Silicon carbide particles having a core-shell structure and a method for producing the same

Abstract

The application belongs to the technical field of inorganic nonmetallic material preparation, and particularly relates to a kind of core-shell structure silicon carbide particles and its preparation method. The method comprises: providing an aluminum-doped silicon carbide core; loading the core into a fluidized bed reactor, and passing a fluidizing gas to make it fluidized; passing a second reaction gas containing a silicon source and a carbon source but no aluminum source at a temperature T2 to deposit a pure silicon carbide shell layer on the surface of the core; cooling, passivating, and collecting. The particle is composed of an aluminum-doped silicon carbide core and a pure silicon carbide shell, and the aluminum content in the shell is less than 1% of the aluminum content in the core. The application encapsulates aluminum in the core by a pure silicon carbide shell, effectively inhibits the re-evaporation of aluminum in subsequent high-temperature processing (such as physical vapor transport method for growing silicon carbide crystals), and increases the aluminum retention rate from 45% to more than 87%. The shell thickness is adjustable in the range of 5-500 nm, realizing the controlled release of aluminum.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic material preparation technology, specifically relating to a core-shell structured silicon carbide particle and its preparation method. Background Technology

[0002] Silicon carbide (SiC) is widely used in abrasives, structural ceramics, high-temperature coatings, and semiconductor devices due to its high hardness, high thermal conductivity, good chemical stability, and wide bandgap semiconductor properties. Aluminum is a common p-type dopant in SiC, which can effectively control its electrical properties and promote the densification of SiC during sintering.

[0003] Patent document CN108675300B discloses a fluidized bed chemical vapor deposition method for preparing core-shell structured nanocomposite particles. Using fluidized bed chemical vapor deposition, by controlling the temperature distribution within the reactor and the inlet method of the precursor vapor, core and shell materials are obtained separately, achieving in-situ coating to obtain core-shell structured nanocomposite particles. The core material can be a metallic element, a non-metallic element, or an inorganic non-metallic compound (such as silicon carbide, alumina, etc.), and the shell material can be a metallic element, a non-metallic element, or an organic or inorganic non-metallic compound (such as carbon, alumina, copper, iron, etc.). This method achieves uniform coating of the core particles by the shell material, and the particle size is adjustable within the range of 5–300 nm.

[0004] However, the technical solution of CN108675300B has the following problems: First, this technical solution only involves the coating of the undoped shell, and does not involve the preparation of aluminum-doped silicon carbide particles, let alone the distribution control of aluminum in the particles; Second, when aluminum-doped silicon carbide particles are used in subsequent high-temperature processing (e.g., as raw materials for growing silicon carbide crystals by physical vapor transport, or as high-temperature ceramic sintering aids), the aluminum in the particles will volatilize in large quantities from the particle surface to the gas phase at high temperatures (usually exceeding 1500℃). This volatilization leads to the loss of aluminum dopant, making the doping concentration uncontrollable during crystal growth, resulting in uneven axial resistivity of the crystal, and causing equipment contamination and material waste. Literature reports indicate that during the growth of silicon carbide crystals by physical vapor transport, the consumption of aluminum dopant decreases exponentially with growth time. The existing solution is to add a continuous flow of aluminum atoms during the growth process, but this method requires equipment modification, is complex to control, and the aluminum utilization efficiency is still not high. CN108675300B does not address the aforementioned technical issues at all, nor does it provide any technical means to suppress aluminum volatilization from aluminum-doped particles during subsequent high-temperature use. Summary of the Invention

[0005] The purpose of this invention is to provide a core-shell structured silicon carbide particle and its preparation method to solve the technical problem of dopant loss caused by the easy volatilization of aluminum during subsequent high-temperature processing of existing aluminum-doped silicon carbide particles; and to further solve the technical problem of uncontrollable aluminum release and inability to match the different requirements of aluminum release rate for different application scenarios.

[0006] The technical solution of the present invention is as follows: On one hand, a method for preparing core-shell structured silicon carbide particles is provided, comprising the following steps: (1) Provide an aluminum-doped silicon carbide core; (2) The aluminum-doped silicon carbide core is loaded into a fluidized bed reactor and fluidizing gas is introduced to make the aluminum-doped silicon carbide core fluidized. (3) At temperature T2, a second reaction gas containing silicon source and carbon source but without aluminum source is introduced into the fluidized bed reactor to deposit a pure silicon carbide shell layer on the surface of the aluminum-doped silicon carbide core, thereby obtaining core-shell structured silicon carbide particles. (4) Cool, passivate and collect the core-shell structured silicon carbide particles.

[0007] Preferably, the method of providing the aluminum-doped silicon carbide core in step (1) is as follows: using pure silicon carbide particles as the starting core, a first reaction gas containing silicon source, carbon source and aluminum source is introduced into a fluidized bed reactor at a temperature T1, and an aluminum-doped silicon carbide layer is deposited on the surface of the pure silicon carbide particles to form an aluminum-doped silicon carbide core.

