SiO2 aerogel multiscale reinforced by glass fibers and SiC nanowhiskers for thermal insulation

SiO2 aerogel attracts much interest as thermal insulation material due to ultra-low density and excellent thermal performance at room temperature. However, the poor mechanical property and a mass of heat transfer by radiation in high temperature limit application of aerogels. Herein, a novel aerogel composite multiscale reinforced by glass fibers felt with SiC nanowhiskers (SiCnw) (AFS) was prepared. SiCnw were evenly distributed in glass fibers felts by freeze-drying method to form a uniform multiscale felt. The SiCnw inside felts provided more contact point with aerogel to increase the interfacial adhesion force so that compressive stress of aerogel composites with 4% volume fraction SiCnw was increased to 0.29 MPa. SiCnw blocked infrared radiation to decrease the heat transfer. Therefore, thermal conductivity at 500℃ of aerogel composite with 4% volume fraction SiCnw was only 0.040 W/(m·K). In a word, SiCnw reduced the sensitivity of thermal conductivity to temperature. AFS showed potential in the field of medium and high temperature insulation.

to strengthen aerogel and reduce the sensitivity of thermal conductivity to temperature. In-situ growth process enabled uniform distribution of mullite whiskers in aerogel composites to avoid thermal stress caused by uneven temperature distribution [22,23]. Besides, these nanoscale 1D opacifiers brought more interface contact significantly to improve the mechanical property of aerogel composites [21]. However, sintering was necessary to in-situ synthesized these 1D opacifiers over surface of fibers and it would damage fibers for high-temperature.
We proposed a novel method to introduce SiC nanowhiskers (SiC nw ) in glass fibers felt by freeze-drying method to realize even distribution of nanowhiskers without hightemperature process, and prepared aerogel composites multiscale reinforced by glass fibers and SiC nw (AFS) by sol-gel and supercritical drying technology. The added SiC nw made more interface contact between reinforcement and aerogels, so that compressive property of aerogel composites was improved. Besides, thermal conductivity of AFS at high temperature was reduced by SiC nw . Thermal mechanism was studied and the effect of infrared radiation suppression of SiC nw in aerogel was analyzed.

Materials
The glass fiber felt (density was 119.94 mg/cm 3 ) was obtained from Nantong Yuanshun Fiber Co., Ltd, China. The NH 3 ·H 2 O, acetic acid and hexane were purchased from Tianjin Yuanli Chemical Co., Ltd., China. The ethyl orthosilicate (TEOS), N, N-dimethylformamide (DMF), and trimethylchlorosilane (TMCS) which applied for preparing silica aerogel were purchased from Shanghai Macklin Biochemical Technology Co., Ltd., China. Deionized water was used throughout the experiment, and all raw chemicals above are of analytical grade (AR) without further purification. SiC nw were purchased from Nangong Bole Metal Materials Co., Ltd. and silica sol were purchased from Shandong Peak-tech New Material Co., Ltd.

Uniform dispersion of SiCnw into glass fibers felt
Glass fibers felts were dipped in slurry with SiC nw and silica sol which were uniformly dispersed by a homogenizer. The mass fraction of silica sol was 0.5%. In order to avoid uneven distribution of SiC nw in the fibers due to sedimentation, liquid nitrogen was used to rapidly freeze the slurry. The frozen slurry was dried by sublimation at low pressure and 50℃ so that the dispersion state of SiC nw in the original slurry was well preserved in the fiber mat. After 100℃ heat treatment, The glass fibers felts with SiC nw were obtained. The glass fibers felts with SiC nw volume fraction of 0, 1%, 2%, 3% and 4% were named FW-0, FW-1, FW-2, FW-3, FW-4 respectively.

