A wave-absorbing and heat-insulating integrated SiC fiber hard foam and a preparation method thereof

Abstract

This invention provides an integrated microwave absorption and thermal insulation SiC fiber rigid foam and its preparation method. Polycarbosilane is used as the main precursor, tetrahydrofuran and ethanol as solvents, and polyvinylpyrrolidone as a spinning aid. Aluminum nitrate nonahydrate is doped into the foam as an aluminum source to control the conductivity and compositional uniformity of the precursor spinning solution. A continuous and uniform precursor fiber membrane is prepared by electrospinning, followed by thermosetting and high-temperature ceramicization to obtain a structurally stable and high-performance SiC fiber rigid foam. The introduction of aluminum promotes the formation of the graphitic carbon phase and Al-O-C and Al-O-Si structures, improving the material's dielectric loss capacity and thermal insulation performance. Test results show that the material has a high dielectric constant, low thermal conductivity, and good compressive strength, making it suitable for high-temperature electromagnetic shielding, thermal insulation, and other fields. This method has a simple preparation process, and the properties of the obtained material are adjustable, showing good engineering application prospects.

Description

Technical Field

[0001] This invention belongs to the field of integrated structural and functional ceramic materials, specifically relating to a rigid SiC fiber foam with integrated wave absorption and heat insulation and its preparation method. Background Technology

[0002] The urgent need for stealth technology and thermal protection materials in extreme environments for advanced equipment has driven the development of functional materials towards an integrated approach of wave absorption, thermal insulation, and structural load-bearing. Currently, electromagnetic wave absorbing materials mainly focus on carbon-based materials (such as graphene and carbon nanotubes) and magnetic metal filler systems. These materials achieve good wave absorption capabilities through dielectric or magnetic losses, but they generally suffer from defects such as poor thermal stability, insufficient structural rigidity, low strength, and high density, making it difficult to maintain stable operation in high-temperature or high-load environments for extended periods.

[0003] Existing research has made some progress in improving the flexibility, wrappability, and impact resistance of materials, showing potential applications in fields such as electrically insulating films and flexible heating elements. However, the core of research on this type of flexible ceramic material focuses on "flexibility" and "deformability," lacking systematic solutions for the overall structural rigidity, load-bearing capacity, and strength performance under high-temperature service. In composite structures, it still suffers from low shear strength and weak structural support, making it difficult to meet the engineering application requirements in high-pressure, high-load environments.

[0004] Currently, there are methods to improve the compressive strength of SiC foam by adding a reinforcing coating to the outer wall. However, this method relies on a complex impregnation and re-sintering process and can only improve local mechanical properties. It cannot fundamentally enhance the overall structural rigidity of the foam and does not take into account its electromagnetic and thermal properties, so its application range remains limited.

[0005] Existing methods also involve the preparation of high-purity α-Al₂O₃ ceramic fibers. Although these materials possess good thermal stability and insulation, they generally suffer from high brittleness and poor impact resistance. This method even requires the introduction of a second-phase material (such as SiO₂ or B₂O₃) to improve its toughness; however, the introduction of the second phase may adversely affect its high-temperature performance, revealing the inherent limitations of single-component ceramic fibers in controlling structural properties.

[0006] Bulk ceramic materials also suffer from insufficient mechanical properties. Existing research indicates that while porous or foam-based ceramic materials (such as SiC foam) used for microwave absorption and thermal insulation exhibit excellent thermal stability and low thermal conductivity (as low as 0.05–0.2 W / m·K), their mechanical strength still falls short of structural requirements. Their compressive strength is only about 1.44 MPa, far below the strength required for structural protection materials. These materials are prone to brittle fracture under high temperatures or loads, resulting in poor reliability and limiting their application in multifunctional integrated materials.

[0007] Therefore, in the existing technology system, flexible ceramic materials are too soft, bulk foam ceramics are too brittle, and single-component oxide ceramics have low performance, making it difficult to balance structural strength, thermal stability, and wave absorption performance.

[0008] While the polymer precursor conversion (PDC) method for preparing SiC nanofibers is widely used in the field of functional ceramics, problems often arise during electrospinning due to the low conductivity of PCS itself. These problems include Taylor cone instability, uneven fiber thickness, and even fiber breakage and droplet formation. Furthermore, pure PCS, during high-temperature ceramization, results in fibers with a loose structure and high porosity but poor overall formability and weak inter-fiber bonding. In addition, undoped SiC fibers exhibit limited dielectric properties and insufficient microwave absorption in the high-frequency electromagnetic band, failing to meet the comprehensive requirements of integrated infrared and electromagnetic compatibility microwave absorption and insulation materials. Summary of the Invention

[0009] 1. The technical problem to be solved:

[0010] To develop a SiC-based nanofiber system with a dense structure, good adaptability to heat treatment, and controllable doping, so as to achieve the control of fiber connection and density at the microscale, improve the overall rigidity and mechanical support performance of the material at the macroscale, and take into account multifunctional properties such as wave absorption and heat insulation.

