A polycrystalline alumina fiber and a preparation method thereof

Abstract

The present invention relates to the field of fiber materials, and discloses a polycrystalline alumina fiber and a preparation method thereof. The preparation method comprises the following steps: taking crystalline aluminum chloride, adding deionized water and mixing evenly, then adding a mixed solution of aluminum powder, tartaric acid, nitric acid and lactic acid, and heating under reflux to obtain a mother liquor of polyaluminum chloride; taking basic zirconium carbonate, glacial acetic acid and ethanol and mixing evenly to obtain a zirconium-containing solution, mixing the mother liquor of polyaluminum chloride, the zirconium-containing solution and tetraethyl orthosilicate evenly, then adding a fiber-forming aid, and concentrating by vacuum distillation to obtain a spinning colloid; centrifugally spinning the spinning colloid to form a fiber blank; drying the fiber blank and then performing heat treatment, and depositing a boron nitride nanolayer on its surface by chemical vapor deposition to prepare a polycrystalline alumina fiber with good high-temperature heat insulation, acid and alkali corrosion resistance and high tensile strength.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber materials, and particularly relates to a polycrystalline alumina fiber and a preparation method thereof. Background Art

[0002] Alumina fiber is a polycrystalline inorganic fiber mainly composed of alumina, and its main component is α-Al2O3. Alumina fiber is one of the new ultra-light high-temperature thermal insulation materials at home and abroad today. The "sol-gel" method can be used to make a colloidal solution with a certain viscosity from soluble aluminum and silicon salts. The solution is spun into a fiber embryo by high-speed centrifugal spinning, and then through processes such as dehydration, drying, and crystallization by medium and high-temperature heat treatment, it is transformed into Al-Si alumina polycrystalline fiber, which can be used as a high-temperature thermal insulation material, a catalyst or catalyst carrier for high-temperature reactions, a reinforcement for metal matrix, ceramic matrix and other composite materials, etc., and has wide applications in the fields of aerospace, mechanical chemical engineering, etc.

[0003] Alumina fiber is relatively brittle. The fiber grain boundaries are prone to become oxygen diffusion channels at high temperatures, resulting in grain coarsening and inelastic deformation, thus affecting its performance stability. At the same time, when the heat preservation temperature increases or in a water vapor environment, the degree of grain coarsening of alumina fiber intensifies, resulting in a significant decline in the mechanical properties of the fiber. The high-porosity fiber aggregate composed of fiber stacking has strong hygroscopicity, affecting its storage and thermal insulation stability. In addition, in a corrosive atmosphere, especially above 1000 °C, as the temperature increases, the fiber strength of alumina fiber gradually decreases, severely restricting its applications in the fields of thermal protection and many cutting-edge fields such as aerospace. Summary of the Invention

[0004] To solve the deficiencies mentioned in the above background art, the purpose of the present invention is to provide a polycrystalline alumina fiber and a preparation method thereof. Using crystalline aluminum chloride and aluminum powder as aluminum sources, tartaric acid, nitric acid and lactic acid as stabilizers and spinning aids, tetraethyl orthosilicate as a silicon source, and a zirconium-containing solution as a strengthening aid, a fiber blank is formed by the sol-gel method and centrifugal spinning technology. After heat treatment and sintering of the fiber blank, a boron nitride nanolayer is deposited on its surface by chemical vapor deposition to prepare a polycrystalline alumina fiber.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A preparation method of a polycrystalline alumina fiber, comprising the following steps:

[0007] S1. Take crystalline aluminum chloride, add deionized water and mix evenly, then add a mixed solution of aluminum powder, tartaric acid, nitric acid and lactic acid, and place it under heating and reflux at 80-100 °C for 2-4 h to obtain a polyaluminum chloride mother liquor;

[0008] S2. Mix zirconium basic carbonate, glacial acetic acid and ethanol uniformly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and tetraethyl orthosilicate uniformly, then add a fiber-forming aid, and concentrate by vacuum distillation to obtain a spinning colloid;

[0009] S3. Use a centrifugal spinning machine to centrifugally spin the spinning colloid. The spinning colloid forms a fiber blank under the action of centrifugal force and the secondary traction of a hot air stream, and is collected by a fiber-forming hood;

[0010] S4. After drying the fiber blank, perform heat treatment, and deposit a boron nitride nano-layer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition to prepare the polycrystalline alumina fiber.

