IntroductionSilica aerogels are highly porous materials with interconnected three-dimensional networks of silica particles, which are typically obtained via removing the liquid in gels under supercritical conditions. Silica aerogels are widely applied in the fields of thermal insulation, adsorption, catalysis, and energy storage due to their ultralow density, large surface area, and unique nanoporous structure. However, silica aerogels are fragile at relatively low stresses due to the existence of rigid skeleton consisting of weak linking of silica particles. The inherent brittleness and poor reliability restrict the application of aerogel materials to a certain extent. In this paper, the preparation and properties of SiO₂ nanotube aerogels (SiO₂-NTA) with a three-dimensional nanotubular network structure were investigated.MethodsFor the prepatation of carbon aerogels (CA) as sacrificial templates, 3.24 g of resorcinol and 4.4 mL of formaldehyde solution were dissolved in 95.5 mL of deionized water. The solution was stirred for 1 h, and 3.9 mg of sodium carbonate was added. After further stirring for 30 min, the precursor solution was poured into a container, sealed and cured in an oven at 85 ℃ for 72 h to obtain a resorcinol-formaldehyde wet gel. The gel was washed with ethanol for several times to completely remove the residual solvents. The wet gel was dried using supercritical carbon dioxide to obtain the resorcinol-formaldehyde aerogel, which was subsequently placed in a tubular furnace and carbonized in a nitrogen atmosphere to obtain the CA templates.SiO₂-NTA were prepared via chemical vapor deposition (CVD) with argon as carrier gas, tetraethoxysilane and aqueous ammonia solution as silicon and oxygen sources, respectively. Firstly, tetraethoxysilane source tank was heated to 60 ℃. Then, CA templates were placed in CVD chamber, and was heated to different deposition temperatures (i.e., 100, 150, 200, 250 ℃ and 300 ℃), respectively. C/SiO₂ composite aerogels (CA@SiO₂) were prepared via introducing tetraethoxysilane (5 mL/min) and aqueous ammonia (15 mL/min) into the chamber, and depositing for 20 h at a target pressure. Finally, CA@SiO₂ was calcined in air at 500 ℃, and SiO₂-NTA was obtained after removing the templates.The microstructural features and pore morphology of aerogels were determined by a model SEM4000Pro scanning electron microscope (SEM) and a model TH-F120 transmission electron microscope (TEM). The specific surface area and total pore volume were measured based on Nitrogen adsorption-desorption isotherms in a model V-Sorb X800 N₂ adsorption analyzer. The compressive strength and Young's modulus of SiO₂-NTA specimens were measured by a model UTM-4103 electronic universal testing machine at a crosshead speed of 0.5 mm/min.Results and discussionSiO₂-NTA prepared by CVD method has a three-dimensional nanotubular network structure. The inner part of the skeleton is hollow, and the average thickness of tube wall ranges from 3 nm to 7 nm. The deposition temperature and pressure have a significant influence on the microstructure as well as density of SiO₂-NTA. The deposition rate and weight gain of aerogels increase gradually, and accompany the skeleton coarsening process as the deposition temperature increases from 100 ℃ to 250 ℃, which inhibits the shrinkage of the SiO₂-NTA after calcination. Silica preferentially deposits on the outside of the template as the deposition temperature further increases to 300 ℃, resulting in the blocking of SiO₂-NTA surface. The density of SiO₂-NTA firstly decreases and then increases as the temperature increases, which is strongly dependent on both weight gain and linear shrinkage. Similarly, SiO₂-NTA density ranges from 65 mg/cm³ to 51 mg/cm³ with the increase of pressure. The preparation of SiO₂-NTA based on CVD method is essentially a reaction transport process of multi-component fluids in a porous template. It is thus necessary to balance the mass transfer rate and reaction rate to obtain SiO₂-NTA with a homogeneous microstructure.Compared with CA, the specific surface area and total pore volume of CA@SiO₂ decrease significantly because of the preferential deposition of silica at defects and junctions of framework. After calcination in air, the feature mesopore is formed via removal of the carbon skeleton, and the specific surface area and total pore volume of SiO₂-NTA can reach 592.717 m²/g and 1.047 cm³/g, respectively. The high pore volume and nanotubular skeletons lead to the low density of SiO₂-NTA. SiO₂-NTA shows relatively excellent mechanical properties. The elastic modulus and the stress at 60% strain of nanotube aerogels are 0.70 MPa and 0.83 MPa, respectively, and the specimens have no obvious fatigue damage after 100 cycles of compression, further indicating that SiO₂-NTA has an outstanding structural reliability in a certain range of strain. In addition, SiO₂-NTA also exhibits an excellent machinability while keeping a low density, compared with conventional silica aerogels, and a thin-walled aerogel specimen with a wall thickness of 200 μm can be obtained via processing.ConclusionsSiO₂-NTA with a three-dimensional nanotubular network structure could be prepared via CVD of silica coating on the skeleton of CA sacrificial templates. The deposition temperature and pressure had a significant influence on the microstructure as well as density of SiO₂-NTA. Within a certain range, the density of SiO₂-NTA reduced as the deposition temperatures and pressure increased because of the decrease of linear shrinkage after calcination. However, as the temperature further increased, silica preferentially deposited on the outside of the template, resulting in the blocking of SiO₂-NTA surface. The preparation of SiO₂-NTA based on CVD method was essentially a reaction transport process of multi-component fluids in porous template. It is necessary to balance the mass transfer rate and reaction rate to obtain SiO₂-NTA with a homogeneous microstructure. The specific surface area and total pore volume of SiO₂-NTA could reach 592.717 m²/g and 1.047 cm³/g, respectively. The high pore volume and nanotubular skeletons led to the low density of SiO₂-NTA. SiO₂-NTA had relatively excellent mechanical properties. The elastic modulus and the stress at 60% strain of nanotube aerogels were 0.70 MPa and 0.83 MPa, respectively, and the specimen could be significantly compressed without obvious brittle fracture. Moreover, SiO₂-NTA had a superior machinability due to its outstanding structural reliability, which could be processed by turning, milling and laser.