Preparation method of silicon carbide powder

Abstract

The present invention relates to the technical field of silicon carbide powder preparation, and discloses a method for preparing silicon carbide powder, which solves the technical problem of effectively controlling the crystal form of silicon carbide. The method 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, wherein 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; and heat-treating the core-shell structure precursor under a preset atmosphere so that the carbon source precursor and the silicon core undergo a carbothermal reduction reaction to form silicon carbide. At the same time, the crystal form control agent precursor decomposes and releases active control elements comprising any one or more of nitrogen, aluminum, and boron, which in situ regulate the silicon carbide crystal structure during the silicon carbide formation process to obtain a silicon carbide product containing the target crystal form of silicon carbide. The method can effectively control and obtain the target crystal form of silicon carbide.

Description

Technical Field

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

[0002] As a key industrial raw material and advanced functional material, silicon carbide (SiC) powder plays an indispensable role in many fields such as abrasives, refractories, structural ceramics, and semiconductors, especially 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, outstanding oxidation resistance and high-temperature strength, as well as excellent chemical stability and wear resistance.

[0003] Currently, the primary method for large-scale industrial production of commercial-grade silicon carbide powder remains the Acheson process, developed in the late 19th century (see U.S. Patent No. 492,767, granted in 1893). This process typically uses quartz sand (SiO2) and petroleum coke (or other carbonaceous materials) as the main raw materials. After mixing, the materials are placed in a large resistance furnace. Electricity is applied to a graphite resistor core, generating high temperatures (core temperatures exceeding 2500°C). Silicon carbide is produced through a carbothermal reduction reaction (SiO2 + 3C → SiC + 2CO). After the reaction, the charge undergoes cooling, crushing, sorting, grinding, and purification to produce the silicon carbide product.

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

[0005] 1) Low product purity. The raw materials used in the Acheson process (such as quartz sand and petroleum coke) typically contain a certain amount of metallic impurities (such as Fe, Al, and Ca) and non-metallic impurities. Although additives such as salt may be added to the process to promote the volatilization and removal of some impurities, and subsequent purification steps such as acid washing are performed, the extremely high reaction temperatures, the complex furnace atmosphere, and the limited purity of the raw materials themselves make it easy for impurity elements to diffuse and dissolve in the SiC lattice or form independent impurity phases. This makes it difficult to cost-effectively achieve the high purity required for electronic-grade applications (for example, a total impurity content of less than 10 ppm, or a purity of ≥99.999 wt%) in the final product. The presence of impurities can severely degrade the electrical properties (such as carrier lifetime and mobility), optical properties, and thermal properties of silicon carbide materials.

[0006] 2) Coarse particle size, irregular morphology, and broad distribution. The Acheson process is essentially a bulk material synthesis process, and its direct product is large aggregates of sintered or bonded SiC crystals. These crystals require multiple mechanical crushing steps and lengthy grinding processes such as ball milling to obtain a powder of the desired particle size. This process is not only energy-intensive and inefficient, but also highly susceptible to the introduction of new contaminants from the grinding media and the environment (such as wear from grinding balls). More importantly, this "top-down" crushing method makes it difficult to obtain micron- or even nanometer-scale SiC powders with a narrow particle size distribution and regular morphology (such as spherical shape or exposed specific crystal faces). Coarse, irregular particles with a broad particle size distribution are detrimental to densification and microstructural control during subsequent ceramic sintering, and are also unsuitable for applications requiring fine powders (such as catalyst supports and high-performance composite fillers).

[0007] 3) Crystalline polymorphs are complex and difficult to control. This is one of the most prominent bottlenecks of the Acheson method in meeting specific application requirements. Silicon carbide exhibits significant polytypism, with multiple crystal structures, primarily cubic β-SiC (the most common being 3C-SiC) and various hexagonal or rhombohedral α-SiC structures (such as 2H-SiC, 4H-SiC, 6H-SiC, and 15R-SiC). Silicon carbide of different polymorphs exhibits significant differences in key physical properties such as bandgap, carrier mobility, thermal conductivity, and fracture toughness. For example, 4H-SiC, due to its wider bandgap, higher electron mobility, and breakdown field strength, has become the preferred polymorph for the manufacture of high-performance, high-voltage, and high-temperature power electronic devices (such as MOSFETs, IGBTs, and Schottky diodes). 6H-SiC has also been widely used in LED substrates and certain power devices. However, the large, difficult-to-precisely control temperature gradients and complex local atmosphere within the Acheson furnace result in the resulting silicon carbide product typically being a physical mixture of various α-SiC polytypes (primarily 6H-SiC and 4H-SiC, as well as other α phases), often accompanied by β-SiC formed in lower temperature zones or areas of incomplete reaction. Operators are unable to actively select or precisely control the production of a single, pure, specific SiC crystal form by adjusting macro-process parameters. This uncontrollable and heterogeneous nature of the crystal form severely limits the direct application of Acheson-processed SiC in applications with stringent requirements for the material's crystal form, particularly semiconductor device manufacturing, requiring 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-mentioned method for preparing silicon carbide powder, further: the aluminum-containing precursor includes any one or more of aluminum acetylacetonate, aluminum triisopropoxide, aluminum nitrate, and a soluble organic aluminum compound.

