A high-temperature resistant magnesium oxide electrothermal insulation material and its preparation method

Abstract

This invention relates to the field of insulating materials, specifically a high-temperature resistant magnesium oxide electrothermal insulating material and its preparation method, comprising high-purity nano-magnesium oxide, modified silica, modified cellulose nanofibers, nano-alumina, and zinc borate. The borate groups on the surface of the modified silica undergo a reversible reaction with the ortho-dihydroxy groups of the modified cellulose nanofibers to form dynamic borate ester bonds, automatically repairing cracks. A borate ester performance agent is grafted onto the silica surface; when the outer shell is stress-triggered, the borate ester repair agent is released, achieving cyclic crack repair. The modified cellulose nanofibers exhibit excellent toughness at low temperatures, improving the fracture toughness of the insulating material. The addition of nano-alumina improves the thermal conductivity of the insulating material, and the nano-alumina and zinc borate prevent the modified cellulose nanofibers from decomposing at high temperatures. This enables the magnesium oxide insulating material to automatically repair cracks at low temperatures, extending its service life.

Description

Technical Field

[0001] This invention relates to the field of insulating materials technology, specifically a high-temperature resistant magnesium oxide electrothermal insulating material and its preparation method. Background Technology

[0002] Magnesium oxide (MgO) possesses excellent high-temperature insulation, thermal conductivity, chemical stability, mechanical strength, and voltage breakdown resistance, and is often used as an insulating material in the manufacture of high-temperature fire-resistant cables. Compared to ordinary plastic insulated cables, it has many advantages such as fire resistance, smokelessness, non-toxicity, high temperature resistance, corrosion resistance, high mechanical strength, and long service life.

[0003] However, while cables made with MgO as a traditional insulating material exhibit stable performance, they are also brittle, and their fracture toughness decreases sharply with decreasing temperature. At -20℃, the fracture toughness is less than 60% of that at room temperature. Therefore, low-temperature cracking is a problem when laying cables in cold winter environments. In cold winter environments, if cables are laid at corners or bends, stress exceeding a critical value can easily cause brittle fracture and crack formation. Furthermore, because the fracture toughness of MgO decreases sharply at low temperatures, once microcracks form, they will propagate rapidly under bending stress, leading to overall material cracking or even fragmentation. This results in a sharp decrease in the insulation resistance and dielectric strength of the insulating material, leading to insulation failure. Cracks can also interrupt the heat conduction path within the MgO powder, reducing local thermal conductivity and posing a risk of overheating. Summary of the Invention

[0004] (1) Technical problems to be solved

[0005] The purpose of this invention is to provide a high-temperature resistant magnesium oxide electrothermal insulation material and its preparation method, which can significantly improve the toughness of magnesium oxide insulation material, enabling it to automatically repair brittle fracture cracks, avoid insulation failure, and improve the service life of cables.

[0006] (2) Technical solution

[0007] To achieve the above objectives, on the one hand, the present invention provides a high-temperature resistant magnesium oxide electrothermal insulation material, characterized in that it comprises, by weight, the following raw materials: 150-200 parts of high-purity nano magnesium oxide, 2-5 parts of modified silicon dioxide, 2-5 parts of modified cellulose nanofibers, 1-3 parts of nano alumina, and 1-3 parts of zinc borate.

[0008] The modified silica includes 4 to 10 parts of borate ester performance agent.

[0009] Furthermore, the preparation method of the borate ester performance agent includes the following steps:

[0010] S11. Catechol boric acid and pentaerythritol tetraacrylate are heated and mixed in a water bath, and then magnetically stirred until uniform. Perfluoropolyether oil is added and ultrasonically dispersed. Pt@ZIF-8 catalyst is added and centrifuged and mixed under nitrogen protection. Trichloromethane is added and ultrasonically mixed to obtain the oil phase.

[0011] S12. Polyvinyl alcohol is dissolved in deionized water, sodium dodecyl sulfate is added, the mixture is magnetically stirred, and then deionized water is added to obtain an aqueous phase.

[0012] S13. The oil phase is added dropwise to the aqueous phase, sheared and emulsified, and then transferred to a high-pressure homogenizer for circulation processing to obtain the first compound;

[0013] S14. Urea and formaldehyde are added to a reaction vessel, heated and stirred in a water bath, and NaOH solution is added to adjust the pH to obtain the second compound;

[0014] S15. Add the second compound to the first compound, add citric acid dropwise, stir until the reaction is complete, add melamine, continue heating and stirring, wash and collect the solid precipitate, wash with a mixture of ethanol and water, freeze dry to obtain the borate ester performance agent.