[0008] Preferably, the aluminum-doped silicon carbide core in step (1) is a pre-prepared aluminum-doped silicon carbide particle.

[0009] Preferably, the temperatures T1 and T2 are each independently between 1000 and 1400°C.

[0010] Preferably, the thickness of the pure silicon carbide outer shell layer is 5 to 500 nm.

[0011] Preferably, the silicon source is selected from one or more of silane, dichlorosilane, and trichlorosilane; the carbon source is selected from one or more of methane, ethane, propane, ethylene, and acetylene.

[0012] Preferably, a purging step is included between step (2) and step (3): before introducing the second reaction gas, the fluidized bed reactor is purged with an inert gas for 1 to 10 minutes.

[0013] Preferably, the fluidizing gas in step (2) is one or a mixture of argon, hydrogen, and helium; the apparent gas velocity of the fluidizing gas is 0.5 to 3 times the minimum fluidization velocity.

[0014] Preferably, after step (3), an annealing process is performed at the temperature T2 for 0.5 to 2 hours.

[0015] On the other hand, a core-shell structured silicon carbide particle is provided, which is prepared by the method described in any of the above-mentioned methods; the particle consists of, from the inside out: an aluminum-doped silicon carbide core and a pure silicon carbide outer shell; the aluminum content in the pure silicon carbide outer shell is less than 1% of the aluminum content in the aluminum-doped silicon carbide core.

[0016] Compared with the prior art, the advantages of the present invention are: (1) Aluminum is encapsulated within the particle by depositing a dense pure silicon carbide shell on the surface of the aluminum-doped silicon carbide core. This shell acts as a physical barrier layer at high temperatures (such as the pre-sintering stage of physical vapor transport crystal growth, above 1500°C), significantly reducing the volatilization loss of aluminum into the gas phase. Experimental data show that, compared with aluminum-doped silicon carbide particles without a shell, the aluminum retention rate of the core-shell structure of the present invention increases from 45% to over 87% after heat treatment at 1500°C in an argon atmosphere for 2 hours.

[0017] (2) By controlling the thickness of the outer shell (adjustable in the range of 5 to 500 nm), the release rate of aluminum under different temperature environments can be precisely controlled. The aluminum retention rate of a thin outer shell (e.g., 15 nm) is about 70% after 1 hour at 1500 °C, which is suitable for applications that require faster aluminum release (e.g., high-temperature sintering aids); the aluminum retention rate of a thick outer shell (e.g., 180 nm) is higher than 95% after 1 hour under the same conditions, and is still higher than 85% after 8 hours, which is suitable for applications that require long-term stable packaging (e.g., semiconductor crystal growth materials).

[0018] (3) The core-shell particles obtained by this invention are made entirely of silicon carbide, with a high purity shell that does not introduce additional impurities such as oxygen, chlorine, or metals. The intrinsic properties of the particles, such as chemical stability and thermal conductivity, are not affected. At the same time, the dense shell avoids wear of the aluminum doped layer during mechanical mixing, improving the storage and transportation stability of the particles. These particles can be directly used as raw materials for growing low-absorption silicon carbide crystals by physical vapor transport or as high-temperature sintering aids, solving the problem of aluminum volatilization without modifying existing equipment.

[0019] (4) The preparation process of the present invention can be completed continuously in the same fluidized bed reactor (in-situ deposition method), or the pre-made core can be directly used for coating (two-step method). It is flexible in operation, simple in equipment, does not involve toxic and harmful solvents, is environmentally friendly, and is easy to scale up for industrial production. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1This is a schematic diagram of the fluidized bed reactor described in this invention; Figure 2 This is a schematic diagram of the core-shell structure of the aluminum-doped silicon carbide particles described in this invention. Figure 3 This is a schematic diagram of the radial concentration distribution of aluminum in the aluminum-doped silicon carbide particles described in this invention. Figure 4 This is a scanning electron microscope (SEM) image of the core-shell structure morphology of the aluminum-doped silicon carbide particles prepared in Example 1 of the present invention; Figure 5 This is a scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) image of the core region of the product of Example 2 of the present invention.

[0021] The components are: 1. Fluidized bed reactor; 2. Gas distribution plate; 3. Preheater; 4. Aluminum source; 5. Mass flow meter; 6. Fluidized particles; 7. Fluidized gas inlet; 8. Exhaust port; 9. Carrier gas chamber; 10. First reaction gas; 11. Second reaction gas; 12. Aluminum-doped silicon carbide core; 13. Pure silicon carbide outer shell. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments.

[0023] Example 1

[0024] In this embodiment, pre-prepared aluminum-doped silicon carbide particles are used as the core, and a pure silicon carbide shell is deposited on its surface.

