Ceramic fiber, a lightweight refractory material in fibrous form, boasts advantages such as low weight, high-temperature resistance, excellent thermal stability, low thermal conductivity, minimal specific heat, and superior mechanical vibration resistance.
Currently, the more widely recognized ceramic fibers include traditional varieties like alumina-silicate fiber, mullite fiber, and alumina fiber, as well as advanced types such as quartz fiber, silicon carbide fiber, zirconia fiber, and nitride fiber.
1. Ceramic Fiber Production Method
Ceramic fibers are categorized into two major groups based on their composition: vitreous (amorphous) fibers and polycrystalline (crystalline) fibers.
1) Production Methods for Vitreous Fibers
Vitreous ceramic fibers are produced by melting raw materials in a resistance furnace. The high-temperature melt is then extruded through a spinneret and flung by high-speed rotating rollers of a multi-roller centrifuge, transforming the melt into fibrous material. Alternatively, the melt can be rapidly quenched by high-speed airflow, forming fibers through a blowing process.
2) Production Methods for Polycrystalline Fibers
Polycrystalline ceramic fibers are manufactured using two primary methods: the sol-gel method and the precursor method.
Sol-Gel Method: Soluble aluminum and silicon salts are converted into a colloidal solution of a specific viscosity. This solution is then formed into fibers via compressed air blowing or centrifugal spinning, followed by high-temperature heat treatment to yield aluminum-silicate oxide crystalline fibers.
Precursor Method: Similar to the sol-gel method, a colloidal solution is prepared from soluble salts. Organic fibers are uniformly impregnated with this solution and, through subsequent heat treatment, are transformed into aluminum-silicate oxide crystalline fibers.
2. 9 Properties of Ceramic Fiber
Ceramic fiber products are exceptional refractory materials, characterized by their lightweight nature, high-temperature resilience, low thermal capacity, excellent insulation properties, and non-toxicity. Key characteristics of ceramic fibers include:
3. 8 Typical Ceramic Fibers
1) Alumina Fiber (Al₂O₃)
Alumina fiber is a high-performance inorganic fiber divided into long fibers, short fibers, and whiskers.
Long Fibers: Also known as continuous fibers, they exhibit superior tensile strength, high-temperature resistance, corrosion resistance, and low thermal conductivity. The production process is simple, requiring basic equipment and raw materials such as metal oxide powders, inorganic salts, water, and polymers.
Short Fibers: Composed of microcrystals, they combine the advantages of both crystalline and fibrous materials, offering excellent resistance to thermal shock, making them suitable for high-temperature insulation.
Preparation Methods for Al₂O₃ Nanofibers:
Melt Spinning: Inorganic oxides are electrically heated to form a melt, which is then spun into Al₂O₃ nanofibers. This method is simple, cost-effective, and avoids the need for high-temperature calcination, thus preventing crystal growth. However, increased alumina content in the melt can raise viscosity, complicating fiber formation and resulting in lower alumina content in the final product.
Sol-Gel Method: Aluminum alkoxides or inorganic salts are used as raw materials, with an organic acid as a catalyst to form a sol. The sol is then spun into fibers and heat-treated to produce alumina ceramic fibers.
Impregnation Method: Hydrophilic viscose fibers are immersed in an inorganic aluminum salt solution, dried, sintered, and woven to form alumina fibers. This method yields fibers of varying morphologies with high strength but at a higher cost.
Slurry Method (DuPont Method): Alumina powder is dispersed in water with additives to form a uniform slurry, which is then extruded, dried, and sintered to produce alumina fibers. This method yields fibers with uniform but relatively large diameters.
Electrospinning: A classic method combining electrospinning with high-temperature calcination to produce high-purity α-Al₂O₃ fibers with diameters around 150 nm.
2) Silicon Carbide Ceramic Fiber (SiC)
SiC nanofibers are non-oxide ceramic fibers composed primarily of carbon and silicon. They are categorized into continuous fibers and whiskers.
SiC fibers exhibit high tensile strength, chemical corrosion resistance, high-temperature resilience, and high modulus. Fiber-reinforced ceramic matrix composites are increasingly used in high-tech fields such as aerospace and military equipment.