[0015] Furthermore, the method for preparing the modified silica includes the following steps:

[0016] S21. Place silica in a muffle furnace and activate for 4 hours. Cool to room temperature and remove. Add a mixed solution of γ-aminopropyltriethoxysilane / toluene. Reflux under nitrogen protection. Wash with toluene, ethanol and water by centrifugation, and dry in a vacuum drying oven to obtain the third compound.

[0017] S22. The third compound was mixed and dispersed with 4-(6-aldehydehexyl)phenylboronic acid in anhydrous ethanol and refluxed in the dark. The product was placed in a Soxhlet extractor and extracted with ethanol to obtain the fourth compound.

[0018] S23. The borate ester performance agent was treated with NaOH to generate carboxyl groups, mixed with the fourth compound, ultrasonically dispersed, and catalyzed by carbodiimide. The reaction was carried out at 4°C to obtain the fifth compound.

[0019] S24. The fifth compound was placed in a fluidized bed CVD reactor, and perfluorooctyltriethoxysilane vapor was introduced. The reaction was carried out at 120°C to obtain modified silica.

[0020] Furthermore, the preparation method of the modified cellulose nanofibers includes the following steps:

[0021] S31. Pass rice husk powder through an 80-mesh sieve, add HCl, heat and stir, centrifuge and wash until neutral, add NaOH and H2O2, continue heating and stirring to react, wash with deionized water until neutral, and obtain the first mixture;

[0022] S32. Tetramethylpiperidine oxide, NaBr, NaClO, and deionized water are added to the first mixture. The mixture is magnetically stirred in a NaOH buffer solution with pH=10. HCl is added dropwise until pH=7 to terminate the reaction. The mixture is centrifuged and filtered to collect the precipitate. The precipitate is prepared into a suspension with deionized water, processed three times by a high-pressure homogenizer, ultrasonically broken, centrifuged, washed with ethanol, and vacuum dried to obtain the second mixture.

[0023] S33. Disperse the second mixture in water, mix ultrasonically, slowly add NaIO4, adjust the pH with acetic acid, stir the reaction in the dark at 4°C, add excess ethylene glycol, stir the reaction until the conductivity of deionized water is <5μS / cm, and obtain the third mixture;

[0024] S34. After diluting the third mixture, cool it in an ice bath, add NaBH4 in batches, stir continuously and maintain pH=9~10. After the reaction is completed, heat to room temperature, continue stirring, neutralize to neutral with dilute hydrochloric acid, dialyze to remove borate, centrifuge and wash, and freeze dry to obtain modified cellulose nanofibers.

[0025] Furthermore, in the tetramethylpiperidine oxide oxidation reaction, the pH is measured every 30 minutes, and the reaction is terminated when the pH is stable above 9.5 and the NaOH consumption rate is <0.1 mL / min.

[0026] Furthermore, the high-pressure homogenizer has a first processing pressure of 50 MPa and a second and third processing pressure of 100 MPa.

[0027] On the other hand, based on the same inventive concept, the present invention also provides a method for preparing a high-temperature resistant magnesium oxide electrothermal insulation material, applied to the aforementioned high-temperature resistant magnesium oxide electrothermal insulation material, comprising the following steps:

[0028] S1. The modified cellulose nanofibers were dispersed in an ethanol solution, ultrasonically dispersed, magnetically stirred, and the pH was adjusted to neutral with ammonia to obtain a modified cellulose nanofiber dispersion.

[0029] S2. Disperse the modified silica in anhydrous ethanol, let it stand to defoam, and then process it three times in a high-pressure homogenizer. Adjust the pH to neutral with acetic acid to obtain the modified silica dispersion.

[0030] S3. Mix the modified cellulose nanofiber dispersion and the modified silica dispersion in equal proportions, stir magnetically, shear with a high-speed shear machine, and then process with a high-pressure homogenizer. Add NaCl and a mixed solution of 1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide. After the reaction is complete, add glycine to consume the residual 1-ethyl-(3-dimethylaminopropyl)carbodiimide, add hydroxyethyl cellulose, and stir until the viscosity is 480~500 mPa·s. Filter with a PVDF filter membrane to obtain the fourth mixture.

[0031] S4. High-purity nano magnesium oxide is added to deionized water and sodium hexametaphosphate, ball-milled and mixed, pH is adjusted with ammonia, and the fourth mixture, zinc borate pre-dispersed in ethanol solution and nano aluminum oxide are added dropwise. The mixture is placed in a high-speed shearing machine for shearing, and the product is placed in a centrifugal atomizer and dried under nitrogen protection at an inlet temperature of 160℃ and an outlet temperature of 85℃ to obtain the fifth mixture.

[0032] S5. Place the fifth mixture in an isostatic press, pressurize it to 200MPa, hold the pressure for 10 minutes, and then cool and demold it with liquid nitrogen to obtain the sixth mixture.

