SiBCN aerogel as well as normal-pressure drying preparation method and application thereof

Abstract

The invention relates to SiBCN aerogel as well as a normal-pressure drying preparation method and application thereof, and belongs to the technical field of functional ceramic aerogel. In order to solve the problems of high equipment requirement, high cost and large energy consumption of the existing SiBCN aerogel preparation method, the invention provides a normal-pressure drying preparation method of SiBCN aerogel, which comprises the steps of precursor solution preparation, hydrothermal crosslinking reaction, solvent replacement, normal-pressure drying and high-temperature ceramization heat treatment. Through step-by-step solvent replacement and normal-temperature and normal-pressure drying treatment, the destructive effect of capillary force on a gel skeleton in the solvent evaporation process is effectively weakened, and the integrity of the gel skeleton can be guaranteed. The method can be implemented only under common room-temperature and normal-pressure conditions, and the preparation process is simple, convenient, efficient and easy to accurately control. The preparation method disclosed by the invention is low in energy consumption and low in preparation cost, meets actual production requirements, is suitable for large-scale production, and provides a technical basis for popularization and application of the SiBCN aerogel.

Description

Technical Field

[0001] The present invention belongs to the technical field of advanced inorganic non-metallic materials, particularly to the technical field of functional ceramic aerogels, and specifically relates to a SiBCN aerogel, its atmospheric pressure drying preparation method and application. Background Art

[0002] During the service process of hypersonic aircraft, they will experience severe aerodynamic heating in the presence of oxygen, which poses a serious threat to the overall safe service of the aircraft. To ensure the normal operation of the internal electronic equipment of the aircraft and the integrity of the airframe structure, thermal protection materials are usually used to protect the aircraft. With the continuous development of hypersonic aircraft technology, their service environment has become increasingly severe, posing higher requirements for the antioxidant performance, heat insulation ability, temperature resistance ability, etc. of thermal protection materials.

[0003] Aerogel is a material with a three-dimensional nano-network structure and a rich pore structure. It has advantages such as low density, high porosity, high specific surface area, and low thermal conductivity, and thus has broad application prospects in fields such as catalysis, heat insulation, sound absorption, and environmental protection. However, currently widely used oxide aerogels generally have disadvantages such as insufficient temperature resistance and easy phase change under high-temperature conditions, while carbon aerogels mainly face the problem of being extremely prone to failure in a high-temperature aerobic environment. As a kind of ceramic aerogel, SiBCN aerogel has good antioxidant ability, usually does not undergo phase separation at 1500 °C, and the crystallization temperature in an inert atmosphere is as high as 1800 °C. It has better temperature resistance than common ceramic aerogels such as SiC, SiOC, and SiCN, and is expected to become a candidate thermal protection material for hypersonic aircraft.

[0004] The invention patent application document with the application number CN113979753A discloses a preparation method of SiBCN aerogel, that is, a precursor gel is obtained through a hydrosilylation reaction, a polyborosilazane aerogel is obtained through freeze-drying, and then a SiBCN ceramic aerogel with a developed pore structure is obtained through a precursor conversion method. However, the ice crystal growth process of freeze-drying usually has a certain impact on the pore structure of the material, and it generally has the problem of high requirements for equipment.

[0005] The invention patent application document with the application number CN110818431A discloses a zirconium-containing polyborosilazane precursor aerogel, a silicon boron carbon nitrogen / zirconium dioxide ceramic aerogel, and their preparation methods and applications, and combines the hydrosilylation reaction, CO2 supercritical drying, and precursor conversion method to synthesize SiBCN-ZrO2 ceramic aerogel. Although supercritical drying method can usually effectively preserve the pore structure of aerogel materials, it generally has problems such as high cost, high energy consumption, and high requirements for equipment.

[0006] Regarding the limitations of the above methods, there is an urgent need to develop a more economical, simple, and batch-preparable drying method to better meet the actual application requirements of SiBCN aerogels in fields such as hypersonic aircraft, and to provide a technical basis for their large-scale application in industrial heat insulation, building insulation, and other fields in the future. Summary of the Invention

[0007] To solve the problems of high equipment requirements, high cost, and high energy consumption in the existing preparation methods of SiBCN aerogels, the present invention provides a SiBCN aerogel, an atmospheric pressure drying preparation method thereof, and an application.