Composition of silica-based aerogel and fibers
Firstly, silica sol was prepared. A mixture of TEOS, ethanol, H 2 O and DMF were stirred by molar ratio of 1:8:6:1.5. Then the pH value of mixture was adjusted to 3-4 by acetic acid to promote the hydrolysis of TEOS. After that, the mixture sealed by plastic wrap was stirred for 3 h. Next, 0.4 wt‰ NH 3 ·H 2 O was added into the stirring mixture dropwise to adjust pH value to 7-8 to gel. The reinforcement was vacuum impregnated in the mixture and the silica sol was absorbed into the porous reinforcement. Then 48 h aging process was performed for strengthening of the skeleton of the silica aerogel. After that, the solvent replacement process was executed by immersing gel composite in hexane for 24 h, and the process was repeated for twice in total. Then surface modification by soaking the gel composite in 0.8 mol/L TMCS hexane solution lasted for 24 h. Then the same solvent replacement process using hexane was carried out to remove excessive TMCS, which was repeated for 3 times. Finally, aerogel composites reinforced by glass fibers and SiC nw were obtained by supercritical drying. Table 1 shows the information of all samples.

Characterization
The densities (ρ) of glass fibers felt, FW and AFS were calculated by the ratio of measured mass (m) and volume (V) of bulk samples (ρ = m/V). The scanning electron microscopy (SEM, TESCAN BRNO, Czech) was performed to analyze the microscopic morphology and structure of samples. The macropore and mesopore size distribution of aerogel were tested by the mercury intrusion porosimetry (MIP, Autopore V9620, USA) and Brunauer-Emmett-Teller (BET, Micromeritics ASAP 2460, USA), respectively. Thermal conductivities of all samples were measured employing the hot disk (TPS2500S, Sweden). The compressive strength  Figure 1 showed the prepared process of FW and AFS. Due to the long time for sol-gel process, to keep SiC nw uniformly distributed in silica sol was difficult. Under the condition of high cooling rate, the distribution of solid phase in the suspension after being frozen by liquid nitrogen could be kept as the state before being frozen [24,25]. Drying the frozen slurry via sublimation under low pressure could maximize the retention of the dispersed state of the slurry before freezing [26]. Therefore, combining with binder, a porous fibers felt and nano whiskers could form a homogeneous composite by immersing and freeze-drying. Figure 2a-b showed structure of glass fibers without or with SiC nw . The raw glass fibers felts had a microscale pore size. After frozen drying in SiC nw suspension, some whiskers were attached to the face of glass fibers, and they interlaced with each other, forming 3D network structure. Since diameter of SiC nw was around 100 nm, the size of pores enclosed were reduced significantly. There were some solid phase between the fibers and whiskers, which were silica particles produced after heat treatment of silica sol. To evaluate the homogeneity of composite mats prepared by the freeze-drying method, a raw glass fibers felt and FW-4 were cut into 25 equal parts, and each part were weighed. The coefficients of variation (CV) were obtained, which were the ratio of the standard deviation to the mean, and could be used to determine the degree of dispersion of data with different means [27]. Figure 2 (c-d) illustrated the contour plots and 3D density uniformity of raw glass fibers and FW-4. The average density of parts of raw glass fibers was 119.94 mg/cm 3 , and the CV was 0.0063. After freezedrying, the average density was 247.96 mg/cm 3 , and the CV was 0.0115, which was still at a low level. It could be inferred that the FW-4 prepared by freeze-drying method is homogeneous.

