High-performance silicon-carbon rod and preparation method thereof

Abstract

The application provides a high-performance silicon-carbon rod and a preparation method thereof. The silicon-carbon rod adopts a hot-end embryo, a transition-zone embryo and a cold-end embryo to form an integrated structure through cold isostatic pressing and sintering, and a Y2O3 / SiO2 composite coating is arranged on the surface. The hot-end embryo is composed of silicon carbide particles with different particle sizes, carbon black and Y2O3 powder. The transition-zone embryo is a five-layer SiC and nano-ZrO2 gradient composite structure. The cold-end embryo is SiC and nano-Al2O3 composite powder. The preparation process comprises segmented embryo preparation, integrated forming, sintering and double-layer coating preparation. Through material and structure optimization, the service life of the silicon-carbon rod in air is increased from 2000-3000 hours to more than 5000 hours, the thermal shock resistance cycle performance is increased by more than 75%, the preparation process is compatible with the existing production line, the cost increase is less than 30%, and the silicon-carbon rod has excellent comprehensive performance and market competitiveness.

Description

Technical Field

[0001] This invention relates to the field of silicon carbide rod preparation technology, and in particular to a high-performance silicon carbide rod and its preparation method. Background Technology

[0002] Silicon carbide rods, as a type of high-temperature electric heating element with silicon carbide as the main material, have significant advantages such as fast heating rate, strong oxidation resistance, high thermal efficiency, and simple operation. They are widely used in high-temperature industrial fields such as metal smelting, glass manufacturing, waste gas treatment, and ceramic sintering.

[0003] However, existing silicon carbide rods still face several technical bottlenecks during long-term high-temperature use: First, regarding high-temperature oxidation, when heated to above 1000℃ in air, the SiC surface oxidizes to form a SiO2 protective layer. However, in high-temperature environments above 1550℃, this protective layer fails due to excessive volatilization, leading to a continuous reduction in the effective cross-sectional area of ​​the element and uncontrollable drift in resistance. Second, regarding thermal stability, frequent start-ups and shutdowns or rapid temperature changes (heating rate > 10℃ / min) can cause internal thermal stress concentration, especially in the transition area between hot and cold ends, where the mismatch in the thermal expansion coefficients of the materials can easily lead to cracking. Third, regarding material structure, long-term high-temperature operation can cause abnormal growth and phase transformation of SiC grains, resulting in deterioration of the material's microstructure and a 30%-50% decrease in mechanical strength. Furthermore, regarding electrical performance, due to oxidation erosion and grain boundary evolution, the conductive network between SiC grains may experience local interruptions, causing abnormal fluctuations in resistance and affecting the stability of heating power output and temperature control accuracy. These factors combined result in the actual service life of conventional silicon carbide rods in air typically being only 2,000-3,000 hours, and even shorter to less than 1,000 hours in corrosive atmospheres.

[0004] Currently, most improvement methods in the industry focus on optimizing single anti-oxidation coatings or conventional sintering processes. While these methods can temporarily alleviate specific problems, they are difficult to achieve a systematic breakthrough from the perspective of material system design and structural optimization. They cannot simultaneously solve multiple technical challenges such as high-temperature oxidation, thermal shock failure, grain coarsening, and electrical resistance stability. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a high-performance silicon carbide rod and its preparation method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first aspect is to provide a high-performance silicon carbide rod, including a hot-end preform, a transition zone preform, and a cold-end preform. The hot-end preform, the transition zone preform, and the cold-end preform are formed into an integral structure by cold isostatic pressing and sintering, and the surface of the silicon carbide rod is provided with a Y2O3 / SiO2 composite coating.

[0008] The hot-end preform is composed of the following components by mass percentage: 50-60% coarse silicon carbide particles, 20-30% medium silicon carbide particles, 10-20% fine silicon carbide particles, 2-5% carbon black, and 0.3-1.0% Y2O3 powder.

[0009] The transition zone embryo is a five-layer gradient composite structure, with each layer consisting of SiC powder and nano ZrO2 powder by mass percentage.

