Preparation method and application of flexible high-emissivity coating on surface of continuous alumina fiber fabric
By preparing a LaAlO3-doped coating on the surface of a continuous alumina fiber fabric, the problems of coating thickness and oxidation were solved, achieving high emissivity and low back surface temperature at high temperatures, improving the thermal protection and reliability of the aircraft, and extending its service life.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANDONG UNIV
- Filing Date
- 2026-03-25
- Publication Date
- 2026-05-15
AI Technical Summary
In the preparation of high emissivity coatings on the surface of continuous fiber fabrics, the existing technology results in excessive coating thickness, which leads to increased weight, accumulation of internal stress, easy cracking and peeling. Furthermore, the oxides are easily oxidized at high temperatures, reducing infrared emissivity and making it difficult to effectively reduce the temperature on the back of the fiber fabric, thus affecting the thermal protection capability and service life of the aircraft.
A LaAlO3-doped coating with a thickness of 200-600 nm was prepared on the surface of continuous alumina fiber fabric using the sol-gel method. The coating formed high emissivity through the lattice defects of LaAlO3, which enhanced the thermal radiation capability in the near-mid-infrared band. The coating was well bonded and not easy to crack.
It achieves high emissivity at high temperatures, significantly reduces the back temperature of fiber fabrics, improves thermal protection capabilities and reliability, and extends service life, making it suitable for thermal protection systems in non-ablative zones of aerospace vehicles.
Smart Images
Figure CN122039411A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of high emissivity coating technology, and particularly relates to a method for preparing a flexible high emissivity coating on the surface of a continuous alumina fiber fabric and its application. Background Technology
[0002] Hypersonic vehicles fly at speeds exceeding Mach 5. During supersonic flight, the surface of the aircraft undergoes complex and significant aerodynamic heating due to intense air compression and friction, generating instantaneous high temperatures on the material surfaces. Studies show that at speeds exceeding Mach 5, the highest temperature of the aircraft skin reaches 1177 K (904°C). The thermal protection system (TPS) maintains the structural temperature of hypersonic aircraft within an acceptable range, which is one of the key technologies and challenges for hypersonic vehicles and directly affects their safety. Flexible ceramic fiber fabrics (such as silicon carbide, aluminum silicate, high-silica, and alumina fiber fabrics) have become indispensable flexible thermal insulation blankets in the non-ablative zone thermal protection system (TPS) of aerospace vehicles due to their low density, low thermal conductivity, excellent flexibility, and high-temperature stability. In practical applications, silicon carbide fibers may undergo oxidative decomposition in extremely high-temperature (>1200°C) oxygen-containing environments; high-silica fibers have relatively low mechanical properties, and aluminum silicate fibers are prone to crystallization and have poor mechanical properties. Compared with the fibers mentioned above, alumina fibers have strong high-temperature oxidation stability and balanced mechanical properties.
[0003] Continuous alumina fibers are composed of high-purity alumina (typically >70%), exhibiting low thermal conductivity, excellent thermal insulation, superior high-temperature resistance, and flexibility. They are commonly used in TPS systems to block aerodynamic heating. However, the increased Mach numbers of hypersonic aircraft can cause local temperatures to exceed 1500℃, even reaching over 2000℃, accompanied by high-temperature, oxygen-rich, and moisture-laden airflow. This necessitates flexible alumina fiber fabrics with higher melting points, more efficient thermal insulation capabilities, surface oxidation resistance, and the ability to balance lightweight design with high strength. According to Planck's radiation law, at high temperatures, thermal radiation rapidly becomes dominant among the three heat transfer modes. Therefore, developing a high-temperature stable, oxidation-resistant, high-emissivity coating on the surface of fiber fabrics to enhance surface thermal radiation, thereby improving heat dissipation efficiency and blocking external heat flow, has become a core technological approach to improving the performance of flexible fiber fabrics.
[0004] Chinese invention patent CN115787285B discloses a three-dimensional alumina fiber fabric with a lanthanum phosphate coating, its preparation method, and its application. The preparation method includes: (1) pretreating the continuous alumina fiber three-dimensional fabric to remove the surface wetting agent; (2) pretreating the continuous alumina fiber fabric with an organic cationic polymeric flocculant to obtain an alumina fiber fabric with a positive charge on the surface; (3) preparing a negatively charged lanthanum phosphate precursor solution; (4) immersing the alumina fiber three-dimensional fabric in the lanthanum phosphate precursor solution for vacuum impregnation; (5) performing a heating treatment after vacuum impregnation, followed by a series of steps including cleaning, drying, and high-temperature treatment; (6) repeating the vacuum impregnation and high-temperature sintering treatment steps multiple times to form a lanthanum phosphate coating of a predetermined thickness on the surface of the alumina fiber. The lanthanum phosphate coating prepared by this invention adheres tightly to the surface of alumina fibers and can be used to prepare alumina fiber-reinforced alumina matrix composites with lanthanum phosphate interfaces. However, there is a lack of specific disclosure regarding some performance aspects such as lightweight, high heat dissipation performance, and temperature reduction performance on the back side of the fiber fabric. Therefore, there may still be some shortcomings and room for improvement in its practical application in aircraft.
