Three-Dimensional Graphene Hybrid SiO2 Hierarchical Dual-Network Aerogel with Low Thermal Conductivity and High Elasticity

We describe lightweight three-dimensional (3D) graphene hybrid SiO2 aerogels (GSAs) with hierarchically robust interconnected networks fabricated via an in situ deposition procedure after a hydrothermal assembling strategy with graphene oxide sheets. The nano-/micron-thick SiO2 coating conformably grew over porous graphene templates with two constituents (e.g., graphene and SiO2) and formed chemically bonded interfaces. In addition, it significantly refined the primary graphene pores by hundreds of microns into smaller porous patterns. Studies of its mechanical properties verified that the graphene interframework made the ceramic composites elastic, while SiO2 deposition enhanced the strength required it to resist deformation. The higher SiO2 contents resulted in lower elasticity but larger strength because of the apparent nanosize effect of SiO2 ceramic thickness; GSAs with a density of 82.3–250.3 mg/cm3 (corresponding to SiO2 sol with concentration ranging from 5 to 20 wt %) could reach a good balance of strength and elasticity. Benefiting from hierarchical micronetworks consisting of semiclosed or closed pores, GSAs offer excellent thermal-insulation performance, with thermal conductivity as low as 0.026 W/(m·K). GSAs offer improved fire-resistant capacity rather than that of pure carbon-based aerogels via the synergic protection of SiO2 ceramic accretion. This highlights the promising applications of GSAs as lightweight thermal-shielding candidates for industrial equipment, civil architectures, and defense transportation vehicles.


Introduction
Thermal-insulation materials are increasingly needed, and have been derived from natural resources or synthetic products [1]. The most popular thermal insulators are currently organic (e.g., expanded polystyrene foam, phenolic foam, and polystyrene foam) or inorganic (e.g., carbon foam, foam glass board, vermiculite, foam concrete, and foam ceramic) with highly porous structures that generated low thermal conductivity (0.032-0.10 W·m −1 ·K −1 ) [2][3][4][5][6]. In general, aircraft require into a Teflon mold and hydrothermally assembled at 120 • C for 6 h with micron-scale GO sheets self-constructed into a 3D elastic hydrogel. The 3D porous GO sample with hierarchical microstructures was obtained after 24 h of dialysis in ethanol solution (20 vol %), followed by freeze-drying. The SiO 2 sol with six different concentrations (0 wt %, 1 wt %, 5 wt %, 10 wt %, 20 wt %, 30 wt %, and 40 wt %) was then selected to fully absorb into the porous frameworks of 3D GO architecture under vacuum conditions, respectively. Subsequently, the elastic hydrogel-like composite of 3D GO architecture and SiO 2 sol was treated by freeze-drying process at −60 • C for 24 h under vacuum conditions (1 Pa) with the contained water removed. After further thermal annealing at 600 • C for 8 h, three-dimensional (3D) lightweight GSAs (size: diameter × thickness = 30 mm × 11 mm) were fabricated with two constituents (i.e., reduced graphene oxide and SiO 2 ) tightly composed together; the corresponding samples were marked as GSA-x (where x = 0, 1, 5, 10, 20, 30, and 40, respectively). Similarly, the reference pure SiO 2 sample was prepared with SiO 2 sol (concentration 40 wt %) by direct freeze-drying process at -60 • C for 24 h under vacuum conditions followed by thermal treatment at 600 • C for 8 h.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 13 sample with hierarchical microstructures was obtained after 24 h of dialysis in ethanol solution (20 vol %), followed by freeze-drying. The SiO2 sol with six different concentrations (0 wt %, 1 wt %, 5 wt %, 10 wt %, 20 wt %, 30 wt %, and 40 wt %) was then selected to fully absorb into the porous frameworks of 3D GO architecture under vacuum conditions, respectively. Subsequently, the elastic hydrogel-like composite of 3D GO architecture and SiO2 sol was treated by freeze-drying process at −60 C for 24 h under vacuum conditions (1 Pa) with the contained water removed. After further thermal annealing at 600 C for 8 h, three-dimensional (3D) lightweight GSAs (size: diameter × thickness = 30 mm × 11 mm) were fabricated with two constituents (i.e., reduced graphene oxide and SiO2) tightly composed together; the corresponding samples were marked as GSA-x (where x = 0, 1, 5, 10, 20, 30, and 40, respectively). Similarly, the reference pure SiO2 sample was prepared with SiO2 sol (concentration 40 wt %) by direct freeze-drying process at -60C for 24 h under vacuum conditions followed by thermal treatment at 600 C for 8 h.

