Study Of Gaseous Conductivity Variation On Nanoporous Silica Aerogel Thermal Insulation Materials

The aerogel nanoporous super thermal insulation materials have great potential application in aerospace, solar thermal power, phase change energy storage, adsorption catalysis, energy saving, environmental protection and many other areas because of its unique properties, and gaseous thermal conduction is one of the main heat transfer for such materials. Theoretical analysis and experimental verification were taken to analyze the influence factors and variation of mean free path and gaseous thermal conductivity in nanoporous space.Based on the "Transient hot-strip method", a vacuum furnace systems was designed and a thermal measurement test stand was built. The pressure of the vacuum furnace can be adjusted from atmospheric pressure down to 1 Pa, while the available temperature range is from ambient temperature to 900 K. The thermal conductivity of the granular and powdered silica aerogel samples were measured using the THS method at different temperature and pressures. The nitrogen adsorption/desorption isotherms and mesopore size distribution curve were measured by cryogenic nitrogen adsorption method using the automatic surface area and porosity analyzer of a micropore analyzer. The specific surface area and average mesopore diameter were calculated using the BET theory and BJH model separately.Based on the model proposed by Zeng, considering the microstructure characteristics of the materials, used the periodic cubic array of intersecting spherical particles to analyse the influencing factors and mechanism on the mean free path and gaseous thermal conductivity. Based on the microstructure feature of powdered silica aerogel, using an ideal structure with spheres arranged in a spatially periodic structure instead. Different models for thermal conductivity calculation were adopted for the nanopores within the particles and the macropores between the particles, and the coupling effect between different forms of heat transfer was also considered. The total effective thermal conductivity model was build for silica aerogel with two-scale pore size distribution, and compared with the experiment results. The results show that the specific surface area and density are two basic parameters of nanoporous materials, and gas thermal conductivity for materials with higher specific surface area and greater density is lower. The mean free path of gas molecules no longer changes with pressure and gas thermal conductivity is closing to zero when p<104 Pa. The value of mean free path and gas thermal conductivity is closing to that in free space when p>106 Pa. The model results are in good agreement with the experimental value when T<400 K, but lower than the experimental value in high temperature range as the contribution of radiation heat transfer increases.