A high-temperature resistant silicon carbide electrothermal element and its preparation method

Abstract

The present invention relates to the field of electrothermal elements, and particularly to a high-temperature resistant silicon carbide electrothermal element and a preparation method thereof. In the present invention, silicon carbide powder, silicon carbide fiber, graphite, metal silicon powder and a binder are mixed and pressed into a preliminary product; using aluminum chloride hexahydrate, tetraethyl orthosilicate and 1,2-epoxypropane as raw materials, a mullite precursor sol is prepared; a lanthanum cerate shell layer is prepared on the outer layer of yttrium oxide powder by a precipitation method to obtain a composite powder; the composite powder is added to the mullite precursor sol to obtain a mixed slurry; the mixed slurry is coated on the surface of the preliminary product and calcined to obtain the high-temperature resistant silicon carbide electrothermal element. The silicon carbide electrothermal element prepared by the present invention has good thermal stability and can effectively resist thermal oxygen aging.

Description

Technical Field

[0001] The present invention relates to the technical field of electrothermal elements, and particularly to a high-temperature resistant silicon carbide electrothermal element and a preparation method thereof. Background Art

[0002] Silicon carbide electrothermal elements are a very common but particularly important high-temperature heating element, with characteristics such as good high-temperature stability, strong chemical inertness, low expansion coefficient, and good thermal conductivity. They are widely used in high-temperature furnaces, electric furnaces, and other industrial heating equipment. Traditional silicon carbide electrothermal elements are prone to problems such as oxidation, corrosion, and fatigue damage under high-temperature conditions, which seriously affect the service life and efficiency of the elements. Therefore, researching how to improve the high-temperature performance of silicon carbide electrothermal elements has become the focus of attention of researchers. Due to the problems of oxidation, corrosion, and fatigue damage that traditional silicon carbide electrothermal elements are prone to under high-temperature conditions, it seriously affects the service life and efficiency of the elements. Therefore, new methods and technical means are needed for the research and development of silicon carbide electrothermal elements. Summary of the Invention

[0003] The purpose of the present invention is to provide a high-temperature resistant silicon carbide electrothermal element and a preparation method thereof to solve the problems raised in the above background art.

[0004] To solve the above technical problems, the present invention provides the following technical solutions: A high-temperature resistant silicon carbide electrothermal element and a preparation method thereof, including the following steps:

[0005] Step 1: Mix silicon carbide powder, silicon carbide fiber, graphite, metal silicon powder, and binder evenly; press the mixture into a mold to obtain a silicon carbide electrothermal element blank; cool the blank with water, perform green firing, and roasting; obtain a preliminary product;

[0006] Step 2: Mix water, ethanol, and aluminum chloride hexahydrate, add hydrochloric acid to adjust the pH value to 2-3, and react with stirring in a water bath to obtain aluminum sol; mix ethanol, tetraethyl orthosilicate, and water, use hydrochloric acid to adjust the pH value to 2-3, and stir to react to obtain silica sol; mix the aluminum sol and the silica sol, and add 1,2-epoxypropane to obtain a mullite precursor sol;

[0007] Step 3: Disperse yttrium oxide powder in absolute ethanol, add sodium dodecylbenzenesulfonate to obtain dispersion liquid A; stir and dissolve lanthanum nitrate in deionized water, then add cerium nitrate and stir, and then dropwise add sodium phosphate solution while stirring, and continue to stir to form dispersion liquid B; mix dispersion liquid A and dispersion liquid B and react to prepare a composite powder;

[0008] Step 4: Mix and stir the composite powder and the mullite precursor sol to obtain a mixed slurry. Coat the mixed slurry on the surface of the semi-finished product and calcine it at 1200 - 1500 °C for 3 - 5 h to obtain a high-temperature resistant silicon carbide heating element.

[0009] Further, in Step 1, the dosages of each component, by weight percentage, are 70 - 80% silicon carbide powder, 5 - 20% silicon carbide fiber, 3 - 5% graphite, 1 - 2% metallic silicon powder, and 0.5 - 3% binder.

[0010] Further, in Step 1, the water cooling time is 10 - 20 min.

