Applications of high silica glass fiber in aerospace
1. What is High-Silica Fiber?
High-silica fiber is short for high-purity silica amorphous continuous fiber. Its silica content is 96-98%, with continuous temperature resistance up to 1000°C and short-term temperature resistance up to 1400°C. Finished products mainly include continuous yarn, ropes, sheaths, mesh fabrics, and woven products. It is primarily used for ultra-high temperature fireproofing and heat insulation up to 1000°C. The single fiber diameter is greater than 5 micrometers, and it contains no asbestos or ceramic wool, making it completely harmless to health.
2. Production Process
High-silica glass fiber is produced by using suitable raw glass components and following the ordinary glass fiber production process to create yarns, fabrics, and other products. After acid leaching and hot sintering, high-silica products with high-temperature resistance close to quartz fiber are obtained. The raw glass components mainly include E-glass and binary systems of SiO2-B2O3-Na2O and SiO2-B2O3. my country mainly uses three-component sodium borosilicate glass.
In production, high-silica products are acid-leached, utilizing the phase separation of their structure to leach out B2O3 and Na2O components into a solution, resulting in a microporous silica skeleton enriched with SiO2 exceeding 96%. This skeleton is then sintered at 600-800℃ to close the micropores and create a denser skeleton structure, thus producing high-performance high-silica glass fiber products.
3. Performance Characteristics
(1) Softening point close to 1700℃, can be used long-term at 900℃, and can withstand instantaneous airflow of several thousand degrees.
(2) Maintains good performance stability in organic and inorganic acids (except hydrofluoric acid, phosphoric acid, and hydrochloric acid), even at high temperatures, and in weak alkalis.
(3) High stability against thermal shock and ultra-high radiation; excellent insulation performance under high temperature and high humidity conditions; and good adhesion to high-temperature adhesives.
(4) Resistant to moisture and sunlight radiation, and shock-resistant; suitable for various sealing materials; the product structure is robust, and it maintains flexibility under many high-temperature conditions. (5) Stable structure and performance, harmless to the human body, and can replace ceramic fibers and asbestos fibers (which undergo a phase transformation at high temperatures).
(6) High fire resistance temperature; does not oxidize or discolor when burning in gasoline; high strength; and does not cause secondary pollution.
(7) Excellent wear resistance and insulation properties; competitively priced in most cases.
(8) Its skeleton can be appropriately controlled to manufacture various membrane components for liquid filtration, gas separation, or as a catalyst or enzyme carrier.
Due to these numerous advantages and characteristics, high-silica fiberglass products have received significant attention and extensive research, and have been widely used in many demanding working environments. Developing and researching high-performance high-silica fiberglass products holds great market potential.
4. Process Theory Research
Due to the lower cost and safer use of high-silica glass fiber compared to high-temperature resistant inorganic fibers such as quartz, several companies in Europe and America have developed a series of products based on the high performance of high-silica glass fiber. Notable companies include ACIT High-Temperature Composite Materials Company (USA), Dako South Company (USA), Kelevers Company (Germany), and H.I. Thompson Company (USA). However, my country's research and development has largely followed a follow-up approach, generally lagging behind international levels. Domestically, only Tonghua New Materials, Shaanxi Huate, and Nanjing Glass Fiber Research and Design Institute have conducted long-term research and achieved large-scale production and application technology.
High-Silica Glass Structure Analysis
The process principle of high-silica glass fiber utilizes the phase separation of two or more immiscible liquid phases during the melting or cooling process of glass, creating a slight inhomogeneity. The high-silica glass composition falls within the phase separation region. When the glass cools or is reheated, it separates into two phases: one phase is almost entirely SiO2, and the other is rich in B2O3 and Na2O, which are easily dissolved by acids. From a glass structure perspective, when the SiO2 content is high and the molecular ratio Na2O : B2O3 < 1, the glass simultaneously contains [SiO4] tetrahedra, [BO4] tetrahedra, and [BO3] trigonometric bodies. Some of the [BO4] tetrahedra and [SiO4] tetrahedra form a uniform, unified, and continuous network structure, while others form an independent layered network structure, thus exhibiting a certain degree of phase separation within the glass. SiO2 is relatively stable in acids, while B2O3-Na2O is readily soluble in acids. The production of high-silica glass fibers utilizes this principle. Yarn or fabric containing the basic glass components is acid-leached under certain conditions, causing the B2O3-Na2O phase to transfer into the solution, leaving behind a microporous silica skeleton. This is then subjected to high-temperature treatment at 600-800℃ to close the micropores and compact the skeleton structure, resulting in a highly stable fiber material. In this way, the three-component sodium borosilicate glass is transformed into high-silica glass fiber.
