Flexible Ceramic Fibers: Recent Development in Preparation and Application

Flexible ceramic fibers (FCFs) have been developed for various advanced applications due to their superior mechanical flexibility, high temperature resistance, and excellent chemical stability. In this article, we present an overview on the recent progress of FCFs in terms of materials, fabrication methods, and applications. We begin with a brief introduction to FCFs and the materials for preparation of FCFs. After that, various methods for preparation of FCFs are discussed, including centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, and polymer conversion. Recent applications of FCFs in various fields are further illustrated in detail, including thermal insulation, air filtration, water treatment, sound absorption, electromagnetic wave absorption, battery separator, catalytic application, among others. Finally, some perspectives on the future directions and opportunities for the preparation and application of FCFs are highlighted. We envision that this review will provide readers with some meaningful guidance on the preparation of FCFs and inspire them to explore more potential applications. Graphical abstract


Introduction
In recent years, ceramic fiber materials have been paid more and more attention, as can be seen from the number of published literatures. By searching the literature published on "ceramic fiber" in the core database of Web of Science in recent 10 years (2011-2020), it is found that the number of literatures on ceramic fiber materials is increasing (Fig. 1a). It should be noted that the reduction of literature in 2020 could be due to the COVID-19 outbreak. China is the origin of potteries and using ceramic materials. In addition, Chinese scientific technology and research centers are extremely focused on materials science and engineering, particularly in ceramic related subjects (Fig. 1b). During the past 10 years, China has published nearly 3000 publications in the field of ceramic fibers, which has been over 40% of the total number of universal publications in the same period. This rate of development stands far higher than that in United States, Germany, Japan, and other countries.
Traditional ceramic fiber materials are usually prepared from ceramic oxide particles, and the inherent brittleness of ceramic materials greatly limits their application. Flexible ceramic fibers (FCFs) are kind of fibrous lightweight refractory material, and they possess superior mechanical flexibility, high temperature resistance, and excellent chemical stability. Various methods have been developed for preparing FCFs, different benefits and characteristics. FCFs two-dimensional ceramic fiber, films and three-dimensional ceramic fiber aerogels usually possess low density, low thermal conductivity, large specific surface area and many other excellent properties. Due to the superior properties of FCFs, they have been widely used in the fields of thermal insulation [1][2][3], air filtration [4,5], water treatment [6][7][8], sound absorption [3,9], etc.
In this review, we focus on the latest advances in FCFs, including their material system, preparation methods and applications (Fig. 1c). We will first briefly introduce the materials used in the preparation of FCFs, including oxides, carbides, nitrides and so on. Various methods for fabrication of FCFs, including centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, and polymer conversion are introduced. We also highlight the most recent applications of FCFs in thermal insulation, air filtration, water treatment, sound absorption, electromagnetic wave absorption, battery separator, and catalytic application. Finally, some perspectives on the future development of FCFs are provided.
Boron nitride (BN) has been widely used in fiber form [8,[88][89][90]. In terms of atomic arrangement and crystalline structure, BN is similar to carbon, as can form a hexagonal, cubic or tubular structure. Owing to these variable but strong structural atomic connections BN possesses a high thermal oxidation resistivity, low dielectric constant and high radiation absorption capacity. Moreover, BN exhibits a high thermal conductivity, a desired mechanical property and is electrically insulator, so can be widely used for structural, thermal, electrical, and optical applications. Silicon nitride (Si 3 N 4 ) is another nitride ceramic compound which is widely applied in fiber preparation [91][92][93]. GaN fiber, as an important functional fiber, shows great application potential in semiconductor devices [94].
Other ceramic fibers such as ZnS nanofibers [95], boehmite nanofibers [96], hydroxyapatite fibers [97,98], Cu 2 ZnSnS 4 fibers [99,100], etc are known for their excellent insulating or semi-conductor properties. The precursors and calcination conditions of various ceramic fibers are presented on Table 1. For preparation of ceramic fibers, usually a polymer is used as a support or fiber forming material. Poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO), poly(vinyl butyral) (PVB) are the best candidate for this purpose. Oxide ceramic fibers are usually prepared by calcinating the precursor fibers in air, while carbide and nitride fibers are calcinated inert atmosphere. In general, it is necessary to select an appropriate processing method to process different ceramics into fibers depending on their chemical composition, resistivity, structure and physical properties.

Methods for Preparation of FCFs
Fabrication of FCFs is usually conducted by one of these typical methods; centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, polymer conversion, etc.

