Direct synthesis of highly stretchable ceramic nanofibrous aerogels via 3D reaction electrospinning

Ceramic aerogels are attractive for many applications due to their ultralow density, high porosity, and multifunctionality but are limited by the typical trade-off relationship between mechanical properties and thermal stability when used in extreme environments. In this work, we design and synthesize ceramic nanofibrous aerogels with three-dimensional (3D) interwoven crimped-nanofibre structures that endow the aerogels with superior mechanical performances and high thermal stability. These ceramic aerogels are synthesized by a direct and facile route, 3D reaction electrospinning. They display robust structural stability with structure-derived mechanical ultra-stretchability up to 100% tensile strain and superior restoring capacity up to 40% tensile strain, 95% bending strain and 60% compressive strain, high thermal stability from −196 to 1400 °C, repeatable stretchability at working temperatures up to 1300 °C, and a low thermal conductivity of 0.0228 W m−1 K−1 in air. This work would enable the innovative design of high-performance ceramic aerogels for various applications.


Introduction
Ultralight ceramic aerogels have been widely applied for thermal, electrical, magnetic, medical, optical and chemical applications owing to their low density and thermal conductivity, high speci c surface area, high porosity, and chemical and thermal inertness [1][2][3][4] . However, conventional ceramic aerogels with typical pearl necklace-like structures are brittle and often tend to structurally collapse under large external stresses or strains caused by ine cient structural continuity and connection [5][6][7][8] . Prior efforts to improve mechanical properties of ceramic aerogels by manufacturing ceramic aerogel materials with continuous structural units [9][10][11][12][13] , such as nano bre or nanosheet. Due to the absence of necklace-like structures and defects, these continuous elements could yield additional degrees of freedom to avoid brittle failure of building blocks for improving compressive brittleness of ceramic aerogels, but cannot address the stretchable brittleness. When used as the main medium for protecting the human body and equipment, the poor stretchable properties of ceramic aerogels have resulted in limited applications in extreme environments such as aerospace and defense.
As a prerequisite for their e cient application, ceramic aerogels must exhibit excellent mechanical properties including both stretchability, compressibility and exibility. Owing to their relatively simple and straight building blocks, fragile connections or lamellar structures 14,19−21 , all existing aerogel-like 3D materials undergo transient structural fracture or slipping under even extremely small tensile strain, and they are far from meeting the needs of composite processing and practical applications, for instance, boron nitride ceramic aerogels prepared by chemical vapour deposition, ceramic aerogels developed by the freeze-shaping method (silica nano brous aerogels 14,15 , hexagonal boron nitride aerogels with stacked nanosheet structures1 and SiC@SiO 2 nanowire aerogels 16 ) and lamellar ceramic brous aerogels directly obtained by spinning 17,18 . In addition, these methods generally have the problems of complicated preparation processes, high cost and di cult mass production. Alternatively, a natural luffa stem with crimped and entangled structures has provided us material design inspirations for improving the stretchability and structural stability of ceramic aerogels. When subjected to tensile stress or strain, crimped structures can transform stretching into unbending of the structures, and dense entanglements are conducive to ensuring that tension transmits along the element and to many other elements, effectively avoiding stress concentration [22][23][24][25][26] . However, ceramics are more di cult to process than metals and polymers, especially into complex shapes 27 , and thus, improvement in stretchable properties of ceramic aerogels via structural design being a major unresolved challenge. Therefore, exploiting a simple, rapid, and low-cost mass-production strategy for the preparation of ceramic nano bres with complex shapes is the key to making a breakthrough in ceramic aerogels with stretchable properties.
Here, we designed a unique electrohydrodynamic methodology, 3D reaction electrospinning, to directly manufacture ceramic nano brous aerogels with interwoven crimped-nano bre structures. The 3D interwoven crimped-nano bre structured ceramic aerogels (ICCAs) exhibited superior performance such as ultralight weight, exibility, stretchability, compressibility, fatigue-tolerance, and ultralow thermal conductivity. The ICCAs could be stretched from their original morphology to 100% tensile strain without fracture and simultaneously exhibited superior restoring capacity in response to large deformations of more than 40% tensile, 60% compressive or 90% bucking strain, as well as robust fatigue-tolerance for 100,000 cycles. Moreover, the mullite phase provided the ceramic aerogel with thermal stability from -196 to 1400 ℃, and the aerogels still had repeatable stretchability after calcination at 1300 ℃ for 1 h. Finally, we demonstrated that ICCAs can be applied as high-performance thermal insulation materials.
