A nano-ceramic protective coating for photovoltaic diffusion furnace tubes containing boron nitride and a method for producing the same

Abstract

This invention discloses a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes and its preparation method. The coating has a three-layer structure, comprising a base layer, a transition layer, and a top layer from the inside out. The base layer consists of hexagonal boron nitride powder and silica sol. The base layer also includes rare earth oxides selected from one or more of yttrium oxide, lanthanum oxide, and praseodymium oxide. The transition layer consists of hexagonal boron nitride powder, alumina sol, and cerium oxide. The top layer consists of hexagonal boron nitride powder and borosilicate glass powder. The total coating thickness is 95-110 μm. During preparation, each layer of slurry is sequentially sprayed onto the inner wall of a sandblasted quartz tube and dried. The base layer and transition layer are sintered at 900℃ in an ammonia atmosphere, and the top layer is sintered at 1300℃ in a nitrogen atmosphere. The coating of this invention exhibits strong adhesion to the quartz tube wall, high density, excellent thermal shock resistance and POCl3 corrosion resistance, and its cost is lower than that of the CVD method.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic equipment protection and ceramic coating technology, specifically to a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes and its preparation method. Background Technology

[0002] The diffusion process in photovoltaic cell manufacturing is a crucial step in forming the PN junction. In this process, the quartz reaction tube (also known as the quartz reaction tube used in the photovoltaic diffusion process, commonly called the diffusion furnace tube, or simply the quartz tube) is a core component that needs to operate continuously in a high-temperature environment (typically 800-1200℃) containing corrosive gases such as phosphorus oxychloride (POCl3). At high temperatures, byproducts such as phosphosilicate glass adhere to and corrode the quartz tube, causing it to fog and crack, severely affecting diffusion uniformity and equipment lifespan.

[0003] Currently, existing technologies primarily employ boron nitride (BN)-based coatings to protect the inner wall of quartz tubes. Boron nitride possesses extremely high heat resistance, chemical inertness, and a coefficient of thermal expansion similar to quartz, making it an ideal protective material. For example, existing technologies deposit boron nitride coatings on the substrate surface via chemical vapor deposition (CVD). While this method produces high-purity coatings, CVD equipment is expensive, deposition rates are low, and the complex geometry of the quartz tube's inner wall makes it difficult to obtain a uniform coating. Other technologies utilize direct coating and sintering of boron nitride slurry; however, coatings prepared using this method often exhibit high porosity and insufficient bonding strength between the coating and the quartz tube wall, making them prone to peeling under frequent temperature fluctuations.

[0004] A search revealed that patent CN119038881A discloses a ceramic coating for increasing the lifespan of quartz tubes. Its components include cobalt sulfate, nickel sulfate, and a small amount of boron nitride (only 1-2 parts), falling under the category of composite coatings. BN is used only as an additive, not as the main protective phase, and no optimization is made for the interface bonding between BN and the quartz substrate. Patent CN119841650A discloses a boron nitride insulating ceramic material containing BN, aluminum nitride, zirconium oxide, etc., prepared by hot pressing and sintering. However, this material is a bulk ceramic and does not involve the preparation of a coating on the inner wall of a quartz tube. Furthermore, patent CN121021170B discloses a multi-layer gradient structure composite side sealing plate, confirming the value of gradient structures in ceramic composite materials. However, its application is in the field of thin strip continuous casting, with the substrate being the ceramic plate itself, rather than a coating applied to the surface of the quartz tube.

[0005] Therefore, developing a nano-ceramic protective coating with high bonding strength, dense structure, excellent thermal shock resistance, and relatively low cost has significant industrial value. Summary of the Invention

[0006] The purpose of this invention is to overcome the deficiencies in the prior art and provide a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes. This coating adopts a gradient composite structure and solves the problem of interface mismatch between the BN coating and the quartz substrate by introducing in-situ generated silicon nitride or aluminum nitride as a bonding phase. Another objective of this invention is to provide a method for preparing the above-mentioned coating.

[0007] To achieve the above objectives, the technical solution of this invention is to design a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes. The coating has a three-layer structure, comprising a bottom layer, a transition layer, and a top layer from the inside out. The bottom layer is made of hexagonal boron nitride powder and silica sol. The bottom layer also includes rare earth oxides selected from one or more of yttrium oxide, lanthanum oxide, and praseodymium oxide. The transition layer is made of hexagonal boron nitride powder, aluminum sol, and rare earth oxides selected from one or more of cerium oxide, lanthanum oxide, and praseodymium oxide. The top layer is made of hexagonal boron nitride powder and borosilicate glass powder. The total thickness of the coating is 95-110 μm.

[0008] A further technical solution is that the dry film thickness of the bottom layer is 35±4μm; the dry film thickness of the transition layer is 30±4μm; and the dry film thickness of the top layer is 30±4μm.

[0009] A further technical solution is that, in the bottom layer raw material, the mass ratio of hexagonal boron nitride powder, silica sol, and yttrium oxide is 40:50:10; in the transition layer raw material, the mass ratio of hexagonal boron nitride powder, aluminum sol, and cerium oxide is 60:30:10; and in the surface layer raw material, the mass ratio of hexagonal boron nitride powder to borosilicate glass powder is 80:20.

[0010] A further technical solution is that the hexagonal boron nitride powder has a flake diameter of 200 nm, a thickness of 2 nm, a purity of 99.9%, and a surface carbon impurity content of <0.05%; the silica sol is a methyl-modified silica sol with a solid content of 30%; the aluminum sol is a carbon-containing aluminum oxane-modified aluminum sol with a solid content of 25%; and the softening point of the borosilicate glass powder is 800℃.

[0011] A further technical solution is that the bottom layer is formed by sintering modified precursor powder A in an ammonia atmosphere. The modified precursor powder A is obtained by mixing hexagonal boron nitride powder and methyl modified silica sol, followed by spray drying and pretreatment in a nitrogen atmosphere at 700°C. After sintering, an interface bonding layer containing an amorphous Si-OC structure is formed. The transition layer is formed by sintering modified precursor powder B in an ammonia atmosphere. The modified precursor powder B is obtained by mixing hexagonal boron nitride powder and carbon-containing aluminum oxane modified aluminum sol, followed by the same process. After sintering, an interface bonding layer containing an amorphous Al-OC structure is formed.

