Preparation method of flexible high-entropy fluorite type oxide ceramic aerogel

Abstract

The invention discloses a preparation method of flexible high-entropy fluorite type oxide ceramic aerogel, and belongs to the technical field of advanced ceramic materials. The method comprises the following steps: firstly, selecting at least five metal elements including Zr, Hf, Ce and at least two other rare earth elements, taking oxychlorides or chlorides of the metal elements as initial raw materials, and synthesizing a metal organic polymer precursor through reaction of acetylacetone and triethylamine; dissolving the precursor and a polymer template agent in a solvent to prepare a spinning solution, and preparing a three-dimensional fiber network precursor by adopting a gas-electricity dual-action electrostatic spinning technology; and finally, carrying out programmed heat treatment to obtain the ceramic aerogel. Through combination of molecular-level uniform precursor design and an electrostatic spinning forming process, the problems that a high-entropy system is prone to phase separation and traditional ceramic aerogel is high in brittleness are successfully solved, and the prepared product has the advantages of being of a single fluorite phase structure, uniform in element distribution, good in flexibility and high in thermal stability; wide application prospects are realized in the fields of high-temperature thermal protection, flexible thermal insulation, energy catalysis and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of advanced inorganic non-metallic materials, and particularly relates to a ceramic aerogel and a preparation method thereof, in particular to a high-entropy fluorite oxide nanofiber aerogel with flexibility and a preparation method thereof. BACKGROUND

[0002] As a kind of porous material with three-dimensional nanometer network structure, aerogel has great potential in the fields of thermal protection and energy catalysis due to its extremely low density, high specific surface area and excellent thermal insulation performance. Among them, ceramic aerogel has become a research hotspot for extreme environment application because of its higher thermal stability and chemical stability. However, most traditional ceramic aerogels are constructed by brittle ceramic nanoparticles or lamellas, which have the problems of large intrinsic brittleness, poor flexibility, easy pulverization and the like, which seriously restricts their application in mechanical scenarios such as bending, vibration or impact.

[0003] In recent years, high-entropy ceramics as a new material system have attracted widespread attention. It is formed by four or more main metal elements in equimolar or near equimolar ratio, and exhibits mechanical properties, thermal stability and functional properties beyond traditional ceramics by virtue of its significant high-entropy effect, lattice distortion effect and slow diffusion effect. Fluorite structure (such as CeO2) is an important oxide crystal structure, which is combined with the design concept of high entropy, and is expected to obtain a new type of ceramic material with stable structure and excellent performance.

[0004] At present, the research and development of high-entropy ceramics are mostly concentrated in coatings, powders and dense blocks. For example, high-entropy ceramic coatings or blocks are prepared by thermal spraying or sintering process to improve the wear resistance, corrosion resistance or thermal insulation performance of the substrate. However, it still faces great challenges to prepare high-entropy ceramic aerogel materials with macro three-dimensional structure, especially with flexibility. The main difficulties are as follows: first, element segregation or phase separation is easy to occur in the synthesis and heat treatment process of multi-component system, and it is difficult to obtain a single and uniform solid solution phase; second, the brittleness of traditional ceramic aerogel still exists in high-entropy system, and it is difficult to realize the flexibility; third, there is a lack of effective preparation method which can realize the accurate control of high-entropy ceramic components, the controllable preparation of nanofiber structure and the construction of three-dimensional flexible network at the same time.

[0005] Therefore, it is of great significance to develop a new method for preparing aerogel materials with excellent performance of high-entropy ceramics and macro flexibility, which can promote its application in the fields of flexible thermal management, flexible electronic device thermal insulation, wearable protection and the like. SUMMARY

[0006] In view of the problems that high-entropy ceramic materials are difficult to form macro three-dimensional flexible structure in the prior art, and traditional ceramic aerogels are brittle and have poor flexibility, the present application aims to provide a preparation method of flexible high-entropy fluorite-type oxide ceramic aerogel. The method aims to solve the two key technical problems that phase separation easily occurs in the preparation process of a multi-component system, it is difficult to obtain a uniform single-phase solid solution, and how to convert brittle ceramic materials into a three-dimensional fiber network structure with good flexibility and deformability.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: The core of the preparation method of the flexible high-entropy fluorite-type oxide ceramic aerogel is to combine high-entropy material design and electrospinning fiber construction technology, and the method specifically comprises the following steps: (1) providing a metal organic polymer precursor: the metal organic polymer precursor contains at least five metal elements selected from Zr, Hf, Ce, Er, Tm, Dy, Y and Ho, and must contain Zr, Hf and Ce elements; (2) preparing a spinning solution: the metal organic polymer precursor obtained in step (1), a spinning solvent and a high molecular template agent are mixed in a mass ratio of (1-3):(1-2):(0.003-0.008) to prepare a spinning solution; wherein the spinning solvent is anhydrous ethanol, anhydrous methanol or a mixture of the two; the high molecular template agent is polyvinylpyrrolidone, polyethylene oxide or a mixture of the two; (3) electrospinning: the spinning solution is spun under the action of gas and electricity, and a high-entropy fluorite-type oxide ceramic aerogel precursor is collected; the parameters of the gas-electric dual action are: air flow pressure 5-10 psi, voltage 8-15 KV; (4) heat treatment: the precursor is subjected to heat treatment to obtain the flexible high-entropy fluorite-type oxide ceramic aerogel; the heat treatment temperature is 600-1000℃.