[0011] Further, in Step 1, the green sintering method is under vacuum conditions, at a temperature of 800 - 1300 °C, and for a time of 15 - 30 min.

[0012] Further, in Step 1, the roasting temperature is 1500 - 1800 °C.

[0013] Further, in Step 2, the molar ratio of the dosages of tetraethyl orthosilicate, aluminum chloride hexahydrate, and 1,2 - epoxypropane is (10 - 14):(2 - 3):1.

[0014] Further, in Step 2, the water bath temperature is 50 - 65 °C, and the stirring reaction time is 1 - 2 h.

[0015] Further, in Step 2, in the silica sol, the molar ratio of the dosages of ethanol, tetraethyl orthosilicate, and water is (3 - 4):4:(3 - 4).

[0016] Further, in Step 2, the stirring reaction time of the silica sol is 0.5 - 1 h.

[0017] Further, in Step 2, in the mullite precursor sol, the molar ratio of silicon element, aluminum element, and 1,2 - epoxypropane is 1:3:(6 - 9).

[0018] Further, in Step 3, the dosages of each component in Dispersion A, by weight, are 0.3 - 0.5 parts of yttrium oxide powder, 10 - 20 parts of absolute ethanol, and 0.1 - 0.2 parts of sodium dodecylbenzenesulfonate.

[0019] Further, in Step 3, the dosages of each component in Dispersion B, by weight, are 2 - 3 parts of lanthanum nitrate, 30 - 40 parts of deionized water, 0.2 - 0.4 parts of cerium nitrate, and 50 - 60 parts of trisodium phosphate solution.

[0020] Further, in step 3, the preparation method of the composite powder is as follows: Mix dispersion liquid A and dispersion liquid B according to a mass ratio of 1:(1-1.5), carry out hydrothermal reaction at 160-200°C for 18-24 h, wash several times with ethanol and water, and after drying, calcine at 400-500°C for 2-3 h to obtain the composite powder.

[0021] Further, in step 4, in the mixed slurry, by weight percentage, 5-10% is the composite powder and 90-95% is the mullite precursor sol.

[0022] Further, in step 4, the calcination temperature is 1200-1500°C and the calcination time is 3-5 h.

[0023] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: In the present invention, silicon carbide powder, silicon carbide fiber, graphite, metal silicon powder and binder are mixed and pressed into an initial product. In order to further improve the anti-thermal oxygen aging performance of the product, the present invention prepares a mixed slurry and coats it on the surface of the initial product. The coated slurry includes a mullite precursor sol and a composite powder. The mullite precursor sol is prepared from aluminum chloride hexahydrate, tetraethyl orthosilicate and 1,2-epoxypropane; the composite powder uses the precipitation method to prepare a lanthanum cerate shell layer on the outer layer of yttrium oxide powder; when the mixed slurry is coated on the surface of the initial product and calcined, the mullite precursor sol will first form a mullite layer and adhere to the surface of the silicon carbide material. As the calcination time increases, due to the poor thermal stability of the lanthanum cerate shell layer of the composite powder, a large number of cracks will gradually appear, releasing the internal yttrium oxide; at high temperatures, yttrium oxide, as a sintering aid, contacts with mullite and further forms a eutectic phase at low temperature, reducing the temperature at which the liquid phase is generated, enabling the liquid phase to participate in the sintering process that was originally mainly a solid-phase reaction. Under the action of the capillary force and surface tension of the liquid phase, the solid-phase grains are rearranged, making the surface coating densified. Therefore, it effectively blocks the reaction between the silicon carbide material and external oxidation factors, thereby effectively improving the anti-thermal oxygen aging performance.