[0012] In the bottom layer, the silica sol-modified precursor is sintered in ammonia to generate Si3N4 in situ, which forms a chemical bond with SiO2 on the quartz tube wall; in the transition layer, the aluminum sol-modified precursor is sintered in situ to generate AlN, which plays a role in the transition of composition; the surface layer is mainly composed of high-content BN, combined with borosilicate glass phase to achieve dense surface protection.

[0013] The present invention also provides a method for preparing a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes, comprising the following steps: (1) Sandblast the inner wall of the quartz tube to make its roughness Ra reach 3-5μm; (2) The bottom layer slurry, the transition layer slurry and the top layer slurry are sprayed sequentially on the inner wall of the quartz tube by air spraying, and each layer is dried after spraying; (3) After spraying the base layer, the first stage of sintering is carried out: under an ammonia atmosphere, the temperature is increased to 900℃ at 5℃ / min and held for 1h; (4) After spraying the transition layer, the second stage of sintering is carried out: under an ammonia atmosphere, the temperature is increased to 900℃ at 5℃ / min and held for 30min; (5) After spraying the top layer, the third stage of sintering is carried out: under nitrogen atmosphere, the temperature is raised to 1300℃ at 5℃ / min, held for 20min, and then cooled to obtain the finished product.

[0014] A further technical solution is that the preparation method of the bottom layer slurry, the transition layer slurry and the top layer slurry is as follows: the corresponding raw materials are mixed with anhydrous ethanol at a mass ratio of 1:0.8, and 2% of polyvinyl butyral is added as a binder, and the mixture is ultrasonically dispersed for 30 minutes to obtain a slurry with a solid content of 45%.

[0015] A further technical solution is that the parameters for the air spraying method are: spray gun nozzle diameter 0.8mm, spraying pressure 0.4MPa, and spraying distance 20cm; The base layer is sprayed in 3 coats, each coat having a wet film thickness of 15-20μm. After each coat, it is dried at 100℃ for 10 minutes, and finally the whole thing is dried at 100℃ for 30 minutes. The transition layer and the top layer are each sprayed in two coats.

[0016] A further technical solution is that the flow rate of ammonia in steps (3) and (4) is 5 L / min; and the flow rate of nitrogen in step (5) is 5 L / min.

[0017] The advantages and beneficial effects of this invention are as follows: The bottom layer utilizes in-situ generated Si3N4 / AlN as a transition layer, forming a chemical bond with the inner wall surface of the quartz tube, while being compatible with the BN coating, significantly reducing thermal stress.

[0018] The surface layer, with a high content of BN, combines with a borosilicate glass phase to form a dense glass / ceramic composite layer, effectively blocking POCl3 corrosion.

[0019] Unlike the expensive CVD method, this method uses liquid-phase modification combined with reaction sintering to transform the amorphous precursor into silicon nitride or aluminum nitride reinforcing phases under an ammonia atmosphere.

[0020] The exposed BN nanosheets on the coating surface can reduce the adhesion of byproducts such as phosphosilicate glass at high temperatures. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of an embodiment of the present invention, a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes, applied to a quartz tube; Figure 2 This is a schematic diagram of the coating structure of Embodiment 3 of the present invention; Figure 3 yes Figure 2 A schematic diagram of coating applied to a quartz tube; Figure 4 This is a schematic diagram of Embodiment 4 of the present invention; Figure 5 This is a schematic diagram of step 1 in embodiment 5 of the present invention; Figure 6 This is a schematic diagram of steps 2 and 3 in Embodiment 5 of the present invention; Figure 7 This is a schematic diagram of step 4 in embodiment 5 of the present invention; Figure 8 This is a schematic diagram of step 5 in embodiment 5 of the present invention; Figure 9 This is a schematic diagram of step 7 in embodiment 5 of the present invention. Detailed Implementation

[0022] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0023] Example 1: As Figure 1 As shown, the present invention is a boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes, with a total thickness controlled at 95-110 μm.

[0024] The raw materials are hexagonal boron nitride powder with a diameter of 200 nm and a thickness of 2 nm, silica sol, yttrium oxide, alumina sol, cerium oxide, and borosilicate glass powder. The hexagonal boron nitride powder has a purity of 99.9% and a surface carbon impurity content of <0.05%; the silica sol has a solid content of 30%; the alumina sol has a solid content of 25%; and the borosilicate glass powder has a softening point of approximately 800℃. The silica sol is methyl-modified silica sol, and the alumina sol is carbon-containing aluminum oxane-modified alumina sol.

[0025] The raw materials required for the bottom layer are 40g BN, 50g silica sol, and 10g yttrium oxide. The raw materials required for the transition layer are 60g BN, 30g aluminum sol, and 10g cerium oxide; the raw materials required for the top layer are 80g BN and 20g borosilicate glass powder.

[0026] The preparation steps are as follows: (1) BN was mixed with modified silica sol and spray-dried. The spray-drying parameters were: inlet temperature 210±10℃, outlet temperature 110±10℃, atomization pressure 0.2MPa, and feed rate 5mL / min. Then, it was pretreated at 700℃ under a nitrogen atmosphere for 1h to obtain modified precursor powder A, which was used as the bottom layer. During the heat treatment, an interface bonding layer containing an amorphous Si-OC structure was formed. BN was mixed with modified aluminum sol (solid content 25%, containing carbon aluminum oxane structure) and treated in the same way to obtain modified precursor powder B, which was used as the transition layer. During the heat treatment, an interface bonding layer containing an amorphous Al-OC structure was formed. The surface layer raw material BN was mixed evenly with borosilicate glass powder to obtain powder C.

[0027] (2) Mix precursor powder A with anhydrous ethanol at a mass ratio of 1:0.8, add 2% polyvinyl butyral (PVB) as a binder, and ultrasonically disperse for 30 min to obtain a primer slurry with a solid content of about 45%. Prepare a transition layer slurry for spraying using precursor powder B in the same way. Mix mixed powder C with anhydrous ethanol at a mass ratio of 1:0.8, add 2% PVB, and ultrasonically disperse for 30 min to obtain a topcoat slurry for spraying.

