Preparation method of multi-scale structure high-temperature protection composite material

Abstract

The invention relates to a preparation method of a high-temperature protection composite material with a multi-scale structure. The preparation method specifically comprises the following steps: (1) theoretically calculating the concentration of counter ions with soft agglomeration among colloidal particles; (2) preparing ceramic slurry; (3) self-assembling the ceramic particles; (4) drying the ceramic biscuit; and (5) carrying out heat treatment on the ceramic biscuit. According to the preparation method disclosed by the invention, the in-situ controllability of the counter ion concentration in the slurry is realized by controlling the hydrolysis rate of urea, and a nano-porous structure with the porosity greater than 80% is constructed. The self-assembled microstructure of the ceramic particles is kept to the maximum extent through methods of freeze drying, solvent replacement, supercritical drying, low-temperature long-time heat preservation and passivation of the sintering activity of a biscuit before a sintering shrinkage point and the like. Compared with an Al2O3 pure-phase unitary porous system, the surface and back temperature difference of the material is further increased by 50%, and the radiation heat flux density penetrating through a sample is reduced by 24%.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous ceramic materials, and particularly relates to a preparation method of a multi-scale structure high-temperature protection composite material. Background Art

[0002] With the development of hypersonic aircraft, in order to maintain hypersonic long-endurance flight in an aerobic atmosphere, the aircraft must have a sharp windward surface shape with low aerodynamic drag, and the windward surface is also the area where aerodynamic heating is the most severe. Traditional decelerating reentry aircraft can use ablative materials to take away heat through the ablation of the material itself (latent heat of phase change) to achieve thermal protection in this area. However, for hypersonic aircraft, the ablation of the windward surface material will inevitably lead to the blunting of the sharp aerodynamic shape. Therefore, ablative-resistant materials must be used. In this way, the heat that cannot be taken away by ablation will inevitably be transferred to the interior of the aircraft. Therefore, in addition to ultra-high temperature ablative-resistant materials, another essential material for the thermal protection material system of hypersonic aircraft is high-efficiency high-temperature thermal insulation materials. Since the aircraft has high requirements for material weight and volume, traditional high-temperature thermal insulation materials cannot meet the requirements. This high-temperature thermal insulation material must solve three scientific and technological problems at the same time: First, construct a micro-nano pore structure with a relatively high porosity and a relatively uniform pore structure; Second, the pore structure has good high-temperature stability (for example: when the temperature rises above 1500 °C, the porosity can still be maintained above 80%); Third, the high-efficiency radiation heat insulation performance of high-porosity materials should be prominent.

[0003] As early as 1992, American scholars Hunt A J et al. proposed the concept of super insulation materials at the International Materials Engineering Conference. Generally speaking, super insulation materials refer to insulation materials with a thermal conductivity lower than that of "non-convective air" (0.023 W / m·k) under predetermined use conditions. The most typical super insulation material is aerogel. Through the sol-gel method and supercritical drying technology, the problem of constructing a high-porosity nano-pore structure can be well solved. At present, aerogel materials with a porosity as high as 99% can be prepared. However, due to the extremely high activity and specific surface area of this aerogel material, it is difficult to resist high temperatures above 1000 °C and loses its nano-pore structure due to sintering; on the other hand, ceramic materials with extremely high porosity are almost transparent to thermal rays, so it is difficult to be used as an insulation material mainly based on radiative heat transfer under ultra-high temperature conditions. In order to solve these two problems of aerogel, a large number of studies have been carried out at home and abroad. For example, in addition to silica aerogel, aerogels with better high-temperature resistance such as alumina, zirconia, carbon, and binary systems have also been studied. However, the aerogel prepared by the sol-gel method is a thermodynamically unstable amorphous material and has an extremely high specific surface area. Therefore, it is difficult to fundamentally solve the problem of high-temperature stability.

[0004] At present, high-temperature thermal insulation materials mainly include ceramic fiber felts or ceramic fiber-reinforced aerogels. Many domestic research institutions such as the Ceramic Research Institute of China National Building Materials Academy and the Aerospace Research Institute of Materials and Processes have conducted extensive research in this area. On the one hand, the problem of preparing high-performance high-temperature-resistant alumina fibers still needs to be solved. On the other hand, even with the filling of nanoparticles, the one-dimensionality of the fibers will make it difficult to ensure that the pore diameter is below the mean free path of air molecules (70 nm). The present invention does not use fibers with high refractoriness, but uses high-purity α-Al2O3 particles with a certain grain size and much higher high-temperature stability than aerogels to directly construct a high-porosity, gradient micro-nano porous structure. The simultaneous realization of high porosity and micro-nano pore diameter mainly depends on the particle size and the connection state of the particles. In terms of high-temperature resistance, the particle size should be as large as possible, but the larger the grain size, the higher the requirement for controlling the particle connection state to achieve a high-porosity nano-pore structure. Most of the domestic and foreign research reported so far focuses on preparing porous ceramics with micron- and sub-micron-sized particles. The prepared porous ceramics have a low porosity and a pore diameter much larger than 70 nm, which is not an ideal choice for high-temperature thermal insulation materials. Therefore, selecting particles of appropriate size (about 140 nm in the present invention) and studying a method for controlling the particle connection state will be a breakthrough for this high-temperature thermal insulation material.

