Method and device for synthesizing low-nitrogen-content high-purity silicon carbide powder

Abstract

The application relates to the field of silicon carbide synthesis, in particular to a synthesis method and device of low-nitrogen-content high-purity silicon carbide powder, which comprises the following steps: (1) mixing high-purity carbon powder and silicon powder and then loading into a reaction container, the inner wall of the reaction container being coated with a dense high-temperature-resistant coating; (2) performing vacuum degassing treatment on the reaction container through a gas guide pipe and introducing inert gas to clean the reaction environment; (3) heating the reaction container to a silicon carbide synthesis temperature, forming a directional gas flow discharged from the reaction container along the gas guide pipe; (4) under high-temperature conditions, realizing plugging of the gas guide pipe through silicon carbide deposition in the gas guide pipe, and isolating external gas pollution; and (5) completing crystal type conversion of the silicon carbide powder and obtaining the low-nitrogen-content high-purity silicon carbide powder. The application solves the problem that nitrogen impurities are difficult to remove in the traditional process by coating the dense high-temperature-resistant coating and setting the gas guide pipe to physically isolate external nitrogen pollution.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of silicon carbide synthesis, in particular to a method and device for synthesizing low-nitrogen-content high-purity silicon carbide powder. BACKGROUND

[0002] Silicon carbide (SiC) is a third-generation wide-bandgap semiconductor material with excellent properties such as high thermal conductivity, high breakdown field strength, and high saturation electron drift rate, which has shown great application potential in high-temperature, high-frequency, high-power, and radiation-resistant device fields. In particular, in the field of AR (Augmented Reality) glasses, colorless and transparent high-purity semi-insulating silicon carbide materials have become an ideal choice to replace traditional materials due to their ultra-thin, high heat dissipation, large field of view, and no "rainbow stripe" interference. However, the preparation of such materials requires low-nitrogen-content (nitrogen concentration less than 1×10 16 cm 3 ) and high-purity (≥6N) silicon carbide powder, which poses a great challenge to existing synthesis techniques.

[0003] Currently, the preparation of silicon carbide powder mainly uses self-propagating high-temperature synthesis (SHS), which has the advantages of simple process, energy efficiency, and controllable crystal type. However, this process has a significant bottleneck in nitrogen impurity control: first, the absorption of nitrogen gas from the environment by porous materials such as carbon powder, silicon powder, and graphite crucible in the reaction system cannot be avoided; second, traditional processes rely on vacuum degassing, high-temperature heating, and inert gas cleaning to remove nitrogen impurities, but due to the low temperature range of the insulation material and the pore structure of the graphite crucible, the nitrogen desorption efficiency is insufficient, resulting in a nitrogen concentration in the final powder as high as 5×10 16 cm 3 , which cannot meet the needs of high-end applications. In addition, there is a lack of effective isolation measures for nitrogen absorption by insulation materials in existing technologies, and nitrogen impurities easily penetrate into the powder from the crucible wall during the reaction process, further exacerbating pollution.

[0004] Although existing technologies attempt to reduce nitrogen content by mixing organic volatile nitrogen gas and optimizing vacuum treatment parameters, these methods still cannot completely block external nitrogen pollution, and the process complexity and cost are significantly increased. Therefore, how to physically isolate the reaction system from external nitrogen and optimize the gas flow path to suppress nitrogen impurity backflow has become the key to breaking through the preparation technology of high-purity silicon carbide powder. SUMMARY

[0005] The present application is to overcome the defects in the prior art that the synthesis method of silicon carbide powder cannot completely block external nitrogen pollution, and the process complexity and cost are significantly increased, therefore a method and device for synthesizing low-nitrogen-content high-purity silicon carbide powder are provided to overcome the above shortcomings.

[0006] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution:

[0007] In a first aspect, the present invention provides a method for synthesizing high-purity silicon carbide powder with low nitrogen content, comprising the following steps:

[0008] (1) High-purity carbon powder and silicon powder are mixed and then loaded into a reaction vessel. The inner wall of the reaction vessel is coated with a dense high-temperature resistant coating.

[0009] (2) Vacuum degassing is performed on the reaction vessel through the gas delivery pipe, and inert gas is introduced to clean the reaction environment;

[0010] (3) Heat the reaction vessel to the silicon carbide synthesis temperature to form a directional gas flow that is discharged outward from the inside of the reaction vessel along the gas guide pipe;

[0011] (4) Under high temperature conditions, the gas duct is sealed by silicon carbide deposition inside the gas duct to isolate external gas pollution;

[0012] (5) Complete the crystal transformation of silicon carbide powder to obtain high-purity silicon carbide powder with low nitrogen content.

[0013] While the self-propagating high-temperature synthesis method can achieve efficient preparation of silicon carbide powder in existing technologies, its nitrogen impurity control relies on traditional methods such as vacuum degassing, high-temperature heating, and inert gas cleaning. However, these methods have significant limitations: on the one hand, the adsorption of nitrogen by porous media such as graphite crucibles and insulation materials in the reaction system is difficult to completely remove, and even after multiple argon cleanings, residual nitrogen still permeates into the powder through the pores; on the other hand, the thermal field design of traditional processes results in the insulation layer being in a low-temperature region, inhibiting effective nitrogen desorption, and the final nitrogen concentration in the powder is still as high as 5 × 10⁻⁶. 16 pcs / cm 3 The scale is significant. Furthermore, existing technologies lack physical isolation mechanisms for nitrogen pollution sources. During the reaction, external nitrogen gas can flow back through the crucible wall or gas pipes, further exacerbating pollution. Although some improvements attempt to reduce nitrogen content by adding volatile organic compounds or optimizing vacuum parameters, these measures often introduce additional impurities or increase process complexity, making it difficult to balance purity and efficiency. Therefore, systematically blocking nitrogen pollution pathways while maintaining the high efficiency of the self-propagating method has become a key challenge in the preparation of high-purity silicon carbide powder.