[0028] (3) The quartz tube is a quartz tube for photovoltaic diffusion furnace, with an inner diameter of 300 mm and a length of 2000 mm. The inner wall of the quartz tube is sandblasted with 60-mesh white corundum sand, and the roughness Ra is required to reach 3-5 μm. The surface dust is blown away with compressed air.

[0029] (4) The air spraying method is adopted, with a spray gun nozzle diameter of 0.8 mm, a spraying pressure of 0.4 MPa, and a spraying distance of 20 cm. The base layer is sprayed in 3 coats, with the wet film thickness of each coat controlled at 15-20 μm. After each coat, it is dried in an oven at 100℃ for 10 min, and finally the whole layer is dried at 100℃ for 30 min. The total dry film thickness of the base layer is 35±4 μm.

[0030] (5) Place the quartz tube with the sprayed base layer in a tube furnace and heat it to 900°C at a rate of 5°C / min. Introduce ammonia gas at a flow rate of 5L / min and hold for 1 hour. This allows the amorphous Si-OC structure in the precursor to nitridate under the action of ammonia gas, generating the Si3N4 crystalline phase in situ. This forms an interface bonding layer dominated by Si-N covalent bonds. XRD can detect the characteristic peaks of Si3N4, thus improving the bonding strength with the quartz matrix. Cool the furnace to room temperature.

[0031] (6) The transition layer was sprayed using the same method as the base coat, in two coats, with a total thickness of 30±4μm after each coat dried. The mixture was then placed in a tube furnace and heated to 900℃ at 5℃ / min, sintered for 30min under an ammonia atmosphere. This nitrided the amorphous Al-OC structure in the precursor under the action of ammonia, generating an in-situ AlN crystalline phase and forming an interface bonding layer dominated by Al-N covalent bonds. The mixture was then cooled to room temperature. The topcoat was sprayed using the same method, in two coats, with a total thickness of 30±4μm. The mixture was placed in a tube furnace and heated to 1300℃ at 5℃ / min, with nitrogen gas introduced at a flow rate of 5L / min. The mixture was held at this temperature for 20min to melt the borosilicate glass powder. The liquid glass flowed and filled the pores in the coating, forming a dense, continuous glassy phase upon cooling, thus achieving complete densification of the coating. Under nitrogen protection, BN remained stable below 1300℃ without significant decomposition. The mixture was then cooled to room temperature with the furnace to obtain the finished product.

[0032] The prepared coating has a dense, crack-free surface, and the critical load for adhesion by the scratch test is >60N, which is significantly higher than that of the uncoated quartz tube (<20N). After corrosion at 1000℃ in a POCl3 atmosphere for 100 hours, the coating did not peel off, and the light transmittance of the quartz tube did not decrease significantly.

[0033] A quartz tube coated with the coating of the present invention was heated to 1100°C and kept at that temperature for 30 minutes. Then it was quickly removed and immersed in room temperature water at 25°C for rapid cooling. This is one thermal shock cycle. After repeating this cycle 50 times, the coating surface showed no cracks, no peeling, and no flaking, indicating that the coating has excellent thermal shock resistance.

[0034] Example 2: The difference from Example 1 is that the bottom layer requires 30g BN, 60g methyl-modified silica sol, and 10g yttrium oxide; the transition layer requires 70g BN, 20g carbon-containing aluminum oxane-modified aluminum sol, and 10g cerium oxide; and the top layer requires 90g BN and 10g borosilicate glass powder. The preparation method is the same as in Example 1, except that the bottom layer precursor forms an Al-N-based structure under an ammonia atmosphere, and the transition layer precursor forms a Si-N-based structure. The thermal shock resistance reaches over 55 cycles.

[0035] It should be noted that the modified silica sol and modified aluminum sol mentioned above are carbon-containing organic-inorganic hybrid sols commonly used in the field, and can be prepared by conventional methods such as methyltriethoxysilane and aluminoxane. The specific source does not constitute a limitation on the present invention.

[0036] Example 3: The difference from Example 1 is that, as shown in Example 3... Figure 2 , Figure 3 As shown, the raw materials are: hexagonal boron nitride nanosheets, 200 nm in diameter, 2 nm in thickness, 99.9% purity, and less than 0.05% carbon content; silica sol, methyl-modified silica sol, with a solid content of 30%; alumina sol, carbon-containing aluminum oxane-modified alumina sol, with a solid content of 25%; yttrium oxide as the sintering aid; cerium oxide as the rare earth oxide; borosilicate glass powder with a softening point of approximately 800℃; and polymethyl methacrylate microspheres (PMMA microspheres), with a particle size of 200-400 nm, as the pore-forming agent.

[0037] The formulation is as follows: The bottom layer consists of 35g hexagonal boron nitride, 55g silica sol, 10g yttrium oxide, and 5g PMMA microspheres. The transition layer consists of 55g hexagonal boron nitride, 30g aluminum sol, 10g cerium oxide, and 3g PMMA microspheres. The top layer consists of 80g hexagonal boron nitride and 20g borosilicate glass powder. No pore-forming agent is added. During sintering at 1300℃, the liquid phase formed by the melting of the borosilicate glass powder mainly fills the pores of the top layer itself. Due to its high viscosity, it does not penetrate into the micro-nano pores of the bottom and transition layers.

[0038] The preparation steps are as follows: (1) Preparation of precursor The preparation method of the underlying precursor is as follows: BN is mixed with silica sol, and PMMA microspheres accounting for 5% of the BN mass are added as a pore-forming agent. Spray drying is then performed, with the inlet temperature set at 210±10℃, the outlet temperature at 110±10℃, the atomization pressure at 0.2MPa, and the feed rate at 5mL / min. After collecting the powder, it is first kept at 350℃ in a nitrogen atmosphere for 1h to allow the PMMA microspheres to completely decompose and escape, forming controllable micro-nano pores inside the particles; subsequently, it is heat-treated at 700℃ in a nitrogen atmosphere for 1h to stabilize the amorphous Si-OC interface layer, thus obtaining the modified precursor powder Al.

