Preparation method and device of cross-scale porous material

Abstract

The invention relates to a preparation method and device of a cross-scale porous material, based on freeze casting and surface acoustic wave cooperative regulation and control, through dynamic multi-scale regulation and control, real-time matching of process parameters, and directional construction and morphology optimization of macroscopic to microscopic multistage pores, precise construction of the multistage pores and improvement of structural stability are achieved, and the preparation method and device of the cross-scale porous material are suitable for industrial production. The material is suitable for high-performance scenes such as fuel cell electrodes and artificial bones.

Description

Technical Field

[0001] This application relates to the field of porous material preparation technology, and in particular to a method and apparatus for preparing multi-scale porous materials. Background Technology

[0002] Multiscale porous materials have attracted much attention due to their unique properties in catalysis, filtration, and energy storage. In existing technologies, cryocasting forms ice crystal structures through directional solidification of solvents, followed by sublimation to obtain materials with oriented pore structures. However, traditional cryocasting processes have significant drawbacks: firstly, the growth direction and size of ice crystals during solidification are constrained by parameters such as temperature gradient and cooling rate, making it difficult to achieve the synergistic construction of submicron to millimeter-scale multiscale pores; secondly, pore morphology control relies on pre-set molds or external physical field interventions, such as using electromagnetic fields to control ice crystal orientation, which involves complex equipment and makes it difficult to dynamically adjust the branched pore structure. Existing acoustic-assisted freezing processes mainly utilize bulk acoustic waves to generate homogenization disturbances, resulting in a homogenized pore structure and making it impossible to precisely control local cavitation effects and stress fields at the solidification interface. Furthermore, the gradient design of multiscale pores also faces challenges, restricting the controllable preparation of high-performance porous materials. Summary of the Invention

[0003] The purpose of this application is to provide a method and apparatus for preparing multi-scale porous materials to overcome the shortcomings of the prior art. Based on the synergistic control of cryogenic casting and surface acoustic waves, it aims to achieve precise construction, dynamic control and improved structural stability of multi-level pores.

[0004] To address the aforementioned technical problems, this application discloses a method for preparing multi-scale porous materials. This method employs a synergistic control based on cryogenic casting and surface acoustic waves, and specifically includes the following steps: Step 1: Prepare a slurry containing a dispersed phase and a solvent, wherein the dispersed phase includes at least one of ceramic powder, polymer precursor or metal particles; Step 2: Inject the slurry into the freezing chamber containing the freezing substrate, and apply surface acoustic waves with a frequency of 10-100 MHz to induce periodic cavitation effect and fluid shear stress inside the slurry. Step 3: Apply directional cooling along the direction perpendicular to the frozen substrate, controlling the cooling rate to be 0.1-5 K / s, so that the solvent is directionally solidified to form an ice crystal structure; Step 4: Synchronously adjust the wavelength and standing wave mode of the surface acoustic wave to regulate the growth direction and pore size distribution of the ice crystal branch structure. The wavelength of the surface acoustic wave is 40-400 μm. Step 5: Vacuum freeze drying to remove the ice crystal structure after freezing the slurry, and then solidification treatment to obtain a multi-scale porous material.

[0005] Preferably, the method of applying the surface acoustic wave in step 2 specifically includes: A traveling wave mode surface acoustic wave is generated by an interdigitated electrode array, which induces the formation of micron-scale main pore channels inside the slurry. Switching from traveling wave mode surface acoustic wave to standing wave mode surface acoustic wave generates nanoscale cavitation bubbles at the solidification interface, thereby controlling the nucleation density of submicron pores. Dynamic matching of surface acoustic wave frequency and ice crystal growth rate enables the acoustic stress field to suppress dendrite coarsening in real time.

[0006] Preferably, the directional cooling in step 3 specifically includes: A two-step freezing method was adopted, with an initial cooling rate of 1-5 K / s, which reduced the slurry temperature to -10℃ to -30℃ within 1 minute; then the cooling was switched to a low-speed cooling method, with a cooling rate of less than 1 K / s maintained for 1 hour, until the temperature dropped to -40℃.

