Temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device and synthesis method

Abstract

The application discloses a temperature response self-closed low-nitrogen silicon carbide powder synthesis device and a synthesis method, and belongs to the technical field of semiconductor material preparation. The synthesis device comprises a crucible body, a crucible upper cover, a tapered plug and a thermal expansion pushing piece. The crucible upper cover is provided with at least one tapered hole. The tapered plug has an outer tapered surface matched with the tapered hole, and an initial exhaust gap is left between the tapered plug and the tapered hole. The thermal expansion pushing piece is arranged above the crucible upper cover and is limited by the side wall of the crucible body. The lower end of the thermal expansion pushing piece is fixedly connected with the tapered plug. When heated, the thermal expansion pushing piece expands axially to push the tapered plug to move towards the tapered hole, thereby reducing or closing the initial exhaust gap. The device can keep the exhaust unobstructed at a low temperature stage to realize sufficient nitrogen exhaust, and can be automatically closed at a high temperature stage to reduce the escape of effective gas phase components, so that the low-temperature nitrogen exhaust and the high-temperature gas preservation are considered, and the obtained silicon carbide powder has a low nitrogen content.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material preparation technology, and in particular to a temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis apparatus and synthesis method. Background Technology

[0002] In the process of growing silicon carbide single crystals by physical vapor transport, high-purity silicon carbide powder is usually transported to the seed crystal after high-temperature sublimation to achieve crystal growth. The impurity content and composition stability of the powder directly affect the electrical properties and quality stability of the crystal.

[0003] During the high-temperature synthesis of high-purity silicon carbide powder, the silicon source, carbon source, graphite crucible, and insulation material easily adsorb or release gases such as N2, CO, H2O, and nitrogen-containing volatiles during the loading, transfer, and heating stages. If these nitrogen-containing gases remain inside the crucible, it will increase the nitrogen partial pressure inside the crucible, increase the nitrogen impurity content in the silicon carbide powder, and is detrimental to the preparation of low-nitrogen, high-purity silicon carbide powder.

[0004] In existing silicon carbide powder synthesis processes, some methods employ closed crucibles combined with furnace atmosphere replacement to reduce impurity gas content. However, nitrogen-containing gases released from the raw material stacking gaps and inside graphite components are difficult to fully escape. Another approach uses open-pore crucibles or fixed holes. While this facilitates nitrogen gas escape at low temperatures, effective silicon-carbon gaseous components such as Si, Si₂C, and SiC₂ generated inside the crucible during high-temperature synthesis can easily escape through these holes, leading to silicon source loss and localized compositional imbalances in the powder. Furthermore, adding metal nitrogen absorbers to reduce nitrogen content may introduce metal impurities at high temperatures, failing to meet the requirement of low metal impurity content for high-purity silicon carbide powder. None of these methods resolves the contradiction between sufficient nitrogen removal at low temperatures and minimizing the escape of silicon-carbon gaseous components at high temperatures. Summary of the Invention

[0005] In view of this, the present invention provides a temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device and synthesis method, aiming to solve the problem of simultaneously achieving sufficient nitrogen removal in the low-temperature stage and suppressing the escape of silicon-carbon gas phase components in the high-temperature stage.

[0006] In a first aspect, the present invention provides a temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis apparatus, comprising: The crucible body, the interior of which is used to hold silicon and carbon sources; A crucible cover is disposed inside the crucible body and above the silicon source and carbon source; the crucible cover has at least one conical hole, the diameter of which gradually increases from bottom to top; A cone plug is disposed at the cone-shaped hole, has an outer cone surface that matches the cone-shaped hole, and an initial venting gap is left between the cone plug and the cone-shaped hole; A thermal expansion pusher is disposed above the crucible cover and limited by the side wall of the crucible body, and the lower end of the thermal expansion pusher is fixedly connected to the cone plug; The thermal expansion pusher is used to expand axially when heated to push the cone plug toward the cone orifice, thereby reducing or closing the initial exhaust gap.

[0007] Preferably, the initial exhaust gap is 0.3~1.8 mm.

[0008] Preferably, the linear expansion rate of the thermal expansion pusher is higher than that of the crucible cover.

[0009] Preferably, the thermal expansion pusher has a limiting part and a pushing part. The limiting part extends radially outward from the upper end of the pushing part along the crucible body and is embedded in the top of the side wall of the crucible body. The pushing part extends axially downward along the crucible body, and the lower end of the pushing part is fixedly connected to the cone plug.

[0010] Preferably, the number of conical holes is 1 to 8, and the multiple conical holes are distributed circumferentially along the upper cover of the crucible.

[0011] Preferably, the thermal expansion pusher, the cone plug, and the crucible cover are made of graphite or graphite with a SiC coating; the crucible body is made of graphite.

