Preparation method of flexible SiC nanofiber membrane

Abstract

The invention discloses a preparation method of a flexible SiC fiber membrane. The preparation method comprises the steps of preparation of monodisperse SiO2 spheres, preparation of a spinning solution, electrostatic spinning, pre-oxidation treatment, heat treatment and the like. According to the preparation method, the self-made monodisperse SiO2 spheres are used as a silicon source instead of commercial products, and the particle size of the silicon source is regulated and controlled by virtue of a nano toughening mechanism, so that the flexible SiC fiber membrane is successfully prepared. The method provided by the invention overcomes the problems of high brittleness, relatively large wire diameter and fiber fracture in preparation of pure SiC fibers by using a commercial silicon source. The obtained flexible SiC fiber membrane is controllable in morphology, crystal grains are uniformly distributed in fibers, and the fiber membrane has the characteristics of being bendable, elastic, resistant to high temperature and strong in electromagnetic wave absorption and has high applicability. The preparation method disclosed by the invention is simple in process, high in controllability and easy to realize large-scale production, and has practicability.

Description

Technical Field

[0001] The present invention relates to a method for preparing a flexible silicon carbide fiber membrane for electromagnetic wave absorption and high-temperature heat insulation, belonging to the technical field of material synthesis and preparation. Background Art

[0002] With the progress of technology, electromagnetic pollution poses an increasingly serious threat to human health and electronic devices, so the research on electromagnetic wave protection has attracted much attention. Absorbing materials can convert electromagnetic waves into other forms of energy, fundamentally solving the problem of electromagnetic pollution, and thus have received extensive attention. However, with the diversified requirements in application fields, absorbing materials no longer only focus on strong attenuation, but also have higher requirements in aspects such as light weight and wide frequency band, which makes it particularly important to develop new absorbing materials with excellent properties. At present, there have been a large number of research reports on electromagnetic wave absorbing materials. The flexibility and hydrophobicity of materials are two important factors affecting the actual application performance of absorbing materials. Especially to meet complex and extreme application environments, the electromagnetic wave absorbing materials used must have excellent properties such as flexibility, hydrophobicity, acid and alkali corrosion resistance, and high-temperature stability.

[0003] Silicon carbide (SiC), as a wide-bandgap semiconductor material, has great applications in the field of electromagnetic wave absorption due to its many excellent physical and chemical properties such as adjustable conductivity, high temperature resistance, oxidation resistance, high modulus, high thermal conductivity, and good chemical stability. Compared with metal materials, polymer materials or carbon materials, SiC has many advantages, but its brittleness greatly limits its application. In the past few decades, silicon carbide has been prepared into materials in different forms, including thin films, rods, fibrous, etc. Among them, flexible SiC fibers have received extensive attention due to their combination of flexibility, adjustable resistance, electromagnetic wave absorption, and high temperature resistance. However, the traditional preparation methods of SiC fibers include: (1) chemical vapor deposition (CVD), (2) activated carbon fiber conversion method, (3) ultra-fine powder high-temperature sintering method, (4) polymer precursor conversion method. The SiC fibers synthesized by these methods generally have defects such as high equipment and production costs, large fiber brittleness, relatively thick wire diameters, and poor oxidation resistance. There is an urgent need to develop an economical and practical new method for preparing flexible and high-quality SiC fibers.

[0004] In recent years, electrospinning technology has attracted much attention due to its relatively low cost and simple operating device, and has been widely used in the preparation of various flexible oxide ceramic nanofibers with controllable diameters and sizes, such as TiO 2 , SiO 2 , Al 2 O 3etc. However, there are still many challenges in preparing flexible carbide ceramic nanofibers by electrospinning technology. This is because carbide ceramic nanofibers usually require a relatively high firing temperature, generally above 1400 °C. The ultra-high temperature preparation process is prone to cause problems such as grain growth of electrospun fibers, increased brittleness, and difficulty in controlling the stoichiometry. Therefore, obtaining flexible carbide ceramic fibers by electrospinning technology places higher requirements on the chemical composition and structure of the spinning solution precursor material, electrospinning process parameters, fiber material heat treatment technology, and process flow. Taking SiC fibers as an example, the pure SiC fibers currently prepared by electrospinning technology have problems such as uneven radial size and grain distribution, relatively thick filament diameter, low aspect ratio, resulting in large brittleness and easy fracture.