Microstructural Characterizations
The morphologies of the GSA samples with different SiO2 contents were studied by SEM and TEM observations. Figure 2a-d shows that the primary GA exhibited typical honeycomb-like and interconnected microstructures with pore dimensions that ranged from hundreds of nanometers to tens of microns. After processing the two constituents, GSA microstructures not only maintained the honeycomb patterns of GA template, but also had changes in pore shape and size; GSA pore changed Figure 1. Schematic of graphene hybrid SiO 2 aerogel (GSAs) fabrication including grapheme oxide (GO) architecture formation by self-assembling GO sheets via hydrothermal chemical reaction. SiO 2 sol absorbing into porous GO architecture and in situ freeze-drying combined with subsequently thermal annealing to form GSAs with graphene templates and SiO 2 ceramics tightly integrated.

Microstructural Characterizations
The morphologies of the GSA samples with different SiO 2 contents were studied by SEM and TEM observations. Figure 2a-d shows that the primary GA exhibited typical honeycomb-like and interconnected microstructures with pore dimensions that ranged from hundreds of nanometers to tens of microns. After processing the two constituents, GSA microstructures not only maintained the honeycomb patterns of GA template, but also had changes in pore shape and size; GSA pore changed to quadrangle from the primary circle shape of the graphene template, with the initial larger size divided into dual networks on a smaller scale. Figure 2e-h shows the sandwiched pore walls with the SiO 2 layer conformably accreted over the rippled graphene sheets that served as basic construction elements to assemble into Y-shaped microjoints that then propagated into monolithic GSAs. The cracks that appeared in Figure 2c,g possibly were induced by shrinkage process during the freeze-drying and thermal annealing treatments with both free and bound water removed. By increasing SiO 2 sol concentrations, the wrinkled coverage of the SiO 2 ceramics wrapped around the graphene sheets. Related thickness was enlarged from several nanometers up to a few microns with the possible cracks on SiO 2 layer synchronously suppressed. Figure 2i-k demonstrates the highly coincidental elemental patterns of EDS mappings corresponding to carbon (C) and silicon (Si) for the surface and internal positions of the GSAs, respectively. The weight percentage of Si in the internal regions was 38.60 wt %, while the surface content presented a slight increase of 41.73 wt %. Both silica-percentage and element-mapping results indicated the uniform attachment of SiO 2 over 3D interconnected GA templates. Moreover, Figure 2l shows a typical high-resolution image of TEM and the selected area electron-diffraction pattern of the SiO 2 ceramic; the corresponding distances for the (420) and (111) crystal planes were 0.32 and 0.41 nm, respectively. These two phases of SiO 2 normally have catastrophic effects on thermal cycling. The sandwiched graphene sheets serving as reinforcing units were designed to improve the brittleness nature of SiO 2 under thermal stresses [8]. to quadrangle from the primary circle shape of the graphene template, with the initial larger size divided into dual networks on a smaller scale. Figure 2e-h shows the sandwiched pore walls with the SiO2 layer conformably accreted over the rippled graphene sheets that served as basic construction elements to assemble into Y-shaped microjoints that then propagated into monolithic GSAs. The cracks that appeared in Figure 2c,g possibly were induced by shrinkage process during the freeze-drying and thermal annealing treatments with both free and bound water removed. By increasing SiO2 sol concentrations, the wrinkled coverage of the SiO2 ceramics wrapped around the graphene sheets. Related thickness was enlarged from several nanometers up to a few microns with the possible cracks on SiO2 layer synchronously suppressed. Figure 2i-k demonstrates the highly coincidental elemental patterns of EDS mappings corresponding to carbon (C) and silicon (Si) for the surface and internal positions of the GSAs, respectively. The weight percentage of Si in the internal regions was 38.60 wt %, while the surface content presented a slight increase of 41.73 wt %. Both silica-percentage and element-mapping results indicated the uniform attachment of SiO2 over 3D interconnected GA templates. Moreover, Figure 2l shows a typical high-resolution image of TEM and the selected area electron-diffraction pattern of the SiO2 ceramic; the corresponding distances for the (420) and (111) crystal planes were 0.32 and 0.41 nm, respectively. These two phases of SiO2 normally have catastrophic effects on thermal cycling. The sandwiched graphene sheets serving as reinforcing units were designed to improve the brittleness nature of SiO2 under thermal stresses [8].

