Ultralight and fire-resistant ceramic nanofibrous...

SC I ENCE ADVANCES | R E S EARCH ART I C L E

MATER IALS SC I ENCE

1Key Laboratory of Textile Science and Technology, Ministry of Education, Collegeof Textiles, Donghua University, Shanghai 201620, China. 2Innovation Center forTextile Science and Technology, Donghua University, Shanghai 200051, China.*Corresponding author. Email: [email protected]

Si et al., Sci. Adv. 2018;4 : eaas8925 27 April 2018

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Ultralight and fire-resistant ceramic nanofibrousaerogels with temperature-invariant superelasticityYang Si,1,2 Xueqin Wang,1,2 Lvye Dou,1,2 Jianyong Yu,1,2 Bin Ding1,2*

Ultralight aerogels that are both highly resilient and compressible have been fabricated from various materials in-cluding polymer, carbon, and metal. However, it has remained a great challenge to realize high elasticity in aerogelssolely based on ceramic components. We report a scalable strategy to create superelastic lamellar-structured ceramicnanofibrous aerogels (CNFAs) by combining SiO2 nanofibers with aluminoborosilicate matrices. This approach causesthe random-deposited SiO2 nanofibers to assemble into elastic ceramic aerogels with tunable densities and desiredshapes on a large scale. The resulting CNFAs exhibit the integrated properties of flyweight densities of >0.15mg cm−3,rapid recovery from 80% strain, zero Poisson’s ratio, and temperature-invariant superelasticity to 1100°C. The integralceramic nature also provided the CNFAswith robust fire resistance and thermal insulationperformance. The successfulsynthesis of these fascinating materials may provide new insights into the development of ceramics in a lightweight,resilient, and structurally adaptive form.

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INTRODUCTIONUltralow density ceramic aerogels are attractive for their fascinatingcharacteristics of low density and thermal conductivity, chemical andthermal inertness, high porosity, and large surface area, and they havebeenwidely studied for thermal, catalytic, electrical, environmental, andenergy applications (1–3). In the ultralight regime below 10 mg cm−3,very few ceramic aerogels currently exist (4–8): silicon carbide foams(>9 mg cm−3), ZnO/silica aerogels (>8 mg cm−3), silica aerogels(>3 mg cm−3), boron nitride aerogels (>1.4 mg cm−3), and boehmitesponges (>1.2 mg cm−3). All of the above aerogels have a similarthree-dimensional (3D) pearl necklace–like microstructure: The net-work consists of ceramic nanoparticles that are connected through nar-row interparticle necks (6, 9). This random porous structure leads tosome beneficial properties (that is, large surface area and restrictionof liquid and gas flow), but generally, the inefficient structure continuityand connection result in poor mechanical properties (10, 11). As withmost of the present porous ceramic materials, these ceramic aerogelsare usually rigid and brittle, with only a slight elastic deformationbefore fracture unless they are hybrid with polymers or grown on pre-existing carbon networks (12, 13). The superelasticity that has beenachieved in aerogelsmade of polymer or carbonhas not been observedin aerogels solely based on ceramic components (14, 15). Because thespecific elastic bending strain of ceramic is known to be intrinsicallyinferior to that of polymer or carbon, previous analyses suggest that itwould be quite challenging to achieve superelasticity in porous ceramicnetworks (3, 16).

The properties of aerogels are generally dictated by the intrinsicproperties of the solid components, the density, and the cellular archi-tectures (3, 17). To develop an appropriate structure that can allow theceramic networks to maintain structural integrity upon large deforma-tion, we examined a variety of existing cellular materials and were par-ticularly impressed by the high mechanical efficiency of nanofibrousnetworks (11, 18). For instance, natural cellular fibrous structures suchas spider webs and the silkworm cocoon have a relatively low amount ofsolid constituents, similar to that of aerogels, but are structurally robust

(19, 20). In these structures, nanofibers are closely connected to form auniform cell geometry, a structure that is highly helpful in optimizingcellular-specific mechanical strength (17, 21). Our previous work alsorevealed that intrinsically brittle ceramics including silica, titania, andzirconia could become soft and flexible when hierarchically structuredas nanofibrousmembranes (22–24). Ceramic nanofibers, in this regard,show great promise as exceptional candidates for constructingmechan-ically robust ceramic aerogels. Despite their outstanding potential, themajor problem associated with ceramic nanofibers is their directspinning-deposition character; the resulting nanofibers usually assem-ble into random-deposited fluffy nonwovens rather than into organizedaerogels. Most recently, ceramic nanofiber–based sponges have beenfabricated by using specially designed cage-like collectors (25–27).How-ever, the principle of these methods remains the direct deposition ofnanofibers; thus, most of the products are not true 3D bulk aerogelsbut rather fluffy cotton-like nanofiber deposits without precise controlof the density and shape (28, 29). Moreover, thesematerials usually lackorganized cellular structure and stable cross-linking between nanofibersand so exhibit poormechanical strength with a weak elastic resilience of<30% strain. The challenge, therefore, is to construct continuous, 3D-structured, andmechanically robustCNFAs capable of forming organizedcellular architectures without compromising the properties of the intrinsicceramic nanofibers.