5. Analysis of High-Silica Glass Fiber Production Process
The production processes for high-silica glass fibers are largely the same both domestically and internationally, involving suitable raw material drawing, spinning, and acid asphalt treatment.
The raw materials for high-silica glass fiber production consist of two components. Some manufacturers, in order to reduce costs, also use E-glass for subsequent processing to produce high-silica glass fiber products. In my country, a ternary system is generally used to produce high-quality fiber products. However, during melting, the significant volatilization of B2O3 and Na2O can lead to compositional instability and severe erosion of refractory materials. Compared to traditional flame furnaces, using an all-electric melting furnace at around 1450℃ to melt high-silica spheres can greatly reduce the volatilization rate and minimize refractory corrosion.
High-Silica Glass Fiber Molding Process
High-silica glass has a low hardening rate, with a drawing temperature of around 1150℃ and a crystallization upper limit of only 983℃, so crystallization rarely occurs during the drawing process. However, the wetting angle between the molten glass and the platinum stencil is small, so the stencil design must consider the influence of various factors, and high requirements are placed on the control of forced cooling airflow and the use of special wetting agents during drawing.
High-Silica Glass Fiber Acid Leaching Process
Acid leaching precipitates the non-silica phase in the structure under the action of acid, so that the SiO2 content in the structure reaches more than 96%. The phase migration rate of ions is affected by factors such as acid concentration, leaching time, temperature, and the amount and rate of H3BO3 in the acid solution. The setting of each process and the reasonable control of each parameter are key to the production of high-silica glass fiber products, especially the H3BO3 in the solution. The content of silica content has a significant impact on the tensile strength of the product.
Hot Sintering Process: In high-silica glass fiber products that have undergone acid leaching, easily soluble components such as B₂O₃ and Na₂O are filtered out, leaving a continuous porous skeleton rich in SiO₂. Hot sintering at 600-800℃ or higher is required to close the micropores and restore some strength. However, excessively high temperatures can cause microcracks on the glass surface to expand under internal stress, thus reducing the product's strength. Similar to acid leaching, this process significantly affects the physical and mechanical properties of the product, as detailed in Table 2. Therefore, during production, the temperature regime and sintering time must be carefully controlled to ensure maximum strength recovery while maintaining a linear shrinkage rate of no more than 3% under application conditions.
6. Current Development Status
In recent years, the state has attached great importance to the development of the high-silica glass fiber industry and has introduced numerous policies to support it. In October 2022, the National Development and Reform Commission and the Ministry of Commerce jointly released the "Catalogue of Industries Encouraging Foreign Investment (2022 Edition)," which explicitly included high-silica glass fiber in the non-metallic mineral products industry catalogue. In November 2023, the Raw Materials Industry Department of the Ministry of Industry and Information Technology released the "Guidance Catalogue for the First Batch of Application Demonstration of Key New Materials (2024 Edition) (Draft for Comments)," which included high-silica glass fiber products in the key strategic materials catalogue.
High-silica glass fiber has broad application prospects in fields such as construction engineering, aerospace, automobile manufacturing, and military industry. In construction engineering, high-silica glass fiber can be used as exterior wall insulation material; in aerospace, it can be used as structural material, insulation material, and thermal insulation material for aerospace vehicles; in automobile manufacturing, it can be used to manufacture heat insulation materials for automobile engines and heat insulation sleeves for automobile exhaust pipes. With the future growth of downstream demand, the development speed of the high-silica glass fiber industry is expected to accelerate.
7. Application Prospects in Aerospace
High-silica materials have wide applications in the aerospace field, primarily due to their excellent high-temperature resistance, thermal insulation, and lightweight properties.
In rockets and spacecraft, high-silica materials are used for thermal protection of critical components, such as rocket engine nozzle liners, combustion chamber insulation linings, and spacecraft reentry capsule thermal insulation tiles and gap insulation strips. They can withstand the impact of high-temperature exhaust gases exceeding 2000°C or the aerodynamic heating of thousands of degrees Celsius during atmospheric reentry, effectively protecting structural safety. Typical applications include the Long March series of launch vehicles and the Shenzhou series of spacecraft.
In aero-engines, high-silica materials are used in fireproof and thermal insulation components, such as firewalls, nacelle insulation layers, and cable sheathing materials in civil aircraft (Boeing, Airbus series) and military fighter jet engines, blocking flame spread and reducing cabin temperature in case of fire.
Furthermore, high-silica materials are used in aerospace ground testing equipment, such as thermal baffles and high-temperature airflow guiding devices for rocket engine test stands, capable of withstanding the impact of high-temperature exhaust gases and ensuring test safety.