Si3N4 nanofiber aerogel material and preparation method thereof

Abstract

The invention relates to the technical field of aerogel materials, in particular to a Si3N4 nanofiber aerogel material and a preparation method thereof. The aerogel material is formed by stacking and assembling multiple layers of Si3N4 nanofiber films, the upper surface of each Si3N4 nanofiber film is of a high-density network structure formed by interweaving Si3N4 nanofibers, and the lower surface of each Si3N4 nanofiber film is of a pearl necklace-shaped structure formed by interweaving Si3N4 nanofibers and Si3N4 nanoparticles. The preparation method disclosed by the invention provides a new thought for developing the structure-enhanced nanofiber aerogel, and the Si3N4 nanofiber aerogel material prepared by the method disclosed by the invention is expected to meet the development requirements of a wave-transparent antenna housing of an ultra-high-speed aircraft on a high-temperature-resistant / heat-insulating / wave-transparent integrated material.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogel materials, and more specifically, to a Si3N4 nanofiber aerogel material and a preparation method thereof. Background Art

[0002] Traditional aerogel materials are usually formed by nanoparticles through chain-like assembly to form a pearl-chain structure, and further cross-linked to form a three-dimensional high-porosity network. Although such materials have excellent thermal insulation performance, their mechanical strength is extremely low, severely limiting their practical applications. Ceramic aerogels represented by silicon nitride (Si3N4) and silicon carbide (SiC) usually need to be prepared from precursor aerogels (such as C / SiO2 binary aerogels) through high-temperature physical and chemical transformations (such as carbothermal reduction, nitridation, etc.). However, the high-temperature process is prone to cause sintering of nanoparticles, resulting in coarsening of the skeleton, a decrease in porosity, and a significant increase in the solid-phase thermal conductivity, thereby reducing the thermal insulation performance.

[0003] To improve the mechanical properties, researchers have proposed ceramic nanofiber aerogels, that is, using flexible ceramic nanofibers to replace nanoparticles as the basic building units. While maintaining a high porosity (>90%), such materials exhibit excellent compressive resilience (>80% strain recovery). Among them, Si3N4 nanofiber aerogels possess the characteristics of high strength, corrosion resistance, oxidation resistance, thermal shock resistance, and moderate dielectric constant of Si3N4, and at the same time have good thermal insulation performance, and are expected to be applied to wave-transparent thermal insulation components (such as radomes / windows) of high Mach number aircraft. However, the pore sizes of existing Si3N4 nanofiber aerogels are usually in the micrometer range (several to dozens of micrometers), much larger than the nanopores of traditional particulate aerogels, resulting in their thermal insulation performance being highly dependent on ultra-low density. In an air flow disturbance environment, the convective heat transfer of gas is enhanced, and the thermal insulation performance is significantly reduced; in addition, the ultra-low density also makes its mechanical properties insufficient to meet the actual needs.

[0004] In recent years, some studies have attempted to in-situ grow SiOx nanowires on the surface of Si3N4 nanofibers through low-temperature chemical vapor deposition (CVD) to divide micrometer-sized pores, reduce the pore size, and improve the thermal insulation performance. However, SiOx nanowires are prone to ablation at high temperatures (>1000°C), resulting in degradation of the pore structure, a sharp deterioration of the high-temperature thermal insulation performance, and the mechanical properties still need to be improved. Therefore, developing Si3N4-based aerogel materials with nanoscale pores, excellent high-temperature stability, and high mechanical strength still faces major challenges. Summary of the Invention

[0005] 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 nanofiber aerogel material, which is assembled by stacking multiple layers of Si3N4 nanofiber films. The upper surface of the Si3N4 nanofiber film is a high-density network structure intertwined with Si3N4 nanofibers, and the lower surface is a pearl necklace-like structure intertwined with Si3N4 nanofibers and Si3N4 nanoparticles.