Preparation Methods for SiC Nanofibers:
Chemical Vapor Deposition (CVD): Produces high-purity SiC fibers with excellent high-temperature stability and creep resistance, though they are challenging to use in composite materials.
Precursor Conversion Method: Comprises four steps: precursor synthesis, melt spinning, infusibilization, and high-temperature sintering, yielding fibers with outstanding mechanical properties.
Activated Carbon Fiber Conversion Method: Organic fibers are treated to form activated carbon fibers, which react with gaseous silicon oxide to transform into SiC fibers. Heat treatment yields the final SiC nanofibers with tensile strengths exceeding 1000 MPa.
Electrospinning: Produces fibers with uniform dimensions and excellent morphology, utilizing polyvinylpyrrolidone as a spinning aid to create continuous SiC fibers with diameters around 200 nm.
3) Zirconia Nanofibers (ZrO₂)
ZrO₂ nanofibers are oxide ceramic fibers with a melting point of 2700°C. They exhibit exceptional chemical stability, remaining unreactive with molten metals like aluminum, platinum, iron, and nickel even at 1900°C.
ZrO₂ is widely used in insulation and ceramic insulation materials due to its high resistivity, refractive index, corrosion resistance, and low thermal expansion coefficient.
At low temperatures, ZrO₂ exhibits a monoclinic crystal structure, transitioning to tetragonal above 1100°C and cubic above 1900°C. To prevent phase transformation, stabilizers like Y₂O₃, CaO, and MgO are added during production.
A novel method combining electrospinning and the sol-gel process produces ZrO₂ nanofibers with fine diameters and continuous structure.
4) Silicon Nitride Fiber (Si₃N₄)
Si₃N₄ fibers are high-temperature, high-strength ceramic fibers. In oxidizing environments, their maximum service temperature is 1300°C, while in non-oxidizing environments, it reaches 1800°C. They exhibit tensile strengths up to 1000 MPa, elastic moduli of 300 GPa, low thermal expansion coefficients, and excellent wear resistance, making them ideal for reinforcing metals and ceramics.
5) Silicon Carbonitride Fiber (SiCN)
Si-B-N-C ceramic fibers are highly heat-resistant and strong, with room-temperature elastic moduli exceeding 250 GPa and creep values of 0.4–1 (measured under BSR standards at 1400°C for 1 hour). These fibers combine high-temperature resistance, oxidation resistance, creep resistance, high strength, and wave absorption, making them versatile structural/functional materials.
6) Alumina-Silicate Fiber
Resembling cotton in shape and color, alumina-silicate fibers are amorphous ceramic fibers composed primarily of alumina and silica, occasionally including trace amounts of iron oxide, titanium dioxide, and calcium oxide. They are classified into four types based on composition and purity: standard (common) alumina-silicate fibers, high-purity alumina-silicate fibers (mullite fibers), high-purity aluminum-containing alumina-silicate fibers, and high-purity zirconia-containing alumina-silicate fibers. These fibers exhibit excellent insulation properties, acid and alkali resistance, chemical stability, and sound absorption, with over 80% absorption of mid-to-high frequency waves above 500 Hz.
7) Mullite Fiber
Mullite fibers are polycrystalline fibers with alumina content between 72–75%. The primary crystalline phase is mullite microcrystals, which are stable in the alumina-silica binary system. These fibers exhibit excellent high-temperature resistance, with service temperatures of 1500–1700°C. However, above 1500°C, crystal growth reduces their mechanical properties, and at 1830°C, they decompose into alumina and a liquid phase.
8) Quartz Fiber
Quartz fibers are high-purity silica glass fibers with impurity levels below 0.1% and diameters ranging from 0.7–15 μm. They exhibit high thermal resistance, with a stable long-term service temperature of 1050°C and a short-term tolerance up to 1700°C. However, their strength begins to decline at 600°C.
Quartz fibers retain some properties of solid quartz, making them excellent high-temperature materials and reinforcement for advanced composites. Their purity exceeds 99.9%, imparting strong ablation resistance, high thermal stability, low thermal conductivity, and excellent chemical and dielectric properties.
