The thermal conductivity of a ceramic fiber board is an intrinsic physical property that characterizes its ability to conduct heat. A lower thermal conductivity indicates a lower heat transfer rate, which means better thermal insulation performance.
Heat transfer occurs through three primary mechanisms: conduction in solids, convection in fluids (gases or liquids), and radiation. The thermal conductivity of a ceramic fiber board is the sum of these three heat transfer modes: conduction through the solid fibers and at their contact points, convection through the air within the pores, and radiation between the pore walls composed of solid fibers. Therefore, it is often referred to as the equivalent thermal conductivity or apparent thermal conductivity.
Several factors influence the thermal conductivity of ceramic fiber boards, including operating temperature, porosity, pore structure, bulk density, shot content, fiber diameter, fiber moisture content, atmosphere, and fiber orientation. A brief analysis of these eight factors is presented below.
The thermal conductivity of ceramic fiber boards generally increases with increasing temperature. This is because the radiative heat transfer between pore walls, convective heat transfer through the pores, and conductive heat transfer within the solid fibers and at their contact points all increase proportionally with temperature due to the enhanced thermal motion of gas and solid molecules. Particularly above 800°C, radiative heat transfer becomes the dominant mode of heat transfer within ceramic fiber boards, and its contribution increases with increasing temperature.
The porosity of a ceramic fiber board is defined as the ratio of the volume of pores to the total volume, expressed as a percentage. High porosity implies that more air is contained within the pores. Since air has a low thermal conductivity (only 0.025 W/m·K at room temperature), it effectively inhibits heat conduction. Ceramic fiber boards are primarily composed of solid fibers and air, with a porosity exceeding 80%. The abundance of low thermal conductivity air within the pores, combined with the disrupted continuous network structure of solid molecules, contributes to their excellent thermal insulation properties. The analysis above indicates that the thermal insulation performance of ceramic fiber boards is primarily attributed to the insulating effect of the air within the pores.
The pore structure and properties significantly influence the convective heat transfer within the ceramic fiber board. Larger pore sizes, often associated with lower bulk density, result in increased convective heat transfer within the pores, and the thermal conductivity of the ceramic fiber board becomes more sensitive to temperature variations. The pores within a ceramic fiber board can be classified into three types: continuous (open), semi-continuous (partially open), and isolated (closed). Ceramic fiber boards with a higher proportion of isolated pores exhibit the lowest thermal conductivity.
Bulk density is defined as the ratio of the weight of a ceramic fiber to its volume. Research has shown a certain correlation between the thermal conductivity of ceramic fibers and their density, as illustrated in Figure 1:
Figure 1: Relationship between thermal conductivity and density of ceramic fibers
From the figure, two trends can be observed: ① As the density increases, the thermal conductivity of ceramic fibers generally decreases, but the rate of decrease gradually diminishes. When the density reaches a certain value, the thermal conductivity may no longer decrease and may even increase. ② At different temperatures, there exists a corresponding minimum thermal conductivity and the associated minimum density. As the temperature increases, the density corresponding to the minimum thermal conductivity also increases.
During the production of ceramic fibers, molten liquid that fails to form fibers forms spherical particles called shots. Shot content is defined as the percentage of non-fibrous material remaining on a 75-micrometer standard sieve relative to the total sample weight. As the shot content increases, the fiber content per unit volume decreases, leading to an increase in the thermal conductivity of the fiber product and a decrease in its insulation performance. Additionally, an increase in shot content can reduce the strength and elasticity of the fiber product. It is worth noting that the impact of shot content on thermal conductivity becomes more pronounced at higher temperatures.
For fiber products with the same density, finer fibers result in smaller pore sizes, which enhances the damping effect on heat conduction. Moreover, finer fibers have a greater total length, further increasing the damping of heat conduction and thus reducing the thermal conductivity. However, excessively fine fibers can lead to increased shrinkage during heating and a decrease in heat resistance. To achieve optimal overall performance, the fiber diameter should be controlled within a suitable range, generally recommended to be between 2 and 4 micrometers.
At 0°C, the thermal conductivity of water is 0.522 W/(m·K), which is more than 20 times that of air at the same temperature (0.0247 W/(m·K)). This means that water conducts heat much more easily than air. When fiber materials absorb water, water molecules fill the internal voids, forming numerous tiny “thermal bridges” that accelerate heat transfer and thus reduce the material’s thermal insulation performance. If water freezes inside the fibers, the situation becomes even worse. Since the thermal conductivity of ice at 0°C is 2.32 W/(m·K), nearly 100 times that of air. Therefore, to ensure good insulation performance, it is essential to strictly control the moisture content of fiber materials and take measures to prevent moisture absorption. Whether for pipe insulation or other insulation applications, attention should be paid to moisture-proofing the materials to ensure their insulation effectiveness.
Generally, the pores in ceramic fiber products are filled with air. Air is an excellent insulating material, so these pores act as small insulating layers, effectively preventing heat transfer. However, in special environments such as vacuum or environments filled with hydrogen, oxygen, or other gases, the thermal insulation performance of ceramic fibers can change. This is because different gases have different thermal conductivities. The smaller and simpler the molecule, the higher the thermal conductivity.
Figure 2: Thermal conductivity formula of ceramic fiber products in gas A
Ceramic fibers exhibit significant anisotropic thermal conductivity, meaning their thermal conductivity varies in different directions. When the heat flow is perpendicular to the fiber direction, heat transfer primarily occurs through the contact points between fibers and collisions of gas molecules within the pores. Due to the relatively small contact area between fibers and the restricted movement of gas molecules in the perpendicular direction, the thermal resistance is higher, resulting in a lower thermal conductivity.
Conversely, when the heat flow is parallel to the fiber direction, heat can be directly transmitted through the interior of the fibers, and gas molecules can move more freely in the parallel direction, leading to lower thermal resistance and higher thermal conductivity.
Generally, in layered ceramic fiber products, the fiber direction is nearly perpendicular to the heat flow direction, resulting in lower thermal conductivity and better thermal insulation performance. In stacked ceramic fiber products, the fiber direction is nearly parallel to the heat flow direction, resulting in higher thermal conductivity and relatively poorer thermal insulation performance.
Experimental data show that under the same material and bulk density conditions, the thermal conductivity of stacked ceramic fiber products is 20% to 30% higher than that of layered structures. This difference is primarily due to the influence of fiber arrangement on the heat transfer path.
By comprehensively considering the above factors, the thermal conductivity of ceramic fiber boards can be effectively controlled, thereby improving their thermal insulation performance and meeting the needs of various applications.