A method for preparing carbon-coated zirconia-based ceramic fibers

Abstract

This invention relates to the field of ceramic fibers and aims to provide a method for preparing carbon-coated zirconia-based ceramic fibers. The method includes: preparing a zirconia-based precursor solution, stirring it evenly, adding an appropriate amount of polymer to obtain a spinning solution, and electrospinning to obtain a zirconia-based ceramic fiber precursor film; drying the precursor film, and then stretching it at a temperature higher than the carbon pyrolysis temperature of the polymer, causing the pyrolytic carbon inside the fiber to overflow through micropores on the fiber surface and oriented under radial force, thus forming a continuous and uniform carbon layer in situ on the fiber surface; calcining the stretched precursor film to obtain carbon-coated zirconia-based ceramic fibers. This invention generates a high-modulus shell layer in situ on the surface of the ceramic fiber, achieving a strong and tough bond between the fiber matrix and the high-modulus shell layer; while retaining the original excellent high-temperature resistance of the fiber, it greatly improves the resistance to brittle fracture and fatigue under high stress conditions.

Description

Technical Field

[0001] This invention belongs to the field of ceramic fibers, and specifically relates to a method for preparing carbon-coated zirconia-based ceramic fibers. Background Technology

[0002] Ceramic fibers, with their excellent high-temperature resistance, extremely low thermal conductivity, and good chemical stability, have become the preferred thermal protection material for extreme environments in recent years. Furthermore, the low density and ease of fabrication of ceramic fibers have led to their widespread application in supersonic aircraft and deep-sea submersibles, replacing traditional high-density, low-efficiency thermal insulation tiles. Among the many types of ceramic fibers, oxide ceramic fibers have attracted widespread attention due to their outstanding thermal insulation performance and good mechanical properties. Typical oxide ceramic fibers include silicon oxide, magnesium oxide, and zirconium oxide, all of which are prepared using electrospinning technology to obtain nanoscale oxide ceramic fibers.

[0003] High-temperature resistant ceramic fibers, when used in high-temperature protection and other fields, often require materials that possess both high-temperature resistance and good mechanical properties to withstand the thermal shock caused by drastic temperature changes in extreme environments, thereby extending the service life of thermal protection materials. However, ceramic fibers inherently suffer from susceptibility to cracking and brittle fracture under stress, which significantly limits their application in high-temperature fields. This phenomenon arises because during the heat treatment process, the volatilization of polymers within the fiber and abnormal increases in grain size create defects on the fiber surface. These defects become stress concentration points, leading to the brittle fracture and poor fatigue resistance of the ceramic fibers.

[0004] To address this defect in ceramic fibers, researchers have focused on controlling the fiber composition and designing the macroscopic structure. The former primarily involves doping with a second phase to suppress grain growth at high temperatures, while the latter optimizes the load-bearing capacity of the ceramic fiber by effectively dispersing stress within the fiber membrane. For example, by introducing elements such as Al and Si into the doping process, they are evenly distributed around the fiber. As the temperature rises, the doped phase effectively pins the grain boundaries, creating a reverse pulling force on grain growth and inhibiting the growth of fiber grain size to some extent. However, doping with a second phase also presents the challenge of controlling the doping ratio. If the amount of the second phase is too small, it cannot be evenly distributed around the grains; if the amount is too large, it will cause the second phase to accumulate, forming new defects on the fiber surface. Regarding the control of the fiber's own structure, with the continuous development of electrospinning technology, ceramic fibers have evolved from the original single continuous fibers to ceramic fibers with hollow, core-shell, and other structures. While hollow structures further reduce the thermal conductivity of fibers, they do not effectively improve their mechanical properties. Core-shell fibers, on the other hand, are produced by simultaneously spinning two or more precursor solutions using coaxial needles, resulting in a core-shell coating structure during the fiber formation process. To obtain a uniform coating structure, composite ceramic fibers with a high-temperature resistant outer shell and a flexible core are prepared by controlling the spinning rate of the precursor solution. This strong and tough design improves the mechanical properties of ceramic fibers to some extent. However, during electrospinning, the different inner and outer shells lead to inconsistent spinnability conditions in the precursor solution. This results in only a small portion of the obtained ceramic fibers achieving a composite structure with inner and outer shells; most fibers do not achieve uniform coating of the outer shell. This significantly reduces the overall mechanical properties of the fiber membrane. Furthermore, the complex preparation of precursor solutions and the stringent requirements for electrospinning of core-shell ceramic fibers limit its applicability as a universally applicable method for improving the mechanical properties of ceramic fibers.

