Multichannel and super-flexible SiZrOC ultrafine fibers for high temperature thermal insulation

: Light but robust porous ceramic fibers with combined properties of super flexibility, excellent thermal stability and thermal insulation performance are attractive for use in extreme conditions, especially in the field of aerospace. However, the practical application of traditional porous ceramic fibers are usually limited by their brittle nature and poor mechanical properties. Herein, we designed a multichannel SiZrOC ultrafine fiber (MSUF) composed of ZrO 2 , SiO x C y and free carbon phases by electrospinning technique. The resulting fibers exhibited integrated properties of excellent fire resistance, high temperature stability, thermal shock resistance and temperature-invariant flexibility. More importantly, the fancy multichannel structure and components of the fiber provides it with outstanding thermal insulation performance with low thermal conductivity (0.041 W m -1 ·K -1 at 25 ºC and 0.141 W m -1 ·K -1 at 1000 ºC). The successful fabrication of such flexible porous MSUFs may provide a new approach to design high performance thermal insulators for high temperature thermal insulation.

fabrication of such flexible porous MSUFs may provide a new approach to design high performance thermal insulators for high temperature thermal insulation.

Introduction
In recent years, materials with robust mechanical properties, excellent thermal stability and low thermal conductivity are highly demanded in aerospace and deep space exploration fields, for personal and aircrafts protection [1][2][3]. While the aircrafts or vehicles reentry atmosphere at high speed, the temperature of the leading edge of the wing and other parts will increase rapidly up to ~1000 ºC in a short time [1][2]. Thus, the thermal insulating materials applied to those aircrafts are always in service under extreme conditions, such as sharp temperature changes and long-period high temperature exposure [4,5]. In addition, these thermal insulating materials should not only be mechanical robust but also be light in order to conserve energy.
Ceramic fibers are considered as promising high temperature thermal insulators due to their outstanding fire resistance, high strength, as well as thermal and chemical resistance [6][7][8][9]. They can be used as both individual thermal insulators and reinforcements and opacifier of aerogel thermal insulators, which are both urgently demanded in the high temperature thermal insulation fields [9][10][11][12]. However, the thermal conductivities of the current ceramic fiber membranes are still high at high temperatures (>0.20 W m -1 ·K -1 at 1000 ºC), because of these kinds of fibers are transparent in the infrared (IR) region or have high intrinsic solid thermal conductivity [13][14][15][16][17].
Recently, we designed a multi-phase SiZrOC fiber contained both oxide and non-oxide phases, which exhibited excellent flexibility and thermal stability [9]. The unique component enhanced the phonon scattering and infrared shielding performance of the fibers, which endowed the fibers with a good thermal insulation performance at high temperatures (0.169 W m -1 ·K -1 at 1000 ºC).
However, the thermal insulation performance of the SiZrOC fibers needed to be further improved in order to meet the steep demands in aerospace field.
Recently, it has been found that constructing porous structure is an effective way to reduce the thermal conductivity of fibers [18][19][20][21][22]. On the one hand, the porous structure will lengthen the path of heat transfer and hinder the gaseous heat conduction, leading to a low solid and gas heat transfer [18,19]. On the other hand, the multiscale porous structure contributes to enhance multiple reflection-absorption of the IR radiation, which can decrease radiation heat transfer [16,20,21].
Moreover, the porous structure is beneficial to reduce the density of ceramic fiber membranes.
Therefore, ceramic fibers with proper porosity and pore structure may endow thermal insulating materials with high thermal insulation performance and lightweight simultaneously.
On the basis of this, many kinds of porous ceramic fibers have been widely developed.
Gbewonyo et al. [18] prepared porous carbon fibers with multi-scale (micro, submicro, and nano) porous structure delivered by electrospinning method. Compared with carbon fibers, the thermal conductivity of the porous carbon fibers with nanoscale structure decreased 98 % to 0.15 W m -1 ·K -1 at ambient temperature. Wang et al. [19] reported SiO2-TiO2 and SiO2-ZrO2 fibers with highly porous structure. The numerous pores within the fiber could effectively reduce the solid heat transfer, confine the gaseous heat transfer and enhance the reflection of IR. Therefore, the SiO2-TiO2 and SiO2-ZrO2 porous fibers had great potential to be used in the thermal insulation fields. However, despite these advances, ceramic fibers with highly porous structure usually suffered from the poor mechanical properties, which was mainly due to the brittle nature of ceramics and numerous pore defects [14,18,19]. Therefore, it was still a long-standing challenge in designing porous structured ceramic fibers with both high flexibility and thermal insulation performance.
Inspired by this, we designed a flexible multichannel SiZrOC ultrafine fiber (MSUF) fabricated via electrospinning technique by selecting polystyrene as the pore-forming template to regulate the pore structure in this work. The prepared MSUFs had multichannel structure and composed of ZrO2, SiOxCy and free carbon phases, which exhibited excellent fire resistance, thermal shock resistance, thermal stability (up to 1400 ºC in Ar) and temperature-invariant super flexibility (-196~1000 ºC). Remarkably, the unique multichannel structure and components provided the MSUFs with high thermal insulation performance with low thermal conductivity (0.041 W m -1 ·K -1 at 25 ºC and 0.141 W m -1 ·K -1 at 1000 ºC). The developed MSUF membranes could be considered as promising high temperature thermal insulators under extreme conditions.