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

[0017] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the amount of the crystal form control agent precursor added to the composite shell layer, measured as the mass of the active control elements released after decomposition of the crystal form control agent precursor, relative to the theoretical mass of the final silicon carbide product, is 0.001%-0.1% by mass.

[0018] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the amount of the carbon source precursor added to the composite shell layer can ensure that the molar ratio of the carbon element that the carbon source precursor can provide 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-mentioned method for preparing silicon carbide powder, further: the specific method of coating the surface of the silicon core with a composite shell layer to form a 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 drying it; performing a carbon source precursor pre-carbonization treatment on the solid particulate matter obtained after the drying treatment to obtain the core-shell structure precursor, and the carbon source precursor pre-carbonization treatment is carried out under an inert atmosphere, the temperature is controlled at 200℃-800℃, the heating rate is 1℃ / min-10℃ / min, and the holding time is 0.5 hours-4 hours.

[0020] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the specific method of 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 particulate matter through a spray drying process.

[0021] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the specific method of heat-treating the core-shell structure precursor under a preset atmosphere includes subjecting the core-shell structure precursor to a multi-stage high-temperature treatment in a high-temperature reaction device; the multi-stage high-temperature treatment includes: a first-stage treatment: under the protection of an inert atmosphere, heating the temperature to 800°C-1000°C at a heating rate of 5°C / min-10°C / min, and keeping the temperature for 1 hour-2 hours to deeply carbonize the carbon source precursor; a second-stage treatment: continuing to heat the temperature at a heating rate of 5°C / min-10°C / min under the protection of an inert atmosphere, and keeping the temperature for 1 hour-2 hours to deeply carbonize the carbon source precursor; min to 1600°C-1800°C and keep warm for 2-4 hours to allow the silicon nuclei to react with carbon to generate β-silicon carbide; the third stage treatment: continue to heat up to 2000°C-2200°C at a heating rate of 3°C / min-5°C / min, and keep warm for 1-3 hours under the condition of a vacuum degree not higher than 1 Pa, and promote the conversion of β-silicon carbide to the target crystalline form α-silicon carbide under the in-situ regulation of the active control element; after the multi-stage high-temperature treatment, slowly cool to room temperature under the protection of an inert atmosphere to obtain a crude silicon carbide rich in the target crystalline form α-silicon carbide.

[0022] As an optimization and / or instantiation of the above-mentioned method for preparing silicon carbide powder, further: the crude silicon carbide product is post-treated to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-silicon carbide; the post-treatment comprises: pre-pickling the crude silicon carbide product with a hot hydrochloric acid solution to remove soluble metal impurities; the crude silicon carbide product after pre-pickling is passed through a mixed gas flow of water vapor and inert gas or a mixed gas flow of carbon dioxide and inert gas at 650°C-800°C for gas phase decarbonization; using hydrofluoric acid and nitric acid to remove the carbon dioxide; The method comprises the following steps: using a mixed acid solution of 2,4-dimethyl-1,2-dioxane-2-propene, 2,4-dimethyl-1,2-dioxane-3 ...

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

[0024] The method for preparing silicon carbide powder of the present invention provides silicon particles as silicon cores and coats the surface of the silicon cores with a composite shell layer comprising a carbon source precursor and a crystal form control agent precursor to form a core-shell structure precursor. The core beneficial effect is that in the subsequent heat treatment process in which the carbon source precursor and the silicon cores undergo a carbothermal reduction reaction to form silicon carbide, the active control elements (including any one or more of nitrogen-containing precursors, aluminum-containing precursors, and boron-containing precursors) are decomposed and released, which in situ regulate the silicon carbide crystal structure during the silicon carbide formation process, thereby effectively solving the key technical problem of precisely controlling the silicon carbide crystal form. In addition, this reaction method, which starts from "silicon particles as silicon cores" and "coats a composite shell on the surface of the silicon cores", provides a way to prepare silicon carbide powder with easier-to-control particle size and morphology compared to the Acheson method of bulk material synthesis and "top-down" crushing. It also creates more favorable conditions for improving product purity due to the precise control of reactants at the microscale and the ability to select high-purity "silicon particles".

