Porous silicon nitride substrate with holes sealed on surface and hole sealing treatment method thereof

Abstract

The invention discloses a surface-sealed porous silicon nitride substrate and a hole sealing treatment method thereof, and belongs to the field of material surface treatment. Aiming at the problems of high porosity and high cost in the existing porous silicon nitride substrate coating treatment process, the invention provides the hole sealing treatment method for the surface-sealed porous silicon nitride substrate, which comprises the following steps of: smearing hole sealing slurry on the surface of the porous silicon nitride substrate; filling holes in the surface of the porous silicon nitride substrate with the hole sealing slurry on the surface of the porous silicon nitride substrate; drying the porous silicon nitride substrate to obtain a porous silicon nitride substrate with holes sealed on the surface; the hole sealing slurry is mixed liquid of silica sol, fumed silica and silicon nitride powder. In the whole process, the porous silicon nitride substrate is endowed with an excellent surface layer hole sealing effect while low dielectric and low density of the porous silicon nitride substrate are ensured, conditions are provided for subsequent coating treatment, the process is simple, the equipment energy consumption is low, the cost is low, the operability of hole sealing treatment is high, and industrial production is facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of material surface treatment, and more particularly relates to a porous silicon nitride substrate with surface sealing and a sealing treatment method thereof. Background Art

[0002] The radome, located at the front end of a missile, is both a structural component and a crucial element of the guidance system. It is a multifunctional component that integrates high-performance requirements such as heat protection, wave transmission, load-bearing, weather resistance, airtightness, and impact resistance. With technological advancements and evolving demands, missiles are evolving towards faster penetration speeds and greater strike accuracy, placing increasingly stringent demands on wave-transmitting materials for radomes. Low-density porous silicon nitride, with its advantages such as high wave transmittance across a wide frequency band, lightweight and high strength, high-temperature and thermal shock resistance, low thermal conductivity, and functional-structural integration, meets the stringent radome requirements of modern hypervelocity weapons and communication systems, making it an ideal choice for missile radomes. However, the high porosity (≥50%) of low-density porous silicon nitride ceramics leads to susceptibility to moisture absorption, poor rain erosion resistance, and airtightness, which in turn affects the stability of the material's dielectric properties and limits its further application in aerospace. The current mainstream method is to form a dense, high-temperature resistant coating on the surface of a low-density silicon nitride substrate. This not only prevents the dielectric properties of the material from degrading due to moisture absorption and improves the material's resistance to rain erosion, but also gives it excellent airtightness, preventing the harsh external environment from interfering with the internal antenna.

[0003] Prior to the filing date of this application, the inventors of this application understood and concluded as follows: Currently, high-temperature resistant coatings formed on the surface of low-density silicon nitride substrates can be divided into two categories based on the material: ceramic coatings (sol-gel method, reaction sintering method, vapor deposition method, etc.) and silicone resin coatings (spray method, solution impregnation method, etc.). Ceramic coatings have higher hardness and superior high-temperature and wear resistance, but are more complex to process and have high production costs. To achieve the airtightness of the radome, additional external surface sealing treatment is required. Silicone resin coatings are easy to operate, have a simple production process, are low-cost, and can achieve excellent moisture resistance and airtightness, but their wear resistance and temperature resistance are relatively poor.

[0004] For example, Chinese patent application number CN201510225006.3, published on December 7, 2016, discloses a porous silicon nitride-based sealing coating, a preparation method, and an application thereof. The silicon nitride-based sealing coating is deposited on a porous silicon nitride substrate using a magnetron sputtering deposition process. The sealing coating has good adhesion to the porous silicon nitride substrate and has advantages such as a dense structure, controllable thickness, high hardness, and high wave transmittance. However, the patent has high requirements on equipment conditions, a long process time, high cost, and is not easy to industrialize. For example, some scholars such as Liu Jian et al. immersed porous silicon nitride materials in a silica dispersion system, and then dried and sintered them to prepare a fused silica coating. The coating bonded well with the porous silicon nitride ceramic substrate and significantly reduced the water absorption rate of the substrate material, thereby improving the strength of the substrate material. However, it also seriously affected the density and dielectric properties of the substrate material. Wang Shubin et al. used a sol-gel method and used a low-dielectric composite oxide system as a binder and sintering aid for silicon nitride to prepare a silicon nitride sealing and reinforcing coating below the phase change temperature. The sealing and moisture-proof coating significantly reduced the water absorption rate of the substrate and improved the strength of the substrate with little effect on the density, dielectric constant and dielectric loss of the substrate. However, the coating is prone to cracking and failure during the subsequent high-temperature densification process, and it also has the disadvantages of complex process, high cost, and is not conducive to industrial production. Summary of the Invention

[0005] 1. Problems to be solved To address the problems of high porosity and high cost in existing porous silicon nitride substrate coating processes, the present application provides a surface-sealed porous silicon nitride substrate and a sealing method thereof. The entire sealing process achieves excellent surface sealing effect on the porous silicon nitride substrate while ensuring low dielectric and low density, providing conditions for subsequent coating treatment. The process is simple, the equipment consumes little energy, the cost is low, and the sealing process is highly operable, making it conducive to industrial production.