[0014] To address the aforementioned problems, this invention proposes a low-nitrogen synthesis method based on physical isolation and dynamic airflow control. The technical solution is designed based on an in-depth analysis of nitrogen contamination pathways. First, the inner wall of the reaction vessel is coated with a dense, high-temperature resistant coating (such as TaC, HfC, etc.). Through the material's low gas permeability, nitrogen adsorbed by external insulation materials is completely blocked from permeating into the reaction system through the crucible wall. This design directly addresses the nitrogen doping problem caused by the pores of graphite crucibles in existing technologies, transforming the traditional passive denitrification process into active isolation. Second, the structure and function of the gas guide pipe are redefined: during the vacuum degassing and inert gas cleaning stages, the gas guide pipe serves as a channel for nitrogen exhaust; while during the high-temperature reaction stage, by controlling the axial temperature gradient to form a directional airflow from the inside out, unreacted gases and volatiles are forced to exit along the gas guide pipe, preventing external gas backflow. More importantly, the gas guide pipe gradually becomes blocked at high temperatures due to silicon carbide deposition. This phenomenon is creatively transformed into a self-sealing mechanism—as the reaction proceeds, deposits accumulate in the gas guide pipe and eventually seal the channel, completely blocking the possibility of external nitrogen backflow in subsequent stages. This "dynamic closure" mechanism not only requires no additional operation, but also cleverly utilizes reaction byproducts to achieve system self-protection, significantly improving the reliability and automation of the process.

[0015] From a technical perspective, this method achieves multi-level blocking of nitrogen pollution through the synergistic effect of coating isolation, directional airflow, and gas duct sealing. The dense coating prevents the physical penetration of external nitrogen at the source, the directional airflow optimizes the discharge efficiency of reactant gases through thermal field design, and the self-sealing mechanism of the gas duct fundamentally eliminates the possibility of nitrogen backflow. Compared to the limitations of traditional processes that rely on a single denitrification method, this scheme elevates denitrification efficiency to a new level through systematic design. Furthermore, the gas duct sealing mechanism eliminates the need for complex external control devices, reducing equipment costs and operational complexity; the selection of coating materials balances high-temperature resistance and economy (e.g., the stability of TaC coatings at high temperatures and the low-cost compatibility of SiC coatings), making the scheme potentially suitable for industrial application. Ultimately, this method can reduce the nitrogen concentration of silicon carbide powder to 1×10⁻⁶ while maintaining the high efficiency and energy-saving advantages of the self-propagating method. 16 pcs / cm 3 The following technology meets the stringent purity requirements of high-end applications such as AR glasses, and its process stability is significantly superior to existing technologies. This technological approach not only solves the long-standing problem of nitrogen contamination in the preparation of high-purity silicon carbide powder, but also provides reliable technical support for the large-scale application of third-generation semiconductor materials.

[0016] Preferably, the dense high-temperature resistant coating is selected from at least one of tantalum carbide, hafnium carbide, and zirconium carbide.

[0017] Preferably, the inner wall of the air guide tube is coated with a high-temperature resistant coating, and the diameter of the air guide tube is 2~5 mm.

[0018] The innovative design of the gas conduit, featuring a high-temperature resistant coating on its inner wall and a pore size of 2-5 mm, represents a breakthrough in both precise intervention in nitrogen contamination pathways and adaptive optimization of the process. In existing technologies, gas conduits typically serve only as gas emission channels, with structural parameters (such as pore size) largely based on empirical settings. Furthermore, the wall material is often graphite or uncoated porous media, leading to two core problems: first, at high temperatures, the graphite gas conduit itself may release impurities or adsorb nitrogen from the environment, becoming a secondary source of pollution; second, conventional gas conduit pore size design lacks synergistic consideration of airflow dynamics and deposition / blocking mechanisms—if the pore size is too large, external nitrogen can easily enter the reaction system through backflow, while if the pore size is too small, gas emission may be impeded, affecting the reaction process.

[0019] This technology, by introducing a high-temperature resistant coating (such as TaC, SiC, etc.) and defining a specific pore size range, transforms the gas delivery tube from a single-function component into an intelligent module that combines contamination isolation and dynamic sealing for the first time. The high-temperature resistant coating not only prevents the gas delivery tube's own materials (such as graphite) from releasing impurities or adsorbing nitrogen at high temperatures, but also reduces gas turbulence by decreasing the surface roughness of the tube wall, thereby optimizing the stability of the directional airflow. The 2-5 mm pore size range, as verified experimentally, achieves a balance between two stages of function: in the initial stage of the reaction, this pore size ensures the containment of unreacted gases (such as residual nitrogen) and volatiles (such as Si, SiC). x (etc.) efficiently discharged, avoiding pressure fluctuations caused by airflow blockage; in the later stages of the reaction, as SiC at high temperatures... x The directional deposition of atmosphere within the gas delivery tube, within this pore size range, ensures that the deposits accumulate within a reasonable time to form an effective seal (avoiding premature blockage due to excessively small pore size or seal failure due to excessively large pore size), while also enabling dynamic control of the sealing effect through the self-limiting growth of the deposits.

