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

Abstract

The invention 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, and the method comprises the following steps: (1) mixing high-purity carbon powder and silicon powder, then putting the mixture into a reaction container, and coating the inner wall of the reaction container with a compact high-temperature-resistant coating; (2) carrying out 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 to form a directional air flow which is discharged outwards from the reaction container along the air guide pipe; (4) under a high-temperature condition, blocking the gas-guide tube through deposition of silicon carbide in the gas-guide tube, and isolating external gas pollution; and (5) completing crystal transformation of the silicon carbide powder to obtain the low-nitrogen-content high-purity silicon carbide powder. External nitrogen pollution is physically isolated by coating the compact high-temperature-resistant coating and arranging the gas guide pipe, and the problem that nitrogen impurities are difficult to remove in a traditional process is systematically solved.

Description

Technical Field

[0001] The present invention relates to the field of silicon carbide synthesis, and particularly to a method and device for synthesizing high-purity silicon carbide powder with low nitrogen content. Background Art

[0002] As a third-generation wide-bandgap semiconductor material, silicon carbide (SiC) exhibits great application potential in the fields of high-temperature, high-frequency, high-power, and radiation-resistant devices due to its excellent properties such as high thermal conductivity, high breakdown field strength, and high saturated electron drift velocity. Especially 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 characteristics such as ultrathin, high heat dissipation, large field of view, and no "rainbow pattern" interference. However, the preparation of such materials requires silicon carbide powder with low nitrogen content (nitrogen concentration below 1×10 16 atoms / cm 3 ) and high purity (≥6N), which poses extremely high challenges to existing synthesis technologies.

[0003] Currently, the preparation of silicon carbide powder mainly uses the self-propagating high-temperature synthesis method (SHS), which has the advantages of simple process, energy saving and high efficiency, and controllable crystal form. However, there are significant bottlenecks in nitrogen impurity control in this process: First, it is inevitable for porous materials such as carbon powder, silicon powder, and graphite crucibles in the reaction system to adsorb nitrogen in the environment; Second, traditional processes rely on means such as vacuum degassing, high-temperature heating, and inert gas cleaning to remove nitrogen impurities. However, due to the low-temperature region of the thermal insulation material and the pore structure of the graphite crucible, the nitrogen desorption efficiency is insufficient, resulting in a nitrogen concentration as high as 5×10 16 atoms / cm 3 in the final powder, which cannot meet the requirements of high-end applications. In addition, there is a lack of effective isolation measures for the thermal insulation material to adsorb nitrogen in the existing technology, and nitrogen impurities are likely to penetrate into the powder through the crucible wall during the reaction process, further aggravating the pollution.

[0004] Although existing technologies have tried to reduce the nitrogen content by means of volatilization of mixed organic matter carrying nitrogen and optimizing vacuum treatment parameters, these methods still cannot completely block external nitrogen pollution, and the process complexity and cost increase significantly. Therefore, how to achieve physical isolation between the reaction system and external nitrogen and optimize the gas flow path to inhibit the reflux of nitrogen impurities has become the key to breaking through the preparation technology of high-purity silicon carbide powder. Summary of the Invention

[0005] The present invention aims to overcome the defects that the existing silicon carbide powder synthesis method cannot completely block external nitrogen pollution, and the process complexity and cost increase significantly. Therefore, a method and device for synthesizing high-purity silicon carbide powder with low nitrogen content are provided to overcome the above deficiencies.

[0006] To achieve the above-mentioned invention object, the present invention is realized through the following technical solutions: In the first aspect, the present invention first provides a method for synthesizing high-purity silicon carbide powder with low nitrogen content, comprising the following steps: (1) Mix high-purity carbon powder and silicon powder and load them into a reaction vessel, the inner wall of which is coated with a dense high-temperature-resistant coating; (2) Perform vacuum degassing treatment on the reaction vessel through a gas pipe, and introduce an inert gas to clean the reaction environment; (3) Heat the reaction vessel to the silicon carbide synthesis temperature to form a directional gas flow that discharges outward along the gas pipe from inside the reaction vessel; (4) Under high-temperature conditions, block the gas pipe through silicon carbide deposition inside the gas pipe to isolate external gas pollution; (5) Complete the crystal form transformation of the silicon carbide powder to obtain high-purity silicon carbide powder with low nitrogen content.

[0007] In the prior art, although the self-propagating high-temperature synthesis method can achieve the efficient preparation of silicon carbide powder, its control of nitrogen impurities relies on traditional means such as vacuum degassing, high-temperature heating, and inert gas cleaning. However, these methods have significant limitations: on the one hand, it is difficult to completely remove the adsorption of nitrogen by porous media such as graphite crucibles and thermal insulation materials in the reaction system. Even after multiple argon cleanings, the residual nitrogen still penetrates into the powder through the pores; on the other hand, the thermal field design of the traditional process results in the thermal insulation layer being in a low-temperature area, which inhibits the effective desorption of nitrogen. Finally, the nitrogen concentration of the powder is still as high as 5×10 16 per cm 3 order of magnitude. In addition, the prior art lacks a physical isolation mechanism for nitrogen pollution sources, and external nitrogen can flow back through the crucible wall or gas pipe during the reaction process, further aggravating the pollution. Although some improvement schemes attempt to reduce the nitrogen content by adding organic matter volatilization or optimizing vacuum parameters, these measures often introduce additional impurities or increase the process complexity, and it is difficult to balance purity and efficiency. Therefore, how to systematically block the nitrogen pollution path while maintaining the high efficiency of the self-propagating method has become the key challenge for breaking through the preparation of high-purity silicon carbide powder.

