A method for preparing alumina fibers by sol-gel-electrospinning

Abstract

The application discloses a sol-gel-electrospinning method for preparing alumina fibers and belongs to the technical field of alumina fiber preparation. The alumina fibers are prepared by the following method: stirring and uniformly mixing aluminum nitrate solution and aluminum isopropyl alcohol solution, adjusting pH to 6-7 by using lye, obtaining crystal seeds through hydrothermal reaction, filtration, washing, drying and calcination; adding the crystal seeds into deionized water, adjusting pH to 2.5-3.5 by using acid solution, stirring and ball milling, and obtaining a crystal seed suspension; dissolving a spinning aid in deionized water, adding an aluminum source, a yttrium source, citric acid and the crystal seed suspension, stirring and uniformly mixing, and obtaining a fiber precursor through electrospinning; and performing calcination treatment on the fiber precursor to obtain alumina fibers. The alumina fibers prepared by the application have high tensile strength and tensile modulus, and the fiber diameter distribution is relatively uniform.

Description

Technical Field

[0001] This invention relates to the field of alumina fiber preparation technology, and specifically to a method for preparing alumina fibers using a sol-gel-electrospinning method. Background Technology

[0002] Alumina fiber is an inorganic fiber with excellent properties such as high strength, high modulus, and corrosion resistance, and is widely used in aerospace, industrial high-temperature furnace linings, filtration catalysis, and composite material reinforcement. The main preparation methods for alumina fibers include the Brunner Mann process, melt spinning, impregnation, prepolymerization, slurry method, sol-gel method, and electrospinning. Among these, the sol-gel-electrospinning method has become an important technical route for alumina fiber preparation in recent years. It typically uses aluminum salts or aluminum alkoxides as the aluminum source, which are hydrolyzed, complexed, and polymerized to form an aluminum-containing sol. This sol is then compounded with spinning aids such as polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide, and continuous precursor fibers are obtained through electrospinning. Finally, the fibers are dried, pre-oxidized, and calcined at high temperature to remove organic components and transform them into alumina fibers. This method has advantages such as mild process, adjustable fiber diameter, high specific surface area, and easy morphology design, but it still has problems such as insufficient storage stability of the spinning solution, high dosage of polymeric aids, significant fiber shrinkage and adhesion during calcination, high crystal transformation temperature, and insufficient mechanical strength and flexibility.

[0003] Chinese invention patent CN111074357A discloses an alumina fiber and its preparation method. The method includes: (A) mixing an aluminum source, a silicon source, and other oxide sources; (B) adding a first spinning aid to (A) to react and obtain an inner spinning solution; (C) preparing a second spinning aid into an outer spinning solution; (D) using a combination of jet spinning and electrospinning to obtain precursor fibers; and (E) heat-treating the precursor fibers to obtain alumina fibers. This invention, by controlling the mixing process of the aluminum source, silicon source, other oxide sources, and the first spinning aid, as well as the form and amount of the second spinning aid added, ultimately obtains soft and smooth fibers; and improves fiber yield while ensuring fiber quality; however, tensile strength and tensile modulus still need improvement. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for preparing alumina fibers using a sol-gel electrospinning method.

[0005] A method for preparing alumina fibers using a sol-gel-electrospinning method includes the following steps: (1) Add aluminum isopropoxide solution to aluminum nitrate solution and stir to mix well. Adjust the pH to 6-7 with alkaline solution. After hydrothermal reaction, filtration, washing, drying and calcination, seed crystals are obtained. (2) Add the seed crystals to deionized water, adjust the pH to 2.5-3.5 with acid, stir and ball mill to obtain a seed crystal suspension; (3) Dissolve the spinning aid in deionized water, add aluminum source, yttrium source, citric acid and seed crystal suspension, stir and mix well, and obtain fiber precursor by electrospinning; (4) The fiber precursor is calcined to obtain alumina fiber.

