A pressureless sintered silicon carbide refractory ceramic thin-walled tube and its preparation method

Abstract

This invention relates to the field of silicon carbide refractory ceramic thin-walled tube preparation technology, specifically to a pressureless sintered silicon carbide refractory ceramic thin-walled tube and its preparation method. The ceramic tube has a wall thickness of 0.5-3 mm and a bulk density ≥3.05 g / cm³ after sintering. The main phase of this invention adopts a bimodal structure of fine and coarse powder. The fine powder fills the pores to increase density, while the coarse powder inhibits abnormal grain growth. Simultaneously, nano-SiO2 binder and Si3N4 particles are introduced to pin the grain boundaries. Through the stress transfer mechanism, the room temperature flexural strength and bulk density are increased. Furthermore, a ZrO2 / SiO2 composite interface layer is formed by high-temperature pre-oxidation of hemp fiber and lotus leaf stalk mixed fiber impregnated with ethyl silicate solution. Combined with high-speed eddy current vibration, the fiber volume fraction is increased and the dispersion is reduced. This reduces the microcrack propagation rate and flexural strength fluctuation rate of the thin-walled tube under thermal cycling conditions, improves the bonding strength between the fiber and the matrix, effectively inhibits the propagation of microcracks in the thin-walled structure, and enhances the strength of the thin-walled tube.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of preparation of silicon carbide refractory ceramic thin-walled pipe, and particularly relates to a pressureless sintering silicon carbide refractory ceramic thin-walled pipe and a preparation method thereof. BACKGROUND

[0002] The pressureless sintering silicon carbide refractory ceramic thin-walled pipe is a silicon carbide ceramic tubular product prepared by a pressureless sintering process, has the characteristics of thin wall, light weight, high strength and excellent extreme environment resistance. The core value lies in solving the material failure problem under harsh working conditions such as high temperature, strong corrosion and high wear. The pressureless sintering silicon carbide refractory ceramic thin-walled pipe breaks through the cost limitation of traditional silicon carbide ceramics through the pressureless sintering process, realizes light weight and efficient heat management by combining with the thin-walled design, becomes a revolutionary solution under high temperature, strong corrosion and high wear conditions, and especially promotes the energy efficiency upgrading and service life improvement of chemical industry, new energy and high-end equipment manufacturing

[0003] For example, the application number is CN202210622212.8, the publication date is 20221014, a pressureless sintering high thermal shock resistance silicon carbide ceramic heat exchanger pipe and a processing method thereof. The material of the pressureless sintering high thermal shock resistance silicon carbide ceramic heat exchanger pipe comprises, by mass percentage, 50%-80% of silicon carbide powder, 0%-20% of zirconium oxide powder, 3-9% of carbon black, 3%-7% of titanium diboride, 2%-6% of adhesive, 0.5-2% of lubricant and 8%-25% of deionized water. The particle size of the silicon carbide powder is 0.45-0.5 microns, the particle size of the carbon black is 1-25 nanometers, the purity of the zirconium oxide powder is 4n, and the specification is 5um. The specification of the titanium diboride is 1um. The pressureless sintering high thermal shock resistance silicon carbide ceramic heat exchanger pipe improves the high thermal shock resistance of the ceramic heat exchanger pipe. The processing method has the advantages of simple preparation process, high production efficiency, low cost, high thermal conductivity, corrosion resistance, high temperature resistance, good thermal shock resistance and low thermal expansion coefficient of the produced heat exchanger pipe.

[0004] The traditional pressureless sintering silicon carbide refractory ceramic thin-walled pipe generally lacks a nanoscale synergistic toughening mechanism, and does not introduce a fiber reinforced material, resulting in insufficient bending strength of the thin-walled structure, and the existing technology also has the problems of low coating bonding strength and poor fiber dispersion uniformity, which causes the pipe to easily produce micro-crack propagation under extreme thermal cycle conditions. Therefore, it is urgent to design a pressureless sintering silicon carbide refractory ceramic thin-walled pipe and a preparation method thereof to solve the above problems. SUMMARY

[0005] The present application aims to provide a pressureless sintering silicon carbide refractory ceramic thin-walled pipe and a preparation method thereof to solve the above problems in the prior art.

