Preparation method of bottom heater for monocrystalline silicon furnace

Abstract

The invention discloses a preparation method of a bottom heater for a monocrystalline silicon furnace, and relates to the technical field of material preparation.The method comprises the steps that graphite powder, petroleum coke, asphalt powder, an adhesive, fibers and a dispersing agent are added into water according to the proportion and mixed to be uniform, and a premix is obtained; carrying out primary mould pressing on the premix in a bottom heater mould, and then drying in the shade to form a blank; the blank is subjected to high-temperature secondary mold pressing in a bottom heater mold, a prefabricated blank is prepared, and the prefabricated blank is prepared; carrying out carbonization treatment on the prefabricated green body to obtain a carbonized green body; performing high-temperature heat treatment on the carbonized green body to obtain a bottom heater; a traditional needling method is replaced with a two-time mold pressing process, the problem of gas escape caused by excessive use of an adhesive is avoided, the blank forming period is shortened through combination of shade drying and staged mold pressing, internal stress concentration points are eliminated, the three-dimensional reinforcing effect of the material is achieved through fibers, and the heat conduction stability of a final product is improved through carbonization and high-temperature treatment; and the service life of the bottom heater is prolonged.

Description

Technical Field

[0001] This application relates to the technical field of material preparation, and particularly relates to a preparation method of a bottom heater for a single crystal furnace. Background Art

[0002] The bottom heater is an important thermal field component in a single crystal pulling furnace. It is used in conjunction with the heater to adjust the temperature in the furnace by changing the power, melt the silicon material, and at the same time provide a suitable thermal field environment for the growth of single crystals. During the production process of single crystals, the bottom heater needs to withstand extremely high temperatures and severe thermal cycles, and its performance directly affects the growth quality and production efficiency of single crystals.

[0003] Currently, most bottom heaters are made of graphite materials. However, traditional graphite has defects such as low strength and poor thermal shock resistance, and is prone to cracks and deformation in a long-term high-temperature working environment, resulting in a short service life of the bottom heater. In the prior art, the preparation method of the bottom heater mainly uses the needle punching method to make a preform, and then repeatedly impregnates and carbonizes with resin for densification treatment, and finally obtains a finished bottom heater through high-temperature graphitization. The above method has many defects: the multiple impregnation processes require a large amount of resin materials, increasing the production cost; secondly, a large amount of gas is generated when the resin is carbonized and decomposed, which is likely to cause cracking and peeling on the surface of the bottom heater, seriously affecting the product appearance and performance; at the same time, the production cycle of this process usually exceeds 60 days, with low efficiency; finally, due to uneven impregnation, the prepared product is prone to problems such as uneven density distribution or even internal delamination, reducing the service life of the bottom heater. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a preparation method of a bottom heater for a single crystal furnace, aiming to improve the production efficiency and service life of the bottom heater.

[0005] A preparation method of a bottom heater for a single crystal furnace includes the following steps: Add graphite powder, petroleum coke, asphalt powder, binder, fiber, and dispersant into water in proportion, and mix evenly to obtain a premix; Perform primary molding on the premix in a bottom heater mold, and then air-dry to form a blank; Perform high-temperature secondary molding on the blank in a bottom heater mold to obtain a preform; Perform carbonization treatment on the preform to obtain a carbonized blank; Perform high-temperature heat treatment on the carbonized blank to obtain a bottom heater.

[0006] Each component in the premix realizes uniform distribution in the system through a dispersant. The fibers form a multi-level gradient distribution through two-stage molding, effectively improving the bending strength of the ligand. Among them, the first-stage molding constructs the basic skeleton structure of the material, and the air-drying process can avoid the stress concentration problem caused by the rapid volatilization of moisture. The second-stage molding combines with the thermal curing effect to make the binder fully penetrate into the particle gaps, forming a three-dimensional grid structure. The carbonization treatment converts the binder into a carbonaceous connector, and the high-temperature heat treatment promotes the ordering of the material crystal structure. Finally, a bottom heater with high density and low porosity is obtained, effectively solving the problems of uneven density and surface cracking of the bottom heater, and significantly shortening the production cycle.

[0007] Optionally, the premix is made from the following raw materials in parts by weight: 40-70 parts of graphite powder, 5-20 parts of petroleum coke, 5-10 parts of asphalt powder, 22-50 parts of binder, 5-25 parts of fiber, 1.1-10 parts of dispersant, and 30-60 parts of deionized water.