[0005] Therefore, there is an urgent need to develop a simple, efficient, and widely applicable method for improving the mechanical properties of ceramic fibers. This method would enable the in-situ formation of a high-modulus shell on the surface of the ceramic fibers, achieving a strong and tough bond between the fiber matrix and the high-modulus shell. While retaining the original excellent high-temperature resistance of the fibers, this method would greatly enhance the resistance to brittle fracture and fatigue under high stress conditions, making it a robust and reliable high-temperature thermal protection material. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing carbon-coated zirconia-based ceramic fibers.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] A method for preparing carbon-coated zirconia-based ceramic fibers is provided, comprising the following steps:

[0009] (1) Prepare a zirconia-based precursor solution, stir it evenly, add an appropriate amount of polymer to prepare a spinning solution, and obtain a zirconia-based ceramic fiber precursor membrane by electrospinning.

[0010] (2) After drying the precursor film, the precursor film is stretched under conditions higher than the carbon pyrolysis temperature of the polymer, so that the pyrolytic carbon inside the fiber overflows through the micropores on the fiber surface and is oriented under the action of radial force, and a continuous and uniform carbon layer is generated in situ on the fiber surface.

[0011] (3) The precursor film after hot stretching is calcined to obtain carbon-coated zirconia-based ceramic fibers.

[0012] As a preferred embodiment of the present invention, the molar ratio of zirconium, aluminum and yttrium in the precursor solution is 20:3~5:1~3.

[0013] As a preferred embodiment of the present invention, the polymer is any one of polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide, and its mass percentage in the spinning solution is 2-5 wt%.

[0014] As a preferred embodiment of the present invention, the process conditions for controlling electrospinning are: ambient temperature 20-30℃, ambient humidity 20-30%, working voltage 12-18 KV, and spinning speed 1-1.5 ml / h.

[0015] As a preferred embodiment of the present invention, the drying treatment of the zirconia-based ceramic fiber precursor film refers to placing it in a vacuum oven and drying it at 60°C for 1 hour.

[0016] As a preferred embodiment of the present invention, the hot stretching treatment of the ceramic fiber precursor membrane refers to: suspending the precursor membrane in an oven and applying a radial stretching force at the bottom; setting the oven temperature to 150~300℃ and the radial stretching force to 0.5~1.5N; maintaining the hot stretching for 2~8 hours and then naturally cooling to room temperature.

[0017] As a preferred embodiment of the present invention, the calcination process includes: transferring the ceramic fiber precursor membrane into a muffle furnace, heating it to 700~1400℃ at 5℃ / min, holding it at that temperature for 2~4h; then cooling it to 200℃ at 5℃ / min, and cooling it to room temperature with the furnace.

[0018] As a preferred embodiment of the present invention, the method for preparing the zirconium oxide precursor solution is as follows: zirconium acetate, as a zirconium source, is added to deionized water and stirred evenly; aluminum isopropoxide and aluminum chloride hexahydrate, which provide aluminum sources, and yttrium nitrate hexahydrate, as a stabilizer, are then added and stirred evenly at room temperature to obtain the zirconium oxide precursor solution.

[0019] As a preferred embodiment of the present invention, the molar ratio of aluminum isopropoxide and aluminum chloride hexahydrate is 1 to 3:1.

[0020] Description of the invention principle:

[0021] First, this invention proposes a novel hot-stretching process to address the shortcomings of existing ceramic fiber mechanical properties. This process enables the in-situ growth of a carbon layer on the surface of the ceramic fiber. The ceramic fiber obtained using this method is coated with a dense, continuous, and highly uniform high-modulus carbon layer; this achieves a core-shell structure with uniform carbon coating, thereby comprehensively improving the mechanical properties of the ceramic fiber. Simultaneously, this hot-stretching process is simple, efficient, and widely applicable. The ceramic fiber treated with this process retains its excellent high-temperature resistance, significantly improving upon the brittleness and inability to withstand large strains inherent in traditional ceramic fibers. The improved ceramic fiber exhibits good tensile and buckling strain resistance, enhancing its fatigue resistance and making it better suited for applications in thermal protection.

