A stepped porosity silicon carbide substrate and method of making the same

Abstract

The present application is suitable for the technical field of silicon carbide material, and provides a stepped porosity silicon carbide substrate and a preparation method thereof, which comprises the following steps: selecting a plurality of silicon carbide powders with a particle size gradient distribution, wherein the particle size range covers 0.3-50 microns; the particle size of each layer of powder increases from top to bottom, and the particle size ratio of adjacent layers of powder is 1:2 to 1:4.5; 0.2-0.6 wt% of B4C, 1-3 wt% of amorphous carbon and 2-5 wt% of PVA binder are added to each layer of powder respectively, and after ball milling and spray granulation, the spherical particle mixed powder required for forming and pressing is prepared; layered filling and composite pressing; sintering process control: including a dewaxing stage and a multi-stage precise temperature control sintering stage; and the sintered substrate is subjected to fine grinding, ultrasonic cleaning and drying. The present application realizes accurate and controllable porosity gradient, accurately realizes a high porosity interval of 25-40% and a smooth stepped distribution in the thickness direction, has excellent mechanical properties and wide adaptability.

Description

Technical Field

[0001] This invention belongs to the field of silicon carbide material technology, and particularly relates to a stepped porosity silicon carbide substrate and its preparation method. Background Technology

[0002] In existing technologies, methods for preparing porous silicon carbide ceramics mainly include partial sintering, the addition of pore-forming agents, and organic foam impregnation. For gradient pore structures, common techniques include: gradient arrangement of pore-forming agents, organic precursor impregnation, film preparation technology, and atmospheric pressure solid-state sintering. Existing technologies mostly focus on the low to medium porosity range (8-18%), aiming to adapt to the thermal conductivity and interfacial bonding of AlSiC substrates. Their processes (such as high-pressure pressing and high-temperature sintering) are not suitable for preparing high porosity gradient materials of 25-40%, which can easily lead to over-densification or poor interlayer bonding.

[0003] The existing technology has the following problems:

[0004] Poor porosity and gradient controllability: Existing methods are difficult to accurately achieve a high porosity range of 25-40% and its smooth step distribution in the thickness direction (e.g., 35-40% in the upper layer, 30-35% in the middle layer, and 25-30% in the lower layer).

[0005] Insufficient strength and bonding force under high porosity: When a large amount of pore-forming agent is introduced to achieve high porosity, traditional pressing and sintering processes cannot guarantee sufficient neck bonding and interlayer bonding strength between particles, and the product is prone to cracking or has low strength.

[0006] The sintering parameters are vague and lack dynamic coordinated control of temperature, pressure and atmosphere, resulting in poor pore gradient stability and uncontrolled grain growth.

[0007] Complex processes or high energy consumption: Some gradient preparation methods (such as freeze drying, photocuring, CVI method, etc.) are complex, require high-end equipment, or rely on extremely high sintering temperatures, making them unsuitable for large-scale production;

[0008] Disconnected from specific application scenarios: Most technologies have not been optimized for pore gradient design for high porosity filtration, catalyst carriers or specific aluminizing requirements, and the performance does not match the application requirements well.

[0009] To address the aforementioned shortcomings, there is an urgent need to provide a powder metallurgy method with precise process parameters and scalable production capabilities. This method would solve the comprehensive technical challenges of achieving precise and controllable pore gradient, ensuring strong interlayer bonding, maintaining sufficient mechanical strength, and meeting the performance requirements of high-end applications (such as filtration, catalysis, and aluminizing) under high porosity (25-40%) targets. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing a silicon carbide substrate with stepped porosity, thereby addressing the problems mentioned in the background art.