[0006] 2. Technical solution To solve the above problems, the present invention adopts the following technical solutions.

[0007] A method for sealing a porous silicon nitride substrate with surface sealing comprises applying a sealing slurry to the surface of the porous silicon nitride substrate; then filling the sealing slurry on the surface of the porous silicon nitride substrate into the pores on the surface of the porous silicon nitride substrate; and finally drying the porous silicon nitride substrate to obtain a surface-sealed porous silicon nitride substrate. The sealing slurry is a mixed liquid of silica sol, fumed silicon dioxide and silicon nitride powder.

[0008] Using the above technical solution, the core innovations of this application are as follows: 1. Setting of sealing slurry: a mixture of silica sol, fumed silica and silicon nitride powder is creatively used; on the one hand, the fumed silica component in the sealing slurry has a low dielectric constant and is less affected by frequency and temperature, and filling it into the surface pores of the silicon nitride matrix will not have much impact on its broadband wave transmission performance; on the other hand, under heating and stirring conditions close to the isoelectric point of the silica sol particles (pH 2~3), the silica sol particles and the silanol groups on the surface of the fumed silica particles will undergo strong hydrogen bonding, and some of the silica sol particles and the fumed silica particles will undergo dehydration condensation reaction to form a semi-crosslinked network structure, which is present in the sealing slurry. The viscosity of the system increases and its fluidity decreases, allowing it to remain in the shallow surface layer of the pores of the silicon nitride matrix, ensuring that the density of the silicon nitride matrix does not increase significantly. Finally, the addition of silicon nitride powder can improve the wettability and adhesion between the sealing slurry and the silicon nitride matrix. In other words, the sealing slurry prepared from silica sol, fumed silica and silicon nitride powder can achieve the purpose of sealing by controlling the rheological properties of the sealing slurry before gelation, while not significantly affecting the density and dielectric properties of the porous silicon nitride matrix. It can then form a three-dimensional network gel after high-temperature curing to achieve the purpose of sealing. Second, sealing operation: The sealing slurry is embedded into the pores of the silicon nitride substrate by applying it first and then filling it. Before the sealing slurry gel network is completely solidified and locked, its remaining plasticity is utilized to forcibly embed it deep into the pores on the substrate surface. Subsequent gelation and curing shrinkage occur within the confined pore space, forming a strong three-dimensional mechanical interlocking structure. This structure can effectively resist shrinkage stress and ensure the integrated bonding of the sealing layer and the silicon nitride substrate, thereby achieving stable, long-lasting and effective surface sealing. In summary, the sealing operation of the porous silicon nitride substrate in this application is simple, and the preparation process of the sealing slurry is simple, the equipment has low energy consumption, low cost, and the sealing treatment is easy to operate, which is conducive to industrial production; the entire sealing process achieves the goal of ensuring the low dielectric and low density of the porous silicon nitride substrate while giving the substrate an excellent surface sealing effect.

[0009] Furthermore, the following steps are included: S1: Preparation: Silica sol, fumed silica, and silicon nitride powder are mixed and stirred to obtain a mixed solution; the mixed solution is then heated and stirred, and cooled to obtain a sealing slurry; S2: Sealing: Applying the sealing slurry to the surface of the porous silicon nitride substrate, and then wiping the sealing slurry on the surface of the porous silicon nitride substrate to fill the pores on the surface of the porous silicon nitride substrate; S3: Drying: The porous silicon nitride substrate after step S2 is placed in an oven for drying to obtain a porous silicon nitride substrate with sealed pores on the surface.

[0010] Using the above technical solution, a specific sealing slurry preparation process and a sealing treatment process are given. It is worth noting that the sealing process adopts a wiping method to embed the sealing slurry into the pores of the silicon nitride substrate. In addition to forcing the deep filling of the pores and forming an anchoring structure by applying pressure and utilizing thixotropy, wiping can also promptly wipe off the excess slurry layer remaining on the macroscopic surface of the silicon nitride substrate, preventing the formation of an independent thick film that is prone to cracking and peeling, ensuring the smoothness of the silicon nitride substrate surface and greatly improving the bonding reliability between the sealing layer and the silicon nitride substrate. The entire technical solution prepares a high-viscosity, highly thixotropic, and stable sealing slurry through step S1; achieves deep sealing, surface smoothing, and firm bonding through smearing and wiping in step S2; and gently removes moisture through step S3, allowing the sealing slurry in the pores to completely solidify and compact, forming a final firm sealing structure. The entire process is simple, does not require complex equipment and tedious procedures, and is conducive to industrial production.

[0011] Furthermore, the solid content of the silica sol in the sealing slurry is greater than 40%, and the pH of the silica sol is 2-3; the nanoparticle size of the fumed silica in the sealing slurry is 10 nm-30 nm.

