Si3N4@SiO2 Core-Shell Structure Foamed Ceramic Material and Preparation Method

Abstract

The present invention relates to the technical field of foam ceramic materials, and more specifically, to Si3N4@SiO2 core-shell structured foam ceramic materials and preparation methods. The microscopic morphology of the Si3N4@SiO2 core-shell structured foam ceramic materials is a honeycomb structure, and the pore walls are composed of Si3N4 and SiO2. The inner side of the pore wall is a Si3N4 layer, and the outer side is a SiO2 layer. The SiO2 between adjacent pore walls is sintered into one body. The Si3N4 foam ceramic materials prepared by the method provided by the present invention can simultaneously have the following performance indicators: porosity of 88% - 90%, compressive strength of 7.5 MPa, room temperature thermal conductivity of 0.0808 W·m ‑1 ·K ‑1 , dielectric constant less than 1.32, dielectric loss less than 0.009, and the service temperature in an oxidizing atmosphere can reach 1100°C, which can meet the urgent needs of aerospace and other fields for high-temperature resistant / heat insulation / wave-transparent integrated materials.

Description

Technical Field

[0001] The present invention relates to the technical field of foamed ceramic materials, and more specifically, to a Si3N4@SiO2 core-shell structured foamed ceramic material and a preparation method thereof. Background Art

[0002] A radome is generally located at the very front end of an aircraft and is a key component of the aircraft's microwave antenna system. During the high-speed flight of the aircraft, it will face extreme environments such as high temperature, sand, humidity, and corrosion. The main function of the radome is to ensure that the microwave antenna system inside it can still work efficiently in the harsh environment. In recent years, the performance requirements for flight equipment in various countries have been continuously improved, and thus more comprehensive requirements have also been put forward for the performance of radome materials. On the one hand, the flight Mach number of high-end flight equipment has been continuously increasing, requiring that the high-temperature resistance of the material must be correspondingly improved, which has promoted the development of organic material radomes towards inorganic material radomes. On the other hand, when the aircraft is in a hypersonic flight state, extremely high aerodynamic heat is bound to be generated on the surface. If the surface heat is rapidly conducted to the interior, it will cause the microwave antenna system to malfunction or even be completely damaged. Therefore, while improving the high-temperature resistance of the radome material, it is also necessary to further improve its high-temperature heat insulation ability.

[0003] Porous ceramics have characteristics such as a high specific surface area, a high open porosity, a low thermal conductivity, and a designable pore structure, and are widely used in many fields such as electromagnetic wave transmission, heat insulation, sound insulation, filtration, and catalysis, and have received widespread attention and research from researchers in the field of materials science. High-porosity materials not only have lower density and thermal conductivity, but also have better dielectric properties than the corresponding dense materials, and have significant application potential in the field of high-temperature heat insulation and wave-transparent integrated materials.

[0004] Aerogel is a porous material with the highest porosity. Traditional aerogels are assembled from nanoparticles and interconnected pores, and their microstructure is in the shape of a pearl necklace, with excellent heat insulation performance. However, the high drying cost, cumbersome operation, extremely poor mechanical properties of aerogels, and high-temperature induced crystallization behavior and other disadvantages limit the practical application of aerogels in high-temperature heat insulation. In recent years, a large number of research reports have been made on three-dimensional flexible ultra-lightweight materials assembled from ceramic nanofibers, which are called a new generation of aerogel materials by researchers. Nanofiber aerogels have unique properties such as excellent high-temperature stability, compression resilience, low thermal conductivity, and strong structure and function designability, making up for the application limitations of traditional aerogels. However, when in a compressed working condition, flexible nanofiber aerogels will almost lose all the advantages relying on high porosity. Therefore, it is still necessary to develop rigid ultra-high porosity ceramic materials with excellent mechanical properties and high-temperature stability to solve the application limitations of traditional aerogels and new nanofiber aerogels.

[0005] Porous silicon nitride ceramics (Si3N4) have characteristics such as thermal shock resistance, oxidation resistance, high strength, low dielectric constant, and low loss coefficient, and are considered to be one of the most promising wave-transparent materials. In the past few decades, dozens of methods for preparing porous Si3N4 ceramics have been developed, such as gel casting method, pore-forming agent addition method, freeze-drying method, and incomplete sintering method, etc. However, the porous Si3N4 ceramics prepared by these methods have large pore sizes, low porosity, and difficult-to-control pore structures. In recent years, some researchers have prepared Si3N4 aerogels by carbothermal reduction nitridation of C / SiO2 binary aerogels. This process cannot avoid the complex process required for preparing C / SiO2 precursor aerogels, and a large amount of SiC components are contained in this aerogel material, which affects the wave-transparent performance; other researchers have prepared Si3N4 foam ceramics composed of in-situ grown Si3N4 nanofibers intertwined by wet forming process, but this research work focuses on highlighting the innovation of the preparation process and does not characterize the key properties of Si3N4 foam ceramics, and cannot meet the requirements of actual application indicators. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, an object of one aspect of the present invention is to provide a Si3N4@SiO2 core-shell structure foam ceramic material. The microstructure of the Si3N4@SiO2 core-shell structure foam ceramic material is a honeycomb structure, which has excellent geometric mechanical properties, thereby ensuring that the Si3N4@SiO2 foam ceramic material has excellent compressive resistance. The pore walls are composed of Si3N4 and SiO2. The inner side of the pore wall is a Si3N4 layer, and the outer side is a SiO2 layer. The SiO2 between adjacent pore walls is sintered into one body.