[0011] The present invention is implemented as follows: a stepped porosity silicon carbide substrate and its preparation method include the following steps:

[0012] S1. Powder Proportioning and Pretreatment: Select 3-5 kinds of silicon carbide powder with a particle size gradient distribution, covering a particle size range of 0.3-50μm; the particle size of each layer of powder increases from top to bottom, and the particle size ratio of adjacent layers of powder is 1:2 to 1:4.5. Add 0.2-0.6wt% B4C, 1-3wt% amorphous carbon and 2-5wt% PVA binder to each layer of powder respectively. After ball milling and spray granulation, obtain spherical granular powder with a flowability ≥15g / min.

[0013] S2. Layered filling and composite pressing: In a mold preheated to 50-80℃, powders of different particle sizes are filled in layers multiple times, with a single filling thickness of 0.2-0.5mm. After filling, nitrogen is purged under a vacuum of -0.08-0.09MPa. After single-gradient layer filling, medium-low synchronous step pressing is performed for pre-pressing, and then the next layer of powder is filled, i.e., synchronous pressing. After the overall filling is completed, asynchronous pressing is used to press the billet into the required size and shape, and then the pressure is released in stages.

[0014] S3. Sintering process control: including dewaxing stage and multi-stage precise temperature control sintering stage. The dewaxing stage adopts a three-stage heating method, which is held at 200℃, 400℃ and 600℃ respectively, and is carried out under argon protection throughout the process. The multi-stage precise temperature control sintering stage includes: (1) Low temperature densification stage 1000-1600℃: heating rate 40-80℃ / h, furnace pressure linearly increased from 0.1MPa to 0.2MPa; (2) Medium temperature grain growth stage 1600-2100℃: heating rate 20-30℃ / h, furnace pressure increased from 0.2MPa to 0.3MPa, and held at 1600-2100℃ for 1-2h; (3) Gradient cooling stage: from 2100℃ to room temperature, the cooling rate and furnace pressure are controlled in stages according to different temperature ranges, and finally cooled to room temperature.

[0015] S4. Post-processing: The sintered substrate is finely ground, ultrasonically cleaned and dried to obtain the final product.

[0016] Another objective of this invention is to provide a stepped porosity silicon carbide substrate, which is prepared by the above-described preparation method. The silicon substrate has at least three layers, and its open porosity is distributed in a stepped manner in the thickness direction. The top layer porosity is 35-40%, the middle layer porosity is 30-35%, and the bottom layer porosity is 25-30%.

[0017] The preparation method provided in this invention has the following advantages:

[0018] 1. High gradient control precision: Achieves precise and controllable pore gradient, accurately realizing a high porosity range of 25-40% and its smooth step distribution in the thickness direction;

[0019] 2. Excellent mechanical properties: flexural strength ≥400MPa, interlaminar bond strength ≥23MPa, residual stress ≤45MPa, and finished product qualification rate increased to over 98%;

[0020] 3. Strong technological innovation: No post-processing is required for hole making, energy consumption is reduced by 25-30%, production cycle is shortened by 20%, and process parameter accuracy reaches ±1℃ and ±0.1MPa, which can be directly mass-produced.

[0021] 4. Wide adaptability: It is suitable for high-end AlSiC composite substrate fields such as semiconductor packaging and new energy devices, and can be extended to the preparation of other graded functional ceramic materials. Attached Figure Description

[0022] Figure 1 A flowchart illustrating a method for preparing a silicon carbide substrate with stepped porosity, provided as an embodiment of the present invention;

[0023] Figure 2 This is a flowchart illustrating a method for preparing a stepped porosity silicon carbide substrate according to an embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0026] Example 1: A stepped porosity silicon carbide substrate, the preparation method of which includes the following steps:

[0027] (1) Powder proportioning and pretreatment:

[0028] Select 3-5 types of silicon carbide powders with a particle size of 0.3-50 μm (20-40% submicron grade) and a gradient distribution. The average particle size (D50) of the top layer powder is 0.3-7 μm, the bottom layer is 30-50 μm, and the particle size of the middle layer increases (adjacent layer particle size ratio 1:2-1:4.5). The purity is ≥99.9%, and the oxygen content is ≤0.1wt%. Each layer is supplemented with sintering aids (0.2-0.6wt% boron carbide, 1-3wt% amorphous carbon with a particle size ≤0.5 μm) and binders (PVA). 2-5 wt%, degree of polymerization 1700-1800; granulated by planetary ball milling for 2-4 hours (ball-to-material ratio 6:1, rotation speed 300-400 r / min, silicon carbide ball diameter 5-10 mm), followed by spray granulation (inlet air 180-220℃, outlet air 80-100℃, atomization pressure 0.3-0.5 MPa) to form 30-80 μm spherical particles, requiring a flowability ≥15 g / min and a bulk density ≥1.2 g / cm³. 3 Moisture content ≤0.5wt%;

[0029] (2) Layered filling and composite pressing:

[0030] A servo molding machine (displacement accuracy ±1μm, pressure control accuracy ±0.1MPa, response time ≤10ms, punch parallelism ≤0.005mm / m) is used for layered powder filling. The mold surface is nitrided and laser surface modified (hardness ≥HV900, surface roughness Ra≤0.2μm). The mold is preheated to 50-80℃ (heating rate 10℃ / min, temperature fluctuation ≤±2℃, temperature measuring points are distributed on the upper and lower molds and the side wall of the mold cavity, a total of 4 temperature measuring points) to improve powder flowability. The filling thickness of each layer is 0.3-0.8mm (filling accuracy ±5μm, filling thickness is monitored in real time using a laser displacement sensor). After powder filling, the mold is purged with 0.3-0.5MPa nitrogen for 10-15s (purging angle 45°, nozzle distance from the blank surface 20mm, nozzle orifice diameter 0.8mm, purging path is spiral, pitch 5mm) to eliminate interlayer gaps (gap ≤2μm, verified by ultrasonic testing).

[0031] The pressing process is divided into two stages, with the mold temperature maintained at 60±5℃ throughout the process, and a pressure-displacement dual closed-loop control mode is adopted.

[0032] Synchronous pressing: After single-gradient layer filling, pre-pressing is performed using a medium-low synchronous step. The upper and lower punches simultaneously apply pressure at a loading rate of 5-10 MPa / s (loading rate fluctuation ≤ ±10%), increasing the pressure to 10-20 MPa, and holding the pressure for 30-60 seconds (pressure fluctuation ≤ ±2 MPa, displacement change ≤ 0.01 mm during holding, with real-time feedback using piezoelectric pressure sensors and grating displacement sensors), ensuring initial interlayer bonding (density difference at the bonding surface ≤ 0.1 g / cm³). 3 (Detected by a micro density meter)

[0033] Asynchronous pressing: The pressure loading rate is dynamically adjusted according to the powder particle size (8-12MPa / s for fine powder layer, 3-5MPa / s for coarse powder layer), (fine powder) is pressurized to 120-150MPa, (medium-sized powder) is pressurized to 90-120MPa, (coarse powder) is pressurized to 50-80MPa. The pressure difference is compensated in real time by a servo system with an accuracy of 0.5MPa / step (compensation response time ≤5ms, compensation trigger condition is pressure difference between adjacent layers ≥3MPa). The pressing speed is controlled at 0.5-5mm / s (fine powder layer speed 1-2mm / s, coarse powder layer speed 3-5mm / s, speed fluctuation rate ≤±5%). The holding time is 40-80s (the larger the particle size, the longer the holding time. For every 10μm increase in particle size, the holding time is extended by 10s. During the holding period, a constant temperature control system is used to maintain the mold temperature stability).

[0034] Then, depressurize: A segmented depressurization mode is used. First, depressurize at 10-15 MPa / s to 50% of the set pressure (depressurization pause for 5 seconds, monitoring billet rebound ≤0.02 mm). Then, slowly depressurize at 3-5 MPa / s to atmospheric pressure. During depressurization, the punch should follow the billet rebound at a speed ≤0.1 mm / s to prevent billet cracking (rebound rate ≤0.8%, calculated through dimensional measurements). The billet density is controlled at 1.8-2.2 g / cm³. 3 (Density difference between layers ≤ 0.15 g / cm³) 3 (Measured using the water displacement method).

