Advancements in Flexible Ceramic Fibers for High-Temperature Applications: A Comprehensive Review

FCFs have demonstrated significant potential across a variety of high-temperature applications due to their unique combination of thermal stability, mechanical flexibility, and chemical resistance. These applications span multiple fields, including thermal insulation, high-temperature flue gas filtration, catalysis, electromagnetic wave absorption, energy storage, and high-temperature sensing. Each application leverages the distinct advantages of FCFs to address specific challenges in extreme environments. 4.1. Thermal Insulation The demand for advanced thermal insulation materials has surged in recent years due to the increasing complexity of aerospace, energy, and defense applications. High-temperature environments, such as those encountered in aero-engines, scramjet engines, and articulated warheads, require materials that resist extreme heat and maintain structural integrity under mechanical stress and thermal cycling. With their unique combination of low density, high thermal stability, and mechanical flexibility, FCFs have emerged as a promising solution for these challenges [72]. By leveraging novel material designs and fabrication techniques, these materials have demonstrated their potential to provide robust thermal insulation in extreme environments, ensuring the safety and functionality of critical systems. Refractory materials, including alumina silica, magnesia oxide and other ceramic materials, are widely used in high-temperature environments due to their melting point of thousands of degrees Celsius and resistance to chemical attack. These materials are essential for industrial and aerial equipment that usually faces high-temperature challenges. FCFs made from these refractory materials like silica fibers can often have flame retardants, which are extremely suitable as building materials for roofs and walls to extend the escape time [73]. In the context of thermal insulation, refractory materials play a crucial role in protecting structures and equipment from extreme heat and thermal shock. For instance, FCFs are selective materials for thermal insulation due to their low thermal conductivity, low density and high-temperature resistance to thermal decomposition, deformation, and brittleness (Figure 3a) [12]. Compared with traditional ceramics, the thermal insulation performance of micro-nano FCFs has been significantly improved [74]. Thanks to high NIR reflection, low density, and high porosity, FCFs can reduce heat convection in the heat transfer process (Figure 3b) [75]. Cheng et al. [76] developed 3D interwoven crimped-nanofiber structured ceramic aerogels (ICCAs), which are fabricated from mullite sol by 3D reaction electrospinning (Figure 3c). These ICCAs exhibited robust fire resistance with a low thermal conductivity of 22.8 mW m⁻¹ K⁻¹ and mechanical flexibility with 40% tensile strain, even when exposed to a butane flame (about 800~1300 °C). Zhang et al. [77] further advanced this field by engineering a stretchable SiO₂-based ceramic meta-aerogel with low thermal conductivity (33.01 mW m⁻¹ K⁻¹), using kirigami stacking and nanofiber freeze-drying techniques (Figure 3d). When tested in a heat pipe was rapidly heated to 1000 °C, the ceramic meta-aerogels maintained stability, with a maximum surface temperature of approximately 260 °C, effectively insulating electronic components inside an articulated warhead (below 900 °C). This development underscores the potential of FCFs for thermal insulation in compact systems where weight and space are critical constraints. Dang et al. [78] introduced a two-component off-axial electrospinning method to create zirconia-silica structured ceramic aerogels, with ultra-large stretchability (150% tensile strain), high thermal stability (from −196 °C to 1000 °C) and low thermal conductivity (106.7 mW m⁻¹ K⁻¹ at 1000 °C) (Figure 3e). These aerogels were tested in a morphing wing, maintaining an internal temperature of only 35 °C after 100 cycles of heating to 1000 °C for 10 minutes. This application demonstrates the versatility of FCFs in dynamic environments where materials must endure repeated thermal and mechanical loading. Besides aerospace applications, thermal insulation is one of the most fundamental applications of FCFs. Extensive research has been conducted in this area. However, achieving effective thermal insulation often requires thick membranes, typically on the order of centimeters, which can significantly increase the weight of the material and limit its use in compact systems. Reducing the thickness of these membranes while maintaining thermal insulation efficiency is a significant challenge, as thinner membranes often struggle to balance heat conduction suppression and mechanical strength. To address this issue, innovative structural designs and composite material strategies are needed to enhance performance under reduced thickness conditions. 