[0007] Preferably, the adjustment method in step 4 specifically includes: During the macroscopic porosity formation stage, low-frequency surface acoustic waves are used to generate a large vortex flow field to guide ice crystals to grow along a predetermined direction. In the mesoscale porosity control stage, switching high-frequency surface acoustic waves excites local cavitation effects to generate secondary branched pores. In the microscale pore modification stage, a high-frequency standing wave with an amplitude ≤0.5μm is applied to refine the surface morphology of the pore wall through the acoustic flow effect. Wherein, the macroscopic scale is 100-1000 μm, the low-frequency surface acoustic wave is 10-40 MHz, the mesoscopic scale is 1-100 μm, the high-frequency surface acoustic wave is 40-100 MHz, and the microscopic scale is 0.1-1 μm.

[0008] Preferably, the slurry in step 1 also includes a dispersant and a binder.

[0009] Preferably, the viscosity of the slurry in step 1 is 3-10 Pa·s.

[0010] This application also discloses an apparatus for preparing multi-scale porous materials, which includes a cryogenic casting unit, a surface acoustic wave control unit, and a multi-scale monitoring unit; The cryogenic casting unit includes a cryogenic chamber, a cryogenic substrate, and a circulating cooling system. The cryogenic substrate is disposed in the cryogenic chamber, and a piezoelectric substrate is disposed on the cryogenic substrate. The circulating cooling system is connected to the cryogenic substrate. The surface acoustic wave control unit includes a piezoelectric transducer array, a radio frequency signal generator, and a power amplifier. The piezoelectric transducer array is integrated on the side and bottom of the cryogenic cavity, and the radio frequency signal generator and the power amplifier are both connected to the piezoelectric transducer. The cross-scale monitoring unit includes a high-speed microscopic camera, an ultrasonic flaw detector, and an embedded sensor, wherein the embedded sensor is a temperature sensor and / or a stress sensor.

[0011] Preferably, the piezoelectric transducer array is composed of periodically arranged interdigitated electrodes, and the electrode material is copper or an aluminum-copper alloy.

[0012] Preferably, the piezoelectric transducer array includes a first subarray and a second subarray, wherein the electrode finger spacing of the first subarray is 25-100 μm and the electrode finger spacing of the second subarray is 10-25 μm.

[0013] Preferably, the temperature of the freezing chamber is controllable, the freezing substrate is made of copper, and the piezoelectric substrate is made of high-purity single-crystal silicon or lithium niobate.

[0014] Compared with the prior art, the advantages of this application are as follows: First, by employing a synergistic control method based on cryogenic casting and surface acoustic waves, the precise construction, dynamic control, and structural stability enhancement of multi-level pores were achieved.

[0015] Second, by switching the surface acoustic wave mode (traveling wave / standing wave) and frequency, periodic cavitation effect and fluid shear stress are induced at the ice crystal growth interface, thereby achieving dynamic multi-scale control through the directional construction and morphology optimization of multi-level pores from macroscopic (100-1000μm) to microscopic (0.1-1μm).

[0016] Third, by using embedded sensors and a high-speed microscopy system to provide real-time feedback on the ice crystal growth rate, the acoustic wavelength (40-400μm) and cooling rate (0.1-5K / s) are dynamically adjusted to suppress dendrite coarsening and improve the uniformity of the pore structure.

[0017] Fourth, by refining the pore wall roughness (to less than 200 nm) through the acoustic flow effect, the interlayer thermal stress is reduced, the porosity is increased to over 85%, the tortuosity is reduced to below 1.2, and the interlayer bonding strength is increased by 30-50%.

[0018] Fifth, through dynamic multi-scale control, real-time matching of process parameters, and gradient structure enhancement, it is possible to customize the fabrication of porous structures with high connectivity and cross-scale gradient distribution, which are suitable for high-performance applications such as fuel cell electrodes (to increase catalytic activity area) and artificial bones (to promote cell infiltration and nutrient transport). Attached Figure Description

[0019] The advantages of this application, as described above and / or in other aspects, will become clearer from the following detailed description in conjunction with the accompanying drawings and specific embodiments.

[0020] Figure 1This is a schematic diagram of the structure of a multi-scale porous material preparation device in one embodiment of this application.