Centrifugal Spinning
Centrifugal spinning employs the centrifugal force which is generated by a high-speed rotation, to convert a melt or a solution into fibers [101]. The schematic of a typical centrifugal spinning equipment, which is utilised for fiber preparation, is shown in Fig. 2a. Molten ceramic raw material is added in the spinning head, which is usually a metal container with multiple spinneret orifices evenly distributed around the sidewall of the spinning head. In order to keep the ceramic raw material in a molten state, the temperature of the spinning head is usually maintained at more than 1000 °C. The spinning head rotates at a very high speed and the rotating speed is usually higher than 2000 rpm [102]. When the centrifugal stress exceeds the surface tension of the ceramic melt, the ceramic melt will be injected from the spinneret orifices, forming bunch of ceramic melt jets. The molten ceramic flowing out of the sidewall is stretched by the centrifugal force and air frictional force, and reform into fine ceramic fibers with several centimeters length and overall diameter of 1 μm and higher.
A crucial advantage of centrifugal spinning is its high throughput. The average production rate of a centrifugal spinning equipment with two nozzles is 50 g/h, which is two orders of magnitude higher than the lab-scale electrospinning apparatus [102]. More importantly, ceramic fibers with oriented structure can be obtained through rational design of configuration device (Fig. 2c, d) [18,19]. Ceramic fibers can be oriented along the tangential direction in respect to the spinning head, while the centrifugal force is applied. As a result the ceramic fibers are formed with homogenous dimensions and desired mechanical property.

Electrospinning
Electrospinning is one of the most commonly used methods for fabrication of ceramic fibers [110][111][112][113][114][115][116][117]. Figure 3a shows the schematic of a typical electrospinning system consisted of a syringe, a metal nozzle, a power supply, and a conductive collector [117]. In electrospinning chamber, a high voltage electrical field is applied between the nozzle and the collector. When the electrostatic force is higher than the surface tension of the spinning solution, a liquid jet is formed at the nozzle. The electrified jet is stretched and whipped, and further split into multiple jets in an instable region (Fig. 3b, c) [118]. As the spinning solution moves to the collector, the solvent is evaporated, and the fiber diameter is reduced. Finally, the ultrafine fibers are obtained and wrapped around a rotating drum or on a flat collector, such as an aluminum paper or a metal sheet.
The typical steps in electrospinning of FCFs are explained as follows (Fig. 3d) [16]: (1) Preparation of a spinning solution containing polymers and ceramic precursors; (2) spinning the solution into ceramic precursor fibers by under electrical field; (3) calcination at high temperature in order to eliminate the organic components and residuals electrospun. Moreover, ceramic precursor sols can be used to directly produce FCFs by electrospinning without additional polymers [119,120]. A polymer-free electrospinning significantly reduces the volume shrinkage of the ceramic fibers during calcination and cuts the production cost.
The main advantages of electrospinning is that the structure and morphology of the FCFs can be easily controlled by tuning the composition of the precursor solution, or alternation of spinning and calcination conditions. So far, electrospinning of FCFs has mostly remained in the research level, which can be basically because of the low rate of production and hard process and parameters controlling in a potential high scale pilot.

Solution Blow Spinning
Solution blow spinning is an economical and effective fiber preparation technology, which can produce a variety of fiber materials with high efficiency and low cost, including ceramic fibers, metal fibers and polymer fibers [3,4,[121][122][123][124][125]. Figure 4a is a schematic diagram of a typical solution blow spinning device, which consists of a compressed air source, a collector and a solution conveying device. Solution blow spinning device contains a pair of coaxial nozzles. The spinning solution is extruded through the inner nozzle, and the high-speed gas flow is ejected through the outer nozzle (Fig. 4b). The spinning solution forms a jet under the shear action of the high-speed gas flow. The jet is further split, drawn and refined, and the solvent is volatilized in the process of jet movement (Fig. 4c, d). Finally, the fibers are formed, solidified, and collected on the collector.
The process of FCFs fabrication by solution blow spinning is partly similar to electrospinning, as it starts with preparation of spinning solution, which contains a polymer phase and a ceramic disperse or particles phase. The polymer-ceramic precursor solution then forms the composite fibers by being subjected to blow spinning and finally calcination in high temperature is applied to form the ceramic permission from Ref. [69]. Copyright 2015, Springer Nature BV. c Reproduced with permission from Ref. [19]. Copyright 2014, Elsevier. d Reproduced with permission from Ref. [18]. Copyright 2014, Elsevier 1 3 fibers. Here we consider the preparation of SiO 2 fibers by solution blow spinning as an example. The PVA-TEOS solution is first prepared (Fig. 4e), and the PVA-SiO 2 composite microfibers are formed via blow spinning of the solution (Fig. 4f). Finally, the composite microfibers are processed into SiO 2 fibers, through calcination at high temperature (Fig. 4g). After removing the organic components and residual, the average diameter of the calcinated fibers decreases significantly.
Solution blow spinning is a simple procedure and spends less spinning solution compared with electrospinning. This is one of the reason that makes this method quite popular for the fabrication of FCFs [2-4, 39, 53, 57, 62, 68, 70, 71, 128]. The blow spinning equipment uses high-speed gas flow as the driving force, and does not need a high voltage electrostatic field, so it has a higher spinning efficiency. In addition, samples as the fibers can be collected by any type of collector, such as plastic or metal meshes or nonwoven fabrics. Also, unlike electrospinning, solution blow spinning can produce both two-dimensional films or webs (Fig. 4h), as well as three-dimensional sponges (Fig. 4i).

