Preparation method of mullite ceramic fiber with hollow hierarchical structure

Abstract

The invention discloses a preparation method of a mullite ceramic fiber with a hollow hierarchical structure, and belongs to the technical field of ceramic fiber material preparation. The preparation method comprises the following steps: firstly, preparing hollow mullite fibers by utilizing coaxial electrostatic spinning, and then growing a hierarchical structure of mullite whiskers on the hollow mullite fibers. The preparation method of the hierarchical structure comprises the following steps: by taking hollow mullite fibers as a matrix, carrying out vacuum impregnation with an aluminum-silicon nano powder solution, then carrying out vacuum impregnation with a fluoride solution, and calcining to obtain the hollow hierarchical mullite ceramic fibers. The preparation method of the porous mullite fiber is simple, the pore structure is easy to control, and the heat insulation performance is excellent. The mullite graded fiber prepared by the method has excellent high-temperature resistance and light weight, can be used for preparing a high-temperature-resistant heat insulation piece, and can provide a novel light high-temperature-resistant heat insulation material for the fields of spaceflight and military industry and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of ceramic fiber materials, and particularly relates to a method for preparing hollow mullite fibers and hollow hierarchical structure mullite ceramic fibers and their applications. Background Art

[0002] As a kind of oxide ceramic, mullite, compared with other oxide ceramics, does not have polymorphic transformation, avoiding the generation of stress during the sintering process. Therefore, it has excellent thermal shock resistance, low thermal expansion, and good mechanical properties. Mullite has a wide range of applications in high-temperature fields. In the fields of high-temperature structural materials and thermal protection materials, especially in military, aerospace industrial production, and civilian products, etc., mullite-based thermal insulation materials have more prominent advantages.

[0003] At present, most of the mullite fibers prepared by electrospinning are solid structures, with relatively high thermal conductivity, and it is difficult to meet the new requirements of high-temperature thermal insulation in modern military and civilian fields. Compared with traditional solid ceramic fibers, the hollow structure changes the single heat transfer mode of solid materials. The fiber material has a higher porosity and finer pore size, increasing its ability to lock air. The presence of gas in the fibers separates the solid phases from each other, slowing down the heat transfer rate, reducing the thermal conductivity of the ceramic material, improving the thermal insulation performance of the material, and making the material lighter in weight.

[0004] On the framework built by mullite fibers, amorphous oxide SiO 2 -Al 2 O 3 aerogel particles are in-situ transformed into high-temperature resistant crystalline mullite whiskers to form fiber aerogel, realizing the mullite fiber / mullite whisker hierarchical pore structure. A porous mullite ceramic with a whisker framework is prepared through a gas-solid reaction mechanism. The in-situ formed mullite whiskers can significantly improve the mechanical properties of the porous mullite ceramic. This three-dimensional porous framework structure not only endows the material with the characteristics of low density and low thermal conductivity, but also ensures that the three-dimensional pores existing in the material will not collapse at high temperatures, thereby improving the high-temperature mechanical properties of the porous mullite ceramic.

[0005] Due to its special hierarchical fiber structure, eiderdown is light and has a good heat preservation effect. There is little research on eiderdown in the field of thermal insulation compared with polar bear hair. If a mullite fiber-based material is prepared by imitating the hierarchical fiber structure of eiderdown, it will have better heat preservation and insulation effects and better mechanical properties. The eiderdown structure consists of a thick main hollow fiber and a large number of thin branch fibers. The main fiber provides mechanical support to ensure the structural stability of the eiderdown, while the branch fibers can fix more static air, playing a heat preservation effect and having a better thermal insulation effect. According to the idea of morphological bionics, the hollow plus hierarchical structure improves the mechanical properties and the thermal insulation performance. Summary of the Invention