[0021] Figure 2 This is a partially enlarged schematic diagram of a piezoelectric transducer array in one embodiment of this application.

[0022] Figure 3 This is a SEM image of a multi-level porous silica material produced by surface acoustic wave-assisted cryogenic casting in one embodiment of this application.

[0023] Illustrations: 1. Freezing chamber; 2. Freezing substrate; 3. Freezing circulation system; 4. Piezoelectric transducer array; 4a. First subarray; 4b. Second subarray; 5. Radio frequency signal generator; 6. Power amplifier; 7. Piezoelectric substrate; 8. High-speed microscopic camera; 9. Ultrasonic flaw detector; 10. Embedded sensor. Detailed Implementation

[0024] The features and exemplary embodiments of various aspects of this application will now be described in detail. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only configured to explain this application and are not configured to limit this application. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples of this application.

[0025] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0027] In existing technologies, multi-scale pore structures can be fabricated using methods such as template method, template-free method, additive manufacturing method, cryogenic casting method, and external field control. However, the aforementioned methods are all limited by the passive coupling of heat transfer and mass transfer, which cannot decouple the requirements of orderliness of micron pores and stability of nanopores. Furthermore, the dynamic response of external field control is insufficient (e.g., acoustic field phase adjustment requires seconds), making it difficult to match the rapid kinetics (microsecond level) of ice crystal growth, resulting in poor customization capability of multi-scale pore structures.

[0028] To address the aforementioned issues, this application proposes a method for preparing multi-scale porous materials based on the synergistic control of cryogenic casting and surface acoustic waves. The following is a detailed description of an embodiment of the method for preparing multi-scale porous materials in this application, with reference to the accompanying drawings.

[0029] Reference Figure 1 , Figure 1 This diagram illustrates the structure of a multi-scale porous material preparation apparatus according to an embodiment of this application. The multi-scale porous material preparation apparatus provided in one embodiment of this application includes a cryogenic casting unit, a surface acoustic wave control unit, and a multi-scale monitoring unit. The cryogenic casting unit includes a temperature-controllable cryogenic chamber 1, a copper cryogenic substrate 2, and a circulating cooling system 3. The cryogenic substrate 2 is disposed in the cryogenic chamber 1, and a piezoelectric substrate 7 is disposed on the cryogenic substrate 2. The circulating cooling system 3 is connected to the cryogenic substrate 2. The piezoelectric substrate 7 is made of high-purity single-crystal silicon or lithium niobate. The surface acoustic wave control unit includes a piezoelectric transducer array 4, a radio frequency signal generator 5, and a power amplifier 6. The piezoelectric transducer array 4 is integrated on the side and bottom of the cryogenic cavity 1, and the radio frequency signal generator 5 and the power amplifier 6 are both connected to the piezoelectric transducer. The cross-scale monitoring unit includes a high-speed microscopic camera 8, an ultrasonic flaw detector 9, and an embedded sensor 10, wherein the embedded sensor 10 is a temperature sensor and / or a stress sensor.

[0030] In one embodiment, the piezoelectric transducer array 4 is further composed of periodically arranged interdigitated electrodes, the electrode material being copper or an aluminum-copper alloy. The piezoelectric transducer array 4 includes a first subarray 4a and a second subarray 4b. The electrode finger spacing of the first subarray 4a is 25-100 μm, used to generate low-frequency traveling waves to control macroscopic pore orientation. The electrode finger spacing of the second subarray 4b is 10-25 μm, spaced apart at intervals in the first subarray, used for high-frequency standing waves to generate mesoscopic branched pores. Figure 2 As shown, the first subarray 4a and the second subarray 4b can be selected according to different needs.

[0031] An embodiment of this application also provides a method for preparing a multi-scale porous material, the method comprising: A slurry containing a dispersed phase and a solvent is prepared. The dispersed phase includes at least one of ceramic powder, polymer precursor, or metal particles. For example, ZrO2, SiO2, or hydroxyapatite can be used as the solute particles, and water or a polymer solution can be used as the matrix. A dispersant (such as PVA) and a binder are added to adjust the viscosity to 3-10 Pa·s. Material selection and substrate preparation: High-purity single-crystal silicon or lithium niobate (LiNbO3) was selected as the piezoelectric substrate. Interdigital transducers (IDTs) were prepared by photolithography. The electrode material was copper or aluminum-copper alloy, and the width and spacing were 1 / 4 of the target acoustic wavelength (λ=v / f, where λ is the wavelength of the acoustic wave propagating in the piezoelectric substrate, v is the sound velocity of the specific mode acoustic wave in the piezoelectric substrate, and f is the operating frequency of the interdigital electrodes).