[0006] In order to achieve the above object, the present application provides the following technical solutions:

[0007] A pressureless sintered silicon carbide refractory ceramic thin-walled tube, the ceramic tube wall thickness is 0.5-3mm, the bulk density after sintering is ≥3.05g / cm³, the room temperature bending strength is ≥350MPa, and the thermal conductivity is ≥120W / (m·K);

[0008] The ceramic thin-walled tube comprises four materials, i.e., a main phase, a reinforcing phase, a sintering aid, and a reinforcing material, and the four materials are specifically as follows:

[0009] The main phase material is selected from α-SiC powder with a purity of ≥99.3%, and the total proportion is 82-90%; the α-SiC powder is composed of 0.5-1μm fine powder and 80-100μm coarse powder in a mass ratio of 7:3;

[0010] The reinforcing phase material is selected from nano-SiO2 binder and silicon nitride nanoparticles, wherein the total proportion of the nano-SiO2 binder is 3-5%, and the total proportion of the silicon nitride nanoparticles is 1-2%;

[0011] The sintering aid material adopts an Al2O3-Y2O3 composite system and a Y2O3-Al2O3 coating, wherein the mass ratio of Al2O3-Y2O3 is 3:1, the total proportion is 2.5-4.5%, and the total proportion of the Y2O3-Al2O3 coating is 0.5-1.2%;

[0012] The reinforcing material is a mixed fiber composed of hemp fibers and lotus stem fibers, and the mixed fiber needs to be impregnated with ethyl silicate solution, wherein the mass ratio of the hemp fibers to the lotus stem fibers is 3:2, and the total proportion of the mixed fiber is 1-2%.

[0013] A pressureless sintered silicon carbide refractory ceramic thin-walled tube preparation method, comprising the following steps:

[0014] Step S1. Raw material pretreatment: α-SiC coarse powder and fine powder are mixed in a three-dimensional mixer in a mass ratio of 3:7 for 2h, and after screening with a 200 mesh sieve, the surface adsorbed impurities are removed by 40kHz ultrasonic cleaning for 30 minutes, and the powder one is obtained by vacuum drying at 80℃;

[0015] Al2O3 (purity ≥99.9%) and Y2O3 (purity ≥99.5%) are mixed in a planetary ball mill in a mass ratio of 3:1 for 4h, and then placed in a corundum crucible for precalcination at 1200℃ for 2h, and then broken to D50=0.8μm after cooling to 200℃ in the furnace, to obtain powder two;

[0016] Wherein, the oxygen content of the α-SiC coarse powder is ≤0.3wt%, and the specific surface area of the fine powder is ≥8m² / g;

[0017] The Al2O3-Y2O3 pre-calcined powder two needs to be air flow milled to D90≤1.2 μm, and the cubic phase in the phase composition of the Y2O3 pre-calcined powder is ≥95%.

[0018] Step S2. Pretreatment of ceramic material; the powder one, the powder two, nano-SiO2 (specific surface area 180 m² / g), and silicon nitride nanoparticles (particle size 50 nm) are added into a zirconia ball mill jar according to the formula proportion, dehydrated ethanol with a solid content of 45 vol% is injected, 0.5 wt% polyvinylpyrrolidone is added, and ball milling is performed for 6-8 h, the ball-to-material ratio is 2:1, and the rotation speed is 250 rpm;

[0019] The slurry viscosity is monitored in real time by using a rotary viscometer, the pH is adjusted to 9.5±0.5 by using triethanolamine, and finally a uniform slurry with a viscosity of 380±50 mPa·s is obtained.

[0020] In the ball milling slurry, 0.1-0.3 wt% triethyl borate is added as a hydrolysis inhibitor, and the particle dispersion degree (PDI≤0.2) is monitored by using a dynamic light scattering instrument.

[0021] Step S3. Pretreatment and vibration mixing of fibers; hemp fibers (length 2-3 mm) / lotus stem fibers (length 3-5 mm) are immersed in a 15 wt% ethyl silicate solution at a mass ratio of 3:2, ultrasonic oscillation treatment is performed at 60°C for 1 h, and drying is performed at 120°C until the water content is <1%;

[0022] The mixed fibers are cut into 0.3-0.8 mm short fibers by using a fiber cutting machine, and the fibers are mixed with the ceramic slurry by using a high-speed vortex vibrator (frequency 800 rpm, amplitude 2 mm) for 15 min to form a premix with a fiber volume fraction of 8-12%;

[0023] The ethyl silicate solution contains 5 wt% zirconia sol, and after immersion, the fibers are pre-oxidized at 800°C for 15 min to form a ZrO2 / SiO2 composite interface layer; the slurry temperature is maintained at 20±2°C during vibration mixing.

[0024] Step S4. Defoaming and injection molding; the premix is placed in a vacuum defoaming machine at -0.095 MPa for 30 min, and a graphite centrifugal mold is preheated to above 50°C;

[0025] The centrifuge is started, the rotation speed is increased to 1000 rpm at 10°C / min, the slurry viscosity is adjusted by adding 0.2 wt% polyethylene glycol, and the gel injection molding is completed after 30 min, the water content of the green body is monitored in real time, and the water content is reduced to 6.8-7.7% when the process is stopped, and a pre-sintered pipe is obtained.