Preparation of the MSUF membranes
The electrospinning solution (ES) was prepared by dissolving PR, PS, PVP and Zr(acac)4 in the DMF with stirring speed of 600 r·min -1 at 40 ºC for 6 h. To tune the pore structure of the MSUFs, the various ES with PS contents from 2 wt%, 6 wt%, 10 wt% to 18 wt% was prepared for electrospinning, respectively. The corresponding ES with various PS was named as ES-2, ES-6, ES-10 and ES-18, respectively. The detail composition of the various ES was shown in Table S1.
Then, the precursor fiber (PF) was electrospun on aluminum foil collector from various ES. The distance between the syringe and the collector was fixed at 15 cm, and the voltage of 15 kV was applied with a flow rate of 1 ml·h -1 . The corresponding PFs prepared by various ES were named as PF-2, PF-6, PF-10 and PF-18, respectively. The as-spun PFs were thermal cured in air at 310 ºC for 2 h with heating rate of 1 ºC·min -1 . Then, the MSUFs were obtained by pyrolyzing the thermal cured PFs at 1200 ºC for 1 h with heating rate of 5 ºC·min -1 under Ar atmosphere. The corresponding MSUF membranes were named as MSUF-2, MSUF-6, MSUF-10 and MSUF-18, respectively.

Characterization
The microscopic morphologies of the MSUFs were characterized by field-emission scanning electron microscopy (SEM, Hitachi S4800, Japan). The microstructure of the MSUFs was examined by a transmission electron microscopy (TEM, Titan G2 60-300, USA). The elemental composition was performed with an energy dispersive X-ray spectroscopy (EDS) equipped on the TEM. X-ray diffraction (XRD) pattern from the sample was collected in the range of 10º~80º using an X-ray diffractometer (Bruker AXS D8, Germany) equipped with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was analyzed with an XPS machine (Escalab 250Xi, Thermo Nitrogen (N2) adsorption-desorption isotherms were tested by a specific surface & pore size analysis instrument (BeiShiDe 3H-2000PS1). The Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods were used to calculate the specific surface area and pore size distribution, respectively. The volume density (ρv) of the samples was calculated with the mass divided by volume. The true density (ρt) of the samples was tested by a liquid densimeter (XPE205, Mettler Toledo). Then the porosity of the samples was calculated by formula 1: (1)

Mechanical properties measurements
Mechanical tensile tests were conducted using a materials testing machines (Testometric Micro 350, Testometric, England) with a loading rate of 1 mm·min -1 . The test samples were prepared in size of 20 mm × 3 mm. The flexibility test of the samples was measured on a flexibility tester (FlexTest, Hunan Nanoup Printed Electronics Technology Co., Ltd, Hunan, China).