[0006] To improve the heat insulation performance and mechanical properties of mullite ceramic fibers, the present invention provides a method for preparing hollow mullite fibers and hollow hierarchical structure mullite ceramic fibers and their applications. The present invention uses coaxial electrospinning technology to prepare mullite fibers with a porous structure inside, a wide pore size range, a low thermal conductivity, and improved heat insulation performance. At the same time, high-temperature resistant crystalline mullite whiskers are in-situ generated on the basis of the prepared mullite fibers to prepare hollow hierarchical mullite ceramic fibers with a goose-down-like structure, realizing the improvement of the heat insulation performance and mechanical properties of mullite ceramic fibers.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing hollow hierarchical structure mullite ceramic fibers, comprising the following steps: (1) Prepare an impregnating solution using silica and alumina nanopowders; (2) Add hollow mullite fibers to the impregnating solution, impregnate under vacuum, and dry; then impregnate under vacuum in a fluoride solution and dry to obtain precursor fibers; (3) Calcinate the precursor fibers, cool, and obtain hollow hierarchical structure mullite ceramic fibers.

[0008] Furthermore, in the preparation method, the mass ratio of the hollow mullite fibers to the silica and alumina nanopowders is 0.5-4:1.

[0009] Furthermore, the particle sizes of the silica and alumina nanopowders are 20-50 nm.

[0010] Furthermore, in step (2), the fluoride is one or more of aluminum fluoride, ammonium fluoride, lithium fluoride, zirconium fluoride, silicon fluoride, and calcium fluoride.

[0011] Furthermore, in step (2), the hollow mullite fibers are impregnated under vacuum in the impregnating solution and the fluoride solution for 2 h-6 h and dried at 60°C-80°C.

[0012] Furthermore, the concentration of the fluoride solution is 3 mol / L-5 mol / L, and the molar ratio of Si:F is 1:1-6.

[0013] Furthermore, the calcination temperature is 1000-1300°C, the heating rate is 1-10°C / min, and the holding time is 1-4 h.

[0014] Furthermore, the method for preparing the hollow mullite fibers comprises the following steps: (1) Prepare a mullite sol using an aluminum-silica sol according to the mullite ratio; (2) Mix absolute ethanol and polyvinylpyrrolidone (PVP) / polyvinyl alcohol (PVA) / polyethylene glycol (PEG) in a certain proportion to prepare a PVP / PVA / PEG ethanol solution. Dissolve the mullite sol in the PVP / PVA / PEG ethanol solution, and then add a certain amount of N,N-dimethylformamide (DMF) / dimethyl sulfoxide (DMSO) / N,N-dimethylacetamide (DMAc) to obtain a mullite precursor solution; (3) Dissolve PVP / PVA / PEG in ethanol and water as the inner spinning solution, and use the mullite precursor solution as the outer spinning solution. Prepare mullite fibers by coaxial electrospinning; (4) Obtain hollow porous mullite fibers after high-temperature calcination of the mullite fibers.

[0015] Further, the process of step (1) is specifically as follows: Mix the aluminum sol and the silicon sol in an Al:Si molar ratio of 3:2 to generate a mullite sol.

[0016] Further, the mass percentage of absolute ethanol in the PVP / PVA / PEG ethanol solution is 75% - 95%, and the mass percentage of PVP / PVA / PEG is 5% - 25%.

[0017] Further, the composition of the mullite precursor solution is: the mass percentage of the mullite sol is 5% - 10%, the mass percentage of the PVP / PVA / PEG ethanol solution is 80% - 85%, and the mass percentage of DMF / DMSO / DMAc is 5% - 10%.

[0018] Further, the composition of the inner spinning solution is: the mass percentage of PVP / PVA / PEG is 4% - 20%, the mass percentage of ethanol is 40% - 50%, and the mass percentage of water is 40% - 50%.

[0019] Further, the parameters of the coaxial electrospinning process are: the flow rate of the inner spinning solution is 0.05 - 1 mL / h, the flow rate of the outer spinning solution is 0.5 - 2 mL / h, the spinning voltage is 10 - 20 kV, the curing distance is 5 - 16 cm, and the flow rate of the inner solution is lower than that of the outer solution.

[0020] Further, the calcination temperature is 1000 - 1300 °C, the heating rate is 1 - 10 °C / min, and the holding time is 1 - 4 h.

[0021] Further, the hollow hierarchical structure mullite ceramic fibers prepared by the above preparation method are used as thermal insulation materials in, but not limited to, the inner lining and coatings of high-temperature industrial kilns.