Meanwhile, we prepared a huge nano brous aerogel with 170 cm long, 130 cm wide, and 12 cm high via the unique method. These results have broad technological and engineering implications for personal protective equipment, thermal protection systems in space vehicles and exible wearable-electronics.
3D reaction electrospinning of ceramic nano brous aerogels. We designed and fabricated a ceramic nano bre aerogel with an interwoven crimped-nano bre structure by controlling the coagulation rate of the jet during electrospinning. In conventional electrospinning, a charged uid jet is formed in a conical droplet by an electrostatic force and accelerates towards the collector with stretching slenderization. Many investigations have indicated that the travelling direction of charged uid jets lacking dampening viscoelastic forces change from the centerline under the action of the applied electric potential during electrospinning 28,29 , inducing the onset of bending instability and the formation of a crimped-bre structure. Ultimately, the crimped-bre structure collapses or is straightened under the action of surface tension and electrostatic forces, and randomly oriented 2D brous membranes are formed by deposition of the unsolidi ed jet. Creating 3D nano brous structures by electrospinning has been intractably di cult thus far. To directly assemble ceramic brous aerogels with interwoven crimped-nano bre structures, we designed a 3D reaction electrospinning method controlled by a sol-gel reaction in the jet ow. The rate of gelation of the sol jet was controlled to achieve precise control of the jet shape within milliseconds by tailoring the extent of protonation of colloidal particles. Figure 1a-c shows the unique 3D reaction electrospinning process, which was performed using a sol solution with high conductivity (12910 µs/cm) and low viscosity (18.76 cp). Our previous work suggested that excess charge density of the liquid would result in the formation of spherical droplets instead of cylindrical jets 30,31 . More excitingly, we changed the ejection mode of the conical droplet to a multijet mode (Supplementary Movie 1) by adding extremely low amounts (0.1 wt%) of high-molecular-weight polymers while the condensation between colloidal particles was not impacted. The speed of a single spinneret hole in this method was 5~10 times higher than that in the fabrication of ceramic nano bres by conventional electrospinning 32 , enabling industrial scale production. The stability of the jet noticeably decreased, and the jet experienced whipping instability after being elongated into a slender straight jet due to the high surface potential, forming a 3D crimped structure ( Supplementary Fig. 1). With rapid solvent evaporation, the distance between colloidal particles and the repulsive coulombic interaction caused by the electrical double layer was reduced, resulting in aggregation of colloidal particles [33][34][35] .
Subsequently, the highly reactive colloidal particles tend to form highly crosslinked and robust skeletons via condensation and jets solidi ed (Fig. 1b), suppressing deformation and collapse of the 3D crimped nano brous structures. We proved that the distance between the protonated oxygen atom of ≡M-OH and the metal atom increased, creating a good leaving group ( Fig. 1c and Supplementary Table 1). In addition, the central metal atom of the protonated group became more electrophilic due to withdrawal of electron density. The electrophilic properties of the central metal atom rendered it more susceptible to attack by nucleophilic groups, indicating that protonation provided a more reactive site for condensation. In contrast, beaded nano bres formed when the extent of protonation was too low, while micro bres formed when the extent of protonation was too high ( Supplementary Fig. 2).
