Anti-falling material for descending cylinder expansion joint

Abstract

The invention belongs to the technical field of metal surface functional coating composite materials, and particularly relates to an anti-falling material for a descending cylinder expansion joint. A metal matrix ceramic coating composite material is selected as an anti-falling material for the descending cylinder expansion joint, components in the coating are modified, and by means of the combined action of modified aluminum oxide, surface treatment silicon carbide and a silica sol composite material, the adhesive force of the material is effectively increased, the hardness is improved, and the service life of the descending cylinder expansion joint is prolonged. And meanwhile, good high-temperature resistance is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal surface functional coating composite materials, and particularly relates to an anti-detachment material for a downcomer expansion joint. Background Art

[0002] The downcomer expansion joint (a key component in industrial equipment such as metallurgy, chemical industry, and electric power) is an important component used to absorb the dimensional changes caused by thermal expansion and contraction in the pipeline system. To ensure the reliability and safety of the expansion joint, it is crucial to select a suitable anti-detachment material. The anti-detachment material for the downcomer expansion joint needs to maintain structural stability and functionality under complex working conditions. The selection of the anti-detachment material requires comprehensive consideration of mechanical properties, environmental tolerance, and economy, and composite structures or surface modification technologies are adopted when necessary. Coating a ceramic coating on a metal matrix can effectively extend the service life of metal instruments. Ceramic materials have characteristics such as high heat resistance, high hardness, and stable chemical properties, and have an ionic bond and covalent bond structure with a relatively high bond energy.

[0003] Chinese Patent (publication number CN107747083B) discloses a metal matrix ceramic coating and its preparation method. Using the dip-coating method technology, with self-made aluminum dihydrogen phosphate as the binder, ceramic aggregate as the main base material, and nano-curing agent as the auxiliary base material, the optimal dip-coating process ratio is achieved by adjusting the slurry ratio, and finally, it is cured at high temperature to obtain a ceramic coating with a certain thickness and dense phase. The metal matrix ceramic coating provided by this invention not only has advantages such as controllable thickness, high temperature resistance, acid and corrosion resistance, wear resistance, radiation resistance, high density, and high insulation strength. However, when applying the metal matrix ceramic coating composite material to the anti-detachment material in the prior art, there are problems such as insufficient adhesion force, low hardness, and poor high temperature resistance, which seriously affect its actual use.

[0004] Therefore, there is an urgent need for an anti-detachment material for a downcomer expansion joint, which selects a metal matrix ceramic coating composite material and modifies the components in the coating to increase the adhesion force of the material, improve the hardness, and simultaneously obtain good high temperature resistance. Summary of the Invention

[0005] The purpose of the present invention is to provide an anti-detachment material for a downcomer expansion joint. By selecting a metal matrix ceramic coating composite material and modifying the components in the coating, through the combined action of modified alumina, surface-treated silicon carbide, and silica sol composite materials, the adhesion force of the material is effectively increased, the hardness is improved, and good high temperature resistance is simultaneously obtained.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] The present invention provides an anti - shedding material for a downcomer expansion joint. The anti - shedding material includes a superalloy and a ceramic coating on the surface of the alloy. Among them, the ceramic coating is made by the following steps:

[0008] Step S1: Surface pretreatment of the superalloy substrate;

[0009] Step S2: Prepare the slurry;

[0010] Step S3: Form a ceramic coating on the surface of the pretreated superalloy substrate by the dip - coating method with the slurry;

[0011] The preparation method of the slurry includes: by weight, mixing 30 - 36 parts of modified alumina, 2 - 6 parts of surface - treated silicon carbide, 1 - 3 parts of nano - metal oxide, 8 - 12 parts of aluminum dihydrogen phosphate, 2 - 4 parts of silica sol composite material and 20 - 30 parts of deionized water, and ball - milling to obtain the slurry;

[0012] The preparation method of the modified alumina includes: by weight, mixing 2 - 6 parts of activated alumina and 2 - 6 parts of nitric acid for acidification treatment to obtain acidified alumina; adding 2 - 6 parts of the acidified alumina to 100 - 120 parts of lanthanum nitrate solution for impregnation treatment to obtain modified alumina.

[0013] As a preferred scheme, the conditions of the acidification treatment include: stirring at a speed of 80 - 100 r / min for 30 - 40 min, then washing with deionized water, and drying at 102 - 106 °C for 4 - 6 h.

