A technique for modifying boron-doped diamond by vapor deposition on a foamed silicon carbide surface

Abstract

The present application relates to the technical field of vapor deposition, and especially relates to a kind of boron-doped diamond modification technology of vapor deposition on the surface of foamed silicon carbide; the present application proposes a kind of boron-doped diamond modification technology of vapor deposition on the surface of foamed silicon carbide, which is prepared with polycarbosilane and polyborosiloxane hybrid sol complexing as precursor, nano boron carbide as induced crystal seed, cage-type silsesquioxane as toughening phase of transition layer impregnation solution; after foamed silicon carbide is impregnated, gradient boron-silicon transition layer is generated in situ on the surface of pore wall by subsection program heat treatment; double-grain-size nano diamond is realized by collaborative adsorption to realize uniform nucleation in all pore channels; functional gradient boron-doped diamond coating is formed by combining normal pressure wide temperature subsection vapor deposition with three-stage temperature-boron concentration coupling gradient doping; residual stress is eliminated by low-temperature inert annealing.The present application solves the key problems in the prior art, such as interface thermal stress concentration of porous substrate, poor film forming uniformity, inability to mass-produce under normal pressure and difficulty in balancing coating performance.

Description

Technical Field

[0001] This invention relates to the field of vapor deposition technology, and more particularly to a vapor deposition technique for modifying silicon carbide foam surfaces with boron-doped diamond. Background Technology

[0002] Silicon carbide foam (SiC) is a porous ceramic material with a three-dimensional interconnected network structure. Due to its low density, high specific strength, excellent high-temperature stability, and chemical inertness, it has broad application prospects in high-temperature filtration, catalyst supports, electrochemical electrodes, and aerospace thermal protection. However, when used as an electrochemical electrode material, silicon carbide foam faces fundamental limitations such as a narrow electrochemical window, low oxygen evolution potential, and insufficient corrosion resistance. Furthermore, its intrinsic brittleness and strong surface inertness make it difficult to achieve uniform functional coating modification on the inner walls of complex three-dimensional channels using traditional methods.

[0003] Boron-doped diamond (BDD) films possess an extremely wide electrochemical window, very low background current, excellent chemical inertness, and anti-fouling ability, making them ideal high-performance electrode materials. Depositing BDD coatings onto a three-dimensional silicon carbide foam framework promises to yield porous electrodes with both high specific surface area and excellent electrochemical performance. However, significant thermal expansion coefficient and lattice mismatch exist between diamond and silicon carbide, leading to low interfacial bonding strength, easy cracking, and peeling of the coating during direct deposition. Furthermore, the three-dimensional porous structure places stringent requirements on gas mass transfer and temperature uniformity during vapor deposition: to achieve uniform deposition of the BDD coating inside and outside the pores, sufficient penetration of the reactant gas and a uniform thermal field distribution within the complex pores must be ensured. However, traditional low-pressure chemical vapor deposition (LPCVD) processes tend to exhibit uneven deposition on three-dimensional porous substrates, with denser deposits on the outside and sparser deposits on the inside.

[0004] To address the aforementioned issues, several improved solutions have emerged in the existing technology. One solution involves pre-depositing a metal transition layer such as Ti, Cr, or W on a planar SiC substrate using magnetron sputtering or ion beam assisted deposition, followed by the deposition of a diamond film. However, this method is only suitable for planar or simple curved substrates and cannot handle the internal pore surfaces of foamed silicon carbide. Furthermore, the metal transition layer is prone to carbonization at high temperatures, resulting in a brittle metal carbide layer that is easily damaged during thermal cycling. Another solution is to use conventional hot-wire chemical vapor deposition (HFCVD) to deposit diamond at lower gas pressures, improving the coverage inside the pores by extending the deposition time. However, this method has a low deposition rate, and the increased mean free path of gas molecules under low pressure conditions leads to a reduced gas collision frequency within the pores, insufficient precursor decomposition, and significantly lower internal deposition quality than external deposition. A coating gradient with a thicker outer layer and a thinner inner layer still exists, failing to meet process requirements.

[0005] In summary, the existing technologies suffer from the following problems: ① Cracks and peeling occur between the BDD coating and the SiC substrate due to the mismatch in their coefficients of thermal expansion; ② Uniform gas mass transfer and temperature distribution are difficult to achieve within the three-dimensional porous structure, resulting in poor deposition uniformity; ③ In traditional BDD deposition, the boron doping concentration is uniformly or randomly distributed along the thickness direction, failing to meet process requirements. Therefore, the existing technologies lack a systematic solution that can comprehensively address the issues of interface bonding, uniform deposition of three-dimensional pores, and boron content gradient control, severely restricting the practical application of foamed silicon carbide-based BDD composite electrode materials. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a boron-doped diamond modification technique for the surface of foamed silicon carbide via vapor deposition. This method involves constructing a boron-siloxane / polycarbosilane composite precursor—in-situ embedding of nano-boron carbide seeds—a boron-silicon transition layer formed by segmented heat treatment microcrystallization, combined with a full-chain process of dual-size diamond synergistic adsorption—aperture-pressure wide-temperature segmented vapor deposition—three-stage temperature-boron concentration coupled gradient doping. This enables the preparation of a large-area, highly uniform, and highly bonded boron-doped diamond coating on the surface of three-dimensional porous foamed silicon carbide under ambient pressure.

[0007] A vapor deposition technique for modifying silicon carbide foam surfaces with boron-doped diamond includes the following steps: Step S1: Preparation of transition layer impregnation solution: Polycarbosilane and xylene are mixed, stirred at 50°C, and polyborosiloxane hybrid sol, boric acid and yttrium nitrate hexahydrate are added. After stirring for 1 hour, xylene is added to adjust the solid content and ultrasonically dispersed to obtain the transition layer impregnation solution. Step S2: Impregnation of foamed silicon carbide substrate: The foamed silicon carbide was ultrasonically cleaned sequentially with acetone, anhydrous ethanol and deionized water, and dried at 100°C. The dried foamed silicon carbide was then impregnated in the transition layer impregnation solution. The vacuum was drawn to 200 Pa and maintained for 30 min. Nitrogen gas was introduced to pressurize to 0.8 MPa and maintained for 60 min. After depressurization, the substrate was centrifuged at 200 rpm for 60 s to obtain impregnated silicon carbide. Step S3: Heat treatment of transition layer: The impregnated silicon carbide is aged at 25°C and 70% relative humidity for 12 hours, then dried at 60°C for 6 hours, and then subjected to segmented heat treatment to form a transition layer on the surface of silicon carbide, thus obtaining pretreated foamed silicon carbide. Step S4: Diamond suspension preparation and adsorption: Prepare a mixed suspension of 5-10nm and 15-20nm dual-size nanodiamonds, immerse the pretreated foamed silicon carbide in the mixed suspension of dual-size nanodiamonds and disperse it ultrasonically, and dry it at 80℃ to achieve in-situ uniform adsorption of nucleation seeds inside and outside the pores. Step S5: Atmospheric pressure wide temperature zone segmented vapor phase deposition: The foamed silicon carbide adsorbed in step S4 is vapor-phase deposited using an HFCVD / MPCVD composite deposition equipment, maintaining an atmospheric pressure of 100 kPa, setting a zoned temperature field of 750-850℃ for the outer layer and 550-650℃ for the inner layer, introducing mixed reactive gas, and depositing for 10 hours to form a boron-doped diamond coating. Step S6: Low-temperature annealing post-treatment: Place the deposited silicon carbide foam obtained in step S5 in an argon atmosphere, heat it to 550℃ at 5℃ / min and hold it at that temperature for 1 hour, then cool it to room temperature in the furnace to complete the modification process of silicon carbide foam.

