Preparation method of bionic mullite fiber sound absorption material with density gradient

Abstract

The invention discloses a preparation method of a bionic mullite fiber sound absorption material with density gradient, and relates to a preparation method of a mullite fiber sound absorption material. The invention aims to solve the technical problems that the existing sound absorption material is insufficient in low and medium frequency sound absorption performance, unstable in structure, poor in high-temperature performance, limited in mechanical strength and easy to deform and damage. The mullite fiber layered porous material with moth wing bionic structure characteristics is successfully constructed by taking mullite fibers as a base material and adopting a sedimentation-layer-by-layer assembly process in combination with a freezing injection molding technology, and the material shows regular layer-by-layer assembly characteristics on a macroscopic scale, and has the characteristics of good mechanical properties, good mechanical properties and good mechanical properties. And a three-dimensional pore network structure formed by staggered lap joint of chopped fibers is formed on the microscopic scale. Meanwhile, accurate design of structural parameters of the material is achieved by regulating and controlling interlayer density, thickness and a fiber sedimentation mode, so that the sound absorption capacity of the material in middle and high frequency bands is remarkably improved, and the material has good compression resilience performance.

Description

Technical Field

[0001] The invention relates to a method for preparing a mullite fiber sound-absorbing material. Background Art

[0002] With the acceleration of urbanization and the widespread use of industrial equipment, noise pollution is becoming increasingly serious and has become a significant factor affecting human health and the production environment. In fields such as transportation, industrial manufacturing, and aerospace, higher demands are being placed on efficient, durable, and structurally stable sound-absorbing materials. Currently, widely used porous sound-absorbing materials, such as polyurethane foam, mineral wool, and fiberglass mat, while capable of absorbing mid- and high-frequency sound, have mature manufacturing processes, and are relatively low in cost. However, they generally suffer from poor mid- and low-frequency sound absorption, weak thermal stability, and structural collapse, making them difficult to adapt to complex or high-temperature operating conditions.

[0003] In recent years, biomimetic structures have become an important area of ​​research in functional material design. Natural layered structures, such as moth wings, exhibit an orderly, stacked distribution of scales at the microscopic level, resulting in excellent broadband sound absorption properties. This layered structure enables multiple scattering and energy dissipation of sound waves, providing insights into the development of high-performance sound-absorbing materials.

[0004] In the field of fiber materials, most existing studies use a high-fiber content system combined with direct compression molding to construct porous sound-absorbing structures, mainly by inducing fiber orientation in the flow through high pressure, thereby obtaining a certain structural integrity and mechanical strength. For example, Song (Perez, C., Song, Y., Gandhi, U., Yang, A., Vallury, S., & Osswald, T. (2017). Method to account for the fiber orientation of the initial charge on the fiber orientation of finished part in compression molding simulation. Composites Part A-applied Science and Manufacturing, 100, 244-254) et al. used X-ray tomography to model the molding process of high volume fraction fiber-reinforced thermoplastics, pointing out that the initial orientation of the fiber in the compression molding stage significantly affects the final structure; Saeed (Saeed, U., & Rizvi, G. (2015). Three-dimensional orientation of compression-molded high-densitypolyethylene / wood fibersusing X-ray micro-tomography. Journal of CellularPlastics, 51, 45 - 57) et al. used micro-CT to study the effect of different wood fiber contents on fiber orientation in thermoplastic composites, also relying on high concentration slurry to force compression molding in the mold. These processes generally ignore the natural orientation and structural control of fibers during the sedimentation stage, resulting in poor structural stratification and uneven pore distribution, making it difficult to achieve effective regulation of the sound absorption frequency band.