[0008] In view of the above problems, the present invention proposes a low-nitrogen content synthesis method based on physical isolation and dynamic air flow regulation. The design of its technical solution stems from an in-depth analysis of the nitrogen pollution path. First, the inner wall of the reaction vessel is coated with a dense high-temperature resistant coating (such as TaC, HfC, etc.). Through the low gas permeability characteristics of the material, the nitrogen adsorbed by the external thermal insulation material is completely blocked from penetrating into the reaction system through the crucible wall. This design directly addresses the nitrogen doping problem caused by the pores of the graphite crucible in the prior art, transforming the passive denitrification of the traditional process into active isolation. Secondly, the structure and function of the gas pipe are redefined: during the vacuum degassing and inert gas cleaning stages, the gas pipe serves as a channel for nitrogen discharge; while during the high-temperature reaction stage, by controlling the axial temperature gradient, a directional air flow from the inside to the outside is formed, forcing the unreacted gas and volatiles to be discharged along the gas pipe to avoid the reverse flow of external gas. More critically, the gas pipe is gradually blocked due to the deposition of silicon carbide at high temperature. This phenomenon is creatively transformed into a self-sealing mechanism - as the reaction progresses, the deposits accumulate in the gas pipe and eventually block the channel, completely blocking the possibility of the reflux of external nitrogen in the subsequent stages. This "dynamic sealing" mechanism not only requires no additional operation, but also cleverly utilizes the reaction by-products to achieve system self-protection, significantly improving the reliability and automation of the process.

[0009] In terms of technical effects, this method realizes multi-level blocking of nitrogen pollution through the synergistic effects of coating isolation, directional air flow, and gas pipe blocking. The dense coating isolates the physical penetration of external nitrogen from the source, the directional air flow optimizes the discharge efficiency of the reaction gas through the thermal field design, and the self-sealing mechanism of the gas pipe fundamentally eliminates the possibility of nitrogen reflux. Compared with the limitations of the traditional process relying on a single denitrification method, this solution improves the denitrification efficiency to a new level through systematic design. In addition, the gas pipe blocking mechanism does not require complex external control devices, reducing the equipment cost and operation difficulty; the selection of coating materials takes into account both high-temperature resistance and economy (such as the high-temperature stability of TaC coating and the low-cost adaptability of SiC coating), making the solution have the potential for industrial promotion. Finally, this method can reduce the nitrogen concentration of silicon carbide powder to 1×10 16 pcs / cm 3 while maintaining the high efficiency and energy saving advantages of the self-propagating method, meeting the stringent requirements for material purity in high-end applications such as AR glasses, and having significantly better process stability than the prior art. This technical path not only solves the long-term nitrogen pollution problem that restricts the preparation of high-purity silicon carbide powder, but also provides reliable technical support for the large-scale application of third-generation semiconductor materials.

[0010] Preferably, the dense high-temperature resistant coating is selected from at least one of tantalum carbide, hafnium carbide, and zirconium carbide.

[0011] Preferably, the inner wall of the gas pipe is coated with a high-temperature resistant coating, and the aperture of the gas pipe is 2 - 5 mm.

[0012] The inner wall of the gas duct is coated with a high-temperature resistant coating and combined with a design with a pore size of 2 - 5 mm. Its creativity lies in the dual breakthroughs of the precise intervention of the nitrogen pollution path and the optimization of process self-adaptability. In the prior art, the gas duct is usually only used as a gas discharge channel, and its structural parameters (such as pore size) are mostly set based on experience, and the wall material directly uses graphite or uncoated porous media, resulting in two core problems: First, the graphite gas duct itself may release impurities or adsorb nitrogen in the environment at high temperatures, becoming a secondary pollution source; Second, the conventional gas duct pore size design lacks the coordinated consideration of gas flow dynamics and deposition plugging mechanism - when the pore size is too large, external nitrogen is likely to enter the reaction system through countercurrent, while when the pore size is too small, it may lead to poor gas discharge and affect the reaction process.

[0013] This technical feature transforms the gas duct from a single-functional component into an intelligent module with both pollution isolation and dynamic plugging for the first time by introducing a high-temperature resistant coating (such as TaC, SiC, etc.) and defining the pore size range. The high-temperature resistant coating not only blocks the possibility of the gas duct's own material (such as graphite) releasing impurities or adsorbing nitrogen at high temperatures, but also reduces gas turbulence by decreasing the surface roughness of the pipe wall, thereby optimizing the stability of the directional gas flow; The pore size range of 2 - 5 mm achieves the balance of two-stage functions through experimental verification: In the initial stage of the reaction, this pore size can ensure the efficient discharge of unreacted gases (such as residual nitrogen) and volatiles (such as Si, SiC x etc.), avoiding pressure fluctuations caused by gas flow blockage; In the later stage of the reaction, with the directional deposition of SiC x atmosphere in the gas duct, this pore size range can not only ensure that the deposits accumulate to form an effective plug in a reasonable time (avoiding premature blockage caused by too small pore size or plugging failure caused by too large pore size), but also achieve dynamic control of the sealing effect through the self-limiting growth of the deposits.