[0008] By limiting the mass ratio of each component in the premix, while ensuring the mechanical strength and thermal shock resistance of the bottom heater, the production cycle is shortened and the problem of surface cracking is avoided. The improvement of the dispersion uniformity of the premix reduces the internal defects of the green body. The continuous bonded carbon network formed during the carbonization process improves the material density. The finally prepared bottom heater has a more uniform microstructure and a longer service life. The graphite powder ensures the carbon content of the material, and the petroleum coke supplements the proportion of the carbon source to optimize the pore structure of the material during carbonization. The asphalt powder is converted into a bonded carbon phase at high temperature to improve the integrity of the green body. The proportion of the binder ensures that the green body has sufficient forming strength during the molding stage. The fiber realizes the three-dimensional strengthening effect of the material. The dispersant ensures the uniform dispersion of each component and reduces particle sedimentation. The deionized water balances the rheological properties of the slurry and the molding efficiency. This ratio makes the premix reduce the density unevenness or layering defects caused by the imbalance of component ratios during the molding and carbonization processes, and at the same time avoid the problem of the escape of thermal decomposition gases caused by excessive resin impregnation.

[0009] Optionally, the binder includes phenolic resin and industrial ethanol; The mass ratio of the phenolic resin to the industrial ethanol is 2-20:1.

[0010] The phenolic resin and industrial ethanol are mixed in proportion to form an adhesive system, and solid particles such as graphite powder are uniformly coated in the premix. During the curing process, the industrial ethanol gradually volatilizes, and the phenolic resin undergoes a cross-linking reaction under heating conditions to form a continuous adhesive network. The mass ratio is controlled within the range of 2-20:1, which not only ensures the sufficient wetting of the adhesive to the powder, avoids local stress concentration caused by uneven mixing, but also prevents excessive solvent residue from affecting the curing effect, and ensures that the preform has sufficient initial strength before carbonization treatment; when there is too much phenolic resin, the viscosity of the premix is too high to be uniformly dispersed, and when there is too little phenolic resin, the material strength is insufficient. Through the optimization of the ratio of the two, the mixing uniformity is improved while ensuring the adhesive strength, effectively avoiding the microcrack propagation caused by solvent residue during the carbonization process.

[0011] Optionally, the dispersant includes carboxymethyl cellulose and sodium dodecylbenzenesulfonate; The mass ratio of the carboxymethyl cellulose to the sodium dodecylbenzenesulfonate is 1-80:1.

[0012] Carboxymethyl cellulose delays the particle sedimentation by increasing the viscosity of the aqueous phase, while sodium dodecylbenzenesulfonate reduces its agglomeration tendency by adsorbing on the surface of graphite powder and petroleum coke. After being combined in proportion, they can achieve the uniform distribution of high-solid-content materials such as graphite powder and petroleum coke under low-shear mixing conditions, avoid stress concentration or density difference inside the subsequent molded preform caused by uneven dispersion, effectively prevent crack propagation caused by local stress release during the carbonization process of the preform, shorten the mixing time and reduce the equipment energy consumption at the same time, and improve the production efficiency of the bottom heater; Optionally, the particle size of the graphite powder is 200-1000 mesh, the particle size of the petroleum coke is 500-800 mesh, and the particle size of the asphalt powder is 200-500 mesh; the fiber is carbon fiber, and the length of the carbon fiber is 5-50 mm; The mixing time of the premix is 2-5 h.

[0013] By limiting the particle size ranges of the graphite powder, petroleum coke, and asphalt powder, and combining with the mixing time of the premix, a stable dispersion system can be formed in the raw material mixing stage. The above-mentioned fine-grained petroleum coke and asphalt powder help to fill the voids between large particles, and sufficient mixing time can avoid local composition segregation; during the subsequent molding and curing processes, the internal structure uniformity of the material is improved, thereby reducing the cracking risk caused by density unevenness in the carbonization and high-temperature treatment stages. The mixing time of 2-5 h can ensure the full dispersion of each component and form a uniform slurry system, thereby improving the production efficiency and service life of the bottom heater.

[0014] Optionally, the carbon fiber includes 30 - 50% carbon fiber with a length of 5 - 15 mm, 30 - 40% carbon fiber with a length of 16 - 35 mm, and 10 - 30% carbon fiber with a length of 36 - 50 mm.

[0015] During the molding process, short fibers can fill the material gaps to reduce porosity, medium - long fibers enhance the toughness of the blank by staggered arrangement, and long fibers form a continuous skeleton structure along the pressing direction. Carbon fibers of different lengths form a gradient transition interface during the thermal curing stage, avoiding crack propagation caused by stress concentration. By adjusting the proportion of the length range, the density and flexural strength of the material can be balanced. The cooperation of fibers of different lengths enables the blank to maintain a complete shape during the carbonization and high - temperature treatment processes, reducing the risk of internal stress cracking and shortening the preform molding cycle at the same time.

[0016] Optionally, the first molding is carried out at room temperature; The pressure of the first molding is 0.05 - 0.5 MPa, and the time of the first molding is 10 - 20 min.