[0025] 1. Raw material preparation and fluidization Pre-prepared aluminum-doped silicon carbide particles were used as the starting material. These particles were α-SiC crystals with an average particle size of 50 μm (D50 = 50.2 μm), an aluminum content of 0.65 wt% (determined by inductively coupled plasma atomic emission spectrometry, after acid digestion and analysis), and a purity ≥ 99.9% (the main impurity was oxygen, with a content of approximately 0.05%). 100 g of these particles were weighed and loaded into a vertical fluidized bed reactor 1.

[0026] like Figure 1As shown, the fluidized bed reactor 1 is a vertical quartz tube structure with an inner diameter of 50 mm and a total height of 600 mm. A fluidizing gas inlet 7 is located at the bottom, and a gas distribution plate 2 (20 μm aperture, 5% porosity) is installed above the inlet to uniformly distribute the incoming gas into the particle bed. The central part of the fluidized bed reactor 1 is the main reaction zone, which accommodates the fluidized particle bed. The fluidized bed reactor 1 is externally encased in a heating furnace (three independently temperature-controlled sections, total heating length 300 mm), with each section having a heating power of 1.5 kW, allowing for precise control of the temperature distribution within the fluidized bed reactor 1. A second reaction gas inlet 11 is located on the side wall of the fluidized bed reactor 1, situated in the center of the heating zone. An independent preheater 3 (100 W heating power) is externally encased in the inlet pipe, allowing for preheating temperature control within the range of 100–300 °C. The aluminum source 4 is stored in a source bottle and fed into the fluidized bed reactor 1 via a carrier gas chamber 9, with the carrier gas flow rate controlled by a mass flow meter 5. The top of the fluidized bed reactor 1 is equipped with an exhaust port 8, which is connected to a bag filter and a negative pressure suction device (the suction volume is adjustable to maintain a slight negative pressure of -100 to -500 Pa in the system) to collect products and prevent the leakage of reaction gases.

[0027] High-purity argon gas (99.999% purity) was introduced as the fluidizing gas. The gas velocity was adjusted to 1.5 L / min using a mass flow meter 5. Preliminary measurements showed that the minimum fluidization velocity for silicon carbide particles with a particle size of 50 μm was approximately 0.75 L / min at room temperature using the pressure drop-gas velocity curve method. Therefore, a gas velocity of 1.5 L / min is twice the minimum fluidization velocity. At this gas velocity, the particle bed was visually observed to be in a uniform tumbling state, with pressure drop fluctuations of less than 5%, and no channeling or throttling phenomena, confirming that the particles were in a stable fluidized state (i.e.,...). Figure 1 The fluidized particles shown in Figure 6).

[0028] The heating furnace was started, and the fluidized bed reactor 1 was heated from room temperature to T2 = 1200℃ at a heating rate of 10℃ / min. Argon gas was continuously introduced (1.5L / min) during the heating process to maintain fluidization. The outer wall temperature of the fluidized bed reactor 1 was monitored in real time using an infrared thermometer (accuracy ±2℃), and the heating power was adjusted using a PID controller. After the temperature stabilized at 1200℃ (held for 10 minutes, with temperature fluctuations within ±5℃), the next step was performed.

[0029] 2. Deposition of pure silicon carbide outer shell layer Maintain the argon flow rate (1.5 L / min) and the temperature of fluidized bed reactor 1 at 1200 °C. Introduce the second reaction gas 11 into fluidized bed reactor 1.

[0030] The second reaction gas 11 consists of a silicon source, a carbon source, and a dilution gas: the silicon source is silane (SiH4) with a purity of 99.99%, output from a steel cylinder via a pressure reducing valve, and its flow rate is set to 30 sccm by mass flow meter 5; the carbon source is acetylene (C2H2) with a purity of 99.9%, also with its flow rate set to 20 sccm by mass flow meter 5; high-purity argon is used as the dilution gas, and its flow rate is adjusted by mass flow meter 5 to maintain the total gas flow rate in the reaction zone at 1.2 L / min. This gas mixture does not contain any aluminum source. The three gases are premixed in a stainless steel mixing chamber before entering the fluidized bed reactor 1, and then enter the fluidized bed reactor 1 through the side wall inlet.

[0031] At a high temperature of 1200℃, silane and acetylene undergo a thermal decomposition reaction: SiH4(g) → Si(s, or adsorbed atoms) + 2H2(g)↑ C2H2(g) → 2C(s, or adsorbed atoms) + H2(g)↑ The active silicon and carbon atoms generated from the pyrolysis migrate to the surface of the fluidized aluminum-doped silicon carbide particles via gas-phase diffusion. As the particles continuously tumble and mix in the fluidized bed, each particle surface is uniformly exposed to the reactive gas. Active atoms adsorb and migrate on the particle surface, then react with each other to form silicon carbide (Si + C → SiC). This process is a chemical vapor deposition (CVD) mechanism, and the deposition rate is controlled by the surface reaction. Since there are no aluminum atoms in the gas phase (the aluminum source is completely shut off), the deposited silicon carbide is pure silicon carbide, free of aluminum impurities. This pure silicon carbide outer shell 13 is uniformly epitaxially grown along the particle surface using aluminum-doped silicon carbide particles as a substrate.