[0033] S6. Place the sixth mixture in a high-temperature atmosphere furnace, introduce a mixture of argon and H2, sinter by programmed temperature rise, and cool to room temperature to obtain a high-temperature resistant magnesium oxide electrothermal insulation material.

[0034] The mechanism of action of the above raw material components is as follows:

[0035] High-purity nano magnesium oxide: High-purity nano magnesium oxide is the main component of insulating materials, possessing excellent high-temperature insulation, thermal conductivity, chemical stability, mechanical strength, and voltage breakdown resistance.

[0036] Modified silica: Silica fills the grain boundaries of MgO, restricting MgO grain growth and inhibiting its brittle fracture at low temperatures. Furthermore, the silica surface undergoes borate treatment, introducing borate groups that react reversibly with the ortho-dihydroxy groups provided by modified cellulose nanofibers to form dynamic borate ester bonds. When external force causes localized fractures and microcracks in the magnesium oxide insulation material, the borate ester bonds break, absorbing energy and reverting to borate and ortho-dihydroxy groups. Because the microcracks allow moisture intrusion, the broken borate ester bonds spontaneously re-esterify, leading to molecular chain recombination and the reconstruction of the dynamic bond network, thus repairing the cracks. However, due to cracks in the magnesium oxide insulation material, moisture invades along the cracks, forming an alkaline environment in which boric acid groups are easily hydrolyzed. This leads to the loss of boric acid groups during repeated freeze-thaw cycles, resulting in the failure of the repair function. Therefore, it is necessary to graft borate ester performance agents onto the silica surface. When the microcapsule shell is triggered by stress, the borate ester repair agent inside is released to wet the crack interface and recombine with the ortho-dihydroxy group to form a dynamic bond, thus forming a new borate ester bond to repair the crack. Furthermore, the aldehyde component in the repair agent can react with the residual amino group to regenerate the boric acid groups in situ.

[0037] Modified cellulose nanofibers: The introduction of ortho-dihydroxy groups on the surface of cellulose nanofibers provides reaction sites for dynamic borate ester bonds. Furthermore, modified cellulose nanofibers are natural toughening agents with excellent toughness at low temperatures, which can significantly improve the fracture toughness of magnesium oxide insulation materials at low temperatures.

[0038] Nano-alumina: Modified cellulose nanofibers reduce the thermal conductivity of magnesium oxide insulation materials. Therefore, adding nano-alumina improves the thermal conductivity of the insulation material. Furthermore, nano-alumina fills the interface between modified cellulose nanofibers and magnesium oxide, inhibiting grain growth and preventing the structure of modified cellulose nanofibers from collapsing at high temperatures.

[0039] Zinc borate: Modified cellulose nanofibers are easily decomposed at high temperatures. Adding zinc borate allows the modified cellulose nanofibers to be melt-encapsulated at high temperatures, preventing their decomposition.

[0040] (3) Beneficial effects

[0041] Compared with the prior art, the beneficial effects of the present invention are:

[0042] 1. Boric acid groups introduced on the surface of modified silica react reversibly with the ortho-dihydroxy groups on the surface of modified cellulose nanofibers to form dynamic borate ester bonds, which repair brittle cracks in magnesium oxide insulation materials at low temperatures. Furthermore, borate ester performance agents are grafted onto the silica surface to prevent the boric acid groups from hydrolyzing during repeated freeze-thaw cycles, thereby enabling multiple repairs of cracks and extending the service life of magnesium oxide insulation materials.

[0043] 2. Modified cellulose nanofibers, as toughening agents, improve the fracture toughness of magnesium oxide insulation materials, but reduce the thermal conductivity of magnesium oxide insulation materials. Adding nano-alumina can improve the thermal conductivity of the material, and adding zinc borate can alleviate the decomposition of modified cellulose nanofibers at high temperatures. Attached Figure Description

[0044] Figure 1 This is a SEM image of the magnesium oxide insulating material in Example 1 of the present invention at -20°C;

[0045] Figure 2 This is a SEM image of the magnesium oxide insulating material in Example 1 of the present invention at a temperature of 25°C. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] The experimental equipment and formulations used in the embodiments described below are as follows:

[0048] Electronic balance (Sartorius, Germany), electrically heated constant temperature dryer (Shanghai Fuma Experimental Equipment), pH meter (Shanghai Precision Scientific Instruments), magnetic stirrer (Shanghai Meiyingpu), Soxhelt extractor (Qingdao Juchuang), high pressure homogenizer (Kaibaisi), muffle furnace (Tianjin Zhonghuan), vacuum drying oven (Shanghai Bowen), fluidized bed CVD reactor (Hangzhou Qianjiang), ball mill (Xuxinshengke), isostatic press (Tianjin Zhongtuo Technology), centrifugal nebulizer (Hangzhou Xinhai), high temperature atmosphere furnace (Bangshida), universal testing machine (Tianshi Ousen); chemicals and reagents were purchased from Sigma-Aldrich.