[0011] 2. Technical Solution:

[0012] To address the above problems, this invention provides a method for preparing an integrated microwave absorption and thermal insulation SiC fiber rigid foam, comprising the following steps:

[0013] Step S01: Preparation of precursor spinning solution: Mix ethanol and aluminum nitrate nonahydrate and stir until completely dissolved. Then add tetrahydrofuran and polyvinylpyrrolidone and stir until completely dissolved. Then add polycarbosilane and continue stirring to obtain a uniform precursor spinning solution.

[0014] Step S02: Electrospinning: The precursor spinning solution is loaded into a syringe and electrospinned through a needle to obtain electrospinned fiber sheets.

[0015] Step S03: Thermal curing treatment: Curing the obtained electrospun fiber sheet to obtain the precursor fiber sheet;

[0016] Step S04: Lamination and Pressurization: Cut several precursor fiber sheets into uniform sizes, stack them to form a fiber felt of a certain thickness, cover the upper surface with a high-purity graphite plate or ceramic plate, and apply constant pressure.

[0017] Step S05: Ceramicization treatment: Place the fiber felt in a nitrogen atmosphere, heat it to 1200-1500℃, keep it at the temperature and then cool it to complete the preparation of SiC fiber rigid foam.

[0018] In step S01, in every 10mL-15mL of precursor spinning solution, there are 2-5mL of ethanol, 0.005-0.02g of aluminum nitrate nonahydrate, 0.5-2g of polyvinylpyrrolidone, 1-3g of polycarbosilane, and the remainder is tetrahydrofuran.

[0019] In step S02, the electrospinning voltage is 10-16.5 kV, the injection speed is 3-5 mL / h, the collection distance is 10-20 cm, and the collector rotation speed is 5-500 rad / min.

[0020] The needle model is one of G20, G21, G22, and G24.

[0021] In step S03, the obtained electrospun fiber sheet is cured at 180-220℃ for 1-3 hours.

[0022] In step S04, a constant external pressure of 20-40 Pa is applied to the precursor fiber sheet.

[0023] In step S05, the temperature is increased at 2-8℃ / min and held for 1-3 hours.

[0024] The present invention also provides an integrated SiC fiber rigid foam, which is prepared by the aforementioned preparation method.

[0025] 3. Beneficial effects:

[0026] This invention introduces an aluminum-containing electrolyte (aluminum nitrate nonahydrate) into a SiC precursor system, and constructs an aluminum-modified fiber structure through electrospinning and PDCs. During sintering, aluminum promotes the ordering and graphitization of the carbon phase interface structure, forming Al-O-Si or Al-OC bridging structures, thus improving the inter-fiber connectivity and overall stability. This method significantly improves the overall rigidity and mechanical support capacity of the material while maintaining its lightweight and porous characteristics, overcoming the bottleneck of insufficient structural performance in traditional flexible microwave absorbing materials. Attached Figure Description

[0027] Figure 1 This is a flowchart of the preparation method of the present invention.

[0028] Figure 2 The compressive stress-strain curves of Examples 1-4 after high-temperature heat treatment under nitrogen atmosphere;

[0029] Figure 3The images show 3D diagrams of X-band reflection loss after high-temperature heat treatment in a nitrogen atmosphere for Examples 1-4. Figure 3 (a) represents the case where aluminum nitrate nonahydrate is not doped; Figure 3 (b) represents the doping of 0.005g of aluminum nitrate nonahydrate; Figure 3 (c) represents the doping of 0.01g of aluminum nitrate nonahydrate; Figure 3 (d) represents the doping of 0.02g aluminum nitrate nonahydrate, all of which are in 10-15mL of precursor spinning solution. Detailed Implementation