[0025] Research has demonstrated that active control elements (including any one or more of nitrogen, aluminum, and boron) can in situ modulate the silicon carbide crystal structure during its formation. During the carbothermal reduction reaction between the carbon source precursor and the silicon nucleus, particularly during critical crystallization and phase transitions such as the transformation from β-SiC to the target α-SiC form, the active control elements' in-situ presence and interaction with the developing SiC lattice can alter the relative thermodynamic stabilities or growth kinetics of different SiC polytypes. This atomic or microscopic influence favors the crystal growth and phase transition processes toward the specific target SiC form guided by the active control elements, enabling the method of the present invention to effectively control and obtain the target SiC form.

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. 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 learned through practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings constituting a part of this specification are used to assist in understanding the present invention. The contents provided in the drawings and their related descriptions in the present invention are used to explain the present invention, but do not constitute improper limitations on the present invention.

[0028] Figure 1 This is a scanning electron microscope (SEM) photograph of the high-purity nano-silicon carbide product of Example 1.

[0029] Figure 2This is the X-ray diffraction (XRD) pattern of the high-purity nano-silicon carbide product of Example 1.

[0030] Figure 3 This is a high-resolution N 1s orbital XPS spectrum of the high-purity nano-silicon carbide product in Example 1.

[0031] Figure 4 This is the X-ray diffraction (XRD) pattern of the high-purity nano-silicon carbide product of Example 2.

[0032] Figure 5 This is the X-ray diffraction (XRD) pattern of the high-purity nano-silicon carbide product of Example 3.

[0033] Figure 6 The figure is the X-ray diffraction (XRD) pattern of the high-purity nano-silicon carbide product of the comparative example. DETAILED DESCRIPTION

[0034] The present invention is described clearly and completely below with reference to the accompanying drawings. A person skilled 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 noted that:

[0035] The technical solutions and technical features provided in each section, including the following description, may be combined with each other unless they conflict. In addition, where possible, these technical solutions, technical features, and related combinations may be assigned specific technical themes and protected by relevant patents.

[0036] The embodiments of the present invention involved in the following description are generally only a part of the embodiments rather than all the embodiments. Based on these embodiments, all other embodiments obtained by ordinary technicians in this field without making any creative work should fall within the scope of patent protection.

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

[0038] The following examples of the present invention provide methods for preparing silicon carbide powder. A composite shell layer comprising a carbon source precursor and a crystal form control agent precursor is coated on the surface of a silicon core to form a core-shell structure precursor. The core-shell structure precursor is then heat-treated under a predetermined atmosphere. Active control elements (nitrogen, aluminum, and boron) released by the decomposition of the crystal form control agent precursor are then used to in-situ regulate the silicon carbide formation process, thereby obtaining the target crystal form of silicon carbide (e.g., 4H-SiC or 6H-SiC). This is described in detail below with reference to specific experimental examples.

[0039] 1. Raw materials and characterization methods

[0040] 1.1 Silicon Core

[0041] Examples 1 to 3 used high-purity silicon powder (purity ≥ 99.999 wt %, D50 particle size of approximately 1 µm).

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

[0043] 1.2 Carbon source precursor

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

[0045] 1.3 Crystal form control agent precursor

[0046] Nitrogen-containing precursor: melamine (analytical grade).

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

[0048] Boron-containing precursor: triisopropyl borate (analytical grade).

[0049] 1.4 Dispersants

[0050] Anhydrous ethanol (analytical grade).

[0051] 1.5 Atmosphere gas

[0052] High-purity argon (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 β-SiC, 4H-SiC, 6H-SiC and other crystal forms.

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

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

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

[0058] High-resolution XPS (HR-XPS): Used to analyze the elemental composition, chemical state, and bonding environment of the product surface. In this study, its key role was to provide direct evidence of chemical bond changes (e.g., from C-N bonds to Si-N bonds) before and after the reaction of crystal-forming control elements (e.g., nitrogen) to verify their in-situ regulation mechanism.

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

[0060] 2. Example

[0061] Example 1

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

[0063] (1) Providing silicon particles as silicon cores.

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

[0065] (2) coating the surface of the silicon core with a composite shell layer to form a core-shell structure precursor.