4.2. High-Temperature Flue Gas Filtration With the development of human society and technology, the problem of air pollution has increasingly gained widespread attention. Nowadays, half the world’s population is exposed to increasing air pollution, leading to millions of people’s deaths every year [79]. Airborne particulate matter (PM), a main pollutant with a wide range of particle sizes and typical chemical composition, seriously threatens environmental protection and human health [80]. According to data analysis, about 58% of PM emissions originate from high-temperature exhaust such as coal combustion, industrial activities, and vehicle exhaust [81]. An efficient method to reduce PM in the air is high-temperature flue gas filtration. To achieve high-temperature flue gas filtration, filter materials should be efficient and high-temperature resistant. Traditional materials that withstand high temperatures are mostly ceramic filters, renowned for their exceptional chemical inertness, enabling them to withstand corrosion from acidic and alkaline gases in hot exhaust streams. Benefiting from a larger specific surface area and smaller pores, ceramic fiber materials are suitable for PM filters of high-temperature flue gas filtration [82]. Wang et al. [83] developed an elastic, high-temperature resistant, and high-efficiency air filter based on yttrium-stabilized zirconia (YSZ) nanofiber sponges using solution blow spinning technology (Figure 4a). The YSZ nanofiber sponges exhibit a filtration efficiency of 99.4% and a pressure drop of only 57 Pa for 20–600 nm NaCl particles at a flow velocity of 4.8 cm s⁻¹ at room temperature. Even though at a high temperature of 750 °C, the YSZ nanofiber sponges maintained a high filtration efficiency of 99.97% for PM_0.3–2.5 under a high airflow velocity of 10 cm s⁻¹, proving its potential application in filtration of automobile exhaust gas. Jia et al. [84] created a flexible and thermally stable Al₂O₃-stabilized ZrO₂ (ASZ) submicron fiber air filter paper for efficient filtration using a solution blow spinning method followed by calcination (Figure 4b). This flexible ASZ paper can be thermally stable up to 1100 °C and is very flexible. At an area density of 56 mg cm^–2, ASZ paper demonstrated a 99.56% filtration efficiency and a pressure drop of only 108 Pa when filtering NaCl particles (15–615 nm) at an airflow speed of 5.4 cm s⁻¹. Besides, the material maintained over 99.3% removal efficiency in a 6-h test, showcasing its potential for sustained high-temperature filtration applications. Tan et al. [85] developed flexible Zr-doped TiO₂ nanofibrous membranes (Zr–TiO₂ NFMs) using electrospinning and calcination for high-efficiency removal of oily particulate matter (PM) from high-temperature flue gas (Figure 4c). The incorporation of Zr⁴⁺ ions improved the mechanical strength, thermal stability, and specific surface area of the TiO₂ nanofibers by reducing nanocrystal size and surface defects. The Zr–TiO₂ NFMs demonstrated outstanding filtration efficiency (>99.98%) for PM2.5 at 350 °C with low-pressure drop and great regeneration performance over five filtration cycles. This work shows the potential of Zr–TiO₂ NFMs for advanced flue gas filtration under extreme conditions, offering insights into temperature-dependent PM capture mechanisms. Li et al. [86] utilized the solution blow spinning technique to develop mullite nanofiber (MNF) films featuring both thick and thin fibers with average diameters of 606 nm and 186 nm, respectively (Figure 4d). Because of this dual-scale structure, the MNF films enhanced filtration efficiency of 98.23%, a low areal density of 3.8 mg cm⁻² and achieved a pressure drop of 141 Pa for PM_0.3 at an airflow velocity of 5.3 cm s⁻¹, exhibiting flexibility (winding radius can be up to 9 mm) and high-temperature resistance (no significant changes at 1300 °C for 5 min) for use in high-temperature flue gas filtration. The same as other fibers or materials, the filtering efficiency of ceramic fiber materials is seriously decided by their fiber diameter [87]. Smaller fiber diameter and areal density result in higher efficiency of filtrate and removal particles, but it increases the difficulty and cost of manufacturing. In the future, the development of ceramic fiber materials with smaller fiber diameter, higher porosity, and greater thermal stability under conditions of lower areal density, higher gas temperatures, and faster airflow rates will be a key research direction of high-temperature flue gas filtration. Additionally, fiber diameter has a significant effect on the filtration property [87]. To improve the filtration property of ceramic fiber materials, we need to overall prepare smaller-diameter fibers. Besides, the cost-effective ceramic fiber aerogels with excellent porosity can be used to develop high dust capacity filtering materials to increase their dust capacity. 