[0039] The transition layer precursor is made by mixing BN with aluminum sol, adding PMMA microspheres accounting for 3% of the mass of BN, and processing it according to the same process: first removing the pore-forming agent at 350℃, and then heat-treating it at 700℃ under a nitrogen atmosphere for 1 hour to stabilize the amorphous Al-OC interface layer, thus obtaining the modified precursor powder B1.

[0040] The surface powder is obtained by uniformly mixing BN with borosilicate glass powder without adding a pore-forming agent, resulting in powder C1.

[0041] (2) Slurry preparation The three-layer slurry was prepared according to the method in Example 1, with a solid content of 45% and a PVB binder content of 2%.

[0042] (3) Spraying and sintering The spraying parameters were the same as in Example 1. The thickness of the base layer was 35±4μm, the thickness of the transition layer was 30±4μm, and the thickness of the top layer was 30±4μm. The first stage of sintering was carried out in an ammonia atmosphere at a temperature of 900℃ and an ammonia flow rate of 5L / min for 1 hour; this caused partial nitriding of the amorphous Si-OC and Al-OC structures in the precursor under the action of ammonia, forming an interface layer mainly composed of Si-N and Al-N bonds. The second stage of sintering was carried out in a nitrogen atmosphere at a temperature of 1300℃ and a nitrogen flow rate of 5L / min for 20 minutes.

[0043] The structural features of the nano-ceramic protective coating prepared in this embodiment are as follows: the bottom layer and the transition layer contain micro- and nano-pores with a volume fraction of about 8% to 12%. Due to the slight sintering shrinkage during the 700℃ heat treatment process, the average pore size is reduced by about 20%-30% compared with the original PMMA particle size, and finally stabilizes at 100-300nm. Its porosity can be controlled by the amount of PMMA added. The top layer is a dense, non-porous structure, completely filled by the glass phase, and the total thickness of the entire coating is maintained at 95 to 110μm.

[0044] Performance test results show that, compared to Example 1 without pore-forming agent, the critical load for bonding force in Example 3, although slightly reduced from >60N to >58N, remains within an acceptable range. In the severe thermal shock cycling from 1100℃ to 25℃ with water cooling, the thermal shock resistance of Example 3 increased significantly from approximately 50 cycles to 80 cycles. Furthermore, both exhibited dense, crack-free surface density and showed no spalling during the 100h POCl3 corrosion test, demonstrating considerable corrosion resistance. The thermal shock resistance of Example 3 increased from approximately 50 cycles to 80 cycles, an improvement of 60%, while corrosion resistance remained largely unaffected. This demonstrates that the micropores in the bottom / transition layer effectively buffer stress, while the dense glassy phase of the surface layer ensures a barrier effect.

[0045] As a control, the same raw material ratio and spraying process as in Example 3 were used, only the sintering regime was changed: the fully densified control used the same raw material ratio and spraying process as in Example 3, but the bottom layer and transition layer were sintered at 1300°C in a nitrogen atmosphere to completely close the pores and form a dense structure; the top layer was still sintered at 1300°C in a nitrogen atmosphere using the same process. The results showed that the coating peeled off after only 15 thermal shock cycles. The reason was that the overall stiffness of the coating was too high, which could not coordinate the strain generated by thermal shock, ultimately leading to brittle cracking.

[0046] The inventors unexpectedly discovered that the appropriate amount of micro- and nano-pores retained in the bottom and transition layers can effectively absorb and buffer the difference in thermal expansion between the coating and the quartz substrate. During the intense thermal shock of water cooling from 1100°C to room temperature, these pores act as stress-relieving zones, preventing stress concentration at the interface. Conversely, if the bottom / transition layer is completely dense, sintering at higher temperatures may lead to spalling due to excessively high elastic modulus and an inability to coordinate strain. This embodiment uses reverse thinking, utilizing the non-dense structure of the bottom / transition layer to solve the problem of thermal expansion coefficient mismatch between the coating and the substrate.

[0047] Those skilled in the art generally believe that ceramic coatings should be as dense as possible, with porosity considered a defect. This invention takes the opposite approach, actively retaining porosity in the bottom / transition layer. Introducing porosity increases thermal shock resistance from approximately 50 cycles to 80 cycles, an increase of 60%, without significantly affecting corrosion resistance. The porosity in the bottom / transition layer buffers stress, while the dense glassy phase in the top layer blocks corrosion; the two layers each perform their specific functions and work together to achieve excellent results. In particular, comparative verification using a fully densed coating shows that the thermal shock life of a non-porous coating is only 15 cycles, fully demonstrating the necessity of retaining porosity to improve thermal shock resistance.

[0048] Example 4: The inner wall of a quartz tube, when untreated, has an extremely smooth surface with Ra < 0.5 μm and low surface energy. BN-based slurries are difficult to wet and spread on this surface, resulting in poor coating adhesion and even the inability to form a continuous coating. Existing technologies typically employ hydrofluoric acid etching to increase the surface roughness of the quartz tube's inner wall, or plasma treatment to improve surface energy. The problems with these methods are that HF ​​etching is highly hazardous to the environment and operators, and the etching depth is difficult to control, easily leading to thinning of the quartz tube wall and a decrease in strength. Plasma treatment equipment is expensive, and the treatment effect is uneven for large-sized tubes.

[0049] Quartz glass begins to soften above 1200℃, with a softening point of approximately 1650℃, but its long-term service temperature typically does not exceed 1200℃. During the second-stage sintering at 1300℃, the inner surface of the quartz tube undergoes slight softening. If the thermomechanical properties of the coating, such as its elastic modulus and creep behavior, do not match those of the softened quartz, the coating may be compressed or stretched during cooling due to the quartz's contraction, leading to cracking. Existing technologies generally address this by lowering the sintering temperature to prevent quartz softening or by using low-temperature sintering aids. The problem with this approach is that lowering the sintering temperature results in insufficient coating densification and reduced corrosion resistance.

