A method for preparing a coating material for a thermocouple surface

Abstract

This invention relates to a method for preparing a coating material for long-life boiler thermocouples, belonging to the field of power plant boiler temperature measurement technology. The preparation steps are as follows: (1) degumming treatment of alumina fiber cloth; (2) sandblasting treatment of the surface of the thermocouple protective sleeve; (3) winding alumina fiber cloth around the thermocouple protective sleeve; (4) preparing a ceramic slurry with moderate viscosity and high solid content as an impregnation liquid; (5) vacuum impregnation; (6) drying and sintering to obtain a coating material on the surface of the thermocouple protective sleeve. The alumina fiber cloth wound around the thermocouple protective sleeve of this invention has excellent properties, ensuring high strength and corrosion resistance when used in boilers, effectively improving the thermocouple's erosion resistance, and making the actual service life of the thermocouple exceed 2 years; at the same time, the addition of thermally conductive filler in the coating ensures the thermal response speed of the thermocouple, which is beneficial for timely judgment of the furnace temperature.

Description

Technical Field

[0001] This invention belongs to the field of power plant boiler temperature measurement technology, specifically relating to a method for preparing coating materials for the surface of thermocouples used in boilers. Background Technology

[0002] For power plant boilers such as waste incinerators, fluidized bed boilers, and pulverized coal boilers, bed temperature is a key parameter for stable and economical operation, used to monitor and protect the unit's operation. It is primarily determined through measurement and calculation using multiple thermocouples arranged in the combustion chamber. In waste incinerators, the average number of thermocouples per unit is 60-90. When used in waste incinerators, thermocouples must withstand both the high-temperature erosion from fly ash particles and the severe high-temperature corrosion from flue gas. Under the combined effects of these two factors, armored thermocouples quickly become unusable due to erosion and perforation. Currently, the armored thermocouples used in boilers lack surface protection, relying solely on the material of the protective sheath to resist corrosion. However, the protective sheath is typically made of materials such as stainless steel and nickel-based alloys, which cannot be used for extended periods in the boiler environment. Therefore, the thermocouple lifespan is approximately 6 months. Since thermocouples are consumables requiring frequent replacement, this increases the operating costs of the power plant. Improving the lifespan of boiler thermocouples would contribute to improving the economic efficiency of the power plant. Summary of the Invention

[0003] To improve the service life of thermocouples in power plant boilers, this invention provides a method for preparing a coating material for the surface of thermocouples.

[0004] The preparation steps of a coating material for a thermocouple surface are as follows:

[0005] (1) Degumming treatment of alumina fiber cloth

[0006] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive, and then cooled with the furnace to obtain the de-adhesive alumina fiber cloth.

[0007] The alumina fiber cloth is made of 95% alumina and 5% silicon dioxide by mass.

[0008] (2) Sandblasting treatment of thermocouple surface

[0009] The surface of the protective sheath of the thermocouple is sandblasted with a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15-25µm, thus obtaining a clean thermocouple.

[0010] (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple.

[0011] Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 2-4 hours; then evenly wrap the soaked degummed alumina fiber cloth around the protective sleeve of the clean thermocouple, wrapping 2-5 layers to obtain the wound fiber cloth thermocouple.

[0012] The mixed solution is prepared by uniformly mixing deionized water and a desiccant.

[0013] (4) Preparation of ceramic impregnation solution

[0014] 2-8 parts by weight of sintering aid, 80-120 parts by weight of thermally conductive filler and 80-100 parts by weight of inorganic binder are slowly added to 80-110 parts by weight of dispersion, stirred thoroughly and then ball-milled to obtain ceramic impregnation solution;

[0015] (5) Vacuum impregnation

[0016] The thermocouple wrapped with fiber cloth is completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution is filled into the pores of the degummed alumina fiber cloth under vacuum conditions to obtain a coated thermocouple.

[0017] (6) Sintering treatment

[0018] The coated thermocouple is dried in a constant temperature drying process; the dried coated thermocouple is sintered in a muffle furnace; and cooled in the furnace to obtain a thermocouple with a coating material on the surface of the protective sleeve.