[0005] In recent years, from a civilian perspective, highly porous ceramics with a high gas porosity (>60 vol%) have received increasing attention in fields such as chemical engineering, biopharmaceuticals, catalyst carriers, and gas filtration. With the rapid development of technology, the ultimate goal of heat insulation and energy-saving technology, as one of the energy-saving measures, is to reduce heat loss, save fuel consumption, improve the working environment, ensure production safety, and increase work efficiency. In short, the research and development and application of lightweight porous thermal insulation materials are of extremely important significance in both national defense, aerospace, and military fields, as well as industrial and civilian sectors. The present invention is proposed under such a background, and while meeting the requirements for high-performance high-temperature-resistant rigid thermal insulation materials in the national defense and aerospace fields, it is also of great significance for the research and development of new, efficient, and lightweight porous thermal insulation materials in China's civilian use. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a multi-scale structure high-temperature protection composite material, so as to provide a method for preparing a lightweight porous thermal insulation material to meet the requirements for high-performance high-temperature-resistant rigid thermal insulation materials.

[0007] To achieve the above purpose, the technical solution of the present invention is as follows.

[0008] A method for preparing a multi-scale structure high-temperature protection composite material specifically includes the following steps: (1)Theoretical calculation: According to the DLVO theory, calculate the experimental conditions for forming the second minimum value on the DLVO potential energy curve of ceramic particles, providing a theoretical basis and guidance for subsequent assembly experiments; (2)Preparation of ceramic slurry: Add deionized water into the ball milling tank. Weigh the required alumina and silicon carbide powders according to 10 Vol% of the total volume of the slurry for later use. Among them, the SiC powder accounts for 5% of the total volume of the powders. Weigh 0.08 wt% of the dispersant poly(dimethyldiallylammonium chloride) based on the mass of the powders and drop it into the deionized water. Stir evenly with a stirrer to prepare a premixed solution. Then weigh urea according to the ratio of 0.2 g / 100 g of the slurry and add it into the premixed solution. After ultrasonic dispersion until completely dissolved, slowly add the weighed powders in portions. Subsequently, add zirconia grinding balls with a weight 1.5 times that of the powders. The weight ratio of zirconia grinding balls with radii of 1 mm, 3 mm, and 5 mm is 2:2:1. Place the ball milling tank into a planetary ball mill and perform isothermal ball milling at a speed of 350 r / min for 8 h. After ball milling, sieve the slurry to filter out the grinding balls, and evacuate the air mixed into the slurry during ball milling with a vacuum pump, then the alumina slurry with good fluidity, uniform mixing, and uniform and saturated adsorption of poly(dimethyldiallylammonium chloride) on the surface of ceramic particles can be prepared; (3)"Self-assembly" of ceramic particles: Adopt two methods, namely urease-induced urea hydrolysis or temperature-induced urea hydrolysis, to "self-assemble" ceramic particles; (4)Drying of the ceramic green body: Adopt a process combining alcohol-water replacement and high-temperature ethanol supercritical drying for the green body obtained after "self-assembly". The time for solvent replacement is 7 days. Before loading the sample, carefully clean the autoclave first and wipe it several times with a towel dipped in alcohol to avoid the existence of residual debris in the autoclave. When loading the sample, first add a certain amount of anhydrous ethanol into the autoclave, then wrap the sample replaced with anhydrous ethanol with a ceramic fiber cloth and put it into the drying autoclave in sequence. After that, add an appropriate amount of anhydrous ethanol to submerge the sample, and it is appropriate to keep the sum of the volume of the liquid and the sample accounting for about 60% of the total volume in the autoclave. Tighten the autoclave lid for sealing, then connect the power supply and fill it with nitrogen to replace the residual air in the autoclave, and purge it three times repeatedly. Finally, continuously introduce nitrogen until the pressure reaches 2 MPa and keep the pressure for 20 minutes. If the pressure does not change within 20 minutes, connect the cooling water, insert the thermocouple and turn on the temperature control switch, set the heating temperature and adjust the voltage, and then run the program. Heat the autoclave to 255 °C at a rate of 2 °C per minute and maintain the pressure in the autoclave between 7.5 - 8.5 MPa. Carry out heat preservation and pressure maintenance at this temperature and pressure, and control the time for 2 hours. Then, while keeping the temperature unchanged, lead the exhaust pipe into the coolant and open the exhaust valve at the same time to slowly release the system pressure, and its speed is usually controlled at 0.05 MPa / min; (5)Heat treatment of the ceramic green body: The heat treatment equipment mainly consists of three parts: a sintering system, an optical imaging system, and a data recording and processing system; the heat treatment process adopted is to heat the specimen from room temperature to 950 °C in a muffle furnace at a temperature rise rate of 3 °C / min, and then keep it at this temperature for 6 h, 12 h, 24 h, 36 h respectively. Subsequently, it is heated to 1250 / 1650 °C at a temperature rise rate of 1 °C / min and kept at this temperature for 30 min.

