The thermal conductivity of ceramic fiber board is the sum of the three heat transfer effects of conduction heat transfer inside the solid fiber and the fiber contact part in the ceramic fiberboard, air convection heat transfer in the pores, and radiation heat transfer between the pore walls composed of solid fibers, etc., so it is also called equivalent thermal conductivity or apparent thermal conductivity. The following is a brief analysis of the above 8 factors that affect the thermal conductivity of ceramic fibre board.
1. Use temperature
Generally, the thermal conductivity of ceramic fiberboard increases with increasing temperature. The reason is that the radiation heat transfer between the pore walls, the convection heat transfer of air in the pores, and the heat conduction inside the solid fiber and the fiber contact part all increase proportionally due to the increase in temperature and the enhanced thermal motion of gas and solid molecules. When the temperature rises to above 800℃, the ceramic fiberboard is mainly radiation heat transfer, and the higher the temperature, the greater the proportion of radiation heat transfer.
2. Porosity and pore structure and properties
Porosity refers to the ratio of the pore volume in the ceramic fiber board to the total volume of the ceramic fiberboard, expressed as a percentage. The pores of the ceramic fibre board are filled with air, and the thermal conductivity of air at room temperature is only 0.025w/(m.k), which is much lower than the conduction heat transfer of the ceramic fiber solid. The ceramic fiber plate is a mixed structure composed of solid fibers and air, with a porosity of more than 80%. A large amount of low thermal conductivity air is filled in the pores, destroying the continuous network structure of the solid molecules, thereby obtaining excellent insulation performance. The above analysis shows that the thermal insulation and energy-saving function of the ceramic fiber sheet is mainly to utilize the insulation effect of the air in the pores.
The pore structure and properties mainly affect the convective heat transfer of the air in the ceramic fiberboard. The larger the pore diameter, the smaller the corresponding volume density of the ceramic fiberboard, and the greater the convective heat transfer of the air in the pores, and the greater the influence of the thermal conductivity value of the ceramic fiber board with the increase of temperature. The pores inside the ceramic fiberboard have three forms: continuous pores (open), semi-continuous pores (open and closed) and isolated pores (closed). The thermal conductivity of the isolated pore (closed) structure is the smallest.
3. Volume density
① The thermal conductivity of ceramic fiber decreases with the increase of density, but the decrease gradually decreases, so that when the density exceeds a certain range, the thermal conductivity no longer decreases and tends to increase.
② At different temperatures, there is a minimum thermal conductivity and a corresponding minimum density value. The density corresponding to the minimum thermal conductivity increases with the increase of temperature.
4. Slag ball content
Slag balls are spherical particles that cannot be fiberized in the high-temperature molten liquid during the fiberization process. Slag ball content refers to the percentage of the non-fiberized matter in the refractory ceramic fiber and products after passing through the 75-micron standard sieve hole, and the sieve residue accounts for the total amount of the sample. As the content of slag balls increases, the amount of solid fibers will decrease, and the density of the fibers themselves will decrease. Therefore, the thermal conductivity of fiber products will increase, the thermal insulation performance of fiber products will deteriorate, and the strength and elasticity of fiber products will decrease. The effect of slag ball content on the thermal conductivity of fiber products increases with increasing temperature.
5. Fiber diameter
When the density of ceramic fiber board is the same, the finer the fiber diameter, the smaller the pore size, and the greater the damping effect on heat transfer; secondly, the finer the fiber and the longer the total fiber length, the greater the damping of heat conduction, so the thermal conductivity decreases. On the other hand, the finer the diameter of ceramic fibers, the greater the shrinkage of the heating line of the product, and the lower the heat resistance index. In order to obtain the best comprehensive technical performance, the fiber should have a suitable fineness (diameter), generally 2 to 4 microns.
6. Fiber humidity
The thermal conductivity of water at 0℃ is 0.522w/(m.k), which is more than 20 times greater than the thermal conductivity of air under the same conditions of 0.0247w/(m.k). Therefore, the increase in fiber humidity or moisture content will inevitably increase the thermal conductivity of fiber products. For example, water in the pores of the fiber freezes into ice, because the thermal conductivity of ice under the same conditions is 2.32w/(m.k), which is close to 100 times the thermal conductivity of air under the same conditions. For this reason, for pipeline insulation projects, the humidity of pipeline insulation materials must be controlled at the lowest content, and at the same time, there should be corresponding strict moisture-proof requirements on the outer protective layer materials and structures of the pipeline to ensure the thermal insulation performance of the fiber insulation structure.
7. Use atmosphere
Usually, ceramic fiber boards are used in atmospheric environment, and the gas in the pores is air. Therefore, the role of the gas phase in ceramic fiber products is actually the thermal insulation role of air. However, in some cases, ceramic fiber products are used under vacuum, protective atmosphere or various conditions requiring controlled atmosphere, such as using atmospheres such as hydrogen, carbon monoxide, carbon dioxide, hydrocarbons and inert gases. At this time, the thermal conductivity value of ceramic fibers changes. The thermal conductivity of a gas is related to the composition and structure of the gas. Generally speaking, the smaller the molecular weight of the gas and the simpler the structure, the greater its thermal conductivity.
8. Fiber direction
When the material and volume density are the same, the thermal conductivity when the heat flow direction is perpendicular to the fiber is less than the thermal conductivity when the heat flow direction is parallel to the fiber. Generally, the heat flow direction of the layered structure is close to perpendicular to the fiber direction, and the thermal conductivity of the fiber product is small; while the heat flow direction of the stacked structure is close to parallel to the fiber direction, and the thermal conductivity of the fiber product is large. Under the same conditions of material and volume density, the thermal conductivity of the stacked structure fiber product is 20% to 30% higher than that of the layered structure fiber product.