[0014] This design breaks the conventional thinking pattern that "the gas duct only serves as a passive channel" in traditional processes. It creatively utilizes the physical properties of reaction by-products (such as the tendency of SiC deposition) and thermal field conditions (such as temperature gradient), deeply coupling the "structural parameters" of the gas duct with the "process", forming a closed-loop control. This collaborative design of the microscale mass transfer mechanism and the macroscale process timing enables the gas duct to automatically achieve the function switch of "open for exhaust in the early stage - closed for isolation in the later stage" without external intervention, significantly improving the process reliability and automation level. Compared with the existing solutions that control the gas path through complex valves or external sealing devices, this feature realizes functional self-adaptability through the intrinsic properties of materials and structures, reducing both the equipment complexity and cost, and avoiding the leakage risks introduced by mechanical seals. Therefore, this technical feature is not a simple improvement to the existing gas duct design, but rather a systematic solution to the inherent contradiction between nitrogen pollution control and process stability through cross-scale (microscale coating - macroscale structure) and cross-process (aerodynamic - deposition kinetic) collaborative innovation.

[0015] Preferably, in step (2), the vacuum degree of the vacuum degassing treatment is lower than 5×10 -6 mbar, and the inert gas is argon with a purity of not less than 6N.

[0016] Preferably, in step (3), the directional gas flow is formed by controlling the axial temperature gradient of the reaction vessel to be 100~200°C.

[0017] During the synthesis of silicon carbide powder, the precise control of the gas flow path is a key factor determining the nitrogen impurity removal efficiency. However, the existing technologies mainly focus on uniform heating or simple upper and lower temperature zone division for the regulation of the thermal field temperature distribution, failing to deeply couple the temperature gradient design with the gas dynamic behavior.

[0018] In traditional processes, the temperature distribution in the reaction vessel usually shows a static mode with low temperature at the top and high temperature at the bottom. Although this design is beneficial for heat preservation and energy conservation, it leads to two core problems: First, the high temperature region at the bottom promotes the upward diffusion of reaction gases (such as Si vapor, SiC x intermediate), but due to the existence of the low temperature region at the top, the kinetic energy of gas molecules decreases, easily forming disordered eddies in the vessel, prolonging the gas residence time and increasing the contact probability between nitrogen and the powder. Second, the nitrogen adsorption capacity of the external heat insulation material (such as graphite felt) increases in the low temperature region, and the desorption efficiency decreases, and the residual nitrogen may reverse infiltrate into the reaction system through heat convection. Although existing technologies have tried to improve gas fluidity by increasing the vacuum degree or the frequency of argon cleaning, these methods can only passively reduce the nitrogen concentration, unable to actively guide the gas to discharge directionally, and the excessive dependence on external equipment leads to increased energy consumption and cost.

[0019] The axial temperature gradient control (100 - 200 °C) proposed by the present invention fundamentally reconstructs the interaction logic between the thermal field and the gas flow: by precisely setting the axial temperature difference of the reaction vessel, a stable thermodynamic driving force is formed in the vertical direction, enabling the reaction gas generated in the high-temperature zone (bottom) to naturally rise due to the density difference. At the same time, the moderate cooling in the low-temperature zone (top) is not complete cooling, but maintains sufficient kinetic energy to drive the gas to continuously flow towards the gas conduit. The selection of this gradient range (100 - 200 °C) is not an empirical value, but is based on the critical threshold of the mean free path of gas molecules and the viscous flow state - when the temperature gradient is lower than 100 °C, the thermal motion energy of gas molecules is insufficient to overcome the viscous resistance, and local turbulence is likely to form; when the gradient exceeds 200 °C, the excessively low temperature at the top will cause premature gas condensation or deposition, blocking the gas conduit or interfering with the crystal form transformation process. By strictly limiting the gradient within this range, it can ensure that the gas migrates directionally in the form of laminar flow and avoid the out-of-control phase change caused by temperature mutation. More importantly, this gradient design forms a synergistic effect with the structure of the gas conduit (such as aperture, coating): the directional gas flow maintains a stable flow rate in the gas conduit due to the temperature gradient, enabling the unreacted gas and nitrogen to be efficiently carried away from the reaction system, while the SiC x volatile matter is concentrated and deposited in a specific area of the gas conduit under the guidance of the gas flow, accelerating the blocking process.

[0020] Compared with the temperature field designs of "uniform high temperature throughout the region" or "simple upper and lower zoning" in traditional processes, this technical feature converts the thermodynamic potential energy into gas kinetic energy through gradient quantization control, enabling the gas flow direction, velocity, and deposition behavior to form a dynamic balance. This balance not only solves the problem of nitrogen retention caused by disordered flow but also avoids the risk of external contamination introduced by forced gas flow (such as mechanical pump suction). In addition, the setting of the gradient range also implicitly considers the compatibility with the thermal expansion coefficient of the material - the deformation differences of the graphite crucible and the coating material under a 100 - 200 °C gradient are controllable, avoiding coating cracking or sealing failure caused by thermal stress concentration. Therefore, this temperature gradient control is not a simple optimization of the existing heating mode, but through the multi-physical field coupling design of the thermal field - gas flow - deposition, integrates the mutually separated "heating", "exhaust", and "sealing" links in traditional processes into a closed-loop self-regulating system, significantly improving the robustness of the process and the nitrogen removal efficiency, and having a substantial creative breakthrough.

