A surface-sealed porous silicon nitride substrate and a sealing treatment method thereof

Abstract

The application discloses a surface-sealed porous silicon nitride base and a sealing treatment method thereof, and belongs to the field of material surface treatment. In view of the problems of high porosity and high cost in the coating treatment process of the existing porous silicon nitride base, the application provides a sealing treatment method of the surface-sealed porous silicon nitride base. The sealing slurry is applied on the surface of the porous silicon nitride base. Then, the sealing slurry on the surface of the porous silicon nitride base is filled into the pores on the surface of the porous silicon nitride base. Then, the porous silicon nitride base is subjected to drying treatment, so as to obtain the surface-sealed porous silicon nitride base. The sealing slurry is a mixed liquid of silica sol, fumed silicon dioxide and silicon nitride powder. The whole process realizes the low dielectric and low density of the porous silicon nitride base while giving the porous silicon nitride base excellent surface sealing effect, provides conditions for subsequent coating treatment, and has the advantages of simple process, small equipment energy consumption, low cost, strong sealing treatment operability and facilitation of industrialized production.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of material surface treatment, and more particularly relates to a surface-sealed porous silicon nitride substrate and a sealing treatment method thereof. BACKGROUND

[0002] The radome is located at the front end of the missile, and is not only a structural part of the missile body but also an important part of the guidance system. It is a multifunctional component integrating heat resistance, wave transmission, load bearing, weather resistance, airtightness, impact resistance and other high-performance requirements. With the progress of science and technology and the needs, missiles are developing towards faster penetration speed and higher strike accuracy, which puts forward more stringent requirements for wave-transmitting materials for radomes. Low-density porous silicon nitride materials, with their advantages of wide-band high-transmission rate, lightweight and high-strength, high-temperature resistance and thermal shock resistance, low thermal conductivity, and function-structure integrated design, meet the stringent requirements of modern hypersonic weapons and communication systems for radomes, and become the ideal choice for missile radome materials. However, the high porosity (≥50%) of low-density porous silicon nitride ceramics leads to easy moisture absorption, poor rain erosion resistance and airtightness, which in turn affects the stability of the dielectric properties of the material and limits its further application in the field of aerospace. The current mainstream way is to form a dense high-temperature-resistant coating on the surface of the low-density silicon nitride substrate, which not only prevents the decline of dielectric properties caused by moisture absorption and improves the rain erosion resistance of the material, but also gives it excellent airtightness to prevent external harsh environments from interfering with the internal antenna.

[0003] The present application is understood and summarized by the present inventor before the present application date: Currently, the high-temperature-resistant coating formed on the surface of the low-density silicon nitride substrate can be divided into two categories according to the material: ceramic (sol-gel method, reaction sintering method, vapor deposition method, etc.) and organic silicon resin (spraying method, solution impregnation method, etc.). Ceramic coatings have higher hardness and more excellent high-temperature resistance and wear resistance, but the processing is more complex and the preparation cost is high. In order to achieve the airtightness of the radome, additional surface sealing treatment is required. Organic silicon resin has strong operability, simple preparation process and low cost, and can achieve excellent moisture resistance and airtightness, but the 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

[0006] 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.

[0007] 2. Technical solution

[0008] To solve the above problems, the present invention adopts the following technical solutions.

[0009] 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.

[0010] The sealing slurry is a mixed liquid of silica sol, fumed silicon dioxide and silicon nitride powder.

[0011] With the above technical solution, the core innovation points of the present application have the following aspects:

[0012] I. The setting of the hole sealing slurry: creatively uses the mixed solution of silica sol, fumed silica and silicon nitride powder; on the one hand, the dielectric constant of fumed silica in the hole sealing slurry is low, and it is less affected by frequency and temperature, so that when it is filled into the surface layer pores of the silicon nitride substrate, it will not have too much impact on the wideband wave transmission performance; on the other hand, under the conditions of heating and stirring, and close to the isoelectric point (pH 2-3) of the silica sol colloidal particles, the silica hydroxyl groups on the surface of the silica sol colloidal particles and the fumed silica particles will have strong hydrogen bonding, and part of the silica sol colloidal particles and the fumed silica particles will undergo dehydration condensation reaction to form a semi-crosslinked network structure, which will increase the viscosity of the system and reduce the fluidity of the hole sealing slurry, so that the hole sealing slurry can remain in the shallow layer of the pores of the silicon nitride substrate, and the density of the silicon nitride substrate will not increase too much; finally, the addition of silicon nitride powder can improve the wettability and adhesion between the hole sealing slurry and the silicon nitride substrate; in other words, the hole sealing slurry prepared from the three components of silica sol, fumed silica and silicon nitride powder can realize the remaining of the hole sealing slurry in the shallow layer of the pores of the porous silicon nitride substrate under the premise that the density and dielectric performance of the porous silicon nitride substrate are not affected too much, and by controlling the rheological properties of the hole sealing slurry before gelation, a three-dimensional network gel can be formed through high-temperature solidification to achieve the purpose of hole sealing;