[0010] The cold-end preform is composed of SiC powder and nano-Al2O3 powder, wherein the mass fraction of nano-Al2O3 in the composite powder is 3-8%;

[0011] The Y2O3 / SiO2 composite coating comprises an inner Y2O3 coating and an outer SiO2 coating, with a total thickness of 15-17 μm.

[0012] Furthermore, the five-layer gradient composite structure of the transition region embryo is specifically as follows: the first layer is 100% SiC powder, the second layer is 95% SiC powder and 5% nano ZrO2 powder, the third layer is 90% SiC powder and 10% nano ZrO2 powder, the fourth layer is 85% SiC powder and 15% nano ZrO2 powder, and the fifth layer is 80% SiC powder and 20% nano ZrO2 powder.

[0013] Furthermore, the amount of Y2O3 powder added to the hot-end preform is 0.5 wt.%.

[0014] Furthermore, the amount of nano-Al2O3 added to the cold-end preform is 4.5 wt.%.

[0015] The second aspect is to provide a method for preparing the aforementioned high-performance silicon carbide rod, including the following steps:

[0016] Step 1, Hot end preform preparation: Weigh the powder according to the mass percentage, including coarse silicon carbide particles, medium silicon carbide particles, fine silicon carbide particles, carbon black and Y2O3 powder, mix the powder with anhydrous ethanol, ball mill for 4-8 hours to form a uniform slurry, fill it into the hot end area of ​​the mold, and dry at room temperature to form the hot end preform segment.

[0017] Step 2, Transition zone green body preparation: Five gradient composite layers, each layer of powder is mixed with anhydrous ethanol, ball milled and then formed into green ceramic tape with a thickness of 0.5-1.0 mm by a casting machine. The layers are stacked in sequence and then subjected to a pressure of 5-10 MPa and a temperature of 80-100℃ in a hot press for 10-20 minutes to form the transition zone green body segment.

[0018] Step 3, Cold end preform preparation: Mix nano Al2O3 powder with anhydrous ethanol, sonicate for 20-30 minutes to prepare slurry, then mix with SiC powder, and make the mass fraction of nano Al2O3 in the composite powder 3-8%. After ball milling for 4-8 hours, fill the cold end area of ​​the mold and dry at room temperature to form the cold end preform.

[0019] Step four: Forming the entire embryo;

[0020] Step 5: Sintering and processing;

[0021] Step 6, Coating Preparation:

[0022] The silicon carbide rod was immersed in Y2O3 sol at a speed of 1-3 mm / s, held for 30-60 seconds and then pulled out. It was gelled at room temperature and kept at 400℃ for 1 hour to obtain an amorphous Y2O3 coating with a thickness of 5-7 μm.

[0023] On a silicon carbide rod already coated with Y2O3, a SiO2 coating is pulled over at a speed of 1-2 mm / s. After gelation at room temperature, the temperature is raised to 1000-1300℃ in air at 4℃ / min and held for 1-2 hours to allow Y2O3 to crystallize into a cubic phase and SiO2 to sinter into a dense protective layer. The total coating thickness is 15-17 μm.

[0024] Step seven, final annealing.

[0025] Furthermore, the integral preform forming process specifically involves: placing the hot-end preform segment, the transition zone preform segment, and the cold-end preform segment into the mold in sequence, applying a pressure of 130-150 MPa using cold isostatic pressing, and holding the pressure for 5-10 minutes to form a rod-shaped integral preform.

[0026] Furthermore, the sintering and processing process specifically involves: placing the entire green blank into a spark plasma sintering furnace, holding it at 1700-1850℃ for 5-15 minutes under vacuum or inert atmosphere, cooling it to room temperature after sintering to obtain a silicon carbide rod blank, and then cleaning and grinding it.

[0027] Furthermore, the preparation method of the Y2O3 sol is as follows: Yttrium isopropoxide is dissolved in anhydrous ethanol at a concentration of 0.5 mol / L, and magnetically stirred until completely dissolved. An ethanol solution containing an inhibitor and deionized water is slowly added dropwise, and stirring is continued for 2-4 hours to obtain the Y2O3 sol. The inhibitor is acetylacetone with a volume fraction of 1-2%.

