Porous graphite flake deposited with composite coating as well as preparation method and application of porous graphite flake

Abstract

The invention relates to the technical field of filtering materials for silicon carbide crystal growth, in particular to a porous graphite flake deposited with a composite coating and a preparation method and application of the porous graphite flake deposited with the composite coating. S2, heating the carbon precursor until the carbon precursor is completely molten, then adding the pore-forming microspheres and the reinforcement into the melt to form a mixture, carrying out pre-pressing treatment, and pressing the mixture into a green body; s3, putting the green body into heating equipment, and carrying out heating curing treatment and carbonization treatment to obtain a reinforced porous graphite sheet; s4, continuously inputting and outputting a first mixed gas into and out of the deposition device, and carrying out pre-deposition; and continuously inputting and outputting the second mixed gas into the deposition device, and performing deposition to obtain the porous graphite sheet deposited with the composite coating. By adding the reinforcer, the mechanical property of the porous graphite flake is improved, and the phenomenon that the porous graphite flake is corroded and pulverized by a silicon carbide atmosphere to reduce the single crystal quality can be effectively avoided.

Description

Technical Field

[0001] The invention relates to the technical field of filter materials for silicon carbide crystal growth, and in particular to a porous graphite sheet deposited with a composite coating, a preparation method and an application thereof. Background Art

[0002] In recent years, the integrated circuit industry has experienced rapid growth. Against this backdrop, traditional silicon single crystals, constrained by their inherent performance limitations, have struggled to meet the increasingly stringent performance requirements of cutting-edge applications such as high-frequency, high-voltage, and high-temperature power, as well as radio frequency semiconductors. Silicon carbide (SiC) crystalline materials, with their unique physical and chemical properties, offer broader application prospects in these areas. Currently, the most commonly used method for growing SiC crystals is physical vapor transport (PVT). This method uses SiC powder as the core growth source and achieves crystal growth through a series of sophisticated physical processes. At high temperatures, due to the significant difference in vapor pressure between silicon and carbon (Si having a lower vapor pressure than C), SiC powder loses significantly more silicon than carbon during the middle and late stages of single crystal growth, leading to a gradual imbalance in the powder composition and ultimately a carbon powder. Furthermore, due to the specific temperature gradient required for single crystal growth, carbon powder is affected by buoyancy and floats within the chamber. Some carbon powder then enters the growing crystal surface, causing carbon encapsulation defects that negatively impact the crystal's quality and performance.

[0003] To solve the above problem, the Chinese patent No. 202022554955.5 discloses an air flow filter assembly for the thermal field of silicon carbide crystal growth. It specifically discloses placing porous graphite between the powder and the substrate to filter carbon powder, thereby reducing the probability of carbon powder entering the crystal growth interface and ensuring the quality of crystal growth; however, in the actual crystal growth process, as the crystal growth time increases, the porous graphite is easily corroded by the silicon-rich atmosphere, and then burn-through occurs, and the pores are completely blocked due to recrystallization; this will not only seriously affect the normal function and use effect of the filter assembly, but also affect the growth rate of the single crystal. Chinese patents No. 202411957499.5 and No. 202411240211.2 both propose the preparation of porous tantalum carbide to replace porous graphite as a filter material for use in the preparation of silicon carbide single crystals; based on the high-temperature stability of tantalum carbide, the problem of porous graphite being prone to burn-through or complete pore blockage due to recrystallization when used as a filter material is solved; however, since these porous tantalum carbide ceramic materials are extremely fragile, they are easy to break during installation, transportation or processing; more importantly, in order to ensure the permeability of the porous tantalum carbide ceramic material, a porous tantalum carbide ceramic material with a thickness of 2 to 5 mm is usually required during the crystal growth process, which will further reduce its mechanical properties.

[0004] The present invention provides a porous graphite sheet deposited with a composite coating, and a preparation method and application thereof, in order to solve the problems existing in the prior art, such as the easy burn-through and complete pore blockage due to recrystallization of the existing porous graphite when used as a filter material, and the easy breakage and poor mechanical properties of the porous tantalum carbide ceramic material. Summary of the Invention

[0005] The purpose of the present invention is to provide a porous graphite sheet deposited with a composite coating, and a preparation method and application thereof, so as to solve the problems existing in the prior art that the existing porous graphite is prone to burn-through when used as a filter material, the pores are completely blocked due to recrystallization, and the porous tantalum carbide ceramic material is extremely easy to break and has poor mechanical properties.

