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Ultralight, Recoverable, and HighTemperature-Resistant SiC Nanowire Aerogel Lei Su, Hongjie Wang,* Min Niu, Xingyu Fan, Mingbo Ma, Zhongqi Shi, and Sheng-Wu Guo State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China S Supporting Information *
ABSTRACT: Ultralight ceramic aerogels with the property combination of recoverable compressibility and excellent high-temperature stability are attractive for use in harsh environments. However, conventional ceramic aerogels are usually constructed by oxide ceramic nanoparticles, and their practical applications have always been limited by the brittle nature of ceramics and volume shrinkage at high temperature. Silicon carbide (SiC) nanowire offers the integrated properties of elasticity and flexibility of onedimensional (1D) nanomaterials and superior high-temperature thermal and chemical stability of SiC ceramics, which makes it a promising building block for compressible ceramic nanowire aerogels (NWAs). Here, we report the fabrication and properties of a highly porous three-dimensional (3D) SiC NWA assembled by a large number of interweaving 3C-SiC nanowires of 20−50 nm diameter and tens to hundreds of micrometers in length. The SiC NWA possesses ultralow density (∼5 mg cm−3), excellent mechanical properties of large recoverable compression strain (>70%) and fatigue resistance, refractory property, oxidation and hightemperature resistance, and thermal insulating property (0.026 W m−1 K−1 at room temperature in N2). When used as absorbents, the SiC NWAs exhibit an adsorption selectivity of low-viscosity organic solvents with high absorption capacity (130−237 g g−1). The successful fabrication of such an attractive material may provide promising perspectives to the design and fabrication of other compressible and multifunctional ceramic NWAs. KEYWORDS: ceramic nanowire aerogel, ultralow density, recoverable compressibility, heat resistance, oil absorption under extreme conditions.8,9 However, for further applications, their brittleness should be overcome. SiC nanowires exhibit not only excellent properties of SiC ceramics but also outstanding elasticity, flexibility, high tensile strength, and high Young’s modulus of 1D nanomaterials,10−12 which are usually used for toughening dense and porous ceramics.13,14 Recently, nanoscale one- and two-dimensional structure based aerogels and ultralight materials such as carbon nanotube aerogels,15,16 carbon nanofiber aerogels,17−19 SiO2 nanofiber/polyacrylonitrile (PAN) nanofiber aerogels,20 graphene aerogels,21−23 hollow alumina nanolattices,24 multinanolayer graphene/ alumina metamaterials,25 oxide ceramic (TiO2, ZrO2, yttriastabilized ZrO2, and BaTiO3) nanofiber sponges,26 and BN nanosheet aerogels27 are simultaneously highly compressible and recoverable under large compression strain, owing to their highly porous elastic interconnected three-dimensional (3D) networks and strong yet flexible building blocks. Inspired by this idea, fabricating a 3D SiC nanowire aerogel (NWA) could be one approach to achieve promising properties for widespread applications.
C
eramic aerogels are well known for their low density, high porosity, large surface area, and excellent thermal and chemical stability, showing promising potential to be used as high-temperature thermal insulators, catalyst supports, filters, lightweight structural materials, and hosts for functional materials for various applications.1−5 However, conventional ceramic aerogels are usually constructed by oxide ceramic (e.g., silica and alumina) nanoparticles, and their practical applications have always been limited by the brittle nature of ceramics and volume shrinkage at elevated temperature (e.g., 600 °C for silica aerogels2 and 1000 °C for alumina aerogels5). For example, heat sealing materials used for the door of hypersonic reentry vehicles must withstand reversible compression under large cyclic strain and a hightemperature (>800 °C) oxidation environment owing to the complex aerodynamic heating conditions.6 And catalytic gas combustion also requires a catalyst support to work at temperatures higher than 1000 °C.7 Therefore, ceramic aerogels with the combination of recoverable compressibility and outstanding high-temperature heat resistance are in strong demand. Silicon carbide (SiC) aerogels have excellent high-temperature chemical stability and better heat resistance than oxide ceramic aerogels, which are promising aerogels for applications © XXXX American Chemical Society
Received: December 3, 2017 Accepted: March 2, 2018
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DOI: 10.1021/acsnano.7b08577 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Fabrication process and macroscopic and microscopic structure of the SiC NWA. (a) Schematic diagrams of the SiC NWA growth. Paths of gas flow are marked with pink arrows. The green inner layer is the location where SiC NWA grows. SiO gas is marked in yellow, and CO gas in blue. Siloxane xerogel is marked in brown. Step 1: Nucleation and growth on the graphite substrate. Step 2: Nucleation and growth on the surface of pre-existing nanowires and assembly into a highly porous 3D network layer by layer. Step 3: Detaching the SiC NWA from the graphite. (b) Digital photograph of a piece of SiC NWA with an area of more than 150 cm2. (c) Highly porous 3D nanowire architecture of SiC NWAs and the EDS of the nanowires (inset in (c)). (d) TEM image of SiC nanowires and the SAED pattern (inset in (d)). (e) Corresponding HRTEM image of SiC nanowires in (d). (f, g) SEM image (f), TEM image (g), and HRTEM image (inset in (g)) of a nanowire bundle. (h, i) SEM image (h), TEM image (i), and HRTEM image (inset in (i)) of a nanowire branch.