To enhance the reaction rate for condensation of colloidal particles, we produced a sol spinning solution with both high gelation reactivity during the spinning process and excellent stability before spinning by two-step acid catalytic processes. First, colloidal particles with low condensation reactivity were prepared by hydrolyzing a metal alkoxide under a low concentration of H + . Subsequently, we acidi ed the sol solution to a lower pH to increase the extent of protonation of colloidal particles by dropwise addition of ethanol-diluted hydrochloric acid solution. The nal sol solution exhibited excellent stability and spinnability even after one week, as an electrical double layer was formed around the colloidal particles again and the repulsive barrier was increased, which was proven by the zeta potential ( Supplementary  Fig. 3). To gain deeper insight into the different spinning processes of three sol spinning solutions with different extents of protonation, we obtained Fourier transform infrared (FTIR) spectra of untreated precursor nano brous aerogels and characterized the particle sizes of untreated aerogels dissolved in water via dynamic light scattering (DLS). The condensation reaction occurred during the spinning process after the extent of protonation increased, as con rmed by the new bands near 813 cm −1 and 1090 cm −1 in the FTIR spectra 36 (Supplementary Fig. 4) and the increase in the particle size from 8 nm to 140 nm ( Supplementary Fig. 5). Moreover, the untreated aerogels fabricated with the least reactive sol could promptly dissolve in water, and the solution was transparent, indicating that no reaction occurred between the low-reactivity colloidal particles in the spinning process. The other two aerogels obtained from a more reactive sol were insoluble in water and the dispersion solution exhibited white haze ( Supplementary Fig. 6), which further proved that the reaction occurred during the spinning process. Subsequently, we interknitted the crimped nano bre to obtain a 3D interwoven crimped-bre structured nano brous aerogel precursor by tailoring the vertical movement of the spinning nozzle and the collector.
Finally, the aerogel precursors were calcined at 1000°C for 1 h in air, and the ICCAs were obtained (Fig. 1d). The nal ceramic nano brous aerogels exhibited outstanding thermal stability and superior stretchability. The integral ceramic nature of the ICCAs allows them to withstand even high-temperature ames 1300°C (butane blowlamp) 37 without any damage or deformation (Fig. 1e) and be stretched from their original morphology to 100% tensile strain without fracture (Fig. 1f, Supplementary Fig. 7 and Supplementary Movie 2).
Material characterization of ICCAs. We rst chose mullite as a sample for our proof-of-concept study in consideration of its outstanding thermal stability. The obtained ICCAs display distinctive structures, as shown in the scanning electron microscope (SEM) images ( Fig. 2a-d). In a marked contrast to the lamellar structures of traditional ceramic brous materials in cross section and the pearl necklace-like structure of traditional ceramic aerogels, the ICCAs showed a 3D interwoven crimped-nano bre structure based on the randomly entangled crimped-bre framework including crimped bres locked with each other and bonding points, which could be attributed to adhesion and fusion between two precursor bres next to each other during calcination.
We also characterized the chemical composition and crystallinity of a single ceramic bre. From the relevant energy-dispersive X-ray spectroscopy (EDS) mapping results, Al, Si, and O elements were homogenously distributed in the ceramic nano bres (Supplementary Fig. 8). The scanning transmission electron microscopy (STEM) and selected-area electron diffraction (SAED) pattern observations revealed that the obtained ceramic nano bres were composed of numerous ne mullite grains with grain boundaries and glass phases (Fig. 2e). To better investigate the crystallization process of ICCAs, we performed thermogravimetric analysis (TGA), and the date revealed the thermal decomposition of the precursors with a weight loss of 50% and the formation of ceramic phases at approximately 1000 ℃ ( Supplementary Fig. 9). The characteristic diffraction peaks of mullite in the X-ray diffraction (XRD) pattern further indicated the formation of mullite after calcination at 1000 ℃. The crystalline structure was still a single mullite crystal even after heating at 1400 ℃, indicating that the ICCAs exhibited excellent thermal stability (Fig. 2f). Moreover, the pronounced Al 2p, Si 2p, and O 1s peaks in the X-ray photoelectron spectroscopy (XPS) spectrum showed a single peak distribution, which con rmed the formation of pure mullite 38,39 ( Supplementary Fig. 10).