[0022] In the electrospinning process using a precursor solution, the precursor solution is continuously stretched under the influence of an electric field and solidifies on the rollers as the solvent rapidly evaporates. The fiber surface contains numerous micropores caused by solvent evaporation. Furthermore, a certain amount of polymer needs to be added during the preparation of the fiber precursor solution. The long chains of the polymer entangle the hydrolyzed and condensed oxides in the solution, thereby achieving the required viscosity for electrospinning. In traditional manufacturing processes, one of the purposes of high-temperature calcination is to remove the polymer from the fiber interior. Upon heating to the polymer decomposition temperature, the volatilization of a large amount of polymer further leads to defects such as pores on the fiber surface. These surface defects significantly affect the mechanical properties of zirconia fibers in practical applications.

[0023] This invention innovatively utilizes the micropores on the fiber surface. By heating the precursor fiber to the carbon pyrolysis temperature and applying a certain radial tensile force to the fiber membrane, under the combined effects of thermodynamics and mechanics, the pyrolytic carbon inside the fiber overflows from the surface micropores and aligns under the radial force, thereby generating a continuous and uniform carbon layer in situ. This carbon layer effectively coats the fiber surface, preventing significant carbon volatilization and decomposition during subsequent high-temperature calcination, ensuring a smooth fiber surface. Simultaneously, the high-modulus carbon layer acts as a stress buffer, absorbing most of the stress while the fiber bears it. The smooth surface prevents stress concentration, distributing stress evenly throughout the fiber and greatly enhancing its tensile strength. Furthermore, the carbon layer and the fiber itself are bonded by weak interactions, preventing bending and breakage due to significant modulus differences between the inner and outer layers under large strain. This strong bond acts as a lubricant for the fiber matrix, and combined with the high modulus of the carbon layer, achieves a strong and tough bond, significantly improving the strain resistance of the ceramic fiber. The surface carbon layer coating did not significantly increase the fiber diameter. Zirconia fibers, with their nanoscale diameter and extremely high aspect ratio, greatly shortened the heat conduction path, preserving the excellent high-temperature resistance of ceramic fibers. Therefore, this invention utilizes the micropores on the fiber surface and residual carbon inside to generate a continuous and dense carbon layer in situ on the fiber surface after hot stretching treatment. This not only preserves the fiber's good thermal insulation properties but also greatly improves the mechanical properties of zirconia ceramic fibers, significantly mitigating the defects of ceramic fibers such as brittleness and poor mechanical properties.

[0024] The zirconia-based ceramic fibers prepared by the method of this invention are coated with a dense and continuous amorphous carbon layer. This carbon layer coating results in a smooth surface after high-temperature calcination, free from significant porosity defects. The uniform coverage of the carbon layer eliminates crack tips in the fibers, ensuring even distribution of stress and minimizing stress concentration points. Furthermore, the in-situ grown carbon layer originates from carbon stored within the fiber itself. Through a hot-stretching process, it is oriented onto the fiber surface, significantly reducing surface defects caused by polymer volatilization during calcination, thus ensuring a smooth and defect-free fiber surface. The carbon layer has a higher modulus than the ceramic fiber matrix; this high-modulus shell coating ensures stress is evenly distributed throughout the fiber, reducing the risk of brittle fracture. The excellent bonding between the carbon layer and the fiber achieves a strong and tough bond, greatly improving the mechanical performance of the ceramic fiber in high-strain fatigue tests such as buckling and compression.