Chemical Composition and Structural Analysis
The GSAs were constituted with two different components, graphene sheets and SiO2 ceramic; the graphene sheets formed a 3D interconnected porous architecture with layered SiO2 conformably deposited over it. The chemical composition and structural characterization of GSAs were further examined with XRD and FT-IR investigations. As shown in Figure 3a, compared with GO typically peaked at around 10° [12], a wide peak of GSA-0 around 20° was generated through two peaks overlapping the graphitic peak at 26.52° and graphitic oxide less than 20°, because rGO sheets were

Chemical Composition and Structural Analysis
The GSAs were constituted with two different components, graphene sheets and SiO 2 ceramic; the graphene sheets formed a 3D interconnected porous architecture with layered SiO 2 conformably deposited over it. The chemical composition and structural characterization of GSAs were further examined with XRD and FT-IR investigations. As shown in Figure 3a, compared with GO typically peaked at around 10 • [12], a wide peak of GSA-0 around 20 • was generated through two peaks overlapping the graphitic peak at 26.52 • and graphitic oxide less than 20 • , because rGO sheets were partially reduced with a certain amount of oxygen-containing functional groups removed during thermal annealing at 600 • C. With the increase of SiO 2 contents, GASs presented a sharp characteristic peak at 2θ = 21.5 • due to SiO 2 dominantly covering the peak information of the rGO component, which corresponded to the (002) plane of the graphitic structure and overlapped with the (222) plane of SiO 2 [23]. This implied that the graphene component within the GSAs was partly reduced with a portion of oxygen functional groups retained on GO sheets during the thermal-annealing process at 600 • C versus 2θ = 26.52 • for the intrinsic graphite (002) crystal plane [24].
Coatings 2020, 10, x FOR PEER REVIEW 5 of 13 partially reduced with a certain amount of oxygen-containing functional groups removed during thermal annealing at 600 °C . With the increase of SiO2 contents, GASs presented a sharp characteristic peak at 2θ = 21.5° due to SiO2 dominantly covering the peak information of the rGO component, which corresponded to the (002) plane of the graphitic structure and overlapped with the (222) plane of SiO2 [23]. This implied that the graphene component within the GSAs was partly reduced with a portion of oxygen functional groups retained on GO sheets during the thermal-annealing process at 600 C versus 2θ = 26.52° for the intrinsic graphite (002) crystal plane [24].  Figure 3b demonstrates that the peak at 1560 cm −1 of the FT-IR spectra corresponded to the vibration of C-C/C=C, which was generated from the graphene sheets. The other characteristic peaks of 1080 and 460 cm −1 represent the symmetric and asymmetric stretching vibrations of Si-O bonds, respectively. The peak at 3450 cm −1 verified the existence of -OH stretching bonds, while the 2350 cm −1 peak was related to residual CO 2 . The peak at around 800 cm −1 corresponded to the stretching mode of Si-C bond. As shown in Figure 3c, the wide XPS spectrum peaks at 520, 280, and 103 eV were coincident with oxygen (O 1s), carbon (C 1s), and silicon (Si 2p), indicating the existence of 3D graphene architectures and the SiO 2 component. The weight percentage of silica was 39.5 wt %, which was coincident with EDS results. The peak at 400 eV belongs to nitrogen (N 1s) formed during the crosslinking process of EDA with the GO sheet. The deconvolution of the C 1s spectrum in Figure 3d gives three typical peaks centered at 281.9, 284.5, and 285.4 eV, corresponding to dominant components within GSAs such as the Si-C bonds, C-C/C=C for aromatic carbon on GA, and slightly residual oxygen-containing groups of C-OH on reduced GO sheets, respectively. Similarly, both peaks in the Si 2p spectrum, located at 103.4 and 100 eV, are attributed to O-Si and C-Si, respectively [25,26] ( Figure 3e). Both the C1s and Si 2p spectra agreed well with XRD and FT-IR data, and even further verified the formation of a covalent bond C-Si at the interface between graphene architecture and SiO 2 ceramic layer. This interface facilitated the structural robustness and co-operative reinforcement of mechanical properties. Moreover, the deconvolution XPS spectrum of C 1s for GAS-0 gave typical peaks of 284. 6

Investigation of Thermal-Conductivity Properties