[0013] II. Hole sealing operation: the hole sealing slurry is embedded into the pores of the silicon nitride substrate by the method of first smearing and then filling, so that before the complete solidification and locking of the gel network of the hole sealing slurry, it can be forcibly embedded into the deep pores on the surface of the substrate by using its remaining plasticity, so that the subsequent gelation and solidification shrinkage occur in the limited pore space, and a strong three-dimensional mechanical interlocking structure is formed, which can effectively resist shrinkage stress and ensure the integration of the hole sealing layer and the silicon nitride substrate, thereby realizing stable, durable and effective surface hole sealing;

[0014] In summary, the present application has the advantages of simple operation for hole sealing of the porous silicon nitride substrate, simple preparation process of the hole sealing slurry, low energy consumption of the equipment, low cost, strong operability of the hole sealing treatment, and is conducive to industrialized production; the whole hole sealing process realizes the low dielectric and low density of the porous silicon nitride substrate while giving the substrate excellent surface hole sealing effect.

[0015] Furthermore, it specifically includes the following steps:

[0016] S1: Preparation: mix and stir silica sol, fumed silica and silicon nitride powder to obtain a mixed solution; then heat and stir the mixed solution, and after cooling, obtain a hole sealing slurry;

[0017] S2: hole sealing: hole sealing slurry is applied to the surface of the porous silicon nitride substrate, and after the application is completed, the hole 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;

[0018] S3: drying: the porous silicon nitride substrate after step S2 is placed in an oven for drying treatment to obtain a porous silicon nitride substrate with a sealed surface.

[0019] The above technical solution provides a specific hole sealing slurry preparation process and hole sealing process. It is worth noting that the hole sealing process uses a wiping method to embed the hole sealing slurry into the pores of the silicon nitride substrate. In addition to forcing the depth filling of the pores and forming an anchoring structure by applying pressure and using thixotropy, the wiping method can also timely remove 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 of the hole sealing layer and the silicon nitride substrate.

[0020] The entire technical solution prepares a high-viscosity, strong-thixotropy, and stable hole sealing slurry through step S1. Through the application and wiping of step S2, deep hole sealing and surface smoothing, as well as firm bonding, are achieved. Through step S3, the moisture is removed gently, allowing the hole sealing slurry in the pores to fully solidify and become dense, forming a final firm hole sealing structure. The entire process is simple and does not require complex equipment or tedious procedures, which is conducive to industrial production.

[0021] Furthermore, the solid content of the silicon sol in the hole sealing slurry is more than 40%, and the pH of the silicon sol is 2-3. The nanometer particle size of the fumed silica in the hole sealing slurry is 10-30 nm.

[0022] With the above technical solution, since the silicon sol is the main slurry of the hole sealing slurry, in order to provide a basic viscosity to the hole sealing slurry and prevent too much hole sealing slurry from entering the pores of the silicon nitride substrate during hole sealing processing, thereby affecting the density of the substrate and failing to achieve the hole sealing effect, the solid content of the silicon sol is limited to more than 40%. At the same time, in terms of the surface charge and hydroxyl activity of the silicon sol particles, an acidic silicon sol with a pH of 2-3 is preferred.

[0023] Since fumed silica is hydrophilic, in order to obtain a higher specific surface area and reactivity, the nanometer particle size of the fumed silica is limited to 10-30 nm, and preferably 10-15 nm.

[0024] Furthermore, the weight ratio of fumed silica to silicon sol in the hole sealing slurry is (0.03-0.06):1, and the weight ratio of silicon nitride powder to silicon sol in the hole sealing slurry is (0.005-0.015):1.