[0035] Auxiliary control: Vacuum assistance (vacuum degree -0.08--0.09MPa) is used during the pressing process to remove residual air in the powder. The vacuum holding time is synchronized with the powder filling time. After the powder filling is completed, the vacuum is broken and pressing is carried out again.

[0036] (3) Sintering process control, specifically including the following processes:

[0037] Furnace preparation: Place the pressed green body into a graphite crucible (purity ≥ 99.9%, density ≥ 1.8 g / cm³). 3 In the process, the distance between the billet and the crucible wall is ≥10mm, the distance between billets is ≥8mm, a 5-10mm thick layer of silicon carbide fine powder (D50=1-3μm, purity ≥99.9%) is laid on the bottom and around the crucible, and the gap between the crucible lid and the crucible is ≤2mm to reduce the influence of atmosphere convection during sintering.

[0038] Dewaxing stage (using a three-stage heating dewaxing method to precisely control the binder decomposition rate):

[0039] Room temperature → 200℃: heating rate 5℃ / min, hold for 30min (removal of free water, dehydration amount ≤0.3wt%).

[0040] 200℃→400℃: heating rate 5℃ / min, hold for 1h (decomposition of PVA oligomers, decomposition gas emission rate ≤0.5L / min);

[0041] 400℃→600℃: Heating rate 5℃ / min, hold for 2h (completely remove residual binder, residual carbon content ≤0.1wt%).

[0042] The argon flow rate is 5-10 L / min throughout the process (flow rate fluctuation ≤ ±0.5 L / min), the furnace pressure is maintained at atmospheric pressure (pressure fluctuation ≤ ±0.01 MPa), and the oxygen content in the furnace atmosphere is ≤10 ppm to avoid gas retention caused by binder decomposition leading to billet bubbling (bubbling rate 0).

[0043] Multi-stage precision temperature-controlled sintering (temperature control accuracy ±1℃, pressure control accuracy ±0.005MPa):

[0044] ① Low-temperature densification stage (1000-1600℃):

[0045] 1000-1300℃: heating rate 80℃ / h (rate fluctuation ≤ ±5℃ / h), argon flow rate 10-15L / min, furnace pressure 0.1MPa (pressure kept constant), promoting the formation of solid solution between sintering aid B4C and C (solid solution formation rate ≥ 95%).

[0046] 1300-1600℃: heating rate 40℃ / h, argon flow rate 15-20L / min (flow rate increases linearly with temperature), furnace pressure increases linearly from 0.1MPa to 0.2MPa (pressure increase rate 0.033MPa / h), accelerating SiC particle surface diffusion and initially forming neck bonding (neck diameter ≥ 15% of particle diameter).

[0047] ②Medium-temperature grain growth stage (1600-2100℃):

[0048] 1600-1800℃: heating rate 30℃ / h, argon flow rate 20-25L / min, furnace pressure linearly increased from 0.2MPa to 0.25MPa (pressure increase rate 0.025MPa / h), controlling slow grain growth (growth rate ≤0.05μm / h).

[0049] 1800-2100℃: Heating rate 20℃ / h, argon flow rate 25-30L / min, furnace pressure maintained at 0.3MPa (pressure fluctuation ≤ ±0.01MPa), holding for 1-2h (precisely adjusted according to substrate thickness: 0.5mm thickness, holding for 1h; 1mm thickness, holding for 1.5h; 2-3mm thickness, holding for 2h), ensuring that layers of different particle sizes complete sintering and densification simultaneously (difference in sintering and densification rate between layers ≤ 10%), controlling grain size to 1-3μm (fine powder layer grains 1-2μm, coarse powder layer grains 2-3μm, grain size variation coefficient ≤ 15%).

[0050] During the heat preservation period, the furnace temperature, pressure and atmosphere composition were recorded every 30 minutes, and the oxygen content was monitored in real time and maintained at ≤5ppm.

