The continuous development of new high-speed space vehicles has put forward an urgent demand for high-temperature resistant， lightweight and high-efficiency thermal insulations. Carbon aerogel not only has the excellent characteristics of low density， high porosity， high specific surface area and low thermal conductivity due to its nanoscale structure， but also has the advantages of ultra-high temperature resistance under inert atmosphere and high infrared specific extinction coefficient.Therefore， it has significant advantages in its application as a thermal insulation for the thermal protection of ultrahigh-temperature parts.In this paper， the research status of the thermal insulation properties of carbon aerogel from the aspects of aerogel density and microstructure （pore size， particle size） based on heat transfer theory were summarized firstly.Then， the researches on the preparation technologies and properties optimization of carbon aerogels for thermal insulations in recent years， which mainly included four major aspects of mechanical property enhancement，ablative resistance/oxidation resistance， low-cost， and superlight and superelastic were reviewed， and the advantages and disadvantages of the four aspects were also summarized. Finally， the future research directions of low-cost and high-performance carbon aerogel thermal insulations are proposed， and its development and application in civil and other special fields are prospected.
A CdS/reduced graphene oxide (rGO) composite aerogel with CdS nanosheets (CdS NSs) grown on an interconnected three-dimensional rGO-based porous network was prepared by one-pot hydrothermal method using L-lysine as a reducing agent and the cross-linker. The results show that its enhanced adsorption toward pollutants owing to its large Brunauer-Emmett-Teller specific surface area and spongy nature, and improved light absorption due to its extremely light weight nature. Moreover, the rGO promotes the photogenerated charge separation. Thus, the CdS/rGO composite aerogel exhibits enhanced activity for photocatalytic degradation.It can be observed that the tetracycline hydrochloride (TC) has been degraded totally after 45 min by irradiated CdS/rGO composite aerogel, and almost all TC has been mineralized after 1.5 h. Moreover, the CdS/rGO composite aerogel also shows high stability and can be easily separated from the reaction systems for recycling. The photocatalytic reduction activity of CdS/rGO composite aerogel shows no obvious decrease after 5 cycles.
Aerogel/fiber composites have shown a wide range of applications in aerospace， defence， environmental management and biomedicine due to their high porosity， low bulk density， high specific surface area and low thermal conductivity.The thermal and mechanical properties and interfacial compatibility of aerogel/fiber composites was reviewed， the heat transfer mechanism， mechanical property enhancement mechanism and interfacial compatibility bonding mechanism was introduced， and the effect of different fiber embedding on the final properties of the composites considering different fiber volume fractions and fiber multiscale i.e. different fiber diameters （aspect ratio） and pore diameters between fibers was summarized in this paper. Finally， the prospective future of the research directions of aerogel/fiber composites were proposed， including heat resistance， improved mechanical properties and material interface bonding.
The pollution of heavy metal ions in wastewater has caused serious harm to human health, and the adsorption method has attracted much attention because of its high efficiency, economy, simplicity, and good selectivity. SiO2 aerogel is a potential adsorbent for removal of heavy metal ions in wastewater due to its high specific surface area (>500 m2/g), high porosity (>80%), controllable surface group and good physical/chemical stability. Herein, the preparation methods of SiO2 aerogel and its effect on microstructure were briefly introduced, focusing on the functionalization methods of SiO2 aerogel and the adsorption performance and factors of functionalized SiO2 aerogel for the adsorption of heavy metal ions in wastewater, and the adsorption mechanism and adsorption kinetics process of functionalized SiO2 aerogel as heavy metal ions adsorbent were analyzed. It was pointed out that the controllable preparation with low cost and short process, effective functionalization and efficient adsorption of various heavy metal ions are the future development directions of SiO2 aerogels as absorbent.