[0050] The inventors unexpectedly discovered that the slight softening of the quartz tube wall during the second stage of sintering at 1300℃ could enable surface atoms to migrate. In this state, the Si3N4 / AlN crystal nuclei formed in the early stages of the coating substrate grow rapidly at 1300℃, their tips directly penetrating the softened quartz substrate surface during growth. After cooling, the quartz regains its rigidity, while the in-situ grown Si3N4 / AlN whiskers, which bind the coating to the substrate, are firmly fixed to the quartz surface, forming a mechanical interlocking structure similar to tree roots embedded in soil. This achieves an anchoring effect far exceeding that of simple sandblasting. This mechanism transforms the softening deformation, which could otherwise lead to coating failure, into a key factor enhancing interfacial bonding strength.

[0051] The difference between this embodiment and Embodiment 1 is that, as Figure 4 As shown, instead of using sandblasting roughening or only using light roughening, the surface softening effect of the quartz tube wall during high-temperature sintering at 1300℃ is utilized to achieve in-situ mechanical interlocking between the coating and the substrate.

[0052] The raw materials and formulation, precursor preparation and slurry preparation in this embodiment are the same as those in Example 1. The main difference lies in the matrix treatment method and the high-temperature sintering process.

[0053] During the substrate treatment stage, the inner wall of the quartz tube is lightly sandblasted using 120-mesh white corundum sand to control the surface roughness Ra at 1-2 μm; or in a very few cases where the surface finish requirement is extremely high, sandblasting may not be performed, and only ethanol cleaning may be used. In this case, the inner wall roughness Ra < 0.5 μm, and the in-situ interlocking effect is used to compensate for the lack of low roughness.

[0054] After the precursor preparation is completed, a base coat is first applied, with the thickness controlled at 35±4μm, and then dried at 100℃. Subsequently, it is placed in an ammonia atmosphere, heated to 900℃ at 5℃ / min and held for 1h to promote the transformation of amorphous Si-OC to Si3N4 phase, forming silicon nitride nuclei in the coating; at this temperature, the quartz tube wall has not yet softened, and the nuclei exist stably on the rigid substrate.

[0055] Next, a transition layer is sprayed with a thickness of 30±4μm. After drying, it is placed in an ammonia atmosphere and heated to 1000℃ at 5℃ / min and held for 30min to transform the amorphous Al-OC phase into the AlN phase and form aluminum nitride crystal nuclei.

[0056] The surface coating thickness was also controlled at 30±4μm. Subsequently, the temperature was increased to 1300℃ at a rate of 5℃ / min and held for 20min in a nitrogen atmosphere. During sintering, a special high-temperature resistant support was used to provide multi-point support for the quartz tube to prevent high-temperature deformation; and the short holding time at 1300℃ (20min) meant that only the surface layer of the quartz tube underwent viscoelastic deformation at a micrometer-level depth, maintaining the overall structural integrity. At this temperature, borosilicate glass powder melted and filled the pores, densifying the coating; simultaneously, the inner surface of the quartz tube softened slightly, and surface atoms gained migration capabilities. The resulting Si3N4 / AlN whiskers were nanoneedle-shaped, approximately 500-800nm ​​in length and 20-50nm in diameter, exhibiting a large aspect ratio. In the 1300℃ interfacial reaction, the crystal nuclei rapidly grew into whiskers driven by the concentration gradient and interfacial energy, with their tips directly penetrating the softened quartz matrix surface during growth, achieving mechanical locking. After cooling in the furnace, the quartz regains its rigidity, and the whiskers are mechanically locked to the quartz surface.

[0057] The final coating has a total thickness of 95-110 μm and a dense surface layer. There is no obvious boundary line at the interface between the coating and the quartz substrate; instead, a gradient interlocking region with a thickness of 0.5-2 μm is formed. One end of the Si3N4 / AlN whisker is located inside the coating, and the other end is embedded in the quartz tube wall, achieving a strong bond between the coating and the substrate.

[0058] Performance test results show that Example 1 (roughened by sandblasting and subjected to only 900℃ heat treatment) and Example 4 (lightly roughened and formed in-situ interlocking using a softening layer at 1300℃) exhibit significant differences in several key indicators. Regarding the interface bonding morphology, Example 1 primarily relies on mechanical interlocking, exhibiting a microscopic uneven structure; while Example 4 forms a nanoscale in-situ mechanical interlocking structure, characterized by whisker embedding in the matrix, resulting in a denser and stronger interface bonding. Correspondingly, the critical load for bonding force increases from 60N in Example 1 to 75N in Example 4, an increase of approximately 25%. In terms of thermal shock resistance, Example 1 can withstand 50 thermal shock cycles without peeling, while Example 4 can withstand 65 cycles, an increase of approximately 30%. Regarding the strength of the quartz tube itself, Example 1 experiences a slight decrease in strength due to surface microcracks introduced by sandblasting; Example 4, by avoiding strong sandblasting, maintains the strength of the quartz tube intact, demonstrating a significant additional advantage. In terms of long-term corrosion resistance, neither exhibited spalling after a 100-hour POCl3 corrosion test, showing comparable performance. The results indicate that, under mild sandblasting (Ra 1-2 μm) conditions, Example 4 utilized the in-situ mechanical interlocking effect formed by high-temperature softening to significantly increase the bonding strength from >60N in Example 1 to >75N, and the thermal shock resistance increased from 50 cycles to 65 cycles. Even under conditions of cleaning only without sandblasting, the bonding strength remained above >65N, verifying the dominant role of the in-situ interlocking effect. This is attributed to the nanoscale in-situ interlocking structure formed by softening, while simultaneously avoiding damage to the quartz tube wall strength caused by sandblasting.

[0059] Example 5: The difference from Example 1 is that, as shown in Example 5... Figures 5 to 9 As shown in the figure, in this embodiment, the quartz tube is segmented and cut according to the length of the furnace during the preparation process.

[0060] The specific steps are as follows: Step 1: Quartz tube segment cutting (actively adapting to furnace length).