[0006] Preferably, the aerogel material spontaneously forms a pore gradient structure from the high-density network structure on the upper surface to the pearl necklace-like structure on the lower surface.

[0007] Another object of the present invention is to provide a preparation method of a Si3N4 nanofiber aerogel material. The specific operation steps of the preparation method are as follows: S1. Prepare an organosilane aerogel precursor; S2. Adopt a high-temperature carbothermal reduction nitridation process to in-situ transform the organosilane aerogel precursor prepared in S1 into a Si3N4 nanoparticle aerogel, and at the same time form a layer of Si3N4 nanofiber film on the upper surface of the Si3N4 nanoparticle aerogel. Peel the Si3N4 nanofiber film from the upper surface of the Si3N4 nanoparticle aerogel, and repeat this step to obtain several Si3N4 nanofiber films; S3. Stack and composite the Si3N4 nanofiber films peeled in S2 to obtain a three-dimensional Si3N4 nanofiber film composite; S4. Heat-treat the three-dimensional Si3N4 nanofiber film composite in an oxygen-containing atmosphere to achieve interlayer oxidation bonding and obtain a Si3N4 nanofiber aerogel material.

[0008] Preferably, the silane for preparing the organosilane aerogel precursor in S1 is one or a combination of methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, vinyltriethoxysilane, and / or phenyltrimethoxysilane.

[0009] Preferably, the temperature of the high-temperature carbothermal reduction nitridation process in S2 is 1000 o °C ~ 1500 o °C.

[0010] Preferably, the heating rate of the high-temperature carbothermal reduction nitridation process in S2 is 1 o °C / min ~ 10 o °C / min, and the holding time after reaching the highest temperature is 1 hour ~ 5 hours.

[0011] Preferably, in the step S2, the high-temperature carbothermal reduction nitridation process requires nitrogen protection, and the nitrogen flow rate is 50 ml / min to 300 ml / min.

[0012] Preferably, during the superposition and composite process of the Si3N4 nanofiber film in the step S3, the peeling side of the Si3N4 nanofiber film must always face the same direction.

[0013] Preferably, the heat treatment temperature in the step S4 is 500 o °C to 1100 o °C.

[0014] Preferably, the heat treatment time in the step S4 is 0.5 hours to 5 hours.

[0015] The beneficial effects of the present invention are as follows: The method for preparing the Si3N4 nanofiber aerogel material of the present invention is based on the principle of chemical vapor deposition, which in-situ transforms the organosilane aerogel into a Si3N4 nanoparticle aerogel while forming Si3N4 nanofibers on its surface. That is, two practical materials can be obtained in one step, which can avoid the waste of reaction gas sources and raw materials existing in the preparation of Si3N4 nanofibers based on the principle of chemical vapor deposition.

[0016] In the preparation method of the present invention, the Si3N4 nanofiber film is in-situ generated on the upper surface of the Si3N4 nanoparticle aerogel based on the principle of chemical vapor deposition. When the Si3N4 nanofiber film is peeled off, the Si3N4 aerogel nanoparticles intertwined with the Si3N4 nanofibers are peeled off together, forming a new structure of Si3N4 nanoparticles in-situ reinforcing Si3N4 nanofibers, which helps to improve the mechanical properties.

[0017] The microscopic morphology of the upper surface of each layer of Si3N4 nanofiber aerogel prepared by the method of the present invention is a high-density network structure interwoven only by Si3N4 nanofibers, and the microscopic morphology of the lower surface is a pearl necklace structure interwoven by Si3N4 nanofibers and Si3N4 nanoparticles, and has a gradient pore structure from the upper surface to the lower surface; the beneficial effect of this structure is that the high-density nanofiber network on the upper surface further restricts the movement of gas molecules through the Knudsen effect, and its dense structure can also reflect infrared radiation. The composite structure of the lower surface (nanofibers and nanoparticles twisted) synergistically inhibits heat transfer through two mechanisms: (i) the micron-scale pores are secondary divided by the nanoparticle aerogel to form nanoscale pores and submicron-scale pores, which significantly extend the mean free path of gas molecules and inhibit air convection heat transfer; (ii) the Si3N4 fiber / Si3N4 particle heterogeneous interface and surface defects can enhance phonon scattering and reduce solid thermal conductivity. More importantly, the pore gradient distribution from top to bottom forms a continuously changing refractive index interface, which effectively weakens the Fresnel reflection loss of thermal radiation and realizes wide-spectrum radiation heat shielding. This multi-scale structural design gradedly regulates the three heat transfer paths of gas, solid and radiation, ultimately enabling the material to exhibit excellent thermal insulation properties over a wide temperature range.