[0032] The deposition reaction time was 20 minutes. During this process, argon gas flow was maintained continuously (1.5 L / min) and the temperature was kept stable (1200 ± 5 °C). The negative pressure suction device at the top outlet of fluidized bed reactor 1 was kept open to remove the tail gas (mainly hydrogen, unreacted silane and acetylene, and diluted argon) and send it to the alkaline absorption tower for treatment.

[0033] After the reaction is complete, stop the flow of silane and acetylene (close the corresponding mass flow meter 5), and continue to purge with argon gas (1.5 L / min) for 5 minutes. The purpose of purging is to remove residual unreacted gases and byproduct hydrogen from the fluidized bed reactor 1, and to prevent unnecessary side reactions or product contamination during the cooling process.

[0034] 3. Cooling, passivation and collection After purging, turn off the power to the heating furnace and maintain argon flow (1.5 L / min) to allow the fluidized bed reactor 1 to cool naturally to room temperature. During the cooling process, the cooling rate gradually slows down due to thermal radiation and convection, taking approximately 2–3 hours to drop from 1200°C to room temperature. During this period, argon gas is continuously introduced to prevent air backflow.

[0035] After the temperature of fluidized bed reactor 1 drops to room temperature (no heat felt when touching the outer wall, or the thermocouple reading <40℃), the argon gas is turned off, and a small amount of air (flow rate 0.1 L / min, manually adjusted via a needle valve) is introduced for passivation treatment for 30 minutes. The purpose of passivation is to form an extremely thin oxide layer (approximately 1–2 nm) on the particle surface to prevent the nanoscale active surfaces from being severely oxidized or adsorbing moisture and organic pollutants upon subsequent exposure to air. After passivation, the air is turned off, the top flange of fluidized bed reactor 1 is opened, and the product is removed with a stainless steel spatula and stored in a desiccator. The obtained product is designated as sample S1.

[0036] 4. Product Characterization (1) Morphological and structural characterization The cross-section of sample S1 was observed using scanning electron microscopy (SEM). The cross-sectional sample was prepared by embedding particles in epoxy resin, curing, and then mechanically grinding and polishing. The SEM observation results are as follows: Figure 4 As shown.

[0037] like Figure 4 As shown, the particle cross-section exhibits a clear contrast between light and dark areas, revealing a core-shell bilayer structure: the core (dark area) is aluminum-doped silicon carbide with a particle size of approximately 50 μm (consistent with the original particle, showing no significant growth); the outer shell (light-colored edge) is pure silicon carbide with uniform thickness. Ten particles were randomly selected from the image, and the thickest and thinnest points of the outer shell for each particle were measured. The average thickness of the outer shell was approximately 50 nm (measured values ​​ranged from 48 to 53 nm, with a standard deviation of 2.1 nm). The outer shell is continuous and dense, with no obvious interface gaps or cracks between it and the core, indicating a good bond between the shell and core, without delamination or detachment. A schematic diagram of the overall particle structure is shown below. Figure 2 As shown, it includes an aluminum-doped silicon carbide core 12 and a pure silicon carbide outer shell layer 13.

[0038] (2) Aluminum element distribution analysis X-ray energy dispersive spectroscopy (EDS, used in conjunction with SEM, with a detection depth of approximately 1 μm) was used for line scan analysis of individual particles. The line scan path was radially from the particle center (i.e., the center of the cross-section) outwards to the outer surface, with a step size of 0.5 μm and a sampling time of 5 seconds per point.

[0039] like Figure 3As shown, the signal intensity of aluminum (Kα line, energy 1.486 keV) varies with radial position, where the horizontal axis represents the radial position of the particle (0 corresponds to the center, increasing outwards), and the vertical axis represents the relative concentration of aluminum (with the signal intensity in the core stable region normalized to 100%). Figure 3 It can be seen that from the particle center to the core edge (approximately 0–25 μm region), the aluminum element signal is stable at around 100%, corresponding to an aluminum content of 0.65 wt% in the core. When the scanning path enters the outer shell region (approximately 25–25.05 μm region, i.e., a region with a thickness of approximately 50 nm), the aluminum element signal drops sharply to near the background level (below 1% of the core Al signal intensity, with an EDS background noise of approximately 0.5%), proving that the outer shell is basically free of aluminum.