We implemented measurements of sample mass (m) by digital high-precision balance (accuracy 0.001 mg), while the volume (V) of the pancake-shaped sample was obtained using a digital micrometer (accuracy 0.01 mm). The apparent density of the sample was calculated directly by m/V. As shown in Figure 4a and Table 1, the bulk densities, relative density, and SiO 2 content of the GSAs were linearly dependent on SiO 2 sol concentration from 0 to 40 wt %. This led to porous GAS architectures via a perfusion process under vacuum conditions. They were enlarged from 6.5 mg/cm 3 of GA towards 532 mg/cm 3 . To understand the thermal-insulation performance of the GSAs, thermal conductivity (κ, W m -1 K −1 ) was investigated on the basis of the hot-wire method. κ significantly depended on the bulk density of GSAs as value increased from 0.026 to 0.11 W·m -1 ·K −1 at room temperature (RT; Figure 4b,c). The value at 300 K corresponded to the bulk-density range of 6.5-532 mg/cm 3 . The higher concentration meant that more SiO 2 was deposited over the GA template, leading to a thicker SiO 2 layer and more pathways for thermal transfer.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 13 Figure 3b demonstrates that the peak at 1560 cm −1 of the FT-IR spectra corresponded to the vibration of C-C/C=C, which was generated from the graphene sheets. The other characteristic peaks of 1080 and 460 cm −1 represent the symmetric and asymmetric stretching vibrations of Si-O bonds, respectively. The peak at 3450 cm −1 verified the existence of -OH stretching bonds, while the 2350 cm −1 peak was related to residual CO2. The peak at around 800 cm −1 corresponded to the stretching mode of Si-C bond. As shown in Figure 3c, the wide XPS spectrum peaks at 520, 280, and 103 eV were coincident with oxygen (O 1s), carbon (C 1s), and silicon (Si 2p), indicating the existence of 3D graphene architectures and the SiO2 component. The weight percentage of silica was 39.5 wt %, which was coincident with EDS results. The peak at 400 eV belongs to nitrogen (N 1s) formed during the crosslinking process of EDA with the GO sheet. The deconvolution of the C 1s spectrum in Figure 3d gives three typical peaks centered at 281.9, 284.5, and 285.4 eV, corresponding to dominant components within GSAs such as the Si-C bonds, C-C/C=C for aromatic carbon on GA, and slightly residual oxygen-containing groups of C-OH on reduced GO sheets, respectively. Similarly, both peaks in the Si 2p spectrum, located at 103.4 and 100 eV, are attributed to O-Si and C-Si, respectively [25,26] (Figure 3e). Both the C1s and Si 2p spectra agreed well with XRD and FT-IR data, and even further verified the formation of a covalent bond C-Si at the interface between graphene architecture and SiO2 ceramic layer. This interface facilitated the structural robustness and co-operative reinforcement of mechanical properties. Moreover, the deconvolution XPS spectrum of C 1s for GAS-0 gave typical peaks of 284. 6

Investigation of Thermal-Conductivity Properties
We implemented measurements of sample mass (m) by digital high-precision balance (accuracy 0.001 mg), while the volume (V) of the pancake-shaped sample was obtained using a digital micrometer (accuracy 0.01 mm). The apparent density of the sample was calculated directly by m/V. As shown in Figure 4a and Table 1, the bulk densities, relative density, and SiO2 content of the GSAs were linearly dependent on SiO2 sol concentration from 0 to 40 wt %. This led to porous GAS architectures via a perfusion process under vacuum conditions. They were enlarged from 6.5 mg/cm 3 of GA towards 532 mg/cm 3 . To understand the thermal-insulation performance of the GSAs, thermal conductivity (κ, W m -1 K −1 ) was investigated on the basis of the hot-wire method. κ significantly depended on the bulk density of GSAs as value increased from 0.026 to 0.11 W·m -1 ·K −1 at room temperature (RT; Figure 4b,c). The value at 300 K corresponded to the bulk-density range of 6.5-532 mg/cm 3 . The higher concentration meant that more SiO2 was deposited over the GA template, leading to a thicker SiO2 layer and more pathways for thermal transfer.   At RT conditions in air, the thermal-insulation capacities of GSAs were obviously divided into three categories according to their densities. At densities lower than 100 mg/cm 3 , the minimal κ was about 0.026 W·m -1 ·K −1 for bulk density, implying outstanding thermal insulation for this lightweight graphene/SiO2 hybrid aerogel versus most natural or artificial insulation candidates [3], such as expanded polystyrene foam (0.04 W·m -1 ·K −1 ), vermiculite (0.044 W·m -1 ·K −1 ), phenolic foam board (0.032 W·m -1 ·K −1 ), polystyrene foam (0.041 W·m -1 ·K −1 ), foam glass board (0.062 W·m -1 ·K −1 ), fiberreinforced composite (0.07 W·m -1 ·K −1 ), foam concrete (0.24 W·m -1 ·K −1 ), and foam ceramic board (0.10 W·m -1 ·K −1 ). κ continuously increased up to 0.07 W·m -1 ·K −1 on the basis of density values (100-400 mg/cm 3 ). However, this slow increase of κ versus density tended to markedly depend on κ being larger than 0.1 W·m -1 ·K −1 , as well as density beyond 400 mg/cm 3 , suggesting a slight decay in the thermal-insulation ability of the denser GSA structure. The impact of density on the material structure, including pore geometric shape and size, did not follow a linear relationship. Thermal conductivity was mainly determined by structural characteristics, so it could not realize ideal linear fitting curves of density vs. κ. As illustrated in Figure 2f-h, the abrupt increase of thermal conductivity for GASs with densities above 400 mg/cm 3 was due to the remarkable enlargement of the SiO2 layer's thickness deposited over the GA framework, which significantly improved the continuity of the microbranches of graphene-and SiO2-composited walls. In addition, due to the combined effects of hot electron activation, radiation conduction, and convection conduction [12], κ increased 18.2%-34.1% with the surrounding temperature increasing from 300 to 500 K.  