[0008] Further, in step S1: the weight ratio of polycarbosilane, polyborosiloxane hybrid sol, boric acid, and yttrium nitrate hexahydrate is 12-14:70:9-10:0.3; the solid content is adjusted to 15-20 wt%. In step S3: the segmented heat treatment process is as follows: the temperature is increased to 200℃ at 1℃ / min and held for 1h, then increased to 600℃ at 0.5℃ / min and held for 1h, and finally increased to 1150℃ at 2℃ / min and held for 2h. After the holding is completed, the temperature is cooled to 500℃ at 3℃ / min and then cooled to room temperature with the furnace. In step S4, the mass ratio of 5-10nm and 15-20nm nanodiamonds is 2:1, and the mass concentration is 0.02-0.04g / L.

[0009] In step S5: H2, CH4, trimethylboron (TBH) and CO2 are used as mixed reaction gases, wherein the volume ratio of H2 to CH4 is 8-12:1 and the volume ratio of CO2 to CH4 is 1:20; the deposition process is divided into three stages: initial stage, intermediate stage and final stage, and the B / C ratio is controlled. The B / C ratio in the initial stage is 32000ppm, the B / C ratio in the intermediate stage decreases linearly to 18000ppm, and the B / C ratio in the final stage is 10000ppm; the deposition time ratio of the initial stage, intermediate stage and final stage is 1:3:1.

[0010] The polyborosiloxane hybrid sol was prepared by the following steps: Step A1: Mix boron carbide nanoparticles with anhydrous ethanol, ultrasonically disperse at room temperature for 30 min, then add phenyltrimethoxysilane and continue ultrasonic dispersion for 60 min, then dry at 60°C until a viscous slurry is obtained to prepare modified boron carbide slurry. Step A2: Mix tetraethyl orthosilicate, methyltrimethoxysilane and diphenyldimethoxysilane, and add them to a citric acid aqueous solution with a pH of 3. Stir at 40°C for 45 min, then add phenylboronic acid and heat to 60°C. Continue stirring for 4 h to obtain polyborosiloxane sol. Step A3: Mix the polyborosiloxane sol and the modified boron carbide slurry and disperse them ultrasonically. React at 50°C for 2 hours, then add the monoepoxy cage-type silsesquioxane and continue stirring for 30 minutes to obtain the polyborosiloxane hybrid sol.

[0011] Furthermore, in step A1: the ratio of boron carbide nanopowder, anhydrous ethanol, and phenyltrimethoxysilane is 0.8-1g: 10mL: 0.1-0.12g; In step A2: the ratio of tetraethyl orthosilicate, methyltrimethoxysilane, diphenyldimethoxysilane, tetrahydrofuran, citric acid aqueous solution, and phenylboronic acid is 20-21g: 27-28g: 24g: 15mL: 10.4g: 9.5-10g; In step A3, the ratio of polyboron siloxane sol, modified boron carbide paste, and monoepoxy cage-type silsesquioxane is 55-60g: 5g: 1.5-2g.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention discloses a boron-doped diamond modification technology for the surface of foamed silicon carbide by vapor deposition; the method constructs a boron-siloxane / polycarbosilane composite precursor—in-situ embedding of nano-boron carbide seeds—segmented heat treatment microcrystallization boron-silicon transition layer, combined with dual-size diamond synergistic adsorption—atmospheric pressure wide-temperature segmented vapor deposition—three-stage temperature-boron concentration coupled gradient doping, to achieve the preparation of a large-area, highly uniform, and highly adhesive boron-doped diamond coating on the surface of three-dimensional porous foamed silicon carbide under atmospheric pressure conditions.

[0013] In step A1, after the boron carbide nanoparticles are ultrasonically dispersed with anhydrous ethanol, phenyltrimethoxysilane is added for surface modification. As a surface modifier, the methoxy groups in phenyltrimethoxysilane hydrolyze under the influence of trace amounts of hydroxyl groups or adsorbed water on the boron carbide surface, generating silanol groups (Si-OH). These silanol groups further condense with the B-OH groups on the surface of the boron carbide particles, grafting an organosilane molecular layer onto the surface of the boron carbide particles. This surface modification transforms the boron carbide nanoparticles from inorganic-philic to organic-philic, enabling them to disperse stably in the subsequent organic-water mixture of polyborosiloxane sol, preventing particle agglomeration and sedimentation caused by van der Waals forces.

[0014] In step A2, citric acid is used to adjust the pH of the system, enabling the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS), methyltrimethoxysilane (MTMS), and diphenyldimethoxysilane (DPDMS) under acidic catalytic conditions. Then, phenylboronic acid (PBA) is added, and its B-OH groups undergo a co-condensation reaction with the silanols (Si-OH) in the system at 60°C to form Si-OB bridging bonds, chemically embedding boron atoms into the polysiloxane network framework. Compared to physically mixing methods of introducing boron sources, this in-situ chemical bonding allows for atomic-level dispersion of boron in the final pyrolysis products, laying the foundation for the subsequent uniform precipitation of boron carbide microcrystals.

[0015] In step A3, the PTMS-modified boron carbide nanoparticles undergo co-condensation with the silanol groups in the polyboron siloxane sol through the residual organosilane chains on their surface, achieving chemical bond anchoring between the boron carbide seed crystals and the boron siloxane network. This pre-embedded network structure locks the boron carbide particles within the gel framework, preventing migration or aggregation during subsequent drying and pyrolysis, ensuring a uniform distribution of boron carbide microcrystals in the final transition layer. Based on this, a monoepoxy cage-type silsesquioxane (POSS) is added. Under heating conditions, its epoxy groups open, reacting with the hydroxyl groups on the siloxane chains to form covalent cross-linking points, embedding the rigid cage-type Si-O core into the flexible polysiloxane network. During subsequent pyrolysis, the organic arms of the POSS break, but the cage-like core structure remains, transforming into nano-SiO2 clusters dispersed throughout the glass matrix, serving a dual purpose of nano-toughening and regulating the coefficient of thermal expansion.