maintain a pressure balance inside and outside of the crucible, the gases flowed out of the crucible along paths as shown by the pink arrows in Figure 1a and provided Si and C sources for the nucleation and growth of SiC nanowires on the inner surface of the graphite lid. The growth of SiC nanowires might be a catalyst-free procedure, as there is no detectable catalyst at the nanowire tip.28,29 The formation of SiC NWAs could be summarized into three stages (shown by steps 1−3 in Figure 1a). First, SiO gas arrived on the surface of the graphite substrate and reacted with the active graphite site (eq 1) to form the SiC nanowire nucleus.30,31 The nucleus grew into SiC nanowires with a continuous supply and reaction of the SiO and CO gases (eq 2).32,33 Second, subsequent generated nanowires nucleated and grew continuously on the surface of pre-existing nanowires (eq 2). Under the influence of highpressure gas flow, these nanowires assembled into a highly porous nanowire architecture. Third, by detaching the products from the substrate after the furnace cooled naturally, a piece of free-standing paper-like SiC NWA was obtained (Movie S1). The SiC NWA was then treated in a furnace at 800 °C for 10 min to remove the graphite sticking to the SiC nanowire.
Here we prepared a SiC NWA via a facile chemical vapor deposition (CVD) method. The macroscopic-assembled SiC NWA exhibits integrated properties of ultralow density (∼5 mg cm−3), large strain (>70%) recoverable compressibility, excellent fatigue resistance (1000 cycles with a set strain of 60%), energy dissipation performance (an energy loss coefficient of 0.40), refractory performance, high-temperature oxidation and heat resistance, thermal insulation (0.026 W m−1K−1 at room temperature in N2), and high absorption capacity (130−237 g g−1) for organic solvents, all originating from the well-interconnected highly porous 3D nanowire architecture and superior mechanical and chemical nature of SiC nanowires.
RESULTS AND DISCUSSION Preparation and Characterization of SiC NWAs. The preparation of the SiC NWA was conducted in a furnace with a constant argon gas pressure of 0.25 MPa (Figure 1a). A siloxane xerogel was used as the starting material. The xerogel was placed into a cylindroid graphite crucible with a graphite lid as NWA growth substrate. The furnace was heated to 1450 °C at 5 °C min−1 and held at the maximum temperature for 2 h. During this process, the xerogel decomposed, resulting in the generation of SiO and CO gases in the crucible. In order to B
SiO(g) + 2C(s) → SiC(s) + CO(g)
(1)
SiO(g) + 3CO(g) → SiC(s) + 2CO2 (g)
(2)
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Figure 2. Macroscopic morphology and mechanical properties of the SiC NWA. (a) Piece of SiC NWA standing on a dandelion. (b) High flexibility of the SiC NWA. (c) High compressibility of the SiC NWA. (d) σ−ε curve of the SiC NWA at set strains of 20%, 40%, 60%, and 76%. (e) One-thousand loading−unloading fatigue cycles at set ε = 60%. (f) History of the Young’s modulus, maximum stress, and energy loss coefficient as a function of the compressive test cycles.