[0024] In the present invention, the composite powder with a core-shell structure can prevent the reaction between the internal yttrium oxide and the mullite precursor sol at the initial stage of calcination, thus effectively ensuring the formation of the mullite layer. It should be added that the dosage of the composite powder needs to be controlled at 5-10%. When the dosage of the composite powder is too high, the content of lanthanum cerate also increases. In actual application, high temperature will cause lanthanum cerate to continuously split, resulting in a large number of voids inside the coating, seriously reducing the densification of the coating. In actual use, oxygen will penetrate into the coating through the voids and directly contact with the electrothermal element body to occur an oxidation reaction, causing performance degradation and shortening of service life. Detailed implementation mode

[0025] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0026] The raw materials used in the present invention and their sources: The silicon carbide powder comes from Huai'an Lita Silicon Carbide Micropowder Co., Ltd., model 1000#; the silicon carbide fiber comes from Saifei Group; the graphite comes from Qingdao Tianyuan Graphite Co., Ltd., TS series, particle size 1 - 38μm; the metallic silicon powder comes from Hanxin Zun Micro Materials, particle size 45 - 425μm; the binder is aluminum dihydrogen phosphate; the concentration of trisodium phosphate is 0.02mol / L.

[0027] Example 1

[0028] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into a shape to obtain a green body of the silicon carbide electrothermal element; cool it with water for 10 min, and carry out green sintering and roasting on the green body under a vacuum state. The green sintering temperature is 800°C, and the green sintering time is 30 min. Roast and form it at 1500°C to obtain a preliminary product.

[0029] Step 2: Mix water, ethanol, and aluminum chloride hexahydrate in a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 2, and stir and react at 50°C for 1 h to obtain an aluminum sol; mix ethanol, tetraethyl orthosilicate, and water in a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 0.5 h to obtain a silica sol; mix the aluminum sol, silica sol, and 1,2 - epoxypropane to obtain a mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate, and 1,2 - epoxypropane is 1:3:9.

[0030] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion liquid A; stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 30 min, and then dropwise add 50 g of trisodium phosphate solution drop by drop while stirring, and continue to stir for 30 min to form dispersion liquid B; mix dispersion liquid A and dispersion liquid B in a mass ratio of 1:1.5, carry out hydrothermal reaction at 180°C for 24 h, wash it several times with ethanol and water, dry it, and calcine it at 400°C for 2 h to obtain a composite powder.

[0031] Step 4: Mix 5% composite powder and 95% mullite precursor sol by stirring to obtain a mixed slurry. Coat the surface of the semi-finished product with the mixed slurry, with a coating thickness of 5 μm, and calcine at 1200 °C for 3 h to obtain a high-temperature resistant silicon carbide heating element.

[0032] Example 2

[0033] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into shape to obtain a silicon carbide heating element blank; cool it with water for 12 min, and subject the blank to green sintering and roasting under a vacuum state. The green sintering temperature is 900 °C and the green sintering time is 20 min. Roast and form at 1550 °C to obtain a semi-finished product;

[0034] Step 2: Mix water, ethanol and aluminum chloride hexahydrate in a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 2, and stir and react at 55 °C for 2 h to obtain an aluminum sol; mix ethanol, tetraethyl orthosilicate and water in a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 1 h to obtain a silicon sol; mix the aluminum sol, the silicon sol and 1,2-epoxypropane to obtain a mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate and 1,2 epoxypropane is 1:3:9;

[0035] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion liquid A; stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 40 min, and then dropwise add 50 g of trisodium phosphate solution drop by drop while stirring, and continue to stir for 30 min to form dispersion liquid B; mix dispersion liquid A and dispersion liquid B in a mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash several times with ethanol and water, dry, and calcine at 450 °C for 2 h to obtain a composite powder;

[0036] Step 4: Mix 5% composite powder and 95% mullite precursor sol by stirring to obtain a mixed slurry. Coat the surface of the semi-finished product with the mixed slurry, with a coating thickness of 5 μm, and calcine at 1200 °C for 4 h to obtain a high-temperature resistant silicon carbide heating element.