[0021] Secondly, the present invention also provides an apparatus for implementing the synthesis method, including: A reaction vessel, the inner wall of which is coated with a dense high-temperature resistant coating; A gas conduit communicating with the reaction vessel, the inner wall of which is coated with a high-temperature resistant coating; A vacuum system for vacuum degassing the reaction vessel; An inert gas circulation system for purifying the reaction environment; A temperature control system for regulating the axial temperature gradient of a reaction vessel.

[0022] The design of high-purity preparation devices for silicon carbide powder has long been limited by the contradiction between nitrogen pollution control and process complexity. In existing technologies, reaction vessels mostly use graphite crucibles to directly load raw materials. Although their porous structure has good high-temperature resistance, it has become the main path for nitrogen penetration - the adsorption of nitrogen in the environment by graphite materials at high temperatures is difficult to completely remove through traditional vacuum or inert gas cleaning, and the nitrogen desorption efficiency of thermal insulation materials (such as graphite felt) in low-temperature regions is low, resulting in the reaction system always being exposed to the risk of nitrogen pollution. Although some improvement schemes attempt to block nitrogen penetration by adding multiple layers of isolation cavities or complex sealing structures, these designs often greatly increase the equipment complexity and manufacturing cost, and it is difficult to adapt to the high-temperature dynamic environment required by the self-propagating method. For example, although multiple cavities can physically isolate the external environment, they hinder the free flow of reaction gases, resulting in a decrease in synthesis efficiency; while the durability of mechanical sealing devices at extreme temperatures is insufficient, and leakage is likely to occur due to thermal expansion differences. In addition, in traditional devices, the gas guide tube usually serves as a passive exhaust channel, and its structural design (such as straight tubes, uniform pore diameters) does not consider the correlation between gas flow patterns and deposition behaviors, resulting in two core defects: First, external nitrogen may flow back into the reaction vessel through the gas guide tube, especially when there are pressure fluctuations; Second, the disordered gas discharge path makes it difficult for SiC x volatile substances to be directionally deposited, and an effective self-sealing mechanism cannot be formed. These problems together lead to a vicious cycle of "insufficient desorption efficiency - difficult to prevent secondary pollution" in nitrogen impurity control of existing devices, seriously restricting the industrial production of high-purity silicon carbide powder.

[0023] In response to the above technical dilemmas, the design concept of the device of the present invention stems from the dual needs of systematically blocking the nitrogen pollution path and improving process self-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 penetration problem caused by the pores of the graphite crucible - the high density of the coating material (gas permeability is lower than 1×10 -6 cm 2 / s) is chemically inert, which not only blocks the diffusion path of nitrogen adsorbed by the external thermal insulation material but also prevents the graphite substrate from releasing impurities at high temperatures to contaminate the powder. This design breaks through the passive mode of "denitrification relying only on gas replacement" in traditional processes and elevates physical isolation to the core prevention and control means. The co - operating gas duct (b) realizes functional reconstruction through the specific combination of the high - temperature - resistant coating on the inner wall and the pore diameter of 2 - 5 mm: in the initial stage of the reaction, the coating blocks the adsorption of nitrogen by the material of the gas duct itself (such as graphite), and at the same time optimizes the smoothness of the pipe wall surface to reduce air flow turbulence; the pore diameter design ensures the efficient discharge of unreacted gases through the matching of viscous flow state and molecular mean free path. More importantly, as the reaction progresses, the directional deposition of SiC in the gas duct forms a self - sealing structure on the coating surface, and this process is strengthened by the precise control of the axial temperature gradient (100 - 200 °C) by the temperature control system (e) - the gradient thermal field drives the gas to flow directionally along the gas duct, and at the same time, moderate cooling at the top promotes the accumulation of deposits in a specific area, ultimately realizing the dynamic sealing of the gas duct. This "structure - process - material" trinity design upgrades the gas duct from a single exhaust component to an intelligent module with both pollution isolation and process adaptability. x The directional deposition of the SiC atmosphere in the gas duct forms a self - sealing structure on the coating surface. This process is strengthened by the precise control of the axial temperature gradient (100 - 200 °C) by the temperature control system (e) - the gradient thermal field drives the gas to flow directionally along the gas duct, and at the same time, moderate cooling at the top promotes the accumulation of deposits in a specific area, ultimately realizing the dynamic sealing of the gas duct.

[0024] The collaborative innovation of each subsystem in 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 realizes complementary functions through timing control: in the vacuum degassing stage, large - molecular - weight impurities (such as adsorbed O2, H2O) are preferentially removed, while the high - purity argon gas circulation cleaning is used to replace light - molecular - weight molecules such as nitrogen. Implementing them in stages can minimize the total amount of residual gas. The axial gradient control (100 - 200 °C) of the temperature control system (e) is not an isolated parameter setting. It forms a deep coupling with the gas duct structure and coating characteristics - for example, when the gradient is set to 150 °C, the high temperature at the bottom (2100 °C) accelerates the carbon - silicon reaction to generate β - SiC, and the moderate cooling at the top (1950 °C) inhibits the excessive volatilization of Si vapor, while maintaining sufficient thermodynamic driving force to promote the gas to flow towards the gas duct. This thermal field design not only optimizes the reaction kinetics but also enhances the efficiency of gas directional migration through natural convection generated by the temperature difference, avoiding the risk of external pollution introduced by forced exhaust of mechanical pumps. More notably, the device realizes process self - optimization through the closed - loop feedback of each subsystem: the vacuum sensor monitors the pressure of the reaction vessel in real - time and dynamically adjusts the injection amount of inert gas to balance the air flow velocity; the temperature sensor feeds back the axial temperature difference data and automatically adjusts the heating power to maintain the preset gradient. This intelligent control mechanism integrates the isolated operation units in the traditional device into an organic whole, significantly improving the process stability and repeatability.