[0005] Chinese invention patent CN107573731B discloses a high-temperature infrared radiation coating, its preparation method, and its application. The invention involves mixing nickel-chromium spinel, quartz, zirconium oxide, silica sol, water, and bentonite at room temperature to obtain a high-temperature infrared radiation coating. An emissivity coating with a thickness of more than 200 μm is prepared on the surface of aluminosilicate fibers by brushing or spraying. Its emissivity in the 1-22 μm band is 0.92. However, the problem is that the adhesion of the excessively thick coating decreases, the internal stress increases at high temperatures, and the coating is prone to cracking and peeling.
[0006] Chinese invention patent CN104805681B discloses a method for preparing a surface coating of flexible thermal insulation felt. This invention uses aluminum sec-butoxide and dilute nitric acid solution to prepare an alumina sol, incorporating short quartz fibers with a diameter not exceeding 2 mm into the sol. High-speed stirring ensures uniform dispersion. SiB4 powder and ZrB2 powder are also incorporated into the sol. Finally, an atmospheric spraying process is used to coat the alumina sol containing the quartz fibers and high-emissivity filler onto the surface of the flexible thermal insulation felt to form a coating with an emissivity of not less than 0.85. However, a problem exists: SiB4, ZrB2, and the short quartz fibers are all easily oxidized at high temperatures, causing radiation attenuation. The glassy phase formed by Si, B, and quartz transfers fracture energy to the fiber cloth, leading to brittle fracture and other problems, making it unsuitable for prolonged high-temperature use.
[0007] Chinese invention patent CN115852706B discloses a gradient coating on the surface of a flexible thermal insulation felt and its preparation method. The technology involves mixing alumina particles, silicon carbide particles, and deionized water using a ball mill to obtain a spare slurry 1; mixing glass powder, silicon carbide particles, alumina particles, and deionized water using a ball mill to obtain a spare slurry 2; mixing slurry 1 with methylphenyl silicone resin and mechanically stirring to obtain a first slurry; mixing slurry 2 with methylphenyl silicone resin and mechanically stirring to obtain a second slurry; cleaning the surface of the high-silica fiber cloth; and using a spraying method to spray the first slurry onto the surface of the flexible thermal insulation felt to obtain a first coating, followed by spraying the second slurry onto the surface of the first coating to obtain a gradient coating on the flexible thermal insulation felt. The fiber fabric with the gradient coating maintains a safe temperature, and the back temperature of the flexible thermal insulation felt is reduced by 80°C (from 470°C to 386°C). However, the problem is that methylphenyl silicone resin is unstable at high temperatures and is prone to failure. At high temperatures, the porous structure will shrink and densify, generating additional shrinkage stress. The surface coating is more prone to cracking due to internal stress in the porous transition layer structure, and the coating is easy to fall off from the interface.
[0008] Chinese invention patent CN108950456A discloses a method for preparing a high-temperature resistant, high-infrared emissivity coating. This invention involves grinding MoSi2, SiC, deionized water, binder, defoamer, and dispersant in a colloid mill, then pumping the mixture into a spray granulation tower to complete the granulation process, obtaining raw powder. The raw powder is then placed in an alumina crucible for atmosphere-protected calcination, followed by sieving to obtain thermally sprayable powder. This thermally sprayable powder is then sprayed onto a pretreated substrate using atmospheric plasma spraying technology to prepare a high-temperature resistant, high-infrared emissivity coating. The prepared high-infrared emissivity coating achieves an emissivity of 0.887 in the 0.76-2.5 μm wavelength range. However, a problem exists regarding the composition of MoSi2, SiC, and Mo... 4.8 Si3C 0.6 They are easily oxidized in high-temperature and oxygen-containing environments, which greatly reduces their infrared emission capability and causes the coating to fail.
[0009] Chinese invention patent CN120137431B discloses a method for preparing a long-lasting, oxidation-resistant, high-emissivity alumina-silica bilayer coating using a pre-oxidation method. This invention involves preparing a slurry from NiAl and a phosphate binder, which is then coated onto the surface of a pretreated high-temperature alloy substrate to obtain an aluminum-containing coating substrate. Alternatively, a slurry from ZrSi2 and a phosphate binder is prepared and coated onto the aluminum-containing coating substrate surface, forming an in-situ bilayer structure of "inner Al2O3 - outer SiO2" through pre-oxidation. After pre-oxidation, the outer layer is mainly composed of SiO2, achieving an emissivity of 0.9. However, SiO2 has a narrow emission band, and its emissivity decreases at high temperatures.
[0010] In summary, the following problems still exist in the preparation of high emissivity coatings on the surface of continuous fiber fabrics: (1) The coating thickness prepared by the slurry method is more than 200 μm, and even increased to 500 μm. The excessive thickness of the coating will directly and seriously offset the weight reduction results and cause a series of performance and reliability problems. It is not just a simple "weight increase", but also has higher requirements for power, fuel and support structure, consumes more fuel, and may become a weak link in the entire thermal protection system. The coating generates internal stress during preparation and cooling. The greater the thickness, the higher the accumulated stress. Excessive stress is very likely to cause the coating to crack, peel or delaminate. The excessively thick coating will also make the composite material brittle and hard, lose its flexibility, reduce the effective adhesion, and the coating is easy to fall off when bent or under stress.
