Preparation method of silicon carbide powder

Abstract

The invention relates to the technical field of preparation of silicon carbide powder, discloses a preparation method of silicon carbide powder, and solves the technical problem of effectively controlling the crystal form of silicon carbide. Comprising the following steps: providing silicon particles as silicon cores; the surface of the silicon core is coated with a composite shell layer to form a core-shell structure precursor, the composite shell layer comprises a carbon source precursor and a crystal form control agent precursor, and the crystal form control agent precursor comprises any one or more of a nitrogen-containing precursor, an aluminum-containing precursor and a boron-containing precursor; the core-shell structure precursor is subjected to heat treatment in a preset atmosphere, so that the carbon source precursor and the silicon core are subjected to a carbon thermal reduction reaction to form silicon carbide, and meanwhile, the crystal form control agent precursor is decomposed to release an active control element containing any one or more of nitrogen, aluminum and boron, the silicon carbide crystal structure is regulated in situ in the silicon carbide forming process, so that a silicon carbide product containing the target crystal form silicon carbide is obtained. And the target crystal form silicon carbide can be effectively controlled and obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide powder preparation, and particularly relates to a method for preparing silicon carbide powder. Background Art

[0002] Silicon carbide (SiC) powder, as a key industrial raw material and advanced functional material, plays an indispensable role in many fields such as abrasive tools, refractory materials, structural ceramics, and especially semiconductors represented by power electronic devices due to its excellent physical and chemical properties - such as high melting point, high hardness, excellent thermal conductivity, high breakdown field strength, high electron saturation drift rate, excellent oxidation resistance and high temperature strength, as well as excellent chemical stability and wear resistance.

[0003] Currently, the main method for large-scale industrial production of commercial-grade silicon carbide powder is still the Acheson method developed since the late 19th century (refer to U.S. Patent US492767 authorized in 1893). This method usually uses quartz sand (SiO2) and petroleum coke (or other carbonaceous materials) as the main raw materials. After mixing, they are placed in a large resistance furnace, and high temperature is generated by energizing the graphite resistance core (the central temperature can reach above 2500 °C). Silicon carbide is prepared through a carbothermal reduction reaction (SiO2 + 3C → SiC + 2CO). After the reaction ends, the furnace charge undergoes processes such as cooling, crushing, sorting, grinding, and purification to obtain the silicon carbide product.

[0004] However, the traditional Acheson method has many inherent defects in preparing high-quality, specific crystal form silicon carbide, and it is difficult to meet the increasingly stringent requirements of modern high-tech fields for silicon carbide materials. Specifically, the traditional Acheson method mainly has the following problems:

[0005] 1) The product purity is relatively low. The raw materials used in the Acheson method (such as quartz sand, petroleum coke) usually contain a certain amount of metal impurities (such as Fe, Al, Ca, etc.) and non-metal impurities. Although additives such as table salt may be added in the process to try to promote the volatilization and removal of some impurities, and there are subsequent purification steps such as pickling, due to the reaction being carried out at extremely high temperatures, the furnace atmosphere is complex, and the purity of the raw materials themselves is limited, impurity elements are still prone to diffusion and solid solution in the SiC lattice or form independent impurity phases. This results in the final product purity being difficult to economically and effectively meet the high purity requirements for electronic applications (for example, the total impurity content is less than 10 ppm, that is, the purity ≥ 99.999 wt%). The presence of impurities will seriously deteriorate the electrical properties (such as carrier lifetime, mobility), optical properties, and thermal properties of silicon carbide materials.

[0006] 2) The particle size is large, the morphology is irregular, and the distribution is broad. The Acheson method is essentially a bulk material synthesis process, and its direct product is large chunks of aggregates sintered or bonded by SiC crystals. These large chunks need to go through multiple mechanical crushing and long-term ball milling and other grinding processes to obtain powders with the required particle size. This process not only has high energy consumption and low efficiency, but also is extremely likely to introduce new pollution from grinding media and the environment (such as wear debris of grinding balls). More importantly, it is difficult to obtain micron- and even nano-scale silicon carbide powders with a narrow particle size distribution and regular morphology (such as spherical or specific crystal plane exposure) through this "top-down" crushing method. Coarse, irregular and wide particle size distribution particles are not conducive to densification and microstructure control in the subsequent ceramic sintering process, and also cannot meet some applications with special requirements for fine powders (such as catalyst carriers, high-performance composite material fillers, etc.).

[0007] 3) The crystal forms are mixed and difficult to control. This is one of the most prominent bottlenecks of the Acheson method in meeting specific application requirements. Silicon carbide has significant polytypism, with multiple crystal structures, mainly divided into cubic β-SiC (the most common is 3C-SiC) and multiple hexagonal or rhombohedral α-SiC (such as 2H-SiC, 4H-SiC, 6H-SiC, 15R-SiC, etc.). Different crystal forms of silicon carbide have significant differences in key physical properties such as band gap width, carrier mobility, thermal conductivity, and fracture toughness. For example, 4H-SiC has become the preferred crystal form for manufacturing high-performance, high-voltage, and high-temperature power electronic devices (such as MOSFETs, IGBTs, Schottky diodes) due to its wider band gap width, higher electron mobility, and breakdown field strength; while 6H-SiC has also been widely used in LED substrates and some power devices. However, there are huge and difficult-to-precisely-control temperature gradients and complex local atmospheres in the Acheson furnace, resulting in the silicon carbide products generated usually being a physical mixture of multiple α-SiC polytypes (mainly 6H-SiC and 4H-SiC, and other α-phases), and often accompanied by β-SiC generated in the lower temperature region or incomplete reaction regions. Operators can hardly actively select or precisely control the generation of a single, pure specific SiC crystal form by adjusting macroscopic process parameters. This uncontrollability and mixing of crystal forms severely limit the direct application of Acheson method SiC in those fields with strict requirements for material crystal forms (especially semiconductor device manufacturing), and usually require subsequent expensive and complex purification and separation processes.

