A hafnium oxide inorganic fiber and a method for producing the same

Abstract

The present application relates to a kind of hafnium oxide inorganic fibers and its preparation method, the crystal phase of the described the hafnium oxide inorganic fiber is monoclinic phase, fiber diameter is 0.2 μm~7 μm, and keep fiber form integrity and crystal phase stability at high temperature 1500 ℃.Preparation method is with inorganic hafnium salt as hafnium source, organic ligand is used as hafnium ion stabilizer, by organic amine to remove hafnium salt anion, obtain hafnium oxide precursor, again with high molecular polymer as spinning aid, by electrostatic spinning technology or centrifugal fiber forming technology obtain hafnium oxide precursor fiber, precursor fiber is obtained hafnium oxide inorganic fiber after heat treatment.The present application has the advantages such as simple process, good sol stability and spinnability, high hafnium oxide solid content, low cost, no environmental pollution in heat treatment and the like.The prepared inorganic fiber crystal phase and microstructure are stable, fiber form is complete under high temperature condition, and the present application has application prospect in the fields such as high temperature gas filtration, high temperature heat insulation and the like.

Description

Technical Field

[0001] This invention relates to a hafnium oxide inorganic fiber and its preparation method, belonging to the field of inorganic non-metallic materials. Background Technology

[0002] Hafnium oxide, with its ultra-high melting point (~2900℃), low thermal conductivity, good chemical stability, and antioxidant properties, shows significant application prospects in ultra-high temperature thermal protection and supersonic thermal protection. However, dense hafnium oxide inorganic materials suffer from drawbacks such as high density and high cost. Among various material structures, one-dimensional ceramic fiber structures not only possess advantages such as high temperature resistance, low thermal conductivity, and thermal shock resistance, but also feature lightweight and low bulk density. Therefore, combining hafnium oxide with fiber structures can overcome the inherent high density of hafnium oxide and improve its utilization rate, giving it a more significant advantage in ultra-high temperature thermal protection.

[0003] The application of hafnium oxide inorganic fibers mainly depends on the economic efficiency of the preparation technology and their stability during use. In recent years, researchers have conducted relevant research on the preparation technology of hafnium oxide inorganic fibers. Due to the high melting point of hafnium oxide, its preparation technology mainly involves the preparation of hafnium oxide precursors and high-temperature heat treatment. Therefore, the economic efficiency and spinnability of the precursor sol, the hafnium oxide solid content in the precursor fiber, and the ease or difficulty of the preparation process are key factors affecting the engineering development and application of hafnium oxide fibers. In 2020, patent CN112010656A provided a method for preparing hafnium oxide fibers. This method uses hafnium acetylacetonate, hafnium dichlorocerocene, etc. as hafnium sources, polymers as spinning aids, and toluene, xylene, etc. as solvents to prepare hafnium oxide fibers by electrospinning. This method is simple and easy to implement, but it uses a large amount of organic solvents with certain toxicity and expensive hafnium sources. In addition, some hafnium sources will produce corrosive or toxic gases during heat treatment. This problem needs to be further solved in engineering preparation applications. In 2021, an article titled "Effect of calcination temperatures on HfO2 fibers via electrospinning" was published. Hafnium oxide precursor fibers were prepared by electrospinning using hafnium acetylacetonate (Hf(acac)4) as the hafnium source, polyvinylpyrrolidone (PVP) as the spinning aid, and DMF and CHCl3 as solvents. Hafnium oxide fibers with good thermal shock resistance were obtained by high-temperature heat treatment. The yield of hafnium oxide ceramics was 50.38 wt%, and a phase transformation from monoclinic to cubic phase occurred at 1351 °C. This method effectively avoids the environmental pollution problem caused by the thermal decomposition of hafnium source. In 2022, patent CN114804860A disclosed a hafnium oxide ceramic nanofiber and its preparation method. This method uses hafnium tetrachloride, hafnium sulfate and other non-spinnable hafnium sources as hafnium sources and polymers as spinning aids to prepare hafnium oxide fibers by electrospinning. The preparation process is simple and low cost, but the low spinnability of the hafnium source leads to the use of a large amount of spinning aids. At the same time, some hafnium sources also have environmental pollution problems in the post-processing process that have not been effectively solved.In 2022, an article titled "High strength, low thermal conductivity and collapsible of Y₂O₃-stablized HfO₂ crystalline fibrous membranes" was published, describing the preparation of yttrium-stabilized hafnium oxide fibers using hafnium oxychloride octahydrate as the hafnium source, yttrium chloride as the yttrium source and crystal phase stabilizer for hafnium oxide, acetylacetone as the hafnium ion ligand stabilizer, triethylamine as the chloride ion precipitant, and acetone as the hafnium extractant. The precursor exhibited high spinnability and improved the ceramic yield of hafnium oxide to 62.0%. Yttrium-stabilized tetragonal phase hafnium oxide fibers stable at 1200℃ were also obtained. However, the precursor preparation process was relatively complex. Furthermore, the microstructural stability and crystal phase stability of hafnium oxide fibers are key factors limiting their use at high temperatures. The stability of the microstructure, the reduction of grain size, and the increase of the formation temperature of "bamboo-like" hafnium oxide fibers depend on the stability of the microstructure of the hafnium oxide fibers. Regarding the stability of the crystal phase, although yttrium-stabilized hafnium oxide can obtain a stable tetragonal crystal phase, the operating temperature and time of this crystal phase are limited. High temperature and long-term use will inevitably lead to the release of the crystal phase stabilizer in the tetragonal hafnium oxide fibers, resulting in a phase transition and reducing the high-temperature performance of the hafnium oxide fibers. In contrast, the monoclinic hafnium oxide itself has a phase transition temperature as high as 1700℃. Therefore, obtaining high-temperature stable monoclinic hafnium oxide can give full play to the advantages of the hafnium oxide crystal phase itself. As can be seen from the above methods and processes for preparing hafnium oxide fibers, the current methods still have problems in simultaneously satisfying the requirements of precursor sol spinnability, precursor fiber solid content, precursor preparation process, post-processing environmental pollution, and high-temperature crystal phase stability. Further optimization is needed to simplify the preparation process, reduce the cost of hafnium oxide fiber preparation, improve sol spinnability, reduce environmental pollution, and improve the high-temperature crystal phase and microstructure stability of the fiber, thus providing technical support for the industrial application of hafnium oxide fibers.