[0005] To address these challenges, the present invention provides a layered porous sound-absorbing material with mullite fibers that mimics the structure of a moth's wing. Prepared from a low-fiber slurry, this material is assembled using a layered process that combines natural sedimentation-induced orientation with reverse pressure drainage. While ensuring both thermal stability and mechanical properties, this method achieves precise control over structural order, interlayer density gradient, and sound absorption performance. This technological approach transcends the traditional pressing method's reliance on disordered fiber stacking, fully demonstrating the enhanced sound absorption performance of a biomimetic layered structure, promising broad engineering applications. Summary of the Invention

[0006] The present invention aims to solve the technical problems of existing sound-absorbing materials, such as insufficient mid- and low-frequency sound absorption performance, unstable structure, poor high-temperature performance, limited mechanical strength and easy deformation and damage, and provides a method for preparing a bionic mullite fiber sound-absorbing material with a density gradient.

[0007] The preparation method of the bionic mullite fiber sound-absorbing material with a density gradient of the present invention is carried out according to the following steps:

[0008] 1. Mullite fiber removal and short cutting:

[0009] The mullite fibers are cut to a length of 200 μm to 500 μm and sieved to remove the larger diameter slag balls. The mullite fibers are then placed in deionized water and allowed to settle due to the higher density of the slag balls than water. The mullite fibers are then removed from the water and dried to obtain pure chopped mullite fibers.

[0010] 2. Preparation of mullite fiber dispersion slurry containing silica sol:

[0011] The pure short-cut mullite fiber prepared in step 1 is added to deionized water, silica sol is added as an inorganic binder, and a uniform slurry is prepared by high-speed stirring;

[0012] The mass of the mullite fiber in the slurry is 1.5% to 2% of the mass of the deionized water, and the silica sol accounts for 5% to 6% of the total mass of the slurry, ensuring good fiber bonding and stability of subsequent drying and molding;

[0013] 3. Layered construction and pressing:

[0014] ①. Pour the slurry prepared in step 2 into a mold and then let it stand for 3 to 5 minutes to allow the fibers to gradually transform into horizontal sedimentation under the action of fluid-solid coupling; then apply a gauze-wrapped pressing sheet to the surface of the slurry, manually apply slow pressure to drain free water from the pressing sheet and pour it out, so that the fibers overlap to form a dense layered structure, remove the pressing sheet, and obtain the slurry after the bottom layer is drained; the pressing sheet is evenly provided with multiple through holes;

[0015] ② Repeat the process of step ①, adding different amounts of slurry layer by layer on the drained slurry at the bottom layer to build a multi-layer structure. The thickness of each layer of drained slurry is 5 cm to 10 cm, and a multi-layer density gradient wet sample is obtained;

[0016] In this step, multi-layer molding in the wet state is beneficial to the bonding between layers, reducing layer peeling and achieving integrated molding of gradient structure;

[0017] 4. Freezing and drying treatment: The multi-layer wet sample prepared in step 3 is frozen together with the mold to completely crystallize the internal moisture, and then transferred to a freeze dryer for sublimation drying to obtain a bionic mullite fiber sound-absorbing material with a density gradient.

[0018] In step three of the present invention, by adjusting the amount of slurry added to each layer, the density gradient difference between layers can be controlled, thereby adjusting and optimizing the sound absorption frequency band of the material. The mass in the density calculation defaults to the mass of the dry short-cut mullite fiber in the slurry (the mass of the silica sol is negligible, and the water is eventually removed and therefore does not count). The volume can be controlled by the downward pressure position of the tablet. When pressing down, water is discharged from the through hole on the tablet to the top of the tablet and then poured out.

[0019] The fiber content of the slurry in step 2 of the present invention is low in order to make the fibers evenly dispersed and settled and overlapped with each other. If the fiber content is too high, it will cause fiber agglomeration, which is not conducive to the construction of the network structure and performance improvement.

[0020] The wet layer-by-layer assembly in step three of the present invention improves interlayer bonding and prevents layer peeling, thereby constructing an integrated structure with gradient density. Free settling and press drainage control enable precise density of each layer simply by controlling thickness along the pressing direction, laying a solid technical foundation for gradient density control.

