阅读说明：本技术 一种具有定向孔结构的耐高温碳化硅气凝胶的制备方法 (Preparation method of high-temperature-resistant silicon carbide aerogel with directional pore structure ) 是由 张和平 潘月磊 程旭东 龚伦伦 于 2021-09-22 设计创作，主要内容包括：本发明公开具有定向孔结构的耐高温碳化硅气凝胶的制备方法,包括以下步骤S10、将纳米纤维素分散液与硅源混合搅拌以得到湿凝胶；S20、将湿凝胶导入模具,并进行定向冷冻处理,以得到冷冻成型的块状凝胶；S30、对块状凝胶进行真空干燥处理,以得到纳米纤维素/硅源复合材料；S40、将纳米纤维素/硅源复合材料进行高温烧结,以得到具有定向孔结构的耐高温碳化硅气凝胶材料,如此使得制备成本低,工序简单,可控程度高,准确率高,操作简便,可规模化生产,制备得到的定向孔结构碳化硅气凝胶材料能够直接应用于保温隔热领域,碳化硅气凝胶室温导热系数<0.02W/m·K,高温800℃下的导热系数<0.07W/m·K,密度<0.15g/cm～(3),孔隙率>90％,热稳定>1200℃。(The invention discloses a preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure, which comprises the following steps of S10, mixing and stirring nano-cellulose dispersion liquid and a silicon source to obtain wet gel; s20, introducing the wet gel into a mould, and performing directional freezing treatment to obtain a frozen block gel; s30, carrying out vacuum drying treatment on the block gel to obtain the nano cellulose/silicon source composite material; s40, sintering the nano-cellulose/silicon source composite material at high temperature to obtain the high-temperature-resistant silicon carbide aerogel material with the directional pore structure, so that the preparation cost is low, the process is simple, the controllability is high, the accuracy is high, the operation is simple and convenient, the large-scale production can be realized, the prepared silicon carbide aerogel material with the directional pore structure can be directly applied to the field of heat preservation and heat insulation, and the silicon carbide aerogel chamber can be used for preparing silicon carbide aerogel materials with the directional pore structureThermal conductivity coefficient of temperature<0.02W/m.K, heat conductivity coefficient at high temperature of 800 DEG C<0.07W/m.K, density<0.15g/cm 3 Porosity of>90% thermal stability>1200℃。)

1. A preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure is characterized by comprising the following steps:

s10, mixing and stirring the nano-cellulose dispersion liquid and a silicon source to obtain wet gel;

s20, introducing the wet gel into a mold, and performing directional freezing treatment to obtain a frozen block gel;

s30, carrying out vacuum drying treatment on the massive gel to obtain a nano cellulose/silicon source composite material;

s40, sintering the nano-cellulose/silicon source composite material at high temperature to obtain the high-temperature-resistant silicon carbide aerogel material with the directional pore structure.

2. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S10:

the mass fraction of the nano-cellulose dispersion liquid is 0.1-7%; and/or the presence of a gas in the gas,

the length of the nanofiber in the nano-cellulose dispersion liquid is 50-1000 nm; and/or the presence of a gas in the gas,

the solvent of the nano-cellulose dispersion liquid comprises one or more of water, 2-methyl-2-propanol, diethyl ether and glycerol.

3. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S10:

in the nano-cellulose dispersion liquid, the nano-fibers comprise one or more of cotton nano-fibers, wood pulp nano-fibers, sisal nano-fibers, bacterial nano-fibers and carboxylated nano-fibers.

4. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S10:

the silicon source comprises SiO powder and SiO₂One or more of powder, water glass, tetraethyl orthosilicate, methyltrimethoxysilane, dimethyldimethoxysilane, tetramethylsilane, and hexamethyldisiloxane.

5. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S10:

the mass ratio of the nano-cellulose dispersion liquid to the silicon source is 0.01-15.5: 1.