[0008] In order to overcome some of the above-mentioned shortcomings of the Acheson method, especially in the preparation of high-purity, specific crystal materials, researchers have developed a variety of advanced silicon carbide preparation technologies. For example, the chemical vapor deposition (CVD) method uses high-purity gaseous precursors (such as silane, chlorosilanes and carbon-containing gases) and, under strictly controlled reaction conditions, can epitaxially grow single crystal thin films or thick films of high purity, low defect density, and specific crystal forms (such as 4H-SiC or 6H-SiC) on suitable substrates. This is crucial for the manufacture of semiconductor devices. However, the CVD method is mainly used to prepare thin films or perform bulk single crystal growth. If it is used to directly, large-scale, and low-costly prepare silicon carbide powder with a specific target crystal form, it may face problems such as limited output, high cost, or complex powder collection and morphology control. Summary of the Invention

[0009] The object of the present invention is to provide a method for preparing silicon carbide powder to solve the technical problem of effectively controlling the crystal form of silicon carbide.

[0010] The preparation method of silicon carbide powder of the present invention comprises: providing silicon particles as silicon cores; coating the surface of the silicon cores with a composite shell layer to form a core-shell structure precursor, wherein the composite shell layer comprises a carbon source precursor and a crystal form control agent precursor, and the crystal form control agent precursor comprises any one or more of a nitrogen-containing precursor, an aluminum-containing precursor, and a boron-containing precursor; heat-treating the core-shell structure precursor under a preset atmosphere so that the carbon source precursor and the silicon core undergo a carbon thermal reduction reaction to form silicon carbide, and at the same time, the active control elements comprising any one or more of nitrogen, aluminum, and boron released by decomposition of the crystal form control agent precursor in situ regulate the silicon carbide crystal structure during the silicon carbide formation process to obtain a silicon carbide product containing target crystal form silicon carbide.

[0011] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the silicon particles are nano-scale particles or micron-scale particles.

[0012] As an optimization and / or embodiment of the above method for preparing silicon carbide powder, further: the silicon particles form silicon powder with a purity of ≥99.999wt%.

[0013] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the carbon source precursor includes any one or more of phenolic resin, polyacrylonitrile, polyvinyl alcohol, sucrose, glucose, asphalt, soluble organic polymer, graphene, and carbon nanotubes.

[0014] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the nitrogen-containing precursor includes any one or more of urea, melamine, polyacrylonitrile, nitrogen-containing organic amine compounds, and nitrogen-containing heterocyclic compounds.

[0015] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the aluminum-containing precursor includes any one or more of aluminum acetylacetonate, aluminum triisopropoxide, aluminum nitrate, and soluble organic aluminum compounds.

[0016] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the boron-containing precursor includes any one or more of boric acid, borate ester, organic boron compound, and borane ammonia complex.

[0017] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the addition amount of the crystal form control agent precursor in the composite shell layer, calculated by the mass of the active control element released after the decomposition of the crystal form control agent precursor, is 0.001% - 0.1% by mass percentage relative to the theoretical mass of the finally formed silicon carbide product.

[0018] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the addition amount of the carbon source precursor in the composite shell layer can ensure that the molar ratio of the carbon element provided by the carbon source precursor during the heat treatment to the silicon element in the silicon core is 1.02:1 to 1.05:1.

[0019] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the specific method for coating the composite shell layer on the surface of the silicon core to form the core-shell structure precursor includes: dissolving or uniformly dispersing the carbon source precursor and the crystal form control agent precursor in the same liquid dispersant to form a mixed precursor solution or a mixed precursor suspension; attaching the mixed precursor solution or the mixed precursor suspension to the surface of the silicon core, and then performing a drying treatment; performing a pre-carbonization treatment on the carbon source precursor on the solid particles obtained after the drying treatment to obtain the core-shell structure precursor, and the pre-carbonization treatment of the carbon source precursor is carried out in an inert atmosphere, the temperature is controlled at 200°C - 800°C, the heating rate is 1°C / min - 10°C / min, and the heat preservation time is 0.5 hour - 4 hours.

[0020] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: the specific method for attaching the mixed precursor solution or the mixed precursor suspension to the surface of the silicon core and then performing a drying treatment includes: dispersing the silicon core in the mixed precursor solution or the mixed precursor suspension to form a slurry, and then preparing the slurry into the solid particles by a spray drying process.

[0021] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: The specific method of heat-treating the core-shell structure precursor in a preset atmosphere includes performing multi-stage high-temperature treatment on the core-shell structure precursor in a high-temperature reaction device; the multi-stage high-temperature treatment includes: The first-stage treatment: Under the protection of an inert atmosphere, heating at a heating rate of 5 °C / min - 10 °C / min to 800 °C - 1000 °C and holding for 1 hour - 2 hours to deeply carbonize the carbon source precursor; The second-stage treatment: Continuing under the protection of an inert atmosphere, heating at a heating rate of 5 °C / min - 10 °C / min to 1600 °C - 1800 °C and holding for 2 hours - 4 hours to react silicon nuclei with carbon to form β-silicon carbide; The third-stage treatment: Continuing to heat at a heating rate of 3 °C / min - 5 °C / min to 2000 °C - 2200 °C and holding for 1 hour - 3 hours under the condition that the vacuum degree is not higher than 1 Pa. Under the in-situ regulation effect of the active control element, promoting the transformation of β-silicon carbide to the target crystal form α-silicon carbide; After the multi-stage high-temperature treatment, slowly cooling to room temperature under the protection of an inert atmosphere to obtain a silicon carbide crude product rich in the target crystal form α-silicon carbide.