[0012] In the above technical solution, since silica sol is used as the main slurry of the sealing slurry, in order to provide the sealing slurry with basic viscosity and prevent excessive sealing slurry from entering the pores of the silicon nitride substrate during the sealing process due to too low viscosity, thereby affecting the density of the substrate and failing to achieve the sealing effect, the solid content of the silica sol is limited to above 40%. At the same time, in order to consider the surface charge and hydroxyl activity of the silica sol particles, an acidic silica sol with a pH of 2 to 3 is preferred. Since fumed silica is hydrophilic, in order to obtain a higher specific surface area and reaction activity, the nanoparticle size of fumed silica is limited to 10 nm to 30 nm, preferably 10 nm to 15 nm.

[0013] Furthermore, the weight ratio of fumed silica to silica sol in the sealing slurry is (0.03-0.06):1; the weight ratio of silicon nitride powder to silica sol in the sealing slurry is (0.005-0.015):1.

[0014] The above technical solution is used to limit the weight ratio of the raw materials in the sealing slurry. When an excessive amount of fumed silica is added, it aggregates with the silica sol particles in the silica sol due to strong hydrogen bonding to form a three-dimensional network structure, which can easily lead to gelation failure of the solution, or cause the viscosity of the sealing slurry system to be too high, and the bonding force between the slurry and the substrate is poor, resulting in peeling. In addition, excessive addition of fumed silica will cause the particles to aggregate to form large agglomerates, affecting the uniformity of the sealing slurry. When the amount of fumed silica added is too little, the viscosity of the sealing slurry system is low, and it is easy to penetrate too much into the pores of the porous silicon nitride substrate, affecting the density of the substrate. When silicon nitride powder is added in excess, its strong water absorption makes the silicon nitride particles adsorb a large amount of free water in the system, hindering the process of condensation of silica sol particles and fumed silica particles to form a complete and continuous network structure. In addition, the addition of a large amount of hard fillers makes the interface a weak point of stress concentration. During the drying process, due to the difference in shrinkage rate, a large internal stress is generated at the interface, causing the coating to crack or directly peel off from the substrate. When too little silicon nitride powder is added, the surface energy difference between the sealing slurry and the substrate is large, resulting in poor spreading of the slurry on the substrate surface.

[0015] Furthermore, the step S1 specifically includes the following steps: S11: placing silica sol in a container, and adding fumed silica and silicon nitride powder into the container under mechanical stirring to obtain a first mixed solution; S12: placing the first mixed solution into an ultrasonic instrument for dispersion to obtain a second mixed solution; S13: placing the second mixed liquid in a water bath and heating and stirring it, then taking out the second mixed liquid and stirring it until it cools down for later use, thereby obtaining a sealing slurry.

[0016] According to the above technical solution, step S1 is to prepare a high-viscosity, strong thixotropic and stable sealing slurry; step S11 is to perform mechanical stirring for initial mixing, and mechanical stirring provides a strong shear force, first ensuring that solid powders such as fumed silica and silicon nitride powders are initially wetted by the silica sol liquid to avoid dry powder agglomeration and achieve initial uniform mixing; step S12 is to perform ultrasonic dispersion to avoid nano-scale agglomeration, so that the fumed silica and silicon nitride powders are evenly distributed in the silica sol continuous phase as single particles or smaller agglomerates as much as possible, thereby improving the uniformity of the system; step S13 is to heat and stir and then cool, and heating provides energy to accelerate the dehydration condensation reaction between the silanol groups on the surface of the nano-SiO2 particles in the silica sol, induce the pre-crosslinking reaction of the silica sol, greatly increase the viscosity, and optimize the thixotropy, while promoting the full fusion of the components, exhausting the gas, and achieving slurry maturation and stabilization; In summary, through the above steps, a sealing slurry that is highly uniform, stable, not easy to settle or separate, and has suitable high viscosity and good thixotropy is finally obtained, which lays a good foundation for subsequent operations.

[0017] Furthermore, the stirring speed of the mechanical stirring in step S11 is 1000 r / min~2000 r / min, and the stirring time is 10 min~30 min; the ultrasonic dispersion time in step S12 is 30 min~60 min; the heating temperature in the water bath in step S13 is 45°C~65°C, and the heating time is 15 min~35 min.

[0018] In the above technical solution, mechanical stirring is used to ensure uniform mixing of the three raw materials. Therefore, the stirring speed and stirring time are strictly controlled to avoid agglomeration of the slurry. Ultrasonic dispersion is used to break up nano-scale agglomerations. Therefore, the ultrasonic dispersion time is reasonably controlled to achieve fine dispersion of the sealing slurry at the nanoscale while maximizing efficiency and reducing equipment energy consumption. Water bath heating promotes the formation of a stronger hydrogen bond network between the silica sol particles and the fumed silica particles, and causes some of the silica sol particles to undergo dehydration condensation reactions with the fumed silica particles to form chemical bonds. The system presents a semi-cross-linked network structure, which increases the viscosity of the sealing slurry and reduces its fluidity, allowing a small amount of sealing slurry to remain in the shallow surface layer of the pores of the silicon nitride matrix, thereby reducing the effect of the sealing treatment on the density of the low-density porous silicon nitride matrix. Therefore, in order to control the viscosity and cross-linking degree of the sealing slurry system, the temperature and time of the water bath heating are limited.