[0066] This step specifically includes:

[0067] The phenolic resin was weighed in an amount to ensure that the molar ratio of carbon element to silicon element in the silicon core provided during the subsequent multi-stage high-temperature treatment was 1.03:1.

[0068] Melamine was weighed as a nitrogen-containing precursor (crystal form control agent precursor). The amount of melamine added was calculated as the mass of the active control element nitrogen (N) released after decomposition, with a mass percentage of 0.0086% relative to the theoretical mass of the final high-purity nano-silicon carbide product.

[0069] Phenolic resin and melamine are dispersed in an appropriate amount of anhydrous ethanol to form a mixed precursor suspension.

[0070] The silicon powder is dispersed in the above mixed precursor suspension and formed into a uniform slurry by mechanical stirring.

[0071] The slurry is prepared into solid particles by a spray drying process, and the dried solid particles are collected.

[0072] The dried solid particles were placed in a tube furnace, and under the protection of a nitrogen atmosphere, the temperature was increased to 600°C at a heating rate of 5°C / min and kept at this temperature for 2 hours to perform a pre-carbonization treatment of the carbon source precursor to obtain a core-shell structure precursor.

[0073] In this example, the spray drying process is a key step in preparing a high-quality core-shell structure precursor. Its main functions are: First, by atomizing a slurry containing silicon cores, a carbon source precursor, and a crystal form control agent precursor into tiny droplets and drying them instantly, it ensures that the components of the composite shell (phenolic resin and melamine) are uniformly deposited and coated on the surface of the silicon cores. Second, spray drying facilitates the formation of solid particles with a regular morphology (typically spherical or near-spherical) and a narrow particle size distribution, making it an important means of controlling the structural uniformity of the target crystalline α-silicon carbide.

[0074] In this embodiment, the primary significance of the carbon source precursor pre-carbonization treatment lies in its conversion to amorphous carbon, a more structurally stable carbon, through a controlled pyrolysis process. This process prematurely releases a significant amount of gases formed by non-carbon elements (such as hydrogen and oxygen), preventing the explosive decomposition of these gases during subsequent, higher-temperature heat treatment. This effectively protects the physical integrity of the composite shell, preventing its rupture and ensuring a stable and structurally controllable composite shell that provides an excellent carrier and environment for the crystal-controlling elements to function more effectively at subsequent high temperatures.

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

[0076] The core-shell structure precursor obtained after pre-carbonization is placed in a high-temperature reaction device and subjected to a multi-stage high-temperature treatment. The multi-stage high-temperature treatment includes:

[0077] The first stage of treatment: Under a high-purity argon atmosphere, the temperature is raised to 900°C at a rate of 8°C / min and held for 1.5 hours to deeply carbonize the carbon source precursor. Through this stage, the carbon source precursor is completely converted into stable carbon, providing a pure reactant for subsequent reactions.

[0078] The second stage of treatment: continue to heat the sample to 1700°C at a heating rate of 8°C / min under the protection of high-purity argon atmosphere and keep it at that temperature for 3 hours to make the silicon nuclei react with carbon to form β-silicon carbide.

[0079] The third stage of treatment: continue to heat up to 2100℃ at a heating rate of 4℃ / min and keep it warm for 2 hours under the condition of vacuum not higher than 1Pa. Under the in-situ regulation of active control element nitrogen, β-silicon carbide is promoted to transform into the target crystal form α-silicon carbide.

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

[0081] The primary thermal decomposition of melamine occurs between 350°C and 600°C, and by 900°C it is completely decomposed. The nitrogen is retained along with the carbon atoms, forming thermodynamically more stable carbon nitride polymers or embedding them into the amorphous carbon formed by the decomposition of phenolic resin. This forms stable CN chemical bonds in the form of pyridinic and pyrrolic nitrogen, thereby fixing the nitrogen element "in situ" in the carbon in a solid phase. When entering the target crystalline α-silicon carbide conversion stage at 1700°C to 2100°C, some of the previously formed CN bonds break, releasing highly chemically active nitrogen atoms or nitrogen-containing radicals. These active nitrogen species can guide the preferential conversion of β-SiC to the thermodynamically favorable 4H-SiC, enabling precise control of the silicon carbide crystal form.

[0082] (4) Post-processing the crude silicon carbide product to obtain a high-purity nano-silicon carbide product rich in target crystalline α-silicon carbide.

[0083] This step specifically includes:

[0084] The crude silicon carbide is added to a 10wt% hot hydrochloric acid solution (60°C) and stirred for 2 hours for pre-pickling. The mixture is then filtered and washed with water until neutral. Pre-pickling is primarily aimed at 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 can usually react with hydrochloric acid to form soluble chlorides and be removed.