Microstructure and distribution of pore size
The photographs and micro-morphology of as-prepared AFS-0 ( Fig. 3 (a-c)) and AFS-4 ( Fig. 3 (d-f)) were shown in Fig. 3. After being compounded with silica aerogel, the soft glass fibers felt became hard and had a density ranged from 159.70 mg/cm 3 to 294.64 mg/cm 3 . SiC nw made composites green (shown in Fig. 3 (d)). As shown in Fig. 3. (b) and (e), nano aerogels were filled in skeleton of fibers. In Fig. 3 (f), it was noticeable that some whiskers were inserted inside Fig. 1 (a-d) prepared process of FW and (e-g) prepared process of AFS than the mean free path of gas molecules (69 nm) at ambient temperature and 1 atm, and it limited air molecules collision with others [29]. Therefore, the heat transfer by gas was reduced by the mesopores structure of aerogels. Figure 4 (c) showed N 2 adsorption desorption curve of pure aerogel. It could be seen that the isotherm of aerogel composites was typical IV isotherms, showing obvious mesoporous characteristics. The curve had a typical H3-type hysteresis loop, indicating that there was a plate-like pore structure in aerogels. When the relative pressure was at a low level, the sample mainly adsorbed N 2 in a monolayer, and the adsorption amount increased slowly, indicating that there were fewer micropores in aerogel. As the pressure increased, when the relative pressure was greater than 0.7, the adsorption of nitrogen gas by the sample gradually aerogels, which provided more contact points compared with AFS-0. Figure 3 (g) and (h) showed the microstructure of pure silica aerogel by SEM and TEM. Obviously, nano particles formed a network, and the size of particles was below 10 nm, which leaded to the mesoporous structure in AFS.
MIP and BET method were used to study pore structure of AFS. Figure 4 (a) showed pore size distribution curve obtained by MIP. There were two peaks at 2.5 and 70 μm, which was caused by cracks between porous aerogel and fibers, and it could be observed in Fig. 3 (e) and (f) [28]. AFS-4 had a mesoporous structure due to silica aerogels. Figure 4 (b) presented BJH pore size distribution curve of pure aerogel obtained by BET. The pore size of silica aerogel was mainly ranged from 2.5 to 35 nm. It was smaller  Where l m was the mean free path of gas molecules, which was 69 nm at at ambient temperature and 1 atm ; δ was the mean pore diameter of porous materials. Based on Eqs. (2) and (3), it could be inferred that pore diameter was crucial to the difference of gas thermal conductivity of materials prepared. In a word, the smaller pore brought heat conductivity by gas molecular collision. It was obvious that some cracks existed in AFS. Table 2 showed average pore size of samples tested by BET. All average pore size tested by BET of samples were lower than 69 nm, and the added nanowhisker reduced the average pore size. Besides, as Fig. 3 (e-f) showed, nanoscale whiskers in aerogel reinforced by fibers composites could divide the microscale cracks between fibers and aerogel to smaller ones. The phenomenon could be resulted from space constraints [21,23]. Therefore, SiC nw could weaken heat transfer by gas phase effectively.
Radiative conductivity could be calculated by the following equation [32]: Where σ was the Stefan-Boltzmann constant; T was the mean temperature (K); E(T) was Rossland mean extinction coefficient of materials. and could be expressed as following: converted to multi-molecular layer adsorption, and a hysteresis loop appeared, indicating that the aerogel had a typical mesoporous structure with a narrow pore size distribution. When the pressure was extremely high (P/P 0 > 0.8), the adsorption capacity increased sharply, indicating that the multi-molecular adsorption of N 2 by the aerogel sample was transformed into the capillary condensation of N 2 between the aerogel particles.