4. Applications of Ceramic Fiber
Ceramic fiber products are processed into various forms and widely used across industries. Their most significant application lies in high-temperature insulation, spanning fields such as metallurgy, machinery, electronics, ceramics, glass, chemicals, automotive, construction, light industry, military, marine, and aerospace.
1) Thermal Insulation Materials
Ceramic fibers exhibit excellent high-temperature resistance, enduring up to 1500°C, while their mixed structure of solid fibers and air provides superior thermal insulation. This makes ceramic fiber products indispensable for insulating industrial furnace walls and building materials.
Ceramic fibers are premium refractory insulation materials, aligning with the demand for energy efficiency, environmental protection, and safety in downstream industries. Annual production in China is around 700,000 tons, accounting for approximately 2.9% of the refractory materials market. With a relatively small base, future growth in each application area is expected to be substantial. As non-standardized products, ceramic fiber materials require tailored formulations, processes, and technical support for different applications, driving their expansion alongside industrial development.
Ceramic fibers are highly efficient, energy-saving insulation materials. Combining the characteristics of conventional fibers with exceptional high-temperature resistance, corrosion resistance, and oxidation resistance, they also overcome the brittleness of traditional refractory materials. This makes them a preferred alternative to heavy refractory bricks for industrial kiln linings.
The most significant advantage of ceramic fiber in kiln construction is energy efficiency. For example, polycrystalline mullite fiber products are used as insulation materials in high-temperature equipment (up to 1600°C), such as silicon carbide furnaces, molybdenum disilicide furnaces, steel heating furnaces, and forging furnaces. They significantly improve thermal efficiency, save energy, enhance productivity, and improve product quality. Applications include insulation for high-temperature industrial kilns, ceramic kilns, mechanical and metallurgical heating furnaces, heat treatment furnaces, and other industrial kilns, as well as high-temperature fire barriers, kiln doors, kiln cars, expansion joints, and glass furnace insulation.
2) High-Temperature Filtration Materials
Ceramic fibers’ large surface area enables the production of highly pure filtration materials. Their superior thermal stability, chemical stability, and thermal shock resistance make them ideal for environmental applications such as air purification, wastewater treatment, and flue gas filtration.
With high strength, excellent thermal shock resistance, and chemical corrosion resistance, ceramic fibers are widely used in air purification, high-temperature flue gas filtration, diesel engine exhaust filtration, chemical filtration, and molten metal filtration. Ceramic fiber filters include composite membranes, papers, meshes (fabrics), and filter bodies.
Ceramic Fiber Cotton: Produced by melting raw materials in resistance furnaces and spinning them through blowing or centrifugal processes, ceramic fiber cotton typically has diameters of 2–5 μm and lengths of 30–250 mm.
The fibers have smooth cylindrical surfaces with circular cross-sections and high porosity (usually over 90%). The size and properties of the pores (open or closed) significantly influence thermal conductivity. The internal structure of ceramic fibers consists of a blend of solid fibers and air, with solid fibers forming a continuous skeletal framework and air filling the interstitial spaces. This structure gives ceramic fibers high porosity, large surface area, excellent insulation properties, and low bulk density.
Ceramic fiber cotton is slightly acidic and resistant to weak acids, alkalis, water, oil, and steam. It does not wet lead, aluminum, or copper and exhibits excellent flexibility and elasticity. Its density is 75% lower than lightweight insulation bricks and 90–95% lower than lightweight castable linings, with thermal conductivity approximately one-eighth that of lightweight clay bricks and one-tenth that of lightweight refractory linings. Its low thermal capacity significantly reduces energy loss and enhances energy efficiency. Additionally, ceramic fiber is easy to install, requires no curing, shortens construction cycles, and simplifies installation. Ceramic fiber cotton is the primary raw material for other ceramic fiber products and is directly used in industrial furnace expansion joint fillers, wall insulation, and sealing materials.