[0028] Furthermore, the SiO2 sol is prepared by mixing tetraethyl orthosilicate, ethanol, deionized water and catalyst in a molar ratio of 1:10:2:0.01 and stirring under reflux at 60-70°C for 2-4 hours to form a stable SiO2 sol; the catalyst is hydrochloric acid.

[0029] Furthermore, the final annealing specifically involves heating the furnace in air at a rate of 2-5°C / min to 800-1000°C, holding the temperature for 1-2 hours, and then cooling it to room temperature in the furnace.

[0030] The present invention adopts the above technical solution and has the following technical effects compared with the prior art:

[0031] This invention, through the synergistic effect of optimizing the preform formulation, constructing a gradient transition structure, and applying a double-layer protective coating, successfully extended the service life of silicon carbide rods in air from the conventional 2000-3000 hours to over 5000 hours, an increase of more than 100%. It effectively alleviated the thermal stress concentration caused by rapid temperature changes, and improved the cycle life of silicon carbide rods under harsh thermal shock test conditions by more than 75% compared with traditional products.

[0032] The preparation process used in this invention is highly compatible with existing silicon carbide rod production lines, enabling industrial-scale production without large-scale equipment modifications. Although the introduction of high-performance materials and special coating processes increases costs, the overall cost increase is controlled within 30%, far lower than the performance improvement, resulting in excellent cost-effectiveness and market competitiveness. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments, but these are not intended to limit the invention. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0034] Experimental methods in the following examples, unless otherwise specified, were performed under standard conditions or as recommended by the manufacturer. Unless otherwise stated, all reagents and materials used in the following examples were commercially available.

[0035] Example 1

[0036] This embodiment provides a method for preparing high-performance silicon carbide rods. By optimizing the design of the preform composition (including the hot end, transition zone and cold end), adopting a gradient composite structure and double-layer coating treatment, the thermal stress of the silicon carbide rods is relieved, densification is improved and surface protection is achieved, thereby extending their service life.

[0037] 1. Materials

[0038] Silicon carbide particles: coarse particles (~100μm), medium particles (~20μm), fine particles (<5μm), purity ≥99%;

[0039] Carbon black: purity ≥99%, used to react with SiO2 on the SiC surface during sintering to generate secondary SiC;

[0040] Y2O3 powder: purity ≥ 99.9%;

[0041] Nano ZrO2 powder: purity ≥99.9%, used in gradient layers in transition regions;

[0042] Nano alumina powder: purity ≥99.9%, used for cold-end preforms;

[0043] Anhydrous ethanol: analytical grade, used as a dispersion medium and solvent;

[0044] Yttrium isopropoxide: chemically pure, used in the preparation of Y2O3 sol;

[0045] Tetraethyl orthosilicate: chemically pure, used in the preparation of SiO2 sol;

[0046] Deionized water: used in sol preparation;

[0047] Catalysts: such as hydrochloric acid or ammonia (used for the hydrolysis of SiO2 sol);

[0048] Inhibitors: such as acetylacetone (used for stabilizing Y2O3 sol).

[0049] 2. Specific preparation steps of high-performance silicon carbide rods

[0050] 2.1 Fabrication of hot-end preform

[0051] Weigh the following powders by mass percentage: 50-60% coarse silicon carbide particles, 20-30% medium silicon carbide particles, 10-20% fine silicon carbide particles, 2-5% carbon black, and 0.3-1.0% Y2O3 powder. Place the powders in a ball mill, add anhydrous ethanol (powder to ethanol mass ratio of 1:1), and ball mill for 4-8 hours to form a uniform slurry. Fill the slurry into the "hot end" area of ​​the mold, and pre-dry it at room temperature to remove some of the solvent, forming a hot end green segment.