[0049] Example 1

[0050] This embodiment discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 150 parts of high-purity nano magnesium oxide, 2 parts of modified silicon dioxide, 2 parts of modified cellulose nanofibers, 1 part of nano aluminum oxide, and 1 part of zinc borate.

[0051] The modified silica includes 4 parts of borate ester performance agent.

[0052] The preparation method of the borate ester performance agent includes the following steps:

[0053] S11. Catechol boric acid and pentaerythritol tetraacrylate are heated and mixed in a water bath, and then magnetically stirred until uniform. Perfluoropolyether oil is added and ultrasonically dispersed. Pt@ZIF-8 catalyst is added and centrifuged and mixed under nitrogen protection. Trichloromethane is added and ultrasonically mixed to obtain the oil phase.

[0054] S12. Polyvinyl alcohol is dissolved in deionized water, sodium dodecyl sulfate is added, the mixture is magnetically stirred, and then deionized water is added to obtain an aqueous phase.

[0055] S13. The oil phase is added dropwise to the aqueous phase, sheared and emulsified, and then transferred to a high-pressure homogenizer for circulation processing to obtain the first compound;

[0056] S14. Urea and formaldehyde are added to a reaction vessel, heated and stirred in a water bath, and NaOH solution is added to adjust the pH to obtain the second compound;

[0057] S15. Add the second compound to the first compound, add citric acid dropwise, stir until the reaction is complete, add melamine, continue heating and stirring, wash and collect the solid precipitate, wash with a mixture of ethanol and water, freeze dry to obtain the borate ester performance agent.

[0058] It should be noted that the Pt@ZIF-8 catalyst is a catalyst in which Pt nanoparticles are encapsulated using ZIF-8 as a metal-organic framework.

[0059] It should be noted that the aqueous phase was prepared by adding deionized water at a volume of 4 times that of the oil phase.

[0060] The method for preparing the modified silica includes the following steps:

[0061] S21. Place silica in a muffle furnace and activate for 4 hours. Cool to room temperature and remove. Add a mixed solution of γ-aminopropyltriethoxysilane / toluene. Reflux under nitrogen protection. Wash with toluene, ethanol and water by centrifugation, and dry in a vacuum drying oven to obtain the third compound.

[0062] S22. The third compound was mixed and dispersed with 4-(6-aldehydehexyl)phenylboronic acid in anhydrous ethanol and refluxed in the dark. The product was placed in a Soxhlet extractor and extracted with ethanol to obtain the fourth compound.

[0063] S23. The borate ester performance agent was treated with NaOH to generate carboxyl groups, mixed with the fourth compound, ultrasonically dispersed, and catalyzed by carbodiimide. The reaction was carried out at 4°C to obtain the fifth compound.

[0064] S24. The fifth compound was placed in a fluidized bed CVD reactor, and perfluorooctyltriethoxysilane vapor was introduced. The reaction was carried out at 120°C to obtain modified silica.

[0065] It should be noted that after cracking, moisture penetrates along the cracks in magnesium oxide insulation, creating an alkaline environment. Boric acid groups are easily hydrolyzed in an alkaline environment, leading to the loss of boric acid groups on the silica surface through repeated freeze-thaw cycles, thus rendering the dynamic bond repair function of the borate ester ineffective. Therefore, a borate ester performance agent needs to be grafted onto the silica surface to supply the repair agent. When the magnesium oxide insulation cracks and the stress at the crack tip exceeds 15 MPa, the outer layer of the borate ester performance agent ruptures, releasing the repair agent and catalyst. These react with ortho-dihydroxy groups to form new borate ester bonds, and the aldehyde component in the repair agent can react with residual amino groups, regenerating boric acid groups in situ, thereby achieving multiple repairs of the dynamic borate ester bonds. Placing the third compound in a fluidized bed CVD reactor and introducing perfluorooctyltriethoxysilane vapor is to form a superhydrophobic layer on the modified silica surface, mitigating the hydrolysis of the borate ester bonds and preventing moisture penetration into the magnesium oxide insulation, which could lead to insulation failure.