[0025] Secondly, yttrium and aluminum play a role in stabilizing the zirconia crystal structure and refining the grain size. Yttrium suppresses the ZrO2 phase transformation during high-temperature processes, thus obtaining tetragonal zirconia fibers at room temperature and preventing surface defects such as cracking caused by volume changes during phase transformation. Aluminum, on the other hand, acts as a pinning agent for the zirconia grains within the fiber, inhibiting excessive grain growth at high temperatures. Simultaneously, this invention achieves uniform doping by selecting two aluminum salts and combining them in a specific ratio, enabling uniform aluminum hydrolysis. The principle is that aluminum isopropoxide has high hydrolytic activity, while aluminum chloride hexahydrate can neutralize the rapid hydrolysis rate while providing an acidic environment to ensure aluminum hydrolysis. Through the synergistic effect of aluminum and yttrium, this invention endows zirconia fibers with a certain mechanical strength, meeting the strength requirements of the fibers under hot stretching conditions. Subsequently, combined with a hot stretching process, a uniform carbon layer is achieved on the fiber surface, thereby comprehensively improving the mechanical properties of the zirconia fibers.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1. This invention provides a simple, efficient, and widely applicable method for improving the mechanical properties of ceramic fibers. This method enables the in-situ formation of a high-modulus shell layer on the surface of the ceramic fibers, achieving a strong and tough bond between the fiber matrix and the high-modulus shell layer. While retaining the original excellent high-temperature resistance of the fibers, it greatly enhances the resistance to brittle fracture and fatigue under high stress conditions, making it a robust and reliable high-temperature thermal protection material.

[0028] 2. This invention obtains carbon-coated zirconia-based ceramic fibers through in-situ growth on the fiber surface. This process is suitable for improving the mechanical properties of ceramic fibers that can withstand large strains in extreme environments. The improved zirconia ceramic fibers can better meet the mechanical requirements of thermal protection materials in extreme environments.

[0029] 3. The heat treatment process of the present invention utilizes the micropores on the fiber surface, which allows residual carbon inside the fiber to escape under pyrolysis, greatly reducing the phenomenon of fiber surface defects caused by the volatilization of residual carbon during the high-temperature calcination process.

[0030] 4. The heat treatment process of this invention, under the application of radial tensile force, causes the residual carbon on the fiber surface to oriented into a continuous and uniform carbon layer. This uniform carbon layer eliminates surface defects while the high-modulus shell effectively buffers and disperses stress. The zirconia ceramic fiber, after hot stretching treatment, exhibits excellent mechanical properties and good thermal insulation, making it a thermally protective material with good mechanical properties that can withstand thermal shock and large strain in high-temperature environments. Attached Figure Description

[0031] Figure 1The images are scanning electron microscope (SEM) images of the fiber membranes in Examples 1 to 6; (a) to (f) in the images correspond to Examples 1 to 6, respectively.

[0032] Figure 2 This is a transmission electron microscope image of the fiber membrane in Example 4.

[0033] Figure 3 The graph shows the surface modulus distribution of the fiber membranes in Example 4 and Comparative Example 1; (a) in the graph is Comparative Example 1 and (b) is Example 4.

[0034] Figure 4 This is a bending resistance test diagram of the fiber membrane in Example 4.

[0035] Figure 5 This is a compression strain diagram of the fiber membrane in Example 4. Detailed Implementation

[0036] The present invention will be described in further detail with reference to the following embodiments, but these embodiments are not intended to limit the scope of the invention.

[0037] I. Overview of the Implementation Schemes of the Invention

[0038] The method for preparing carbon-coated zirconia-based ceramic fibers provided by the present invention includes the following steps:

[0039] (1) Prepare a zirconium oxide-based precursor solution, stir it evenly, and then add an appropriate amount of polymer to obtain a spinning solution;

[0040] The method for preparing the zirconia-based precursor solution is as follows: using deionized water as a solvent, zirconium acetate as a zirconium source, aluminum isopropoxide and aluminum chloride hexahydrate as aluminum sources, and yttrium nitrate hexahydrate as a stabilizer are added to the solution; the mixture is stirred evenly at room temperature to obtain the zirconia-based precursor solution.

[0041] In this precursor solution, the molar ratio of zirconium, aluminum, and yttrium is 20:3~5:1~3; the molar ratio of aluminum isopropoxide and aluminum chloride hexahydrate is 1~3:1. The polymer is any one of polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide, and its addition amount is controlled so that its mass percentage in the spinning solution is 2~5 wt%.

[0042] The zirconia-based ceramic fiber precursor membrane was then obtained by electrospinning. The electrospinning process conditions were controlled as follows: ambient temperature 20–30℃, ambient humidity 20–30%, working voltage 12–18 KV, and spinning speed 1–1.5 ml / h.

[0043] (2) Place the precursor film in a vacuum oven and dry it at 60°C for 1 h.