[0037] Example 3

[0038] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into shape to obtain a green body of silicon carbide electrothermal element; cool it with water for 20 min, and carry out green sintering and roasting on the green body under vacuum state. The green sintering temperature is 1000 °C and the green sintering time is 20 min. Roast and form it at 1600 °C to obtain a preliminary product;

[0039] Step 2: Mix water, ethanol and aluminum chloride hexahydrate according to the molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 2, and stir and react at 65 °C for 1 h to obtain aluminum sol; mix ethanol, tetraethyl orthosilicate and water according to the molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 0.5 h to obtain silica sol; mix aluminum sol, silica sol and 1,2-epoxypropane to obtain mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate and 1,2-epoxypropane is 1:3:9;

[0040] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, and add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion liquid A; stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 45 min, and then dropwise add 50 g of trisodium phosphate solution drop by drop while stirring, and continue to stir for 30 min to form dispersion liquid B; mix dispersion liquid A and dispersion liquid B according to the mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash it several times with ethanol and water, dry it, and calcine it at 500 °C for 2 h to obtain composite powder;

[0041] Step 4: Mix 5% composite powder and 95% mullite precursor sol and stir to obtain a mixed slurry. Coat the mixed slurry on the surface of the preliminary product with a coating thickness of 5 μm, and calcine it at 1200 °C for 5 h to obtain a high-temperature resistant silicon carbide electrothermal element.

[0042] Example 4

[0043] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into shape to obtain a green body of silicon carbide electrothermal element; cool it with water for 10 min, and carry out green sintering and roasting on the green body under vacuum state. The green sintering temperature is 1100 °C and the green sintering time is 15 min. Roast and form it at 1650 °C to obtain a preliminary product;

[0044] Step 2: Mix water, ethanol, and aluminum chloride hexahydrate in a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 3, and stir and react at 50 °C for 1 h to obtain aluminum sol; mix ethanol, tetraethyl orthosilicate, and water in a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 3, and stir for 0.5 h to obtain silica sol; mix the aluminum sol, silica sol, and 1,2-epoxypropane to obtain a mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate, and 1,2-epoxypropane is 1:3:9;

[0045] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion A; stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 30 min, and then dropwise add 50 g of trisodium phosphate solution drop by drop while stirring, and continue to stir for 30 min to form dispersion B; mix dispersion A and dispersion B in a mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash several times with ethanol and water, dry, and calcine at 400 °C for 3 h to obtain a composite powder;

[0046] Step 4: Mix 5% of the composite powder and 95% of the mullite precursor sol and stir to obtain a mixed slurry. Coat the mixed slurry on the surface of the semi-finished product, with a coating thickness of 5 μm, and calcine at 1200 °C for 3 h to obtain a high-temperature resistant silicon carbide heating element.

[0047] Example 5

[0048] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder, and 3% binder evenly; press the mixture into a shape to obtain a silicon carbide heating element blank; cool with water for 10 min, and carry out green sintering and roasting of the blank under a vacuum state. The green sintering temperature is 1200 °C, and the green sintering time is 25 min. Roast and form at 1500 °C to obtain a semi-finished product;

[0049] Step 2: Mix water, ethanol, and aluminum chloride hexahydrate in a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 3, and stir and react at 60 °C for 1 h to obtain aluminum sol; mix ethanol, tetraethyl orthosilicate, and water in a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 0.5 h to obtain silica sol; mix the aluminum sol, silica sol, and 1,2-epoxypropane to obtain a mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate, and 1,2-epoxypropane is 1:3:9;

[0050] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion A; Stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 30 min, and then dropwise add 50 g of trisodium phosphate solution while stirring, continue to stir for 30 min to form dispersion B; Mix dispersion A and dispersion B according to a mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash several times with ethanol and water, dry, and then calcine at 450 °C for 3 h to obtain the composite powder;

[0051] Step 4: Mix and stir 5% of the composite powder and 95% of the mullite precursor sol to obtain a mixed slurry. Coat the mixed slurry on the surface of the semi-finished product with a coating thickness of 5 μm, and calcine at 1400 °C for 4.5 h to obtain the high-temperature resistant silicon carbide heating element.

