Aerogel is a solid material with three-dimensional nano porous structure. It has the characteristics of low density, high porosity, high specific surface area and ultra-low thermal conductivity. It is divided into two categories: inorganic and organic. At present, the aerogel on the market is the SiO2 aerogel which was first studied, and the following is a typical structural sketch of it.
Thermal conductivity is a parameter to measure the thermal insulation effect of materials. The lower the parameter, the better the thermal insulation effect. SiO2 aerogel is the lowest thermal conductivity material in the world, and its thermal conductivity at room temperature can be 0.018W/m. K.
Then, why can its thermal conductivity be so low? Then we need to analyze it from the perspective of heat transfer. Heat transfer methods include: 1. Radiation: objects use radiation and absorption of each other’s infrared to achieve temperature balance; 2. Conduction: direct contact between objects transfers heat in the form of atomic vibration, from high temperature to low temperature; 3. Convection: heat transfer between objects takes fluid (such as air and water) as the medium. For the first time, the radiation heat transfer is negligible at room temperature, and aerogels are also airtight without convection. Therefore, heat transfer is the main factor leading to the heat transfer of aerogel materials. Heat conduction can be divided into gas phase heat conduction and solid phase heat conduction. Because the pore size of aerogels is mainly distributed in the range of 2-50nm, and the average free path of air molecules is 70nm, the mesoporous structure of aerogels greatly restricts the gas phase heat conduction caused by collisions of gas molecules. Let’s look at the solid phase heat transfer. The way of transmission depends on the contact between solid materials, and the specific surface area of aerogels is as high as 1200m2/g. Therefore, in the aerogels, the contact degree of SiO2 particles is very low, and the three-dimensional skeleton also increases the propagation path of solid phase heat conduction to a certain extent. In summary, aerogels have set up obstacles and almost reached the utmost in all heat transfer pathways, so this is what makes their thermal conductivity so low.
Insulation capacity test
We use thermal conductivity to quantify the thermal insulation effect of materials. At present, there are three main methods for measuring thermal conductivity: flat plate method, heat flow meter method and laser flash method. The first two can be used to measure the thermal conductivity at room temperature and low temperature. Generally, more than 5cm samples are required. Laser flash method, after many operations, is not suitable for measuring aerogel materials. These tests can be done by basically all colleges and universities or relevant scientific research institutes. If you want to issue an authoritative test report, you need to find a professional institution to do it.
There is also a device to assess the high-temperature heat insulation effect of materials, that is, the cold surface thermometer. Give a constant temperature below, put the sample on it, measure the temperature on the upper surface, and record the temperature rise curve and equilibrium temperature.