[0008] The technical solution of the present invention:

[0009] An atmospheric pressure drying preparation method of a SiBCN aerogel includes the following steps:

[0010] Step 1. Prepare a precursor solution:

[0011] Using polyborosilazane as a precursor, add it to an organic solvent and stir evenly. Then add divinylbenzene as a cross-linking agent and a catalyst to the obtained mixed system, and stir evenly to obtain a wet gel precursor solution.

[0012] Step 2. Hydrothermal cross-linking reaction:

[0013] Place the wet gel precursor solution obtained in Step 1 at a certain temperature for hydrothermal cross-linking reaction to obtain a polyborosilazane wet gel.

[0014] Step 3. Solvent replacement and atmospheric pressure drying:

[0015] Prepare a concentration gradient mixed solution of a replacement solvent and the organic solvent. Immerse the polyborosilazane wet gel obtained in Step 2 successively in the concentration gradient mixed solution with gradually increasing concentration of the replacement solvent to gradually replace the organic solvent in the polyborosilazane wet gel with the replacement solvent. Then perform room temperature atmospheric pressure drying treatment on the replaced polyborosilazane wet gel to obtain a polyborosilazane dry gel.

[0016] Step 4. High-temperature ceramization heat treatment:

[0017] Place the polyborosilazane dry gel obtained in Step 3 under a protective atmosphere for high-temperature heat treatment to obtain a SiBCN aerogel.

[0018] Furthermore, the organic solvent in Step 1 is one of n-hexane, cyclohexane, or toluene, the catalyst is Karstedt catalyst, and the concentration of the catalyst in the wet gel precursor solution is 20 ppm.

[0019] Further, the mass ratio of the crosslinking agent divinylbenzene to polyborosilazane in the wet gel precursor solution obtained in Step 1 is 1:0.5 - 1.5; the mass percentage content of polyborosilazane in the obtained wet gel precursor solution is 7.5 - 12.5 wt.%.

[0020] Further, the temperature of the hydrothermal crosslinking reaction described in Step 2 is 120 - 150 °C, and the reaction time is 20 h.

[0021] Further, the volume concentrations of the displacement solvents in the concentration gradient mixed solution described in Step 3 are 20%, 40%, 60%, 80%, and 100% in sequence, and the displacement solvent is ethanol.

[0022] Further, the immersion time of the polyborosilazane wet gel in each concentration gradient mixed solution in Step 3 is 24 h.

[0023] Further, the time for the room temperature and atmospheric pressure drying treatment described in Step 3 is 4 days.

[0024] Further, the protective atmosphere described in Step 4 is a nitrogen atmosphere, the heating rate of the high-temperature heat treatment is 2 - 3 °C / min, the heat treatment temperature is 800 - 1000 °C, and the heat treatment time is 2 h.

[0025] A SiBCN aerogel prepared by the atmospheric pressure drying preparation method provided by the present invention, the density of the SiBCN aerogel is 0.20 - 0.26 g / cm 3 .

[0026] An application of the SiBCN aerogel prepared by the present invention in the fields of aircraft thermal protection, industrial heat insulation, or building thermal insulation.

[0027] Advantages of the present invention:

[0028] The atmospheric pressure drying preparation method of the SiBCN aerogel provided by the present invention effectively weakens the destructive effect of capillary force on the gel skeleton during the solvent evaporation process through stepwise solvent replacement and room temperature and atmospheric pressure drying treatment, and can ensure the integrity of the gel skeleton. The polyborosilazane xerogel prepared by this method does not crack and its average drying size shrinkage rate < 5%.