Thermal conductivity measurements
The thermal conductivity of the samples was measured by a thermal conductivity tester ( where l was the thickness of sample, respectively.

Morphology and microstructures
The preparation process and formation mechanism of the MSUFs were illustrated in Fig. 1a.
The preparation process of the MSUFs mainly included four procedures: preparation of ES, electrospinning of the ES, thermal curing of the as-spun PFs and pyrolysis of the thermal cured PFs (Fig. S1). Due to the simplicity of electrospinning technique, scaling up the fabrication of MSUF membranes was feasible. As shown in Fig. S1, a large piece of flexible MSUF membrane with size of 29×29 cm 2 was prepared. In order to investigate the transformation mechanism of the multichannel structure, the TG analysis of the PS and cured PF-10 fibers was carried. As shown in Fig. S2, the TG curve of PS delivers a significant mass loss of about 100 % between 30 ºC and 430 ºC, indicating that the PS decomposed completely during the pyrolysis process. Noticeably, there was a two-stage thermal degradation (300~430 ºC and 430~600 ºC) in the TG curve of cured PF-10, which should be ascribed to the decomposition of PS and ceramic transformation of the PR and Zr(acac)4. Therefore, the formation mechanism of the MSUFs could be concluded as follows: the PS was firstly stretched into nanowires within the PFs during the electrospinning process, and then decomposed to generate multichannel pores during the pyrolysis process (Fig. 1a).
As shown in Fig. 1b-e, the prepared MSUFs showed a long continuous fiber morphology and relatively uniform diameter. It could be found that the diameter of MSUFs increased from 0.57±0.07 μm to 2.54±0.28 μm with the PS content increased from 2 wt% to 18 wt%, as demonstrated in Fig. S3. This phenomenon could be contribute to the increasing viscosity and decreasing electrical conductivity of the corresponding ES (Fig. S4), which resulted in the stretching and whipping of the jet by the electrical field force decreased [23].
The cross-section SEM images of a single fiber confirmed the successful introduction of abundant channel pores into the fibers (Fig. 1f-i). It could be found that the pore structure of the 8 MSUFs was easily controlled by changing the content of PS from 2 wt% to 18 wt%. With the PS content increasing, both channel size and channel numbers inside each MSUF increased accordingly ( Fig. 1f-i). Of note, when the PS content increased up to 18 wt%, there were some incomplete pores appeared on the cross section of fiber, and some grooves could be observed on the surface of fibers (Fig. 1i). This might be resulted from the high PS content, which could not be completely converted to nanowires within the PFs but agglomerated on the surface of the PFs or directly formed PS fibers. As a result, some grooves formed on the surface of the MSUFs after high temperature pyrolysis. Therefore, proper PS content (<18 wt%) might contributed to form multichannel pores within the MSUFs. The TEM image of MSUF further demonstrated that the channel-like pore structures were well generated throughout the single fiber (Fig. 2a). Moreover, the corresponding HRTEM image (Fig.  2b) revealed that the amorphous phase and t-ZrO2 crystalline structure of the fiber, with a characteristic lattice fringe distance of ∼0.297 nm, corresponding to the (101) plane of t-ZrO2 phase [24]. The XRD pattern showed that a broad peak located at about 2θ=22.5º, corresponding to the amorphous SiOxCy phase (Fig. 2d) [25]. However, the ZrO2 peaks could not be identified from the XRD pattern, which could be due to the inability of the XRD to detect the component with low contents. The EDS element mapping images showed that the Si, Zr, O and C elements were homogeneously distributed throughout the fibers, confirming fibers were mainly composed of these elements (Fig. 2c). The results were also verified by the XPS analysis. As shown in Fig.   2e, the characteristic peaks corresponding to Si2p, Zr3d, O1s and C1s were clearly observed in the XPS survey. Further, the microstructure of the fibers was characterized by the Raman analysis. As shown in Fig. 2f, two main strong peaks were detected at 1350 cm -1 and 1580 cm -1 , which correspond to the D and G bands of free carbon, respectively. Moreover, there were two small peaks located at 464 cm -1 and 840 cm -1 , which belonged to the Si-O-Si and Si-C mode of the SiOxCy phase, respectively [26]. The results indicated that the MSUFs mainly composed of SiOxCy, ZrO2 and free carbon phases.  (Fig. 3b). As these results indicated, the proper PS content (~10 wt%) in the 11 ES would endow the resultant MSUF membranes with enhanced pore structure and large specific surface area, which were beneficial to decreasing density and thermal conductivity of fibers.
Furthermore, the volume density and porosity of the resultant MSUF membranes were also tested to further investigate the pore structure, as shown in Fig. 3c. Notably, the MSUF-10 membranes showed lower density (0.044 g·cm -3 ) and higher porosity (97.7 %) than other membranes. The results also evidenced that the MSUF-10 with higher pore structure and porosity than other fibers, which corresponded to the BET analysis. The detailed specific surface area, volume density and porosity parameters of the MSUF membranes were shown in Table S2.