Self-assembly
Nanofibers can be prepared by binding the small molecules together through intermolecular interactions. This chemical process is generally known as self-assembly. Nanofibers can be assembled by different mechanisms depending on the chemical structure of their unit small molecules. A common mechanism for nanofibers assembly, is the formation of hydrogels, which consist of two mutually permeating phases, namely liquid phase (water is the most common) and solid phase (a network of nanofibers formed by self-assembly of hydrogel molecules). The dried nanofibers can be achieved by removing the liquid phase of the hydrogels. Li et al. [88] prepared BN nanoribbon aerogels using hydrogen bond self-assembly method (Fig. 5). The preparation of BN nanoribbon aerogels requires four serial steps ( Fig. 5a): (1) Boric acid (B) and melamine (M) are mixed in a hot distilled water/tertiary butanol (TBA) co-solvent; (2) The mixed solution is cooled to room temperature (25 °C) by ultrasonic treatment to obtain M·2B hydrogel; (3) M·2B hydrogel is freeze-dried to obtain aerogel-like M·2B precursor; (4) After pyrolysis of M·2B in an ammonia atmosphere at 1200 °C for 3 h, lightweight, thermal insulating and hydrophobic BN nanoribbon aerogel are finally obtained. The BN nanoribbons with high aspect ratio are intertwined with each other to form an aerogel network structure (Fig. 5b). In addition, these BN nanoribbons are able to withstand deformation due to stresses, such as bending and twisting, showing excellent flexibility (Fig. 5c).

Atomic Layer Deposition
Atomic layer deposition (ALD) is a method for producing thin films by deposition of solid phase from a vapor precursor, on the surface of a deposition matrics. Tubular ceramic materials have been prepared by ALD [41,129,130]. Since the atoms are deposited layer by layer, the resultant materials forms a homogenous structure and a controllable thickness. Xu et al. [130] prepared Al 2 O 3 fiber sponges with hollow fibrous structure by ALD (Fig. 6a). Polyvinyl propylene (PVP) fiber sponges were first prepared by solution blow spinning technique (Fig. 6b), and then a layer of Al 2 O 3 was coated on the surface of the PVP fibers using ALD (Fig. 6c), and finally the PVP fiber scaffold was removed by calcination, and a high resilience Al 2 O 3 fiber sponges with an ultralow density and thermal conductivity remained (Fig. 6d).  In addition to synthetic fibers, natural fibers have also been used as template to prepare FCFs using the ALD process. Korhonen et al. [41] prepared ZnO, TiO 2 and Al 2 O 3 nanotube aerogels by ALD, where nanocellulose were employed as the scaffold fibers.

Chemical Vapor Deposition
Chemical vapor deposition (CVD) is used for preparation of uniform layers (films) from inorganic precursors. This process occurs together with chemical reactions between ingredients in vapor phase, leaving a uniform solid layer. In recent years, CVD method has been frequently used for preparation of carbide and nitride FCFs, such as SiC fibers, Si 3 N 4 fibers, etc. [6-8, 91, 131, 132].
Su et al. [6] prepared highly porous three-dimensional SiC nanowire aerogels by CVD (Fig. 7). The process in general requires three sequential steps: (1) Initial materials are nucleated and grown on the graphite substrate; (2) raw materials are nucleated and grown on the surface of the existing nanowires layer by layer; (3) highly porous threedimensional SiC nanowire aerogels are detached from the graphite substrate (Fig. 7a). The SiC nanowire aerogels obtained in this work has an ultralow density of 5 mg cm −3 (Fig. 7b, c), and a diameter of 20-50 nm and a length of tens to hundreds micrometers (Fig. 7d, e). Such tiny ceramic fibers are practically hard to be prepared by bow spinning or electrospinning.

Polymer Conversion
Polymer conversion is a method for preparing high temperature resistant FCFs from polymers containing silicon (Si), carbon (C), nitrogen (N) and other elements [133]. The polymer precursors are first processed into fibrous materials by fiber processing methods, such as electrospinning, solution Polymer conversion method is often used to prepare non-oxide ceramic fiber materials, and SiC fiber is the most studied [72,73,76,[134][135][136][137][138][139][140]. Hou et al. [134][135][136][137]139] developed SiC fibers and a series of modified SiC fibers by electrospinning combined with polymer precursor conversion: First, the spinning solutions containing polymers, ceramic precursors and other ingredients such as modifiers are prepared, then the composite fibers are prepared by electrospinning process, and eventually the SiC fibers or modified SiC fibers are obtained by thermal curing and pyrolysis (Fig. 8).