[0005] In the existing literature and patents, the products obtained by preparing flexible silicon carbide fibers by electrospinning technology mainly consist of silicon carbide particles compounded with carbon fibers, while it is difficult for pure SiC fibers to maintain the continuity and flexibility of the fibers. Summary of the Invention

[0006] The technical problem to be solved by the present invention is: the defects existing in the preparation process of traditional pure SiC fibers, such as uneven radial size and grain distribution of the fibers, difficulty in obtaining continuous fiber filaments, and easy breakage of the fibers.

[0007] To solve the above technical problems, the present invention provides the following technical solutions:

[0008] A method for preparing a flexible SiC fiber membrane, characterized by comprising the following steps:

[0009] Step 1), preparing monodisperse SiO 2 microspheres: dissolving tetraethyl orthosilicate and ammonia water in absolute ethanol respectively, mixing, and stirring at a constant temperature to obtain a white suspension; centrifuging, washing with water, and freeze-drying;

[0010] Step 2), preparing a spinning solution: dissolving a spinning aid, monodisperse SiO 2 microspheres and a surfactant in a spinning solvent, and stirring until evenly mixed to obtain a spinning solution;

[0011] Step 3), electrospinning: using the spinning solution for electrospinning to prepare SiC precursor nanofibers, and then drying in an oven at 50-100 °C for 3-12 h;

[0012] Step 4), pre-oxidation treatment: performing heat treatment on the SiC precursor nanofibers in an air atmosphere to complete curing and cross-linking;

[0013] Step 5), heat treatment: putting the pre-oxidized SiC precursor nanofibers into a tubular furnace, first performing low-temperature carbonization under an inert atmosphere condition, and then raising the temperature for carbothermal reduction, and flexible SiC fiber membranes can be obtained by using monodisperse SiO2 Flexible SiC fiber membrane prepared from small balls.

[0014] Preferably, in the step 1), the concentration of tetraethyl orthosilicate in the white suspension is 0.1 - 1 mol / L, and the concentration of the ammonia water solution is 0.1 - 1.1 mol / L; the volume fraction of the ammonia water solution in the white suspension is 10 - 30%, and monodisperse SiO 2 The particle size is 1 - 2000 nm.

[0015] Preferably, in the step 1), the constant temperature is 30 - 90 °C; the stirring time is 3 - 24 h, and the freeze-drying time is 12 - 48 h.

[0016] Preferably, in the step 2), the spinning aid is any one or more of polyvinyl alcohol, polyacrylonitrile, polystyrene, polyester, and polyamide; the surfactant is any one or more of sodium dodecyl sulfate (SDS), polysorbate, alkylphenol polyoxyethylene ether (APEO), and polyoxyethylene fatty acid ester; the spinning solvent is one or more of N,N-dimethylformamide, sodium thiocyanate, formic acid, tetrahydrofuran, dimethyl sulfoxide, and acetone.

[0017] Preferably, in the step 2), the mass ratio of the spinning aid, monodisperse SiO 2 small balls, and surfactant is 1 - 20:1 - 40:1 - 10; the temperature of the stirring is 30 - 90 °C, and the time is 5 - 24 h.

[0018] Preferably, in the step 3), the spinning process parameters are: spinning voltage 10 - 30 kV, spinning solution propulsion speed 0.005 - 0.5 mL / min, rotation speed 100 - 500 r / min, and the distance between the collection drum and the spinneret needle is 15 - 25 cm.

[0019] Preferably, in the step 4), the temperature of the heat treatment for curing and crosslinking is 150 - 300 °C, the heating rate is 1 - 5 °C / min, and the constant temperature time is 1 - 5 h.

[0020] Preferably, in the step 5), the temperature of the low-temperature carbonization is 300 - 1000 °C, the heating rate is 1 - 10 °C / min, and the constant temperature time is 1 - 5 h; the heating rate of the carbothermal reduction is 1 - 10 °C / min, the heating target is 1200 - 1600 °C, and the constant temperature time is 1 - 10 h.