Mechanical properties
The mechanical properties of the ceramic fiber membranes were closely related to the strength and morphology of single fiber and bonding structure among fibers [23,28]. Thus, the transformation of fibers morphology and microstructure could significantly influence the mechanical properties of the prepared MSUF membranes. The mechanical properties of the prepared MSUF membranes were investigated by uniaxial tensile test. The typical tensile stressstrain curves of the membranes were shown in Fig. S5a. The membranes exhibited typically elastic-perfect plastic like response, which attributed to the obviously slipping among fibers during the tensile process. As shown in Fig. S5b 3c), which resulted from the multichannel pores within the fiber increased. In the practical application, thermal insulation materials must be mechanically robust to bear external mechanical forces suffered in the course of mounting, transportation and applications. As can be seen from the In stark contrast to the brittle nature of conventional ceramic fibers and porous ceramic fibers, the MSUF membranes exhibited super flexibility. They could be easily bended, twisted, knotted and rolled without any cracks (Fig. 4a). The excellent flexibility of the membranes could be further demonstrated by their fast elastic resilience. A movie taken by a high-speed camera showed that a piece of bent MSUF membrane could rebound with a fast recovery speed (Fig. S7), revealing excellent flexibility and elastic performance of the membrane.
In order to further investigate the super flexibility, robust mechanical properties and durability of the membranes, we bended, twisted and rolled the MSUF membranes at various cycles (10~100).
Then the tensile strength was measured and the tensile strength retention was calculated. The schematic of side view of bending, twisting and rolling test was shown in Fig. S8a. Remarkably, the membranes could endure 100 cycles bending, twisting and rolling, no cracks were observed on them (Fig. S8b-d), except for a slight tensile strength retention decrease (Fig. 4b). The tensile strength retention was up to 87.1%, 79.2%, and 62.1% after 100 cycles twisting, bending and rolling tested (Fig. 4b), respectively. The results highlighted the membranes with high flexibility, strength and durability. The bending deformation of a piece of MSUF membrane and a single fiber was further analyzed by the SEM. As shown in Fig. 4c, the membrane could tolerate extreme deformation without generating fractures. Moreover, a single fiber was able to bear large bending deformation with a bending angle up to 145º without any cracks (Fig. 4d), revealing excellent bending properties of the fiber.
Flexibility was a long-standing challenge in ceramic materials, especially in porous ceramic materials because the cracks were easily formed from the pores and propagate rapidly [23,29].
Benefiting from the unique composition and microstructure, the prepared MSUF membranes exhibited outstanding flexibility. Based on the systematical analysis of the fiber's composition and microstructure, the plausible flexibility mechanism of the MSUF membranes could be understood in multiscale (Fig. 4e). At macroscopic level, the membranes composed of entangled and random fibers, which could freely move and bear large deformation when the external force loaded on them. Therefore, the bending stress would be easily released by the slipping and bending of fibers during the bending process of the membranes [24,30]. At microscale level, there were numerous t-ZrO2 nanograins wrapped in the amorphous SiOxCy phase. Theses nanograins could inhibit the 14 initiation and propagation of cracks or lead to crack redirection [29]. Moreover, the stress could induce the transfer of t-ZrO2 to m-ZrO2 accompanied with volume expansion, which could also heal some cracks [31]. At nanoscale level, the amorphous SiOxCy phase play a major factor to the flexible MSUFs, which lack microstructural defects and can dispersed stress and bear enormous deformation [24]. It had been found that the network structure composed of SiO4 tetrahedron were tetrahedron could vibrate at a wide distribution range to release some external stress [24].
Correspondingly, the SiOxCy was formed when some carbon atoms replaced the oxygen atom in the SiO4 tetrahedron. Therefore, the amorphous SiOxCy phase inherited the excellent flexibility of the SiO4 tetrahedron. Owing to the above mechanisms, the MSUFs could avoid stress concentration and yield large deformation.

