A method for rapidly preparing hafnium oxide ceramic fibers

Abstract

The application belongs to the technical field of ceramic material preparation, and specifically relates to a method for rapidly preparing hafnium oxide ceramic fibers, which comprises the following steps: dissolving hafnium-containing organic precursors in an organic solvent, preparing organic precursor fibers through an electrostatic spinning process, performing solidification treatment on the organic precursor fibers, placing the organic precursor fibers in a reaction chamber of a microwave plasma device, introducing treatment gas, applying microwave power, performing plasma treatment on the organic precursor fibers to convert the organic precursor fibers into hafnium oxide ceramic fibers, turning off the microwave source after the treatment is completed, naturally cooling the obtained product to room temperature, and obtaining hafnium oxide ceramic fibers. The hafnium oxide ceramic fibers prepared by the method are prepared by using microwave-excited plasma, have excellent toughness and elasticity, and are not prone to brittle fracture when subjected to bending or tensile stress. The process flow of the method is short, and the energy consumption is low, so that the method provides a new technical path for the rapid preparation of high-performance hafnium oxide ceramic fibers.

Description

Technical Field

[0001] This invention belongs to the field of advanced ceramic material preparation technology, specifically relating to a method for rapidly preparing hafnium oxide ceramic fibers. Background Technology

[0002] Ceramic fibers, as a type of high-performance inorganic fiber, possess excellent properties such as high temperature resistance, corrosion resistance, good thermal stability, and high strength, and are widely used in cutting-edge fields such as high-temperature insulation, catalyst carriers, composite reinforcement, and aerospace. Among them, hafnium oxide (HfO2) ceramic fibers, due to their extremely high melting point (over 2700℃), excellent high-temperature phase stability, low thermal conductivity, and good creep resistance, are considered one of the most promising materials for application in ultra-high temperature extreme environments. They are key materials for developing thermal protection systems for next-generation hypersonic aircraft, new aero engines, and high-end industrial furnaces.

[0003] However, the preparation of hafnium oxide ceramic fibers still faces significant technical challenges, severely restricting their practical application and development. Traditional methods for preparing ceramic fibers mainly include the sol-gel method and precursor conversion method. These methods typically require multiple steps, such as lengthy precursor synthesis, spinning, cross-linking, and high-temperature sintering at no less than 1500℃, with the entire process taking tens of hours or even days. This process is not only energy-intensive and inefficient, but the high-temperature, long-duration sintering also easily leads to excessive grain growth within the fiber, forming coarse, brittle grain boundaries. This results in fibers with high brittleness, poor toughness, and a lack of flexibility and elasticity, making them prone to brittle fracture under bending or tensile stress. This inherent brittleness makes subsequent weaving and composite processing extremely difficult and greatly limits its application in flexible thermal protection components that need to withstand deformation.

[0004] In addition, traditional methods often require the introduction of complex sintering aids or second-phase substances in order to suppress grain growth and improve fiber performance. This may introduce impurities, reduce fiber purity and high-temperature performance, which contradicts the original intention of obtaining high-performance intrinsic hafnium oxide fibers.

[0005] Microwave-excited plasma technology, as a highly efficient method for material synthesis and processing, features rapid heating rates, concentrated energy, and strong activation capabilities. However, currently, there are no mature reports on its application in the preparation of high-melting-point, high-purity oxide ceramic fibers, especially hafnium oxide fibers. Summary of the Invention

[0006] To address the above problems, this invention provides a method for rapidly preparing hafnium oxide ceramic fibers. The method utilizes microwave-excited plasma to rapidly prepare hafnium oxide ceramic fibers, resulting in a short process flow and low energy consumption. This provides a novel technical path for the rapid preparation of high-performance hafnium oxide ceramic fibers.

[0007] This invention is specifically achieved through the following technical solution: a method for rapidly preparing hafnium oxide ceramic fibers according to this invention includes the following steps:

[0008] (1) Dissolve the hafnium-containing organic precursor in an organic solvent to prepare a spinning solution, and prepare organic precursor fibers by electrospinning process. Place the obtained organic precursor fibers in a muffle furnace and keep them at 260~290℃ for 15~30 min to complete the curing treatment.