[0021] This invention uses mullite fiber as a matrix material, employs a sedimentation-layered assembly process, and combines it with freeze-injection molding technology to successfully construct a mullite fiber-based porous material with the biomimetic structural characteristics of a moth wing. On a macroscale, the material exhibits a regular layered assembly pattern, while on a microscale, it forms a three-dimensional pore network composed of interlaced chopped fibers. Simultaneously, by regulating interlayer density, thickness, and fiber sedimentation, the material's structural parameters are precisely designed, significantly enhancing its sound absorption in the mid- and high-frequency ranges while also exhibiting excellent compression and rebound properties.

[0022] The bionic layered sound-absorbing material constructed by the present invention provides a new path for high-efficiency, wide-band, and structurally adjustable noise control materials. It is particularly suitable for noise control needs under high temperature, high vibration, and complex working conditions, and has broad engineering application prospects and industrial value.

[0023] The biomimetic mullite fiber sound-absorbing material with a density gradient prepared by the present invention has significant characteristics in terms of structural construction and functional performance: on a macroscopic scale, the material presents a multilayer structure distributed along the thickness direction, and each layer is constructed through a pressurized drainage process to form a dense and uniform fiber layer; on a microscopic scale, a three-dimensional pore network is formed by the staggered overlap of short-cut mullite fibers, and the fibers are mainly oriented along the horizontal plane, effectively increasing the propagation path of sound waves within the material, enhancing multiple scattering and energy dissipation capabilities, and thus improving the sound absorption performance in the medium and low frequency bands. By regulating the mass of the fibers in different layers, the present invention can achieve a gradient design of interlayer density, further improving the transmission behavior and frequency band response of sound waves in the material, and enhancing the broadband sound absorption and directional adjustability of the material. The present invention adopts a freeze-drying method during the preparation process, avoiding the structural collapse and pore closure that may be caused by high-temperature sintering, retaining the original layered pore structure to the greatest extent, while simplifying the process flow and reducing energy consumption. The sound-absorbing material prepared by the present invention has both excellent thermal stability and compression resilience, and is suitable for noise control applications under complex working conditions such as high temperature and high vibration, and exhibits comprehensive advantages in structural hierarchy, functional integration and processability.

[0024] The biomimetic mullite fiber sound-absorbing material with a density gradient, prepared by this invention, draws inspiration from the natural layered structure of moth wings. Combining fiber sedimentation with a pressurized drainage process, it achieves a multi-scale ordered structure with macroscopic stratification and microscopic interlaced fiber stacking. This effectively extends the sound wave propagation path within the material, enhancing the scattering and dissipation of sound energy, and significantly improving mid- and low-frequency sound absorption. The material exhibits excellent sound absorption across a wide frequency range of 500Hz to 4000Hz, with a noise reduction coefficient (NRC) of up to 0.47, particularly in the 1000Hz to 1600Hz band. Compared to traditional polymer-based sound-absorbing materials, this material, based on mullite ceramic fibers, possesses excellent thermal stability and structural integrity, withstanding operating temperatures up to 1300°C, addressing the problem of carbonization and degradation of sound-absorbing materials in high-temperature environments. Furthermore, the material exhibits excellent compression resilience and deformation resistance, promising long-term service under complex loads and frequent vibration conditions. The freeze-drying method used during the preparation process solidifies the structure, eliminating the need for high-temperature sintering. This simplifies the process and reduces energy consumption and costs. By adjusting the slurry composition, settling time, and pressing conditions, the number of layers, density, and thickness of the material can be precisely controlled, enabling customized performance. Furthermore, this biomimetic design approach provides an innovative path for novel structurally and functionally integrated sound-absorbing materials, demonstrating strong engineering adaptability and industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A photo of the bionic mullite fiber sound-absorbing material with density gradient prepared for Experiment 1;