[0029] The atmospheric pressure drying preparation method of the SiBCN aerogel provided by the present invention can be implemented only under ordinary room temperature and atmospheric pressure conditions, solves the problem of high equipment requirements for the existing SiBCN aerogel preparation method, and makes the preparation process simple, efficient, and easy to accurately control. The preparation method of the present invention has low energy consumption and low preparation cost, meets the actual production requirements, is suitable for large-scale production, and provides a technical basis for the popularization and application of SiBCN aerogels. Description of the Drawings

[0030] Figure 1 SEM image of the SiBCN aerogel prepared in Example 1;

[0031] Figure 2 TEM image of the SiBCN aerogel prepared in Example 1;

[0032] Figure 3 Physical photos of the polyborosilazane xerogel prepared in Example 1 and the SiBCN aerogel obtained by high-temperature ceramization heat treatment. Detailed implementation manners

[0033] The technical solutions of the present invention will be further described below in conjunction with the embodiments, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention. The process equipment or devices not specifically noted in the following embodiments are all conventional equipment or devices in the art. If not specifically indicated, the raw materials used in the embodiments of the present invention can be obtained commercially; if not specifically indicated, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0034] Example 1

[0035] This example provides a method for preparing SiBCN aerogel by atmospheric drying, which includes the following steps:

[0036] Step 1. Prepare a precursor solution:

[0037] Polyborosilazane is added to toluene and stirred evenly. Divinylbenzene is added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane of 1:1. Finally, Karstedt catalyst is added. After stirring evenly, a wet gel precursor solution is obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution is 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution is 20 ppm.

[0038] Step 2. Hydrothermal crosslinking reaction:

[0039] The wet gel precursor solution obtained in Step 1 is added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy liner, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it is taken out to obtain a polyborosilazane wet gel;

[0040] Step 3. Solvent replacement and atmospheric drying:

[0041] Prepare a concentration gradient mixed solution of ethanol and toluene. First, immerse the obtained polyborosilazane wet gel in an anhydrous ethanol-toluene mixed solution with an ethanol volume concentration of 20% for 24 hours; then immerse it in an anhydrous ethanol-toluene mixed solution with an ethanol volume concentration of 40% for 24 hours; then immerse it in an anhydrous ethanol-toluene mixed solution with an ethanol volume concentration of 60% for 24 hours; then immerse it in an anhydrous ethanol-toluene mixed solution with an ethanol volume concentration of 80% for 24 hours; finally, immerse it in 100% anhydrous ethanol for 24 hours. Each time the solvent is changed, ensure that the ratio of the wet gel to the solvent is consistent to ensure the effect of gradual replacement.

[0042] Transfer the replaced polyborosilazane wet gel into a drying dish and dry it at normal pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate is 4.1%.

[0043] Step Four: High-temperature ceramization heat treatment:

[0044] Place the polyborosilazane dry gel obtained in Step Three into a high-temperature tube furnace. Under a flowing nitrogen atmosphere, heat it to 800 °C at a rate of 2 °C / min and hold for 2 h to obtain a SiBCN ceramic aerogel. The density of the obtained SiBCN ceramic aerogel is 0.22 g / cm 3 。

[0045] Figure 1 Figure SEM of the SiBCN aerogel prepared in Example 1. As can be seen from Figure 1 it, as can be seen from Figure 1 it, the surface of the aerogel presents a uniform pore structure, and the pore distribution is relatively uniform, showing good network structure characteristics.

[0046] Figure 2 Figure TEM of the SiBCN aerogel prepared in Example 1. As can be seen from Figure 2 it, the inside of the aerogel presents a delicate nano-scale structure, the crystal particles are evenly distributed, and the pore structure is consistent with the network structure observed in the scanning electron micrograph.

[0047] Figure 3 Figure of the physical photos of the polyborosilazane dry gel prepared in Example 1 and the SiBCN aerogel obtained by high-temperature ceramization heat treatment. As can be seen from Figure 3 it, both the polyborosilazane dry gel and the SiBCN aerogel obtained by ceramization heat treatment have extremely low density and weight.