[0044] Then, the precursor film is stretched under conditions higher than the carbon pyrolysis temperature of the polymer. Specifically, the precursor film is suspended in an oven and a radial stretching force is applied to the bottom. The oven temperature is set to 150~300℃ and the radial stretching force is 0.5~1.5 N. After holding the hot stretching for 2~8 hours, it is naturally cooled to room temperature.

[0045] Through hot stretching treatment, the pyrolytic carbon inside the fiber overflows through the micropores on the fiber surface and aligns under the action of radial force, forming a continuous and uniform carbon layer in situ on the fiber surface.

[0046] (3) The precursor film after hot stretching is subjected to calcination treatment. The specific operations include:

[0047] The ceramic fiber precursor film was transferred to a muffle furnace and heated to 700~1400℃ at 5℃ / min, and held for 2~4h; then cooled to 200℃ at 5℃ / min and cooled to room temperature with the furnace to finally obtain carbon-coated zirconia-based ceramic fibers.

[0048] The zirconia ceramic fiber membrane prepared by this invention is white, has good mechanical properties, and combines flexibility with excellent thermal insulation performance.

[0049] II. Examples and Comparative Examples

[0050] To explore the optimal hot stretching process for zirconia-based ceramic fibers, this invention conducts gradient experiments by setting different stretching times, stretching forces, and stretching temperatures.

[0051] Example 1:

[0052] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, and add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate at the same time. The molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 1:1. Stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:3:1.

[0053] Step (2): Polyvinylpyrrolidone is added in batches to the solution from step (1), with the polymer accounting for 2 wt% of the total solution mass. After stirring evenly, a homogeneous spinnable precursor solution is obtained.

[0054] Step (3): Electrospin the precursor solution obtained in step (2) with the spinning voltage maintained at 12 KV, the spinning speed at 1 ml / h, the ambient temperature at 20℃, the ambient humidity at 30%, and the spinning amount at 10 ml.

[0055] Step (4): After the precursor fiber membrane is dried in a vacuum oven at 60°C for 1 hour, the precursor fiber membrane is suspended in the oven and a radial tensile force of 0.5N is applied at the bottom by using a weight. The oven temperature is maintained at 150°C for 2 hours. After the oven cools down, the fiber membrane is taken out.

[0056] Step (5): The fiber membrane obtained in step (4) is then transferred to a muffle furnace and heated to 700°C at 5°C / min. After holding at 700°C for 2 hours, it is cooled to 200°C at 5°C / min and then cooled to room temperature with the furnace. A dense and uniform ceramic fiber membrane is obtained, which exhibits good flexibility and mechanical properties.

[0057] Example 2:

[0058] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 2:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:4:3.

[0059] Step (2): Add polyvinylpyrrolidone to the precursor solution obtained in step (1), with the polymer accounting for 3 wt% of the solution mass. Stir at room temperature to obtain a uniform, transparent, spinnable precursor solution.

[0060] Step (3): Electrospin the precursor solution obtained in step (2). Process conditions: spinning amount 10 ml, spinning speed 1.2 ml / h, spinning voltage 14 KV, ambient temperature 20℃, ambient humidity 30%.

[0061] Step (4): After drying the precursor fiber membrane obtained in step (3) at 60°C for 1 h in a vacuum oven, the precursor fiber membrane is suspended in the oven and a radial tensile force of 0.8 N is applied at the bottom by means of a hanging weight. The oven temperature is maintained at 250°C for 3 h. After the oven cools down, the fiber membrane is taken out.

[0062] Step (5): The precursor ceramic fiber membrane obtained in step (4) was transferred to a muffle furnace and heated to 800°C at 5°C / min. After holding at 800°C for 3 h, it was cooled to 200°C at 5°C / min and cooled to room temperature with the furnace. A dense and uniform ceramic fiber membrane was obtained, and the fiber membrane exhibited good flexibility and mechanical properties.

[0063] Example 3:

[0064] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 3:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:5:3.

[0065] Step (2): Add polyvinylpyrrolidone to the precursor solution obtained in step (1), with the polymer accounting for 5 wt% of the solution mass, and stir until a uniform, transparent, spinnable precursor solution is obtained.

[0066] Step (3): Electrospin the precursor solution obtained in step (2) with a spinning volume of 10 ml, a spinning speed of 1.5 ml / h, a spinning voltage of 18 KV, an ambient temperature of 25℃, and an ambient humidity of 25%.