[0007] Another object of the present invention is to provide a preparation method of a Si3N4@SiO2 core-shell structure foam ceramic material. The specific steps of the preparation method are as follows:

[0008] S1. Take a carbon source and synthesize spherical carbon by hydrothermal method;

[0009] S2. Take a silicon source and use a low-temperature chemical vapor deposition process to deposit SiO2 on the surface of the spherical carbon prepared in S1 to form a SiO2 shell layer, and obtain a C@SiO2 core-shell structure powder;

[0010] S3. Use a pressure forming process to prepare the C@SiO2 core-shell structure powder prepared in S2 into a C@SiO2 green body;

[0011] S4. Sinter the C@SiO2 green body prepared in S3 in a nitrogen atmosphere to form a C@Si3N4@SiO2 intermediate;

[0012] S5. Heat-treat the C@Si3N4@SiO2 intermediate prepared in S4 in an oxygen-containing atmosphere to remove the carbon template therein, and obtain a Si3N4@SiO2 core-shell structured foamed ceramic material.

[0013] Preferably, the carbon source in S1 is one of glucose, fructose, chitosan, sucrose, cellulose, or starch.

[0014] Preferably, the temperature used in the hydrothermal method in S1 is 150°C to 200°C, the time is 4 hours to 8 hours, and the spherical carbon sphere diameter is 50 nm to 500 nm.

[0015] Preferably, the silicon source in S2 is one of methyl orthosilicate, ethyl orthosilicate, propyl orthosilicate, butyl orthosilicate, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, or ethyltriethoxysilane.

[0016] Preferably, the temperature required for the low-temperature chemical vapor deposition process in S2 is room temperature to 300°C, and the required time is 6 hours to 48 hours.

[0017] Preferably, the SiO2 shell thickness in S2 is 20 nm to 60 nm.

[0018] Preferably, the molding pressure in S3 is 10 MPa to 100 MPa.

[0019] Preferably, the sintering temperature in S4 is 1400°C to 1600°C; the sintering time is 0.5 hours to 3 hours.

[0020] Preferably, the heat-treatment temperature in S5 is 600°C to 900°C, and the heat-treatment time is 0.5 hours to 5 hours.

[0021] The beneficial effects of the present invention are as follows:

[0022] The preparation method provided by the present invention belongs to a dry forming process, avoiding the solvent waste and pollution, complex rheology problems, and long drying process involved in the wet forming process, and having the characteristics of simple operation, low energy consumption, and convenient batch production.

[0023] The preparation method provided by the present invention can design and regulate parameters such as the porosity, pore diameter, and pore wall thickness of the Si3N4 foamed ceramic material according to different application scenarios, so as to meet the requirements of actual application indicators.

[0024] The Si3N4 foamed ceramic material prepared by the method provided by the present invention can simultaneously have the following performance indicators: porosity 88% - 90%, compressive strength 7.5 MPa, room temperature thermal conductivity 0.0808 W·m -1 ·K -1, with a dielectric constant less than 1.32 and a dielectric loss less than 0.009, can be used at a temperature up to 1100°C in an oxidizing atmosphere. It is currently the Si3N4 foam ceramic material with the best comprehensive performance. The Si3N4 foam ceramic material prepared by the method provided by the present invention has high porosity, low thermal conductivity, high compressive strength, low dielectric constant and loss value, and excellent oxidation resistance, which can meet the urgent needs of high-temperature resistant / heat-insulating / wave-transparent integrated materials for parts such as radomes and antenna windows of equipment such as missiles, rockets, space shuttles, and extraterrestrial exploration vehicles.