[0025] By adopting the technical scheme, the raw materials in the hole sealing slurry are limited in weight proportion. When the fumed silica is added in excess, the silica sol colloidal particles in the silica sol and the fumed silica are aggregated to form a three-dimensional network structure due to strong hydrogen bonding, which easily leads to solution gelation failure or causes the hole sealing slurry system to have excessively high viscosity and poor adhesion between the slurry and the substrate, resulting in peeling. In addition, the excessive addition of the fumed silica causes the particles to aggregate to form large clusters, which affects the uniformity of the hole sealing slurry. When the fumed silica is added in too small an amount, the hole sealing slurry system has small viscosity, and the hole sealing slurry easily penetrates into the pores of the porous silicon nitride substrate, affecting the density of the substrate.

[0026] When the silicon nitride powder is added in excess, the strong water absorption of the silicon nitride particles causes the particles to adsorb a large amount of free water in the system, hinders the condensation of the silica sol colloidal particles and the fumed silica particles to form a complete and continuous network structure, and the addition of a large amount of hard filler causes the interface to become 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, leading to the generation of coating cracking or direct peeling from the substrate. When the silicon nitride powder is added in too small an amount, the surface energy difference between the hole sealing slurry and the substrate is large, causing the slurry to have poor spreading performance on the surface of the substrate.

[0027] Further, the step S1 specifically comprises the following steps:

[0028] S11: The silica sol is placed in a container, and the fumed silica and the silicon nitride powder are added into the container under mechanical stirring to obtain a first mixed solution;

[0029] S12: The first mixed solution is placed in an ultrasonic instrument for dispersion to obtain a second mixed solution;

[0030] S13: The second mixed solution is placed in a water bath kettle for heating and stirring, and then the second mixed solution is taken out and stirred until it is cooled for standby to obtain the hole sealing slurry.

[0031] By adopting the technical scheme, step S1 is required to prepare a hole sealing slurry with high viscosity, strong thixotropy and stability. Step S11 is preliminary mixing by mechanical stirring. The mechanical stirring provides strong shear force, which first ensures that the solid powders such as fumed silica and silicon nitride powder are preliminarily wetted by the silica sol liquid, avoids dry powder aggregation, and realizes preliminary uniform mixing. Step S12 is ultrasonic dispersion, which avoids nanoscale aggregation, makes the fumed silica and the silicon nitride powder as uniformly distributed as possible in the silica sol continuous phase in the form of single particles or smaller aggregates, and improves the uniformity of the system. Step S13 is heating and stirring followed by cooling. The heating provides energy, accelerates the dehydration condensation reaction between the silicon hydroxyl groups on the surface of the nano-SiO2 particles in the silica sol, induces the pre-crosslinking reaction of the silica sol, greatly improves the viscosity and optimizes the thixotropy, and at the same time promotes the full fusion of the components, the discharge of gas, the ripening and stabilization of the slurry.

[0032] In summary, through the above steps, a highly uniform, stable, not easy to settle or phase separation, with a suitable high viscosity and good thixotropy of the sealing slurry is finally obtained, which lays a good foundation for subsequent operation.

[0033] Further, the stirring speed of the mechanical stirring in the step S11 is 1000r / min-2000r / min, and the stirring time is 10min-30min; the ultrasonic dispersion time in the step S12 is 30min-60min; the heating temperature in the water bath in the step S13 is 45℃-65℃, and the heating time is 15min-35min.

[0034] By adopting the technical scheme, the mechanical stirring is to ensure the uniform mixing of the three raw materials, so the stirring speed and stirring time are strictly controlled to avoid the agglomeration of the slurry; the ultrasonic dispersion is to break the nano-level agglomeration, so the ultrasonic dispersion time is reasonably controlled to realize the fine dispersion of the sealing slurry in the nanometer scale while improving the efficiency as much as possible and reducing the equipment energy consumption.

[0035] The water bath heating promotes the formation of a stronger hydrogen bond network between the silica sol particles and the fumed silica particles, and causes part of the silica sol particles to undergo dehydration condensation reaction with the fumed silica particles to form chemical bonds, so that the system presents a semi-crosslinked network structure, the viscosity of the sealing slurry is increased and the fluidity is reduced, the sealing slurry is left in a small amount on the surface layer of the silicon nitride substrate pores, and the influence of the sealing treatment on the density of the low-density porous silicon nitride substrate is reduced; therefore, in order to control the viscosity and crosslinking degree of the sealing slurry system, the temperature and time of the water bath heating are limited.

[0036] Further, in the step S2, a brush is used to apply the sealing slurry on the surface of the porous silicon nitride substrate; a non-water-absorbing and non-falling wool 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 cycled at least twice.