[0052] 3.2 Preparation of embryos in the transition zone

[0053] The design incorporates five gradient composite layers, with each layer containing the following powder composition:

[0054] First layer: 100% SiC powder;

[0055] Second layer: 95% SiC + 5% nano ZrO2 powder;

[0056] Third layer: 90% SiC + 10% nano ZrO2 powder;

[0057] Fourth layer: 85% SiC + 15% nano ZrO2 powder;

[0058] Fifth layer: 80% SiC + 20% nano ZrO2 powder;

[0059] Each layer of powder is mixed with anhydrous ethanol and ball-milled until uniform. The mixture is then formed into a green ceramic tape (thickness controlled at 0.5-1.0 mm) using a casting machine. The green ceramic tapes are then stacked in sequence (from the first to the fifth layer). Pressure (5-10 MPa) and temperature (80-100℃) are applied in a hot press and kept at that temperature for 10-20 minutes to ensure that the layers are tightly bonded together, forming a transition zone green segment.

[0060] 3.3 Cold-end preform fabrication

[0061] Nano-Al2O3 powder and anhydrous ethanol are mixed at a mass ratio of 1:1 and subjected to high-intensity ultrasonic treatment for 20–30 minutes to prepare a uniformly dispersed slurry. The slurry is then mixed with SiC powder at a mass ratio of approximately (32:1) to (11.5:1) (to ensure that the final mass fraction of Al2O3 in the composite powder is 3–8%). The mixture is placed in a ball milling jar made of SiC material and ball-milled for 4–8 hours at a ball-to-powder ratio of 5:1 to 10:1. The ball-milled slurry is then filled into the "cold end" area of ​​the mold and dried at room temperature to form a cold end green segment.

[0062] 3.4 Integral green embryo forming

[0063] Place the hot end green section, the transition zone green section, and the cold end green section into the mold in a precise sequence (the interfaces between the hot end and the transition zone, and between the transition zone and the cold end must be in close contact); apply a pressure of 150 MPa using cold isostatic pressing and hold the pressure for 5-10 minutes to form a dense rod-shaped integral green section (the diameter can be determined according to the mold design, for example, 20-50 mm).

[0064] 3.5 Sintering and Processing

[0065] The above-mentioned rod-shaped green blank is placed in a spark plasma sintering furnace and held at 1700-1850℃ for 5-15 minutes under vacuum or inert atmosphere. After sintering, it is naturally cooled to room temperature, and the silicon carbide rod blank is taken out. The blank is then cleaned and ground.

[0066] 3.6 Coating Preparation

[0067] Preparation of Y2O3 sol: Dissolve yttrium isopropoxide in anhydrous ethanol (concentration 0.5 mol / L), stir magnetically until completely dissolved, slowly add an ethanol solution containing an inhibitor (such as acetylacetone, volume fraction 1-2%) and a small amount of deionized water (water to yttrium isopropoxide molar ratio 1:1), and continue stirring for 2-4 hours to obtain a clear and transparent Y2O3 sol.

[0068] Preparation of the inner Y2O3 coating: Immerse the silicon carbide rod into the Y2O3 sol at a constant speed (1-3 mm / s), hold for 30-60 seconds, and then pull it up at the same speed; place the coated workpiece at room temperature to allow the solvent to evaporate and the sol to transform into a gel; then keep it at 400℃ for 1 hour to remove organic matter and obtain an amorphous Y2O3 coating (thickness of about 5-7 μm).

[0069] Preparation of SiO2 sol: Mix tetraethyl orthosilicate, ethanol, deionized water and catalyst (such as hydrochloric acid, pH adjusted to 2-3) in a molar ratio of 1:10:2:0.01, and reflux and stir at 60-70℃ for 2-4 hours to form a stable sol.

[0070] Preparation of outer SiO2 coating: On the silicon carbide rod coated with Y2O3, SiO2 coating is applied by pulling method (speed 1-2 mm / s) and gelled at room temperature; the coated silicon carbide rod is heated to 1000-1300℃ in air at a heating rate of 4℃ / min and held for 1-2 hours to allow the amorphous Y2O3 to crystallize into a dense cubic phase and the SiO2 gel to sinter into a dense protective layer (total coating thickness is about 15-17μm).

[0071] 3.7 Final Annealing

[0072] Heat the furnace to 800-1000℃ in air at a rate of 2-5℃ / min, hold for 1-2 hours, and then cool it to room temperature in the furnace.