[0006] In step (1), the molar ratio of aluminum nitrate to aluminum isopropoxide is 1:(0.1-0.2).

[0007] In step (1), the solvent of the aluminum nitrate solution is deionized water with a concentration of 1-2 mol / L; the solvent of the aluminum isopropoxide solution is isopropanol with a concentration of 0.2-0.5 mol / L.

[0008] In step (1), the hydrothermal reaction temperature is 140-180℃ and the time is 12-16h; the calcination temperature is 1200-1300℃ and the time is 2-3h.

[0009] In step (3), the aluminum source is aluminum nitrate nonahydrate; the yttrium source is yttrium nitrate hexahydrate.

[0010] In step (3), the mass ratio of the aluminum source, yttrium source, citric acid, spinning aid and seed crystal in the seed suspension is 1:(0.01-0.02):(0.2-0.4):(0.3-0.5):(0.002-0.006).

[0011] In step (3), the spinning aid is a mixture of polyvinyl alcohol and polyvinylpyrrolidone in a mass ratio of 1:(1-2).

[0012] In step (3), the temperature for heating and stirring is 60-80℃ and the time is 4-8h.

[0013] In step (3), the voltage of electrospinning is 10-20kV, the feeding rate is 1-2mL / h, the receiving distance is 10-25cm, the temperature is 40-60°C, and the humidity is 30-55%.

[0014] In step (4), the calcination process is as follows: heat up to 300-350℃ at a rate of 1-2℃ / min and hold for 1-2h; heat up to 800-900℃ at a rate of 3-5℃ / min and hold for 0.5-1.5h; heat up to 1200-1300℃ at a rate of 3-5℃ / min and hold for 0.5-1.5h.

[0015] Due to the adoption of the above technical solutions, the beneficial effects of the present invention include: The alumina fibers prepared by this invention have high tensile strength and tensile modulus, and the fiber diameter distribution is relatively uniform. Attached Figure Description

[0016] Figure 1 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared in Example 2.

[0017] Figure 2 X-ray diffraction patterns of the seed crystals and alumina fibers prepared in Example 2.

[0018] Figure 3 The image shows a scanning electron microscope (SEM) image of the alumina fibers prepared in Example 2.

[0019] Figure 4 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared for Comparative Example 1.

[0020] Figure 5 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared for Comparative Example 2.

[0021] Figure 6 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared for Comparative Example 3. Detailed Implementation

[0022] Example 1: Preparation of alumina fibers (1) Slowly add 100ml of 0.2mol / L aluminum isopropoxide isopropanol solution to 200ml of 1mol / L aluminum nitrate aqueous solution and stir for 20min. Adjust the pH to 6 with 1M ammonium bicarbonate solution. Add to 500ml of polytetrafluoroethylene reaction vessel, heat to 140℃ and hydrothermally react for 16h. Cool to room temperature, filter, wash with deionized water (3×50ml), vacuum dry to constant weight at 90℃, place in muffle furnace, heat to 1200℃ at a rate of 3℃ / min under air atmosphere and calcine for 3h. Cool naturally to room temperature to obtain seed crystals. (2) Add 0.1g of seed crystals to 10ml of deionized water, adjust the pH to 2.5 with 0.5M nitric acid solution, stir at 500rpm for 24h and then ball mill for 24h, during which the pH is controlled to 2.5 with 0.5M nitric acid solution to obtain seed crystal suspension; (3) Add 15g of spinning aid (7.5g of polyvinyl alcohol and 7.5g of polyvinylpyrrolidone) to 200ml of deionized water, heat to 80℃ and stir for 1h, cool to room temperature, add 50g of aluminum nitrate nonahydrate, 0.5g of yttrium nitrate hexahydrate, 10g of citric acid and all the seed crystal suspension prepared in step (2), stir for 8h, filter with a 200-mesh sieve to obtain spinning solution; electrospin the obtained spinning solution, the spinning voltage is 10kV, the feeding rate is 1mL / h, the receiving distance is 10cm, the temperature is 40℃, the humidity is 30%, and vacuum dry at 80℃ for 8h to obtain fiber precursor; (3) The fiber precursor is placed in a muffle furnace and heated to 300°C at a rate of 1°C / min under an air atmosphere and held for 2 hours; heated to 800°C at a rate of 3°C / min and held for 1.5 hours; heated to 1200°C at a rate of 3°C / min and held for 1.5 hours; and then cooled naturally to room temperature with the furnace to obtain alumina fiber.