More interestingly, we found that the crimped mullite nano bre is stretchable, as revealed by using focused ion beam-SEM (FIB-SEM). As shown in Fig. 2g and Supplementary Movie 3, the two bonding points between the crimped ceramic nano bre and FIB probe could be stretched from their original distance to 120% tensile strain without fracture of the nano bre. Fig. 2h-i shows the tensile stress-strain curves of the straight nano bre. The tensile strength of the mullite nano bre reached 1.74 GPa, rendering the ceramic nano bres robust for preventing breakage while the aerogels were stretched. Therefore, based on the structure of ICCAs and the intrinsic performances of the single ceramic nano bre, the ICCAs exhibited a series of superior features. (i) Ultralight weight: A piece of an 18-cm 3 ICCA could be placed on the petal of a ower, and the minimum density of ICCAs was approximately 1.5 mg cm −3 corresponding to a porosity of 99.95% (Supplementary Fig. 11). (ii)Superior stretchability, exibility and elasticity: The fabricated ICCAs exhibited excellent stretchability and could be stretched and knotted without any visible breakage/fracture (Fig. 2j-k). In particular, ICCAs (density of 6 mg cm −3 ) with a weight of 0.06 g could withstand up to ~3300 times their own weight without fracture.
Temperature-invariant stretchability, exibility and compressibility. We further evaluated the superior mechanical behavior of ICCAs over a wide range of temperatures by carrying out a series of tests on a dynamic mechanical analyser (DMA) instrument. Fig. 3a shows that the ICCAs could be stretched from their original morphology to 100% tensile strain without fracture. The tensile fracture stress was 12.7 kPa. According to the tensile stress-strain curve, we observed three characteristic stages in the loadingunloading process: a relatively linear elastic regime when ε < 50%, a subsequent nearly at stress plateau when 50% < ε < 80%, and a nal regime when ε > 80% in which σ increased sharply. To gain deeper insight into the peculiar stretchability, we examined the microstructural evolution by performing an in situ SEM tensile test. The ICCAs without tensile strain exhibited a 3D interwoven crimped-nano bre structure. When the tensile strain reached 90%, declinations, deformations, interlockings and orientations caused by straightening appeared in the crimped nano bres of the ICCAs, preventing structural collapse ( Fig. 3b and Supplementary Fig. 12). Upon further investigation, we found that the perfect stretchability is due to several factors. First, dense entanglements are conducive to maintaining the network con guration, and tension is transmitted along the nano bres and to many other nano bres by entanglements when the ceramic aerogel is stretched (Supplementary Fig. 13). Second, ceramic nano bres are long, crimped, and exible and do not easily break before the aerogel fractures. Third, the low density and high porosity of the ICCAs provide su cient space for ceramic nano bres to de ect rather than fracture due to excessive twisting and bending when the aerogel suffers large strain. In contrast, the ceramic membrane with lamellar structure and straight nano bres had smaller nal strain (5%) (Fig. 3c), and it experienced a fracture that appeared to be caused by slippage of the straight nano bres (Fig. 3c insert), con rming the importance of the 3D interwoven crimped-nano bre structure.
Subsequently, we demonstrated that the ICCAs could withstand 1000 tensile-release cycles at a strain of 40% (Fig. 3d). The aerogels displayed invisible plastic deformation, and the maximum stress remained at 60% of that in the rst cycle. The constant energy loss coe cient (~ 0.47) during cyclic tensile testing indicated the structural robustness (Fig. 3e). More interestingly, the ICCAs exhibited amazing tensilerecovery fatigue resistance behaviour. As presented in Fig. 3f, after being stretched for 100 000 cycles at an oscillatory strain of 5%, storage modulus, loss modulus and damping ratio (~ 0.16) remained almost identical, highlighting an excellent stretching elasticity and fatigue-resistance. Furthermore, a bucklingrecovery test with increasing strain to 90% and a 1,000-cycle buckling-recovery test were performed to demonstrate the exibility of the ICCAs (Supplementary Fig. 14). The aerogels did not obviously fracture even when the buckling ratio was up to 95%, or after buckling deformation for 1,000 cycles at a buckling strain of 90%, indicating a high exibility. Moreover, the ICCAs exhibited large compression recovery and robust compressive fatigue tolerance up to 1000 cycles at a compressive strain of 60% ( Supplementary  Fig. 15).