[0016] In step S1, polycarbosilane (PCS) is dissolved in xylene and then compounded with polyborosiloxane hybrid sol, boric acid and yttrium nitrate hexahydrate in a multi-component formulation. Polyborosiloxane hybrid sol serves as the main precursor, providing a continuous Si-OB inorganic network framework and pre-embedded B4C seeds. Polycarbosilane serves as an auxiliary precursor, its main chain rich in Si-C framework. During pyrolysis, it provides a large amount of active carbon source and transforms itself into a SiC microcrystal precursor. Simultaneously, the active carbon in its molecular chain preferentially reacts with B2O3 and SiO2 in the subsequent carbothermic reduction stage, driving microcrystal formation. Boric acid serves as an additional boron source, generating B2O3 after thermal dehydration and incorporating it into the glass network, increasing the total amount of boron that can participate in carbothermic reduction in the system and ensuring the formation of a sufficient number of boron carbide microcrystals. Yttrium nitrate hexahydrate decomposes and releases Y2O3 during pyrolysis, forming a trace amount of Y-Si-BO liquid phase with B2O3 and SiO2 in the 1100-1150℃ range. This liquid phase wets and rearranges the coating particles, promoting densification sintering.

[0017] Step S3 achieves the transformation from organic gel to gradient microcrystalline ceramic coating through four-stage programmed temperature control: First stage (room temperature → 200℃, keep warm for 1 hour): slowly raise the temperature to remove the residual solvent in the impregnation solution, while the silanol groups in the gel further condense, and the gel network is initially solidified and shaped.

[0018] The second stage (200→600℃, held for 1h): The organic groups of PBS and PCS are decomposed in an inert atmosphere and escape in the form of gaseous small molecules; boric acid is dehydrated and converted into B2O3 and incorporated into the gradually forming SiBOC amorphous network; in this stage, a carbon-rich amorphous SiBOC glass matrix is ​​formed, in which B2O3, SiO2 and carbon from PCS pyrolysis are uniformly distributed, providing ideal reactant contact conditions for subsequent carbothermic reduction.

[0019] The third stage (600→1150℃, holding for 2 hours): This is the core stage where the carbothermic reduction reaction occurs intensively and microcrystals precipitate. Within the temperature window of 1000-1150℃, the following key solid-state reactions occur in the system: B2O3+7C→2B4C+6CO↑, SiO2+3C→SiC+2CO↑. The presence of nano-B4C seeds significantly reduces the nucleation barrier of B4C, allowing boron carbide microcrystals to precipitate efficiently at 1100-1150℃ and grow around the original seeds, without the need for the high temperatures above 1400℃ required in traditional solid-state methods. The trace liquid phase generated by Y2O3 in this temperature range promotes particle rearrangement and densification, and the escape of CO gas products forms micro-nano-scale channels in the coating, preventing the coating from becoming completely dense and closed-pore. Because the CO diffusion escape path is shorter on the surface of the coating, the degree of reduction reaction is slightly higher than that in the inner layer, naturally forming a gradient transition structure with B4C / SiC microcrystals on the surface and amorphous SiBOC glass phase in the inner layer.

[0020] Cooling stage (1150℃→500℃, slow cooling at 3℃ / min): The slow cooling regime allows the thermal stress in the coating to be gradually dissipated through the viscous flow of the amorphous phase and the micro-region stress release of the POSS-derived SiO2 nanoclusters, effectively preventing cooling cracking.

[0021] Step S4 involves preparing a mixed suspension containing 5-10 nm and 15-20 nm dual-size diamond nanoparticles. The pretreated foamed silicon carbide, after heat treatment in step S3, is then immersed in this suspension and subjected to ultrasonic treatment. After heat treatment at 1150 °C, although the density of residual silanol and boronol on the transition layer surface decreases, some negatively charged sites provided by the amorphous SiBOC glass phase are still retained. Simultaneously, the microcrystalline surface forms a rough, uneven morphology at the nanoscale, providing the geometric conditions for mechanical intercalation and physical anchoring of the diamond nanoparticles. Under the combined effects of ultrasonic assistance and electrostatic attraction, positively charged dual-size diamond particles enter the pores of the foamed silicon carbide. The smaller 5-10 nm particles preferentially penetrate and adsorb onto the surface of the transition layer in submicron-level micropores and narrow channels, while the larger 15-20 nm particles form a high-density coverage on the large pore surface. After drying, uniform nucleation and seed adsorption are achieved throughout the entire pore structure.

[0022] If only a single small-diameter seed is used, although it can enter the micropores, the electrostatic repulsion between particles on the surface of the macropores results in insufficient coverage. If only a single large-diameter seed is used, it is difficult to enter the narrow micropores, resulting in uneven nucleation inside. The present invention adopts a 2:1 ratio of large to small particle size, which enables the two types of particles to form an alternating and complementary hydrodynamic behavior in the suspension, jointly constructing a uniform seed layer coverage across the entire scale of the pores.

[0023] Step S5 is the core step in the growth of boron-doped diamond coatings. An HFCVD / MPCVD composite deposition equipment is used under normal pressure conditions. Through the high-temperature pyrolysis generated by the hot filament and the synergistic excitation of microwave plasma, a high concentration of atomic hydrogen (H·) and methyl radicals (CH3·) is maintained on the substrate surface, ensuring preferential growth of the diamond phase and selective etching of non-diamond carbon.

[0024] Initial stage: A higher deposition temperature and high boron concentration are used. Under high temperature conditions, boron atoms can more easily replace carbon sites in the diamond lattice to achieve high boron doping and form a highly conductive bottom layer on the surface of the transition layer. At the same time, the high temperature promotes the generation and surface diffusion of atomic hydrogen, accelerating the initial nucleation and grain fusion.

[0025] In the middle section, the temperature B / C ratio decreases linearly, and the decreasing temperature gradient, combined with the gradual reduction of boron doping concentration, ensures continuous crystal growth while the amount of boron atoms gradually decreases. This achieves a smooth transition in boron doping concentration and avoids stress concentration and electrochemical corrosion weak points that may be caused by abrupt interface changes.