[0019] Furthermore, in step S2, a brush is used to apply the sealing slurry to the surface of the porous silicon nitride substrate; a non-absorbent and non-linting cloth is used to fill the sealing slurry on the surface of the porous silicon nitride substrate into the pores on the surface of the porous silicon nitride substrate; and the application and wiping are repeated at least twice.

[0020] By adopting the above technical solution, the brush and rag tools are relatively simple, easy for operators to use, widely available and low-cost; at the same time, the smearing and wiping cycles are carried out to ensure that the sealing slurry can be firmly filled into the surface pores of the silicon nitride substrate.

[0021] Furthermore, the area of ​​a single application with a brush is less than 0.15m 2 , and the time interval between application and wiping is less than 15s.

[0022] Using the above technical solution, when the sealing slurry is brushed on the surface of the silicon nitride substrate, as the water in the system evaporates, the surface slurry begins to move from a semi-cross-linked structure to a gelled three-dimensional network structure. If the slurry is not immediately filled into the surface pores of the silicon nitride substrate by wiping, the sealing slurry gradually loses water, undergoes a curing reaction, and then detaches from the substrate surface as a whole. Therefore, the time interval between brushing and wiping should be as small as possible, while at the same time limiting the area of ​​single brushing.

[0023] Furthermore, the drying process in step S3 specifically includes: a drying temperature of 100° C. to 150° C., and a drying time of 1.5 h to 3 h.

[0024] By adopting the above technical solution, under high temperature conditions, the water in the system evaporates faster, the distance between the silica sol particles and the fumed silica particles decreases, and a dehydration condensation reaction gradually occurs to form a three-dimensional network gel. The silicon nitride powder particles in the system fill the structural gaps. In order to save process energy consumption and cycle, the drying temperature and drying time are optimized.

[0025] A porous silicon nitride substrate with sealed pores is produced using the sealing method for a porous silicon nitride substrate described in any of the above technical solutions. This method achieves surface sealing of the porous silicon nitride substrate without affecting its density and dielectric properties, laying a good foundation for subsequent spraying of a high-temperature resistant silicone resin coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram showing the comparison between the surface of the porous silicon nitride substrate without and after pore sealing treatment; Figure 1 (a) Schematic diagram of the porous silicon nitride substrate surface without sealing treatment; Figure 1 (b) Schematic diagram of the porous silicon nitride substrate surface after sealing treatment. DETAILED DESCRIPTION

[0027] The present invention is further described below with reference to specific embodiments and accompanying drawings.

[0028] It is also explained that the porous silicon nitride substrate in this application is a low-density porous silicon nitride material used in the field of wave-transmitting antenna covers. In order to achieve broadband high wave transmittance, the porosity is above 50%, preferably 55% to 70%; the density is less than 1.60 g / cm 3 , preferably 1.2~1.50g / cm 3 .

[0029] The reason for sealing the porous silicon nitride substrate surface in this application is that the inventors discovered that, in the mass production of low-density porous silicon nitride radomes, spraying a high-temperature-resistant silicone resin coating on the surface can impart excellent moisture resistance, rain erosion resistance, and airtightness to the radomes, meeting their operational and storage requirements. This process also offers advantages such as a simple process, low equipment requirements, short processing time, and low cost, leading to its widespread use. However, due to the high porosity (≥50%) of low-density porous silicon nitride materials, excessive resin organic matter can easily penetrate into the substrate pores when spraying the high-temperature-resistant silicone resin coating, significantly impacting the density and dielectric properties of the silicon nitride substrate. Therefore, the present application proposes sealing the porous silicon nitride substrate surface before spraying the coating, preventing performance degradation. This facilitates the subsequent spraying of the high-temperature-resistant silicone resin coating, which is of great significance for the application of porous silicon nitride materials in the radome field. This basis led to the development of the present application.

[0030] A method for sealing a surface-sealed porous silicon nitride substrate comprises applying a sealing slurry to the surface of the porous silicon nitride substrate; then filling the sealing slurry on the surface of the porous silicon nitride substrate into the pores on the surface of the porous silicon nitride substrate; and finally drying the porous silicon nitride substrate to obtain a surface-sealed porous silicon nitride substrate; wherein the sealing slurry is a mixed liquid of silica sol, fumed silica and silicon nitride powder.