[0040] (3) Aluminum retention rate test Take 100 mg of sample S1 and place it in an alumina crucible (99.5% purity, 2 mL volume) for high-temperature heat treatment in a tube furnace. Heat treatment conditions: evacuate the tube furnace to <10 Pa, then backfill with high-purity argon (99.999% purity) to atmospheric pressure, repeating three times to replace the air; then continuously introduce argon at a flow rate of 200 sccm; raise the temperature from room temperature to 1500℃ at a heating rate of 10℃ / min; hold at 1500℃ for 2 hours; after holding, turn off the heating power and allow it to cool naturally to room temperature in the argon flow.

[0041] The mass fraction of aluminum in the samples before and after heat treatment was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Sample pretreatment: 10 mg of sample was taken, 0.5 mL of hydrofluoric acid and 0.5 mL of nitric acid were added, and the sample was digested in a microwave digester at 180 °C for 20 minutes. Then, the volume was adjusted to 10 mL with water, filtered, and then analyzed by the instrument.

[0042] Results: Before heat treatment, the aluminum content of sample S1 was 0.65 wt%; after heat treatment, the aluminum content was 0.58 wt%. The aluminum retention rate was calculated using the following formula: Aluminum retention rate (%) = (Aluminum content after heat treatment / Aluminum content before heat treatment) × 100% Substituting the values: Aluminum retention rate = (0.58 / 0.65) × 100% = 89.2%.

[0043] The results show that after treatment at 1500℃ for 2 hours, the pure silicon carbide shell effectively prevents aluminum from volatilizing to the outside, with an aluminum retention rate as high as 89.2%.

[0044] Example 2

[0045] This embodiment starts with a pure silicon carbide core and sequentially deposits an aluminum-doped silicon carbide layer (forming a core) and a pure silicon carbide outer shell layer 13 in the same fluidized bed reactor 1 to achieve continuous production without intermediate unloading.

[0046] 1. Raw material preparation and fluidization Pure α-SiC spherical particles with an average particle size of 30 μm were used as the starting core. These particles had a purity of 99.99% (the main impurity being oxygen, with a content <0.01%). 100 g of these particles were weighed and loaded into fluidized bed reactor 1, identical to that in Example 1. High-purity argon gas was introduced, and the gas rate was adjusted to 1.2 L / min. Preliminary measurements indicated that the minimum fluidization velocity of 30 μm silicon carbide particles in this fluidized bed reactor 1 was approximately 0.6 L / min. Therefore, a gas rate of 1.2 L / min was twice the minimum fluidization velocity, sufficient to form a stable fluidized bed. Fluidized bed reactor 1 was heated to a temperature T1 = 1200 °C at a heating rate of 10 °C / min.

[0047] 2. Deposit an aluminum-doped silicon carbide layer (to form an aluminum-doped core). Once the temperature of fluidized bed reactor 1 stabilizes at 1200℃ (hold for 10 minutes with fluctuations of ±5℃), the first reaction gas 10 is introduced.

[0048] The first reaction gas 10 contains silicon, carbon, and aluminum sources: the silicon source is silane (SiH4) at a flow rate of 30 sccm; the carbon source is acetylene (C2H2) at a flow rate of 20 sccm; and the aluminum source is trimethylaluminum (TMA) saturated vapor. The TMA source bottle (200 mL volume) is heated to 40°C in a water bath, at which temperature the saturated vapor pressure of TMA is approximately 4.8 mmHg. High-purity argon is used as the carrier gas at a flow rate of 15 sccm. As the argon flows across the TMA liquid surface, it carries the TMA vapor into the fluidized bed reactor 1. To prevent TMA condensation in the pipeline, the inlet pipeline from the source bottle to the fluidized bed reactor 1 is externally covered with a heating belt, and the preheating temperature is set to 150°C. Simultaneously, dilution argon is introduced, controlled by another mass flow meter 5, to maintain the total gas flow rate in the reaction zone at 1.2 L / min.

[0049] A chemical vapor deposition reaction was carried out for 40 minutes. During this process, silane, acetylene, and TMA simultaneously underwent pyrolysis. SiH4→Si+2H2 C2H2→2C+H2 Al(CH3)3→Al+3CH3· (reactive free radical) The generated active Si, C, and Al atoms adsorb and migrate on the surface of fluidized pure silicon carbide particles, and then react with each other to form aluminum-doped silicon carbide (Si + C + xAl → Al). x Si 1-x(C solid solution). This layer covers the surface of the original pure silicon carbide particles, transforming the particles into aluminum-doped silicon carbide cores 12. As the deposition time increases, the particle size of the core particles gradually increases, and the aluminum content gradually rises.

[0050] After 40 minutes of reaction, the TMA supply was stopped (carrier gas shut off), and the silane and acetylene flow rates were kept constant. The reactor was then purged with argon for 5 minutes. The purpose of purging was to remove residual aluminum source gas and possible gaseous aluminum species (such as undecomposed TMA, CH3· radicals, etc.) from the fluidized bed reactor 1, ensuring that the subsequently deposited pure silicon carbide outer shell layer 13 was free of aluminum impurities.