At RT conditions in air, the thermal-insulation capacities of GSAs were obviously divided into three categories according to their densities. At densities lower than 100 mg/cm 3 , the minimal κ was about 0.026 W·m -1 ·K −1 for bulk density, implying outstanding thermal insulation for this lightweight graphene/SiO 2 hybrid aerogel versus most natural or artificial insulation candidates [3], such as expanded polystyrene foam (0.04 W·m -1 ·K −1 ), vermiculite (0.044 W·m -1 ·K −1 ), phenolic foam board (0.032 W·m -1 ·K −1 ), polystyrene foam (0.041 W·m -1 ·K −1 ), foam glass board (0.062 W·m -1 ·K −1 ), fiber-reinforced composite (0.07 W·m -1 ·K −1 ), foam concrete (0.24 W·m -1 ·K −1 ), and foam ceramic board (0.10 W·m -1 ·K −1 ). κ continuously increased up to 0.07 W·m -1 ·K −1 on the basis of density values (100-400 mg/cm 3 ). However, this slow increase of κ versus density tended to markedly depend on κ being larger than 0.1 W·m -1 ·K −1 , as well as density beyond 400 mg/cm 3 , suggesting a slight decay in the thermal-insulation ability of the denser GSA structure. The impact of density on the material structure, including pore geometric shape and size, did not follow a linear relationship. Thermal conductivity was mainly determined by structural characteristics, so it could not realize ideal linear fitting curves of density vs. κ. As illustrated in Figure 2f-h, the abrupt increase of thermal conductivity for GASs with densities above 400 mg/cm 3 was due to the remarkable enlargement of the SiO 2 layer's thickness deposited over the GA framework, which significantly improved the continuity of the microbranches of graphene-and SiO 2 -composited walls. In addition, due to the combined effects of hot electron activation, radiation conduction, and convection conduction [12], κ increased 18.2%-34.1% with the surrounding temperature increasing from 300 to 500 K.
Figure 4d-f shows a mathematical fitting operation based on the least-squares method. The κ of all GSA samples with seven different densities nicely followed a linear function format of temperature variations as κ = AT + B (where both A and B were constants). At GSA density of 6.5 mg/cm 3 , A and B values were 1.0 × 10 -6 and 0.022, respectively. Both A and B were enlarged as long as temperature increased. For instance, A and B further increased up to 8.0 × 10 -5 and 0.035, respectively, corresponding to a density of 270 mg/cm 3 . Moreover, the as-obtained densest GSA (532 mg/cm 3 ) resulted in the largest values for A and B of 1.0 × 10 -4 and 0.073, respectively.

Thermal-Insulation, Flame-Resistance, and Thermal-Stability Studies
GSAs are an insulating material in thermal engineering. Figure 5a demonstrates the experiment setup of GSA thermal-insulation performance with a pie-shaped sample placed over an asbestos-free wire gauze that is was directly heated by an alcohol lamp; GSA-10 was selected as the representative sample for followed investigation. To visually illustrate thermal-insulation properties, a fresh chrysanthemum flower was placed over the GSA sample. Temperature distribution throughout the system was captured by a digital infrared camera. Figure 5b shows infrared images of both the GSA sample and the flower, they were almost the same as the room-temperature background. Once the lamp was lighted, temperature at the bottom of the GSAs was rapidly elevated to over 650 • C within 10 min, but the flower maintained the same temperature as the surrounding environment (see Figure 5c,d). Even though the GSAs were heated over 30 min, the gradual increase in temperature over the flower was less than 10 • C, with its original shape remaining fresh and nonshrinkable. This implies that porous GSAs have excellent thermal-insulation capabilities, and block heat transfer from hot to cold. Moreover, GA composition could absorb infrared radiation to suppress the infrared thermal transform and further make up the shortage of infrared transparent properties in normal pure SiO 2 ceramic aerogels [27,28]. Figure 4d-f shows a mathematical fitting operation based on the least-squares method. The κ of all GSA samples with seven different densities nicely followed a linear function format of temperature variations as κ = AT + B (where both A and B were constants). At GSA density of 6.5 mg/cm 3 , A and B values were 1.0 × 10 -6 and 0.022, respectively. Both A and B were enlarged as long as temperature increased. For instance, A and B further increased up to 8.0 × 10 -5 and 0.035, respectively, corresponding to a density of 270 mg/cm 3 . Moreover, the as-obtained densest GSA (532 mg/cm 3 ) resulted in the largest values for A and B of 1.0 × 10 -4 and 0.073, respectively.

