Aerogels can be classified into inorganic aerogels, organic aerogels, hybrid aerogels and composite aerogels. Commonly seen aerogels mainly include silica aerogels, carbon aerogels and silica aerogels. Recently developed aerogels mainly include graphene oxide aerogels, fullerene aerogels and fiber/silica aerogels.
Currently, the common and well-researched ones in the market can be divided into oxide aerogel materials, carbon aerogel materials (with a high-temperature resistance up to 3000℃) and carbide aerogel materials.
1. Oxide aerogel materials
Oxide aerogel materials are prone to crystal transformation and particle sintering in high-temperature zones (＞1000℃), and their temperature resistance is relatively poor. However, they have relatively low thermal conductivity in medium and high-temperature zones (＜1000℃). Oxide aerogel materials mainly include SiO2, Al2O3, TiO2, ZrO2, CuO, etc.
1) SiO2 aerogel materials
SiO2 aerogel is the most studied and mature high-temperature resistant aerogel in the field of thermal insulation. Its porosity can reach 80% to 99.8%, the typical size of the pores is 1 to 100 nm, the specific surface area is 200 to 1000 m2/g, and the density can be as low as 3 kg/m3, with a room-temperature thermal conductivity as low as 12 m.W/(m·K). SiO2 aerogel materials are usually combined with infrared blockers and reinforcing agents to improve the thermal insulation and mechanical properties of SiO2 aerogels, making them not only practical nano-porous super-insulating materials but also having good thermal insulation and mechanical properties. They are mainly used in aerospace, military, electronics, construction, household appliances and industrial pipelines for thermal insulation. Common infrared blockers include silicon carbide, TiO2 (anatase and rutile types), carbon black, potassium hexatitanate, etc.; common reinforcing materials include ceramic fibers, alkali-free ultra-fine glass fibers, polycrystalline mullite fibers, aluminosilicate fibers, zirconia fibers, etc.
2) ZrO2 aerogel materials
Compared with SiO2 aerogel materials, ZrO2 aerogels have lower thermal conductivity at high temperatures and are more suitable for thermal insulation applications in high-temperature segments. They have great application potential as high-temperature thermal insulation and heat preservation materials. The pore size of ZrO2 aerogel materials is smaller than the average free path of air molecules, and there is no air convection in the aerogel. The porosity is extremely high, and the volume ratio of the solid is very low, making the thermal conductivity of the aerogel very low. Currently, there are relatively few reports on the application of ZrO2 aerogels in thermal insulation fields, and researchers mainly focus on the study of the preparation process of ZrO2 aerogels.
3) Al2O3 aerogel materials
Alumina aerogel materials have a nano-porous structure, which makes them have a lighter mass and smaller volume to achieve the same thermal insulation effect. They also have high porosity, high specific surface area and open fibrous structure, and have potential application value in catalysts and catalytic carriers. Alumina aerogels can also be used as high-pressure insulating materials, substrates for high-speed or ultra-high-speed integrated circuits, isolation media for vacuum electrodes and supercapacitors.
2. Carbon aerogel and carbide gel materials
The most significant feature of carbon aerogels is their high-temperature resistance of up to 2000℃ in inert and vacuum atmospheres. After graphitization, the temperature resistance can even reach 3000℃. Moreover, the carbon nanoparticles in carbon aerogels themselves have excellent absorption performance for infrared radiation, thus generating an effect similar to that of infrared blockers, resulting in relatively low thermal conductivity at high temperatures. However, in the presence of oxygen, carbon aerogels oxidize above 350℃, which greatly limits their application in high-temperature thermal insulation fields. With the development of high-oxidation-resistant coatings such as SiC, MoSi2, HfSi2, and TaSi2, coating the surface of carbon aerogel materials with a dense oxidation-resistant coating to prevent further oxygen diffusion will greatly enhance the application prospects of this material. Carbide materials have excellent oxidation resistance, but their thermal conductivity is relatively high. Transforming them into aerogels with a three-dimensional network structure can significantly reduce the thermal conductivity of the material and further improve its thermal insulation performance. Currently, research on carbide aerogels at home and abroad is still relatively limited, especially for the study of well-formed bulk carbide aerogels, which is still in its initial stage. As a highly efficient thermal insulation material,