[0006] The technical solution of the present invention is: a method for preparing a porous graphite sheet deposited with a composite coating, comprising the following steps:

[0007] S1, weighing pore-forming microspheres, and immersing them in a surface treatment solution for pretreatment to obtain pretreated pore-forming microspheres;

[0008] The pore-forming microspheres are carbon-based organic microspheres;

[0009] S2. Weighing a carbon precursor and a reinforcement, first heating the weighed carbon precursor until it is completely melted to form a melt, then adding the pretreated pore-forming microspheres and the weighed reinforcement to the melt, stirring evenly to form a mixture, and then performing a pre-pressing treatment to press the mixture into a green body;

[0010] S3, placing the green body into a heating device, first performing a temperature-raising curing treatment in an air atmosphere, and then performing a carbonization treatment in an inert atmosphere. After the treatment is completed, a reinforced porous graphite sheet is obtained;

[0011] S4. Place the obtained reinforced porous graphite sheet in a deposition device, heat it to a first temperature range, and continuously input and output a first mixed gas into the deposition device for pre-deposition; then, continue to increase the temperature in the deposition device to a second temperature range, and continuously input and output a second mixed gas into the deposition device for deposition. After the deposition is completed, a porous graphite sheet deposited with a composite coating is obtained; the first mixed gas includes chlorosilane and hydrogen; the second mixed gas includes tantalum pentachloride and hydrogen.

[0012] Preferably, in the mixture, the mass ratio of the pore-forming microspheres, the carbon precursor and the reinforcement is (5-7): (3-2): (0.1-1);

[0013] The carbon precursor is asphalt;

[0014] The asphalt is coal asphalt or petroleum asphalt;

[0015] The reinforcement is chopped silicon carbide fiber and / or carbon fiber.

[0016] Preferably, the pore-forming microspheres are made of any one or more of polymethyl methacrylate, polystyrene, polyethylene, polycarbonate, and polyurethane foam;

[0017] The diameter of the pore-forming microspheres is 50 to 100 μm;

[0018] The diameters of the chopped silicon carbide fibers and the carbon fibers are both 1 to 50 μm; the fiber lengths of the chopped silicon carbide fibers and the carbon fibers are both 10 to 5000 μm.

[0019] Preferably, in step S2, the pre-pressing treatment has a treatment pressure of 1 to 20 MPa and a holding time of 5 to 30 min; the green body is designed to be sheet-shaped and has a thickness of 2 to 10 mm.

[0020] Preferably, in step S3, the method of the temperature-raising curing treatment is to first raise the temperature inside the heating device to 250-300° C. at a heating rate of 1-3° C. / min, and then maintain the temperature at a constant level for 0.5-3 hours;

[0021] The carbonization treatment includes a first stage of temperature-raising carbonization treatment and a second stage of temperature-raising carbonization treatment;

[0022] The first stage of the carbonization treatment is as follows: the temperature inside the heating device is increased to 300-500°C at a heating rate of 0.5-1.5°C / min, and the temperature is maintained constant for 1-2 hours;

[0023] The second stage of the carbonization treatment is to increase the temperature inside the heating device to 500-800° C. at a heating rate of 3-8° C. / min and maintain the constant temperature for 1-2 hours.

[0024] Preferably, the carbonization treatment further includes a third stage of temperature-raising carbonization treatment; the treatment method of the third stage of temperature-raising carbonization treatment is: raising the temperature inside the heating equipment to 800-1200°C at a heating rate of 2-4°C / min, and maintaining the constant temperature for 1-4 hours.

[0025] Preferably, in step S4, the delivery flow ratio of the chlorosilane to the hydrogen in the first mixed gas is 1:(10-30);

[0026] In the second mixed gas, the delivery flow ratio of the tantalum pentachloride to the hydrogen is 1:(10-50).

[0027] Preferably, in step S4, the inlet pressure of the first mixed gas and the second mixed gas are both 0.1-1 bar, and the outlet pressure is not higher than 0.1 bar.

[0028] The present invention also provides a porous graphite sheet deposited with a composite coating, and the porous graphite sheet is prepared by the above-mentioned preparation method.

[0029] The present invention also provides applications of the porous graphite sheet, including application as a filter material in the preparation process of silicon carbide single crystals.