[0022] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: Post-treating the silicon carbide crude product to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-silicon carbide; The post-treatment includes: Pre-pickling the silicon carbide crude product with a hot hydrochloric acid solution to remove easily soluble metal impurities; Passing a mixed gas stream of water vapor and an inert gas or a mixed gas stream of carbon dioxide and an inert gas through the pre-pickled silicon carbide crude product at 650 °C - 800 °C for gas-phase carbon removal; Using a mixed acid solution of hydrofluoric acid and nitric acid to perform main pickling and impurity removal on the silicon carbide crude product after gas-phase carbon removal at room temperature to 60 °C with the assistance of ultrasonic waves; Washing the silicon carbide crude product after main pickling and impurity removal with high-purity water until neutral and drying at low temperature to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-silicon carbide with a purity ≥ 99.9 wt%; Crushing and classifying the high-purity nano-silicon carbide product through a supersonic airflow mill with an internal classification wheel to obtain a high-purity nano-silicon carbide product with a D90 particle size ≤ 500 nm.

[0023] As an optimization and / or instantiation of the above method for preparing silicon carbide powder, further: The target crystal form of silicon carbide is 4H-SiC or 6H-SiC.

[0024] The preparation method of silicon carbide powder of the present invention involves providing silicon particles as silicon nuclei and coating a composite shell layer containing a carbon source precursor and a crystal form control agent precursor on the surface of the silicon nuclei to form a core-shell structure precursor. Its core beneficial effect lies in that during the subsequent heat treatment when the carbon source precursor reacts with the silicon nuclei through carbothermal reduction to form silicon carbide, the active control elements (including any one or several of nitrogen, aluminum, and boron) released by the decomposition of the crystal form control agent precursor (including any one or several of nitrogen-containing precursors, aluminum-containing precursors, and boron-containing precursors) in-situ regulate the crystal structure of silicon carbide during the formation process of silicon carbide, thus effectively solving the key technical problem of precisely controlling the crystal form of silicon carbide. In addition, this reaction method starting from "silicon particles as silicon nuclei" and "coating a composite shell layer on the surface of the silicon nuclei" provides a way to prepare silicon carbide powder with more easily controllable particle size and morphology compared with the bulk material synthesis of the Acheson method and the "top-down" crushing, and creates more favorable conditions for improving the product purity due to the precise control at the microscale of the reactants and the ability to select high-purity "silicon particles".

[0025] Research has proven that active control elements (including any one or several of nitrogen, aluminum, and boron) can in-situ regulate the crystal structure of silicon carbide during the formation process of silicon carbide. When silicon carbide is formed by the carbothermal reduction reaction of the carbon source precursor and the silicon nuclei, especially during key crystallization and phase transition stages such as the transformation from β-silicon carbide to the target crystal form α-silicon carbide, through their in-situ presence and interaction with the forming SiC lattice, they can change the relative thermodynamic stability or growth kinetic conditions for the formation of different silicon carbide polytypes. This influence at the atomic or microscale makes the crystal growth and phase transition processes more conducive to evolving towards a specific target crystal form of silicon carbide guided by the active control elements, so that the method of the present invention can effectively control and obtain the target crystal form of silicon carbide.

[0026] The following further describes the present invention in conjunction with the drawings and specific embodiments. The additional aspects and advantages provided by the present invention will be partially given in the following description, partially become apparent from the following description, or be understood through practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings forming a part of this specification are used to assist in understanding the present invention. The content provided in the drawings and the related explanations in the present invention are used to explain the present invention, but do not constitute an improper limitation to the present invention.

[0028] Figure 1 Scanning electron microscope (SEM) photograph of the high-purity nano-silicon carbide product for Example 1.

[0029] Figure 2XRD pattern of the high-purity nanosized silicon carbide product of Example 1.

[0030] Figure 3 High-resolution XPS spectrum of the N 1s orbital of the high-purity nanosized silicon carbide product of Example 1.

[0031] Figure 4 XRD pattern of the high-purity nanosized silicon carbide product of Example 2.

[0032] Figure 5 XRD pattern of the high-purity nanosized silicon carbide product of Example 3.

[0033] Figure 6 XRD pattern of the high-purity nanosized silicon carbide product of the comparative example. Detailed implementation manners

[0034] The present invention will be clearly and completely described below with reference to the accompanying drawings. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. Before describing the present invention with reference to the accompanying drawings, it should be particularly noted that:

[0035] In each part including the following description, the technical solutions and technical features provided can be combined with each other without conflict. In addition, if possible, these technical solutions, technical features and related combinations can be given specific technical themes and be protected by relevant patents.

[0036] The embodiments of the present invention involved in the following description are usually only some embodiments rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on these embodiments should fall within the scope of patent protection.

[0037] The terms "comprising", "including" and any variations thereof in the present specification, the corresponding claims and the relevant parts are intended to cover non-exclusive inclusion. Other related terms and units can be reasonably interpreted based on the relevant content provided by the present invention.

[0038] The preparation method of the silicon carbide powder in the following embodiments of the present invention forms a core-shell structure precursor by coating a composite shell layer containing a carbon source precursor and a crystal form control agent precursor on the surface of a silicon core, and then heat-treats the core-shell structure precursor in a preset atmosphere. The active control elements (nitrogen, aluminum, boron) released by the decomposition of the crystal form control agent precursor are used for in-situ regulation during the formation of silicon carbide to obtain a target crystal form of silicon carbide (such as 4H-SiC or 6H-SiC). The following will be described in detail with specific experimental examples.