Comparison of Different FCF Preparation Methods
Based on the above discussions, we can see that different FCF preparation methods are associated with advantages and disadvantages. The structure and properties of different ceramic fibers are summarized in Table 2. Out of the above-mentioned methods, only centrifugal spinning is suitable for processing the ceramic melts [102]. Preparation of ceramic fibers by centrifugal spinning, from a melt precursor is an efficient approach, but highly energy consuming. However, electrospinning necessarily works with a colloidal and polymeric solution as a precursor [117,120]. So far various polymeric fibers have been prepared by melt electrospinning [141,142], but it is still hard to fabricate the ceramic fibers by melt electrospinning.
Ceramic fiber materials with different structures and morphologies can be obtained from different preparation methods. Hollow ceramic fibers can be obtained by electrospinning [143] and ALD method [130]. Ribbon structural ceramic fiber materials can be prepared by self-assembly method [88]. Centrifugal spinning and electrospinning can be used to fabricate FCFs with aligned structure [18,144]. Compared with other methods, the FCFs produced by CVD method exhibit smaller and more uniform diameter [6]. Non-oxide ceramic fibers are usually prepared by chemical vapor deposition and polymer conversion [6,74].
In terms of material choices, centrifugal spinning, electrospinning and solution blow spinning can be used to prepare various ceramic fiber materials. However, the material choices for other ceramic fiber preparation methods, including self-assembly, ALD, CVD, and polymer conversion, are limited. For example, the self-assembly method can only use small molecules to assemble and form supramolecular hydrogels through weak interactions to prepare fiber materials, especially for the preparation of ceramic fiber materials, the choice of raw materials is very limited. From the perspective of production efficiency of ceramic fibers, centrifugal spinning has higher spinning efficiency and industrial application prospect, especially centrifugal melting spinning. In addition to centrifugal spinning, compared with other ceramic fibers, solution blow spinning has attracted more and more attention in recent years for its higher efficiency and better fiber material properties, showing a broad prospect of industrial application [123].

Thermal Insulation
Because ceramic fiber materials have the advantages of low density, high temperature resistance, low thermal conductivity, and good chemical stability, they are particularly suitable for thermal insulation purposes, especially for aerospace and other fields that need lightweight thermal insulation materials. Ceramic fiber materials are composed of solid fibers and air between the fibers, in which the solid fibers are a skeleton and endow the material with a specific shape. The thermal insulation property of ceramic fiber materials is mainly attributed to the air component between the solid fibers. In addition, the ceramic fiber materials usually have excellent flexibility, which makes it better than ordinary bulk insulation materials. Therefore, application of ceramic fibers material, as high temperature insulation materials, has attracted increasing attention in recent years [1,2,6,10,73,91,[148][149][150].
We prepared SiO 2 -Al 2 O 3 composite fiber sponges with anisotropic lamellar structure and low density [3]. Due to the low density and layered structure, the sponges with a density of 13 mg cm −3 have a low thermal conductivity of 0.034 W m −1 K −1 perpendicular to the fiber layer direction at 20 °C. The higher the density of the sponges, the higher the thermal conductivity (Fig. 9a). The anisotropic structure endows the sponges with anisotropic thermal properties, and the thermal conductivity parallel to the fiber web is significantly higher than that perpendicular to the fiber layer. In addition, the sponges along the two different fiber layers have greater thermal conductivity at higher temperatures (Fig. 9b). It should be emphasized that the sponges are able to withstand high temperature of around 1200 °C and liquid nitrogen temperature of − 196 °C, and maintain excellent compression resilience under these temperature conditions (Fig. 9c).
Su et al. [6] prepared three-dimensional SiC nanowire aerogels by CVD method. The aerogels demonstrated excellent thermal insulation property, as a 10 mm-thick SiC nanowire aerogels can protect fresh petals from withering, when it is heated by an alcohol lamp in 10 min (Fig. 9d). In a nitrogen atmosphere, the thermal conductivity of the SiC nanowire aerogels at room temperature is only 0.026 W m −1 K −1 . With the increase of temperature, the thermal conductivity of the SiC nanowire aerogels gradually  [3]. Copyright 2020, Springer Nature. d-e Reproduced with permission from Ref. [6]. Copyright 2018, American Chemical Society. f, g Reproduced with permission from Ref. [73]. Copyright 2017, Royal Society of Chemistry increases due to the increasing gas thermal radiation, at high temperatures (Fig. 9e). Liu et al. [73] prepared nitrogendoped hollow SiC fiber mats by electrospinning method combined with polymer precursor conversion method. The hollow SiC fiber mats have good thermal insulation property (Fig. 9f), and the thermal conductivity at 600 °C is only 0.039 W m −1 K −1 (Fig. 9g).
In order to achieve good thermal insulation property of ceramic fiber materials, one of the most important factors is having the low density ceramic fibers. In general, the lower the density, the lower the thermal conductivity. For instance, an ultralow density of approximately 0.2 mg cm −3 has been achieved for SiO 2 nanofibrous aerogels, endowing them with an ultralow thermal conductivity of 23.27 mW m −1 K −1 [148]. The special structural design can effectively reduce the density of ceramic fiber materials, and thus decrease their thermal conductivity, such as lamellar ceramic fiber sponges obtained by solution blow spinning [3] and oriented ceramic nanofibrous aerogels obtained by directional freezing [1].