[0019] The coating material on the surface of the thermocouple's protective sheath has a thickness of 200–600 μm, a porosity of 3%–6%, a bonding strength of 15.1–26.2 MPa, and a thermal conductivity of 16.3–33.7 W / (m·K).

[0020] The further defined technical solution is as follows:

[0021] In step (1), the degumming heating conditions of the muffle furnace are: temperature 500℃~600℃, holding time 1h~3h.

[0022] In step (3), the mixed solution is prepared by uniformly mixing 100-150 parts by weight of water and 50-100 parts by weight of water absorbent.

[0023] In step (3), the absorbent is two of glycerol, propylene glycol, and butylene glycol, and the mass ratio of the two absorbents is 1:1.

[0024] In step (4), 1 to 5 parts by mass of dispersant are added to 80 to 110 parts by mass of deionized water and mixed evenly by electromagnetic stirring to obtain a dispersion.

[0025] In step (4), the dispersant is polyacrylic acid or ammonium polyacrylate.

[0026] In step (4), the sintering aid is magnesium oxide; the thermally conductive filler is at least one of boron nitride, silicon carbide, and aluminum nitride, and the average particle size of the thermally conductive filler is 1-5µm; the inorganic binder is at least one of aluminum dihydrogen phosphate, silica sol, and water glass; and the modulus of the water glass is 3.2.

[0027] In step (4), the ball milling time is 6h to 9h.

[0028] In step (5), the vacuum conditions are: temperature 20℃~25℃, vacuum degree 0.5~1MPa, and time 50min~60min.

[0029] In step (6), the drying conditions are: heating rate of 4℃~8℃ / min, temperature below 100℃, and time of 2h~3h; the sintering conditions are: heating rate of 4℃~8℃ / min, temperature of 1000℃~1300℃, and time of 2h~3h.

[0030] Compared with the prior art, the beneficial technical effects of the present invention are reflected in the following aspects:

[0031] 1. The thermocouple coating material prepared by this invention for power plant boilers solves the problem of short service life caused by high-temperature corrosion and erosion of thermocouples when used in boilers. The coating material is made of alumina fiber cloth, which has a service temperature of 1450℃~1600℃ and possesses high strength and excellent corrosion resistance. Due to its superior properties, it ensures high strength and corrosion resistance when used in boilers, effectively improving the thermocouple's erosion resistance. The actual service life of the thermocouple exceeds 2 years, significantly reducing the economic cost of maintenance and replacement.

[0032] 2. The coating material prepared by this invention has high thermal conductivity. Boron nitride ceramics have a high thermal conductivity similar to that of metals, approximately 80 W / (m·K), and also have a low coefficient of thermal expansion of 20 × 10⁻⁶. -6 The temperature range is [temperature value missing]℃, indicating good thermal shock resistance. Silicon carbide has a very high theoretical thermal conductivity, reaching 270 W / (m·K); aluminum nitride has a thermal conductivity greater than 170 W / (m·K). Using boron nitride, silicon carbide, and aluminum nitride as raw materials to prepare the ceramic coating can effectively improve the thermal response speed of the thermocouple while enhancing its erosion resistance. The thermally conductive filler used in preparing the coating material is a ceramic powder with high thermal conductivity. The sintered coated thermocouple also exhibits high thermal conductivity, ensuring a fast thermal response speed, which is beneficial for timely furnace temperature determination and improves the accuracy of thermocouple temperature measurement. Attached Figure Description

[0033] Figure 1 This is a structural diagram of the thermocouple of the present invention.

[0034] Figure 2 for Figure 1 A sectional view. Detailed Implementation

[0035] To better understand the technical solution of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0036] Example 1

[0037] The preparation steps of a coating material for thermocouples are as follows:

[0038] (1) Degumming treatment of alumina fiber cloth

[0039] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive. The temperature is raised to 600°C in the air and held for 3 hours. The cloth is then cooled in the furnace to obtain the de-adhesive alumina fiber cloth.

[0040] Alumina fiber cloth is made of 95% alumina and 5% silica by mass.

[0041] (2) Sandblasting treatment of thermocouple surface

[0042] The surface of the thermocouple's protective sheath is sandblasted using a sandblasting machine. The protective sheath is 120 cm long. This process ensures that the surface cleanliness of the thermocouple reaches Sa3.0 or higher, and the surface roughness reaches 15-25 µm, resulting in a clean thermocouple.