[0004] Therefore, in order to optimize the preparation of hafnium oxide precursor sol and precursor fiber, improve the spinnability of the sol, reduce the content of toxic organic solvents used in the sol, increase the solid content of hafnium oxide, avoid the use of environmentally polluting anions, simplify the preparation process, obtain monoclinic phase high-temperature stable hafnium oxide fiber, and promote the industrial application of hafnium oxide fiber, this invention is proposed. Summary of the Invention

[0005] To address the shortcomings of existing technologies, such as sol spinnability, low-toxicity organic solvents, solid content, environmental pollution from anions, and preparation processes, this invention provides a high-temperature stable monoclinic hafnium oxide preparation technology, offering a new approach for the engineering application of hafnium oxide fiber preparation. Invention Overview

[0007] This invention utilizes the coordination of hafnium ions with carbonyl and carboxyl groups to prepare a high-solids-content precursor sol free of chloride ions via a one-step precipitation method. Hafnium oxide precursor fibers are then prepared by electrospinning or centrifugal spinning, and the precursor fibers are heat-treated to obtain monoclinic hafnium oxide fibers. Compared with existing methods for preparing hafnium oxide precursor sols, this method uses inexpensive raw materials, has a simple process, and produces precursor fibers with high solids content that are free of environmentally polluting anions. Secondly, the hafnium oxide fibers prepared by this invention have a monoclinic crystal phase. Compared with yttrium-stabilized hafnium oxide, the single-phase hafnium oxide can maintain its crystal phase stability at a high temperature of 1500℃, avoiding phase transformation and structural deterioration caused by the precipitation of stabilizers. Finally, the monoclinic hafnium oxide fibers prepared by this invention can maintain their morphological integrity at 1500℃, ensuring the stability of hafnium oxide fibers under high-temperature conditions. Invention Details

[0009] The technical solution of the present invention is as follows:

[0010] According to the present invention, a method for preparing monoclinic hafnium oxide fiber includes the following steps:

[0011] (1) Preparation of hafnium oxide precursor

[0012] (a) A hafnium source ethanol solution is obtained by dissolving the hafnium source in anhydrous ethanol at a mass ratio of hafnium source: anhydrous ethanol = 1:1 to 50.