[0014] As a preferred scheme, the conditions of the impregnation treatment include: the mass concentration fraction of the lanthanum nitrate solution is 4 - 8%, the impregnation time is 10 - 12 h, and after impregnation, it is dried at 100 - 105 °C for 6 - 8 h.

[0015] The modified alumina is first activated by nitric acid, and then impregnated with lanthanum nitrate. The introduced lanthanum element will occupy the lattice interstitial or substitution positions of alumina, causing lattice distortion and inhibiting grain growth, thereby improving the hardness of the final ceramic coating.

[0016] As a preferred scheme, the preparation method of the surface - treated silicon carbide includes: by weight, mixing 10 - 12 parts of deionized water and 20 - 24 parts of absolute ethanol evenly, then adding 2 - 4 parts of γ - glycidoxypropyltrimethoxysilane for ultrasonic dispersion, and then adding 2 - 6 parts of silicon carbide for surface treatment to obtain surface - treated silicon carbide.

[0017] As a preferred scheme, the conditions of the surface treatment include: stirring at 40 - 50 °C for 20 - 40 min, and then drying at 80 - 90 °C for 3 - 5 h.

[0018] As a preferred embodiment, the average particle size of the silicon carbide is 5-10 μm.

[0019] The surface-treated silicon carbide introduces epoxy groups through a coupling agent, can form a high-density structure during the high-temperature curing process and act as a buffer layer, effectively relieving the thermal stress generated during the temperature change process, preventing crack propagation, and thus improving the high-temperature resistance of the material.

[0020] As a preferred embodiment, the preparation method of the silica sol composite material includes: by weight, mixing 40-50 parts of silica sol and 10-20 parts of polyvinyl alcohol and stirring, then adding 1-3 parts of non-ionic low-foaming organophosphate and 1-3 parts of defoaming agent for stirring treatment to obtain the silica sol composite material.

[0021] As a preferred embodiment, the conditions of the stirring treatment include: the stirring speed is 300-400 rpm, the temperature is 36-40 °C, and the time is 2-4 h.

[0022] The polyvinyl alcohol introduced into the silica sol composite material has good adhesion and film-forming properties, can form a uniform transition layer on the metal surface, improve the interfacial bonding between the ceramic coating and the metal, and the nano-silica particles in the silica sol fill the interfacial micropores, improving the uniformity of the interface, and the combined effect increases the adhesion force.

[0023] As a preferred embodiment, the superalloy is a nickel-based alloy.

[0024] As a preferred embodiment, the conditions of the surface pretreatment include: first ultrasonically cleaning with acetone for 30-40 min, then ultrasonically degreasing with a sodium hydroxide solution with a mass fraction of 2-6% for 10-20 min, and finally polishing with 100-120 mesh sandpaper.

[0025] As a preferred embodiment, the nano metal oxide is selected from any one or a combination of at least two of nano magnesium oxide, nano copper oxide, and nano zinc oxide.

[0026] As a preferred embodiment, the ball milling time is 4-6 h.

[0027] As a preferred embodiment, the conditions of the dip-coating method include: immersing the pretreated metal substrate in the precursor slurry, slowly pulling it out of the solution, and air-drying and curing at room temperature to form a precursor coating; subjecting the precursor coating to high-temperature curing treatment to obtain the ceramic coating.

[0028] As a preferred embodiment, the temperature of the high-temperature curing treatment is 160-220 °C, and the time is 2-4 h.

[0029] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0030] 1. The lanthanum element introduced by chemical impregnation modification of the modified alumina of the present invention is loaded on the surface of activated alumina. During the high-temperature curing process, part of the lanthanum element will migrate to the surface and enter the bulk phase of the nickel-based alloy, thereby increasing the adhesion between the ceramic coating and the nickel-based alloy. At the same time, polyvinyl alcohol in the silica sol composite can form a uniform transition layer on the metal surface, improving the interfacial bonding between the ceramic coating and the metal. The synergistic effect improves the adhesion of the material; at the same time, the surface-treated silicon carbide introduces epoxy groups through a coupling agent, and the epoxy groups can combine with the hydroxyl groups of polyvinyl alcohol. At the same time, the free H remaining after the acidification modification of the modified alumina + can promote the reaction to proceed, form a network interconnection structure, and improve the hardness and high-temperature resistance of the material.

[0031] 2. The modified alumina of the present invention is first activated by nitric acid and then impregnated with lanthanum nitrate. The introduced lanthanum element will occupy the lattice interstitial or substitution sites of alumina, causing lattice distortion and inhibiting grain growth, thereby improving the hardness of the final ceramic coating.