[0066] The method for preparing the modified cellulose nanofibers includes the following steps:

[0067] S31. Pass rice husk powder through an 80-mesh sieve, add HCl, heat and stir, centrifuge and wash until neutral, add NaOH and H2O2, continue heating and stirring to react, wash with deionized water until neutral, and obtain the first mixture;

[0068] S32. Tetramethylpiperidine oxide, NaBr, NaClO, and deionized water are added to the first mixture. The mixture is magnetically stirred in a NaOH buffer solution with pH=10. HCl is added dropwise until pH=7 to terminate the reaction. The mixture is centrifuged and filtered to collect the precipitate. The precipitate is prepared into a suspension with deionized water, processed three times by a high-pressure homogenizer, ultrasonically broken, centrifuged, washed with ethanol, and vacuum dried to obtain the second mixture.

[0069] S33. Disperse the second mixture in water, mix ultrasonically, slowly add NaIO4, adjust the pH with acetic acid, stir the reaction in the dark at 4°C, add excess ethylene glycol, stir the reaction until the conductivity of deionized water is <5μS / cm, and obtain the third mixture;

[0070] S34. After diluting the third mixture, cool it in an ice bath, add NaBH4 in batches, stir continuously and maintain pH=9~10. After the reaction is completed, heat to room temperature, continue stirring, neutralize to neutral with dilute hydrochloric acid, dialyze to remove borate, centrifuge and wash, and freeze dry to obtain modified cellulose nanofibers.

[0071] In the oxidation reaction of tetramethylpiperidine oxide, the pH was measured every 30 minutes. The reaction was terminated when the pH was stable above 9.5 and the NaOH consumption rate was <0.1 mL / min.

[0072] The high-pressure homogenizer has a first processing pressure of 50 MPa and a second and third processing pressure of 100 MPa.

[0073] The preparation method of the high-temperature resistant magnesium oxide electrothermal insulation material includes the following steps:

[0074] S1. The modified cellulose nanofibers were dispersed in an ethanol solution, ultrasonically dispersed, magnetically stirred, and the pH was adjusted to neutral with ammonia to obtain a modified cellulose nanofiber dispersion.

[0075] S2. Disperse the modified silica in anhydrous ethanol, let it stand to defoam, and then process it three times in a high-pressure homogenizer. Adjust the pH to neutral with acetic acid to obtain the modified silica dispersion.

[0076] S3. Mix the modified cellulose nanofiber dispersion and the modified silica dispersion in equal proportions, stir magnetically, shear with a high-speed shear machine, and then process with a high-pressure homogenizer. Add NaCl and a mixed solution of 1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide. After the reaction is complete, add glycine to consume the residual 1-ethyl-(3-dimethylaminopropyl)carbodiimide, add hydroxyethyl cellulose, and stir until the viscosity is 480~500 mPa·s. Filter with a PVDF filter membrane to obtain the fourth mixture.

[0077] S4. High-purity nano magnesium oxide is added to deionized water and sodium hexametaphosphate, ball-milled and mixed, pH is adjusted with ammonia, and the fourth mixture, zinc borate pre-dispersed in ethanol solution and nano aluminum oxide are added dropwise. The mixture is placed in a high-speed shearing machine for shearing, and the product is placed in a centrifugal atomizer and dried under nitrogen protection at an inlet temperature of 160℃ and an outlet temperature of 85℃ to obtain the fifth mixture.

[0078] S5. Place the fifth mixture in an isostatic press, pressurize it to 200MPa, hold the pressure for 10 minutes, and then cool and demold it with liquid nitrogen to obtain the sixth mixture.

[0079] S6. Place the sixth mixture in a high-temperature atmosphere furnace, introduce a mixture of argon and H2, sinter by programmed temperature rise, and cool to room temperature to obtain a high-temperature resistant magnesium oxide electrothermal insulation material.

[0080] It should be noted that modified silica and modified cellulose nanofibers undergo a chemical reaction through premixing to form amide bonds, which enhances the interfacial bonding ability and greatly improves the flexural strength and fracture toughness of the material.

[0081] Example 2

[0082] This embodiment discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 170 parts of high-purity nano magnesium oxide, 3 parts of modified silicon dioxide, 3 parts of modified cellulose nanofibers, 2 parts of nano aluminum oxide, and 2 parts of zinc borate.

[0083] The modified silica includes 6 parts of borate ester performance agent.

[0084] The preparation methods for the modified silica and modified cellulose nanofibers in this embodiment are the same as in Example 1. The preparation method for the high-temperature resistant magnesium oxide electrothermal insulation material in this embodiment is the same as in Example 1.

[0085] Example 3

[0086] This embodiment discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 190 parts of high-purity nano magnesium oxide, 4 parts of modified silicon dioxide, 4 parts of modified cellulose nanofibers, 2 parts of nano aluminum oxide, and 2 parts of zinc borate.

[0087] The modified silica includes 8 parts of borate ester performance agent.

[0088] The preparation methods for the modified silica and modified cellulose nanofibers in this embodiment are the same as in Example 1. The preparation method for the high-temperature resistant magnesium oxide electrothermal insulation material in this embodiment is the same as in Example 1.