[0009] Furthermore, the step of using urease to induce the "self-assembly" of ceramic particles in step (2) is as follows: Place the prepared alumina and silicon carbide slurries in a beaker, and mix urease into it by magnetic stirring. The addition amount of urease is 200 U / 100 g slurry, and the stirring time is controlled within 2 min. After the stirring stops, quickly pour the slurry into a cylindrical metal mold for drying; the time for urease to induce the full hydrolysis of urea at room temperature is 5 min. After the full hydrolysis is completed, the ceramic particles can in-situ solidify the dispersing medium deionized water in the assembled structure to form a "gel" in a semi-solid state with a pH of 9.5.

[0010] Furthermore, the method of using temperature to induce the "self-assembly" of ceramic particles in the said step (2) is carried out by two schemes: a two-step method or a one-step method.

[0011] Furthermore, the two-step self-assembly method is as follows: Place the dispersed ceramic slurry in a sealable container, and then place the sealed container in a water bath. Under the condition of 85 °C, keep it at a constant temperature for 2 h. After that, take out the container, cool it to room temperature, and then move it into the water bath again. Under the condition of 100 °C, keep it at a constant temperature for 2 h to obtain the assembled ceramic green body.

[0012] Furthermore, the so-called one-step self-assembly method is to directly place the slurry in a water bath pot at 100 °C and keep it at a constant temperature for 2 h, allowing urea to slowly hydrolyze and release counterions, gradually compressing the electric double layer of ceramic particles. The ceramic particles attract each other due to the opposite charge polarities and form the final assembly.

[0013] The beneficial effects of this invention are as follows: The beneficial effects of this invention are as follows: (1) Through the theoretical calculation of the interaction between colloidal particles, two possibilities of particle self-assembly and their key control factors are revealed. Adjusting the interaction energy between uniformly charged particles to the second minimum value and using the surface that has not adsorbed a dispersant or has a low charge density as the assembly interface are the specific ways to achieve assembly. The counterion concentration is the most important factor for realizing the self-assembly of ceramic particles. The interaction between particles can be regulated by in-situ controlling the counterion concentration in the slurry, thereby controlling the dispersion and aggregation states of the particles.

[0014] (2)It is proposed to achieve in-situ controllability of the counterion concentration in the slurry by controlling the urea hydrolysis rate. According to the results of theoretical calculations, different self-assembly experiments are designed for different adsorption rates of cationic dispersants on the ceramic particle surface, and a nanoporous structure with a porosity of 80% is constructed.

[0015] (3)By methods such as freeze-drying, dissolution replacement combined with supercritical drying, and low-temperature long-time heat preservation before the sintering shrinkage point to passivate the sintering activity of the green body, the microstructure of the self-assembled ceramic particles is maintained to the greatest extent. Under the conditions of 1250 °C / 0.5 h, the porosity of the material is about 86.5%. This structure shows good high-temperature stability under the high-temperature resistance test conditions of 1650 °C / 0.5 h, and the pore retention rate is as high as 83.75%.

[0016] (4)The homogeneous / gradient SiC-Al2O3 porous ceramics prepared by particle size design combined with the self-assembly method have good high-temperature radiation heat insulation performance. Compared with pure Al2O3 porous ceramics, the front-back temperature difference of the homogeneous SiC-Al2O3 porous ceramic material increases by about 50%, and the transmitted radiant heat flux density decreases by about 24%. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of the principle for studying the high-temperature heat insulation performance of the Al2O3 / SiC porous composite ceramic material in the embodiment of the present invention.

[0018] Figure 2 Schematic diagram of the preparation technical route of the multi-scale structure high-temperature protection composite material in the embodiment of the present invention.

[0019] Figure 3 It is a photo of the well-dispersed ceramic slurry in the embodiment of the present invention.

[0020] Figure 4 It is a photo of the "gel" assembly in the semi-solid state in the embodiment of the present invention.

[0021] Figure 5 It is a graph of the pressure change with temperature during the autoclave heating process in the embodiment of the present invention.