[0025] The additional aspects and advantages of the present invention will become obvious in the following description or be understood through the practice of the present invention. Brief Description of the Drawings

[0026] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, wherein:

[0027] Figure 1 is a schematic diagram of the low-temperature chemical vapor deposition device and process of the embodiment of the present invention;

[0028] Figure 2 is the green body of C@SiO2 prepared by the pressure forming process in the embodiment of the present invention;

[0029] Figure 3 is the C@Si3N4@SiO2 intermediate obtained after sintering the green body of C@SiO2 in the N2 atmosphere in the embodiment of the present invention;

[0030] Figure 4 is the Si3N4@SiO2 foam ceramic material obtained by removing the carbon template after heat treatment of the C@Si3N4@SiO2 intermediate in an oxygen-containing atmosphere in the embodiment of the present invention;

[0031] Figure 5 is the XRD pattern of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention;

[0032] Figure 6 is the SEM image of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention;

[0033] Figure 7 is the TEM image of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention;

[0034] Figure 8 is the stress-strain curve of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention;

[0035] Figure 9It is the dielectric constant and loss value of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention tested in the range of 8 GHz to 18 GHz;

[0036] Figure 10 It is the isothermal oxidation curve graph of the Si3N4@SiO2 foam ceramic material in the embodiment of the present invention. Detailed implementation manners

[0037] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0038] Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0039] Embodiment 1

[0040] Step 1: Using glucose as a carbon source, adopting the hydrothermal method to keep warm in an environment of 170 °C for 3 hours to prepare spherical carbon with a sphere diameter of 200 nm;

[0041] Step 2: Using the low-temperature chemical vapor deposition device as shown in Figure 1 , using tetraethyl orthosilicate as a silicon source, performing vapor deposition for 12 hours in an environment of 180 °C to form a SiO2 shell layer with a thickness of 20 nm on the surface of the spherical carbon, and obtaining C@SiO2 core-shell structure powder;

[0042] Step 3: Pressing the C@SiO2 powder into a green body under a pressure condition of 40 MPa, as shown in Figure 2 ;

[0043] Step 4: Sintering the C@SiO2 green body in an N2 atmosphere at 1450 °C for 2 hours to form a C@Si3N4@SiO2 intermediate, as shown in Figure 3 ;

[0044] Step 5: Heat-treating the C@Si3N4@SiO2 intermediate in an oxygen-containing atmosphere at 600 °C for 5 hours to remove the carbon template therein, and obtaining the Si3N4@SiO2 core-shell structure foam ceramic material, as shown in Figure 4 ;

[0045] Embodiment 2

[0046] Step 1: Using glucose as a carbon source, adopting the hydrothermal method to keep warm in an environment of 180 °C for 5 hours to prepare spherical carbon with a sphere diameter of 300 nm;

[0047] Step 2: Using the one as shown inFigure 1 The low-temperature chemical vapor deposition device shown, using tetramethoxysilane as the silicon source, performs vapor deposition for 24 hours in an environment of 150 °C to form a 40-nm SiO2 shell layer on the spherical carbon surface, obtaining C@SiO2 core-shell structure powder;

[0048] Step 3: Press the C@SiO2 powder into a green body under a pressure condition of 60 MPa, as Figure 2 shown;

[0049] Step 4: Sinter the C@SiO2 green body in an N2 atmosphere at 1500 °C for 2 hours to form a C@Si3N4@SiO2 intermediate, as Figure 3 shown;

[0050] Step 5: Heat-treat the C@Si3N4@SiO2 intermediate in an oxygen-containing atmosphere at 700 °C for 4 hours to remove the carbon template therein, obtaining the Si3N4@SiO2 core-shell structure foam ceramic material, as Figure 4 shown.

[0051] Example 3

[0052] Step 1: Using glucose as the carbon source, adopt the hydrothermal method to keep warm in an environment of 150 °C for 3 hours to prepare spherical carbon with a ball diameter of 100 nm;

[0053] Step 2: Using the low-temperature chemical vapor deposition device as Figure 1 shown, using methyltrimethoxysilane as the silicon source, perform vapor deposition for 36 hours in an environment of 120 °C to form a 50-nm SiO2 shell layer on the spherical carbon surface, obtaining C@SiO2 core-shell structure powder;

[0054] Step 3: Press the C@SiO2 powder into a green body under a pressure condition of 70 MPa, as Figure 2 shown;

[0055] Step 4: Sinter the C@SiO2 green body in an N2 atmosphere at 1550 °C for 2 hours to form a C@Si3N4@SiO2 intermediate, as Figure 3 shown;

[0056] Step 5: Heat-treat the C@Si3N4@SiO2 intermediate in an oxygen-containing atmosphere at 800 °C for 2 hours to remove the carbon template therein, obtaining the Si3N4@SiO2 core-shell structure foam ceramic material, as Figure 4 shown.