[0008] Preferably, the metal organic polymer precursor is selected from six or more than six of Zr, Hf, Ce, Er, Tm, Dy, Y and Ho. The preparation steps are as follows: (a) metal oxychloride and / or rare earth chloride containing the at least five metal elements are used as starting materials, and are reacted with acetylacetone and triethylamine in anhydrous methanol in a molar ratio of 1:(0.8-1.5):(2.5-3.2), and then are dried under reduced pressure at 40-50℃ to obtain an intermediate product containing triethylamine salt; wherein the molar ratio of the starting materials to anhydrous methanol is 1:(20-80); (b) soaking the intermediate product obtained in step (a) in acetone, filtering out the triethylamine salt after standing for 24-72 h, and drying the filtrate under reduced pressure at 30-40 DEG C to obtain the metal-organic polymer precursor.

[0009] Preferably, in step (3), the electrospinning parameters further include: a jetting speed of the spinning solution of 0.8-1.2 mm / min, a distance between the spinning needle and the collecting device of 35-50 cm, and an ambient humidity of 40-55%; and the collecting device is a roller, a grounded flat plate or a metal mesh cage.

[0010] Preferably, in step (4), the heat treatment process specifically comprises: heating at a heating rate of 0.5-1 DEG C / min to 500 DEG C, holding at 500 DEG C for 30-60 min, then heating at a heating rate of 1-3 DEG C / min to the final heat treatment temperature and holding for 1-2 h, and finally cooling with the furnace.

[0011] Compared with the prior art, the present application has the following remarkable advantages and beneficial effects: 1. Flexibility and macro three-dimensional forming of high-entropy ceramics are realized: the present application creatively combines the high-entropy material system with the electrospinning technology, and successfully prepares a three-dimensional fiber network-like high-entropy ceramic aerogel with self-supporting, bendable and compressible properties, thus breaking through the limitation of high-entropy ceramics which can only be used in the form of coating, powder or brittle bulk.

[0012] 2. Uniformity of components and singleness of phase structure are ensured: the strategy of first synthesizing a metal-organic polymer precursor ensures the uniform mixing of multiple metal elements from the molecular source, effectively inhibits the segregation of elements and the precipitation of second phases during high-temperature treatment, and obtains a pure-phase high-entropy fluorite-type oxide solid solution.

[0013] 3. The product has excellent comprehensive performance: the obtained ceramic aerogel not only inherits the inherent high thermal stability and good chemical stability of high-entropy materials, but also has excellent flexibility and fatigue resistance due to its unique flexible nanofiber network structure, while maintaining low density, high porosity and good thermal insulation performance.

[0014] 4. The process is controllable and has wide application prospect: the preparation method has clear and controllable process parameters, which is conducive to repeated production and large-scale preparation. The flexible high-entropy ceramic aerogel has great application potential in fields requiring lightweight and flexible design, such as flexible thermal protection systems of new-generation aerospace vehicles, thermal insulation and packaging of flexible electronic devices, high-efficiency energy catalyst carriers and special radiation protection clothing, etc. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1XRD pattern and refinement result of high-entropy fluorite oxide ceramic aerogel prepared in Example 1 of the present application.

[0016] Figure 2 SEM image and corresponding elemental mapping of high-entropy fluorite oxide ceramic aerogel prepared in Example 1 of the present application.

[0017] Figure 3 XRD pattern and refinement result of high-entropy fluorite oxide ceramic aerogel prepared in Example 2 of the present application.

[0018] Figure 4 Mechanical property test curve of high-entropy fluorite oxide ceramic aerogel prepared in Example 1 of the present application.