[0030] Compared with the prior art, the advantages of the present invention are:

[0031] (1) The present invention provides a porous graphite sheet deposited with a composite coating, and its preparation method and application. The preparation method is to firstly subject a mixture containing pore-forming microspheres, a carbon precursor and a reinforcement to pre-pressing, temperature curing, carbonization and other processes to prepare a reinforced porous graphite sheet with a certain porosity and uniform pore size distribution, and then deposit a composite coating on the reinforced porous graphite sheet to obtain a porous graphite sheet deposited with a composite coating; since the pores on the porous graphite sheet mainly come from the holes left after the decomposition of the pore-forming microspheres, the particle size of the pore-forming microspheres is directly related to the pore size of the pores on the porous graphite sheet, and the use of carbon-based organic microspheres as a template for the pores will not affect the purity of the porous graphite sheet, but can also make the pores on the porous graphite sheet smaller. The diameter distribution is uniform; and, by adding a reinforcement, the mechanical properties of the porous graphite sheet can be improved to a certain extent, preventing the porous graphite sheet from being easily broken during subsequent applications; at the same time, the composite coating deposited on the porous graphite sheet can effectively prevent the porous graphite sheet from being corroded by the silicon carbide atmosphere, producing carbon powder after pulverization, and reducing the quality of the single crystal, and avoid the phenomenon of completely blocking the pores on the porous graphite sheet due to recrystallization of gas components such as silicon carbide, carbon, and silicon in the silicon carbide atmosphere, and preventing the porous graphite sheet from being burned through, etc.; the problems existing in the prior art such as the easy occurrence of burn-through when the existing porous graphite is used as a filter material, the complete blocking of pores due to recrystallization, and the problems of the porous tantalum carbide ceramic material being extremely easy to break and having poor mechanical properties are solved.

[0032] (2) The present invention provides a porous graphite sheet deposited with a composite coating, and a preparation method and application thereof. The preparation method is to prepare a porous graphite sheet deposited with a composite coating by using a specific deposition device in combination with a specific deposition method, that is, by first using chlorosilane and hydrogen as a first mixed gas to pre-deposit a silicon carbide coating on the reinforced porous graphite sheet, and then using tantalum pentachloride and hydrogen as a second mixed gas to deposit a carbon carbide coating on the reinforced porous graphite sheet, and during the deposition process, no hydrocarbon gas required for depositing tantalum carbide coating by existing vapor deposition (CVD) method is added, thereby achieving uniform deposition of the composite coating on the surface of the reinforced porous graphite sheet and on the inner wall of its pores; By simultaneously and continuously inputting and outputting the first mixed gas or the second mixed gas into the deposition device, a certain pressure difference is created between the upper and lower sides of the reinforced porous graphite sheet during the pre-deposition and deposition processes, thereby ensuring that the first mixed gas or the second mixed gas has a certain diffusion driving force inside the reinforced porous graphite sheet, so that the silicon carbide and tantalum carbide coatings can be gradually deposited from the outside to the inside; in addition, since only tantalum pentachloride is used as the tantalum source for deposition, the thickness of the tantalum carbide coating will be limited by the transport of the matrix carbon material. The longer the deposition time is in the area where the coating is present, the slower the deposition rate is, which is more conducive to achieving uniform deposition of the tantalum carbide coating on the inner wall of the pores of the reinforced porous graphite sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0034] Figure 1 This is a process flow chart of the method for preparing a porous graphite sheet deposited with a composite coating according to the present invention;

[0035] Figure 2 Schematic diagram of the structure of the deposition device of the present invention;

[0036] Figure 3 This is a photograph of the porous graphite sheet P1 prepared in Example 1 of the present invention;

[0037] Figure 4 This is an electron microscope image of the reinforced porous graphite sheet prepared in Example 2 of the present invention;

[0038] Wherein: 1. deposition furnace; 2. deposition device; 3. reinforced porous graphite sheet; 4. air inlet; 5. air outlet; 6. support member. DETAILED DESCRIPTION

[0039] The present invention will be described in further detail below with reference to specific embodiments:

[0040] like Figure 1 As shown, a method for preparing a porous graphite sheet deposited with a composite coating comprises the following steps:

[0041] S1. Weigh the pore-forming microspheres according to the mass ratio, immerse the weighed pore-forming microspheres into the surface treatment liquid, pre-treat the pore-forming microspheres at 30-60°C for 1-3 hours, and obtain the pre-treated pore-forming microspheres. The pore-forming microspheres are carbon-based organic microspheres; and the material of the pore-forming microspheres can be one or more of polymethyl methacrylate (PMMA), polystyrene (PS), polyethylene (PE), polycarbonate (PC), polyurethane foam (PU), etc.; the diameter of the pore-forming microspheres needs to be controlled within the range of 50-100um; the surface treatment liquid is an alcohol solution of a silane coupling agent; and in the surface treatment liquid, the concentration of the silane coupling agent is 0.05-2%; in addition, the surface treatment liquid is preferably a silane coupling agent-ethanol solution, and most preferably a silane coupling agent-ethanol solution with a concentration of 1%. The silane coupling agent may be selected from commercially available vinyltriethoxysilane (A151), vinyltrimethoxysilane (A171), vinyltri(β-methoxyethoxy)silane (A172), and the like.