[0023] Example 2 Preparation of alumina fibers (1) Add 100 ml of 0.4 mol / L aluminum isopropoxide solution to 200 ml of 1.5 mol / L aluminum nitrate aqueous solution. After 50 min, adjust the pH to 6.5 with 1 M ammonium bicarbonate solution. Add the solution to 500 ml of polytetrafluoroethylene reaction vessel, heat to 160 °C and hydrothermally react for 14 h. Cool to room temperature, filter, wash with deionized water (3 × 50 ml), vacuum dry to constant weight at 90 °C, place in muffle furnace, heat to 1250 °C at a rate of 4 °C / min under air atmosphere and calcine for 2.5 h. Cool naturally to room temperature to obtain seed crystals. (2) Add 0.2g of seed crystals to 10ml of deionized water, adjust the pH to 3 with 0.5M nitric acid solution, stir at 500rpm for 24h, and then ball mill for 24h. During this period, use 0.5M nitric acid solution to control the pH to 3 to obtain a seed crystal suspension. (3) Add 20g of spinning aid (8g of polyvinyl alcohol and 12g of polyvinylpyrrolidone) to 200ml of deionized water, heat to 80℃ and stir for 1h, cool to room temperature, add 50g of aluminum nitrate nonahydrate, 0.8g of yttrium nitrate hexahydrate, 15g of citric acid and all the seed crystal suspension prepared in step (2), stir for 9h, filter with a 200-mesh sieve to obtain spinning solution; electrospin the obtained spinning solution, the spinning voltage is 15kV, the feeding rate is 1.5mL / h, the receiving distance is 20cm, the temperature is 50℃, the humidity is 45%, and vacuum dry at 80℃ for 8h to obtain fiber precursor; (4) The fiber precursor is placed in a muffle furnace and heated to 320°C at a rate of 2°C / min under an air atmosphere and held for 1.5 h; heated to 850°C at a rate of 4°C / min and held for 1 h; heated to 1250°C at a rate of 4°C / min and held for 1 h; and then cooled naturally to room temperature with the furnace to obtain alumina fiber.

[0024] Figure 1 The image shows a scanning electron microscope (SEM) image of the seed crystal. As can be seen from the image, the seed crystal exhibits a spindle-shaped particle with pointed ends and a slightly wider middle, and the particles are relatively uniformly dispersed. The seed crystal surface has a certain degree of roughness and small protrusions, indicating that a date-shaped alumina seed crystal was formed after hydrothermal reaction and calcination.

[0025] Figure 2The X-ray diffraction patterns of the seed crystal and alumina fiber are shown. As can be seen from the figure, both exhibit typical α-Al₂O₃ (corundum phase) characteristic diffraction peaks. Obvious diffraction peaks appear at 25.6°, 35.1°, 37.8°, 43.4°, 52.5°, 57.5°, 61.3°, 66.5°, and 68.2°, corresponding to the α-Al₂O₃ phase, respectively. Compared with the XRD pattern of the seed crystal, the alumina fiber shows weak peaks at 33.3°, 36.6°, 41.1°, 55.1°, 57.4°, and 61.8°, corresponding to the yttrium aluminum garnet phase. This indicates that the introduced yttrium source reacts with some aluminum oxide species during high-temperature calcination to form a small amount of yttrium aluminum garnet phase.