Air Filtration
Particulate matter (PM) pollution cause serious harm to human health, and effective removal of PMs has become an urgent need in various fields. So far, various filtration materials have been developed, and ceramic fiber-based filters are offer a advanced filtration technology developed in recent years, comparatively [4,5,7,20,98]. Because ceramic fibers have larger specific surface area and smaller pore diameter, they exhibit higher filtration efficiency and lower gas resistance to PMs compared to traditional granular filters and foam ceramic filter materials. Compared with polymer fiber filters, the ceramic fiber filters offer a better chemical stability, and high temperature resistance, demonstrating more advantages in the field of high temperature flue gas filtration.
In 2018, Wang et al. [5] developed an elastic, hightemperature resistant and high-efficiency air filter based on yttrium-stabilized zirconia (YSZ) nanofiber sponges using solution blow spinning technology. The YSZ nanofiber sponges exhibit a filtration efficiency of 99.4% and a pressure drop of only 57 Pa for NaCl particles in the range of 20-600 nm at a flow velocity of 4.8 cm s −1 at room temperature (Fig. 10a). They also tested the filtration properties of the YSZ nanofiber sponges at 750 °C. After filtration, the concentration of PMs with different particle sizes decreased significantly (Fig. 10b). When the airflow velocity was 10 cm s −1 , the filtration efficiency of the sponges to PM 0.3-2.5 and PM 2.5-10 was 99.97% and 99.98%, respectively (Fig. 10c). Therefore, ceramic nanofiber sponges have a potential application prospect in the field of high temperature PM filtration.
In the same year, Zhang et al. [98] developed hydroxyapatite nanowire-based aerogels with excellent elasticity, high porosity and low density. The aerogels demonstrated good biocompatibility and mechanical properties, high PM 2.5 filtration efficiency, and low pressure drop. They designed a series of breathing masks based on the hydroxyapatite  [98]. Copyright 2018, American Chemical Society. e Reproduced with permission from Ref. [4]. Copyright 2020, American Chemical Society nanowire aerogels (Fig. 10d), indicating that the aerogels promise great application in the field of personal protection.
An important progress about the preparation of FCFs was made in 2020, and an Al 2 O 3 -stabilized ZrO 2 (ASZ) fiber filter paper, with excellent flexibility and high temperature resistance of up to 1100 °C was developed via solution blow spinning (Fig. 4h) [4]. The filter papers with an areal density of 56 mg cm −2 possess a high filtration efficiency of 99.56% and a low pressure drop of 108 Pa to NaCl particles with a size of 15-615 nm at an airflow velocity of 5.4 cm s −1 , which combined with the high working temperature make the ceramic fiber papers promising in the field of high-temperature flue gas filtration (Fig. 10e).
Fiber materials have been widely used in the field of air filtration due to their excellent filtration properties. PMs can be removed from polluted gas by fiber materials through the mechanisms of direct interception, Brownian diffusion, inertial impacts, gravity settling, and electrostatic deposition. Additionally, fiber diameter has a significant effect on the filtration property [151], so in order 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.