[0011] Preferably, the molar ratio of the aluminum powder, tartaric acid, lactic acid, nitric acid and tetraethyl orthosilicate is 1: 0.3-0.5: 0.6-0.7: 0.3-0.4: 0.15-0.3.

[0012] Preferably, the mass ratio of zirconium basic carbonate, glacial acetic acid and ethanol is 2.5-3: 2-2.5: 7-10.

[0013] Preferably, the fiber-forming aid is polyvinylpyrrolidone.

[0014] Preferably, the viscosity of the spinning colloid is 40-80 Pa·s.

[0015] Preferably, the process parameters of the heat treatment are specifically: heating at a rate of 1-3 °C / min to 500-550 °C, holding for 0.5-1 h, then heating at a rate of 3-5 °C / min to 600-800 °C, holding for 0.5-1 h, and then heating at a rate of 5-10 °C / min to 1000-1400 °C, holding for 1-1.5 h.

[0016] Preferably, the process parameters of the chemical vapor deposition method are specifically: using boron trichloride and ammonia as reaction gas sources, using argon as a carrier gas and a dilution gas, the flow rate ratio of ammonia, boron trichloride and argon is 3: 1: 5, the reaction system pressure is 5 KPa, the deposition temperature is 950-1050 °C, and the deposition time is 45-60 min.

[0017] Preferably, the rotation speed of the spinning disk during the centrifugal spinning process in step S3 is 5000-6000 r / min.

[0018] Preferably, the temperature in the fiber-forming hood is controlled at 50-70 °C, and the relative humidity is 30-50%.

[0019] A polycrystalline alumina fiber is made by the preparation method as described above.

[0020] The beneficial effects of the present invention:

[0021] The present invention uses crystalline aluminum chloride and aluminum powder as aluminum sources, tartaric acid, nitric acid and lactic acid as stabilizers and spinning aids, tetraethyl orthosilicate as a silicon source, and a zirconium-containing solution as a reinforcing aid. The sol-gel method and centrifugal spinning technology are used to form a fiber blank. After heat treatment and sintering of the fiber blank, a boron nitride nano-layer is deposited on its surface by chemical vapor deposition to prepare polycrystalline alumina fibers. Introducing silicon dioxide in the form of tetraethyl orthosilicate can inhibit grain growth. The zirconium-containing solution is prepared from basic zirconium carbonate, glacial acetic acid and ethanol, and is doped into the polyaluminum chloride mother liquor to enhance the high-temperature heat insulation performance of the alumina fibers. Moreover, the lattice distortion caused by Si-O-Zr increases the configurational entropy of the overall fiber structure, which can increase the activation energy of grain growth and inhibit its crystallization. Boron nitride has a multi-layer flaky structure, has good interfacial bonding strength, and has good antioxidant properties. Its oxidation onset temperature is relatively high, and a protective condensed oxide boron trioxide is formed at high temperatures. It covers and fills the voids in the alumina fibers, inhibits the migration and diffusion of oxygen, thereby maintaining the passivation of the alumina fibers, and can eliminate internal pore structures such as pores, avoiding becoming stress concentration points. When the fiber is subjected to an external load, crack propagation can be prevented. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 It is the SEM image of the polycrystalline alumina fiber prepared in Example 1 of the present invention after being treated with water vapor, alkali vapor and acid vapor for 24 hours.