[0022] Figure 6 It is a schematic diagram of the high-temperature on-line test device for heat insulation performance in the embodiment of the present invention. Detailed Embodiments

[0023] The following describes the specific embodiments of the present invention in conjunction with the drawings and embodiments for better understanding of the present invention. Embodiment

[0024] In the embodiments of the present invention, from the perspective of colloid science, the finite element analysis method is used to systematically explore the influence of various factors on the interaction potential energy between ceramic particles. The research results show that for uniformly charged Al2O3 colloidal particles saturated with an adsorbed cationic dispersant (Pdadmac), compared with the Zeta potential, the valence state of counterions, and the pH value of the dispersion medium, the interaction potential energy between colloidal particles is most sensitive to the concentration of counterions in the dispersion medium. By changing the concentration value of counterions, the experimental conditions for forming the second minimum on the DLVO potential curve are obtained (c = 1.5×10 -2 mol / L); for non-uniformly charged Al2O3 colloidal particles that are not saturated with an adsorbed dispersant, by changing the Zeta potential and ionic strength of the "bare" surface, the morphology of the non-uniform electric double layer can be effectively adjusted, and the degree of "display" of the heterogeneous electric double layer can be controlled, thereby providing a theoretical basis and guidance for subsequent assembly experiments, such as Figure 1 is a schematic diagram of the principle for studying the high-temperature heat insulation performance of Al2O3 / SiC porous composite ceramic materials.

[0025] In the embodiments of the present invention, a new method for preparing a highly porous ceramic material by combining the "self-assembly" of ceramic particles with freeze-drying technology is proposed. In the method of the present invention, during the "self-assembly" process of ceramic particles, urea acts as a release agent for counterions and is hydrolyzed in the slurry through the catalytic action of urease. The electrostatic repulsion between ceramic particles is weakened due to the sudden increase in OH- in the slurry, and the particles thus complete the transformation from the initially highly dispersed state to the loose soft agglomerate state after rapid assembly. The macroscopic manifestation of this microscopic phenomenon is that the slurry changes from the initially "pure milk" state with good fluidity to the semi-solid "gel" state without fluidity. The pore structure strength after the "self-assembly" of ceramic particles is relatively low and difficult to withstand the surface tension of the liquid during the drying process, so the freeze-drying technology is used to preserve it. Compared with the specimens prepared by the traditional freeze-drying method without undergoing the "self-assembly" process, the microscopic characteristics and thermal stability of the assembled specimens are significantly superior. The unassembled specimens have a lamellar channel pore structure formed by ice crystals, and the pores of the assembled specimens are three-dimensionally interconnected "honeycomb structures" with a micro-nano gradient arrangement of pore diameters (the pore walls of the micron-sized pores are composed of nano-sized pores with uniform distribution and a narrow pore size distribution (105-135 nm)). The high-temperature resistance test results show that the assembled specimens have good thermal stability. After being kept at 1650°C for two hours, the porosity of the specimens is still as high as 74%, and the pore retention rate is about 83.75%, which is about 68% higher than the pore retention rate of the unassembled specimens.

[0026] In the embodiments of the present invention, in order to achieve the controllability of the "self-assembly" of ceramic particles and further optimize the obtained nano-porous structure, temperature and urease are used to induce the hydrolysis of urea to control the nature and magnitude of the interaction between ceramic particles, thereby realizing the control of nanoparticle assembly. Three typical assembly processes are adopted in the implementation of the present invention: "one-step method" ("D"), "two-step method" ("G") and urease-induced "rapid assembly" ("U"), so as to control the connection speed of ceramic particles and their coordination numbers. The microstructure after the controllable assembly of particles is in-situ maintained by the process of supercritical drying. The results show that the "one-step method" ("D") specimen has the highest porosity (∼80%), the largest cumulative pore volume (0.21 ml / g), the narrowest pore size distribution (160 - 180 nm), and the smallest coordination number of ceramic particles (2 - 3). The porosity and cumulative pore volume of the urease-induced "rapid assembly" "U" specimen are slightly lower than those of the "D" specimen. However, the microstructure is uneven due to the too-fast assembly rate, and the gas generated by urea hydrolysis has no time to escape and is sealed inside the specimen. On the one hand, these gases will generate large pores inside the material, and on the other hand, they will also cause extrusion and damage to the original assembled structure. During the "two-step method" assembly process, the "G" specimen experiences a preliminary assembly process, which increases the coordination number of ceramic particles, ultimately resulting in a decrease in the porosity and cumulative pore volume of the specimen, while the pore size distribution becomes larger accordingly. In addition, by keeping the material at a certain temperature for a long time before the obvious shrinkage temperature point of the material, the activity of the powder can be effectively reduced, and its high-temperature anti-sintering performance can be improved. In particular, taking the "D" specimen as an example, after heat preservation at a low temperature for 24 h and then high-temperature heat treatment (1250 °C), the sacrifice rate of its cumulative pore volume is reduced from 40% under the condition of no heat preservation to 14%.