[0022] A preparation method of a high-temperature resistant thermal insulation coating, comprising the following steps: Mix the raw materials for preparing the heat-insulating coating, stir at 1000 r / min for 30 min to 60 min, and then ultrasonicate for 30 min to 60 min to prepare a coating solution; coat the coating solution onto a cylindrical steel sheet with a diameter of 30 mm and a height of 2 to 4 mm, and dry at 60 °C to obtain a high-temperature heat-insulating coating.

[0023] Furthermore, the raw materials of the heat-insulating coating are composed of the following substances: 40 to 80 m / g of pure acrylic emulsion, 20 to 40 m / g of deionized water, 20 to 40 m / g of hollow hierarchical structure mullite ceramic fibers prepared by the above preparation method, 0.2 to 0.3 m / g of thickener, 0.2 to 0.4 m / g of defoamer, 0.2 to 0.5 m / g of dispersant, 0.05 to 0.1 m / g of wetting agent, 0.05 to 0.1 m / g of film-forming aid, and 20 to 40 m / g of silica sol.

[0024] Furthermore, the thickener is preferably HBR250, the defoamer is preferably NXZ, the dispersant is preferably Fs10, the wetting agent is preferably X405, and the film-forming aid is preferably dodecyl alcohol ester.

[0025] Furthermore, the thickness of the coating is 1 to 3 mm, preferably 2 mm.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The main hollow mullite fibers provide mechanical support to ensure the structural stability of the eiderdown. This hollow structure makes the material lighter and more heat-insulating. The mullite hierarchical structure imitating the branched fibers of the eiderdown can fix more static air, and the presence of whiskers makes the mechanical properties more excellent. According to the idea of bionic morphology, the hollow and hierarchical structure improves the mechanical properties and the heat-insulating performance.

[0027] 2. The hollow hierarchical structure mullite ceramic fibers prepared by the method of the present invention have excellent high-temperature resistance and are light in weight, and can be used to prepare high-temperature heat-insulating parts, providing a new type of lightweight high-temperature heat-insulating material for the fields of aerospace and military industry, etc.

[0028] 3. The present invention prepares hollow hierarchical structure mullite ceramic fibers by coaxial electrospinning to prepare hollow structure mullite fibers and vacuum impregnation method, which is a very excellent heat-insulating material. Applying it to the coating greatly reduces the energy loss. Description of the Drawings

[0029] Figure 1 It is the SEM image of the hollow hierarchical structure mullite ceramic fibers prepared in Example 2 of the present invention. Figure 1 b is Figure 1 The partial enlarged view of a.

[0030] Figure 2XRD pattern of the hollow hierarchical structure mullite ceramic fiber prepared in Example 2 of the present invention. Detailed implementation manners

[0031] The technical solutions and effects of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto. Example 1

[0032] The following method for preparing hollow mullite fibers was used to prepare the hollow hierarchical structure mullite ceramic fibers in the following examples, including the following steps: (1) Prepare a mullite sol using an aluminum-silica sol according to the mullite ratio. The specific process used in this example is as follows: First, an aluminum sol was synthesized using Al 2 (SO 4 ) 3 ·18H 2 O and (CH 2 ) 6 N 4 as raw materials. Then, a commercially available silica sol was added to the aluminum sol at an Al:Si molar ratio of 3:2 to synthesize a mullite sol.

[0033] (2) Mix absolute ethanol and polyvinylpyrrolidone (PVP, molecular weight 1,300,000) at a mass ratio of 4:1 to prepare a PVP ethanol solution. Dissolve the mullite sol in the PVP ethanol solution, and then add a certain amount of N,N-dimethylformamide (DMF) to obtain a mullite precursor solution; the mass percentage of the mullite sol in the mullite precursor solution is 10%, the mass percentage of the PVP ethanol solution is 80%, and the mass percentage of DMF is 10%.

[0034] (3) Dissolve PVP in ethanol and water as the inner spinning solution, and use the mullite precursor solution as the outer spinning solution. Prepare mullite fibers using a coaxial electrospinning process; the composition of the inner spinning solution is: the mass percentage of PVP is 10%, the mass percentage of ethanol is 40%, and the mass percentage of water is 50%; the coaxial electrospinning process parameters are: the flow rate of the inner spinning solution is 1 mL / h, the flow rate of the outer spinning solution is 2 mL / h, the spinning voltage is 20 kV, the curing distance is 10 cm, and the flow rate of the inner solution is lower than that of the outer solution.