[0026] Wherein: the inner surface of the centrifugal mold is sprayed with a 0.1mm water-based acrylic gel system, the temperature gradient of the mold during the gel injection stage is ≤5℃ / cm, and the green blank is placed in a constant temperature room with 50% humidity for 24h immediately after demolding.

[0027] Step S5. Drying and sintering:

[0028] Step S5.1. Drying and degreasing:

[0029] First stage: The obtained pre-sintered pipe fittings are sent into the sintering furnace and heated to 400℃ at 2℃ / min under nitrogen atmosphere, and held for 12h to remove free water;

[0030] Second stage: Switch to argon atmosphere, heat to 800℃ at 5℃ / min, and hold for 4 hours to decompose organic matter;

[0031] in:

[0032] During the first stage of defatting, the nitrogen flow rate should be ≥20L / min;

[0033] In the second stage, an argon gas mixture containing 5 vol% hydrogen is introduced, and the decomposition rate of organic matter is monitored to ≤50 ppm by an online mass spectrometer.

[0034] Step S5.2. Pressureless sintering:

[0035] Under the protection of 99.999% high-purity argon, the temperature is increased to 2050-2150℃ at 10℃ / min, the furnace pressure is maintained at 60-100Pa, and the temperature is held for 90-120min to achieve complete densification;

[0036] During the sintering and heat preservation stage, an axial pressure of 0.5-1 MPa is applied to reduce the axial shrinkage rate of the pipe to 3±0.5%, and the temperature uniformity is monitored by an infrared thermal imager (ΔT≤15℃).

[0037] Step S5.3. Gradient cooling:

[0038] (1) After sintering, cool rapidly at 8℃ / min at 2150-1600℃;

[0039] (2) When cooling to 1600-800℃, slow cooling at 2℃ / min;

[0040] (2) After cooling to 800℃, cool with the furnace;

[0041] Specifically, during the gradient cooling process, Y3Al5O was isothermally held at 1750℃ for 45 minutes to complete the process. 12 (YAG) phase precipitation, cooling rate below 800℃ via PID control curve:

[0042] Vc = 0.6 + 0.04 × (800 - T)0.5 ;

[0043] Where T is the temperature, T < 800℃, and Vc is the cooling rate.

[0044] Step S6. Post-processing and inspection: A Y2O3-Al2O3 composite coating with a thickness of 10-20 μm is prepared on the outer surface of the tube by plasma spraying, followed by heat treatment at 1200℃ for 2 h to form a YAG / Al2O3 composite oxide layer, thus obtaining a thin-walled tube;

[0045] Then, the bulk density, room temperature flexural strength, and thermal conductivity of the thin-walled tube are tested. The bulk density is tested according to GB / T6569-2010, and the density needs to be greater than 3.05 g / cm³. The room temperature flexural strength is tested according to ASTM C1161, and the flexural strength is ≥350 MPa. The thermal conductivity is measured using a laser thermal conductivity meter and the thermal conductivity is ≥120 W / (m·K).

[0046] Wherein: the Y2O3-Al2O3 composite coating contains 30-40wt% Y2O3, the plasma spraying power is 32-38kW, the carrier gas flow rate is 45-50SCFH, and the porosity of the oxide layer after heat treatment is ≤1.2%.

[0047] During testing, industrial CT scanning was used to analyze the uniformity of pipe wall density distribution (standard deviation ≤ 0.03 g / cm³). Thermal conductivity testing included radial and axial directions, with the ratio controlled between 0.95 and 1.05.

[0048] During testing, a 20kV / 30min withstand voltage test (leakage current ≤1μA) and a 1000h accelerated corrosion test in a 10% ozone environment (mass loss rate ≤0.02mg / cm³) are required. 2 ·h).

[0049] In the above technical solution, the present invention provides a pressureless sintered silicon carbide refractory ceramic thin-walled tube and its preparation method, which have the following beneficial effects:

[0050] (1) The main phase of this invention adopts a bimodal structure of fine powder and coarse powder. The fine powder fills the pores to increase the density, while the coarse powder inhibits abnormal grain growth. At the same time, nano-SiO2 binder and Si3N4 particles are introduced to pin the grain boundaries. Through the stress transfer mechanism, the room temperature flexural strength and bulk density are increased. Furthermore, the mixed fibers of hemp fiber and lotus leaf stalk impregnated with ethyl silicate solution are pre-oxidized at high temperature to form a ZrO2 / SiO2 composite interface layer. Combined with high-speed eddy current vibration, the fiber volume fraction is increased and the dispersion is reduced, which reduces the microcrack propagation rate of the thin-walled tube under thermal cycling conditions, reduces the flexural strength fluctuation rate, improves the bonding strength between the fiber and the matrix, effectively inhibits the propagation of microcracks in the thin-walled structure, and enhances the strength of the thin-walled tube.