[0026] Final stage: The B / C ratio continues to decrease, and at lower temperatures, the secondary nucleation rate of diamond increases while the grain growth rate decreases. Combined with a low boron concentration, this ultimately results in a fine-grained, low-boron-doped, sp... 3 High-purity sealing layer.

[0027] In step S6, the deposited foamed silicon carbide is placed in a high-purity argon atmosphere, heated to 550°C at a rate of 5°C / min, held at that temperature for 1 hour, and then cooled in the furnace. During the inert atmosphere annealing process at 550°C, the minute cooling stress caused by the difference in thermal expansion coefficients between the diamond coating and the borosilicate transition layer is gradually released through interfacial atomic diffusion and lattice relaxation. At the same time, some unstable hydrogen terminals formed on the surface of the diamond film during CVD deposition are reconstructed during annealing, transforming into more stable CC or CH bond configurations, thereby improving the chemical stability and durability of the coating in subsequent service environments. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram of the structure of the present invention; 1 is a foamed silicon carbide matrix, 2 is a borosilicate co-infiltration gradient transition layer, 3 is a high boron-doped diamond bonding layer, 4 is a medium boron-doped diamond intermediate layer, and 5 is a low boron-doped diamond outer layer. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] In the following preparation examples, embodiments, and comparative examples: the average particle size of the boron carbide nanoparticles is 30 nm, and the purity is ≥98%; the average molecular weight of the polycarbosilane is 1500; and the monoepoxy cage-type silsesquioxane is Glycidyl-POSS.

[0031] Preparation Example 1: Step A1: Mix 0.9g of boron carbide nanoparticles with 10mL of anhydrous ethanol and ultrasonically disperse at 180W for 30min at room temperature; add 0.11g of phenyltrimethoxysilane (PTMS) and continue ultrasonic dispersion for 60min; dry and concentrate the resulting suspension in an oven at 60℃ to a viscous slurry to obtain modified boron carbide slurry.

[0032] Step A2: Mix 20.80g of tetraethyl orthosilicate (TEOS), 27.20g of methyltrimethoxysilane (MTMS), and 24.20g of diphenyldimethoxysilane (DPDMS) evenly, and add them to a citric acid aqueous solution with a pH of 3 (10.40g of deionized water mixed with citric acid solution), using 15mL of tetrahydrofuran as a co-solvent; hydrolyze by stirring at 300rpm for 45min in a 40℃ water bath; add 9.76g of phenylboronic acid, raise the temperature to 60℃, and continue stirring for 4h to obtain polyborosiloxane sol.

[0033] Step A3: Take 58g of the polyborosiloxane sol obtained in step A2 and mix it with 5g of the modified boron carbide slurry obtained in step A1. Disperse the mixture by ultrasonication at 180W for 20min. Stir and react at 50℃ for 2h. Add 1.8g of monoepoxy cage-type silsesquioxane and continue stirring for 30min to obtain the polyborosiloxane hybrid sol.

[0034] Preparation Example 2: Compared with Preparation Example 1, the amount of boron carbide nanopowder in step A1 was adjusted from 0.9g to 0.8g, and the amount of phenyltrimethoxysilane was adjusted from 0.11g to 0.10g, while other steps and amounts remained unchanged.

[0035] Preparation Example 3: Compared with Preparation Example 1, the amount of boron carbide nanopowder in step A1 was adjusted from 0.9g to 1.0g, and the amount of phenyltrimethoxysilane was adjusted from 0.11g to 0.12g, while other steps and amounts remained unchanged.

[0036] Preparation Example 4: Compared with Preparation Example 1, the amount of polyborosiloxane sol in step A3 was adjusted from 58g to 55g, while other steps and amounts remained unchanged.

[0037] Preparation Example 5: Compared with Preparation Example 1, the amount of polyborosiloxane sol in step A3 was adjusted from 58g to 60g, while other steps and amounts remained unchanged.

[0038] Preparation Example 6: Compared with Preparation Example 1, the amount of monoepoxy cage-type silsesquioxane in step A3 was adjusted from 1.8g to 1.5g, while other steps and amounts remained unchanged.

[0039] Preparation Example 7: Compared with Preparation Example 1, the amount of monoepoxy cage-type silsesquioxane in step A3 was adjusted from 1.8g to 2.0g, while other steps and amounts remained unchanged.

[0040] Comparative Preparation Example 1: Compared with Preparation Example 1, step A1 is omitted in Comparative Preparation Example 1, and modified boron carbide slurry is no longer added in step A3. Instead, polyborosiloxane sol and POSS are directly mixed and reacted. Other steps and dosages are the same as in Preparation Example 1.

[0041] Comparative Preparation Example 2: Compared with Preparation Example 1, the addition of monoepoxy cage-type silsesquioxane in step A3 was omitted in Comparative Preparation Example 2, while the other steps and dosages were the same as in Preparation Example 1.

[0042] Example 1: Step S1: Mix 13g of polycarbosilane (PCS) and 50mL of xylene, and stir at 50°C until completely dissolved; add 70g of the polyborosiloxane hybrid sol prepared in Example 1, 9.5g of boric acid and 0.3g of yttrium nitrate hexahydrate in sequence, and stir for 1h; add xylene to adjust the solid content to 18wt%, and ultrasonically disperse for 15min to obtain the transition layer impregnation solution.

[0043] Step S2: Take a piece of foamed silicon carbide with dimensions of 100mm×100mm×10mm (porosity 85%, pore size 0.55mm), and ultrasonically clean it sequentially with acetone, anhydrous ethanol, and deionized water for 15min each. Dry it at 100℃ for 2h. Place the dried foamed silicon carbide in an impregnation tank, evacuate it to 200Pa and maintain it for 30min; under vacuum, draw in the transition layer impregnation liquid obtained in step S1 until the foamed silicon carbide is completely submerged; close the vacuum valve, introduce nitrogen gas to pressurize it to 0.8MPa and maintain it for 60min; after depressurization, remove it and centrifuge it at 200rpm for 60s to obtain impregnated silicon carbide.

[0044] Step S3: Place the impregnated silicon carbide in a constant temperature and humidity chamber and age it for 12 hours at 25℃ and 70% relative humidity; then dry it in a 60℃ forced-air drying oven for 6 hours. Transfer the dried sample to a tube furnace and introduce high-purity argon gas for segmented heat treatment according to the following procedure: heat up from room temperature to 200℃ at 1℃ / min and hold for 1 hour; heat up from 200℃ to 600℃ at 0.5℃ / min and hold for 1 hour; heat up from 600℃ to 1150℃ at 2℃ / min and hold for 2 hours; after the holding period, cool down to 500℃ at 3℃ / min and allow it to cool naturally to room temperature with the furnace to obtain pretreated foamed silicon carbide.