[0019] Figure 5 Comparison of macroscopic optical photographs of high-entropy fluorite oxide ceramic aerogel precursor (left) and ceramic aerogel (right) obtained after heat treatment prepared in Example 3 of the present application.

[0020] Figure 6 XRD pattern of product obtained in Comparative Example 1 (using element combination not meeting the limitation of the present application).

[0021] Figure 7 SEM image of product collected after electrospinning in Comparative Example 2 (using inappropriate polymer template). DETAILED DESCRIPTION

[0022] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and examples. It should be noted that the following examples are only used to explain the present application, but not limit the present application. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0023] Example 1: Preparation of six-element (Zr, Hf, Ce, Lu, Er, Y) high-entropy oxide ceramic aerogel (1) Synthesis of metal-organic polymeric precursor: Accurately weigh 0.01 mol of zirconium oxychloride hexahydrate, hafnium dichloride octahydrate, cerium trichloride heptahydrate, lutetium chloride hexahydrate, erbium chloride hexahydrate, and yttrium chloride hexahydrate, respectively, and dissolve them together in 96 g of anhydrous methanol under magnetic stirring until completely clear. Add 7.2 g of acetylacetone to the above solution and continue stirring for 2 hours. Transfer the mixed solution to an ice water bath for cooling. Slowly add 18.22 g of triethylamine using a constant pressure dropping funnel under vigorous stirring, and control the dropping rate to prevent local overheating. After the addition is completed, remove the ice bath and continue stirring the reaction at room temperature for 4 hours. Transfer the reaction liquid to a rotary evaporator and distill under reduced pressure at a water bath temperature of 50°C to remove most of the solvent and triethylamine hydrochloride salt byproduct volatile components, obtaining a viscous salt-containing precursor mixture.

[0024] (2) Purification of the precursor: Add 240 mL of acetone to the mixture obtained in step 1 and stand at room temperature for 24 hours to allow the triethylamine hydrochloride to fully crystallize and precipitate. Remove the white triethylamine hydrochloride crystals by suction filtration and collect the filtrate. Transfer the filtrate to a rotary evaporator again and dry under reduced pressure at 50°C to finally obtain a light yellow metal-organic polymer precursor solid powder.

[0025] (3) Preparation of the spinning solution and electrospinning: Weigh 10.0 g of the above precursor powder and 0.045 g of polyethylene oxide with a molecular weight of about 1,000,000, and dissolve them together in 12.5 g of anhydrous methanol. Stir at 60°C for 2 hours until completely dissolved, and then cool to room temperature to obtain a uniform and viscous spinning solution. Inject the spinning solution into a 10 mL plastic syringe with a 20G needle. Set the electrospinning parameters: the positive electrode of the high-voltage power supply is connected to the needle, the voltage is 12 kV; use airflow assistance with a pressure of 10 psi; the injection pump injection speed is 2 mm / min; the distance between the needle and the cylindrical rotating metal mesh collector is about 45 cm; and the environmental humidity is controlled at about 50%. Turn on the equipment to spin, and obtain a thick, yellow flocculent fiber mat, i.e., a high-entropy oxide ceramic aerogel precursor, on the collector.

[0026] (4) Heat treatment: Place the collected fiber mat precursor in a forced air drying oven and dry at 50°C for 2 hours to completely remove residual solvents. Transfer the dried precursor fiber mat to a muffle furnace for heat treatment. The heat treatment program is set as follows: increase the temperature from room temperature to 500°C at a rate of 1°C / min, and keep at 500°C for 30 minutes; then continue to increase the temperature to 900°C at a rate of 2°C / min, and keep at 900°C for 60 minutes; finally, turn off the power and let the sample cool to room temperature naturally with the furnace, obtaining a self-supporting flexible high-entropy oxide ceramic aerogel block that maintains the original three-dimensional shape.

[0027] Characterization of the prepared materials: Phase analysis: such as Figure 1 As shown, the XRD pattern, after refinement, perfectly matches the cubic fluorite structure (PDF#34-0394). All diffraction peaks are broadened and there are no impurity phase peaks, proving that a single-phase (Zr, Hf, Ce, Lu, Er, Y) hexa-membered high-entropy oxide solid solution has been successfully synthesized.

[0028] Microscopic morphology and elemental distribution: such as Figure 2 As shown in the SEM images, the aerogel is composed of a large number of uniformly sized, interwoven, continuous nanofibers, forming abundant three-dimensional pores. The corresponding elemental distribution maps show that the six elements Zr, Hf, Ce, Lu, Er, and Y are uniformly distributed on the fibers, without elemental segregation.