Herein, we demonstrate a robust method for creating superelasticlamellar-structured ceramic nanofibrous aerogels (CNFAs) by combiningflexible SiO2 nanofibers with aluminoborosilicate (AlBSi) matrices. Theadvantage of the design is that random-deposited SiO2 nanofibers arereconstructed into elastic ceramic aerogels with tunable densities andshapes on a large scale. The resulting CNFAs exhibit the integratedproperties of ultralow density (minimum of 0.15 mg cm−3), completerecovery from large deformation, zero Poisson’s ratio, temperature-invariant superelasticity, low thermal conductivity, and fire resistance,all resulting from the synergistic effects of organized nanofibrous archi-tectures and well-bonded ceramic nanofibers.

RESULTSFabrication and hierarchical cellular structureWe prepared the CNFAs based on three criteria: (i) the ceramic nano-fibersmust be flexible andmechanically robust, (ii) the nanofibers must

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assemble into 3D aerogels with controllable porous structure and shape,and (iii) the nanofibers must cross-link to form elastic and thermallystable networks. The first two requirements are satisfied by usingmicro/nanotextured structures; we combined the unique propertiesof electrospun ceramic nanofibers (soft and flexible) with the fibrousfreeze-shaping method (isotropic and controllable assembly) to yield3D ceramic nanofibrous networks. To satisfy the last criterion—theformation of stable cross-linking—our material design is based onnonalkaline AlBSi, which is widely used as high-temperature ceramicmatrices in the glass industry and inorganic synthesis (30). The cross-linking activity came from the formation of silicate bonding (X-O-Si)


with silica nanofiberswhen theywere calcined in the presence of oxygen(30, 31).

The synthetic pathway mainly involved four components: water,SiO2 nanofibers, polyacrylamide, and AlBSi sol, as illustrated in Fig. 1A.The fabrication process began with the preparation of flexible SiO2

nanofibers using a sol-gel electrospinning method. The resulting nano-fibers had an average fiber diameter of 206 nm (fig. S1 and Supple-mentaryMethods). Upon homogenizationwith polyacrylamide inwater,the SiO2 nanofibers became highly entangled with each other and welldispersed because of mutual repulsion, which was caused by the site-specific wrapping of the fiber surface with cationic polyacrylamide

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Fig. 1. Structuredesign and cellular architectures of CNFAs. (A) Schematic illustration of the fabrication of CNFAs. (B) XPS spectrum of CNFAs for all elements. a.u., arbitraryunit. (C) A CNFA heated by a butane blowtorch without any damage. (D) An optical image of CNFAs with diverse shapes. (E) An optical image showing a 20-cm3 CNFA (r =0.15 mg cm−3) standing on the tip of a feather. (F to H) Microscopic structure of CNFAs at different magnifications demonstrating the hierarchical nanofibrous cellulararchitecture. (I) STEM-EDS images of a single nanofiber with corresponding elemental mapping images of Si, O, Al, and B, respectively. (J) Schematic showing the three levelsof hierarchy of the relevant structures.

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(32, 33). The resulting homogenized fiber dispersion could remain stablewithout gravity sedimentation (figs. S2 and S3 and SupplementaryMethods). Subsequently, AlBSi sol was prepared by the mixing and hy-dration of tetraethyl orthosilicate, aluminum chloride, and boric acid.This mixture was added to the above nanofiber dispersion for furtherhomogenization. After degassing in a vacuum chamber, the dispersionwas frozen in a dry ice/acetone bath and freeze-dried into AlBSi gel/polyacrylamide/SiO2 nanofibers composite pre-aerogels (PNFAs) (seedetails in fig. S4 and Supplementary Methods). The freshly preparedPNFAswere fragile and heat-intolerant due to the lack of effective bond-ing. To build robust bonding among the fibers, the obtained PNFAswere calcined at 900°C to form cross-linked ceramic nanofibrous net-works, endowing the resultant CNFAs with elastic resilience and heatresistance. As a result of the calcination in air, the organic polyacrylamidewas decomposed and removed; simultaneously, the in situ sintering ofthe AlBSi gel in the fiber surface led to the formation of amorphousAlBSi glass ceramics and finally cemented the adjacent silica nanofibers(31, 34). The densities of the CNFAs could be readily regulated by chang-ing the concentrations of the precursor dispersions. Except as noted, allthe structure and property investigations were performed by using theCNFAswith a density of 5mg cm−3. Evidence of the formation of AlBSiceramics was obtained from the x-ray photoelectron spectroscopy(XPS) analysis (Fig. 1B and a magnified spectrum in fig. S5); the char-acteristic peaks ofAl2p, B1s, Si2p3, and Si2swere assigned toAl- and B-dopedsilica tetrahedron structures (34, 35). Significantly, the integral ceramicnature of both the silica nanofibers and the AlBSi surface layer allowsthe CNFAs to withstand even 1100°C of high-temperature flames (bu-tane blowtorch) without any visible damage or deformation (Fig. 1C),demonstrating the robust fire resistance.