[0045] Step S4: Prepare a mixed suspension of nanodiamonds with a mass concentration of 0.03 g / L (the mass ratio of 5-10 nm to 15-20 nm nanodiamonds is 2:1). Immerse the pretreated foam silicon carbide obtained in step S3 in the suspension, sonicate at 180 W for 12 min, and dry in an oven at 80 °C to achieve uniform adsorption of nucleated seeds inside and outside the pores.

[0046] Step S5: Place the foamed silicon carbide after seed adsorption in step S4 into an HFCVD / MPCVD composite deposition apparatus and maintain a normal pressure of 100 kPa. Use H2, CH4, trimethylboron (TMB), and CO2 as mixed reaction gases, where the volume ratio of H2 to CH4 is 10:1 and the volume ratio of CO2 to CH4 is 1:20.

[0047] The deposition process is carried out in three stages: Initial stage (0-2h): deposition temperature 870℃, TMB flow rate adjusted to B / C ratio 32000ppm; Intermediate stage (2-8h): The deposition temperature is reduced at a constant rate from 870℃ to 820℃, and the TMB flow rate decreases linearly, causing the B / C ratio to decrease linearly from 32000ppm to 18000ppm; Final stage (8-10h): The deposition temperature is further reduced to 780℃, and the TMB flow rate is adjusted to stabilize the B / C ratio at 10000ppm.

[0048] Step S6: Place the foamed silicon carbide deposited in step S5 into a tube furnace, introduce high-purity argon gas, heat to 550℃ at 5℃ / min and hold for 1 hour, and allow it to cool naturally to room temperature to complete the vapor deposition of boron-doped diamond on the surface of the foamed silicon carbide.

[0049] Example 2: Compared with Example 1, the amount of polycarbosilane used in step S1 is adjusted from 13g to 12g. Other steps and parameters are the same as in Example 1.

[0050] Example 3: Compared with Example 1, the amount of polycarbosilane used in step S1 is adjusted from 13g to 14g. Other steps and parameters are the same as in Example 1.

[0051] Example 4: Compared with Example 1, the amount of boric acid in step S1 is adjusted from 9.5g to 9g. Other steps and parameters are the same as in Example 1.

[0052] Example 5: Compared with Example 1, the amount of boric acid in step S1 is adjusted from 9.5g to 10g. Other steps and parameters are the same as in Example 1.

[0053] Example 6: Compared with Example 1, the solid content in step S1 of Example 6 is adjusted from 18wt% to 15wt%, while the other steps and parameters are the same as those of Example 1.

[0054] Example 7: Compared with Example 1, the solid content in step S1 of Example 7 is adjusted from 18wt% to 20wt%, while the other steps and parameters are the same as in Example 1.

[0055] Example 8: Compared with Example 1, Example 8 replaces the polyborosiloxane hybrid sol prepared in Preparation Example 1 in step S1 with an equal amount of polyborosiloxane hybrid sol prepared in Preparation Example 2. Other steps and parameters are the same as in Example 1.

[0056] Example 9: Compared with Example 1, Example 9 replaces the polyborosiloxane hybrid sol prepared in Preparation Example 1 in step S1 with an equal amount of polyborosiloxane hybrid sol prepared in Preparation Example 3. Other steps and parameters are the same as in Example 1.

[0057] Example 10: Compared with Example 1, Example 10 replaces the polyborosiloxane hybrid sol prepared in Preparation Example 1 in step S1 with an equal amount of the polyborosiloxane hybrid sol prepared in Preparation Example 6. Other steps and parameters are the same as in Example 1.

[0058] Example 11: Compared with Example 1, Example 11 replaces the polyborosiloxane hybrid sol prepared in Preparation Example 1 in step S1 with an equal amount of the polyborosiloxane hybrid sol prepared in Preparation Example 7. Other steps and parameters are the same as in Example 1.

[0059] Example 12: Compared with Example 1, the volume ratio of H2 to CH4 in step S5 is adjusted from 10:1 to 8:1 in Example 12, while the other steps and parameters are the same as in Example 1.

[0060] Example 13: Compared with Example 1, the volume ratio of H2 to CH4 in step S5 is adjusted from 10:1 to 12:1 in Example 13. Other steps and parameters are the same as in Example 1.

[0061] Comparative Example 1: Compared with Example 1, Comparative Example 1 completely omits steps S1-S3 and directly uses raw foam silicon carbide that has only undergone cleaning and drying in step S2 but has not undergone any impregnation and heat treatment for seed adsorption in step S4 and subsequent vapor deposition and annealing in steps S5 and S6.

[0062] Comparative Example 2: Compared with Example 1, the polyborosiloxane hybrid sol prepared in preparation example 1 in step S1 was replaced with an equal amount of the polyborosiloxane hybrid sol prepared in comparative preparation example 1. Other steps and parameters were the same as in Example 1.

[0063] Comparative Example 3: Compared with Example 1, the polyborosiloxane hybrid sol prepared in Preparation Example 1 in step S1 was replaced with an equal amount of the polyborosiloxane hybrid sol prepared in Comparative Preparation Example 2. Other steps and parameters were the same as in Example 1.

[0064] Comparative Example 4: Compared with Example 1, Comparative Example 4 modifies the segmented gradient doping in step S5 to uniform doping throughout the process, maintains a constant deposition temperature of 850°C throughout the process, maintains a constant B / C ratio of 20000ppm throughout the process, and the total deposition time is still 10h; other steps and parameters are the same as in Example 1.

[0065] Comparative Example 5: Compared with Example 1, Comparative Example 5 replaced steps S1 and S3 entirely with the traditional borosilicate co-infiltration embedding process. The foamed silicon carbide that was cleaned and dried in step S2 was completely embedded in a mixed powder of B4C and Si powder (mass ratio 3:1), placed in a vacuum tube furnace, and kept at 1400°C for 50 minutes. It was then allowed to cool naturally to room temperature in the furnace. The resulting foamed silicon carbide with the traditional borosilicate co-infiltration transition layer was then processed according to steps S4-S6 of Example 1.