[0032] 3. The surface-treated silicon carbide of the present invention introduces epoxy groups through a coupling agent, which can form a high-density structure and act as a buffer layer during the high-temperature curing process, effectively alleviating the thermal stress generated during the temperature change process, preventing crack propagation, and thus improving the high-temperature resistance of the material.

[0033] 4. The polyvinyl alcohol introduced in the silica sol composite of the present invention has good adhesion and film-forming properties, can form a uniform transition layer on the metal surface, improve the interfacial bonding between the ceramic coating and the metal, and the nano-silica particles in the silica sol fill the interfacial micropores, improving the uniformity of the interface. The comprehensive effect increases the adhesion. Specific Embodiments

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described 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 any creative work shall fall within the protection scope of the present invention.

[0035] The sources of some components in the examples and comparative examples are as follows:

[0036] Nickel-based alloy, model KF-Ni60A, purchased from Beijing General Research Institute of Mining and Metallurgy New Materials Technology Co., Ltd.;

[0037] Activated alumina, product number A102878, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0038] Silicon carbide I, grade 66556, average particle size 8 μm, purchased from Hebei Wenlun Metal Materials Co., Ltd.;

[0039] Silicon carbide II, model DH-06, average particle size 38 μm, purchased from Ningxia Dehui Carbon Co., Ltd.;

[0040] Silicon carbide III, item number S121696, average particle size 700 nm, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0041] Nano magnesium oxide, item number M489765, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0042] Nano copper oxide, item number C299304, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0043] Nano zinc oxide, item number Z112848, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0044] Aluminum dihydrogen phosphate, CAS No. 13530-50-2, purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0045] Silica sol, model HW-02, purchased from Jinan Haiwei Chemical Co., Ltd.;

[0046] Nitric acid, CAS No. 7697-37-2, purchased from Sinopharm Chemical Reagent Co., Ltd.;

[0047] Lanthanum nitrate, CAS No. 100587-94-8, purchased from Sinopharm Chemical Reagent Co., Ltd.;

[0048] γ-Glycidoxypropyltrimethoxysilane, CAS No. 2530-83-8, purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0049] Polyvinyl alcohol, item number P139540, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0050] Non-ionic low-foaming organophosphate ester, model PPE 1040, purchased from Lakeland;

[0051] Defoaming agent, grade XPJ-S225, purchased from Changzhou Shunhua Chemical Co., Ltd.

[0052] Example 1

[0053] This example provides an anti-shedding material for a downcomer expansion joint. The anti-shedding material includes a superalloy and a ceramic coating on the alloy surface. Among them, the ceramic coating is made by the following steps:

[0054] Step S1: Pretreat the surface of the nickel-based alloy matrix. First, ultrasonically clean it with acetone for 40 min, then ultrasonically degrease it with a 6% sodium hydroxide solution by mass for 20 min, and finally polish it with 120-mesh sandpaper.

[0055] Step S2: Mix 36 parts of modified alumina, 6 parts of surface-treated silicon carbide, 3 parts of nano-magnesium oxide, 12 parts of aluminum dihydrogen phosphate, 4 parts of silica sol composite material, and 30 parts of deionized water by weight and ball-mill them (the ball-milling time is 6 h) to obtain a slurry.

[0056] Step S3: Immerse the pretreated nickel-based alloy matrix in the precursor slurry, slowly lift it out of the solution, and air-dry and cure it at room temperature to form a precursor coating. Subject the precursor coating to high-temperature curing treatment (temperature: 220 °C, time: 2 h) to obtain the ceramic coating.

[0057] Preparation of the modified alumina: Mix 6 parts of activated alumina and 6 parts of nitric acid by weight for acidification treatment, stir at a speed of 100 r / min for 40 min, then wash with deionized water and dry at 106 °C for 4 h to obtain acidified alumina. Add 6 parts of the acidified alumina to 120 parts of lanthanum nitrate solution for impregnation treatment. The mass concentration fraction of the lanthanum nitrate solution is 8%, the impregnation time is 12 h, and after impregnation, dry it at 105 °C for 6 h to obtain the modified alumina.