[0017] Room - temperature molding can avoid the difference in thermal expansion of materials caused by high temperature, helping to maintain the shape stability of the blank. The combination of pressure and time enables the graphite powder and fibers to be evenly distributed with the assistance of moisture, avoiding fiber damage caused by the traditional needling method and reducing internal stress concentration in the blank through the uniformity of pressure distribution, providing a structurally stable blank foundation for subsequent carbonization treatment.

[0018] Optionally, the pressing temperature of the second molding is between 120 - 250 °C; The pressure of the second molding is 10 - 15 MPa, and the molding time is 20 - 120 min.

[0019] By adjusting the temperature of the second molding, the binder in the pre - formed blank softens but does not completely decompose, thus promoting material flow and enhancing density; the pressure range can effectively remove internal pores in the blank and improve material density; the time range ensures that the material fully flows to fill the mold cavity and completes structural shaping. During this process, the remaining pores inside the blank are compressed and eliminated, and the component materials are densely arranged under the combined action of heat and force. This operation completes the final shaping of the blank structure through a single process. The present application effectively solves the problems of long production cycle, uneven product density, and surface cracking caused by multiple impregnation carbonizations in traditional processes. By optimizing the process parameters of the second molding, while ensuring the density of the blank, the molding efficiency is significantly improved, and structural damage caused by gas escape inside the material is avoided.

[0020] Optionally, the temperature of the carbonization treatment is 850 - 1050 °C, and the time of the carbonization treatment is 5 - 8 h.

[0021] The preform is heated to the target temperature range in a closed environment, and the temperature inside the preform is evenly distributed by gradient heating. The gas generated during the carbonization process is directionally discharged through a negative pressure exhaust system. The carbonized preform forms a carbon matrix with a three-dimensional network structure, effectively reducing the gas aggregation phenomenon during the carbonization process and improving the surface integrity of the bottom heater preform.

[0022] Optionally, the temperature of the high-temperature heat treatment is 1800 - 2300 °C, and the time of the high-temperature heat treatment is 1 - 2 h.

[0023] During the high-temperature heat treatment, carbon atoms are orderly arranged through diffusion to form a three-dimensional network graphite structure, suppressing abnormal grain growth while ensuring the density of the material, thereby obtaining a bottom heater with high thermal stability.

[0024] The beneficial effects of the present invention are as follows: By replacing the traditional needle punching and repeated impregnation steps with two molding processes, the problem of gas escape caused by excessive use of adhesives is avoided. The combination of air drying and staged molding shortens the preform forming cycle, eliminates internal stress concentration points at the same time. The directional arrangement and multi-level gradient distribution of fibers enhance the thermal shock resistance of the material. The synergistic effect of carbonization and high-temperature treatment improves the thermal conductivity stability of the final product and increases the service life of the bottom heater. Description of the Drawings

[0025] Figure 1 is a flowchart of a preparation method of a bottom heater for a single crystal silicon furnace provided by an embodiment of the present application. Detailed Embodiments

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Figure 1 is a flowchart of a preparation method of a bottom heater for a single crystal silicon furnace provided by an embodiment of the present application, as Figure 1 shown, the method includes the following steps: S10. Add graphite powder, petroleum coke, asphalt powder, binder, fiber, and dispersant into water in proportion, and mix evenly to obtain a premix; S20. Perform a first molding on the premix at room temperature, and then air dry to form a blank; S30. Perform a high-temperature second molding on the blank in a bottom heater mold to obtain a preform; S40. Perform carbonization treatment on the preform to obtain a carbonized blank; S50. Perform high-temperature heat treatment on the carbonized green body to obtain a bottom heater.

[0028] In the technical solution provided by this application, each component in the premix realizes uniform distribution in the system through a dispersant, and the fibers form a multi-level gradient distribution through two-stage molding, effectively improving the bending strength of the ligand. Among them, the first-stage molding refers to the preliminary densification of the premix under normal temperature conditions, using a hydraulic-activated mechanical press to apply pressure to make the material initially formed. The green body after the first-stage molding is placed in an environment with controllable humidity for slow dehydration to prevent cracking caused by rapid drying. The first-stage molding constructs the basic skeleton structure of the material, and the air-drying process avoids stress concentration caused by rapid water evaporation. The heat-curing treatment causes the binder to undergo a cross-linking reaction by raising the temperature. A higher pressure is applied to the green body through the second-stage molding to further eliminate the internal pores of the green body and enhance the structural density. The second-stage molding combined with the heat-curing effect enables the binder to fully penetrate into the particle gaps to form a three-dimensional grid structure. The carbonization treatment converts the binder into a carbonaceous connector, and the high-temperature heat treatment promotes the ordering of the crystal structure of the material. Finally, a bottom heater with high density and low porosity is obtained. By performing graphitization on the carbonized green body at high temperature through high-temperature heat treatment, the thermal stability of the material is improved, effectively solving the problems of uneven density and surface cracking of the bottom heater, and significantly shortening the production cycle.