[0051] (2) The present invention improves thermal shock stability and reduces the standard deviation of density distribution uniformity by using three-stage gradient cooling and YAG phase precipitation at high temperature, combined with axial pressure control. This is superior to the density non-uniformity caused by non-directional precipitation in the prior art.

[0052] (3) The present invention uses Y2O3-Al2O3 plasma spraying combined with high temperature heat treatment to form a low porosity oxide layer. The mass loss rate is greatly reduced by ozone accelerated corrosion test, and the coating bonding strength is improved compared with traditional coating.

[0053] (4) This invention overcomes industry bottlenecks such as insufficient strength, easy peeling of coating and thermal cycling failure of traditional silicon carbide thin-walled tubes through material composition innovation and precise process control. Attached Figure Description

[0054] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0055] Figure 1 This is a schematic diagram of a ceramic thin-walled tube provided in an embodiment of the pressureless sintering silicon carbide refractory ceramic thin-walled tube and its preparation method according to the present invention.

[0056] Figure 2 This is a schematic diagram of the process flow for an embodiment of the pressureless sintering silicon carbide refractory ceramic thin-walled tube and its preparation method according to the present invention.

[0057] Figure 3 This is a schematic diagram of a scanning electron microscope (SEM) provided for an embodiment of the pressureless sintering silicon carbide refractory ceramic thin-walled tube and its preparation method according to the present invention. Detailed Implementation

[0058] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0059] like Figure 1 As shown in the embodiment of the present invention, a pressureless sintered silicon carbide refractory ceramic thin-walled tube is provided. The ceramic tube has a wall thickness of 0.5-3 mm, a bulk density after sintering of ≥3.05 g / cm³, a room temperature flexural strength of ≥350 MPa, and a thermal conductivity of ≥120 W / (m·K).

[0060] Ceramic thin-walled tubes consist of four materials: the main phase, the reinforcing phase, the sintering aid, and the reinforcing material. The specific details of these four materials are as follows:

[0061] The main phase material is α-SiC powder with a purity of ≥99.3%, accounting for 82-90% of the total. The α-SiC powder is composed of 0.5-1μm fine powder and 80-100μm coarse powder in a mass ratio of 7:3.

[0062] The reinforcing phase material is composed of nano-SiO2 binder and silicon nitride nanoparticles, with the total proportion of nano-SiO2 binder being 3-5% and the total proportion of silicon nitride nanoparticles being 1-2%.

[0063] The sintering aid material adopts an Al2O3-Y2O3 composite system and a Y2O3-Al2O3 coating, wherein the mass ratio of Al2O3 to Y2O3 is 3:1, accounting for 2.5-4.5% of the total, and the total proportion of Y2O3-Al2O3 coating is 0.5-1.2%.

[0064] The reinforcing material is a mixture of hemp fiber and lotus stalk fiber. The mixture needs to be impregnated with ethyl silicate solution. The mass ratio of hemp fiber to lotus stalk fiber is 3:2, and the total proportion of the mixture is 1-2%.

[0065] A method for preparing pressureless sintered silicon carbide refractory ceramic thin-walled tubes, such as... Figures 2-3 The process, as shown, includes the following steps:

[0066] Step S1. Raw material pretreatment: α-SiC coarse powder and fine powder are mixed in a three-dimensional mixer at a mass ratio of 3:7 for 2 hours. After being classified by a 200-mesh sieve, the powder is cleaned with 40kHz ultrasonic for 30 minutes to remove surface adsorbed impurities. The powder is then vacuum dried at 80℃ to obtain powder one.

[0067] Al2O3 (purity ≥99.9%) and Y2O3 (purity ≥99.5%) were mixed in a planetary ball mill at a mass ratio of 3:1 for 4 hours, then placed in a corundum crucible for pre-calcination at 1200℃ for 2 hours. After cooling to 200℃ in the furnace, the mixture was crushed to D50=0.8μm to obtain powder II.

[0068] Among them, the oxygen content of the coarse α-SiC powder is ≤0.3wt%, and the specific surface area of ​​the fine powder is ≥8m² / g;

[0069] The Al2O3-Y2O3 precalcined powder needs to be pulverized by airflow to D90≤1.2μm, and the cubic phase accounts for ≥95% of the phase composition of the Y2O3 after precalcination.

[0070] Step S2. Ceramic material pretreatment: Powder 1, Powder 2, and nano-SiO2 (specific surface area 180m² / g) and silicon nitride nanoparticles (particle size 50nm) are added to a zirconia ball mill jar according to the formula ratio. Anhydrous ethanol with a solid content of 45vol% is injected, and 0.5wt% polyvinylpyrrolidone is added. The mixture is ball-milled for 6-8 hours at a ball-to-material ratio of 2:1 and a rotation speed of 250rpm.