[0048] Example 2

[0049] This example provides a method for preparing SiBCN aerogel by atmospheric pressure drying, including the following steps:

[0050] Step 1. Prepare the precursor solution:

[0051] Add polyborosilazane into cyclohexane and stir evenly. Add divinylbenzene to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane being 1:1. Finally, add Karstedt catalyst and stir evenly to obtain a wet gel precursor solution. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution is 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution is 20 ppm.

[0052] Step 2. Hydrothermal crosslinking reaction:

[0053] Add the wet gel precursor solution obtained in Step 1 into a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy inner lining, react at 120 °C for 20 h, take it out after natural cooling to room temperature to obtain a polyborosilazane wet gel;

[0054] Step 3. Solvent replacement and atmospheric drying:

[0055] Prepare a concentration gradient mixed solution of ethanol and cyclohexane. First, immerse the obtained polyborosilazane wet gel in an anhydrous ethanol - cyclohexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immerse it in an anhydrous ethanol - cyclohexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immerse it in an anhydrous ethanol - cyclohexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immerse it in an anhydrous ethanol - cyclohexane mixed solution with an ethanol volume concentration of 80% for 24 hours; finally, immerse it in 100% anhydrous ethanol for 24 hours. Each time when replacing the solvent, ensure that the ratio of the wet gel to the solvent is consistent to ensure the effect of gradual replacement.

[0056] Transfer the replaced polyborosilazane wet gel into a drying dish and perform atmospheric drying at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate is 4.0%.

[0057] Step 4. High - temperature ceramization heat treatment:

[0058] Place the polyborosilazane dry gel obtained in Step 3 in a high - temperature tube furnace, under a flowing nitrogen atmosphere, heat it to 800 °C at a rate of 2 °C / min, and hold for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel is 0.22 g / cm 3 .

[0059] Example 3

[0060] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0061] Step 1. Prepare the precursor solution:

[0062] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane being 1:1. Finally, Karstedt catalyst was added. After stirring evenly, a wet - gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet - gel precursor solution was 10 wt.%, and the concentration of Karstedt catalyst in the wet - gel precursor solution was 20 ppm.

[0063] Step Two: Hydrothermal cross - linking reaction:

[0064] The wet - gel precursor solution obtained in Step One was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy inner lining, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0065] Step Three: Solvent replacement and atmospheric drying:

[0066] A concentration - gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, it was ensured that the ratio of the wet gel to the solvent was consistent to ensure the effect of gradual replacement.

[0067] The replaced polyborosilazane wet gel was transferred into a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 3.5%.

[0068] Step Four: High - temperature ceramization heat treatment:

[0069] The polyborosilazane dry gel obtained in Step Three was placed in a high - temperature tubular furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel was 0.20 g / cm 3 .

[0070] Example 4

[0071] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0072] Step One: Preparation of precursor solution:

[0073] The polyborosilazane was added to n-hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane being 1:1. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 7.5 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0074] Step 2. Hydrothermal crosslinking reaction:

[0075] The wet gel precursor solution obtained in Step 1 was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy liner, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0076] Step 3. Solvent replacement and atmospheric drying:

[0077] A concentration gradient mixed solution of ethanol and n-hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an anhydrous ethanol-n-hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an anhydrous ethanol-n-hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an anhydrous ethanol-n-hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an anhydrous ethanol-n-hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, it was ensured that the ratio of the wet gel to the solvent was consistent to ensure the effect of gradual replacement.

[0078] The replaced polyborosilazane wet gel was transferred to a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 4.4%.

[0079] Step 4. High-temperature ceramization heat treatment:

[0080] The polyborosilazane dry gel obtained in Step 3 was placed in a high-temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel was 0.26 g / cm 3 .

[0081] Example 5

[0082] This example provides a method for preparing a SiBCN aerogel by atmospheric drying, including the following steps:

[0083] Step 1. Prepare a precursor solution:

[0084] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane of 1:1. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 12.5 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0085] Step 2. Hydrothermal cross - linking reaction:

[0086] The wet gel precursor solution obtained in Step 1 was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy inner lining, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel.

