Thermal Conductivity Analysis of High Porosity Structures with Open and Closed Pores

Abstract

Developing a robust, cost-effective, super-thermal insulation material with an ultralow thermal conductivity, i.e., below that of stationary air (0.026 W/(mK), 300 K, 1.0 atm), is challenging. A significant number of building blocks (e.g., polymers, oxides, ceramics, and carbon materials) have been explored to fabricate super-thermal insulators with various pore structures (i.e., interconnected “open” pores/channels or sealed “closed” pores, separated by matrix material) over the past decades. Understanding heat transfer in highly porous structures is essential for both geometric design and practical thermal insulation applications. Herein, we adopted two analytical models to analyze the influence of both geometric (e.g., pore size, pore type and porosity) and thermophysical parameters (e.g., solid thermal conductivity, thermal accommodation coefficient, and pressure) on the effective thermal conductivity of porous structures with open and closed pores. Our results indicate that, as porosity increases, the effective thermal conductivity of porous structures, regardless of pore type (open or closed), approaches the thermal conductivity of the gas inside the pores. Effective techniques to diminish the effective thermal conductivity of a porous structure to less than that of stationary air include decreasing gaseous thermal conductivity by implementing nm-scale pores in the materials or by reducing the pressure inside existing pores. Additionally, the thermal conductivity of porous structures with µm-scale pores may also be lower than that of stationary air, if the thermal conductivity of the solid matrix is reduced to much lower than that of the air inside the pores. Decreasing the thermal accommodation coefficient helps to reduce the effective thermal conductivity of porous structure if the nm-scale pores are included. This work is expected to prove helpful in understanding heat transfer in high porosity structures, analyzing experimental measurements, and guiding the design and fabrication of super-thermal insulation materials.