[0027] In order to further improve the high-temperature radiation heat insulation performance of the prepared ceramic materials, in the embodiments of the present invention, high-emissivity SiC particles are added to the Al2O3 skeleton structure with low thermal conductivity. By changing the particle size of the added SiC particles, Al2O3 / SiC two-phase composite materials with different microstructures are constructed. When the heat source temperature is constant at 1650 °C, the two-phase composite structure with uniformly dispersed SiC particles constructed by SiC particles with a particle size of 800 nm and matrix Al2O3 particles has a good scattering effect on thermal rays. In the range of 0-20 vol%, the scattering effect on thermal rays becomes more significant with the increase of the SiC addition amount. In particular, when the content of SiC particles in the material is 20 vol%, the front-back temperature difference of the material is the largest, about 469 °C, which is 50% higher than that of the pure Al2O3 porous ceramic material, and the radiative heat flux density passing through the material is correspondingly reduced by 24%. On the other hand, adding two kinds of SiC particles with different particle sizes (800 nm / 5-10 μm) to the Al2O3 matrix material can further improve the high-temperature radiation heat insulation performance of the material. For this special structure composed of a deposition layer rich in coarse-grained SiC and a fine-grained uniformly dispersed layer, when the precipitation layer rich in coarse-grained SiC faces the constant-temperature heat source at 1650 °C, with the increase of the proportion of coarse-grained SiC particles in the total addition amount, the front-back temperature difference of the material shows a gradually increasing trend. On the contrary, the radiative heat flux density passing through the material is significantly reduced. In particular, when the addition amount of coarse-grained SiC rises to 50% of the total addition amount by volume fraction, the material shows the best high-temperature heat insulation performance. At this time, the front-back temperature difference of the material rises to 612 °C, and the radiative heat flux density passing through the material is reduced to 205 W / m 2 , compared with the two-phase composite material added with pure fine-grained SiC, the front-back temperature difference of the sample with 50 vol% of the volume fraction of coarse and fine-grained SiC increases by 30.5%, and the radiative heat flux density passing through the sample is reduced by 37.9%.

[0028] Example 2 As Figure 2 shown, the preparation method of the multi-scale structure high-temperature protection composite material in this embodiment specifically includes the following steps: (1) Prepare ceramic slurry: The solid content of the "self-assembly" slurry was selected to be 10 vol%, that is, the volume of the powder accounted for 10 Vol% of the total volume of the slurry, and the SiC powder accounted for 5% of the total volume of the powder. Deionized water was added to the ball mill, and the required silicon carbide and aluminum oxide powders were weighed in a ratio of 1:19 for standby use. The dispersant polydiallyl dimethyl ammonium chloride solution (Pdadmac) accounting for 0.08 wt% of the total mass of the powder was weighed and dropped into the deionized water. The stirring rod was stirred evenly to prepare a premixed solution, and then urea was weighed at a ratio of 0.2g / 100g slurry and added to the premixed solution (this concentration ratio just meets the concentration of counterions in the medium after the urea is fully hydrolyzed to 1.5×10 -2 mol / L), ultrasonically disperse until completely dissolved, slowly add the weighed alumina powder in portions, and then add alumina grinding balls 1.5 times the weight of the powder (D1mm: D3mm: D5mm = 2:2:1). Finally, put the ball mill into a planetary ball mill and ball mill at a constant temperature of 350 r / min for 8 h. After the ball milling, the slurry is sieved to filter out the grinding balls, and the air mixed into the slurry during the ball milling process is discharged by a vacuum pump, so that the alumina slurry with good fluidity, uniform mixing, and uniform saturated adsorption of Pdadmac on the surface of ceramic particles can be prepared.

[0029] (2) “Self-assembled” ceramic particles: In order to explore the effect of the assembly speed of ceramic particles on the microstructure and properties of the final porous ceramic material, this study took two routes in the "self-assembly" experiment. First, urease induces urea hydrolysis; second, temperature induces urea hydrolysis. The ultimate goal that can be achieved by the two paths is the same, that is, by inducing urea hydrolysis, the strength of the counterions in the slurry is increased, thereby compressing the double electric layer of the ceramic particles that have adsorbed the dispersant, so that the surface of the ceramic particles that have adsorbed a small amount of dispersant or are completely exposed is exposed, so that the ceramic particles form a weak contact-like soft agglomerate structure due to the reduction of repulsive potential energy or the opposite potential polarity. The specific experimental process is as follows: 1) Urease induces ceramic particle "self-assembly" The method of urease-induced ceramic particle "self-assembly" is as follows: the prepared alumina slurry is placed in a beaker, such as Figure 3 As shown, urease is mixed into it by magnetic stirring. The amount of urease added is 200 U / 100g slurry. The stirring speed should not be too fast during the stirring process, otherwise air will be mixed in again. The stirring time is controlled within 2 minutes. After the stirring stops, the slurry is quickly poured into a cylindrical metal mold for drying. The time for urease to induce full hydrolysis of urea at room temperature is 5 minutes. After the hydrolysis is fully completed, the ceramic particles can solidify the deionized water of the dispersion medium in situ in the assembled structure to form a semi-solid "gel" with a pH of 9.5, as shown in FIG. Figure 4 shown.