[0029] Optionally, the premix is made of raw materials with the following mass ratios: 40 - 70 parts of graphite powder, 5 - 20 parts of petroleum coke, 5 - 10 parts of asphalt powder, 22 - 50 parts of binder, 5 - 25 parts of fiber, 1.1 - 10 parts of dispersant, and 30 - 60 parts of deionized water.

[0030] Specifically, artificial graphite powder can be used as the matrix material for the graphite powder to ensure the high-temperature resistance of the material. Petroleum coke is used as a carburizer, and specifically needle coke can be used, which can increase the carbon content of the green body after high-temperature treatment. The asphalt powder forms a bonded carbon network during the carbonization process to enhance the strength of the green body. The binder plays a temporary bonding role during the green body forming stage. The fibers are distributed inside the green body to achieve stress dispersion. The dispersant prevents solid particles from agglomerating and improves the fluidity of the slurry. By controlling the proportion of the premix components, a uniform carbon skeleton structure can be directly constructed during the green body forming stage, reducing subsequent process steps. On the premise of ensuring the mechanical strength and thermal shock resistance of the bottom heater, the production cycle is shortened and the surface cracking problem is avoided. The improvement of the slurry dispersion uniformity reduces the internal defects of the green body, and the continuous bonded carbon network formed during the carbonization process improves the material density. Finally, the obtained bottom heater has a more uniform microstructure and a longer service life.

[0031] Optionally, the binder includes phenolic resin and industrial ethanol; the mass ratio of phenolic resin to industrial ethanol is 2 - 20:1.

[0032] Phenolic resin has thermosetting characteristics and forms a three-dimensional network structure through curing reaction to ensure the bonding strength; industrial ethanol is used to adjust the viscosity and fluidity of the phenolic resin system. When the mass ratio of phenolic resin to industrial ethanol is in the range of 2 - 20:1, it can balance the wettability of the binder to the powder material in the premix and the stability of the product structure. When the addition amount of phenolic resin is small, the viscosity of the binder is small, which is not conducive to the displacement filling of powder particles during the subsequent molding process, affecting the strength of the green body before carbonization treatment; when the addition amount of phenolic resin is large, the viscosity of the premix is too high, and the other components are difficult to disperse; this ratio realizes the uniform distribution of the binder in the premix, reduces the internal defects caused by the concentrated volatilization of the solvent during subsequent treatment, improves the structural density of the bottom heater, shortens the time required for the binder to cure, and further improves the production efficiency of the bottom heater.

[0033] Optionally, the dispersants are carboxymethyl cellulose and sodium dodecylbenzenesulfonate; the mass ratio of carboxymethyl cellulose to sodium dodecylbenzenesulfonate is 1 - 80:1.

[0034] Carboxymethyl cellulose is a substance with thickening and stabilizing effects. Specifically, powdered sodium carboxymethyl cellulose can be used. It forms a steric hindrance effect by adsorbing on the surface of system particles, reducing the phenomenon of particle agglomeration in the premix; sodium dodecylbenzenesulfonate promotes the wetting and dispersion of solid particles by reducing the liquid surface tension.

[0035] Carboxymethyl cellulose delays particle sedimentation by increasing the viscosity of the aqueous phase, while sodium dodecylbenzenesulfonate reduces its agglomeration tendency by adsorbing on the surface of graphite powder and petroleum coke. After being combined in proportion, they can achieve the uniform distribution of high-solid-content materials such as graphite powder and petroleum coke under low-shear mixing conditions, avoiding stress concentration or density difference inside the subsequent molded green body caused by uneven dispersion, effectively preventing crack propagation caused by local stress release during the carbonization process of the green body, while shortening the mixing time and reducing equipment energy consumption.

[0036] Optionally, the fiber is carbon fiber, the length of the carbon fiber is 5 - 50 mm and shows a multi-level length gradient distribution. The carbon fiber includes 30 - 50% of carbon fibers with a length of 5 - 15 mm, 30 - 40% of carbon fibers with a length of 16 - 35 mm, and 10 - 30% of carbon fibers with a length of 36 - 50 mm.

[0037] Carbon fiber is an inorganic polymer fiber material composed of carbon elements. It has high strength and heat resistance at high temperatures. By mixing carbon fibers of different lengths in different proportions, carbon fibers with a length of 5 - 15 mm in the green body can fill the material gaps, reducing the porosity. Carbon fibers with a length of 16 - 35 mm enhance the toughness of the green body through staggered arrangement. Carbon fibers with a length of 36 - 50 mm form a continuous skeleton structure, improving the overall strength of the green body. By adjusting the proportion of the length range, the density and flexural strength of the green body can be balanced. Carbon fibers of different lengths form a gradient transition interface during the thermal curing stage, which can avoid crack propagation caused by stress concentration. By precisely controlling the multi-level length gradient distribution, the fibers are complementarily strengthened in different directions, which not only avoids the carbonization cracking problem caused by excessive impregnation of adhesives in traditional processes but also improves the dimensional stability of the green body during the high-temperature treatment stage.