[0085] The pre-acid-washed product was subjected to gas-phase decarbonization at 700°C for 2 hours while passing a mixture of water vapor and argon (water vapor volume fraction 20%). The purpose of gas-phase decarbonization is to remove excess carbon. In carbothermal synthesis, the carbon feed is typically in excess to ensure complete silicon reaction. After the reaction, this unreacted carbon forms as amorphous carbon or graphitic carbon, encapsulating the surface of the SiC particles or existing between the particles. The chemical principles of gas-phase decarbonization are: C + H2O(g) → CO(g) + H2(g); C + CO2(g) → 2CO(g).

[0086] The product after gas-phase decarbonization was subjected to a primary pickling process for 2 hours using a mixed solution of hydrofluoric acid (40wt%) and nitric acid (65wt%) (volume ratio HF:HNO3 = 1:1) at 50°C, supplemented by ultrasonic waves. This primary pickling process thoroughly removes almost all impurities (primarily SiO2 and stubborn metal impurities) encapsulating the SiC particle surface. The addition of ultrasonic waves enhances mass transfer and improves pickling efficiency.

[0087] The product after the main acid washing and impurity removal is repeatedly washed with high-purity water until it is neutral, and then dried in a vacuum drying oven at 80°C for 12 hours to obtain a high-purity nano-silicon carbide product rich in the target crystalline α-silicon carbide with a purity of ≥99.9wt%.

[0088] The obtained high-purity target crystalline α-silicon carbide is crushed and classified by a supersonic airflow mill with an internal classifying wheel to obtain a high-purity nano-silicon carbide product with a D90 particle size of ≤500nm.

[0089] Figure 1 The scanning electron microscope (SEM) photo of the high-purity nano-silicon carbide product of Example 1 is shown in FIG. Figure 1 As shown in the figure, SEM observation shows that the powder particles have regular morphology and good dispersion.

[0090] Figure 2 The X-ray diffraction (XRD) diagram of the high-purity nano-silicon carbide product of Example 1 is shown in FIG. Figure 2 As shown in the figure, XRD analysis results show that the high-purity nano-silicon carbide product is mainly 4H-SiC crystal type, and its relative content is about 90wt% through quantitative analysis by RIR method, and the rest is a small amount of 6H-SiC and residual β-SiC.

[0091] To further verify the in-situ regulation mechanism of nitrogen in Example 1, high-resolution XPS analysis was performed on the high-purity nano-silicon carbide product, and its N 1s spectrum was as follows: Figure 3 As shown. The overall signal intensity of the spectrum is low, which is consistent with the low doping level of nitrogen. After deconvolution analysis of the spectrum, it was found that its dominant signal peak is located at about 397.6 eV, which is attributed to the Si-N chemical bond formed in the SiC lattice. At the same time, the signal peaks of pyridinic nitrogen (about 398.8 eV) and pyrrolic nitrogen (about 400.6 eV), which serve as the main carriers of nitrogen in the pre-carbonization stage, have been greatly attenuated to weak residual peaks. This chemical state transition from CN-dominated to Si-N-dominated confirms that after nitrogen completes the guidance and regulation of the transformation of β-SiC to 4H-SiC, it eventually remains in the SiC lattice in the form of a small amount of solid solution.

[0092] In order to accurately determine the bulk residual amount of active control element nitrogen (N) in the high-purity nano-silicon carbide product, the bulk chemical composition analysis of the high-purity nano-silicon carbide product obtained in Example 1 was carried out by inert gas fusion (IGF). This method melts the sample at high temperature in an inert atmosphere to completely release all forms of nitrogen elements into nitrogen (N2), which is then accurately measured by a high-sensitivity thermal conductivity detection cell. The analysis results revealed that the residual nitrogen element in the high-purity nano-silicon carbide product was 0.0086wt%. This proves that most of the nitrogen elements contained in melamine, a precursor of the crystal form control agent, have successfully escaped from the system after completing the guidance of the transformation of β-silicon carbide to the target crystal form α-silicon carbide, and only a trace amount is dissolved in the SiC lattice. This macroscopic quantitative result is consistent with Figure 3 The results show that they confirm each other.

[0093] To further characterize the comprehensive performance of the final product, particle size and purity analysis were performed. Using a laser particle size analyzer, the D90 particle size of the high-purity nano-silicon carbide product from Example 1 was measured to be ≤500 nm, demonstrating excellent pulverization and classification. Furthermore, inductively coupled plasma mass spectrometry (ICP-MS) analysis revealed a total metal impurity content of less than 10 ppm, meeting high purity standards.