[0067] Step (4): After drying the precursor fiber membrane obtained in step (3) at 60°C for 1 h in a vacuum oven, the precursor fiber membrane is suspended in the oven and a radial tensile force of 1 N is applied at the bottom by means of a hanging weight. The oven temperature is maintained at 300°C for 4 h. After the oven cools down, the fiber membrane is taken out.

[0068] Step (5): The precursor ceramic fiber membrane obtained in step (4) was transferred to a muffle furnace and heated to 1000℃ at 5℃ / min. After holding at 1000℃ for 3 h, it was cooled to 200℃ at 5℃ / min and cooled to room temperature with the furnace. A dense and uniform ceramic fiber membrane was obtained, and the fiber membrane exhibited good flexibility and mechanical properties.

[0069] Example 4:

[0070] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 2:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:4:2.

[0071] Step (2): Add polyvinylpyrrolidone to the precursor solution obtained in step (1), with the polymer accounting for 4 wt% of the solution mass. Stir until homogeneous to obtain a uniform, transparent, spinnable precursor solution.

[0072] Step (3): Electrospin the precursor solution obtained in step (2) with a spinning volume of 10 ml, a spinning speed of 1.2 ml / h, a spinning voltage of 18 KV, an ambient temperature of 30℃, and an ambient humidity of 20%.

[0073] Step (4): After drying the precursor fiber membrane obtained in step (3) at 60°C for 1 h in a vacuum oven, the precursor fiber membrane is suspended in the oven and a radial tensile force of 1.2 N is applied at the bottom by means of a hanging weight. The oven temperature is maintained at 200°C for 4 h. After the oven cools down, the fiber membrane is taken out.

[0074] Step (5): The precursor ceramic fiber membrane obtained in step (4) was transferred to a muffle furnace and heated to 800°C at 5°C / min. After holding at 800°C for 2 h, it was cooled to 200°C at 5°C / min and cooled to room temperature with the furnace. A dense and uniform ceramic fiber membrane was obtained, and the fiber membrane exhibited good flexibility and mechanical properties.

[0075] Example 5:

[0076] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 1.5:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:3:2.

[0077] Step (2): Add polyethylene oxide to the precursor solution obtained in step (1), with the polymer accounting for 3 wt% of the solution mass. Stir at room temperature to obtain a uniform, transparent, spinnable precursor solution.

[0078] Step (3): Add the spinning solution obtained in step (2) to the electrospinning machine, with a spinning amount of 10 ml, a spinning speed of 1.5 ml / h, a spinning voltage of 18 KV, an ambient temperature of 30℃, and an ambient humidity of 20%.

[0079] Step (4): After drying the precursor fiber membrane obtained in step (3) at 60°C for 1 h in a vacuum oven, the precursor fiber membrane is suspended in the oven and a radial tensile force of 1.5 N is applied at the bottom by means of a hanging weight. The oven temperature is maintained at 300°C for 4 h. After the oven cools down, the fiber membrane is taken out.

[0080] Step (5): The precursor ceramic fiber membrane obtained in step (4) was transferred to a muffle furnace, heated to 1200°C at 5°C / min, held for 4 h and then cooled to 200°C at 5°C / min, and cooled to room temperature with the furnace; a dense and uniform ceramic fiber membrane was obtained.

[0081] Example 6:

[0082] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 3:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:3:3.

[0083] Step (2): Add polyvinyl alcohol to the precursor solution obtained in step (1), with the polymer accounting for 2 wt% of the solution mass, and stir to obtain a uniform, transparent, spinnable precursor solution.

[0084] Step (3): Electrospin the precursor solution obtained in step (2) with the spinning voltage maintained at 12 KV, ambient temperature at 30℃, ambient humidity at 20%, spinning speed at 1 ml / h, and spinning amount at 10 ml.

[0085] Step (4): After drying the precursor fiber membrane obtained in step (3) at 60°C for 1 h in a vacuum oven, the precursor fiber membrane is suspended in the oven and a radial tensile force of 1.5 N is applied at the bottom by means of a hanging weight. The oven temperature is maintained at 300°C for 8 h. After the oven cools down, the fiber membrane is taken out.

[0086] Step (5): The precursor ceramic fiber membrane obtained in step (4) was transferred to a muffle furnace, heated to 1400℃ at 5℃ / min, held for 4 h and then cooled to 200℃ at 5℃ / min, and cooled to room temperature with the furnace; a dense and uniform ceramic fiber membrane was obtained.

