Silica Aerogel Powder Thermal Conductivity Determination And Analysis

The unique nanoscale skeleton network structure of silica aerogel leads to its characteristics of extremely low density,high specific surface area and high porosity.Silica aerogel apparent thermal conductivity is much lower than that of air,which makes it a new type of lightweight and efficient nanoscale super adiabatic material.The high thermal insulation performance makes silica aerogel a good core material for vacuum insulation panels(VIP)which have a wide application prospect in the field of buildings envelope insulation.In this paper,the micro and nano scale morphologies of silica aerogel powder were characterized by scanning electron microscopy and specific surface area micropore analyser.It was observed that the microstructures of silica aerogel can be considered as tertiary structure:the primary structure is spherical particles from several to scores of nanometers;the secondary structure is particle clusters that are aggregated by primary particles with a scale of about hundreds of nanometers;The third structure is composed of secondary structure particles aggregated into larger particle clusters with a scale of approximately scores of microns.A fractal model was set up based on the silica aerogel tertiary structure.According to this physical model,a multiscale-fractal thermal conductivity mathematical model was proposed to predict silica aerogel thermal conductivity.Different gas thermal conductivity calculation models were considered to describe different scales of pores.By comparing with the experimental data of literatures and theoretical models,it was verified that this model has high accuracy for predicting the total effective thermal conductivity of the silica aerogel.The paper has also studied the influence law and mechanism of pressure,density,specific surface area on the total effective thermal conductivity.The results indicated that the effective thermal conductivity of aerogel declines sharply while pressure decreases from atmospheric pressure.This is due to the fact that as the pressure decreases,the mean free path of the gas molecules increases,and the free movement of molecules is greatly restrained in pores,which leads to a significant reduction in the gas thermal conductivity.When the pressure decreases to about 10~3Pa,the mean free path of gas molecules is similar to or even larger than the aerogel pore-scale.The gas molecules are already severely bound by the pore space at about10~3Pa and the reduction of pressure cannot significantly enhance the inhibition of the free movement of gas molecules anymore with pressure decreases continually,therefore the silica aerogel thermal conductivity decrease becomes slowly.At this condition,the heat transfer in silica aerogel is mainly by solid heat conduction and radiation.The specific surface area and density of materials are two basic parameters to characterize aerogel structure.Density increase means porosity decrease,which indicates the contribution of solid thermal conductivity increases while that of gas thermal conductivity decreases.The solid thermal conductivity accounts for a large proportion when the density is large,and the gas thermal conductivity is significant when the density is small.Results show that there exists an optimal aerogel density at which the silica aerogel keeps the minimum thermal conductivity.When the density is constant,increasing the specific surface area of aerogel can improve its thermal insulation performance.but the The reason is that particle size and pore scale decreases with specific surface area increases at given density,,which enhances,the pores restriction on the free movement of gas molecules and reduce the gas thermal conductivity.As a result,the total effective thermal conductivity of aerogel decreases.