[0024] Among them, a is the SEM image after being treated with 150 °C water vapor for 24 hours, b is the SEM image after being treated with sodium nitrite alkali vapor for 24 hours, and c is the SEM image after being treated with hydrochloric acid vapor for 24 hours. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0026] Example 1 A method for preparing polycrystalline alumina fibers, comprising the following steps:

[0027] S1. Take 50 g of crystalline aluminum chloride, add it to 350 mL of deionized water and mix evenly. Then add a mixed solution of 5 g of aluminum powder, 8.3 g of tartaric acid, 3.5 g of nitric acid and 10 g of lactic acid, and place it under reflux at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0028] S2. Take 3 g of zirconium basic carbonate, 2.2 g of glacial acetic acid and 10 g of ethanol and mix evenly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and 5.8 g of tetraethyl orthosilicate evenly, and then add 3.5 g of the fiber-forming aid polyvinylpyrrolidone. Concentrate by vacuum distillation until the viscosity reaches 60 Pa·s to obtain a spinning colloid;

[0029] S3. Use a centrifugal spinning machine to centrifugally spin the spinning colloid. The spinning colloid forms a fiber blank under the secondary traction of centrifugal force and hot air flow, and is collected with a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally. The rotation speed of the spinning disk is 6000 r / min, the temperature in the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0030] S4. After drying the fiber blank, perform heat treatment. Heat it to 500 °C at a rate of 1 °C / min, hold for 0.5 h, then heat it to 650 °C at a rate of 4 °C / min, hold for 0.5 h, and then heat it to 1300 °C at a rate of 6 °C / min, hold for 1 h. And deposit a boron nitride nanolayer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition. Use boron trichloride and ammonia as reaction gas sources, use argon as the carrier gas and dilution gas, the ammonia flow rate is 60 mL / min, the boron trichloride flow rate is 20 mL / min, and the argon flow rate is 100 mL / min. The reaction system pressure is 5 KPa, the deposition temperature is 950 °C, and the deposition time is 60 min to prepare polycrystalline alumina fiber.

[0031] Please refer to Figure 1 , a is the SEM image after being treated with 150 °C water vapor for 24 h, b is the SEM image after being treated with sodium nitrite alkali vapor for 24 h, c is the SEM image after being treated with hydrochloric acid vapor for 24 h. After the polycrystalline alumina fiber is exposed to water vapor, alkali vapor and acid vapor, the sample fiber morphology remains intact, the surface morphology is smooth, and there are no obvious crack or hole defects, indicating that the sample fiber has good acid and alkali resistance.

[0032] Example 2 A method for preparing polycrystalline alumina fiber, comprising the following steps:

[0033] S1. Take 50 g of crystalline aluminum chloride, add it to 350 mL of deionized water and mix evenly. Then add a mixed solution of 5 g of aluminum powder, 11.1 g of tartaric acid, 3.5 g of nitric acid and 11.7 g of lactic acid, and place it under reflux at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0034] S2. Take 2.5 g of zirconium basic carbonate, 2.4 g of glacial acetic acid and 9.2 g of ethanol and mix evenly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and 7.7 g of tetraethyl orthosilicate evenly, then add 3.5 g of the fiber-forming aid polyvinylpyrrolidone, and concentrate by vacuum distillation until the viscosity reaches 50 Pa·s to obtain a spinning colloid;

[0035] S3. Use a centrifugal spinning machine to spin the spinning colloid. The spinning colloid forms a fiber blank under the action of centrifugal force and the secondary traction of the hot air flow, and is collected by a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally, the rotation speed of the spinning disk is 6000 r / min, the temperature in the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0036] S4. After drying the fiber blank, perform heat treatment. Heat it at a rate of 1 °C / min to 550 °C, hold for 0.5 h, then heat it at a rate of 5 °C / min to 720 °C, hold for 1 h, and then heat it at a rate of 8 °C / min to 1300 °C, hold for 1.5 h, and deposit a boron nitride nanolayer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition. Use boron trichloride and ammonia as reaction gas sources, argon as the carrier gas and diluent gas, the ammonia flow rate is 60 mL / min, the boron trichloride flow rate is 20 mL / min, and the argon flow rate is 100 mL / min, the reaction system pressure is 5 KPa, the deposition temperature is 950 °C, and the deposition time is 60 min to prepare polycrystalline alumina fibers.