ICCAs that combine the peculiar properties of the interwoven crimped-nano bre structure with a mullite nature are anticipated to provide temperature-invariant stretching elasticity. As shown in Supplementary  Fig. 16, the dynamic storage tensile modulus, the dynamic loss tensile modulus and damping ratio remained nearly invariable from -100 to 500°C. For further investigation of the mechanical stability at extreme temperatures, we heated the ICCAs at temperatures of 1300°C and 1400°C for 1 h. After calcination at 1300°C, the ICCAs showed superb ductility with a tensile strain up to 48.3%. The aerogels calcinated at 1400°C had a smaller nal strain (23.2%) (Fig. 3g), which might be attributed to the increase in defects in nano bres produced by excessive crystallite growth during high-temperature treatment ( Fig. 3h and Supplementary Fig. 17). The results revealed that the aerogels can preserve superior stretchability after treatment at deep cryogenic or ultrahigh temperatures. Subsequently, we explored the stretching elasticity of the sample annealed at 1300°C for 1 h. More importantly, this aerogel can also sustain up to 1,000 tensile-release fatigue cycles with a 20% tensile strain (Fig. 3i). Moreover, the aerogels can be completely restored to their original shape when bent or compressed while being either exposed to the ame of a butane blowtorch or immersed in liquid nitrogen ( Supplementary Fig. 18-19). Thus, the temperature-invariant stretchability, exibility and compressibility of ICCAs from -196 to 1400°C have been demonstrated.
Thermal insulation and re resistance. In our previous studies, we found that the total thermal conductivity (λ t ) is mostly affected by the solid thermal conductivity (λ s ) and the gaseous thermal conductivity (λ g ), which depends on the porosity of materials at ambient pressure and room temperature 14,40 . Generally, porous materials can exhibit remarkably increased thermal resistance through the combination of restricted convection and decreased conduction at room temperature provided by the nano brous structure and ultralight characteristics 41,42 . As a result, our ultralight nano brous aerogels exhibited outstanding thermal insulating performance. Fig. 4a shows a plot of λ t as a function of the density of ICCAs. λ t was as low as 0.0228 W m −1 K −1 with a density of 6 mg cm −3 under ambient conditions, which could be attributed to the ultralow diameter of the ceramic nano bre and extremely high porosity (99.8%). This value is lower than the thermal conductivity of air. When the density was increased to 22 mg cm −3 , λ t was increased slightly to 0.0274 W m −1 K −1 by virtue of the almost constant porosity (99.27%). This low thermal conductivity in combination with the high degree of temperature-invariant stretchability and exibility allows ICCAs to be applied as excellent thermal insulation materials under extreme environments beyond the reach of conventional porous insulation materials (Fig. 4b). (i) Most porous polymeric insulation materials, such as polybenzazole aerogels and polysilsesquioxane aerogels usually exhibit a relatively low thermal conductivity, but cannot withstand temperatures above 500°C (green zone) 40,43,44 . (ii) Carbon materials show a relatively high heatresistance, but still cannot withstand a high-temperature oxidizing atmosphere (orange zone) [45][46][47] . (iii) Compared with previously reported advanced ceramic insulators, the ICCAs exhibited a λ value among the lowest values reported with robust structural stability up to 1400°C, indicating that they are more attractive for extreme thermal insulation 1,15,17,21 . Moreover, the macroscopic product can be easily and quickly prepared by pilot equipment. As shown in Fig. 4c and Supplementary Fig. 15, a ceramic nano brous aerogel precursor 170 cm long, 130 cm wide, and 12 cm high weighing 313 g was produced in one hour ( Fig. 4c and Supplementary Fig. 20).
The thermal insulation materials used to protect equipment and fuel tanks in aero-engines or scramjet engines should withstand high temperatures or even direct contact with a ame 48 . Therefore, we used infrared camera observations to monitor the dynamic temperature distribution of ICCAs upon heating and exposure to ames to further evaluate the practical thermal insulation performance under extreme conditions. First, we wrapped ICCAs that had thickness of 10 mm and a density of 8 mg/cm 3 around the nozzle of a butane blowlamp (Fig. 4d). When the butane blowlamp was turned on, we took thermalinfrared images in real time to characterize the time dependent high-temperature thermal insulation property. After heating for 5 min, the temperature of the nozzle rapidly exceeded the limitation of the equipment (800°C), while the temperature of the ICCAs exhibited a slow rise to a maximum temperature of approximately 137°C. Remarkably, the temperature outside of the ICCAs remained nearly constant even after 30 min, indicating that the aerogels are superior thermal insulators. Subsequently, a piece of ICCA with a density of 22 mg/cm 3 and a thickness of 20 mm was directly heated by a butane blowlamp ame (approximately 1300°C 37 ). Similarly, the backside temperature slowly increased to 121°C after 10 min, demonstrating the robust re resistance of the ICCAs (Fig. 4e). These results indicated that our aerogels present a combination of excellent thermal insulation and robust structural stability and stretchability that offers extensive potential applicability in the thermal insulation eld under extreme conditions, such as re suits for emergency rescue and in aviation and aerospace industries.