4.3. Catalysis Catalysis of ceramic fibers have garnered significant attention due to their unique advantages, such as high thermal stability, mechanical strength, and large surface areas, which make them indispensable candidates for high-temperature and complex catalytic reactions. FCFs can be the carriers of catalysts by loading catalysts on the surface or inside of the FCFs. When it comes to reactions with high temperature or corrosion, such as gas synthesis and reduction reactions, it is usually better to use FCFs rather than organic films or fibers [88]. FCFs can be endowed with photothermal catalysis. For example, Zhang et al. [89] developed elastic TiO₂ nanofibrous aerogels with a hierarchically ordered structure and enhanced conductivity (38.2 mS cm⁻¹) via sol-gel electrospinning and lithium reduction (Figure 5a). These aerogels, which calcined at 600 °C in argon, have an obvious photothermal effect and served as self-supported electrocatalysts for nitrogen fixation, achieving an ammonia yield of 4.19 × 10⁻¹⁰ mol s⁻¹ cm⁻² and Faradaic efficiency of 20.3%. Also, FCFs can be used to catalyze synthetic reactions with the help of catalytically active particles. Fu et al. [90] meticulously crafted loofah-like γ-Al₂O₃ mesoporous nanofibers (MNFs) via the sol-gel electrospinning technique, subsequently depositing Pt nanocrystals onto the MNFs using an impregnation method (Figure 5b). The synthesized catalyst exhibited exceptional thermal stability at 500 °C and achieved a remarkable reduction reaction rate of 6.8 S⁻¹·mg⁻¹ for the thermo-catalytic reduction of p-nitrophenol, which was a significant fourfold increase in speed compared to the use of net-like Pt nanocrystals. Compared with photocatalysis, thermal catalysis can better reflect the advantages of FCFs, especially for gas synthesis reactions at temperatures of hundreds or thousands of degrees Celsius. Sánchez et al. [91] investigated ceramic fiber-based structures as catalyst supports for CO₂ methanation, focusing on mass and heat transport behavior. They found ceramic fibers with small diameters provided high surface areas, enabling efficient external mass and heat transfer, but radial heat dispersion remained a bottleneck (Figure 5c). Yan et al. [92] developed ultra-flexible Al₂O₃ fibers using the blow-spinning method, demonstrating remarkable mechanical flexibility and thermal stability from −196 °C to 1200 °C (Figure 5d). By loading nickel nanoparticles onto the fibers, Ni/Al₂O₃ fibers can demonstrate great catalytic performance for dry reforming of methane (DRM), achieving stable CH₄ and CO₂ conversions (~87% and ~92%, respectively) and high H₂ selectivity (~96%) at 800 °C over 150 h. These advancements have demonstrated their potential in critical applications such as nitrogen fixation, CO₂ methanation, and methane reforming. However, it is a challenge to enhance the compatibility between active catalyst particles and supports. By overcoming these challenges, FCFs can play a transformative role in sustainable and high-performance catalytic systems. 4.4. Electromagnetic Wave Absorption With the rapid development of modern communication technologies and electronic devices, electromagnetic wave (EMW) pollution has become a pressing issue, leading to the demand for high-performance EMW absorption materials [93]. FCFs have garnered significant attention for this application due to their unique combination of lightweight structure, thermal stability, and EMW attenuation capabilities [94]. The inherent advantages of FCFs, such as their porous 3D structures, high specific surface areas, and tunable dielectric properties, make them ideal candidates for EMW absorption in extreme environments, including aerospace and stealth technologies [95]. In recent years, advances have shown that SiC-based FCFs are highly effective for EMW absorption. Their 3D porous structures and core-shell configurations optimize impedance matching and enhance dielectric loss. For instance, Zhang et al. [96] developed SiC@SiO₂ nanofiber aerogels that achieved a minimum reflection loss (RLmin) of −50.36 dB and an effective absorption bandwidth (EAB) of 8.6 GHz at a thickness of only 1.6 mm (Figure 6a). This demonstrates the potential of FCF-based aerogels for lightweight and efficient