[0042] S2. Weigh the carbon precursor and reinforcement according to the mass ratio, first heat the weighed carbon precursor to 200-300°C to completely melt it to form a melt, then add the pretreated pore-forming microspheres and the weighed reinforcement to the melt in turn, and mechanically stir them at a speed of 100-1000 rpm to form a mixture after stirring evenly. Thereafter, pre-press the mixture under a pressure of 1-20 MPa and a holding time of 5-30 minutes, and press the mixture into a blank of a preset shape. Among them, in the formed mixture, the mass ratio of pore-forming microspheres, carbon precursor and reinforcement is (5-7): (3-2): (0.1-1); the carbon precursor can be selected from resin, asphalt, polystyrene, etc., preferably asphalt; the asphalt is preferably coal tar or petroleum asphalt; the reinforcement is chopped silicon carbide fiber and / or carbon fiber; the diameter of the chopped silicon carbide fiber and carbon fiber is controlled within the range of 1-50um; the fiber length of the chopped silicon carbide fiber and carbon fiber is 10-5000um; the blank is preferably pressed into a sheet shape, and the thickness is controlled within the range of 2-10mm.

[0043] S3. Place the green body in a heating device and first perform a temperature curing treatment under air atmosphere conditions, that is, increase the temperature in the heating device from room temperature to 250-300°C at a heating rate of 1-3°C / min and maintain the constant temperature for 0.5-3 hours to partially crosslink and cure the carbon precursor in the green body to prevent defects such as melting and collapse on the green body; then, switch the atmosphere in the heating device to an inert atmosphere, such as nitrogen or argon atmosphere, and perform a carbonization treatment under inert atmosphere conditions. After the treatment is completed, a reinforced porous graphite sheet is obtained. The carbonization treatment includes two stages: a first stage temperature carbonization treatment and a second stage temperature carbonization treatment. The specific treatment method of the first stage of temperature carbonization treatment is to increase the temperature inside the heating device to 300-500°C at a heating rate of 0.5-1.5°C / min and maintain the temperature for 1-2 hours; this is used to decompose the pore-forming microspheres into gas; in addition, the cavities left after the decomposition of the pore-forming microspheres can form the initial pores of the porous graphite sheet, and the channels formed when the decomposed gas overflows serve as secondary pores of the porous graphite sheet. The treatment method of the second stage of temperature carbonization treatment is to first increase the temperature inside the heating device to 500-800°C at a heating rate of 3-8°C / min, and then maintain the temperature for 1-2 hours; this is used to carbonize the pitch to form a rigid carbon skeleton. The carbonization treatment can also include a third stage of temperature carbonization treatment; this third stage of temperature carbonization treatment is to increase the temperature inside the heating device to 800-1200°C at a heating rate of 2-4°C / min and maintain the temperature for 1-4 hours; this is used to enhance the degree of graphitization of the pitch.