[0012] Secondly, the present invention provides a method for synthesizing low-nitrogen silicon carbide powder, employing the aforementioned temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis apparatus, comprising the following steps: The silicon source and carbon source are loaded into the crucible body; Evacuate the furnace cavity; The temperature is increased, and during the heating process, vacuuming and gas filling are alternately cyclically replaced. After heating to 1100~1300℃, keep warm in a vacuum or inert atmosphere; Then the temperature is raised to 2100~2400℃ for high-temperature synthesis, and the temperature is maintained under an inert atmosphere; After synthesis, the mixture is cooled and the silicon carbide powder is removed.

[0013] Preferably, the number of times the vacuuming and gas filling cycles are alternated is 1 to 5; the gas filling is high-purity Ar or a mixture of Ar and H2.

[0014] Preferably, after heating to 1100~1300℃, the temperature is maintained in a vacuum or inert atmosphere for 2~5 hours.

[0015] Preferably, the high-temperature synthesis pressure is 50~100 mbar, and the high-temperature synthesis holding time is 2~6 h.

[0016] Compared with the prior art, the present invention has achieved the following beneficial effects: This invention utilizes the initial venting gap maintained between the conical plug and the conical orifice to allow nitrogen-containing gases released from the silicon source, carbon source, and graphite components to be promptly discharged through the conical orifice during the low-temperature degassing stage. This prevents nitrogen-containing gases from stagnating inside the crucible and increasing the nitrogen partial pressure, thereby reducing the nitrogen impurity content in the silicon carbide powder. As the temperature rises, the thermal expansion pusher expands axially due to heat, pushing the conical plug fixedly connected to it towards the conical orifice. The outer conical surface of the conical plug and the inner conical surface of the conical orifice cooperate with each other, causing the conical orifice to gradually decrease or close. This suppresses the escape of effective silicon-carbon gaseous components generated inside the crucible through the conical orifice during the high-temperature synthesis stage, reducing the risk of silicon source loss and local component imbalance in the powder. By automatically controlling the opening and closing of the conical orifice with temperature changes through the thermal expansion pusher, the conical orifice gradually changes from an open state to a flow-limited or closed state, ensuring sufficient nitrogen removal during the low-temperature stage and effective gas retention during the high-temperature stage, thus solving the problem of simultaneously achieving low-temperature nitrogen removal and high-temperature gas retention. Attached Figure Description

[0017] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the spacing or dimensions between parts are exaggerated to show their positions; the schematic diagrams are for illustrative purposes only.

[0018] Figure 1 This is a schematic diagram of the temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device provided by the present invention; Figure 2 These are macroscopic images of the low-nitrogen silicon carbide powder prepared in Example 1 and Comparative Example 1 of the present invention, wherein a is a macroscopic image of the low-nitrogen silicon carbide powder prepared in Comparative Example 1; and b is a macroscopic image of the low-nitrogen silicon carbide powder prepared in Example 1. In the figure, 1 is the limiting structure; 2 is the thermal expansion pusher; 3 is the cone plug; 4 is the cone-shaped hole; 5 is the crucible body; 6 is the raw material; 7 is the top insulation layer; 8 is the crucible cover; 9 is the outer insulation layer; 10 is the induction heating coil; and 11 is the bottom insulation layer. Detailed Implementation

[0019] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0020] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0021] Furthermore, in the description of this invention, it should be understood that the terms "upper," "lower," "left," "right," "top," "bottom," "inner," "outer," "axial," "radial," "circumferential," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a communication connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0023] In this invention, unless otherwise explicitly specified and limited, the first feature being "on" or "below" the second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium.

[0024] High-purity silicon carbide powder is the core raw material for growing silicon carbide single crystals using the physical vapor transport method. Its impurity content and compositional stability directly affect the electrical properties and quality stability of the crystal. During high-temperature synthesis, the silicon source, carbon source, graphite crucible, and insulation materials easily adsorb or release gases such as N2, CO, H2O, and nitrogen-containing volatiles. If nitrogen-containing gases remain inside the crucible, it will increase the nitrogen partial pressure within the crucible, increasing the nitrogen impurity content in the silicon carbide powder. Simultaneously, if effective silicon-carbon gaseous components such as Si, Si2C, and SiC2 generated inside the crucible during the high-temperature stage escape, it will lead to silicon source loss and local compositional imbalance in the powder. Therefore, this invention provides a temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device and method.

[0025] See Figure 1The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device provided by this invention includes a crucible body 5, a crucible cover 8, a conical plug 3, and a thermal expansion pusher 2. The crucible body 5 is externally wrapped with an insulation layer, including a top insulation layer 7, an outer insulation layer 9, and a bottom insulation layer 11, to reduce heat loss during the high-temperature synthesis process and ensure the uniformity and stability of the internal temperature field of the crucible. An induction heating coil 10 is disposed around the outer insulation layer 9, which is used to inductively heat the crucible body 5, providing the necessary heat for the high-temperature synthesis reaction of the silicon carbide powder.