[0009] (2) The organic precursor fiber obtained after curing is placed in the reaction chamber of the microwave plasma device, the processing gas is introduced into the reaction chamber, the microwave source is started, the microwave power is applied, and the organic precursor fiber is plasma treated by adjusting the gas type, gas flow rate and microwave power to convert it into hafnium oxide ceramic fiber.

[0010] (3) Post-processing: After the plasma treatment is completed, the microwave source is turned off and the obtained product is naturally cooled to room temperature in the treatment gas atmosphere to obtain hafnium oxide ceramic fiber.

[0011] In the aforementioned method for rapid preparation of hafnium oxide ceramic fibers, the hafnium-containing organic precursor in step (1) is hafnium chloride.

[0012] In the aforementioned method for rapid preparation of hafnium oxide ceramic fibers, the organic solvent in step (1) is N,N-dimethylformamide (DMF).

[0013] In the aforementioned method for rapid preparation of hafnium oxide ceramic fibers, the mass concentration of hafnium organic precursor in the spinning solution in step (1) is 10-25%.

[0014] The aforementioned method for rapidly preparing hafnium oxide ceramic fibers, the electrospinning process parameters in step (1) are: voltage 10~20 kV, receiving distance 10~20 cm, and feed rate 0.3~0.8 mL / h.

[0015] The aforementioned method for rapid preparation of hafnium oxide ceramic fibers includes a plasma treatment stage and a ceramicization stage.

[0016] During the pre-oxidation stage, the processing gas introduced into the reaction chamber is oxygen or air, with a gas flow rate of 20~100 sccm, a microwave power of 2~4 kW, and a processing time of 5~30 min.

[0017] During the ceramization stage, the processing gas introduced into the reaction chamber is one or more mixed gases selected from argon, nitrogen, and hydrogen, with a gas flow rate of 50~200 sccm, a microwave power of 5~10 kW, and a processing time of 10~60 min.

[0018] Preferably, in the pre-oxidation stage, the gas flow rate is 40~60 sccm, the microwave power is 2.5~3.5 kW, and the processing time is 10~20 min; in the ceramization stage, the gas flow rate is 80~150 sccm, the microwave power is 6~8 kW, and the processing time is 20~40 min.

[0019] Furthermore, during the ceramization stage, the mixed gas is a mixture of argon and hydrogen, with the volume percentage of hydrogen in the mixed gas being 1-10%; or the mixed gas is a mixture of argon and nitrogen, with the volume percentage of nitrogen in the mixed gas being 10-50%.

[0020] The aforementioned method for rapidly preparing hafnium oxide ceramic fibers produces hafnium oxide ceramic fibers that are mainly composed of monoclinic hafnium oxide, with a diameter of 400-500 nm.

[0021] The present invention also provides a hafnium oxide ceramic fiber obtained by any of the foregoing methods. The hafnium oxide ceramic fiber has excellent toughness and elasticity and is not prone to brittle fracture when subjected to bending or tensile stress. It can be used to prepare ultra-high temperature flexible thermal insulation materials, composite material reinforcements or flexible ceramic fabrics.

[0022] Compared with existing technologies, this invention has significant advantages and beneficial effects. Through the above technical solution, this invention achieves considerable technological advancement and practicality, and has broad application value, possessing at least the following advantages:

[0023] This invention provides a method for rapidly preparing hafnium oxide ceramic fibers using microwave-excited plasma. Hafnium-containing organic precursor fibers are placed in a microwave plasma environment. By precisely controlling three key process parameters—the type of processing gas, gas flow rate, and microwave power—the hafnium-containing organic precursor fibers are subjected to plasma treatment in stages, successfully achieving rapid conversion from precursor fibers to ceramic fibers. Specifically, the pre-oxidation stage is carried out in an oxidizing atmosphere and at a relatively low microwave power, enabling the hafnium-containing organic precursor fibers to form a stable cross-linked backbone and preventing melting during subsequent processing. The ceramization stage is completed under an inert or specific reaction atmosphere and at a higher microwave power, promoting the formation of the ceramic phase.