[0051] After deposition, a small sample was taken from fluidized bed reactor 1, and the particle size distribution was determined using a laser particle size analyzer: the average particle size increased from the original 30 μm to approximately 45 μm (D50 = 45.3 μm). The aluminum content in the core was determined using ICP-OES (method as in Example 1), and the result was 0.63 wt%. Simultaneously, point-to-point analysis of the core region was performed using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and the results are as follows... Figure 5 As shown, the energy dispersive spectroscopy (EDS) spectrum exhibits distinct characteristic peaks for carbon (C), silicon (Si), and aluminum (Al). Quantitative analysis reveals an Al content of 0.63 wt%, a C content of 23.92 wt%, and a Si content of 63.55 wt%, with a total content of 100%. This result indicates that the core is aluminum-doped silicon carbide, and aluminum has been successfully incorporated into the core.

[0052] 3. Deposition of pure silicon carbide outer shell layer The temperature of fluidized bed reactor 1 was maintained at 1200°C, and the fluidizing gas velocity was kept constant at 1.2 L / min. The reaction gas was switched to a second reaction gas 11 without an aluminum source: silane 30 sccm, acetylene 20 sccm, and argon diluted to a total flow rate of 1.2 L / min. The reaction time was 15 minutes. Under these conditions, following the same deposition principle as step 2 of Example 1, a pure silicon carbide outer shell layer 13 was deposited on the surface of the aluminum-doped silicon carbide core 12.

[0053] 4. Cooling, passivation and collection It is exactly the same as step 3 in Example 1.

[0054] 5. Product Characterization The resulting product was designated as sample S2. Scanning electron microscopy (SEM) cross-sectional observation (using the same method as in Example 1) showed that sample S2 had a clear core-shell structure: the core was aluminum-doped silicon carbide (average particle size approximately 45 μm), and the outer shell was pure silicon carbide with a thickness of approximately 50 nm (measured from 10 particles, ranging from 47 to 54 nm). EDS line scan analysis (using the same method as in Example 1) showed that the Al elemental signal was only present in the core region, and the Al signal in the outer shell region was below the background noise (less than 0.8% of the Al signal intensity in the core), proving that the outer shell did not contain aluminum.

[0055] Aluminum retention rate test (1500℃, 2 hours, same method as Example 1) results: Aluminum content before heat treatment was 0.63wt%, aluminum content after heat treatment was 0.55wt%, aluminum retention rate = (0.55 / 0.63)×100% = 87.3%.

[0056] The results show that the in-situ deposition method can also prepare high-quality core-shell structured silicon carbide particles, and the process is continuous and does not require unloading in the middle, making it more suitable for large-scale industrial production.

[0057] Example 3

[0058] This embodiment aims to quantitatively study the relationship between the thickness of the pure silicon carbide shell and the aluminum retention rate, and to verify that the shell thickness can control the aluminum release rate.

[0059] Based on Example 2, all process parameters were kept constant (initial core particle size 30 μm, reaction temperature 1200 °C, silane 30 sccm, acetylene 20 sccm, TMA carrier gas 15 sccm deposition for 40 minutes to form the core), only the reaction time of the pure silicon carbide shell deposition step was changed. Five time points were set: 5 minutes, 10 minutes, 20 minutes, 30 minutes, and 40 minutes.

[0060] At the end of each time point, the reaction gas was stopped, the mixture was purged with argon for 5 minutes, and then cooled, passivated, and the product was collected. Five types of core-shell structured particles were prepared and designated as samples S3, S4, S5, S6, and S7.

[0061] The shell thickness was measured using scanning electron microscopy (SEM) cross-sectional images (10 particles were randomly selected from each sample; the thickest and thinnest points of the shell on the cross-section were measured, and the average value was taken). Thickness measurement results: S3 (5-minute deposition): Shell thickness approximately 15nm (range 13–18nm) S4 (10-minute deposition): Shell thickness approximately 30 nm (range 27–34 nm) S5 (20-minute deposition): Shell thickness approximately 60 nm (range 55–66 nm) S6 (30-minute deposition): Shell thickness approximately 110 nm (range 102–122 nm) S7 (40-minute deposition): Shell thickness approximately 180 nm (range 170–195 nm) Samples (S3–S7) were heat-treated at 1500℃ in an argon atmosphere for different durations: 0.5 hours, 1 hour, 2 hours, 4 hours, and 8 hours. Independent samples were used at each time point (5 samples per thickness, for a total of 25 samples) to avoid the cumulative effect of multiple heat treatments on the same batch of samples. The heat treatment method was the same as in Example 1. After heat treatment, the aluminum content was determined using ICP-OES (method as in Example 1), and the aluminum retention rate was calculated.