[0026] The crucible body 5 includes a bottom wall and side walls. The side walls of the crucible body 5 extend upwards to form a receiving space. The bottom of this receiving space is used to hold the silicon source and the carbon source (i.e., raw material 6). The crucible body 5 is made of graphite. Graphite has advantages such as high temperature resistance, good chemical stability, and high thermal conductivity, and is a commonly used crucible material in the field of silicon carbide powder synthesis.

[0027] The crucible cover 8 is disposed inside the crucible body 5 and above the raw material 6. The edge of the crucible cover 8 is sealed to the inner surface of the side wall of the crucible body 5. The crucible cover 8 has a certain thickness and at least one conical hole 4 is formed thereon. The diameter of the conical hole 4 gradually increases from bottom to top, that is, the diameter of the end of the conical hole 4 facing the lower part of the crucible cover 8 is smaller, and the diameter of the end facing the upper part of the crucible cover 8 is larger. The material of the crucible cover 8 is graphite or graphite with a SiC coating. SiC-coated graphite is formed by chemical vapor deposition or other methods to form a silicon carbide coating on the surface of a graphite substrate, which can further improve the high-temperature oxidation resistance and corrosion resistance of graphite components and reduce the contamination of powder by graphite components at high temperatures.

[0028] A conical plug 3 is positioned at the conical hole 4 and has an outer conical surface that matches the conical hole 4. The conical plug 3 has a truncated conical structure, wider at the top and narrower at the bottom; that is, the outer conical surface of the plug 3 gradually widens from bottom to top, aligning with the taper direction of the conical hole 4. An initial venting gap is maintained between the conical plug 3 and the conical hole 4, forming the initial channel for the gas to escape from the crucible. The conical plug 3 is made of graphite or graphite with a SiC coating.

[0029] The lower end of the thermal expansion pusher 2 is fixedly connected to the cone plug 3. The thermal expansion pusher 2 and the cone plug 3 can be integrally molded or separately fixedly connected, for example, by means of threaded connection, welding, or bonding. The thermal expansion pusher 2 is made of graphite or graphite with a SiC coating.

[0030] The thermal expansion pusher 2 is positioned above the crucible cover 8 and within the accommodating space enclosed by the side walls of the crucible body 5. The limiting structure 1 is positioned on the uppermost side wall of the crucible body 5. The thermal expansion pusher 2 has a limiting part and a pushing part. The limiting part extends radially outward from the upper end of the pushing part and is embedded in the limiting structure 1. The pushing part extends axially downward, and its lower end is fixedly connected to the cone plug 3. The top insulation layer 7 is positioned above the thermal expansion pusher 2 and works in conjunction with the limiting structure 1 to limit the thermal expansion pusher 2 axially (i.e., prevent it from moving upward). When the thermal expansion pusher 2 expands axially due to heat, because the limiting part is blocked by the limiting structure 1 and the top insulation layer 7 presses it down, its axial expansion cannot be released upward. Instead, it is converted into a downward pushing force, pushing the cone plug 3 towards the conical hole 4, thereby reducing or closing the initial exhaust gap.

[0031] At low temperatures (e.g., within the temperature range of room temperature to approximately 1200°C), an initial venting gap is maintained between the cone plug 3 and the cone-shaped hole 4. This initial venting gap keeps the interior of the crucible body 5 connected to the external environment. Gases such as N2, CO, H2O, and nitrogen-containing volatiles released from the silicon source, carbon source, graphite crucible, and insulation material during loading, transfer, and heating can be promptly discharged through the cone-shaped hole 4, preventing nitrogen-containing gases from stagnating inside the crucible and increasing the nitrogen partial pressure, thereby reducing the nitrogen impurity content in the silicon carbide powder.

[0032] The initial exhaust gap is 0.3~1.8 mm, for example, it can be 0.3 mm, 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.4 mm, 1.6 mm or 1.8 mm, preferably 0.6~1.4 mm. If the initial exhaust gap is too large, the cone plug 3 needs to move a longer distance to close the conical hole 4 in the high-temperature stage, which places higher demands on the expansion amount of the thermal expansion pusher 2; if the initial exhaust gap is too small, exhaust will be poor in the low-temperature stage, and nitrogen-containing gas may not be able to be fully discharged. Controlling the initial exhaust gap within the range of 0.3~1.8 mm ensures sufficient exhaust in the low-temperature stage, and also allows the thermal expansion pusher 2 to push the cone plug 3 to close the conical hole 4 with a reasonable expansion amount in the high-temperature stage.