[0051] ③ Gradient cooling stage (to avoid cracking caused by thermal shock):

[0052] 2100℃→1500℃: cooling rate 5℃ / min (rate fluctuation ≤±0.5℃ / min), argon flow rate 20-25L / min, furnace pressure linearly decreases from 0.3MPa to 0.2MPa (pressure reduction rate 0.017MPa / min).

[0053] 1500℃→1200℃: cooling rate 5℃ / min, argon flow rate 15-20L / min, furnace pressure maintained at 0.2MPa;

[0054] 1200℃→800℃: cooling rate 2℃ / min, argon flow rate 10-15L / min, furnace pressure linearly decreases from 0.2MPa to 0.15MPa (pressure reduction rate 0.001MPa / min).

[0055] 800℃→room temperature: cooling rate 3℃ / min, argon flow rate 5-10L / min, furnace pressure linearly reduced from 0.15MPa to atmospheric pressure (pressure reduction rate 0.0008MPa / min).

[0056] The surface temperature of the billet was monitored in real time using an infrared thermometer (temperature accuracy ±2℃), and the residual stress was monitored online using an X-ray diffractometer. The residual stress was eventually reduced to below 45MPa (stress distribution uniformity ≤ ±5MPa).

[0057] Atmosphere control: Argon purity ≥ 99.999%, oxygen content ≤ 5 ppm, moisture content ≤ 3 ppm, to avoid SiC oxidation during sintering to generate SiO2 (oxide layer thickness ≤ 0.5 μm) which affects the pore structure and interfacial bonding with Al;

[0058] (3) Post-processing:

[0059] After sintering, the substrate is finely ground using diamond grinding wheels (1000# grit, resin binder) at a speed of 30-50 m / s (40-50 m / s for fine powder and 30-40 m / s for coarse powder) and a feed rate of 5-10 μm / pass (5 μm for the first pass and increasing to 10 μm thereafter) to remove the surface oxide layer (thickness ≤ 5 μm). The final substrate thickness is controlled at 0.5-3 mm with a thickness tolerance of ±0.02 mm (thickness uniformity ≤ ±0.01 mm).

[0060] Ultrasonic cleaning: Deionized water (conductivity ≤10μS / cm) is used as the cleaning medium, with a frequency of 40kHz (power density 0.5W / cm²). 2 The cleaning time is 15-20 minutes (divided into two cleaning sessions, 10 minutes each, with water changed once in between), to remove surface grinding dust (dust residue ≤0.01mg / cm³). 2 );

[0061] Drying: A hot air circulating oven is used, with a drying temperature of 120℃ (temperature fluctuation ≤ ±5℃), and the temperature is maintained for 30 minutes. After drying, the moisture content of the substrate is ≤ 0.1wt%.

[0062] Performance testing:

[0063] 1. Sample preparation: Based on the preparation parameters of Example 1, the following sample was prepared:

[0064] S1 sample: top layer D50=0.5μm, middle layer 1 D50=2.0μm, middle layer 2 D50=12μm, bottom layer D50=40μm, particle size ratio of adjacent layers 1:4, submicron-sized powder content 30wt%;

[0065] S2 sample: top layer D50=0.3μm, middle layer 1 D50=1.5μm, middle layer 2 D50=10μm, bottom layer D50=30μm, particle size ratio of adjacent layers 1:5, submicron powder content 30wt%;

[0066] S3 sample: top layer D50=0.7μm, middle layer 1 D50=2.5μm, middle layer 2 D50=15μm, bottom layer D50=50μm, particle size ratio of adjacent layers 1:3.6, submicron powder content 30wt%.

[0067] 2. Performance testing methods:

[0068] Porosity testing: Archimedes' drainage method (GB / T 25995-2010) was used.

[0069] Compressive strength test: A universal testing machine (Instron 5592) was used, with a sample size of 10mm×10mm×10mm and a loading rate of 0.5mm / min (refer to the test standard in the PMC literature).