[0037] By adopting the technical scheme, the brush and the cloth tool are relatively simple, easy for the operator to operate, and widely available and low in cost; at the same time, the application and wiping are cycled to ensure that the sealing slurry can be stably filled into the surface layer pores of the silicon nitride substrate.

[0038] Further, the area of a single application of the brush is less than 0.15m 2 , and the time interval between the application and wiping is less than 15s.

[0039] With the above technical scheme, when the hole sealing slurry is brushed on the surface of the silicon nitride substrate, the surface layer slurry starts to approach from the semi-crosslinked structure to the three-dimensional network structure of gelation with the volatilization of water in the system. If the slurry is not immediately filled into the surface layer pores of the silicon nitride substrate by wiping, the hole sealing slurry will gradually lose water and separate from the surface of the substrate after the solidification reaction, so the time interval between brushing and wiping is as small as possible, and at the same time, the single brushing area is limited.

[0040] Further, the drying treatment in step S3 specifically includes: the drying temperature is 100℃-150℃, and the drying time is 1.5h-3h.

[0041] With the above technical scheme, under high temperature conditions, the water in the system volatilizes at a faster rate, the distance between the silica sol colloidal particles and the gas phase silica particles is reduced, and the dehydration condensation reaction gradually occurs to form a three-dimensional network gel. In order to save process energy consumption and cycle, the drying temperature and drying time are optimized.

[0042] A porous silicon nitride substrate with surface hole sealing is made by using the hole sealing treatment method of the porous silicon nitride substrate according to any one of the above technical schemes. The surface hole sealing of the porous silicon nitride substrate is realized without affecting its density and dielectric properties, which lays a good performance foundation for subsequent spraying of high-temperature resistant organic silicon resin coating. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 Figure 1 is a schematic diagram of a porous silicon nitride substrate surface without hole sealing treatment and a porous silicon nitride substrate surface with hole sealing treatment; wherein Figure 1 (a) is a schematic diagram of a porous silicon nitride substrate surface without hole sealing treatment; Figure 1 (b) is a schematic diagram of a porous silicon nitride substrate surface with hole sealing treatment. DETAILED DESCRIPTION

[0044] The application will be further described below in conjunction with specific embodiments and drawings.

[0045] At the same time, it is explained that the porous silicon nitride substrate in this application is a low-density porous silicon nitride material applied in the field of wave-transparent radome. In order to realize wide frequency and high wave-transparency, the porosity is more than 50%, preferably 55%-70%; the density is less than 1.60g / cm 3 , preferably 1.2-1.50g / cm 3 .

[0046] And the reason why the surface of the porous silicon nitride substrate is sealed in the present application is that the inventors of the present application found that in the mass production of low-density porous silicon nitride radomes, spraying a high-temperature resistant silicone resin coating on the surface can not only give the radome excellent moisture resistance, rain erosion resistance and air tightness, but also meet the use and storage requirements of the radome, and has the advantages of simple process, low requirement for equipment conditions, short process time and low cost, so it is widely used. However, due to the high porosity (≥50%) of the low-density porous silicon nitride material, when spraying the high-temperature resistant silicone resin coating, the resin organic matter is easy to penetrate into the pores of the substrate, which greatly affects the density and dielectric properties of the silicon nitride substrate material. Therefore, the surface of the porous silicon nitride substrate is sealed before coating in the present application, so that the performance will not be affected. This provides conditions for subsequent spraying of high-temperature resistant silicone resin coating, and has important significance for the application of porous silicon nitride material in the field of radomes. On this basis, the present application is derived.

[0047] A sealing treatment method of a surface-sealed porous silicon nitride substrate, the method comprising the following steps: applying a sealing slurry to the surface of the porous silicon nitride substrate; 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.

[0048] In the present embodiment, the silica sol is one of acidic, neutral and basic; the fumed silica is hydrophilic; and the silicon nitride powder is preferably the raw material powder used for preparing the porous silicon nitride substrate. The three play the following roles respectively:

[0049] Silica sol: as the main liquid carrier and solid source of the sealing slurry, it constitutes the main skeleton of the sealing material; the main component of silica sol is silicon dioxide, which has a low dielectric constant and is less affected by frequency and temperature changes, and when filled into the surface layer pores of the silicon nitride substrate, it will not significantly deteriorate the inherent wideband wave transmission performance of the substrate;