[0038] Optionally, the particle size of the graphite powder is 200 - 1000 mesh, the particle size of the petroleum coke is 500 - 800 mesh, and the particle size of the asphalt powder is 200 - 500 mesh; the mixing time of the premix is 2 - 5 h.

[0039] The particle size of the graphite powder within the range of 200 - 1000 mesh is beneficial to improving the uniformity of the premix and the density of the bottom heater; the particle size of the petroleum coke within the range of 500 - 800 mesh helps to enhance the thermal conductivity of the material during the high-temperature treatment process; the particle size of the asphalt powder within the range of 200 - 500 mesh can promote the dispersion of the adhesive during the mixing process. Through 2 - 5 h of stirring, each component is fully dispersed to form a uniform premix system. By precisely controlling the raw material particle size and mixing parameters, the structural uniformity of the premix is optimized, avoiding the problem of extended production cycle caused by repeated impregnation in traditional processes, and at the same time reducing the surface defects caused by the escape of resin decomposition gas; effectively reducing the internal stress concentration in the bottom heater green body during the forming and heat treatment stages, reducing the risk of product cracking, shortening the mixing process time and improving the batch consistency, thereby enhancing the production efficiency and finished product quality stability of the bottom heater.

[0040] Optionally, the first molding is carried out at room temperature; the pressure of the first molding is 0.05 - 0.5 MPa, and the time of the first molding is 10 - 20 min.

[0041] Specifically, after the premix is loaded into the mold, a primary molding is performed at room temperature, effectively avoiding the material thermal expansion differences caused by high temperature, which helps to maintain the shape stability of the blank. The primary molding pressure range is 0.05 - 0.5 MPa, which can not only ensure the initial compaction of the blank but also prevent the internal fibers of the blank from breaking due to excessive pressure. The pressing time is 10 - 20 min to ensure that the premix particles fully fill the mold cavity, and at the same time avoid excessive moisture precipitation caused by long-term pressing. The combination of the primary molding pressure and time enables the graphite powder and fibers to form a uniform arrangement with the assistance of moisture, avoiding fiber damage caused by the traditional needle punching method and reducing the internal stress concentration in the blank through the pressure distribution uniformity, providing a blank basis with stable structure for the subsequent carbonization treatment.

[0042] Optionally, the pressing temperature of the secondary molding is between 120 - 250 °C; the pressure of the secondary molding is 10 - 15 MPa, and the time of the secondary molding is 20 - 120 min.

[0043] Specifically, the blank is subjected to secondary molding on a press. The temperature of the secondary molding is controlled within the range of 120 - 250 °C, which can soften but not completely decompose the binder in the blank, thus promoting material flow and increasing the material density; the pressure of the secondary molding is within the range of 10 - 15 MPa, which can effectively discharge the internal pores of the blank and improve the material density, and fill the mold cavity and complete the structure shaping within 20 - 120 min. The traditional method relies on repeated impregnation with resin and carbonization to increase density. Each carbonization process takes several hours and is prone to generating gas resulting in cracks. However, in this application, by precisely controlling the temperature, pressure, and time parameters of the secondary molding, material densification and structure forming are synchronously achieved in a single hot pressing process, not only shortening the production cycle but also avoiding surface defects caused by the release of resin decomposition gas.

[0044] Optionally, the temperature of the carbonization treatment is 850 - 1050 °C, and the time of the carbonization treatment is 5 - 8 h.

[0045] The temperature range for carbonization treatment is 850 - 1050 °C. This temperature range can effectively control the decomposition rate of organic matter, avoid the cracking of the green body caused by the violent overflow of gas. The carbonization time is 5 - 8 h, ensuring the full decomposition of organic matter and avoiding excessive energy consumption. In the initial stage of carbonization, the temperature rises to 600 °C at a rate of 50 - 80 °C per hour to complete the coking of the binder, and then rises to the target temperature at a rate of 20 - 30 °C per hour to complete the carbonization of asphalt. The gas generated during the carbonization process is directed out through a negative pressure exhaust system. The carbonized green body forms a carbon matrix with a three-dimensional network structure. The temperature in the prior art is about 1200 °C, which enables the rapid carbonization of the green body and the rapid decomposition of resin to generate a large amount of gas. While the solution of this application reduces the carbonization temperature and adjusts the carbonization time, allowing the gas to slowly release through the pores of the material, making the internal stress distribution of the green body more uniform. At the same time, the carbonization temperature forms a gradient connection with the subsequent high-temperature heat treatment, avoiding structural damage to the material due to excessive temperature difference.