[0030] 2) Temperature-induced "self-assembly" of ceramic particles: Since the rate of urea hydrolysis induced by urease is very rapid, this method controls the "self-assembly" rate of ceramic particles by temperature-induced urea hydrolysis. The pH change curve of the urea solution with temperature was experimentally measured, and the temperature for rapid urea hydrolysis was found to be 100 °C, with a hydrolysis equilibrium time of 2 h.

[0031] Based on the characteristics of urea hydrolysis, two schemes, namely the "two-step method" and the "one-step method", were designed for the experiment of temperature-induced slow "self-assembly" of ceramic particles.

[0032] The so-called "two-step method" means placing the dispersed ceramic slurry in a sealable container, then putting the sealed container in a water bath kettle. Under the condition of 85 °C, keep it at a constant temperature for 2 h. After that, take out the container, cool it to room temperature, and then move it into the water bath kettle again. Under the condition of 100 °C, keep it at a constant temperature for 2 h, and then the assembled ceramic green body can be obtained. The reason for doing this is that at 85 °C, urea has been partially hydrolyzed, and the concentration of counterions in the slurry has increased significantly. Microscopically, the double electric layer of ceramic particles has been compressed to a certain extent, the influence range of the positive surface potential has gradually become smaller, and the double electric layer on the part with a lower positive charge density or the part without adsorbed dispersant has gradually started to play a role. Driven by thermal motion, the ceramic particles have changed from the previous strong repulsion state to a weak repulsion or mutual attraction state. In this case, compared with the ceramic particles in the dispersed slurry, the distance between them will be significantly shortened to form a "pre-assembly body". After cooling, then put it in the water bath kettle and continue to heat it to 100 °C and keep it at this temperature for insulation, in order to let urea further hydrolyze, thereby generating more counterions in the slurry, and further compressing the positive double electric layer of ceramic particles, weakening the electrostatic repulsion potential energy between them, so that the exposed surfaces of ceramic particles are completely exposed, and the particles form the final assembly due to opposite electric charges.

[0033] The so-called "one-step method" means directly placing the slurry in a water bath kettle at 100 °C and keeping it at a constant temperature for 2 h, allowing urea to slowly hydrolyze and release counterions, gradually compressing the double electric layer of ceramic particles, the repulsive force between the particles gradually decreases, the double electric layer on the negatively charged surface gradually plays a role, and the ceramic particles attract each other due to opposite charge polarities and form the final assembly.

[0034] In order to minimize the volume shrinkage of the material caused by the drying process and the resulting changes in the material's microstructure, and to reduce interference factors, a process combining alcohol-water replacement and high-temperature ethanol supercritical drying is adopted for the green body obtained after "self-assembly". The time for solvent replacement is 7 days. The high-temperature and high-pressure autoclave used for supercritical drying is the FYXD permanent magnet rotary stirring autoclave manufactured by Dalian Tongchan High-Pressure Autoclave Container Manufacturing Co., Ltd. It belongs to the first category of pressure vessels, with a design pressure of 22 MPa, a maximum allowable working pressure of 20 MPa, a heating power of 2 kW, a design temperature of 380 °C, and a volume of 2 L. Before loading the sample, the autoclave should be carefully cleaned and wiped several times with a towel dipped in alcohol to avoid any residual debris inside the autoclave.

[0035] When the autoclave is also used as other experimental equipment, before drying the "self-assembled" body of alumina ceramics, only anhydrous ethanol should be added first to raise the temperature and pressure and maintain the pressure for several hours, and then quickly release the gas to reduce the pressure to thoroughly purge the organic matter attached to the inner wall of the autoclave to avoid contaminating the sample. When loading the sample, first add a certain amount of anhydrous ethanol into the autoclave, then wrap the sample replaced with anhydrous ethanol with ceramic fiber cloth and put it into the drying autoclave in sequence. After that, add an appropriate amount of anhydrous ethanol to submerge the sample, and it is appropriate to keep the sum of the liquid and sample volume accounting for about 60% of the total volume in the autoclave. Tighten the autoclave lid for sealing, then turn on the power supply and fill it with nitrogen to replace the residual air in the autoclave, and purge it three times repeatedly; finally, continuously introduce nitrogen until the pressure reaches 2 MPa and maintain the pressure for 20 minutes. This can not only check the airtightness of the autoclave to determine whether it is well sealed, but also act as an inert gas to avoid the explosion risk of ethanol vapor under high temperature and high pressure. If the pressure does not change within 20 minutes, then turn on the cooling water, insert the thermocouple and turn on the temperature control switch, set the heating temperature and adjust the voltage, and then run the program. Heat the autoclave to 255 °C at a rate of 2 °C per minute and maintain the pressure in the autoclave between 7.5 - 8.5 MPa. The heating curve is as Figure 5 shown. Although the supercritical state of anhydrous ethanol (240 °C, 6.3 MPa) has been reached at this time, it takes a certain amount of time for the liquid in the sample pores to be completely converted into the supercritical state. Therefore, it is necessary to keep the temperature and pressure constant for 2 hours. Then, while keeping the temperature unchanged, insert the exhaust pipe into the coolant, and at the same time open the exhaust valve to slowly release the system pressure, and its speed is usually controlled at 0.05 MPa / min. After the pressure in the autoclave drops to the ambient pressure, purge it with nitrogen for a certain period of time to ensure that there is no residual ethanol vapor in the autoclave to prevent condensation and liquefaction in the later stage of cooling and damage the sample. After the exhaust is completed, turn off the heating switch and the power supply. Finally, keep the exhaust valve and the cooling water open for natural cooling, and at the same time pull out the exhaust pipe from the coolant to avoid backflow phenomenon after the temperature drops.

