Theoretical study on thermal conductivities of silica aerogel composite insulating material

This paper shows a complete computing procedure for calculating the thermal conductivities of silica aerogel composite insulating materials by considering the heat conduction and thermal radiation simultaneously. A fractal-intersecting sphere model was proposed for the nano-porous silica aerogel in which the scale effect on gas conduction and solid-matrix conduction were both considered. While for micro-scale composite insulating materials, Rossland approximation and a mixing model were used to account for the multiphase material's radiative thermal conductivity and conductive thermal conductivity respectively. The radiative properties of additives (opacifier particles and fibers), which are needed for calculating the radiative conductivity in Rossland equation, are determined by Mie theory. Based on the theoretical model, the conductivities of silica aerogel with opacifiers and fibers were calculated, and the contributing factors, such as the doping concentration, size (particle diameter or fiber diameter), different opacifiers (carbon(C), carbide (SiC), titanium dioxide (TiO₂)), fiber orientation, and temperature on thermal conductivities were systematically investigated. The results show that the total conductivity decreases first and then increases as the mass fraction of additive increases. The best doped mass fraction of additive that corresponds to the lowest value of total conductivity is found to be the function of temperature. Temperature has very significant effect on radiative heat transfer. As temperature increases, radiative conductivity rises rapidly which leads to the deterioration of insulating performance at high-temperature environment. Fiber diameter and opacifier particle diameter also influence the insulating performance significantly. With small fiber diameter and particle diameter, the extinction effect of additive on radiative heat transfer becomes more significant and the insulating performance becomes better.