[0032] The surface acoustic wave (SAW) system is constructed by connecting a piezoelectric transducer to a radio frequency signal generator and a power amplifier, outputting an AC signal with a frequency of 10-60 MHz and a power of 0-33 dBm to excite SAW waves. A cryogenic freezing platform is built, with the freezing chamber forming a freezing mold. The mold has a thickness of 1-5 mm, and a copper freezing substrate (cooled to -196℃ with liquid nitrogen) is installed at the bottom of the mold. A temperature control module (accuracy ±0.1℃) is configured, and a high-speed camera system and an ultrasonic flaw detector are integrated simultaneously. Embedded temperature / stress sensors monitor ice crystal growth and temperature field distribution in real time.

[0033] The acoustic-thermal field parameters are set as follows: acoustic field parameters include frequency (10-100 MHz), power (0-33 dBm), and waveform (traveling wave / standing wave), and the acoustic flow velocity (0.1-10 mm / s) is calibrated using a laser Doppler vibrometer. Temperature field parameters include cooling rate (0.1-5 K / s) and temperature gradient direction (unidirectional / radial), and the thermal resistance between the freezing substrate and the mold is adjusted using a PID controller.

[0034] Surface acoustic waves (SAWs) are applied by injecting a slurry into a freezing chamber containing a freezing substrate. SAWs with frequencies of 1-100 MHz are applied to induce periodic cavitation effects and fluid shear stress within the slurry. Specific methods of applying SAWs include: A traveling-wave mode surface acoustic wave (SAW) is generated using an interdigitated electrode array, converting electrical energy into mechanical vibration. This vibration propagates along the substrate surface and is transmitted to the slurry. The slurry, covering the substrate surface, is then subjected to the same vibration, causing mechanical vibration within the slurry. This vibration leads to pressure fluctuations, forming acoustic waves, including positive and negative pressure cycles. During the negative pressure cycle, the pressure is lower than the vapor pressure of the liquid, causing bubble formation. During the positive pressure cycle, the bubbles rapidly collapse, generating shock waves and high temperatures, forming microjets. The localized high shear forces generated by the microjets and bubble closure disrupt particle aggregation and promote particle dispersion. The fluid motion within the slurry increases shear stress, promoting particle rearrangement. The combined effect of shear stress and cavitation improves the slurry's fluidity and mixing efficiency, reducing particle agglomeration. This induces the formation of micron-sized master pore channels within the slurry. Switching from traveling wave mode surface acoustic waves (SAWs) to standing wave mode involves several key changes. In traveling wave mode, the SAWs propagate continuously with gradually decreasing energy. In standing wave mode, by introducing a reflecting surface or adjusting the acoustic wave frequency, interference between the acoustic wave and its reflected wave is created, forming a standing wave. Standing waves have fixed nodes and antinodes, resulting in a more concentrated energy distribution. Changing the acoustic wave frequency ensures sufficient energy density to induce cavitation, and adjusting the wavelength to several micrometers or higher generates sufficient pressure fluctuations. Ensuring a smooth material surface at the solidification interface prevents impurities from interfering with acoustic wave propagation. Controlling the temperature gradient and cooling rate during solidification promotes the formation of a uniform solidification interface. Inducing nanoscale cavitation bubbles at the solidification interface regulates the nucleation density of submicron pores. Dynamically matching the surface acoustic wave (SAW) frequency with the ice crystal growth rate enables the acoustic stress field to suppress dendrite coarsening in real time. This is achieved by monitoring the ice crystal growth rate in real time using high-speed imaging technology to capture the dynamic process of ice crystal growth. Monitoring data includes the ice crystal growth rate, shape changes, and the degree of dendrite coarsening. This data is used in a feedback control system to dynamically adjust the acoustic wave frequency. The frequency is dynamically adjusted based on the monitored ice crystal growth rate. When the ice crystal growth rate increases, the acoustic wave frequency is appropriately increased to enhance the effect of the stress field. Through the feedback control system, real-time matching of the acoustic wave frequency and the ice crystal growth rate is achieved, ensuring that the stress field effectively suppresses dendrite coarsening. The intensity and distribution of the stress field need to be precisely controlled to avoid unnecessary damage to the ice crystal structure. The intensity of the stress field should be matched with the ice crystal growth rate to achieve the best suppression effect.

