Thermal conductivity investigations of granular and powdered silica aerogels at different temperatures and pressures

This paper describes the experimental determination of thermal conductivity of powdered and granular silica aerogels using the transient hot-strip (THS) method. A vacuum furnace was designed for the THS method for measuring thermal conductivity at different temperatures and pressures. The specific surface area and average mesopore diameter of the two samples were also measured by cryogenic nitrogen adsorption method. The results show that the thermal conductivity of the powdered sample reached a constant only when the pressure was less than 20 Pa, while the thermal conductivity of the granular sample became constant when the pressure was less than 3 Pa due to the macropore effect. The measured samples demonstrated similar patterns in thermal conductivity as gas pressure decreased, with the thermal conductivity first declining slowly when p > 1000 Pa, than with an accelerated decline once p < 1000 Pa. The thermal conductivity of the measured samples distinctly increased with elevation of temperature, indicating that silica aerogel materials with larger macropores exhibit higher thermal conductivity than monolithic silica aerogel materials at elevated temperatures.