Figure 1b shows a piece of the paper-like aerogel with an area of more than 150 cm2. The scanning electron microscopy (SEM) image in Figure 1c displays the highly porous nanowire architecture of the SiC NWA. The nanowires are 20−50 nm in diameter and tens to hundreds of micrometers in length. The large bending curvature of the nanowires indicates their excellent flexibility. An energy dispersive spectrum (EDS) (inset in Figure 1c) shows the nanowires are mainly composed of Si and C, confirming the SiC nature of the nanowire, while a small amount of O is due to slight oxidation of the nanowire surface. Transmission electron microscopy (TEM) in Figure 1d, accompanied by its corresponding selected area electron diffraction (SAED) pattern (inset of Figure 1d) and highresolution TEM (HRTEM, Figure 1e), reveals the dense (111) stacking faults distributed in the 3C-SiC single-crystalline structure of the nanowire. Amplified SEM images show two forms of junctions among the nanowires’ architecture: nanowire bundle (Figure 1f) and nanowire branch (Figure 1h). The bundles possibly formed as a result of minimizing the system energy, and the two nanowires in the bundle are tightly adherent to each other by an amorphous oxide layer ∼2 nm in thickness (Figure 1g and Figure S1). The formation of the branch is possibly due to the nucleation of a nanowire on a pre-
existing nanowire (Figure 1i). The large amount of connections at the atomic level makes the present SiC NWA a wellinterconnected, highly porous 3D architecture instead of a simple nanowires stack. Ultralow Density of the SiC NWA. We further explored the macroscopic properties of the SiC NWA. Interestingly, the NWA shows good self-adhesive behavior, which might be due to the physical bond formation between the nanowires by van der Waals forces and electrostatic attraction. The self-adherent bulk aerogel (a detail description of the fabrication process and microstructure is presented in the Supporting Information) is robust enough to keep its original shape after a freefall (Movie S2). Even though we repeated this procedure several times, its shape and size remained the same. A self-adherent bulk aerogel is shown in Figure 2a; it is so light that it can stand on a dandelion. The measured result (Movie S3) verifies its ultralow density of ∼5 mg cm−3 (yielding a porosity of ∼99.8%), which is comparable to those of other ultralight aerogels15,17−20,22,34 and only about half the value of the lightest reported SiC structure.31 High Flexibility and Compressibility of the SiC NWA. Another attractive property is its high flexibility. In stark contrast to the brittle nature of conventional SiC materials or C
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Figure 3. Microstructure evolution of the SiC NWA during the compression and releasing process. (a−d) Compression and recovery process. (a) Before deformation. (b, c) Under deformation. The red dashed lines show the outline of the moving and buckling of the nanowires. (d) Full recovery after removal of the load. (e−h) The deformation process of two nanowire bundles. (e) Original shape. (f, g) Under deformation. The red arrow shows the bending position. (h) Full recovery after removal of the load. The yellow dashed circles show the junctions in the aerogel. (i) Schematic model showing compression and recovery of a single nanowire to illustrate the elastic mechanism of the SiC NWA.
ceramic aerogels, the NWA is flexible enough to be rolled up without any breakage (Figure 2b). This property makes the aerogel easy to collect and store for further use (inset in Figure 2b). The SiC NWA can also be compressed into a tablet and then recovered after releasing the compression (Figure 2c). Figure 2d presents the stress−strain (σ−ε) curves at set strains of 20%, 40%, 60%, and 76%, respectively. For moderate strain (ε < 60%), it shows a complete recovery; with an increase in the compression strain, there is a slight permanent deformation (∼5% at ε = 76%). Like the deformation behavior of some other nanofiber aerogels,17,18,20 the loading process of the NWA contains two different stages: a linear elastic regime at ε < 40% with an elastic modulus of ∼20.6 kPa and a nonlinear regime with σ and slope rising steeply. The aerogel also endured 1000 loading−unloading fatigue cycles at a set ε = 60% with a loading rate of 80 mm min−1 (Figure 2e and Movie S4). This high loading−unloading speed shows a rapid elastic recovery property of the NWA. After 1000 cycles, the NWA almost kept its macroscopic shape, except for a slight permanent deformation of only 6.5%. Meanwhile, a nearly constant maximum stress and Young’s modulus were also observed (Figure 2f). These observations highlight the robust mechanical properties of the SiC NWA. Hysteresis loops between the loading and unloading curves indicate substantial energy dissipation. For the first cycle, the calculated compressive work and energy dissipation are 0.76 and 0.30 mJ mg−1, respectively, yielding an energy loss coefficient of 0.40. After 1000 cycles, a nearly constant energy loss coefficient of ∼0.38 was calculated, which shows the same level as that of a SiO2 nanofiber/PAN nanofiber aerogel and indicates the promising potential for our SiC NWA to be used as energy damping materials.20 To clarify the mechanism of the elasticity, we used SEM to in situ observe the microstructure evolution of our SiC NWA during the compression and releasing process. Figure 3a to d and Motion Graph S1 show a series of SEM images of SiC