[0039] I. Raw Materials and Characterization Methods

[0040] 1.1 Silicon Core

[0041] In Examples 1 - 3, high-purity silicon powder (purity ≥ 99.999 wt%, D50 particle size is about 1 µm) was used.

[0042] In Example 4, high-purity nano-silicon powder (purity ≥ 99.999 wt%, D50 particle size is about 100 nm) was used.

[0043] 1.2 Carbon Source Precursor

[0044] Phenolic resin (solid, industrial grade).

[0045] 1.3 Crystal Form Control Agent Precursor

[0046] Nitrogen-containing precursor: Melamine (analytical pure).

[0047] Aluminum-containing precursor: Aluminum acetylacetonate (Al(acac)3, analytical pure).

[0048] Boron-containing precursor: Triisopropyl borate (analytical pure).

[0049] 1.4 Dispersant

[0050] Absolute ethanol (analytical pure).

[0051] 1.5 Atmosphere Gas

[0052] High-purity argon gas (Ar, ≥ 99.999 vol%).

[0053] 1.6 Characterization Methods

[0054] X-ray diffraction (XRD): Used to analyze the crystal structure and phase composition of the product, and determine the types and relative contents of crystal forms such as β-SiC, 4H-SiC, 6H-SiC, etc.

[0055] Scanning electron microscope (SEM): Used to observe the microscopic morphology, particle size, and the formation of core-shell structure of the powder.

[0056] Laser particle size analyzer: Used to measure the particle size distribution of the powder and determine parameters such as D10, D50, D90, etc. (based on volume distribution).

[0057] Inductively coupled plasma mass spectrometry (ICP-MS): Used to analyze the chemical purity of the powder and detect the content of impurity elements.

[0058] High-resolution XPS (HR-XPS): It is used to analyze the elemental composition, chemical state, and bonding environment of the product surface. In this study, its core role is to provide direct evidence of the chemical bond changes of crystal form control elements (such as nitrogen) before and after the reaction (such as the transformation from C-N bond to Si-N bond), so as to verify its in-situ regulation mechanism.

[0059] Inert Gas Fusion (IGF): It is used to quantitatively analyze the final bulk residual amount of the crystal form control element nitrogen (N) in silicon carbide products.

[0060] II. Examples

[0061] Example 1

[0062] Prepare high-purity nano-silicon carbide products with α-silicon carbide of 4H-SiC as the target crystal form. The steps include:

[0063] (I) Provide silicon particles as silicon nuclei.

[0064] Take 100 g of high-purity silicon powder (purity ≥ 99.999 wt%, D50 particle size is about 10 µm).

[0065] (II) Coating a composite shell layer on the surface of the silicon nuclei to form a core-shell structure precursor.

[0066] This step specifically includes:

[0067] Weigh phenolic resin, and its amount ensures that the molar ratio of carbon element provided during subsequent multi-stage high-temperature treatment to silicon element in the silicon nuclei is 1.03:1.

[0068] Weigh melamine as a nitrogen-containing precursor (crystal form control agent precursor), and its addition amount, calculated by the mass of the active control element nitrogen (N) released after decomposition, is 0.0086% by mass relative to the theoretical mass of the finally produced high-purity nano-silicon carbide product.

[0069] Disperse phenolic resin and melamine in an appropriate amount of absolute ethanol to form a mixed precursor suspension.

[0070] Disperse the silicon powder in the above mixed precursor suspension, and form a homogeneous slurry by mechanical stirring.

[0071] Use the spray drying process to prepare the slurry into solid particles, and collect the dried solid particles.

[0072] Place the dried solid particles in a tube furnace, under the protection of nitrogen atmosphere, heat up to 600 °C at a heating rate of 5 °C / min, and keep warm for 2 hours for pre-carbonization treatment of the carbon source precursor to obtain a core-shell structure precursor.

[0073] In this embodiment, the spray drying process is a key step in preparing the high-quality core-shell structure precursor. Its main functions are as follows: First, by atomizing the slurry containing silicon cores, carbon source precursors, and crystal form control agent precursors into tiny droplets and completing drying instantaneously, it can ensure that the components of the composite shell layer (phenolic resin and melamine) are evenly deposited and coated on the surface of the silicon cores. Second, spray drying is conducive to forming solid particles with regular morphologies (usually spherical or near-spherical) and narrow particle size distributions, and it is an important means to control the uniformity of the target crystal form of α-silicon carbide structure.

[0074] In this embodiment, the main significance of the pre-carbonization treatment of the carbon source precursor lies in that the carbon source precursor (phenolic resin) is transformed into more structurally stable amorphous carbon through a controlled pyrolysis process. This process will release a large amount of gases formed by non-carbon elements (such as hydrogen and oxygen) in advance, avoiding the explosive airflow generated by the violent decomposition of these gases during subsequent heat treatment at higher temperatures, thereby effectively protecting the physical integrity of the composite shell layer, preventing the composite shell layer from cracking, and ensuring a stable and structurally controllable composite shell layer to provide a good carrier and environment for the crystal form control elements to play a more effective role at high temperatures subsequently.

[0075] (3) Heat-treat the core-shell structure precursor under a preset atmosphere.

[0076] Place the core-shell structure precursor obtained after pre-carbonization in a high-temperature reaction device for multi-stage high-temperature treatment. The multi-stage high-temperature treatment includes:

[0077] The first-stage treatment: Under the protection of a high-purity argon atmosphere, heat up to 900 °C at a heating rate of 8 °C / min and hold for 1.5 hours to deeply carbonize the carbon source precursor. Through this stage, the carbon source precursor is completely transformed into carbon with stable properties, providing pure reactants for subsequent reactions.