[0089] Example 4

[0090] This embodiment discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 200 parts of high-purity nano magnesium oxide, 5 parts of modified silicon dioxide, 5 parts of modified cellulose nanofibers, 3 parts of nano aluminum oxide, and 3 parts of zinc borate.

[0091] The modified silica includes 10 parts of borate ester performance agent.

[0092] The preparation methods for the modified silica and modified cellulose nanofibers in this embodiment are the same as in Example 1. The preparation method for the high-temperature resistant magnesium oxide electrothermal insulation material in this embodiment is the same as in Example 1.

[0093] Example 5

[0094] The difference between this embodiment and Embodiment 1 is that the modified silica is not grafted with borate ester performance agent.

[0095] This embodiment discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 150 parts of high-purity nano magnesium oxide, 2 parts of modified silicon dioxide, 2 parts of modified cellulose nanofibers, 1 part of nano alumina, and 1 part of zinc borate.

[0096] The method for preparing the modified silica includes the following steps:

[0097] S21. Place silica in a muffle furnace and activate for 4 hours. Cool to room temperature and remove. Add a mixed solution of γ-aminopropyltriethoxysilane / toluene. Reflux under nitrogen protection. Wash with toluene, ethanol and water by centrifugation, and dry in a vacuum drying oven to obtain the third compound.

[0098] S22. The third compound was mixed and dispersed with 4-(6-aldehydehexyl)phenylboronic acid in anhydrous ethanol and refluxed in the dark. The product was placed in a Soxhlet extractor and extracted with ethanol to obtain the fourth compound.

[0099] S23. The fourth compound was placed in a fluidized bed CVD reactor, and perfluorooctyltriethoxysilane vapor was introduced. The reaction was carried out at 120°C to obtain modified silica.

[0100] The preparation method of the modified cellulose nanofibers in this embodiment is the same as that in Example 1. The preparation method of the high-temperature resistant magnesium oxide electrothermal insulation material in this embodiment is the same as that in Example 1.

[0101] Control group 1

[0102] The difference between this control group and Example 1 is that it does not contain modified silica.

[0103] This control group discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 150 parts of high-purity nano magnesium oxide, 2 parts of modified cellulose nanofibers, 1 part of nano alumina, and 1 part of zinc borate.

[0104] The preparation method of the modified cellulose nanofibers in this control group is the same as that in Example 1. The preparation method of the high-temperature resistant magnesium oxide electrothermal insulation material in this control group is also the same as that in Example 1.

[0105] Control group 2

[0106] The difference between this control group and Example 1 is that it does not contain modified cellulose nanofibers.

[0107] This control group discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 150 parts of high-purity nano magnesium oxide, 2 parts of modified silicon dioxide, 1 part of nano aluminum oxide, and 1 part of zinc borate.

[0108] The modified silica includes 4 parts of borate ester performance agent.

[0109] The preparation method of the modified silica in this control group is the same as that in Example 1. The preparation method of the high-temperature resistant magnesium oxide electrothermal insulation material in this control group is also the same as that in Example 1.

[0110] Control group 3

[0111] The difference between this control group and Example 1 is that it does not contain nano-alumina or zinc borate.

[0112] This control group discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes the following raw materials by mass: 150 parts of high-purity nano magnesium oxide, 2 parts of modified silicon dioxide, and 2 parts of modified cellulose nanofibers.

[0113] The modified silica includes 4 parts of borate ester performance agent.

[0114] The preparation methods for the modified silica and modified cellulose nanofibers in this control group are the same as in Example 1. The preparation method for a high-temperature resistant magnesium oxide electrothermal insulation material in this control group is also the same as in Example 1.

[0115] Control group 4

[0116] This control group is a blank control group.

[0117] This control group discloses a high-temperature resistant magnesium oxide electrothermal insulation material, which includes, by mass, 150 parts of raw material: high-purity nano magnesium oxide.

[0118] Experimental verification:

[0119] Sample preparation: The magnesium oxide insulating materials prepared in the experimental and control groups were made into rectangular strips with a length, width, and thickness of 50 mm × 10 mm × 1 mm, with 5 samples prepared for each group. The fracture toughness of the materials was tested at room temperature, and the average value of each group was calculated and recorded as K0.

[0120] Low-temperature fracture toughness test: An initial notch is cut on one side of the middle of the specimen with a blade, and a sharp crack is created by lightly pressing the blade tip to prepare a specimen with a pre-notched notch. The specimen with the pre-notched notch is placed at -20℃ and kept at that temperature for 12 hours. The surface condition of the specimen is observed using a scanning electron microscope, and the low-temperature fracture toughness of the material is tested using a universal testing machine. The average value of each group is calculated and recorded as K1.