[0011] (2) In order to improve the infrared emissivity of the coating, materials such as MoSi2, SiC, TaSi2, SiB4, and ZrB2 are commonly used in emissivity coatings. However, these materials are easily oxidized in high-temperature oxygen-containing environments, which reduces the infrared emissivity. If ordinary oxides are used to improve the infrared emissivity of the coating, they will not be oxidized at high temperatures. However, as the temperature increases, according to Wien's displacement law, the peak wavelength of radiation shifts to the short-wave direction. Ordinary oxides themselves have a narrow radiation band and low near-infrared emission, which leads to a decrease in the emissivity of the coating at high temperatures.
[0012] (3) Although some technical solutions can reduce the back temperature of continuous fiber fabric by 80°C, if the back temperature is further reduced to above 100°C, it can not only further improve the safety and reliability of the aircraft, but also significantly improve the survivability and mission success rate when facing unexpected situations such as extreme aerodynamic heating, local heat shield damage or extended mission time, and also help extend the service life of the aircraft. Controlling the back temperature at a low level can effectively slow down the thermal oxidation, creep and fatigue damage of internal materials, thereby extending the service life of key components (such as structural beams, electronic equipment, fuel tanks, etc.), reducing maintenance costs and replacement frequency. It enhances structural stability, reduces the damage of thermal stress to the aircraft structure, reduces the temperature difference between the inside and outside of the structure, reduces the thermal stress level, and improves the fatigue resistance and overall stability of the structure. Therefore, the coating technology of continuous fiber fabric still has great room for improvement and potential demand. Summary of the Invention
[0013] This invention addresses the technical problems existing in the surface coatings of continuous fiber fabrics mentioned above, and proposes a method for preparing a flexible high emissivity coating on the surface of continuous alumina fiber fabrics with good bonding performance and flexibility, which can further reduce the temperature of the back side of the fiber fabric and is conducive to improving the thermal protection capability, reliability, safety and service life of aircraft, as well as its application.
[0014] To achieve the above objectives, the technical solution adopted by the present invention is as follows: the method for preparing a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric provided by the present invention includes the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the continuous alumina fiber fabric in a muffle furnace and heat it to 600℃ at 5℃ / min and hold it for 1 h. S2. Preparation of precursor sol: Weigh lanthanum nitrate hexahydrate, aluminum nitrate nonahydrate, metal nitrate, complexing agent, co-solvent and solvent according to a certain molar ratio. Dissolve the metal nitrate in the mixture of co-solvent and solvent to obtain solution A. Dissolve the complexing agent in the solvent to obtain solution B. Pour solution A into solution B, stir and heat to obtain precursor sol. The concentration of the precursor sol was 0.1–0.5 mol / L, and its chemical formula was La. x A 1-x Al y B 1-y O3, A is Ca 2+ 、Sr 2+ Bi 3+ One or more of them, where B is Fe 3+ Cr 3+ Ni 2+ One or more of the following; where x = 0.1~0.5, y = 0.1~0.5; The solvents are deionized water and ethanol, and the volume ratio of deionized water to ethanol is (7:1) to (1:1). S3. Preparation of coated continuous alumina fiber fabric: S3-1. The precursor sol prepared in step S2 is coated on the surface of the continuous alumina fiber fabric pretreated in step S1 to obtain the sol-continuous alumina fiber fabric. S3-2, Drying treatment to obtain gel-continuous alumina fiber fabric; S3-3. Calcination treatment to obtain coated continuous alumina fiber fabric; the specific method of calcination treatment is to pre-calcine the gel continuous alumina fiber fabric in a tube furnace at 400~700℃, and then calcine it at 700~1100℃, with a heating rate of 2℃ / min and a compressed air flow rate of 5~40 mL / min.
[0015] Preferably, the complexing agent is one of citric acid monohydrate, tartaric acid, and glycine.
[0016] Preferably, the co-solvent is one of polyethylene glycol, polyvinyl alcohol, and isopropanol.
[0017] Preferably, the coating method of the precursor sol in S3 is one of dip coating, vacuum coating, or vacuum filtration.
[0018] Preferably, the drying process in S3 is one of room temperature drying, 60°C drying, and 100°C drying.
[0019] A flexible, high-emissivity coating on the surface of a continuous alumina fiber fabric is used as a high-temperature insulation material.
[0020] As a preferred application, it is used in the thermal protection system for non-ablative zones of aerospace vehicles.
[0021] Compared with the prior art, the advantages and positive effects of the present invention are as follows: 1. The method for preparing a flexible high emissivity coating on the surface of continuous alumina fiber fabric provided by the present invention uses the sol-gel method to prepare a high emissivity coating on the surface of a single fiber in the continuous alumina fiber fabric. The coating thickness is 200-600nm, which fully utilizes the lightweight advantage of the fiber fabric and achieves stronger heat dissipation capacity per unit weight.
[0022] 2. The method for preparing a flexible high emissivity coating on the surface of continuous alumina fiber fabric provided by the present invention not only produces a coating with low internal stress and strong adhesion, but also shows good bonding between the coating and the continuous alumina fiber fabric, with no glass phase appearing, thus improving the flexibility of the coated continuous alumina fiber fabric.