[0020] This design breaks away from the traditional mindset that "the gas guide pipe is merely a passive channel." It creatively utilizes the physical properties of reaction byproducts (such as SiC deposition tendency) and thermal conditions (such as temperature gradients) to deeply couple the "structural parameters" of the gas guide pipe with the "process," forming a closed-loop control. This synergistic design of microscopic mass transfer mechanisms and macroscopic process timing allows the gas guide pipe to automatically switch between "open exhaust in the early stage and closed isolation in the later stage" without external intervention, significantly improving process reliability and automation. Compared to existing technologies that control gas paths through complex valves or external sealing devices, this feature achieves functional adaptability through the intrinsic properties of materials and structures, reducing equipment complexity and cost while avoiding the leakage risks introduced by mechanical seals. Therefore, this technical feature is not a simple improvement on existing gas guide pipe designs, but rather a systematic solution to the inherent contradiction between nitrogen pollution control and process stability through synergistic innovation across scales (microscopic coatings - macroscopic structures) and processes (airflow dynamics - deposition dynamics).

[0021] Preferably, in step (2), the vacuum degree of the vacuum degassing process is lower than 5 × 10⁻⁶. -6 mbar, the inert gas is argon with a purity of not less than 6N.

[0022] Preferably, in step (3), the directional airflow is formed by controlling the axial temperature gradient of the reaction vessel to be 100~200℃.

[0023] In the process of silicon carbide powder synthesis, the precise control of gas flow path is the key factor that determines the efficiency of nitrogen impurity removal. However, the existing technology for regulating the temperature distribution of the thermal field is mostly limited to uniform heating or simple division of upper and lower temperature zones, and fails to deeply couple the temperature gradient design with the gas dynamics behavior.

[0024] In traditional processes, the temperature distribution inside the reaction vessel typically exhibits a static pattern of low temperature at the top and high temperature at the bottom. While this design is beneficial for heat preservation and energy saving, it leads to two core problems: First, the high-temperature region at the bottom promotes the reaction of gases (such as Si vapor, SiC...) x The intermediate diffuses upwards, but due to the presence of the low-temperature region at the top, the kinetic energy of gas molecules decreases, easily forming disordered vortices within the container, prolonging the gas residence time and increasing the probability of contact between nitrogen and powder. Secondly, the nitrogen adsorption capacity of external insulation materials (such as graphite felt) is enhanced in the low-temperature region, while the desorption efficiency decreases, and residual nitrogen may seep back into the reaction system through thermal convection. Although existing technologies have attempted to improve gas flowability by increasing the vacuum level or increasing the frequency of argon purging, these methods can only passively reduce the nitrogen concentration and cannot actively guide the gas to be discharged in a specific direction. Furthermore, excessive reliance on external equipment leads to increased energy consumption and costs.

[0025] The axial temperature gradient control (100~200℃) proposed in this invention fundamentally reconstructs the interaction logic between the thermal field and the airflow: by precisely setting the axial temperature difference of the reaction vessel, a stable thermodynamic driving force is formed in the vertical direction, causing the reactant gas generated in the high-temperature zone (bottom) to rise naturally due to the density difference. Simultaneously, the moderate cooling in the low-temperature zone (top) is not complete cooling, but rather maintains sufficient kinetic energy to propel the gas continuously towards the guide pipe. This gradient range (100~200℃) is not selected empirically, but is based on the critical threshold of the gas molecule mean free path and viscous flow state—when the temperature gradient is below 100℃, the thermal energy of the gas molecules is insufficient to overcome viscous resistance, easily forming local turbulence; while when the gradient exceeds 200℃, the excessively low temperature at the top can cause premature gas condensation or deposition, blocking the guide pipe or interfering with the crystal transformation process. By strictly limiting the gradient within this range, it ensures that the gas migrates directionally in a laminar flow manner while avoiding uncontrolled phase transitions caused by sudden temperature changes. More importantly, this gradient design works synergistically with the gas delivery pipe structure (such as pore size and coating): the directional airflow maintains a stable velocity within the gas delivery pipe due to the temperature gradient, allowing unreacted gases and nitrogen to be efficiently carried away from the reaction system, while SiC... x Volatile substances are concentrated and deposited in specific areas of the air duct under the guidance of airflow, accelerating the sealing process.

[0026] Compared to the traditional temperature field design of "uniform high temperature across the entire domain" or "simple upper and lower partitioning" in traditional processes, this technology uses gradient quantization control to convert thermodynamic potential energy into gas kinetic energy, achieving a dynamic balance between airflow direction, velocity, and deposition behavior. This balance solves the nitrogen retention problem caused by disordered flow and avoids the risk of external contamination introduced by forced airflow (such as mechanical pump suction). Furthermore, the gradient range setting implicitly considers the compatibility of the material's thermal expansion coefficient—the deformation difference between the graphite crucible and the coating material at a gradient of 100~200℃ is controllable, preventing coating cracking or sealing failure due to thermal stress concentration. Therefore, this temperature gradient control is not a simple optimization of existing heating modes, but rather integrates the previously fragmented "heating," "venting," and "sealing" stages into a closed-loop self-regulating system through a multi-physics field coupling design of the thermal field, airflow, and deposition. This significantly improves the robustness of the process and nitrogen removal efficiency, representing a substantial creative breakthrough.