[0033] As the temperature rises, the thermal expansion pusher 2 undergoes axial thermal expansion. Since the limiting structure 1 restricts the upward displacement of the thermal expansion pusher 2, the axial expansion of the thermal expansion pusher 2 is converted into a downward pushing force, propelling the cone plug 3, which is fixedly connected to it, towards the conical hole 4. As the cone plug 3 moves downward, the gap between the outer conical surface of the cone plug 3 and the inner conical surface of the conical hole 4 gradually decreases, causing the conical hole 4 to gradually change from an open state to a flow-limiting state, and finally enter a closed state.

[0034] The linear expansion rate of the thermal expansion pusher 2 is higher than that of the crucible cover 8. The thermal expansion pusher 2 is made of high-purity graphite material with a high linear expansion rate, and its effective length is determined according to the actual dimensions of the crucible body 5, typically ranging from 20 to 150 mm, for example, 20 mm, 30 mm, 50 mm, 80 mm, 100 mm, 120 mm, or 150 mm, preferably 40 to 120 mm. The effective length of the thermal expansion pusher 2 refers to the length along which it can generate effective thermal expansion in the axial direction. The longer the effective length, the greater the absolute expansion amount generated under the same linear expansion rate, which is more beneficial for pushing the cone plug 3 to close the cone hole 4.

[0035] The number of conical holes 4 is 1 to 8, for example, 1, 2, 3, 4, 5, 6, 7, or 8, preferably 2 to 4. Multiple conical holes 4 are distributed circumferentially along the crucible cover 8. Providing multiple conical holes 4 helps improve the efficiency of nitrogen-containing gas venting from the crucible during the low-temperature stage, allowing gases released from different areas inside the crucible to find a nearby venting channel, avoiding localized gas stagnation. Simultaneously, the uniform circumferential distribution of multiple conical holes 4 facilitates the balanced release of gas pressure within the crucible across different areas.

[0036] When multiple conical orifices 4 and corresponding conical plugs 3 are provided, the initial exhaust gaps corresponding to the multiple conical plugs 3 can be the same or different. The effective lengths of the thermal expansion actuators 2 corresponding to the multiple conical plugs 3 can be the same or different. Specifically, the larger the initial exhaust gap, the greater the distance the conical plug 3 needs to move, and the higher the temperature required for the conical orifice 4 to enter the closed state; the longer the effective length of the thermal expansion actuator 2, the greater the axial expansion at the same temperature, and the lower the temperature required for the conical orifice 4 to enter the closed state. By setting different initial exhaust gaps or different effective lengths of the thermal expansion actuators 2, multiple conical orifices 4 can be synchronously limited or closed during the heating process, or multiple conical orifices 4 can be limited or closed in stages during the heating process. For example, some conical orifices 4 can enter the limited flow state first at a lower temperature, and other conical orifices 4 can enter the limited flow or closed state at a higher temperature, thereby realizing the staged control of the exhaust channel and further optimizing the synergistic effect of low-temperature nitrogen removal and high-temperature gas retention.

[0037] Secondly, the present invention provides a method for synthesizing low-nitrogen silicon carbide powder, employing the aforementioned temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis apparatus, comprising the following steps: A silicon source and a carbon source are loaded into the crucible body 5. The silicon source and the carbon source can be powder, granules, blocks, or a combination thereof. More preferably, the silicon source can be high-purity silicon powder, and the carbon source can be high-purity carbon powder or high-purity graphite powder. The silicon source and the carbon source are mixed according to the stoichiometric ratio for silicon carbide synthesis and then loaded into the crucible body 5.

[0038] Evacuate the furnace chamber. The furnace chamber is the internal space of the furnace body used to house the crucible. Evacuate the furnace chamber to a vacuum level ≤1×10⁻⁶. - 3 Pa, preferably evacuated to ≤1×10 Pa, -4 Pa. The purpose of evacuation is to remove residual air from the furnace cavity and crucible, and to prevent impurities such as oxygen and nitrogen in the air from affecting subsequent synthesis reactions.

[0039] The system is heated, and during the heating process, vacuuming and gas filling are alternately cyclical. The number of vacuuming and gas filling cycles is 1 to 5, for example, 1, 2, 3, 4, or 5. Each filling cycle involves introducing high-purity Ar or a mixture of Ar and H2. Each filling cycle is performed to 100 to 800 mbar, for example, 100 mbar, 200 mbar, 300 mbar, 400 mbar, 500 mbar, 600 mbar, 700 mbar, or 800 mbar; each vacuuming cycle is performed to 1 to 50 mbar, for example, 1 mbar, 5 mbar, 10 mbar, 20 mbar, 30 mbar, 40 mbar, or 50 mbar. When the Ar and H2 mixture is introduced, the volume fraction of H2 in the mixture is 1 to 10%, for example, 1%, 2%, 3%, 5%, 7%, 8%, or 10%. The purpose of alternating vacuuming and gas filling is to promptly remove gases such as N2, CO, H2O, and nitrogen-containing volatiles continuously released from the silicon source, carbon source, graphite crucible, and insulation material during the heating process, preventing these gases from accumulating inside the crucible. The addition of H2 helps to form a reducing atmosphere, which can promote the removal of oxygen-containing impurities.