[0030] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0031] This invention proposes introducing an aluminum-containing electrolyte (aluminum nitrate nonahydrate) into the PCS spinning precursor solution. By controlling the conductivity and component ratio of the solution system, spinning performance is improved from the solution stage, and stable interfacial structures such as Al-O-Si or Al-OC are guided to form during heat treatment, thereby enhancing the mechanical properties and electromagnetic response of ceramic fibers. Polyvinylpyrrolidone (PVP) is introduced into the system as a spinning aid, which both increases the viscosity of the solution and provides dispersibility in the precursor. Specifically, a method for preparing rigid SiC fiber foam doped with an aluminum-containing electrolyte is described below. Figure 1 As shown, it includes the following steps:

[0032] Step S01: Solution preparation: Mix 2-5 mL of ethanol with 0.02 g of aluminum nitrate nonahydrate and stir until completely dissolved, then add tetrahydrofuran; add 0.5-2 g of polyvinylpyrrolidone (PVP) to the resulting solution and stir until completely dissolved; add PCS: add 1-3 g of polycarbosilane (PCS) and continue stirring for 24-48 hours to obtain a uniform precursor spinning solution.

[0033] Step S02: Electrospinning: The precursor spinning solution is loaded into a syringe and electrospinned using one of the G20, G21, G22, or G24 needles. The voltage is 10-16.5 kV, the injection speed is 3-5 mL / h, the collection distance is 10-20 cm, and the collector rotation speed is 5-500 rad / min, forming electrospinned fiber sheets.

[0034] Step S03: Thermal curing treatment: Place the obtained fibers at 180-220℃ for 1-3 hours to stabilize the structure and obtain precursor fiber sheets.

[0035] Step S04: Lamination and Pressurization: Cut multiple cured precursor fiber sheets into uniform sizes and laminate them to form a fiber mat with a thickness of 2-4 cm. Apply a constant external pressure of 20-40 Pa to the fiber mat. The uniform size can be customized.

[0036] Step S05: Ceramicization treatment: Place the fiber felt in a nitrogen atmosphere, heat it to 1200-1500℃ at 2-8℃ / min, keep it at that temperature for 1-3 hours and then cool it to complete the preparation of SiC fiber rigid foam.

[0037] Example 1

[0038] A method for preparing SiC fiber rigid foam without aluminum nitrate nonahydrate doping. Specifically, the method includes: mixing 2 mL of anhydrous ethanol with 9 mL of tetrahydrofuran, adding 0.5 g of polyvinylpyrrolidone (PVP) and dissolving it completely, then adding 1 g of polycarbosilane (PCS), and stirring continuously for 36 hours to obtain a precursor spinning solution. This solution is then used for electrospinning under the following conditions: voltage 16.5 kV, injection rate 3 mL / h, collection distance 15 cm, needle type G22, and collector rotation speed 300 rad / min to form an electrospun fiber sheet. The obtained electrospun fiber sheet is cured at 220°C for 1 hour to obtain a precursor fiber sheet. The precursor fiber sheet is cut and stacked to obtain a 3 cm thick fiber mat, and then subjected to a pressure of 25 Pa and heat-treated at 1500°C at a heating rate of 2 °C / min under a nitrogen atmosphere for 2 hours to complete ceramization. The obtained SiC fiber rigid foam has a compact overall structure. XRD analysis showed that the SiO2 amorphous peak was the main feature, while Raman spectroscopy showed that the degree of graphitization was low, the dielectric loss was small, and the microwave absorption performance was limited.

[0039] Example 2

[0040] A rigid SiC fiber foam doped with 0.005 g of aluminum nitrate nonahydrate. Specifically, the process involves mixing 3 mL of anhydrous ethanol with 8 mL of tetrahydrofuran, adding 1 g of polyvinylpyrrolidone (PVP) to dissolve it completely, then adding 3 g of polycarbosilane (PCS), and stirring continuously for 24 hours to obtain a precursor spinning solution. This solution is then used for electrospinning under the following conditions: voltage 12 kV, injection rate 3 mL / h, collection distance 15 cm, needle type G21, and collector rotation speed 200 rad / min to form an electrospun fiber sheet. The obtained electrospun fiber sheet is cured at 180 °C for 2 hours to obtain a precursor fiber sheet. The precursor fiber sheet is then cut and stacked to obtain a 4 cm thick fiber mat, and subjected to a pressure of 20 Pa. The mat is then heat-treated at 1200 °C with a heating rate of 5 °C / min under a nitrogen atmosphere for 3 hours to complete ceramization. The resulting SiC fiber rigid foam structure is more porous than the undoped aluminum sample, with enhanced graphite phase and a significantly improved D / G intensity ratio in the Raman spectrum. XRD analysis still shows a predominantly amorphous state but with some SiC formation. This material exhibits a low thermal conductivity of approximately 0.05 W / m·K, improved electromagnetic properties, an imaginary dielectric part of 7.6, and good microwave absorption and thermal insulation properties.