[0044] S4, such as Figure 2As shown, the obtained reinforced porous graphite sheet 3 is first placed on the needle-shaped support 6 in the deposition device 2, and then the deposition device 2 is placed in the deposition furnace 1; then, the temperature in the deposition device 2 is raised to the first temperature range, that is, the temperature in the deposition furnace is raised to the range of 800-1200°C, and the first mixed gas is continuously input and output into the deposition device 2 through the air inlet 4 and the air outlet 5 respectively, to perform pre-deposition, and a layer of silicon carbide coating is deposited on the surface of the reinforced porous graphite sheet 3 and the inner wall of its pores; then, continue The temperature in the deposition device 2 is raised to a second temperature range, that is, the temperature in the deposition furnace is raised to a range of 1500-2000°C. Simultaneously, a second mixed gas is continuously fed into and out of the deposition device 2 through the gas inlet 4 and the gas outlet 5, respectively, to perform deposition. A layer of silicon carbide coating is then deposited on the surface of the reinforced porous graphite sheet 3 and on the inner walls of its pores, forming a composite coating. After the deposition is completed, the temperature in the deposition device 2 is naturally cooled to room temperature, and the porous graphite sheet with the composite coating is removed from the deposition device 2 to obtain the porous graphite sheet with the composite coating. The porous graphite sheet with the composite coating has a porosity of 20%-70%. The first mixed gas includes chlorosilane and hydrogen; and, in the first mixed gas, the delivery flow ratio of chlorosilane and hydrogen is 1:(10~30); during the pre-deposition process, the inlet pressure when the first mixed gas is delivered to the deposition device through the air inlet is 0.1~1 bar; the outlet pressure when the first mixed gas in the deposition device is output through the air outlet is not higher than 0.1 bar; the deposition time of the pre-deposition is controlled in the range of 0.1~5h; the purpose of the pre-deposition is to deposit a layer of silicon carbide coating on the surface of the reinforced porous graphite sheet and on the inner wall of its pores as the first coating layer. The second mixed gas includes tantalum pentachloride and hydrogen; and in the second mixed gas, the delivery flow ratio of tantalum pentachloride to hydrogen is 1:(10-50); during the deposition process, the inlet pressure when the second mixed gas is delivered to the deposition device through the air inlet is also 0.1-1 bar, and the outlet pressure when the first mixed gas in the deposition device is output through the air outlet is no higher than 0.1 bar; and the deposition time also needs to be controlled within the range of 0.1-5 hours. In addition, during the pre-deposition and deposition processes, the inlet pressure of the air inlet gradually decreases from the bottom of the deposition device to the top; and the pressure difference between the two adjacent air inlets is 0.005-0.05 bar; that is, the inlet pressure of the air inlet closer to the bottom of the deposition device is greater than the inlet pressure of the air inlet farther from the bottom of the deposition device. The lower the outlet pressure, the better the gas permeability will be, and the more uniform the deposition of the silicon carbide and tantalum carbide coatings on the reinforced porous graphite sheet will be. However, the lower the outlet pressure is, the better. When the outlet pressure is too low, the deposition rate of the silicon carbide and tantalum carbide coatings will slow down.In the first mixed gas and the second mixed gas, the higher the hydrogen content, the faster the pre-deposition and deposition rates; however, when the hydrogen content is too high, it will lead to uneven deposition of the silicon carbide and tantalum carbide coatings on the reinforced porous graphite sheet; therefore, it is necessary to reasonably control the hydrogen content in the first mixed gas and the second mixed gas. The present invention also provides a porous graphite sheet deposited with a composite coating, which is prepared using the above-mentioned preparation method. During the deposition process, the temperature in the deposition device is increased to 1500-2000°C in order to increase the decomposition rate of tantalum pentachloride and the diffusion rate of carbon atoms, thereby increasing the deposition rate; it also helps to graphitize the porous graphite sheet itself.

[0045] The present invention also provides applications of the porous graphite sheet deposited with the composite coating, including application of the porous graphite sheet as a filter material in the preparation process of silicon carbide single crystals.

[0046] Example 1

[0047] S1: Weigh pore-forming microspheres, a carbon precursor, and a reinforcement in a mass ratio of 5:2:0.2, immerse the weighed pore-forming microspheres in a 1% vinyltrimethoxysilane-ethanol solution, and pretreat them at 50°C to obtain pretreated pore-forming microspheres; wherein the pore-forming microspheres are made of polymethyl methacrylate with a diameter of 80 μm; the carbon precursor is natural asphalt; and the reinforcement is chopped silicon carbide fiber with a fiber diameter of 1 μm;

[0048] S2: First, the carbon precursor weighed in step S1 is heated to 200°C and maintained at a constant temperature to completely melt it to form a melt; then, the pretreated pore-forming microspheres and the reinforcement weighed in step S1 are added to the melt in sequence, and mechanically stirred at a speed of 500 rpm to form a mixture; then, the mixture is placed in a pre-pressing device and pre-pressed at a pressure of 12 MPa. After holding the pressure for 20 minutes, a sheet-like green body with a thickness of 4 mm is obtained;