[0025] In terms of technical effects, the device achieves a simultaneous improvement in nitrogen pollution control and process efficiency through multi-level innovation. The dense coating completely blocks more than 90% of the external nitrogen penetration paths. The directional air flow design increases the unreacted gas emission efficiency by more than 40%, and the self-sealing mechanism of the gas guide pipe reduces the nitrogen reflux risk to a negligible level. Compared with traditional devices, this solution can reduce the powder nitrogen concentration from 5×10 16 per cm 3 to 1×10 16 per cm 3 or less under the same process conditions without the need to add complex external sealing or purification equipment. In addition, the synergistic effect of the temperature gradient and gas flow dynamics makes the SiC polymorphic transformation more uniform, significantly reducing the dispersion of the powder particle size distribution and remarkably improving the yield of subsequent single crystal growth. At the industrial level, the device adopts a standardized module design (such as replaceable coating crucibles, graded gas guide pipe components), which not only reduces the maintenance cost but also adapts to flexible production with different purity requirements.

[0026] In summary, the device is not a partial improvement of existing equipment, but rather redefines the technical paradigm for the preparation of high-purity silicon carbide powder through interdisciplinary technology integration (materials science, thermodynamics, fluid mechanics) and system-level innovation. Its creativity lies in the comprehensive leap from "passive defense" to "active isolation", from "experience-driven" to "model optimization", and from "single function" to "adaptive coordination", providing disruptive equipment support for the large-scale application of third-generation semiconductor materials.

[0027] Preferably, the reaction vessel includes a thermal field insulation base with a lower temperature measurement hole, an isostatic graphite crucible disposed above the thermal field insulation base, and a graphite crucible cover matching therewith, and further includes insulation components on the side and top of the isostatic graphite crucible; The gas guide pipe is disposed upward on the top of the graphite crucible cover and is connected to the isostatic graphite crucible.

[0028] Preferably, a graphite paper sandwich is provided between the crucible cover and the isostatic graphite crucible.

[0029] During the high-temperature synthesis of silicon carbide powder, volatile substances (such as Si vapor, SiC xThe deposition of (intermediate) is likely to cause physical erosion and chemical corrosion to the inner wall coating of the crucible lid. In the traditional process, the direct contact between the crucible lid and the crucible may lead to seal failure due to the difference in thermal expansion coefficients, and once the coating surface is covered by deposits, its dense isolation function will be significantly weakened. In the present invention, a graphite paper sandwich layer is added between the crucible lid and the static pressure graphite crucible, and triple optimizations are achieved through the high-temperature stability and flexible deformation characteristics of the graphite paper: Firstly, the graphite paper, as a sacrificial layer, preferentially adsorbs volatile substances, preventing them from directly contacting the surface of the crucible lid coating, and greatly reducing the risk of peeling of the coating caused by sediment impact or chemical erosion; Secondly, the elastic compression characteristics of the graphite paper can adaptively compensate for the small gap generated by the thermal expansion and contraction of the crucible and the lid body, maintaining the dynamic sealing effect at high temperature and preventing external nitrogen from infiltrating into the reaction system through the gap; Thirdly, the high-purity graphite paper (≥6N) itself does not introduce additional impurities, and its porous structure can partially adsorb residual gas molecules at high temperature, further purifying the reaction environment in cooperation with vacuum and inert gas cleaning.

[0030] Preferably, the gas guide tube is eccentrically connected to the reaction vessel, and the length of the gas guide tube is 1 / 3 to 1 / 2 of the height of the reaction vessel.

[0031] The gas guide tube is eccentrically connected to the reaction vessel and its length is limited to 1 / 3 to 1 / 2 of the height of the reaction vessel. This design significantly improves the efficiency of nitrogen pollution control and the process stability by optimizing the gas flow dynamics and deposition behavior. The central symmetric layout of the traditional gas guide tube is prone to forming eddies of gas at the top of the vessel, resulting in an extended residence time of unreacted gas (such as residual nitrogen) and increasing the risk of secondary contact with the powder; while the eccentric connection breaks the symmetry of gas flow through the asymmetric flow channel design, forcing the gas to migrate directionally along one side of the gas guide tube, reducing turbulence and accelerating the discharge of impurities.

[0032] The setting of the length of the gas guide tube (1 / 3 to 1 / 2 of the height of the reaction vessel) balances the gas flow resistance and the deposition blocking efficiency - when the length is too short, the gas flow rate is too fast, which is likely to cause uneven distribution of deposits and difficult to form an effective seal; when the length is too long, it increases the gas flow resistance and affects the degassing efficiency in the initial stage of the reaction. For example, when the length of the gas guide tube is 40% of the height of the reactor, its physical position extending to the middle and upper parts of the vessel can make full use of the axial temperature gradient of the thermal field (high temperature at the bottom and moderate cooling at the top), so that the SiC x volatiles generated in the high-temperature zone are naturally condensed during the upward process due to the temperature gradient and are directionally deposited along the inner wall of the gas guide tube, and the eccentric layout further guides the deposits to concentrate in specific areas of the lumen, accelerating the blocking process.