[0043] (3) Mix glycerol and propylene glycol evenly at a mass ratio of 1:1 to obtain a water absorbent.

[0044] Mix 150g of deionized water and 80g of absorbent to make a mixed solution. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 4 hours. The absorbent will prevent moisture loss and keep the alumina fiber cloth soft, making it suitable for cutting, laying and wrapping. Wrap the soaked degummed alumina fiber cloth evenly around the protective sleeve of the clean thermocouple, wrapping 5 layers to obtain the wound fiber cloth thermocouple.

[0045] (4) Preparation of ceramic impregnation solution

[0046] (4.1) Add 4 g of polyacrylic acid to 100 g of deionized water and mix evenly by electromagnetic stirring to obtain a dispersion;

[0047] (4.2) 6 g of magnesium oxide, 50 g of boron nitride, 50 g of silicon carbide, 50 g of aluminum dihydrogen phosphate and 50 g of silica sol were slowly added to 104 g of dispersion, stirred thoroughly and uniformly, and ball-milled for 8 h to obtain ceramic impregnation solution.

[0048] The ceramic impregnation solution has good fluidity and stability, which allows it to fill the fiber pores of the alumina fiber cloth smoothly.

[0049] Magnesium oxide is a sintering aid. Adding an appropriate amount of magnesium oxide can lower the sintering temperature, inhibit grain growth, reduce the porosity of alumina ceramics, and improve their density.

[0050] Boron nitride and silicon carbide are used as thermally conductive fillers, and the average particle size of the thermally conductive fillers is 2μm.

[0051] Aluminum dihydrogen phosphate and silica sol are inorganic binders.

[0052] (5) Vacuum impregnation

[0053] The thermocouple wrapped with fiber cloth was completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution was filled into the pores of the degummed alumina fiber cloth under vacuum conditions: temperature 25℃, vacuum degree 0.9MPa, time 60min, to obtain coated thermocouple.

[0054] (6) Sintering treatment

[0055] The coated thermocouple was dried in a constant temperature drying process, with the drying conditions being a heating rate of 5℃ / min, a temperature below 100℃, and a time of 3 hours.

[0056] In a muffle furnace, the dried coated thermocouples were sintered under the following conditions: heating rate 5°C / min, temperature 1300°C, and time 3 hours. After furnace cooling, thermocouples with a coating material on the protective sheath surface were obtained. (See [link to relevant documentation]). Figure 1 and Figure 2 .

[0057] The coating material on the protective sheath surface of the thermocouple in Example 1 has a thickness of 600 μm as measured by scanning electron microscopy; a porosity of 3%; a composite ceramic coating bonding strength of 25.9 MPa as tested by T 8642-2002; a coating thermal conductivity of 33.7 W / m·K; and a strength of 3 mg / cm³ at 650℃. 2 The high-temperature corrosion resistance of the composite ceramic coating was tested by incubating it with a mixed salt of KCl and Na2SO4 for 60 hours. The mass change was 0.127 mg / mm. 2 .

[0058] Example 2

[0059] The preparation steps of a coating material for thermocouples are as follows:

[0060] (1) Degumming treatment of alumina fiber cloth

[0061] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive. The temperature is raised to 600°C in the air and held for 3 hours. The cloth is then cooled in the furnace to obtain the de-adhesive alumina fiber cloth.

[0062] Alumina fiber cloth is made of 95% alumina and 5% silica by mass.

[0063] (2) Sandblasting treatment of thermocouple surface

[0064] The surface of the protective sheath of the thermocouple is sandblasted using a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15-25µm, thus obtaining a clean thermocouple.

[0065] (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple.

[0066] Glycerol and propylene glycol are mixed evenly at a mass ratio of 1:1 to obtain a water-absorbing agent.

[0067] Mix 150g of deionized water and 60g of absorbent to make a mixed solution. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 4 hours. The absorbent will prevent moisture loss and keep the alumina fiber cloth soft, making it suitable for cutting, laying and wrapping. Wrap the soaked degummed alumina fiber cloth evenly around the protective sleeve of the clean thermocouple, wrapping 5 layers to obtain the wound fiber cloth thermocouple.