[0021] The preparation conditions of the present invention are easy to achieve and the operation is simple. The electrospinning method and the carbothermal reduction process are mainly used to obtain a flexible SiC nanofiber membrane material with good nanofiber morphology and excellent electromagnetic wave absorption and heat insulation properties. The present invention prepares uniform monodisperse nano-SiO 2Microspheres are used as the silicon source, and electrospinning and carbothermal reduction technologies are integrated to prepare a pure SiC flexible nanofiber membrane. The prepared pure SiC flexible nanofiber membrane not only inherits the excellent electromagnetic absorption and heat insulation properties of the SiC material, but also endows the SiC fiber membrane material with good flexibility and elasticity. This SiC nanofiber membrane with both flexibility and electromagnetic absorption and heat insulation characteristics shows great application potential in many fields such as electromagnetic shielding, stealth technology, flexible electronic devices and high-temperature heat insulation.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a method for preparing a flexible SiC nanofiber membrane, which is prepared by combining electrospinning and carbothermal reduction. A spinning aid is used as the carbon source, and self-made monodisperse SiO 2 microspheres are used as the silicon source. The auxiliary surfactant is dissolved in the spinning solvent, and a polymer nanofiber membrane precursor containing SiO 2 is prepared through electrospinning technology, and then is transformed into a pure SiC nanofiber membrane through pre-oxidation and high-temperature heat treatment. The pure SiC nanofiber membrane prepared by this method exhibits good continuity and flexibility due to the excellent mechanical properties and uniform nanoscale diameter distribution of single nanofibers. Compared with the SiC fibers prepared by commercial silicon sources, this method overcomes the problems of large brittleness, relatively thick diameter and fiber fracture of pure SiC nanofibers. The obtained flexible SiC nanofiber membrane has bendability, elasticity, heat resistance and strong electromagnetic wave absorption ability, and these characteristics endow it with broad application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a process schematic diagram for preparing the flexible SiC nanofiber membrane of the present invention;

[0025] Figure 2 is an SEM image of the flexible SiC nanofibers prepared in Example 1;

[0026] Figure 3 is an XRD schematic diagram of the flexible SiC nanofibers prepared in Example 1;

[0027] Figure 4Appearance diagrams of SiC samples obtained in the examples and comparative examples. In the figures: a represents the appearance diagram of the flexible SiC nanofiber membrane synthesized using the self-made silicon source (Example 1); b represents the appearance diagram of the flexible SiC nanofiber membrane aerogel synthesized using the self-made silicon source (Example 1); c represents the brittle SiC sample synthesized using the commercial hydrophobic silicon source (Example 2); d represents the SiC powder sample synthesized using the commercial hydrophobic silicon source (Example 2); e represents the brittle SiC sample synthesized using the commercial hydrophilic silicon source (Example 3); f represents the collapsed SiC sample synthesized with reference to CN113097469A (Comparative Example 1).

[0028] Figure 5 Schematic diagram of the compression mechanical experiment of the flexible SiC nanofiber membrane prepared in Example 1;

[0029] Figure 6 Infrared thermal imaging diagram of the heat insulation performance of the flexible SiC nanofiber membrane aerogel sample prepared in Example 1;

[0030] Figure 7 Three-dimensional electromagnetic wave absorption diagrams of the SiC fiber membranes obtained in the examples and comparative examples: (a) Flexible SiC nanofibers in Example 1; (b) Brittle SiC sample in Example 2; (c) Brittle SiC sample in Example 3; (d) SiC powder sample in Comparative Example 1. Detailed implementation mode

[0031] To make the present invention more obvious and understandable, preferred embodiments are described in detail below in conjunction with the accompanying drawings.

[0032] In the present invention, materials and reagents used without special instructions can be obtained from commercial channels. In the present invention, unless otherwise stated, the numerical range "a~b" represents the abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "5~12" means that all real numbers between "5~12" have been fully listed in this article, and "5~12" is only the abbreviated representation of these numerical combinations.

[0033] Example 1

[0034] (1) 0.5 mol / L TEOS and 0.5 mol / L ammonia water were respectively dissolved in an appropriate amount of absolute ethanol and then mixed. Then, the mixture was stirred at a constant temperature of 60 °C for 10 h, and finally centrifuged, washed with water 2 - 3 times, and freeze-dried for 24 h to obtain monodisperse SiO 2 microspheres with a particle size between 1 and 2000 nm.

[0035] (2) PAN and the monodisperse SiO 2The small balls and alkylphenol polyoxyethylene ether surfactant were sampled in a mass ratio of 6:14:3 and dissolved in an appropriate amount of tetrahydrofuran. They were stirred at 55 °C for 12 h to obtain a uniform spinning solution.

