What are the factors that affect the thermal conductivity of thermal conductive materials

1、 Moisture content ratio of substance

Generally, thermal insulation materials have porous structures that are prone to moisture absorption. After the material absorbs moisture, its thermal conductivity increases. When the moisture content is greater than 5% -10%, the increase in thermal conductivity is most pronounced in porous materials.

The principle is that when moisture (including water vapor) is present in the pores of the material, the diffusion of vapor and the movement of water molecules in the pores will play a major role in heat transfer. The thermal conductivity of water is about 20 times higher than that of air, resulting in a significant increase in its effective thermal conductivity. If the water in the pores forms ice, the thermal conductivity of the ice will be higher, resulting in an increase in the thermal conductivity of the material.

2、 Working temperature

Temperature has a direct impact on the thermal conductivity of thermal conductive materials. As the temperature increases, the thermal conductivity of the material will also rise. Because according to physics knowledge, when the temperature increases, the motion of solid molecules in the material will intensify, abbreviated as "thermal motion". The thermal conductivity of air in the pores of the material and the radiation between the pore walls have also increased. However, its impact is not significant within the temperature range of 0-50 ℃, and only for materials at high or negative temperatures should the effect of temperature be considered.

Internal factors affecting thermal conductivity

1. Material type

Different types of thermal conductive materials have different thermal conductivity coefficients. For solid insulation materials with low porosity, the thermal conductivity of the crystalline structure is the highest, followed by the microcrystalline structure, and the glass structure is the smallest. However, for insulation materials with high porosity, the influence of gas (air) on thermal conductivity is the main factor, and the solid part, whether in crystalline or glassy structure, has little effect on thermal conductivity.

2. Bulk density size

Bulk density (or specific gravity, density) is a direct reflection of the porosity of the material itself. Due to the fact that the thermal conductivity of the gas phase is usually lower than that of the solid phase, insulation materials often have a high porosity, that is, a small bulk density. In general, increasing porosity or reducing bulk density will lead to a decrease in thermal conductivity.

3. Heat flow direction

The relationship between thermal conductivity and heat flow direction only exists in anisotropic materials, that is, in materials constructed in different directions.

Fibrous materials can be classified into two types based on their arrangement: those with a direction perpendicular to the heat flow direction and those with a fiber direction parallel to the heat flow direction. The insulation performance is better when the heat transfer direction is perpendicular to the fiber direction 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 conditions, their thermal conductivity is much lower than that of other forms of porous insulation materials.

For anisotropic materials such as wood, when the heat flow is parallel to the fiber direction, the resistance is relatively small; When perpendicular to the fiber direction, the resistance is greater. Taking pine wood as an example, when the heat flow is perpendicular to the wood grain, the thermal conductivity is 0.17W/(m · K), and when it is parallel to the wood grain, the thermal conductivity is 0.35W/(m · K).

Porous materials are divided into two types: solid materials with bubbles and solid materials with slight contact between particles. Thermal insulation materials with a large number or countless multi opening pores have poorer insulation performance than materials with a large number of closed pores because the direction of pore connectivity is closer to parallel to the heat transfer direction.

4. Specific heat capacity

Thermal conductivity=coefficient of thermal diffusion x specific heat x 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 materials is related to the calculation of the required cooling (or heating) for insulation structures during cooling and heating. At low temperatures, the specific heat of all solids varies greatly. At normal temperature and pressure, the mass of air does not exceed 5% of the insulation material, but as the temperature decreases, the proportion of gas increases. Therefore, this factor should be taken into account when calculating insulation materials that work under normal pressure.

For commonly used insulation materials, the impact of apparent density and humidity is the greatest among the above factors.