Discussion
The successful synthesis of ICCAs using 3D reaction electrospinning provides a novel methodology to explore the formation of 3D interwoven crimped-nano bre structured ceramic aerogel materials. Here, mullite-based nano brous aerogels served as model systems for a proof of concept. The rate of gelation of the sol jet was controlled to achieve precise control of the jet shape. The nal ICCAs exhibit superior performance such as ultralight weight, exibility, stretchability, compressibility, fatigue tolerance, and ultralow thermal conductivity over a very-wide temperature range. Considering the ease of optimization of sol solutions based on the sol-gel method and the diversity of the nano bres, our work will provide a versatile platform for designing new types of nano brous aerogels for various applications. Moreover, we showed that the ICCAs can be easily scaled up in pilot equipment. Due to their excellent integrated properties, we can expect that the ICCAs will have broad technological and engineering implications for personal protective equipment, thermal protection systems in space vehicles and exible wearableelectronics.
During electrospinning, the spinning solution was injected at a speed of 10 ml/h and stretched by an applied voltage of 25 kV. The mullite nano bres were collected on the surface of the metallic rotating covered by aluminium foil at a 220-mm recieving distance. Furthermore, the humidity (40 ± 2%) and relevant temperature (22 ± 2 ℃) during electrospinning were controlled. Moveover, the spun specimen was immediately calcined in a mu e furnace. The process of calcination can be divided into four stages: rapid heating at 15 ℃ min −1 from 20 to 1000 ℃; maintenance at 1000 ℃ for 1 h; heating at 10 ℃ min −1 from 1000 ℃ to the set temperature (1100~1500 ℃); and maintenance at the set temperature for 1 h. Other mullite nano bre aerogels with different densities were manufactured via adjusting the addition of water and ethanol. Except as noted, mullite nano bre aerogels with density of 6 mg cm −3 were used to performe the structural and physical property tests.
Characterization. The microstructure of ICCAs was characterized by FE-SEM (S-4800) and TEM (JEM-2100). Optical images of ICCAs and sol solution were recorded by a digital video camera (Canon M50). An infrared thermal camera (Fluke TiX560) was used to take the infrared images of ICCAs. The chemical structure of ICCAs were tested via XPS (PHI 5000C ESCA), TGA(TA SDT Q600 TG-DSC analyzer) and XRD (Bruker D8 ADVANCE). The single bre tensile test involves the following parts: (a) preparing the dispersion of ICCAs to obtain single nano bre; (b) a thin layer of ceramic binder is applied to the bre placement area of the xed sample table. Then the bre samples are received and left to cure for 12h; (c) put the sample stage gently into the xture and start the test. The size of particles was characterized by DLS (Malvern zetasizer nano ZEN3700). The thermal conductivity of ICCAs was characterized by a Hot Disk instrument (TPS2500S, Switzerland). All the mechanical tests were performed using a DMA instrument (TA-Q850). The temperature and the frequency dependent tensile properties of ICCAs were assessed with an oscillatory strain of 3%.

Data availability
The experimental data that support the ndings of this study are available from the corresponding author upon reasonable request.   (e) Young's modulus, energy loss coe cient, and maximum stress versus tensile cycle. (f) Storage modulus, loss modulus, and damping ratio of ICCAs during 100,000 tensile-release fatigue cycles; the oscillatory strain was 5%. (g) Tensile stress-strain curves of ICCAs after holding them at extremely hightemperature conditions for 1 h. (i) SEM images of ICCAs after treatment at 1300 °C and 1400 °C for 1 h.
(J) One-thousand-cycle tensile test with 20% tensile strain after holding at 1300 °C for 1 h. Figure 4