[0046] Optionally, the temperature for high-temperature heat treatment is 1800 - 2300 °C, and the time for high-temperature heat treatment is 1 - 2 h.

[0047] Through high-temperature heat treatment, the rearrangement of carbon atoms is promoted to form a graphite crystal structure. At a temperature of 1800 - 2300 °C, it helps to eliminate the residual stress of the carbonized green body during the carbonization process and promotes the transformation of carbon materials into highly crystalline graphite. Among them, 1 - 2 h of high-temperature heat treatment can ensure the completion of the graphitization transformation of the material while avoiding excessive energy consumption. By precisely controlling the upper limit of temperature and the holding time, the abnormal growth of grains can be inhibited on the premise of ensuring the density of the material, thereby obtaining a bottom heater with uniform conductivity and high thermal stability, shortening the total production cycle to less than 1 / 4 of the original process.

[0048] The following specifically describes the preparation method of the bottom heater involved in this application in combination with specific examples and comparative examples: Example 1 A preparation method of a bottom heater for a single-crystal silicon furnace includes the following steps: Add 40 g of 200-mesh graphite powder, 5 g of 500-mesh petroleum coke, 5 g of 200-mesh asphalt powder, 20 g of phenolic resin, 2 g of industrial ethanol, 5 g of fiber, 1 g of sodium carboxymethylcellulose, and 0.1 g of sodium dodecylbenzenesulfonate into 30 g of deionized water, and stir for 2 h at 200 r / min to obtain a premix. The fiber is composed of 30% carbon fiber with a length of 5 - 15 mm, 30% carbon fiber with a length of 16 - 35 mm, and 40% carbon fiber with a length of 36 - 50 mm; Add the premix into the bottom heater mold and perform a first molding at room temperature and 0.05 MPa, keep the pressure for 10 minutes and then air-dry to form a blank; Heat the blank in the bottom heater mold to 120 °C and perform secondary molding at 10 MPa. The heating and molding time is 20 min to obtain a preform; Put the preform into a carbonization furnace and carbonize it at 850 °C for 5 h to obtain a carbonized blank; After heating the carbonized blank at 1800 °C for 1 h, machine it to the required size and polish the surface to obtain the bottom heater.

[0049] Example 2 A preparation method of a bottom heater for a single crystal silicon furnace includes the following steps: Add 70 g of 200-mesh graphite powder, 20 g of 500-mesh petroleum coke, 10 g of 200-mesh asphalt powder, 40 g of phenolic resin, 10 g of industrial ethanol, 25 g of fiber, 9 g of sodium carboxymethyl cellulose, and 1 g of sodium dodecylbenzenesulfonate to 60 g of deionized water, and stir at 200 r / min for 5 h to obtain a premix. The fiber consists of 50% carbon fiber with a length of 5 - 15 mm, 40% carbon fiber with a length of 16 - 35 mm, and 10% carbon fiber with a length of 36 - 50 mm; Add the premix to the bottom heater mold and perform primary molding at room temperature and 0.5 MPa. After maintaining the pressure for 20 minutes, air-dry to form a blank; Heat the blank in the bottom heater mold to 250 °C and perform secondary molding at 15 MPa. The heating and molding time is 120 min to obtain a preform; Put the preform into a carbonization furnace and carbonize it at 1050 °C for 8 h to obtain a carbonized blank; After heating the carbonized blank at 2300 °C for 2 h, machine it to the required size and polish the surface to obtain the bottom heater.

[0050] Example 3 A preparation method of a bottom heater for a single crystal silicon furnace includes the following steps: Add 60 g of 200-mesh graphite powder, 10 g of 500-mesh petroleum coke, 8 g of 200-mesh asphalt powder, 30 g of phenolic resin, 10 g of industrial ethanol, 15 g of fiber, 5 g of sodium carboxymethyl cellulose, and 0.6 g of sodium dodecylbenzenesulfonate to 50 g of deionized water, and stir at 200 r / min for 2 h to obtain a premix. The fiber consists of 40% carbon fiber with a length of 5 - 15 mm, 35% carbon fiber with a length of 16 - 35 mm, and 25% carbon fiber with a length of 36 - 50 mm; Add the premix to the bottom heater mold and perform primary molding at room temperature and 0.3 MPa. After maintaining the pressure for 15 minutes, air-dry to form a blank; Heat the blank in the bottom heater mold to 180 °C and perform secondary molding at 12 MPa. The heating and molding time is 70 min to obtain a preform; Put the preform into a carbonization furnace and carbonize it at 950 °C for 6 h to obtain a carbonized preform; After heating the carbonized preform at 2000 °C for 1.5 h, process it to the required size and polish the surface to obtain the bottom heater.