[0070] Thermal conductivity test: Laser flash method (GB / T 10297-2015) was used, and the test temperature was 25℃;

[0071] Impregnation and filling rate test: After pressure impregnation of aluminum alloy (Al-7Si-2Mg), the aluminum alloy filling area was observed with a metallographic microscope and the proportion of the pore area to the aluminum alloy filling area was calculated.

[0072] Surface roughness test: The Ra value of the top surface is tested using a roughness meter.

[0073] 3. Performance test results:

[0074] The performance test results of the three groups of samples were taken as the average value of parallel samples, and the specific data are shown in Table 1:

[0075] Table 1

[0076] Sample number Top layer porosity Bottom porosity Compressive strength (MPa) Thermal conductivity (W / (m·K)) Impregnation and filling rate (%) Top surface Ra (μm) S1 50% 28% 22.5±1.2 85.3±3.1 98.2±0.5 0.21±0.03 S2 55% 30% 20.3±0.9 82.1±2.8 97.8±0.6 0.18±0.02 S3 45% 25% 24.7±1.5 88.6±3.5 96.5±0.8 0.23±0.04 average value 50% 27.7% 22.5 85.3 97.5 0.21

[0077] 4. Performance Comparison:

[0078] Comparative Example 1: Silicon carbide substrate produced by single-porosity powder metallurgy (Purchaser: Sansheng Technology (Shenzhen) Co., Ltd., Model: SSiC-P-20 (Pressureless Sintered Porous SiC Substrate))

[0079] Comparative Example 2: Silicon carbide substrate with disordered pore gradient sol-gel method (Purchaser: Pingxiang Zhongyuan Ceramics Co., Ltd., Model: IKTS-SiC-Grad)

[0080] Table 2 shows the comparison data of sample S1 and comparative examples 1-2 in terms of core parameters and performance indicators:

[0081] Table 2

[0082] Comparison Projects S1 Comparative Example 1 (Single Porosity) Comparative Example 2 (Disordered Pore Gradient) Advantages Pore ​​structure type Continuous stepped porosity (particle size ratio 1:3-1:5) Single pore size (D50=20μm) Disordered pore gradient (no fixed particle size ratio) Pore ​​gradient is controllable, adaptable to infiltration requirements. Submicron powder ratio 30wt% 0wt% 15wt% Optimize pore connectivity and reduce surface roughness Compressive strength (MPa) 22.5±1.2 15.8±1.0 7.0±0.8 Strength increased by 42.4% (compared to Comparative Example 1) Thermal conductivity (W / (m·K)) 85.3±3.1 62.5±2.5 48.2±3.0 Thermal conductivity improved by 36.5% (compared to Comparative Example 1) Impregnation and filling rate (%) 97.5±0.7 82.3±1.2 75.6±1.5 The fill rate was increased by 18.5% (compared to Comparative Example 1). Maximum thickness of a single layer (mm) ＞3.5 2 (Prone to cracking) 3 (Uneven filling) Higher preparation efficiency and more uniform filling

[0083] Compared with existing technologies, the embodiments of the present invention have the following core advantages:

[0084] By employing a continuously controllable stepped pore structure, combined with precise particle size distribution and process parameters, the impregnation and filling rate is increased to 97.5%, significantly better than the 75.6%-82.3% of existing technologies.

[0085] The compressive strength and thermal conductivity are significantly improved, by 42.4% and 36.5% respectively compared to Comparative Example 1, meeting the performance requirements of high-end metal matrix composite substrates;

[0086] With a maximum single-layer thickness of >3.5mm, it balances preparation efficiency and structural stability, solving the problems of low preparation efficiency caused by excessively small single-layer thickness and uneven filling caused by excessively large single-layer thickness in existing technologies.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a silicon carbide substrate with stepped porosity, characterized in that, Includes the following steps: S1. Powder Proportioning and Pretreatment: Select 3-5 kinds of silicon carbide powder with a particle size gradient distribution, covering a particle size range of 0.3-50μm; the particle size of each layer of powder increases from top to bottom, and the particle size ratio of adjacent layers of powder is 1:2 to 1:4.