The selectively etch nature balsa wood, leading to a wood sponge with three-dimensional(3D) lamellar structure. After loading a certain proportion of reduced graphene oxide(rGO) and graphene nanosheets(GNP), the graphene-wood sponge(G-WS)/epoxy resin composite was prepared by vacuum impregnation and curing with epoxy resin. After loading graphene via hydrothermal reduction, graphene oxide(GO) is reduced to rGO. During the self-assembly of rGO sheets, GNP are wrapped and connected by rGO sheets by π-π interactions. Meanwhile, G-WS can also maintain the good 3D structure after vacuum impregnation. The lamellar structure inherited from the wood stock that can lead to anisotropic conductivity G-WS with epoxy resin, at loading of 1.45%(mass fraction, the same below), represents a high through-plane thermal conductivity of 1.59 W·m-1·K-1, compared to the neat epoxy matrix, which is equivalent to a significant enhancement 457% of per 1% loading. The lamellar structure made the G-WS can achieve 80% compression and 40% deformation and compression cycles for 100 times without significant deformation.
To optimize the gas transfer, adsorption and photo-generated charge separation in the process of photocatalytic CO2 reduction by g-C3N4, the photocatalytic materials were designed from the aspects of foam pore structure and heterojunction construction. The typical g-C3N4 foam was first constructed using surfactant foaming method, and then Cu(OH)2 nanosheets were loaded to prepare the Cu(OH)2/CNF composites with projects of electroless copper plating and hydrogen oxidation treatment. The structure and photocatalytic properties of the as-prepared samples were investigated. The results show that g-C3N4 foam and Cu(OH)2/CNF all demonstrate developed structures with 3D micron pore frameworks, which is conducive to improving CO2 diffusion and adsorption at dynamics during gas-solid catalytic process. The adsorption amounts of CO2 for g-C3N4 foam and Cu(OH)2/CNF are respectively 3.97 cm3/g and 3.59 cm3/g, which are 2.96 times and 2.68 times respectively higher than that of pure g-C3N4 powder. Moreover, many Cu(OH)2 nanosheets are also formed in the Cu(OH)2/CNF samples which provide a way to simultaneously broaden light absorption and form heterojunction between g-C3N4 and Cu(OH)2. This heterojunction can accelerate the separation of photo-generated e--h+ and make photo-generated electrons transfer from g-C3N4 to Cu(OH)2. As a result, the Cu(OH)2/CNF has demonstrated optimal photocatalytic activity with CO production rate at 11.041 μmol·g-1·h-1, which is 2.76 times and 6.83 times respectively higher than that of g-C3N4 foam and g-C3N4 powder.
With graphite oxide (GO) as the main raw material, combining with carboxymethyl cellulose (CMC), hydrothermal reduction combined ice template method was used to prepare graphene/carboxymethyl cellulose composite aerogel (HGA/CMC), through drying under the environmental pressure and hydrophobic modification. The HGA/CMC was characterized through SEM, FT-IR, XPS and microcomputer controlled electronic universal testing machine, which proves the successful combination between GO and CMC and the effective hydrophobic modification. HGA/CMC can absorb pure oil because of its abundant pore structure, the adsorption capacity of oil is 70.28-172.78 g·g-1, and the higher the oil density is, the greater the oil mass can be adsorbed by aerogel per unit mass. Furthermore, HGA/CMC shows good selective adsorption capacity for floating oil on water, heavy oil on water bottom and emulsified oil in water. HGA/CMC can be recycled by mechanical extrusion, and its adsorption capacity loss is only 15% after 10 times of extrusion regeneration. It is an oily wastewater treatment material with application potential.