[0071] The viscosity of the slurry was monitored in real time using a rotational viscometer, and the pH was adjusted to 9.5±0.5 using triethanolamine, ultimately obtaining a uniform slurry with a viscosity of 380±50 mPa·s.

[0072] Among them, 0.1-0.3 wt% triethyl borate was added to the ball milling slurry as a hydrolysis inhibitor, and the particle dispersion (PDI≤0.2) was monitored by a dynamic light scattering instrument.

[0073] Step S3. Fiber pretreatment and vibration mixing: Immerse hemp fibers (2-3 mm in length) and lotus stalk fibers (3-5 mm in length) in a 3:2 mass ratio in a 15 wt% ethyl silicate solution, ultrasonically vibrate at 60℃ for 1 hour, and dry at 120℃ until the moisture content is <1%;

[0074] The mixed fibers are cut into short fibers of 0.3-0.8mm using a fiber cutter, and then mixed with ceramic slurry for 15 minutes using a high-speed vortex vibrator (frequency 800rpm, amplitude 2mm) to form a premix with a fiber volume fraction of 8-12%.

[0075] The solution contains 5 wt% zirconium oxide sol. After impregnation, the fibers are pre-oxidized at 800℃ for 15 min to form a ZrO2 / SiO2 composite interface layer. The slurry temperature is maintained at 20±2℃ during vibration mixing.

[0076] Step S4. Degassing and Injection Molding: Place the premixed material in a -0.095MPa vacuum degassing machine for 30 minutes and inject it into a graphite centrifugal mold preheated to above 50°C.

[0077] Start the centrifuge and increase the speed to 1000 rpm at 10℃ / min. Adjust the viscosity of the slurry by adding 0.2wt% polyethylene glycol. Maintain this for 30 minutes to complete the gel casting. Monitor the moisture content of the green blank in real time. Stop when the moisture content drops to 6.8-7.7% to obtain the pre-sintered pipe fitting.

[0078] Among them: the inner surface of the centrifugal mold is sprayed with a 0.1mm water-based acrylic gel system, the temperature gradient of the mold during the gel injection stage is ≤5℃ / cm, and the green blank is placed in a constant temperature room with 50% humidity for 24h immediately after demolding.

[0079] Step S5. Drying and sintering:

[0080] Step S5.1. Drying and degreasing:

[0081] First stage: The obtained pre-sintered pipe fittings are sent into the sintering furnace and heated to 400℃ at 2℃ / min under nitrogen atmosphere, and held for 12h to remove free water;

[0082] Second stage: Switch to argon atmosphere, heat to 800℃ at 5℃ / min, and hold for 4 hours to decompose organic matter;

[0083] in:

[0084] During the first stage of defatting, the nitrogen flow rate should be ≥20L / min;

[0085] In the second stage, an argon gas mixture containing 5 vol% hydrogen is introduced, and the decomposition rate of organic matter is monitored to ≤50 ppm by an online mass spectrometer.

[0086] Step S5.2. Pressureless sintering:

[0087] Under the protection of 99.999% high-purity argon, the temperature is increased to 2050-2150℃ at 10℃ / min, the furnace pressure is maintained at 60-100Pa, and the temperature is held for 90-120min to achieve complete densification;

[0088] During the sintering and heat preservation stage, an axial pressure of 0.5-1 MPa is applied to reduce the axial shrinkage rate of the pipe to 3±0.5%, and the temperature uniformity is monitored by an infrared thermal imager (ΔT≤15℃).

[0089] Step S5.3. Gradient cooling:

[0090] (1) After sintering, cool rapidly at 8℃ / min at 2150-1600℃;

[0091] (2) When cooling to 1600-800℃, slow cooling at 2℃ / min;

[0092] (2) After cooling to 800℃, cool with the furnace;

[0093] Specifically, during the gradient cooling process, Y3Al5O was isothermally held at 1750℃ for 45 minutes to complete the process. 12 (YAG) phase precipitation, cooling rate below 800℃ via PID control curve:

[0094] Vc = 0.6 + 0.04 × (800 - T) 0.5 ;

[0095] Where T is the temperature, T < 800℃, and Vc is the cooling rate.

[0096] Step S6. Post-processing and inspection: A Y2O3-Al2O3 composite coating with a thickness of 10-20 μm is prepared on the outer surface of the tube by plasma spraying, followed by heat treatment at 1200℃ for 2 h to form a YAG / Al2O3 composite oxide layer, thus obtaining a thin-walled tube;

[0097] Then, the bulk density, room temperature flexural strength, and thermal conductivity of the thin-walled tube are tested. The bulk density is tested according to GB / T6569-2010, and the density needs to be greater than 3.05 g / cm³. The room temperature flexural strength is tested according to ASTM C1161, and the flexural strength is ≥350 MPa. The thermal conductivity is measured using a laser thermal conductivity meter and the thermal conductivity is ≥120 W / (m·K).