[0026] Figure 3 The image shows a scanning electron microscope (SEM) image of alumina fibers. As can be seen from the image, the fibers are relatively straight, with a relatively uniform diameter distribution and a relatively smooth surface. No breakage or agglomeration was observed.

[0027] Example 3 Preparation of alumina fibers (1) Slowly add 160 ml of 0.5 mol / L aluminum isopropoxide isopropanol solution to 200 ml of 2 mol / L aluminum nitrate aqueous solution and stir for 20 min. Adjust the pH to 7 with 1 M ammonium bicarbonate solution. Add to 500 ml of polytetrafluoroethylene reaction vessel, heat to 180 °C and hydrothermally react for 12 h. Cool to room temperature, filter, wash with deionized water (3 × 50 ml), vacuum dry to constant weight at 90 °C, place in muffle furnace, heat to 1300 °C at a rate of 5 °C / min under air atmosphere and calcine for 3 h. Cool naturally to room temperature to obtain seed crystals. (2) Add 0.3g of seed crystals to 10ml of deionized water, adjust the pH to 3.5 with 0.5M nitric acid solution, stir at 500rpm for 24h and then ball mill for 24h, during which the pH is controlled to 3.5 with 0.5M nitric acid solution to obtain seed crystal suspension; (3) Add 25g of spinning aid (8.33g of polyvinyl alcohol and 16.67g of polyvinylpyrrolidone) to 200ml of deionized water, heat to 80℃ and stir for 1h, cool to room temperature, add 50g of aluminum nitrate nonahydrate, 1g of yttrium nitrate hexahydrate, 20g of citric acid and all the seed suspension prepared in step (2), stir for 10h, filter with a 200-mesh sieve to obtain spinning solution; electrospin the obtained spinning solution, the spinning voltage is 20kV, the feeding rate is 2mL / h, the receiving distance is 25cm, the temperature is 60℃, the humidity is 55%, and vacuum dry at 80℃ for 8h to obtain fiber precursor; (4) The fiber precursor is placed in a muffle furnace and heated to 350°C at a rate of 2°C / min under an air atmosphere and held for 1 hour; heated to 900°C at a rate of 5°C / min and held for 0.5 hours; heated to 1300°C at a rate of 5°C / min and held for 0.5 hours; and then cooled naturally to room temperature with the furnace to obtain alumina fiber.

[0028] Comparative Example 1 The preparation method of alumina fibers is basically the same as that in Example 2, except that the seed crystals in step (2) are replaced with an equal weight of seed crystals prepared by the following method: 100 ml of isopropanol was added dropwise to 200 ml of 1.5 mol / L aluminum nitrate aqueous solution over 50 min. The pH was adjusted to 6.5 using 1 M ammonium bicarbonate solution. The solution was then added to a 500 ml polytetrafluoroethylene reaction vessel and heated to 160 °C for hydrothermal reaction for 14 h. After cooling to room temperature, the solution was filtered, washed with deionized water (3 × 50 ml), and dried under vacuum at 90 °C to constant weight. The solution was then placed in a muffle furnace and calcined at 1250 °C for 2.5 h in air at a rate of 4 °C / min. After naturally cooling to room temperature, seed crystals were obtained. Figure 4 The image shown is a scanning electron microscope image of the seed crystals prepared in Comparative Example 1. The overall morphology of the crystals is a near-spherical particle morphology with uneven particle size distribution.

[0029] Comparative Example 2 The preparation method of alumina fiber is basically the same as that in Example 2, except that the pH adjustment of 1M ammonium bicarbonate solution to 6.5 in step (1) is changed to pH adjustment of 1M ammonium bicarbonate solution to 4.

[0030] Figure 5 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared in Comparative Example 2. As can be seen from the image, the seed crystals are generally in the form of short rods or spindle-shaped short fibers with a relatively rough surface. The individual seed crystals are short in length, have a low aspect ratio, and have uneven particle size distribution.