[0031] In this embodiment, the silica sol is one of acidic, neutral, and alkaline; the fumed silica is hydrophilic; and the silicon nitride powder is preferably the raw material powder used to prepare the porous silicon nitride matrix. These three components play the following roles: Silica sol: Serving as the main liquid phase carrier and solid phase source of the sealing slurry, it forms the main skeleton of the sealing material. Silica sol is mainly composed of silicon dioxide, which has a low dielectric constant and is less affected by frequency and temperature changes. When filled into the surface pores of the silicon nitride substrate, it does not significantly degrade the substrate's inherent broadband wave transmission performance. Fumed silica: Its extremely high specific surface area and nanometer-scale particle size enable it to effectively fill the gaps between silica sol particles and smaller pores, increasing the density of the sealing layer. Simultaneously, strong hydrogen bonds occur between the silica sol particles and the silanol groups on the surface of the fumed silica particles, and some of the silica sol particles undergo dehydration condensation with the fumed silica particles to form a semi-crosslinked network structure. Fumed silica provides key thickening and thixotropic properties, ensuring that the sealing slurry remains and densely fills shallow pores. Silicon nitride powder: Silicon nitride powder has the same chemical properties as the porous silicon nitride matrix. Adding it to the sealing slurry significantly improves the chemical compatibility and physical similarity between the slurry as a whole and the surface of the porous silicon nitride matrix, thereby greatly improving the wettability of the sealing slurry on the pore surface of the porous silicon nitride matrix and providing strong adhesion; The mixed liquid of these three can form an effective sealing layer on the shallow surface of the pores on the surface of the silicon nitride substrate, which is dense, firmly adhered, has excellent dielectric properties, and does not affect the overall density and wave transmission properties of the substrate.

[0032] After preparing a sealing slurry with excellent performance, it is also crucial to use the sealing slurry on the surface of the porous silicon nitride substrate to ensure a good sealing effect. Therefore, the inventors of this application also differ from conventional operations in the sealing process. Specifically, the sealing slurry is first applied to the surface of the porous silicon nitride substrate, and after the application is completed, the sealing slurry on the surface of the porous silicon nitride substrate is immediately and quickly filled into the pores on the surface of the porous silicon nitride substrate. The purpose of filling is that the inventors found that if the sealing slurry is not filled into the surface pores of the porous silicon nitride substrate in time, as the water evaporates, the silica sol and the fumed silica are triggered to transform from semi-crosslinked to a three-dimensional gel network, and the surface slurry will quickly form an independent, continuous gel film; this film is connected to the substrate only by weak surface adhesion, and cannot resist the huge shrinkage stress generated during its own gelation and drying process, so it gradually loses water and undergoes a curing reaction and detaches from the substrate surface as a whole, and eventually the whole will warp, peel off, and fall off, resulting in sealing failure. Therefore, the filling operation embeds the sealing slurry into the pores of the silicon nitride matrix, so that the subsequent gelation and curing shrinkage occur within the confined pore space, ensuring the integrated bonding of the sealing layer and the silicon nitride matrix, thereby achieving stable, durable and effective surface sealing.

[0033] Therefore, the sealing slurry provided in this embodiment gives the substrate an excellent surface sealing effect while ensuring the low dielectric and low density of the porous silicon nitride substrate. At the same time, the sealing slurry preparation process is simple, the equipment energy consumption is low, and the cost is low; and the sealing treatment is highly operational, which is conducive to industrial production.

[0034] like Figure 1 As shown, Figure 1 The picture on the middle left is a microscope photo of the surface of the low-density porous silicon nitride substrate before sealing treatment; Figure 1 The picture on the right is a microscope photo of the surface of the low-density porous silicon nitride substrate after the sealing treatment method of the porous silicon nitride substrate described in this embodiment; Figure 1 It can be seen intuitively that the surface of the material is rough and porous before treatment, and a smooth and dense layer is formed on the surface after treatment.

[0035] In a specific embodiment, the method comprises the following steps: S1: Preparation: Silica sol, fumed silica, and silicon nitride powder are mixed and stirred to obtain a mixed solution; the mixed solution is then heated and stirred, and cooled to obtain a sealing slurry; S2: Sealing: Apply the sealing slurry to the surface of the porous silicon nitride substrate. After the application is completed, the sealing slurry on the surface of the porous silicon nitride substrate is filled into the pores on the surface of the porous silicon nitride substrate by wiping. Note that the porous silicon nitride substrate must be cleaned with deionized water, dried and polished in advance. The surface must be clean and free of oil, dust and other impurities before the sealing operation can be carried out. S3: Drying: The porous silicon nitride substrate after step S2 is placed in an oven for drying to obtain a porous silicon nitride substrate with sealed pores on the surface.

[0036] In this embodiment, the specific operations of each step are given. Step S1 is used to prepare a high-viscosity, strong thixotropic, and stable sealing slurry; step S2 is used for spreading and wiping to achieve deep sealing, surface smoothing, and firm bonding; step S3 is used to gently remove moisture so that the sealing slurry in the pores is completely solidified and dense, forming a final firm sealing structure; the entire process is simple, does not require complex equipment and tedious procedures, and is conducive to industrial production.