[0026] Figure 2 This is the SEM image of the bionic mullite fiber sound-absorbing material prepared in Experiment 1;

[0027] Figure 3 Multi-scale model diagram for the biomimetic design of moth wings;

[0028] Figure 4 is the cyclic compression mechanical behavior diagram;

[0029] Figure 5 The sound absorption performance control law diagram of the bionic mullite fiber sound absorbing material prepared in Experiments 1 to 4;

[0030] Figure 6 This is a photo of the bionic mullite fiber sound-absorbing material with density gradient prepared in experiment one. DETAILED DESCRIPTION

[0031] Specific embodiment 1: This embodiment is a method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient, which is specifically carried out in the following steps:

[0032] 1. Mullite fiber removal and short cutting:

[0033] The mullite fibers are cut to a length of 200 μm to 500 μm and sieved to remove the larger diameter slag balls. The mullite fibers are then placed in deionized water and allowed to settle due to the higher density of the slag balls than water. The mullite fibers are then removed from the water and dried to obtain pure chopped mullite fibers.

[0034] 2. Preparation of mullite fiber dispersion slurry containing silica sol:

[0035] The pure short-cut mullite fiber prepared in step 1 is added to deionized water, silica sol is added as an inorganic binder, and a uniform slurry is prepared by high-speed stirring;

[0036] The mass of the mullite fiber in the slurry is 1.5% to 2% of the mass of the deionized water, and the silica sol accounts for 5% to 6% of the total mass of the slurry, ensuring good fiber bonding and stability of subsequent drying and molding;

[0037] 3. Layered construction and pressing:

[0038] ①. Pour the slurry prepared in step 2 into a mold and then let it stand for 3 to 5 minutes to allow the fibers to gradually transform into horizontal sedimentation under the action of fluid-solid coupling; then apply a gauze-wrapped pressing sheet to the surface of the slurry, manually apply slow pressure to drain free water from the pressing sheet and pour it out, so that the fibers overlap to form a dense layered structure, remove the pressing sheet, and obtain the slurry after the bottom layer is drained; the pressing sheet is evenly provided with multiple through holes;

[0039] ② Repeat the process of step ①, adding different amounts of slurry layer by layer on the drained slurry at the bottom layer to build a multi-layer structure. The thickness of each layer of drained slurry is 5 cm to 10 cm, and a multi-layer density gradient wet sample is obtained;

[0040] 4. Freezing and drying treatment: The multi-layer wet sample prepared in step 3 is frozen together with the mold to completely crystallize the internal moisture, and then transferred to a freeze dryer for sublimation drying to obtain a bionic mullite fiber sound-absorbing material with a density gradient.

[0041] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the sieving in step 1 is 300 mesh. Other aspects are the same as specific embodiment 1.

[0042] Specific embodiment 3: This embodiment differs from specific embodiment 1 or 2 in that the drying temperature in step 1 is 60° C. to 90° C. The rest is the same as specific embodiment 1 or 2.

[0043] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: the silica sol described in step 2 is prepared by acid-catalyzed hydrolysis, and the specific process is: ethyl orthosilicate is mixed with anhydrous ethanol, deionized water and dilute hydrochloric acid are added, and then reacted in a water bath at 40°C to 50°C for 1 hour to obtain silica sol; the molar ratio of ethyl orthosilicate: deionized water: anhydrous ethanol: hydrogen chloride in dilute hydrochloric acid is 1:4:4:7.5×10 -4 The concentration of the dilute hydrochloric acid is 0.2 mol / L. Other embodiments are the same as those of the first to third embodiments.

[0044] Specific embodiment 5: This embodiment differs from specific embodiment 4 in that the rotation speed of the high-speed stirring in step 2 is 1000 r / min and the stirring time is 30 min. Other aspects are the same as specific embodiment 4.

[0045] Specific embodiment 6: This embodiment differs from specific embodiment 5 in that the diameter of the through hole on the pressing sheet in step 3 is 2 mm. Other aspects are the same as specific embodiment 5.