[0035] Directional cooling is applied along the direction perpendicular to the frozen substrate, with the cooling rate controlled at 0.1-5 K / s, causing the solvent to solidify directionally and form an ice crystal structure. Further, the directional cooling specifically includes: using a liquid nitrogen circulating cooling system to cool the frozen substrate to -15°C to -40°C, using a two-step freezing method, with an initial cooling rate of 1-5 K / s, to reduce the slurry temperature to -10°C to -30°C within 1 minute; then switching to low-speed cooling, with a cooling rate of less than 1 K / s maintained for 1 hour, to reduce the temperature to -40°C, thereby reducing the accumulation of thermal stress.

[0036] The wavelength and standing wave mode of the surface acoustic wave are simultaneously adjusted to regulate the growth direction and pore size distribution of the ice crystal branching structure. The wavelength of the surface acoustic wave is 40-400 μm. The specific adjustment methods include: During the macroscopic porosity formation stage, the vibration of sound waves in the slurry causes the fluid to rotate, forming a vortex flow field. This vortex flow field can be stabilized by the formation of standing waves. Standing waves are the result of the interference between sound waves and their reflected waves, possessing fixed nodes and antinodes, and exhibiting a more concentrated energy distribution. Using low-frequency surface acoustic waves to generate a large vortex flow field, the vortex flow field influences the growth direction of ice crystals by altering the temperature and solute concentration gradients in the liquid. The movement of the flow field can redistribute heat and solute, thereby promoting the formation and growth of ice crystals in a specific direction. By controlling the parameters of the sound waves, such as frequency and wavelength, the vortex flow field can be controlled, guiding ice crystals to grow along a predetermined direction. In the mesoscale porosity control stage, high-frequency surface acoustic waves (SAWs) are used to excite local cavitation effects. A high-frequency transducer converts high-frequency electrical signals into mechanical vibrations, thus exciting SAWs. The sound waves propagate on the material surface, generating high-frequency vibrations. These vibrations have small amplitudes but high frequencies and concentrated energy, producing significant mechanical effects on the material surface and within the material. Due to their high-frequency characteristics, the high-frequency sound waves can generate more cavitation bubbles in a smaller area. The collapse of these cavitation bubbles generates shock waves and high temperatures, causing minor damage or alterations to the material surface or interior. These localized changes promote the softening or decomposition of the material, leading to the formation of secondary branched pores around the main pores. The shock wave and high-temperature effects can further promote changes in the microstructure within the material, forming a complex pore network and generating secondary branched pores. In the microscale pore modification stage, high-frequency standing waves with an amplitude ≤0.5μm are applied to ensure that the modification of the pore walls is not too drastic, maintaining the integrity of the material. When the standing wave propagates within the material, it causes pressure fluctuations and particle vibrations, generating minute fluid motions, i.e., acoustic flow. The intensity and direction of the acoustic flow depend on the frequency and amplitude of the standing wave, as well as the physical properties of the material. By adjusting these parameters, the magnitude and distribution of the acoustic flow effect can be controlled. The acoustic flow effect refines the surface morphology of the pore walls; it alters the surface microstructure through shear and impact forces on the pore wall surface. This shear force can remove protrusions and fill depressions, thereby refining the surface morphology of the pore walls, making them smoother and more uniform. The refined surface morphology can improve the material's mechanical strength, thermal conductivity, and corrosion resistance.

[0037] Wherein, the macroscopic scale is 100-1000μm, the low-frequency surface acoustic wave is 1-10 MHz, the mesoscopic scale is 1-100μm, the high-frequency surface acoustic wave is 20-50 MHz, and the microscopic scale is 0.1-1μm.