NWA during the loading and unloading process. Upon loading, the SiC NWA is largely compressed, and the nanowires move along the compressive direction and deform under the load (Figure 3b and c), resulting in the densification of the aerogel. After releasing the load, the compressed aerogel recovers to its original shape without any breakage with the nanowires recovering their original shapes and positions (Figure 3d). Zooming in on the individual nanowire bundles, elastic buckling (Figure 3e to h and Movie S5) is observed. Because of the large aspect ratio of the nanowires, the nanowires possess more than one deforming position (Figure 3f), resulting in the out-of-plane buckling behavior (Figure 3g). After releasing the load, the nanowires recover their original morphology (Figure 3h), showing the good elasticity and flexibility of the SiC nanowires. Remarkably, the junctions (marked with the yellow dashed circle) among the 3D nanowire architecture is well retained during the loading and unloading process, holding the relative positions of the nanowires in the 3D architecture. Therefore, the reversible compression of the SiC NWA is attributed to the moving and buckling of the SiC nanowires (Figure 3i), originating from the highly porous 3D architecture and strong yet flexible building blocks of the SiC NWA. Tensile measurement was further conducted to characterize the mechanical properties of the SiC NWA. The tensile stress− strain curve in Figure S2 shows the elastomeric tension followed by fast fracture.35 The difference in deformation behavior between compression and tension is due to the different mechanisms of yielding and fracturing, which is universal in highly porous materials.20,35 A detailed discussion is present in the Supporting Information. High-Temperature Chemical and Thermal Stability of the SiC NWA. We further investigated the high-temperature performance of the SiC NWA. Figure 4a illustrates the excellent refractory property of the SiC NWA. A piece of NWA was nonflammable in fire and stable enough to resist red-hot heating of an alcohol lamp for more than 10 min, which makes D
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Figure 4. Thermal and chemical stability tests of SiC NWA. (a) Fire-resistance performance of the SiC NWA. (b) TGA curve of the SiC NWA in air. (c) HRTEM image of a SiC nanowire after oxidation at 900 °C for 1 h. (d) Macroscopic images of a SiC NWA and a 900-NWA. (e) SEM image and EDS (inset in (e)) of 900-NWA. (f) One-hundred loading−unloading fatigue cycles at a set ε = 60% of a 900-NWA. (g) Macroscopic images of a SiC NWA and a 1500-NWA. (h) SEM image of a 1500-NWA. (i) One-hundred loading−unloading fatigue cycles at a set ε = 60% of a 1500-NWA.
Figure 5. Thermal insulation performance of the SiC NWA. (a) Fresh petal protected by a piece of aerogel with a thickness of 10 mm from withering or carbonization under the heating of an alcohol lamp for 10 min. (b) Thermal conductivity of the SiC NWA in N2 at different temperatures.
the aerogel an attractive candidate for fireproofing materials. Simultaneously, accompanied by its highly porous 3D architecture, the NWA is promising for use as a hightemperature filter for the filtration of soot produced by fossil fuel burning (Figure S3). The thermal stability of the SiC NWA in air was further investigated by thermogravimetric analysis (TGA, Figure 4b). Below 100 °C, there is a weight loss of ∼5
wt %, which is mainly induced by the evaporation of absorbed water; from 100 to 900 °C, the weight shows no obvious change; after 900 °C, the weight ratio increases rapidly, owing to the passive oxidation (eq 3) induced amorphous SiO2 layer, which was verified by the HRTEM image of SiC nanowires after being treated at 900 °C in air for 1 h (Figure 4c). The well-coated oxide layer can protect the nanowires from further E
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Figure 6. Oil and organic solvent absorption properties of the SiC NWA. (a) Absorption process of kerosene (colored with Sudan III for clear presentation) within 5 s. (b) Recyclability of the absorption process. (c) Absorption capability of the SiC NWA for various organic liquids. (d) Recyclability of the SiC NWA for absorption of kerosene when using the squeeze method. (e) Recyclability of the SiC NWA for absorption of ethanol when using the direct combustion method.