[0046] Specific embodiment 7: This embodiment differs from specific embodiment 6 in that the pressing sheet in step 3 is an acrylic sheet. Other aspects are the same as specific embodiment 6.

[0047] Specific embodiment eight: This embodiment differs from specific embodiment seven in that the density of each layer of the multi-layer wet sample prepared in step three gradually decreases from top to bottom. Other aspects are the same as specific embodiment seven.

[0048] Specific embodiment 9: This embodiment differs from specific embodiment 8 in that the density of each layer in the multi-layer wet sample prepared in step 3 gradually increases from top to bottom. Other aspects are the same as specific embodiment 8.

[0049] Specific embodiment 10: This embodiment differs from specific embodiment 9 in that, in step 4, the multilayer wet sample prepared in step 3 is frozen together with the mold at -20°C for 24 hours to completely crystallize the internal moisture. The sample is then transferred to a freeze dryer for sublimation drying for 72 hours at -50°C and a vacuum of 50 Pa to 100 Pa, resulting in a biomimetic mullite fiber sound-absorbing material with a density gradient. Other aspects are the same as specific embodiment 9.

[0050] The present invention is verified by the following test:

[0051] Experiment 1: This experiment is a method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient, which is specifically carried out in the following steps:

[0052] 1. Mullite fiber removal and short cutting:

[0053] The mullite fibers were cut to a length of 200 μm to 500 μm and sieved through a 300-mesh sieve to remove the larger-diameter slag balls. The mullite fibers were then placed in deionized water and allowed to settle due to the higher density of the slag balls than in water. The mullite fibers were then removed from the water and dried at 60°C to obtain pure chopped mullite fibers.

[0054] 2. Preparation of mullite fiber dispersion slurry containing silica sol:

[0055] The pure short-cut mullite fiber prepared in step 1 was added to deionized water, and silica sol was added as an inorganic binder. A uniform slurry was prepared by high-speed stirring at a speed of 1000 r / min for 30 min.

[0056] The mass of the mullite fiber in the slurry is 1.5% of the mass of the deionized water, and the silica sol accounts for 5% of the total mass of the slurry, ensuring good fiber bonding and stability of subsequent drying and molding;

[0057] The silica sol is prepared by an acid-catalyzed hydrolysis method. The specific process is as follows: ethyl orthosilicate is mixed with anhydrous ethanol, deionized water and dilute hydrochloric acid are added, and then the mixture is reacted in a water bath at 40°C to 50°C for 1 hour to obtain the silica sol. The molar ratio of ethyl orthosilicate: deionized water: anhydrous ethanol: hydrogen chloride in dilute hydrochloric acid is 1:4:4:7.5×10 -4 ; The concentration of the dilute hydrochloric acid is 0.2 mol / L;

[0058] 3. Layered construction and pressing:

[0059] 1. Pour the slurry prepared in step 2 into a mold and then let it stand for 5 minutes to allow the fibers to gradually transform into horizontal sedimentation under the action of fluid-solid coupling; then apply a gauze-wrapped pressing sheet to the surface of the slurry, manually apply slow pressure to drain free water from the pressing sheet and pour it out, so that the fibers overlap to form a dense layered structure, remove the pressing sheet, and obtain the slurry after the bottom layer is drained; the pressing sheet is evenly provided with a plurality of through holes with a diameter of 2 mm; the pressing sheet is an acrylic disc with a diameter of 100 mm;

[0060] 2. Repeat step 1 twice, adding different amounts of slurry layer by layer on the drained slurry at the bottom layer to build a three-layer structure. The thickness of each layer of drained slurry is equal (by controlling the pressing depth of the tablets at each layer to be equal), to obtain a three-layer wet sample. The density of each layer in the three-layer wet sample gradually increases from top to bottom.