[0049] S3: placing the obtained green body into a heating device, first raising the temperature in the heating device from room temperature to 280°C at a heating rate of 2°C / min under air atmosphere to perform a temperature curing treatment; after maintaining the constant temperature for 2 hours, switching the atmosphere in the heating device to a nitrogen atmosphere, and performing a carbonization treatment under nitrogen atmosphere: first raising the temperature inside the heating device to 400°C at a heating rate of 1°C / min and maintaining the constant temperature for 2 hours to decompose the pore-forming microspheres into gas; then further raising the temperature inside the heating device to 650°C at a heating rate of 5°C / min and maintaining the constant temperature for 2 hours to carbonize the asphalt to form a rigid carbon skeleton; after the carbonization treatment is completed, a reinforced porous graphite sheet is obtained;

[0050] S4: placing the obtained reinforced porous graphite sheet into a deposition device, raising the temperature in the deposition device to 1200°C, and continuously inputting a mixed gas of chlorosilane and hydrogen with a flow rate ratio of 1:20 into the deposition device at an inlet pressure of 0.5 bar, and at the same time discharging the mixed gas of chlorosilane and hydrogen in the deposition device at an outlet pressure of 0.05 bar, for pre-deposition. After deposition for 2 hours, a layer of silicon carbide coating is deposited on the surface of the reinforced porous graphite sheet and on the inner wall of its pores; thereafter, the temperature in the deposition device is further raised to 1800°C, and continuously inputting a mixed gas of tantalum pentachloride and hydrogen with a flow rate ratio of 1:30 into the deposition device at an inlet pressure of 0.5 bar, and at the same time discharging the mixed gas of tantalum pentachloride and hydrogen in the deposition device at an outlet pressure of 0.05 bar, for deposition; after deposition for 2 hours, as shown in FIG. Figure 3 As shown, a porous graphite sheet with a composite coating deposited thereon is obtained, P1.

[0051] Example 2

[0052] S1: Weigh pore-forming microspheres, a carbon precursor, and a reinforcement in a mass ratio of 6:2:0.1, immerse the weighed pore-forming microspheres in a 1% vinyltriethoxysilane-ethanol solution, and pretreat them at 50°C to obtain pretreated pore-forming microspheres; wherein the pore-forming microspheres are made of polystyrene with a diameter of 50 μm; the carbon precursor is natural asphalt; and the reinforcement is carbon fiber with a fiber diameter of 3 μm;

[0053] S2: First, the carbon precursor weighed in step S1 is heated to 250° C. and maintained at a constant temperature to completely melt it to form a melt; then, the pretreated pore-forming microspheres and the reinforcement weighed in step S1 are sequentially added to the melt, and mechanically stirred at a speed of 300 rpm to form a mixture; then, the mixture is placed in a pre-pressing device and pre-pressed at a pressure of 10 MPa. After holding the pressure for 30 minutes, a sheet-like green body with a thickness of 6 mm is obtained;

[0054] S3: The obtained green body is placed in a heating device, and the temperature in the heating device is first raised from room temperature to 300°C at a heating rate of 1°C / min under air atmosphere to perform a heating and curing treatment; after maintaining the constant temperature for 2 hours, the atmosphere in the heating device is switched to an argon atmosphere, and a carbonization treatment is performed under argon atmosphere: first, the temperature inside the heating device is raised to 400°C at a heating rate of 1°C / min, and maintained at a constant temperature for 2 hours to decompose the pore-forming microspheres into gas; then, the temperature inside the heating device is further raised to 650°C at a heating rate of 5°C / min, and maintained at a constant temperature for 2 hours to carbonize the asphalt to form a rigid carbon skeleton; then, the temperature inside the heating device is further raised to 1000°C at a heating rate of 3°C / min, and maintained at a constant temperature for 3 hours to improve the degree of graphitization of the asphalt; after the carbonization treatment is completed, as shown in FIG. Figure 4 As shown, a reinforced porous graphite sheet is obtained;

[0055] S4: The obtained reinforced porous graphite sheet is placed in a deposition device, the temperature in the deposition device is increased to 1200°C, and a mixed gas of chlorosilane and hydrogen with a flow rate ratio of 1:15 is continuously input into the deposition device at an inlet pressure of 1 bar, and the mixed gas of chlorosilane and hydrogen in the deposition device is discharged at an outlet pressure of 0.1 bar for pre-deposition. After 2 hours of deposition, a layer of silicon carbide coating is deposited on the surface of the reinforced porous graphite sheet and the inner wall of its pores; thereafter, the temperature in the deposition device is further increased to 2000°C, and a mixed gas of tantalum pentachloride and hydrogen with a flow rate ratio of 1:25 is continuously input into the deposition device at an inlet pressure of 1 bar, and the mixed gas of tantalum pentachloride and hydrogen in the deposition device is discharged at an outlet pressure of 0.1 bar for deposition; after 2 hours of deposition, a porous graphite sheet with a composite coating is obtained, P2.