[0066] The modified foamed silicon carbide prepared by the reference embodiment and comparative process was subjected to performance testing in the form of electrode application to verify the modification effect achieved by this technical solution. Modified foamed silicon carbide samples were taken and cut into cross-sections perpendicular to the hole walls using a diamond wire saw. After being cold-mounted with epoxy resin and polished, the samples were observed using a scanning electron microscope in backscattered electron mode. Five fields of view were randomly selected on both the outer surface of the foamed silicon carbide and inside the pores (at a depth of about 3 mm from the surface) to measure the film thickness. The average value was taken and the difference between the inner and outer thicknesses was calculated. At the same time, the surface microcracks were observed and graded from 0 to 5. Grade 0: No cracks, the coating in the field of view is dense and continuous, and there are no discernible cracks. Grade 1: Very slight cracks, isolated microcracks appear in the field of view, non-penetrating; Grade 2: Slight cracks, microcracks appear in the field of view, occasionally microcracks deflected along grain boundaries, no penetrating cracks; Grade 3: Moderate cracks, microcracks appear in the field of view, a small number of cracks penetrating a single film layer appear; Grade 4: More severe cracks, more cracks appear in the field of view, cracks penetrating the coating appear; Grade 5: Severe cracks, dense crack network appears in the field of view, cracks penetrate the coating, local peeling or fragmentation of the coating appears.

[0067] 2) Using a four-probe resistivity meter (probe spacing 1 mm), the modified foamed silicon carbide was processed into small samples of 10 mm × 10 mm × 5 mm. Resistivity was measured at 5 sites on the outer surface and 5 sites on the inner surface of the pores. Three samples were prepared from the same batch, for a total of 30 test sites. The average resistivity and relative standard deviation (RSD) were calculated. RSD was used to characterize the batch resistivity fluctuation.

[0068] 3) A three-electrode system was used, with modified foamed silicon carbide as the working electrode (exposed area 1 cm²), a platinum sheet (20 mm × 20 mm) as the auxiliary electrode, and Ag / AgCl (saturated KCl) as the reference electrode. The electrolyte was 0.5 M H₂SO₄ solution, which was bubbled with high-purity N₂ for 30 min before testing to remove dissolved oxygen. Linear sweep voltammetry (LSV) was performed using an electrochemical workstation, with a scan range of 0-3 V (vs Ag / AgCl) and a scan rate of 100 mV / s. A current density of 1 mA / cm² was used. 2The potential corresponding to this time is the oxygen evolution potential, and the electrochemical window is the difference between the oxygen evolution potential and 0V.

[0069] 4) A three-electrode system is adopted, with modified foamed silicon carbide as the working electrode (exposed area 1 cm²). 2 A platinum sheet was used as the auxiliary electrode, Ag / AgCl as the reference electrode, and a 1M H₂SO₄ solution was used as the electrolyte. A galvanometer was used to apply an electrode at a rate of 1 A / cm². 2 Electrolysis was performed at a constant current density. The potential change of the working electrode relative to the reference electrode was monitored at regular intervals (every 10 hours in the initial stage and every 50 hours in the later stage). The total time it took for the working electrode potential to rise to 10V (vs Ag / AgCl) was defined as the accelerated lifetime.

[0070] 5) The open-cell porosity of the original and modified foamed silicon carbide was determined using the drainage method. Pore retention rate = (modified porosity / original porosity) × 100%.

[0071] The test results are shown in the table below: Table 1. Coating performance test results

[0072] Table 2 Electrochemical performance test results

[0073] Analysis of the data obtained from the examples and comparative examples: Example 1 achieved optimal performance in terms of coating properties, electrochemical window, and accelerated lifetime. This is because the ratio of PCS carbon source supply to boron source precisely meets the stoichiometric requirements of carbothermal reduction; the sufficient number of B4C seed crystals and the adequate PTMS modification ensure uniform dispersion and effective induction of the seed crystals; the continuous and dense POSS toughening network is sufficient to absorb the thermal mismatch stress at the microcrystalline-amorphous interface; and the moderate solid content balances coating thickness and shrinkage stress. The three-stage gradient doping and temperature-dependent coupling enable the BDD coating to form a complete functional gradient structure from the bottom layer to the outer layer.

[0074] Example 2 reduced the PCS dosage to the lower limit. The reduced carbon source supply directly led to incomplete carbothermic reduction, decreased B4C and SiC crystallite formation, and a lower crystallite integral number in the transition layer. The reduced crystallite content shifted the thermal expansion coefficient of the transition layer towards the amorphous glass end, weakening the thermal matching with the BDD coating. Simultaneously, unreduced B2O3 remained in the glass matrix and was slowly reduced in the CVD hydrogen plasma environment, slightly weakening the interfacial stability during long-term service. However, since the PBS main network and POSS toughening structure remained intact, the performance degradation was limited. This indicates that the lower limit of PCS dosage still meets the basic microcrystallization requirements, but it deviates from the optimal stoichiometry for carbothermic reduction.

[0075] Example 3 increased the PCS dosage to the maximum. Excessive carbon source resulted in free carbon remaining in the transition layer after carbothermal reduction. This required additional atomic hydrogen for etching and removal during the initial stages of CVD, prolonging the effective nucleation induction time. 3 The purity decreased slightly. Simultaneously, excess CO gas escaped and formed dense but small microporous channels within the transition layer, slightly reducing the coating's density. SEM cross-sections revealed occasional micropores and isolated microcracks in the coating. This indicates that observable performance degradation occurred at the upper limit of PCS dosage, and excessive carbon source negatively impacted both the density of the transition layer and subsequent nucleation efficiency.

[0076] Example 4 reduced the amount of boric acid to the lower limit. The reduction in the additional boron source decreased the total amount of B2O3 that can be converted to B4C. With sufficient carbon source, crystallization is limited by boron supply, resulting in a decrease in the microcrystal integral number of the transition layer. The reduced microcrystal content weakens the ability to regulate the thermal expansion coefficient of the transition layer, but the basic gradient structure and POSS toughening network of the transition layer remain intact, with only a slight degree of performance degradation. This demonstrates that the amount of boric acid is an effective lever for controlling the microcrystal content, and the lower limit still meets basic requirements.

[0077] Example 5 increased the amount of boric acid to its maximum. Increasing the boron source increased the microcrystal integral number, but also increased the difference in thermal expansion coefficients between the B4C microcrystals and the amorphous glass matrix, leading to a rise in cooling stress at the crystalline-amorphous interface. While the POSS toughening network could partially absorb this additional stress, it was nearing its compensation limit—SEM cross-sections showed microcracks deflecting along grain boundaries in the microcrystal-rich region. This indicates that at the maximum boric acid dosage, the microcrystal content was close to the critical value for POSS toughening capability.

[0078] Example 6 reduced the solid content to the lower limit. Lowering the impregnation solution concentration reduced the amount of material deposited in a single impregnation, resulting in a thinner overall transition layer. Simultaneously, the total number of nucleation active sites in the transition layer decreased with decreasing thickness. This indicates that while a lower solid content limit can yield a defect-free transition layer, the thickness is insufficient to provide adequate thermal expansion buffering.

