Main Factors Affecting Thermal Conductivity of Thermal Insulation Materials
I. Material Type Different types of thermal insulation materials (thermal insulation materials) have different thermal conductivity coefficients. The physical and thermal properties of thermal insulation materials vary with their material composition. There are differences in thermal insulation mechanism, and their thermal conductivity or coefficient of thermal conductivity are also different. Even for thermal insulation materials made of the same material, the internal structure is different, or the production control process is different, the thermal conductivity is sometimes very different. For solid thermal insulation materials with low porosity, the thermal conductivity of crystalline structure is the largest, followed by micro-crystalline structure, and the vitreous structure is the smallest. However, for thermal insulation materials with high porosity, because gas (air) plays a major role in the thermal conductivity, the solid part, whether crystalline or glassy, has little effect on the thermal conductivity. Second, the working temperature Temperature has a direct influence on the thermal conductivity of all kinds of thermal insulation materials. As the temperature increases, the thermal conductivity of materials increases. As the temperature rises, the thermal motion of the solid molecules of the material increases, and at the same time, the heat conduction of air in the pores of the material and the radiation effect between the pore walls also increase. However, this effect is not significant in the temperature range of 0-50℃. Only for materials at high temperature or negative temperature should the effect of temperature be considered. Iii. moisture content ratio The vast majority of thermal insulation materials have porous structure and are easy to absorb moisture. The thermal conductivity of the material increases when it absorbs moisture. When the moisture content is more than 5%-10%, the increase of thermal conductivity is most obvious in porous materials. This is because when there is water (including water vapor) in the pores of the material, the diffusion of steam and the movement of water molecules in the pores will play a major role in heat transfer, and the thermal conductivity of water is about 20 times greater than that of air, thus causing the effective thermal conductivity to increase significantly. If the water in the pores forms ice, the thermal conductivity of the ice is greater, and the result is that the thermal conductivity of the material is even greater. Therefore, non-hydrophobic heat insulation materials must be waterproof and moisture-proof when applied. IV. Pore Characteristics Under the same porosity, the larger the pore size, the larger the thermal conductivity. Interconnected pores have higher thermal conductivity than closed pores. The higher the closed porosity, the lower the thermal conductivity. Five, bulk density size Bulk density (or specific gravity and density) is a direct reflection of the porosity of the material. As the thermal conductivity of the gas phase is usually lower than that of the solid phase, thermal insulation materials often have very high porosity, i.e. smaller bulk density. In general, increasing porosity or decreasing bulk density will lead to a decrease in thermal conductivity. However, for materials with very low apparent density, especially fibrous materials (such as ultra-fine glass fibers), when their apparent density is lower than a certain limit value, the thermal conductivity will increase instead. This is because the interconnected pores increase greatly when the porosity increases, thus enhancing the convection effect. Therefore, such materials have an optimal apparent density, i.e. the minimum thermal conductivity at this apparent density. Six, material particle size At room temperature, the thermal conductivity of loose granular materials decreases with the decrease of material particle size. When the particle size is large, the gap size between the particles increases, and the thermal conductivity of the air between the particles must increase. In addition, the smaller the particle size, the less the thermal conductivity is affected by temperature changes. VII. Direction of Heat Flow The relationship between thermal conductivity and heat flow direction only exists in anisotropic materials, i.e. materials with different structures in different directions. From the arrangement state, fibrous materials can be divided into two situations: the direction perpendicular to the heat flow direction and the fiber direction parallel to the heat flow direction. The adiabatic performance when the heat transfer direction is perpendicular to the fiber direction is better than when the heat transfer direction is parallel to the fiber direction. In general, the fiber arrangement of fiber insulation materials is the latter or close to the latter. Under the same density condition, its thermal conductivity is much smaller than that of other forms of porous insulation materials. For anisotropic materials (such as wood, etc.), when the heat flow is parallel to the fiber direction, the resistance is small. However, when perpendicular to the fiber direction, the resistance is greater. Take pine as an example, when the heat flow is perpendicular to the wood grain, the thermal conductivity is 0.17 w/(m k), and when parallel to the wood grain, the thermal conductivity is 0.35 w/(m k). Porosity materials are divided into two types: bubble-like solid materials and solid materials with particles slightly contacting each other. The thermal insulation material with a large number or countless open pores has a lower thermal insulation performance than the material with a large number of closed pores because the communication direction of pores is closer to the parallel to the heat transfer direction. VIII. Filling Gas In thermal insulation materials, most of the heat is conducted from the gas in the pores. Therefore, the thermal conductivity of thermal insulation materials is largely determined by the type of filling gas. If helium or hydrogen is filled in cryogenic engineering, it can be regarded as a first-order approximation. It is believed that the thermal conductivity of thermal insulation materials is equivalent to that of these gases, because the thermal conductivity of helium and hydrogen are both relatively large. IX. Specific Heat Capacity Thermal conductivity = thermal diffusion coefficient × specific heat × density. Under the same conditions of thermal diffusion coefficient and density, the higher the specific heat, the higher the thermal conductivity. The specific heat of insulation material is related to the cooling capacity (or heat) required for calculating the cooling and heating of the insulation structure. At low temperatures, the specific heat of all solids varies greatly. Under normal temperature and pressure, the quality of air does not exceed 5% of the heat insulation material, but with the decrease of temperature, the proportion of gas becomes larger and larger. Therefore, this factor should be taken into account when calculating the thermal insulation materials working under normal pressure. For commonly used thermal insulation materials, apparent density and humidity have the greatest influence among the above factors. Therefore, when measuring the thermal conductivity of the material, the apparent density of the material must be measured simultaneously. As for humidity, for most insulation materials, the equilibrium humidity of the material when the relative humidity of air is 80%-85% can be taken as the reference state, and the thermal conductivity of the material should be measured under this humidity condition as much as possible.