[0068] (4) Preparation of ceramic impregnation solution

[0069] (4.1) Add 4 g of ammonium polyacrylate to 100 g of deionized water and mix evenly by electromagnetic stirring to obtain a dispersion;

[0070] (4.2) 6 g of magnesium oxide, 50 g of boron nitride, 50 g of aluminum nitride, 50 g of aluminum dihydrogen phosphate and 50 g of water glass were slowly added to 104 g of dispersion, stirred thoroughly, and ball-milled for 9 h to obtain ceramic impregnation solution. The ceramic impregnation solution has good fluidity and stability, which enables it to fill the fiber pores of alumina fiber cloth smoothly.

[0071] Magnesium oxide is a sintering aid. Adding an appropriate amount of magnesium oxide can lower the sintering temperature, inhibit grain growth, reduce the porosity of alumina ceramics, and improve their density.

[0072] Boron nitride and aluminum nitride are used as thermally conductive fillers, and the average particle size of the thermally conductive fillers is 2μm.

[0073] Aluminum dihydrogen phosphate and water glass are inorganic binders, with the modulus of water glass being 3.2.

[0074] (5) Vacuum impregnation

[0075] The thermocouple wrapped with fiber cloth was completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution was filled into the pores of the degummed alumina fiber cloth under vacuum conditions, namely: temperature 25℃, vacuum degree 0.9MPa, and time 60min, to obtain the coated thermocouple.

[0076] (6) Sintering treatment

[0077] The coated thermocouple was dried in a constant temperature drying process, with the drying conditions being a heating rate of 5℃ / min, a temperature below 100℃, and a time of 3h; the dried coated thermocouple was sintered in a muffle furnace, with the sintering conditions being a heating rate of 5℃ / min, a temperature of 1300℃, and a time of 3h; and cooled in the furnace to obtain a thermocouple with a coating material on the surface of the protective sleeve.

[0078] The coating material on the protective sheath surface of the thermocouple in Example 2, measured by scanning electron microscopy, has a thickness of 570 μm and a porosity of 4%. The composite ceramic coating bonding strength, tested according to GB / T 8642-2002, is 26.2 MPa. The coating thermal conductivity is 28.6 W / m·K. The coating also exhibits a thermal conductivity of 3 mg / cm³ at 650℃. 2 The high-temperature corrosion resistance of the composite ceramic coating was tested by incubating it with a mixed salt of KCl and Na2SO4 for 60 hours. The mass change was 0.279 mg / mm. 2 .

[0079] Example 3

[0080] The preparation steps of a coating material for thermocouples are as follows:

[0081] (1) Degumming treatment of alumina fiber cloth

[0082] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive. The temperature is raised to 550°C in the air and held for 2 hours. The cloth is then cooled in the furnace to obtain the de-adhesive alumina fiber cloth.

[0083] Alumina fiber cloth is made of 95% alumina and 5% silica by mass.

[0084] (2) Sandblasting treatment of thermocouple surface

[0085] The surface of the thermocouple protective sheath is sandblasted using a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15–25 µm, thus obtaining a clean thermocouple.

[0086] (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple.

[0087] Glycerol and butanediol are mixed evenly in a mass ratio of 1:1 to obtain a water-absorbing agent.

[0088] Mix 130g of deionized water and 80g of absorbent to make a mixed solution. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 4 hours. The absorbent will prevent moisture loss and keep the alumina fiber cloth soft, making it suitable for cutting, laying and wrapping. Wrap the soaked degummed alumina fiber cloth evenly around the protective sleeve of the clean thermocouple, wrapping 5 layers to obtain the wound fiber cloth thermocouple.

[0089] (4) Preparation of ceramic impregnation solution

[0090] (4.1) Add 2 grams of polyacrylic acid to 100 grams of deionized water and mix evenly by electromagnetic stirring to obtain a dispersion;

[0091] (4.2) 6 g of magnesium oxide, 100 g of boron nitride and 80 g of aluminum dihydrogen phosphate were slowly added to 102 g of dispersion, stirred thoroughly and evenly, and ball-milled for 9 h to obtain ceramic impregnation solution. The ceramic impregnation solution has good fluidity and stability, which allows it to fill the fiber pores of alumina fiber cloth smoothly.