[0073] In subsequent embodiments, in order to test the performance of the silicon carbide rods prepared by the present invention, the key performance indicators and testing methods include:

[0074] (1) Bending strength test

[0075] Test objective: To verify the mechanical load-bearing capacity of the material under working conditions at room temperature and high temperature, and to evaluate its strength retention rate.

[0076] Temperature: Ambient temperature (25°C, as a baseline) and 1400°C (simulating common operating temperature).

[0077] Time: Keep warm at 1400°C for 10-30 minutes to ensure uniform temperature inside and outside the sample.

[0078] Environment: Vacuum, to avoid high-temperature oxidation affecting the sample surface and to ensure the authenticity of the strength test results.

[0079] Failure Criteria:

[0080] Absolute strength value: The measured fracture strength (MPa).

[0081] Strength retention rate: (high temperature strength / room temperature strength) × 100%.

[0082] (2) Thermal shock life test

[0083] Test objective: To verify the durability of gradient structure design and material toughness under drastic temperature changes and to simulate equipment start-up and shutdown conditions.

[0084] Hot end temperature: 1550°C (90% of the rated maximum temperature of silicon carbide rod 1700°C).

[0085] Cold end temperature: room temperature (25°C), or use compressed air cooling to create more demanding conditions.

[0086] Heating: Keep warm in a high-temperature furnace for 5-15 minutes.

[0087] Cooling: Quickly transfer to room temperature environment, complete the transfer within ≤30 seconds, and record the total time to cool to room temperature (e.g., 60 seconds).

[0088] Environment: Static air. This is the most demanding condition because oxidation during the cooling process will exacerbate the damage.

[0089] Failure Criteria:

[0090] Macroscopic failure: Cracks visible to the naked eye or under a magnifying glass appear at the hot end or transition zone.

[0091] Microscopic / performance failure: Measure the resistance at room temperature after every 5-10 cycles. Failure is defined as a resistance change rate >10%.

[0092] (3) Oxidation mass loss test

[0093] Test objective: To directly verify the long-term antioxidant protection capability of the Y2O3+SiO2 double-layer coating at high temperatures.

[0094] Temperature: 1550°C (select the temperature for accelerated aging).

[0095] Time: Long-term constant temperature test (100, 200, 500 hours). Periodically (e.g., every 24 / 48 hours), remove and cool, weigh, and plot the "mass change - time" curve.

[0096] Dry air: standard oxidation conditions.

[0097] Moist oxygen (such as O2 + 10% H2O): a more severe condition, water vapor will accelerate the destruction of the SiO2 protective layer.

[0098] Failure Criteria:

[0099] Mass loss per unit area (Δm / S): Calculate the final mass loss per unit surface area (mg / cm²). Ideally, there would be a slight weight gain (formation of protective oxidation), but we are more concerned with slow mass loss (coating evaporation).

[0100] Oxidation rate: The oxidation rate is calculated based on the steady-state phase of the mass change curve.

[0101] (4) Service life test:

[0102] Test objective: To comprehensively evaluate the durability and ultimate lifespan of silicon carbide rods under conditions closest to actual use.

[0103] Temperature: Operating above the rated temperature, for example, making the bar surface temperature 50-100°C higher than the maximum operating temperature (for bars rated at 1600°C, test at 1650-1700°C).

[0104] Cyclic mode: An intermittent cycle of "power-on heating → heat preservation → power-off cooling" is adopted (e.g., power-on for 2 hours, power-off for 0.5 hours). This thermal cycle is more demanding than continuous heating and can accelerate the exposure of defects.

[0105] Environment: Air.

[0106] Failure Criteria:

[0107] Complete failure: Silicon carbide rod melts.

[0108] Performance failure: Periodically measure its cold resistance. When the resistance increase is >20%, the life is considered to have ended (because excessive resistance will cause power runaway and fail to meet process requirements).

[0109] Example 2: Preparation method of high-performance silicon carbide rods with optimized Y2O3 addition amount

[0110] Based on Example 1, this embodiment optimizes the amount of Y2O3 added to the hot-end preform and systematically studies the effects of different Y2O3 contents (0.3wt.%, 0.5wt.%, 0.7wt.%, 1.0wt.%) on the high-temperature stability, mechanical properties and oxidation resistance of silicon carbide rods, and determines the optimal addition amount to further improve product life and reliability.