[0033] In addition, this length range is also adapted to the aperture of the gas guide tube (2 - 5 mm) and the coating properties (such as the surface energy of the SiC coating). By adjusting the matching relationship between the deposition rate of the deposit and the gas flow rate, it is ensured that the plugging action is precisely synchronized with the crystal form transformation stage. Compared with the traditional centrally symmetric gas guide tube design, the combined effect of the eccentric connection and the length limitation not only reduces the probability of nitrogen backflow, but also enhances the reliability of the self-sealing mechanism through directional deposition, achieving the adaptive optimization of the process without external control devices, while reducing the equipment manufacturing cost and maintenance complexity.

[0034] Preferably, the heat insulation component is carbon fiber felt or graphite felt; The high-temperature resistant coating applied on the inner wall of the gas guide tube is silicon carbide.

[0035] Therefore, the present application has the following beneficial effects: In the present invention, the reaction vessel with a dense high-temperature resistant coating and the gas guide tube physically isolate the external nitrogen pollution. By combining the axial temperature gradient control to form a directional gas flow to efficiently discharge the unreacted gas, and using the silicon carbide deposition in the gas guide tube to achieve self-sealing and block the nitrogen backflow, the problem that it is difficult to remove nitrogen impurities in the traditional process is systematically solved. Finally, silicon carbide powder with a nitrogen concentration lower than 1×10 16 pcs / cm 3 and a purity of more than 6N is obtained; while maintaining the high efficiency and energy saving characteristics of the self-propagating high-temperature synthesis method, through the innovation of the device structure and the collaborative optimization of the process parameters, the production stability and material consistency are significantly improved, and the strict requirements for high-purity silicon carbide materials in high-end fields such as AR glasses can be met without relying on complex external equipment, and it has the industrial application advantages of low cost and high reliability. Description of the Drawings

[0036] Figure 1 It is a schematic structural diagram of the synthesis crucible device for low-nitrogen-content high-purity silicon carbide powder in Embodiment 1 of the present invention.

[0037] Figure 2 It is a picture of the powder obtained in Example 2.

[0038] Figure 3 It is a picture of the powder obtained in Comparative Example 1.

[0039] Figure 4 It is a picture of the powder obtained in Comparative Example 2.

[0040] Figure 5 It is a picture of the powder obtained in Comparative Example 3.

[0041] Among them: reaction vessel 10, thermal field heat preservation base 11, isostatic graphite crucible 12, graphite crucible cover 13, reaction cavity 14, dense high-temperature resistant coating 15, graphite paper sandwich 16, gas guide pipe 17, high-temperature resistant coating 18, heat preservation component 19, temperature measuring hole 20, temperature control system 21, vacuum system 22, inert gas circulation system 23. Detailed implementation mode

[0042] The present invention will be further described below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are usually only a part of the embodiments of the present invention, rather than all the embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the scope of protection of the present invention.

[0043] Embodiment 1 As Figure 1 shown, a synthesis crucible device for low-nitrogen-content high-purity silicon carbide powder includes a group of reaction vessels 10, and the reaction vessels 10 sequentially include a thermal field heat preservation base 11, an isostatic graphite crucible 12, and a graphite crucible cover 13 matching the isostatic graphite crucible 12 from bottom to top.

[0044] Among them, the area between the isostatic graphite crucible 12 and the graphite crucible cover 13 forms a reaction cavity 14, and high-purity carbon powder and silicon powder are stored inside the reaction cavity 14. When the isostatic graphite crucible 12 is heated, the high-purity carbon powder and silicon powder inside the reaction cavity 14 react to form silicon carbide powder.

[0045] In order to control the nitrogen content inside the reaction cavity 14 during the reaction of carbon powder and silicon powder, in this embodiment, a dense high-temperature resistant coating 15 is coated on the inner wall of the reaction cavity 14. 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 be used for a long time under the volatilization of SiC powder and Si atmosphere, and maintain the structural strength and density, so as to prevent nitrogen impurities outside the isostatic graphite crucible 12, such as nitrogen, from penetrating into the reaction cavity 14 through its interior, thereby contaminating the SiC powder.

[0046] In addition, in order to prevent the volatiles formed during the reaction of carbon powder and silicon powder from depositing on the surface of the graphite crucible cover 13, thereby having a deposition and destructive effect on the dense high-temperature resistant coating 15 on the surface of the graphite crucible cover 13, in a preferred solution of this embodiment, a graphite paper sandwich 16 is also provided between the graphite crucible cover 13 and the isostatic graphite crucible 12, thereby achieving the blocking effect on gas volatiles.

[0047] 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 made of graphite and communicating with the reaction chamber 14 is further provided at a relatively eccentric position of the graphite crucible cover 13. The length of the gas guide pipe 17 is 1 / 3 to 1 / 2 of the height of the reaction vessel 10.