[0013] (b) Add the organic ligand to the solution obtained in step (a) at a molar ratio of organic ligand:Hf = 1:0.5 to 1.5 to obtain an organic ligand hafnium ethanol solution;

[0014] (c) According to the molar ratio of organic amines:Cl - Anhydrous ethanol = 1:1 to 3:1 to 50, organic amine is dissolved in anhydrous ethanol to obtain an organic amine ethanol solution;

[0015] (d) The organic amine ethanol solution obtained in step (c) is slowly added dropwise to the organic ligand hafnium ethanol solution obtained in step (b) while the solution is stirred; after the addition is complete, the precipitate is filtered to obtain the hafnium oxide precursor solution.

[0016] (e) The hafnium oxide precursor solution obtained in step (d) is evaporated to dryness by vacuum distillation to obtain the hafnium oxide precursor;

[0017] (2) Preparation of hafnium oxide precursor fibers

[0018] Hafnium oxide precursor fiber preparation by electrospinning: The hafnium oxide precursor obtained in step (1) is dissolved in the solvent by stirring at a mass ratio of hafnium oxide precursor: solvent = 1:2~6, and then 0.5%~10% of spinning aid is added to obtain precursor sol. Hafnium oxide precursor fiber is obtained by electrospinning.

[0019] Preparation of hafnium oxide precursor fibers by centrifugal spinning: The hafnium oxide precursor obtained in step (1) is dissolved in the solvent by stirring at a mass ratio of hafnium oxide precursor: solvent = 1:2~50. Then, a spinning aid with a mass fraction of 0.5%~10% is added, and a spinnable precursor sol with a viscosity of 1~30 Pa·s is obtained by vacuum distillation. Hafnium oxide precursor fibers are obtained by centrifugal spinning technology.

[0020] (3) Preparation of hafnium oxide fibers

[0021] The hafnium oxide precursor fiber obtained in step (2) is heat-treated under atmospheric conditions to obtain hafnium oxide inorganic fiber.

[0022] According to a preferred embodiment of the present invention, the mass ratio of hafnium source to anhydrous ethanol in step (a) is 1:5 to 20;

[0023] According to a preferred embodiment of the present invention, the molar ratio in step (b) is organic ligand:Hf = 1:0.6 to 1.0;

[0024] According to a preferred embodiment of the present invention, the organic amine in step (c) is ethylenediamine, and the preferred molar ratio is organic amine:Cl. - Anhydrous ethanol = 1:(2-2.2):(5-20).

[0025] According to a preferred embodiment of the present invention, the solvent in step (2) is ethanol, water, or a combination thereof.

[0026] According to a preferred embodiment of the present invention, the spinning aid in step (2) is one or a combination of polyvinylpyrrolidone, polyethylene oxide, and polyvinyl butyral.

[0027] According to a preferred embodiment of the present invention, the mass fraction of the spinning aid in step (2) is 1% to 5%.

[0028] According to a preferred embodiment of the present invention, the process conditions for the electrospinning method in step (b) are as follows: sol injection rate of 0.6-1.5 mL / h, spinning voltage of 12-20 kV, fiber receiving distance of 10-30 cm, spinning ambient temperature of 20-45 °C, and spinning ambient humidity of 20-55%.

[0029] According to a preferred embodiment of the present invention, the process conditions for the centrifugal spinning method in step (2) are as follows: the spinning environment temperature is 25-60℃, the spinning environment humidity is 25-40%, the centrifuge speed is 15000-24000r / min, the spinning hole linear velocity is 25-45m / s, and the spinning hole diameter is 0.2-0.4mm.

[0030] According to a preferred embodiment of the present invention, the atmosphere described in step (3) is an air atmosphere.

[0031] According to a preferred embodiment of the present invention, the heat treatment process in step (3) is as follows: heating to 400-600°C at a heating rate of 1-5°C / min and holding for 60-120 min; then heating to 800-1500°C at a heating rate of 2-5°C / min and holding for 1-3 h.

[0032] The superior effects of this invention are as follows:

[0033] 1. The raw materials used in this invention are inexpensive and readily available, the process is simple and the cost is low, and it can achieve the preparation of highly stable, high-viscosity, and environmentally friendly anionic hafnium oxide precursor sol.