[0111] The preparation steps in this embodiment are basically the same as those in Example 1, except that the amount of Y2O3 added in the hot end preform preparation is different. The other steps (transition zone preform preparation, cold end preform preparation, overall green preform forming, sintering and processing, coating preparation, and final annealing) remain unchanged.

[0112] The following focuses on the adjustments made during the fabrication of the hot-end preform:

[0113] Sample 1: 55% coarse silicon carbide particles, 25% medium silicon carbide particles, 15% fine silicon carbide particles, 4.7% carbon black, and 0.3% Y2O3 powder;

[0114] Sample 2: 55% coarse silicon carbide particles, 25% medium silicon carbide particles, 15% fine silicon carbide particles, 4.5% carbon black, and 0.5% Y2O3 powder;

[0115] Sample 3: 55% coarse silicon carbide particles, 25% medium silicon carbide particles, 15% fine silicon carbide particles, 4.3% carbon black, and 0.7% Y2O3 powder;

[0116] Sample 4: 55% coarse silicon carbide particles, 25% medium silicon carbide particles, 15% fine silicon carbide particles, 4.0% carbon black, and 1.0% Y2O3 powder.

[0117] Place the powder in a ball mill, add anhydrous ethanol (powder to ethanol mass ratio 1:1), and ball mill for 4-8 hours to form a uniform slurry. Fill the slurry into the "hot end" area of ​​the mold and allow it to dry initially at room temperature to form a hot end green segment.

[0118] Performance tests were conducted on silicon carbide rod samples with four Y₂O₃ additions (0.3 wt.%, 0.5 wt.%, 0.7 wt.%, and 1.0 wt.%). The results show that Y, as a typical grain boundary strengthening dopant, has a significant impact on performance. The optimal addition of 0.5 wt.% resulted in the best overall performance, significantly enhancing the intergranular bonding of SiC grains and thus improving the overall mechanical strength and high-temperature stability of the material. Compared to the substrate material without Y₂O₃, the flexural strength of this formulation was increased by 18%, the thermal shock lifetime was increased by 10 cycles (taking a 1350℃ thermal shock test as an example), the oxidation mass loss was reduced from approximately 4.5% to about 2.0%, and the average service life was extended by 600 hours. However, when the Y₂O₃ addition was too low (0.3 wt.%), the strengthening effect was insufficient; when the addition was too high (0.7 wt.% and 1.0 wt.%), the material toughness decreased, which was detrimental to thermal shock lifetime. Therefore, the present invention determines that 0.5 wt.% is the optimal addition amount of Y2O3, and this formulation is particularly suitable for high-frequency heating and aerobic environment operation.

[0119] Example 3

[0120] Based on Example 2 (i.e., selecting: 55% coarse silicon carbide particles, 25% medium silicon carbide particles, 15% fine silicon carbide particles, 4.5% carbon black, and 0.5% Y2O3 powder), and adding 3wt.%, 4.5wt.%, 6wt.%, and 8wt.% nano-Al2O3 to the cold-end preform respectively, with other preparation steps being the same as in Example 1, high-performance silicon carbide rods were prepared, and performance tests and results analysis were performed.

[0121] Experimental results show that introducing nano-Al2O3 particles into the SiC matrix can effectively prevent crack propagation and significantly improve the fracture toughness of the material at high temperatures. The strengthening mechanism mainly includes crack deflection, bridging, and toughening effects, thereby significantly improving the material's properties. Compared with SiC materials without nano-Al2O3, the flexural strength of the material with added nano-Al2O3 increased by approximately 25%, the thermal shock life increased by 18 cycles, the oxidation mass loss was controlled within 1.8%, and the service life was extended by more than 1000 hours, making it particularly suitable for high-temperature heating equipment requiring frequent start-ups and shutdowns.

[0122] It is worth noting that the amount of nano-Al2O3 added has a significant impact on the material properties. When the addition amount is 4.5 wt.%, the overall material performance is optimal, with significantly improved high-temperature strength and hardness. However, when the addition amount is too low (e.g., 3 wt.%), the strengthening effect is insufficient; when the addition amount is too high (e.g., 6 wt.% and 8 wt.%), it will excessively disrupt the conductive pathways of SiC, leading to abnormally high resistance of the silicon carbide rod, uneven heating, and material embrittlement.