Thermal-Insulation, Flame-Resistance, and Thermal-Stability Studies
GSAs are an insulating material in thermal engineering. Figure 5a demonstrates the experiment setup of GSA thermal-insulation performance with a pie-shaped sample placed over an asbestos-free wire gauze that is was directly heated by an alcohol lamp; GSA-10 was selected as the representative sample for followed investigation. To visually illustrate thermal-insulation properties, a fresh chrysanthemum flower was placed over the GSA sample. Temperature distribution throughout the system was captured by a digital infrared camera. Figure 5b shows infrared images of both the GSA sample and the flower, they were almost the same as the room-temperature background. Once the lamp was lighted, temperature at the bottom of the GSAs was rapidly elevated to over 650 °C within 10 min, but the flower maintained the same temperature as the surrounding environment (see Figure  5c,d). Even though the GSAs were heated over 30 min, the gradual increase in temperature over the flower was less than 10 C, with its original shape remaining fresh and nonshrinkable. This implies that porous GSAs have excellent thermal-insulation capabilities, and block heat transfer from hot to cold. Moreover, GA composition could absorb infrared radiation to suppress the infrared thermal transform and further make up the shortage of infrared transparent properties in normal pure SiO2 ceramic aerogels [27,28]. Furthermore, the combination of SiO2 ceramic with GA could improve thermal stability and flame resistance under air conditions because the pure carbon nanomaterial could not tolerate the oxidizing reaction with direct contact of oxygen at temperatures over 350 °C [29,30]. Figure 6 shows that the GSA sample was directly ablated by alcohol lamp fire with the outer flame temperature over 650 °C. Figure 6a shows the primary black and regular geometric configuration of GSA-10. The sample retained its original color and regular shape within the first 10 min. After 20 min, there was a slight color change at the edges from black to white; this indicated that a very small portion of the GA was thermally etched. At 50 min, about 20% of the black region became white, and the sample did not burn. This is attributed to the synergetic protection of SiO2 ceramic accretions over GA microframework. The flame resistance of GSA was significantly improved versus that of pure carbon Furthermore, the combination of SiO 2 ceramic with GA could improve thermal stability and flame resistance under air conditions because the pure carbon nanomaterial could not tolerate the oxidizing reaction with direct contact of oxygen at temperatures over 350 • C [29,30]. Figure 6 shows that the GSA sample was directly ablated by alcohol lamp fire with the outer flame temperature over 650 • C. Figure 6a shows the primary black and regular geometric configuration of GSA-10. The sample retained its original color and regular shape within the first 10 min. After 20 min, there was a slight color change at the edges from black to white; this indicated that a very small portion of the GA was thermally etched. At 50 min, about 20% of the black region became white, and the sample did not burn. This is attributed to the synergetic protection of SiO 2 ceramic accretions over GA microframework.
The flame resistance of GSA was significantly improved versus that of pure carbon nanomaterials that are flammable at 350 • C [8]. Moreover, despite the GA that was partially oxidized by flames, the pielike GSA sample always retained its regular geometric configuration and robust structure without any visible volumetric shrinkage or structural fractures.
Coatings 2020, 10, x FOR PEER REVIEW 9 of 13 nanomaterials that are flammable at 350 °C [8]. Moreover, despite the GA that was partially oxidized by flames, the pielike GSA sample always retained its regular geometric configuration and robust structure without any visible volumetric shrinkage or structural fractures. The improvement in GSA thermal stability versus that of pure GA under the thermal-protective deposition of SiO2 ceramic was evaluated by TGA investigation as temperature increased from RT to 1000 C. Figure 7a shows the weight losses for GSAs. They decreased from 11.03% for the pure GA to 7.04% for GSA-1 at N2 atmosphere conditions. There was an increase in SiO2 ceramic contents with eventual weight losses at 1000 C, less than 4% for GSA-30. This indicated that the SiO2 ceramic combination could enhance the thermal stability of GA-based functional insulation materials. In contrast, for GSA samples exposed to air conditions, both pure GA and GSA-1 exhibited typical TGA curves, with the first weight-loss stage occurring at 250 °C because of the decomposition of oxygencontaining residues remaining on the rGO. Subsequently, the most dramatic weight drops appeared at temperatures from 500 to 600 C due to the partial oxidation of the carbon components in GSAs. However, for GSA-10 and GSA-30, the final weight losses at 1000 C decreased to 6.13% and 3.7%, respectively. This clearly demonstrated the existence of the SiO2 ceramic constituent that gives the GSAs outstanding thermal stability for either air or inert atmosphere conditions-there was no direct contact between GA and oxygen. Comparatively, 10 wt % SiO2 sol-derived GSA-10 already presented excellent thermal stability with a weight drop in air conditions less than 7% at 1000 °C . The improvement in GSA thermal stability versus that of pure GA under the thermal-protective deposition of SiO 2 ceramic was evaluated by TGA investigation as temperature increased from RT to 1000 • C. Figure 7a shows the weight losses for GSAs. They decreased from 11.03% for the pure GA to 7.04% for GSA-1 at N 2 atmosphere conditions. There was an increase in SiO 2 ceramic contents with eventual weight losses at 1000 • C, less than 4% for GSA-30. This indicated that the SiO 2 ceramic combination could enhance the thermal stability of GA-based functional insulation materials. In contrast, for GSA samples exposed to air conditions, both pure GA and GSA-1 exhibited typical TGA curves, with the first weight-loss stage occurring at 250 • C because of the decomposition of oxygen-containing residues remaining on the rGO. Subsequently, the most dramatic weight drops appeared at temperatures from 500 to 600 • C due to the partial oxidation of the carbon components in GSAs. However, for GSA-10 and GSA-30, the final weight losses at 1000 • C decreased to 6.13% and 3.7%, respectively. This clearly demonstrated the existence of the SiO 2 ceramic constituent that gives the GSAs outstanding thermal stability for either air or inert atmosphere conditions-there was no direct contact between GA and oxygen. Comparatively, 10 wt % SiO 2 sol-derived GSA-10 already presented excellent thermal stability with a weight drop in air conditions less than 7% at 1000 • C.