[0092] Magnesium oxide is a sintering aid. Adding an appropriate amount of magnesium oxide can lower the sintering temperature, inhibit grain growth, reduce the porosity of alumina ceramics, and improve their density.

[0093] Boron nitride is used as a thermally conductive filler, and the average particle size of the thermally conductive filler is 2 μm.

[0094] Aluminum dihydrogen phosphate is an inorganic binder.

[0095] (5) Vacuum impregnation

[0096] The thermocouple wrapped with fiber cloth was completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution was filled into the pores of the degummed alumina fiber cloth under vacuum conditions, namely: temperature 20℃, vacuum degree 0.6MPa, and time 50min, to obtain the coated thermocouple.

[0097] (6) Sintering treatment

[0098] The coated thermocouple was dried in a constant temperature drying process, with the drying conditions being a heating rate of 5℃ / min, a temperature below 100℃, and a time of 3h; the dried coated thermocouple was sintered in a muffle furnace, with the sintering conditions being a heating rate of 5℃ / min, a temperature of 1300℃, and a time of 3h; and cooled in the furnace to obtain a thermocouple with a coating material on the surface of the protective sleeve.

[0099] The coating material on the protective sheath surface of the thermocouple in Example 3, as measured by scanning electron microscopy, has a thickness of 550 μm and a porosity of 6%. The composite ceramic coating bonding strength, as tested according to GB / T 8642-2002, is 22.3 MPa. The coating thermal conductivity is 24.6 W / m·K. The coating also exhibits a thermal conductivity of 3 mg / cm³ at 650℃. 2 The high-temperature corrosion resistance of the composite ceramic coating was tested by incubating it with a mixed salt of KCl and Na2SO4 for 60 hours. The mass change was 0.317 mg / mm. 2 .

[0100] Example 4

[0101] The preparation steps of a coating material for thermocouples are as follows:

[0102] (1) Degumming treatment of alumina fiber cloth

[0103] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive. The temperature is raised to 550°C in the air and held for 2 hours. The cloth is then cooled in the furnace to obtain the de-adhesive alumina fiber cloth.

[0104] Alumina fiber cloth is made of 95% alumina and 5% silica by mass.

[0105] (2) Sandblasting treatment of thermocouple surface

[0106] The surface of the thermocouple protective sheath is sandblasted using a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15–25 µm, thus obtaining a clean thermocouple.

[0107] (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple.

[0108] Glycerol and butanediol are mixed evenly in a mass ratio of 1:1 to obtain a water-absorbing agent.

[0109] Mix 110g of deionized water and 60g of absorbent to make a mixed solution. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 2 hours. The absorbent will prevent moisture loss and keep the alumina fiber cloth soft, making it suitable for cutting, laying and wrapping. Wrap the soaked degummed alumina fiber cloth evenly around the protective sleeve of the clean thermocouple, wrapping it 3 times to obtain the wound fiber cloth thermocouple.

[0110] (4) Preparation of ceramic impregnation solution

[0111] (4.1) Add 2 g of ammonium polyacrylate to 100 g of deionized water and mix evenly by electromagnetic stirring to obtain a dispersion;

[0112] (4.2) Slowly add 2 g of magnesium oxide, 100 g of aluminum nitride and 80 g of water glass to 102 g of dispersion, stir thoroughly and evenly, ball mill for 6 h to obtain ceramic impregnation solution. The ceramic impregnation solution has good fluidity and stability, which enables it to fill the fiber pores of alumina fiber cloth smoothly.

[0113] Magnesium oxide is a sintering aid. Adding an appropriate amount of magnesium oxide can lower the sintering temperature, inhibit grain growth, reduce the porosity of alumina ceramics, and improve their density.

[0114] Aluminum nitride is used as the thermally conductive filler, and the average particle size of the thermally conductive filler is 3μm.

[0115] Water glass is an inorganic binder with a modulus of 3.2.

[0116] (5) Vacuum impregnation

[0117] The thermocouple wrapped with fiber cloth was completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution was filled into the pores of the degummed alumina fiber cloth under vacuum conditions, namely: temperature 20℃, vacuum degree 0.6MPa, and time 60min, to obtain the coated thermocouple.