[0036] To maximize the retention of the "self-assembled" structure, it is necessary to minimize the sintering shrinkage of the material during the heat treatment process. In the heat treatment step, long-term heat preservation is carried out before the shrinkage point of the material to reduce the surface activity of the powder material, thereby suppressing the linear shrinkage during the subsequent sintering process. The method used in this process is optical non-contact measurement, aiming to provide guidance and reference for the formulation and optimization of the Al2O3 ceramic sintering process.

[0037] This equipment mainly consists of three parts: a sintering system, an optical imaging system, and a data recording and processing system. The temperature range measured by the instrument is from room temperature to 1600 °C, the temperature uniformity of the sample heating zone is ±6 °C, and the size measurement accuracy reaches 10 µm. The method for measuring the sintering shrinkage curve of the material is as follows: Place the sample on the test bracket, heat the sample to 1600 °C at a heating rate of 10 °C / min, then stop heating and let the sample cool naturally. During the sintering process, a telecentric blue light source continuously emits parallel light beams to irradiate the sample, generating a projection. After passing through a high-temperature filter, the image is received by a high-speed camera, and then the data recording and processing system analyzes and processes the image. Finally, the sintering shrinkage data of the material is obtained online. When the temperature is below 950 °C, there is no obvious linear shrinkage in the specimen. When the heat treatment temperature is higher than this temperature, the specimen begins to shrink significantly. Therefore, to control the linear shrinkage of the prepared "self-assembled" specimen, it is necessary to keep the temperature low for a long time before 950 °C to reduce the activity of the material, thereby suppressing the shrinkage that occurs during the heat treatment process. In view of this, the heat treatment process adopted in this embodiment is to heat the specimen from room temperature to 950 °C in a muffle furnace at a temperature rise rate of 3 °C / min, and then keep it at this temperature for 6 h, 12 h, 24 h, and 36 h respectively. Subsequently, it is heated to 1250 / 1650 °C at a temperature rise rate of 1 °C / min and kept at this temperature for 30 min.

[0038] Figure 6 This is a schematic diagram of the high-temperature on-line test device for heat insulation performance in the embodiment of the present invention. The composite material obtained by the above process maximizes the retention of the microscopic structure of the self-assembled ceramic particles through methods such as freeze-drying, dissolution replacement, combination with supercritical drying, and low-temperature long-term heat preservation before the sintering shrinkage point to passivate the sintering activity of the green body. Under the condition of 1250 °C, the porosity of the material is about 88%. Under the heat resistance test conditions of 1650 °C / 0.5 h, this structure exhibits good high-temperature stability, and the pore retention rate is as high as 83.75%.

[0039] Compared with pure Al2O3 porous ceramics, the surface-back temperature difference of homogeneous SiC-Al2O3 porous ceramic materials increases by about 50%, while the radiative heat flux density transmitted therethrough decreases by about 24%. For gradient SiC-Al2O3 porous ceramics, when the coarse-grained SiC-rich layer faces the high-temperature heat source, the surface-back temperature difference of the material can be further reduced. In particular, when the addition amount of coarse-grained SiC accounts for 50 vol% of the total SiC, compared with the Al2O3 / SiC homogeneous binary porous system, the surface-back temperature difference of the material further increases by 30.5%, and the radiative heat flux density transmitted through the specimen decreases by 37.9%.