Coatings 2020, 10, x FOR PEER REVIEW 9 of 13 nanomaterials that are flammable at 350 °C [8]. Moreover, despite the GA that was partially oxidized by flames, the pielike GSA sample always retained its regular geometric configuration and robust structure without any visible volumetric shrinkage or structural fractures. The improvement in GSA thermal stability versus that of pure GA under the thermal-protective deposition of SiO2 ceramic was evaluated by TGA investigation as temperature increased from RT to 1000 C. Figure 7a shows the weight losses for GSAs. They decreased from 11.03% for the pure GA to 7.04% for GSA-1 at N2 atmosphere conditions. There was an increase in SiO2 ceramic contents with eventual weight losses at 1000 C, less than 4% for GSA-30. This indicated that the SiO2 ceramic combination could enhance the thermal stability of GA-based functional insulation materials. In contrast, for GSA samples exposed to air conditions, both pure GA and GSA-1 exhibited typical TGA curves, with the first weight-loss stage occurring at 250 °C because of the decomposition of oxygencontaining residues remaining on the rGO. Subsequently, the most dramatic weight drops appeared at temperatures from 500 to 600 C due to the partial oxidation of the carbon components in GSAs. However, for GSA-10 and GSA-30, the final weight losses at 1000 C decreased to 6.13% and 3.7%, respectively. This clearly demonstrated the existence of the SiO2 ceramic constituent that gives the GSAs outstanding thermal stability for either air or inert atmosphere conditions-there was no direct contact between GA and oxygen. Comparatively, 10 wt % SiO2 sol-derived GSA-10 already presented excellent thermal stability with a weight drop in air conditions less than 7% at 1000 °C .

Investigation of Mechanical Properties
Lightweight insulation materials are mostly expected to have good elasticity to make up for possible deformation under complex thermal-mechanical coupling fields [7,8]. As schematically illustrated in Figure 8a, to improve the intrinsic brittleness of SiO 2 ceramics, 3D graphene frameworks were used as a highly porous template for SiO 2 deposition. Here, the sandwiched graphene skeletons served as the reinforcing components to suppress the possible crack propagation of the SiO 2 ceramic, making GSAs synergistically realize good elasticity and high strength. Studies of GSA mechanical properties were conducted via a static compressive machine with a step-loading scheme (compression strain = 20%, 40%, 60%, and 80%) and a loading rate of 1 mm/min.