[0061] 4. Freezing and drying treatment: The three-layer wet sample prepared in step 3 and the mold were frozen together in a -20°C environment for 24 hours to completely crystallize the internal water. Then, the sample was transferred to a freeze dryer for sublimation drying for 72 hours. The drying conditions were: -50°C, vacuum degree of 50Pa~100Pa, and a bionic mullite fiber sound-absorbing material with a density gradient was obtained. The total thickness was 20mm, and the density of the upper layer was 0.12g / cm 3 Simulate the surface scales of the wings, the density of the middle layer is 0.16g / cm 3 Corresponding to the biological transition layer, the density of the lower layer is 0.2g / cm 3 Bionic base layer.

[0062] Figure 1 and Figure 6 This is a photo of the bionic mullite fiber sound-absorbing material with density gradient prepared in Experiment 1. Figure 1 The red line in the figure was added artificially later. It can be seen that the cylindrical sample clearly presents a macroscopic three-layer gradient structure, and the interface between the layers is flat and non-permeable, which intuitively confirms the feasibility of the gradient design and meets the thickness requirements for low-frequency sound absorption in engineering applications.

[0063] Figure 2 This is the SEM image of the bionic mullite fiber sound-absorbing material prepared in experiment 1. Figure 2 Figure a shows a cross-sectional micrograph of the sample, clearly showing the material's three-dimensional fiber network structure. The fibers are evenly distributed in this image, with good interlacing and overlapping between them, exhibiting a certain porosity that facilitates sound wave scattering and energy dissipation, thereby enhancing sound absorption. This structure provides the material with a high porosity, enhancing its sound absorption performance. Figure 2(b) A microscopic photograph of the material surface shows the relatively smooth and regular arrangement of the fibers, with no noticeable agglomerates or impurities. This uniform fiber distribution improves the material's stability and durability and demonstrates the superiority of the fabrication process in controlling fiber distribution and structure. Surface structural features also play a crucial role in the reflection and transmission of sound waves, effectively enhancing the absorption of sound waves of varying frequencies. Figure 2 c is Figure 2 The enlarged image in the red dashed box in a, Figure 2 Figure c shows the material's layered structure. Clear stratification is observed in this image, indicating the material's layered assembly process, where layers of varying density are effectively superimposed through processes such as sedimentation and drainage. The density differences between layers play a key role in regulating the material's acoustic properties, optimizing its sound absorption at different frequencies. Figure 2 d is Figure 2 The enlarged image in the red dashed box in b, Figure 2 Figure d shows the presence of silica sol between the fibers. High-magnification micrographs reveal that the silica sol forms a coating on the fiber surface, further enhancing the material's adhesion and stability. The presence of silica sol not only improves the bond strength between the fibers but also provides the material with improved high-temperature resistance, extending its service life in high-temperature environments. Overall, through precise process design and optimized layered structure, this material effectively enhances sound absorption while maintaining excellent mechanical properties and stability in high-temperature environments.

[0064] Figure 3 A multi-scale model diagram of a moth wing biomimetic design. The moth wing scale layered structure achieves 40kHz ultra-wideband sound absorption through the periodic stacking of 5µm single layers and 20-50µm cavities. The material of this invention replicates this multi-scale mechanism: in the macroscopic three-layer gradient design, 0.12g / cm 3 The upper layer simulates the surface scales of the wings, 0.16g / cm 3 The middle layer corresponds to the biological transition layer, 0.2g / cm 3 The lower layer is a biomimetic substrate. At the mesoscale, fibers are arranged in layers along the z-direction, forming a 20-50 μm thick layer, extending the acoustic wave propagation path by 3-5 times. At the microscale, a random network in the xy plane creates pores of 1-10 μm, which, combined with the nanopores of the silica sol, expands high-frequency dissipation capabilities. This three-scale synergy overcomes the frequency band limitations of traditional materials. Figure a is an SEM image of the upper structure in Figure c, Figure b is an enlarged view of the area within the yellow dashed box in Figure a, and Figure d is an SEM image of the side surface in Figure c.