6. The method for preparing high temperature resistant silicon carbide aerogel having a directional pore structure as claimed in claim 1, wherein step S20 comprises:

providing a directional freezing device and a mold, wherein the directional freezing device comprises a base and a heat-conducting cover plate, the base is provided with a cavity, liquid nitrogen is filled in the cavity, and the heat-conducting cover plate is covered on the cavity so as to conduct the temperature of the liquid nitrogen to the heat-conducting cover plate;

and guiding the wet gel into the mold, and then placing the mold on the surface of the heat-conducting cover plate to perform directional freezing from the bottom of the mold upwards to obtain the frozen and molded block gel.

7. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S20:

the directional freezing treatment time is 0.2-10 h.

8. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S30:

the pressure of the vacuum drying treatment is less than 1000 Pa; and/or the presence of a gas in the gas,

when vacuum drying treatment is carried out, the drying treatment time is 0.1-48 h.

9. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S40:

and (2) performing high-temperature sintering through a high-temperature sintering furnace, and introducing inert gas during the high-temperature sintering, wherein the inert gas is one or more of helium, neon, argon, krypton and xenon.

10. The method for preparing high temperature resistant silicon carbide aerogel with oriented pore structure as claimed in claim 1, wherein in step S40:

when high-temperature sintering is carried out, the high-temperature sintering time is 0.2-8 h, the heating rate is 2-8 ℃/min, and the heat preservation temperature is 1000-1700 ℃.

Technical Field

The invention relates to the technical field of aerogel preparation, in particular to a preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure.

Background

The silicon carbide aerogel integrates the excellent high temperature resistance and oxidation resistance of the silicon carbide substance and the light and porous characteristics of the aerogel, so that the silicon carbide aerogel has the advantages of small density, high porosity, excellent high-temperature stability and the like. At present, there are two main methods for preparing silicon carbide aerogel, the first method is to polymerize carbosilane and vinyl chemical under the action of catalyst and curing agent, and then to obtain silicon carbide aerogel by high temperature sintering. The silicon carbide aerogel obtained by the method has high density, low porosity and high cost because an expensive catalyst is required. The second method is to directly mix a silicon source with a carbon source or form a precursor and sinter the precursor at high temperature to obtain the silicon carbide aerogel. The density of the silicon carbide aerogel material prepared by the method is obviously reduced, the porosity is obviously improved, but the room-temperature heat conductivity coefficient of the silicon carbide aerogel prepared by the method is more than 0.03W/m.K, and the heat insulation performance still needs to be optimized. Therefore, a method for preparing silicon carbide aerogel is needed to solve the above problems.

Disclosure of Invention

The invention mainly aims to provide a preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure, and aims to solve the problems of high preparation cost, complex working procedures, high aerogel thermal conductivity coefficient and the like of the preparation method of the silicon carbide aerogel in the prior art.

In order to achieve the purpose, the invention provides a preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure, which comprises the following steps:

s10, mixing and stirring the nano-cellulose dispersion liquid and a silicon source to obtain wet gel;

s20, introducing the wet gel into a mold, and performing directional freezing treatment to obtain a frozen block gel;

s30, carrying out vacuum drying treatment on the massive gel to obtain a nano cellulose/silicon source composite material;

s40, sintering the nano-cellulose/silicon source composite material at high temperature to obtain the high-temperature-resistant silicon carbide aerogel material with the directional pore structure.

Alternatively, in step S10:

the mass fraction of the nano-cellulose dispersion liquid is 0.1-7%; and/or the presence of a gas in the gas,

the length of the nanofiber in the nano-cellulose dispersion liquid is 50-1000 nm; and/or the presence of a gas in the gas,

the solvent of the nano-cellulose dispersion liquid comprises one or more of water, 2-methyl-2-propanol, diethyl ether and glycerol.

Alternatively, in step S10:

in the nano-cellulose dispersion liquid, the nano-fibers comprise one or more of cotton nano-fibers, wood pulp nano-fibers, sisal nano-fibers, bacterial nano-fibers and carboxylated nano-fibers.

Alternatively, in step S10:

the silicon source comprises SiO powder and SiO₂One or more of powder, water glass, tetraethyl orthosilicate, methyltrimethoxysilane, dimethyldimethoxysilane, tetramethylsilane, and hexamethyldisiloxane.