[0040] After heating to 1100~1300℃, for example, to 1100℃, 1150℃, 1200℃, 1250℃, or 1300℃, the material is held at this temperature under a vacuum or inert atmosphere. Within this temperature range, the silicon and carbon sources have not yet undergone a vigorous silicon carbide synthesis reaction, but the gases adsorbed inside the raw materials and graphite components are released relatively fully. The holding time is 2~5 hours, for example, 2 hours, 3 hours, 4 hours, or 5 hours. It should be noted that the holding time can be adjusted according to the amount of raw material, the thickness of the material layer, the gas replacement conditions, and the thermal field state. When the amount of material is small, the material layer is thin, or the gas is released relatively fully, the holding time can be appropriately shortened; when the amount of material is large, the material layer is thick, or the gas release path is long, the holding time can be appropriately extended to promote the full removal of adsorbed gases and volatile impurities from the raw materials and the inside of the crucible. Furthermore, the holding time can be adaptively adjusted according to the actual thermal field conditions as the usage status of the graphite crucible and insulation components changes. The holding process allows sufficient time for nitrogen-containing gases to diffuse out from the raw material and graphite components, and then exit through the conical orifice 4. At this time, an initial venting gap is still maintained between the conical plug 3 and the conical orifice 4, and the conical orifice 4 is in the open state, allowing N2, CO, H2O, and nitrogen-containing volatiles released from inside the crucible to be promptly discharged through the conical orifice 4.

[0041] During the heating process to 1100-1300℃ and the subsequent holding period, the axial expansion of the thermal expansion pusher 2 is insufficient to overcome the initial exhaust gap, so the conical orifice 4 remains open, ensuring the full exhaust of nitrogen-containing gas. As the temperature continues to rise to 1300-1800℃ (i.e., the intermediate temperature transition stage), the axial expansion of the thermal expansion pusher 2 gradually increases. When the temperature rises to approximately 1700-1800℃, the axial expansion of the thermal expansion pusher 2 reaches or exceeds the size of the initial exhaust gap, and the conical plug 3 is pushed to form a conical surface fit with the conical orifice 4. The conical orifice 4 gradually changes from an open state to a flow-limiting state, and eventually essentially enters a closed state. That is, when the temperature rises to approximately 1800℃, the conical orifice 4 has completed the transition from open to closed, thus preparing for the subsequent high-temperature synthesis stage.

[0042] Then, the temperature is raised to 2100~2400℃ for high-temperature synthesis, for example, to 2100℃, 2150℃, 2200℃, 2250℃, 2300℃, 2350℃, or 2400℃, preferably to 2200~2350℃. The pressure for high-temperature synthesis is 50~100 mbar, for example, 50 mbar, 60 mbar, 70 mbar, 80 mbar, 90 mbar, or 100 mbar. High-temperature synthesis is carried out in an inert atmosphere (e.g., high-purity Ar atmosphere), and the holding time is 2~6 h, for example, 3 h, 4 h, 5 h, or 6 h. It should be noted that the holding time can be adjusted according to the synthesis temperature, synthesis pressure, raw material charge amount, and thermal field conditions. When the synthesis temperature is higher or the pressure conditions are favorable for the transport of silicon-containing gaseous components, the holding time can be appropriately shortened; when the synthesis temperature is lower, the pressure is higher, or the charge amount is larger, the holding time can be appropriately extended to promote the full reaction between the silicon source and the carbon source. The holding time for high-temperature synthesis can be adjusted adaptively based on the actual usage of the graphite crucible and insulation components. During this high-temperature stage, the silicon and carbon sources react fully to generate silicon carbide powder. Because the thermal expansion pusher 2 has already expanded axially during the heating process (especially within the 1300~1800℃ range) and pushed the cone plug 3 towards the cone-shaped hole 4, the cone-shaped hole 4 is already in a flow-limited or closed state. Therefore, during the high-temperature synthesis stage, the cone-shaped hole 4 is in a closed state, effectively reducing the escape of effective silicon-carbon gaseous components such as Si, Si2C, and SiC2.

[0043] After synthesis, the mixture is cooled and the silicon carbide powder is removed. Cooling can be carried out under a high-purity Ar protective atmosphere or in a vacuum to prevent the silicon carbide powder generated at high temperature from being oxidized during the cooling process. After cooling to a temperature suitable for unloading from the furnace, the furnace chamber is opened, the crucible is removed, and the synthesized silicon carbide powder is collected from the crucible body 5.