[0018] The preparation method of the present invention provides a new idea for the development of structurally enhanced nanofiber aerogels. The Si3N4 nanofiber aerogel material prepared by the method of the present invention is expected to meet the development needs of high-temperature resistant / heat-insulating / wave-transparent integrated materials for ultra-high-speed aircraft wave-transparent antenna covers.

[0019] Additional aspects and advantages of the invention will become apparent from the following description, or may be learned by practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1 This is a schematic diagram of the preparation process of Si3N4 nanofiber aerogel material according to an embodiment of the present invention; Figure 2 This is the XRD spectrum of the Si3N4 nanofiber aerogel material of the embodiment of the present invention; Figure 3 is a SEM image of the upper surface of the Si3N4 nanofiber aerogel material of an embodiment of the present invention; Figure 4 is a SEM image of the lower surface of the Si3N4 nanofiber aerogel material of an embodiment of the present invention; Figure 5 is a stress-strain curve diagram of the Si3N4 nanofiber aerogel material under different deformation conditions in an embodiment of the present invention; Figure 6It is the cyclic compression stress-strain curve of the Si3N4 nanofiber aerogel material in the embodiment of the present invention under the condition of 40% strain; Figure 7 They are the dielectric constant and loss value of the Si3N4 nanofiber aerogel material in the embodiment of the present invention in the frequency range of 8 GHz to 12 GHz; Figure 8 They are the dielectric constant and loss value of the Si3N4 nanofiber aerogel material in the embodiment of the present invention in the frequency range of 12 GHz to 18 GHz; Figure 9 It is the test result of the high-temperature heat insulation performance of the Si3N4 nanofiber aerogel material in the embodiment of the present invention. Detailed implementation manners

[0021] In order to more clearly understand the above objects, features and advantages of the present invention, the present invention will be further described in detail below with reference to the drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments may be combined with each other.

[0022] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention may 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.

[0023] The preparation process route diagram of the Si3N4 nanofiber aerogel material in the embodiment of the present invention is as Figure 1 shown.

[0024] Embodiment 1 S1: Using methyltrimethoxysilane as the silicon source, a methyltrimethoxysilane aerogel precursor is prepared by the sol-gel process; S2: Place the methyltrimethoxysilane aerogel at the bottom of a graphite crucible, and carry out a carbothermal reduction nitridation reaction in an atmosphere furnace. Heat it to 1450 o °C at a rate of 3 o °C / min, hold for 3 hours, set the N2 flow rate to 100 ml / min, the methyltrimethoxysilane aerogel is transformed into a Si3N4 nanoparticle aerogel, and at the same time, a Si3N4 nanofiber film is formed on its surface. Peel off the Si3N4 nanofiber film, and repeat step two to obtain 10 Si3N4 nanofiber films; S3: Always keep the peeled side of the Si3N4 nanofiber film facing the same direction, stack and compound the Si3N4 nanofiber films in sequence to obtain a three-dimensional Si3N4 nanofiber film composite; S4: Heat the three-dimensional Si3N4 nanofiber film composite to 900 oOxidize for 3 hours in C to cause interlayer oxidation bonding and obtain the Si3N4 nanofiber aerogel material.