[0038] Vacuum freeze-drying removes the ice crystal structure of the slurry after freezing, followed by sintering or curing to obtain a multi-scale porous material. The vacuum freeze-drying temperature is -50℃ for 48 hours. After sintering or curing, the material is impregnated with epoxy resin, then mechanically polished, and the exposed cross-sections are reconstructed by SEM (resolution 1μm).

[0039] In this application, the preparation of multi-scale porous materials involves injecting a suspension into a mold, initiating a surface acoustic wave and cooling process, and using a liquid nitrogen circulating cooling system to lower the temperature of the frozen substrate to -15°C to -40°C. A two-step freezing method is employed, with an initial cooling rate of 1-5 K / s to lower the slurry temperature to -10°C to -30°C within 1 minute. The process is then switched to low-speed cooling, less than 1 K / s, maintained for 1 hour, until the temperature drops to -40°C to reduce thermal stress accumulation. Simultaneously, the dynamics of the ice crystal growth interface are recorded (a high-speed microscopic camera captures the dendrite morphology, and an embedded temperature / stress sensor monitors the undercooling distribution). Finally, the ice crystals are removed by vacuum freeze-drying, followed by impregnation with epoxy resin and mechanical polishing. The exposed cross-sections are then subjected to SEM three-dimensional reconstruction (resolution 1 μm).

[0040] SEM measurements were performed on micron-sized pores (1-500 μm), pore orientation (angular deviation ≤5°), and tortuosity (≤1.2). For nano-sized pores, TEM was used to statistically analyze the particle aggregation index (<0.2) and nanopore density (1.0×10¹²-1.5×10¹² pores / m³). Micro-CT analysis was performed on pore connectivity (Thomson parameter >0.85) and gradient distribution (micron-sized pore layer thickness 100-200 μm, nanopore wall roughness 50-200 nm). Figure 3The image shown is a SEM image of the hierarchical porous silica material prepared by surface acoustic wave-assisted cryogenic casting. The table below compares the relevant parameters of the multi-scale porous material prepared by the method of this application with those prepared by the traditional cryogenic casting (single temperature field) method: Table 1 Comparison of Relevant Parameters

[0041] Regarding micron-scale pore size, the method in this application achieves precise control of micron-scale pore size across the entire range of 1-500 μm through coordinated acoustic-thermal field control (surface acoustic wave frequency 10-100 MHz, cooling rate 0.1-5 K / s). Specifically, macroscopic-scale (100-1000 μm) pores are formed by guiding the directional growth of ice crystals using low-frequency surface acoustic waves (10-40 MHz), while mesoscopic-scale (1-100 μm) pores are constructed by exciting secondary branches using high-frequency surface acoustic waves (40-100 MHz). In contrast, traditional cryogenic casting (single temperature field) methods are limited by the cooling rate and cannot achieve full-range control of pore size. When freezing rapidly (cooling rate 1-5 K / s), the ice crystal growth rate exceeds the particle diffusion rate, resulting in only fine layered pores of 1-50 μm. Furthermore, the pores are prone to local closure due to ice crystal branching. When freezing slowly (cooling rate <1 K / s), the particles diffuse sufficiently, forming wide layered pores of 100-200 μm. However, the pore wall surface is prone to cracking due to particle coarsening, making it difficult to meet the diverse pore size requirements of different scenarios.

[0042] Regarding the channel orientation, this application employs a dual-mechanism synergy: the temperature field provides the basic directional freezing trend, and surface acoustic waves precisely drive the particles to align along the sound pressure node / sound flow direction through high-frequency acoustic flow and acoustic radiation force, achieving decoupling of heat transfer and mass transfer, actively enhancing channel orientation, with an orientation angle deviation ≤5°; while traditional methods are dominated by a single mechanism, relying solely on the ice crystal growth orientation dominated by the temperature gradient, promoting competitive growth of ice crystals through unidirectional heat flow, with particles passively squeezed to the grain boundary to form channels, and the orientation is entirely passively determined by the heat transfer process, with an orientation angle deviation of 15°-30° during rapid freezing and ≤8° during slow freezing.