[0023] 3. The method for preparing a flexible high-emissivity coating on the surface of continuous alumina fiber fabric provided by this invention involves preparing a LaAlO3-doped coating on the surface of the continuous alumina fiber fabric. LaAlO3 is a perovskite oxide with a high melting point and good high-temperature stability. The lattice defects introduced by LaAlO3 doping form defect energy levels in the band gap. The relatively low energy is sufficient to excite electrons on the impurity or defect energy levels to undergo transitions. Compared with general oxides such as SiO2, it significantly enhances the emissivity in the near-mid-infrared band (especially 0.76-2.5 μm), enabling the coated continuous alumina fiber fabric to maintain high emissivity at high temperatures. The continuous alumina fiber fabric was tested using a butane spray gun capable of providing a flame exceeding 1300°C. The temperature of the uncoated continuous alumina fiber fabric dropped from 1300°C on the front side to 349°C on the back side, while the temperature of the coated continuous alumina fiber fabric dropped from 1300°C on the front side to 223°C on the back side. The emissivity coating reduced the temperature of the back side of the fiber fabric by more than 100°C (from 349°C to 223°C).
[0024] 4. The application of the flexible high emissivity coating on the surface of the continuous alumina fiber fabric provided by this invention in high-temperature thermal insulation of aircraft can not only significantly improve the thermal protection capability and reliability of aircraft and extend their service life, but also ensure flight safety and lay the foundation for the future application of aircraft in a wider range and more severe environments; its stronger heat dissipation capacity per unit weight can provide a basis for aircraft to have a longer range or flight range, higher maneuverability and faster speed, which is conducive to saving a lot of fuel. Attached Figure Description
[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0026] Figure 1 Thermogravimetric analysis (TGA) of co-doped lanthanum aluminate prepared in Example 1 of this invention; Figure 2 The XRD patterns of co-doped lanthanum aluminate and undoped lanthanum aluminate prepared in Example 1 of this invention are shown below. Figure 3 Macroscopic views of uncoated continuous alumina fiber fabric (left) and coated continuous alumina fiber fabric prepared in Example 1 of the present invention (right); Figure 4 SEM images of uncoated continuous alumina fiber fabric (left) and coated continuous alumina fiber fabric prepared in Example 1 of the present invention (right) at different magnifications (from top to bottom); Figure 5 This is a SEM image of a single fiber cross-section of the coated continuous alumina fiber fabric prepared in Example 1 of the present invention; Figure 6 Thermogravimetric curves of gel-coated continuous alumina fibers and uncoated alumina fibers prepared in Example 1 of this invention are shown. Figure 7 The XRD patterns of the coated continuous alumina fiber fabric and the uncoated alumina fiber fabric prepared in Example 1 of this invention are shown below. Figure 8 Macroscopic bending images of uncoated continuous alumina fiber fabric and coated continuous alumina fiber fabric prepared in Example 1 of the present invention; Figure 9 The displacement-load force curves are shown for the uncoated continuous alumina fiber fabric and the coated continuous alumina fiber fabric prepared in Example 1 of this invention. Figure 10 The infrared emissivity spectrum of the coated continuous alumina fiber fabric prepared in Example 1 of this invention in the 0.8-2.5 μm band; Figure 11 The infrared emissivity spectrum of the coated continuous alumina fiber fabric prepared in Example 1 of the present invention in the range of 2.5-14 μm. Figure 12 Infrared imaging spectra of uncoated continuous alumina fiber fabric (left) and coated continuous alumina fiber fabric prepared in Example 1 of the present invention (right); Figure 13 The graphs show the changes in back surface temperature over time for uncoated continuous alumina fiber fabric and coated continuous alumina fiber fabric prepared in Example 1 of this invention. Figure 14 Macroscopic images of the coated continuous alumina fiber fabric prepared in Example 1 of the present invention after a tape adhesion test and the unadheded surface. Detailed Implementation
[0027] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.
[0028] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification. Example 1
[0029] This embodiment provides a method for preparing a flexible high-emissivity coating on a continuous alumina fiber fabric, comprising the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the alumina fiber fabric in a muffle furnace and heat it to 600℃ at 5℃ / min and hold it for 1 h. S2. Preparation of precursor sol: Weigh the components according to the following molar ratios: lanthanum nitrate hexahydrate: strontium nitrate: aluminum nitrate nonahydrate: chromium nitrate nonahydrate = 0.9:0.1:0.9:0.1, total metal ions: citric acid monohydrate = 1:1.2, citric acid monohydrate: polyethylene glycol = 1:0.2. Dissolve the metal salt and polyethylene glycol in the solvent to obtain solution A, and dissolve the citric acid monohydrate in the solvent to obtain solution B. Pour solution A into solution B and stir for 1 h, heat in an oven at 80℃ for 10 h, and age at room temperature for 10 h to obtain the precursor sol. The volume ratio of deionized water to ethanol in the solvent is 7:1, and the concentration of the precursor sol is 0.2 mol / L. S3. Preparation of coated continuous alumina fiber fabric: S3-1. The sol prepared in step S2 is coated onto the continuous alumina fiber fabric in step S1 using a vacuum impregnation method to obtain a sol-coated continuous alumina fiber fabric. Vacuum impregnation is performed for 30 min at a pressure of -0.06 MPa. S3-2. Drying treatment: Drying in an oven at 60℃ to obtain gel-coated continuous alumina fiber fabric; S3-3, Calcination treatment: The gel-coated continuous alumina fiber fabric obtained in S3-2 is placed in a tube furnace with an air flow rate of 10 mL / min and a heating rate of 2℃ / min. It is pre-calcined at 600℃ and held for 2 h. Then, the temperature is raised to 800℃ to obtain the coated continuous alumina fiber fabric.