[0123] Example 4

[0124] Building upon Example 3 (where the content of nano-Al2O3 in the cold-end blank was optimized to 4.5 wt.%), this example further investigates the effect of the coating on the performance of the silicon carbide rod. Specifically, the sintered silicon carbide rod blank was treated with a Y2O3 / SiO2 composite coating, and the coating preparation steps were performed according to the method described in "3.6 Coating Preparation". The enhancing effect of the coating on oxidation resistance and service life was evaluated by comparing the performance of the coated and uncoated samples.

[0125] 4.1 Coating Preparation Process

[0126] Y2O3 sol preparation: Dissolve yttrium isopropoxide in anhydrous ethanol (concentration 0.5 mol / L), stir magnetically until completely dissolved, then slowly add an ethanol solution containing inhibitor (acetylacetone, volume fraction 1.5%) and deionized water (water to yttrium isopropoxide molar ratio 1:1), and continue stirring for 3 hours to obtain a clear and transparent Y2O3 sol.

[0127] Preparation of inner Y2O3 coating: The silicon carbide rod was immersed in Y2O3 sol at a constant speed of 2 mm / s, held for 45 seconds, and then pulled up at the same speed. The solvent evaporated at room temperature to form a gel, and then the gel was kept at 400℃ for 1 hour to obtain an amorphous Y2O3 coating (thickness of about 6 μm).

[0128] SiO2 sol preparation: Tetraethyl orthosilicate, ethanol, deionized water and catalyst (hydrochloric acid, pH adjusted to 2.5) are mixed in a molar ratio of 1:10:2:0.01 and stirred under reflux at 65°C for 3 hours to form a stable sol.

[0129] Preparation of outer SiO2 coating: On a silicon carbide rod coated with Y2O3, a SiO2 coating is applied at a pulling speed of 1.5 mm / s. After gelation at room temperature, the temperature is raised to 1200℃ in air at 4℃ / min and held for 1.5 hours to allow Y2O3 to crystallize into a dense cubic phase and SiO2 to gel and sinter into a dense protective layer (total coating thickness is about 16 μm).

[0130] 4.2 Results and Analysis

[0131] The composite coating formed by Y2O3 / SiO2 can exist stably at high temperatures, forming a dense glassy barrier that effectively prevents oxygen from diffusing into the SiC substrate. The oxidation mass loss of the double-coated sample is 1.6%, significantly lower than the 5.1% oxidation mass loss of the uncoated sample. Compared with the uncoated sample, the thermal shock life is improved by 12 cycles, and the service life is increased by more than 900 hours.

[0132] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the content of the present invention specification should be included within the protection scope of the present invention.

Claims

1. A high-performance silicon carbide rod, characterized in that, It includes a hot-end preform, a transition zone preform, and a cold-end preform. The hot-end preform, the transition zone preform, and the cold-end preform are formed into an integral structure by cold isostatic pressing and sintering, and the surface of the silicon carbide rod is provided with a Y2O3 / SiO2 composite coating. The hot-end preform is composed of the following components by mass percentage: 50-60% coarse silicon carbide particles, 20-30% medium silicon carbide particles, 10-20% fine silicon carbide particles, 2-5% carbon black, and 0.3-1.0% Y2O3 powder. The transition region preform has a five-layer gradient composite structure, with each layer composed of SiC powder and nano ZrO2 powder by mass percentage. Specifically, the five-layer gradient composite structure of the transition region preform is as follows: the first layer is 100% SiC powder, the second layer is 95% SiC powder and 5% nano ZrO2 powder, the third layer is 90% SiC powder and 10% nano ZrO2 powder, the fourth layer is 85% SiC powder and 15% nano ZrO2 powder, and the fifth layer is 80% SiC powder and 20% nano ZrO2 powder. The cold-end preform is composed of SiC powder and nano-Al2O3 powder, wherein the mass fraction of nano-Al2O3 in the composite powder is 3-8%; The Y2O3 / SiO2 composite coating comprises an inner Y2O3 coating and an outer SiO2 coating, with a total thickness of 15-17 μm.