[0058] Preparation of the surface-treated silicon carbide: Mix 12 parts of deionized water and 24 parts of absolute ethanol by weight evenly, then add 4 parts of γ-glycidoxypropyltrimethoxysilane for ultrasonic dispersion, and then add 6 parts of silicon carbide I (grade: 66556, average particle size: 8 μm) for surface treatment. Stir at 50 °C for 20 min, and then dry at 90 °C for 3 h to obtain the surface-treated silicon carbide.

[0059] Preparation of the silica sol composite material: Mix 50 parts of silica sol and 20 parts of polyvinyl alcohol by weight and stir, then add 3 parts of non-ionic low-foam organophosphate and 3 parts of defoamer for stirring treatment (stirring speed: 400 rpm, temperature: 40 °C, time: 2 h) to obtain the silica sol composite material.

[0060] Example 2

[0061] This example provides an anti-detachment material for a downcomer expansion joint. The anti-detachment material includes a superalloy and a ceramic coating on the surface of the alloy. Among them, the ceramic coating is made by the following steps:

[0062] Step S1: Pretreat the surface of the nickel-based alloy matrix. First, ultrasonically clean it with acetone for 30 min, then ultrasonically degrease it with a 2% sodium hydroxide solution by mass for 10 min, and finally polish it with 100-mesh sandpaper.

[0063] Step S2: Mix 30 parts of modified alumina, 2 parts of surface-treated silicon carbide, 1 part of nano-copper oxide, 8 parts of aluminum dihydrogen phosphate, 2 parts of silica sol composite material, and 20 parts of deionized water by weight and ball-mill them (the ball-milling time is 4 h) to obtain a slurry.

[0064] Step S3: Immerse the pretreated nickel-based alloy matrix in the precursor slurry, slowly lift it out of the solution, and air-dry and cure it at room temperature to form a precursor coating; subject the precursor coating to high-temperature curing treatment (temperature is 160 °C, time is 4 h) to obtain the ceramic coating.

[0065] Preparation of the modified alumina: Mix 2 parts of activated alumina and 2 parts of nitric acid by weight for acidification treatment, stir at a speed of 80 r / min for 40 min, then wash with deionized water and dry at 102 °C for 6 h to obtain acidified alumina; add 2 parts of the acidified alumina to 100 parts of lanthanum nitrate solution for impregnation treatment, the mass concentration fraction of the lanthanum nitrate solution is 4%, the impregnation time is 12 h, and after impregnation, dry it at 100 °C for 8 h to obtain modified alumina.

[0066] Preparation of the surface-treated silicon carbide: Mix 10 parts of deionized water and 20 parts of absolute ethanol by weight evenly, then add 2 parts of γ-glycidoxypropyltrimethoxysilane and ultrasonically disperse it, and then add 2 parts of silicon carbide I (grade 66556, average particle size 8 μm) for surface treatment, stir at 40 °C for 40 min, and then dry at 80 °C for 5 h to obtain surface-treated silicon carbide.

[0067] Preparation of the silica sol composite material: Mix 40 parts of silica sol and 10 parts of polyvinyl alcohol by weight and stir, then add 1 part of non-ionic low-foam organophosphate and 1 part of defoaming agent for stirring treatment (stirring speed is 300 rpm, temperature is 36 °C, time is 4 h) to obtain the silica sol composite material.

[0068] Example 3

[0069] This example provides an anti-shedding material for a downcomer expansion joint. The anti-shedding material includes a superalloy and a ceramic coating on the alloy surface. Among them, the ceramic coating is made by the following steps:

[0070] Step S1: Pretreat the surface of the nickel-based alloy substrate. First, ultrasonically clean it with acetone for 35 min, then ultrasonically degrease it with a 4% sodium hydroxide solution by mass for 15 min, and finally polish it with 120-mesh sandpaper.

[0071] Step S2: Mix 32 parts of modified alumina, 4 parts of surface-treated silicon carbide, 2 parts of nano-zinc oxide, 10 parts of aluminum dihydrogen phosphate, 3 parts of silica sol composite, and 25 parts of deionized water by weight and ball-mill them (the ball-milling time is 5 h) to obtain a slurry.

[0072] Step S3: Immerse the pretreated nickel-based alloy substrate in the precursor slurry, slowly lift it out of the solution, and air-dry and cure it at room temperature to form a precursor coating; subject the precursor coating to high-temperature curing treatment (temperature: 190 °C, time: 3 h) to obtain the ceramic coating.

