A thermal conductivity study of double-pore distributed powdered silica aerogels

Abstract

Powdered nanoporous silica aerogel is a typical double-pore distributed material. This paper develops a total effective thermal conductivity model that considers the coupling effect between different forms of heat transfer for the microstructure features of powdered silica aerogel by using an ideal structure with nanoporous spheres arranged in a spatially periodic structure. Performance of the developed model is compared to experimental results, with the effects of various parameters on thermal conductivity analyzed. The results show that the thermal conductivity of powdered silica aerogel decreased with decreasing gas pressure, and stabilized at its lowest value at pressures below 20Pa. A turning point existed at approximately 103Pa when looking at the logarithmic scale of gas pressure. The thermal conductivity decreased quite slowly with decreasing pressure when p>103Pa, but more quickly when gas pressure was lower than 103Pa. The thermal conductivity reduced with increased specific surface area, but did not change with powder diameter D when the pressure was higher than 103Pa. On the other hand, the thermal conductivity increased with increasing powder diameter D, but did not change with specific surface area when the pressure was less than 103Pa. As the powder diameter D decreased, the lowest stable point of the thermal conductivity increased. The radiation heat transfer contribution was very small when the temperature was less than 400K. With elevating temperature, the thermal conductivity of powdered silica aerogel distinctly increased. Thus, silica aerogel samples with large macro pores achieve greater thermal conductivity at elevated temperatures.