[0052] Example 6

[0053] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; Press the mixture into shape to obtain the blank of the silicon carbide heating element; Cool with water for 15 min, carry out green sintering and roasting on the blank under vacuum conditions. The green sintering temperature is 1300 °C and the green sintering time is 15 min, and roast and form at 1800 °C to obtain the semi-finished product;

[0054] Step 2: Mix water, ethanol and aluminum chloride hexahydrate according to a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 3, and stir and react at 65 °C for 2 h to obtain aluminum sol; Mix ethanol, tetraethyl orthosilicate and water according to a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 3, and stir for 1 h to obtain silicon sol; Mix the aluminum sol, silicon sol and 1,2-epoxypropane to obtain the mullite precursor sol; The molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate and 1,2-epoxypropane is 1:3:9;

[0055] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion A; Stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 60 min, and then dropwise add 50 g of trisodium phosphate solution while stirring, continue to stir for 30 min to form dispersion B; Mix dispersion A and dispersion B according to a mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash several times with ethanol and water, dry, and then calcine at 500 °C for 3 h to obtain the composite powder;

[0056] Step 4: Mix and stir 5% composite powder and 95% mullite precursor sol to obtain a mixed slurry. Coat the mixed slurry on the surface of the semi-finished product with a coating thickness of 5 μm, and calcine at 1500 °C for 5 h to obtain a high-temperature resistant silicon carbide heating element.

[0057] Comparative Example 1

[0058] Do not use the mixed slurry for coating, and the other parameters are the same as those in Example 1.

[0059] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into a shape to obtain a blank of silicon carbide heating element; cool with water for 10 min, and perform green sintering and roasting on the blank under a vacuum state. The green sintering temperature is 800 °C and the green sintering time is 30 min, and roast and form at 1500 °C to obtain a high-temperature resistant silicon carbide heating element.

[0060] Comparative Example 2

[0061] Do not add the composite powder, and the other parameters are the same as those in Example 2.

[0062] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into a shape to obtain a blank of silicon carbide heating element; cool with water for 12 min, and perform green sintering and roasting on the blank under a vacuum state. The green sintering temperature is 900 °C and the green sintering time is 20 min, and roast and form at 1550 °C to obtain a semi-finished product;

[0063] Step 2: Mix water, ethanol and aluminum chloride hexahydrate in a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 2, and stir and react at 55 °C for 2 h to obtain an aluminum sol; mix ethanol, tetraethyl orthosilicate and water in a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 1 h to obtain a silicon sol; mix the aluminum sol, the silicon sol and 1,2-epoxypropane to obtain a mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate and 1,2-epoxypropane is 1:3:9;

[0064] Step 3: Coat the mullite precursor sol on the surface of the semi-finished product with a coating thickness of 5 μm, and calcine at 1200 °C for 4 h to obtain a high-temperature resistant silicon carbide heating element.

[0065] Comparative Example 3

[0066] Increase the dosage of the composite powder, and the other parameters are the same as those in Example 3.

[0067] Step 1: Mix 80% silicon carbide powder, 10% silicon carbide fiber, 5% graphite, 2% metallic silicon powder and 3% binder evenly; press the mixture into shape to obtain a green body of silicon carbide electrothermal element; cool it with water for 20 min, and carry out green sintering and roasting on the green body under vacuum state. The green sintering temperature is 1000 °C and the green sintering time is 20 min. Roast and form it at 1600 °C to obtain a preliminary product;

[0068] Step 2: Mix water, ethanol and aluminum chloride hexahydrate according to a molar ratio of 12:3:1, add hydrochloric acid to adjust the pH value to 2, and stir and react at 65 °C for 1 h to obtain aluminum sol; mix ethanol, tetraethyl orthosilicate and water according to a molar ratio of 4:4:3, use hydrochloric acid to adjust the pH value to 2, and stir for 0.5 h to obtain silica sol; mix the aluminum sol, silica sol and 1,2-epoxypropane to obtain mullite precursor sol; the molar ratio of tetraethyl orthosilicate, aluminum chloride hexahydrate and 1,2-epoxypropane is 1:3:9;

[0069] Step 3: Disperse 0.4 g of yttrium oxide powder in 20 g of absolute ethanol, and add 0.2 g of sodium dodecylbenzenesulfonate to obtain dispersion liquid A; stir and dissolve 2.92 g of lanthanum nitrate (0.009 mol) in 30 g of deionized water, then add 0.326 g of cerium nitrate (0.001 mol), stir for 45 min, and then dropwise add 50 g of trisodium phosphate solution dropwise while stirring, and continue to stir for 30 min to form dispersion liquid B; mix dispersion liquid A and dispersion liquid B according to a mass ratio of 1:1.5, carry out hydrothermal reaction at 180 °C for 24 h, wash it several times with ethanol and water, dry it, and calcine it at 500 °C for 2 h to obtain composite powder;

