Silica-aerogel granulate – Structural, optical and thermal properties

Understanding the heat transport process is significant for all thermal applications using granular silica aerogels. We studied the influence of gas pressure, external pressure load and temperature on the heat transfer through a packed aerogel bed. For example, an evacuated packed silica aerogel bed (with gas pressure between 10−4 mbar and 10−2 mbar) has a thermal conductivity of about 11 × 10−3 W m−1 K−1 at 50 °C if highly reflective boundaries are employed; the evacuation of an enclosed sample results in an external pressure load of 1 bar. The thermal conductivity of the aerogel granulate was calculated with a packed bed model adapted from reactor physics. Here, the ballistic radiative transport was particularly significant because the silica aerogel granulate transports more heat through radiation than do materials normally examined with this model. The deviations between the experimental data and the theoretical predictions are discussed here. In addition, we investigated the solar transmittance and reflectance of packed beds of various granular silica aerogels. Translucent silica aerogel in a 10 mm thick packed bed has a solar transmittance of 88%, the semi-translucent granulate yields a solar transmittance of 72%. To elucidate the structure/transmittance relationship of these two types of samples, we measured the scattering behavior of individual granules, employing light scattering as well as SAXS and USAXS. The structural information derived from the scattering data and its relationship to the macroscopic optical properties is also discussed here. The cluster size of the semi-translucent material is ∼10 nm and only ∼2 nm in the highly translucent granulate, which is the reason for the high transparency of the granulate.