Alternatively, in step S10:

the mass ratio of the nano-cellulose dispersion liquid to the silicon source is 0.01-15.5: 1.

Optionally, step S20 includes:

providing a directional freezing device and a mold, wherein the directional freezing device comprises a base and a heat-conducting cover plate, the base is provided with a cavity, liquid nitrogen is filled in the cavity, and the heat-conducting cover plate is covered on the cavity so as to conduct the temperature of the liquid nitrogen to the heat-conducting cover plate;

and guiding the wet gel into the mold, and then placing the mold on the surface of the heat-conducting cover plate to perform directional freezing from the bottom of the mold upwards to obtain the frozen and molded block gel.

Alternatively, in step S20:

the directional freezing treatment time is 0.2-10 h.

Alternatively, in step S30:

the pressure of the vacuum drying treatment is less than 1000 Pa; and/or the presence of a gas in the gas,

when vacuum drying treatment is carried out, the drying treatment time is 0.1-48 h.

Alternatively, in step S40:

and (2) performing high-temperature sintering through a high-temperature sintering furnace, and introducing inert gas during the high-temperature sintering, wherein the inert gas is one or more of helium, neon, argon, krypton and xenon.

Alternatively, in step S40:

when high-temperature sintering is carried out, the high-temperature sintering time is 0.2-8 h, the heating rate is 2-8 ℃/min, and the heat preservation temperature is 1000-1700 ℃.

According to the technical scheme provided by the invention, the nano-cellulose dispersion liquid and a silicon source are mixed and stirred to obtain wet gel, and the wet gel is sequentially subjected to directional freezing, vacuum drying and high-temperature sintering treatment, so that the reaction condition is mild, the preparation period is short, and the yield is stable. The directional freezing can control the growth direction of the ice crystals and the size of the ice crystals in the formed wet gel, so that the growth of the ice crystals in the gel can be controlled by regulating and controlling the temperature, the heat-insulating material, the gel components and the like of the directional freezing process. After vacuum drying, the directionally grown ice crystals can be removed by sublimation on the premise of not damaging the internal microstructure of the gel, thereby obtaining the nano-cellulose/silicon source aerogel material with the directional pore structure. Under the action of high temperature, the nanocellulose is carbonized during the temperature rise period of room temperature to 500 ℃ and is converted into a carbon source. When the temperature is further increased, the carbon source and the silicon source generate a carbothermic reduction reaction to generate gas phase SiO and grow along the directional hole, and finally the silicon carbide aerogel with the directional hole structure is obtained, so that the preparation cost is low, the process is simple, the controllable degree is high, the accuracy is high, the operation is simple and convenient, the large-scale production can be realized, the prepared silicon carbide aerogel material with the directional hole structure can be directly applied to the field of heat preservation and heat insulation, the room-temperature heat conductivity coefficient of the silicon carbide aerogel is less than 0.02W/m.K, the heat conductivity coefficient at the high temperature of 800 ℃ is less than 0.07W/m.K, the density is less than 0.15g/cm3, the porosity is more than 90%, and the thermal stability is more than 1200 ℃.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other related drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a schematic flow chart illustrating one embodiment of a method for preparing a refractory silicon carbide aerogel having a directional pore structure according to the present invention;

FIG. 2 is the structure of the product after the directional freezing process;

FIG. 3 is the structure of the product after treatment by vacuum drying;

FIG. 4 is a structure of a product after a high-temperature sintering process;

FIG. 5 is the structure of the product of FIG. 1 after the directional freezing process;

FIG. 6 is an SEM image of silicon carbide aerogel prepared according to embodiments of the present invention at different magnifications;

FIG. 7 is an SEM image of a silicon carbide aerogel prepared by a comparative example;

FIG. 8 is an infrared photograph of a silicon carbide aerogel produced according to an example of the present invention, taken at different times under a butane torch;

FIG. 9 is a comparison of the thermal conductivity of silicon carbide aerogel produced by examples of the present invention at various temperatures and air;

FIG. 10 is a thermogravimetric analysis plot of a silicon carbide aerogel prepared according to an example of the present invention under air conditions.