2. The high-performance silicon carbide rod according to claim 1, characterized in that, The amount of Y2O3 powder added to the hot-end preform is 0.5 wt.%.

3. The high-performance silicon carbide rod according to claim 1, characterized in that, The amount of nano-Al2O3 added to the cold-end preform is 4.5 wt.%.

4. The method for preparing high-performance silicon carbide rods according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1, Hot end preform preparation: Weigh the powder according to the mass percentage, including coarse silicon carbide particles, medium silicon carbide particles, fine silicon carbide particles, carbon black and Y2O3 powder, mix the powder with anhydrous ethanol, ball mill for 4-8 hours to form a uniform slurry, fill it into the hot end area of ​​the mold, and dry at room temperature to form the hot end preform segment. Step 2, Transition zone green body preparation: Five gradient composite layers, each layer of powder is mixed with anhydrous ethanol, ball milled and then formed into green ceramic tape with a thickness of 0.5-1.0 mm by a casting machine. The layers are stacked in sequence and then subjected to a pressure of 5-10 MPa and a temperature of 80-100℃ in a hot press for 10-20 minutes to form the transition zone green body segment. Step 3, Cold end preform preparation: Mix nano Al2O3 powder with anhydrous ethanol, sonicate for 20-30 minutes to prepare slurry, then mix with SiC powder, and make the mass fraction of nano Al2O3 in the composite powder 3-8%. After ball milling for 4-8 hours, fill the cold end area of ​​the mold and dry at room temperature to form the cold end preform. Step four: Forming the entire embryo; Step 5: Sintering and processing; Step 6, Coating Preparation: The silicon carbide rod was immersed in Y2O3 sol at a speed of 1-3 mm / s, held for 30-60 seconds and then pulled out. It was gelled at room temperature and kept at 400℃ for 1 hour to obtain an amorphous Y2O3 coating with a thickness of 5-7 μm. On a silicon carbide rod already coated with Y2O3, a SiO2 coating is pulled over at a speed of 1-2 mm / s. After gelation at room temperature, the temperature is raised to 1000-1300℃ in air at 4℃ / min and held for 1-2 hours to allow Y2O3 to crystallize into a cubic phase and SiO2 to sinter into a dense protective layer. The total coating thickness is 15-17 μm. Step seven, final annealing.

5. The preparation method according to claim 4, characterized in that, The integral preform forming process is as follows: the hot end preform segment, the transition zone preform segment, and the cold end preform segment are placed into the mold in sequence, and a pressure of 130-150MPa is applied by cold isostatic pressing and held for 5-10 minutes to form a rod-shaped integral preform.

6. The preparation method according to claim 4, characterized in that, The sintering and processing process is as follows: the whole green blank is placed in a spark plasma sintering furnace and held at 1700-1850℃ for 5-15 minutes under vacuum or inert atmosphere. After sintering, it is cooled to room temperature to obtain silicon carbide rod blanks, which are then cleaned and ground.

7. The preparation method according to claim 4, characterized in that, The Y2O3 sol is prepared by dissolving yttrium isopropoxide in anhydrous ethanol at a concentration of 0.5 mol / L, stirring magnetically until completely dissolved, slowly adding an ethanol solution containing an inhibitor and deionized water, and continuing stirring for 2-4 hours to obtain the Y2O3 sol; the inhibitor is acetylacetone with a volume fraction of 1-2%.

8. The preparation method according to claim 4, characterized in that, The SiO2 sol is prepared by mixing tetraethyl orthosilicate, ethanol, deionized water and catalyst in a molar ratio of 1:10:2:0.01 and stirring under reflux at 60-70°C for 2-4 hours to form a stable SiO2 sol; the catalyst is hydrochloric acid.

9. The preparation method according to claim 4, characterized in that, The final annealing process specifically involves heating the furnace in air at a rate of 2-5°C / min to 800-1000°C, holding the temperature for 1-2 hours, and then cooling it to room temperature in the furnace.