[0056] Example 3

[0057] The difference between this embodiment and embodiment 1 is that: in step S4, no pre-deposition is performed, and the tantalum carbide coating is directly deposited on the enhanced porous graphite sheet; thus, a porous graphite sheet with a tantalum carbide coating deposited thereon is obtained, P3.

[0058] Comparative Example 1

[0059] A porous graphite sheet purchased on the market is taken, and a tantalum carbide coating is deposited on the surface and in the pores of the porous graphite sheet to obtain a porous graphite sheet with a tantalum carbide coating, Q1.

[0060] The porosity, pore size distribution uniformity, and other properties of the porous graphite sheets P1-P3 prepared in Examples 1-3 and the porous graphite sheet Q1 in Comparative Example 1 were tested. The porous graphite sheets P1-P3 prepared in Examples 1-3 and the porous graphite sheet Q1 in Comparative Example 1 were used as filter materials in preparing silicon carbide single crystals by physical vapor deposition to obtain silicon carbide single crystals, which were then processed into silicon carbide wafers. The resulting silicon carbide wafers were then subjected to defect detection using a silicon carbide substrate defect detection device. The test data are shown in Table 1.

[0061] Table 1. Performance of porous graphite sheets P1-P3 and Q1 and defect detection results of corresponding silicon carbide wafers

[0062]

[0063] As shown in Table 1, compared with the porous graphite sheet Q1 with a tantalum carbide coating deposited in Comparative Example 1, the pores on the porous graphite sheets P1-P3 prepared in Examples 1-3 have a more uniform pore size distribution, and there is no blockage inside the pores, and the airways are smooth; when the porous graphite sheets P1-P3 prepared in Examples 1-3 and the porous graphite sheet Q1 in Comparative Example 1 are respectively used as filter materials in the process of preparing silicon carbide single crystals by physical vapor deposition, the silicon carbide single crystals prepared by using the porous graphite sheets P1-P3 as filter materials are wrapped with tantalum carbide. The results show that the porous graphite sheets P1-P3 are significantly lower in content than the silicon carbide and microtubes, resulting in higher quality silicon carbide single crystals. This further demonstrates that the use of porous graphite sheets P1-P3 as filter materials can better filter out carbon particles in the silicon carbide atmosphere generated by the sublimation of silicon carbide powder. Furthermore, during the crystal growth process, the porous graphite sheets are not corroded by the silicon carbide atmosphere, and impurities entering the single crystals after ashing, or defects such as carbon encapsulation, which could reduce the quality of the single crystal, are prevented. Furthermore, the recrystallization of gaseous components such as silicon carbide, carbon, and silicon in the silicon carbide atmosphere completely blocks the pores in the porous graphite sheets, effectively achieving a filtering effect. Furthermore, the addition of a certain amount of reinforcement during the preparation of the porous graphite sheets significantly improves their mechanical properties.

[0064] Comparing the porous graphite sheets P1 and P2 prepared in Example 1 and Example 2 with the porous graphite sheet P3 prepared in Example 3, it can be seen that the porosity and pore size distribution uniformity of the porous graphite sheets P1, P2 and the porous graphite sheet P3 are comparable; however, a composite coating of silicon carbide and tantalum carbide is deposited on the porous graphite sheets P1 and P2, while only a tantalum carbide coating is deposited on the porous graphite sheet P3; the crystal quality of the silicon carbide single crystals prepared when the porous graphite sheets P1 and P2 are used as filter materials is significantly higher than the crystal quality of the silicon carbide single crystals prepared when the porous graphite sheet P3 is used as the filter material; this further indicates that depositing a composite coating on the reinforced porous graphite sheet can effectively prevent the porous graphite sheet from being corroded by the silicon carbide atmosphere and improve the quality of the crystals. Furthermore, due to the significant difference in thermal expansion coefficients between the tantalum carbide coating and graphite, minor cracks inevitably exist between the tantalum carbide coating and the reinforced porous graphite sheet. Depositing the silicon carbide coating on the reinforced porous graphite sheet first, followed by the tantalum carbide coating, effectively mitigates this difference in thermal expansion coefficients, making the tantalum carbide coating deposited on the reinforced porous graphite sheet more compact. Furthermore, during the crystal growth process, especially in the early stages, the atmosphere in the growth chamber significantly corrodes the porous graphite sheet. The composite coating deposited on the porous graphite sheet can provide a partial silicon carbide source to balance the atmosphere's corrosive effects on the graphite, thereby preventing the porous graphite sheet from burning through.

