Microscopic revelation of the solid–gas coupling and Knudsen effect on the thermal conductivity of silica aerogel with inter-connected pores

Abstract

As a star insulation material, aerogel plays a significant role in saving energy and meeting temperature requirements in industry due to its extremely low thermal conductivity. The prediction of aerogel’s thermal conductivity is of great interest in both research and industry, particularly because of the difficulty in measuring the separated gas conductivities directly by experiment. Hence, the proportions of separated gas conduction and solid–gas coupling conduction are debatable. In this work, molecular dynamics simulations were performed on porous silica aerogel systems to determine their thermal conductivities directly. The pore size achieved in the present study was improved significantly, making it possible to include the gas phase in the investigation of aerogel thermal conductivity. The separated solid conductivity {lambda }_{s} and the separated gas thermal conductivity {lambda }_{g} as well as the effective solid conductivity {lambda }_{s}^{e} and the effective gas conductivity {lambda }_{g}^{e} were calculated. The results suggest that the solid–gas coupling effect is negligible in rarefied gas because the enhancement of thermal conduction due to the short cut bridging effect by gas between gaps in the solid is limited. The gas pressure is the most significant factor that affects the solid–gas coupling effect. The large differential between the prediction and the actual value of the thermal conductivity is mainly from the underestimate of {lambda }_{g}, and not because of ignoring the coupling effect. As a conclusion, the solid–gas coupling effect can be neglected in the prediction of silica aerogel’s thermal conductivity at low and moderate gas pressure, i.e., decreasing the gas pressure is the most efficient way to suppress the coupling effect. The findings could be used in multi-scale simulations and be beneficial for improving the accuracy of predictions of aerogel thermal conductivity.