The implementation, functional features and advantages of the objects of the present invention will be further explained with reference to the accompanying drawings.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

In addition, the meaning of "and/or" appearing throughout includes three juxtapositions, exemplified by "A and/or B" including either A or B or both A and B. In addition, technical solutions between various embodiments may be combined with each other, but must be realized by a person skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination should not be considered to exist, and is not within the protection scope of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The silicon carbide aerogel integrates the excellent high temperature resistance and oxidation resistance of the silicon carbide substance and the light and porous characteristics of the aerogel, so that the silicon carbide aerogel has the advantages of small density, high porosity, excellent high-temperature stability and the like. At present, there are two main methods for preparing silicon carbide aerogel, the first method is to polymerize carbosilane and vinyl chemical under the action of catalyst and curing agent, and then to obtain silicon carbide aerogel by high temperature sintering. The silicon carbide aerogel obtained by the method has high density, low porosity and high cost because an expensive catalyst is required. The second method is to directly mix a silicon source with a carbon source or form a precursor and sinter the precursor at high temperature to obtain the silicon carbide aerogel. The density of the silicon carbide aerogel material prepared by the method is obviously reduced, the porosity is obviously improved, but the room-temperature heat conductivity coefficient of the silicon carbide aerogel prepared by the method is more than 0.03W/m.K, and the heat insulation performance still needs to be optimized. Therefore, a method for preparing silicon carbide aerogel is needed to solve the above problems.

In view of this, the invention provides a preparation method of high-temperature-resistant silicon carbide aerogel with a directional pore structure. Fig. 1 is a schematic flow chart of an embodiment of a method for preparing a high-temperature-resistant silicon carbide aerogel with a directional pore structure according to the present invention.

In one embodiment, as shown in fig. 1, the method for preparing the high temperature resistant silicon carbide aerogel with a directional pore structure includes the following steps:

s10, mixing and stirring the nano-cellulose dispersion liquid and a silicon source to obtain wet gel;

specifically, in step S10, the mass fraction of the nanocellulose dispersion is 0.1% to 7%, the length of the nanofibers in the nanocellulose dispersion is 50 to 1000nm, the solvent of the nanocellulose dispersion includes one or more of water, 2-methyl-2-propanol, ethyl ether and glycerol, the nanofibers in the nanocellulose dispersion include one or more of cotton nanofibers, wood pulp nanofibers, sisal nanofibers, bacterial nanofibers and carboxylated nanofibers, the silicon source includes one or more of SiO powder, SiO2 powder, water glass, tetraethyl orthosilicate, methyltrimethoxysilane, dimethyldimethoxysilane, tetramethylsilane and hexamethyldisiloxane, and the mass ratio of the nanocellulose dispersion to the silicon source is 0.01 to 15.5: 1.

The invention has the advantages that the density of the obtained aerogel can be greatly reduced and the porosity can be improved by taking the nano-fibers as the carbon source, the three-dimensional fiber space structure formed by overlapping the nano-fibers is mainly benefited, the density can be finely regulated according to the solid content of the cellulose, and the method is simple, convenient and low in cost.

It is understood that the conditions in the preparation of the wet gel may be satisfied simultaneously or only one or more of them, and as a preferred embodiment of the present invention, the conditions are satisfied simultaneously to make the wet gel perform better.

S20, introducing the wet gel into a mold, and performing directional freezing treatment to obtain a frozen block gel;

specifically, step S20 includes providing a directional freezing device and a mold, where the directional freezing device includes a base and a heat-conducting cover plate, the base has a cavity, liquid nitrogen is filled in the cavity, the heat-conducting cover plate is covered on the cavity to conduct the temperature of the liquid nitrogen to the heat-conducting cover plate, the wet gel is guided into the mold, and then the mold is placed on the surface of the heat-conducting cover plate to perform directional freezing from the bottom of the mold upwards to obtain a frozen block gel, and the directional freezing time is 0.2-10 h.