[0036] (3) The solution from step (2) was transferred into a spinning machine and spun at a propulsion rate of 0.1 mL / min. The distance between the collecting drum and the spinneret needle was 20 cm, the spinning voltage was 26 kV, and the rotational speed of the collecting drum was 400 r / min. After the spun fiber membrane was removed from the receiver, it was placed in a drying oven and vacuum-dried at 90 °C for 10 h.

[0037] (4) In an air atmosphere, the dried fiber membrane was heated to 200 °C at a rate of 5 °C / min and held for 4 h to complete pre-oxidation.

[0038] (5) The pre-oxidized fiber membrane was placed in a tube furnace for carbonization treatment. It was heated to 900 °C at a rate of 5 °C / min and held for 4 h. Subsequently, it was heated to 1500 °C at a rate of 3 °C / min and held for 7 h. A flexible SiC fiber membrane sample was obtained through carbothermal reduction, and the entire pyrolysis process was protected by argon.

[0039] (6) The pre-oxidized fiber cloth was stacked and placed in a tube furnace for carbonization treatment. It was heated to 900 °C at a rate of 5 °C / min and held for 4 h. Subsequently, it was heated to 1500 °C at a rate of 3 °C / min and held for 7 h. A SiC fiber membrane aerogel sample was obtained through carbothermal reduction, and the entire pyrolysis process was protected by argon.

[0040] The microscopic morphology of the flexible SiC nanofibers prepared in this example is shown in Figure 2 , and the XRD crystal structure is shown in Figure 3 . The appearance diagrams of the SiC flexible nanofiber membrane and the aerogel are shown in Figure 4 a and 4b. It can be seen that the prepared silicon carbide fiber membrane has good flexibility. The SEM image shows that the nanofibers maintain uniform size and continuity. The nanofiber membrane aerogel prepared by multi-layer stacking has good elasticity and excellent heat insulation performance as shown in Figure 5 and 6 . The wave absorption performance of the prepared flexible SiC fiber membrane is shown in Figure 7 a. When the thickness is 9.6 mm, it has the minimum reflection loss of -48.1 dB, and the effective absorption bandwidth EAB is 5.0 GHz. It can be seen that the continuous and flexible fibers prepared in this invention have good wave absorption performance.

[0041] Example 2

[0042] The difference between this example and Example 1 is only that: the hydrophobic nano-SiO 2 used is a commercially purchased product, with a diameter ranging from 20 to 2000 nm.

[0043] All the other contents are exactly the same as those described in Example 1.

[0044] The appearance diagram of the SiC fiber membrane prepared in this example is shown in Figure 4 c. It can be seen that the synthesized SiC fiber membrane using commercial hydrophobic nano-SiO with a series of sizes 2 is brittle and lacks flexibility, and some products show a crumbly granular shape as Figure 4 d.

[0045] In addition, the wave absorption performance of this brittle SiC, as can be seen from Figure 7 b, when the thickness is 7.2 mm, has the minimum reflection loss of -11.8 dB, and the effective absorption bandwidth EAB is 0.9 GHz. Therefore, the wave absorption performance is poor. Example 3

[0046] The difference between this example and Example 1 is only that: the hydrophilic nano-SiO of a commercially purchased product is used 2 , and the diameter is between 20 and 2000 nm.

[0047] All the other contents are exactly the same as those described in Example 1.

[0048] The appearance diagram of the SiC fiber membrane prepared in this example is shown in Figure 4 e. It can be seen that the synthesized SiC fiber membrane using commercial hydrophilic nano-SiO with a series of sizes 2 shows an overall brittle and easily breakable characteristic. In addition, the wave absorption performance of this brittle SiC, as can be seen from Figure 7 c, when the thickness is 6.2 mm, has the minimum reflection loss of -27.3 dB, and the effective absorption bandwidth EAB is 3.0 GHz. Therefore, the wave absorption performance is poor. Comparative Example 1

[0049] The technical solution of this example refers to CN113097469A.

[0050] (1) At 70 °C, polyacrylonitrile, commercial nano-silica, and N,N-dimethylformamide (DMF) were weighed according to a mass ratio of 1:3:5, and stirred evenly for 12 h to obtain a spinning solution. (2) The above spinning solution was placed in a high-voltage electrospinning machine, and electrospun at a voltage of 15 kV and a feeding speed of 0.8 mL / min to obtain a precursor film, and the precursor film was placed in a vacuum oven at 60 °C for 12 h. (3) In an air atmosphere, the above precursor film was heated to 280 °C at a rate of 1 °C / min and held for 2 h to obtain a SiO 2 / C nanofilm.