[0041] Example 3

[0042] Rigid SiC fiber foam was prepared by doping with 0.01 g of aluminum nitrate nonahydrate. Specifically, 3 mL of anhydrous ethanol and 10 mL of tetrahydrofuran were mixed, followed by the addition of 2 g of polyvinylpyrrolidone (PVP) for complete dissolution, and then 1.5 g of polycarbosilane (PCS) was added. The mixture was stirred continuously for 48 hours to obtain a precursor spinning solution. This solution was electrospun into electrospun fiber sheets under the following conditions: voltage 10 kV, injection rate 5 mL / h, collection distance 10 cm, needle type G24, and collector rotation speed 500 rad / min. The obtained electrospun fiber sheets were cured at 200 °C for 3 hours to obtain precursor fiber sheets. These precursor fiber sheets were cut and stacked to obtain a 2 cm thick fiber mat, which was then subjected to a pressure of 35 Pa and heat-treated at 1300 °C at a heating rate of 8 °C / min under a nitrogen atmosphere for 1 hour to complete ceramization. The obtained SiC fiber rigid foam has a high overall porosity, XRD analysis shows a high degree of crystallization, Raman spectroscopy shows that the degree of graphitization increases in low-doped samples, the Raman D / G intensity ratio decreases to 1.257, the dielectric loss is reduced, and the microwave absorption performance is further enhanced to -9.1dB.

[0043] Example 4

[0044] Rigid SiC fiber foam was prepared by doping with 0.02 g of aluminum nitrate nonahydrate. Specifically, 5 mL of anhydrous ethanol and 8 mL of tetrahydrofuran were mixed, followed by the addition of 1 g of polyvinylpyrrolidone (PVP) for complete dissolution, and then 1.5 g of polycarbosilane (PCS) was added. The mixture was stirred continuously for 36 hours to obtain a precursor spinning solution. This solution was electrospun into electrospun fiber sheets under the following conditions: voltage 12 kV, injection rate 4 mL / h, collection distance 20 cm, needle type G20, and collector rotation speed 5 rad / min. The obtained electrospun fiber sheets were cured at 180°C for 1 hour to obtain precursor fiber sheets. These precursor fiber sheets were cut and stacked to obtain a 4 cm thick fiber mat, which was then subjected to a pressure of 40 Pa and heat-treated at 1400°C with a heating rate of 2°C / min under a nitrogen atmosphere for 3 hours to complete ceramization. The resulting rigid SiC fiber foam exhibited decreased structural integrity, particle aggregation, and fiber coarsening. EDS analysis showed uneven aluminum distribution. The XRD diffraction peak intensity decreased, and the Raman D / G intensity ratio decreased overall, indicating an increase in carbon phase structural defects and a reduction in graphitization. The dielectric properties of this sample degraded compared to the 1% doped sample, with an imaginary part of approximately 5.4. The porosity decreased, but the density did not translate into performance advantages.

[0045] The compressive stress-strain curves of Examples 1-4 after high-temperature heat treatment under nitrogen atmosphere are as follows: Figure 2 As shown, AL-0 represents 0% doping of aluminum nitrate nonahydrate, AL-0.5 represents 0.005g doping of aluminum nitrate nonahydrate, AL-1 represents 0.01g doping of aluminum nitrate nonahydrate, and AL-2 represents 0.02g doping of aluminum nitrate nonahydrate. All of these figures represent doping of aluminum nitrate nonahydrate in 10-15mL of precursor spinning solution. Figure 2 It can be seen that the compressive stress of aluminum nitrate nonahydrate is the lowest without doping.

[0046] Best Practices

[0047] Add 5 mL of anhydrous ethanol to a flask, and weigh 0.01 g of aluminum nitrate nonahydrate into it and stir to dissolve, forming an aluminum-containing transparent solution. Then add 8 mL of tetrahydrofuran (THF) and mix well. Next, add 1 g of PVP and stir until completely dissolved. Finally, slowly add 1.5 g of polycarbosilane (PCS) and continue stirring for 24 hours to obtain a homogeneous and stable precursor spinning solution. Compared with the aluminum-free electrolyte system, the conductivity of the aluminum nitrate nonahydrate solution in this example is increased to 6.73 μS / cm, the spinning current is more stable, the spray morphology is more stable, and the fiber formability is significantly improved.