[0061] First, the effective length of the furnace chamber of the standard tubular furnace used is measured. In this embodiment, a furnace chamber length of 1000mm is selected. A 3000mm long photovoltaic diffusion furnace quartz tube (this invention applies to quartz tubes of 2000–4000mm) is cut into 4 sections, each 750mm long. A gap of approximately 200mm is left between the top of the tube section and the top of the furnace chamber (furnace chamber length 1000mm minus tube section length 750mm, minus approximately 50mm for the bottom groove). This gap is used for lance operation, airflow circulation, thermal expansion compensation, and loading / unloading operations. All cut ends are chamfered with a chamfer size of 1mm × 45° to reduce stress concentration and prevent cracking. At the same time, the larger top space facilitates the safe and convenient removal of the tube section after sintering. The bottom of the tube section is placed in the positioning groove of the furnace bottom plate. The groove depth is 3mm, which can achieve reliable positioning without occupying additional furnace chamber height space.

[0062] Step 2: Stand upright and fix it to the process tray (gravity self-stabilizing, no clamps required). Each section of quartz tube is placed vertically on the bottom plate of the tubular furnace. The bottom plate of the furnace is a flat surface, and the lower end of the quartz tube is in direct contact with the bottom plate, maintaining its vertical position by its own weight, without the use of any external clamps.

[0063] Multiple quartz tubes are arranged in an array inside the furnace, for example, in a three-row, three-column arrangement, with a gap of 10 to 20 mm between adjacent tube sections to ensure that they do not come into contact with each other, so as to avoid heat transfer and mechanical interference.

[0064] To further enhance stability, a shallow groove is provided on the furnace bottom plate, with a depth of 3mm and a diameter slightly larger than the outer diameter of the quartz tube. During furnace loading, the bottom end of the quartz tube section is placed into the groove for positioning. The groove only provides circumferential restraint and does not apply rigid clamping force, avoiding local stress concentration. Therefore, the quartz tube will not break due to stress concentration under high-temperature conditions.

[0065] The self-stability analysis of the vertically standing tube is as follows: When the quartz tube remains in a complete tube state (length 3000mm), its center of gravity height is approximately 1500mm, and its base area is approximately 0.07m². 2The quartz tube has a height-to-diameter ratio close to 10:1, resulting in poor overall stability and a high risk of tipping over without additional constraints. Cutting the quartz tube into 750mm segments lowers the center of gravity to approximately 375mm, while maintaining the same base area. The height-to-diameter ratio decreases to approximately 2.5:1, significantly lowering the center of gravity, making the supporting foundation more stable, and enhancing its self-stabilizing ability. It can remain upright under gravity without clamps. This embodiment uses an integrated process tray to support the quartz tube segments. The tray is made of heat-resistant stainless steel, measuring 800mm × 600mm, and can accommodate six segments simultaneously. The tray bottom plate has six positioning grooves, 3mm deep, with an inner diameter 1mm larger than the tube's outer diameter, to limit horizontal slippage of the tube segments at the bottom. Vertical guide rods are provided at the four corners of the tray. The internal space enclosed by the guide rods is slightly larger than the outer rectangular dimension occupied by the six tube segments arranged as a whole (a more preferable option is that a rectangular ring is fixed at the middle of the guide rods, and the internal space enclosed by the rectangular ring is slightly larger than the outer rectangular dimension occupied by the six tube segments arranged as a whole). This can act as an overall limit to prevent the tube segments from tilting. Handles are provided on both sides of the tray for easy handling. The quartz tube segments remain within the tray from the time they are clamped until sintering, without needing to be moved individually.

[0066] Step 3: Spraying in a vertical position The tray supporting the quartz tube segment is placed at the spraying station (outside the furnace). The quartz tube segment is kept vertically upright. An air spray gun is inserted vertically into the tube from above and sprayed from bottom to top. The spray gun descends along the central axis of the tube, and after reaching a position approximately 50 mm from the bottom, it is raised upwards at a uniform speed of 10 mm / s (preferably 8–12 mm / s) while spraying. During spraying, the slurry is atomized and radially deposited on the tube wall. The deposited wet film flows downwards under gravity. By controlling the matching relationship between the upward speed of the spray gun and the downward speed of the slurry, a dynamic deposition-flow balance is formed on the tube wall, making the net thickness at each location tend to be consistent, thus obtaining a coating with uniform thickness. By controlling the atomized particle size, slurry viscosity, and lifting speed, the wet film of each spray is made extremely thin and pre-cured before gravity leveling is completed, thus forming a coating with a uniform thickness in the axial direction.

[0067] The spray gun used for the spraying operation has a nozzle diameter of 0.8mm, the spraying pressure is controlled at 0.4MPa, and the distance between the nozzle and the pipe wall is maintained at 100mm. To ensure the coating quality, three coats are applied: the first coat forms the base coat, the second coat forms the transition coat, and the third coat forms the top coat. Each coat is applied only after the previous coat has fully dried.

[0068] The principle of uniformity in vertical spraying is as follows: In traditional horizontal spraying, the quartz tube is placed horizontally and rotates around its axis, while the spray gun moves along the axis. Due to gravity, the slurry accumulates towards the bottom of the tube during rotation, resulting in a coating thickness difference of 10-15% between the top and bottom of the tube. The vertical spraying of this invention achieves uniform deposition using the following mechanism: the spray gun moves upward from the bottom of the tube section at a uniform speed of 10 mm / s (preferably 8–12 mm / s); the slurry is atomized and radially deposited on the tube wall; the deposited wet film flows downward under gravity; the upward speed of the spray gun and the downward speed of the slurry are optimized and matched to achieve a dynamic deposition-flow balance at various points on the tube wall during the spraying process, ultimately resulting in a more uniform net thickness. Experimental data show that when the spray gun lifting speed is 10 mm / s and the slurry viscosity is 300 mPa.s, the difference in coating thickness between the top and bottom of the pipe section can be controlled within ±3% (absolute difference ≤4 μm), with the top being 98 μm and the bottom being 102 μm. This level is better than the ±12% of horizontal spraying, where the top is about 85 μm and the bottom is 115 μm.

[0069] Step 4: Drying After spraying, the entire tray carrying the quartz tube segment is moved into a hot air circulating drying oven, with the quartz tube segment remaining upright for drying. The drying temperature is 100℃, and the time is 30 minutes. Throughout the process, the quartz tube segment is kept upright to avoid uneven coating due to changes in posture.