Thermal Insulation Performance of SiC NWA. The SiC NWA also exhibits an excellent thermal insulation performance. As shown in Figure 5a and Movie S6, a piece of aerogel with a thickness of 10 mm can effectively protect the fresh petal from withering or carbonization under the heating of alcohol lamp for 10 min. The thermal conductivity of the SiC NWA in N2 at different temperatures is shown in Figure 5b. The roomtemperature thermal conductivity is ∼0.026 W m−1 K−1. This value is comparable to those of other thermal insulators.20,38 We attributed this low thermal conductivity to the low density of the NWA, as well as the size confinement enhanced boundary scattering and phono-defect scattering caused by dense stacking faults along the nanowire’s length.20,39,40 With the increase in temperature, the thermal conductivity increases, mainly due to the increasing thermal radiation of the gas at high temperature.31,38 These results suggest that our SiC NWA is a promising high-temperature thermal insulator. High Absorption Capacities for Organic Solvents and Oils. The wettability of SiC NWA can transform from hydrophilicity to hydrophobicity by fabricating an oilimpregnated surface (Figure S4). The oil-modified NWA exhibits an adsorption selectivity of low-viscosity organic solvents. As shown in Figure 6a and Movie S7, a piece of oilmodified SiC NWA absorbed the kerosene (dyed with Sudan III) completely in 5 s, resulting in clean water originally polluted by the oil. Due to its low density and hydrophobicity,
oxidation, resulting in the gradually smaller slope ranging from 1100 to 1200 °C. Above 1200 °C, the negative slope is due to the gradually destroyed surface oxide layer and active oxidation (eq 4) of SiC at higher temperature.36,37 2SiC(s) + 3O2 (g) → 2SiO2 (s) + 2CO(g)
(3)
SiC + O2 (g) → SiO(g) + CO(g)
(4)
The NWA was also well preserved without any visible damage after being treated for 2 h at 900 °C in air (Figure 4d, marked as 900-NWA) or at 1500 °C in argon (Figure 4g, marked as 1500-NWA). As shown in Figure 4e and h, their microstructures were still highly porous networks similar to that of the as-prepared aerogel. Cyclic compressive tests at a set ε = 60% were further conducted on the 900-NWA (Figure 4f) and the 1500-NWA (Figure 4i). Both the 900-NWA and 1500NWA possess excellent cyclic compressive stability, although the max stresses decreased after heat treatment. After 100 cycles, no permanent deformation was found in the 900-NWA and only ∼5.5% in the 1500-NWA, which is comparable to that of the as-prepared SiC NWA. The excellent high-temperature stability of the NWA is attributed to the intrinsic hightemperature chemical and thermal stability of the SiC ceramic. These observations indicate the potential of our SiC NWA to be used in high-temperature conditions. F
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ACS Nano the SiC NWA can float on the water surface after absorbing all the kerosene, which indicates a simple yet useful approach for cleaning up water contaminated by oils. Figure 6c presents the absorption capacities (Q, the ratio of the weight after full absorption to the original weight of the SiC NWA) of the SiC NWA to various organic pollutants in daily life or industry, such as commercial petroleum products, hydrocarbons, alcohols, and amines. The SiC NWA exhibits very high absorption capacities for all of these organic liquids, with 130−237 g g−1, which is higher than those of many other typical carbon nanostructures based absorbents.15,17,18,41,42 The specific BET surface area of the NWA is ∼78 m2 g−1 (Figure S5), which is comparable to those of one-dimensional nanomaterials but lower than those of graphene-based aerogels (for example, 272 m2 g−1 for ultraflyweight graphene aerogels).17,21 Thus, the absorption mechanism of our SiC NWA might be similar to that of a carbon nanofiber aerogel, that is, physical absorption of organic molecules.18 And the absorbed organic pollutants are stored in the nanowire-constructed pores. Thus, the high absorption capacity of the SiC NWA is due to its high porosity and low density. Due to the excellent elasticity and fire resistance of our NWA, the absorbed organic solvents could be collected by squeezing the NWA (Figure 6b) and direct combustion in air for heating (inset in Figure 6e). As shown in Figure 6b and Movie S8, the squeezed NWA deformed into a tablet and can recover almost its original size and shape after reabsorbing. This process was repeated 10 times without any decrease in absorption capacity (Figure 6d). Direct combustion of the absorbed ethanol in air was also applied for 10 cycles (Figure 6e), after which no obvious absorption capacity changes were observed. In comparison, an 11% decrease of the absorption capacity occurred to carbon nanofiber aerogels due to their mass loss during combustion.18 The high absorption and excellent recyclability make the SiC NWA a promising candidate for practical applications in the area of sewage treatment and environmental protection.