[0035] (4) Obtain hollow porous mullite fibers by high-temperature calcination of the mullite fibers. The calcination temperature is 1300 °C, the heating rate is 5 °C / min, and the holding time is 3 h.

[0036] The hollow mullite fibers prepared by coaxial electrospinning in the present invention are characterized in that the fiber diameter distribution is relatively uniform, the interior of the fibers has a porous structure, and the pore size range is wide. Compared with the prior art, the hollow structure greatly improves the heat insulation performance of the fibers. The porous mullite fibers prepared in the present invention can be used as high-temperature heat insulation materials in the fields of aerospace and military industry, etc.

[0037] Examples 2-5 elaborate in detail on the preparation method of the hollow hierarchical mullite ceramic fibers of the present invention. Among them, the mass ratio of the hollow mullite fibers to the nanopowder is 0.5-4:1, and the molar ratio of Si:F in the impregnating silica is 1:1-6. Example 2

[0038] The method for preparing the hollow hierarchical structure mullite ceramic fibers in-situ generated based on the hollow mullite fibers in this example includes the following steps: (1) Mix the silica nanopowder and alumina nanopowder according to a molar ratio of 2:1 to prepare 10wt% solutions respectively, and then mix them to make an impregnating solution; weigh ammonium fluoride according to a molar ratio of Si:F of 2:3 to prepare a 3mol / L solution; (2) Add the mullite fibers in an amount twice the mass of the nanopowder to the above impregnating solution, impregnate under vacuum for 2h, and dry at 60°C; then impregnate in the 3mol / L ammonium fluoride solution under vacuum for 2h and dry at 60°C to obtain the precursor fibers; (3) Heat the above precursor fibers from room temperature to 1300°C at a heating rate of 5°C / min, hold for 2h, and then cool with the furnace to obtain the hollow hierarchical structure mullite ceramic fibers.

[0039] The SEM micrograph of the hollow hierarchical mullite ceramic fibers prepared in this example is as Figure 1 shown. It can be seen that each fiber in the figure has uniformly grown a hierarchical structure. After further magnification, it is observed that the aspect ratio of the hierarchical structure is very high and the hierarchical structure is very dense. Such a structure determines its excellent performance. The XRD phase analysis is as Figure 2 shown. The phase purity of the hierarchical mullite ceramic fibers is very high, being the mullite phase. Example 3

[0040] The method for preparing the hollow hierarchical structure mullite ceramic fibers in-situ generated based on the hollow mullite fibers in this example includes the following steps: (1) Mix the silica nanopowder and alumina nanopowder according to a molar ratio of 3:2 to prepare 5wt% solutions respectively, and then mix them to make an impregnating solution; weigh silicon fluoride according to a molar ratio of Si:F of 2:3 to prepare a 1mol / L solution; (2) Add mullite fibers to the above impregnating solution at 1.5 times the mass of the nanopowder, impregnate under vacuum for 2 h, and dry at 60 °C; then impregnate in 1 mol / L silicon fluoride solution under vacuum for 2 h and dry at 60 °C to obtain precursor fibers. (3) Heat the above precursor fibers from room temperature to 1200 °C at a heating rate of 5 °C / min, hold for 3 h, and then cool in the furnace to obtain the hollow hierarchical structure mullite ceramic fibers. Example 4

[0041] The method for preparing hollow hierarchical structure mullite ceramic fibers based on in-situ generation of hollow mullite fibers in this example includes the following steps: (1) Prepare 15 wt% solutions of silica nanopowder and alumina nanopowder respectively according to a molar ratio of 5:1, mix them to make an impregnating solution; weigh aluminum fluoride according to a molar ratio of Si:F of 1:2 to make a 1 mol / L solution. (2) Add mullite fibers to the above impregnating solution at 1 times the mass of the nanopowder, impregnate under vacuum for 3 h, and dry at 60 °C; then impregnate in 1 mol / L aluminum fluoride solution under vacuum for 2 h and dry at 60 °C to obtain precursor fibers. (3) Heat the above precursor fibers from room temperature to 1250 °C at a heating rate of 5 °C / min, hold for 2 h, and then cool in the furnace to obtain the hollow hierarchical structure mullite ceramic fibers. Example 5