[0070] Step 5: Sintering After drying, the quartz tube segments are removed from the tray and placed directly into the positioning grooves on the bottom plate of the tubular furnace for vertical segmented sintering. Sintering is carried out in two stages: the first stage is conducted in an ammonia atmosphere, heated to 900℃ at a heating rate of 5℃ / min and held for 1 hour, with the ammonia flow rate controlled at 5L / min; the second stage is conducted in a nitrogen atmosphere, heated to 1300℃ at the same heating rate and held for 20 minutes, with the nitrogen flow rate also at 5L / min.

[0071] Step 6: Cooling and Removal After sintering, the tube sections are cooled to room temperature in the furnace and then removed from the furnace. At this point, a complete boron nitride gradient coating has been formed on the inner wall of each quartz tube section.

[0072] Step 7: Pipe Segment Assembly After removing the quartz tubes from the furnace, they are assembled into a complete furnace tube using flange connections: each tube end is pre-machined with an annular flange, the flange face has a sealing groove, and a metal sealing ring (such as an oxygen-free copper gasket) or a high-temperature ceramic sealing ring is installed, with a temperature resistance not lower than the sintering temperature, and is locked and fixed with stainless steel clamps. During assembly, ensure that the coating of adjacent tube sections overlaps by 20-30mm, and ensure that there is no exposed quartz at the joint.

[0073] The process involves placing the pipe segment into the positioning groove of the tray, then spraying it sequentially, and finally moving it into the drying oven for drying. After drying, the pipe segment is removed from the tray and placed directly into the furnace chamber of the tubular furnace for segmented sintering. After sintering, it is removed after cooling with the furnace.

[0074] Compared to traditional horizontal clamping, this embodiment adopts a vertical clamping method without clamps, which significantly improves the clamping effect. Traditional horizontal clamping requires 2 to 3 clamps for each quartz tube segment, with a clamping time of 10 to 15 minutes per tube, a clamping breakage rate of 3% to 5%, and a clamping cost of approximately 500 yuan per set. It also requires two operators to assist in clamping. In contrast, this embodiment does not require any clamps, has a clamping time of 0, and has been verified through 100 repeated tests to have a clamping breakage rate of 0%. It requires no additional clamping cost and only one operator to complete the operation.

[0075] Regarding coating quality, the vertical spraying combined with gravity leveling technology used in this embodiment has significant advantages over traditional horizontal spraying. In traditional horizontal spraying, the coating thickness difference between the top and bottom of the same section of quartz tube is typically between 10% and 15%, and the inter-section coating thickness mainly relies on manual control, resulting in large fluctuations in consistency. In contrast, this embodiment, through the synergistic effect of vertical spraying and gravity leveling, controls the coating thickness difference within the same section to within 5%, and further reduces the inter-section thickness difference to within 3% (absolute difference ≤ 4μm), mainly due to the automatic adjustment effect of gravity on slurry flow. In addition, the slurry deposits more densely under the action of gravity during vertical spraying, increasing the critical load for coating adhesion from over 60N in traditional horizontal spraying to over 65N; at the same time, the coating's thermal shock cycle resistance (1100℃ and alternating cold water cycles) also improves from 50 cycles without peeling to 55 cycles without peeling.

[0076] The segmented assembly method also brings significant engineering benefits. Traditional whole-pipe transportation requires specialized long vehicles, which is difficult; while segmented short pipes can be directly packed and transported, significantly reducing the difficulty. In terms of replacement costs, traditional whole-pipe transportation requires the entire pipe to be replaced, which is costly; in this embodiment, only the faulty section needs to be replaced, reducing costs by 75%. Regarding maintenance convenience, traditional whole-pipe transportation requires the entire pipe to be disassembled for maintenance, while in this embodiment, only the faulty section needs to be disassembled, greatly shortening maintenance time.

[0077] Quartz glass is extremely hard but also very brittle, with a wall thickness of only 3-5 mm. During processes such as spraying, handling, rotating, and sintering, clamps or support devices are required to hold the quartz tube in place. Traditional clamps use metal claws or V-blocks; even slightly excessive clamping force in certain areas can cause the quartz tube to break, while insufficient clamping force causes the tube to wobble during spraying, affecting the uniformity of the coating. Current solutions generally involve wrapping the claws with soft materials such as rubber or PTFE, or using multi-point supports or integral roller supports. However, soft materials cannot be used in high-temperature sintering processes because they will melt or decompose, necessitating metal clamps at high temperatures, thus maintaining the risk of breakage. While the overall roller support is safe, the quartz tube may move axially, i.e., move left and right, during rotary spraying, requiring end limiting. However, the end limiting will partially block the tube opening, affecting spraying and airflow. The roller and the outer wall of the quartz tube are in line contact, and there may be minor wear at the contact point. For thin-walled quartz tubes, although the local pressure at the roller support point is much smaller than the clamping force of the fixture, it still exists.

[0078] Furthermore, quartz tubes can reach lengths of 2000-4000 mm, while the furnace length of a standard tube furnace is typically 1000-1500 mm. Long quartz tubes require custom-made extra-long furnace chambers, resulting in extremely high equipment costs; a 4-meter-long tube furnace costs several times more than a standard furnace, and its heating / cooling rates are slow and energy consumption is high. If a pit furnace or box furnace is used with the tubes placed vertically, there is a risk of the quartz tubes tipping over, requiring complex high-temperature support fixtures. Current solutions include custom-made extra-long horizontal tube furnaces, segmented sintering (firing one end first, then the other), or cutting the tubes into sections. However, custom-made equipment is expensive and time-consuming, and ensuring temperature uniformity in an extra-long furnace chamber is difficult. Segmented sintering can create overlap marks in the middle of the quartz tube, resulting in uneven coating and stress concentration points. Cutting long quartz tubes into short segments is usually seen as a necessary compromise or a last resort, because segmentation creates splicing joints that increase the risk of leakage; segmentation requires reassembly, adding to the process; and after segmentation, each segment is still placed horizontally, still requiring clamps.