[0094] Example 1 successfully produced a high-purity nano-silicon carbide product with a predominantly targeted crystal form (4H-SiC) and a regular micromorphology. Further performance characterization confirmed that the product had a D90 particle size of ≤500nm and a total metal impurity content of less than 10ppm, meeting the stringent material particle size and purity requirements for high-end applications such as semiconductors.

[0095] Example 2

[0096] Prepare a high-purity nano-silicon carbide product 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:

[0097] In step (2), the crystal form control agent precursor does not use melamine, but instead uses aluminum acetylacetonate as the aluminum-containing precursor. The amount of aluminum acetylacetonate added, calculated as the mass of the active control element aluminum (Al) released after decomposition, is 0.067% by mass relative to the theoretical mass of the final high-purity nano-silicon carbide product.

[0098] Thermal decomposition of aluminum acetylacetonate typically occurs at relatively low temperatures (approximately 200-400°C). During the pre-carbonization step (2) and the initial heating phase (3), aluminum acetylacetonate completely decomposes, its organic ligand (acetylacetone) undergoes thermal decomposition, and the aluminum element is converted into highly dispersed nano-aluminum oxide (Al2O3) particles, which are "in situ" fixed and embedded in the amorphous carbon matrix derived from the phenolic resin. During the conversion phase to the target crystalline α-silicon carbide at temperatures between 1700°C and 2100°C, these previously formed nano-Al2O3 particles, which are in close contact with the carbon matrix, undergo a carbothermal reduction reaction: Al2O3 + 3C → 2Al(g) + 3CO(g). This reaction releases highly chemically active gaseous aluminum atoms (Al). These gaseous aluminum atoms readily diffuse and replace silicon atoms in the SiC lattice, forming p-type doping. This doping behavior of aluminum atoms significantly changes the stacking fault energy of SiC and thermodynamically favors the formation of the 6H-SiC polytype. Therefore, these in-situ released active aluminum atoms can guide the preferential transformation of β-SiC to the thermodynamically stable 6H-SiC polytype, thereby achieving precise control of the silicon carbide crystal form.

[0099] Figure 4 The X-ray diffraction (XRD) diagram of the high-purity nano-silicon carbide product of Example 2 is shown in FIG. Figure 4 As shown in the figure, XRD analysis results show that the high-purity nano-silicon carbide product is mainly 6H-SiC crystal type, and its relative content is about 85wt% by quantitative analysis by RIR method, and the rest is 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-SiC product, the chemical composition analysis of the high-purity nano-SiC product obtained in Example 2 was performed. First, microwave digestion technology was used to completely dissolve the sample in a mixed acid system of hydrofluoric acid and nitric acid. Subsequently, 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 aluminum element in the high-purity nano-SiC product was 0.067%.

[0101] Example 3

[0102] Prepare a high-purity nano-silicon carbide product 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:

[0103] In step (2), the crystal form control agent precursor does not use melamine (Example 1) or aluminum acetylacetonate (Example 2). Instead, triisopropyl borate is used as the boron-containing precursor. The amount of triisopropyl borate added, calculated as the mass of the active control element boron (B) released after complete decomposition, is 0.092% by mass relative to the theoretical mass of the final high-purity nano-silicon carbide product.

[0104] Triisopropyl borate is a liquid organoboron compound that readily decomposes upon heating. During the pre-carbonization phase (step (2)) and the initial heating phase (step (3)), triisopropyl borate undergoes hydrolysis and thermal decomposition, transforming into highly dispersed nano-sized boron oxide (B2O3) particles, which are uniformly embedded in the carbon matrix converted from the phenolic resin. When the reaction enters the target crystal transformation phase at temperatures between 1700°C and 2100°C, these in-situ generated nano-sized B2O3 particles undergo a vigorous carbothermal reduction reaction with the surrounding carbon matrix: B2O3 + 3C → 2B + 3CO(g). This reaction releases highly chemically active elemental boron (B) in situ. Similar to aluminum atoms, elemental boron, a typical p-type dopant, readily diffuses and replaces silicon atoms in the SiC lattice. This doping behavior also significantly alters the stacking fault energy of SiC, thermodynamically favoring the formation of the 6H-SiC polytype over other polytypes. Therefore, these in-situ released active boron atoms can effectively guide β-SiC to preferentially transform into the most thermodynamically stable 6H-SiC crystal form, thereby achieving precise control of the silicon carbide crystal form.

