What are the factors that affect the thermal conductivity of insulating refractory bricks?

Thermal conductivity is one of the important indicators of thermal insulation refractory bricks. Thermal conductivity is the thermal conductivity λe, which mainly reflects the thermal conductivity of materials. At high temperatures, the thermal conductivity of most materials will increase with temperature. In order to improve the working efficiency of the kiln and reduce the heat loss, people have been devoted to the research on thermal insulation refractory bricks with small thermal conductivity for many years. Because most insulating refractory bricks are composed of gas phase and solid phase, the thermal conductivity of insulating refractory bricks is related to the chemical composition, crystalline state, porosity and pore diameter of the raw materials used.
 

(1) Effect of porosity on thermal conductivity
 

It is generally believed that the thermal conductivity of gas is much smaller than that of solid. The high porosity of the heat insulation material effectively blocks the heat transfer and makes a major contribution to reducing the thermal conductivity of the material. The size of the porosity plays an important role in the thermal insulation performance of the material. The higher the porosity, the smaller the area occupied by the pore walls in the same volume of insulating refractory bricks, and the lower the thermal conductivity of the insulating refractory bricks. However, the increase in porosity is also limited. If the porosity is too high, the solid content of the insulating refractory bricks in the same volume is less, the mechanical properties of the material are poor, and the use requirements cannot be met.
 

In addition, the properties of the pores also have a great influence on the thermal conductivity of the material. Thermal insulation refractory bricks with high open porosity and connected porosity have higher thermal conductivity than materials with high closed porosity at the same volume density.
 

(2) The effect of pore size on the thermal conductivity of thermal insulation materials
 

When the porosity is constant, the smaller the pore size, the lower the thermal conductivity of the material and the better the thermal insulation performance. This is because at the same porosity, the smaller the pore size inside the material, the greater the number of pores within the same volume of material. -As the pore size decreases, the movement of gas molecules inside the pores is restricted, the amount of heat convection heat generated inside the pores decreases, and the thermal conductivity of the material naturally decreases. On the other hand, the increase in the number of pores increases the area of ​​the pore walls in the same volume, increases the heat transfer distance in the solid phase, that is, extends the heat transfer time, and reduces the thermal conductivity of the material. When the pore diameter is greater than 0.1μm, the thermal conductivity of the material conforms to the Loeb model.
 

When the pore size is greater than 1μm, the thermal conductivity is proportional to the cubic factor of the pore shape factor, pore size, radiation constant, thermal emissivity, and temperature. When the pore size is less than 0.1 μm, the change in thermal conductivity of the material will no longer follow the Loeb model. If the pore diameter is as small as the free path of the gas molecule movement is 50mm, the movement of the gas molecule is restricted, and it will be adsorbed on the pore wall without movement, and heat transfer will no longer occur.

(3) The effect of solid relative to the thermal conductivity of the material
 

The mode of heat transfer that occurs in the solid phase of the material is mainly conduction heat transfer, which is the thermal vibration of the crystal lattice. If the thermal conductivity and heat capacity of the raw material are small, the thermal insulation capacity of the thermal insulation refractory brick will increase. Below 1000C, as the temperature increases, the thermal conductivity of most materials generally shows a downward trend; above 1000C, as the temperature increases, the thermal conductivity of most materials shows an upward trend. In addition, the thermal conductivity of non-oxides is generally higher than that of oxides. Among oxides, the thermal conductivity of aluminosilicate materials is smaller than that of pure oxide materials. Because substances with a complicated lattice structure generally have a messy internal atom distribution and slow heat transfer, the thermal conductivity is small.
 

At low temperature, the thermal conductivity of the glass phase is lower than that of the crystalline phase, because the disorder of the arrangement of atoms in the glass phase is higher, and the resistance to thermal vibration is greater. However, the viscosity of the glass phase decreases with increasing temperature, the resistance of atomic thermal vibration decreases, and the thermal conductivity increases accordingly. For the crystalline phase, after the temperature rises, the amplitude of atomic vibration increases, which reduces the free path of motion, but reduces the thermal conductivity.