[0078] The second-stage treatment: Continue to heat up to 1700 °C at a heating rate of 8 °C / min under the protection of a high-purity argon atmosphere and hold for 3 hours to react the silicon cores with carbon to form β-silicon carbide.

[0079] The third-stage treatment: Continue to heat up to 2100 °C at a heating rate of 4 °C / min and hold for 2 hours under the condition that the vacuum degree is not higher than 1 Pa. Under the in-situ regulation of the active control element nitrogen, promote the transformation of β-silicon carbide into the target crystal form of α-silicon carbide.

[0080] After multi-stage high-temperature treatment, slowly cool to room temperature under the protection of an argon atmosphere to obtain a crude silicon carbide product rich in the target crystal form of α-silicon carbide.

[0081] The main thermal decomposition of melamine occurs between 350°C and 600°C and is completely decomposed at 900°C. The nitrogen therein is retained together with carbon atoms, forming a thermodynamically more stable carbon nitride polymer or embedding into the amorphous carbon formed by the decomposition of phenolic resin, forming stable C-N chemical bonds in the form of pyridine nitrogen and pyrrole nitrogen, etc., thereby "in-situ" fixing the nitrogen element in carbon in a solid phase form. When entering the target crystal form α-SiC conversion stage at 1700°C to 2100°C, some of the originally formed C-N bonds will break, releasing highly chemically active nitrogen atoms or nitrogen-containing free radicals. These active nitrogen species can guide the preferential conversion of β-SiC to the thermodynamically favorable 4H-SiC, achieving precise control of the crystal form of silicon carbide.

[0082] (IV) Post-treat the crude silicon carbide to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-SiC.

[0083] This step specifically includes:

[0084] Add the crude silicon carbide to a 10wt% hot hydrochloric acid solution (60°C), stir for 2 hours for pre-acid washing, then filter and wash with water until neutral. Pre-acid washing is mainly aimed at the soluble metal impurities and their oxides that may be introduced or remain during the high-temperature synthesis process. For example, metal impurities such as iron (Fe), nickel (Ni), and chromium (Cr) introduced by high-temperature reaction devices, etc., usually react with hydrochloric acid to form soluble chlorides and are removed.

[0085] Pass a mixed gas stream of steam and argon (steam volume fraction 20%) through the product after pre-acid washing at 700°C for 2 hours for gas-phase carbon removal. The purpose of gas-phase carbon removal is to remove excess carbon. In the carbothermal synthesis, to ensure complete reaction of silicon, the carbon feed amount is usually in excess. After the reaction, these unreacted carbons will wrap around the SiC particles in the form of amorphous carbon or graphite carbon or exist between the particles. The chemical principle of gas-phase carbon removal is: C + H2O(g) → CO(g) + H2(g); C + CO2(g) → 2CO(g).

[0086] Use a mixed acid solution of hydrofluoric acid (40wt%) and nitric acid (65wt%) (volume ratio HF:HNO3 = 1:1), and perform main acid washing and impurity removal on the product after gas-phase carbon removal at 50°C with the assistance of ultrasonic waves for 2 hours. Main acid washing can more thoroughly remove almost all impurities (mainly SiO2 and stubborn metal impurities) wrapped on the surface of SiC particles. The assistance of ultrasonic waves can enhance mass transfer and improve the acid washing efficiency.

[0087] The product after main pickling and impurity removal was repeatedly washed with high-purity water until neutral, and then dried at a low temperature in a vacuum drying oven at 80 °C for 12 hours to obtain a high-purity nanometer silicon carbide product with a purity ≥ 99.9 wt% and rich in the target crystal form of α-silicon carbide.

[0088] The obtained high-purity target crystal form of α-silicon carbide was pulverized and classified by a supersonic airflow pulverizer with an internal classification wheel to obtain a high-purity nanometer silicon carbide product with a D90 particle size ≤ 500 nm.

[0089] Figure 1 This is a scanning electron microscope (SEM) photograph of the high-purity nanometer silicon carbide product of Example 1. As Figure 1 shown, SEM observation shows that the powder particle morphology is relatively regular and the dispersibility is good.

[0090] Figure 2 This is an X-ray diffraction (XRD) pattern of the high-purity nanometer silicon carbide product of Example 1. As Figure 2 shown, the XRD analysis results show that the high-purity nanometer silicon carbide product is mainly of the 4H-SiC crystal form. Its relative content was quantitatively analyzed by the RIR method and reached about 90 wt%. The rest are a small amount of 6H-SiC and residual β-SiC.

[0091] To further verify the in-situ regulation mechanism of nitrogen element in Example 1, high-resolution XPS analysis was carried out on the high-purity nanometer silicon carbide product, and its N 1s spectrum is as Figure 3 shown. The overall signal intensity of the spectrum is relatively low, which is consistent with the low doping amount of nitrogen element. After deconvolution analysis of the spectrum, it is found that its dominant signal peak is located at about 397.6 eV, which belongs to the Si-N chemical bond formed in the SiC lattice. At the same time, the signal peaks of pyridine nitrogen (about 398.8 eV) and pyrrole nitrogen (about 400.6 eV), which are the main carriers of nitrogen element in the pre-carbonization stage, have been greatly attenuated into weak residual peaks. This chemical state transformation from C-N dominance to Si-N dominance confirms that after nitrogen element completes the guiding regulation of the transformation from β-SiC to 4H-SiC, it finally remains in the SiC lattice in the form of a small amount of solid solution.