[0027] Secondly, the present invention also provides an apparatus for implementing the synthesis method, comprising:

[0028] The reaction vessel has a dense, high-temperature resistant coating on its inner wall.

[0029] A gas delivery pipe connected to the reaction vessel, the inner wall of which is coated with a high-temperature resistant coating;

[0030] Vacuum system, used for vacuum degassing of reaction vessels;

[0031] An inert gas circulation system is used to clean the reaction environment;

[0032] Temperature control system, used to adjust the axial temperature gradient of the reaction vessel.

[0033] The design of high-purity silicon carbide powder preparation equipment has long been constrained by the contradiction between nitrogen contamination control and process complexity. Existing technologies often use graphite crucibles to directly load raw materials. While their porous structure provides good high-temperature resistance, it becomes the primary pathway for nitrogen permeation. The adsorption of nitrogen by graphite materials at high temperatures is difficult to completely remove using traditional vacuum or inert gas cleaning methods. Furthermore, insulation materials (such as graphite felt) have low nitrogen desorption efficiency at low temperatures, leaving the reaction system constantly exposed to nitrogen contamination risks. Although some improvements attempt to block nitrogen permeation by adding multi-layered isolation chambers or complex sealing structures, these designs often significantly increase equipment complexity and manufacturing costs, and are difficult to adapt to the high-temperature dynamic environment required for self-propagating methods. For example, while multi-layered chambers can physically isolate the external environment, they hinder the free flow of reaction gases, leading to a decrease in synthesis efficiency; and mechanical seals lack durability at extreme temperatures, easily causing leaks due to differences in thermal expansion. Furthermore, in traditional devices, the gas duct is typically used as a passive exhaust channel. Its structural design (e.g., straight pipe, uniform aperture) does not consider the correlation between gas flow patterns and deposition behavior, leading to two core defects: first, external nitrogen may flow back into the reaction vessel through the gas duct, especially under pressure fluctuations; second, the disordered gas emission path makes SiC... x Volatile substances are difficult to deposit in a directional manner, making it impossible to form an effective self-sealing mechanism. These problems together lead to a vicious cycle of "insufficient desorption efficiency - difficulty in preventing secondary pollution" in the control of nitrogen impurities in existing equipment, which seriously restricts the industrial production of high-purity silicon carbide powder.

[0034] To address the aforementioned technical challenges, the design concept of this invention stems from the dual requirements of systematically blocking nitrogen contamination pathways and improving process adaptability. The innovative design of coating the inner wall of the reaction vessel (a) with a dense, high-temperature resistant coating (such as TaC, HfC, etc.) directly addresses the nitrogen permeation problem caused by the pores of the graphite crucible—the high density of the coating material (gas permeability less than 1×10⁻⁶) is crucial. -6 cm 2The chemical inertness of the graphite substrate (S) not only blocks the diffusion path of nitrogen adsorbed by the external insulation material, but also prevents the graphite substrate from releasing impurities that contaminate the powder at high temperatures. This design breaks through the passive mode of "denitrification relying solely on gas replacement" in traditional processes, elevating physical isolation to a core control measure. The accompanying gas guide tube (b) achieves functional reconstruction through a specific combination of a high-temperature resistant coating on its inner wall and a pore size of 2-5 mm: in the initial stage of the reaction, the coating blocks the adsorption of nitrogen by the gas guide tube's own material (such as graphite), while optimizing the smoothness of the tube wall surface to reduce airflow turbulence; the pore size design, through matching the viscous flow state with the molecular free path, ensures efficient discharge of unreacted gas. More importantly, as the reaction proceeds, the SiC inside the gas guide tube... x The directional deposition of the atmosphere forms a self-sealing structure on the coating surface. This process is enhanced by precise control of the axial temperature gradient (100~200℃) through a temperature control system (e). The gradient thermal field drives the gas to flow directionally along the duct, while moderate cooling at the top promotes the accumulation of deposits in specific areas, ultimately achieving dynamic sealing of the duct. This integrated design of "structure-process-materials" upgrades the duct from a single exhaust component into an intelligent module that combines pollution isolation and process self-adaptation.

[0035] The synergistic innovation of the various subsystems within the device further highlights its technological breakthrough. The combination of the vacuum system (c) and the inert gas circulation system (d) is not a simple superposition, but rather achieves functional complementarity through sequential control: the vacuum degassing stage prioritizes the removal of high molecular weight impurities (such as adsorbed O2 and H2O), while the high-purity argon circulation cleaning targets and replaces light molecules such as nitrogen. The phased implementation of both minimizes the total amount of residual gas. The axial gradient control (100~200℃) of the temperature control system (e) is not an isolated parameter setting, but is deeply coupled with the gas guide tube structure and coating characteristics. For example, when the gradient is set to 150℃, the high temperature at the bottom (2100℃) accelerates the carbon-silicon reaction to generate β-SiC, while the moderate cooling at the top (1950℃) suppresses excessive Si vapor volatilization, while maintaining sufficient thermodynamic driving force to propel the gas through the gas guide tube. This thermal field design not only optimizes reaction kinetics but also enhances the directional migration efficiency of gas through natural convection generated by the temperature difference, avoiding the risk of external contamination introduced by forced exhaust by mechanical pumps. More notably, the device achieves process self-optimization through closed-loop feedback from its various subsystems: a vacuum sensor monitors the pressure in the reaction vessel in real time, dynamically adjusting the amount of inert gas injected to balance the gas flow rate; a temperature sensor provides feedback on the axial temperature difference, automatically adjusting the heating power to maintain the preset gradient. This intelligent control mechanism integrates isolated operating units in traditional devices into an organic whole, significantly improving process stability and repeatability.