[0087] Step 3. Solvent replacement and atmospheric drying:

[0088] A concentration gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first soaked in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then soaked in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then soaked in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then soaked in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally soaked in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, the ratio of the wet gel to the solvent was ensured to be consistent to guarantee the effect of gradual replacement.

[0089] The replaced polyborosilazane wet gel was transferred into a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 3.8%.

[0090] Step 4. High - temperature ceramization heat treatment:

[0091] The polyborosilazane dry gel obtained in Step 3 was placed in a high - temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel was 0.24 g / cm 3 .

[0092] Example 6

[0093] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0094] Step 1. Preparation of precursor solution:

[0095] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane of 1:0.5. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0096] Step Two, hydrothermal cross - linking reaction:

[0097] The wet gel precursor solution obtained in Step One was added into a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy liner, and reacted at 120 °C for 20 h. After natural cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0098] Step Three, solvent replacement and atmospheric drying:

[0099] A concentration gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, the ratio of the wet gel to the solvent was ensured to be consistent to guarantee the effect of gradual replacement.

[0100] The replaced polyborosilazane wet gel was transferred into a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 3.9%.

[0101] Step Four, high - temperature ceramization heat treatment:

[0102] The polyborosilazane dry gel obtained in Step Three was placed in a high - temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel was 0.23 g / cm 3 .

[0103] Example 7

[0104] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0105] Step One, preparing a precursor solution:

[0106] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane of 1:1. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0107] Step 2. Hydrothermal cross - linking reaction:

[0108] The wet gel precursor solution obtained in Step 1 was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy inner lining, and reacted at 150 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0109] Step 3. Solvent replacement and atmospheric drying:

[0110] A concentration gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, the ratio of the wet gel to the solvent was ensured to be consistent to guarantee the effect of gradual replacement.

[0111] The replaced polyborosilazane wet gel was transferred to a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 4.4%.

[0112] Step 4. High - temperature ceramization heat treatment:

[0113] The polyborosilazane dry gel obtained in Step 3 was placed in a high - temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding obtained SiBCN ceramic aerogel was 0.23 g / cm 3 .

[0114] Example 8

[0115] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0116] Step 1. Preparation of precursor solution:

[0117] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane of 1:1.5. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0118] Step 2: Hydrothermal cross - linking reaction:

[0119] The wet gel precursor solution obtained in Step 1 was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy liner, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0120] Step 3: Solvent replacement and atmospheric drying:

[0121] A concentration gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, it was ensured that the ratio of the wet gel to the solvent was the same to ensure the effect of gradual replacement.

[0122] The replaced polyborosilazane wet gel was transferred to a drying dish and dried at atmospheric pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 3.4%.

[0123] Step 4: High - temperature ceramization heat treatment:

[0124] The polyborosilazane dry gel obtained in Step 3 was placed in a high - temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 800 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel. The density of the corresponding SiBCN ceramic aerogel was 0.24 g / cm 3 .

[0125] Example 9

[0126] This example provides a method for preparing SiBCN aerogel by atmospheric drying, including the following steps:

[0127] Step 1: Preparation of precursor solution:

[0128] The polyborosilazane was added to n - hexane and stirred evenly. Divinylbenzene was added to the obtained mixed system according to the mass ratio of divinylbenzene to polyborosilazane being 1:1. Finally, Karstedt catalyst was added. After stirring evenly, a wet gel precursor solution was obtained. The mass percentage content of polyborosilazane in the obtained wet gel precursor solution was 10 wt.%, and the concentration of Karstedt catalyst in the wet gel precursor solution was 20 ppm.

[0129] Step 2: Hydrothermal cross - linking reaction:

[0130] The wet gel precursor solution obtained in Step 1 was added to a hydrothermal reaction kettle with a polytetrafluoroethylene magnetic therapy inner lining, and reacted at 120 °C for 20 h. After naturally cooling to room temperature, it was taken out to obtain a polyborosilazane wet gel;

[0131] Step 3: Solvent replacement and atmospheric drying:

[0132] A concentration gradient mixed solution of ethanol and n - hexane was prepared. The obtained polyborosilazane wet gel was first immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 20% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 40% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 60% for 24 hours; then immersed in an anhydrous ethanol - n - hexane mixed solution with an ethanol volume concentration of 80% for 24 hours; and finally immersed in 100% anhydrous ethanol for 24 hours. Each time the solvent was changed, the ratio of the wet gel to the solvent was ensured to be consistent to guarantee the effect of gradual replacement.

