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Thermal properties and applications of silica aerogel

2019-03-18
Thermal properties and applications of silica aerogel
 
Thermal properties and applications of silica aerogel _ silica aerogel is a synthetic amorphous silica gel, which is significantly different from crystalline silica gel.A silicon molecule consists of one silicon atom and two oxygen atoms.As shown in the figure below, there are two basic forms of silica gel: amorphous silica gel and crystalline silica gel.Crystalline silica gel is defined as one in which the molecules of silica gel are arranged neatly and form a repeatable pattern.If the silica gel molecules are not ordered, it is amorphous silica gel.


 
Silica aerogel, also known as blue smoke and solid smoke, is the lightest solid material known so far and the material with the best thermal insulation performance.Because of its nano porous structure (1 ~ 100 nm), low density (3 ~ 250 kg/m3), low dielectric constant (1.1 ~ 2.5), low coefficient of thermal conductivity (0.013 ~ 0.025 W/(m k)), high porosity (80 ~ 99.8%), high specific surface area (500 ~ 1000 m2 / g), and other characteristics, such as mechanics, acoustics, thermal, optical aspects show the unique nature, in the aerospace, military, petroleum, chemical, mining, communications, medical, building materials, electronics, metallurgy, and many other fields has been widely and are of great application value, known as the magic to change the world.
 
Thermal characteristics and applications of silica aerogel
 
Thermal characteristics of silica aerogel


 
The thermal conductivity of air under normal temperature and pressure is 0.026w /(mk), while the thermal conductivity of aerogel under normal temperature and pressure is generally less than 0.020w /(mk). Under vacuum, the thermal conductivity can be as low as 0.004w /(mk).The reason why aerogel has such good adiabatic properties is related to its high porosity.There are three main ways of heat conduction, gas conduction, solid conduction and radiation conduction.In all three cases, the heat transferred through the gas is very small, so most gases have very low thermal conductivity.The commonly used thermal insulation materials are porous structures, which use air to occupy a part of the volume of solid materials, thus reducing the overall thermal conductivity of the material.

The porosity of aerogel is much larger than that of ordinary adiabatic material, and more than 95% of it is composed of air, which determines that it will have the same low thermal conductivity as air.Moreover, the aerogel contains a large number of holes with the pore diameter less than 70nm, which is the free path of nitrogen and oxygen (the statistical average of the distance traveled by gas molecules in the time between two collisions), so it means that air cannot realize convection in the aerogel, further reducing the gaseous thermal conductivity.The solid framework with very little content in the aerogel is also composed of nanoparticles, and its contact area is very small, so that the aerogel also has a very small solid thermal conductivity.

The thermal radiation conduction of aerogel is mainly the infrared thermal radiation occurring in the 3-5um region. It can effectively block the infrared thermal radiation at room temperature, but with the increase of temperature, the infrared thermal radiation transmissibility increases.In order to further reduce the high-temperature infrared thermal radiation, shading agents, such as carbon black and titanium dioxide, are usually added into the aerogel. The use of shading agents can greatly reduce the infrared thermal radiation at high temperature.