[0037] Example 3 A method for preparing polycrystalline alumina fibers, comprising the following steps:

[0038] S1. Take 50 g of crystalline aluminum chloride, add it to 350 mL of deionized water and mix evenly. Then add a mixed solution of 5 g of aluminum powder, 13.5 g of tartaric acid, 3.8 g of nitric acid and 10.8 g of lactic acid, and place it under reflux at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0039] S2. Take 2.7 g of zirconium basic carbonate, 2.5 g of glacial acetic acid and 10 g of ethanol and mix evenly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and 9.6 g of tetraethyl orthosilicate evenly, then add 5 g of the fiber-forming aid polyvinylpyrrolidone, and concentrate by vacuum distillation until the viscosity reaches 80 Pa·s to obtain a spinning colloid;

[0040] S3. Centrifugally spin the spinning colloid using a centrifugal spinning machine. The spinning colloid forms a fiber blank under the secondary traction of centrifugal force and hot air flow, and is collected by a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally, and the rotation speed of the spinning disk is 6000 r / min. The temperature inside the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0041] S4. After drying the fiber blank, perform heat treatment. Heat it up to 550 °C at a rate of 2 °C / min, hold for 1 h, then heat it up to 800 °C at a rate of 5 °C / min, hold for 1 h, and then heat it up to 1300 °C at a rate of 10 °C / min, hold for 1 h. And deposit a boron nitride nano-layer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition. Using boron trichloride and ammonia as reaction gas sources, argon as the carrier gas and dilution gas, the ammonia gas flow rate is 60 mL / min, the boron trichloride flow rate is 20 mL / min, and the argon gas flow rate is 100 mL / min. The reaction system pressure is 5 KPa, the deposition temperature is 950 °C, and the deposition time is 60 min to prepare polycrystalline alumina fibers.

[0042] Comparative Example 1 A method for preparing polycrystalline alumina fibers, comprising the following steps:

[0043] S1. Take 50 g of crystalline aluminum chloride, add 350 mL of deionized water and mix evenly, then add 5 g of aluminum powder and stir to mix. Place it under reflux at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0044] S2. Take 3 g of zirconium basic carbonate, 2.2 g of glacial acetic acid and 10 g of ethanol and mix evenly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and 5.8 g of tetraethyl orthosilicate evenly, then add 3.5 g of the fiber-forming aid polyvinylpyrrolidone, and concentrate by vacuum distillation until the viscosity reaches 60 Pa·s to obtain a spinning colloid;

[0045] S3. Centrifugally spin the spinning colloid using a centrifugal spinning machine. The spinning colloid forms a fiber blank under the secondary traction of centrifugal force and hot air flow, and is collected by a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally, and the rotation speed of the spinning disk is 6000 r / min. The temperature inside the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0046] S4. After drying the fiber preform, perform heat treatment. Heat it up to 500 °C at a rate of 1 °C / min, hold for 0.5 h, then heat it up to 650 °C at a rate of 4 °C / min, hold for 0.5 h, then heat it up to 1300 °C at a rate of 6 °C / min, hold for 1 h, and deposit a boron nitride nanolayer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition. Using boron trichloride and ammonia as reaction gas sources, argon as the carrier gas and diluent gas, the ammonia flow rate is 60 mL / min, the boron trichloride flow rate is 20 mL / min, and the argon flow rate is 100 mL / min, the reaction system pressure is 5 KPa, the deposition temperature is 950 °C, and the deposition time is 60 min to prepare polycrystalline alumina fiber.