[0025] Example 2 S1: Use vinyltriethoxysilane as the silicon source and prepare the vinyltriethoxysilane aerogel precursor by the sol-gel process. S2: Place the vinyltriethoxysilane aerogel at the bottom of a graphite crucible and carry out carbothermal reduction nitridation reaction in an atmosphere furnace. Heat to 1500 o C at a rate of 5 o C / min, hold for 2 hours, set the N2 flow rate to 150 ml / min. The vinyltriethoxysilane aerogel is transformed into the Si3N4 nanoparticle aerogel, and at the same time, a layer of Si3N4 nanofiber film is formed on its surface. Peel off the Si3N4 nanofiber film and repeat step two to obtain 10 Si3N4 nanofiber films. S3: Always keep the peeled side of the Si3N4 nanofiber film facing the same direction, stack and compound the Si3N4 nanofiber films in sequence to obtain a three-dimensional Si3N4 nanofiber film composite. S4: Oxidize the three-dimensional Si3N4 nanofiber film composite at 1000 o C for 2 hours to cause interlayer oxidation bonding and obtain the Si3N4 nanofiber aerogel material.

[0026] Example 3 S1: Use methyltrimethoxysilane and vinyltriethoxysilane as the silicon sources and prepare the methyltrimethoxysilane / vinyltriethoxysilane composite aerogel precursor by the sol-gel process. S2: Place the methyltrimethoxysilane / vinyltriethoxysilane composite aerogel at the bottom of a graphite crucible and carry out carbothermal reduction nitridation reaction in an atmosphere furnace. Heat to 1450 o C at a rate of 5 o C / min, hold for 4 hours, set the N2 flow rate to 100 ml / min. The methyltrimethoxysilane / vinyltriethoxysilane composite aerogel is transformed into the Si3N4 nanoparticle aerogel, and at the same time, a layer of Si3N4 nanofiber film is formed on its surface. Peel off the Si3N4 nanofiber film and repeat step two to obtain 10 Si3N4 nanofiber films. S3: Always keep the peeled side of the Si3N4 nanofiber film facing the same direction, stack and compound the Si3N4 nanofiber films in sequence to obtain a three-dimensional Si3N4 nanofiber film composite. S4: Oxidize the three-dimensional Si3N4 nanofiber film composite at 1000 oOxidize for 1 hour in C to cause oxidation bonding between layers and obtain the Si3N4 nanofiber aerogel material.

[0027] Example 4 S1: Using phenyltrimethoxysilane as the silicon source, prepare a phenyltrimethoxysilane aerogel precursor by the sol-gel process; S2: Place the phenyltrimethoxysilane aerogel at the bottom of a graphite crucible and carry out a carbothermal reduction nitridation reaction in an atmosphere furnace. Heat it from 8 o C / min to 1400 o C, hold for 5 hours, set the N2 flow rate to 150 ml / min. The phenyltrimethoxysilane aerogel is transformed into a Si3N4 nanoparticle aerogel, and at the same time, a Si3N4 nanofiber film is formed on its surface. Peel off the Si3N4 nanofiber film and repeat step two to obtain 10 Si3N4 nanofiber films; S3: Always keep the peeled side of the Si3N4 nanofiber film facing the same direction, stack and composite the Si3N4 nanofiber films in sequence to obtain a three-dimensional Si3N4 nanofiber film composite; S4: Oxidize the three-dimensional Si3N4 nanofiber film composite in 900 o C for 4 hours to cause oxidation bonding between layers and obtain the Si3N4 nanofiber aerogel material.

[0028] Detection test Conduct a thermal conductivity test on Examples 1 to 4 of the present invention. The test results show that the thermal conductivity of the Si3N4 nanofiber aerogel material is 0.027 W·m -1 ·K -1 .

[0029] Conduct XRD analysis on Examples 1 to 4 of the present invention. The analysis results are as Figure 2 shown.

[0030] Take SEM photos of Examples 1 to 4 of the present invention. The image results are as Figure 3 and Figure 4 shown.