[0043] Regarding tortuosity, the tortuosity of the micron-sized directional channels (e.g., 10 μm) in this application is ≤1.2 (close to straight channels), and low tortuosity can be maintained at different pore sizes (1-500 μm) by adjusting the acoustic flow direction; the cross-scale channels (micron + nano) have smooth connectivity paths, no coarsening or blockage of nanopores, and the overall tortuosity is stable and controllable; even when preparing 100-200 μm wide-layer pores by slow freezing, the acoustic field can still avoid channel distortion, and the tortuosity does not increase significantly. In contrast, when the traditional method is used for rapid freezing (1-50 μm fine pores), the ice crystals are disordered and branches, and the particles are quickly captured, resulting in many channel turns and a tortuosity of 1.8-2.5; when slow freezing (100-200 μm wide-layer pores), the tortuosity of the micron-sized pores decreases to 1.3-1.5, but particle coarsening leads to blockage of nanopores, and the overall tortuosity of the cross-scale channels rises back to 1.6-2.0; the tortuosity fluctuates greatly with the freezing rate and cannot be stably maintained at a low level.

[0044] Regarding the particle aggregation index, the aggregation index of nanoscale particles (such as ZrO2 and SiO2) in this application is ≤0.15, and the particles are in a monodisperse or small agglomerate state (≤5 particles / agglomerate). By adjusting the acoustic field parameters (frequency 10-60MHz, power 1-100W), the aggregation index can be stably controlled in the range of 0.1-0.2, unaffected by the freezing rate (0.1-10°C / min). The particles are uniformly distributed across the pore wall across scales, with no local dense agglomeration areas. The stability of the nanopores (10-100nm) is improved, and the specific surface area is increased by more than 30% compared with traditional technologies. In traditional methods, rapid freezing (1-50μm fine pores) results in rapid particle capture, with an aggregation index of 0.4-0.6, forming a large number of dense aggregates (≥10 particles / aggregate), which block the nanopores. In slow freezing (100-200μm wide pores), the particle diffusion time is prolonged, and the aggregation index drops to 0.25-0.35, but local aggregation still exists, leading to coarsening of the nanopores (pore size increases to 100-200nm). The aggregation index fluctuates drastically with the freezing rate, making stable control impossible and directly restricting the consistency of material properties.

[0045] Regarding nanopore density, this application takes ZrO2 and SiO2 suspensions as examples. The nanopore density (10-100nm) is stable at 1.2×10¹²-1.5×10¹² pores / m³ (statistics by TEM+ImageJ), and the pore size distribution is concentrated (deviation ≤10nm). By adjusting the acoustic-thermal parameters (frequency 10-60MHz, cooling rate 0.1-10°C / min), the nanopore density fluctuation is ≤5%. Even when preparing 100-200μm wide-layer micropores by slow freezing, the density is still maintained above 1.0×10¹² pores / m³. In the multi-scale channels, the nanopores are uniformly distributed on the micropore walls, without local voids or dense areas, and the specific surface area is stably maintained at 80-100m² / g, which is more than 40% higher than that of traditional technology. In traditional methods, rapid freezing (1-50μm fine pores) results in particles quickly capturing and clogging pores, with a nanopore density of only 0.3×10¹²-0.5×10¹² pores / m³, and the pore size distribution is disordered (deviation ≥30nm). In slow freezing (100-200μm wide pores), particle coarsening leads to pore merging, and the nanopore density drops to 0.6×10¹²-0.8×10¹² pores / m³, with some areas showing nanopore vacancies due to agglomeration. The nanopore density fluctuates drastically with the freezing rate (fluctuation amplitude ≥50%), making it impossible to form a stable nanopore network, which directly affects the number of active sites in the material.