[0065] Figure 4 This is a cyclic compression mechanical behavior diagram. The cyclic compression test reveals that the material has excellent dynamic rebound performance and energy management capabilities. The bottom layer material prepared in the first test was tested, that is, the density is 0.2g / cm3 The sample showed highly repeatable mechanical response in four consecutive loading-unloading cycles: when the strain increased step by step from the initial state to 20%, 30%, 40% and finally 50%, each unloading path closely followed the loading curve, forming a narrow hysteresis loop that accounted for only 8.7% of the total loading work, indicating that the viscoelastic dissipation inside the material was controlled at an extremely low level; the residual strain at each unloading to zero stress was always less than 2%, confirming that the fibrous layered network bonded by silica sol has almost completely reversible deformation ability; under the condition of a maximum strain of 50%, the material still maintained a storage modulus of 286.93kPa, and the loss factor of 0.12 confirmed its high-efficiency vibration attenuation potential; after four large deformation cycles, the material stiffness decay rate was less than 3%, and the stress-strain curve overlap exceeded 95%, highlighting the excellent durability of the layered structure under repeated compression.

[0066] Experiment 2: The difference between this experiment and Experiment 1 is that in step 3, the density difference between the two adjacent layers is controlled to be 0.02g / cm 3 The density of the upper layer is 0.14g / cm 3 Simulate the surface scales of the wings, the density of the middle layer is 0.16g / cm 3 Corresponding to the biological transition layer, the density of the lower layer is 0.18g / cm 3 Bionic base layer. Others are the same as Experiment 1.

[0067] Experiment 3: The difference between this experiment and Experiment 1 is that in step 3, the density difference between the two adjacent layers is controlled to be 0.06g / cm 3 , the density of the upper layer is 0.1g / cm 3 Simulate the surface scales of the wings, the density of the middle layer is 0.16g / cm 3 Corresponding to the biological transition layer, the density of the lower layer is 0.22g / cm 3 Bionic base layer. Others are the same as Experiment 1.

[0068] Test 4: The difference between this test and test 1 is that in step 3, the density difference between the two adjacent layers is controlled to be 0.08g / cm 3 The density of the upper layer is 0.08g / cm 3 Simulate the surface scales of the wings, the density of the middle layer is 0.16g / cm 3 Corresponding to the biological transition layer, the density of the lower layer is 0.24g / cm 3 Bionic base layer. Others are the same as Experiment 1.

[0069] Figure 5 The sound absorption performance control diagram of the bionic mullite fiber sound absorbing material prepared in Experiments 1 to 4 shows that as the density difference between adjacent layers increases, the NRC value shows a certain fluctuation trend and has an obvious change pattern. Specifically, when the density difference between the layers is 0.02g / cm3 When the interlayer density difference increases to 0.04 g / cm 3 When the density difference between the layers increases to 0.06 g / cm3, the NRC value rises to 0.43, showing the best sound absorption performance. This shows that a moderate density difference between the layers can effectively enhance the sound absorption effect of the material. 3 (corresponding to test 3) and 0.08g / cm 3 (corresponding to test four), the NRC values ​​dropped to 0.4 and 0.36 respectively, indicating that as the density difference becomes too large, the sound absorption performance of the material decreases slightly. This shows that a reasonable density difference between layers can establish a gradually changing sound wave propagation path inside the material, causing the sound wave to be reflected and scattered multiple times between layers of different densities, thereby achieving efficient energy dissipation. However, a density difference between layers that is too small may result in a shorter propagation path for sound waves inside the material, insufficient energy dissipation, and lower sound absorption performance. On the contrary, a large density difference between layers may cause sound waves to be reflected back to the outside at the interface with large density changes, weakening the material's ability to absorb sound waves. This may be related to the fact that when the interlayer structure and density difference of the material are too large, the bonding and structural stability between the layers are affected, resulting in a reduction in sound absorption efficiency. Overall, 0.04g / cm 3 The interlayer density difference (corresponding to test 1) shows the best sound absorption performance and has significant engineering application potential.