[0036] From a technical perspective, this device achieves simultaneous improvements in nitrogen pollution control and process efficiency through multi-level innovation. The dense coating completely blocks over 90% of external nitrogen permeation paths, the directional airflow design increases unreacted gas emission efficiency by over 40%, and the self-sealing mechanism of the gas guide pipe reduces the risk of nitrogen backflow to a negligible level. Compared to traditional devices, this solution can reduce the nitrogen concentration of powdered materials from 5 × 10⁻⁶ under the same process conditions. 16 pcs / cm 3 Reduced to 1×10 16 pcs / cm 3 Furthermore, the process eliminates the need for complex external sealing or purification equipment. Moreover, the synergistic effect of temperature gradient and airflow dynamics results in a more uniform SiC crystal transformation, significantly reducing the dispersion of powder particle size distribution and substantially improving the yield of subsequent single crystal growth. At the industrial level, the device employs a standardized modular design (such as replaceable coated crucibles and graded gas guide pipe assemblies), reducing maintenance costs and adapting to flexible production with varying purity requirements.

[0037] In summary, this device is not a partial improvement on existing equipment, but rather a redefinition of the technological paradigm for the preparation of high-purity silicon carbide powder through interdisciplinary technological integration (materials science, thermodynamics, and fluid mechanics) and system-level innovation. Its creativity is reflected in the comprehensive leap from "passive defense" to "active isolation," from "experience-driven" to "model optimization," and from "single function" to "adaptive collaboration," providing disruptive equipment support for the large-scale application of third-generation semiconductor materials.

[0038] Preferably, the reaction vessel includes a thermal insulation base with a lower temperature measuring hole, an isostatic graphite crucible disposed above the thermal insulation base, and a matching graphite crucible cover, and also includes insulation components for the sides and top of the isostatic graphite crucible.

[0039] The gas guide pipe is positioned upwards at the top of the graphite crucible lid and is connected to the isostatic graphite crucible.

[0040] Preferably, a graphite paper interlayer is provided between the crucible lid and the static pressure graphite crucible.

[0041] During the high-temperature synthesis of silicon carbide powder, volatiles (such as Si vapor, SiC) are released. xThe deposition of intermediates can easily cause physical and chemical corrosion to the coating on the inner wall of the crucible lid. In traditional processes, the difference in thermal expansion coefficients between the crucible lid and the crucible in direct contact may lead to sealing failure, and once the coating surface is covered by deposits, its dense isolation function will be significantly weakened. This invention adds a graphite paper interlayer between the crucible lid and the static pressure graphite crucible, achieving triple optimization through the high-temperature stability and flexible deformation characteristics of graphite paper: First, the graphite paper, as a sacrificial layer, preferentially adsorbs volatiles, avoiding direct contact with the coating surface of the crucible lid, greatly reducing the risk of coating peeling due to deposit impact or chemical corrosion; Second, the elastic compression characteristics of graphite paper can adaptively compensate for the small gaps between the crucible and the lid caused by thermal expansion and contraction, maintaining a dynamic sealing effect at high temperatures and preventing external nitrogen from seeping into the reaction system through gaps; Third, high-purity graphite paper (≥6N) itself does not introduce additional impurities, and its porous structure can partially adsorb residual gas molecules at high temperatures, further purifying the reaction environment in conjunction with vacuum and inert gas cleaning.

[0042] Preferably, the gas guide pipe is eccentrically connected to the reaction vessel, and the length of the gas guide pipe is 1 / 3 to 1 / 2 of the height of the reaction vessel.

[0043] The gas duct is eccentrically connected to the reaction vessel, and its length is limited to 1 / 3 to 1 / 2 of the vessel's height. This design significantly improves the efficiency of nitrogen contamination control and process stability by optimizing airflow dynamics and deposition behavior. The centrally symmetrical layout of traditional gas ducts easily causes vortices to form at the top of the vessel, resulting in prolonged residence time of unreacted gases (such as residual nitrogen) and increasing the risk of secondary contact with powder. The eccentric connection, through its asymmetrical flow channel design, breaks the symmetry of gas flow, forcing the airflow to migrate directionally along one side of the gas duct, reducing turbulence and accelerating impurity removal.

[0044] The setting of the gas duct length (1 / 3 to 1 / 2 of the reaction vessel height) balances airflow resistance and deposition sealing efficiency. If the length is too short, the gas flow rate is too high, easily leading to uneven deposition distribution and difficulty in forming an effective seal; if the length is too long, it increases airflow resistance and affects the degassing efficiency in the initial stage of the reaction. For example, when the gas duct length is 40% of the reactor height, its physical position extending to the upper part of the vessel can fully utilize the axial temperature gradient of the thermal field (high temperature at the bottom, moderate cooling at the top), allowing the SiC generated in the high-temperature zone to... x As the volatiles rise, they condense naturally due to the temperature gradient and are deposited directionally along the inner wall of the gas duct. The eccentric layout further guides the deposits to concentrate in specific areas of the duct, accelerating the sealing process.