[0065] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it accordingly, and they are not intended to limit the scope of protection of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present invention.

Claims

1. A method for preparing a porous graphite sheet deposited with a composite coating, characterized in that: The following steps are involved: S1, weighing pore-forming microspheres, and immersing them in a surface treatment solution for pretreatment to obtain pretreated pore-forming microspheres; The pore-forming microspheres are carbon-based organic microspheres; S2. Weighing a carbon precursor and a reinforcement, first heating the weighed carbon precursor until it is completely melted to form a melt, then adding the pretreated pore-forming microspheres and the weighed reinforcement to the melt, stirring evenly to form a mixture, and then performing a pre-pressing treatment to press the mixture into a green body; S3, placing the green body into a heating device, first performing a temperature-raising curing treatment in an air atmosphere, and then performing a carbonization treatment in an inert atmosphere. After the treatment is completed, a reinforced porous graphite sheet is obtained; S4. Place the obtained reinforced porous graphite sheet into a deposition device, heat it to a first temperature range, and continuously input and output a first mixed gas into the deposition device for pre-deposition; then, continue to increase the temperature in the deposition device to a second temperature range, and continuously input and output a second mixed gas into the deposition device for deposition. After the deposition is completed, a porous graphite sheet deposited with a composite coating is obtained; the first mixed gas includes chlorosilane and hydrogen; the second mixed gas includes tantalum pentachloride and hydrogen.

2. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 1, wherein: In the mixture, the mass ratio of the pore-forming microspheres, the carbon precursor and the reinforcement is (5-7): (3-2): (0.1-1); The carbon precursor is asphalt; The asphalt is coal asphalt or petroleum asphalt; The reinforcement is chopped silicon carbide fiber and / or carbon fiber.

3. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 2, wherein: The pore-forming microspheres are made of any one or more of polymethyl methacrylate, polystyrene, polyethylene, polycarbonate, and polyurethane foam; The diameter of the pore-forming microspheres is 50 to 100 μm; The diameters of the chopped silicon carbide fibers and the carbon fibers are both 1 to 50 μm; the fiber lengths of the chopped silicon carbide fibers and the carbon fibers are both 10 to 5000 μm.

4. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 2, wherein: In step S2, the pre-pressing treatment is performed at a pressure of 1 to 20 MPa and a holding time of 5 to 30 minutes; The blank is designed to be sheet-shaped and has a thickness of 2 to 10 mm.

5. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 2, wherein: In step S3, the temperature-raising curing treatment is performed by first raising the temperature inside the heating device to 250-300° C. at a heating rate of 1-3° C. / min, and then maintaining the temperature at a constant level for 0.5-3 hours; The carbonization treatment includes a first stage of temperature-raising carbonization treatment and a second stage of temperature-raising carbonization treatment; The first stage of the carbonization treatment is as follows: the temperature inside the heating device is increased to 300-500°C at a heating rate of 0.5-1.5°C / min, and the temperature is maintained constant for 1-2 hours; The second stage of the carbonization treatment is to increase the temperature inside the heating device to 500-800° C. at a heating rate of 3-8° C. / min and maintain the constant temperature for 1-2 hours.

6. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 5, wherein: The carbonization treatment also includes a third stage of temperature-raising carbonization treatment; the treatment method of the third stage of temperature-raising carbonization treatment is: raising the temperature inside the heating equipment to 800-1200°C at a heating rate of 2-4°C / min, and maintaining the constant temperature for 1-4 hours.

7. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 2, wherein: In step S4, in the first mixed gas, the delivery flow ratio of the chlorosilane and the hydrogen is 1:(10-30); In the second mixed gas, the delivery flow ratio of the tantalum pentachloride to the hydrogen is 1:(10-50).

8. The method for preparing a porous graphite sheet deposited with a composite coating according to claim 2, wherein: In step S4, the inlet pressure of the first mixed gas and the second mixed gas are both 0.1-1 bar, and the outlet pressure is not higher than 0.1 bar.

9. A porous graphite sheet deposited with a composite coating, characterized in that: The preparation method is described in any one of claims 1 to 8.

10. The use of the porous graphite sheet deposited with a composite coating as claimed in claim 9, characterized in that: Including the application as a filter material in the preparation process of silicon carbide single crystals.

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