Thermal stability and thermal shock resistance
Thermal stability and thermal shock resistance were also critical criterion to evaluate application safety of thermal insulators. The thermal stability of the MSUF membranes was illustrated by TG analysis from 25 ºC to 1400 ºC in Ar atmosphere. As shown in Fig. 5a, the mass was stable up to 1400 ºC without obvious change, indicating excellent thermal stability of the MSUF membranes.
Furthermore, the MSUF membranes treated at 1400 ºC for 2 h in Ar could folded and then recovered their original shape without any cracks (inset in Fig. 5a). Besides, the surface of the heat-treated MSUFs were still smooth without obvious defects, similar to the as-prepared MSUFs (Fig. 5b). The results also evidenced that the MSUF membranes could withstand high temperature and maintain their high flexibility for long periods. Except for, the MSUF membranes exhibited excellent fire resistance and high flexibility at high temperatures, which was demonstrated by in situ bending tests while heated with a butane blowlamp to ~1000 ºC (Fig. S9a). No obvious structural changes or cracks were observed when the MSUF membranes were exposed to high temperatures (Fig. S9a), illustrating their high thermal stability and flexibility at high temperatures.
Apart from the high temperature flexibility, the MSUF membranes also exhibited high flexibility at low temperature. As shown in Fig. S9b, the MSUF membranes remained flexibility and could be bended in liquid N2 (-196 ºC). When the load was released, the MSUF membranes returned to their original shape without obvious cracks (Fig. S9b), demonstrating their high flexibility at low temperatures.
High thermal shock resistance was a vitally important feature for materials in many application areas, especially for aerospace applications [2,4]. The thermal shock tests were carried by heating the MSUF membranes via butane blowlamp for 30 s, and then immersed into liquid nitrogen (-196 ºC) for 30 s. We endured the thermal shock tests of the MSUF membranes at various (10~100) cycles, and their flexibility and tensile strength retention were investigated. Unexpectedly, after enduring rapid thermal shock for 100 cycles, the MSUF membranes still maintain their original morphology and high flexibility, and no cracks appeared ( Fig. 5c and inset in Fig. 5d). Moreover, the tensile strength retention of the membranes does not change obviously (Fig. 5d). The results indicated that the MSUF membranes with high structural stability and thermal shock resistance at rapid temperature changes, which were vitally critical for practical application in extreme environments.