[0051] Example 4 A preparation method of a bottom heater for a single crystal silicon furnace, comprising the following steps: Put 58 g of 500-mesh graphite powder, 15 g of 600-mesh petroleum coke, 7 g of 300-mesh asphalt powder, 30 g of phenolic resin, 6 g of industrial ethanol, 20 g of fiber, 6 g of sodium carboxymethyl cellulose, 0.6 g of sodium dodecylbenzenesulfonate into 53 g of deionized water, and stir at 200 r / min for 3 h to obtain a premix. The fiber is composed of 40% carbon fiber with a length of 5 - 15 mm, 35% carbon fiber with a length of 16 - 35 mm, and 25% carbon fiber with a length of 36 - 50 mm; Add the premix to the bottom heater mold and perform a first molding at room temperature and 0.5 MPa, keep the pressure for 15 minutes and then air-dry to form a blank; Heat the blank in the bottom heater mold to 200 °C and perform a second molding at 13 MPa, and the heating and molding time is 100 min to obtain a preform; Put the preform into a carbonization furnace and carbonize it at 1000 °C for 6 h to obtain a carbonized preform; After heating the carbonized preform at 2100 °C for 1.5 h, process it to the required size and polish the surface to obtain the bottom heater.

[0052] Example 5 A preparation method of a bottom heater for a single crystal silicon furnace, different from Example 4 in that the addition amount of phenolic resin is 20 g.

[0053] Example 6 A preparation method of a bottom heater for a single crystal silicon furnace, different from Example 4 in that the addition amount of phenolic resin is 40 g.

[0054] Example 7 A preparation method of a bottom heater for a single crystal silicon furnace, different from Example 4 in that the addition amount of carboxymethyl cellulose is 1 g.

[0055] Example 8 A preparation method of a bottom heater for a single crystal silicon furnace, different from Example 4 in that the addition amount of carboxymethyl cellulose is 8 g.

[0056] Comparative Example 1 A preparation method of a bottom heater for a single-crystal silicon furnace, comprising the following steps: a 2.5D carbon fiber preform is prepared by needle punching, and the carbon fiber preform is heated at 1000 °C for 1 h to obtain a preform. 150 g of water is added to the premix prepared in Example 3 to prepare a slurry. After the heated preform is immersed in the slurry, it is dried at 100 °C for 16 h. After repeating this process only 5 times, the impregnated preform is cured at 200 °C for 80 min, and then the carbonization treatment, high-temperature heat treatment and subsequent treatment in Example 3 are carried out.

[0057] Comparative Example 2 A preparation method of a bottom heater for a single-crystal silicon furnace, which is different from Example 3 in that the heating temperature of the one-time molding is 500 °C.

[0058] Comparative Example 3 A preparation method of a bottom heater for a single-crystal silicon furnace, which is different from Example 3 in that the one-time molding time is 2 h.

[0059] Comparative Example 4 A preparation method of a bottom heater for a single-crystal silicon furnace, which is different from Example 3 in that the secondary molding time is 5 h.

[0060] Comparative Example 5 A preparation method of a bottom heater for a single-crystal silicon furnace, which is different from Example 3 in that the secondary molding temperature is 500 °C.

[0061] Comparative Example 6 A preparation method of a bottom heater for a single-crystal silicon furnace, which is different from Example 3 in that the carbonization treatment temperature is 1200 °C.

[0062] The preparation methods of the bottom heaters in Examples 2-8 and Comparative Example 1 are the same as those in Example 1.

[0063] For the bottom heaters provided in Examples 1-8 and Comparative Examples 1-6 of the present invention, the following performance tests are carried out. Table 1 shows the performance test results.

[0064] Density: Samples of the bottom heaters of Examples 1-8 and Comparative Example 1 with a size of 1 cm * 1 cm * 1 cm cube are taken respectively, and their weights are recorded as m1. The samples are immersed in water for more than 30 min, and after taking out the saturated samples and wiping off the surface liquid drops, their weights are recorded as m2. The saturated samples are suspended in water and weighed as m3. The density calculation formula is as follows:

[0065] Density CV value: 5 density values are measured for 5 samples each from Examples 1-8 and Comparative Example 1, and the average value and standard deviation of the 5 density values are calculated. The density CV value is calculated through the following formula:

[0066] Flexural strength: The bottom heater samples of Examples 1-8 and Comparative Example 1 were taken respectively, and the flexural strength of the samples was tested according to GB / T 33501-2017 "Test Method for Flexural Properties of Carbon-Carbon Composites".