[0087] Comparative Example 1:

[0088] Compared to Examples 1-6, this comparative example does not include a hot stretching process; the other operating steps are basically the same as those in the above examples, as follows:

[0089] Step (1): Measure 50 ml of deionized water as a solvent at room temperature, add zirconium acetate as a zirconium source, add aluminum isopropoxide, aluminum chloride hexahydrate and yttrium nitrate hexahydrate, wherein the molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 3:1, stir evenly at room temperature, and maintain the molar ratio of zirconium to aluminum and yttrium at 20:3:1.

[0090] Step (2): Polyvinylpyrrolidone is added to the precursor solution obtained in step (1), with the polymer accounting for 2 wt% of the solution mass. The mixture is stirred at room temperature to obtain a uniform, transparent, spinnable precursor solution.

[0091] Step (3): Electrospin the spinnable precursor solution obtained in step (2) at a spinning speed of 1 ml / h, a spinning voltage of 15 KV, an ambient temperature of 30℃, an ambient humidity of 20%, and a spinning volume of 10 ml.

[0092] Step (4): The precursor ceramic fiber membrane obtained in step (3) was transferred to a muffle furnace, heated to 800°C at 5°C / min, held for 2 h and then cooled to 200°C at 5°C / min, and cooled to room temperature with the furnace; a dense and uniform ceramic fiber membrane was obtained.

[0093] III. Experimental Data and Results Analysis:

[0094] The fiber membrane samples prepared in Examples 1-6 and the comparative examples were subjected to tensile testing in accordance with the national standard GB / T 23805-2009 and thermal conductivity testing using the transient planar source method of ISO 220072:2015.

[0095] Specifically, the tests included: dynamic thermomechanical analysis (DMA) to test the tensile properties of the fiber membrane, thermal conductivity measurement using a thermal conductivity meter, and surface modulus measurement using an atomic force microscope. The specific numerical results are shown in Table 1.

[0096] Table 1. Summary of tensile strength and thermal conductivity data for comparative examples and embodiments.

[0097]

[0098] As shown in Table 1 above, the zirconia-based ceramic nanofibers prepared according to the method of the present invention are all ceramic fibers with both excellent mechanical properties and good high-temperature resistance. The hot stretching process generates a carbon layer on the surface of the zirconia fibers, thereby improving the mechanical properties of the zirconia ceramics.

[0099] To verify the optimal process parameters for hot stretching, untreated zirconia fibers were used as Comparative Example 1. Meanwhile, in Examples 1-6, the tensile force, stretching time, and stretching temperature were varied during the hot stretching process, and tensile strength tests were conducted to explore the optimal hot stretching process parameters. The ultimate tensile stress of each example was used to determine the optimal hot stretching process. The test results showed that the tensile strength of the examples after hot stretching was stronger than that of the comparative example, indicating that the carbon layer introduced by the hot stretching process significantly improved the fiber's mechanical properties.

[0100] Through a comparative analysis of the embodiments, as the radial tensile force on the fiber membrane gradually increased from 0.5 N to 1.5 N, the tensile strength of the embodiments showed a trend of first increasing and then decreasing. When the radial tensile force reached 1.2 N, Embodiment 4 achieved the highest ultimate tensile strength of 8.13 MPa. However, as the mass of the weights added during the stretching process gradually increased, the tensile strength of the fiber membrane decreased to a certain extent; this was due to structural damage to the fiber surface caused by excessive radial tensile force. Simultaneously, to promote the overflow of residual carbon from the fiber through surface micropores, the optimal temperature was determined by changing the oven temperature. When the oven temperature was below 200°C, the carbon was not in a pyrolysis state, and an effective carbon coating layer could not be formed on the fiber surface; if the temperature was too high, the carbon layer would volatilize and decompose in the air. In the calcination process after hot stretching, as the calcination temperature gradually increased, the carbon layer would oxidize and gradually volatilize in the air, thus affecting the mechanical strengthening effect of the carbon layer on the fiber. As can be seen from Examples 5 and 6, when the calcination temperature reaches above 1000℃, the carbon layer continuously volatilizes due to oxidation, causing the fiber surface to no longer be continuously and uniformly coated with a carbon layer, resulting in a relative decrease in the mechanical properties of the fiber.