Thermal insulation property and mechanism
Thermal conductivity of AFS consisted 3 parts: solid thermal conductivity, gas thermal conductivity, and radiation thermal conductivity, and could be expressed as following: k tot = k solid + k gas + k rad (1) k solid was defined by specific heat at constant volume and density of material [30]. Due to similar densities and specific heat, the difference of k solid of AFS was negligible. Thereby, the difference thermal conductivity was caused by k gas and k rad .
k gas of porous materials could be expressed as following [31]: Where, k 0 was thermal conductivity of free still air at temperature T; β was the constant of energy transfer when gas molecules hit the wall of material; K n was Knudsen number, which can be expressed as following:  a superb barrier to heat radiation. Furtherly, specific extinction coefficient (e * ) was calculated based on Eq. 3 and shown in Fig. 5 (b). Obviously, the e * of AFS with SiC nw was much higher than that without nano whiskers. Specially, the e * of AFS-4 was several times of that of aerogel in the wavelength range of 3-5 μm. Thereby, SiC nw weakened heat radiation of aerogel composites. That made a great contribution to excellent thermal insulation performance at high temperature [35]. Figure 5 (c) showed thermal conductivities of all samples. At 25 ℃, AFS-0 had the lowest thermal conductivity (0.0192 W/(m·K)), and thermal conductivities of all samples were positively correlated with the content of SiC nw . As temperature rising, thermal conductivities of AFS rose. Specially, the thermal conductivity of AFS-0 was more sensitive to temperature. And SiC nw made AFS have a superior temperature resistance in thermal conductivity. Thermal conductivity increments from 25 ℃ to 500 ℃of AFS-0, AFS-1, AFS-2, AFS-3 and AFS-4 were 0.047, 0.031, 0.019, 0.013 and 0.006 W/(m·K) respectively. And at 500℃, thermal conductivity of AFS-4 was only 0.040 W/(m·K). Figure 5 (d) showed the insulation mechanism of AFS. In summary, although SiC nw brought a shield to heat transfer Where ρ was the density of sample, and e* was the specific extinction coefficient, which was obtained by following equation [33]: Where τ was the infrared transmittance of the samples, measured at different infrared wavelengths by a Fourier Transform Infrared Spectrometer. W was the total mass of the KBr pellet (kg); P was the mass per-centage of the powder sample in the KBr pellet (%); A was the cross-sectional area of the pellet (m 2 ). Figure 5 (a) showed the infrared transmission spectra (τ) of all samples. It was obvious that AFS-0 was almost transparent in the wavelength of 3-5 μm with 85% transmittance, which was the main region peak wavelength of 300℃-500℃ concentrated in according to Wien displacement law [34]. Therefore, the aerogel without SiC nw was unable to block heat radiation at 300℃-500℃ and radiative heat transfer increased greatly. The added SiC nw decreased the transmittance effectively, which suggested SiC nw made  Figure 7 illustrated the mechanical property of AFS. In general, the stress-strain curves of AFS included 3 stages: linear stage, yielding stage and densification stage and there was no fracture [36]. From 0 to 15% of strain was linear stage, where the stress increased almost linearly with the strain. At this stage, porous aerogel filler was the main force-loading structure. In the yield stage (strain of 15 -30%), the stress caused by strain increased at a lower rate. This was because the porous structure of aerogel began to break down, and the reinforcement acted as the main force-loading structure. In the densification stage (strain >30%), the structure reinforcement was broken and the composite became denser. by radiation, strong property of heat transfer by solid phase contributed to total thermal conductivity at low temperature rising. With temperature increasing, heat transfer by radiation was enhanced greatly because k r was proportional to T 3 according to Eq. (1). SiC nw blocked a mass of heat transfer by radiation at high-temperature, and it could offset the increase of heat transfer by solid, so that SiC nw contributed a better thermal performance of aerogel at high-temperature. Figure 6 visually demonstrated the thermal insulation performance of AFS via a continuous heating test. It could be observed a piece of AFS-4 with thickness of 10 mm could effectively protect fresh flower from the heating of alcohol lamp in Fig. 6 (a). As Fig. 6 (b-d) showed, at first, when the temperature of the heated surface reached 411.8 °C, the temperature of other surface was lower than 50 °C. With continuous heating, the temperature hot surface reached 565.9 °C and the temperature of flower was only 63.6 °C. It suggested that AFS was a promising high-temperature thermal insulation material.

Declarations
of glass fiber provided more contact. Detachment from whiskers was a prerequisite for separation of aerogel from fibers, which significantly increased the interfacial adhesion force and leaded to the higher fracture energy absorbed by fibers pull-out and interfacial debonding. It contributed to the improvement of mechanical properties of aerogel composites.

Conclusion
In summary, SiC nw were introduced into glass fibers felt by freeze-drying, and aerogel composites multiscale reinforced by glass fibers and SiC nw with low high-temperature thermal conductivity and high strength were prepared by sol-gel and supercritical drying method. SiC nw were evenly distributed in glass fibers felt by freeze-drying. At 25℃, AFS without SiC nw has the lowest thermal conductivity (0.0192 W/ (m·K)) but was sensitive to temperature, which caused the higher increment from 25℃ to 500℃. Thermal conductivity at high-temperature of AFS was reduced significantly by SiC nw for the suppression of infrared radiation. Thermal conductivity of AFS-4 at 500℃ was 0.040 W/(m·K). The 4% volume fraction nanowhiskers increased the compressive strength of AFS to 0.29 MPa. AFS showed potential of thermal insulation in extreme environment.