Pore structure of nano-porous thermal insulating materials and thermal transport via gas phase in their pores

The thermal insulation properties of nano-porous thermal insulating materials largely depend on thermal transport via gas phase within their pores, and this process relies on their pore structures. Therefore, investigating pore structures and thermal transport via gas phase is important to understand the heat transfer mechanism. Current research mainly focuses on the theoretical calculation and analysis from the perspective of heat transfer, and special and systematic studies based on actual materials have not been reported yet. In addition, accurate analysis of pore structures using usual techniques is difficult due to the complex pore network and the poor mechanical properties of their solid skeleton. In this study, nano-porous thermal insulating materials with different pore structures were synthesized via a sol-gel process followed by supercritical drying. The materials were then characterized by thermal conductivity tester, nitrogen adsorption-desorption, and helium pycnometer. The pore structures of the resulting materials were obtained, and the relationship between pore structures and thermal transport via gas phase was studied. Results show that the bimodal distribution of pores in the resulting materials, corresponding to gas-contributed thermal conductivity. All pores within the resulting materials can be equivalent to pores with a single diameter when the equivalent size of large pores is 10 times less than that of small pores. Similar to the pure gaseous thermal conductivity, the intrinsic gas-contributed thermal conductivity including gas-solid coupling effects rises with increasing pore diameter of the materials. The ratio of intrinsic gas-contributed thermal conductivity to pure gaseous thermal conductivity is 2.0, 1.5, and 2.0-1.5 for pores smaller than 200 nm, larger than 500 nm, and with size between 200 and 500 nm, respectively. When the equivalent size of large pores is 10 times less than that of small pores or when the equivalent size of large pores is 100-1000 times that of small pores and the contribution of large pores to the total porosity is less than 10%, the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into three stages (steep decreasing stage, slow decreasing stage, and hardly changing stage) according to decreasing rate. When the equivalent size of large pores is 3000 times larger than that of small pores, the gas-contributed thermal conductivity reduction of the resulting material with decreasing gas pressure can be divided into four stages (steep decreasing stage, slow decreasing stage, steep decreasing stage, and hardly changing stage) even if the contribution of large pores to the total porosity is very low (less than 10%).