[0024] The hafnium oxide ceramic fibers prepared by this invention possess excellent toughness and elasticity, and are not prone to brittle fracture under bending or tensile stress. They can be used to prepare ultra-high temperature flexible thermal insulation materials, composite material reinforcements, or flexible ceramic fabrics. This solves the problem of high brittleness and difficulty in applying traditional ceramic fibers to flexible components. The process of this invention has a short flow and low energy consumption, providing a novel technical route for the rapid preparation of high-performance hafnium oxide ceramic fibers, and has broad application prospects. Attached Figure Description

[0025] Figure 1 This is a SEM image of the organic precursor fiber obtained in step (1) of Example 1 before curing, magnified at 2000x.

[0026] Figure 2 yes Figure 1 Diameter size distribution of organic precursor fibers.

[0027] Figure 3 This is a SEM image of the organic precursor fiber obtained in step (1) of Example 1 after curing, magnified at 2000x.

[0028] Figure 4 yes Figure 3 Diameter size distribution of organic precursor fibers after medium curing.

[0029] Figure 5 This is a SEM image of the hafnium oxide ceramic fiber finally prepared in Example 1 at a magnification of 10,000.

[0030] Figure 6 This is the XRD pattern of the hafnium oxide ceramic fiber finally prepared in Example 1.

[0031] Figure 7 This is a physical image of the hafnium oxide ceramic fibers prepared in Example 2.

[0032] Figure 8 This is a photograph of the hafnium oxide ceramic fiber prepared in Example 3.

[0033] Figure 9 It is Figure 8 A picture of hafnium oxide ceramic fibers after they have been wound and curled.

[0034] Figure 10 yes Figure 9 A photograph of hafnium oxide ceramic fibers after their shape has been restored following winding and curling.

[0035] Figure 11 This is a photograph of the ceramic fibers prepared in proportion. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] This invention provides a method for rapidly preparing hafnium oxide ceramic fibers, comprising the following steps:

[0038] (1) Dissolve the hafnium-containing organic precursor in N,N-dimethylformamide (DMF) to prepare a spinning solution, and prepare organic precursor fibers by electrospinning process.

[0039] Preferably, the hafnium-containing organic precursor is hafnium chloride. The mass concentration of the hafnium-containing organic precursor in the spinning solution is preferably 10-25%. The preferred electrospinning process parameters are: voltage 10-20 kV, receiving distance 10-20 cm, and feed rate 0.3-0.8 mL / h.

[0040] Preferably, the organic precursor fibers obtained by electrospinning are further subjected to a curing treatment. The curing treatment involves holding the organic precursor fibers obtained by electrospinning at 260~290°C in a muffle furnace for 15-30 minutes. After the curing treatment, the precursor fiber structure becomes more stable.

[0041] (2) The obtained organic precursor fiber is placed in the reaction chamber of the microwave plasma device, the processing gas is introduced into the reaction chamber, the microwave source is started, the microwave power is applied, and the obtained organic precursor fiber is subjected to plasma treatment by controlling the gas type, gas flow rate and microwave power to convert it into hafnium oxide ceramic fiber.

[0042] Specifically, the plasma treatment includes a pre-oxidation stage and a ceramization stage:

[0043] In the pre-oxidation stage, the introduced processing gas is preferably oxygen or air, and the gas flow rate is preferably 20~100 sccm, more preferably 40~60 sccm; the microwave power is preferably 2~4 kW, more preferably 2.5~3.5 kW; and the processing time is preferably 5~30 min, more preferably 10~20 min.

[0044] During the ceramization stage, the processing gas introduced is preferably one or a mixture of argon, nitrogen, and hydrogen, with a gas flow rate preferably of 50-200 sccm, more preferably 80-150 sccm; the microwave power is preferably 5-10 kW, more preferably 6-8 kW; and the processing time is preferably 10-60 min, more preferably 20-40 min.

[0045] During the ceramization stage, when the processing gas is a mixed gas, it is preferably a mixed gas of argon and hydrogen, and the volume percentage of hydrogen in the mixed gas is 1 to 10%; or, the mixed gas is preferably a mixed gas of argon and nitrogen, and the volume percentage of nitrogen in the mixed gas is 10 to 50%.

[0046] (3) Post-processing: After the plasma treatment is completed, the microwave source is turned off, and the product is naturally cooled to room temperature in the processing gas atmosphere of the ceramicization stage to obtain hafnium oxide ceramic fibers.