[0050] Fumed silica: its extremely high specific surface area and nanoscale particle size enable it to effectively fill the pores between silica sol particles and even smaller pores, improving the density of the sealing layer; at the same time, the silica sol particles and the silica particles on the surface of the fumed silica will form a strong hydrogen bond, and part of the silica sol particles and the fumed silica particles will undergo dehydration condensation reaction to form a semi-crosslinked network structure; providing key thickening and thixotropic properties to ensure that the sealing slurry stays and densely fills the shallow pores;

[0051] 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;

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] In one specific embodiment, the method specifically comprises the following steps:

[0057] S1: Preparation: mixing and stirring silicon sol, fumed silica and silicon nitride powder to obtain a mixed solution; then heating and stirring the mixed solution, and obtaining a sealing slurry after cooling;

[0058] S2: Sealing: applying the sealing slurry on the surface of the porous silicon nitride substrate, and then filling the sealing slurry on the surface of the porous silicon nitride substrate into the pores of the porous silicon nitride substrate by wiping. It should be noted that the porous silicon nitride substrate is pre-cleaned with deionized water, dried and polished, and the surface must be clean without impurities such as oil stains and dust, so that the sealing operation can be performed.

[0059] S3: Drying: placing the porous silicon nitride substrate after step S2 in an oven for drying treatment to obtain a porous silicon nitride substrate with a sealed surface.

[0060] In this embodiment, each step is given. The high-viscosity, strong thixotropy and stable sealing slurry is prepared by step S1; the deep sealing and surface leveling are achieved by the application and wiping of step S2, and the combination is firm; the water is removed by step S3, the sealing slurry in the pores is completely solidified and dense, and the final firm sealing structure is formed; the whole process is simple, without complex equipment and tedious procedures, which is conducive to industrial production.

[0061] In one specific embodiment, the solid content of the silicon sol in the sealing slurry is more than 40%, and the pH of the silicon sol is 2-3; the nanoparticle size of the fumed silica in the sealing slurry is 10-30 nm. The raw materials in the sealing slurry are limited in this embodiment. The silicon sol can be divided into low-concentration and high-concentration silicon sol according to the solid content. In order to provide basic viscosity to the sealing slurry, prevent too much slurry from entering the pores of the silicon nitride substrate during sealing, and thus affect the density of the substrate and fail to achieve the sealing effect, the high-concentration silicon sol with a solid content of more than 40% is preferred, and the high-concentration silicon sol with a solid content of 48%-52% is further preferred. In terms of surface charge and hydroxyl activity of silicon sol particles, the acidic silicon sol with a pH of 2-3 is preferred. In order to obtain higher specific surface area and reactivity, the nanoparticle size of the fumed silica is 10-30 nm, and the nanoparticle size of the fumed silica is further preferably 10-15 nm.

[0062] In one specific embodiment, the weight ratio of fumed silica to silicon sol in the sealing slurry is (0.03-0.06):1; and the weight ratio of silicon nitride powder to silicon sol in the sealing slurry is (0.005-0.015):1.

[0063] The embodiment limits the amount of three raw materials in the hole sealing slurry: the weight ratio of fumed silica and silica sol is (0.03-0.06):1, and is further preferably (0.04-0.055):1; the weight ratio of silicon nitride powder and silica sol is (0.005-0.015):1, and is further preferably (0.075-0.012):1; so as to ensure excellent performance of the hole sealing slurry.

[0064] In one specific embodiment, the step S1 specifically comprises the following steps:

[0065] S11: The silica sol is placed in a container, and fumed silica and silicon nitride powder are added to the container under mechanical stirring to obtain a first mixed solution;

[0066] S12: The first mixed solution is placed in an ultrasonic instrument for dispersion to obtain a second mixed solution;

[0067] S13: The second mixed solution is placed in a water bath for heating and stirring, and then the second mixed solution is taken out and stirred until it is cooled for standby, to obtain a hole sealing slurry.

[0068] Specifically, the stirring speed of the mechanical stirring in the step S11 is 1000-2000 r / min, and the stirring time is 10-30 min; the ultrasonic dispersion time in the step S12 is 30-60 min; and the heating temperature of the water bath in the step S13 is 45-65℃, and the heating time is 15-35 min.

[0069] In one specific embodiment, the step S2 adopts a brush to apply the hole sealing slurry to the surface of the porous silicon nitride substrate; a non-water-absorbing and non-falling wool cloth is used to fill the hole 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 cycled at least twice.