[0070] Step 4: Mix 15% composite powder and 85% mullite precursor sol and stir to obtain a mixed slurry. Coat the mixed slurry on the surface of the preliminary product with a coating thickness of 5 μm, and calcine it at 1200 °C for 5 h to obtain a high-temperature resistant silicon carbide electrothermal element.

[0071] Experiment:

[0072] Place the sample in air at 1500 °C for 200 h of aging, and test the change rate of resistance and the retention rate of flexural strength before and after aging; among them:

[0073] Change rate of resistance = (resistance after aging - resistance before aging) / resistance before aging × 100%;

[0074] Retention rate of flexural strength = flexural strength after aging / flexural strength before aging × 100%;

[0075] The high-temperature oxidation resistance performance of the coating is described by the oxidation weight gain rate under isothermal oxidation conditions. Using the cyclic oxidation method, the sintered sample is placed in an electric resistance furnace, heated to 1200 °C, held for 24 h, and then taken out and air-cooled. The mass of the sample is recorded using a precision electronic balance (accuracy of 0.01 mg). The oxidation weight gain rate G = (weight of the sample after oxidation - weight of the sample before oxidation) / weight of the sample before oxidation × 100%.

[0076] Example Resistance change rate / % Flexural strength retention rate / % Oxidation weight gain rate / % Example 1 9.9 92.4 0.095 Example 2 10.2 92.6 0.092 Example 3 10.0 91.3 0.096 Example 4 9.8 90.6 0.093 Example 5 9.9 90.9 0.094 Example 6 10.1 91.7 0.098 Comparative example 1 20.4 80.7 0.198 Comparative example 2 14.5 86.4 0.136 Comparative example 3 17.8 84.3 0.154

[0077] Conclusion:

[0078] The silicon carbide electrothermal element prepared by the present invention has good high-temperature resistance performance. After aging for 200 h under the high-temperature condition of 1500 °C, the resistivity change is small, and the flexural strength retention rate reaches more than 90%; in the thermal oxidation test, the oxidation weight gain rate is less than 0.1%, showing good antioxidant effect.

[0079] The data of Example 1 and Comparative Example 1 show that the silicon carbide electrothermal element after being coated with the mixed slurry has a smaller change in resistivity during aging, a high flexural strength retention rate, and good antioxidant effect. The data of Example 2 and Comparative Example 2 show that in Example 2, after adding the composite powder to the mullite precursor sol, the performance of the prepared silicon carbide electrothermal element is better. This is mainly because the composite powder has a core-shell structure, with lanthanum cerate as the shell and yttrium trioxide as the core. When the mixed slurry is coated on the surface of the electrothermal element and calcined, the mullite precursor sol will first form a mullite layer and adhere to the surface of the silicon carbide electrothermal element. As the calcination time increases, a large number of cracks will appear in the lanthanum cerate of the shell layer, releasing the internal yttrium trioxide; yttrium trioxide, as a sintering aid, forms a eutectic phase at low temperature after contacting with mullite at high temperature, reducing the temperature at which the liquid phase is generated, so that the sintering process mainly based on solid-phase reaction has the participation of the liquid phase. Under the action of the capillary force and surface tension of the liquid phase, the solid-phase grains are rearranged, making the surface coating densified. Therefore, it effectively blocks the reaction between the silicon carbide electrothermal element and external oxidation factors. Therefore, the silicon carbide electrothermal element prepared in Example 2 has better high-temperature electrothermal performance. The data of Example 3 and Comparative Example 3 show that too large an addition amount of the composite powder will also affect the performance of the final silicon carbide electrothermal element. As the dosage of the composite powder increases, the content of lanthanum cerate also increases. Under the action of high temperature, lanthanum cerate continuously splits into fragments, and a large number of voids appear inside the coating, seriously reducing the densification of the coating. During actual use, oxygen will penetrate into the coating through the voids and react with the electrothermal element body after contacting, resulting in performance degradation.