It should be noted that, referring to fig. 2, the growth direction of ice crystals and the size of ice crystals can be controlled in the formed wet gel by directional freezing, and specifically, the growth of ice crystals in the wet gel can be controlled by controlling the temperature, the heat insulating material, the gel component, and the like of the directional freezing process. Because the directional freezing is carried out on the aerogel to construct a directional hole structure, compared with the traditional disordered hole structure, the directional hole structure limits the radial conduction of heat convection, so that the heat conductivity coefficient can be further reduced, and the heat conductivity coefficient is less than 0.02W/m.K, so that the conditions in the preparation process of the frozen block-shaped gel can be simultaneously met, or only one or more of the conditions can be met, and as a preferred embodiment of the invention, the conditions are simultaneously met, so that the growth size, the size and the length of ice crystals in the frozen block-shaped gel can meet the requirements.

S30, carrying out vacuum drying treatment on the massive gel to obtain a nano cellulose/silicon source composite material;

specifically, in step S30, the pressure of the vacuum drying process is less than 1000Pa, and the drying time is 0.1 to 48 hours when the vacuum drying process is performed.

It should be noted that, referring to fig. 3, after vacuum drying, the directionally-grown ice crystals can be sublimated and removed without destroying the internal microstructure of the gel, so as to obtain the nanocellulose/silicon source aerogel material with the directional pore structure. It is understood that the conditions for performing the vacuum drying treatment may be satisfied simultaneously or only one or more of the conditions may be satisfied simultaneously, and as a preferred embodiment of the present invention, the conditions may be satisfied simultaneously, so that the properties of the prepared nanocellulose/silicon source composite material may be better.

S40, sintering the nano-cellulose/silicon source composite material at high temperature to obtain the high-temperature-resistant silicon carbide aerogel material with the directional pore structure.

Specifically, in step S40, performing high-temperature sintering in a high-temperature sintering furnace, introducing an inert gas during the high-temperature sintering, wherein the inert gas is one or more of helium, neon, argon, krypton, and xenon, and during the high-temperature sintering, the high-temperature sintering time is 0.2-8 hours, the heating rate is 2-8 ℃/min, and the heat preservation temperature is 1000-1700 ℃.

Referring to fig. 4, under the action of high temperature, nanocellulose is carbonized at room temperature to 500 ℃ and converted into a carbon source. When the temperature is further increased, the carbon source and the silicon source are subjected to a carbothermic reduction reaction, which is explained here by taking SiO as the silicon source, gas-phase SiO is generated and grows along the directional holes, and finally the silicon carbide aerogel with the directional hole structure is obtained.

The silicon carbide aerogel prepared by the method adopts an in-situ assembly mode, and compared with the traditional sintering method for paving silicon source powder, the silicon source and the carbon source are uniformly mixed in the method, so that the in-situ growth and uniform distribution of silicon carbide fibers are ensured. However, the traditional method for laying silicon source and performing secretion sintering has the phenomena that silicon carbide fibers are not uniformly distributed, a large amount of silicon carbide fibers are accumulated on the surface, and the interior is less.

It is understood that the conditions for performing the high-temperature sintering may be satisfied simultaneously or only one or more of the conditions may be satisfied simultaneously, and as a preferred embodiment of the present invention, the conditions may be satisfied simultaneously, so that the performance of the prepared silicon carbide aerogel material is better.

According to the technical scheme provided by the invention, the nano-cellulose dispersion liquid and a silicon source are mixed and stirred to obtain wet gel, and the wet gel is sequentially subjected to directional freezing, vacuum drying and high-temperature sintering treatment, so that the reaction condition is mild, the preparation period is short, and the yield is stable. The directional freezing can control the growth direction of ice crystals and the size of the ice crystals in the formed wet gelTherefore, the growth of internal ice crystals can be controlled by regulating and controlling the temperature, the heat-insulating material, the gel component and the like of the directional freezing process. After vacuum drying, the directionally grown ice crystals can be removed by sublimation on the premise of not damaging the internal microstructure of the gel, thereby obtaining the nano-cellulose/silicon source aerogel material with the directional pore structure. Under the action of high temperature, the nanocellulose is carbonized during the temperature rise period of room temperature to 500 ℃ and is converted into a carbon source. When the temperature is further increased, the carbon source and the silicon source are subjected to a carbothermic reduction reaction to generate gas-phase SiO and grow along the directional hole, and finally the silicon carbide aerogel with the directional hole structure is obtained<0.02W/m.K, heat conductivity coefficient at high temperature of 800 DEG C<0.07W/m.K, density<0.15g/cm³Porosity of>90% thermal stability>1200℃。

