the furnace to obtain bismuth oxide@aluminum oxide.

[0077] ④ Dissolve 28.3g MPP in acetone to prepare a solution, place the bismuth oxide@alumina prepared in step ③ into the solution, ultrasonically disperse for 30min, filter to remove the solvent, dry at 60℃, and sinter at 200℃ for 4 hours to obtain bismuth oxide@alumina supported particles.

[0078] ⑤ The bismuth oxide@alumina supported particles prepared in step ④ are mixed with the raw materials at 10% of the total mass of the polymer material system. After melt blending and extrusion, cooling and drying, the particles are granulated to obtain the comparative polymer material.

[0079] Comparative Example 2

[0080] ① Weigh 20g of aluminum chloride to prepare an aluminum salt solution, add 23.3g of bismuth oxide powder and 0.32g of polyacrylic acid to the aluminum salt solution, and stir until homogeneous to obtain a reaction solution;

[0081] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 26.25g of ammonia and 14.4g of citric acid to the reaction solution in sequence, mix well, transfer to a reaction vessel and seal, and hydrothermally react at 170℃ for 3h to obtain a solid-liquid mixture;

[0082] ③ The solid-liquid mixture from step ② is filtered, washed, dried, and then placed in a muffle furnace. The temperature is increased to 600℃ at 1.5℃ / min and held at that temperature for 2 hours. After that, it is cooled to room temperature with the furnace to obtain bismuth oxide@aluminum oxide.

[0083] ④ Mix the bismuth oxide@alumina prepared in step ③ with 28.3g MPP, and mix with the raw materials at a mass ratio of 10% of the total mass of the polymer material system. After melt blending and extrusion, cooling, drying, and pelletizing, the comparative polymer material is obtained.

[0084] Comparative Example 3

[0085] ① Weigh 20g of aluminum chloride to prepare an aluminum salt solution. Add 23.3g of bismuth oxide powder, 0.32g of polyacrylic acid and 1g of polyethylene glycol to the aluminum salt solution and stir until homogeneous to obtain a reaction solution.

[0086] ② Adjust the pH of the reaction solution to 3-5 with HNO3, add 26.25g of ammonia and 14.4g of citric acid to the reaction solution in sequence, mix well, transfer to a reaction vessel and seal, and hydrothermally react at 170℃ for 3h to obtain a solid-liquid mixture;

[0087] ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide;

[0088] ④ Place bismuth oxide@alumina in a muffle furnace, heat to 600℃ at 1.5℃ / min, hold at the temperature for 2 hours, and then cool to room temperature with the furnace to obtain bismuth oxide@porous alumina;

[0089] ⑤ Mix the bismuth oxide@porous alumina prepared in step ④ with 28.3g MPP, and mix it with the raw materials at a mass ratio of 10% of the total mass of the polymer material system. After melt blending and extrusion, cooling, drying, and pelletizing, the comparative polymer material is obtained.

[0090] Comparative Example 4

[0091] Weigh out 23.3g of bismuth oxide, 28.3g of MPP and 15.15g of alumina, mix them together, and then mix them with the raw materials at a mass ratio of 10% of the total mass of the polymer material system. After melt blending and extrusion, cooling and drying, pelletizing, the comparative polymer material is obtained.

[0092] Test example:

[0093] The polymer materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to the following tests:

[0094] For flame retardant performance testing, the material was injection molded into a 120*12*2mm sample strip and tested according to the US UL94 fire resistance standard.

[0095] For the burning test, the material was injection molded into a 100*100*3mm sample. Liquefied petroleum gas was used for the burning test, and the gas flow rate was controlled by setting a pressure reducing valve to 0.5MPa. The flame temperature was set to 1000℃ using an infrared thermometer. The flame gun was kept 50mm away from the sample for continuous burning, and the burning time of the sample was recorded. After cooling to room temperature, the carbon layer hardness was tested using a Shore hardness tester (Type A). The average value of five sets of carbon layer hardness tests was taken.

[0096] For the ultraviolet aging test, the material was injection molded into a 100×50mm sample, ensuring the surface was clean and free of contaminants. The selected lamp type was UVB~313, and the irradiance was 1~1.55W / m. 2 The material's color difference (ΔE), gloss (60° angle), and surface cracks were observed in a cyclic mode (4 hours UV (60°) + 4 hours condensation (50°)).