[0030] like Figure 1 As shown, the thermogravimetric curve of co-doped LaAlO3 changes smoothly, with no significant mass drop in the range of room temperature to 1200℃, indicating that this doped lanthanum aluminate material has good high-temperature thermal stability and is suitable as a substrate material for high-temperature coatings.
[0031] like Figure 2 As shown, the XRD patterns of co-doped LaAlO3 and undoped LaAlO3 exhibit differences in characteristic peaks, indicating that Ca... 2 + / Sr 2+ / Bi 3+ and Fe 3+ / Cr 3+ / Ni 2+ The co-doping successfully introduced lattice defects without changing the perovskite crystal structure of LaAlO3, providing a structural basis for improving infrared emissivity.
[0032] like Figure 3 , Figure 4 and Figure 5 As shown, the macroscopic appearance of the alumina fiber fabric after coating is not significantly changed; SEM images at different magnifications show that the coating is uniformly coated on the surface of a single fiber, the pore structure between fibers is well preserved, and there is no phenomenon of coating agglomeration or pore blockage; the coating thickness is in the range of 200-600 nm, which is a nanoscale coating, meeting the requirements of lightweight design and avoiding problems such as internal stress and reduced flexibility caused by thick coatings.
[0033] like Figure 6 As shown, the thermogravimetric loss rate of gel-coated alumina fiber fabric is lower than that of uncoated fabric, and the mass change is more gradual in the high-temperature range, indicating that the gel coating can effectively delay the high-temperature thermal decomposition of fiber fabric and improve its thermal stability.
[0034] like Figure 7 As shown, the XRD pattern of the coated fiber fabric only shows characteristic peaks of Al2O3 and LaAlO3, with no other impurity peaks. This indicates that the coating is a uniform coating of LaAlO3 on the surface of alumina fiber, and no chemical reaction occurs between the two to generate a new phase. The coating formation process is controllable.
[0035] like Figure 8 and Figure 9As shown in the macroscopic bending images, the coated continuous alumina fiber fabric prepared in Example 1 can still achieve large-angle bending without coating cracking or peeling; the displacement-load force curves show that the mechanical load properties of the coated fabric are similar to those of the uncoated fabric, and the coating does not cause the fabric to become brittle or hard, and its flexibility and mechanical properties are preserved.
[0036] like Figure 10 As shown, in the 0.8-2.5 μm near-infrared band, the infrared emissivity of the coated continuous alumina fiber fabric prepared in Example 1 is much higher than that of the uncoated fabric, and the emissivity value is close to 1.0. This indicates that the LaAlO3 doped coating significantly enhances the thermal radiation capability in the near-infrared band, which is consistent with the heat transfer characteristics dominated by thermal radiation at high temperatures.
[0037] like Figure 11 As shown, in the 2.5-14 μm mid-infrared band, the infrared emissivity of the coated continuous alumina fiber fabric prepared in Example 1 is always much higher than that of the uncoated fabric, indicating that the coating has excellent thermal radiation performance in a wide infrared band and the emissivity does not decrease significantly at high temperatures.
[0038] like Figure 12 As shown, under the same heating conditions, the infrared radiation brightness of the coated continuous alumina fiber fabric prepared in Example 1 is significantly higher than that of the uncoated fabric, which intuitively proves that the coating has a stronger heat radiation dissipation capacity and can dissipate heat in the form of infrared radiation more efficiently.
[0039] like Figure 12 and Figure 13 As shown, with increasing heating time, the back surface temperature of the coated continuous alumina fiber fabric prepared in Example 1 was always much lower than that of the uncoated fabric, and eventually stabilized at around 223℃ (222℃), which is more than 100℃ lower than the 349℃ of the uncoated fabric. This proves that the coating prepared in Example 1 can effectively block heat flow and greatly improve the heat insulation effect.
[0040] like Figure 14 As shown, after the tape adhesion test, there was no coating peeling or flaking on the surface of the coated fabric, which was consistent with the surface state of the unadheded area. This indicates that the nano-scale LaAlO3 coating has excellent bonding force and strong adhesion to the alumina fiber fabric, and can resist coating peeling under external force.
[0041] In this embodiment 1, as shown... Figures 1-14As shown, this invention employs the sol-gel method to prepare a high-emissivity coating on the surface of a single fiber in a continuous alumina fiber fabric. The coating thickness is 200-600 nm, fully leveraging the lightweight advantage of the fiber fabric to achieve stronger heat dissipation per unit weight. This provides a foundation for aircraft to have longer range or flight range, higher maneuverability, and higher speed, while saving a significant amount of fuel. The coating prepared by this invention not only has low internal stress and strong adhesion, but also exhibits good bonding with the continuous alumina fiber fabric, without the appearance of a glassy phase, thus improving the flexibility of the continuous alumina fiber fabric. Example 2
[0042] This embodiment provides a method for preparing a flexible high-emissivity coating on a continuous alumina fiber fabric, comprising the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the alumina fiber fabric in a muffle furnace and heat it to 600℃ at a rate of 5℃ / min and hold it for 1 h.