[0048] After the solution was prepared, it was loaded into a syringe, and electrospinning was performed using a G20 needle. The spinning voltage was set to 12 kV, the injection rate to 4 mL / h, the distance between the spinning needle and the collecting roller to 20 cm, the roller speed to 5 rad / min, the room temperature to 25°C, and the humidity to 35%. The resulting electrospun fiber sheets were dense and continuous, with no obvious droplet defects.

[0049] Electrospun fiber sheets were placed in a hot air drying oven and kept at 200℃ for 1 hour to complete thermosetting and obtain precursor fiber sheets. These were then cut into 45mm × 45mm sheets, stacked to a thickness of approximately 40mm, and covered with graphite paper under a constant load of 40Pa to maintain a smooth morphology and prevent bulging during subsequent high-temperature treatment. The samples were heat-treated at 1300℃ under a nitrogen atmosphere for 2 hours and then cooled to complete the ceramic transformation of the precursor. At high temperatures, aluminum partially forms Al-OC and Al-O-Si network structures, effectively enhancing the cross-linking between fibers and inducing partial graphitic carbon phase formation, further improving dielectric loss performance. Figure 3 As shown, from Figure 3 (a) Figure 3 (b) Figure 3 (c) and Figure 3 (d) It can be seen that the dielectric properties are the best when 0.01 g of aluminum nitrate nonahydrate is doped in 10-15 mL of precursor spinning solution.

[0050] Characterization tests on the heat-treated sample showed that its dielectric constant had a real part of 4.88 and an imaginary part of 9.71 in both the X-band and Ku-band, with a loss tangent as high as 1.99, indicating good dielectric loss capability. Thermal conductivity tests showed a thermal conductivity of 0.097 W / m·K at 100°C, demonstrating excellent thermal insulation performance. The compressive strength was 13.932 MPa, exhibiting good structural stability even under high porosity (72.77%) conditions. These results verify that the SiC fiber rigid foam prepared by doping with aluminum-containing electrolytes in this invention is superior to the undoped system in terms of electromagnetic properties, thermal properties, and mechanical stability, demonstrating its application potential and practical engineering value in the field of integrated microwave absorption and thermal insulation protective materials.

Claims

1. A method for preparing an integrated microwave absorption and heat insulation SiC fiber rigid foam, characterized in that: Includes the following steps: Step S01: Preparation of precursor spinning solution: Mix ethanol and aluminum nitrate nonahydrate and stir until completely dissolved. Then add tetrahydrofuran and polyvinylpyrrolidone and stir until completely dissolved. Then add polycarbosilane and continue stirring to obtain a homogeneous precursor spinning solution. In every 10mL-15mL of precursor spinning solution, there are 2-5mL of ethanol, 0.005-0.02g of aluminum nitrate nonahydrate, 0.5-2g of polyvinylpyrrolidone, 1-3g of polycarbosilane, and the remainder is tetrahydrofuran. Step S02: Electrospinning: The precursor spinning solution is loaded into a syringe and electrospinned through a needle to obtain electrospinned fiber sheets. Step S03: Thermal curing treatment: Curing the obtained electrospun fiber sheet to obtain the precursor fiber sheet; Step S04: Lamination and Pressurization: Several precursor fiber sheets are cut into uniform sizes, stacked to form a fiber felt of a certain thickness, and covered with a high-purity graphite plate or ceramic plate. A constant external pressure of 20-40 Pa is applied to the SiC fiber rigid foam to maintain a smooth morphology and prevent bulging during subsequent high-temperature treatment. Step S05: Ceramicization treatment: Place the fiber felt in a nitrogen atmosphere, heat it to 1200-1500℃, keep it at the temperature and then cool it to complete the preparation of SiC fiber rigid foam.

2. The preparation method according to claim 1, characterized in that: In step S02, the electrospinning voltage is 10-16.5 kV, the injection speed is 3-5 mL / h, the collection distance is 10-20 cm, and the collector rotation speed is 5-500 rad / min.

3. The preparation method according to claim 2, characterized in that: The needle model is one of G20, G21, G22, and G24.

4. The preparation method according to claim 1, characterized in that: In step S03, the obtained electrospun fiber sheet is cured at 180-220℃ for 1-3 hours.

5. The preparation method according to claim 1, characterized in that: In step S05, the temperature is increased at 2-8℃ / min and held for 1-3 hours.

6. An integrated SiC fiber rigid foam, characterized in that: It is prepared by the preparation method according to any one of claims 1-5.

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