Silica aerogel has many unique properties in the three-dimensional nanonetwork structure. It has broad application prospects in optics, heat, acoustics, microelectronics, high-power lasers, and other fields. It is another new type of silicon compound functional material after silica.
Normal pressure drying technology
Supercritical drying technology has shortcomings such as high energy consumption, high risks, complex equipment, and difficulty in achieving continuous large-scale production. Compared with supercritical drying technology, atmospheric drying technology has the advantages of simple equipment and low operating costs. Therefore, atmospheric drying technology is the development direction of airtel drying technology. By adding a medium with low surface tension and a surface modifier to the solvent under normal pressure, the gel network structure is enhanced, the capillary force of the gel network is reduced, and the collapse of the gel removal solvent is avoided as much as possible.
According to the drying mechanism of aerogel, the non-supercritical nature of aerogel can be prepared by increasing the strength of the gel network skeleton, improving the uniformity of pores in the aerogel, changing the surface of the aerogel, and reducing the surface tension of the solvent. During gel drying, the increase in capillary pressure is directly related to the surface tension of the solvent in the capillary. Generally speaking, alcohol gel is hydroly zed and condensed, and its network is mainly filled with solvents such as water and ethanol. Due to the high surface tension of water, the additional pressure on the capillaries during the drying process is very high, which is the direct cause of cracks and breaks in aerogel preparation. If changing solvents, use low surface tension solvents instead of water and alcohol. When a solvent with low surface tension evaporates and dries, the pressure is greatly reduced, which is very beneficial for preparing non-supercritically dried aerogels.
Therefore, surfactant solutions can be used instead of surfactant solutions with low surface tension to reduce the additional pressure in the capillary pores. Reduce gel shrinkage during drying and gel collapse during drying. Secondly, various controlled source substances are introduced, the formation conditions of alcohol gel are strictly screened, the raw material ratio and preparation process are optimized, and the density, strength and flexibility of the gel network skeleton are improved. As long as the network structure of the gel is relatively intact and has sufficient strength and elasticity, it is sufficient to resist the damage to the gel caused by the additional pressure of the capillary during the drying process, thereby achieving the drying of SiO2 aerogels.
Freeze drying
There are some problems with the formation of nanostructured aerogels during the freezing process: the fluid solvent is frozen, and the network structure of the aerogel is destroyed as the crystallinity and pressure increase. When ethanol is used as a solvent, ethanol freezes at 160K, nanopores freeze ultrafine particles, and supercooling is the basic factor for freezing liquids; when the solvent freezes, the pressure must be reduced to allow it to sublime. When the solvent is removed, the purity is very high, but high fluidity cannot be obtained because the temperature is low and the vapor pressure is too small, so the solvent takes a long time to evaporate. Freeze drying is a new airgel drying technology that can effectively avoid the collapse of the nanopore structure during drying if the surface temperature of the gel is relatively stable and the surface is enhanced by cold gas convection at the melting point of the liquid. Since freeze-drying of nano silica gel may result in nanopore collapse or even powder formation, it is not possible to prepare aerogels by freeze-drying.
Conductive drying technology
Smith reports a new method of soaking gels in a solvent that does not penetrate the gel structure and heats and dries the gel structure, i.e., conductively dries it. Conductive drying regulates the heating rate by controlling the temperature of the external fluid. As the gel dries, the density of the external fluid decreases even to the fluid surface during post-drying. Research by Smith's team shows that thermal drying is faster and more energy-efficient than traditional drying methods. Still, it isn't easy to select a suitable heating solution, which requires chemical treatment of the gel surface before heating. Therefore, there are few studies on the preparation of aerogel materials by this method. Evaporative drying technology is well suited to prepare silica aerogels in batches at a moderate price. However, evaporative drying is still immature, and materials prepared by evaporative drying are generally limited to silica xerogels.
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