Compared with traditional methods for preparing ceramic aerogels,our synthetic method combined the simplicity of fibrous freeze shapingwith the large-scale applicability of electrospun ceramic nanofibers,enabling the quick, easy, and reproducible preparation and wide struc-tural scalability of the products. Preparation can be finished within aday, and the resulting CNFAs can be formed in any shape desired, suchas a cylinder, cuboid, triangular prism, or even the premolded shapes of“D,” “H,” and “U” (Fig. 1D). The advantages of the structurally robustCNFAs can be further demonstrated by constructing ultralight and elas-tic aerogels with a minimum density of 0.15 mg cm−3 (figs. S6 and S7),corresponding to a porosity of 99.993%, which is significantly superior tothat of the previously lightest ceramic aerogels (1.2 mg cm−3) (8). Figure1E showed that a typical 20-cm3 CNFA (0.15 mg cm−3) could freelystand on the tip of a feather, highlighting the ultralight feature. In amarked contrast to the pearl necklace–like structure of traditional ce-ramic aerogels, the fibrous freeze shaping also concurrently caused thenanofibers to organize into a hierarchical and lamellar cellular structureon both the nanometer and micrometer scales. The scanning electronmicroscopy (SEM) images in Fig. 1 (F to J) demonstrate that the cellulararchitecture of CNFAs can be distilled to three levels of hierarchy: unitcells (10 to 30mm), cell walls (1 to 3 mm), andnanofibers (100 to 250 nm).Ceramic nanofibers in the cell walls are tightly bonded and entangled in ahighly packed morphology to maximize strength. Each fibrous cell isthen closely connected to form a lamellar-cell geometry, a structurethat is helpful to withstand cellular-specific deformation (11, 36). Asignificant difference between CNFAs and other cellular aerogels is theunique fibrous cell walls that consist of numerous minor pores (100 to1000 nm) due to the bonding of nanofibers, as shown in fig. S8. Thebasic principle of forming the organized cellular structure could beattributed to the phase separation induced by the crystallization ofwater


(37, 38), as illustrated in Fig. 1A. When the mixed nanofiber dispersionwas quickly frozen, ice formation can lead to the rejection of solid nano-fibers from the growing crystal front; the nanofibers were then accumu-lated among the laminar crystals and locked into a 3D solid network.Simultaneously, the precipitated AlBSi also aggregated on the fiber sur-face. After freeze drying and calcination, the hierarchical cellular struc-ture was obtained, whichwas a direct replica of the frozen and solidifiednanofiber networks. It should also be noted that the cellular structureof CNFAs exhibited a special micro-orientation and macro-isotropiccharacter, resulting in the isotropicmechanical properties (figs. S9 andS10 and the Supplementary Discussions). The highly cross-linkedstructure was further supported by the scanning transmission electronmicroscopy (STEM) observation and the relevant energy-dispersive x-rayspectroscopy (EDS) mapping results. As shown in Fig. 1I, the Al and Belements were almost exclusively covering the Si and O elements, reveal-ing that the AlBSi components were homogenously wrapped on thesurface of silica nanofibers. The junctions between the SiO2 nanofiberswere also examined by the EDS mapping, which confirmed the robustcross-linkingby theAlBSimatrices (fig. S11). Moreover, the EDS sam-ples were prepared by homogenization and sonication for a long time(20min), whereas theAlBSi layerwas still observed to attach to the surfaceof nanofibers, indicating the robust silicate bonding with nanofibers.