5. Add 0.2-0.6wt% B4C, 1-3wt% amorphous carbon and 2-5wt% PVA binder to each layer of powder respectively. After ball milling and spray granulation, obtain a spherical particle mixed powder with a flowability ≥15g / min. S2. Layered filling and composite pressing: In a mold preheated to 50-80℃, powders of different particle sizes are filled in layers multiple times, with a single filling thickness of 0.2-0.5mm. After filling, nitrogen is purged under a vacuum of -0.08-0.09MPa. After single-gradient layer filling, medium-low synchronous step pressing is performed for pre-pressing, and then the next layer of powder is filled, i.e., synchronous pressing. After the overall filling is completed, asynchronous pressing is used to press the billet into the required size and shape, and then the pressure is released in stages. S3. Sintering process control: including dewaxing stage and multi-stage precise temperature control sintering stage. The dewaxing stage adopts a three-stage heating method, which is held at 200℃, 400℃ and 600℃ respectively, and is carried out under argon protection throughout the process. The multi-stage precise temperature control sintering stage includes: (1) Low temperature densification stage 1000-1600℃: heating rate 40-80℃ / h, furnace pressure linearly increased from 0.1MPa to 0.2MPa; (2) Medium temperature grain growth stage 1600-2100℃: heating rate 20-30℃ / h, furnace pressure increased from 0.2MPa to 0.3MPa, and held at 1600-2100℃ for 1-2h; (3) Gradient cooling stage: from 2100℃ to room temperature, the cooling rate and furnace pressure are controlled in stages according to different temperature ranges, and finally cooled to room temperature. S4. Post-processing: The sintered substrate is finely ground, ultrasonically cleaned and dried to obtain the final product.

2. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S1, the average particle size D50 of each layer of powder, from top to bottom, ranges as follows: top layer 0.3-7μm, middle layer 10-30μm, bottom layer 30-50μm.

3. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S1, the ball mill rotates at a speed of 300-400 r / min for 2-4 hours.

4. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S1, the inlet air temperature of the spray granulation is 180-220℃, the outlet air temperature is 80-100℃, the atomization pressure is 0.3-0.5MPa, and the particle size of the mixture is 30-80μm.

5. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S2, the two-stage suppression includes: Synchronous pressing: After single-gradient powder filling, the upper and lower punches are simultaneously pressurized at a rate of 5-10MPa / s to 10-20MPa and held at pressure for 30-60s; Asynchronous pressing: The pressure loading rate is dynamically adjusted according to the powder particle size. For silicon carbide particles with a minimum particle size of <10μm, the pressure is increased to 120-150MPa; for 10-30μm, the pressure is increased to 90-120MPa; and for 30-50μm, the pressure is increased to 50-80MPa. During the pressing process, the pressure difference between adjacent layers with a pressure difference of ≥3MPa is compensated in real time. The entire pressing process adopts a pressure-displacement dual closed-loop control mode and is maintained at 60±5℃.

6. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S2, the segmented pressure relief is implemented after the asynchronous compression process. Specifically, it involves first depressurizing at 10-15 MPa / s to 50% of the set pressure, and then slowly depressurizing at 3-5 MPa / s to atmospheric pressure.

7. The method for preparing a stepped porosity silicon carbide substrate according to claim 1, characterized in that, In step S3, the heating rate during the dewaxing stage is 1-5℃ / min.

8. A silicon carbide substrate with stepped porosity, characterized in that, It is prepared by any one of the preparation methods described in claims 1-7. The silicon carbide substrate has at least three layers, and its porosity is distributed in a stepped manner in the thickness direction. The porosity of the top layer is 35-40%, the porosity of the middle layer is 30-35%, and the porosity of the bottom layer is 25-30%.

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