Investigation of Mechanical Properties
Lightweight insulation materials are mostly expected to have good elasticity to make up for possible deformation under complex thermal-mechanical coupling fields [7,8]. As schematically illustrated in Figure 8a, to improve the intrinsic brittleness of SiO2 ceramics, 3D graphene frameworks were used as a highly porous template for SiO2 deposition. Here, the sandwiched graphene skeletons served as the reinforcing components to suppress the possible crack propagation of the SiO2 ceramic, making GSAs synergistically realize good elasticity and high strength. Studies of GSA mechanical properties were conducted via a static compressive machine with a step-loading scheme (compression strain = 20%, 40%, 60%, and 80%) and a loading rate of 1 mm/min.  Figure 8b shows that the GA could elastically deform with a maximal strain as large as 80%; the primary structure was not damaged, indicating outstanding elasticity to overcome a large deformation of the hierarchical structure on the multiscale. However, pure GA showed a relatively low Young's modulus of 0.023 MPa, the stepped increases of compressive strength (0.0026, 0.0041, 0.0067, and 0.0170 MPa corresponding to strains of 20%, 40%, 60%, and 80%) demonstrated a typical intensification effect as similar as most insulation materials (Table 2) Figure 8b shows that the GA could elastically deform with a maximal strain as large as 80%; the primary structure was not damaged, indicating outstanding elasticity to overcome a large deformation of the hierarchical structure on the multiscale. However, pure GA showed a relatively low Young's modulus of 0.023 MPa, the stepped increases of compressive strength (0.0026, 0.0041, 0.0067, and 0.0170 MPa corresponding to strains of 20%, 40%, 60%, and 80%) demonstrated a typical intensification effect as similar as most insulation materials (Table 2) [31,32]. Comparatively (Figure 8c), with a combination of SiO 2 constituents, GSA-1 showed a nearly 300% increase in Young's modulus (0.074 MPa) and over 200% enhancement of compressive strength (e.g., 0.006 MPa for strain of 20%). However, the elastic region for GSA-1 decreased to 50% elastic strain (Table 3). Both Young's modulus and compressive strength increased with more SiO 2 , but elasticity conversely decreased (Figure 8c,d). For example, the Young's modulus and compressive strength of GSA-20 were 0.307 and 0.0376 MPa, respectively, but the maximal elastic strain dropped to 12% (Figure 8d). Such balance for strength and elasticity attributed to collaborative contributions between elastic contributor GA and strengthening component SiO 2 . Therefore, once the SiO 2 content was larger than that of GSA-20, GSA elasticity was dominated by the intrinsic brittle ceramic with a maximal elastic strain lower than 10% (Table 3). Nevertheless, the strength of GSA-40 at 60% strain was larger than 1 MPa, facilitating promising applications in structural materials with good thermally protective functions ( Table 2).

Strain
Graphene Hybrid SiO 2 Aerogels (GSA-x)  Furthermore, both elastic strain and Young's modulus of the GSAs were directly dependent on the contents of SiO 2 and bulk density (Figure 8f). These are essentially determined by the thickness of the SiO 2 layer deposited over the graphene sheets within GSA microstructures ( Figure 2). Specifically, the density for GSA-1 was about 15.3 mg/cm 3 corresponding to SiO 2 layer thickness of a few nanometers (Figure 2f). In this case, GSAs exhibited the highest elasticity of over 50% but the lowest Young's modulus (less than 0.1 MPa). As density ranged from 82.3 for GSA-5 to 250.3 mg/cm 3 for GSA-20, GSAs presented moderate elasticity (12%-30%) and Young's modulus (0.202-0.307 MPa; the thickness of SiO 2 deposition was enlarged from 9.5 to 52 nm; Figure 2f,g). In contrast, at a density of over 300 mg/cm 3 , GSA-30 elasticity severely dropped to less than 10%, while the mechanical Young's modulus significantly increased up to the 1 MPa order. This corresponded to SiO 2 deposition of up to 3 microns (Figure 2h).
This three-scheme characteristic of elastic strain and Young's modulus relying on bulk densities emphasizes the size effect of the microelement scale on mechanical properties. Fundamentally, versus either pure GA or SiO 2 aerogel, GSAs presented higher elasticity and Young's modulus because of the coordinated enhancement of the two-composite constituents. GA is the reinforcing unit that can suppress crack propagation of brittle SiO 2 . This makes GSAs more elastic under large-scale deformation. Simultaneously, SiO 2 deposition over graphene sheets is the protective layer that can enlarge the inertia moment of the basic pore wall in the microstructure. This can markedly enhance compression resistance via microelement bending deformation. Results suggested a feasible way to create a graphene-based ceramic composite with expected mechanical properties via a multiscale structural design and rational composition process [7,8].

Conclusions
We reported a three-dimensional lightweight GSA aerogel with hierarchical and robust structures via two constituents (i.e., graphene and SiO 2 ) tightly composited together by chemical bonds at the Si-C interface bond. Due to the highly porous architectures and protective SiO 2 accretion, the GSAs showed good thermal-insulation properties, with thermal conductivity as low as 0.026 W m −1 K −1 , and significant improvements in thermal stability (weight losses < 10% at 1000 • C) and flame resistance. The sandwiched graphene interskeletons made GSAs more elastic (recoverable compression strain up to 50%) than pure SiO 2 aerogels under direct crack-propagation suppression; SiO 2 deposition obviously increased strength (up to 1 MPa) by enlarging the bending resistance of the micropore walls. Moreover, the mechanical properties confirmed the size effects on GSA elasticity and compressive strength of the SiO 2 layer thickness from a few nanometers to several microns. This indicated that rational structure design and controllable fabrication on a multiscale create graphene-based ceramic composites with expected mechanical and thermal properties. They show promise for use in industrial equipment, civil architectures, and defense transportation vehicles.