Thermal Conductivity of Evacuated Highly Transparent Silica Aerogel

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Abstract
The thermal loss coefficient k of evacuated load bearing transparent silica aerogel tiles is determined for temperatures between 280 K and 400 K. The measurements were performed with the large guarded hot plate high vacuum system LOLA I, the reference plates of which can be pressed onto the samples with an adjustable load (0... 1 bar). For a 22mm layer of aerogel (density ρ ≈ 100...110g/liter) the loss coefficient varied between about k = 0.36 W//m2 K) at 280 K and k = 1.45 W/(m2.K) at 400 K, corresponding to a pseudo-conductivity λ = 8. 10−3 W/(m. K) and λ = 32. 10−3 W/(m. K), respectively. Thus at typical winter temperatures (inside 20 °C, outside −10 °C to 0 °C), evacuated transparent aerogel could provide excellent thermal insulation as a spacer in double pane windows. It also could be used in passive solar systems, for example as a cover for Trombe walls or as a transparent insulating structure around house walls. The dramatic increase of thermal losses at higher temperatures is due to a transmission window for infrared radiation between 3 and 7 μm, Additional data reveal the influence of water adsorption and of air pressure on the thermal conductivity of aerogel.

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