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Aerogel refers to a Nano-scale porous solid material formed by the sol-gel method, using a certain drying method to replace the liquid phase in the gel with gas. Such as gelatin, gum Arabic, silica aerogel , hair, nails, etc. Aerogels also have the properties of gels, that is, they have swelling, thixotropy, and de-sizing effects. Aerogel is a solid material form, the smallest dense solid in the world. The density is 3 kilograms per cubic meter. The common aerogel is silicon aerogel. There are many types of aerogels, including silicon, carbon, sulfur, metal oxide, metal, and so on. Aerogel is a compound word, where aero is an adjective, which means flying, and gel is obviously a gel. Literally means a flying gel. The gel of any substance can be called an aerogel as long as it can be dried and removed from the internal solvent, but can basically keep its shape unchanged, and the product has high porosity and low density.
Preparation of silica aerogel
Aerogel was originally named by S. Kistler. Because he successfully prepared silica aerogel by supercritical drying method, he defined aerogel as the material obtained by supercritical drying of wet gel, called it is aerogel. In the mid to late 1990s, with the emergence and development of atmospheric drying technology, the generally accepted definition of aerogel in the mid to late 1990s is: no matter what drying method is used, as long as the liquid in the wet gel is absorbed by the gas Instead, while the network structure of the gel remains basically unchanged, the resulting materials are all called aerogels. The structural feature of aerogel is a cylindrical multi-branched Nano porous three-position network structure with high permeability, extremely high porosity, extremely low density, high specific surface area, and ultra-high pore volume rate. Its bulk density is Adjustable within the range of 0.003-0.500 g / cm-3. (The density of air is 0.00129 g / cm-3).

The preparation of aerogel usually consists of a sol-gel process and a supercritical drying process. In the sol-gel process, by controlling the hydrolysis and polycondensation reaction conditions of the solution, nanoclusters of different structures are formed in the solution, and the clusters adhere to each other to form a gel, and around the solid skeleton of the gel Full of liquid reagents remaining after the chemical reaction. In order to prevent the damage of the material structure caused by the surface tension in the micropores during the gel drying process, the supercritical drying process is used to treat the gel, and the gel is placed in a pressure vessel to increase the temperature and pressure to make the liquid in the gel phase change for the supercritical fluid, the gas-liquid interface disappears and the surface tension no longer exists. At this time, the supercritical fluid is released from the pressure vessel, and a porous, disordered, low-density gas with a Nano-scale continuous network structure can be obtained. Material gel.

As a thermal insulation material
The slender Nano network structure of silicon aerogel effectively limits the propagation of local thermal excitation, and its solid-state thermal conductivity is 2-3 orders of magnitude lower than that of the corresponding glassy materials. The Nano-pores suppress the contribution of gas molecules to heat conduction. The refractive index of silicone aerogel is close to 1, and the ratio of the annihilation coefficient to infrared and visible light is more than 100. It can effectively transmit sunlight and prevent infrared heat radiation from ambient temperature, making it an ideal transparent thermal insulation material . , Has been applied in solar energy utilization and building energy saving. By means of doping, the radiant heat conduction of silicon aerogel can be further reduced. The thermal conductivity of carbon-doped aerogel can be as low as 0.013 w / m K at room temperature and pressure, which is the solid material with the lowest thermal conductivity. It is expected to replace polyurethane foam as a new type of refrigerator insulation material. Incorporating titanium dioxide can make silicon aerogel become a new type of high-temperature thermal insulation material. The thermal conductivity at 800K is only 0.03w / mK, and it will be further developed as a new material for military products.
Due to its low sound velocity, silicon aerogel is also an ideal acoustic delay or high-temperature sound insulation material. The material has a large acoustic impedance variable range (103-107 kg / m2 s), and is an ideal acoustic resistance coupling material for ultrasonic detectors. For example, the commonly used acoustic resistance turns Zp = 1. 5 x l07 kg / Piezoelectric ceramics of m2 * s are used as ultrasonic generators and detectors, while the acoustic resistance of air is only 400 kg / m2 * s. Silicon aerogel with a thickness of 1/4 wavelength is used as the acoustic resistance coupling material between piezoelectric ceramics and air. It can improve the transmission efficiency of sound waves and reduce the signal-to-noise ratio in device applications. Preliminary experimental results show that silica aerogel with a density of about 300 kg / m3 as a coupling material can increase the sound intensity by 30 dB. If a silica aerogel with a density gradient is used, a higher sound intensity gain can be expected.
In environmental protection and chemical industry. Nano-structured aerogel can also be used as a new type of gas filtration. The difference from other materials is that the material has uniform pore size distribution and high porosity. It is a high-efficiency gas filtration material. Because the material is particularly large than the table and accumulate. Aerogels also have broad application prospects as new catalysts or catalyst carriers.

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