[0045] Furthermore, this length range is also compatible with the duct aperture (2~5 mm) and coating properties (such as the surface energy of SiC coatings). By adjusting the matching relationship between the deposition growth rate and the gas flow velocity, precise synchronization between the sealing action and the crystal transformation stage is ensured. Compared with the traditional centrosymmetric duct design, the synergistic effect of the eccentric connection and length limitation not only reduces the probability of nitrogen backflow but also enhances the reliability of the self-sealing mechanism through directional deposition. This enables adaptive optimization of the process without the need for external control devices, while reducing equipment manufacturing costs and maintenance complexity.

[0046] Preferably, the insulation component is carbon fiber felt or graphite felt;

[0047] The high-temperature resistant coating on the inner wall of the air duct is silicon carbide.

[0048] Therefore, this application has the following beneficial effects:

[0049] This invention physically isolates the reaction vessel from external nitrogen contamination by coating it with a dense, high-temperature-resistant coating and a gas delivery pipe. Combined with axial temperature gradient control to create a directional airflow for efficient removal of unreacted gases, and the self-sealing effect achieved by silicon carbide deposition within the gas delivery pipe to prevent nitrogen backflow, this systematically solves the problem of difficult nitrogen impurity removal in traditional processes, ultimately achieving a nitrogen concentration below 1×10⁻⁶. 16 pcs / cm 3 Silicon carbide powder with a purity of 6N or higher; while maintaining the high efficiency and energy-saving characteristics of the self-propagating method, this process significantly improves production stability and material consistency through device structure innovation and synergistic optimization of process parameters. It can meet the stringent requirements of high-end fields such as AR glasses for high-purity silicon carbide materials without relying on complex external equipment, and has the advantages of low cost and high reliability for industrial applications. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the structure of the crucible device for synthesizing high-purity silicon carbide powder with low nitrogen content in Embodiment 1 of the present invention.

[0051] Figure 2 The image shows the powder obtained in Example 2.

[0052] Figure 3 The image shows the powder obtained in Comparative Example 1.

[0053] Figure 4 The image shows the powder obtained in Comparative Example 2.

[0054] Figure 5 The image shows the powder obtained in Comparative Example 3.

[0055] The components include: reaction vessel 10, thermal insulation base 11, isostatic graphite crucible 12, graphite crucible cover 13, reaction chamber 14, dense high-temperature resistant coating 15, graphite paper interlayer 16, gas guide pipe 17, high-temperature resistant coating 18, insulation component 19, temperature measuring hole 20, temperature control system 21, vacuum system 22, and inert gas circulation system 23. Detailed Implementation

[0056] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0057] Example 1

[0058] like Figure 1 As shown, a crucible device for synthesizing high-purity silicon carbide powder with low nitrogen content includes a set of reaction containers 10. The reaction containers 10 include, from bottom to top, a thermal insulation base 11, an isostatic graphite crucible 12, and a graphite crucible cover 13 that matches the isostatic graphite crucible 12.

[0059] The area between the isostatic graphite crucible 12 and the graphite crucible cover 13 forms a reaction chamber 14. The reaction chamber 14 is used to store high-purity carbon powder and silicon powder. When the isostatic graphite crucible 12 is heated, the high-purity carbon powder and silicon powder inside the reaction chamber 14 react to form silicon carbide powder.

[0060] In order to control the nitrogen content inside the reaction chamber 14 during the reaction of carbon powder and silicon powder, in this embodiment, the inner wall of the reaction chamber 14 is coated with a dense high-temperature resistant coating 15. The material of the dense high-temperature resistant coating 15 can be selected from at least one of tantalum carbide, hafnium carbide, and zirconium carbide, so as to have the ability to maintain structural strength and density during the volatilization of SiC powder and long-term use in Si atmosphere, so as to prevent nitrogen impurities such as nitrogen gas outside the isostatic graphite crucible 12 from penetrating into the interior of the reaction chamber 14 and contaminating the SiC powder.

[0061] In addition, in order to prevent the volatiles formed by carbon powder and silicon powder during the reaction process from depositing and damaging the dense high-temperature resistant coating 15 on the surface of the graphite crucible lid 13, in a preferred embodiment, a graphite paper interlayer 16 is also provided between the graphite crucible lid 13 and the isostatic graphite crucible 12, thereby achieving the barrier effect against gaseous volatiles.

[0062] In order to better guide the impurities and volatile gases inside the reaction chamber 14 to flow out of the reaction chamber 14, in this embodiment, a hollow gas guide pipe 17 connected to the reaction chamber 14 is provided at a relatively eccentric position of the graphite crucible cover 13. The gas guide pipe 17 is made of graphite material and its length is 1 / 3 to 1 / 2 of the height of the reaction container 10.

[0063] In a preferred embodiment, similar to the reaction chamber 14, the inner wall of the gas guide tube 17 is also coated with a high-temperature resistant coating 18. The material of this high-temperature resistant coating 18 includes, but is not limited to, TaC or other high-temperature resistant and corrosion-resistant carbide materials, such as hafnium carbide and zirconium carbide. Here, to reduce the preparation cost of the high-temperature resistant coating 18, the coating inside the gas guide tube 17 can preferably be made of SiC coating material. During the SiC powder synthesis process described in this embodiment, the temperature at the location of the gas guide tube 17 is relatively low, and the SiC coating material will not volatilize significantly. However, the inner wall of the aforementioned reaction chamber 14 is not suitable for using a SiC coating due to the higher ambient temperature.