[0047] This invention utilizes the synergistic processing of the two plasma stages, particularly the precise control of gas type, gas flow rate, and microwave power, to maintain a highly active and stable microwave plasma environment within the reaction chamber. This allows for rapid heat treatment of organic precursor fibers, completing the conversion of hafnium-containing organic precursors into hafnium oxide ceramic fibers within tens of minutes.

[0048] The present invention will be described in detail below with reference to specific embodiments. Unless otherwise specified, all conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Raw materials and reagents whose manufacturers are not specified are all commercially available products. The microwave plasma device used in the embodiments below is a GHXN-01 microwave plasma device manufactured by Henan Guanghe New Energy Technology Co., Ltd. However, this description is not intended to limit the invention.

[0049] Example 1

[0050] (1) Preparation of organic precursor fibers: Hafnium chloride was dissolved in N,N-dimethylformamide to prepare a spinning solution with a concentration of 13wt%. Organic precursor fibers were obtained by electrospinning (process conditions: voltage 15 kV, receiving distance 15 cm, feed rate 0.5 mL / h). The organic precursor fibers were placed in a muffle furnace and cured at 270℃ for 18 min.

[0051] (2) Plasma treatment: The cured precursor fiber is placed on the sample stage of the reaction chamber of the microwave plasma device, the treatment gas is introduced into the reaction chamber, the microwave source is started, microwave power is applied, and the precursor fiber is subjected to plasma treatment. The plasma treatment includes a pre-oxidation stage and a ceramicization stage, specifically:

[0052] Pre-oxidation stage: Oxygen is introduced into the reaction chamber at a flow rate of 50 sccm. The microwave source is started and the microwave power is set to 3 kW. The process is carried out for 10 min under these conditions. After completion, the process proceeds to the ceramization stage.

[0053] Ceramification stage: switch the gas to argon, adjust the gas flow rate to 80 sccm, increase the microwave power to 6 kW, and process under these conditions for 30 min;

[0054] (3) Post-processing: After the plasma treatment is completed, the microwave source is turned off and the sample is naturally cooled to room temperature in an argon atmosphere (flow rate remains unchanged). The sample is then removed to obtain hafnium oxide ceramic fibers.

[0055] Figure 1 This is a SEM image of the precursor fiber obtained in step (1) of this embodiment before curing, at a magnification of 2000x. Figure 2 yes Figure 1 Diameter size distribution diagram of organic precursor fibers, from Figure 1 and Figure 2 It can be seen that the organic precursor fibers have a distinct fiber structure, with an average fiber diameter of about 1 μm.

[0056] Figure 3 This is a SEM image of the precursor fiber obtained in step (1) of this embodiment after curing, magnified at 2000x. Figure 4 yes Figure 3 Diameter size distribution of organic precursor fibers after medium curing, from Figure 3 and Figure 4 It can be seen that after curing, the fiber diameter increases slightly, with an average diameter of about 1.5 μm.

[0057] Figure 5 This is a SEM image of the hafnium oxide ceramic fiber finally prepared in this embodiment at a magnification of 10,000. It can be clearly seen that the diameter of the hafnium oxide ceramic fiber is about 400-500 nm.

[0058] Figure 6 This is the XRD pattern of the hafnium oxide ceramic fiber finally prepared in this embodiment. It can be seen that the PDF card corresponding to the measured sample is consistent with monoclinic hafnium oxide, indicating that the main component of the fiber sample finally prepared in this embodiment is monoclinic hafnium oxide.

[0059] Example 2

[0060] (1) Preparation of organic precursor fibers: Hafnium chloride was dissolved in N,N-dimethylformamide to prepare a spinning solution with a concentration of 15wt%. Organic precursor fibers were obtained by electrospinning (process conditions: voltage 18 kV, receiving distance 20 cm, feed rate 0.8 mL / h). The organic precursor fibers were placed in a muffle furnace and cured at 280℃ for 15 min.