Water Treatment
In order to solve the problems of oil spills and industrial organic solvent emissions, it is necessary to develop threedimensional absorbent materials with excellent selectivity to absorb and remove organic pollutants from polluted water. Due to the excellent chemical and thermal stability, FCFs as  [6]. Copyright 2018, American Chemical Society a new absorption material have been widely used to remove various organic pollutants in recent years [6-8, 89, 98].
Su et al. [6] prepared highly porous three-dimensional SiC nanowire aerogels via CVD method and used them as absorbents. Due to the low density and good hydrophobicity of the SiC nanowires aerogels, they can float on the surface of water after absorbing all the stained kerosene (Fig. 11a). Due to the excellent elasticity of the SiC nanowire aerogels, the absorbed organic solvents can be collected by squeezing the aerogel materials (Fig. 11b). The SiC nanowire aerogels exhibited a desired absorption selectivity and high absorption capacity (130-237 g g −1 ), toward low viscosity organic solvents (Fig. 11c). The high absorption capacity of the SiC nanowire aerogels can be attributed to their high porosity and low density. In addition, the SiC nanowire aerogels can be reused either by squeezing or burning the absorbed organic solvents (Fig. 11d, e).
Ren et al. [7] prepared SiC/SiO x core-shell nanofiber aerogels with three-dimensional porous structure by CVD method and layer-by-layer self-assembly. The aerogel materials can quickly absorb various organic solvents and oils with large absorption capacity and robust reusability. Song et al. [89] demonstrated that BN aerogels are highly hydrophobic and can absorb oil equivalent to 160 times their weight, and the aerogels can be reused by burning the absorbed oil in air. Xue et al. [8] developed a multifunctional cellular-network foam, which is made of interconnected nanotubular hexagonal BN architectures, using in situ CVD assisted by carbothermal reduction. The foams have a high capacity for absorption, separation and removal of various oils and organic chemicals in oil/water systems, and show a feasible recycling. In addition, hydroxyapatite nanowire aerogels are also an ideal oil-water separation material, and they can be used as an absorbing material to achieve continuous oil-water separation [98].
Based on the analysis of reported literature, ceramic fiber materials used for water treatment are mostly nonoxide ceramic fibers, including SiC fibers, BN fibers, etc. In addition, these ceramic fiber materials mainly absorb organic matters in polluted water in the form of aerogel to achieve the purpose of water treatment. Currently, common oxide ceramic fibers are rarely used in water treatment. Therefore, more efforts should be made to develop oxide ceramic fiber absorption materials or filtration materials to achieve low-cost wastewater treatment.

Sound Absorption
With the rapid development of urbanization and traffic, noise pollution from motor vehicles, mechanical equipment and engineering construction has become one of the most serious environmental problems. All kinds of noise greatly affect people's working efficiency and living standard. Sound absorption materials can be used to consume sound energy in the process of noise transmission, so the development of efficient sound absorption materials to eliminate or reduce noise pollution is of great significance. In addition, with the improvement of safety requirements, the flame retardancy requirements to sound absorption materials also improve continually, especially for the indoor and vehicle use, so it is urgent to develop efficient sound absorption materials with excellent flame retardant property.
Ceramic fiber materials have the characteristics of lightweight, non-combustibility, high temperature resistance and good chemical stability, so they are suitable for use as sound absorption material. We developed a superelastic SiO 2 -Al 2 O 3 composite fiber sponges with superior thermal insulation and sound absorption properties using solution blow spinning technology (Fig. 12a) [3]. The sound absorption coefficient and noise reduction coefficient (NRC) increased, by increasing the thickness of the sponges (Fig. 12b, c), as an sponge with an areal density of 58 mg cm −2 possesses an NRC value of 0.77, greater than most of the reported sound absorption materials (Fig. 12d). The superior sound absorption property of the sponges can be ascribed mainly to their low density and layered structure, and the sound waves can be absorbed multiple times in the sponges (Fig. 12e). In addition, the rough surface of the microfibers and the vibration of the microfibers caused by sound waves are also conducive to the sound energy absorption.

Electromagnetic Wave Absorption
With the rapid development of modern information technology, electromagnetic wave interference pollution has become more and more serious. It is of great importance to develop an absorbers with an acceptable level of electromagnetic wave absorption property. SiC fibers have become the most used electromagnetic wave absorption material because of their adjustable microstructure and dielectric property [72, 85, 131, 132, 134-137, 139, 152-155].
Hou et al. [136] developed SiC nanofiber mats with and without fiber alignment, via electrospinning and polymer precursor conversion (Fig. 13a). The 2.3 mm thick silicone resin composites with 5 wt% aligned SiC nanofibers exhibit a reflection loss (RL) of − 53 dB at the frequency of 17.6 GHz and the same composites with a thickness of 3 mm have an effective absorption bandwidth (EAB) of 8.6 GHz (Fig. 13b). The electromagnetic wave absorption properties of the composites with aligned SiC nanofibers outperforms the one with randomly decorated SiC nanofibers.
In addition to the orientation of the fibers, adding appropriate modifiers to the SiC fibers can also improve the electromagnetic wave absorption properties of the materials.  [136]. Copyright 2017, American Chemical Society. c, d Reproduced with permission from Ref. [137]. Copyright 2018, American Chemical Society. e-g Reproduced with permission from Ref. [132]. Copyright 2020, American Chemical Society Currently, iron (Fe) [139], Fe 3 Si [135], Fe 3 O 4 [154], hafnium carbide (HfC) [137], cobalt (Co) [134] have been reported as modifiers of SiC fibers. Taking HfC as an example, the addition of a small amount of HfC component in the SiC fibers can significantly improve the electrical conductivity of the fibers, thus improving the dielectric property and electromagnetic wave absorption property (Fig. 13c) [137]. The HfC/SiC nanofibers with 2.5wt% HfC in the 3 mm thick silicon resin composites show a minimal RL of -33.9 dB at 12.8 GHz and an EAB of 7.4 GHz (Fig. 13d).
Like the polymer precursor conversion, CVD is also a common procedure, for preparing the SiC fibers [6,131,132]. Cai et al. [132] developed three-dimensional porous SiC@C nanowire foams (SCNFs) by infiltrating glucose solution in CVD-constructed SiC nanowire aerogels and subsequent carbonization (Fig. 13e). The foams with a density of 108 mg cm −3 and a thickness of 3 mm exhibited a minimum RL value of -52.5 dB at the frequency of 11.5 GHz and a broad EAB of 10.1 GHz in the frequency range of 7.9-18 GHz (Fig. 13f). The excellent electromagnetic wave absorption properties of the foams can be attributed to their improved impedance match and multiscale energy dissipation mechanism (Fig. 13g).