Characterization of superelasticity at room temperatureIn dramatic contrast to the stiff and brittle character of traditionalceramic aerogels, the CNFAs exhibited robust mechanical properties,tolerating large compression without cracking (insets in Fig. 2A andmovie S1). The stress-strain (s-e) curves (Fig. 2A) obtained in theloading process exhibit the three characteristic stages typically foundin cellular networks (13, 17): a linear elastic regime for e < 8%, a sub-sequent plateau stage for 8% < e < 65%, and a densification regime ofe > 65%with s rising sharply. Themaximum s at 80% strain was foundto be 10.5 kPa, indicating that the CNFAs can support over 7000 timestheir own weight without fracture, a property that has rarely beenobserved in other porous ceramics. The highly elastic CNFAs also ex-hibited durable cycling performance. Hysteresis curves (Fig. 2B) for500 loading-unloading cycles at large e (60%) showed slight plasticdeformation (8% at the 100th cycle and 12% at the 500th cycle). Nosignificant decay in strength or stiffness was found for the CNFAs after500 cyclic compressions; they retained over 70% of the initial Young’smodulus and maximum stress (Fig. 2C), highlighting their structuralrobustness. Moreover, the CNFAs also exhibited promising resistanceto shear and tensile deformations, which achieved robust shear andtensile fracture stresses of 6.79 and 4.17 kPa, respectively (see detailsin figs. S12 and S13 and the Supplementary Discussions).

The elasticity observed in the CNFAs was a surprise to us. Nearly allthe existing ceramic aerogels and porous ceramics are easy to crack in abrittle and stiff manner upon slight deformation (27, 39, 40). Previousstudies also found that when an aerogel made of ceramic nanoparticlesis severely compressed, the “neck-limited” connection of particles willcause the serious collapse of networks, preventing elastic recovery (3, 6, 41).Slight elasticity (e < 25%) has recently been observed in ceramic net-works by increasing the structure continuity within lattice and fibermicro-units (25, 27, 29); however, these structures fail to bear largecyclic deformation due to the lack of organized cellular geometry andstable bonding. We also found that CNFAs with a random organizedstructure or a nonbonding network fabricated from the same precursorall collapsed when subjected to large compression (see fig. S14 and Sup-plementary Discussions). We thus believe that it is the unique cellular

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structure and well-bonded nanofibers that have endowed the CNFAswith structurally robustness and high elasticity. To provide insight intothe elasticity, we investigated the dependence of the Poisson’s ratio ofthe CNFAs on applied compression strain up to 80% by in situ SEMobservation. As shown in Fig. 2D, the Poisson’s ratios remained nearzero (from 0 to −0.1) and showed little strain dependence for boththe loading and unloading processes. These slightly negative Poisson’sratios indicated a cell inversion–dominated deformation behavior simi-lar to that of cellular-structured foams (13, 42). The ultralight CNFAsstill maintained a high porosity of 99.1% even at 80% of e, whichmeansthat the fibrous cell walls largely do not come into contact to providelateral expansion (43). The compressive strain is thus absorbed at a highlevel by the buckling and inversion of the nanofibrous cells, which sug-gests that the hierarchical cellular structure is significantly important forelastic recovery, as illustrated in Fig. 2E.

At the top level, the cellular structure of the CNFAs is organized in alamellar-cell geometry to maximize the elastic modulus and strength.At the meta-scale level, the cell walls consisting of tangled and bondednanofibers bend freely with a typical curvature radius of 5 to 10 mm(Fig. 2F), and the cell inversion feature has also proven to be highlybeneficial for transferring loading strain with minimum cell wall


bending (17, 42). At the lowest level, a single silica nanofiber was ableto bear a large bending deformation at a curvature radius of less than0.5 mm without generating any cracks (Fig. 2G). The key factor in theflexible ceramic nanofiber is the amorphous atomic structure of boththe silica fiber matrix and the AlBSi cross-linking components (fig.S15). The dispersed defects in the glassy silica tetrahedron networksact as a “lubrication region” that can dissipate external stress andyield large deformations (23, 44), as shown in Fig. 2H. Consider thatthe allowable bending radius of a single nanofiber is only one-tenth ofthat of the cell walls, which has enabled a high compliance in the straindirection and the ability of the CNFAs to accommodate a severe elasticbending deformation. Therefore, the efficient combination of organizednanofibrous cellular networks and stable AlBSi cross-linking is the keyaspect for realizing high specific elasticity and strength. A detaileddiscussion for the effect of the hierarchical cellular structure (fiber diam-eters and lamellar spacing) on the mechanical properties of CNFAs ispresented in figs. S16 to S18 and Supplementary Discussions.