[0057] Example 4

[0058] Step 1: Using glucose as the carbon source, adopt the hydrothermal method to keep warm in an environment of 150 °C for 3 hours to prepare spherical carbon with a ball diameter of 100 nm;

[0059] Step 2: Using the Figure 1 low-temperature chemical vapor deposition device shown in Figure 1 , using ethyltrimethoxysilane as the silicon source, performing vapor deposition at 150 °C for 48 hours, forming a 60-nm SiO2 shell layer on the surface of spherical carbon, and obtaining C@SiO2 core-shell structure powder;

[0060] Step 3: Pressing the C@SiO2 powder into a green body under a pressure condition of 80 MPa, as shown in Figure 2 Figure 2 ;

[0061] Step 4: Sintering the C@SiO2 green body in an N2 atmosphere at 1550 °C for 3 hours to form a C@Si3N4@SiO2 intermediate, as shown in Figure 3 Figure 3 ;

[0062] Step 5: Heat-treating the C@Si3N4@SiO2 intermediate in an oxygen-containing atmosphere at 900 °C for 1.5 hours to remove the carbon template therein, thereby obtaining the Si3N4@SiO2 core-shell structure foam ceramic material, as shown in Figure 4 Figure 4 .

[0063] Detection Test

[0064] The thermal conductivity of Examples 1 to 4 of the present invention was detected, and the detection results showed that the lowest thermal conductivity of the Si3N4@SiO2 core-shell structure foam ceramic material could reach 0.0808 W·m -1 ·K -1 .

[0065] The XRD analysis of Examples 1 to 4 of the present invention was carried out, and the results are as follows Figure 5 Figure 5 .

[0066] The SEM and TEM photographs of Examples 1 to 4 of the present invention were taken, and the results are as follows Figure 6 and Figure 7 Figure 7 .

[0067] The anti-compression performance and dielectric performance of Examples 1 to 4 of the present invention were detected, and the detection results are as follows Figure 8 and Figure 9 Figure 9 .

[0068] The isothermal oxidation test of Examples 1 to 4 of the present invention was carried out, and the detection results are as follows Figure 10 Figure 10 .

[0069] Experimental Conclusion

[0070] The Si3N4 foam ceramic material prepared by the method provided by the present invention can simultaneously have the following performance indexes: porosity 88% - 90%, anti-compression strength 7.5 MPa, room temperature thermal conductivity 0.0808 W·m -1 ·K -1, with a dielectric constant less than 1.32 and a dielectric loss less than 0.009, can be used at a temperature up to 1100 °C in an oxidizing atmosphere, and is currently the Si3N4 foam ceramic material with the best comprehensive performance.

[0071] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and changes can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material, characterized in that: The microscopic morphology of the Si3N4@SiO2 core-shell structured foam ceramic material is a honeycomb structure. The pore walls are composed of Si3N4 and SiO2. The inner side of the pore wall is a Si3N4 layer, and the outer side is a SiO2 layer. The SiO2 between adjacent pore walls is sintered into one body. The specific steps of the preparation method are as follows: S1. Take a carbon source and synthesize spherical carbon by a hydrothermal method. S2. Take a silicon source and use a low-temperature chemical vapor deposition process to deposit SiO2 on the surface of the spherical carbon prepared in S1 to form a SiO2 shell layer, obtaining a C@SiO2 core-shell structured powder. S3. Use a pressure molding process to prepare the C@SiO2 core-shell structured powder prepared in S2 into a C@SiO2 green body. S4. Sinter the C@SiO2 green body prepared in S3 in a nitrogen atmosphere to form a C@Si3N4@SiO2 intermediate. S5. Heat-treat the C@Si3N4@SiO2 intermediate prepared in S4 in an oxygen-containing atmosphere to remove the carbon template therein, obtaining the Si3N4@SiO2 core-shell structured foam ceramic material.

2. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The carbon source in S1 is one of glucose, fructose, chitosan, sucrose, cellulose or starch.

3. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The temperature used in the hydrothermal method in S1 is 150 o °C to 200 o °C, the time is 4 hours to 8 hours, and the diameter of the spherical carbon spheres is 50 nm to 500 nm.

4. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The silicon source in S2 is one of methyl orthosilicate, ethyl orthosilicate, propyl orthosilicate, butyl orthosilicate, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane or ethyltriethoxysilane.

5. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The temperature required for the low-temperature chemical vapor deposition process in S2 is from room temperature to 300 °C, and the required time is from 6 hours to 48 hours.

6. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The thickness of the SiO2 shell layer in S2 is 20 nm to 60 nm.

7. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The molding pressure in S3 is 10 MPa to 100 MPa.

8. The preparation method of a Si3N4@SiO2 core-shell structured foam ceramic material according to claim 1, characterized in that: The sintering temperature in S4 is 1400 °C to 1600 °C; the sintering time is 0.5 hour to 3 hours.

9. The preparation method of a Si3N4@SiO2 core-shell structured foamed ceramic material according to claim 1, characterized in that: The heat-treatment temperature in S5 is 600 °C to 900 °C, and the heat-treatment time is 0.5 hour to 5 hours.

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