FIG. 6 is an SEM image of silicon carbide aerogel prepared by the embodiment of the present invention at different magnifications, from which it can be seen that the aerogel has a directional pore structure inside, the diameter of the directional pore is 1-2 μm, and the aerogel is formed by disorderly assembling a large amount of in-situ grown silicon carbide fibers.

Fig. 8 is an infrared image of the silicon carbide aerogel prepared by the embodiment of the present invention under a butane torch at different times, and it can be seen from the image that the temperature of the hot end of the silicon carbide aerogel in direct contact with the butane torch is 1200 ℃, even after continuous ablation for 140s, the temperature of the back surface of the aerogel is only 210 ℃ at most, and the high temperature heat insulation performance is excellent.

FIG. 9 is a comparison of the thermal conductivity of silicon carbide aerogel prepared by the present invention with that of air at different temperatures, and the thermal conductivity of silicon carbide aerogel at room temperature is only < 0.019W/m.K, which is much lower than the thermal conductivity of air, which is 0.026W/m.K. In addition, the thermal conductivity at 800 ℃ at high temperature is less than 0.07W/m.K, and excellent medium-high temperature heat insulation performance is shown.

Fig. 10 is a thermogravimetric analysis curve of the silicon carbide aerogel prepared by the embodiment of the invention under the air condition, and it can be seen that the thermal stability of the silicon carbide in the air is more than 1200 ℃, and the high temperature resistance is good.

An example of a method of preparing a high temperature resistant silicon carbide aerogel having a directional pore structure according to the present invention is given below:

(1) mixing and stirring a nano-cellulose dispersion liquid and a silicon source to obtain wet gel, wherein the mass fraction of the nano-cellulose dispersion liquid is 0.1-7%; and/or the length of the nanofiber in the nano-cellulose dispersion liquid is 50-1000 nm; and/or the solvent of the nano-cellulose dispersion liquid comprises one or more of water, 2-methyl-2-propanol, diethyl ether and glycerol, the nano-fibers in the nano-cellulose dispersion liquid comprise one or more of cotton nano-fibers, wood pulp nano-fibers, sisal nano-fibers, bacterial nano-fibers and carboxylated nano-fibers, the silicon source comprises one or more of SiO powder, SiO2 powder, water glass, tetraethyl orthosilicate, methyltrimethoxysilane, dimethyldimethoxysilane, tetramethylsilane and hexamethyldisiloxane, and the mass ratio of the nano-cellulose dispersion liquid to the silicon source is 0.01-15.5: 1.

(2) And guiding the wet gel into a mold, performing directional freezing treatment to obtain frozen and molded block gel, and providing a directional freezing device and the mold, wherein the directional freezing device comprises a base and a heat-conducting cover plate, the base is provided with a containing cavity, liquid nitrogen is filled in the containing cavity, the heat-conducting cover plate is covered on the containing cavity to conduct the temperature of the liquid nitrogen to the heat-conducting cover plate, the wet gel is guided into the mold, then the mold is placed on the surface of the heat-conducting cover plate to perform directional freezing from the bottom of the mold upwards to obtain the frozen and molded block gel, and the directional freezing treatment time is 0.2-10 h.

(3) Carrying out vacuum drying treatment on the massive gel to obtain a nano cellulose/silicon source composite material; the pressure of the vacuum drying treatment is less than 1000 Pa; and/or when vacuum drying treatment is carried out, the drying treatment time is 0.1-48 h.

(4) The nano-cellulose/silicon source composite material is sintered at high temperature to obtain a high-temperature-resistant silicon carbide aerogel material with a directional pore structure, the high-temperature sintering is carried out through a high-temperature sintering furnace, inert gas is introduced during the high-temperature sintering, the inert gas is one or more of helium, neon, argon, krypton and xenon, the high-temperature sintering time is 0.2-8 hours during the high-temperature sintering, the heating rate is 2-8 ℃/min, and the heat preservation temperature is 1000-1700 ℃.