[0047] Comparative Example 2 A method for preparing polycrystalline alumina fiber, comprising the following steps:

[0048] S1. Take 50 g of crystalline aluminum chloride, add it to 350 mL of deionized water and mix evenly, then add a mixed solution of 5 g of aluminum powder, 8.3 g of tartaric acid, 3.5 g of nitric acid and 10 g of lactic acid, and place it under reflux heating at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0049] S2. Mix the polyaluminum chloride mother liquor and 5.8 g of tetraethyl orthosilicate evenly, then add 3.5 g of the fiber-forming aid polyvinylpyrrolidone, and concentrate by vacuum distillation until the viscosity reaches 60 Pa·s to obtain a spinning colloid;

[0050] S3. Use a centrifugal spinning machine to centrifugally spin the spinning colloid. The spinning colloid forms a fiber preform under the secondary traction of centrifugal force and hot air flow, and is collected by a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally, the rotation speed of the spinning disk is 6000 r / min, the temperature in the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0051] S4. After drying the fiber preform, perform heat treatment. Heat it up to 500 °C at a rate of 1 °C / min, hold for 0.5 h, then heat it up to 650 °C at a rate of 4 °C / min, hold for 0.5 h, then heat it up to 1300 °C at a rate of 6 °C / min, hold for 1 h, and deposit a boron nitride nanolayer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition. Using boron trichloride and ammonia as reaction gas sources, argon as the carrier gas and diluent gas, the ammonia flow rate is 60 mL / min, the boron trichloride flow rate is 20 mL / min, and the argon flow rate is 100 mL / min, the reaction system pressure is 5 KPa, the deposition temperature is 950 °C, and the deposition time is 60 min to prepare polycrystalline alumina fiber.

[0052] Comparative Example 3 A preparation method of polycrystalline alumina fiber, comprising the following steps:

[0053] S1. Take 50 g of crystalline aluminum chloride, add it to 350 mL of deionized water and mix evenly, then add a mixed solution of 5 g of aluminum powder, 8.3 g of tartaric acid, 3.5 g of nitric acid and 10 g of lactic acid, place it under reflux at 85 °C for 3 h to obtain a polyaluminum chloride mother liquor;

[0054] S2. Take 3 g of zirconium basic carbonate, 2.2 g of glacial acetic acid and 10 g of ethanol and mix evenly to obtain a zirconium-containing solution. Mix the polyaluminum chloride mother liquor, the zirconium-containing solution and 5.8 g of tetraethyl orthosilicate evenly, then add 3.5 g of the fiber-forming aid polyvinylpyrrolidone, and concentrate by vacuum distillation until the viscosity reaches 60 Pa·s to obtain a spinning colloid;

[0055] S3. Use a centrifugal spinning machine to centrifugally spin the spinning colloid. The spinning colloid forms a fiber blank under the secondary traction of centrifugal force and hot air flow, and is collected by a fiber-forming hood. The diameter of the spinning disk is 12 cm, the height is 3 cm, and there are three rows of round holes with a diameter of 0.5 mm drilled on the side wall. The interval between the holes is 2 mm both vertically and horizontally. The rotation speed of the spinning disk is 6000 r / min, the temperature in the fiber-forming hood is controlled at 60 °C, and the relative humidity is 40%;

[0056] S4. After drying the fiber blank, perform heat treatment. Heat it to 500 °C at a rate of 1 °C / min, hold for 0.5 h, then heat it to 650 °C at a rate of 4 °C / min, hold for 0.5 h, and then heat it to 1300 °C at a rate of 6 °C / min, hold for 1 h to prepare polycrystalline alumina fiber.

[0057] Performance detection

[0058] Perform performance detection on the polycrystalline alumina fibers prepared in Examples 1-3 and Comparative Examples 1-3: Refer to GB / T31290-2014 and ASTM C1557-14, and use the XS(08)XT-3 type single fiber tensile testing machine of Shanghai Xusai Company to perform single filament tensile testing on continuous alumina fibers. At least 20 samples are tested for each group of fibers. The tensile strength of the fiber is calculated by the following formula:

[0059]

[0060] In the formula: σ - average single filament tensile strength, unit MPa; F - maximum tensile force when the fiber breaks, unit N; S - cross-sectional area of the fiber, unit m 2 ; d - diameter of the fiber, unit m; The thermal conductivity of the sample at 25 °C is measured by the Hot Disk method; The specific surface area of the fiber is tested by an ASAP2000 type specific surface area tester, and the data results are shown in Table 1.