[0044] The present invention does not impose any special restrictions on the heating rate of the above heating process, which is preferably 2~5℃ / min, for example, it can be 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, etc.

[0045] The entire synthesis method described above automatically controls the opening and closing of the conical orifice 4 by temperature changes, which ensures sufficient nitrogen removal at low temperatures and effective gas retention at high temperatures, thus solving the problem of the difficulty in simultaneously achieving nitrogen removal at low temperatures and gas retention at high temperatures.

[0046] The technical solution of the present invention will be further described below with reference to specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments of the present invention were all purchased through commercial channels.

[0047] In the following embodiments of the present invention, the crucible assemblies (including the crucible body 5, the crucible cover 8, the conical plug 3, and the thermal expansion pusher 2) are all made of high-purity graphite, and the purity of each graphite component is not less than 99.99%. The linear expansion rate of the high-purity graphite material used in the thermal expansion pusher 2 is higher than that of the materials used in the crucible cover 8 and the conical plug 3. The purity of the high-purity silicon source and the high-purity carbon source is not less than 99.999%. The purity of the high-purity Ar is not less than 99.999%. In the following embodiments, the thermal expansion pusher 2 and the conical plug 3 are integrally formed.

[0048] To illustrate the axial expansion effect of the thermal expansion actuator 2 during the heating process, thermal expansion tests were conducted on the selected high-purity graphite material. The test results are shown in Table 1, where the linear expansion rate is the relative length change rate of the material at the corresponding temperature relative to the initial length, and the theoretical expansion length of the actuator is calculated using the formula ΔL = L0 × (dL / L0).

[0049] Table 1. Linear expansion rate and theoretical expansion length of high-purity graphite thermal expansion actuator at different temperatures.

[0050] Example 1 The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device provided in this embodiment is as follows: Figure 1 As shown. In this embodiment, the crucible cover 8 has two conical holes 4, and the initial venting gap between the conical plug 3 and the conical hole 4 is 1.0 mm, and the effective length of the thermal expansion pusher 2 is 80 mm.

[0051] The method for synthesizing low-nitrogen silicon carbide powder in this embodiment includes the following steps: (1) Mix high-purity silicon powder and high-purity carbon powder in a molar ratio of 1:1 and put them into the crucible body 5.

[0052] (2) Install the crucible cover 8, and leave an initial venting gap of 1.0 mm between the cone plug 3 and the cone hole 4.

[0053] (3) Evacuate the furnace cavity to ≤1×10 -4 Pa.

[0054] (4) Heat to 1200℃ at a heating rate of 3℃ / min, and perform three alternating cycles of vacuuming and gas filling during the heating process. Each time, fill with high-purity Ar to 500 mbar and then vacuum to 10 mbar.

[0055] (5) After heating to 1200℃, keep it at a low pressure atmosphere of 10 mbar for 3 h. At this time, the initial exhaust gap is maintained between the cone plug 3 and the cone hole 4. The cone hole 4 is in the open state, and the N2, CO and nitrogen-containing volatiles released inside the crucible are discharged through the cone hole 4.

[0056] (6) Continue heating at a heating rate of 2.5℃ / min to 1700℃. The thermal expansion pusher 2 expands axially due to the heat. As shown in Table 1, the theoretical expansion length generated by the thermal expansion pusher 2 is greater than the initial exhaust gap between the cone plug 3 and the cone hole 4 at this temperature. The cone hole 4 enters the closed state.

[0057] (7) Continue heating at a rate of 3℃ / min to 2200℃ for high-temperature synthesis, and keep warm for 4 h in a high-purity Ar atmosphere of 70 mbar to allow silicon powder and carbon powder to react fully to generate silicon carbide powder. At this time, the conical hole 4 is closed to prevent the effective silicon-carbon gas phase components such as Si, Si2C, and SiC2 from escaping.

[0058] (8) After the synthesis is completed, cool to room temperature under a high-purity Ar protective atmosphere and take out the silicon carbide powder.

[0059] The silicon carbide powder obtained in this embodiment is yellowish-white in color, such as... Figure 2 As shown in b, the nitrogen content in the obtained silicon carbide powder was 7.8 × 10⁻⁶ as determined by secondary ion mass spectrometry (SIMS). 15 atom / cm 3 .

[0060] Example 2 The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device provided in this embodiment is as follows: Figure 1 As shown. In this embodiment, the crucible cover 8 has four conical holes 4, and the initial venting gap between the conical plug 3 and the conical hole 4 is 1.4 mm, and the effective length of the thermal expansion pusher 2 is 100 mm.

[0061] The method for synthesizing low-nitrogen silicon carbide powder in this embodiment includes the following steps: (1) Mix high-purity silicon powder and high-purity carbon powder in a molar ratio of 1:1 and put them into the crucible body 5.