[0101] Therefore, the applicant explored the optimal hot stretching process through Example 4, namely: by applying a radial tension of 1.2 N for 4 h by suspending weights at the bottom of the fiber membrane in an oven at 200°C, and then transferring it to a muffle furnace after the oven has cooled down, and then heating it to 800°C in the muffle furnace at 5°C / min and holding it for 2 h, zirconia fibers with good mechanical properties with a continuous and uniform carbon layer on the surface are obtained.

[0102] like Figure 2 As shown, the fiber membrane treated with the optimal hot stretching process in Example 4 was characterized microscopically. TEM results show that the red box in the image represents a continuous and uniform carbon layer on the fiber surface, and the area below the red box is the zirconium oxide fiber matrix. Figure 3 As shown, the surface modulus of the fiber after hot stretching treatment is higher than that of the comparative fiber, and the good coating of the high-modulus carbon layer on the surface improves the overall surface modulus of the fiber. Figure 4 and Figure 5 As shown, the fiber membrane was subjected to bending and compressive fatigue tests using a dynamic thermomechanical apparatus. The results showed that the excellent coating of the carbon layer greatly improved the strain resistance of the zirconia fiber. After 500 cycles of 80% bending strain, the fiber membrane still maintained its intact shape; after 100 cycles of 50% compressive strain, the fiber membrane did not experience structural collapse. The zirconia ceramic fiber prepared by the method of this invention possesses excellent mechanical properties while maintaining the good thermal insulation ability of ceramic fibers.

[0103] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing carbon-coated zirconia-based ceramic fibers, characterized in that, Includes the following steps: (1) Prepare a zirconia-based precursor solution, stir it evenly, add an appropriate amount of polymer to prepare a spinning solution, and obtain a zirconia-based ceramic fiber precursor membrane by electrospinning. (2) After drying the precursor film, the precursor film is stretched under conditions higher than the carbon pyrolysis temperature of the polymer, so that the pyrolytic carbon inside the fiber overflows through the micropores on the fiber surface and is oriented under the action of radial force, and a continuous and uniform carbon layer is generated in situ on the fiber surface. (3) The precursor film after hot stretching is calcined to obtain carbon-coated zirconia-based ceramic fibers.

2. The method according to claim 1, characterized in that, In the precursor solution, the molar ratio of zirconium, aluminum, and yttrium is 20:3~5:1~3.

3. The method according to claim 1, characterized in that, The polymer is any one of polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide, and its mass percentage in the spinning solution is 2-5 wt%.

4. The method according to claim 1, characterized in that, The process conditions for controlling electrospinning are: ambient temperature 20-30℃, ambient humidity 20-30%, working voltage 12-18 KV, and spinning speed 1-1.5 ml / h.

5. The method according to claim 1, characterized in that, The drying treatment of zirconia-based ceramic fiber precursor membranes refers to placing them in a vacuum oven and drying them at 60°C for 1 hour.

6. The method according to claim 1, characterized in that, The hot stretching treatment of ceramic fiber precursor membranes refers to: suspending the precursor membrane in an oven and applying a radial tensile force at the bottom; setting the oven temperature to 150~300℃ and the radial tensile force to 0.5~1.5 N; maintaining the hot stretching for 2~8 hours and then allowing it to cool naturally to room temperature.

7. The method according to claim 1, characterized in that, The calcination process includes: transferring the ceramic fiber precursor membrane into a muffle furnace, heating it to 700~1400℃ at 5℃ / min, holding it at that temperature for 2~4h; then cooling it to 200℃ at 5℃ / min, and then cooling it to room temperature with the furnace.

8. The method according to any one of claims 1 to 7, characterized in that, The method for preparing the zirconia-based precursor solution is as follows: Zirconium acetate, which serves as the zirconium source, is added to deionized water and stirred until homogeneous; aluminum isopropoxide and aluminum chloride hexahydrate, which provide the aluminum source, and yttrium nitrate hexahydrate, which serve as the stabilizer, are then added and stirred until homogeneous at room temperature to obtain the zirconia-based precursor solution.

9. The method according to claim 8, characterized in that, The molar ratio of aluminum isopropoxide to aluminum chloride hexahydrate is 1~3:1.

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