Thermal insulation performance
Generally, the total thermal conductivity (λ) of ceramic fiber membranes is mainly composed of three components: solid thermal conductivity (λs), gaseous thermal conductivity (λg) and radiation thermal conductivity (λr) [18,20]. At ambient temperature, the contribution of λr to λ is small and can be neglected. Therefore, the λ can be approximately equal to the sum of λs and λg at ambient temperature. However, on the basis of Stefan-Boltzmann law, the λr is proportional to the third power of the temperature, which indicates that the λr plays an important role in heat transfer at high temperatures [32]. The λr can be calculated by the formula 3 [32]: where n, σ, ρ and T are the effective refractive index, Stefan-Boltzmann constant, sample density, and temperature. In general, λs can be reduced by decreasing the bulk density, λg can be decreased by narrowing the pore size of the material and λr can be reduced by enhancing the e * [18]. Owing to the unique multichannel structure and components, the as-prepared MSUF membranes exhibited excellent thermal insulation performance, which makes it a promising thermal insulating material for application in the field of aerospace and aircrafts.
As shown in Fig. S10, we put a fresh flower on a piece of MSUF membrane with 8 mm thickness and heated it using an alcohol lamp. After 3 minutes of heating, the flower remained fresh, revealing high thermal insulation performance of the membrane (Fig. S10). The corresponding IR images also illustrate the excellent thermal insulation performance of the membrane (Fig. S10).
Benefiting from the high strength and flexibility of the MSUF membranes, our MSUF membranes have remarkable advantage in practical applications, especially in the field of protecting complex curved surface. As shown in Fig. 6a, a piece of MSUF membrane with a thickness of 5 mm could be wrapped around the nozzle of a butane torch. The IR images showed that the temperature of the exposed nozzle increases rapidly up to ~700 ºC after 1 min, while the temperature of the membrane increased slowly to ~43.7 ºC. The results also indicated that MSUF membranes possessed superior thermal insulation performance. Fig. 6b showed the λ at ambient temperature of the prepared various MSUF membranes.
Obviously, the MSUF-10 membranes presented a lower λ (0.041 W m -1 ·K -1 ) than other membranes. The low λ of the membranes could be ascribed to their low density and high porosity structure (Fig. 3c), which decreased λs and λg. At high-temperatures, the λr plays a major role in heat transfer. The e * could be used to evaluate the λr at high-temperatures. Higher e * means lower λr at high-temperatures according to the formula 3. Therefore, the e * versus IR wavelength from 2.5~7 μm of various MSUF membranes were tested, as shown in Fig. 6c. Of note, the MSUF-10 membranes presented higher e * over wide IR wavelength range than the other membranes, revealing low λr at high temperatures. The lower λr could be the numerous channels enhance the reflecting and absorption of IR radiation [21].
Based on the above analysis, the thermal insulation mechanisms of MSUFs could be attributed to three aspects (Fig. 6d). First, the multichannel pores lengthen the path of solid heat transfer and restrict the gaseous heat conduction, which lead to the decline of λs and λg [18,19]. Second, the multichannel structure makes the IR radiation multiple reflect and absorb, resulting in the lowering of λr [21]. Third, the phase interfaces among the SiOxCy, ZrO2 and free carbon phases enhance the phonon scattering and the SiOxCy and free carbon phases have high e * , which also contribute to the decrease of λs and λr, respectively [32].  insulation performance than other reported ceramic fiber membranes [14,16,[33][34][35]. Furthermore, the prepared MSUF membranes possess high temperatures thermal stability up to 1400 ºC.
Therefore, in addition to the super flexibility, the resulting MSUF membranes present a 20 combination of low thermal conductivity and high temperature stability, indicating their promising applications in high temperature thermal insulation.

Fig. 7
Thermal conductivity versus temperature of ceramic fiber membranes.

Conclusions
In summary, we designed multichannel structured MSUF membranes composed of ZrO2, SiOxCy and free carbon phases through electrospinning technique by selecting polystyrene as the pore-forming template to regulate the pore structure. Owing to the unique components, the asprepared MSUFs exhibits integrated properties of excellent fire resistance, thermal shock resistance, temperature-invariant flexibility over a wide temperature range (-196~1000 ºC) and high temperature stability up to 1400 ºC. Remarkably, the fancy multichannel structure and components endow the MSUF membranes with excellent thermal insulation performance with low thermal conductivity (0.041 W m -1 ·K -1 at ambient and 0.141 W m -1 ·K -1 at 1000 ºC). In conclusion,