[0040] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

Claims

1. A method for preparing a multi-scale structure high temperature protection composite material, characterized in that: The specific steps include: (1) Theoretical calculation: Based on the DLVO theory, the experimental conditions for forming the second minimum (the counterion concentration that enables soft agglomeration between particles) on the DLVO potential energy curve of ceramic particles are calculated to provide a theoretical basis and guidance for subsequent assembly experiments; (2) Preparation of ceramic slurry: Deionized water was added into a ball mill, and the required alumina and silicon carbide powders were weighed according to 10 vol% of the total volume of the slurry, of which SiC powder accounted for 5% of the total volume of the powders. 0.08 wt% of the dispersant polydimethyldiallyl ammonium chloride was weighed and dropped into the deionized water. The mixture was stirred evenly with a stirring rod to prepare a premixed solution. Urea was weighed at a ratio of 0.2 g / 100 g of slurry and added to the premixed solution. After ultrasonic dispersion until all the slurry was dissolved, the weighed powders were slowly added in portions. Subsequently, zirconia grinding balls with a weight of 1.5 times the weight of the powders were added. The weight ratio of zirconia grinding balls with a radius of 1 mm, 3 mm, and 5 mm was 2:2:

1. The ball mill was placed into a planetary ball mill and ball milled at a speed of 350 r / min for 8 hours at a constant temperature. h; after the ball milling is completed, the slurry is sieved to filter out the grinding balls, and the air mixed into the slurry during the ball milling process is discharged through a vacuum pump, so that an alumina slurry with good fluidity, uniform mixing, and uniform saturated adsorption of polydimethyldiallylammonium chloride on the surface of ceramic particles can be prepared; (3) "Self-assembled" ceramic particles: "Self-assembled" ceramic particles are produced by urease-induced urea hydrolysis or temperature-induced urea hydrolysis. (4) Drying the ceramic blank: The blank obtained after "self-assembly" is subjected to a process combining alcohol-water replacement and high-temperature ethanol supercritical drying. The solvent replacement time is 7 days. Before loading the sample, the autoclave should be carefully cleaned and wiped several times with a towel dipped in alcohol to avoid residual debris in the autoclave. When loading the sample, a certain amount of anhydrous ethanol is first added to the autoclave, and then the sample replaced with anhydrous ethanol is wrapped with ceramic fiber cloth and placed in the drying autoclave in order. Then, an appropriate amount of anhydrous ethanol is added to immerse the sample, and the sum of the volume of the liquid and the sample is kept at about 60% of the total volume of the autoclave. The autoclave cover is tightened to seal, and then the power is turned on and nitrogen is filled in to replace the residual air in the autoclave. The purge is repeated three times. Finally, nitrogen is continuously introduced to make the pressure reach 2MPa and the pressure is maintained for 20 minutes. If the pressure does not change within 20 minutes, connect the cooling water, insert the thermocouple and turn on the temperature control switch, set the heating temperature and adjust the voltage, and then run the program; heat the autoclave to 255 °C at a rate of 2 °C per minute, and maintain the pressure in the autoclave between 7.5 and 8.5 MPa; keep the temperature and pressure at this temperature and pressure for 2 hours, then pass the vent pipe into the coolant while keeping the temperature unchanged, and open the vent valve at the same time to slowly release the system pressure, and the speed is usually controlled at 0.05 MPa / min; (5) Heat treatment of ceramic blanks: The heat treatment equipment mainly consists of three parts: sintering system, optical imaging system and data recording and processing system. The heat treatment process adopted is to heat the sample from room temperature to 950℃ in a muffle furnace at a temperature rise rate of 3℃ / min, and then keep it at this temperature for 6h, 12h, 24h and 36h respectively, and then heat it to 1250 / 1650℃ at a temperature rise rate of 1℃ / min and keep it at this temperature for 30min.

2. The method for preparing the multi-scale structure high temperature protection composite material according to claim 1, characterized in that: The step (2) of using urease to induce the "self-assembly" of ceramic particles is as follows: the prepared alumina and silicon carbide slurry is placed in a beaker, and urease is mixed therein by magnetic stirring. The amount of urease added is 200 U / 100g slurry, and the stirring time is controlled within 2 minutes. After the stirring stops, the slurry is quickly poured into a cylindrical metal mold for drying; the time for urease to induce full hydrolysis of urea at room temperature is 5 minutes. After the hydrolysis is fully completed, the ceramic particles can solidify the dispersion medium deionized water in situ in the assembled structure to form a semi-solid "gel" with a pH of 9.

5.

3. The method for preparing the multi-scale structure high temperature protection composite material according to claim 1, characterized in that: The temperature-induced ceramic particle "self-assembly" method in step (2) is carried out by using a two-step method or a one-step method.

4. The method for preparing the multi-scale structure high temperature protection composite material according to claim 3, characterized in that: The two-step self-assembly method is as follows: the dispersed ceramic slurry is placed in a sealable container, and the sealed container is placed in a water bath, kept at a constant temperature of 85°C for 2 hours, then the container is taken out, cooled to room temperature, and then moved into a water bath, kept at a constant temperature of 100°C for 2 hours to obtain an assembled ceramic blank.

5. The method for preparing the multi-scale structure high temperature protection composite material according to claim 3, characterized in that: The so-called one-step self-assembly method is to directly place the slurry in a water pot at 100°C and keep it warm for 2 hours, allowing the urea to slowly hydrolyze and release counter ions, gradually compressing the double layer of the ceramic particles. The ceramic particles attract each other due to their opposite charge polarity and form the final assembly.

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