[0061] (2) Plasma treatment: The cured precursor fiber is placed on the sample stage of the reaction chamber of the microwave plasma device, the treatment gas is introduced into the reaction chamber, the microwave source is started, microwave power is applied, and the precursor fiber is subjected to plasma treatment. The plasma treatment includes a pre-oxidation stage and a ceramicization stage, specifically:

[0062] Pre-oxidation stage: Air is introduced into the reaction chamber at a flow rate of 60 sccm. The microwave source is started and the microwave power is set to 2.5 kW. The process is carried out for 15 min under these conditions. After completion, the process proceeds to the ceramization stage.

[0063] Ceramification stage: The gas was switched to a mixture of argon and hydrogen (the volume percentage of hydrogen in the mixture was 5%), the total gas flow rate was set to 100 sccm, the microwave power was set to 7 kW, and the process was carried out for 20 min under these conditions.

[0064] (3) Post-processing: After the plasma treatment is completed, the microwave source is turned off and the sample is naturally cooled to room temperature in a mixed atmosphere of argon and hydrogen (flow rate remains unchanged). The sample is then removed to obtain hafnium oxide ceramic fibers.

[0065] Figure 7 This is a physical image of the hafnium oxide ceramic fiber prepared in this embodiment. As can be seen, the prepared hafnium oxide ceramic fiber is black.

[0066] Example 3

[0067] (1) Preparation of organic precursor fibers: Hafnium chloride was dissolved in N,N-dimethylformamide to prepare a spinning solution with a concentration of 20wt%. Organic precursor fibers were obtained by electrospinning (process conditions: voltage 20 kV, receiving distance 20 cm, feed rate 0.5 mL / h). The organic precursor fibers were placed in a muffle furnace and cured at 260℃ for 20 min.

[0068] (2) Plasma treatment: The cured precursor fiber is placed on the sample stage of the reaction chamber of the microwave plasma device, the treatment gas is introduced into the reaction chamber, the microwave source is started, microwave power is applied, and the precursor fiber is subjected to plasma treatment. The plasma treatment includes a pre-oxidation stage and a ceramicization stage, specifically:

[0069] Pre-oxidation stage: Oxygen is introduced into the reaction chamber at a flow rate of 40 sccm. The microwave source is started and the microwave power is set to 3 kW. The process is carried out for 15 min under these conditions. After completion, the process proceeds to the ceramization stage.

[0070] Ceramification stage: The gas was switched to argon, the gas flow rate was set to 150 sccm, and the microwave power was set to 5 kW. The treatment was carried out under these conditions for 25 min. The high flow rate of argon effectively removes reaction byproducts and has a slight "cooling" effect on the fiber surface, which can inhibit grain growth.

[0071] (3) Post-processing: Same as in Example 1.

[0072] Figure 8 This is a physical image of the hafnium oxide ceramic fiber prepared in this embodiment. The hafnium oxide ceramic fiber is black.

[0073] Figure 9 It is Figure 8The image shows a real product after hafnium oxide ceramic fibers were wound and coiled. It can be seen that the hafnium oxide ceramic fibers did not break after being wound and coiled, indicating that they have excellent toughness and elasticity.

[0074] Figure 10 yes Figure 9 The image shows the actual product after the hafnium oxide ceramic fiber was wound and coiled and its shape was restored. It is clear that after being wound and coiled and restored to its original shape, the hafnium oxide ceramic fiber did not undergo brittle fracture, which further demonstrates its excellent toughness, elasticity and flexibility.

[0075] Example 4

[0076] (1) Preparation of organic precursor fibers: Hafnium chloride was dissolved in N,N-dimethylformamide to prepare a spinning solution with a concentration of 20wt%. Organic precursor fibers were obtained by electrospinning (process conditions: voltage 15 kV, receiving distance 18 cm, feed rate 0.8 mL / h). The organic precursor fibers were placed in a muffle furnace and cured at 270℃ for 18 min.

[0077] (2) Plasma treatment: The cured precursor fiber is placed on the sample stage of the reaction chamber of the microwave plasma device, the treatment gas is introduced into the reaction chamber, the microwave source is started, microwave power is applied, and the precursor fiber is subjected to plasma treatment. The plasma treatment includes a pre-oxidation stage and a ceramicization stage, specifically:

[0078] Pre-oxidation stage: Oxygen is introduced into the reaction chamber at a flow rate of 50 sccm. The microwave source is started and the microwave power is set to 3.5 kW. The processing time is 10 min. After completion, the ceramization stage begins.