[0043] S2. Preparation of precursor sol: Lanthanum nitrate hexahydrate: calcium nitrate tetrahydrate: aluminum nitrate nonahydrate: ferric nitrate nonahydrate = 0.8:0.2:0.8:0.2, total metal ions: glycine = 1:1.2, glycine: polyvinyl alcohol = 1:0.2 were weighed. The metal salt and polyvinyl alcohol were dissolved in the solvent to obtain solution A, and glycine was dissolved in the solvent to obtain solution B. Solution A was poured into solution B and stirred for 1 h, heated in an oven at 80℃ for 10 h, and aged at room temperature for 10 h to obtain the precursor sol. The volume ratio of deionized water to ethanol in the solvent was 3:1, and the concentration of the precursor sol was 0.1 mol / L.
[0044] S3. Preparation of coated continuous alumina fiber fabric: S3-1. The sol prepared in step S2 is coated onto the continuous alumina fiber fabric in step S1 using the vacuum filtration method to obtain the sol-coated continuous alumina fiber fabric. S3-2. Drying treatment: Drying in an oven at 60℃ to obtain gel-coated continuous alumina fiber fabric.
[0045] S3-3 Calcination treatment: The gel-coated alumina fiber fabric obtained in step S3-2 is placed in a tube furnace with an air flow rate of 40 mL / min and a heating rate of 2℃ / min. It is pre-calcined at 400℃ and held for 2 h. Then, the temperature is raised to 700℃ to obtain a continuously coated alumina fiber fabric. Example 3
[0046] This embodiment provides a method for preparing a flexible high-emissivity coating on a continuous alumina fiber fabric, comprising the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the alumina fiber fabric in a muffle furnace and heat it to 600℃ at a rate of 5℃ / min and hold it for 1 h.
[0047] S2. Preparation of precursor sol: Lanthanum nitrate hexahydrate: bismuth nitrate pentahydrate: aluminum nitrate nonahydrate: nickel nitrate hexahydrate = 0.7:0.3:0.7:0.3, total metal ions: tartaric acid = 1:1.2, tartaric acid: isopropanol = 1:0.2 were weighed. The metal salt and isopropanol were dissolved in the solvent to obtain solution A, and tartaric acid was dissolved in the solvent to obtain solution B. Solution A was poured into solution B and stirred for 1 h, heated in an oven at 80℃ for 10 h, and aged at room temperature for 10 h to obtain the precursor sol. The volume ratio of deionized water to ethanol in the solvent was 5:3, and the concentration of the precursor sol was 0.4 mol / L.
[0048] S3. Preparation of coated continuous alumina fiber fabric: S3-1. The sol prepared in step S2 is coated onto the continuous alumina fiber fabric in step S1 using the dip-coating method to obtain the sol-coated continuous alumina fiber fabric. S3-2, Drying treatment: Drying in an oven at 100℃ to obtain gel-coated continuous alumina fiber fabric.
[0049] S3-3 Calcination treatment: The gel-coated alumina fiber fabric obtained in step S3-2 is placed in a tube furnace with an air flow rate of 20 mL / min and a heating rate of 2℃ / min. It is pre-calcined at 700℃ and held for 2 h. Then, the temperature is raised to 900℃ to obtain a continuously coated alumina fiber fabric. Example 4
[0050] This embodiment provides a method for preparing a flexible high-emissivity coating on a continuous alumina fiber fabric, comprising the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the alumina fiber fabric in a muffle furnace and heat it to 600℃ at 5℃ / min and hold it for 1 h. S2. Preparation of precursor sol: Lanthanum nitrate hexahydrate: calcium nitrate tetrahydrate: aluminum nitrate nonahydrate: chromium nitrate nonahydrate = 0.6:0.4:0.6:0.4, total metal ions: tartaric acid = 1:1.2, tartaric acid: polyethylene glycol = 1:0.2 were weighed. The metal salt and polyethylene glycol were dissolved in the solvent to obtain solution A, and tartaric acid was dissolved in the solvent to obtain solution B. Solution A was poured into solution B and stirred for 1 h, heated in an oven at 80℃ for 10 h, and aged at room temperature for 10 h to obtain the precursor sol. The volume ratio of deionized water to ethanol in the solvent was 1:1, and the concentration of the precursor sol was 0.5 mol / L. S3. Preparation of coated continuous alumina fiber fabric: S3-1. The sol prepared in step S2 is coated onto the continuous alumina fiber fabric in step S1 using a vacuum impregnation method to obtain a sol-coated continuous alumina fiber fabric. Vacuum impregnation is performed for 30 min at a pressure of -0.06 MPa. S3-2, Drying treatment: Drying at room temperature to obtain gel-coated continuous alumina fiber fabric; S3-3 Calcination treatment: The gel-coated alumina fiber fabric obtained in step S3-2 is placed in a tube furnace with an air flow rate of 5 mL / min and a heating rate of 2℃ / min. It is pre-calcined at 500℃ and held for 2 h. Then, the temperature is raised to 1000℃ to obtain a continuously coated alumina fiber fabric. Example 5
