THEORETICAL MODELING OF CARBON CONTENT TO MINIMIZE HEAT-TRANSFER IN SILICA AEROGEL

Silica aerogel has a small absorption coefficient over the range 3-8 m u m where significant thermal energy is transferred by radiation. Addi ng carbon to silica aerogel reduces thermal radiation but increases so lid conduction. Whether the total energy transfer increases or decreas es depends on the carbon content. This paper presents a theoretical me thod for determining the optimal carbon-loading level in silica aeroge l to minimize the energy transfer. This method includes calculation of heat transport by coupled conduction and radiation through aerogel wh ich is optically thin in some spectral ranges and thick in others, and the calculation of solid conductivity and spectral absorption coeffic ient, both of which vary with the carbon content. At ambient temperatu re, about 8% carbon in silica aerogel can lower the total energy trans fer by about 1/3. At temperatures as high as 600 K, non-opacified aero gel has a total energy transfer that is 10 times bigger than that of o pacified aerogel with optimal carbon content. The optimal carbon conte nt that minimizes total energy transfer increases linearly with temper ature.