[0133] The replaced polyborosilazane wet gel was transferred into a drying dish and dried at normal pressure at room temperature for 4 days to obtain a polyborosilazane dry gel, and the gel drying shrinkage rate was 3.5%.

[0134] Step 4: High - temperature ceramization heat treatment:

[0135] The polyborosilazane dry gel obtained in Step 3 was placed in a high - temperature tube furnace. Under a flowing nitrogen atmosphere, it was heated to 1000 °C at a rate of 2 °C / min and held for 2 h to obtain a SiBCN ceramic aerogel, and the density of the corresponding SiBCN ceramic aerogel was 0.25 g / cm 3 .

Claims

1. A method for preparing SiBCN aerogel by drying at normal pressure, characterized in that: The steps include: Step 1: Prepare precursor solution: Adding polyborosilazane as a precursor into an organic solvent and stirring evenly, adding a crosslinking agent divinylbenzene and a catalyst to the obtained mixed system, stirring evenly to obtain a wet gel precursor solution; Step 2: Hydrothermal cross-linking reaction: placing the wet gel precursor solution obtained in step 1 at a certain temperature for hydrothermal crosslinking reaction to obtain a polyborosilazane wet gel; Step 3: Solvent replacement and atmospheric pressure drying: Prepare a concentration gradient mixed solution of a replacement solvent and the organic solvent, immerse the polyborosilazane wet gel obtained in step 2 in a concentration gradient mixed solution with gradually increasing concentrations of the replacement solvent, gradually replace the organic solvent in the polyborosilazane wet gel with the replacement solvent, and dry the replaced polyborosilazane wet gel at room temperature and pressure to obtain a polyborosilazane dry gel; Step 4: High temperature ceramic heat treatment: The polyborosilazane xerogel obtained in step 3 is placed in a protective atmosphere for high-temperature heat treatment to obtain SiBCN aerogel.

2. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 1, characterized in that: In step 1, the organic solvent is one of n-hexane, cyclohexane or toluene, the catalyst is a Karstedt catalyst, and the concentration of the catalyst in the wet gel precursor solution is 20 ppm.

3. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 1 or 2, characterized in that: In the wet gel precursor solution obtained in step 1, the mass ratio of the crosslinking agent divinylbenzene to polyborosilazane is 1:0.5-1.5; the mass percentage of polyborosilazane in the wet gel precursor solution obtained is 7.5-12.5wt.%.

4. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 3, characterized in that: The temperature of the hydrothermal crosslinking reaction in step 2 is 120-150° C., and the reaction time is 20 hours.

5. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 4, characterized in that: In step 3, the volume concentrations of the replacement solvent in the concentration gradient mixed solution are 20%, 40%, 60%, 80% and 100% respectively, and the replacement solvent is ethanol.

6. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 5, characterized in that: In step 3, the immersion time of the polyborosilazane wet gel in each concentration gradient mixed solution is 24 hours.

7. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 6, characterized in that: The drying time at room temperature and normal pressure in step 3 is 4 days.

8. The method for preparing SiBCN aerogel by drying at normal pressure according to claim 7, characterized in that: The protective atmosphere in step 4 is a nitrogen atmosphere, the heating rate of the high-temperature heat treatment is 2-3°C / min, the heat treatment temperature is 800-1000°C, and the heat treatment time is 2h.

9. A SiBCN aerogel prepared by the atmospheric pressure drying preparation method according to any one of claims 1 to 8, characterized in that: The density of the SiBCN aerogel is 0.20-0.26 g / cm 3 .

10. Use of the SiBCN aerogel according to claim 9 in the fields of aircraft thermal protection, industrial thermal insulation or building thermal insulation.

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