[0105] Figure 5 The X-ray diffraction (XRD) diagram of the high-purity nano-silicon carbide product of Example 3 is shown in FIG. Figure 5 As shown in the figure, the XRD analysis results show that the high-purity nano-silicon carbide product obtained by boron regulation is mainly 6H-SiC crystal type, and its relative content is quantitatively analyzed by RIR method, reaching about 90wt%, and the rest is mainly residual β-SiC, which proves that boron element also has excellent crystal regulation 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, the chemical composition of the product obtained in Example 3 was analyzed using microwave digestion-inductively coupled plasma mass spectrometry (ICP-MS). The analysis results revealed that the residual boron content in the high-purity nano-silicon carbide product was 0.092%.

[0107] Comparative Example

[0108] Repeat steps (1), (2), (3), and (4) of Example 1. The only difference is that in step (2), no crystal form control agent precursor is added.

[0109] Figure 6 The X-ray diffraction (XRD) diagram of the high-purity nano-silicon carbide product of the comparative example is shown in FIG. Figure 6 As shown, XRD analysis results show that without the guidance of a crystal-forming agent, the product has a very mixed phase composition. Quantitative analysis by RIR reveals that the main phases are: residual β-SiC content up to 40wt%, and the remainder is a mixture of various α-SiC polytypes, of which 6H-SiC accounts for approximately 30wt%, 4H-SiC accounts for approximately 20wt%, and 15R-SiC accounts for approximately 10wt%.

[0110] As can be seen, nitrogen (N), aluminum (Al), and boron (B) are suitable doping elements for silicon carbide. Through in-situ doping, they can directly and effectively alter the stacking fault energy (SFE) during SiC crystal growth at the atomic level, thereby thermodynamically directing the preferential transformation of β-SiC to a specific α-SiC polytype (such as 4H-SiC or 6H-SiC). Specifically, the atomic size and electronic structure of these three elements enable them to readily diffuse and replace silicon (Si) or carbon (C) atomic sites in the SiC lattice at high temperatures. As shown in Example 1, nitrogen, as an n-type dopant, effectively reduces the formation energy of 4H-SiC, thereby directional production of a product dominated by the 4H crystal form. Furthermore, as shown in Examples 2 and 3, aluminum and boron, as p-type dopants, facilitate the formation of the more thermodynamically stable 6H-SiC crystal form. Therefore, by selectively introducing these different active control elements, the present invention enables the synthesis of the target silicon carbide crystal form.

[0111] In the present invention, nitrogen (N), aluminum (Al) and boron (B) are cleverly introduced in the form of a composite shell. The carbon source precursor and the crystal form control agent precursor are evenly 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 evenly distributed at the interface where the reaction occurs. Therefore, with trace amounts of 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 embodiment does not reach a level close to 100%, and the high-purity nano-silicon carbide product contains doping elements, from the perspective of the engineering application of semiconductor materials, this is precisely the core advantage of the technology of the present invention in being able to solve practical problems and having certain industrial value.

[0112] Specifically, the present invention achieves absolute dominance of the target crystal form through in-situ control. In materials science, this means the dominant crystal form constitutes a continuous matrix phase, whose macroscopic physical properties (such as bandgap and thermal conductivity) are completely determined by this matrix phase. Compared to existing products with uncontrollable crystal forms and random properties, the present invention provides a foundation for engineered materials with highly consistent and predictable performance.

[0113] The residual active control elements are not "impurities" or "contaminants" in the traditional sense. In semiconductor device manufacturing, introducing controllable background doping into the material is a critical and necessary process. The present invention achieves uniform doping at the atomic level during the raw material synthesis stage, providing an ideal platform with highly uniform electrical properties for downstream processes such as epitaxial growth. This avoids the problem of device failure caused by subsequent uneven doping at the source, greatly improving the yield and reliability of the final product.

[0114] The silicon carbide prepared by this method can be used as an ideal raw material for growing large, high-quality silicon carbide single crystals. This "prefabricated" high-grade raw material significantly improves the stability and yield of the crystal growth process, ensuring that the final cut wafer substrate has highly consistent electrical properties and lower defect density. This provides a high-quality material foundation for the manufacture of high-efficiency 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 describes the relevant contents of the present invention. Based on this description, a person skilled in the art will be able to implement the present invention. Based on the above content of this specification, all other embodiments obtained by a person skilled in the art without making any creative efforts should fall within the scope of the present invention.