The technical solutions of the present invention are further described in detail with reference to the following specific examples, which should be understood as merely illustrative and not limitative.

Example 1

(1) Taking 20g of cotton nanocellulose dispersion with the mass fraction of 1% and the fiber length of 100nm, mixing the cotton nanocellulose dispersion with 2.1g of SiO powder, stirring uniformly, and standing to obtain wet gel;

(2) guiding the wet gel into a mold, placing the mold on a directional freezing device, and performing directional freezing treatment for 0.5h through liquid nitrogen to obtain blocky gel;

(3) taking out the blocky gel after freezing, and transferring the blocky gel into a vacuum drier, wherein the pressure of the vacuum drying treatment is 15Pa, and the drying treatment time is 10h, so as to obtain the nano cellulose/SiO aerogel;

(4) and (3) sintering the obtained nano-cellulose/SiO aerogel at high temperature in argon for 2h, wherein the heating rate is 4 ℃/min, and the heat preservation temperature is 1200 ℃, so as to finally obtain the directional pore structure silicon carbide aerogel material.

It should be noted that the aerogel material obtained by the method for preparing the high temperature resistant silicon carbide aerogel with the directional pore structure in the above embodiments has the directional pore structure, the pore diameter is 1.2 micrometers, the thermal conductivity of the aerogel at room temperature is 0.019W/m.k, the thermal conductivity at high temperature of 800 ℃ is 0.064W/m.k, and the density is 0.13g/cm³Porosity 92%, maximum heat resistance temperature 1247 ℃.

Example 2

(1) Taking 20g of cotton nanocellulose dispersion with the mass fraction of 6% and the fiber length of 1000nm, mixing the cotton nanocellulose dispersion with 5.5g of TEOS solution, stirring uniformly, and standing to obtain wet gel;

(2) guiding the wet gel into a mold, placing the mold on a directional freezing device, and performing directional freezing treatment for 4 hours by using liquid nitrogen to obtain blocky gel;

(3) after freezing, taking out the blocky gel and transferring the blocky gel into a vacuum drier, wherein the pressure of the vacuum drying treatment is 20Pa, and the drying treatment time is 10h, so as to obtain the nano-cellulose/TEOS aerogel;

(4) and (3) sintering the obtained nano-cellulose/TEOS aerogel at high temperature in argon for 3h, wherein the heating rate is 4 ℃/min, the heat preservation temperature is 1400 ℃, and finally the silicon carbide aerogel material with the directional pore structure is obtained.

It should be noted that the aerogel material obtained by the method for preparing the high-temperature resistant silicon carbide aerogel with a directional pore structure in the above embodiments has a directional pore structure, a pore diameter of 1.5 micrometers, a thermal conductivity of the aerogel at room temperature of 0.018W/m.k, a thermal conductivity at high temperature of 800 ℃ of 0.065W/m.k, and a density of 0.13g/cm³Porosity 91%, maximum heat resistance temperature 1235 ℃.

Example 3

(1) Taking 10g of wood pulp nano cellulose dispersion liquid with the mass fraction of 0.1% and the fiber length of 50nm, mixing the wood pulp nano cellulose dispersion liquid with 1000g of tetramethylsilane, stirring uniformly, and standing to obtain wet gel;

(2) guiding the wet gel into a mold, placing the mold on a directional freezing device, and performing directional freezing treatment for 0.1h through liquid nitrogen to obtain blocky gel;

(3) taking out the blocky gel after freezing, and transferring the blocky gel into a vacuum drier, wherein the pressure of the vacuum drying treatment is 8Pa, and the drying treatment time is 48h, so as to obtain the nano-cellulose/tetramethylsilane aerogel;

(4) and sintering the obtained nano-cellulose/tetramethylsilane aerogel in helium at a high temperature for 8h, wherein the heating rate is 2 ℃/min, the heat preservation temperature is 1700 ℃, and finally the silicon carbide aerogel material with the directional pore structure is obtained.