Ceramic Fiber Blanket Systems: These are produced by naturally settling bulk ceramic fibers onto a web-forming machine’s belt, creating a uniform fiber mat. Needle-punching produces binder-free, dry-process needle-punched blankets, which are soft, elastic, and highly processable, making them one of the most widely used ceramic fiber products. Based on production methods, ceramic fiber blankets are categorized into spun and blown types.
Ceramic fiber blanket systems are suitable for sealing kiln doors, furnace curtains, and roof insulation in various industrial kilns; high-temperature flue and duct linings; expansion joints; insulation for petrochemical equipment, vessels, and pipelines; protective clothing, gloves, headgear, helmets, and boots for high-temperature environments; automotive engine insulation covers, heavy oil engine exhaust wraps, high-speed racing composite brake friction pads; insulation for nuclear power and steam turbines; heat treatment insulation; sealing gaskets and packing for pumps, compressors, and valves handling high-temperature liquids or gases; high-temperature electrical insulation; fire doors, fire curtains, fire blankets, spark-catching pads, and insulation covers for fire protection; thermal and insulation materials, brake friction pads for aerospace; insulation and wrapping for cryogenic equipment, vessels, and pipelines; and heat insulation and fire protection layers for archival vaults, safes, and important spaces in high-end office buildings.
Ceramic Fiber Shapes: Made from high-quality ceramic fiber cotton using vacuum molding, these products are rigid, self-supporting, and maintain low shrinkage, high insulation, lightweight, and impact resistance within their service temperature range. Unburned materials are easily cut or machined, and the finished products exhibit excellent wear and spalling resistance, as well as resistance to wetting by most molten metals.
Ceramic fiber shapes include tubes, cones, domes, and rectangular boxes, often custom-produced to meet specific client requirements.
3) Sound Absorption and Insulation Materials
Ceramic fibers provide excellent sound absorption and insulation. When sound waves penetrate the material, viscous interactions with air in the pores and frictional resistance between fibers convert part of the sound energy into heat. Additionally, heat conduction in the compressed air within the pores dissipates sound energy, resulting in effective sound absorption. These properties make ceramic fibers widely used in construction and transportation sectors.
4) Catalyst Carrier Materials
Ceramic fibers’ high surface area and porosity make them ideal catalyst carriers. Their low diffusion resistance enhances catalytic efficiency in diffusion-controlled reactions, showcasing their potential in catalytic applications.
5) Reinforcement and Toughening Materials
Ceramic fibers are a proven solution to the inherent brittleness of ceramic materials. Commonly used fibers for reinforcement include Al₂O₃ and SiC fibers, which are also applied in toughening metal materials.
6) Advanced Ceramic Fiber Applications
Advanced ceramic fibers differ from traditional ceramic fibers in their application focus. Beyond leveraging their high-temperature resistance, insulation, and fireproofing qualities, advanced ceramic fibers amplify their other functional characteristics, such as wave absorption, corrosion resistance, and weather resistance.
Ceramic fibers are inherently semiconducting, making them critical materials for radar wave absorption. They also possess ideal structural properties, including lightweight, high strength, high-temperature resistance, and oxidation resistance. By modifying their crystal structure during production, the resistivity of the fibers can be adjusted. Multi-directional, multi-layer layering enables both wave absorption and transmission. Ceramic fiber-reinforced composites can be directly fabricated into stealth structural components, offering superior strength and high-temperature performance compared to stealth coatings. For example, the F-22 Raptor uses ceramic-based stealth structural materials near its exhaust nozzles. The tail fins of France’s APTGD missile are composed of hexagonal ceramic wave-absorbing tiles with excellent absorption properties. The U.S. Air Force has developed a broadband wave-transparent radome made of Si₃N₄.
Examples:
5. Conclusion
The ceramic fibers described above are primarily used for high-temperature resistance, insulation, and fire resistance. As refractory materials, they have significant potential for expansion, particularly in building insulation panels, high-temperature industrial furnaces, and filtration systems, especially with China’s carbon neutrality goals. Advanced ceramic fibers, with their lightweight, high strength, high modulus, high-temperature resistance, corrosion resistance, erosion resistance, and design flexibility, are indispensable in cutting-edge applications such as rockets, satellites, missiles, fighter jets, and naval vessels.