Claims

1. A method for preparing silicon carbide powder, characterized in that: include: providing silicon particles as silicon cores; A composite shell layer is coated on the surface of the silicon core 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; The core-shell structure precursor is heat-treated under a preset atmosphere, so that the carbon source precursor and the silicon core undergo a carbothermal reduction reaction to form silicon carbide. At the same time, the active control elements including any one or more of nitrogen, aluminum, and boron released by the decomposition of the crystal form control agent precursor in situ regulate the silicon carbide crystal structure during the silicon carbide formation process, so as to obtain a silicon carbide product containing the target crystal form of silicon carbide; The target crystalline silicon carbide is 4H-SiC or 6H-SiC; the nitrogen-containing precursor includes any one or more of melamine, polyacrylonitrile, and polyethyleneimine; the aluminum-containing precursor includes any one or more of aluminum acetylacetonate, aluminum sec-butoxide, and aluminum isopropoxide; and the boron-containing precursor includes any one or more of triisopropyl borate, ammonia borane, and phenylboric acid.

2. The method for preparing silicon carbide powder according to claim 1, wherein: The silicon particles are nano-sized particles or micron-sized particles; and / or, the silicon particles form silicon powder with a purity of ≥99.999 wt%.

3. The method for preparing silicon carbide powder according to claim 1, wherein: The carbon source precursor comprises any one or more of phenolic resin, polyacrylonitrile and polyvinyl alcohol.

4. The method for preparing silicon carbide powder according to claim 1, wherein: The amount of the crystal form control agent precursor added to the composite shell layer, calculated as 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 final silicon carbide product.

5. The method for preparing silicon carbide powder according to claim 1, wherein: The amount of the carbon source precursor added to 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 method for preparing silicon carbide powder according to claim 1, wherein: The specific method of coating the silicon core surface with a composite shell layer 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 process; The solid particles obtained after the drying treatment are subjected to carbon source precursor pre-carbonization treatment to obtain the core-shell structure precursor. The carbon source precursor pre-carbonization treatment is carried out under an inert atmosphere, the temperature is controlled at 200°C-800°C, the heating rate is 1°C / min-10°C / min, and the holding time is 0.5 hour-4 hours.

7. The method for preparing silicon carbide powder according to claim 6, wherein: The specific method of 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 through a spray drying process.

8. The method for preparing silicon carbide powder according to claim 1, wherein: The specific method of heat-treating the core-shell structure precursor under a preset atmosphere comprises subjecting the core-shell structure precursor to a multi-stage high-temperature treatment in a high-temperature reaction device; the multi-stage high-temperature treatment comprises: The first stage of treatment: under the protection of an inert atmosphere, heating the temperature to 800-1000°C at a heating rate of 5-10°C / min and keeping the temperature for 1-2 hours to deeply carbonize the carbon source precursor; Second stage treatment: Continue to heat the sample to 1600-1800°C at a heating rate of 5-10°C / min under inert atmosphere and keep it at that temperature for 2-4 hours to make the silicon nuclei react with carbon to form β-silicon carbide. The third stage of treatment: continuing to heat up to 2000-2200°C at a heating rate of 3°C / min-5°C / min, and maintaining the temperature for 1-3 hours under a vacuum degree not higher than 1 Pa, thereby promoting the conversion of β-silicon carbide to the target crystalline form α-silicon carbide under the in-situ regulation of the active control element; After the multi-stage high-temperature treatment, the mixture is slowly cooled to room temperature under the protection of an inert atmosphere to obtain a crude silicon carbide product rich in the target crystalline form α-silicon carbide.

9. The method for preparing silicon carbide powder according to claim 8, wherein: The crude silicon carbide product is post-processed to obtain a high-purity nano-silicon carbide product rich in target crystalline α-silicon carbide; the post-processing comprises: Pre-pickling the crude silicon carbide product with a hot hydrochloric acid solution to remove soluble metal impurities; The crude silicon carbide after pre-acid washing is subjected to gas phase decarbonization by passing a mixed flow of water vapor and inert gas or a mixed flow of carbon dioxide and inert gas at 650-800°C; The crude silicon carbide product after gas phase decarbonization is subjected to primary acid washing and impurity removal using a mixed acid solution of hydrofluoric acid and nitric acid at room temperature to 60°C and with the aid of ultrasonic waves; The crude silicon carbide product after the main acid washing and impurity removal is washed with high-purity water until it is neutral and then dried at low temperature to obtain a high-purity nano-silicon carbide product rich in the target crystal form α-silicon carbide with a purity of ≥99.9wt%; The high-purity nano-silicon carbide product is crushed and classified by a supersonic airflow mill with an internal classifying wheel to obtain a high-purity nano-silicon carbide product with a D90 particle size of ≤500nm.

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