[0070] Of course, in the embodiment, the brush is one of nylon, polyester, and polypropylene materials; from the perspective of the pH value of the hole sealing slurry, polyester material with excellent acid resistance is preferred; the cloth is one of super-high-precision nylon cloth, polyvinyl chloride coated cloth, chloroprene rubber coated cloth, and polypropylene non-woven cloth; and considering the material cost, one of the polyvinyl chloride coated cloth and the polypropylene non-woven cloth is preferred. The at least two cycles of application and wiping mean that after the application, the cloth is wiped, and then after the re-application, the cloth is wiped again, so that one process step is cycled to ensure that the hole sealing slurry can be stably filled into the surface pores of the silicon nitride substrate.

[0071] In one specific embodiment, the area of a single application of the brush is less than 0.15 m 2, and the interval between the coating and wiping is less than 15s. In order to facilitate operation, the area of single coating and wiping and the interval between the coating and wiping are limited to ensure that the sealing slurry is filled into the surface layer pores of the silicon nitride substrate and the sealing performance is ensured. The area of single coating and wiping is further preferably less than 0.1m 2 The interval between the coating and wiping is further preferably less than 10s.

[0072] In a specific embodiment, the drying treatment in step S3 specifically includes: the drying temperature is 100℃-150℃, and the drying time is 1.5h-3h. Under high temperature conditions, the water in the system volatilizes at a high speed, the distance between the silica sol colloidal particles and the gas-phase silicon dioxide particles is reduced, and the dehydration condensation reaction gradually occurs to form a three-dimensional network gel. The silicon nitride powder particles in the system are filled in the structural voids. In order to save process energy consumption and cycle, the drying temperature and the drying time are specifically optimized in this embodiment. The drying temperature is further optimized to 110℃-130℃, and the drying time is further optimized to 1.5h-2h.

[0073] A porous silicon nitride substrate with a sealed surface is made by using the sealing treatment method of the porous silicon nitride substrate according to any one of the above embodiments. The sealing of the surface of the porous silicon nitride substrate is realized without affecting its density and dielectric properties, which lays a good performance foundation for subsequent spraying of a high-temperature-resistant organic silicon resin coating.

[0074] In order to further intuitively understand the technical solutions of the present application, the following examples and comparative examples are given:

[0075] At the same time, the method for measuring the sealing effect used in the examples and comparative examples is as follows:

[0076] The sample after the sealing treatment is placed on a clean table. The solid content of the organic silicon resin used as the coating material is 30%. The sample surface is sprayed by adjusting the spray gun pressure, spray width and spray gun distance. After spraying, the sample is naturally air-dried for a period of time, and then placed in an oven for curing. The surface state of the sample is observed during the process, and the sealing effect is evaluated by using three stages A-C. A is qualified, and B-C is unqualified.

[0077] A: After air-drying, it can be observed that the surface of the substrate is covered with a continuous resin bright coating. After drying and curing, it can be observed that the surface of the substrate is covered with a continuous resin bright coating.

[0078] B: After air-drying, it can be observed that the surface of the substrate is covered with a continuous resin bright coating. After drying and curing, it can be observed that the surface of the substrate is not covered with a resin bright coating.

[0079] C: After air-drying, it can be observed that the surface of the substrate is not covered with a resin bright coating.

[0080] The method for calculating the relative weight gain rate after the pore sealing treatment used in this application is as follows:

[0081] The test piece with the size of 40mmx40mmx2.8mm is dried and weighed as M1, and then the test piece is subjected to the pore sealing treatment and weighed as M2. The calculation formula of the weight gain rate is as follows:

[0082]

[0083] The weight gain rate is relative to the test piece with the size of 40mmx40mmx2.8mm. If the sample is a thicker silicon nitride substrate material, the weight gain rate is relatively lower.

[0084] Example 1

[0085] S1, 20g of silica sol (pH=2~3, solid content 50wt%) is taken in a beaker, 1.0g of fumed silica with a particle size of 12nm and 0.2g of silicon nitride powder are slowly added under the action of mechanical stirring at 1300r / min, and after stirring for 20min, a first mixed solution is obtained; the first mixed solution is placed in an ultrasonic instrument for dispersion for 40min to obtain a second mixed solution; the prepared second mixed solution is placed in a water bath kettle for heating and stirring at 60℃ for 20min, then taken out and stirred to cool for standby, and a pore sealing slurry is obtained;

[0086] S2, the pore sealing slurry is evenly brushed on the surface of the porous silicon nitride substrate with a polyester brush, and the excess slurry on the surface is immediately wiped off with a polypropylene non-woven fabric, and the operation is repeated twice;

[0087] S3, the treated sample is placed in an oven for drying treatment at 120℃ for 1.5h to obtain a porous silicon nitride substrate with a sealed surface, and subsequent coating spraying treatment is directly carried out.