[0029] Mechanical properties: such as Figure 4 As shown, the ceramic aerogel block can be bent and folded without breaking, and compression tests show that it has good elasticity and the ability to recover deformation.

[0030] Example 2: Preparation of hexa-component (Zr, Hf, Ce, Lu, Er, Ho) high-entropy oxide ceramic aerogels The only difference between this embodiment and Example 1 is that the "yttrium chloride hexahydrate" in the metal source is replaced with an equimolar amount of "holmium chloride hexahydrate (HoCl3·6H2O)". All other steps and parameters are exactly the same.

[0031] Characterization results: The XRD pattern of the obtained ceramic aerogel is as follows Figure 3 As shown, it also exhibits pure fluorite-type diffraction peaks, proving that replacing Y with Ho can still form a stable single-phase high-entropy oxide solid solution.

[0032] Example 3: Preparation of hexa-component (Zr, Hf, Ce, Lu, Dy, Tm) high-entropy oxide ceramic aerogels The difference between this embodiment and Example 1 lies in the combination of metal elements. 0.01 mol each of zirconium oxychloride hexahydrate, hafnium dichloride octahydrate, cerium chloride heptahydrate, lutetium chloride hexahydrate, dysprosium chloride hexahydrate, and thulium chloride hexahydrate were weighed as starting materials to replace the corresponding components in Example 1. All other steps and parameters were exactly the same as in Example 1.

[0033] Characterization results: Self-supporting flexible ceramic aerogel blocks were successfully obtained. Macroscopic optical images before and after heat treatment are shown below. Figure 5 As shown, the shape remains intact. XRD analysis indicates that the product is a single fluorite phase.

[0034] Example 4: Preparation of six-element (Zr, Hf, Ce, Er, Tm, Ho) high-entropy oxide ceramic aerogel The difference between this example and Example 1 is the combination of metal elements. Zirconium oxychloride hexahydrate, hafnium dichloride octahydrate, cerium chloride heptahydrate, erbium chloride hexahydrate, thulium chloride hexahydrate, and holmium chloride hexahydrate were weighed at 0.01 mol each as starting materials. Other steps and parameters are exactly the same as Example 1.

[0035] Characterization results: self-supporting flexible ceramic aerogel blocks were successfully obtained, and XRD tests showed that the product was a single fluorite phase.

[0036] Example 5: Preparation of six-element (Zr, Hf, Ce, Tm, Y, Dy) high-entropy oxide ceramic aerogel The difference between this example and Example 1 is the combination of metal elements. Zirconium oxychloride hexahydrate, hafnium dichloride octahydrate, cerium chloride heptahydrate, thulium chloride hexahydrate, yttrium chloride hexahydrate, and dysprosium chloride hexahydrate were weighed at 0.01 mol each as starting materials. Other steps and parameters are exactly the same as Example 1.

[0037] Characterization results: self-supporting flexible ceramic aerogel blocks were successfully obtained, and XRD tests showed that the product was a single fluorite phase.

[0038] Example 6: Preparation of six-element (Zr, Hf, Ce, Er, Dy, Tm) high-entropy oxide ceramic aerogel This example demonstrates the preparation of a five-element system. Zirconium oxychloride hexahydrate, hafnium dichloride octahydrate, cerium chloride heptahydrate, erbium chloride hexahydrate, dysprosium chloride hexahydrate, and thulium chloride hexahydrate were weighed at 0.01 mol each as starting materials. Other steps and parameters are exactly the same as Example 1 Characterization results: self-supporting flexible ceramic aerogel blocks were successfully obtained, and XRD tests showed that the product was a single fluorite phase.

[0039] Comparative Example 1: Comparison of non-optimal element combination This comparative example aims to illustrate the importance of element selection. The metal source was replaced with a mixture of hafnium oxychloride, zirconium oxychloride, titanium tetrachloride, lanthanum trichloride, and yttrium trichloride (i.e. Ti and La were introduced, and Ce was removed), and a five-element system was attempted to be prepared. The precursor synthesis and subsequent spinning and heat treatment steps refer to Example 1.

[0040] Characterization results: As shown in Figure 6 the XRD pattern of the obtained product showed obvious impurity diffraction peaks near the main peaks of the fluorite phase, indicating that this element combination could not form a single high-entropy fluorite phase solid solution under the process, and phase separation occurred.

[0041] Comparative Example 2: Comparison of non-preferred high molecular template This comparative example is intended to illustrate the criticality of the selection of high molecular template. The high molecular template PEO in Example 1 was replaced by an equal amount of polyvinylpyrrolidone (PVP, molecular weight 1,300,000), with other conditions unchanged.