[0098] The Y2O3-Al2O3 composite coating contains 30-40 wt% Y2O3, the plasma spraying power is 35 kW, and the carrier gas flow rate is 48 SCFH; the porosity of the oxide layer after heat treatment is ≤1.2%.

[0099] During testing, industrial CT scanning was used to analyze the uniformity of pipe wall density distribution (standard deviation ≤ 0.03 g / cm³). Thermal conductivity testing included radial and axial directions, with the ratio controlled between 0.95 and 1.05.

[0100] During testing, it is required to pass a 20kV / 30min withstand voltage test (leakage current ≤1μA) and undergo a 1000h accelerated corrosion test in a 10% ozone environment (mass loss rate ≤0.02mg / cm²·h).

[0101] Example 1:

[0102] 1. Material Formula

[0103] Main phase: α-SiC powder (purity ≥99.3%), accounting for 88% of the total;

[0104] Fine powder (0.8μm): 61.6%;

[0105] Coarse powder (90μm): 26.4%;

[0106] Enhanced phase:

[0107] Nano-SiO2 binder (specific surface area 180m² / g): 4%;

[0108] Silicon nitride nanoparticles (50nm diameter): 1.5%;

[0109] Sintering aids:

[0110] Al2O3-Y2O3 (mass ratio 3:1, D90=1.0μm): 3.5%;

[0111] Y2O3-Al2O3 coating: 0.8%;

[0112] Reinforcing material: hemp fiber / lotus stalk fiber (3:2, length 0.5mm), 1.2% after impregnation with ethyl silicate solution.

[0113] 2. Key Points of the Preparation Process

[0114] Powder pretreatment: α-SiC powder was ultrasonically cleaned at 40kHz for 25min and vacuum dried (80℃); Al2O3-Y2O3 was pre-calcined (1200℃ / 2h) and air-jet pulverized to D90≤1.2μm.

[0115] Slurry control: Add 0.2wt% triethyl borate, slurry pH=10.0, viscosity 380mPa·s (PDI=0.18).

[0116] Fiber treatment: Ethyl silicate solution containing 5 wt% zirconium oxide sol is pre-oxidized at 600℃ for 10 min to form a composite interface layer.

[0117] Centrifugal molding: The inner surface of the graphite mold is sprayed with a 0.1mm water-based acrylic gel system, centrifuged at 1000rpm, and the moisture content of the preform is 7.2%.

[0118] Segmented sintering:

[0119] Degreasing: Nitrogen (20L / min) - 400℃ / 2h; Argon + 5% H2 - 800℃ / 4h (decomposition rate ≤ 50ppm).

[0120] Sintering: 2150℃ / 105min, axial pressure 0.8MPa (ΔT≤12℃).

[0121] Cooling: Isothermal at 1600℃ for 30 minutes; below 800℃, follow the formula Vc = 0.6 + 0.04 × (800 - T). 0.5 cool down.

[0122] Post-treatment: Plasma spraying of Y2O3-Al2O3 coating (Y2O3 content 35%), heat treatment at 1400℃ for 2 hours (porosity 1.2%).

[0123] 3. Performance Results:

[0124]

[0125] Among them, the industrial CT showed a density standard deviation of 0.025 g / cm³; the withstand voltage test leakage current was 0.8 μA.

[0126] Comparative Example 1 (Traditional Sintering Aids):

[0127] Change point: Replace the sintering aid with the B4C-AlN system (B4C 2.5% + AlN 2.5%).

[0128] defect:

[0129] The melting point of the grain boundary phase is increased, and the densification is insufficient - the density is only 2.98 g / cm³;

[0130] Flexural strength 291 MPa (17% lower than the standard).

[0131] Comparative Example 2 (Fiber-free interface modification):

[0132] Changes: Impregnation and pre-oxidation with ethyl silicate solution are eliminated; fibers are added directly to the slurry.

[0133] defect:

[0134] The fiber-matrix bond is weak, and cracking occurs after 10 thermal shock cycles.

[0135] The axial thermal conductivity decreased to 112 W / (m·K) (anisotropy ratio 1.15).

[0136] Comparative Example 3 (Simplified Sintering Process):

[0137] Changes: Gradient cooling and axial pressure are eliminated; cooling is performed directly with the furnace.

[0138] defect:

[0139] Residual stress caused microcracks in the pipe wall - flexural strength 318 MPa;

[0140] The density distribution is uneven (standard deviation 0.08 g / cm³).