[0037] In one specific embodiment, the solids content of the silica sol in the sealing slurry is greater than 40%, and the pH of the silica sol is between 2 and 3. The nanoparticle size of the fumed silica in the sealing slurry is between 10 nm and 30 nm. This embodiment defines the raw materials in the sealing slurry. Silica sols can be divided into low-concentration and high-concentration silica sols based on their solids content. To provide a base viscosity for the sealing slurry and prevent excessive slurry from entering the pores of the silicon nitride matrix during the sealing process, thereby affecting the matrix density and failing to achieve the sealing effect, a high-concentration silica sol with a solids content of greater than 40% is preferred, and a high-concentration silica sol with a solids content of 48% to 52% is more preferred. In terms of the surface charge and hydroxyl activity of the silica sol particles, an acidic silica sol with a pH of 2 to 3 is preferred. To achieve a higher specific surface area and reactivity, the nanoparticle size of the fumed silica is between 10 nm and 30 nm, and more preferably between 10 nm and 15 nm.

[0038] In a specific embodiment, the weight ratio of fumed silica to silica sol in the sealing slurry is (0.03-0.06):1; the weight ratio of silicon nitride powder to silica sol in the sealing slurry is (0.005-0.015):1.

[0039] This embodiment limits the amounts of the three raw materials in the sealing slurry: the weight ratio of fumed silica to silica sol is (0.03-0.06):1, more preferably (0.04-0.055):1; the weight ratio of silicon nitride powder to silica sol is (0.005-0.015):1, more preferably (0.075-0.012):1; to ensure the excellent performance of the sealing slurry.

[0040] In a specific embodiment, step S1 specifically includes the following steps: S11: placing silica sol in a container, and adding fumed silica and silicon nitride powder into the container under mechanical stirring to obtain a first mixed solution; S12: placing the first mixed solution into an ultrasonic instrument for dispersion to obtain a second mixed solution; S13: placing the second mixed liquid in a water bath and heating and stirring it, then taking out the second mixed liquid and stirring it until it cools down for later use, thereby obtaining a sealing slurry.

[0041] Specifically, the stirring speed of the mechanical stirring in step S11 is 1000 r / min~2000 r / min, and the stirring time is 10 min~30 min; the ultrasonic dispersion time in step S12 is 30 min~60 min; the heating temperature in the water bath in step S13 is 45°C~65°C, and the heating time is 15 min~35 min.

[0042] In a specific embodiment, in step S2, the sealing slurry is applied to the surface of the porous silicon nitride substrate using a brush; the sealing slurry on the surface of the porous silicon nitride substrate is filled into the pores on the surface of the porous silicon nitride substrate using a non-absorbent and non-linting cloth; and the application and wiping are repeated at least twice.

[0043] In this embodiment, the brush is made of nylon, polyester, or polypropylene. Considering the pH value of the sealing slurry, polyester is preferred due to its excellent acid resistance. The wipe is made of ultra-high-precision nylon, polyvinyl chloride-coated cloth, neoprene-coated cloth, or polypropylene non-woven fabric. Considering material cost, polyvinyl chloride-coated cloth or polypropylene non-woven fabric is preferred. The application and wiping cycle is repeated at least twice, i.e., applying, wiping, and then applying again. This cyclical operation ensures that the sealing slurry is firmly filled into the surface pores of the silicon nitride substrate.

[0044] In one embodiment, the area of ​​a single application of the brush is less than 0.15 m 2 , and the time interval between smearing and wiping is less than 15s. For the convenience of operation and the ability to quickly wipe after smearing, in this embodiment, the area of ​​a single smearing and the time of smearing and wiping are limited to ensure that the sealing slurry is smoothly filled into the surface pores of the silicon nitride substrate to ensure the sealing performance. The area of ​​a single brushing and wiping is further preferably less than 0.1m 2 The interval between the brushing and wiping steps is further preferably less than 10s.

[0045] In one embodiment, the drying process in step S3 specifically includes a drying temperature of 100°C to 150°C and a drying time of 1.5 to 3 hours. Under high temperature conditions, water in the system evaporates faster, the distance between the silica sol particles and the fumed silica particles decreases, and a dehydration condensation reaction gradually occurs to form a three-dimensional network gel. The silicon nitride powder particles in the system fill the structural gaps. To save process energy and cycle time, the drying temperature and drying time are specifically optimized in this embodiment. The drying temperature is further optimized to 110°C to 130°C, and the drying time is further optimized to 1.5 to 2 hours.

[0046] A porous silicon nitride substrate with sealed pores is produced using the sealing method for a porous silicon nitride substrate described in any of the above embodiments. This method achieves surface sealing of the porous silicon nitride substrate without affecting its density and dielectric properties, laying a good foundation for subsequent spraying of a high-temperature resistant silicone resin coating.

[0047] In order to further understand the technical solution of the present application, the following examples and comparative examples are given: Meanwhile, the method for measuring the sealing effect used in the Examples and Comparative Examples is as follows: Place the sealed sample on a clean table, take a silicone resin with a solid content of 30% as the coating material, adjust the spray gun pressure, spray width and spray gun distance to spray the sample surface, let the sample dry naturally for a period of time after spraying, and then put it into an oven for curing. Observe the surface state of the sample during the process, and use three stages A to C to evaluate its sealing effect, A is qualified, and B to C are unqualified.

[0048] A: After airing, it can be observed that the substrate surface is covered with a continuous resin glossy coating. After drying and curing, it can be observed that the substrate surface is covered with a continuous resin glossy coating.