[0079] Ceramification stage: switch the gas to argon, set the gas flow rate to 50 sccm, set the microwave power to 8 kW, and process under these conditions for 30 min.

[0080] (3) Post-processing: Same as in Example 1.

[0081] Comparative example:

[0082] (1) Preparation of organic precursor fibers: Same as in Example 1.

[0083] (2) Sintering using a traditional muffle furnace: The cured precursor fiber is placed in a muffle furnace, and the furnace is heated to 1400℃ at a rate of 2℃ / min. The temperature is held at 1400℃ for 2 hours, and then naturally cooled to room temperature to obtain the sintered product, such as... Figure 11 As shown, the final product obtained by this process is a powder that lacks elasticity.

[0084] This invention provides a method for rapidly preparing hafnium oxide ceramic fibers using microwave-excited plasma. By synergistically adjusting the gas type, gas flow rate, and microwave power within the reaction chamber, a highly active and stable microwave plasma environment is excited and maintained around the precursor fibers. The organic precursor fibers are then subjected to plasma treatment in stages, successfully achieving the rapid conversion of precursor fibers into ceramic fibers. The entire preparation process is short and energy-efficient, providing a novel technological pathway for the rapid preparation of high-performance hafnium oxide ceramic fibers, with broad application prospects.

[0085] The above description is merely an embodiment of the present invention and is not intended to limit the present invention in any way. The present invention can also have other embodiments based on the above structure and function, which will not be listed hereafter. Therefore, any simple modifications, equivalent changes, and alterations made by those skilled in the art to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for rapidly producing a hafnium oxide ceramic fiber, characterized by, Includes the following steps: (1) Hafnium chloride is dissolved in an organic solvent to prepare a spinning solution. The mass concentration of hafnium chloride in the spinning solution is 10-25%. Organic precursor fibers are prepared by electrospinning. The obtained organic precursor fibers are placed in a muffle furnace and kept at 260-290℃ for 15-30 min to complete the curing treatment. (2) The cured organic precursor fiber is placed in the reaction chamber of the microwave plasma device, the processing gas is introduced into the reaction chamber, microwave power is applied, and the organic precursor fiber is plasma-treated by adjusting the gas type, gas flow rate and microwave power to convert it into hafnium oxide ceramic fiber. The plasma treatment includes a pre-oxidation stage and a ceramization stage: During the pre-oxidation stage, the processing gas introduced into the reaction chamber is oxygen or air, with a gas flow rate of 20~100 sccm, a microwave power of 2~4 kW, and a processing time of 5~30 min. During the ceramization stage, the processing gas introduced into the reaction chamber is one or more of argon, nitrogen, and hydrogen, with a gas flow rate of 50~200 sccm, a microwave power of 5~10 kW, and a processing time of 10~60 min. (3) Post-processing: After the plasma treatment is completed, the microwave source is turned off and the obtained product is cooled to room temperature in the treatment gas atmosphere to obtain hafnium oxide ceramic fiber.

2. The method for rapid preparation of hafnium oxide ceramic fibers as described in claim 1, characterized in that, The organic solvent used in step (1) is N,N-dimethylformamide.

3. The method for rapidly preparing hafnium oxide ceramic fibers as described in claim 1, characterized in that, The electrospinning process parameters in step (1) are: voltage 10~20 kV, receiving distance 10~20 cm, and feed rate 0.3~0.8 mL / h.

4. The method for rapidly preparing hafnium oxide ceramic fibers as described in claim 1, characterized in that, During the ceramization stage, the mixed gas is a mixture of argon and hydrogen, with the volume percentage of hydrogen in the mixed gas being 1-10%; or the mixed gas is a mixture of argon and nitrogen, with the volume percentage of nitrogen in the mixed gas being 10-50%.

5. The method for rapid preparation of hafnium oxide ceramic fibers as described in any one of claims 1-4, characterized in that, The obtained hafnium oxide ceramic fibers are mainly composed of monoclinic hafnium oxide, and the diameter of the hafnium oxide ceramic fibers is 400-500 nm.

6. Hafnium oxide ceramic fibers obtained by any one of claims 1-4.

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