[0067] Coefficient of variation (CV) of flexural strength: Five samples were taken from Examples 1-8 and Comparative Example 1 to measure five flexural strengths. The average value and standard deviation of the five flexural strengths were calculated, and the CV value of flexural strength was calculated by the following formula:

[0068] Resistivity: The bottom heater samples of Examples 1-8 and Comparative Example 1 were taken respectively, and the normal temperature resistivity of the samples was tested according to GB / T 24525-2009 "Determination Method for Resistivity of Carbon Materials".

[0069] Coefficient of variation (CV) of resistivity: Five samples were taken from Examples 1-8 and Comparative Example 1 to measure five resistivities. The average value and standard deviation of the five resistivities were calculated, and the CV value of resistivity was calculated by the following formula:

[0070] Porosity: m1, m2 and m3 of each sample in Examples 1-8 and Comparative Example 1 in density measurement were taken respectively, and the porosity of the bottom heater sample was calculated by the following formula:

[0071] Three samples were taken for each example and comparative example respectively, and the average value was taken for the test results. The specific test results are shown in Table 1.

[0072] Table 1 Performance Results

[0073] In this application, two molding processes are used to replace the traditional needling and repeated impregnation steps, avoiding the problem of gas escape caused by excessive use of adhesives. The combination of air drying and staged molding shortens the blank forming cycle, eliminates internal stress concentration points at the same time. The directional arrangement and multi-level gradient distribution of fibers enhance the thermal shock resistance of the material, and the synergistic effect of carbonization and high-temperature treatment improves the thermal conductivity stability of the final product, and improves the service life of the bottom heater.

[0074] The bottom heater prepared by the preparation method of the present application has the internal porosity of the material reduced to less than 8%. The carbonization time is shortened to 1 / 4 of the traditional process, and the production cycle is compressed from 60 - 90 days to 10 - 15 days. Specifically, the production cycle of Comparative Example 1 is 90 days, and the production cycles of Examples 1 - 8 and Comparative Examples 2 - 6 are 10 - 15 days. Moreover, the product density uniformity of the bottom heater prepared according to the preparation method of the present application reaches more than 97%, effectively improving the service life and production efficiency of the bottom heater.

[0075] Finally, it should be noted that the above are only optional examples of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a bottom heater for a single crystal silicon furnace, characterized in that, It includes the following steps: Add graphite powder, petroleum coke, asphalt powder, binder, fiber and dispersant into water in proportion, and mix evenly to obtain a premix; Perform primary molding on the premix in a bottom heater mold, and then air-dry it to form a blank; Perform high-temperature secondary molding on the blank in a bottom heater mold to obtain a preform; Carry out carbonization treatment on the preform to obtain a carbonized blank; Perform high-temperature heat treatment on the carbonized blank to obtain a bottom heater.

2. The preparation method according to claim 1, characterized in that, The premix is made from the following raw materials in parts by weight: 40-70 parts of graphite powder, 5-20 parts of petroleum coke, 5-10 parts of asphalt powder, 22-50 parts of binder, 5-25 parts of fiber, 1.1-10 parts of dispersant, and 30-60 parts of deionized water.

3. The preparation method according to claim 1, characterized in that, The binder includes phenolic resin and industrial ethanol; The mass ratio of the phenolic resin to the industrial ethanol is 2-20:

1.

4. The preparation method according to claim 1, characterized in that, The dispersant includes carboxymethyl cellulose and sodium dodecylbenzenesulfonate; The mass ratio of the carboxymethyl cellulose to the sodium dodecylbenzenesulfonate is 1-80:

1.

5. The preparation method according to claim 1, characterized in that, The particle size of the graphite powder is 200-1000 mesh, the particle size of the petroleum coke is 500-800 mesh, and the particle size of the asphalt powder is 200-500 mesh; the fiber is carbon fiber, and the length of the carbon fiber is 5-50 mm; The mixing time of the premix is 2-5 h.

6. The preparation method according to claim 5, wherein The carbon fiber includes 30-50% of carbon fiber with a length of 5-15 mm, 30-40% of carbon fiber with a length of 16-35 mm, and 10-40% of carbon fiber with a length of 36-50 mm.

7. The preparation method according to claim 1, characterized in that, The primary molding is normal-temperature pressing; The pressure of the primary molding is 0.05-0.5 MPa, and the time of the primary molding is 10-20 min.

8. The preparation method according to claim 1, characterized in that, The pressing temperature of the secondary molding is between 120-250 °C; The pressure of the secondary molding is 10-15 MPa, and the time of the secondary molding is 20-120 min.

9. The preparation method according to claim 1, characterized in that, The temperature of the carbonization treatment is 850-1050 °C, and the time of the carbonization treatment is 5-8 h.

10. The preparation method according to claim 1, characterized in that, The temperature of the high-temperature heat treatment is 1800-2300 °C, and the time of the high-temperature heat treatment is 1-2 h.

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