[0062] Table 1: Variation of aluminum retention rate with heat treatment time for different shell thicknesses

[0063] Data Analysis and Discussion: As can be seen from Table 1: Under the same heat treatment time, the greater the shell thickness, the higher the aluminum retention rate. For example, after 2 hours of heat treatment, the aluminum retention rate of a 15nm thick shell is 55%, while the aluminum retention rate of a 180nm thick shell is as high as 95%.

[0064] With increasing heat treatment time, the aluminum retention rate of all samples showed a decreasing trend, but the rate of decrease was significantly slower for the thick-shell samples than for the thin-shell samples. The 180nm thick shell still retained 88% aluminum after 8 hours of heat treatment, while the 15nm thick shell retained only 30% after 8 hours.

[0065] This data shows that by selecting different shell thicknesses, the release rate of aluminum at high temperatures can be controlled within a wide range (aluminum retention rate 30% to 98%), thereby matching the needs of different application scenarios.

[0066] Example 4 Based on Example 2, after depositing the pure silicon carbide outer shell layer 13, the temperature was not immediately lowered, but instead held at 1200°C for 1 hour (argon gas flow was maintained in the furnace tube at a flow rate of 1.2 L / min), followed by natural cooling. All other process parameters were identical to those in Example 2. The annealed sample was designated S8. Annealing promotes atomic diffusion and interfacial bonding between the outer shell layer and the core, reduces porosity and defects at the interface, thereby reducing the number of channels for aluminum diffusion along grain boundaries and further improving the barrier effect.

[0067] Characterization results: Cross-sectional observation by scanning electron microscopy: The thickness of the shell of S8 did not change significantly (still about 50 nm), but the interface between the shell and the core became more blurred in the SEM image than that of the unannealed sample (S2), indicating that slight interdiffusion occurred at the interface.

[0068] EDS line scan: with Figure 3In contrast, the Al element in the S8 exhibits slight diffusion in the interface region (diffusion width of approximately 5 nm, from the core edge to the interior of the shell), forming a gradient interface rather than a steep one. However, the main shell region (>5 nm from the interface) remains essentially aluminum-free.

[0069] Aluminum retention rate test (1500℃, 2 hours, same method as Example 1): Aluminum content before heat treatment was 0.63wt%, aluminum content after heat treatment was 0.57wt%, aluminum retention rate = (0.57 / 0.63)×100%=90.1%, which is higher than 87.3% of the unannealed sample S2.

[0070] The results showed that annealing promoted interfacial bonding, further reduced the channels for aluminum diffusion along grain boundaries, and slightly improved the aluminum retention rate. However, the annealing time should not be too long (>2 hours), otherwise it may lead to excessive segregation of aluminum to the outer shell surface, which would reduce the barrier effect. The preferred annealing time is 0.5 to 2 hours. Example 5 (Boundary Temperature Verification) Example 5 This embodiment verifies the feasibility of the lower temperature limit (T1=T2=1000℃) and the upper temperature limit (T2=1400℃).

[0071] 5.1 Low temperature boundary (T1=T2=1000℃) The process is essentially the same as in Example 2, except that the temperatures for depositing the aluminum-doped silicon carbide layer (first reactant gas 10) and the pure silicon carbide outer shell layer 13 (second reactant gas 11) are both set to 1000°C. The initial core particle size is 30 μm, and all other parameters, such as gas flow rate and deposition time, are exactly the same as in Example 2.

[0072] Product characterization: Scanning electron microscopy (SEM) cross-sections showed that the average core particle size was approximately 44 μm (slightly smaller than 45 μm at 1200 °C due to the lower deposition rate), and the shell thickness was approximately 48 nm. EDS line scanning showed that Al was present only in the core. ICP-OES determined the core aluminum content to be 0.60 wt%. Aluminum retention test results (1500 °C, 2 hours): Aluminum content before heat treatment was 0.60 wt%, and after heat treatment, aluminum content was 0.51 wt%, with an aluminum retention rate of 85.6%.

[0073] The results showed that core-shell structures could still be successfully prepared at 1000℃, but the deposition efficiency of aluminum doped layer was slightly lower than that at 1200℃ (aluminum content 0.60wt% vs 0.63wt%), and the density of the shell decreased slightly (aluminum retention rate 85.6% vs 87.3%), but was still much higher than that of the control without shell (44.6%).

[0074] 5.2 High Temperature Boundary (T2=1400℃) The process is essentially the same as in Example 2, except that the temperature for depositing the pure silicon carbide outer shell 13 is set to 1400°C. To prevent excessive particle sintering (the surface activity of silicon carbide particles increases at 1400°C, potentially leading to slight adhesion), the outer shell deposition time is shortened to 10 minutes (because the deposition rate is faster at higher temperatures). The temperature for depositing the aluminum-doped core remains 1200°C, the same as in Example 2. All other parameters remain unchanged.