[0049] B: After airing, a continuous resin glossy coating can be observed on the substrate surface. After drying and curing, no resin glossy coating is observed on the substrate surface.

[0050] C: After air-drying, no resin glossy coating was observed on the substrate surface.

[0051] The method used in this application for the relative weight gain rate after sealing treatment is as follows: The 40mm×40mm×2.8mm test piece is weighed after drying and recorded as M1. The test piece is then weighed after sealing and recorded as M2. The weight gain rate is calculated as follows:

[0052] The weight gain rate is relative to a test piece with a size of 40 mm×40 mm×2.8 mm. If the sample is made of a thicker silicon nitride substrate, the weight gain rate will be relatively lower.

[0053] Example 1 S1. Take 20g of silica sol (pH=2-3, solid content 50wt%) in a beaker, slowly add 1.0g of fumed silica with a particle size of 12nm and 0.2g of silicon nitride powder under mechanical stirring at 1300r / min, and stir for 20min to obtain a first mixed solution; place the first mixed solution in an ultrasonic instrument and disperse it for 40min to obtain a second mixed solution; place the prepared second mixed solution in a water bath at 60°C and heat and stir for 20min, then take out and stir until cooled for use to obtain a sealing slurry; S2. Use a polyester brush to dip the sealing slurry into the porous silicon nitride substrate and evenly apply it on the surface. Immediately wipe off the excess slurry on the surface with a polypropylene non-woven fabric. Repeat twice. S3. The treated sample is placed in an oven and dried at 120° C. for 1.5 h to obtain a porous silicon nitride substrate with sealed pores on the surface, which is then directly subjected to coating spraying.

[0054] The relative weight gain rate of this embodiment is 1.71%, and the sealing effect is A.

[0055] Example 2 S1. Take 20g of silica sol (pH=2-3, solid content 50wt%) in a beaker, slowly add 1.1g of fumed silica with a particle size of 15nm and 0.15g of silicon nitride powder under mechanical stirring at 1400r / min, and stir for 30min to obtain a first mixed solution; place the first mixed solution in an ultrasonic instrument and disperse it for 50min to obtain a second mixed solution; place the prepared second mixed solution in a water bath at 60°C and heat and stir for 10min, then take out and stir until cooled for use to obtain a sealing slurry; S2. Use a polyester brush to dip the sealing slurry into the porous silicon nitride substrate and evenly apply it on the surface. Immediately wipe off the excess slurry on the surface with a polyvinyl chloride coated cloth. Repeat twice. S3. The treated sample is placed in an oven and dried at 120° C. for 2 h to obtain a porous silicon nitride substrate with sealed pores on the surface, which is then directly subjected to coating spraying.

[0056] The relative weight gain rate of this embodiment is 2.52%, and the sealing effect is A.

[0057] Example 3 S1. Take 20g of silica sol (pH=2-3, solid content 52wt%) in a beaker, slowly add 0.9g of fumed silica with a particle size of 12nm and 0.25g of silicon nitride powder under mechanical stirring at 1100r / min, and stir for 20min to obtain a first mixed solution; place the first mixed solution in an ultrasonic instrument and disperse it for 40min to obtain a second mixed solution; place the prepared second mixed solution in a water bath at 60°C and heat and stir for 25min, then take out and stir until cooled for use to obtain a sealing slurry; S2. Use a brush to dip the sealing slurry into the porous silicon nitride substrate and evenly apply it on the surface. Immediately wipe off the excess slurry on the surface. Repeat 3 times. S3. The treated sample is placed in an oven and dried at 110° C. for 1.5 h to obtain a porous silicon nitride substrate with sealed pores on the surface, which is then directly subjected to coating spraying.

[0058] The relative weight gain rate of this embodiment is 2.74%, and the sealing effect is A.

[0059] Example 4 S1. Take 20g of silica sol (pH=2-3, solid content 48wt%) in a beaker, slowly add 0.8g of fumed silica with a particle size of 15nm and 0.2g of silicon nitride powder under mechanical stirring at 1200r / min, and stir for 15 minutes to obtain a first mixed solution; place the first mixed solution in an ultrasonic instrument and disperse it for 40 minutes to obtain a second mixed solution; place the prepared second mixed solution in a water bath at 60°C and heat and stir for 15 minutes, then take out and stir until cooled for use to obtain a sealing slurry; S2. Use a polyester brush to dip the mixed sealing slurry and evenly apply it on the surface of the porous silicon nitride substrate. Immediately wipe off the excess slurry on the surface with a polyvinyl chloride coated cloth. Repeat 3 times. S3. The treated sample is placed in an oven and dried at 120° C. for 1.5 h to obtain a porous silicon nitride substrate with sealed pores on the surface, which is then directly subjected to coating spraying.

[0060] The relative weight gain rate of this embodiment is 4.42%, and the sealing effect is A.