It should be noted that the aerogel material obtained by the method for preparing the high temperature resistant silicon carbide aerogel with a directional pore structure in the above embodiments has a directional pore structure, a pore diameter of 1.4 micrometers, and a thermal conductivity of the aerogel at room temperatureA number of 0.016W/mK, a thermal conductivity at 800 deg.C of 0.068W/mK, and a density of 0.11g/cm³Porosity 92%, maximum heat resistance temperature 1250 ℃.

Example 4

(1) Taking 15.5g of sisal hemp nano cellulose dispersion liquid with the mass fraction of 7% and the fiber length of 525nm, mixing the dispersion liquid with 1g of SiO2 powder, stirring uniformly, and standing to obtain wet gel;

(2) guiding the wet gel into a mold, placing the mold on a directional freezing device, and performing directional freezing treatment for 10 hours by using liquid nitrogen to obtain blocky gel;

(3) taking out the blocky gel after freezing, and transferring the blocky gel into a vacuum drier, wherein the pressure of the vacuum drying treatment is 10Pa, and the drying treatment time is 0.1h, so as to obtain the nano-cellulose/SiO 2 aerogel;

(4) the obtained nano-cellulose/SiO₂And (3) sintering the aerogel in helium at high temperature for 0.2h, wherein the heating rate is 8 ℃/min, the heat preservation temperature is 1000 ℃, and finally the silicon carbide aerogel material with the directional pore structure is obtained.

It should be noted that the aerogel material obtained by the method for preparing the high temperature resistant silicon carbide aerogel with a directional pore structure in the above embodiments has a directional pore structure, a pore diameter of 1.4 μm, a thermal conductivity of 0.016W/m.k at room temperature, a thermal conductivity of 0.068W/m.k at 800 ℃ and a density of 0.14g/cm³Porosity 93%, maximum heat resistance temperature 1255 ℃.

Comparative example 1

The same procedure as in example 1 was followed, except that the step (2) was omitted, to finally prepare a silica aerogel having no oriented pore structure, and referring to fig. 7 and 9, the silicon carbide aerogel having no oriented pore structure had a thermal conductivity of > 0.025W/m.k at room temperature, a thermal conductivity of > 0.085W/m.k at a high temperature of 800 ℃, and a thermal stability of <1200 ℃.

Comparative example 2

With the exception that the vacuum drying process in the step (3) is replaced by the non-vacuum drying process, the remaining steps are the same as those of the example 1, referring to fig. 7 and 9, since the ice crystals generated by the directional freezing process are liquefied and dried during the non-vacuum drying process, the generated directional pore structure collapses, and finally the silicon carbide aerogel without the directional pore structure is prepared, wherein the thermal conductivity of the silicon carbide aerogel without the directional pore structure is greater than 0.025W/m.k at room temperature, greater than 0.085W/m.k at high temperature of 800 ℃, and the thermal stability is less than 1200 ℃.

From the comparison of examples 1 to 4 with comparative examples 1-2 (i.e., FIG. 9), it can be seen that the thermal conductivity of the silicon carbide aerogel having oriented pores is < 0.02W/m.K at room temperature and is thermally stable at >1200 deg.C, whereas the thermal conductivity of the silicon carbide aerogel having no oriented pore structure is > 0.025W/m.K at room temperature, 0.085W/m.K at high temperature of 800 deg.C and is thermally stable at <1200 deg.C, and thus the thermal conductivity of the silicon carbide aerogel having oriented pores is small and is thermally stable.

In conclusion, the silicon carbide aerogel material with the directional hole structure prepared by the preparation method of the high-temperature-resistant silicon carbide aerogel with the directional hole structure can be directly applied to the field of heat preservation and heat insulation, and the room-temperature heat conductivity coefficient of the silicon carbide aerogel<0.02W/m.K, heat conductivity coefficient at high temperature of 800 DEG C<0.07W/m.K, density<0.15g/cm³Porosity of>90% thermal stability>1200℃。

The above is only a preferred embodiment of the present invention, and it is not intended to limit the scope of the invention, and various modifications and changes will occur to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included in the scope of the present invention.