[0034] 2. The hafnium oxide precursor fiber prepared by this invention has low content of organic ligands and spinning aids, and high solid content of hafnium oxide, which can realize the preparation of high solid content hafnium oxide precursor fiber.

[0035] 3. The hafnium oxide fiber precursor fiber prepared by the present invention does not generate environmentally polluting gases during the heat treatment process. The prepared hafnium oxide can maintain the integrity of the fiber morphology at 1500℃, and the crystal phase is monoclinic, which has good high-temperature crystal phase stability and fiber structure stability. Attached Figure Description

[0036] Figure 1 This is the TG diagram of the hafnium oxide precursor fiber obtained in Example 1.

[0037] Figure 2 This is a SEM image of the hafnium oxide fiber obtained in Example 2 after heat treatment to 1400°C.

[0038] Figure 3 This is the XRD pattern of the hafnium oxide fiber obtained in Example 3 after heat treatment to 1400°C. Detailed Implementation

[0039] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0040] All raw materials used in the examples were commercially available.

[0041] Example 1:

[0042] A method for preparing hafnium oxide fiber includes the following steps:

[0043] (a) Hafnium tetrachloride was dissolved in 32.030 g of anhydrous ethanol at a mass ratio of hafnium tetrachloride: anhydrous ethanol = 1:10 to obtain a hafnium tetrachloride ethanol solution. Acetylacetone was added to the hafnium tetrachloride ethanol solution at a molar ratio of acetylacetone: Hf = 1:1 to obtain an acetylacetone-hafnium ethanol solution. Ethylenediamine was added to the hafnium tetrachloride ethanol solution at a molar ratio of ethylenediamine: Cl... - Anhydrous ethanol = 1:2:10. Dissolve the organic amine in anhydrous ethanol to obtain an ethylenediamine ethanol solution; add the ethylenediamine ethanol solution dropwise to an acetylacetone hafnium ethanol solution, filter to remove the precipitate, and evaporate the clear, transparent solution to dryness under low pressure to obtain the acetylacetone hafnium precursor. Figure 1 The thermogravimetric analysis (TGA) of hafnium acetylacetonate precursor shows a weight loss of 34.2%, indicating the production of hafnium oxide precursor fibers with high solids content.

[0044] (b) Dissolve 1.000 g of hafnium oxide precursor in 3.000 g of anhydrous ethanol, add 0.01 g of polyethylene oxide, and obtain hafnium oxide precursor fiber by electrospinning. The electrospinning process conditions are: voltage 16 kV, spinning distance 16 cm, injection rate 1.2 mL / h, ambient temperature 25-35℃, and ambient humidity 35%-45%.

[0045] (c) The hafnium oxide precursor fiber obtained in step (b) is heated to 600°C at a heating rate of 1°C / min under air conditions and held for 30 min; then heated to 800°C at a heating rate of 2°C / min and held for 2 h to obtain monoclinic hafnium oxide fiber.

[0046] Example 2:

[0047] As described in Example 1, except that hafnium tetrachloride in step (a) was replaced with hafnium oxychloride, and the SEM image of the obtained hafnium oxide fiber after heat treatment to 1400°C is shown below. Figure 2 As shown.

[0048] Example 3:

[0049] As described in Example 1, the difference is that in step (a), hafnium tetrachloride is added along with hafnium hydroxide in a molar ratio of hafnium tetrachloride to hafnium hydroxide = 1:0.5. The XRD pattern of the obtained hafnium oxide fiber after heat treatment to 1400°C is shown below. Figure 3 As shown.

[0050] Example 4:

[0051] As described in Example 1, the difference is that in step (a), the sol after precipitation and filtration is concentrated under reduced pressure at 60°C to obtain a precursor sol with a viscosity of 10-20 Pa·s; in step (b), the electrospinning method is replaced by centrifugal spinning to obtain hafnium oxide precursor fibers. The process conditions for centrifugal spinning are: spinning ambient temperature of 25°C, spinning ambient humidity of 35-40%, centrifuge speed of 16000 r / min, and spinning aperture of 0.3 mm.

[0052] Example 5:

[0053] As described in Example 1, except that acetylacetone in step (a) is replaced with ethyl acetoacetate.

[0054] Example 6:

[0055] As described in Example 1, except that in step (b) the polyethylene oxide is replaced with polyvinylpyrrolidone.