[0080] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a high-temperature resistant silicon carbide heating element, characterized in that: The following steps are involved: Step 1: Evenly mix silicon carbide powder, silicon carbide fiber, graphite, metallic silicon powder and a binder; press the mixture into a shape to obtain a silicon carbide electric heating element blank; water-cool the blank, biscuit-sinter, and roast it to obtain a primary product; Step 2: Mix water, ethanol, and aluminum chloride hexahydrate, add hydrochloric acid to adjust the pH value to 2-3, and stir in a water bath to react to obtain aluminum sol; mix ethanol, ethyl orthosilicate, and water, use hydrochloric acid to adjust the pH value to 2-3, and stir to react to obtain silica sol; mix the aluminum sol and silica sol, and add 1,2-propylene oxide to obtain a mullite precursor sol; Step 3: Dispersing yttrium trioxide powder in anhydrous ethanol, adding sodium dodecylbenzenesulfonate to obtain dispersion A; stirring and dissolving lanthanum nitrate in deionized water, adding cerium nitrate, stirring, and then adding trisodium phosphate solution dropwise while stirring, and continuing to stir to form dispersion B; mixing dispersion A and dispersion B to prepare a composite powder; Step 4: Mixing and stirring the composite powder and the mullite precursor sol to obtain a mixed slurry, coating the mixed slurry on the surface of the primary finished product, and calcining to obtain a high-temperature resistant silicon carbide electric heating element; In step 3, the amounts of the components in dispersion A, by weight, are 0.3-0.5 parts of yttrium trioxide powder, 10-20 parts of anhydrous ethanol, and 0.1-0.2 parts of sodium dodecylbenzenesulfonate; the amounts of the components in dispersion B, by weight, are 2-3 parts of lanthanum nitrate, 30-40 parts of deionized water, 0.2-0.4 parts of cerium nitrate, and 50-60 parts of trisodium phosphate solution; In step 3, the composite powder is prepared by mixing dispersion A and dispersion B in a mass ratio of 1: (1 to 1.5), hydrothermally reacting at 160 to 200° C. for 18 to 24 hours, washing with ethanol and water several times, drying, and calcining at 400 to 500° C. for 2 to 3 hours to obtain a composite powder; In step 4, the mixed slurry contains, by weight percentage, 5 to 10% of the composite powder and 90 to 95% of the mullite precursor sol.

2. The preparation method of a high-temperature resistant silicon carbide electrothermal element according to claim 1, characterized in that: In step 1, the amount of each component used, by weight percentage, is 70-80% silicon carbide powder, 10-20% silicon carbide fiber, 3-5% graphite, 1-2% metallic silicon powder, and 0.5-3% binder.

3. The preparation method of a high-temperature resistant silicon carbide electrothermal element according to claim 1, characterized in that: In step 2, the molar ratio of water, ethanol and aluminum chloride hexahydrate in the aluminum sol is (10-14): (2-3): 1; and the molar ratio of ethanol, tetraethyl orthosilicate and water in the silica sol is (3-4): 4: (3-4).

4. The preparation method of a high-temperature resistant silicon carbide electrothermal element according to claim 1, characterized in that: In step 2, the water bath temperature is 50-65° C., the stirring reaction time is 1-2 h, and the stirring reaction time of the silica sol is 0.5-1 h.

5. The preparation method of a high-temperature resistant silicon carbide electrothermal element according to claim 1, characterized in that: In step 2, in the mullite precursor sol, the molar ratio of ethyl orthosilicate, aluminum chloride hexahydrate and 1,2-propylene oxide is 1:3:(6-9).

6. The preparation method of a high-temperature resistant silicon carbide electrothermal element according to claim 1, characterized in that: In step 4, the calcination temperature is 1200-1500° C., and the calcination time is 3-5 hours. 7 . A high-temperature resistant silicon carbide electric heating element prepared by the method for preparing a high-temperature resistant silicon carbide electric heating element according to any one of claims 1 to 6 .