EMW absorbers in high-performance applications. To obtain great EMW absorption, the diameter of fibers should be small. Kang et al. [97] fabricated submicron SiC fibers via electrospinning and high-temperature sintering, achieving a reflection loss of −63.34 dB at 12.85 GHz with a thickness of 1.7 mm (Figure 6b). These fibers demonstrated exceptional thermal stability up to 1800 °C and oxidation resistance at 1400 °C. Besides, the microstructure of fibers also influences EMW absorption performance. SiBCN fibers after heat treatment at 1200–1600 °C were investigated. Ding et al. [98] fabricated SiBCN fibers through electrospinning and studied how heat treatment leads to the microstructure evolution of SiBCN fibers, which ultimately affects the EMW absorbing performance (Figure 6c). Upon heat treatment at 1200 °C, turbostratic carbon precipitated, yielding a minimum reflection loss (RLmin) of −63 dB and an efficient absorption bandwidth (EAB) of 5.2 GHz. At 1400 °C, the simultaneous formation of turbostratic carbon and nano-SiC grains further enhanced the performance, achieving an RLmin of −67 dB. This superior performance is attributed to conductive loss, dipole polarization, and interfacial polarization. Their study highlights the potential of SiBCN fibers as high-performance EMW absorbers for extreme environments. Future efforts should focus on microstructure-function integrated control and developing large-scale manufacturing methods to address these limitations and expand their application in extreme environments. 4.5. Energy Devices Energy storage is a cornerstone of human society, enabling advancements in modern technology and industry [99]. As the demand for efficient, durable, and safe energy devices grows, researchers are exploring new materials and architectures to address critical challenges [100]. Flexibility is emerging as a key requirement, particularly for applications in wearable electronics, foldable devices, and next-generation portable systems [101]. Flexible energy storage devices, typically including batteries and supercapacitors, promise lightweight, adaptable, and multifunctional solutions. However, their development faces several significant hurdles, like aging, dendrite growth, swelling, and self-ignition explosion. Under high current densities or elevated temperatures, it is critical to avoid the failure of materials inside energy devices [102]. Conventional polymer-based components in energy devices, such as separators and electrolytes, struggle to meet the demands of flexibility and safety. For instance, polymer separators often suffer from low porosity, poor thermal stability, and flammability, compromising their performance and reliability. Similarly, liquid electrolytes pose risks of leakage and combustion. To overcome these challenges, researchers are turning to ceramic materials, particularly FCFs, which offer a unique combination of mechanical flexibility, high thermal stability, and electrochemical inertness. Some researchers have reported that FCFs have been used to be electrodes, electrolytes, and separators, which are parts of batteries and the main constructures of batteries. For electrodes, Sang et al. [103] developed graphene-modified SiOC ceramic cloths with hierarchical porous structures via a polymer-derived ceramic approach (Figure 7a). These materials demonstrated fast lithium-ion diffusion and high electrical conductivity, achieving a remarkable reversible capacity of 686 mAh g⁻¹ at 0.5 A g⁻¹ after 500 cycles. Notably, the ceramic cloths retained their mechanical flexibility even under repeated bending, showcasing their potential as flexible anodes for wearable and portable electronics. For electrolytes, Pan et al. [104] developed a garnet-type Li₇La₃Zr₂O₁₂ (LLZO) ceramic fabric composite solid electrolyte (CF-CSE) using a silk template, achieving a unique 3D structure with high ceramic content (70 wt%) for fast ion conduction (Figure 7b). The LLZO CF-CSE exhibited enhanced ionic conductivity (8.89 × 10⁻⁵ S cm⁻¹ at 30 °C), wide electrochemical stability (5.1 V), and great thermal stability (decomposition temperature of 320 °C and low combustibility). Integrated into all-solid-state lithium metal batteries (ASSLBs) enabled stable cycling for 700 h and suppressed lithium dendrite growth. When paired with a LiFePO₄ cathode, the ASSLB delivered high capacities (149.3 mAh g⁻¹ after 100 cycles at 0.2 C) and outstanding safety, making it a promising candidate for high-energy, safe ASSLBs. For separators, Jing et al. [29] developed a flexible ZrO₂ ceramic nanofiber membrane using a sol-assisted electrospinning method followed by high-temperature burnout (Figure 7c). The membrane demonstrated high thermal and