Claims

1. A monoclinic hafnium oxide inorganic fiber, characterized in that, The hafnium oxide inorganic fiber has a monoclinic crystal phase, a fiber diameter of 0.2 μm to 7 μm, and maintains fiber morphology integrity and crystal phase stability at a high temperature of 1500℃. The hafnium oxide inorganic fiber is prepared by the following steps: (1) Preparation of hafnium oxide precursor (a) A hafnium source ethanol solution is obtained by dissolving the hafnium source in anhydrous ethanol at a mass ratio of hafnium source: anhydrous ethanol = 1:1 to 50. (b) Add the organic ligand to the solution obtained in step (a) at a molar ratio of organic ligand:Hf = 1:0.5 to 1.5 to obtain an organic ligand hafnium ethanol solution; (c) According to the molar ratio of organic amines:Cl - Anhydrous ethanol = 1:(1-3):(1-50), organic amine is dissolved in anhydrous ethanol to obtain an organic amine ethanol solution; (d) The organic amine ethanol solution obtained in step (c) is slowly added dropwise to the organic ligand hafnium ethanol solution obtained in step (b) while the solution is stirred; after the addition is complete, the precipitate is filtered to obtain the hafnium oxide precursor solution. (e) The hafnium oxide precursor solution obtained in step (d) is evaporated to dryness by vacuum distillation to obtain the hafnium oxide precursor; (2) Preparation of hafnium oxide precursor fibers Preparation of hafnium oxide precursor fibers by electrospinning: The hafnium oxide precursor obtained in step (1) is dissolved in the solvent by stirring at a mass ratio of hafnium oxide precursor: solvent = 1:2~6. Then, a spinning aid with a mass fraction of 0.5%~10% is added to obtain a precursor sol. Hafnium oxide precursor fibers are obtained by electrospinning. Preparation of hafnium oxide precursor fibers by centrifugal spinning: The hafnium oxide precursor obtained in step (1) is dissolved in the solvent by stirring at a mass ratio of hafnium oxide precursor: solvent = 1:2~50. Then, a spinning aid with a mass fraction of 0.5%~10% is added. Then, a spinnable precursor sol with a viscosity of 1~30 Pa·s is obtained by vacuum distillation. Hafnium oxide precursor fibers are obtained by centrifugal spinning technology. (3) Preparation of hafnium oxide inorganic fibers The hafnium oxide precursor fiber obtained in step (2) is heat-treated under atmospheric conditions to obtain hafnium oxide inorganic fiber; The hafnium oxide inorganic fiber is characterized in that, in step (a), the hafnium source is hafnium oxychloride octahydrate, hafnium tetrachloride, or hafnium tetrachloride and hafnium hydroxide; in step (b), the organic ligand is one or a combination of formic acid, acetic acid, propionic acid, tartaric acid, citric acid, acetylacetone, methyl acetoacetate, ethyl acetoacetate, propyl acetoacetate, isopropyl acetoacetate, and butyl acetoacetate; and in step (c), the organic amine is ethylenediamine.

2. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The solvent mentioned in step (2) is one or a combination of water, methanol, ethanol, N,N-dimethylformamide, and dichloromethane.

3. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The spinning aid mentioned in step (2) is one or a combination of polyvinylpyrrolidone, polyethylene oxide, polyvinyl alcohol, polyacrylonitrile, and polyvinyl butyral.

4. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The process conditions for electrospinning in step (2) are as follows: sol injection rate is 0.3-3.5 mL / h, spinning voltage is 4-50 kV, fiber receiving distance is 5-45 cm, spinning ambient temperature is 5-65℃, and spinning ambient humidity is 10-65%.

5. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The process conditions for centrifugal spinning in step (2) are as follows: the spinning environment temperature is 10-80℃, the spinning environment humidity is 10-70%, the centrifuge speed is 10000-35000 r / min, and the spinning hole diameter is 0.10-0.60 mm.

6. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The atmosphere described in step (3) is one or a combination of air, nitrogen, water vapor, rare gas, and ammonia.

7. The hafnium oxide inorganic fiber as described in claim 1, characterized in that... The heat treatment process described in step (3) is as follows: the temperature is raised to 400-600℃ at a heating rate of 0.5-5℃ / min and held for 30-120min; then the temperature is raised to 800-1500℃ at a heating rate of 2-10℃ / min and held for 1-5h.

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