[0088] The relative weight gain rate of this embodiment is 1.71%, and the pore sealing effect is A.

[0089] Example 2

[0090] S1, 20g of silica sol (pH=2~3, solid content 50wt%) is taken in a beaker, 1.1g of fumed silica with a particle size of 15nm and 0.15g of silicon nitride powder are slowly added under the action of mechanical stirring at 1400r / min, and after stirring for 30min, a first mixed solution is obtained; the first mixed solution is placed in an ultrasonic instrument for dispersion for 50min to obtain a second mixed solution; the prepared second mixed solution is placed in a water bath kettle for heating and stirring at 60℃ for 10min, then taken out and stirred to cool for standby, and a pore sealing slurry is obtained;

[0091] S2, the pore sealing slurry is evenly brushed on the surface of the porous silicon nitride substrate with a polyester brush, and the excess slurry on the surface is immediately wiped off with a polypropylene non-woven fabric, and the operation is repeated twice;

[0092] S3, the treated sample is placed in an oven at 120°C for drying treatment for 2h, to obtain a porous silicon nitride substrate with sealed pores, which is directly subjected to subsequent coating spraying treatment.

[0093] The relative weight gain rate of the present embodiment is 2.52%, and the sealing effect is A.

[0094] Example 3

[0095] S1, 20g of silica sol (pH=2~3, solid content 52wt%) is taken in a beaker, 0.9g of fumed silica with a particle size of 12nm and 0.25g of silicon nitride powder are slowly added under the action of mechanical stirring at 1100r / min, and after stirring for 20min, a first mixed solution is obtained; the first mixed solution is placed in an ultrasonic instrument for dispersion for 40min to obtain a second mixed solution; the prepared second mixed solution is placed in a water bath kettle for heating and stirring at 60°C for 25min, then taken out and stirred to cool for standby, to obtain a sealing slurry;

[0096] S2, the sealing slurry is evenly brushed on the surface of the porous silicon nitride substrate with a brush, and the excess slurry on the surface is immediately wiped off, and the operation is repeated for 3 times;

[0097] S3, the treated sample is placed in an oven at 110°C for drying treatment for 1.5h, to obtain a porous silicon nitride substrate with sealed pores, which is directly subjected to subsequent coating spraying treatment.

[0098] The relative weight gain rate of the present embodiment is 2.74%, and the sealing effect is A.

[0099] Example 4

[0100] S1, 20g of silica sol (pH=2~3, solid content 48wt%) is taken in a beaker, 0.8g of fumed silica with a particle size of 15nm and 0.2g of silicon nitride powder are slowly added under the action of mechanical stirring at 1200r / min, and after stirring for 15min, a first mixed solution is obtained; the first mixed solution is placed in an ultrasonic instrument for dispersion for 40min to obtain a second mixed solution; the prepared second mixed solution is placed in a water bath kettle for heating and stirring at 60°C for 15min, then taken out and stirred to cool for standby, to obtain a sealing slurry;

[0101] S2, the sealing slurry is evenly brushed on the surface of the porous silicon nitride substrate with a polyester brush, and the excess slurry on the surface is immediately wiped off with a polyvinyl chloride coating cloth, and the operation is repeated for 3 times;

[0102] S3, the treated sample is placed in an oven at 120°C for drying treatment for 1.5h, to obtain a porous silicon nitride substrate with sealed pores, which is directly subjected to subsequent coating spraying treatment.

[0103] The relative weight gain rate of the embodiment is 4.42%, and the sealing effect is A.

[0104] Comparative Example 1

[0105] The fumed silica component in Example 1 is removed, and other conditions are the same as in Example 1. The sealing slurry obtained in this example has a lower viscosity. When the slurry is brushed on the surface of the porous silicon nitride material, the slurry quickly penetrates into the pores of the substrate, and after drying, the relative weight gain rate of the substrate is 15.49%, and the sealing effect is C.

[0106] Comparative Example 2

[0107] The particle size of the fumed silica in Example 1 is changed to 0.1 µm, and other conditions are the same as in Example 1. When the sealing slurry obtained in this example is brushed on the surface of the porous silicon nitride material, local particle protrusions and orange peel phenomena can be observed, and after drying, the relative weight gain rate of the substrate is 0.23%, and the sealing effect is C.