[0141] The following table compares the key performance characteristics of the embodiments and comparative examples:

[0142]

[0143] As shown in the table above, Example 1 is the best and has the following advantages:

[0144] Densification advantages: Al2O3-Y2O3 additive system and Y2O 3- Al2O3 coatings reduce sintering temperature and promote liquid phase diffusion, achieving near-full density (>99%).

[0145] Fiber reinforcement effect: The ethyl silicate solution interface layer enhances the bonding strength between the fiber and the SiC matrix and inhibits crack propagation;

[0146] Process stability: Gradient cooling and axial pressure reduce thermal stress and ensure pipe dimensional accuracy (axial shrinkage rate 3.2%).

[0147] Overall performance: High thermal conductivity, high strength, and corrosion resistance meet the requirements of extreme working conditions (such as aerospace thermal protection and chemical reactors).

[0148] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A pressureless sintered silicon carbide refractory ceramic thin-walled tube, characterized in that The ceramic tube has a wall thickness of 0.5-3mm, and a bulk density of ≥3.05g / cm after sintering 3 , a room temperature bending strength of ≥350MPa, and a thermal conductivity of ≥120W / (m·K). The ceramic thin-walled pipe comprises four materials of a main phase, a reinforcing phase, a sintering aid, and a reinforcing material, and the four materials are specifically as follows: The main phase material is selected from α-SiC powder with a purity of ≥99.3%, and the total proportion is 82-90%, and the α-SiC powder is composed of 0.5-1 μm fine powder and 80-100 μm coarse powder at a mass ratio of 7:3; The reinforcing phase material is selected from nano-SiO2 binder and silicon nitride nanoparticles, wherein the total proportion of the nano-SiO2 binder is 3-5%, and the total proportion of the silicon nitride nanoparticles is 1-2%; The sintering aid material adopts an Al2O3-Y2O3 composite system, wherein the mass ratio of Al2O3-Y2O3 is 3:1, and the total proportion is 2.5-4.5%; The reinforcing material is a mixed fiber composed of hemp fibers and lotus stem fibers, the mixed fiber is impregnated with a silicate ethyl solution and subjected to high-temperature pre-oxidation treatment, the silicate ethyl solution contains 5wt% zirconium oxide sol, the mass ratio of the hemp fibers to the lotus stem fibers is 3:2, and the total proportion of the mixed fiber is 1-2%; The outer surface of the ceramic thin-walled pipe has a Y2O3-Al2O3 composite coating, and the total proportion of the coating is 0.5-1.2%, and the proportion of Y2O3 is 30-40wt%.

2. A method for producing a pressureless sintered silicon carbide refractory ceramic thin-walled tube for producing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 1, characterized in that The method comprises the following steps: Step S1. Raw material pretreatment: α-SiC coarse powder and fine powder are mixed in a three-dimensional mixer at a mass ratio of 3:7 for 1.8-2h, and after being classified by a 200-mesh sieve, the surface adsorbed impurities are removed by 40kHz ultrasonic cleaning for 20-30min, and the powder one is obtained by vacuum drying at 60-100℃; Al2O3 and Y2O3 are mixed in a planetary ball mill at a mass ratio of 3:1 for 3-5h, and then are placed in a corundum crucible for pre-calcination at 1100-1300℃ for 1.8-2h, and after cooling to 200℃ in the furnace, they are broken to D50=0.8μm to obtain powder two; Step S2. Ceramic material pretreatment: powder one, powder two, nano-SiO2, and silicon nitride nanoparticles are added to a zirconium oxide ball mill jar according to the formula proportion, 45vol% anhydrous ethanol is injected, 0.3-0.5wt% polyvinylpyrrolidone is added, and ball milling is carried out for 6-8h, the ball-to-material ratio is 2:1, and the rotation speed is 200-300rpm; The slurry viscosity is monitored in real time by using a rotary viscometer, the pH is adjusted to 9.5±0.5 by triethanolamine, and finally a uniform slurry with a viscosity of 380±50mPa·s is obtained; Step S3. Fiber pretreatment and vibration mixing: hemp fibers and lotus stem fibers are immersed in a 15wt% silicate ethyl solution at a mass ratio of 3:2, ultrasonic oscillation treatment is carried out at 55-65℃ for 1-1.2h, and drying is carried out at 100-150℃ until the water content is <1%; The mixed fiber is cut into 0.3-0.8mm short fibers by using a fiber cutting machine, and the short fibers are mixed with the ceramic slurry by using a high-speed vortex vibrator for 15-20min to form a premix with a fiber volume fraction of 8-12%; Step S4. Defoaming and injection molding: the premix is placed in a vacuum defoaming machine at -0.095MPa for 25-35min, and a graphite centrifugal mold is preheated to above 50℃. Start the centrifuge, and increase the speed to 800-1200 rpm at 10 ℃ / min. The viscosity of the slurry is adjusted by adding 0.2 wt% polyethylene glycol, and the gel injection molding is completed in 20-40 min. The green body moisture content is monitored in real time, and the process is stopped when the moisture content is reduced to 6.8-7.7%. The pre-sintered pipe is obtained. Step S5. Drying and sintering: Step S5.

