A method for producing fine-crystal dense alumina ceramic fiber

Abstract

The application belongs to the field of ceramic fibers, and particularly relates to a preparation method of fine-crystal dense alumina ceramic fibers. The application first configures an aluminum sol, an iron sol, a PVP solution and an acidified silica sol, then mixes them according to a set proportion to obtain a mixed sol, concentrates the mixed sol to obtain a spinnable gel, heats the spinnable gel to 500-600 DEG C at a heating rate of 2-10 DEG C / min for pre-sintering after spinning, finally performs microwave sintering at 1000-1400 DEG C at a power of 3000-5000 W and a heating rate of 50-100 DEG C / min to obtain the fine-crystal dense alumina ceramic fibers. The application has simple and controllable process, and the obtained product has excellent performance and is convenient for large-scale industrial application.

Description

Technical Field

[0001] This invention belongs to the field of ceramic fibers, and specifically relates to a method for preparing fine-grained dense alumina ceramic fibers. Background Technology

[0002] Alumina fibers possess advantages such as low density, high tensile strength, and good thermal insulation properties, making them widely used in aerospace fields such as aircraft engine components. Research on alumina fibers began earlier abroad, starting in the 1980s, and various alumina-based fibers were invented in the following one or two decades. The most famous, and still in production today, is the Nextel series of fibers invented by 3M in the United States. Previous research (Hay RS, Fair GE, Tidball T. Fiberstrength after grain growth in Nextel) TM 610 alumina fiber [J]. Journal of the American Ceramic Society, 2015, 98(6): 1907-1914.), alumina fibers with a fine-grained, dense microstructure and pure α-Al2O3 phase exhibit the best tensile strength, reaching up to 3.1 GPa. The fine-grained, dense microstructure is the main reason for the high tensile strength of alumina fibers, while the pure α-Al2O3 phase prevents phase transformation during service, increases internal porosity, and accelerates failure. Preparing alumina fibers with both a fine-grained, dense microstructure and pure α-Al2O3 phase is quite difficult. In the past two decades, researchers have continuously devoted themselves to related research, but so far only Nextel 610 alumina fiber has been achieved, which is prepared using a traditional sintering method.

[0003] Fibers have unique shapes, unlike bulk materials that can be sintered under pressure. Fibers can only be sintered without pressure, resulting in fewer sintering methods for achieving fiber density. Compared to traditional sintering methods, microwave sintering offers advantages such as rapid heating rate, uniform temperature, low sintering temperature, and short sintering time (Cheng J, Agrawal D, Zhang Y, et al. Microwave sintering of transparent alumina[J]. Materials Letters, 2002, 56(4):587-592.), and has been widely used in sintering ceramic materials. The rapid heating rate, low sintering temperature, and short sintering time reduce the energy required for fiber sintering and shorten grain growth time, thus facilitating the preparation of ceramic fiber materials with fine grains. A search of domestic and international papers and patents revealed no reports of using the sol-gel method combined with microwave sintering to prepare alumina fibers. Summary of the Invention

[0004] The application aims to provide a preparation method of fine-crystal dense alumina ceramic fiber, which has short sintering time, high production efficiency, energy saving and environmental protection, and the alumina fiber prepared by the method has fine crystal grains, dense microstructure and alpha-Al2O3 phase.

[0005] The application first realizes the fine-crystal dense alumina ceramic fiber with extremely high tensile strength at a sintering temperature of 1100 DEG C.

[0006] In order to achieve the above-mentioned purpose, the technical scheme of the application is as follows: a preparation method of fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0007] Step one: liquid preparation

[0008] According to the molar ratio, various raw materials are weighed in the proportion of aluminum powder:formic acid:acetic acid:deionized water:nitric acid = 1:(0.5-1):(0.5-1):(10-30):(0.1-3), and the stirring reaction is carried out at 200 r / min for 6-24 hours at 60-105 DEG C, and the aluminum sol is obtained after cooling and filtration;

[0009] According to the molar ratio, various raw materials are weighed in the proportion of aluminum powder:formic acid:acetic acid:deionized water:nitric acid = 1:(0.5-1):(0.5-1):(10-30):(0.1-3), and the stirring reaction is carried out at 200 r / min for 6-24 hours at 60-105 DEG C, and the aluminum sol is obtained after cooling and filtration;

[0010] According to the mass ratio, the PVP solution is prepared in the proportion of polyvinylpyrrolidone:deionized water = 1:9-11;