The properties of mechanically robust, cellular-structured CNFAscan be further demonstrated by their fast elastic resilience. A movietaken by a high-speed camera shows that a typical CNFA can rebounda steel ball (7.5 g, 70 times heavier than itself)with a fast recovery speed

Fig. 2. Multicycle compressive properties of the CNFAs. (A) Compressive s versus e curves during loading-unloading cycles with increasing e amplitude. (B) A 500-cyclefatigue test with compressive e of 60%. (C) Young’s modulus, energy loss coefficient, and maximum stress versus compressive cycles. (D) The Poisson’s ratio of the CNFAsversus e. Inset: SEM observations of the CNFAs under compression and release, focusing on a small piece (<1 mm). (E) Sketch of the inversion of the nanofibrous cell wallsunder compression. SEM images showing the curvature radius of (F) a single cellular cell and (G) a single nanofiber. (H) Schematic illustration of the microstructure of a bentsilica nanofiber. (I) A set of real-time images showing that CNFAs can rebound a steel ball at high speed. (J) The frequency dependence of the storage modulus, loss modulus,and damping ratio for CNFAs (oscillatory e of 3%). (K) The relative Young’s modulus of selected cellular aerogels with low densities.

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(860 mm s−1) (Fig. 2I and movie S2), revealing an elastc performancethat achieved almost 60% of the recovery speed of the commercial poly-urethane (PU) elastic foams (fig. S19). Further dynamic compressivetests by dynamic mechanical analysis (DMA) revealed that the storageand loss modulus were almost stable and independent of the angularfrequency upon three orders of magnitude from 0.2 to 100 Hz (Fig.2J). A damping ratio of 0.1 to 0.2 over the entire tested angular frequen-cy range indicated that the elastic response was predominant for theCNFAs, which could be attributed to the strong and viscoelastic nano-fibrous networks (10). The effect of the hierarchical cellular nanofibrousarchitecture on the mechanical properties is also reflected in the elastic-ity of the CNFAs with a wide range of densities from 0.5 to 10 mg cm−3

(fig. S20). Plotting the relative Young’s modulus (E/Es) of a variety ofCNFAs versus the relative density (r/rs) indicated that the modulusscaled as E/Es ~ (r/rs)

2 (Fig. 2K), revealing a cellular-governed elasticbehavior similar to that of a low-density, open-cell architecture (17, 45).These modulus-density scaling laws correspond well with the observedefficient load transfer to the CNFA struts (presently nanofibrous cellwalls), which undergo reversible transverse bending (17). In contrastto this result, most existing inorganic aerogels exhibit a larger E/Es ~(r/rs) scaling of >2.5 because of the inefficient stress transfer amongrandom structural units (46, 47).

Fire resistance and elasticity at high temperatureThe vast majority of cellular aerogels have been made from polymers,carbon, and metals, which exhibit excellent elasticity at room tempera-ture. However, these constituent materials typically cannot withstandhigh temperatures in air. By contrast, ceramic aerogels are robustheat-resistant materials but have always suffered from poor mechanicalproperties. As a new alternative, CNFAs combining the unique proper-ties of the cellular nanofibrous structure with a ceramic nature areexpected to yield promising temperature-invariant elasticity. As shownin Fig. 3 (A to C), a frequency dependence test (0.1 to 10 Hz) measuredby DMA in ambient air showed stable viscoelastic properties (storagemodulus, loss modulus, and damping ratio) of the CNFAs over a widerange of temperatures from −100° to 500°C. As far as we know, com-mon DMAs are usually limited to 500°C. To extend the temperaturerange studied, we first heated the CNFAs in a furnace at temperaturesranging from 100° to 1400°C for 30 min; then, we estimated the workdone during the compression and recovery processes (see details in theSupplementary Methods). As shown in Fig. 3D, both the compressionand recovery work were nearly constant and independent of the treatingtemperature to 1100°C. Above that temperature, the compression workexhibited a slight decrease, whereas the recovery work decreased mark-edly. These results indicated that the CNFAs didmaintain their originalshape and cellular structure upon the high temperature of 1400°C, andthe elastic resilience was also well retained up to 1100°C.

To gain insight into the thermal stability, the heat-induced structuralreorganization of the CNFAs was studied by x-ray diffraction (XRD)analysis. As shown in Fig. 3E, the XRDpatterns of CNFAs uponheatingat 1000° and 1200°C only exhibited a wide peak around 22°, character-izing the typical amorphous structure of silica. Meanwhile, after heatingat 1400°C, the XRD pattern revealed distinct peaks at 23.9° (210),35.3° (311), 49.6° (421), 54.1° (422), and 62.5° (440), which indicated thecrystalline b-quartz structure of silica (6, 44). As mentioned previously,the amorphous character of the ceramic nanofibers is the critical pre-condition for the bendable properties. The excessive crystallite growthin nanofibers during high-temperature treatment significantly reducesthe “lubrication region,” and the external stress can easily become con-