magnetic stirring to obtain the siloxane sol. Ethanol, deionized water, and nitric acid were used as the solvent, hydrolytic agent, and gelation catalyst, respectively. After cross-linking for 2 h, the sol transformed into silane gel and then dried at 100 °C for approximately 2 h to form the siloxane xerogel. Characterization. The microscopic morphology of samples was observed by field emission scanning electron microscope (FESEM, Quanta 600, FEI, United States) and field emission transmission electron microscope (FETEM, JEM-2100, JEOL, Japan). The elemental composition of samples was detected by an energy dispersive spectrometer equipped on the FESEM. The density of the SiC NWA was calculated from measured geometries and mass of the samples. TGA was performed with a DSC/TGA thermal analyzer (SDT-Q600, TA, USA) at a heating rate of 10 °C min−1 with a temperature rise from 25 to 1400 °C under an air environment. The adsorption and desorption curves were obtained on an ASAP 2020 Plus HD88 instrument (Micromeritics Instrument Corporation, Norcross, USA) at 77 K. Mechanical Property Test. The compression tests of the asprepared SiC NWA at set strains of 20%, 40%, 60%, and 76% and the 100 compression cyclic tests of 900-NWA and 1500-NWA at a set strain of 60% were conducted using a TA-Q800 DMA instrument at 2 mm min−1. The 1000 loading−unloading fatigue test of as-prepared SiC NWA at a set strain of 60% was measured using an Instron 5943 testing system at 80 mm min−1. In situ observation of the microstructure evolution during the compression and releasing process was conducted in SEM. The tensile test was performed on a TA-Q800 DMA instrument at 2 mm min−1. Thermal Conductivity Test. Thermal transport properties were tested by a Hot Disk TPS 2500 S instrument using 130 mW output power in the transient mode. A 9.7 mm mica sensor was used, and the samples were probed in the isotropic mode to obtain information on thermal conductivity at elevated temperatures. The samples used for the test were 40 mm in diameter and 3.5 mm in thickness.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08577. EDS results of the amorphous layer conjunction among nanowire bundles, demonstration of using SiC NWA for high-temperature filtration, tensile behavior, water contact angle measurements, specific BET surface area, detailed preparation process of SiC NWA (PDF) Movie S1. Detaching SiC NWA from graphite substrate (AVI) Movie S2. Macroscopic morphology of self-adherent bulk SiC NWA after a freefall (AVI) Movie S3. Measuring the density of SiC NWA (AVI) Movie S4. Cyclic compression test at set strain of 60% at rate of 80 mm min−1 (AVI) Motion Graph S1. Motion graph for the microstructure evolution of SiC NWA during compression and releasing process (AVI) Movie S5. In situ observation of the microstructure evolution of nanowire bundles during compression and releasing process (AVI) Movie S6. Demonstration of the thermal insulation property of SiC NWA (AVI) Movie S7. Fast absorbing of oil (AVI) Movie S8. Recovering the original size and shape from squeezed stage (AVI)
CONCLUSION In summary, we have prepared a SiC NWA via a facile CVD method under an argon atmosphere with a constant gas pressure of 0.25 MPa. The SiC NWA with a robust highly porous 3D architecture was constructed by a large amount of well-interconnected SiC nanowires. It offers integrated properties of ultralow density (∼5 mg cm−3), high porosity (∼99.8%), large strain (>70%) recoverable compressibility, excellent fatigue resistance, efficient mechanical shock energy dissipation (an energy loss coefficient of 0.40), refractory performance, thermal insulation (0.026 W m−1K−1 at room temperature in N2), high-temperature oxidation and heat resistance, and high absorption capacity (130−237 g