[0011] 2% dilute nitric acid is added dropwise into the silica sol to make the pH value of the silica sol in the range of 3-4, and the appropriate acidified silica sol is obtained;

[0012] Step two: mixing

[0013] According to the mass ratio, the aluminum sol:the iron sol:the PVP solution:the acidified silica sol = (60-99.7):(0.1-20):(0.1-10):(0.1-10), the aluminum sol, the iron sol, the PVP solution and the acidified silica sol are weighed, and then the stirring is carried out at 45-60 r / min for 1-24 hours; and the mixed sol is obtained;

[0014] Step three: concentration: the mixed sol is concentrated at 30-60 DEG C until the spinnable gel with the viscosity value of 10-1000 Pa·s is obtained;

[0015] Step four: spinning: the obtained spinnable gel is poured into the kettle of the dry spinning machine for spinning, and the drum is used for collection at 20-120 r / min, and the alumina precursor fiber is obtained.

[0016] Step five: pre-sintering: the obtained alumina precursor fiber is placed into a sintering furnace, heated to 500-600℃ at a heating rate of 2-10℃ / min, and then cooled to room temperature to obtain alumina pre-sintered fiber.

[0017] Step six: microwave sintering: the above alumina pre-sintered fiber is placed into a microwave sintering furnace, heated to 1000-1400℃ at a heating rate of 50-100℃ / min under air atmosphere and normal pressure, and then microwave sintered for 1-60min to obtain fine-grained dense alumina ceramic fiber.

[0018] In the preparation of the aluminum sol, the molar ratio of aluminum powder: formic acid: acetic acid: deionized water: nitric acid is 1:(0.5-1):(0.5-1):(10-30):(0.1-3), preferably 1:(0.5-0.8):(0.5-0.8):(15-25):(0.1-0.5); the reaction temperature is 60-105℃, preferably 85-95℃.

[0019] In the preparation of the iron sol, the molar ratio of iron nitrate nonahydrate: ammonium bicarbonate: deionized water is 1:(1-3):(573-660), preferably 1:(1.5-2.5):(595-638).

[0020] In the preparation of the mixed sol, the mass ratio of aluminum sol: iron sol: PVP solution: acidified silica sol is (60-99.7):(0.1-20):(0.1-10):(0.1-10), and the mass ratio is preferably (80-99.7):(0.1-10):(0.1-5):(0.1-5), further preferably (87-99.7):(0.1-5):(0.1-5):(0.1-3), and more preferably (87-89):(4.5-5.5):(4.5-5.5):(1.5-2.5).

[0021] In the preparation of the mixed sol, stirring can be carried out at room temperature.

[0022] In the concentration, rotary evaporation can be carried out.

[0023] In step five, the alumina precursor fiber is placed into a box furnace, heated to 500-600℃ at a heating rate of 2-10℃ / min under air atmosphere and normal pressure, and then cooled to room temperature to obtain alumina pre-sintered fiber.

[0024] In the pre-sintering step, the heating rate is 2-10℃ / min, preferably 2-5℃ / min.

[0025] The microwave sintering power is 3000-5000W, preferably 4000-5000W, and further preferably 4000-4100W; the microwave sintering temperature rising rate is 50-100℃ / min, preferably 75-85℃ / min; the microwave sintering heating temperature is 1000-1400℃, preferably 1090-1210℃; and the microwave sintering time is 1-60min, preferably 5-40min, and further preferably 15-35min.

[0026] The diameter of the fine-crystal dense alumina ceramic fiber is 10-30μm, the density is 3.80-3.90g / cm 3 , the microstructure is dense, the average grain size is 110-140nm, and the phase is α-Al2O3 phase.

[0027] The present application can obtain the fine-crystal dense alumina ceramic fiber with single phase (pure α-Al2O3 phase) at 1200℃, even as low as 1000-1100℃ without adding seeds. The present application first obtains the fine-crystal dense alumina ceramic fiber with single phase (pure α-Al2O3 phase) with tensile strength of 3.15GPa-3.2GPa at a heating temperature of 1100℃.

[0028] The tensile strength of the fiber obtained by the present application can reach 2.5-3.3GPa, and can reach 3.2-3.3GPa after optimization.

[0029] The present application has the following advantages:

[0030] (1) The present application uses microwave sintering to prepare alumina ceramic fiber, which is carried out in air atmosphere and at normal pressure, saves raw material input, has short sintering time, high production efficiency, energy saving and environmental protection.