mechanical stability, high porosity (63.45%), superior electrolyte wettability, and high ionic conductivity (3.5 mS cm⁻¹). When used as a separator in lithium-ion and sodium-ion batteries, it enabled superior rate performance, extended cycling stability, and flame resistance compared to commercial separators like Celgard and glass fiber. This study positions the ZrO₂ membrane as a promising fully inorganic separator for next-generation high-power batteries under extreme conditions. The development of FCF-based materials marks a significant step forward in energy technology. By addressing critical issues such as dendrite growth, thermal instability, and flammability, FCFs offer a promising pathway to safer and more efficient energy devices. Recent studies have demonstrated the potential of FCF-based electrodes, electrolytes, and separators to deliver outstanding performance in terms of flexibility, thermal stability, and electrochemical properties. Nowadays, there is little research on the application of FCFs in batteries. Looking ahead, in order to achieve the true lightweight and high safety of flexible batteries, the ultimate goal is to develop fully FCFs-based energy devices. 4.6. Sensors In recent years, the development of flexible high-temperature resistant sensors has gained significant attention with the continuous advancement of material science. These sensors, which combine flexibility with thermal durability, usually rely on innovative materials like carbon fibers, ceramic fibers and so on. Their ability to operate under extreme conditions while maintaining accuracy and sensitivity has made them essential in applications requiring real-time monitoring of strain [105], temperature [106,107], and pressure [27,108,109], at elevated temperatures. For strain sensing, Sun et al. [105] developed a bioinspired flexible strain sensor based on ceramic fiber paper (CFP) coated with gold nanoparticles (Au NPs) and SiO₂ aerogel inspired by scorpions Heterometrus petersii (Figure 8a). The sensor demonstrated ultra-sensitivity and superhydrophobicity, withstanding high temperatures up to 220 °C. Its biomimetic design enabled effective strain detection through the synergistic interaction of the ceramic fibers and coatings, ensuring mechanical durability and stability. This innovation addresses the need for lightweight and flexible strain sensors capable of reliable operation in aerospace, offering a robust alternative to conventional rigid strain gauges. For temperature sensing, Liu et al. [106] developed a flexible thermocouple temperature sensor by screen-printing indium oxide and indium tin oxide (In/In₂O₃) onto an aerogel blanket substrate (Figure 8b). This sensor operated across a wide temperature range, from cryogenic conditions to 1200 °C, enabling real-time monitoring of air flow temperatures in aviation engines. Its lightweight and flexible structure ensured durability in dynamic environments, outperforming traditional rigid sensors. For pressure sensing, Fu et al. [109] developed a high-temperature-resistant pressure sensor by integrating carbon fiber cloth with a dielectric TiO₂ nanofiber film. This sandwich-structured capacitive pressure sensor exhibited exceptional performance, including high sensitivity (≈4.4 kPa⁻¹), an ultralow limit of detection (<0.8 Pa), and a fast response speed (<16 ms) (Figure 8c). Operating at temperatures up to 370 °C, the sensor demonstrated remarkable stability under high-pressure conditions. Besides capacitive pressure sensor, Guo et al. [27] proposed a novel strategy for fabricating piezoresistive pressure sensors using a combination of ceramic fibers and metal components (Figure 8d). By depositing and inducing in situ thermal reactions, stable ceramic-metal contact was achieved between SiO₂ nanofiber films and Au-Cu alloy. The resulting interdigital electrode arrays (Au-Cu@SiO₂) were applied in flexible sensors capable of recognizing human body parts and measuring pressure under temperatures up to 600 °C, with transient operation possible at 1300 °C. Despite significant advancements, high-temperature flexible sensors have relatively simple structures and functions, and their long-term durability under high temperatures presents significant challenges. One major issue is their long-term durability under prolonged exposure to high-temperature. For instance, the interfaces between ceramic and metallic components in hybrid designs, leading to performance deterioration, such as solid-state deweting [110] and thermal expansion [111]. Additionally, these sensors have relatively simple structures and functions, which are still limited to multifunctional monitoring of firefighters, industry devices, and aerospace electronics.