1. Drying and debinding: First stage: The obtained pre-sintered pipe is put into a sintering furnace, and the temperature is increased to 400 ℃ at 2 ℃ / min under a nitrogen atmosphere. The free water is removed by maintaining the temperature for 1.8-2.5 h. Second stage: Switch to an argon atmosphere, and increase the temperature to 800 ℃ at 5 ℃ / min. The organic matter is decomposed by maintaining the temperature for 3-5 h. Step S5.

2. Pressureless sintering: Under the protection of 99.999% high-purity argon, the temperature is increased to 2050-2150 ℃ at 10 ℃ / min. The furnace pressure is maintained at 60-100 Pa, and the complete densification is achieved by maintaining the temperature for 90-120 min. An axial pressure of 0.5-1 MPa is applied during the sintering and holding stage. Step S5.

3. Gradient cooling: (1) After sintering, fast cooling at 8 ℃ / min is performed when the temperature is 2150-1600 ℃. (2) Slow cooling at 2 ℃ / min is performed when the temperature is cooled to 1600-800 ℃. (3) After the temperature is cooled to 800 ℃, the furnace is cooled. Step S6. Post-processing and detection: A Y2O3-Al2O3 composite coating with a thickness of 10-20 μm is prepared on the outer surface of the pipe body by plasma spraying. Then, the pipe is heat-treated at 1000-1400 ℃ for 1.8-2.2 h to form a YAG / Al2O3 composite oxide layer, and a thin-walled pipe is obtained. The volume density, room temperature bending strength, and thermal conductivity of the thin-walled pipe are detected.

3. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S1: The oxygen content of the α-SiC coarse powder is ≤0.3 wt%, and the specific surface area of the fine powder is ≥8 m² / g. The Al2O3-Y2O3 pre-calcined powder is subjected to air flow crushing to D90≤1.2 μm, and the proportion of cubic phase in the phase composition of the Y2O3 pre-calcined powder is ≥95%.

4. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S2: 0.1-0.3 wt% triethyl borate is added to the ball-milled slurry as a hydrolysis inhibitor, and the particle dispersion degree is monitored by a dynamic light scattering instrument.

5. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S3: After impregnation, the fibers are pre-oxidized at 800 ℃ for 15 min to form a ZrO2 / SiO2 composite interface layer. The slurry temperature is maintained at 20±2 ℃ during the vibration mixing.

6. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S4: The inner surface of the centrifugal mold is sprayed with a 0.1 mm water-based acrylate gel system. The mold temperature gradient during the gel injection molding stage is ≤5 ℃ / cm. The green body is immediately placed in a constant-temperature room with a humidity of 50% for aging for 24 h after demolding.

7. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S5.1: The nitrogen flow rate during the first stage debinding is ≥20 L / min. In the second stage, a mixed gas of argon containing 5 vol% hydrogen is introduced. The organic matter decomposition rate is monitored by an online mass spectrometer to be ≤50 ppm. In the step S5.2: An axial pressure of 0.5-1 MPa is applied during the sintering and holding stage to reduce the axial shrinkage of the pipe to 3±0.5%. The temperature uniformity is monitored by an infrared thermal imager. In the step S5.3: The Y3Al5O 12 (YAG) phase precipitates at 1750 °C isothermally for 45 min, and the cooling rate below 800 °C is controlled by a PID curve: V c = 0.6 + 0.04 x (800 - T) 0.5 ; where T is the temperature, T < 800 °C, V c is the cooling rate.

8. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S6: The Y2O3-Al2O3 composite coating has 30-40wt% of Y2O3, a plasma spraying power of 32-38kW, and a carrier gas flow of 45-50SCFH; and the porosity of the oxide layer after heat treatment is less than or equal to 1.2%.

9. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, In the step S6: The uniformity of the density distribution of the pipe wall is analyzed by using an industrial CT scan during detection, and the ratio of the radial thermal conductivity to the axial thermal conductivity is controlled to be 0.95-1.

05.

10. The method for preparing a pressureless sintered silicon carbide refractory ceramic thin-walled tube according to claim 2, characterized in that, During the detection in the step S6, a 20kV / 30min voltage resistance test is required, and a 1000h accelerated corrosion test in a 10% ozone environment is required.

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