[0121] Crack repair performance test: The pre-notched sample was placed at -20℃ and kept at that temperature for 12 hours, then heated to 25℃ and left to stand for 72 hours. The surface condition of the sample was observed using a scanning electron microscope, and the fracture toughness of the material was tested using a universal testing machine. The average value of each group was calculated and recorded as K2.

[0122] Crack repair performance test: The pre-notched sample was placed at -20℃ and kept at that temperature for 12 hours, then heated to 25℃ and left to stand for 72 hours. After the crack in the sample was repaired, a new notch was pre-made near the original crack, and the temperature was lowered and raised again. This process was repeated 5 times. The fracture toughness of the material was tested using a universal testing machine, and the average value of each group was calculated and recorded as K3.

[0123] The pre-notched specimen was placed at -20℃ and kept at that temperature for 12 hours. Then the temperature was raised to 25℃ and left to stand for 72 hours. After the crack in the specimen was repaired, a new notch was pre-made near the original crack. The operation was repeated 10 times. The fracture toughness of the material was tested using a universal testing machine, and the average value of each group was calculated and recorded as K4.

[0124] Table 1. Fracture toughness data of magnesium oxide insulating materials in experimental and control groups.

[0125]

[0126] The fracture toughness of the magnesium oxide insulation materials in the example group and the control group is shown in Table 1. The table shows that the material with modified silica and modified cellulose nanofibers exhibits higher fracture toughness, indicating that modified silica and modified cellulose nanofibers can significantly improve the fracture toughness of magnesium oxide insulation materials. When the magnesium oxide insulation material is subjected to low-temperature treatment and then heated to 25°C, the boric acid groups on the surface of the modified silica and the ortho-dihydroxy groups of the modified cellulose nanofibers undergo a reversible reaction to form dynamic borate ester bonds, repairing cracks and restoring the fracture toughness of the magnesium oxide insulation material. Figure 1 As shown, the pre-notched sample prepared in Example 1 was left to stand at -20°C for 12 hours, and the cracks in the sample were obvious under an electron microscope. Figure 2 As shown, when the temperature rises to 25℃, the borate ester bonds function, repairing the crack, and no cracks are observed in the electron microscope image of the sample. Furthermore, the data clearly demonstrate the significant effect of the borate ester performance agent. After repeated heating and cooling treatments 10 times, comparing Example 1 and Example 5, it is evident that the crack in Example 1, grafted with the borate ester performance agent, still showed good recovery, and the fracture toughness significantly improved. This indicates that grafting the borate ester performance agent onto the silica surface, upon stress triggering, releases the borate ester repair agent, which is captured by the boric acid groups on the silica surface, reforming new borate ester bonds, thus achieving a cyclical multiple-stage crack repair process of "release-capture-re-repair."