[0073] Preparation of the modified alumina: Mix 4 parts of activated alumina and 4 parts of nitric acid by weight for acidification treatment, stir at a speed of 90 r / min for 35 min, then wash with deionized water and dry at 104 °C for 5 h to obtain acidified alumina; add 4 parts of the acidified alumina to 110 parts of lanthanum nitrate solution for impregnation treatment. The mass concentration fraction of the lanthanum nitrate solution is 6%, the impregnation time is 11 h, and after impregnation, dry it at 102 °C for 7 h to obtain the modified alumina.

[0074] Preparation of the surface-treated silicon carbide: Mix 11 parts of deionized water and 22 parts of absolute ethanol by weight evenly, then add 3 parts of γ-glycidoxypropyltrimethoxysilane for ultrasonic dispersion, and then add 4 parts of silicon carbide I (grade number 66556, average particle size 8 μm) for surface treatment. Stir at 45 °C for 30 min, and then dry at 85 °C for 4 h to obtain the surface-treated silicon carbide.

[0075] Preparation of the silica sol composite: Mix 45 parts of silica sol and 15 parts of polyvinyl alcohol by weight and stir, then add 2 parts of non-ionic low-foam organophosphate and 2 parts of defoamer for stirring treatment (stirring speed: 350 rpm, temperature: 37 °C, time: 3 h) to obtain the silica sol composite.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that commercially available activated alumina (product number A102878) is used to replace the modified alumina.

[0078] Comparative Example 2

[0079] The difference between this comparative example and Example 1 is that silicon carbide I is used to replace the surface-treated silicon carbide.

[0080] Comparative Example 3

[0081] The difference between this comparative example and Example 1 is that silicon carbide II is used to replace silicon carbide I for the preparation of surface-treated silicon carbide.

[0082] Comparative Example 4

[0083] The difference between this comparative example and Example 1 is that silicon carbide III is used to replace silicon carbide I for the preparation of surface-treated silicon carbide.

[0084] Comparative Example 5

[0085] The difference between this comparative example and Example 1 is that commercially available silica sol (model HW-02) is used to replace the silica sol composite material.

[0086] Performance Test

[0087] The following tests are carried out on the falling cylinder expansion joints of the above examples and comparative examples with anti-detachment materials:

[0088] (1) Adhesion test: The test is carried out according to the requirements of "GB / T 31586.2-2015 Evaluation and acceptance criteria for the anti-corrosion protection coating adhesion / cohesion (failure strength) of protective coating systems for steel structures - Part 2: Cross-cut test and cross-hatch test".

[0089] (2) Hardness test: The test is carried out according to the requirements of "GB / T 6739-2022 Paints and varnishes - Determination of film hardness by pencil test".

[0090] (3) High-temperature resistance test: The anti-detachment material is heat-treated at 500 °C for 5 h, and whether there is any change in the coating of the material is observed.

[0091] Table 1 Performance test results

[0092] Adhesion Hardness High temperature resistance Example 1 Grade 0 9H The coating has no change Example 2 Grade 0 9H The coating has no change Example 3 Grade 0 9H The coating has no change Comparative example 1 Grade 3 6H Microcracks Comparative example 2 Grade 3 6H Microcracks Comparative example 3 Grade 2 7H Microcracks Comparative example 4 Grade 2 7H Microcracks Comparative example 5 Grade 3 6H Microcracks

[0093] It can be seen from the above performance test results that the anti-detachment materials in Examples 1-3 have the best comprehensive effect, with an adhesion level of 0, a hardness of 9H, and no change in the coating after high-temperature treatment; this is mainly because the components in the coating are modified, and through the combined action of modified alumina, surface-treated silicon carbide, and silica sol composite material, the adhesion of the material is effectively increased, the hardness is improved, and at the same time, good high-temperature resistance performance is obtained.

[0094] In the comparative examples, since the necessary technical solutions were not adopted, their corresponding performance tests were significantly worse than those of the examples. Compared with Example 1, in Comparative Example 1, commercially available activated alumina (product number A102878) was used to replace the modified alumina, resulting in a decrease in adhesion, a reduction in hardness, and a deterioration in high-temperature resistance; compared with Example 1, in Comparative Example 2, silicon carbide I was used to replace the surface-treated silicon carbide, resulting in a decrease in adhesion, a reduction in hardness, and a deterioration in high-temperature resistance; compared with Example 1, in Comparative Example 3, silicon carbide II was used to replace silicon carbide I for the preparation of surface-treated silicon carbide. Due to the too large particle size of silicon carbide II, the modification effect was poor, resulting in a decrease in adhesion, a reduction in hardness, and a deterioration in high-temperature resistance; compared with Example 1, in Comparative Example 4, silicon carbide III was used to replace silicon carbide I for the preparation of surface-treated silicon carbide. Due to the too small particle size of silicon carbide III, the modification effect was poor, resulting in a decrease in adhesion, a reduction in hardness, and a deterioration in high-temperature resistance; compared with Example 1, in Comparative Example 5, commercially available silica sol (model HW-02) was used to replace the silica sol composite, resulting in a decrease in adhesion, a reduction in hardness, and a deterioration in high-temperature resistance. The above experimental results further prove the importance of the technical solutions defined in the present invention for its technical effects.