[0061] Table 1 Test Results of Specimen Performance

[0062]

[0063] As can be seen from the data in Table 1, for the polycrystalline alumina fibers prepared in Examples 1-3 of the present invention, at a high temperature of 1300 °C, their thermal conductivity is 0.03 W˙m -1 ˙K -1 or so, having good high-temperature heat insulation performance, the average tensile strength of a single filament is greater than 660 MPa, having good mechanical properties, and the micropore size of the fibers generally remains at a low level. Among them, in Comparative Example 1, a mixed solution of tartaric acid, nitric acid and lactic acid was not added as a spinning aid, in Comparative Example 2, a zirconium-containing solution was not added, and in Comparative Example 3, a boron nitride nanolayer was not deposited. The measured high-temperature heat insulation and mechanical properties of Comparative Examples 1-3 are worse than those of Examples 1-3.

[0064] In the description of this specification, the description with reference to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0065] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A method for preparing polycrystalline alumina fiber, characterized in that: The following steps are involved: S1. Take crystalline aluminum chloride and add deionized water to mix evenly, then add a mixed solution of aluminum powder, tartaric acid, nitric acid and lactic acid, heat and reflux at 80-100°C for 2-4h to obtain a polyaluminum chloride mother liquor; S2, mixing basic zirconium carbonate, glacial acetic acid and ethanol evenly to obtain a zirconium-containing solution, mixing the polyaluminium chloride mother solution, the zirconium-containing solution and tetraethyl orthosilicate evenly, then adding a fiber-forming aid, and concentrating by vacuum distillation to obtain a spinning colloid; S3, spinning the spinning colloid by centrifugal spinning machine, the spinning colloid forms fiber blanks under the secondary traction of centrifugal force and hot air flow, and the fiber blanks are collected by a fiber forming hood; S4, after drying the fiber blank, heat-treating it, and depositing a boron nitride nanolayer on the surface of the heat-treated and sintered alumina fiber by chemical vapor deposition to prepare the polycrystalline alumina fiber; The molar ratio of aluminum powder, tartaric acid, lactic acid, nitric acid and tetraethyl orthosilicate is 1: 0.3-0.5: 0.6-0.7: 0.3-0.4: 0.15-0.3; the mass ratio of basic zirconium carbonate, glacial acetic acid and ethanol is 2.5-3: 2-2.5: 7-10; The process parameters of the chemical vapor deposition method are as follows: boron trichloride and ammonia are used as reaction gas sources, argon is used as carrier gas and dilution gas, the flow rate ratio of ammonia, boron trichloride and argon is 2~3:1:4~5, the pressure of the reaction system is 4~6KPa, the deposition temperature is 950~1050℃, and the deposition time is 45~60min.

2. The method for preparing polycrystalline alumina fiber according to claim 1, characterized in that: The fiber-forming auxiliary agent is polyvinyl pyrrolidone.

3. The method for preparing polycrystalline alumina fiber according to claim 1, characterized in that: The viscosity of the spinning colloid is 40-80 Pa·s.

4. The method for preparing polycrystalline alumina fiber according to claim 1, characterized in that: The specific process parameters of the heat treatment are: heating to 500-550°C at a rate of 1-3°C / min, keeping warm for 0.5-1h, then heating to 600-800°C at a rate of 3-5°C / min, keeping warm for 0.5-1h, then heating to 1000-1400°C at a rate of 5-10°C / min, keeping warm for 1-1.5h.

5. The method for preparing polycrystalline alumina fiber according to claim 1, characterized in that: During the centrifugal spinning process in step S3, the spinning disk rotates at a speed of 5000-6000 r / min.

6. The method for preparing polycrystalline alumina fiber according to claim 1, characterized in that: The temperature in the fiber-forming hood is controlled at 50~70℃ and the relative humidity is 30~50%.

7. A polycrystalline alumina fiber, made by the preparation method according to any one of claims 1 to 6.

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