[0031] Conduct a compression and rebound performance test on Examples 1 to 4 of the present invention. The test results are as Figure 5 and Figure 6 shown.

[0032] Conduct a dielectric performance test on Examples 1 to 4 of the present invention. The test results are as Figure 7 and Figure 8 shown.

[0033] Conduct a high-temperature heat insulation performance test on Examples 1 to 4 of the present invention. The test results are as Figure 9 shown.

[0034] Experimental conclusion The Si3N4 nanofiber aerogel material prepared by the method provided by the present invention can simultaneously have the following performance indicators: density 0.033 g·cm -3 , thermal conductivity 0.027 W·m -1 ·K -1 , excellent anti-deformation ability, maximum elastic modulus 25.1 kPa. During the 100-cycle compression test, the maximum compressive strength only decreases by 2%. In the frequency range of 8 GHz to 18 GHz, the dielectric constant ε′ = 2.31–2.39, tan δ < 0.08. The results of the high-temperature infrared thermal imaging test show that the temperature of the back-heated surface is about 900 o °C lower than that of the heated surface, and the heat insulation performance is excellent.

[0035] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, 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 Si3N4 nanofiber aerogel material, characterized in that: The aerogel material is assembled by stacking multiple layers of Si3N4 nanofiber films. The upper surface of the Si3N4 nanofiber film is a high-density network structure intertwined with Si3N4 nanofibers, and the lower surface is a pearl necklace-like structure intertwined with Si3N4 nanofibers and Si3N4 nanoparticles.

2. The Si3N4 nanofiber aerogel material according to claim 1, wherein: The aerogel material spontaneously forms a pore gradient structure from the high-density network structure on the upper surface to the pearl necklace-like structure on the lower surface.

3. A preparation method of a Si3N4 nanofiber aerogel material, characterized in that: The specific operating steps of the preparation method are as follows: S1. Prepare an organosilane aerogel precursor. S2. Using a high-temperature carbothermal reduction nitridation process, in-situ transform the organosilane aerogel precursor prepared in S1 into a Si3N4 nanoparticle aerogel, and at the same time form a layer of Si3N4 nanofiber film on the upper surface of the Si3N4 nanoparticle aerogel. Peel the Si3N4 nanofiber film from the upper surface of the Si3N4 nanoparticle aerogel, and repeat this step to obtain several Si3N4 nanofiber films. S3. Stack and composite the Si3N4 nanofiber films peeled in S2 to obtain a three-dimensional Si3N4 nanofiber film composite. S4. Heat-treat the three-dimensional Si3N4 nanofiber film composite in an oxygen-containing atmosphere to achieve interlayer oxidation bonding and obtain a Si3N4 nanofiber aerogel material.

4. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: The silane for preparing the organosilane aerogel precursor in S1 is one or a combination of methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, vinyltriethoxysilane, and / or phenyltrimethoxysilane.

5. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: The temperature of the high-temperature carbothermal reduction nitridation process in S2 is 1000 o °C to 1500 o °C.

6. The preparation method of a Si3N4 nanofiber aerogel material according to claim 5, characterized in that: The heating rate of the high-temperature carbothermal reduction nitridation process in S2 is 1 o °C / min to 10 o °C / min, and the holding time after reaching the highest temperature is 1 hour to 5 hours.

7. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: The high-temperature carbothermal reduction nitridation process in S2 requires nitrogen protection, and the nitrogen flow rate is 50 ml / min to 300 ml / min.

8. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: During the stacking and composite process of the Si3N4 nanofiber films in S3, the peeled side of the Si3N4 nanofiber film must always face the same direction.

9. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: The heat treatment temperature in S4 is 500 o °C to 1100 o °C.

10. The preparation method of a Si3N4 nanofiber aerogel material according to claim 3, characterized in that: The heat treatment time in S4 is 0.5 hours to 5 hours.

Images