[0079] Example 7 increased the solid content to the maximum limit. Increasing the impregnation solution concentration thickened the transition layer, but the absolute amount of volume shrinkage associated with the organic-inorganic conversion also increased. The accumulated tensile stress within the coating exceeded the POSS toughening threshold, and shrinkage microcracks were observed in the cross-section. Isolated microcracks also appeared within the BDD film in the thick coating area.

[0080] Example 8 simultaneously reduced the amounts of B4C seed crystals and PTMS to their lower limits. The reduced seed crystal quantity resulted in insufficient non-uniform nucleation sites for the microcrystals, leading to a decrease in the amount of boron carbide microcrystals precipitated. The reduced PTMS amount weakened the sufficiency of seed crystal surface modification, decreasing the uniformity of seed crystal dispersion in the PBS sol. The combined effect of these two factors resulted in both the degree of microcrystallization and uniformity of the transition layer being weaker than the optimal values. This indicates that when both seed crystals and modifiers are used at their lower limits, both microcrystal induction efficiency and dispersibility are weakened, but still within acceptable ranges.

[0081] Example 9 simultaneously increased the amount of B4C seed crystals and PTMS to their maximum limits. Increasing the number of seed crystals provided more nucleation sites, leading to a higher microcrystal integral number. However, excessively high seed crystal content made it difficult to completely avoid weak agglomeration under the limited viscosity of the impregnation solution. The excessively high crystalline phase density in these microregions resulted in deteriorated thermal matching with the amorphous matrix. This indicates that the uniformity of B4C seed crystal dispersion began to be challenged when the maximum limit was reached.

[0082] Example 10 represents the case with the most significant performance degradation in parameter shift. Reducing the POSS dosage to the lower limit resulted in excessively large spacing between SiO2 nanoclusters after pyrolysis, disrupting the continuity of the toughening network. The thermal mismatch stress between the microcrystalline B4C / SiC and the amorphous glass matrix could not be adequately released at the nanoscale, leading to penetrating microcracks in the transition layer. These cracks further propagated during long-term service, resulting in a greater reduction in accelerated lifespan. Cross-sections showed that the microcrack level in the transition layer increased to level 3. This clearly demonstrates that POSS toughening is a crucial guarantee for maintaining the structural integrity of the transition layer, and the lower limit value is approaching the critical failure point.

[0083] Example 11 increased the amount of POSS to the maximum. The toughening network became denser and the crack resistance of the transition layer was enhanced. However, the increase in POSS content increased the viscosity of the impregnation solution and slightly reduced the penetration depth into the smallest pore size region of the foamed silicon carbide. The thickness of the transition layer at the deepest pore was thinner. This indicates that the toughening effect was sufficient at the upper limit of POSS, but the impregnation penetration began to be slightly affected.

[0084] Example 12 reduced the hydrogen-to-carbon volume ratio to the lower limit. The relatively increased carbon source concentration accelerated the diamond growth rate and simultaneously increased the film thickness. However, the decreased relative concentration of atomic hydrogen led to a weakened etching selectivity for non-diamond carbon, sp 3 Decreasing content narrows the electrochemical window, accelerating lifetime decline. Occasionally, micro-gaps and growth steps are observed within the BDD film in the cross-section. This indicates that measurable degradation of coating quality has already occurred at the lower limit of the hydrogen-to-carbon ratio.

[0085] Example 13 increases the hydrogen-to-carbon volume ratio to its maximum. Sufficient atomic hydrogen concentration ensures extremely thorough etching of non-diamond carbon, sp 3Both purity and electrochemical window reached optimal levels. However, at excessively high hydrogen-to-carbon ratios, the weak etching effect of atomic hydrogen on the diamond surface reduced the effective growth rate, while the film thickness was relatively thin. The film's long-term service tolerance was slightly insufficient, and the accelerated lifetime was slightly lower than the optimal value. This indicates that at the upper limit of the hydrogen-to-carbon ratio, the coating quality is excellent, but the growth rate becomes the limiting factor.

[0086] Comparative Example 1 completely eliminated the construction of the borosilicate gradient transition layer. The difference in thermal expansion coefficients between the SiC substrate and the diamond coating generated huge interfacial thermal stress during CVD cooling, resulting in a dense network of through-cracks in the BDD coating and large-area delamination. The bare SiC surface lacked uniform nucleation sites, and the diamond seeds relied only on weak physical adsorption, leading to extremely large differences in film thickness. Cracks and numerous non-diamond carbon grain boundaries together severely narrowed the electrochemical window, allowing the electrolyte to rapidly penetrate along the cracks to the substrate interface, accelerating the extremely short lifetime. This fully demonstrates that the borosilicate gradient transition layer plays a fundamental and necessary role in achieving high-quality BDD coating modification of foamed silicon carbide surfaces.

[0087] Comparative Example 2 used a hybrid sol without B4C seeds. Without seed induction, the homogeneous nucleation kinetics of B4C and SiC at 1150℃ were almost frozen, resulting in an undesigned microcrystalline-glass gradient composite structure. The ability to adjust the thermal expansion coefficient of the homogeneous transition layer was significantly weakened, and the lack of surface microcrystals caused the loss of the nano-roughness anchoring effect of seed adsorption, relying solely on the electrostatic adsorption of residual hydroxyl groups. This fully demonstrates that the embedding of nano-B4C seeds plays an irreplaceable role in inducing low-temperature microcrystallization and constructing the gradient transition layer.

[0088] Comparative Example 3 employed a hybrid sol without POSS. The chemical composition and crystallite integral number of the transition layer were similar to those of Example 1, but the nano-SiO2 toughening mechanism was completely lost. The thermal mismatch stress between the crystallites and the amorphous glass substrate had nowhere to be released during cooling, forming a high-density, penetrating crack network in the transition layer. The cracks propagated along the crystallite-glass interface and interconnected, penetrating in multiple places and extending into the BDD film. During long-term electrochemical service, the crack network provided electrolyte penetration channels, gradually weakening the interface until complete failure. This clearly demonstrates the crucial role of POSS toughening in ensuring the structural integrity and long-term service stability of the transition layer.

[0089] Comparative Example 4 eliminated the coupling control of segmented gradient doping and temperature variation, employing constant-temperature, constant-boron-concentration deposition throughout. While uniform doping ensured consistent boron content across the BDD coating from the bottom layer to the outermost layer, the lack of the temperature variation synergy between the high-temperature stage promoting initial nucleation and the low-temperature stage densification resulted in uneven grain size distribution and significantly increased batch-to-batch resistivity fluctuations. This clearly demonstrates the necessity of three-stage temperature-boron-concentration coupled gradient doping for achieving a balance between conductivity and corrosion resistance.