[0079] This embodiment takes a different approach, actively acknowledging and utilizing this deficiency—cutting the long quartz tube into short segments to fit the furnace chamber. These short segments (600-1000mm in length) can stand upright within the furnace chamber, maintaining stability under their own weight without any clamps. The segmentation measure, originally forced upon the furnace chamber due to its short length, has actually completely solved the long-standing mechanical structural problem of fragile clamping. The furnace chamber length, usually considered a defect, has been transformed into a driving force for proactive segmentation. After segmentation, the short tubes can stand upright, maintaining stability under their own weight, completely eliminating the need for clamps, and reducing the clamping breakage rate from 3-5% to 0%. The vertical spraying method of this invention uses a spray gun that moves uniformly upwards from the bottom of the tube. By optimizing the matching of the spray gun's lifting speed (8-12mm / s) and the slurry viscosity (200-400mPa.s), a dynamic deposition balance is achieved between the downward flow of the wet film under gravity and the upward movement of the spray gun. Experimental data shows that the thickness difference between the top and bottom of the tube can be controlled within ±2-3% (absolute difference of approximately 4μm), with uniformity improved by 5-7 times compared to horizontal spraying. No customized ultra-long tube furnace is required; a standard 800-1200mm furnace chamber can process 3000-4000mm long tubes. No clamps or specialized equipment are needed, reducing labor, transportation difficulties, and segmented replacement saves maintenance costs. By reverse-engineering the limitation of insufficient furnace chamber length in standard tube furnaces, long quartz tubes are actively cut into short segments and placed vertically. The natural self-stability gained by the shortened length of the short segments completely eliminates the need for clamps. Simultaneously, the dynamic deposition balance mechanism in the vertical state results in coating thickness uniformity significantly superior to traditional horizontal spraying.

[0080] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A nano-ceramic protective coating for a boron nitride-containing photovoltaic diffusion furnace tube, characterized in that, The coating has a three-layer structure, consisting of a base layer, a transition layer, and a top layer from the inside out. The base layer is made of hexagonal boron nitride powder and silica sol. The base layer also includes rare earth oxides selected from one or more of yttrium oxide, lanthanum oxide, and praseodymium oxide. The transition layer is made of hexagonal boron nitride powder, aluminum sol, and rare earth oxides selected from one or more of cerium oxide, lanthanum oxide, and praseodymium oxide. The top layer is made of hexagonal boron nitride powder and borosilicate glass powder. The total thickness of the coating is 95-110 μm.

2. The boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes according to claim 1, characterized in that, The dry film thickness of the bottom layer is 35±4μm; the dry film thickness of the transition layer is 30±4μm; and the dry film thickness of the top layer is 30±4μm.

3. The boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes according to claim 2, characterized in that, The rare earth oxide in the bottom layer is yttrium oxide, and the mass ratio of hexagonal boron nitride powder, silica sol and yttrium oxide is 40:50:10; the rare earth oxide in the transition layer is cerium oxide, and the mass ratio of hexagonal boron nitride powder, aluminum sol and cerium oxide is 60:30:10; the mass ratio of hexagonal boron nitride powder and borosilicate glass powder in the top layer is 80:

20.

4. The boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes according to claim 3, characterized in that, The hexagonal boron nitride powder has a flake diameter of 200 nm, a thickness of 2 nm, a purity of 99.9%, and a surface carbon impurity content of <0.05%; the silica sol is a methyl-modified silica sol with a solid content of 30%; the aluminum sol is a carbon-containing aluminum oxane-modified aluminum sol with a solid content of 25%; and the borosilicate glass powder has a softening point of 800℃.

5. The boron nitride-containing nano-ceramic protective coating for photovoltaic diffusion furnace tubes according to claim 4, characterized in that, The bottom layer is formed by sintering modified precursor powder A in an ammonia atmosphere. Modified precursor powder A is obtained by mixing hexagonal boron nitride powder and methyl modified silica sol, followed by spray drying and pretreatment in a nitrogen atmosphere at 700°C. After sintering, it forms an interface bonding layer containing an amorphous Si-OC structure. The transition layer is formed by sintering modified precursor powder B in an ammonia atmosphere. Modified precursor powder B is obtained by mixing hexagonal boron nitride powder and carbon-containing aluminum oxane modified aluminum sol, followed by the same process. After sintering, it forms an interface bonding layer containing an amorphous Al-OC structure.

6. A method for preparing a boron nitride-containing nano-ceramic protective coating for a photovoltaic diffusion furnace tube as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Sandblast the inner wall of the quartz tube to make its roughness Ra reach 3-5μm; (2) The bottom layer slurry, the transition layer slurry and the top layer slurry are sprayed sequentially on the inner wall of the quartz tube by air spraying, and each layer is dried after spraying; (3) After spraying the base layer, the first stage of sintering is carried out: under an ammonia atmosphere, the temperature is increased to 900℃ at 5℃ / min and held for 1h; (4) After spraying the transition layer, the second stage of sintering is carried out: under an ammonia atmosphere, the temperature is increased to 900℃ at 5℃ / min and held for 30min; (5) After spraying the top layer, the third stage of sintering is carried out: under nitrogen atmosphere, the temperature is raised to 1300℃ at 5℃ / min, held for 20min, and then cooled to obtain the finished product.

7. The preparation method according to claim 6, characterized in that, The preparation method of the bottom layer slurry, transition layer slurry and top layer slurry is as follows: the corresponding raw materials are mixed with anhydrous ethanol at a mass ratio of 1:0.8, and 2% of polyvinyl butyral is added as a binder. The mixture is ultrasonically dispersed for 30 minutes to obtain a slurry with a solid content of 45%.

8. The preparation method according to claim 6, characterized in that, The parameters for the air spraying method are: spray gun nozzle diameter 0.8mm, spraying pressure 0.4MPa, and spraying distance 20cm; The base layer is sprayed in 3 coats, each coat having a wet film thickness of 15-20μm. After each coat, it is dried at 100℃ for 10 minutes, and finally the whole thing is dried at 100℃ for 30 minutes. The transition layer and the top layer are each sprayed in two coats.

9. The preparation method according to claim 6, characterized in that, The flow rate of ammonia in steps (3) and (4) is 5 L / min; the flow rate of nitrogen in step (5) is 5 L / min.

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