[0031] (2) The alumina ceramic fiber prepared by the method has pure α-alumina phase, small grain size, dense microstructure and good mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The figure is the phase structure diagram of the fine-crystal dense alumina ceramic fiber obtained in Example 1.

[0033] Figure 2 The figure is the low-magnification microstructure diagram of the fine-crystal dense alumina fiber obtained in Example 1.

[0034] Figure 3 The figure is the high-magnification microstructure diagram of the fine-crystal dense alumina fiber obtained in Example 1. DETAILED DESCRIPTION

[0035] Example 1

[0036] A method for preparing fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0037] Step one: preparing solution

[0038] Take 27g of aluminum powder, 36.8g of formic acid, 42g of acetic acid, 450g of deionized water, 52.5g of 60% nitric acid, and other raw materials, and stir at 200r / min for 24 hours at 95℃. After cooling, filter to obtain an aluminum sol;

[0039] Take 3.1g of iron nitrate nonahydrate, 1.5g of ammonium bicarbonate, and 86.6g of deionized water, and react for 1 hour in a 70℃ water bath to obtain an iron sol;

[0040] Take 2.4g of polyvinylpyrrolidone and 24.1g of deionized water to prepare a PVP solution;

[0041] Add 2% dilute nitric acid to the silica sol to obtain acidified silica sol 1.9g with pH value = 4.

[0042] Step two: mixing: take the above aluminum sol, iron sol, PVP solution, and acidified silica sol according to the mass ratio of 72:10:10:8, and stir at 50r / min for 24 hours at room temperature.

[0043] Step three: concentration: use a rotary evaporator to concentrate the above mixed sol at 60℃ to obtain a spinnable gel with a viscosity value of 150Pa·s.

[0044] Step four: spinning: pour the above spinnable gel into the kettle of a dry spinning machine for spinning, and collect it with a rotating drum at 100-120r / min to obtain an alumina precursor fiber.

[0045] Step five: pre-sintering: place the above alumina precursor fiber in a box-type furnace, heat to 600℃ at a heating rate of 10℃ / min under air atmosphere and normal pressure, and then cool to room temperature to obtain an alumina pre-sintered fiber.

[0046] Step six: microwave sintering: place the above alumina pre-sintered fiber in a microwave sintering furnace, set the microwave power to 5000W, heat to 1400℃ at a heating rate of 100℃ / min under air atmosphere and normal pressure, and then microwave sinter for 1min to obtain fine-crystal dense alumina ceramic fiber.

[0047] The obtained fine-crystal dense alumina ceramic fiber has a diameter of 10-20μm (see Figure 2 ), a density of 3.80g / cm 3 , a dense microstructure, an average grain size of 110nm, and a grain size range of 50-400nm (see Figure 3 ), and a phase of α-Al2O3 (seeFigure 1 ), and the tensile strength was 3.0 GPa. The tensile strength was determined by a single fiber strength tester (sample gauge length 25 mm, tensile rate 1 mm / min), and 50 fibers were measured and averaged.

[0048] Example 2

[0049] A method for preparing fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0050] Step one: liquid preparation: weigh 270 g of aluminum powder, 253 g of formic acid, 300 g of acetic acid, 2700 g of deionized water, 180 g of 35% nitric acid, and other raw materials, and stir at 200 r / min for 6 hours at 85°C. After cooling, filter to obtain an aluminum sol;

[0051] Weigh 0.31 g of iron nitrate nonahydrate, 0.15 g of ammonium bicarbonate, and 8.7 g of deionized water, and react in a 40°C water bath for 1 hour to obtain an iron sol;

[0052] Weigh 0.24 g of polyvinylpyrrolidone and 2.4 g of deionized water to prepare a PVP solution;

[0053] Add 2% dilute nitric acid to the silica sol to obtain acidified silica sol with a pH value of 3.

[0054] Step two: mixing: take the above aluminum sol, iron sol, PVP solution, and acidified silica sol according to the mass ratio of 99.7:0.1:0.1:0.1, and stir at 50 r / min for 1 hour at room temperature.

[0055] Step three: concentration: use a rotary evaporator to concentrate the above mixed sol at 30°C to obtain a spinnable gel with a viscosity value of 10 Pa·s.