[0064] In addition, in some preferred embodiments of this example, the reaction vessel 10 further includes a heat insulation component 19 located on the side of the isostatic graphite crucible 12 and on the top of the graphite crucible lid 13. The material of the heat insulation component 19 can be selected from carbon fiber felt or graphite felt. The heat insulation component 19 located on the top of the graphite crucible lid 13 usually has a certain thickness, but is generally less than the thickness of the thermal insulation base 11. In order to accurately measure the temperature inside the reaction chamber 14, a temperature measuring hole 20 is provided at both the thermal insulation base 11 and the heat insulation component 19 located on the top of the graphite crucible lid 13. Thus, during the powder synthesis process, the axial temperature gradient of the furnace body can be adjusted based on the temperature values ​​measured by the upper and lower temperature measuring holes 20.

[0065] In order to heat the isostatic graphite crucible 12, a temperature control system 21 is provided outside the reaction vessel 10 in this embodiment. It can be one or more sets of induction heating coils or resistance heaters, thereby realizing temperature control of the reaction chamber 14 inside the isostatic graphite crucible 12.

[0066] In addition, to further remove impurity gases inside the reaction chamber 14 before the reaction, a preferred embodiment of this invention also includes a vacuum system 22 and an inert gas circulation system 23. The vacuum system 22 degasses the impurity gases inside the reaction chamber 14 through the gas guide pipe 17, thereby removing as many impurity gases as possible. The inert gas circulation system 23 introduces inert gas into the reaction chamber 14 to clean the reaction environment. Both the vacuum system 22 and the inert gas circulation system 23 can be connected to the gas guide pipe 17 to achieve the functions of vacuum degassing and cleaning the reaction environment.

[0067] Example 2

[0068] A method for synthesizing high-purity silicon carbide powder with low nitrogen content, comprising the following steps:

[0069] Step (1): Use a powder mixer to uniformly mix carbon powder and silicon powder in a molar ratio of 1:1. The carbon powder and silicon powder must have a purity of not less than 6N, and the experiments and equipment used in the mixing process must be of high purity.

[0070] Step (2): The synthesis crucible device shown in Example 1 is subjected to vacuum degassing at a high temperature of 2100 degrees Celsius for 5 hours, and then cooled down for later use.

[0071] Step (3): The uniformly mixed carbon and silicon powder from step (1) is loaded into the isostatic graphite crucible 12. The dense high-temperature resistant coating 15 on the inner wall of the isostatic graphite crucible 12 and the graphite crucible lid 13 is made of TaC and has a thickness of 50 micrometers. At the same time, the graphite crucible lid 13 and a 6N purity graphite paper interlayer 16 are installed on the graphite crucible, wherein the thickness of the graphite paper interlayer 16 is 1 mm.

[0072] Step (4): Install a gas guide pipe 17 with a high-temperature resistant coating 18 made of silicon carbide on the top of the graphite crucible lid 13. The diameter of the gas guide pipe 17 is preferably 2~5 mm. In this embodiment, the diameter of the gas guide pipe 17 is 3 mm.

[0073] Step (5): Install the thermal insulation base 11 and the insulation component 19, wherein the temperature of the thermal insulation base 11 and the temperature measuring hole 20 of the insulation component 19 located on the top of the graphite crucible cover 13 are measured by an optical thermometer, thereby obtaining the top and bottom temperatures of the isostatic graphite crucible 12.

[0074] Step (6): The above-mentioned synthesis crucible device is loaded into the powder synthesis furnace containing a temperature control system.

[0075] Step (7): Vacuum the reaction vessel 10 using the vacuum system 22 to ensure that the ultimate vacuum level of the furnace body is below 5 × 10⁻⁶.-6 mbar.

[0076] Step (8): Increase the temperature and adjust the position of the induction furnace coil to ensure that the temperatures of the top and bottom thermometers of the isostatic graphite crucible 12 are basically the same, with a temperature difference not exceeding ±20 degrees Celsius and an absolute temperature of 1300 degrees Celsius.

[0077] Step (9): The isostatic graphite crucible 12 is cleaned with high-purity argon (7N) three times.

[0078] Step (10): Pour high-purity argon gas to 5 mbar. Continue heating to 1800 degrees Celsius and hold at that temperature for 5 hours.

[0079] Step (11): Continue heating while controlling the pressure of the isostatic graphite crucible 12 to 100 mbar, change the position of the coil, and make the axial temperature gradient of the isostatic graphite crucible 12 100~200 degrees Celsius. In this embodiment, it is preferably 150°C. The top temperature is 2100 degrees Celsius, thereby forming a directional airflow that is discharged from the reaction vessel 10 along the gas guide pipe 17.

[0080] Step (12): The temperature is kept constant for 20 hours. The gas pipe is sealed by silicon carbide deposition in the gas pipe to isolate external gas pollution. Then, the crystal transformation of SiC powder (beta phase to alpha phase) and particle size increase are completed.

[0081] Step (13): Cool down and set aside to obtain high-purity silicon carbide powder with low nitrogen content.