[0048] In a preferred solution, similar to the reaction chamber 14, a high-temperature resistant coating 18 is also coated on the inner wall of the gas guide pipe 17. The material of the high-temperature resistant coating 18 also includes but is not limited to TaC material or other high-temperature resistant and corrosion-resistant carbide materials, such as hafnium carbide, zirconium carbide, etc. Here, in order to reduce the preparation cost of the high-temperature resistant coating 18, the coating inside the gas guide pipe 17 can preferably use SiC coating material. During the synthesis process of the SiC powder described in this embodiment, the temperature at the position of the gas guide pipe 17 is relatively low, and the SiC coating material will not volatilize significantly. However, due to the relatively high temperature of the inner wall of the reaction chamber 14, it is not suitable to use the SiC coating.

[0049] In addition, in some preferred solutions of this embodiment, the reaction vessel 10 further includes a heat insulation assembly 19 located on the side of the isostatic graphite crucible 12 and on the top of the graphite crucible cover 13. The material of the heat insulation assembly 19 can be selected from carbon fiber felt or graphite felt. Among them, the heat insulation assembly 19 located on the top of the graphite crucible cover 13 usually has a certain thickness, but generally lower than the thickness of the heat field insulation base 11. In order to accurately measure the temperature inside the reaction chamber 14, a temperature measurement hole 20 is provided at both the heat field insulation base 11 and the heat insulation assembly 19 located on the top of the graphite crucible cover 13. Therefore, during the powder synthesis process, the axial temperature gradient of the heat field can be feedback-regulated according to the temperature measurement values of the upper and lower temperature measurement holes 20.

[0050] In order to heat the isostatic graphite crucible 12, a set of temperature control systems 21 is further provided outside the reaction vessel 10 in this embodiment, which can be one or more induction heating coils or resistance heaters, so as to control the temperature of the reaction chamber 14 inside the isostatic graphite crucible 12.

[0051] In addition, in order to further remove the impurity gas inside the reaction chamber 14 before the reaction, in the preferred solution of this embodiment, a vacuum system 22 and an inert gas circulation system 23 are also provided. Among them, the vacuum system 22 can perform vacuum degassing on the impurity gas inside the reaction chamber 14 through the gas guide pipe 17, so as to remove the impurity gas inside the reaction chamber 14 as much as possible, while the inert gas circulation system 23 can introduce inert gas into the reaction chamber 14 to clean the reaction environment. Moreover, both the vacuum system 22 and the inert gas circulation system 23 can be connected to the gas guide pipe 17 to realize the functions of vacuum degassing and cleaning the reaction environment.

[0052] Example 2 A method for synthesizing high-purity silicon carbide powder with low nitrogen content, which includes the following steps: Step (1): Use a powder mixer to uniformly mix carbon powder and silicon powder in a molar ratio of 1:1. Among them, the carbon powder and silicon powder have a purity of not less than 6N, and the experiments and equipment used in the mixing process need to have high purity.

[0053] Step (2): Perform vacuum degassing treatment on the synthesis crucible device shown in Example 1 at a high temperature of 2100 °C for 5 h, and cool it for later use.

[0054] Step (3): Load the uniformly mixed carbon and silicon powder in step (1) into the isostatic graphite crucible 12. The material of the dense high-temperature resistant coating 15 on the inner walls of the isostatic graphite crucible 12 and the graphite crucible cover 13 is TaC, and its thickness is 50 microns. At the same time, install the graphite crucible cover 13 and the graphite paper sandwich 16 with a purity of 6N on the graphite crucible. Among them, the thickness of the graphite paper sandwich 16 is 1 mm.

[0055] Step (4): Install a gas guide pipe 17 with a high-temperature resistant coating 18 made of silicon carbide on the inner wall at the top of the graphite crucible cover 13. The aperture of the gas guide pipe 17 is preferably 2-5 mm, and the diameter of the gas guide pipe 17 selected in this embodiment is 3 mm.

[0056] Step (5): Install the thermal field insulation base 11 and the insulation component 19. Among them, measure the temperature of the thermal field insulation base 11 and the temperature measuring holes 20 of the insulation component 19 located at the top of the graphite crucible cover 13 through an optical thermometer, so as to obtain the temperatures at the top and bottom of the isostatic graphite crucible 12.

[0057] Step (6): Install the above-mentioned synthesis crucible device into a powder synthesis furnace equipped with a temperature control system.

[0058] Step (7): Perform vacuum pumping on the reaction vessel 10 through the vacuum system 22 to meet the requirement that the ultimate vacuum degree of the furnace body is lower than 5×10 -6 mbar.

[0059] Step (8): Raise the temperature and adjust the position of the induction furnace coil to ensure that the temperatures measured by the thermometers at the top and bottom of the isostatic graphite crucible 12 are basically the same, with a temperature difference not exceeding ±20 °C, and reach an absolute temperature of 1300 °C.

[0060] Step (9): Conduct three times of furnace washing treatment on the isostatic graphite crucible 12 with high-purity argon (7N).

[0061] Step (10): Fill with high-purity argon to 5 mbar. Continue to raise the temperature to 1800 °C and keep it at a constant temperature for 5 h.

[0062] Step (11): Continue to raise the temperature, and at the same time control the pressure value of the isostatic graphite crucible 12 to 100 mbar. Change the position of the coil to make the axial temperature gradient of the isostatic graphite crucible 12 be 100 - 200 °C. In this embodiment, it is preferably 150 °C, and the measured temperature at the top is 2100 °C, so as to form a directional gas flow discharged from the inside of the reaction vessel 10 along the gas guide pipe 17 to the outside.