[0090] Comparative Example 5 employed a conventional B4C+Si powder embedding method for borosilicate co-infiltration at 1400℃. At high temperatures, the low-viscosity liquid silicon generated by the melting of silicon powder infiltrates extensively into the pores of the foamed silicon carbide through capillary force and solidifies upon cooling, severely clogging micropores and pores, resulting in a significant loss of effective specific surface area. The mass transfer limitations of the powder physical embedding resulted in an extremely thick outer surface of the transition layer while the deep pores were almost uncoated, leading to extremely poor thickness uniformity. High temperatures caused complete surface ceramization, with the seed material relying solely on physical adsorption and exhibiting very low density. The triple defects of pore blockage, surface inertization, and uneven coating resulted in a loss of effective electrochemical area, decreased adhesion, and shortened lifespan. However, a SiC+B4C ceramic layer was indeed formed, demonstrating the advantages of the liquid-phase impregnation-medium-temperature segmented heat treatment scheme of this invention in avoiding the inherent problems of high-temperature pore blockage and uneven distribution in traditional methods.

[0091] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A technology for modifying silicon carbide foam surface with boron-doped diamond by vapor deposition, characterized in that, The process includes the following steps: Step S1: Preparation of the transition layer impregnation solution: Polycarbosilane and xylene are mixed and heated and stirred to dissolve. Polyboron siloxane hybrid sol, boric acid and yttrium nitrate hexahydrate are added to form a multi-component compound. The solid content is adjusted and ultrasonically dispersed to obtain the transition layer impregnation solution. Step S2: Impregnation of foamed silicon carbide substrate: After cleaning and drying the foamed silicon carbide, it is impregnated in the impregnation solution prepared in step S1 under vacuum-pressure conditions, and the excess liquid is removed by centrifugation to obtain impregnated silicon carbide. Step S3: Segmented heat treatment of transition layer: After aging and drying, the impregnated silicon carbide is subjected to segmented programmed temperature rise heat treatment to generate a borosilicate gradient transition layer on the surface of the foam silicon carbide pore wall, thus obtaining pretreated foam silicon carbide. Step S4: Diamond suspension preparation and adsorption: Prepare a mixed suspension of dual-particle-size nanodiamonds, immerse the pretreated foam silicon carbide obtained in step S3 in the suspension and ultrasonically disperse it, then dry it to achieve uniform adsorption of nucleated seeds inside and outside the pores. Step S5: Atmospheric pressure wide temperature zone vapor phase segmentation deposition: Using a composite deposition equipment, the foam silicon carbide after seed adsorption in step S4 is subjected to staged temperature-stage boron doping vapor phase deposition under atmospheric pressure to form a gradient doped boron-doped diamond coating. Step S6: Low-temperature annealing post-treatment: After the foamed silicon carbide is deposited in step S5, it is placed in an inert atmosphere and subjected to low-temperature annealing to eliminate residual thermal stress and complete the surface modification of the foamed silicon carbide.

2. The technology for modifying foamed silicon carbide surface with boron-doped diamond by vapor deposition according to claim 1, characterized in that: In step S1: the weight ratio of polycarbosilane, polyboronsiloxane hybrid sol, boric acid and yttrium nitrate hexahydrate is 12-14:70:9-10:0.3; the solid content is adjusted to 15-20 wt%.

3. The boron-doped diamond modification technology for the surface of foamed silicon carbide by vapor deposition according to claim 1, characterized in that: In step S3: the segmented heat treatment process is as follows: the temperature is increased to 200℃ at 1℃ / min and held for 1h, then increased to 600℃ at 0.5℃ / min and held for 1h, and finally increased to 1150℃ at 2℃ / min and held for 2h. After the holding is completed, the temperature is cooled to 500℃ at 3℃ / min and then cooled to room temperature with the furnace.

4. The boron-doped diamond modification technology for the surface of foamed silicon carbide by vapor deposition according to claim 1, characterized in that: In step S4: the mass ratio of 5-10nm and 15-20nm nanodiamonds is 2:1, and the mass concentration of the mixed suspension of dual-size nanodiamonds is 0.02-0.04g / L.

5. The technology for modifying foamed silicon carbide surface with boron-doped diamond by vapor deposition according to claim 1, characterized in that: In step S5: H2, CH4, trimethylboron, and CO2 are used as mixed reaction gases, wherein the volume ratio of H2 to CH4 is 8-12:1, and the volume ratio of CO2 to CH4 is 1:20; the deposition process is divided into three stages: initial stage, intermediate stage, and final stage, with the B / C ratio controlled. The initial stage B / C ratio is 32000ppm, the intermediate stage B / C ratio decreases linearly to 18000ppm, and the final stage B / C ratio is 10000ppm; the deposition time ratio of the initial stage, intermediate stage, and final stage is 1:3:

1.

6. The boron-doped diamond modification technology for the surface of foamed silicon carbide by vapor deposition according to claim 1, characterized in that: The polyboron siloxane hybrid sol is prepared by the following steps: Step A1: Mix nano boron carbide powder and anhydrous ethanol, ultrasonically disperse at room temperature for 30 min, then add phenyltrimethoxysilane and continue ultrasonic dispersion for 60 min, and then dry at 60°C to a viscous slurry to obtain modified boron carbide slurry. Step A2: Mix tetraethyl orthosilicate, methyltrimethoxysilane and diphenyldimethoxysilane, and add them to a citric acid aqueous solution with a pH of 3. Stir at 40°C for 45 min, then add phenylboronic acid and heat to 60°C. Continue stirring for 4 h to obtain polyborosiloxane sol. Step A3: Mix the polyborosiloxane sol and the modified boron carbide slurry and disperse them ultrasonically. React at 50°C for 2 hours, then add the monoepoxy cage-type silsesquioxane and continue stirring for 30 minutes to obtain the polyborosiloxane hybrid sol.

7. The boron-doped diamond modification technology for the surface of foamed silicon carbide by vapor deposition according to claim 6, characterized in that: In step A1: the ratio of boron carbide nanopowder, anhydrous ethanol, and phenyltrimethoxysilane is 0.8-1g: 10mL: 0.1-0.12g; In step A2: the ratio of tetraethyl orthosilicate, methyltrimethoxysilane, diphenyldimethoxysilane, tetrahydrofuran, citric acid aqueous solution, and phenylboronic acid is 20-21g: 27-28g: 24g: 15mL: 10.40g: 9.5-10g; In step A3, the ratio of polyboron siloxane sol, modified boron carbide paste, and monoepoxy cage-type silsesquioxane is 55-60g: 5g: 1.5-2g.

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