[0092] To accurately determine the bulk residual amount of the active control element nitrogen (N) in high-purity nanosized silicon carbide products, the bulk chemical composition analysis of the high-purity nanosized silicon carbide products obtained in Example 1 was carried out by the Inert Gas Fusion (IGF) method. In this method, the sample is melted at high temperature in an inert atmosphere to completely release all forms of nitrogen element as nitrogen gas (N2), and then accurately measured by a high-sensitivity thermal conductivity detection cell. The analysis results reveal that the residual amount of nitrogen element in the high-purity nanosized silicon carbide products is 0.0086 wt%. This proves that most of the nitrogen element contained in melamine, which is the precursor of the crystal form control agent, has successfully escaped from the system after guiding the transformation of β-silicon carbide to the target crystal form α-silicon carbide, and only a trace amount is solid-soluted in the SiC lattice. This macroscopic quantitative result is consistent with Figure 3 shown and corroborates each other.

[0093] To further characterize the comprehensive performance of the final product, its particle size and purity were analyzed. The D90 particle size of the high-purity nanosized silicon carbide products in Example 1 was measured by a laser particle size analyzer to be ≤500 nm, showing excellent comminution and classification effects. In addition, the analysis by inductively coupled plasma mass spectrometry (ICP-MS) shows that its total metal impurity content is less than 10 ppm, meeting the high-purity standard.

[0094] Example 1 successfully prepared high-purity nanosized silicon carbide products with the target crystal form (4H-SiC) dominant and regular microtopography. Further performance characterization confirmed that the D90 particle size of this product is ≤500 nm and the total metal impurity content is less than 10 ppm, meeting the stringent requirements of high-end fields such as semiconductors for the particle size and purity of materials.

[0095] Example 2

[0096] Prepare high-purity nanosized silicon carbide products with 6H-SiC as the target crystal form of α-silicon carbide. Repeat steps (1), (2), (3), and (4) of Example 1. The differences are as follows:

[0097] In step (2), instead of using melamine, aluminum acetylacetonate is used as the aluminum-containing precursor of the crystal form control agent. The addition amount of aluminum acetylacetonate is calculated by the mass of the active control element aluminum (Al) released after decomposition, and the mass percentage is 0.067% relative to the theoretical mass of the finally produced high-purity nanosized silicon carbide products.

[0098] The thermal decomposition of aluminum acetylacetonate usually occurs at relatively low temperatures (about 200 - 400 °C). During the pre-carbonization in step (ii) and the initial heating stage in step (iii), aluminum acetylacetonate decomposes completely, its organic ligand (acetylacetone) undergoes pyrolysis, and the aluminum element is transformed into highly dispersed nano-aluminum oxide (Al2O3) particles, which are "in-situ" fixed and embedded in the amorphous carbon matrix transformed from phenolic resin. When entering the target crystal form α-silicon carbide transformation stage at 1700 °C to 2100 °C, these previously formed nano-Al2O3 particles in close contact with the carbon matrix will undergo a carbothermal reduction reaction Al2O3 + 3C → 2Al(g) + 3CO(g). This reaction releases gaseous aluminum atoms (Al) with high chemical activity. These gaseous aluminum atoms can easily diffuse and replace the silicon atoms in the SiC lattice, forming p-type doping. This doping behavior of aluminum atoms will significantly change the stacking fault energy of SiC and make the formation of the 6H-SiC polytype more favorable thermodynamically. Therefore, these in-situ released active aluminum atoms can guide the preferential transformation of β-SiC to the thermodynamically stable 6H-SiC crystal form, thus achieving precise control of the silicon carbide crystal form.

[0099] Figure 4 XRD pattern of the high-purity nano-silicon carbide product of Example 2. As Figure 4 shown, the XRD analysis results show that the high-purity nano-silicon carbide product is mainly in the 6H-SiC crystal form. Its relative content was quantitatively analyzed by the RIR method and reached about 85 wt%, and the rest was a small amount of 4H-SiC and residual β-SiC.

[0100] To accurately determine the final bulk residual amount of the active control element aluminum (Al) in the high-purity nano-silicon carbide product, chemical composition analysis was carried out on the high-purity nano-silicon carbide product obtained in Example 2. First, the microwave digestion technique was used to completely dissolve the sample in a mixed acid system of hydrofluoric acid and nitric acid; then, inductively coupled plasma mass spectrometry (ICP-MS) was used to accurately quantify the aluminum element in the solution. The analysis results revealed that the residual amount of aluminum element in the high-purity nano-silicon carbide product was 0.067%.

[0101] Example 3

[0102] Prepare a high-purity nano-silicon carbide product with 6H-SiC as the target crystal form α-silicon carbide. Repeat steps (i), (ii), (iii), and (iv) of Example 1. The differences are as follows:

[0103] In step (ii), instead of using melamine (Example 1) or aluminum acetylacetonate (Example 2) as the crystal form control agent precursor, triisopropyl borate is used as the boron-containing precursor. The addition amount of triisopropyl borate is calculated based on the mass of the active control element boron (B) released after its complete decomposition, and the mass percentage is 0.092% relative to the theoretical mass of the finally produced high-purity nano silicon carbide product.

[0104] Triisopropyl borate is a liquid organic boron compound that is prone to decomposition during heating. In the pre-carbonization step (ii) and the initial heating stage of step (iii), triisopropyl borate undergoes hydrolysis and thermal decomposition, transforming into highly dispersed nano boron oxide (B2O3) particles and being uniformly embedded in the carbon matrix transformed from phenolic resin. When the reaction enters the target crystal form transformation stage of 1700 °C to 2100 °C, these in-situ generated nano B2O3 particles undergo a violent carbothermal reduction reaction with the surrounding carbon matrix: B2O3 + 3C → 2B + 3CO(g). This reaction in-situ releases highly chemically active element boron (B). Similar to aluminum atoms, element boron, as a typical p-type dopant, can easily diffuse and replace the silicon atom sites in the SiC lattice. This doping behavior will also significantly change the stacking fault energy of SiC, making the formation of the 6H-SiC polytype more thermodynamically favorable than other polytypes. Therefore, these in-situ released active boron atoms can effectively guide the preferential transformation of β-SiC to the thermodynamically most stable 6H-SiC crystal form, thereby achieving precise control of the silicon carbide crystal form.