[0051] This embodiment provides a method for preparing a flexible high-emissivity coating on a continuous alumina fiber fabric, comprising the following specific steps: S1. Pretreatment of continuous alumina fiber fabric: Place the alumina fiber fabric in a muffle furnace and heat it to 600℃ at 5℃ / min and keep it at that temperature for 1 hour. S2. Preparation of precursor sol: Lanthanum nitrate hexahydrate: bismuth nitrate pentahydrate: aluminum nitrate nonahydrate: nickel nitrate hexahydrate = 0.5:0.5:0.5:0.5, total metal ions: citric acid monohydrate = 1:1.2, citric acid monohydrate: isopropanol = 1:0.2 were weighed. The metal salt and isopropanol were dissolved in the solvent to obtain solution A, and citric acid monohydrate was dissolved in the solvent to obtain solution B. Solution A was poured into solution B and stirred for 1 h, heated in an oven at 80℃ for 10 h, and aged at room temperature for 10 h to obtain the precursor sol. The volume ratio of deionized water to ethanol in the solvent was 7:5, and the concentration of the precursor sol was 0.3 mol / L. S3. Preparation of coated continuous alumina fiber fabric: S3-1. The sol prepared in step S2 is coated onto the continuous alumina fiber fabric in step S1 using the dip-coating method to obtain the sol-coated continuous alumina fiber fabric. S3-2, Drying treatment: Drying at 100℃ to obtain gel-coated continuous alumina fiber fabric; S3-3 Calcination treatment: The gel-coated alumina fiber fabric obtained in step S3-2 is placed in a tube furnace with an air flow rate of 30 mL / min and a heating rate of 2℃ / min. It is pre-calcined at 700℃ and held for 2 h. Then, the temperature is raised to 1100℃ to obtain a continuously coated alumina fiber fabric.
[0052] Comparative Example 1 This comparative example provides a method for preparing a coating. The difference from Example 1 is that the ratio of citric acid and polyethylene glycol changes in step S2, with the ratio of total metal ions to citric acid monohydrate being 1:1.5 and the ratio of citric acid monohydrate to polyethylene glycol being 1:0.4.
[0053] Comparative Example 2 This comparative example provides a method for preparing a coating, which differs from Example 1 in that step S3-3 does not include a pre-firing process at 400~700℃.
[0054] Comparative Example 3 This comparative example provides a method for preparing a coating, which differs from Example 1 in that step S1 does not include a 600°C pre-firing process.
[0055] Examples 1-5 show that within the limits of the invention, the concentration of the precursor sol, solvent ratio, and doping ratio can all successfully prepare nanoscale LaAlO3 doped coatings, and the coatings all possess the core properties of high emissivity, high adhesion, and good flexibility. With changes in the doping ratio x / y and an increase in calcination temperature from 800℃ to 1100℃, the LaAlO3 coating prepared by this invention can still be successfully reproduced and a stable coating structure can be obtained. This provides an effective parameter library for the doping system and calcination process of this invention. Supported by this parameter library, it also demonstrates that the LaAlO3 coating has a certain range of process tolerance. Different complexing agents, co-solvents, coating methods, drying methods, and calcination temperatures can meet the preparation requirements of the flexible high-emissivity coating on the surface of continuous alumina fiber fabrics of this invention.
[0056] Examples 1-5: In this invention, a LaAlO3-doped coating was prepared on the surface of a continuous alumina fiber fabric. LaAlO3 is a perovskite oxide with a high melting point and good high-temperature stability. The lattice defects introduced by LaAlO3 doping form defect energy levels in the band gap. The relatively low energy is sufficient to excite electrons on the impurity or defect energy levels to undergo transitions. Compared with common oxides such as SiO2, it significantly enhances the emissivity in the near-mid-infrared band (especially 0.76-2.5 μm), enabling the coated continuous alumina fiber fabric to maintain high emissivity at high temperatures. The continuous alumina fiber fabric was tested using a butane spray gun capable of providing a flame exceeding 1300°C. The temperature of the uncoated continuous alumina fiber fabric dropped from 1300°C on the front side to 349°C on the back side, while the temperature of the coated continuous alumina fiber fabric dropped from 1300°C on the front side to 223°C on the back side. The emissivity coating reduced the temperature of the back side of the fiber fabric by more than 100°C (from 349°C to 223°C).
[0057] This invention provides an application of a flexible, high-emissivity coating on the surface of continuous alumina fiber fabric. Used as a high-temperature thermal insulation material in the thermal protection system of the non-ablative zone of hypersonic vehicles, it exhibits ultra-high temperature stability and aligns with the characteristic that thermal radiation is the primary heat transfer mode at high temperatures. It efficiently dissipates heat generated by aerodynamic heating in the form of infrared radiation, significantly reducing the temperature on the back side of the non-ablative zone. The coating is highly flexible and tough, allowing for large-angle bending and folding along with flexible substrates such as alumina fiber fabric without cracking or peeling, adapting to the structural deformation of the non-ablative zone of the aircraft. It avoids the internal stress accumulation problems associated with thick coatings, while ensuring lightweight design and not increasing the weight load of the thermal protection system, meeting the lightweight requirements of aircraft power and fuel. Therefore, the application of this invention not only significantly improves the thermal protection capability and reliability of hypersonic vehicles, extending their service life, but also ensures flight safety and lays the foundation for future applications of aircraft in broader and more demanding environments.