[0042] The method for preparing hollow hierarchical structure mullite ceramic fibers based on in-situ generation of hollow mullite fibers in this example includes the following steps: (1) Prepare 10 wt% solutions of silica nanopowder and alumina nanopowder respectively according to a molar ratio of 2:3, mix them to make an impregnating solution; weigh lithium fluoride according to a molar ratio of Si:F of 2:5 to make a 3 mol / L solution. (2) Add mullite fibers to the above impregnating solution at 0.5 times the mass of the nanopowder, impregnate under vacuum for 4 h, and dry at 60 °C; then impregnate in 1 mol / L aluminum fluoride solution under vacuum for 2 h and dry at 60 °C to obtain precursor fibers. (3) Heat the above precursor fibers from room temperature to 1150 °C at a heating rate of 5 °C / min, hold for 4 h, and then cool in the furnace to obtain the hollow hierarchical structure mullite ceramic fibers.

[0043] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a hollow hierarchical structure mullite ceramic fiber, characterized in that: The following steps are involved: (1) Using silicon dioxide and aluminum oxide nanopowder to prepare the impregnation solution; (2) adding the hollow mullite fiber to the impregnation solution, vacuum impregnating, and drying; and then vacuum impregnating in a fluoride solution, drying, to obtain a precursor fiber; (3) The precursor fiber is calcined and cooled to obtain the hollow graded mullite ceramic fiber.

2. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: In the preparation method, the mass ratio of the hollow mullite fiber to the nano powder of silicon dioxide and aluminum oxide is 0.5-4:

1.

3. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: In the step (2), the fluoride is one or more of aluminum fluoride, ammonium fluoride, lithium fluoride, zirconium fluoride, silicon fluoride and calcium fluoride.

4. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: In the step (2), the hollow mullite fiber is vacuum impregnated in the impregnation solution and the fluoride solution for 2 hours to 6 hours respectively, and then dried at 60° C. to 80° C.

5. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: The concentration of the fluoride solution is 3 mol / L-5 mol / L, and the molar ratio of Si:F is 1:1-6.

6. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: The calcination temperature is 1000-1300° C., the heating rate is 1-10° C. / min, and the heat preservation time is 1-4h.

7. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 1, characterized in that: The preparation method of the hollow mullite fiber comprises the following steps: (1) preparing mullite sol using aluminum silicate sol according to the mullite ratio; (2) Anhydrous ethanol and PVP / PVA / PEG are mixed in a certain proportion to prepare a PVP / PVA / PEG ethanol solution, mullite sol is dissolved in the PVP / PVA / PEG ethanol solution, and a certain amount of DMF / DMSO / DMAc is added to obtain a mullite precursor solution; (3) PVP / PVA / PEG was dissolved in ethanol and water as the spinning liquid, and the mullite precursor solution was used as the spinning liquid, and the mullite fiber was prepared by coaxial electrospinning process; (4) The mullite fiber is calcined at high temperature to obtain hollow porous mullite fiber.

8. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 7, characterized in that: The composition of the mullite precursor solution is as follows: the mass percentage of mullite sol is 5%-10%, the mass percentage of PVP / PVA / PEG ethanol solution is 80%-85%, and the mass percentage of DMF / DMSO / DMAc is 5%-10%; the mass percentage of anhydrous ethanol in the PVP / PVA / PEG ethanol solution is 75%-95%, and the mass percentage of PVP / PVA / PEG is 5%-25%.

9. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 7, characterized in that: The coaxial electrospinning process parameters are: the internal liquid flow rate is 0.05-1 mL / h, the external liquid flow rate is 0.5-2 mL / h, the spinning voltage is 10-20 kV, the solidification distance is 5-16 cm, and the internal liquid flow rate is lower than the external liquid flow rate.

10. The method for preparing the hollow hierarchical structure mullite ceramic fiber according to claim 7, characterized in that: The calcination temperature is 1000-1300° C., the heating rate is 1-10° C. / min, and the heat preservation time is 1-4h.

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