[0062] (2) Install the crucible cover 8, and leave an initial venting gap of 1.4 mm between the cone plug 3 and the cone hole 4.

[0063] (3) Evacuate the furnace cavity to ≤1×10 -4 Pa.

[0064] (4) Heat to 1200℃ at a heating rate of 3℃ / min, and perform three alternating cycles of vacuuming and gas filling during the heating process. Each time, fill with a mixture of high-purity Ar and H2 to 500 mbar, and then vacuum to 10 mbar, where the volume fraction of H2 is 5%.

[0065] (5) After heating to 1200℃, keep it at a low pressure atmosphere of 10 mbar for 3 h. At this time, the initial exhaust gap is maintained between the cone plug 3 and the cone hole 4. The cone hole 4 is in the open state, and the N2, CO and nitrogen-containing volatiles released inside the crucible are discharged through the cone hole 4.

[0066] (6) Continue heating at a heating rate of 2.5℃ / min to 1750℃. The thermal expansion pusher 2 expands axially due to the heat. As shown in Table 1, the theoretical axial expansion length generated by the thermal expansion pusher 2 is greater than the initial exhaust gap between the cone plug 3 and the cone hole 4 at this temperature. The cone hole 4 enters the flow restriction or closed state.

[0067] (7) Continue heating at a rate of 3℃ / min to 2200℃ for high-temperature synthesis, and keep warm for 4 h in a high-purity Ar atmosphere of 70 mbar to allow silicon powder and carbon powder to react fully to generate silicon carbide powder. At this time, the conical hole 4 is closed to prevent the effective silicon-carbon gas phase components such as Si, Si2C, and SiC2 from escaping.

[0068] (8) After the synthesis is completed, cool to room temperature under a high-purity Ar protective atmosphere and take out the silicon carbide powder.

[0069] The silicon carbide powder obtained in this embodiment is yellowish-white in color. SIMS testing showed that the nitrogen content in the obtained silicon carbide powder was 4.5 × 10⁻⁶. 15 atom / cm 3 .

[0070] Example 3 The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device provided in this embodiment is as follows: Figure 1 As shown. In this embodiment, the crucible cover 8 has four conical holes 4, and the initial venting gap between the conical plug 3 and the conical hole 4 is 1.4 mm, and the effective length of the thermal expansion pusher 2 is 100 mm.

[0071] The method for synthesizing low-nitrogen silicon carbide powder in this embodiment includes the following steps: (1) Mix high-purity silicon powder and high-purity carbon powder in a molar ratio of 1:1 and put them into the crucible body 5.

[0072] (2) Install the crucible cover 8, and leave an initial venting gap of 1.4 mm between the cone plug 3 and the cone hole 4.

[0073] (3) Evacuate the furnace cavity to ≤1×10 -4 Pa.

[0074] (4) Heat to 1300℃ at a heating rate of 3℃ / min, and perform three alternating cycles of vacuuming and gas filling during the heating process. Each time, fill with a mixture of high-purity Ar and H2 to 500 mbar, and then vacuum to 10 mbar, where the volume fraction of H2 is 5%.

[0075] (5) After heating to 1300℃, keep it at 10 mbar low pressure atmosphere for 2 h. At this time, the initial exhaust gap is maintained between the cone plug 3 and the cone hole 4. The cone hole 4 is in the open state, and the N2, CO and nitrogen-containing volatiles released inside the crucible are discharged through the cone hole 4.

[0076] (6) Continue heating at a heating rate of 2.5℃ / min to 1800℃. The thermal expansion pusher 2 expands axially due to the heat. As shown in Table 1, the theoretical axial expansion length generated by the thermal expansion pusher 2 is greater than the initial exhaust gap between the cone plug 3 and the cone hole 4 at this temperature. The cone hole 4 enters the flow restriction or closed state.

[0077] (7) Continue heating at a rate of 3℃ / min to 2350℃ for high-temperature synthesis, and keep warm for 2 h in a high-purity Ar atmosphere of 60 mbar to allow silicon powder and carbon powder to react fully to generate silicon carbide powder. At this time, the conical hole 4 is closed to prevent the effective silicon-carbon gas phase components such as Si, Si2C, and SiC2 from escaping.

[0078] (8) After the synthesis is completed, cool to room temperature under a high-purity Ar protective atmosphere and take out the silicon carbide powder.

[0079] The silicon carbide powder obtained in this embodiment is yellowish-white in color. SIMS testing showed that the nitrogen content in the obtained silicon carbide powder was 4.1 × 10⁻⁶. 15 atom / cm 3 .

[0080] Comparative Example This comparative example uses a common graphite top cover, which does not have a conical hole 4, a conical plug 3, or a thermal expansion pusher 2, meaning it lacks the exhaust structure of a temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device. Except for the structure of the crucible top cover 8, the types of raw materials, the charging method, the heating regime, and the high-temperature synthesis conditions are all the same as in Example 1.