[0063] Step (12): Keep at a high temperature for 20 h. Seal the gas guide pipe through the deposition of silicon carbide inside the gas guide pipe to isolate external gas pollution, and then complete the crystal form transformation (from beta phase to alpha phase) and particle growth of the SiC powder.

[0064] Step (13): Cool down and keep it for later use to obtain high-purity silicon carbide powder with low nitrogen content.

[0065] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the inner wall of the reaction vessel in the synthesis crucible device is not coated with a dense high-temperature resistant coating, and the other conditions are the same.

[0066] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that the gas guide pipe is not provided in the synthesis crucible device, nor is there a gas guide hole, and the other conditions are the same.

[0067] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the inside of the gas guide pipe in the synthesis crucible device is not coated with a high-temperature resistant coating, and the other conditions are the same.

[0068] Test the high-purity silicon carbide powder prepared in Example 2 and Comparative Examples 1 - 3 to measure the nitrogen content inside, and the test results are shown in Table 1 below.

[0069] Table 1 Project Nitrogen content Example 2 <![CDATA[5E15 atom / cm 3 > Comparative Example 1 <![CDATA[2E16 atom / cm 3 > Comparative Example 2 <![CDATA[1E17 atom / cm 3 > Comparative Example 3 <![CDATA[1E16 atom / cm 3 > As can be seen from the data in the above table, in this application, the reaction vessel coated with a dense high-temperature resistant coating is physically isolated from the gas guide pipe to prevent external nitrogen pollution. Combined with the control of the axial temperature gradient, a directional gas flow is formed to efficiently discharge unreacted gases. The silicon carbide deposition in the gas guide pipe is used to achieve self-sealing and block the nitrogen reflux, systematically solving the problem that it is difficult to remove nitrogen impurities in the traditional process. Finally, silicon carbide powder with an extremely low nitrogen concentration and a purity of more than 6N is obtained. While maintaining the high efficiency and energy saving characteristics of the self-propagating method, through the innovation of the device structure and the coordinated optimization of process parameters, the production stability and material consistency are significantly improved.

[0070] However, it should be noted that if a dense high-temperature resistant coating is only introduced on the inner wall of the crucible and no gas guide channel is set, the sealed crucible system formed by this structure will instead inhibit the effect of removing nitrogen adsorbed on the surfaces of carbon powder and silicon powder from the inside of the crucible to the outside, ultimately resulting in a much higher residual amount of nitrogen impurities in Comparative Example 2 than the powder synthesis process results in Examples 1 and 2. Figure 2 - Figure 5 They are pictures of the powders obtained in Example 2 and Comparative Examples 1 - 3 respectively.

[0071] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for synthesizing high-purity silicon carbide powder with low nitrogen content, characterized in that, It includes the following steps: (1) Mix high-purity carbon powder and silicon powder and load them into a reaction vessel, the inner wall of which is coated with a dense high-temperature resistant coating; (2) Conduct vacuum degassing treatment on the reaction vessel through a gas conduit and introduce an inert gas to clean the reaction environment; (3) Heat the reaction vessel to the silicon carbide synthesis temperature to form a directional gas flow that discharges outward along the gas conduit from inside the reaction vessel; (4) Under high-temperature conditions, block the gas conduit through silicon carbide deposition inside the gas conduit to isolate external gas pollution; (5) Complete the crystal form transformation of the silicon carbide powder to obtain high-purity silicon carbide powder with a 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 gas conduit is coated with a high-temperature resistant coating, and the aperture of the gas conduit is 2 - 5 mm.

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

5. The synthesis process according to claim 1, characterized in that in step (3), the directional gas flow is formed by controlling the axial temperature gradient of the reaction vessel to be 100 - 200 °C.

6. An apparatus for implementing the synthesis method according to any one of claims 1 to 5, characterized in that, It includes: a reaction vessel, the inner wall of which is coated with a dense high-temperature resistant coating; a gas conduit connected to the reaction vessel, the inner wall of which is coated with a high-temperature resistant coating; a vacuum system for vacuum degassing the reaction vessel; an inert gas circulation system for cleaning the reaction environment; a temperature control system for adjusting the axial temperature gradient of the reaction vessel.

7. The device according to claim 6, characterized in that the reaction vessel includes a thermal field heat preservation base with a lower temperature measurement hole, an isostatic graphite crucible arranged above the thermal field heat preservation base, and a matching graphite crucible cover, and further includes a heat preservation component located on the side of the isostatic graphite crucible and on the top of the graphite crucible cover; the gas conduit is arranged upward on the top of the graphite crucible cover and is connected to the isostatic graphite crucible.

8. The device according to claim 7, characterized in that a graphite paper sandwich layer is arranged between the graphite crucible cover and the isostatic graphite crucible.

9. The device according to claim 6 or 7, characterized in that the gas conduit is eccentrically connected to the reaction vessel, and the length of the gas conduit is 1 / 3 - 1 / 2 of the height of the reaction vessel.

10. The device according to claim 7, characterized in that the heat preservation component is carbon fiber felt or graphite felt; the high-temperature resistant coating coated on the inner wall of the gas conduit is silicon carbide.

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