Claims

1. A method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient, characterized in that The preparation method of the bionic mullite fiber sound-absorbing material with a density gradient is carried out according to the following steps:

1. Mullite fiber removal and short cutting: The mullite fibers are cut to a length of 200 μm to 500 μm and sieved to remove the larger diameter slag balls. The mullite fibers are then placed in deionized water and allowed to settle due to the higher density of the slag balls than water. The mullite fibers are then removed from the water and dried to obtain pure chopped mullite fibers.

2. Preparation of mullite fiber dispersion slurry containing silica sol: The pure short-cut mullite fiber prepared in step 1 is added to deionized water, silica sol is added as an inorganic binder, and a uniform slurry is prepared by high-speed stirring; The mass of the mullite fiber in the slurry is 1.5% to 2% of the mass of the deionized water, and the silica sol accounts for 5% to 6% of the total mass of the slurry, ensuring good fiber bonding and stability of subsequent drying and molding; 3. Layered construction and pressing: ①. Pour the slurry prepared in step 2 into a mold and then let it stand for 3 to 5 minutes to allow the fibers to gradually transform into horizontal sedimentation under the action of fluid-solid coupling; then apply a gauze-wrapped pressing sheet to the surface of the slurry, manually apply slow pressure to drain free water from the pressing sheet and pour it out, so that the fibers overlap to form a dense layered structure, remove the pressing sheet, and obtain the slurry after the bottom layer is drained; the pressing sheet is evenly provided with multiple through holes; ② Repeat the process of step ①, adding different amounts of slurry layer by layer on the drained slurry at the bottom layer to build a multi-layer structure. The thickness of each layer of drained slurry is 5 cm to 10 cm, and a multi-layer density gradient wet sample is obtained; 4. Freezing and drying treatment: The multi-layer wet sample prepared in step 3 is frozen together with the mold to completely crystallize the internal moisture, and then transferred to a freeze dryer for sublimation drying to obtain a bionic mullite fiber sound-absorbing material with a density gradient.

2. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that The sieving in step 1 is 300 mesh.

3. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that The drying temperature in step 1 is 60°C to 90°C.

4. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that The silica sol described in step 2 is prepared by an acid-catalyzed hydrolysis method. The specific process is as follows: ethyl orthosilicate is mixed with anhydrous ethanol, deionized water and dilute hydrochloric acid are added, and then the mixture is reacted in a water bath at 40°C to 50°C for 1 hour to obtain silica sol; the molar ratio of ethyl orthosilicate: deionized water: anhydrous ethanol: hydrogen chloride in dilute hydrochloric acid is 1:4:4:7.5×10 -4 ; The concentration of the dilute hydrochloric acid is 0.2mol / L.

5. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that In step 2, the speed of high-speed stirring is 1000 r / min and the time is 30 min.

6. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that The diameter of the through hole on the pressing sheet described in step 3 is 2 mm.

7. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that The pressing sheet described in step 3 is an acrylic sheet.

8. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that In the multi-layer wet sample prepared in step 3, the density of each layer gradually decreases from top to bottom.

9. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that In the multi-layer wet sample prepared in step 3, the density of each layer gradually increases from top to bottom.

10. The method for preparing a biomimetic mullite fiber sound-absorbing material with a density gradient according to claim 1, characterized in that In step 4, the multilayer wet sample prepared in step 3 and the mold are frozen together in a -20°C environment for 24 hours to completely crystallize the internal moisture, and then transferred to a freeze dryer for sublimation drying for 72 hours. The drying conditions are: -50°C, vacuum degree of 50Pa~100Pa, to obtain a bionic mullite fiber sound-absorbing material with a density gradient.

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