[0082] Comparative Example 1

[0083] The difference between Comparative Example 1 and Example 2 is that the inner wall of the reaction vessel in the synthesis crucible apparatus is not coated with a dense high-temperature resistant coating, while the other conditions are the same.

[0084] Comparative Example 2

[0085] The difference between Comparative Example 2 and Example 2 is that the synthesis crucible device does not have a gas guide pipe or a gas guide hole, while the other conditions are the same.

[0086] Comparative Example 3

[0087] The difference between Comparative Example 3 and Example 1 is that the gas guide tube in the synthesis crucible device was not coated with a high-temperature resistant coating, while the other conditions were the same.

[0088] The high-purity silicon carbide powders prepared in Example 2 and Comparative Examples 1-3 were tested to determine their internal nitrogen content. The test results are shown in Table 1 below.

[0089] Table 1

[0090] Item Nitrogen content Example 2 5E15 atoms / cm 3 ]]> Comparative Example 1 2E16 atoms / cm 3 ]]> Comparative Example 2 1E17 atom / cm 3 ]] Comparative Example 3 1E16 atom / cm 3 ]]

[0091] As can be seen from the data in the table above, this application physically isolates external nitrogen contamination by coating the reaction vessel with a dense, high-temperature resistant coating and the gas delivery pipe. Combined with axial temperature gradient control, it forms a directional airflow to efficiently discharge unreacted gases. Furthermore, it utilizes silicon carbide deposition within the gas delivery pipe to achieve self-sealing and block nitrogen backflow. This systematically solves the problem of nitrogen impurities being difficult to remove in traditional processes, ultimately obtaining silicon carbide powder with extremely low nitrogen concentration and a purity of over 6N. While maintaining the high efficiency and energy-saving characteristics of the self-propagating method, this process significantly improves production stability and material consistency through innovative device structure and synergistic optimization of process parameters.

[0092] However, it should be noted that if only a dense high-temperature resistant coating is introduced on the inner wall of the crucible without setting a gas guiding channel, the sealed crucible system formed by this structure will actually inhibit the removal of nitrogen adsorbed on the surface of carbon powder and silicon powder from inside the crucible to outside the crucible. Ultimately, this results in the nitrogen impurity residue in Comparative Example 2 being much higher than the powder synthesis process results in Examples 1 and 2. Figures 2-5 The images are of the powders obtained in Example 2 and Comparative Examples 1-3, respectively.

[0093] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for synthesizing low-nitrogen-content, high-purity silicon carbide powder, characterized in that, Includes the following steps: (1) High-purity carbon powder and silicon powder are mixed and then loaded into a reaction vessel. The inner wall of the reaction vessel is coated with a dense high-temperature resistant coating. (2) The reaction vessel is vacuum degassed through the gas guide pipe and the reaction environment is cleaned by inert gas. The gas guide pipe is eccentrically connected to the reaction vessel. The length of the gas guide pipe is 1 / 3 to 1 / 2 of the height of the reaction vessel and the diameter of the gas guide pipe is 2 to 5 mm. (3) Heat the reaction vessel to the silicon carbide synthesis temperature, and control the axial temperature gradient of the reaction vessel to 100~200℃ to form a directional airflow that is discharged from the inside of the reaction vessel along the gas guide pipe. (4) Under high temperature conditions, the gas duct is sealed by silicon carbide deposition inside the gas duct to isolate external gas pollution; (5) Complete the crystal transformation of silicon carbide powder to obtain high-purity silicon carbide powder with low nitrogen content.

2. The method according to claim 1, characterized in that, The dense high-temperature resistant coating is selected from at least one of tantalum carbide, hafnium carbide, and zirconium carbide.

3. The method according to claim 1, characterized in that, The inner wall of the air duct is coated with a high-temperature resistant coating.

4. The method according to claim 1, characterized in that, In step (2), the vacuum degree of the vacuum degassing process is lower than 5×10⁻⁶. -6 mbar, the inert gas is argon with a purity of not less than 6N.

5. An apparatus for implementing the synthesis method according to any one of claims 1 to 4, characterized in that, include: The reaction vessel has a dense, high-temperature resistant coating on its inner wall. A gas delivery pipe connected to the reaction vessel, the inner wall of which is coated with a high-temperature resistant coating; Vacuum system, used for vacuum degassing of reaction vessels; An inert gas circulation system is used to clean the reaction environment; Temperature control system, used to adjust the axial temperature gradient of the reaction vessel.

6. The apparatus according to claim 5, characterized in that, The reaction vessel includes a thermal insulation base with a lower temperature measuring hole, an isostatic graphite crucible disposed above the thermal insulation base, and a matching graphite crucible lid, and also includes insulation components located on the side of the isostatic graphite crucible and on the top of the graphite crucible lid. The gas guide pipe is positioned upwards at the top of the graphite crucible lid and is connected to the isostatic graphite crucible.

7. The apparatus according to claim 6, characterized in that, A graphite paper interlayer is provided between the graphite crucible lid and the static pressure graphite crucible.

8. The apparatus according to claim 5 or 6, characterized in that, The gas delivery pipe is eccentrically connected to the reaction vessel, and the length of the gas delivery pipe is 1 / 3 to 1 / 2 of the height of the reaction vessel.

9. The apparatus according to claim 6, characterized in that, The insulation component is a carbon fiber felt or a graphite felt; The high-temperature resistant coating on the inner wall of the air duct is silicon carbide.

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