[0042] Characterization results: The electrospinning process was unstable and could not form continuous fibers. As shown in FIG. 2B, only a large number of beads and very short discontinuous fiber fragments were obtained on the collector, and a three-dimensional fiber network aerogel structure could not be constructed. Figure 7

[0043] The above examples and comparative examples fully illustrate that the element system, precursor synthesis method, spinning solution formula and process parameters provided by the present application are necessary and effective for the successful preparation of flexible, single-phase high-entropy fluorite-type oxide ceramic aerogels. Those skilled in the art can make appropriate adjustments within the scope of the technical ideas and parameters disclosed in the present application, and these adjustments should be included in the protection scope of the present application.​

Claims

1. A method for preparing a flexible high-entropy fluorite-oxide ceramic aerogel, characterized in that, The method comprises the following steps: (1) providing a metal-organic polymer precursor: the metal-organic polymer precursor comprises at least five metal elements selected from Zr, Hf, Ce, Er, Tm, Dy, Y and Ho, and must comprise Zr, Hf and Ce elements; (2) preparing a spinning solution: the metal-organic polymer precursor obtained in step (1), a spinning solvent and a polymer template agent are mixed in a mass ratio of (1-3):(1-2):(0.003-0.008) to prepare a spinning solution; wherein the spinning solvent is anhydrous ethanol, anhydrous methanol or a mixture of the two; the polymer template agent is polyvinylpyrrolidone, polyethylene oxide or a mixture of the two; (3) electrospinning: the spinning solution is spun under the action of gas and electricity, and a high-entropy fluorite-type oxide ceramic aerogel precursor is collected; the parameters of the action of gas and electricity are as follows: gas flow pressure is 5-10 psi, and voltage is 8-15 KV; (4) heat treatment: the precursor is subjected to heat treatment, and the flexible high-entropy fluorite-type oxide ceramic aerogel is obtained; the heat treatment temperature is 600-1000℃.

2. The production method according to claim 1, characterized by, The metal-organic polymer precursor comprises six or more than six metal elements selected from Zr, Hf, Ce, Er, Tm, Dy, Y and Ho.

3. The preparation method according to claim 1, characterized in that, In step (1), the metal-organic polymer precursor is prepared by a method comprising the following steps: (a) metal oxychloride and / or rare earth chloride containing the at least five metal elements are used as starting materials, and are reacted with acetylacetone and triethylamine in anhydrous methanol in a molar ratio of 1:(0.8-1.5):(2.2-3.2), and then are dried under reduced pressure at 40-50℃ to obtain an intermediate product containing triethylamine salt; wherein the molar ratio of the starting materials to anhydrous methanol is 1:(2-10); (b) the intermediate product obtained in step (a) is soaked in acetone, and after standing for 24-72 h, the triethylamine salt is removed by filtration, and the filtrate is dried under reduced pressure at 30-40℃ to obtain the metal-organic polymer precursor.

4. The production method according to claim 3, characterized by, In step (b), 3000-5000 mL of acetone is used per mole of the starting material for soaking.

5. The method of claim 1, wherein, In step (2), the spinning solvent is anhydrous methanol, and the polymer template agent is polyethylene oxide.

6. The method of claim 1, wherein, In step (3), the parameters of the electrospinning further include: the injection speed of the spinning solution is 0.8-2.5 mm / min, the distance between the spinning needle and the collecting device is 35-50 cm, and the environmental humidity is 40-55%; the collecting device is a roller, a grounded flat plate or a metal mesh cage.

7. The preparation method according to claim 1, characterized in that, In step (4), the heat treatment process specifically comprises: heating at a heating rate of 0.5-1℃ / min to 500℃, holding at 500℃ for 30-60 min; then heating at a heating rate of 1-3℃ / min to the final heat treatment temperature and holding for 1-2 h, and finally cooling with the furnace.

8. A flexible high-entropy fluorite-oxide ceramic aerogel, characterized in that, The flexible high-entropy fluorite-type oxide ceramic aerogel is prepared by the preparation method of any one of claims 1-7.

9. The flexible high-entropy fluorite-oxide aerogel of claim 8, wherein, The basic structural unit of the flexible high-entropy fluorite-type oxide ceramic aerogel is a one-dimensional nanofiber with uniform diameter, and each metal element is uniformly distributed in the fiber.

10. Use of the flexible high-entropy fluorite-oxide ceramic aerogel according to claim 8 or 9 in the field of high-temperature thermal protection, energy catalysis or radiation protection.

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