[0056] Step four: spinning: pour the above spinnable gel into the kettle of a dry spinning machine for spinning, and collect it with a rotating drum at 20-50 r / min to obtain an alumina precursor fiber.

[0057] Step five: pre-sintering: place the above alumina precursor fiber in a box furnace, heat to 500°C at a heating rate of 5°C / min under air atmosphere and normal pressure, and then cool to room temperature to obtain an alumina pre-sintered fiber.

[0058] Step six: microwave sintering: place the above alumina pre-sintered fiber in a microwave sintering furnace, set the microwave power to 3000 W, heat to 1000°C at a heating rate of 50°C / min under air atmosphere and normal pressure, and then microwave sinter for 60 min to obtain fine-crystal dense alumina ceramic fiber.

[0059] The fine-crystal dense alumina ceramic fiber has a diameter of 15-30 μm and a density of 3.83 g / cm 3 , a dense microstructure, an average grain size of 116 nm, a grain size range of 60-270 nm, an alpha-Al2O3 phase, and a tensile strength of 2.5 GPa. The tensile strength is measured by a single fiber strength tester (sample gauge length 25 mm, tensile rate 1 mm / min) and is obtained by averaging the tensile strength of 50 fibers.

[0060] Example 3

[0061] A method for preparing fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0062] Step one: liquid preparation: 27 g of aluminum powder, 27.6 g of formic acid, 33 g of acetic acid, 360 g of deionized water, 47.3 g of 40% nitric acid, and the like are weighed, and a reaction is carried out at 90°C for 12 hours under stirring at a rotation speed of 200 r / min, and then the aluminum sol is obtained by filtering after cooling;

[0063] 1.5 g of iron nitrate nonahydrate, 0.7 g of ammonium bicarbonate, and 43.3 g of deionized water are weighed, and an iron sol is obtained by reacting in a 50°C water bath for 1 hour;

[0064] 1.2 g of polyvinylpyrrolidone and 12.1 g of deionized water are weighed to prepare a PVP solution;

[0065] 2% dilute nitric acid is added dropwise into the silica sol to obtain 0.5 g of acidified silica sol with a pH value of 3.4.

[0066] Step two: mixing: the above aluminum sol, iron sol, PVP solution, and acidified silica sol are weighed according to a mass ratio of 88:5:5:2, and are stirred at a rotation speed of 50 r / min for 2 hours at room temperature.

[0067] Step three: concentration: the above mixed sol is concentrated by using a rotary evaporator at 40°C to obtain spinnable gel with a viscosity value of 100 Pa·s.

[0068] Step four: spinning: the above spinnable gel is poured into a kettle of a dry spinning machine to perform spinning, and a rotating drum is used to collect at a rotation speed of 80-120 r / min to obtain alumina precursor fiber.

[0069] Step five: pre-sintering: the above alumina precursor fiber is placed into a box-type furnace, heated to 600°C at a heating rate of 2°C / min under air atmosphere and normal pressure, and then cooled to room temperature to obtain alumina pre-sintered fiber.

[0070] Step six microwave sintering: the alumina pre-sintered fiber is put into the microwave sintering furnace, heated to 1300℃ at the heating rate of 100℃ / min under the air atmosphere and normal pressure, and then microwave sintered for 10 min, to obtain the fine-crystal dense alumina ceramic fiber.

[0071] The obtained fine-crystal dense alumina ceramic fiber has the diameter of 10-23 μm, the density of 3.90 g / cm 3 , the dense microstructure, the average grain size of 132 nm, the grain size range of 90-350 nm, the phase of α-Al2O3, and the tensile strength of 2.9 GPa. The tensile strength is measured by the single fiber strength tester (the sample gauge length of 25 mm and the tensile rate of 1 mm / min), and the average value of 50 fibers is obtained.

[0072] Example 4

[0073] A preparation method of fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0074] The other conditions are the same as those in Example 3, except that step six microwave sintering: the alumina pre-sintered fiber is put into the microwave sintering furnace, heated to 1200℃ at the heating rate of 80℃ / min under the air atmosphere and normal pressure, and then microwave sintered for 20 min, to obtain the fine-crystal dense alumina ceramic fiber.

[0075] The obtained fine-crystal dense alumina ceramic fiber has the diameter of 10-23 μm, the density of 3.87 g / cm 3 , the dense microstructure, the average grain size of 124 nm, the grain size range of 70-290 nm, the phase of α-Al2O3, and the tensile strength of 3.3 GPa. The tensile strength is measured by the single fiber strength tester (the sample gauge length of 25 mm and the tensile rate of 1 mm / min), and the average value of 50 fibers is obtained.

