First, the type of material
Insulation materials (insulation materials) are of different types and have different thermal conductivity. The material composition of the insulation material is different, and its physical thermal properties are different; the thermal insulation mechanism is different, and its thermal conductivity or thermal conductivity is also different.
Even for insulating materials composed of the same substance, the internal structure is different, or the control process of production is different, and the difference in thermal conductivity is sometimes large. For solid insulation materials with low porosity, the thermal conductivity of the crystalline structure is the largest, followed by the microcrystalline structure, and the smallest of the vitreous structure. However, for the heat insulating material with high porosity, the influence of gas (air) on the thermal conductivity plays a major role. The solid portion, whether it is a crystalline structure or a glassy structure, has little effect on the thermal conductivity.
Second, the working temperature
The temperature has a direct influence on the thermal conductivity of various thermal insulation materials, the temperature is increased, and the thermal conductivity of the material increases. As the temperature increases, the thermal motion of the solid molecules of the material increases, and the heat transfer of the air in the pores of the material and the radiation between the walls of the pores also increase. However, this effect is not significant in the temperature range of 0-50 ° C. Only for materials at high temperature or negative temperature, the influence of temperature should be considered.
Third, the moisture ratio
Most of the thermal insulation materials have a porous structure and are easy to absorb moisture. After the material absorbs moisture and moisture, its thermal conductivity increases. When the moisture content is more than 5% to 10%, the increase in thermal conductivity is most pronounced in the porous material.
This is because when there is moisture (including water vapor) in the pores of the material, the diffusion of steam in the pores and the movement of water molecules will play a major role in heat transfer, and the thermal conductivity of water is about 20 times greater than the thermal conductivity of air. Therefore, it causes a significant increase in its effective thermal conductivity. If the water in the pores forms ice, the thermal conductivity of the ice is greater, and as a result, the thermal conductivity of the material is further increased. Therefore, non-hydrophobic insulation materials must be protected from water and moisture during application.
Fourth, the pore characteristics
Under the same porosity, the larger the pore size, the larger the thermal conductivity; the interconnected pores have higher thermal conductivity than the closed pores, and the higher the closed porosity, the lower the thermal conductivity.
Five, the size of the weight
The bulk density (or specific gravity, density) is a direct reflection of the porosity of the material. Since the thermal conductivity of the gas phase is usually smaller than the thermal conductivity of the solid phase, the thermal insulation material tends to have a high porosity, that is, a small bulk density. In general, increasing the porosity or reducing the bulk density will result in a decrease in thermal conductivity.
However, for materials with low apparent density, especially fibrous materials, when the apparent density is lower than a certain limit value, the thermal conductivity will increase, which is due to the increase in porosity when the porosity increases. Thus, the convection effect is enhanced. Therefore, such materials have an optimum apparent density, that is, the thermal conductivity is the smallest at this apparent density.
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