[0081] The method for synthesizing silicon carbide powder in this comparative example includes the following steps: (1) Mix high-purity silicon powder and high-purity carbon powder in a molar ratio of 1:1 and put them into the crucible body 5.

[0082] (2) Install a regular graphite top cover.

[0083] (3) Evacuate the furnace cavity to ≤1×10 -4 Pa.

[0084] (4) Heat to 1200 °C at a heating rate of 3 °C / min, and perform three alternating cycles of vacuuming and gas filling during the heating process. Each time, fill with high-purity Ar to 500 mbar and then vacuum to 10 mbar.

[0085] (5) After heating to 1200 ℃, keep warm for 3 h under a low pressure atmosphere of 10 mbar.

[0086] (6) Continue heating to 1700℃ at a heating rate of 2.5℃ / min, and then continue heating to 2200℃ at a heating rate of 3℃ / min for high-temperature synthesis, and keep warm for 4 h in a high-purity Ar atmosphere of 70 mbar.

[0087] (7) After the synthesis is completed, cool to room temperature under a high-purity Ar protective atmosphere and take out the silicon carbide powder.

[0088] Because the ordinary graphite top cover does not have a conical hole 4, N2, CO, H2O, and nitrogen-containing volatiles released from the silicon powder, carbon powder, and graphite crucible during the low-temperature heating stage cannot be discharged in time. Nitrogen-containing gases remain inside the crucible body 5, increasing the nitrogen partial pressure within the crucible. After synthesis, the obtained silicon carbide powder appears yellowish-green, such as... Figure 2 As shown in 'a', the nitrogen content in the obtained silicon carbide powder, as determined by SIMS testing, is 9.5 × 10⁻⁶. 16 atom / cm 3 .

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device, characterized in that, include: The crucible body, the interior of which is used to hold silicon and carbon sources; A crucible cover is disposed inside the crucible body and above the silicon source and carbon source; the crucible cover has at least one conical hole, the diameter of which gradually increases from bottom to top; A cone plug is disposed at the cone-shaped hole, has an outer cone surface that matches the cone-shaped hole, and an initial venting gap is left between the cone plug and the cone-shaped hole; A thermal expansion pusher is disposed above the crucible cover and limited by the side wall of the crucible body, and the lower end of the thermal expansion pusher is fixedly connected to the cone plug; The thermal expansion pusher is used to expand axially when heated to push the cone plug toward the cone orifice, thereby reducing or closing the initial exhaust gap.

2. The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device as described in claim 1, characterized in that, The initial exhaust gap is 0.3~1.8 mm.

3. The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device as described in claim 1, characterized in that, The linear expansion rate of the thermal expansion actuator is higher than that of the crucible cover.

4. The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device as described in claim 1, characterized in that, The thermal expansion pusher has a limiting part and a pushing part. The limiting part extends radially outward from the upper end of the pushing part along the crucible body and is embedded in the top of the side wall of the crucible body. The pushing part extends axially downward along the crucible body, and the lower end of the pushing part is fixedly connected to the cone plug.

5. The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device as described in claim 1, characterized in that, The number of conical holes is 1 to 8, and multiple conical holes are distributed circumferentially along the upper cover of the crucible.

6. The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis device as described in claim 1, characterized in that, The thermal expansion pusher, cone plug, and crucible cover are made of graphite or graphite with a SiC coating; the crucible body is made of graphite.

7. A method for synthesizing low-nitrogen silicon carbide powder, characterized in that, The temperature-responsive self-sealing low-nitrogen silicon carbide powder synthesis apparatus according to any one of claims 1 to 6 includes the following steps: The silicon source and carbon source are loaded into the crucible body; Evacuate the furnace cavity; The temperature is increased, and during the heating process, vacuuming and gas filling are alternately cyclically replaced. After heating to 1100~1300℃, keep warm in a vacuum or inert atmosphere; Then the temperature is raised to 2100~2400℃ for high-temperature synthesis, and the temperature is maintained under an inert atmosphere; After synthesis, the mixture is cooled and the silicon carbide powder is removed.

8. The method for synthesizing low-nitrogen silicon carbide powder as described in claim 7, characterized in that, The number of times the vacuuming and gas filling cycles are alternated is 1 to 5; the gas filling is high-purity Ar or a mixture of Ar and H2.

9. The method for synthesizing low-nitrogen silicon carbide powder as described in claim 7, characterized in that, After heating to 1100~1300℃, the temperature is maintained in a vacuum or inert atmosphere for 2~5 hours.

10. The method for synthesizing low-nitrogen silicon carbide powder as described in claim 7, characterized in that, The high-temperature synthesis pressure is 50~100 mbar, and the high-temperature synthesis holding time is 2~6 h.

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