[0076] Example 5

[0077] A preparation method of fine-crystal dense alumina ceramic fiber, comprising the following steps:

[0078] The other conditions are the same as those in Example 3, except that step six microwave sintering: the alumina pre-sintered fiber is put into the microwave sintering furnace, heated to 1100℃ at the heating rate of 80℃ / min under the air atmosphere and normal pressure, and then microwave sintered for 30 min, to obtain the fine-crystal dense alumina ceramic fiber.

[0079] The obtained fine-crystal dense alumina ceramic fiber has the diameter of 10-23 μm, the density of 3.84 g / cm 3, the microstructure is dense, the average grain size is 119 nm, the grain size ranges from 65 nm to 278 nm, the phase is a-Al203, and the tensile strength is 3.2 GPa. The tensile strength is measured by a single fiber strength tester (the sample gauge length is 25 mm, and the tensile rate is 1 mm / min), and the average value of 50 fibers is obtained.

[0080] Comparative Example 1

[0081] The other conditions are the same as those in Example 3, except that 4.6 g of formic acid, 120 g of acetic acid, and 78.8 g of 40% nitric acid are used, and a white precipitate is generated after the reaction. After cooling, a small amount of aluminum sol is obtained by filtration. A gel with a viscosity value of 8 Pa s is obtained by concentration. The gel cannot be spun, and the alumina precursor fiber cannot be obtained or subjected to subsequent sintering, and the tensile strength performance is not available.

[0082] Comparative Example 2

[0083] The other conditions are the same as those in Example 3, except that 92 g of formic acid, 6 g of acetic acid, and 78.8 g of 40% nitric acid are used in step one, and a white precipitate is generated after the reaction. After cooling, a small amount of aluminum sol is obtained by filtration. In step five, the heating rate is 20 ℃ / min to 800 ℃. In step six, the microwave power is 2500 W, the heating rate is 40 ℃ / min to 1500 ℃, and then microwave sintering is performed for 70 min. The obtained alumina ceramic fiber has a density of 3.30 g / cm 3 , the microstructure is not dense, there are many internal pores, the average grain size is greater than 1 μm, the phase is a-Al203, and the tensile strength is 0.5 GPa. The tensile strength is measured by a single fiber strength tester (the sample gauge length is 25 mm, and the tensile rate is 1 mm / min), and the average value of 50 fibers is obtained.

[0084] Comparative Example 3

[0085] The other conditions are the same as those in Example 3, except that the heating is performed to 300 ℃ in step five. The obtained alumina ceramic fiber is broken into a powder, the microstructure is not dense, and the tensile strength performance is not available.

[0086] Comparative Example 4

[0087] The other conditions are the same as those in Example 3, except that the heating is performed to 900 ℃ in step six, and then microwave sintering is performed for 70 min. The obtained alumina ceramic fiber has a density of 2.82 g / cm 3 , the microstructure is not dense, there are many internal pores, the phase is amorphous Al203 and γ-Al203, and the tensile strength is 0.8 GPa. The tensile strength is measured by a single fiber strength tester (the sample gauge length is 25 mm, and the tensile rate is 1 mm / min), and the average value of 50 fibers is obtained.

[0088] Comparative Example 5

[0089] Other conditions are the same as in Example 3, except that step 5 is omitted, i.e. after obtaining the alumina precursor fiber, it is directly subjected to microwave sintering, the microwave sintering is carried out in air atmosphere and at normal pressure, the microwave power is set to 5000 W, the heating rate is set to 100 °C / min, the temperature is heated to 1300 °C, and then the microwave sintering is carried out for 10 min, no fibrous product is obtained, the obtained product is in powder form, and no tensile strength performance.

[0090] Comparative Example 6

[0091] Other conditions are the same as in Example 3, except that in step five, the heating is to 700 °C. The obtained alumina ceramic fiber has a density of 3.50 g / cm 3 , the fiber surface microstructure is dense but the internal porosity is more, forming a layered structure, the phase is α-Al2O3 phase, and the tensile strength is 0.9 GPa. The tensile strength is determined by a single fiber strength tester (the sample gauge length is 25 mm, the tensile rate is 1 mm / min), and the average value of 50 fibers is obtained.