[0031] Comparative Example 3 The preparation method of alumina fiber is basically the same as that in Example 2, except that the pH adjustment of 1M ammonium bicarbonate solution to 6.5 in step (1) is changed to pH adjustment of 1M ammonium bicarbonate solution to 7.5.

[0032] Figure 6 The image shows a scanning electron microscope (SEM) image of the seed crystals prepared in Comparative Example 3. As can be seen from the image, the seed crystals have a short rod-like structure, the fiber surface is relatively rough with small protrusions, the individual seed crystals are relatively short and have a low aspect ratio.

[0033] Comparative Example 4 The preparation method of alumina fiber is basically the same as that in Example 2, except that the pH is adjusted to 4 using 0.5M nitric acid solution instead of 0.5M nitric acid solution in step (2) to adjust pH to 3.

[0034] Comparative Example 5 The preparation method of alumina fiber is basically the same as that in Example 2, except that the seed crystal added in step (2) is replaced with 0.05g.

[0035] Comparative Example 6 The preparation method of alumina fiber is basically the same as that in Example 2, except that the seed crystal added in step (2) is replaced with 0.5g.

[0036] Comparative Example 7 The preparation method of alumina fiber is basically the same as that in Example 2, except that the amount of yttrium nitrate hexahydrate added in step (2) is replaced with 1.5g.

[0037] Comparative Example 8 The preparation method of alumina fiber is basically the same as that in Example 2, except that the spinning aid (8g of polyvinyl alcohol and 12g of polyvinylpyrrolidone) in step (2) is replaced with spinning aid (4g of polyvinyl alcohol and 16g of polyvinylpyrrolidone).

[0038] The polyvinyl alcohol used in this application is model PVA-2499; the polyvinylpyrrolidone is model PVP-K30.

[0039] Unless otherwise stated, all operations involving pH adjustment and maintenance in this invention shall keep the pH of the system within the range of the target value ±0.1.

[0040] The alumina fibers prepared in the examples and comparative examples were tested for tensile strength and tensile modulus according to 6.9 of GB / T 46537-2025 standard.

[0041] Table 1 Performance Test Data As can be seen from the data in Table 1, the alumina fibers prepared by the present invention have high tensile strength and tensile modulus.

[0042] This invention first provides a soluble aluminum source using aluminum nitrate and aluminum isopropoxide. After pH adjustment with ammonium bicarbonate, hydrothermal reaction, and calcination, a date-kernel-shaped alumina seed crystal is obtained. After acidification and dispersion, the surface charge state of this seed crystal is improved, which facilitates its uniform dispersion in the subsequent aluminum-yttrium composite sol system. In the spinning solution system, citric acid complexes with Al³⁺ and Y³⁺, ​​slowing down the hydrolysis rate of metal ions, improving the uniformity and stability of the spinning solution, and promoting the uniform distribution of yttrium in the precursor. Polyvinyl alcohol and polyvinylpyrrolidone impart good spinnability to the spinning solution by increasing the system viscosity and chain entanglement, enabling the inorganic precursor to form a continuous fiber precursor under electrostatic stretching. During the high-temperature calcination of the fiber precursor, the seed crystal can act as a heterogeneous nucleation site, promoting the uniform formation of the alumina crystal phase and reducing abnormal local grain growth caused by disordered crystallization. During the calcination process, yttrium sources can participate in crystal phase regulation through grain boundary enrichment, solid solution regulation, or the formation of a small amount of yttrium aluminum oxide phase, thereby inhibiting grain boundary migration and rapid grain growth and improving the mechanical properties of alumina fibers.