centrated in the crystalline region, resulting in a catastrophic failure asmacrocracks propagate similar to the behavior in typical porousceramics (44, 48). These results were also confirmed by SEM observa-tion (Fig. 3F). Clear adhesion and fusion were observed among nano-fibers in CNFAs treated at 1200°C and especially at 1400°C, whichcould be attributed to the crystallite growth in silica nanofibers. More-over, the high-temperature elasticity and fire resistance of the CNFAswere further assessed by in situ compression testing in the flame of analcohol lamp (~700°C) and a butane blowtorch (~1100°C); themeasured temperatures at the position of the CNFAs upon the alcohollamp and the butane blowtorch were 691° and 1063°C, respectively (fig.S21). As demonstrated in Fig. 3 (G and H) (see also movies S3 and S4),no ignition or structure collapse was observed when the CNFAs wereexposed to those high-temperature flames; moreover, they still exhib-ited good elastic resilience after several compression cycles upon flameburning, highlighting the temperature-invariant superelasticity.

DISCUSSIONThermal insulation plays a significant role in controlling energy effi-ciency of both industry and daily life, and aerogels are among themost well-known thermal insulation materials, with significantly lowthermal conductivity originating from their high porosity (49, 50).Generally, the thermal conductivity of aerogels is mainly made upof two components: thermal transportation by the gas phase andby the solid phase. The as-prepared CNFAs had both high porosityand tortuous porous channels, which could lead to the promisinglow gas and solid thermal transportation. As shown in Fig. 4A, theCNFAs with a density of 10 mg cm−3 exhibited a low thermal con-ductivity (l) of 0.025 W m−1 K−1, which is quite close to that of airunder ambient conditions (0.023 W m−1 K−1). With the density ofthe CNFAs decreased to 0.5 mg cm−3, the l increased slightly to0.032 W m−1 K−1 due to the increase in pore size (50). Combiningthe extraordinarily ultralight feature, the specific thermal insulationperformance per mass of the CNFAs (take the sample of 5 mg cm−3

as an example) was estimated to be one or two orders of magnitudesuperior to that of commercialized insulation materials (table S1) (48).

The CNFAs present a unique combination of low thermal conduc-tivity and temperature-invariant superelasticity, which offer a far supe-rior high-temperature insulation performance to that of currentinsulating materials. As shown in Fig. 4B, polymeric insulationmaterials such as PU foams or cellulose aerogels are easily collapsedat temperatures above 200°C, and their ignitable nature also requiresthe addition of toxic flame retardants (48). It has also been shown thatthe fire retardancy of organic aerogels can be improved to over 400°C bythe addition of inorganic fillers, such as silicate and clays (51, 52). Car-bon aerogels derived from thermal or hydrothermal carbonization ofpolymer aerogels can further increase heat resistance (53). However,structure shrinkage and ignition still occur upon exposure to fire over600°C. All of these polymeric, hybrid, and carbon aerogels were elasticbut were not able to bear high-temperature conditions (>1000°C) orflame ignition. Meanwhile, traditional ceramic aerogels, such as SiO2,Al2O3, and ZrO2, exhibited excellent heat resistance to 1200°C withlow l values even less than 0.2 W m−1 K−1, but the widespread use ofthese aerogels has been limited by their poormechanical properties andsafety concerns related to dust release (3, 9). Significantly, the as-prepared CNFAs show robust heat resistance and low l similar to thoseof common ceramic aerogels while exhibiting superelasticity like that ofa polymeric elastomer. Considering that insulating materials can easily

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encounter squeezing or shaking in their working conditions (48), thisperformance is very promising from the perspective of practical insula-tion applications. Moreover, because of the simplicity of the assemblyprocess in our method, CNFA fabrication can be easily scaled up to ob-tainmeter-sized insulating panels, felts, or even prefabricated pipes (Fig.4C), which could be conveniently used as common insulatingmaterialswithout additional processing.