Claims

1. A method for preparing fine-grained dense alumina ceramic fibers, characterized in that, Includes the following steps: Step 1: Solution preparation Weigh out the various raw materials according to the molar ratio of aluminum powder: formic acid: acetic acid: deionized water: nitric acid = 1: (0.5~1): (0.5~1): (10~30): (0.1~3), stir and react at 200 r / min at 60~105 ℃ for 6~24 hours, and filter after cooling to obtain aluminum sol; Weigh out the iron sol according to the molar ratio of ferric nitrate nonahydrate: ammonium bicarbonate: deionized water = 1: (1~3): (573~660), and react in a water bath at 40~70℃ for 30~120 min. Prepare a PVP solution by mass ratio of polyvinylpyrrolidone:deionized water = 1:9~11; Add 2% dilute nitric acid to the silica sol to adjust the pH value of the silica sol to the range of 3-4, thus obtaining a suitable acidified silica sol. Step 2: Mixing materials When preparing the mixed sol, the aluminum sol, iron sol, PVP solution, and silica sol are prepared according to the mass ratio of aluminum sol: iron sol: PVP solution: silica sol = 80~99.7: 0.1~10: 0.1~5: 0.1~5. The aluminum sol, iron sol, PVP solution, and silica sol are then mixed and stirred at a speed of 45~60 r / min for 1~24 hours to obtain the mixed sol. Step 3 Concentration: Concentrate the mixed sol at 30~60℃ until a spinnable gel with a viscosity of 10~1000 Pa·s is obtained; Step 4: Spinning: The obtained spinnable gel is poured into the kettle of a dry spinning machine for spinning, and collected by a rotating drum at 20~120 r / min to obtain alumina precursor fibers. Step 5: Pre-sintering: The obtained alumina precursor fibers are placed in a sintering furnace and heated to 500-600°C at a heating rate of 2-10°C / min, and then cooled to room temperature to obtain alumina pre-sintered fibers. Step 6: Microwave sintering: Place the obtained alumina pre-sintered fibers into a microwave sintering furnace. Under air atmosphere and normal pressure, set the microwave power to 3000~5000 W and heat to 1000~1400℃ at a heating rate of 50~100℃ / min. Then microwave sinter for 1~60 min to obtain fine-grained dense alumina ceramic fibers. The resulting fine-grained, dense alumina ceramic fibers have a diameter of 10–30 μm and a density of 3.80–3.90 g / cm³. 3 It has a dense microstructure with an average grain size of 110~140 nm and the phase is α-Al2O3.

2. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: When preparing aluminum sol, the molar ratio of aluminum powder: formic acid: acetic acid: deionized water: nitric acid is 1: (0.5~1): (0.5~1): (10~30): (0.1~3); the reaction temperature is 60~105 ℃.

3. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: When preparing iron sol, the molar ratio of ferric nitrate nonahydrate: ammonium bicarbonate: deionized water is 1: (1~3): (573~660).

4. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: When preparing the mixed sol, the aluminum sol, iron sol, PVP solution and silica sol are prepared according to the mass ratio of aluminum sol: iron sol: PVP solution: silica sol = (87~99.7): (0.1~5): (0.1~5): (0.1~3).

5. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: When preparing the mixed sol, the aluminum sol, iron sol, PVP solution and silica sol are prepared according to the mass ratio of aluminum sol: iron sol: PVP solution: silica sol = (87~89): (4.5~5.5): (4.5~5.5): (1.5~2.5).

6. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: During pre-sintering, the heating rate is 2~5 ℃ / min.

7. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: During pre-sintering, alumina precursor fibers are placed in a box furnace and heated to 500-600°C at a heating rate of 2-5°C / min under air atmosphere and normal pressure. Then, they are cooled to room temperature to obtain alumina pre-sintered fibers.

8. The method for preparing fine-grained dense alumina ceramic fibers according to claim 1, characterized in that: The microwave sintering power is 4000~5000 W; the microwave sintering heating rate is 50~100℃ / min; the microwave sintering heating temperature is 1090~1210℃; and the microwave sintering time is 1~60 min.

9. The method for preparing fine-grained dense alumina ceramic fibers according to claim 8, characterized in that: The tensile strength of the obtained fibers is 2.5~3.3 GPa; When the microwave sintering temperature is 1100℃, alumina ceramic fibers with a tensile strength of 3.15GPa~3.2GPa are obtained.

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