[0108] Comparative Example 3

[0109] The amount of fumed silica in Example 1 is changed to 1.4 g, and other conditions are the same as in Example 1. When the sealing slurry obtained in this example is brushed on the surface of the porous silicon nitride material, peeling phenomena can be observed, and after drying, the relative weight gain rate of the substrate is 0.48%, and the sealing effect is C.

[0110] Comparative Example 4

[0111] The amount of silicon nitride powder in Example 1 is changed to 0.4 g, and other conditions are the same as in Example 1. When the sealing slurry obtained in this example is brushed on the surface of the porous silicon nitride material, obvious peeling phenomena can be observed, and after drying, the relative weight gain rate of the substrate is 0.15%, and the sealing effect is C.

[0112] From the above examples and comparative examples, it can be seen that by adjusting the component ratio of the sealing slurry and the water bath heating time and other parameters, the application realizes precise control of the weight gain rate and sealing effect after sealing treatment, and at the same time, the preparation process is simple, the equipment energy consumption is small, the cost is low, and the operability is strong, which lays a foundation for actual industrial production.

[0113] The examples described in the present application are only used to describe the preferred embodiments of the present application, and do not limit the concept and scope of the present application. Without departing from the design idea of the present application, various modifications and improvements of the technical solutions of the present application made by the engineering and technical personnel in the field shall fall within the protection scope of the present application.

Claims

1. A method for sealing the surface of a porous silicon nitride substrate, characterized by: The sealing slurry is applied on the surface of the porous silicon nitride substrate, and then 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, and finally the porous silicon nitride substrate is dried to obtain the porous silicon nitride substrate with sealed surface. The sealing slurry is a mixed liquid of silica sol, fumed silica and silicon nitride powder. The solid content of the silica sol in the sealing slurry is more than 40%, the nano-particle size of the fumed silica in the sealing slurry is 10-30 nm, the weight ratio of the fumed silica to the silica sol in the sealing slurry is (0.03-0.06):1, and the weight ratio of the silicon nitride powder to the silica sol in the sealing slurry is (0.005-0.015):

1.

2. The method of sealing the surface of a porous silicon nitride substrate according to claim 1, wherein: The method comprises the following steps: S1: Preparation: the silica sol, fumed silica and silicon nitride powder are mixed and stirred to obtain a mixed liquid, and the mixed liquid is heated and stirred to obtain the sealing slurry after cooling; S2: Sealing: the sealing slurry is applied on the surface of the porous silicon nitride substrate, and then 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; S3: Drying: the porous silicon nitride substrate after step S2 is placed in an oven for drying treatment to obtain the porous silicon nitride substrate with sealed surface.

3. The method for sealing the surface of a porous silicon nitride substrate according to claim 1 or 2, wherein: The pH of the silica sol is 2-3.

4. The method of claim 2, wherein the surface of the porous silicon nitride body is sealed by a method comprising: The step S1 specifically comprises the following steps: ​ S11: The silica sol is placed in a container, and the fumed silica and silicon nitride powder are added into the container under mechanical stirring to obtain a first mixed liquid; S12: The first mixed liquid is placed in an ultrasonic instrument for dispersion to obtain a second mixed liquid; S13: The second mixed liquid is heated and stirred in a water bath kettle, and then the second mixed liquid is taken out and stirred until cooling for standby to obtain the sealing slurry.

5. The method of sealing the surface of a porous silicon nitride body according to claim 4, wherein: The stirring speed of the mechanical stirring in the step S11 is 1000-2000 r / min, and the stirring time is 10-30 min; the ultrasonic dispersion time in the step S12 is 30-60 min; and the heating temperature of the water bath kettle in the step S13 is 45-65℃, and the heating time is 15-35 min.

6. The method of claim 2, wherein the method is characterized by: In the step S2, the sealing slurry is applied on the surface of the porous silicon nitride substrate by using a brush, and 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 using a wiping cloth which does not absorb water and does not shed hair, and the application and wiping are cycled at least twice.

7. The method of sealing the surface of a porous silicon nitride body according to claim 6, wherein: The area of a single application with a brush is less than 0.15 m 2 and the time interval between application and wiping is less than 15 s.

8. The method of sealing the surface of a porous silicon nitride body according to claim 2, wherein: The drying treatment in the step S3 specifically comprises: the drying temperature is 100-150℃, and the drying time is 1.5-3 h.

9. A surface-sealed porous silicon nitride body, characterized by: The method is used to seal the porous silicon nitride substrate.

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