[0095] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An anti-falling material for a descending tube expansion joint, characterized in that: The anti-shedding material comprises a high-temperature alloy and a ceramic coating on the surface of the alloy, wherein the ceramic coating is made by the following steps: Step S1: pretreatment of the surface of the high-temperature alloy substrate; Step S2: preparing slurry; Step S3: forming a ceramic coating on the surface of the pretreated high-temperature alloy substrate by using the slurry through an immersion and pulling method; The preparation method of the slurry comprises: mixing and ball-milling 30 to 36 parts by weight of modified alumina, 2 to 6 parts by weight of surface-treated silicon carbide, 1 to 3 parts by weight of nano metal oxide, 8 to 12 parts by weight of aluminum dihydrogen phosphate, 2 to 4 parts by weight of silica sol composite material and 20 to 30 parts by weight of deionized water to obtain the slurry; The preparation method of the modified alumina comprises: mixing 2 to 6 parts of active alumina and 2 to 6 parts of nitric acid by weight for acidification to obtain acidified alumina; adding 2 to 6 parts of the acidified alumina into 100 to 120 parts of lanthanum nitrate solution for impregnation to obtain modified alumina.

2. The anti-falling material for the expansion joint of the descending tube according to claim 1 is characterized in that: The conditions of the acidification treatment include: stirring at a speed of 80 to 100 r / min for 30 to 40 minutes, then washing with deionized water, and drying at 102 to 106° C. for 4 to 6 hours.

3. The anti-falling material for the expansion joint of the descending tube according to claim 1 is characterized in that: The conditions for the impregnation treatment include: the mass concentration of the lanthanum nitrate solution is 4-8%, the impregnation time is 10-12 hours, and after the impregnation is completed, the solution is dried at 100-105° C. for 6-8 hours.

4. The anti-falling material for the expansion joint of the descending tube according to claim 1 is characterized in that: The preparation method of the surface-treated silicon carbide comprises: mixing 10 to 12 parts of deionized water and 20 to 24 parts of anhydrous ethanol uniformly by weight, then adding 2 to 4 parts of γ-glycidyloxypropyltrimethoxysilane for ultrasonic dispersion, and then adding 2 to 6 parts of silicon carbide for surface treatment to obtain the surface-treated silicon carbide.

5. The anti-falling material for the expansion joint of the descending tube according to claim 4 is characterized in that: The average particle size of the silicon carbide is 5 to 10 μm.

6. The anti-falling material for the expansion joint of the descending tube according to claim 1 is characterized in that: The preparation method of the silica sol composite material comprises: mixing and stirring 40 to 50 parts of silica sol and 10 to 20 parts of polyvinyl alcohol by weight, and then adding 1 to 3 parts of non-ionic low-foaming organic phosphate and 1 to 3 parts of defoaming agent for stirring to obtain the silica sol composite material.

7. The anti-falling material for the expansion joint of the descending tube according to claim 6 is characterized in that: The stirring treatment conditions include: a stirring speed of 300-400 rpm, a temperature of 36-40° C., and a time of 2-4 hours.

8. The anti-falling material for the expansion joint of the descending tube according to claim 1 is characterized in that: The high temperature alloy is a nickel-based alloy.

9. The anti-falling material for the expansion joint of the descending tube according to claim 1, characterized in that: The surface pretreatment conditions include: firstly using acetone for ultrasonic cleaning for 30 to 40 minutes, then using a sodium hydroxide solution with a mass fraction of 2 to 6% for ultrasonic degreasing for 10 to 20 minutes, and finally using 100 to 120 mesh sandpaper for polishing.

10. The anti-falling material for the expansion joint of the descending tube according to claim 1, characterized in that: The nano metal oxide is selected from any one of nano magnesium oxide, nano copper oxide, and nano zinc oxide, or a combination of at least two thereof.