Gel is a very common substance, and our favorite jelly is a typical gel. Most gels are formed from solutions or sols by gelation. The colloidal particles or polymer particles in the solution crosslink under certain conditions to form a spatial network structure, and the solvent fills the pores of the space network to form a gel. A gel whose solvent is water is called a hydrogel, and a gel whose solvent is a gas is naturally an aerogel.
As early as 1931 , SS Kistler succeeded in preparing silica aerogels by hydrolyzing water glass. Ironically, the original intention of this creation turned out to be a bet between him and his colleague, C. Learned , who had a way to replace the liquid components of the gel with gas but not to change the porous network structure. Although the original intention may seem like a joke, their thinking is indeed correct. Even now, the principle of aerogel preparation remains the same. Finally, Kistler won this bet by extracting the liquid components from the gel by supercritical drying. Since then, he succeeded in preparing aerogel based on aluminum, chromium or tin oxide. Today, aerogel has developed into a huge family. According to the materials constituting the spatial network structure, aerogels can be classified into silicon gas coagulants, carbon aerogels, sulfur aerogels, metal aerogels and oxide aerogels, and the like. But in fact, most of the volume in aerogel is air, a silica aerogel invented by the materials scientists of the â€œJet Propulsion Laboratoryâ€ under NASA. 99.8% of the volume is air. Density is comparable to air and is known as "solidified smoke."
So where does the warmth of aerogel winter clothes come from? In the field of materials science there is a classic quotation - structure determines performance. The aerogel microstructure is shown above. We can describe the aerogel structure with several geometric parameters such as porosity, pore size and fiber diameter. These parameters also determine the gas Thermal conductivity of the gel. There are three types of heat transfer in nature: heat conduction, heat radiation, and thermal convection. Let me take a look at these three kinds of heat transfer methods in aerogel.
First of all, the heat conduction is mainly done along the fibers that make up the spatial network structure, and the diameter of the fibers in the aerogel is usually very fine, basically reaching a hundred nanometer scale or less, and the heat transfer ability is very low; furthermore, the fibers are arbitrary in space. Curved stacks further extend the heat transfer path, resulting in the aerogel's thermal conductivity approaching the lowest limit.
Secondly, thermal radiation in the aerogel is constantly scattered by the interface between the fiber and the pore. Since the pores in the aerogel are all nanopores and the porosity is extremely high, it is equivalent to adding numerous scattering centers to the material. Limit the free path of radiation, resulting in the aerogel's heat radiation ability is close to the lowest limit.
Finally, because most of the aerogels are air, thermal convection should be the main way of heat transfer. However, theoretical studies have shown that when the pore size in aerogel is less than 70 nm , the air molecules lose their ability to flow freely due to the interaction between air and fibers, but instead are relatively attached to the fibers, resulting in aerogels. The thermal convection capacity is also very low. This is also the biggest advantage of aerogel winter clothing compared to conventional sweaters, cotton or down coats. The traditional winter clothes are also loose air porous network structure wrapped in air, but its porosity is significantly lower, the pore size is relatively large, the heat convection ability is significantly stronger than aerogel winter clothing. In addition, when the cotton or down coat is wetted by rain and snow, the spatial network structure composed of cotton wool or down is compressed by the weight and surface tension of the water, and the air therein is squeezed out, which greatly reduces the ability to keep warm. The surface-treated air-curing agent material has a certain degree of hydrophobicity, and can maintain the structure and heat-retaining ability in rain and snow weather. In fact, aerogel winter clothes can maintain the internal temperature at room temperature near liquid nitrogen, which can meet the astronauts' need for warmth in space.
It can be said that aerogel winter protection clothing is a true " NASA technology for civilian use," but aerogel's ability is far more than just winter clothes. In addition to thermal insulation applied to aerospace equipment or high-precision scientific instrument insulation, the ultra-high porosity has strong adsorption capacity and can be used for the absorption and removal of pollutants in air and water. In addition, the mechanical properties brought about by the fiber network structure even allow aerogels to capture the cometary debris in space. At present, research papers that use aerogel as a key word are still developing at a rate of over a thousand articles a year. We will wait and see what aerogel will bring us in the future.
Author: Zhao Meng, edited by the Chinese rescue equipment network.
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