[0105] Figure 5 XRD pattern of the high-purity nano silicon carbide product of Example 3. As Figure 5 shown, the XRD analysis results show that the high-purity nano silicon carbide product obtained by boron regulation is mainly of the 6H-SiC crystal form. Its relative content was quantitatively analyzed by the RIR method and reached about 90 wt%, and the rest is mainly residual β-SiC, proving that boron element also has excellent crystal form control ability.

[0106] To accurately determine the final bulk residual amount of the active control element boron (B) in the high-purity nano silicon carbide product, microwave digestion-inductively coupled plasma mass spectrometry (ICP-MS) was used to analyze the chemical composition of the product obtained in Example 3. The analysis results reveal that the residual amount of boron element in the high-purity nano silicon carbide product is 0.092%.

[0107] Comparative Example

[0108] Repeat steps (i), (ii), (iii), and (iv) of Example 1. The only difference is that: in step (ii), no crystal form control agent precursor is added.

[0109] Figure 6 XRD pattern of the high-purity nanosized silicon carbide product of the comparative example. As Figure 6 shown, the XRD analysis results show that without the guidance of a crystal form control agent, the phase composition of the product is very mixed. By quantitative analysis using the RIR method, its main phase composition is as follows: the content of residual β-SiC is as high as 40 wt%, and the rest is a mixture of various α-SiC polytypes, among which 6H-SiC accounts for about 30 wt%, 4H-SiC accounts for about 20 wt%, and 15R-SiC accounts for about 10 wt%.

[0110] It can be seen that nitrogen (N), aluminum (Al), and boron (B) are suitable doping elements for silicon carbide. They can directly and effectively change the stacking fault energy during the SiC crystal growth process at the atomic level through in-situ doping, thereby thermodynamically guiding the preferential transformation of β-SiC into a specific α-SiC polytype (such as 4H-SiC or 6H-SiC). Specifically, the atomic sizes and electronic structures of these three elements enable them to easily diffuse and replace the silicon (Si) or carbon (C) atomic sites in the SiC lattice at high temperatures: as shown in Example 1, nitrogen element as an n-type dopant can effectively reduce the formation energy of 4H-SiC, thereby obtaining a product mainly in the 4H crystal form; while as shown in Example 2 and Example 3, aluminum and boron elements as p-type dopants make the thermodynamically more stable 6H-SiC crystal form easier to form. Therefore, by selectively introducing these different active control elements, the present invention can achieve the synthesis of the target crystal form of silicon carbide.

[0111] In the present invention, nitrogen (N), aluminum (Al), and boron (B) are ingeniously introduced in the form of a composite shell. The carbon source precursor and the crystal form control agent precursor are uniformly coated on the surface of the silicon core, ensuring that silicon, carbon, and the active control elements (nitrogen, aluminum, boron) are in close contact and uniformly distributed at the reaction interface. Therefore, under the condition of trace doping, precise induction and locking of the final crystal form can be achieved. Although the content of the target crystal form α-silicon carbide in the examples does not reach close to 100%, and the high-purity nanosized silicon carbide product contains doping elements, however, from the perspective of the engineering application of semiconductor materials, this is precisely the core advantage of the technology of the present invention to solve practical problems and have certain industrialization value.

[0112] Specifically, through in-situ regulation, the content of the target crystal form dominates absolutely in the present invention. In materials science, this means that the dominant crystal form has formed a continuous matrix phase, and its macroscopic physical properties (such as bandgap width, thermal conductivity) are completely determined by this matrix phase. Compared with the products with uncontrollable crystal forms and random properties in the prior art, the present invention provides an engineering material basis with highly consistent and stable performance.

[0113] The remaining active control elements are not "impurities" or "pollutants" in the traditional sense. In the manufacture of semiconductor devices, introducing controllable background doping into materials is a crucial and necessary process. In the raw material synthesis stage of the present invention, atomic-level uniform doping is achieved synchronously, providing an ideal platform with highly uniform electrical properties for downstream processes such as epitaxial growth, avoiding the device failure problems that may be caused by subsequent non-uniform doping from the source, and greatly improving the yield and reliability of the final product.

[0114] The silicon carbide prepared by the present invention can be used as an ideal raw material for growing large-size and high-quality silicon carbide single crystals. This "pre-fabricated" advanced raw material can significantly improve the stability and yield of the crystal growth process, ensure that the finally cut wafer substrate has highly consistent electrical properties and lower defect density, thus providing a high-quality material basis for manufacturing high-performance power devices (such as MOSFETs) used in cutting-edge fields such as electric vehicles, photovoltaic inverters, 5G base stations, and data centers.

[0115] The above has described the relevant content of the present invention. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. Based on the above content of this specification, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of the present invention.