Battery Separators
In recent years, with the development of battery technology, the safety of the battery is different systems has attracted more and more attention. Polymeric separators currently in use, are prone to causing serious safety incidents, mostly due to internal short circuits. Especially when thermal runaway occurs, the temperature inside the battery instantly reaches above 500 °C, where the polymer separators cannot withstand and degrades immediately. Therefore, it is important to develop ceramic fiber separators with high temperature resistance, especially for the advanced systems which work with battery.
Yan et al. [16] developed LLZO ceramic nanofiber films with silk-like softness, low density, and robust fire resistance via sol-gel electrospinning technique (Fig. 14a). The separators prepared from LLZO films, exhibit better electrolyte wettability than commercial Celgard 2500 separator and the electrolyte absorption rate is greater than 900% (Fig. 14b). The assembled Li-ion battery exhibited a wide electrochemical window of 2.7-4.6 V. When the termination charging voltage was increased to 4.6 V, the discharge capacity of the battery was increased to 190 mAh g −1 at 0.5 C (Fig. 14c). Compared with Celgard 2500-based battery, the LLZObased battery demonstrated significantly stable capacity retention over 600 cycles (Fig. 14d).
A elastic and well-aligned Li 6.4 La 3 Zr 2 Al 0.2 O 12 (Al-doped LLZO) ceramic nanofiber web (EACN) was developed via sol-gel electrospinning and the composite electrolytes were prepared by combining the ceramic webs with ion-conducting polyvinylidene fluoride (PVDF) (Fig. 14e) [156]. Continuous ion transport channels are formed in the composite electrolytes, which enables them have high ionic conductivity of 1.16 × 10 -4 S cm −1 at 30 °C (Fig. 14f). The solid-state battery NCA (LiNi 0.8 Co 0.15 Al 0.05 O 2 )/EACN/Li based on the Al-doped LLZO exhibited a high termination charging voltage of 4.6 V and large initial discharge capacity of 213 mAh  [16]. Copyright 2019, Elsevier. e-h Reproduced with permission from Ref. [156]. Copyright 2019, Elsevier g −1 at 0.2 C (Fig. 14g). Multi-cycle test results show that the battery has a high Coulombic efficiency of > 95%, demonstrating a good electrochemical stability of the composite electrolytes (Fig. 14h).
Currently, there are few researches on the application of ceramic fibers in battery separator, most of which focus on LLZO fiber material. Given the excellent safety of ceramic fiber materials, more work should be devoted to developing more ceramic fiber-based separators with excellent electrochemical properties. In order to reduce the pore size of ceramic fiber-based battery separators and achieve better electrochemical property, it is an important research direction to develop ceramic fiber materials with smaller diameter.

Catalytic Application
Ceramic fiber materials have great application prospects in the field of catalysis because of their large specific surface area and good chemical and thermal stability [113]. The application of ceramic fibers in the field of catalysis has two forms. On the one hand, some ceramic materials possess good catalytic activity, and they can be applied in catalytic field by processing them into ceramic fibers [35,38,[157][158][159][160]. On the other hand, ceramic fibers can be used as the carrier of catalysts, and ceramic fibers with catalytic activity can be obtained by loading catalysts on/in the ceramic fibers [161][162][163][164].
There are two methods for preparing catalyst materials with ceramic fibers as the catalyst carrier. One method is to directly add catalysts or catalyst precursors to the spinning solution to obtain catalytic ceramic fibers by spinning and calcination [161,163,164]. For example, flexible self-standing Cu-Al 2 O 3 fibrous membranes with high Fenton catalytic activity were prepared by electrospinning a spinning solution containing precursors of alumina and copper (Fig. 15d) [161]. Another alternative method is to obtain ceramic fibers by spinning and calcination first and then loading catalysts on the surface of the ceramic fibers [162]. Cheng et al. [162] first synthesized ZnO/γ-Al 2 O 3 nanofibers via electrospinning and calcination. The Ag/ZnO/γ-Al 2 O 3 nanofibers with efficient photocatalytic degradation property were further prepared by decorating silver nanoparticles on the surface of the ZnO/γ-Al 2 O 3 nanofibers, by photoreduction and hydrothermal processes (Fig. 15e).
The preparation process of the first method for immobilizing catalysts is simple, but the catalysts are mainly distributed inside the fibers, which may affect the catalytic property of the catalysts, especially for these catalytic applications that require the catalysts to be exposed to the fiber surface, such as photocatalysis. In a general comparison, the second method for loading catalysts is slightly more complicated, but the catalysts are distributed on the surface of the fibers. This structure outstandingly improves the catalytic functionality of the catalysts can be fully.

