OPTIMIZATION OF MONOLITHIC SILICA AEROGEL INSULANTS

In this work we systematically investigate thermal transport in opacified monolithic silica aerogels by changing their density and the concentration of infrared opacifier. The goal is to minimize the thermal conductivity of these highly porous inorganic materials. The lowest achieved thermal conductivities at 300 K are about 0.013 W m-1 K-1 for non-evacuated specimens, which have to be compared with values of about 0.020-0.025 W m-1 K-1 for CFC-blown insulating polyurethane foams and with 0.035 W m-1 K-1 for the best fiber insulations. Our investigations allow us to quantitatively determine the gaseous, solid and radiative conductivities lambda(g), lambda(s) and lambda(r), respectively. The derived variations with aerogel density-rho are: lambda(g) is-proportional-to rho--0.6, lambda(s) is-proportional-to rho-1.5 and lambda(r), is-proportional-to (rho . e)-1 in the range 70 < rho/kg m-3 < 230. The total conductivity shows a minimum around 120 kg m-3.