[0051] (4) Under argon conditions, SiO 2The Si / SiC / C nanofiber membrane was obtained by heating the Si / SiC / C nanofilm material to 1500 °C at a heating rate of 5 °C / min and holding for 2 h.

[0052] (5) The obtained Si / SiC / C nanofiber membrane was decarbonized at 800 °C for 2 h to obtain a pure Si / SiC nanofiber membrane.

[0053] During the preparation process of this example, it can be clearly seen that the synthesized SiC fibers do not have flexibility after high-temperature decarbonization, and collapse due to fiber fracture ( Figure 4 f). In addition, the wave absorption performance of the broken fibers is Figure 7 As can be seen from d, when the thickness is 7.1 mm, it has the minimum reflection loss of -31.5 dB, and the effective absorption bandwidth EAB is 4.2 GHz. The wave absorption performance is worse than that of the flexible SiC nanofiber membrane prepared in Example 1 of the present invention.

Claims

1. A method for preparing a flexible SiC fiber membrane, characterized in that: The following steps are involved: Step 1) Prepare monodisperse SiO2 beads: dissolve ethyl orthosilicate and ammonia water in anhydrous ethanol respectively, mix, and stir at a constant temperature to obtain a white suspension; centrifuge, wash with water and freeze-dry; Step 2), preparing a spinning solution: dissolving a spinning aid, monodisperse SiO2 beads and a surfactant in a spinning solvent, and stirring until the mixture is uniform to obtain a spinning solution; Step 3), electrospinning: spinning with a spinning solution to prepare SiC precursor nanofibers, and then drying in an oven at 50-100° C. for 3-12 hours; Step 4), pre-oxidation treatment: heat-treating the SiC precursor nanofibers in an air atmosphere to complete curing and cross-linking; Step 5), heat treatment: the SiC precursor nanofibers after pre-oxidation treatment are placed in a tubular furnace, first subjected to low-temperature carbonization under inert atmosphere conditions, and then subjected to carbon thermal reduction by heating to obtain a flexible SiC fiber membrane prepared using monodisperse SiO2 spheres.

2. The preparation method according to claim 1, characterized in that In the step 1), the concentration of tetraethyl orthosilicate in the white suspension is 0.1-1 mol / L, and the concentration of the ammonia solution is 0.1-1.1 mol / L; the volume fraction of the ammonia solution in the white suspension is 10-30%, and the obtained monodisperse SiO2 particle size is 1-2000 nm.

3. The preparation method according to claim 1, characterized in that: In the step 1), the constant temperature is 30 to 90° C.; the stirring time is 3 to 24 hours; and the freeze-drying time is 12 to 48 hours.

4. The preparation method according to claim 1, characterized in that: In the step 2), the spinning aid is any one or more of polyvinyl alcohol, polyacrylonitrile, polystyrene, polyester, and polyamide; the surfactant is any one or more of sodium dodecyl sulfate, polysorbate, alkylphenol polyoxyethylene ether, and polyoxyethylene fatty acid ester; and the spinning solvent is one or more of N,N-dimethylformamide, sodium thiocyanate, formic acid, tetrahydrofuran, dimethyl sulfoxide, and acetone.

5. The preparation method according to claim 1, characterized in that: In the step 2), the mass ratio of the spinning aid, the monodisperse SiO2 beads, and the surfactant is 1-20:1-40:1-10; the stirring temperature is 30-90° C., and the stirring time is 5-24 hours.

6. The preparation method according to claim 1, characterized in that: In the step 3), the spinning process parameters are: spinning voltage 10-30 kV, spinning solution advancing speed 0.005-0.5 mL / min, rotation speed 100-500 r / min, and the distance between the collecting drum and the spinneret needle is 15-25 cm.

7. The preparation method according to claim 1, characterized in that: In the step 4), the temperature of the heat treatment curing and cross-linking is 150-300° C., the heating rate is 1-5° C. / min, and the constant temperature time is 1-5 hours.

8. The preparation method according to claim 1, characterized in that: In the step 5), the temperature of low-temperature carbonization is 300-1000°C, the heating rate is 1-10°C / min, and the constant temperature time is 1-5h; the heating rate of carbon thermal reduction is 1-10°C / min, the heating target is 1200-1600°C, and the constant temperature time is 1-10h.

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