[0043] Comparative Example 1, without the addition of aluminum isopropoxide and using only aluminum nitrate as the aluminum source, produced spherical particles with relatively smooth surfaces. However, the surface active sites and crystal faces were limited in exposure, reducing the ability to induce directional nucleation of alumina precursors and resulting in insufficient phase transformation and structural strengthening during fiber calcination. Comparative Example 3 increased the pH to 7.5 during seed preparation, increasing the alkalinity of the system. This resulted in larger seed particle sizes, making uniform distribution in the spinning solution difficult. During calcination, this could cause localized stress concentration and fiber structural defects, reducing fiber continuity, density, and phase uniformity, ultimately leading to a decline in alumina fiber performance. Comparative Example 4 raised the pH of the seed suspension to 4, resulting in insufficient acidification and decreased seed suspension stability. After addition to the spinning solution, uniform dispersion was difficult to achieve, leading to uneven alumina grain growth during subsequent calcination. Some areas experienced rapid phase transformation while others underwent insufficient transformation, ultimately resulting in a decrease in the mechanical properties of the alumina fiber. The increased yttrium content in the alumina fibers prepared in Comparative Example 6 easily leads to the formation of local yttrium-rich regions, weakening the densification and uniform crystallization effects of the alumina fibers, resulting in a decrease in the tensile modulus and tensile strength of the fibers.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. However, any modifications, alterations, and variations made by those skilled in the art without departing from the scope of the present invention based on the disclosed technical content are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing alumina fibers using a sol-gel-electrospinning process, characterized in that, Includes the following steps: (1) Add aluminum isopropoxide solution to aluminum nitrate solution and stir to mix well. Adjust the pH to 6-7 with alkaline solution. After hydrothermal reaction, filtration, washing, drying and calcination, seed crystals are obtained. (2) Add the seed crystals to deionized water, adjust the pH to 2.5-3.5 with acid, stir and ball mill to obtain a seed crystal suspension; (3) Dissolve the spinning aid in deionized water, add aluminum source, yttrium source, citric acid and seed crystal suspension, stir and mix well, and obtain fiber precursor by electrospinning; (4) The fiber precursor is calcined to obtain alumina fiber; In step (3), the aluminum source is aluminum nitrate nonahydrate; the yttrium source is yttrium nitrate hexahydrate; the mass ratio of the aluminum source, yttrium source, citric acid, spinning aid and seed crystal in the seed suspension is 1:(0.01-0.02):(0.2-0.4):(0.3-0.5):(0.002-0.006); the spinning aid is a mixture of polyvinyl alcohol and polyvinylpyrrolidone in a mass ratio of 1:(1-2).

2. The method for preparing alumina fibers by sol-gel electrospinning according to claim 1, characterized in that, In step (1), the molar ratio of aluminum nitrate to aluminum isopropoxide is 1:(0.1-0.2).

3. The method for preparing alumina fibers by sol-gel electrospinning according to claim 1, characterized in that, In step (1), the solvent of the aluminum nitrate solution is deionized water with a concentration of 1-2 mol / L; the solvent of the aluminum isopropoxide solution is isopropanol with a concentration of 0.2-0.5 mol / L.

4. The method for preparing alumina fibers by sol-gel electrospinning according to claim 1, characterized in that, In step (1), the hydrothermal reaction temperature is 140-180℃ and the time is 12-16h; the calcination temperature is 1200-1300℃ and the time is 2-3h.

5. The method for preparing alumina fibers by sol-gel electrospinning according to claim 1, characterized in that, In step (3), the voltage of electrospinning is 10-20kV, the feeding rate is 1-2mL / h, the receiving distance is 10-25cm, the temperature is 40-60°C, and the humidity is 30-55%.

6. The method for preparing alumina fibers by sol-gel electrospinning according to claim 1, characterized in that, In step (4), the calcination process is as follows: heat up to 300-350℃ at a rate of 1-2℃ / min and hold for 1-2h; heat up to 800-900℃ at a rate of 3-5℃ / min and hold for 0.5-1.5h; heat up to 1200-1300℃ at a rate of 3-5℃ / min and hold for 0.5-1.5h.

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