[0061] Comparative Example 1 The fumed silica component in Example 1 was removed, and other conditions were the same as in Example 1. The sealing slurry obtained in this example had a low viscosity. When the slurry was applied to the surface of the porous silicon nitride material by brush, the slurry quickly penetrated into the pores of the substrate. After drying, the relative weight gain of the substrate was 15.49%, and the sealing effect was C.

[0062] Comparative Example 2 The 12 nm fumed silica in Example 1 was replaced with 0.1 µm silica powder, with all other conditions remaining the same as in Example 1. When the sealing slurry obtained in this example was applied to the surface of a porous silicon nitride material, localized particle protrusions and an orange peel effect were observed. After drying, the relative weight gain of the substrate was 0.23%, resulting in a sealing performance rating of C.

[0063] Comparative Example 3 The amount of fumed silica in Example 1 was changed to 1.4 g, and other conditions were the same as in Example 1. When the sealing slurry obtained in this example was brush-coated on the surface of a porous silicon nitride material, peeling and flaking were observed. After drying, the relative weight gain of the substrate was 0.48%, and the sealing effect was C.

[0064] Comparative Example 4 The amount of silicon nitride powder used in Example 1 was changed to 0.4 g, and other conditions were the same as in Example 1. When the sealing slurry obtained in this example was brush-coated on the surface of the porous silicon nitride material, obvious peeling and flaking were observed. After drying, the relative weight gain of the substrate was 0.15%, and the sealing effect was C.

[0065] It can be seen from the above embodiments and comparative examples that the present application achieves precise control of the weight gain rate and sealing effect after sealing treatment by regulating parameters such as the proportion of sealing slurry components and water bath heating time. At the same time, the preparation process is simple, the equipment energy consumption is low, the cost is low, and the operability is strong, laying the foundation for actual industrial production.

[0066] The examples described in the present invention are merely descriptions of the preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should fall within the scope of protection of the present invention.

Claims

1. A method for sealing a porous silicon nitride substrate with surface sealing, characterized in that: Applying sealing slurry on the surface of the porous silicon nitride substrate; then filling the sealing slurry on the surface of the porous silicon nitride substrate into the pores on the surface of the porous silicon nitride substrate; finally drying the porous silicon nitride substrate to obtain a surface-sealed porous silicon nitride substrate; The sealing slurry is a mixed liquid of silica sol, fumed silicon dioxide and silicon nitride powder.

2. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 1, characterized in that: The specific steps include: S1: Preparation: Silica sol, fumed silica, and silicon nitride powder are mixed and stirred to obtain a mixed solution; the mixed solution is then heated and stirred, and cooled to obtain a sealing slurry; S2: Sealing: Applying the sealing slurry to the surface of the porous silicon nitride substrate, and then wiping the sealing slurry on the surface of the porous silicon nitride substrate to fill the pores on the surface of the porous silicon nitride substrate; S3: Drying: The porous silicon nitride substrate after step S2 is placed in an oven for drying to obtain a porous silicon nitride substrate with sealed pores on the surface.

3. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 1 or 2, characterized in that: The solid content of the silica sol in the sealing slurry is more than 40%, and the pH of the silica sol is 2-3; the nanoparticle size of the fumed silica in the sealing slurry is 10nm-30nm.

4. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 3, wherein: The weight ratio of fumed silica to silica sol in the sealing slurry is (0.03-0.06):1; the weight ratio of silicon nitride powder to silica sol in the sealing slurry is (0.005-0.015):

1.

5. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 2, wherein: The step S1 specifically includes the following steps: S11: placing silica sol in a container, and adding fumed silica and silicon nitride powder into the container under mechanical stirring to obtain a first mixed solution; S12: placing the first mixed solution into an ultrasonic instrument for dispersion to obtain a second mixed solution; S13: placing the second mixed liquid in a water bath and heating and stirring it, then taking out the second mixed liquid and stirring it until it cools down for later use, thereby obtaining a sealing slurry.

6. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 5, characterized in that: The stirring speed of the mechanical stirring in step S11 is 1000 r / min~2000 r / min, and the stirring time is 10 min~30 min; the ultrasonic dispersion time in step S12 is 30 min~60 min; the heating temperature in the water bath in step S13 is 45°C~65°C, and the heating time is 15 min~35 min.

7. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 2, wherein: In step S2, the sealing slurry is applied to the surface of the porous silicon nitride substrate using a brush; the sealing slurry on the surface of the porous silicon nitride substrate is filled into the pores on the surface of the porous silicon nitride substrate using a non-absorbent and non-linting cloth; and the application and wiping cycle is repeated at least twice.

8. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 7, characterized in that: The area of ​​a single application with a brush is less than 0.15m 2 , and the time interval between application and wiping is less than 15s.

9. The method for sealing a porous silicon nitride substrate with surface sealing according to claim 2, wherein: The drying process in step S3 specifically includes: a drying temperature of 100° C. to 150° C., and a drying time of 1.5 hours to 3 hours.

10. A surface-sealed porous silicon nitride substrate, characterized in that: The porous silicon nitride substrate is manufactured by the sealing treatment method according to any one of claims 1 to 9.

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