g−1) for organic solvents. These excellent properties make our SiC NWA a promising material to be used as a fireproofing material, high-temperature thermal insulator and catalyst support, and organic solvents and oil absorbents for environmental protection and sewage treatment. METHODS Synthesis of the Siloxane Xerogel. Methyltrimethoxysilane (MTMS, ≥98% purity, Meryer Chemial Technology Co. Lth., China) and dimethyldimethoxysilane (DMDMS, ≥98% purity, Meryer Chemical Technology Co. Ltd., China) with a weight ratio of 4:1 were mixed with ethanol, deionized water, and nitric acid with G
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ACS Nano
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AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Su: 0000-0003-1707-3102 Hongjie Wang: 0000-0003-0043-2885 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51472198 and No. 51772237). The authors acknowledge Prof. Junmin Qian for the helpful suggestions to the present work, and Dr. Chaowei Guo from CAMPNANO of Xi’an Jiaotong University for the help in in situ observation of the microstructure evolution of the SiC NWA. REFERENCES (1) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Silica Sol as a Nanoglue: Flexible Synthesis of Composite Aerogels. Science 1999, 284, 622−624. (2) Dmmerling, A.; Gross, J.; Gerlach, J.; Goswin, R.; Reichenauer, G.; Fricke, J.; Haubold, H. G. Isothermal Sintering of SiO2-Aerogels. J. Non-Cryst. Solids 1992, 125, 230−243. (3) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243−4266. (4) Leventis, N.; Sotiriou-Leventis, C.; Zhang, G. H.; Rawashdeh, A. M. M. Nanoengineering Strong Silica Aerogels. Nano Lett. 2002, 2, 957−960. (5) Saliger, R.; Heinrich, T.; Gleissner, T.; Fricke, J. Sintering Behaviour of Alumina-Modified Silica Aerogels. J. Non-Cryst. Solids 1995, 186, 113−117. (6) Dunlap, P. H.; Steinetz, B. M.; Curry, D. M.; DeMange, J. J.; Rivers, H. K.; Hsu, S. Y. Investigations of a Control Surface Seal for Reentry Vehicles. J. Spacecr. Rockets 2003, 40, 570−583. (7) Zarur, A. J.; Ying, J. Y. Reverse Microemulsion Synthesis of Nanostructured Complex Oxides for Catalytic Combustion. Nature 2000, 403, 65−68. (8) Leventis, N.; Sadekar, A.; Chandrasekaran, N.; Sotiriou-Leventis, C. Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks. Chem. Mater. 2010, 22, 2790−2803. (9) Zu, G.; Shen, J.; Zou, L.; Wang, W.; Lian, Y.; Zhang, Z.; Du, A. Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels. Chem. Mater. 2013, 25, 4757−4764. (10) Zekentes, K.; Rogdakis, K. SiC Nanowires: Material and Devices. J. Phys. D: Appl. Phys. 2011, 44, 133001. (11) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Nanobeam Mechanics: Elasticity, Strength, and Toughness of Nanorods and Nanotubes. Science 1997, 277, 1971−1975. (12) Cheng, G.; Chang, T.; Qin, Q.; Huang, H.; Zhu, Y. Mechanical Properties of Silicon Carbide Nanowires: Effect of Size-Dependent Defect Density. Nano Lett. 2014, 14, 754−758. (13) Yang, W.; Araki, H.; Tang, C.; Thaveethavorn, S.; Kohyama, A.; Suzuki, H.; Noda, T. Single-Crystal SiC Nanowires with a Thin Carbon Coating for Stronger and Tougher Ceramic Composites. Adv. Mater. 2005, 17, 1519−1523. (14) Yoon, B. H.; Park, C. S.; Kim, H. E.; Koh, Y. H. In situ Synthesis of Porous Silicon Carbide (SiC) Ceramics Decorated with SiC Nanowires. J. Am. Ceram. Soc. 2007, 90, 3759−3766. (15) Gui, X.; Wei, J.; Wang, K.; Cao, A.; Zhu, H.; Jia, Y.; Shu, Q.; Wu, D. Carbon Nanotube Sponges. Adv. Mater. 2010, 22, 617−621. (16) Kim, K. H.; Oh, Y.; Islam, M. F. Graphene Coating Makes Carbon Nanotube Aerogels Superelastic and Resistant to Fatigue. Nat. Nanotechnol. 2012, 7, 562−566. H
DOI: 10.1021/acsnano.7b08577 ACS Nano XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsnano.7b08577 ACS Nano XXXX, XXX, XXX−XXX