Shape-stabilized SiO2 aerogel phase change composites (PCCs) was prepared by physical adsorption method with SiO2 aerogel as support material, and then used for secondary packaging.The optimal adsorption ratio of SiO2aerogel and phase change materials was explored, and microscopic morphology, chemical composition, pore structure, phase change characteristics, thermal reliability, shape stability and thermal insulation performance of composites were also characterized. The results show that the PCCs with 80%(mass fraction) phase change material (LS-80) has the optimization proportion, the composites exhibit shape-stability during the phase change process, and the melting point and melting enthalpy are -15.6 ℃ and 170.2 J/g respectively. The successful adsorption of SiO2 aerogel makes the specific surface area, pore size and pore volume of LS-80 reduced to 59 m2/g, 13 nm and 0.2 cm3/g. After 20 thermal cycles, the latent heat of packaged phase change material is decreased by 13.4%, while the SL-80 is only decreased by 2.8%, which proves good thermal reliability. Besides, the thermal conductivity of the composites is reduced and the thermal insulation capacity is enhanced due to the addition of SiO2 aerogel. The results provide experimental basis for the application of SiO2 aerogel PCCs in the field of cold chain logistics.
Uniaxial compression tests were carried out on mullite fiber reinforced silica aerogel composites in the out-of-plane direction. Influences of different ultimate strains and thermal exposure temperatures on the compression springback behavior and deformation recovery capability were investigated. Internal mechanisms based on the microstructure morphology changes were explained. Phenomenological mechanical models were established respectively for the deformation behavior in the loading and unloading stages. The results show that the compression springback behavior of mullite fiber reinforced silica aerogel composites exhibits nonlinear characteristics. The greater the ultimate strain, the worse the deformation recovery capability. High temperature thermal exposure pre-treatment has an effect on the compression springback property, the higher the thermal exposure temperature, the worse the deformation recovery capability. The aggregation of matrix particle-cluster structure and the formation and collapse of the large size holes are main causes. The phenomenological mechanical model can be used to describe the stress-strain curve of the composites during loading and unloading. The fitting results are in good agreement with the experimental data.
Lightweight and cellular-structured graphene-pyrrole (G-P) aerogels /epoxy composites were prepared basing on the three-step fabrication process which involving infiltration of epoxy resin into G-P aerogels under vacuum atmosphere. The microstructure of G-P aerogels possesses uniform three-dimensional structure, which can also be preserved well in epoxy composite. The three-dimensional interconnected graphene network serves as fast channels for charge carriers. The conductive property of the composite is improved significantly, 67.1 S/m with only 0.23%(mass fraction) filler content (1G-1%P, 1300 ℃). The electromagnetic interference shielding effectiveness (EMI SE) of the composite (1G-1%P, 1300 ℃) can reach 33 dB in the frequency range of 8-12 GHz. More importantly, the G-P aerogel network also enhances the mechanical properties of epoxy matrix. Flexural strength and flexural modulus are increased by 60.93% and 25.98% respectively (10G-5%P, 180 ℃). Implication of the results suggests that the three-dimensional structure is an effective method for preparing composites with both excellent EMI SE and mechanical properties.
4, 4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) were used as monomers. Carbon nanofiber (CNF) was used as the reinforcing agent. CNF reinforced polyimide (PI) composite aerogels were prepared with acidified CNF (a-CNF) via sol-gel process followed by freeze-drying technology. The morphologies, thermal insulation, microwave absorption as well as compression properties of PI composite aerogels were characterized. The results show that the volume of PI composite aerogels is shrunk from 45.52% to 35.32%, and the density is decreased from 0.084 g/cm3 to 0.069 g/cm3 with the increase of a-CNF content. The composite aerogels exhibit bigger pore sizes and wider pore size distribution after the introduction of a-CNF as well. CNF in PI matrix play roles for reducing the shrinkage of PI composite aerogels, thereby the thermal conductivity is reduced. Additionally, the reflection loss (RL) of PI composite aerogel with 15%(mass fraction) of a-CNF (15% CNF/PI) reaches -9.7 dB at 8.3 GHz. This is due to the fact that the introduction of CNF induced the conduction loss and the porous structure of aerogels provides better impedance matching. The compressive strength and modulus of PI composite aerogels with 15% of a-CNF content are increased by approximately 1.5 times and 2 times compared with pure PI aerogel, respectively.