As a proof of concept for thermal insulation applications, 15-mm-thick plates made of CNFAs, iron, glass, and Al2O3 ceramics wereplaced on a 350°C heating stage, and fresh petals were then furtherloaded on top of each plate. As shown in Fig. 4D, the petals on theCNFAs exhibited only slight wilting after 10 min of heating, whereasthe petals loaded on other plates were significantly scorched and carbon-


ized, highlighting the robust insulating performance of the CNFAs.We further monitored the dynamic temperature distribution of theCNFAs upon heating and flame by infrared camera observation. Asshown in Fig. 4E, a gradient distribution of temperature from theheating plate through the CNFAs was observed. The top surface of theCNFAs (15 mm thick) maintained a relatively low temperature of about50°C after being placed on a 350°C heating stage for 1 min, and the tem-perature increased slightly to 62°C after 10min and then remained nearlyconstant with further heating until 30min. Similarly, the CNFAs (15mmthick) exhibited promising thermal insulation against a high-temperaturebutane blowtorch flame, achieving a significantly low temperature of35°C at the far end of the CNFAs (Fig. 4F). These results indicate thatthe CNFAs could serve as a robust insulation material with potential

Fig. 3. Mechanical properties of the CNFAs over a wide range of temperature. (A to C) Storage modulus, loss modulus, and damping ratio of the CNFAs versusangular frequency (0.1 to 10 Hz) at temperatures from −100° to 500°C, with an oscillatory e of 3%. (D) Compression and recovery work of the CNFAs after treatment atvarious temperatures for 30 min. (E) XRD patterns of CNFAs after treatment at 1000°, 1200°, and 1400°C for 30 min. (F) SEM images of CNFAs after treatment at 1200°and 1400°C for 30 min. Compression and recovery process of the CNFAs in the flame of (G) an alcohol lamp and (H) a butane blowtorch.

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broad applications, such as insulation layers combining flyweight den-sity and elasticity for aviation and aerospace industries or scalable heat-and fire-proof materials for modern buildings.

In summary, we have presented that ultralight ceramic-based aero-gels can bemade superelastic by the combination of hierarchical cellularfibrous architectures and AlBSi-bonded SiO2 nanofibers. With their ul-tralowdensity, resilient compressibility, zeroPoisson’s ratio, temperature-invariant superelasticity, low thermal conductivity, and fire resistance,we anticipate that these exceptional CNFAs will open broad techno-logical implications in thermal insulation, catalyst supports, adsor-bents, flexible electrical devices, and electromagnetic, energy,acoustic, or vibration damping.Moreover, our discovery also providesthe possibility to explore the properties and applications of ceramics ina lightweight, resilient, and structurally adaptive form. Furthermore,the basic principles of the preparation and the resilient mechanism ofCNFAs were demonstrated; thus, we can expect that, similar to SiO2

nanofibers, a variety of other ceramic nanofibers could be involved inthe fabrication of CNFAs, offering ample opportunities to developmany new ceramic-based functional aerogels.

MATERIALS AND METHODSFabrication of CNFAsSoft and flexible SiO2nanofiberswere prepared through a similar sol-gelelectrospinning method as we previously reported (see details in fig. S1and Supplementary Methods) (44). In a typical procedure for the fab-


rication of CNFAs with a density of 5mg cm−3, 1.2 g of SiO2 nanofibersand 0.03 g of polyacrylamide were dispersed in 280 ml of water byhomogenizing (IKA T21) the mixture for 20 min at 13,000 rpm,yielding uniform nanofiber dispersions. An AlBSi sol (with an Al/Si/Bmolar ratio of 2:5:1) was prepared by mixing and hydrating 1.52 g oftetraethyl orthosilicate, 0.39 g of aluminum chloride, and 0.09 g of boricacid in 20 ml of water for 4 hours. The AlBSi sol was then added to thenanofiber dispersions with further homogenization for 5min. After de-gassing in a vacuum chamber, the obtained dispersionswere transferredto the desired mold, frozen in a dry ice/acetone bath, and then freeze-dried for 18 hours to obtain the PNFAs. Next, the PNFAs were calcinedat 900°C under flowing air for 30 min to generate the AlBSi ceramic–bonded CNFAs. Other CNFAs with densities in the range from 0.15 to10 mg cm−3 were prepared by changing the concentrations of the pre-cursor dispersions (with a constant weight ratio of SiO2 nanofibers,polyacrylamide, and AlBSi sol). Except as noted, all the structure andproperty investigations were performed by using the CNFAs with adensity of 5 mg cm−3.

CharacterizationThe compression experiments were tested using an Instron 3365 uni-versalmachine with 100-N load cells at a strain rate of 30mmmin−1 fors-ɛ tests and of 300mmmin−1 for 500 cyclic fatigue tests. The dynamicand temperature-dependent compressive characteristics were evaluatedby a DMA instrument (TA-Q800) equipped with a parallel-plate com-pression clamp. The DMA was also equipped with a heating and

Fig. 4. Thermal insulation properties of the CNFAs. (A) The thermal conductivities of the CNFAs as a function of density. (B) Thermal conductivity versus maximumworking temperature for aerogel-like materials. (C) An optical image of the CNFAs on the large scale for thermal insulation applications. (D) Thermal insulation capacityof the CNFAs compared with those of Fe, SiO2, and Al2O3 materials for protecting fresh petals from withering. (E) Optical and infrared images of CNFAs on a 350°Cheating stage for 30 min. (F) Optical and infrared images of CNFAs exposed to a butane blowtorch for 120 s.