[0075] Product characterization: Scanning electron microscopy (SEM) cross-section showed that the shell thickness was approximately 45 nm, with a significantly improved density (no pores were observed in the SEM image, indicating good intergranular bonding). Aluminum retention rate test results (1500℃, 2 hours): The core aluminum content was 0.63 wt% before heat treatment and 0.57 wt% after heat treatment, with an aluminum retention rate of 90.5%, slightly higher than the 87.3% in Example 2.

[0076] The results show that increasing the shell deposition temperature is beneficial to improving the shell density, thereby enhancing the ability to block aluminum diffusion. However, it should be noted that higher temperatures (>1400℃) may cause intrinsic sintering (particle adhesion) to begin on the surface of silicon carbide particles, disrupting the fluidization state.

[0077] Comparative Example To verify the inhibitory effect of the pure silicon carbide shell on aluminum volatilization, Comparative Example 1 was set up. This comparative example used aluminum-doped silicon carbide particles of the same batch, particle size, and aluminum content (0.65wt%) as in Example 1, without any shell deposition treatment.

[0078] 100 mg of the granules were weighed and subjected to high-temperature heat treatment under the same conditions as in Example 1 (1500 °C, argon atmosphere, 2 hours). After heat treatment, the aluminum content was determined by ICP-OES. The results showed that the aluminum content was 0.65 wt% before heat treatment and 0.29 wt% after heat treatment, with a calculated aluminum retention rate of only 44.6%.

[0079] Comparative analysis Table 2 shows a comparison of the aluminum retention rates between Example 1 and Comparative Example 1. The results clearly show that, under exactly the same precursor feed rate and heat treatment conditions, the aluminum retention rate was 89.2% with a pure silicon carbide shell, while it was only 44.6% without the shell. This indicates that the presence of a pure silicon carbide shell is the key reason for suppressing the high-temperature volatilization of aluminum.

[0080] Summary of key parameters and results for each embodiment and comparative example Table 2: Key parameters and aluminum retention rates of each embodiment and comparative example

[0081] The data in Table 2 show that the pure silicon carbide shell increases the aluminum retention rate from 44.6% (comparative example) to over 87% (Examples 1 and 2), and the greater the shell thickness, the more annealing treatment, and the more appropriate the deposition temperature, the better the aluminum retention rate.

[0082] The above embodiments are merely illustrative of the technical concept and features of the present invention, intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and should not be construed as limiting the scope of protection of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and therefore all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

Claims

1. A method for preparing core-shell structured silicon carbide particles, comprising the following steps: (1) Provide an aluminum-doped silicon carbide core; (2) The aluminum-doped silicon carbide core is loaded into a fluidized bed reactor and fluidizing gas is introduced to make the aluminum-doped silicon carbide core fluidized. (3) At temperature T2, a second reaction gas containing silicon source and carbon source but without aluminum source is introduced into the fluidized bed reactor to deposit a pure silicon carbide shell layer on the surface of the aluminum-doped silicon carbide core, thereby obtaining core-shell structured silicon carbide particles. (4) Cool, passivate and collect the core-shell structured silicon carbide particles.

2. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, The method of providing the aluminum-doped silicon carbide core in step (1) is as follows: using pure silicon carbide particles as the starting core, a first reaction gas containing silicon source, carbon source and aluminum source is introduced into a fluidized bed reactor at a temperature T1, and an aluminum-doped silicon carbide layer is deposited on the surface of the pure silicon carbide particles to form an aluminum-doped silicon carbide core.

3. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, The aluminum-doped silicon carbide core mentioned in step (1) is a pre-prepared aluminum-doped silicon carbide particle.

4. The method for preparing core-shell structured silicon carbide particles according to claim 2, characterized in that, The temperatures T1 and T2 are each independently between 1000 and 1400°C.

5. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, The thickness of the pure silicon carbide outer shell layer is 5–500 nm.

6. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, The silicon source is selected from one or more of silane, dichlorosilane, and trichlorosilane; the carbon source is selected from one or more of methane, ethane, propane, ethylene, and acetylene.

7. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, A purging step is also included between steps (2) and (3): before introducing the second reaction gas, the fluidized bed reactor is purged with inert gas for 1 to 10 minutes.

8. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, The fluidizing gas in step (2) is one or a mixture of argon, hydrogen, and helium; the apparent gas velocity of the fluidizing gas is 0.5 to 3 times the minimum fluidization velocity.

9. The method for preparing core-shell structured silicon carbide particles according to claim 1, characterized in that, After step (3), the process also includes an annealing process at the temperature T2 for 0.5 to 2 hours.

10. A core-shell structured silicon carbide particle, characterized in that, The particles are prepared by the method according to any one of claims 1 to 9; the particles, from the inside out, consist of: an aluminum-doped silicon carbide core and a pure silicon carbide outer shell; the aluminum content in the pure silicon carbide outer shell is less than 1% of the aluminum content in the aluminum-doped silicon carbide core.

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