[0118] (6) Sintering treatment

[0119] The coated thermocouple is dried in a constant temperature drying process, wherein the drying conditions are a heating rate of 8℃ / min, a temperature below 100℃, and a time of 2h; the dried coated thermocouple is sintered in a muffle furnace, wherein the sintering conditions are a heating rate of 8℃ / min, a temperature of 1300℃, and a time of 2h; and cooled in the furnace to obtain a thermocouple with a coating material on the surface of the protective sleeve.

[0120] The coating material on the protective sheath of the thermocouple in Example 4 has a thickness of 360 μm as measured by scanning electron microscopy; a porosity of 3%; a composite ceramic coating bonding strength of 19.2 MPa as tested by GB / T 8642-2002; a coating thermal conductivity of 21.6 W / m·K; and a strength of 3 mg / cm² at 650℃. 2 The high-temperature corrosion resistance of the composite ceramic coating was tested by incubating it with a mixed salt of KCl and Na2SO4 for 60 hours. The mass change was 0.236 mg / mm. 2 .

[0121] Example 5

[0122] The preparation steps of a coating material for thermocouples are as follows:

[0123] (1) Degumming treatment of alumina fiber cloth

[0124] In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive. The temperature is raised to 500°C in the air and held for 2 hours. The cloth is then cooled in the furnace to obtain the de-adhesive alumina fiber cloth.

[0125] Alumina fiber cloth is made of 95% alumina and 5% silica by mass.

[0126] (2) Sandblasting treatment of thermocouple surface

[0127] The surface of the thermocouple protective sheath is sandblasted using a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15–25 µm, thus obtaining a clean thermocouple.

[0128] (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple.

[0129] Glycerol and butanediol are mixed evenly in a mass ratio of 1:1 to obtain a water-absorbing agent.

[0130] Mix 150g of deionized water and 80g of absorbent to make a mixed solution. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 2 hours. The absorbent will prevent moisture loss and keep the alumina fiber cloth soft, making it suitable for cutting, laying and wrapping. Wrap the soaked degummed alumina fiber cloth evenly around the protective sleeve of the clean thermocouple, wrapping it twice to obtain a wound fiber cloth thermocouple.

[0131] (4) Preparation of ceramic impregnation solution

[0132] (4.1) Add 1 gram of ammonium polyacrylate to 80 grams of deionized water and mix evenly by electromagnetic stirring to obtain a dispersion;

[0133] (4.2) 2 g magnesium oxide, 80 g boron nitride, 40 g silicon carbide and 80 g silica sol were slowly added to 81 g dispersion, stirred thoroughly and evenly, and ball milled for 6 h to obtain ceramic impregnation liquid. The ceramic impregnation liquid has good fluidity and stability, which enables it to fill the fiber pores of alumina fiber cloth smoothly.

[0134] Magnesium oxide is a sintering aid. Adding an appropriate amount of magnesium oxide can lower the sintering temperature, inhibit grain growth, reduce the porosity of alumina ceramics, and improve their density.

[0135] Boron nitride and silicon carbide are used as thermally conductive fillers, and the average particle size of the thermally conductive fillers is 5 μm.

[0136] Silica sol is an inorganic binder.

[0137] (5) Vacuum impregnation

[0138] The thermocouple wrapped with fiber cloth was completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution was filled into the pores of the degummed alumina fiber cloth under vacuum conditions, namely: temperature 25℃, vacuum degree 0.9MPa, and time 60min, to obtain the coated thermocouple.

[0139] (6) Sintering treatment

[0140] The coated thermocouple is dried in a constant temperature drying process, wherein the drying conditions are a heating rate of 8℃ / min, a temperature below 100℃, and a time of 2h; the dried coated thermocouple is sintered in a muffle furnace, wherein the sintering conditions are a heating rate of 5℃ / min, a temperature of 1000℃, and a time of 2h; and cooled in the furnace to obtain a thermocouple with a coating material on the surface of the protective sleeve.