[0046] This application provides a method and apparatus for preparing multi-scale porous materials. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method of preparing a cross-scale porous material, characterized by, The preparation method adopts cooperative regulation based on freeze casting and surface acoustic wave, and specifically comprises the following steps. Step 1: preparing a slurry containing a dispersed phase and a solvent, wherein the dispersed phase comprises at least one of ceramic powder, polymer precursor or metal particles; Step 2: injecting the slurry into a freeze cavity containing a freeze substrate, applying a surface acoustic wave with a frequency of 10-100 MHz to induce periodic cavitation effect and fluid shear stress inside the slurry; Step 3: applying directional cooling along the direction perpendicular to the freeze substrate to control the cooling rate at 0.1-5 K / s, so that the solvent is directionally solidified to form ice crystal structures; Step 4: synchronously adjusting the wavelength and standing wave mode of the surface acoustic wave to regulate the growth direction of the ice crystal branch structure and the pore size distribution, wherein the wavelength of the surface acoustic wave is 40-400 μm; Step 5: vacuum freeze drying to remove the ice crystal structure after freezing of the slurry, and then obtaining the cross-scale porous material through solidification treatment.

2. The method of claim 1, wherein, The application mode of the surface acoustic wave in step 2 specifically comprises: Generating a traveling wave mode surface acoustic wave through an interdigital electrode array to induce the formation of micron-level main pore channels inside the slurry; Switching the traveling wave mode surface acoustic wave to a standing wave mode surface acoustic wave to generate nanometer-level cavitation bubbles at the solidification interface to regulate the nucleation density of submicron pores; Dynamically matching the surface acoustic wave frequency and the ice crystal growth rate to make the acoustic wave stress field inhibit dendrite coarsening in real time.

3. The method of claim 1, wherein, The directional cooling in step 3 specifically comprises: Using a two-step freezing method, the initial cooling rate is 1-5 K / s, so that the temperature of the slurry is reduced to-10℃ to-30℃ within 1 min; then switching to low-speed cooling with a cooling rate less than 1 K / s for 1 h to reduce the temperature to-40℃.

4. The method of claim 1, wherein, The adjustment mode of step 4 specifically comprises: In the macro-scale pore formation stage, a low-frequency surface acoustic wave is used to generate a large vortex flow field to guide the growth of ice crystals along the preset direction; In the mesoscale pore regulation stage, a high-frequency surface acoustic wave is switched to excite local cavitation effect to generate secondary branch pores; In the micro-scale pore modification stage, a high-frequency standing wave with an amplitude of ≤0.5 μm is applied to refine the pore wall surface morphology through acoustic streaming effect; Wherein, the macro-scale is 100-1000 μm, the low-frequency surface acoustic wave is 10-40 MHz; the mesoscale is 1-100 μm; the high-frequency surface acoustic wave is 40-100 MHz; the micro-scale is 0.1-1 μm.

5. The method of claim 1, wherein, The slurry in step 1 further comprises a dispersant and a binder.

6. The method of claim 5, wherein, The viscosity of the slurry in step 1 is 3-10 Pa·s.

7. An apparatus for carrying out the process of claim 1, characterized in that The device comprises a freeze casting unit, a surface acoustic wave regulation unit and a cross-scale monitoring unit; The freeze casting unit comprises a freeze cavity, a freeze substrate and a circulating cooling system, the freeze substrate is arranged in the freeze cavity, a piezoelectric substrate is arranged on the freeze substrate, and the circulating cooling system is connected with the freeze substrate; The surface acoustic wave regulation unit comprises a piezoelectric transducer array, a radio frequency signal generator and a power amplifier, the piezoelectric transducer array is integrated on the side and bottom of the freeze cavity, and the radio frequency signal generator and the power amplifier are connected with the piezoelectric transducer; The cross-scale monitoring unit comprises a high-speed microscopic camera, an ultrasonic flaw detector and embedded sensors, which are temperature sensors and / or stress sensors.

8. The apparatus of claim 7, wherein, The piezoelectric transducer array is composed of periodically arranged interdigital electrodes, and the electrode material is copper or aluminum-copper alloy.

9. The apparatus of claim 8, wherein, The piezoelectric transducer array comprises a first sub-array and a second sub-array, the electrode finger spacing of the first sub-array is 25-100 mu m, and the electrode finger spacing of the second sub-array is 10-25 mu m.

10. The apparatus of claim 7, wherein, The temperature of the freezing cavity is controllable, the freezing substrate is made of copper, and the piezoelectric substrate is high-purity single crystal silicon or lithium niobate.

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