[0127] Finally, it should be noted that 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 make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-temperature resistant magnesium oxide electrothermal insulation material, characterized in that, The raw materials, by weight, include: 150-200 parts of high-purity nano magnesium oxide, 2-5 parts of modified silicon dioxide, 2-5 parts of modified cellulose nanofibers, 1-3 parts of nano aluminum oxide, and 1-3 parts of zinc borate. The modified silica includes 4-10 parts of borate ester performance agent; The preparation method of the borate ester performance agent includes the following steps: S11. Catechol borate and pentaerythritol tetraacrylate are heated and mixed in a water bath, and then magnetically stirred until homogeneous. Perfluoropolyether oil is added and ultrasonically dispersed. Pt@ZIF-8 catalyst is added and centrifuged and mixed under nitrogen protection. Trichloromethane is added and ultrasonically mixed to obtain the oil phase. S12. Polyvinyl alcohol is dissolved in deionized water, sodium dodecyl sulfate is added, the mixture is magnetically stirred, and then deionized water is added to obtain an aqueous phase. S13. The oil phase is added dropwise to the aqueous phase, sheared and emulsified, and then transferred to a high-pressure homogenizer for circulation processing to obtain the first compound; S14. Urea and formaldehyde are added to a reaction vessel, heated and stirred in a water bath, and NaOH solution is added to adjust the pH to obtain the second compound; S15. Add the second compound to the first compound, add citric acid dropwise, stir until the reaction is complete, add melamine, continue heating and stirring to react, wash and collect the solid precipitate, wash with a mixed solution of ethanol and water, freeze dry to obtain the borate ester performance agent; The method for preparing the modified silica includes the following steps: S21. Place silica in a muffle furnace and activate for 4 hours. Cool to room temperature and remove. Add a mixed solution of γ-aminopropyltriethoxysilane / toluene. Reflux under nitrogen protection. Wash with toluene, ethanol and water by centrifugation, and dry in a vacuum drying oven to obtain the third compound. S22. The third compound was mixed and dispersed with 4-(6-aldehydehexyl)phenylboronic acid in anhydrous ethanol and refluxed in the dark. The product was placed in a Soxhlet extractor and extracted with ethanol to obtain the fourth compound. S23. The borate ester performance agent was treated with NaOH to generate carboxyl groups, mixed with the fourth compound, ultrasonically dispersed, and catalyzed by carbodiimide. The reaction was carried out at 4°C to obtain the fifth compound. S24. The fifth compound was placed in a fluidized bed CVD reactor, and perfluorooctyltriethoxysilane vapor was introduced. The reaction was carried out at 120°C to obtain modified silica. The method for preparing the modified cellulose nanofibers includes the following steps: S31. Pass rice husk powder through an 80-mesh sieve, add HCl, heat and stir, centrifuge and wash until neutral, add NaOH and H2O2, continue heating and stirring to react, wash with deionized water until neutral, to obtain the first mixture; S32. Add tetramethylpiperidine oxide, NaBr, NaClO, and deionized water to the first mixture, magnetically stir the reaction in NaOH buffer solution at pH=10, add HCl dropwise until pH=7 to terminate the reaction, centrifuge and filter, collect the precipitate, prepare a suspension with deionized water, process it three times with a high-pressure homogenizer, ultrasonically break it up, centrifuge, wash with ethanol, and vacuum dry to obtain the second mixture; S33. Disperse the second mixture in water, mix ultrasonically, slowly add NaIO4, adjust the pH with acetic acid, stir the reaction in the dark at 4°C, add excess ethylene glycol, stir the reaction until the conductivity of deionized water is <5μS / cm, and obtain the third mixture; S34. After diluting the third mixture, cool it in an ice bath, add NaBH4 in batches, stir continuously and maintain pH=9~10. After the reaction is completed, heat to room temperature, continue stirring, neutralize to neutral with dilute hydrochloric acid, dialyze to remove borate, centrifuge and wash, and freeze dry to obtain modified cellulose nanofibers.

2. The high-temperature resistant magnesium oxide electrothermal insulation material according to claim 1, characterized in that, In the oxidation reaction of tetramethylpiperidine oxide, the pH was measured every 30 minutes. The reaction was terminated when the pH was stable above 9.5 and the NaOH consumption rate was <0.1 mL / min.

3. The high-temperature resistant magnesium oxide electrothermal insulation material according to claim 1, characterized in that, The high-pressure homogenizer has a first processing pressure of 50 MPa and a second and third processing pressure of 100 MPa.

4. A method for preparing a high-temperature resistant magnesium oxide electrothermal insulation material, applied to the preparation of the high-temperature resistant magnesium oxide electrothermal insulation material as described in any one of claims 1-3, characterized in that, The method includes the following steps: S1. The modified cellulose nanofibers were dispersed in an ethanol solution, ultrasonically dispersed, magnetically stirred, and the pH was adjusted to neutral with ammonia to obtain a modified cellulose nanofiber dispersion. S2. Disperse the modified silica in anhydrous ethanol, let it stand to defoam, and then process it three times in a high-pressure homogenizer. Adjust the pH to neutral with acetic acid to obtain the modified silica dispersion. S3. Mix the modified cellulose nanofiber dispersion and the modified silica dispersion in equal proportions, stir magnetically, shear with a high-speed shear machine, and then process with a high-pressure homogenizer. Add NaCl and a mixed solution of 1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide. After the reaction is complete, add glycine to consume the residual 1-ethyl-(3-dimethylaminopropyl)carbodiimide, add hydroxyethyl cellulose, and stir until the viscosity is 480~500 mPa·s. Filter with a PVDF filter membrane to obtain the fourth mixture. S4. High-purity nano magnesium oxide is added to deionized water and sodium hexametaphosphate, ball-milled and mixed, pH is adjusted with ammonia, and the fourth mixture, zinc borate pre-dispersed in ethanol solution and nano aluminum oxide are added dropwise. The mixture is placed in a high-speed shearing machine for shearing, and the product is placed in a centrifugal atomizer and dried under nitrogen protection at an inlet temperature of 160℃ and an outlet temperature of 85℃ to obtain the fifth mixture. S5. Place the fifth mixture in an isostatic press, pressurize it to 200MPa, hold the pressure for 10 minutes, and then cool and demold it with liquid nitrogen to obtain the sixth mixture. S6. Place the sixth mixture in a high-temperature atmosphere furnace, introduce a mixture of argon and H2, sinter by programmed temperature rise, and cool to room temperature to obtain a high-temperature resistant magnesium oxide electrothermal insulation material.

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