[0097] The cracks were assessed using a combination of visual inspection (compared to standard grade charts) and optical microscopy (measuring the width and density of microcracks), in accordance with ISO 4628-4:2016.

[0098] Table 1 shows the test statistics for each embodiment and comparative example.

[0099]

[0100] In summary, by coating porous alumina around bismuth oxide powder and loading flame retardants onto the porous alumina, the problem of catalytic decomposition and oxidation of polymer chains during the preparation process can be effectively prevented due to direct contact between the active sites in bismuth oxide and polymer materials. Furthermore, during long-term use, bismuth oxide absorbs and scatters ultraviolet light, effectively delaying the aging of the material. At the same time, the porous alumina adsorbs free bismuth ions, preventing the material from yellowing due to the oxidation reaction between bismuth ions and active groups of polymers. Moreover, under high-temperature conditions, the flame retardant loaded on the shell reacts rapidly with the bismuth oxide in the core to form a dense bismuth phosphate layer, which effectively isolates oxygen and heat, greatly improving the flame retardancy of the material.

[0101] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing alumina-coated bismuth oxide powder, characterized in that, Includes the following steps: ① Prepare an aluminum salt solution by adding bismuth oxide powder, dispersant, and pore-forming agent to the aluminum salt solution and stirring until homogeneous to obtain a reaction solution; The aluminum ion concentration is 0.1–0.5 mol / L, the molar ratio of aluminum ions to bismuth ions is 3–5:1, the mass ratio of bismuth oxide to dispersant is 1:0.01–0.05, and the mass ratio of aluminum salt to pore-forming agent is 1:0.05–0.

1. The dispersant is one or more of sodium dodecyl sulfate, polyacrylic acid, polyethylene glycol, and citric acid; The pore-forming agent is polyethylene glycol or polyvinylpyrrolidone; ② Add the precipitant and complexing agent to the reaction solution in sequence, mix well, transfer to the reaction vessel and seal, and perform hydrothermal reaction at 150-170℃ for 3-6 hours to obtain a solid-liquid mixture; ③ The solid-liquid mixture from step ② is filtered, washed, and dried to obtain bismuth oxide@aluminum oxide; ④ Place bismuth oxide@alumina in a muffle furnace, heat to 400-600℃ at 1-2℃ / min, hold at the temperature for 2-4 hours, and then cool to room temperature with the furnace to form a core-shell structure with a porous outer shell, thus obtaining bismuth oxide@porous alumina; ⑤ Dissolve the flame retardant in the solution, place the bismuth oxide@porous alumina prepared in step ④ in the solution, ultrasonically disperse for 20-30 min, filter to remove the solvent, dry at 60-80℃, and sinter at 200℃ for 2-4 hours to obtain bismuth oxide composite particles.

2. The method for manufacturing alumina-coated bismuth oxide powder according to claim 1, characterized in that: The aluminum salt is one or more of aluminum nitrate, aluminum chloride, and aluminum sulfate.

3. The method for manufacturing alumina-coated bismuth oxide powder according to claim 1, characterized in that: The dispersant is more preferably citric acid.

4. The method for manufacturing alumina-coated bismuth oxide powder according to claim 1, characterized in that: The precipitant is one or more of ammonia, urea, and ammonium carbonate.

5. The method for manufacturing alumina-coated bismuth oxide powder according to claim 4, characterized in that: The precipitant is more preferably urea.

6. The method for manufacturing alumina-coated bismuth oxide powder according to claim 1, characterized in that: The complexing agent is one or more of citric acid, chitosan, EDTA, and PAA.

7. The method for manufacturing alumina-coated bismuth oxide powder according to claim 1, characterized in that: The flame retardant is one of the following: halogenated flame retardant, phosphorus-based flame retardant, or nitrogen-based flame retardant.

8. The method for manufacturing alumina-coated bismuth oxide powder according to claim 7, characterized in that: The flame retardant is more preferably a phosphorus-based flame retardant.

9. An application of alumina-coated bismuth oxide powder, characterized in that, Bismuth oxide composite particles prepared by any one of the methods described in claims 1 to 8 are used as high-functional fillers in the preparation of highly flame-retardant polymer materials.

10. The application of the alumina-coated bismuth oxide powder according to claim 9, characterized in that, The bismuth oxide composite particles constitute 5% to 15% of the total mass of the polymer material system.

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