[0141] The coating material on the protective sheath of the thermocouple in Example 5 has a thickness of 200 μm as measured by scanning electron microscopy; a porosity of 4%; a composite ceramic coating bonding strength of 15.1 MPa as tested by GB / T 8642-2002; a coating thermal conductivity of 16.3 W / m·K; and a strength of 3 mg / cm² at 650℃. 2 The high-temperature corrosion resistance of the composite ceramic coating was tested by incubating it with a mixed salt of KCl and Na2SO4 for 60 hours. The mass change was 0.278 mg / mm. 2 .

Claims

1. A method of preparing a coating material for a thermocouple surface, characterized by, The operation steps are as follows: (1) Degumming treatment of alumina fiber cloth In a muffle furnace, the alumina fiber cloth is heated to remove the adhesive, and then cooled with the furnace to obtain the de-adhesive alumina fiber cloth. The alumina fiber cloth is made of 95% alumina and 5% silicon dioxide by mass. (2) Sandblasting treatment of thermocouple surface The surface of the protective sheath of the thermocouple is sandblasted with a sandblasting machine to achieve a surface cleanliness of Sa3.0 or higher and a surface roughness of 15-25µm, thus obtaining a clean thermocouple. (3) Wrap a de-adhesive alumina fiber cloth around the clean thermocouple. Soak the degummed alumina fiber cloth in the mixed solution and let it stand for 2-4 hours; then evenly wrap the soaked degummed alumina fiber cloth around the protective sleeve of the clean thermocouple, wrapping 2-5 layers to obtain the wound fiber cloth thermocouple. The mixed solution is prepared by uniformly mixing 100-150 parts by weight of deionized water and 50-100 parts by weight of desiccant. The water-absorbing agent is two of glycerol, propylene glycol, and butylene glycol, and the mass ratio of the two water-absorbing agents is 1:

1. (4) Preparation of ceramic impregnation solution 2-8 parts by weight of sintering aid, 80-120 parts by weight of thermally conductive filler and 80-100 parts by weight of inorganic binder are slowly added to 80-110 parts by weight of dispersion, stirred thoroughly and then ball-milled to obtain ceramic impregnation solution; The sintering aid is magnesium oxide; The thermally conductive filler is at least one of boron nitride, silicon carbide, and aluminum nitride. The inorganic binder is at least one of aluminum dihydrogen phosphate, silica sol, and water glass. Add 1-5 parts by weight of dispersant to 80-110 parts by weight of deionized water, and mix evenly by electromagnetic stirring to obtain a dispersion. The dispersant is polyacrylic acid or ammonium polyacrylate; (5) Vacuum impregnation The thermocouple wrapped with fiber cloth is completely immersed in ceramic impregnation solution. In a vacuum drying oven, the ceramic impregnation solution is filled into the pores of the degummed alumina fiber cloth under vacuum conditions to obtain a coated thermocouple. (6) Sintering treatment The coated thermocouple is dried in a constant temperature drying process; In a muffle furnace, the dried coated thermocouples are sintered. The thermocouple with a coating material on the surface of the protective sleeve is obtained by cooling in the furnace. The coating material on the surface of the thermocouple's protective sheath has a thickness of 200–600 μm, a porosity of 3%–6%, a bonding strength of 15.1–26.2 MPa, and a thermal conductivity of 16.3–33.7 W / (m·K).

2. The method for preparing a coating material for thermocouple surfaces according to claim 1, characterized in that: In step (1), the degumming heating conditions of the muffle furnace are: temperature 500℃~600℃, holding time 1h~3h.

3. The method for preparing a coating material for thermocouple surfaces according to claim 1, characterized in that: In step (4), the average particle size of the thermally conductive filler is 1 to 5 µm; the modulus of the water glass is 3.

2.

4. The method for preparing a coating material for thermocouple surfaces according to claim 1, characterized in that: In step (4), the ball milling time is 6h to 9h.

5. The method for preparing a coating material for a thermocouple surface according to claim 1, characterized in that: In step (5), the vacuum conditions are: temperature 20℃~25℃, vacuum degree 0.5~1MPa, and time 50min~60min.

6. The method for preparing a coating material for a thermocouple surface according to claim 1, characterized in that: In step (6), the drying conditions are: heating rate of 4℃~8℃ / min, temperature below 100℃, and time of 2h~3h; the sintering conditions are: heating rate of 4℃~8℃ / min, temperature of 1000℃~1300℃, and time of 2h~3h.

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