[0058] The coatings prepared by Example 1 and Comparative Example 1 showed that changes in the ratio of citric acid monohydrate and polyethylene glycol in Comparative Example 1 could directly lead to a decrease in the gelation properties of the precursor sol, thereby affecting the quality of the coating. This indicates that the molar ratio of complexing agent to cosolvent must match the total metal ion content, which is the key to ensuring the preparation of high-quality coatings by the sol-gel method. An imbalance in the ratio will damage the coating and affect its application.
[0059] The coatings prepared in Example 1 and Comparative Example 2 revealed that Comparative Example 2, by directly subjecting the coating to high-temperature calcination without a pre-firing process, did not produce a flexible, high-emissivity coating for the continuous alumina fiber fabric surface that meets the requirements of this invention. This comparative example may lead to two major problems: first, the organic components (complexing agents, co-solvents) in the gel coating rapidly decompose, generating a large amount of gas, resulting in pores and cracks on the coating surface and a significant decrease in structural density; second, direct high-temperature calcination creates instantaneous thermal stress between the coating and the fiber fabric, preventing the gradual release of internal stress in the coating, ultimately leading to decreased coating adhesion, easy peeling, and a reduction in infrared emissivity due to structural defects. Example 1, through a pre-firing process, removes organic components and releases internal stress, laying the necessary foundation for the formation of a dense and stable coating during subsequent high-temperature calcination.
[0060] The coatings prepared in Example 1 and Comparative Example 3 revealed that Comparative Example 3, which directly applied sol-gel coating without pretreatment of the alumina fiber fabric, did not produce a continuous, flexible, high-emissivity coating for the alumina fiber fabric surface that meets the requirements of this invention. A potential problem with this comparative example is that the lack of pretreatment leads to residual oil, adsorbed water, and weakly bound impurities on the fiber fabric surface, preventing the precursor sol from effectively wetting and bonding with the fiber surface. This results in interfacial separation and poor coating adhesion, such as coating peeling during tape adhesion tests. In contrast, the pretreatment performed in Example 1 is a prerequisite for ensuring coating adhesion and long-term stability.
[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.
Claims
1. A method for preparing a flexible, high-emissivity coating on the surface of a continuous alumina fiber fabric, characterized in that, The specific steps include the following: S1. Pretreatment of continuous alumina fiber fabric: Place the continuous alumina fiber fabric in a muffle furnace and heat it to 600℃ at 5℃ / min and hold it for 1 h. S2. Preparation of precursor sol: Weigh lanthanum nitrate hexahydrate, aluminum nitrate nonahydrate, metal nitrate, complexing agent, co-solvent and solvent according to a certain molar ratio. Dissolve the metal nitrate in the mixture of co-solvent and solvent to obtain solution A. Dissolve the complexing agent in the solvent to obtain solution B. Pour solution A into solution B, stir and heat to obtain precursor sol. The concentration of the precursor sol was 0.1–0.5 mol / L, and its chemical formula was La. x A 1-x Al y B 1-y O3, A is Ca 2+ 、Sr 2+ Bi 3+ One or more of them, where B is Fe 3+ Cr 3+ Ni 2+ One or more of the following; where x = 0.1~0.5, y = 0.1~0.5; The solvents are deionized water and ethanol, and the volume ratio of deionized water to ethanol is (7:1) to (1:1). S3. Preparation of coated continuous alumina fiber fabric: S3-1. The precursor sol prepared in step S2 is coated on the surface of the continuous alumina fiber fabric pretreated in step S1 to obtain the sol-continuous alumina fiber fabric. S3-2, Drying treatment: to obtain gel-continuous alumina fiber fabric; S3-3, Calcination treatment: The coated continuous alumina fiber fabric is obtained; The specific method of calcination treatment is to pre-calcine the gel continuous alumina fiber fabric in a tube furnace at 400~700℃, and then calcine it at 700~1100℃, with a heating rate of 2℃ / min and a compressed air flow rate of 5~40 mL / min.
2. The method for preparing a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric according to claim 1, characterized in that, The complexing agent is one of citric acid monohydrate, tartaric acid, and glycine.
3. The method for preparing a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric according to claim 2, characterized in that, The co-solvent is one of polyethylene glycol, polyvinyl alcohol, and isopropanol.
4. The method for preparing a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric according to claim 3, characterized in that, The precursor sol in S3 is coated using one of the following methods: dip coating, vacuum impregnation, or vacuum filtration.
5. The method for preparing a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric according to claim 4, characterized in that, The drying process in S3 can be one of the following: room temperature drying, 60°C drying, or 100°C drying.
6. An application of a flexible high-emissivity coating on the surface of a continuous alumina fiber fabric, characterized in that, It is used as a high-temperature insulation material.
7. The application of the flexible high emissivity coating on the surface of the continuous alumina fiber fabric according to claim 6, characterized in that, It is used in thermal protection systems for non-ablative zones of aerospace vehicles.