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cooling chamber for transitions from −100° to 500°C with flowing air.The storage modulus, loss modulus, and damping ratio as a function offrequency were measured with a prestrain of 5% and an oscillatorystrain of 3%. The shear and tensile mechanical properties were per-formed using a TA-Q800 DMA instrument with the sandwich shearand tensile clamps, respectively.

The weight of the CNFA samples wasmeasured using an ultra-microbalance (MS105DU, Mettler Toledo) with an accuracy of 0.08 mg onthe basis of the ISO 845:2006 standard (see details in SupplementaryMethods). The microscopic architecture and chemical structure ofthe CNFAs were characterized by SEM (Hitachi S-4800), STEM(JEM-2100F), EDS (Bruker Quantax 400), XPS (PHI 5000C ESCA),and XRD (Bruker D8 ADVANCE). The fast recovery of a compressedCNFA was recorded by a high-speed EX-FA CASIO camera operatedat 1000 frames per second. The thermal conductivitywasmeasured ona Hot Disk instrument (TPS2500S) with the transient-plane sourcemethod. The infrared images of the CNFAs were recorded using aFotric 225 (−10° to 400°C) and an Optris PI 640 (100° to 1500°C) in-frared thermal camera.

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SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaas8925/DC1fig. S1. Fabrication of electrospun SiO2 nanofibers.fig. S2. Homogenization of nanofibers.fig. S3. Morphology of homogenized nanofibers.fig. S4. The freezing of nanofiber dispersions.fig. S5. Magnified XPS spectrum.fig. S6. The ultralow density of the CNFAs.fig. S7. Compressibility of the CNFAs with a density of 0.15 mg cm−3.fig. S8. SEM images of the nanofibrous cell walls.fig. S9. Micro-orientation and macro-isotropic structure of CNFAs.fig. S10. Mechanical properties of the CNFAs upon different orientation.fig. S11. EDS mapping of junctions between nanofibers.fig. S12. Shear mechanical properties of CNFAs.fig. S13. Tensile mechanical properties of CNFAs.fig. S14. Elastic resilience of CNFAs with different structures.fig. S15. Amorphous character of the CNFAs.fig. S16. Effect of fiber diameter on the structure of CNFAs.fig. S17. Effect of fiber diameter on the mechanical properties of CNFAs.fig. S18. Effect of lamellar spacing on the structure and properties of CNFAs.fig. S19. Elasticity of a PU foam.fig. S20. Elasticity of CNFAs with a wide range of densities.fig. S21. The temperature at the position of the CNFAs upon flames.table S1. The relevant densities and thermal conductivities of CNFAs and other insulationmaterials.Supplementary MethodsSupplementary Discussionsmovie S1. Compression and recovery processes of CNFAs.movie S2. Fast recovery of CNFAs by rebounding a steel ball.movie S3. Compression testing in the flame of an alcohol lamp.movie S4. Compression testing in the flame of a butane blowtorch.

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AcknowledgmentsFunding: This work is supported by the National Natural Science Foundation of China(nos. 51673037 and 51473030), the Shanghai Committee of Science and Technology(no. 15JC1400500), and the Innovation Program of Shanghai Municipal Education Commission(no. 2017-01-07-00-03-E00024). Author contributions: B.D. and Y.S. designed the research.Y.S. wrote the paper. X.W. and L.D. synthesized and characterized the samples. J.Y. wasinvolved in the analysis of mechanical mechanism. Y.S. and X.W. contributed equally tothis work. Competing interests: The authors declare that they have no competing interests.Data and materials availability: All data needed to evaluate the conclusions in the paperare present in the paper and/or the Supplementary Materials. Additional data related tothis paper may be requested from the authors.

Submitted 2 January 2018Accepted 8 March 2018Published 27 April 201810.1126/sciadv.aas8925

Citation: Y. Si, X. Wang, L. Dou, J. Yu, B. Ding, Ultralight and fire-resistant ceramic nanofibrousaerogels with temperature-invariant superelasticity. Sci. Adv. 4, eaas8925 (2018).

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superelasticityUltralight and fire-resistant ceramic nanofibrous aerogels with temperature-invariant

Yang Si, Xueqin Wang, Lvye Dou, Jianyong Yu and Bin Ding

DOI: 10.1126/sciadv.aas8925 (4), eaas8925.4Sci Adv

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