Claims

1. A method for preparing silicon carbide powder, characterized in that: Comprising: Providing silicon particles as the silicon core; Coating a composite shell layer on the surface of the silicon core to form a core-shell structure precursor, the composite shell layer comprising a carbon source precursor and a crystal form control agent precursor, the crystal form control agent precursor comprising any one or more of a nitrogen-containing precursor, an aluminum-containing precursor, and a boron-containing precursor; Performing heat treatment on the core-shell structure precursor under a preset atmosphere, such that the carbon source precursor undergoes a carbothermal reduction reaction with the silicon core to form silicon carbide. Meanwhile, active control elements comprising any one or more of nitrogen, aluminum, and boron released by the decomposition of the crystal form control agent precursor in-situ regulate the crystal structure of silicon carbide during the formation of silicon carbide, so as to obtain a silicon carbide product containing silicon carbide with a target crystal form.

2. The preparation method of silicon carbide powder according to claim 1, characterized in that: The silicon particles are nano-scale particles or micro-scale particles; and / or, the silicon particles form silicon powder with a purity ≥ 99.999 wt%.

3. The preparation method of silicon carbide powder according to claim 1, characterized in that: The carbon source precursor comprises any one or more of phenolic resin, polyacrylonitrile, and polyvinyl alcohol; and / or, the nitrogen-containing precursor comprises any one or more of melamine, polyacrylonitrile, and polyethyleneimine; and / or, the aluminum-containing precursor comprises any one or more of aluminum acetylacetonate, aluminum sec-butoxide, and aluminum isopropoxide; and / or, the boron-containing precursor comprises any one or more of triisopropyl borate, ammonia borane, and phenylboronic acid.

4. The preparation method of silicon carbide powder according to claim 1, characterized in that: The addition amount of the crystal form control agent precursor in the composite shell layer, calculated by the mass of the active control elements released after the decomposition of the crystal form control agent precursor, is 0.001% - 0.1% by mass relative to the theoretical mass of the finally produced silicon carbide product.

5. The preparation method of silicon carbide powder according to claim 1, characterized in that: The addition amount of the carbon source precursor in the composite shell layer can ensure that the molar ratio of the carbon element provided by the carbon source precursor during the heat treatment to the silicon element in the silicon core is 1.02:1 to 1.05:

1.

6. The preparation method of silicon carbide powder according to claim 1, characterized in that: The specific method for coating a composite shell layer on the surface of the silicon core to form a core-shell structure precursor comprises: Dissolving or uniformly dispersing the carbon source precursor and the crystal form control agent precursor in the same liquid dispersant to form a mixed precursor solution or a mixed precursor suspension; Attaching the mixed precursor solution or the mixed precursor suspension to the surface of the silicon core, and then performing a drying treatment; Performing pre-carbonization treatment on the solid particles obtained after the drying treatment with the carbon source precursor to obtain the core-shell structure precursor, the pre-carbonization treatment of the carbon source precursor being performed under an inert atmosphere, with the temperature controlled at 200°C - 800°C, the heating rate being 1°C / min - 10°C / min, and the heat preservation time being 0.5 hour - 4 hours.

7. The preparation method of silicon carbide powder according to claim 6, characterized in that: The specific method for attaching the mixed precursor solution or the mixed precursor suspension to the surface of the silicon core and then performing a drying treatment comprises: dispersing the silicon core in the mixed precursor solution or the mixed precursor suspension to form a slurry, and then preparing the slurry into the solid particles by a spray drying process.

8. The preparation method of silicon carbide powder according to claim 1, characterized in that: The specific method for heat-treating the core-shell structure precursor in a preset atmosphere includes performing multi-stage high-temperature treatment on the core-shell structure precursor in a high-temperature reaction device; the multi-stage high-temperature treatment includes: The first-stage treatment: Under the protection of an inert atmosphere, heating at a heating rate of 5°C / min - 10°C / min to 800°C - 1000°C and holding for 1 hour - 2 hours to deeply carbonize the carbon source precursor; The second-stage treatment: Continuing under the protection of an inert atmosphere, heating at a heating rate of 5°C / min - 10°C / min to 1600°C - 1800°C and holding for 2 hours - 4 hours to react silicon core with carbon to form β-silicon carbide; The third-stage treatment: Continuing to heat at a heating rate of 3°C / min - 5°C / min to 2000°C - 2200°C and holding for 1 hour - 3 hours under the condition that the vacuum degree is not higher than 1 Pa. Under the in-situ regulation of the active control element, promoting the transformation of β-silicon carbide to the target crystal form α-silicon carbide; After the multi-stage high-temperature treatment, slowly cooling to room temperature under the protection of an inert atmosphere to obtain a crude silicon carbide product rich in the target crystal form α-silicon carbide.

9. The method for preparing silicon carbide powder according to claim 8, characterized in that: Performing post-treatment on the crude silicon carbide product to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-silicon carbide; the post-treatment includes: Using a hot hydrochloric acid solution to perform pre-acid washing on the crude silicon carbide product to remove easily soluble metal impurities; For the pre-acid-washed crude silicon carbide product, under the condition of 650°C - 800°C, introducing a mixed gas stream of water vapor and inert gas or a mixed gas stream of carbon dioxide and inert gas to perform gas-phase carbon removal; Using a mixed acid solution of hydrofluoric acid and nitric acid, under the condition of room temperature to 60°C and assisted by ultrasonic waves, performing main acid washing on the crude silicon carbide product after gas-phase carbon removal to remove impurities; Washing the crude silicon carbide product after main acid washing with high-purity water until neutral and drying at low temperature to obtain a high-purity nano-silicon carbide product with a purity ≥ 99.9 wt% and rich in the target crystal form α-silicon carbide; Crushing and classifying the high-purity nano-silicon carbide product through a supersonic airflow mill with an internal classification wheel to obtain a high-purity nano-silicon carbide product with a D90 particle size ≤ 500 nm.

10. The preparation method of silicon carbide powder according to claim 1, characterized in that: The target crystal form of silicon carbide is 4H-SiC or 6H-SiC.

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