Other Applications
Using piezoelectric effect to harvest energy from movement and to generate power using small wearable electronic devices is today a hot topic of society. Some ceramic materials can be processed into FCFs with excellent piezoelectric properties, such as BaTiO 3 [165], lead zirconate titanate (PbZr 0.52 Ti 0.48 O 3 , PZT) [166], ZnO [167,168], and (Na, K)NbO 3 [169][170][171]. These ceramic fibers exhibit excellent responses to the impacts, and that is why they found broad applications in piezoelectric nanogenerators, highly functional electromechanical sensors, transducers, and other wearable stress responding electrical devices.

Conclusions and Outlook
In the last decade or so, tremendous progresses have been witnessed in development and application of FCFs. Different types of materials have been processed into FCFs, such as oxide fibers, carbide fibers, nitride fibers and other ceramic fibers from the major categories. Many methods have been developed to prepare FCFs, including centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, and polymer conversion. The rapid development of FCFs preparation has promoted the exploration of their applications in multiple different fields, including high temperature thermal insulation, air filtration, water treatment, sound absorption, electromagnetic wave absorption, battery separator, catalytic application, among others.
Some development directions and opportunities for FCFs should be noted.
1. For some preparation methods of FCFs, such as electrospinning and solution blow spinning, polymers need to be added to the spinning solution as thickeners and templates to assist in fiber forming. These polymer components should be eliminated by calcination, which usually leaves the ceramic fibers, as the final product. Also, elimination of polymers will result in a significant reduction of the fiber diameter, and a significant contraction of the three-dimensional bulk fiber materials can be observed macroscopically. This also dramatically wastes the resources, a dramatic, therefore, using the high molecular weight polymers as the template for the electrospinning is strongly suggested. 2. For some single-component ceramic materials, they are less flexible when processed into ceramic fibers. Flexibility of ceramic fibers can be improved by adding the other ceramic components. For example, adding some yttrium oxide, alumina, etc. to the zirconia fibers can effectively improve their flexibility [4,5]. Therefore, development of multi-component ceramic fibers is an important approach to solve the rigidity of FCFs. The composite of different ceramic components not only can improve the flexibility of ceramic fibers, but also enhances the other important properties, such as temperature resistance [3]. In addition, what we also need to do is to explore the use of low-cost ceramic precursors to prepare FCFs to reduce the cost of materials. 3. A certain shape is required for the practical application of FCFs, including one-dimensional yarns, two-dimensional films, and three-dimensional aerogels. Most of the preparation methods cannot obtain FCFs with regular shapes, and some methods can only obtain FCF films or webs. For those applications that require threedimensional FCFs, such as thermal insulation and sound absorption, the FCF films need to be further processed, as they seriously limit their practical application. Among the discussed methods, solution blow spinning can readily obtain two-dimensional FCF films and three-dimensional FCF sponges, which should be more applied in future research. 4. In order to reduce the consumption of petroleum resources, polymers derived from natural materials can be used in the preparation of FCFs. On the one hand, in methods such as electrospinning and solution blow spinning that use spinning solutions, polymers derived from natural materials can be used as thickeners for spinning solutions and templates for FCFs. On the other hand, natural materials can be used as carbon or nitrogen sources for non-oxide FCFs in methods such as chemical vapor deposition. 5. For the electrospinning and solution blow spinning, the spinning devices with single or several nozzles can only be used for laboratory research, and it's difficult for them to meet the practical application requirements. In recent years, needleless electrospinning technology has been developed as an efficient technique for the large-scale preparation of ultrathin fibers [180]. However, currently the needleless electrospinning is mainly used in the preparation of polymer fibers. Therefore, it is urgent to expand the application of this technology in the largescale production of ceramic fiber materials. In addition, it is of great significance to develop needle-free solution blow spinning technology and explore its application in the mass production of ceramic fibers.